The exact mechanism of IA reactions is not known but is thought to be a combination of systemic effects. IA reactions have not been shown to result from a true IgE antibody immunologic reaction to the contrast media (Dawson, 1999). At least four mechanisms may play a role in IA reactions: (1) release of vasoactive substances including histamine; (2) activation of physiologic cascades including complement, kinin, coagulation, and fibrinolytic systems; (3) inhibition of enzymes including cholinesterase, which may cause prolonged vagal stimulation; and (4) the patient’s own anxiety and fear of the actual procedure. IA reactions are not dose dependent. Severe reactions have been reported after only 1 mL injected at the beginning of the procedure and have also occurred after completion of a full dose despite no reaction to the initial test dose (Nelson et al, 1988; Thomsen et al, 1999; ACR, 2008).

Nonidiosyncratic (NI) reactions are dose dependent and consequently related to the osmolality, concentration, volume, and injection rate of the IRCM. Because the concentration of absorbed or free iodine is low, only patients with an underlying iodine deficiency are at risk for increased intake of iodine during contrast imaging. Patients with endemic goiter may develop thyrotoxicosis after injection of IRCM agents.

The hyperosmolar contrast media (HOCM) have an osmolality that is five times greater than physiologic osmolality of body cells (300 mOsm/kg water). The hyperosmolar agents are associated with erythrocyte damage, endothelial damage, vasodilation, hypervolemia, interruption of the blood-brain barrier, and cardiac depression. Chemotoxic reactions to IRCM include cardiac, vascular, neurologic, and renal toxicity. The low osmotic contrast media (LOCM) have an osmolality the same as or slightly higher than physiologic osmolality and are associated with fewer ADRs and toxic events (Morcos, 1999).

Contrast Complications

All contrast ADRs are more common with HOCM (12%) compared with LOCM (3%) (Katayama et al, 1990; Heinrich et al, 2005). Previous adverse reaction to IRCM and a history of asthma are two of the most concerning predisposing conditions predicating an ADR with use of contrast media. Additional factors that may predict an ADR include history of known allergy to iodine, severe cardiac disease, renal insufficiency, dehydration, sickle cell anemia, anxiety, apprehension, hyperthyroidism, and presence of adrenal pheochromocytoma.

Minor Reactions

It is common to have nausea, flushing, pruritus, urticaria, headache, and even emesis after injection of IRCM. These reactions are usually mild and do not require additional treatment. Arm pain can also result from venous spasm or infiltration of the media outside the vein. An H₁ receptor blocker like diphenhydramine (Benadryl) PO/IM/IV 1 to 2 mg/kg, up to 50 mg may be helpful in this patient population.

Intermediate Reactions

Worsening or more severe minor reactions including hypotension or bronchospasm occur in 0.5% to 2% of patients. These are usually transient and do not need treatment. If necessary, 4 to 10 mg of chlorphenamine administered orally, IV, or IM; 5 mg of diazepam for anxiety; 100 to 500 mg of hydrocortisone IM or IV; or two to three puffs of beta-agonist inhalators for bronchospasm (bronchiolar dilators such as metaproterenol [Alupent], terbutaline [Brethaire], or albuterol [Proventil or Ventolin]) are administered; repeat as needed.

Severe Reactions

Life-threatening reactions occur in approximately 1/1000 uses for high-osmolar agents and are far less for low-osmolar contrast media, with both types of agents resulting in mortality rates of 1/170,000 uses (Spring et al, 1997). Severe reactions include seizure, laryngeal spasm, bronchospasm, pulmonary edema, cardiac arrhythmia, respiratory collapse, or cardiac arrest (Katayama et al, 1990). Ready access to a crash cart and trained personnel are necessary components of any imaging center. If needed, cardiopulmonary resuscitation is started immediately. Many different medications have been used for the treatment of an acute severe life-threatening reaction to IRCM. However, rapid administration of epinephrine is the treatment of choice for severe contrast reactions. Current guidelines recommend immediate delivery of 0.01 mg/kg of body weight to a maximum of 0.5 mg of 1 : 1000 concentration of epinephrine, injected IM in the lateral thigh as first-line treatment. Subcutaneous injection is much less effective. Intravenous injection can be given with more dilute concentration and must be given slowly (ACR, 2008; Lightfoot et al, 2009).

Large-volume extravasation can be seen with power injections not monitored with electrical skin impedance devices that detect extravasation and arrest the injection process. When large-volume extravasation of IRCM occurs, the result can be cellulitis, edema, and compartment syndrome leading to tissue necrosis. The most severe consequences may not be manifest immediately. Consequently, admission to the hospital for observation or frequent follow-up in the clinic may be necessary in some cases of large-volume extravasation.

Premedication Strategies

No known premedication strategy will eliminate the risk of a severe adverse reaction to IRCM. The regimens suggested in the literature include the use of corticosteroids, antihistamines, H₁ and H₂ antagonists, and ephedrine. Patients at high risk should be premedicated with corticosteroids and possibly antihistamines 12 to 24 hours before and after use of IRCM. LOCM can be used in these patients. Several premedication regimens have been proposed to reduce the frequency and/or severity of reactions to contrast media. Two frequently used regimens are outlined in Table 4–4.

Table 4–4 Premedication Strategies to Reduce Severity of Reactions to Contrast Media

Prednisone	50 mg by mouth at 13 hr, 7 hr, and 1 hr before contrast media injection
	Plus diphenhydramine (Benadryl)—50 mg IV, IM, or by mouth 1 hr before contrast medium injection
Methylprednisolone (Medrol)	32 mg by mouth 12 hr and 2 hr before contrast media injection
	Plus diphenhydramine (Benadryl)—50 mg IV, IM, or by mouth 1 hr before contrast medium injection

From American College of Radiology Manual on Contrast Media, version 6, pp. 9-10, <http://www.acr.org/SecondaryMainMenuCategories/quality_safety/contrast_manual.aspx> [accessed 01.10.10].

It has been demonstrated that the use of nonionic contrast media combined with a premedication strategy including corticosteroids results in a reduction in reaction rates compared with other protocols for patients who have experienced a prior contrast media–induced reaction. However, no controlled studies are available to determine whether pretreatment alters the incidence of serious reactions. Oral administration of steroids seems preferable to intravascular administration, and prednisone and methylprednisolone are equally effective. If the patient is unable to take oral medication, 200 mg of hydrocortisone IV may be substituted for oral prednisone. One consistent finding is that the steroids should be given at least 6 hours before the injection of contrast media regardless of the route of steroid administration. It is clear that administration for 3 hours or less before contrast does not decrease adverse reactions (Lasser, 1988).

Supplemental administration of an H₁ antihistamine (e.g., diphenhydramine), orally or IV, may reduce the frequency of urticaria, angioedema, and respiratory symptoms. In emergency situations, intravenous corticosteroid (e.g., 200 mg hydrocortisone) every 4 hours plus an H₁ antihistamine (e.g., 50 mg diphenhydramine) 1 hour before the procedure has been used. In patients who have a prior documented contrast reaction, the use of a different contrast agent has been advocated and may be protective. Switching to a different agent should be in combination with a premedication regimen.

Specific Clinical Considerations

Metformin

Patients with type II diabetes mellitus on metformin oral biguanide hyperglycemic therapy may have an accumulation of the drug after administering IRCM, resulting in biguanide lactic acidosis presenting with vomiting, diarrhea, and somnolence. This condition is fatal in approximately 50% of cases (Wiholm, 1993). Biguanide lactic acidosis is rare in patients with normal renal function. Consequently, in patients with normal renal function and no known comorbidities there is no need to discontinue metformin before IRCM use, nor is there a need to check creatinine following the imaging study. However, in patients with renal insufficiency, metformin should be discontinued the day of the study and withheld for 48 hours. Postprocedure creatinine should be measured at 48 hours and metformin started once kidney function is normal (Bailey and Turner, 1996). It is not necessary to discontinue metformin before gadolinium-enhanced magnetic resonance studies when the amount of gadolinium administered is in the usual dose range of 0.1 to 0.3 mmol per kg of body weight.

Contrast-Induced Nephropathy

Contrast-induced nephropathy (CIN) is defined as a rise in serum creatinine 25% above baseline, or more than 0.5 mg/dL within 3 days following exposure to contrast media, in the absence of an alternative cause. The precise cause of CIN continues to elude investigators but is believed to be a combination of tubular injury and renal ischemia (Katholi et al, 1998). High doses of IRCM can impair renal function in some patients for 3 to 5 days. CIN in patients with normal kidney function is rare (Pannu et al, 2006; Kelly et al, 2008). CIN is the third most common cause of acute kidney failure in the hospitalized patients (Nash et al, 2002). The most common patient-related risk factors are chronic kidney disease (creatinine clearance <60 mL/min), diabetes mellitus, dehydration, congestive heart failure, age, hypertension, low hematocrit, and ventricular ejection fraction less than 40%. The patients at highest risk for developing CIN are those with both diabetes and preexisting renal insufficiency. The most common non–patient-related causes are high-osmolar contrast agents, ionic contrast, increased contrast viscosity, and large-contrast volume infusion (Pannu et al, 2006).

Despite significant discussion on the part of radiologists and urologists, the literature does not support an absolute serum creatinine level that prohibits the use of contrast media. Prevention of CIN has been the subject of many research studies, and the results have been summarized by several different meta-analyses. In these meta-analyses the baseline serum creatinine of study participants ranged from 0.9 to 2.5 mg/dL. In one survey the policies regarding the cutoff value for serum creatinine varied widely among radiology practices. Thirty-five percent of respondents used 1.5 mg/dL, 27% used 1.7 mg/dL, and 31% used 2.0 mg/dL (mean, 1.78 mg/dL) as a cutoff value in patients with no risk factors other than elevated creatinine; threshold values were slightly lower in diabetics (mean 1.68 mg/dL). Patients in end-stage renal disease who have no remaining natural renal function are no longer at risk for CIN and may receive LOCM or IOCM (Elicker et al, 2006).

The summary of the meta-analysis for the prevention of CIN after contrast media use supports using hydration, bicarbonate, iso- or low-osmolar contrast media, and N-acetylcysteine. In one review article, N-acetylcysteine was determined to be more protective than hydration alone. Furosemide was found to increase the risk of developing CIN (Pannu et al, 2006; Kelly et al, 2008).

N-acetylcysteine is inexpensive, readily available, administered orally, and associated with few drug interactions or side effects. Its mechanism of protection against CIN is not understood but may serve as a scavenger of oxygen-free radicals and/or augment the vasodilatory effects of nitric oxide in the kidney (Safirstein et al, 2000). Doses used in the different studies ranged from 600 to 1200 mg orally twice a day for 2 doses before the contrast-enhanced study and 2 doses after the procedure (Fig. 4–2).

Figure 4–2 Preventive strategies for patient with risk factors for contrast-induced nephropathy (CIN).

Magnetic Resonance Imaging Contrast Agents

Because magnetic resonance imaging (MRI) offers previously unseen detailed soft tissue imaging compared with CT, it was initially believed that MRI would not require contrast enhancement. However, by 2005, almost 50% of MRI studies were being performed with contrast media. Extracellular MRI contrast agents contain paramagnetic metal ions. Copper, manganese, and gadolinium were the potential paramagnetic ions for use with MRI. Gadolinium, however, is the most powerful having seven unpaired electrons, but its toxicity required encapsulation by a chelate. Paramagnetic agents like gadolinium are positive enhancers reducing the T1 and T2 relaxation times and increasing tissue signal intensity on T1-weighted images, while having little effect on T2-weighted images.

Although gadolinium chelates can be differentiated on the basis of stability, viscosity, and osmolality, they cannot be differentiated on the basis of efficacy. Acute adverse reactions are encountered less frequently with gadolinium than after administration of iodinated contrast media. The frequency of all acute adverse events after an injection of 0.1 or 0.2 mmol/kg of gadolinium chelate ranges from 0.07% to 2.4%. The vast majority of these reactions are mild including coldness at the injection site, nausea, emesis, headache, warmth or pain at the injection site, paresthesias, dizziness, and itching. Reactions resembling an “allergic” response occur frequently, from 0.004% to 0.7%. Systemic reactions consisting of rash, hives, or urticaria are the most frequent of this group, and rarely bronchospasm. The severe, life-threatening anaphylactoid or nonallergic anaphylactic reactions are exceedingly rare (0.0001% to 0.001%). In a meta-analysis of 687,000 gadolinium doses for MRI, there were only five severe reactions. In another survey based on 20 million administered doses there were 55 cases (0.0003%) of severe reactions. Fatal reactions to gadolinium chelate agents have been reported, but they are extremely rare events (Murphy et al, 1999).

Extracellular MRI agents are known to interfere with some serum chemistry assays. For example, serum calcium tests will often be measured as a false reading of hypocalcemia for 24 hours after MRI with gadolinium enhancement, even though serum calcium is actually in the normal range. Other tests including iron, magnesium, iron binding capacity, and zinc may also have spurious results. Biochemical assessment is more reliable when performed 24 hours after exposure to gadolinium contrast media.

Nephrogenic Systemic Fibrosis

Gadolinium-based contrast media (GBCM) are considered to have no nephrotoxicity at approved dosages for MRI. Magnetic resonance with gadolinium has been used instead of contrast-enhanced CT in those at risk for developing worsening renal failure if exposed to iodinated contrast media. However, nephrogenic systemic fibrosis (NSF) has recently been reported in patients with advanced renal insufficiency (glomerular filtration rate [GFR] <30 mL/min). Nephrogenic systemic fibrosis (NSF) is a fibrosing disease of the skin, subcutaneous tissues, lungs, esophagus, heart, and skeletal muscles. Initial symptoms typically include skin thickening and/or pruritus. Symptoms and signs may develop and progress rapidly, with some affected patients developing contractures and joint immobility within days of exposure. Death may result in some patients, presumably as a result of visceral organ involvement.

In 1997 NSF was described in dialysis patients who had not been exposed to GBCM. The condition was previously known as nephrogenic fibrosing dermopathy. In 2006 and again in 2007 independent reports surfaced defining a strong association with gadolinium-based contrast media (Grobner, 2006; Marckmann et al, 2006). Onset of NSF varies between 2 days and 3 months. Early manifestations include subacute swelling of distal extremities, followed by severe skin induration and later even organ involvement. In a 2007 survey performed by the American College of Radiology, 156 cases of NSF were reported by 27 responding institutions; 140 of these 156 patients were known to have received GBCM. In 78 patients, the specific GBCM was known. Forty-five of them received gadodiamide, 17 gadopentetate dimeglumine, 13 gadoversetamide, and 3 gadobenate dimeglumine. NSF following gadoteridol administration has also been reported. Many of the cases in which agents other than gadodiamide and gadopentetate dimeglumine were used are confounded by the fact that affected patients were injected with other agents as well (ACR, 2008). Patients with chronic kidney disease (CKD) have a 1% to 7% chance of developing NSF after MRI imaging with gadolinium agents (Todd et al, 2007).

Patients with GFR less than 30 mL/min/1.73m² (not on chronic dialysis) are the most difficult patient population in terms of choosing an imaging modality. They are at risk for CIN if exposed to iodinated contrast media for CT imaging and are also at significant risk of developing NSF if exposed to GBCM during MRI. Recent data suggest that the risk of NSF may be greatest in patients with a GFR of less than 15 mL/min/1.73m^² and much less in patients with GFRs that are higher. Patients with severe chronic kidney disease have a 1% to 7% chance of developing NSF after GBCM MRI (Kanal et al, 2008). In the chronic kidney disease patient population, it is recommended that contrast media be avoided if possible. If MRI contrast media is absolutely essential, use of the lowest possible doses (needed to obtain a diagnostic study) of selected GBCM is recommended. In this setting, the patient should be informed of the risks of GBCM administration and must give their consent to proceed. There is no proof that any GBCM is completely safe in this patient group; however, some have suggested avoiding gadodiamide and considering use of macrocyclic agents (Kanal et al, 2008). Patients with CKD but GRF greater than 30 mL/min/1.73m^² are considered to be at extremely low or no risk for developing NSF if a dose of GBCM of 0.1 mmol/kg or less is used. Patients with GFR greater than 60 mL/min/1.73 m^² do not appear to be at increased risk of developing NSF, and the current consensus is that all GBCM can be administered safely to these patients. In their publications, the American College of Radiology stresses that the current information on NSF and its relationship to GBCM administration is preliminary and further research is necessary to better understand this potentially devastating complication.

Key Points

Contrast Media

• Type II diabetics with renal insufficiency on oral metformin biguanide hyperglycemic therapy are at risk for developing biguanide lactic acidosis after exposure to intravascular radiologic contrast media and should stop metformin the day before the procedure and restart 48 hours after if they have a normal or baseline serum creatinine.

• Patients at risk for adverse reaction to contrast include those with previous adverse reactions, a history of asthma, severe cardiac disease, renal insufficiency, dehydration, sickle cell anemia, anxiety, apprehension, hyperthyroidism, and presence of adrenal pheochromocytoma.

• The treatment of choice for a severe adverse drug reaction is 0.01 mg/kg of epinephrine (1:1000 concentration) IM in the lateral thigh.

• Patients at greatest risk for contrast-induced nephropathy are those with diabetes mellitus and dehydration.

• Steroids given to prevent adverse contrast agents should be given at least 6 hours before injection.

Intravenous Urography

Once the mainstay of urologic imaging, the intravenous excretory urographic (IVU) study has essentially been replaced by CT and MRI. With the ability of new scanners to perform axial, sagittal, and coronal reconstruction of the upper tract urinary system, essentially all of the data and information obtained by traditional IVU can be realized with CT imaging. In addition, some parenchymal defects, cysts, and tumors can be better delineated with CT than IVU.

Technique

Bowel prep may help to visualize the entire ureters and upper collecting systems. Patients with chronic constipation may benefit most from complete bowel prep with clear liquids for 12 to 24 hours and an enema 2 hours before the procedure. Most patients will need only clear liquids for 12 hours and an enema 1 hour before the procedure.

Before injection of contrast, a scout radiograph or KUB (kidneys, ureters, bladder) film is taken demonstrating the top of the kidneys and the entire pelvis to the pubic symphysis. This allows determination of adequate bowel prep, confirms correct positioning, and exposes kidney stones or bladder stones.

Contrast is injected as a bolus of 50 to 100 mL of contrast. The nephrogenic phase is captured with a radiograph immediately after injection. In the past, tomograms were used to look for parenchymal defects but now CT or MRI is preferred. A film is taken at 5 minutes and then additional films at 5-minute intervals until the question that prompted the IVU is answered. Abdominal compression may be used to better visualize the ureters. Occasionally oblique films will be used to better define the course of the ureter in the bony pelvis and to precisely differentiate ureteral stones from pelvic calcifications.

Upright films may be helpful in certain situations. In the rare case of suspected symptomatic renal ptosis, IVU can be particularly helpful (Fig. 4–3). Supine films are compared with upright films to measure the degree of ptosis. Such a comparison cannot be made with MRI or CT. In the case of calyceal stones or milk of calcium stones, layering of the contrast can be helpful to evaluate the anatomy of the calyx harboring the stones.

Figure 4–3 Intravenous excretory urogram (IVU) in a 40-year-old female with the complaint of a mobile mass in the right lower quadrant with standing associated with bilateral flank and back pain that resolved in the supine position. A, Supine IVU shows kidneys in the normal position, with normal ureters and proximal collecting systems. B, Standing film shows significant displacement of both kidneys with the right kidney moving onto the pelvis as described by the patient.

Postvoid films are obtained to evaluate the presence of outlet obstruction, prostate enlargement, and bladder filling defects including stones and urothelial cancers.

Plain Abdominal Radiography

The plain abdominal radiograph is a conventional radiography study, which, in urology, is intended to display the kidneys, ureters and bladder. The plain abdominal radiograph may be employed (1) as a primary study or (2) as a scout film in anticipation of contrast media. Plain films are widely used in the management of renal calculus disease. Plain radiography is also useful in evaluation of the trauma patient because it can be performed as a portable study in the trauma unit. Secondary findings on plain radiography such as rib fractures, fractures of the transverse processes of the vertebral bodies, and pelvic fractures may indicate serious associated urologic injuries.

Technique

An abdominal plain radiograph is obtained with the patient in the supine position, using an anterior to posterior exposure. The study typically includes that portion of the anatomy from the level of the diaphragm to the inferior pubic symphysis. It may occasionally be necessary to make two exposures to cover the desired anatomic field. Depending on the indication for the study, oblique films are obtained to clarify the position of structures in relation to the urinary tract. If small bowel obstruction or free peritoneal air is suspected, upright films will be obtained.

Limitations

Although plain film radiography is often used in the evaluation of renal colic, it is unreliable in the demonstration of calculus disease for various reasons: (1) overlying stool and bowel gas may obscure small calculi; (2) stones may be obscured by other structures such as bones or ribs (Fig. 4–4); (3) calcifications in pelvic veins or vascular structures may be confused with ureteral calculi; and (4) stones that are poorly calcified or composed of uric acid may be radiolucent. Nevertheless, plain film radiography is valuable in assessing the suitability of a patient for extracorporeal shock wave lithotripsy because the ability to identify the stone on fluoroscopy is critical to targeting. Furthermore, a KUB film is cost-effective for monitoring residual stone burden after treatment (Fig. 4–5). For complex pathology of the urinary tract, plain abdominal radiography has been supplanted by axial imaging. Plain radiography has a limited role in evaluating soft tissue abnormalities of the urinary tract.

Figure 4–4 A, Right ureteral calculus (arrow) overlying the sacrum is difficult to visualize on the plain film. B, The right posterior oblique study fails to confirm the location of the ureteral calculus. C, Computed tomography confirms this 6-mm calculus in the right ureter at the level of the third sacral segment.

Figure 4–5 Kidney ureter bladder film demonstrating residual stone fragments (arrows) adjacent to a right ureteral stent 1 week following right extracorporeal shock wave lithotripsy.

Retrograde Pyelography

Retrograde pyelograms are performed to opacify the ureters and intrarenal collecting system by the retrograde injection of contrast media. Any contrast media that can be used for excretory urography is also acceptable for retrograde pyelography. Attempts should be made to sterilize the urine before retrograde pyelography because there is a risk of introducing bacteria into the upper urinary tracts or into the bloodstream. Although many studies are able to document the presence or absence of dilation of the ureter, retrograde pyelography has the unique ability to document the normalcy of the ureter distal to the level of obstruction and to better define the extent of the ureteral abnormality.

Technique

Retrograde pyelography is usually performed with the patient in the dorsal lithotomy position. An abdominal plain radiograph (scout film) is obtained to ensure that the patient is in the appropriate position to evaluate the entire ureter and intrarenal collecting system. Cystoscopy is performed and the ureteral orifice is identified.

Contrast may be injected through either a nonobstructing catheter or an obstructing catheter. Nonobstructing catheters include whistle tip, spiral tip, or open-ended catheters. Use of nonobstructing catheters allows passage of the catheter into the ureter and up to the collecting system, over a guidewire if necessary. Contrast can then be introduced directly into the upper collecting system, and the ureters can be visualized by injection of contrast as the catheter is withdrawn.

The other commonly employed method is the use of an obstructing ureteral catheter such as a bulb-tip, cone-tip, or wedge-tip catheter. These catheters are inserted into the ureteral orifice and then pulled back against the orifice to effectively obstruct the ureter. Contrast is then injected to opacify the ureter and intrarenal collecting system. Depending on the indication for the study, it is useful to dilute the contrast material to 50% or less with sterile fluid. This prevents subtle filling defects in the collecting system or ureter from being obscured. Contrast is injected slowly, usually requiring from 5 to 8 mL to completely opacify the ureter and intrarenal collecting system in adults (Fig. 4–6). More or less contrast may be required depending on the size of the patient and the capaciousness of the collecting system. Limited use of fluoroscopy while injecting will help prevent overdistension of the collecting system and reduce the risk of extravasation of contrast. Care should be taken to evacuate air bubbles from the syringe and catheter before injection. Such air bubble artifacts could be mistaken for stones or tumors.

Figure 4–6 A, Right retrograde pyelogram performed using an 8-Fr cone-tipped ureteral catheter and dilute contrast material. The ureter and intrarenal collecting system are normal. B, Left retrograde pyelogram using an 8-Fr cone-tipped ureteral catheter. A filling defect in the left distal ureter (arrow) is a low-grade transitional cell carcinoma. The ureter demonstrates dilation, elongation, and tortuosity, the hallmarks of chronic obstruction.

Historically, when a retrograde pyelogram consisted of a series of radiographs taken at intervals, it was important to document various stages of filling and emptying of the ureter and collecting systems. Because of peristalsis the entire ureter will often not be seen on any given static exposure or view. With current equipment including tables that incorporate fluoroscopy, it is possible to evaluate the ureter during peristalsis in real time, thus reducing the need for static image documentation. Documentary still images or “spot films” may be saved for future comparison. Urologists interpret retrograde pyelograms in real time as they are performed.

Limitations

Retrograde pyelography may be difficult in cases where the bladder is involved with diffuse inflammation or neoplastic changes, especially when bleeding is present. Identification of the ureteral orifices may be assisted by the intravenous injection of indigotindisulfonate sodium or methylene blue in such cases. Changes associated with bladder outlet obstruction may result in angulation of the intramural ureters. This may make cannulation with an obstructing catheter quite difficult. Attempts to cannulate the ureteral orifice may result in trauma to the ureteral orifice and extravasation of contrast material into the bladder wall. The potential for damage to the intramural ureter must be weighed against the potential information to be obtained by the retrograde pyelogram.

Complications

Backflow occurs during retrograde pyelography when contrast is injected under pressure and escapes the collecting system. Contrast may escape the collecting system by one of four routes: Pyelotubular backflow occurs when contrast fills the distal collecting ducts producing opacification of the medullary pyramids (Fig. 4–7A). Pyelosinus backflow occurs when a tear in the calyces at the fornix allows contrast to leak into the renal sinus (Fig. 4–7B). Pyelolymphatic backflow is characterized by opacification of the renal lymphatic channels (Fig. 4–7C). Pyelovenous backflow is seen when contrast enters the venous system, resulting in visualization of the renal vein.

Figure 4–7 Patterns of backflow during retrograde pyelography. A, Pyelotubular backflow. B, Pyelosinus backflow. C, Pyelolymphatic backflow.

Although backflow does not usually cause measurable clinical harm, the potential implications of backflow include (1) introduction of bacteria from infected urine into the vascular system and (2) the absorption of contrast media, which could result in adverse reactions in susceptible patients. It has been demonstrated that the risk of significant urinary tract infection is only about 10% and the risk of sepsis is low when antibiotic prophylaxis therapy is administered before endoscopic procedures (including retrograde pyelography) (Christiano et al, 2000). Although contrast reactions are rare with retrograde pyelography, they have been reported (Johenning, 1980; Weese et al, 1993). In patients with documented severe contrast allergy, prophylactic pretreatment may be appropriate. In those patients considered at risk, care should be taken to inject under low pressures to minimize the probability of backflow and absorption of the contrast into the vasculature system.

Loopography

Loopography is a diagnostic procedure performed in patients who have undergone urinary diversion. Historically the term “loopogram” has been associated with ileal conduit diversion but may be used in reference to any bowel segment serving as a urinary conduit. When imaging patients with a continent diversion involving a reservoir or neobladder, “pouch-o-gram” would be more accurately descriptive. Because an ileal conduit urinary diversion usually has freely refluxing ureterointestinal anastomoses, the ureters and upper collecting systems may be visualized. In other forms of diversion, the ureterointestinal anastomoses may be purposely nonrefluxing. In such circumstances, when opacification of the upper urinary tract is desirable, antegrade ureteral imaging such as IVU, CT, or MRI urography or antegrade nephrostography may be required. When the patient has compromised renal function or is allergic to iodinated contrast material, loopography can be performed with a low risk of systemic absorption (Hudsen et al, 1981).

Technique

The patient is positioned supine. An abdominal plain radiograph is obtained before the introduction of contrast material (Fig. 4–8A). A commonly employed technique is to insert a small-gauge catheter into the ostomy of the loop, advancing it just proximal to the abdominal wall fascia. The balloon on such a catheter can then be inflated to 5 to 10 mL with sterile water. By gently introducing contrast through the catheter, the loop can be distended, usually producing bilateral reflux into the upper tracts. Oblique films should be obtained in order to evaluate the entire length of the loop (Fig. 4–8B). Because of the angle at which many loops are constructed, a traditional anteroposterior (AP) view will often show a foreshortened loop and could miss a substantial pathology. A drain film should be obtained (Fig. 4–8C). This may demonstrate whether there is obstruction of the conduit.

Figure 4–8 Loopography in a patient with epispadias/exstrophy and ileal conduit urinary diversion. The plain film (A) shows wide diastasis of the pubic symphysis. After contrast administration via a catheter placed in the ileal conduit, free reflux of both ureterointestinal anastomoses is demonstrated (B). A postdrain radiograph (C) demonstrates persistent dilation of the proximal loop indicating mechanical obstruction of the conduit (arrows).

Retrograde Urethrography

A retrograde urethrogram is a study meant to evaluate the anterior and posterior urethra. Retrograde urethrography may be particularly beneficial in demonstrating the total length of a urethral stricture, which cannot be negotiated by cystoscopy. Retrograde urethrography also demonstrates the anatomy of the urethra distal to a stricture, which may not be assessable by voiding cystourethrography. Retrograde urethrography may be performed in the office or in the operating room before performing visual internal urethrotomy or formal urethroplasty.

Technique

A plain film radiograph is obtained before injection of contrast. The patient is usually positioned slightly obliquely to allow evaluation of the full length of urethra. The penis is placed on slight tension. A small catheter may be inserted into the fossa navicularis with the balloon inflated to 2 mL with sterile water. Contrast is then introduced via catheter-tipped syringe. Alternatively, a penile clamp (e.g., Brodney clamp) may be used to occlude the urethra around the catheter (Fig. 4–9).

Figure 4–9 Normal retrograde urethrogram demonstrating (A) the balloon technique for retrograde urethrography; (B) Brodney clamp (arrowhead) technique; note the bulbar urethral stricture (arrow). C, Normal structures of the male urethra as seen on retrograde urethrogram.

Indications

1. Evaluation of ureteral stricture disease

2. Assessment for foreign bodies

3. Evaluation of penile or urethral penetrating trauma

4. Evaluation of traumatic gross hematuria

Static Cystography

Static cystography is employed primarily to evaluate the structural integrity of the bladder. The shape and contour of the bladder may give information about neurogenic dysfunction or bladder outlet obstruction. Filling defects such as tumors and stones may be appreciated.

Technique

The patient is positioned supine. A plain radiograph is performed to evaluate for stones and residual contrast and to confirm position and technique. The bladder is filled with 200 to 400 mL of contrast depending on bladder size and patient comfort. Adequate filling is important to demonstrate intravesical pathology or bladder rupture. Oblique films should be obtained because posterior diverticula or fistulae may be obscured by the full bladder. A postdrainage film completes the study (Fig. 4–10).

Figure 4–10 The patient has undergone radical retropubic prostatectomy. A, During bladder filling, contrast is seen adjacent to the vesicoureteral anastomoses (arrow). B, The postdrain film clearly demonstrates a collection of extravasated contrast (arrow).

Limitations

Abdominal and pelvic CT is so commonly used in the evaluation of blunt or penetrating trauma to the abdomen that CT cystography is often performed in conjunction with the trauma evaluation. However, studies have shown that conventional static cystography is as sensitive as CT cystography in detecting bladder rupture (Quagliano et al, 2006; Broghammer and Wessells, 2008).

Voiding Cystourethrogram

A voiding cystourethrogram (VCUG) is performed to evaluate the anatomy and physiology of the bladder and urethra. The study provides valuable information regarding the posterior urethra in pediatric patients. VCUG has long been used to demonstrate vesicoureteral reflux.

Technique

The study may be performed with the patient supine or in a semiupright position using a table capable of bringing the patient into the full upright position. A preliminary pelvic plain radiograph is obtained. In children a 5- to 8-Fr feeding tube is used to fill the bladder to the appropriate volume. Patient comfort should be taken into account when determining the appropriate volume. In the adult population a standard catheter may be placed and the bladder filled to 200 to 400 mL. The catheter is removed and a film is obtained. During voiding, AP and oblique films are obtained. The bladder neck and urethra may be evaluated by fluoroscopy during voiding. Bilateral oblique views may demonstrate low-grade reflux, which cannot be appreciated on the AP film. In addition, oblique films will demonstrate bladder or urethral diverticula, which are not always visible in the straight AP projection. Postvoiding films should be performed (Fig. 4–11).

Figure 4–11 A voiding cystourethrography performed for the evaluation of recurrent urinary tract infection in this female patient. A, An oblique film during voiding demonstrates thickening of the midureteral profile (arrows). B, After interruption of voiding, a ureteral diverticulum is clearly visible extending posteriorly and to the left of the midline (arrows).

Indications

1. Evaluation of structural and functional bladder outlet obstruction

2. Evaluation of reflux

3. Evaluation of the urethra in males and females

Limitations

This study requires bladder filling using a catheter. This may be traumatic in children and difficult in some patients with anatomic abnormalities of the urethra or bladder neck. Filling of the bladder may stimulate bladder spasms at low volumes, and some patients are unable to hold adequate volumes for investigation. Bladder filling in patients with spinal cord injuries higher than T6 may precipitate autonomic dysreflexia (Barbaric, 1976; Fleischman and Shah, 1977; Linsenmeyer et al, 1996).

Ultrasonography

The use of ultrasonography is fundamental to the practice of urology. Ultrasonography is a versatile and relatively inexpensive imaging modality that has the unique feature of being the only imaging modality to provide real-time evaluation of urologic organs and structures without the need for ionizing radiation. In order for urologists to best use this technology on behalf of their patients, they must have a mature understanding of the underlying physical principles of ultrasound. They must also understand how the manipulation of ultrasound equipment can affect the quality of ultrasound images. The technical skills required to perform and interpret urologic ultrasonography represent a combination of practical scanning ability and knowledge of the underlying disease process in organs being imaged. In order to communicate the findings appropriately, urologists should understand the nomenclature of ultrasonography and have a specific plan for documentation of each type of study. Understanding how ultrasonography interacts with human tissues will allow urologists to use this modality effectively, appropriately, and safely.

Principles

All ultrasound imaging is the result of the interaction of sound waves with tissues and structures within the human body. Ultrasound waves are produced by applying short bursts of alternating electrical current to a series of crystals housed in the transducer. Alternating expansion and contraction of the crystals via the piezoelectric effect creates a mechanical wave that is transmitted through a coupling medium to the skin and then into the body. The waves that are produced are longitudinal waves. In a longitudinal wave the particle motion is in the same direction as the propagation of the wave (Fig. 4–12). This motion produces areas of rarefaction and compression of tissue in the direction of travel of the ultrasound wave (Fig. 4–13). A portion of the wave is reflected toward the transducer. The transducer then serves as a receiver and “listens” for the returning sound wave reconverting the mechanical to electrical energy. The transducer must be in direct, secure contact with the subject to transmit and receive the reflected sound waves.

Figure 4–12 The alternating expansion and contraction of the crystal produces longitudinal mechanical waves. In this simplified schematic drawing, the individual molecules (depicted as circles) are displaced in the direction of the propagated wave.

Figure 4–13 Areas of compression alternating with areas of rarefaction are depicted as a sine wave. The wavelength (λ) is the length from peak compression to peak compression in this drawing. This graphic depiction is critical to the understanding of the behavior of sound waves in the human body and of how ultrasound images are generated.

(Adapted from Merritt CRB. Physics of ultrasound. In: Rumack CM, Wilson SR, Charboneau JW, editors: Diagnostic ultrasound. 3rd ed. St Louis: Elsevier; 2005. p. 4.)

The appearance of the image produced by ultrasonography is the result of the interaction of mechanical ultrasound waves with biologic tissues and materials. Because ultrasound waves are transmitted and received at frequent intervals, the images can be rapidly reconstructed and refreshed, providing a real-time image. The frequency of the sound waves used for urologic ultrasound imaging is in the range of 3.5 MHz to 12 MHz.

Mechanical waves are represented graphically as a sine wave alternating between a positive and negative direction from the baseline. Ultrasound waves are described using the standard nomenclature for sine waves. A wavelength (λ) is described as the distance between one peak of the wave and the next peak. The complete path traveled by the wave from one peak to the next is called a cycle. One cycle per second is known as 1 hertz (Hz). The “period” is the time it takes for one complete cycle of the wave.

The “amplitude” of a wave is the maximal excursion in the positive or negative direction from the baseline. Amplitude corresponds to the mechanical energy associated with the sound wave and is a key property in assigning pixel brightness to a gray-scale ultrasound image. The greater the amplitude is, the brighter the corresponding pixel.

Ultrasound Image Generation

The image produced by an ultrasound machine begins with the transducer. In ultrasound imaging, the transducer has a dual function as a sender and receiver. Sound waves are created in short pulses and transmitted into the body and are then at least partially reflected. Reflected mechanical sound waves are received by the transducer and converted back into electrical energy. The transducer acts as a receiver more than 99% of the time. The electrical energy is converted by the ultrasound instrument to an image displayed on a monitor (Fig. 4–14).

Figure 4–14 In this simplified schematic diagram of ultrasound imaging, the ultrasound wave is produced by virtue of a pulse generator controlled by a master clock. The reflected waves received by the transducer are analyzed for amplitude and transit time within the body. The scan converter produces the familiar picture seen on the monitor. The actual image is a series of vertical lines that are continuously refreshed to produce the familiar real-time, gray-scale image.

Resolution

The resolution of an ultrasound image refers to the ability to discriminate two objects in close proximity to one another. Axial resolution refers to the ability to identify as separate two objects in the direction of the traveling sound wave. Axial resolution is directly dependent on the frequency of sound waves. The higher the sound wave’s frequency is, the better the axial resolution. Lateral resolution refers to the ability to identify separately objects that are equidistant from the transducer. Lateral resolution is a function of the focused width of the ultrasound beam and is a characteristic of the transducer. The location of the narrowest beam width can be adjusted by the user. The more focused the beam is, the better the lateral resolution at that location. Thus image quality can be enhanced by locating the narrowest beam width (focus or focal zone) at the depth of the object or tissue of interest (Fig. 4–15).

Figure 4–15 The shape of the ultrasound beam is simulated in this drawing (purple). The focal zone (A) is located to produce the best lateral resolution of the medial renal cortex. The location of the focal zone is designated by the caret (B). The location of the focal zone can be adjusted by the operator.

The velocity with which a sound wave travels through tissue is a product of its frequency and wavelength (Fig. 4–16). The average velocity of sound in human tissues is 1540 meters per second. Because the average velocity of sound in tissue is a constant, changes in frequency will result in changes in wavelength.

Figure 4–16 The relationship between velocity, frequency, and wavelength of sound waves in tissue. Wavelength and frequency vary in an inverse relationship.

The optimal ultrasound image requires tradeoffs between resolution and depth of penetration. High-frequency transducers of 6 to 10 MHz may be used to image structures near the surface of the body (e.g., testis, pediatric kidney) with excellent resolution. However, deeper structures (e.g., right kidney, bladder) require lower frequencies of 3.5 to 5 MHz to penetrate. Such images will have poorer axial resolution.

Mechanisms of Attenuation

As sound waves transit tissues, energy is lost or attenuated. Mechanisms of attenuation include reflection, scattering, interference, and absorption. Reflection is the key physical phenomenon that allows for information to return to the transducer as mechanical energy. Reflection occurs when ultrasound waves strike an object, a surface, or a boundary (called an interface) between unlike tissues. The shape and size of the object and the angle at which the advancing wave strikes the object are critical determinants of the amount of energy reflected. The amount of energy reflected from an interface is also influenced by the impedance of the two tissues at the interface. Impedance is a property that is influenced by tissue stiffness and density. It is the difference in impedance that allows an appreciation of interfaces between different types of tissue (Table 4–5).

Table 4–5 Density and Impedance of Tissues Encountered During Urologic Ultrasound

	DENSITY	IMPEDANCE
Air and other gases	1.2	0.0004
Fat tissue	952	1.38
Water and other clear liquids	1000	1.48
Kidney (average of soft tissue)	1060	1.63
Liver	1060	1.64
Muscle	1080	1.70
Bone and other calcified objects	1912	7.8

The impedance difference between perinephric fat and the kidney allows a sharp visual distinction at the interface. If the impedance difference between tissues is small (such as that between liver and kidney), the interface between the tissues will be more difficult to see (Fig. 4–17A). If impedance differences are large, there will be significant reflection of the sound wave producing an acoustical shadow distal to the interface (Fig. 4–17B).

Figure 4–17 A, In this sagittal view of the right kidney, the paucity of perinephric fat and the small impedance difference make it difficult to distinguish the interface between the kidney and the liver (arrows). B, The large impedance difference at the interface between urine and this bladder stone (arrow) results in significant reflection and attenuation of the sound wave. An acoustic shadow is seen distal to the stone (arrowhead).

Scattering occurs when sound waves strike a small or irregular object. The resulting spherical wave overlaps those of surrounding scattering objects (Fig. 4–18).

Figure 4–18 Scattering is a phenomenon that occurs when sound waves strike small objects. The resulting pattern of energy dispersal often results in interference.

When interacting sound waves are in phase or out of phase, their amplitude will be enhanced or diminished. This pattern of interference is partially responsible for the echo architecture or texture of organs. One pattern of interference, commonly called “speckling” (Fig. 4–19), is seen in organs with fine, internal histology such as the testis.

Figure 4–19 Fine internal echogenicity called “speckle” is caused by scattering of sound waves and the resultant pattern of interference. Note the resulting finely granular, homogenous echogenicity (arrows) of the testicular parenchyma.

Absorption occurs when the mechanical energy of the ultrasound waves is converted to heat. Absorption is directly proportional to frequency. Therefore the higher the frequency of the incident wave, the greater will be the absorption of energy and the more tissue heating that will result. It follows that higher-frequency waves are more rapidly attenuated and therefore have a limited depth of penetration (Fig. 4–20).

Figure 4–20 The relationship between frequency and tissue penetration. High-frequency sound waves are rapidly attenuated and are unable to penetrate deeply. Conversely, low-frequency waves are less attenuated and able to penetrate deeply to internal structures.

Artifacts

The interaction of ultrasound waves with tissues may produce images that do not reflect the true underlying anatomy. These misrepresentations are called “artifacts.” Artifacts may be misleading but, if recognized, may also assist diagnosis. Acoustical shadowing occurs when there is significant attenuation of sound waves at a tissue interface. Echo information posterior to the interface may be obscured or lost. An anechoic or hypoechoic “shadow” is produced. Under these conditions, three-dimensional objects such as stones may appear as crescentic objects, making it difficult to obtain accurate measurements (Fig. 4–21). Important pathology posterior to such an interface may be missed. This problem may often be overcome by changing the angle of insonation, changing the frequency of the transducer, or changing the focal zone of the transducer.

Figure 4–21 In this transverse view of the urinary bladder (B), there are two large bladder diverticula (D). Two stones (arrows) strongly reflect and attenuate the incident sound wave, producing an acoustical shadow. Note that the stones appear crescentic even though they are ovoid in shape.

Increased through transmission is observed when sound waves are less attenuated while passing through a given structure or tissue than by the surrounding tissues. For example, when imaging a simple cyst of the kidney, sound waves passing through the cyst are less attenuated than those passing through the surrounding renal cortex and renal sinus. When the waves transiting the cyst strike the back wall of the cyst and posterior renal tissue, the waves are more energetic on arrival to these tissues. The reflected sound waves are also more energetic and less attenuated as they return to the transducer. The result is that tissue posterior to the cyst appears hyperechoic compared with the surrounding renal tissue, even though the tissues are histologically identical (Fig. 4–22). The effect of this artifact can be mitigated by changing the angle of insonation or adjusting the time-gain compensation (TGC) settings.

Figure 4–22 Increased thru-transmission (also called distal enhancement) is demonstrated in this longitudinal view of the left kidney. The tissue distal to the cyst appears hyperechoic (indicated by arrows) compared with adjacent tissue.

An edging artifact occurs when sound waves strike a curved surface or interface at an incident angle, resulting in refraction of the wave along the plane of the interface (Fig. 4–23).

Figure 4–23 When sound waves strike a surface or interface at a “critical angle,” the wave is refracted without significant reflection.

An incident wave at this angle (the critical angle) will not be directly reflected to the transducer, resulting in a hypoechoic “shadow.” This artifact is commonly seen in testicular ultrasonography and transrectal ultrasonography (Fig. 4–24). It can be overcome by changing the angle of insonation.

Figure 4–24 A, The curved surface of the tunica albuginea of the upper pole of testis creates a critical angle edging artifact (arrows). B, The rounded surfaces of the lateral lobes of the prostate as they meet in the prostatic urethra create an edging artifact (arrows) in this transverse image of the prostate.

A reverberation artifact results when there are large differences in impedance between two adjacent tissues or surfaces with a strong reflection of the incident wave. The ultrasound wave bounces back and forth (reverberates) between the transducer interface and the reflective interface. With the second transit of the sound wave, the ultrasound equipment interprets a second object that is twice as far away as the first. There is ongoing attenuation of the sound wave with each successive reverberation, resulting in a slightly less intense image displayed on the screen. Therefore echoes are produced, spaced at equal intervals from the transducer but progressively less intense (Fig. 4–25).

Figure 4–25 Reverberation artifact: The strongly reflective interface is projected with decreasing amplitude as the incident sound wave makes multiple round trips.

The reverberation artifact can also be seen in cases where the incident sound wave strikes a series of smaller reflective objects (such as the gas-fluid mixture in the small bowel), which results in multiple reflected sound waves of various angles and intensity (Fig. 4–26). The resultant echo pattern is a collection of hyperechoic artifactual reflections distal to the structure with progressive attenuation of the sound wave.

Figure 4–26 When ultrasound strikes a structure such as bowel, which contains gas bubbles (A), the resultant reverberation artifact has a characteristic appearance sometimes called a “comet tail.” B, A comet tail artifact produced by bowel gas (arrows) obscures the lower pole of the kidney.

Modes of Ultrasound

(From Rumack CM, Wilson, SR, Charboneau JW, Johnson J, editors. Diagnostic ultrasound. 3rd ed. St Louis: Elsevier; 2005. Fig. 1–17, p. 17.)

Spatial Compounding

Spatial compounding is a scanning mode whereby the direction of insonation is electronically altered and a composite image is generated. This technique reduces the amount of artifact and noise, producing a scan of better clarity.

Three-Dimensional Scanning

Three-dimensional (3-D) scanning has been used extensively in obstetrics and gynecology but so far has limited application in urology. 3-D scanning produces a composite of images (data set), which can then be manipulated to generate additional views of the anatomy in question (Fig. 4–28). 3-D rendering may be important in procedural planning and precise volumetric assessments (Ghani et al, 2008a, 2008b). 3-D scanning may allow the recognition of some tissue patterns that would otherwise be inapparent on two-dimensional scanning (Mitterberger et al, 2007b; Onik and Barzell, 2008).

Figure 4–28 A three-dimensional image of the testis demonstrating intratesticular blood flow on power Doppler. The image can be virtually rotated and manipulated to produce unique anatomic perspectives.

(Used with permission by BK Medical.)

Contrast Agents in Ultrasound

Intravenous compounds that contain microbubbles have been used for enhancing the echogenicity of blood and tissue. Microbubbles are distributed in the vascular system and create strong echoes with harmonics when struck by sound waves. The bubbles themselves are rapidly degraded by their interaction with the sound waves. Contrast agents may be useful in prostatic ultrasonography by enhancing the ability to recognize areas of increased vasculature. The use of intravenous ultrasound contrast agents is considered investigational but has shown promise in a number of urologic scanning situations (Mitterberger et al, 2007a; Wink et al, 2008).

Documentation

When urologists perform and interpret ultrasound studies, it is important that appropriate nomenclature be used to describe the objects imaged (Fig. 4–29). By convention, the liver is used as a benchmark for echogenicity. If a structure is hypoechoic, it means it is darker than the surrounding tissues. If it is hyperechoic, it means it is brighter than the surrounding tissues. Isoechoic means it is similar to the surrounding tissues. Structures that do not generate echoes are called anechoic. A simple cyst is an example of a structure with an anechoic interior. In general, a high water content causes tissue to appear hypoechoic. In general, a high fat content causes tissue to appear hyperechoic.

Figure 4–29 The nomenclature for describing the appearance of ultrasound images.

By convention, structures imaged by ultrasonography should be oriented so that the superior aspect of the structure is to the left as the image is viewed and the inferior aspect of the structure to the right. With paired structures it is critically important to document right or left. It is useful to use equipment-generated icons to illustrate patient position and the orientation of insonation (Fig. 4–30).

Figure 4–30 In this sagittal image of the right testis, the superior pole of the testis (A) is to the left, the inferior pole of the testis (B) is to the right. The anterior aspect of the testis (C) is at the top of the image and the posterior aspect (D) at the bottom. Without the label, there would be no way to distinguish the right from the left testis.

The appropriate number of images to be captured for documentation is the number necessary to document a systematic and complete examination and to document relevant pathology.

Patient Safety

Diagnostic ultrasonography transmits energy into the patient that has the potential to produce biologic effects. The two main categories of biologic effects are mechanical effects and thermal effects.

The mechanical effects of ultrasonography are torque and streaming. The mechanical effects of an acoustic field may produce a phenomenon called cavitation. Cavitation occurs when small gas-filled bubbles form and then collapse. These collapsing bubbles liberate a large amount of energy, which may cause damage to tissue in certain circumstances. Mechanical effects are most likely to be observed around gas-containing structures such as lung and bowel. The thermal effects of ultrasonography are primarily the result of tissue heating resulting from the absorption of energy. The amount of tissue heating is influenced by several factors including beam focusing, transducer frequency, exposure time, scanning mode, and tissue density.

To assist the sonographer in monitoring the bioeffects of ultrasound, the ultrasonography community has adopted the output display standard (ODS). Two values are typically displayed: the mechanical index (MI) and the thermal index (TI). These indices are calculated estimates of the potential for bioeffects of ultrasonography based on the mode of ultrasonography being used, frequency, power output, and time of insonation. The MI indicates the probability that cavitation will occur. For tissues not containing stabilized gas bodies (lung and intestine), the risk of cavitation is low as long as the MI is less than or equal to 0.7. For structures adjacent to lung or intestine, scanning time should be limited if the MI exceeds 0.4. The TI indicates the probability that tissue temperature within the sonographic field will be increased by 1° C. The precise consequences of tissue heating are not completely understood, but even tissue temperature elevations of up to 6° C are not likely to be dangerous unless exposure time exceeds 60 seconds. The MI and TI are typically displayed on the monitor during ultrasound examinations, and all practitioners should be familiar with the location. It is important to note that these indices are not safety limits.

In general, ultrasonography performed by urologists has a low risk for patient harm as long as standard protocols are followed (Fowlkes, 2008). Although tissue heating may occur, there are no confirmed biologic effects of tissue heating in nonfetal scanning except when they are sustained for extended periods. Users should be aware that for soft tissues not known to contain gas bodies, there is no basis in present knowledge to suggest an adverse nonthermal bioeffect from current diagnostic instruments not exceeding the U.S. Food and Drug Administration output limits (Fowlkes, 2008). Nevertheless, all urologists should endeavor to follow the principles of ALARA, which stands for “As Low As Reasonably Achievable.” The ALARA principle is intended to limit the total energy imparted to the patient during an examination. This can be accomplished by (1) keeping power outputs low, (2) using appropriate scanning modes, (3) limiting examination times, (4) adjusting focus and frequency, and (5) using the cine function during documentation.

In summary, ultrasound scanning offers an excellent, cost-effective modality for diagnosing and treating urologic conditions. The most important factor in ultrasound safety is the informed operator. Urologists should endeavor to perform limited examinations using consistent technique for specific indications. Patient safety and equipment maintenance should be emphasized in all the environments where ultrasound technology is used.

Clinical Urologic Ultrasound

The use of ultrasonography in urology has expanded dramatically because of its profound utility in the clinic and operating room. Long the mainstay of the diagnosis of prostatic disease, ultrasonography is increasingly being used by urologists in the clinical environment for initial diagnosis, interventional management, and longitudinal follow-up of urologic diseases.

Renal Ultrasound

Introduction

Urologists, by virtue of their intimate knowledge of surgical anatomy of the kidneys and retroperitoneum, are uniquely qualified to perform and interpret selected ultrasound examinations of the abdomen. These skills are relevant in both the office and the operating room environment. Urologists generally perform abdominal ultrasonography for a specific clinical indication and less often for general screening of the abdominal contents. Therefore in most clinical situations, a limited retroperitoneal examination is used in urologic practice.

Technique

The transducer normally used for renal ultrasonography is a curved array transducer of 3.5 to 5.0 MHz. Transducers of a higher frequency may be used for pediatric patients. For intraoperative and laparoscopic renal ultrasonography, a linear array transducer of 6 to 10 MHz is typically employed.

Scanning of the right kidney is performed with the patient supine. The kidney is located by beginning in the midclavicular line in the right upper quadrant. In the sagittal plane the transducer is moved laterally until the midsagittal plane of the kidney is imaged. Once the kidney has been imaged anteriorly and posteriorly in the sagittal plane, the probe is rotated 90 degrees counterclockwise. The midtransverse plane will demonstrate the renal hilum containing the renal vein. The kidney is scanned from upper pole to lower pole.

The technique and documentation for left renal ultrasonography is identical to that of the right side. However, the left kidney is slightly more cephalad than the right kidney. Bowel gas is more problematic on the left because of the position of the splenic flexure of the colon. Visualization of the left kidney often requires the patient to be turned into a lateral position. Ultrasound imaging of the left kidney lacks the liver as an acoustic window, and it is sometimes more difficult to image the left kidney in a true sagittal plane.

Normal Findings

It is helpful during scanning of the kidney to understand its anatomic position within the retroperitoneum. This assists identifying the midsagittal plane, which serves as a reference point for a complete examination (Fig. 4–31).

Figure 4–31 The lower pole of the kidney is displaced 15 degrees laterally compared with the upper pole (A). The kidney is rotated 30 degrees posterior to the true coronal plane (B). The lower pole of the kidney is slightly anterior compared with the upper pole.

The adult right kidney in the sagittal view demonstrates a cortex that is usually hypoechoic with respect to the liver. The central band of echoes in the kidney is a hyperechoic area that contains the renal hilar adipose tissue, blood vessels, and collecting system. Acoustic shadowing from ribs overlying the inferior pole can be eliminated by moving the probe to a more lateral position or into the intercostal space. By having the patient take a deep breath, the kidney can be moved inferiorly to assist complete imaging (Fig. 4–32).

Figure 4–32 Midsagittal plane of the kidney. Note the relative hypoechogenicity of the renal pyramids (P) compared with the cortex (C). The central band of echoes (B) is hyperechoic compared with the cortex. The midsagittal plane will have the greatest length measurement pole to pole. A perfectly sagittal plane will result in a horizontal long axis of the kidney.

The echogenicity of the kidney varies with age. The renal cortex of an infant is relatively hyperechoic compared with that of an adult. In addition, there is a smaller and less apparent central band of echoes in the infant. In the adult, the echogenicity of the renal cortex is usually hypoechoic with respect to the liver (Emamian et al, 1993a). In patients with chronic medical renal diseases the renal cortex is often thinned and isoechoic or hyperechoic with respect to the liver (O’Neill, 2001).

Renal size changes over the lifetime of an individual. Nomograms for pediatric renal size should be consulted. These are based on age, height, and weight of the patient. The average adult kidney measures 10 to 12 cm in length and 4 to 5 cm in width. Measurements of renal volume may be appropriate in cases of severe renal impairment. Renal measurements should be obtained in the midsagittal plane and midtransverse plane. Measurements taken in other than the midsagittal plane and midtransverse may be spuriously low. The thickness of the parenchyma is the average distance between the renal capsule and the central band of echoes. The precise location for making this measurement is somewhat subjective. The midlateral renal parenchyma in the sagittal view is a common choice for obtaining this measurement (Fig. 4–33). Although there is no universal standard, the renal cortical thickness should be greater than 7 mm (Roger et al, 1994), and the renal parenchymal thickness should be greater than 15 mm in adults (Emamian et al, 1993b).

Figure 4–33 The distinction between renal cortical thickness and renal parenchymal thickness is that the renal parenchyma is measured from the central band of echoes to the renal capsule. The renal cortex is measured from the outer margin of the medullary pyramid to the renal capsule.

Doppler ultrasound may be helpful in evaluating the renal artery and renal vein and assessing the vascular resistance in the kidney. Doppler modes may also be useful in evaluating neovascularity associated with renal tumors and in correctly characterizing hypoechoic structures in the renal pelvis such as a parapelvic cyst, the renal vein, or the dilated collecting system.

Limitations

Some patients are not favorable candidates for renal ultrasonography. Obesity, intestinal gas, and physical deformity may be impediments to complete renal evaluation. Renal ultrasonography has poor sensitivity for renal masses less than 2 cm (Warshauer et al, 1988). There is a lack of specificity for renal tumor type except for angiomyolipoma. Angiomyolipoma has characteristics that are distinctive on ultrasonography (highly echoic), but some small renal cell carcinomas have been shown to be indistinguishable from angiomyolipoma by ultrasound criteria (Yamashita et al, 1992; Forman et al, 1993).