Hip and Pelvis

• Visibility of hip and pelvis details. An optimal kVp technique, as shown in Table 7-1, sufficiently penetrates the proximal femur, hip, and pelvic structures and provides a contrast scale necessary to visualize the pelvic and femoral details. Use a grid to absorb the scatter radiation produced by the proximal femur, hip, and pelvis, providing a higher contrast image. To obtain optimal density, set a manual milliampere-seconds (mAs) level based on the patient's part thickness or choose the appropriate automatic exposure control (AEC) chamber when recommended. AEC is contraindicated if the patient has hip or pelvic hardware or orthopedic apparatus.

• Fat planes on AP hip and pelvic projections. When evaluating AP hip and pelvis projections, the reviewer not only analyzes the bony structures but also studies the placement of the soft tissue fat planes. Four fat planes are of interest on AP hip projections, and their visualization aids in the detection of intraarticular and periarticular disease: obturator internus fat plane, which lies within the pelvic inlet next to the medial brim; the iliopsoas fat plane, which lies medial to the lesser trochanter; the pericapsular fat plane, which is found superior to the femoral neck; and the gluteal fat plane, which lies superior to the pericapsular fat plane (Figure 7-2).

• An AP projection of the hip is obtained by placing the patient supine on the imaging table with the legs extended (Figure 7-3). To ensure that the pelvis is not rotated, judge the distances from the anterior superior iliac spines (ASISs) to the imaging table. The distances on each side should be equal.

• Detecting pelvis rotation. Rotation on an AP hip projection is initially detected by evaluating the relationship of the ischial spine and the pelvic brim, the alignment of the sacrum and coccyx with the symphysis pubis, and the degree of obturator foramen demonstration. If the patient was rotated toward the affected hip, the ischial spine is demonstrated without pelvic brim superimposition, the sacrum and coccyx are not aligned with the symphysis pubis but are rotated away from the affected hip, and the obturator foramen is narrowed (see Image 1). If the patient has been rotated away from the affected hip, the ischial spine is not aligned with the pelvic brim but is demonstrated closer to the acetabulum, the sacrum and coccyx are not aligned with the symphysis pubis, but are rotated toward the affected hip, and the obturator foramen is widened (see Image 2).

• Accurate leg positioning. To demonstrate an AP hip projection with the femoral neck shown without foreshortening and the greater trochanter in profile, the patient's leg should be internally rotated until the foot is angled 15 to 20 degrees from vertical and the femoral epicondyles are positioned parallel with the imaging table (Figure 7-4; see Figure 7-1). A sandbag or tape may be needed to help the patient maintain this internal leg rotation.

• Poor leg positioning. The relationship of the patient's leg to the imaging table determines how the femoral neck and trochanters are shown on an AP hip projection. In general, when patients are relaxed, their legs and feet are externally (laterally) rotated. On external rotation, the femoral neck declines posteriorly (toward the table) and is foreshortened on an AP hip projection. Increased external rotation increases the degree of posterior decline and foreshortening of the femoral neck on the image. If the patient's leg is externally (laterally) rotated enough to position the foot at a 45-degree angle and an imaginary line connecting the femoral epicondyles at a 60- to 65-degree angle with the imaging table, the femoral neck is demonstrated on end and the lesser trochanter is demonstrated in profile (Figure 7-5; see Image 3). If the patient's leg is positioned with the foot placed vertically and an imaginary line connecting the femoral epicondyles at approximately a 15- to 20-degree angle with the imaging table, the lesser trochanter is demonstrated in partial profile and the femoral neck is only partially foreshortened (see Image 4).

• Positioning for a fractured or dislocated proximal femur. When a patient has a dislocated or fractured proximal femur, the leg should not be internally rotated but left as is. Forced internal rotation of a dislocated or fractured proximal femur may injure the blood supply and nerves that surround the injured area. Because the patient's leg is not internally rotated when a fracture is suspected, the resulting AP hip projection may demonstrate the femoral neck with some degree of foreshortening and the lesser trochanter without femoral shaft superimposition (see Image 5).

• A perpendicular central ray is centered 1.5 inches (4 cm) distal to the midpoint of a line connecting the ASIS and superior symphysis pubis, to center the hip joint in the center of the exposure field, and a perpendicular central ray is centered 2.5 inches (6.25 cm) distal to the midpoint of a line connecting the ASIS and superior symphysis pubis to place the femoral neck in the center of the exposure field (Figure 7-6). Center the IR to the central ray and open the longitudinal collimation enough to include the ASIS and any hip orthopedic apparatus. Transversely collimate to the patient's midsagittal plane and within 0.5 inch (1.25 cm) of the lateral hip skin line. Including half of the sacrum, coccyx, and symphysis pubis within the exposure field provides a way to evaluate pelvic rotation.

• A 10- × 12-inch (24- × 30-cm) IR placed lengthwise should be adequate to include all the required anatomic structures. A larger IR and lower centering point may be necessary to include hip orthopedic apparatus (Figure 7-7).

• Gonadal shielding. Use gonadal shielding on all male patients. Female patients should be shielded, although it is important that no pelvic anatomy be covered by the shield. It is not uncommon for patients with hip fractures to have an associated pelvic fracture. Remember that a shield placed on top of the patient will be greatly magnified.

Anteroposterior Hip Projection Analysis

• An AP oblique projection of the hip is obtained by placing the patient supine on the imaging table, with the unaffected leg extended and the affected leg flexed and abducted (Figure 7-9). To ensure that the pelvis is not rotated, judge the distances from the ASISs to the imaging table. The distance on each side should be equal.

• Detecting pelvis rotation. Rotation of the pelvis on an AP oblique projection is detected by evaluating the relationship of the ischial spine and the pelvic brim, the alignment of the sacrum and coccyx with the symphysis pubis, and the demonstration of the obturator foramen. If the patient was rotated toward the affected hip, the ischial spine is demonstrated without pelvic brim superimposition, the sacrum and coccyx are not aligned with the symphysis pubis but are rotated away from the affected hip, and demonstration of the obturator foramen is decreased (see Image 6). If the patient was rotated away from the affected hip, the ischial spine is not aligned with the pelvic brim but is demonstrated closer to the acetabulum, the sacrum and coccyx are not aligned with the symphysis pubis but are rotated toward the affected hip, and demonstration of the obturator foramen is increased (see Image 7).

• Lauenstein and Hickey methods, lateral hip. The Lauenstein and Hickey methods are modifications of the AP oblique hip projection. For these methods, the patient is positioned as described for the AP oblique hip projection with the femur flexed and abducted, except that the pelvis is rotated toward the affected hip as needed to position the femur against the imaging table (Figure 7-10).

• Accurate femur positioning. To position the greater trochanter accurately beneath the proximal femur and position the lesser trochanter in profile, flex the patient's knee and hip until the femur is angled at 60 to 70 degrees with the imaging table (20 to 30 degrees from vertical) (Figure 7-11).

• Effect of distal femur elevation on proximal femur visualization. For an AP oblique hip projection, the medial and lateral placement of the greater and lesser trochanters are determined when the patient flexes the knee and hip. Use a femoral skeletal bone for a better understanding of how the relationship of the greater and lesser trochanters to the proximal femur changes as the distal femur is elevated with knee and hip flexion. Begin by placing the femoral bone on a flat surface in an AP position. While slowly elevating the distal femur, observe how the greater trochanter rotates around the proximal femur. First, the greater trochanter moves beneath the proximal femur; then, as elevation of the distal femur continues, it moves from beneath the proximal femur and is demonstrated on the medial side of the femur.

• Poor distal femur elevation. If the knee and hip are not flexed enough to place the femur at this angle with the imaging table, the greater trochanter is demonstrated laterally, as it is on an AP projection (see Images 7 and 8). If the knee and hip are flexed too much, placing the femur at an angle greater than 60 to 70 degrees with the imaging table, the greater trochanter is demonstrated medially (see Image 9). The greater trochanter is also demonstrated medially, as shown in Image 9, when the foot and ankle of the affected leg are elevated and placed on top of the unaffected leg. This positioning causes the femur to rotate externally. The foot of the affected leg should remain resting on the imaging table.

• Accurate leg positioning. To demonstrate the femoral neck and proximal femur with only partial foreshortening and the proximal greater trochanter at a transverse level halfway between the femoral head and lesser trochanter on an AP oblique hip projection, abduct the femoral shaft to a 45-degree angle from vertical (Figure 7-12).

• Effect of leg abduction. The degree of femoral abduction determines the amount of femoral neck foreshortening and the transverse level at which the proximal greater trochanter is demonstrated between the femoral head and lesser trochanter.

    Use a femoral skeleton bone to understand how leg abduction determines the visualization of the femoral neck and the position of the greater trochanter. Place the femoral bone on a flat surface in an AP position, with the distal femur elevated until the greater trochanter is positioned beneath the proximal femur and the lesser trochanter is in profile (20 to 30 degrees from vertical or 60 to 70 degrees from flat surface). From this position, abduct the femoral bone (move the lateral surface of the femoral bone toward the flat surface). As the bone moves toward the flat surface, observe how the femoral neck is positioned more on end and the greater trochanter moves proximally (toward the femoral head).

• Poor leg abduction. If the femoral shaft is abducted 20 to 30 degrees from vertical (60 to 70 degree angle with the imaging table; Figure 7-13) the femoral neck is demonstrated without foreshortening and the proximal greater trochanter is at the same transverse level as the lesser trochanter (see Image 10). If the femoral shaft is abducted to the imaging table (Figure 7-14), the proximal femoral shaft is demonstrated without foreshortening, the proximal greater trochanter is at the same transverse level as the femoral head, and the femoral neck is demonstrated on end (see Image 11).

• Center a perpendicular central ray 2.5 inches (6.25 cm) distal to the midpoint of a line connecting the ASIS and superior symphysis pubis to center the femoral neck in the center of the exposure field. Center the IR to the central ray and open longitudinal collimation to include the ASIS. Transversely collimate to the patient's midsagittal plane and transversely collimate to within 0.5 inch (1.25 cm) of the lateral hip skin line.

• Including half of the sacrum, coccyx, and symphysis pubis within the exposure field provides a way to evaluate pelvic rotation.

• A 10- × 12-inch (24- × 30-cm) IR placed lengthwise should be adequate to include all the required anatomic structures.

AP Oblique Hip Projection Analysis

• Tight collimation and placement of a flat lead contact strip or the straight edge of a lead apron over the top, unused half of the IR, as shown in Figure 7-16, also prevent scatter radiation from reaching the IR.

• Compensating filter. Frequently, when an exposure (mAs) is set that adequately demonstrates the hip joint, the proximal femur is overexposed because of the difference in body thickness in these two regions. A wedge-type compensating filter attached to the x-ray tube can be used to obtain uniform image density of the hip joint and proximal femur. Align the thin end of the filter with the femoral neck and the thicker end with the proximal femur.

• An axiolateral projection of the hip is obtained by placing the patient on the imaging table in an AP projection, with the unaffected hip positioned next to the lateral edge of the table. Flex the patient's unaffected leg until the femur is as close to a vertical position as the patient can tolerate, and then abduct the leg as far as the patient will allow. Support this leg position by using a specially designed leg holder or suitable support. Flexion and abduction of the unaffected leg move its bony and soft tissue structures away from the affected hip. Inadequate flexion or abduction of the unaffected leg results in superimposition of soft tissue onto the affected hip, preventing visualization of the affected hip (see Image 12).

• IR placement. Once the patient's unaffected leg has been positioned, place the grid IR against the patient's affected side at the level of the iliac crest (Figure 7-16). To demonstrate the affected femoral neck without foreshortening, align the x-ray tube horizontally with the central ray perpendicular to the femoral neck and adjust the distal end of the IR until the receptor's long axis is perpendicular to the central ray and parallel with the femoral neck.

• Localizing the femoral neck for central ray alignment. To localize the affected femoral neck, first find the center of an imaginary line drawn between the superior symphysis pubis and the ASIS. Then, bisect that line by drawing a perpendicular line distally (Figure 7-17). This imaginary line parallels the long axis of the femoral neck as long as the leg is not abducted. Once the long axis of the femoral neck has been located, align the central ray perpendicular to it and the IR parallel with it.

• Effect of central ray and femoral neck misalignment. Misalignment of the central ray with the femoral neck results in femoral neck foreshortening and a shift in the transverse level at which the greater trochanter is located. If the angle formed between the femur and the central ray is too large, the proximal greater trochanter is demonstrated proximal to the transverse level of the lesser trochanter and is superimposed by a portion of the femoral neck (see Image 13). If the angle between the femur and the central ray is too small, the proximal greater trochanter is demonstrated distal to the transverse level of the lesser trochanter. This mispositioning seldom occurs, because the imaging table and tube position prevent such a small angle.

• Rotation of the patient's affected leg determines the relationship of the lesser and greater trochanter to the proximal femur on an axiolateral hip projection. In general, when a patient is placed on the imaging table and the affected leg is allowed to rotate freely, it is laterally (externally) rotated.

• Effect of leg rotation on proximal femur visualization. To position the proximal femur in a lateral projection (90 degrees from the AP projection), demonstrating the lesser trochanter in profile posteriorly and superimposing the greater trochanter by the femoral shaft, the affected leg must be internally rotated until an imaginary line drawn between the femoral epicondyles is positioned parallel with the imaging table. The patient's foot is angled internally 15 to 20 degrees from a vertical position (Figure 7-18). If the affected leg is not rotated internally, the greater trochanter is demonstrated posteriorly and the lesser trochanter is superimposed over the femoral shaft (see Image 14). How much greater trochanter is demonstrated without femoral shaft superimposition depends on the degree of external rotation. Greater external rotation increases the amount of greater trochanter shown.

• Positioning for a proximal femoral fracture or dislocation. When a patient has a dislocated hip or a suspected or known proximal femoral fracture, the leg should not be internally rotated, but left as is. Forced internal rotation of a dislocated hip or fractured proximal femur may injure the blood supply and nerves that surround the injured area. Because the patient's leg is not internally rotated in such cases, it is acceptable for the greater trochanter to be demonstrated posteriorly and the lesser trochanter to be superimposed over the femoral shaft (see Image 15).

• Center a perpendicular central ray to the patient's midthigh, at the level of the femoral neck, to place it in the center of the exposure field. The center of the femoral neck is located at a level 2.5 inches (6.25 cm) distal to the midpoint of a line connecting the ASIS and superior symphysis pubis. Open the longitudinal collimation the full length of the IR. Transversely collimate to within 0.5 inch (1.25 cm) of the proximal femoral skin line.

• A 10- × 12-inch (24- × 30-cm) IR placed lengthwise should be adequate to include all the required anatomic structures. A larger IR and lower centering point may be necessary to include hip orthopedic apparatus (Figure 7-19).

• IR placement alternative. The level at which the IR is placed along the patient's lateral body surface determines whether the acetabulum and femoral head are included on the IR. For patients with minimal lateral soft tissue thickness, the upper IR edge should be firmly placed in the crease formed at the patient's waist, just superior to the iliac crest (see Figure 7-17). For patients with ample lateral soft tissue thickness, the upper IR edge needs to be positioned superior to the iliac crest (Figure 7-20). This superior positioning will result in magnification because of the increase in the object–image receptor distance (OID) but is necessary if the acetabulum and femoral head are to be included on the axiolateral hip projection.

Axiolateral Hip Projection Analysis

• Regarding the male and female pelves. Be aware of the bony architectural differences that exist between the male and female pelves (Table 7-2). These differences are the result of the need for the female pelvis to accommodate fetal growth during pregnancy and fetal passage during delivery.

• Refer to Figure 7-2 for an illustration of each fat plane location.

• An AP projection of the pelvis is accomplished by placing the patient supine on the imaging table, with the legs extended and the arms drawn away from the pelvic area (Figure 7-23). To ensure that the pelvis is not rotated, judge the distance from the ASIS to the imaging table on each side. The distances should be equal.

• Pelvic rotation. A nonrotated AP pelvis projection demonstrates symmetrical ilia and obturator foramina. Rotation is initially detected by evaluating the relationships of the ischial spines with the pelvic brim and of the sacrum and coccyx with the symphysis pubis. The ischial spines should be aligned with the pelvic brim, and the sacrum and coccyx should be in alignment with the symphysis pubis on a nonrotated pelvis. If the pelvis is rotated into a LPO position, the left ilium is wider than the right, the left obturator foramen is narrower than the right, the left ischial spine is demonstrated without pelvic brim superimposition, and the sacrum and coccyx are not aligned with the symphysis pubis but are rotated toward the right hip (see Image 16).

• If the patient was rotated into a right posterior oblique (RPO) position, the opposite is true. The right ilium is wider than the left, the right obturator foramen is narrower than the left, the right ischial spine is demonstrated without pelvic brim superimposition, and the sacrum and coccyx are rotated toward the left hip.

• Accurate leg positioning. To demonstrate the femoral necks without foreshortening and the greater trochanters in profile on an AP pelvis projection, the patient's leg should be internally rotated until the feet are angled 15 to 20 degrees from vertical and the femoral epicondyles are positioned parallel with the imaging table (Figure 7-24; see Figure 7-21). Sandbags or tape may be needed to help maintain this internal leg rotation. An AP pelvis projection may not demonstrate the proximal femurs with exactly the same degree of internal rotation. How each proximal femur appears will depend on the degree of internal rotation placed on that leg.

• Poor leg positioning. The relationship of the patient's entire leg to the imaging table determines how the femoral necks and trochanters are shown on an AP pelvis projection. In general, when patients are relaxed, their legs and feet are externally (laterally) rotated. On external rotation, the femoral necks decline posteriorly (toward the table) and are foreshortened on an AP pelvis projection. Greater external rotation increases the posterior decline and foreshortening of the femoral necks. If the patient's legs are externally (laterally) rotated enough to position the feet at a 45-degree angle and an imaginary line connecting the femoral epicondyles at a 60- to 65-degree angle with the imaging table, the femoral necks are demonstrated on end and the lesser trochanters are demonstrated in profile (Figure 7-25; see Image 17). If the patient's legs are positioned with the feet placed vertically and an imaginary line connecting the femoral epicondyles at approximately a 15- to 20-degree angle with the imaging table, the lesser trochanter is demonstrated in partial profile and the femoral neck is only partially foreshortened (see Image 18).

• Positioning for a proximal femoral fracture. Often, when a fracture of a proximal femur is suspected, an AP pelvis projection is ordered instead of an AP hip projection because pelvic fractures are frequently associated with proximal femur fractures. If a patient has a suspected fracture or a fractured proximal femur, the leg should not be internally rotated but should be left as is. Forced internal rotation of a fractured proximal femur may injure the blood supply and nerves that surround the injured area. Because the patient's leg is not internally rotated when a fracture is in question, such an AP pelvis projection demonstrates the affected femoral neck with some degree of foreshortening and the lesser trochanter without femoral shaft superimposition.

• Center a perpendicular central ray to the midsagittal plane at a level halfway between the symphysis pubis and the midpoint of an imaginary line connecting the ASIS to place the inferior sacrum in the center of the exposure field. Center the IR to the central ray and open the longitudinal collimation the full 14-inch (35-cm) IR length for most adult patients. Transversely collimate to within 0.5 inch (1.25 cm) of the lateral skin line.

• A 14- × 17-inch (35- × 43-cm) IR placed crosswise should be adequate to include all the required anatomic structures.

• Central ray centering for analysis of hip joint mobility. When an AP pelvis projection is being taken specifically to evaluate hip joint mobility, the central ray should be centered to the midsagittal plane at a level 1 inch (2.5 cm) superior to the symphysis pubis. Such positioning centers the hip joints on the image but may result in clipping of the superior ilia.

Anteroposterior Pelvis Projection Analysis

• An AP projection of the pelvis is accomplished by placing the patient on the imaging table with the legs flexed and abducted (Figure 7-27). To ensure that the pelvis is not rotated, judge the distance from the ASIS to the imaging table on each side. The distances should be equal.

• Detecting pelvic rotation. A nonrotated AP oblique pelvis projection will demonstrate symmetrical ilia and obturator foramina. Rotation can be detected by evaluating the relationships of the ischial spines with the pelvic brim and of the sacrum and coccyx with the symphysis pubis. The ischial spines should be aligned with the pelvic brim and the sacrum and coccyx should align with the symphysis pubis on a nonrotated pelvis. If the pelvis is rotated into an LPO position, the left ilium is wider than the right, the left obturator foramen is narrower than the right, the left ischial spine is demonstrated without pelvic brim superimposition, and the sacrum and coccyx are not aligned with the symphysis pubis but are rotated toward the right hip (see Image 19). If the patient is rotated into an RPO position, the opposite is true. The right ilium is wider than the left, the right obturator foramen is narrower than the left, the right ischial spine is demonstrated without pelvic brim superimposition, and the sacrum and coccyx are rotated toward the left hip.

• Accurate femur positioning. To position the greater trochanters beneath the proximal femurs accurately and position the lesser trochanters in profile, flex the patient's knees and hips until the femurs are angled 60 to 70 degrees with the imaging table (20 to 30 degrees from vertical; Figure 7-28).

• Poor distal femur elevation. For an AP oblique pelvis projection, the relationship of the greater and the lesser trochanters with the proximal femurs is determined when the patient flexes the knees and hips. If the knees and hips are not flexed enough to place the femur at a 60- to 70-degree angle with the imaging table, the greater trochanters are demonstrated laterally, as with an AP projection (see Image 20). If the knees and hips are flexed too much, placing the femurs at an angle greater than 60 to 70 degrees with the imaging table, the greater trochanters are demonstrated medially Image 9).

• Accurate leg positioning. To demonstrate the femoral necks and proximal femora with only partial foreshortening and the greater trochanters at a transverse level halfway between the femoral heads and lesser trochanters on an AP oblique pelvis projection, abduct the femoral shafts to a 45-degree angle from vertical (Figures 7-26 and 7-29).

• Effect of leg abduction. The degree of femoral abduction determines the amount of femoral neck foreshortening and the transverse level at which the greater trochanters are demonstrated between the femoral heads and lesser trochanters.

• Poor leg abduction. If the femoral shafts are abducted to 20 to 30 degrees from vertical (60- to 70-degree angle from the imaging table; Figure 7-30), the femoral necks are demonstrated without foreshortening and the proximal greater trochanters are at the same transverse level as the lesser trochanters on an AP oblique pelvis projection (see Image 21). If the femoral shafts are abducted until they are placed next to the imaging table (Figure 7-31), the proximal femoral shafts are demonstrated without foreshortening, the proximal greater trochanters are at the same transverse level as the femoral heads, and the femoral necks are demonstrated on end on an AP oblique pelvis projection (see Image 22).

• Importance of symmetrical femoral abduction. An AP oblique pelvis projection may not demonstrate the proximal femurs with exactly the same degree of femoral abduction. How each proximal femur appears depends on the degree of femoral abduction placed on that leg. As a standard, unless the AP oblique pelvis projection is ordered to evaluate hip mobility, both femurs should be abducted equally for the image. This symmetrical abduction helps prevent pelvic rotation. It may be necessary to position an angled sponge beneath the patient's femurs to maintain the desired femoral abduction.

• Center a perpendicular central ray to the midsagittal plane at a level 1 inch (2.5 cm) superior to the symphysis pubis. Center the IR to the central ray and open the longitudinal collimation the full 14-inch (35-cm) length for most adult patients. Transversely collimate to within 0.5 inch (1.25 cm) of the lateral skin line.

• A 14- × 17-inch (35- x 43-cm) IR placed crosswise should be adequate to include all the required anatomic structures.

Anteroposterior Oblique Pelvis Projection Analysis

• An AP axial projection of the sacroiliac joints is obtained by positioning the patient supine on the imaging table with the legs extended. Position the patient's shoulders and ASISs at equal distances from the imaging table to prevent rotation (Figure 7-33).

• Detecting sacroiliac joint rotation. Rotation is detected on an AP axial sacroiliac joint projection by evaluating the alignment of the long axis of the median sacral crest with the symphysis pubis and the distance from the sacrum to the lateral pelvic brim. When the patient is rotated away from the supine position, the sacrum moves in a direction opposite from the movement of the symphysis pubis and is positioned next to the lateral pelvic brim situated farther from the imaging table. If the patient is rotated into an LPO position, the sacrum is rotated toward the patient's right pelvic brim. If the patient is rotated into an RPO position, the sacrum rotates toward the patient's left pelvic brim.

• The patient is positioned supine, with the legs extended, and the lumbosacral curve causes the proximal sacrum and sacroiliac joints to be angled 30 to 35 degrees with the imaging table and IR. To demonstrate the sacroiliac joints without foreshortening, a 30-degree cephalic angle should be used for male patients and a 35-degree cephalic angle for female patients. Patients with less or greater lumbosacral curvature will require a decrease or increase, respectively, in cephalic angulation to maintain the 30- to 35-degree alignment of the central ray and sacroiliac joints. If an AP axial sacroiliac joint projection is taken with a perpendicular central ray or without enough cephalad angulation, the sacroiliac joints and the first through third sacral segments are foreshortened (see Image 23). If the AP axial sacroiliac joint projection is taken with too much cephalic angulation, the sacrum and sacroiliac joints demonstrate elongation and the symphysis pubis is superimposed over the inferior aspects of the sacrum and sacroiliac joints (see Image 24).

• Aligning the long axis of the median sacral crest with the long axis of the exposure field allows for tight collimation and ensures that the central ray is angled directly into the sacroiliac joints. To obtain proper alignment, find the point halfway between the patient's palpable ASISs and then align this point and the palpable symphysis pubis with the center of the collimator's longitudinal light line.

• Center the central ray to the patient's midsagittal plane at a level 1.5 inches (3 cm) superior to the symphysis pubis. Center the IR to the central ray, and open the longitudinal exposure field to the symphysis pubis. Transversely collimate to approximately a 9-inch (22-cm) collimated field size.

• A 10- × 12-inch (24- × 30-cm) IR placed lengthwise should be adequate to include all the required anatomic structures.

Anteroposterior Axial Sacroiliac Joint Projection Analysis

• Because the sacroiliac joint of interest is situated farther from the IR when AP oblique projections are taken, the marker used should identify the sacroiliac joint situated farther from the IR. This differs from the way most oblique projections are marked; routinely, the side marked is the one positioned closer to the IR.

• An AP oblique sacroiliac joint projection is obtained by beginning with the patient positioned supine on the imaging table, with the legs extended. From this position, rotate the patient toward the unaffected side until the midcoronal plane is at a 25- to 30-degree angle with the imaging table and IR. The sacral ala and ilium are positioned in profile. Place a radiolucent support beneath the patient's elevated hip and thorax to help maintain the position (Figure 7-35). Both AP oblique projections (RPO and LPO positions) must be obtained to demonstrate the right and left sacroiliac joints. When AP oblique projections are taken, the elevated sacroiliac joint is the joint of interest.

• Determining accuracy of obliquity. The accuracy of an AP oblique sacroiliac joint projection can be determined by the lack of ilium and sacral superimposition. The degree of separation or cavity demonstrated between the ilium and sacrum, which represents the sacroiliac joint, varies from patient to patient. The ilia and sacrum fit very snugly together and in older patients the joint spaces between them may be smaller or even nonexistent because of fibrous adhesions or synostosis. If the patient was not rotated enough to place the ilium and sacral ala in profile, the inferior and superior sacral aspects of the ala are demonstrated without ilium superimposition, whereas the lateral sacral ala is superimposed over the iliac tuberosity (see Image 25). The lateral sacrum is also demonstrated without ilium superimposition. If the patient was rotated more than needed to position the ilium and sacral ala in profile, the ilium is superimposed over the lateral sacral ala and the inferior sacrum (see Image 26).

• Aligning the long axis of the sacroiliac joint with the long axis of the exposure field allows for tight collimation without clipping any portion of the joint.

• Center the central ray 1 inch (2.5 cm) medial to the elevated ASIS to position the sacroiliac joint of interest in the center of the exposure field. Center the IR to the central ray and open the longitudinal collimation to the elevated iliac crest. Transverse collimation should be to the elevated ASIS.

• A 10- × 12-inch (24- × 30-cm) IR placed lengthwise should be adequate to include all the required anatomic structures.

Anteroposterior Oblique Sacroiliac Joint Projection Analysis