Single-Slice Spiral/Helical Computed Tomography: Physical Principles and Instrumentation

In the Beginning

An early pioneer in the development of the technique of volume scanning in CT was Dr. Willi A. Kalender (see Chapter 1) of the Institute of Medical Physics at the University of Erlangen, Germany. Kalender started work on spiral CT in 1988 with Peter Vock from Switzerland, and in 1989 he described the technical details and clinical applications of spiral CT to the Radiological Society of North America (RSNA) meeting in Chicago. Other early investigators included Mori (1987), various Japanese researchers, and Bresler and Skrabecz (1993).

Terminology Controversy

Kalender and Vock’s presentation at the RSNA in 1989 resulted in a flurry of activities related to volume scanning. An interesting debate that surfaced in the early literature was related to the terminology to best describe the method of volume data acquisition. Should volume scanning be called spiral CT or helical CT? In a letter to the editors of the American Journal of Roentgenology (AJR), Towers (1993) used an illustration (Fig. 11-1) to describe the fundamental differences between spiral CT and helical CT. Towers noted that the term helical describes a cylindrical configuration, whereas spiral refers to both cylindrical and conic configurations. He therefore recommended the use of the term spiral CT.

In another letter to the editors of AJR, Silverman et al (1994) argued that the term helical CT “best describes this new CT technology.” Dr. Mark Bahn of the Mallinckrodt Institute of Radiology offered mathematical definitions to support this view. He pointed out that mathematical dictionaries (Baker, 1961; James and James, 1976) provide more technical definitions of these two terms: a spiral describes a curve on a plane surface; a helix describes a curve in 3D space.

In response, Kalender (1994a) submitted a letter (Appendix A) to the editors of Radiology to support use of the term spiral CT, which convinced the editors to accept either term for CT papers published in Radiology. This book uses both terms as synonyms, as suggested by Kalender (1994a).

Scanning Sequence

In conventional CT scanning, the x-ray tube rotates around the patient to collect data from a single slice of tissue, followed by table indexing so that the next contiguous slice can be scanned. This process is repeated until data from several contiguous slices have been collected. The scanning sequence for this type of data acquisition consists of four distinct steps (Fig. 11-2).

In the first step, the x-ray tube must be accelerated to a constant speed of rotation. This means that the cables that supply power to the x-ray tube must be long enough to allow for the full 360-degree rotation. During this rotation (step 2), the x-ray tube produces x-rays that are transmitted through the patient to fall on the detectors, which measure the relative transmission values (data). At this particular point, the patient holds a breath, and data are collected from a specific axial slice. In step 3, the patient resumes breathing while the x-ray tube slows to a stop. In step 4, it is necessary to unwind the cable because of the 360-degree rotation and to move the patient and table so that the next contiguous slice can be scanned.

This four-step process is repeated until all the required contiguous axial slices have been scanned. The time it takes to accomplish steps 1, 3, and 4 is referred to as the interscan delay time (ISD).

Slices are usually collected in groups while the patient holds a breath, with the ISD after each pair of slices. Between two groups of slices is an intergroup delay, and it is at this point that the patient breathes. Additionally, the scan rate is expressed as the following ratio:

If the study requires more slices, its duration would have to be increased. This requirement places an additional burden on the patient to remain still to ensure that images obtained are free of motion artifacts.

Limitations

The limitations imposed by slice-by-slice sequential CT scanning include the following:

This chapter concentrates on methods to remove the ISD with the goal of acquiring the data continuously rather than in slices. This technique is only possible with continuous rotation scanners based on slip-ring technology.

Requirements for Volume Scanning

Because the data in spiral/helical CT are collected in volumes rather than slices, the following requirements must be met:

Data Acquisition

The first step in volume scanning is data acquisition (Fig. 11-7). The x-ray tube traces a spiral/helical path with a radius equal to the distance from the focal spot to the center of rotation. This results in an entire volume of tissue being scanned during a single breath-hold compared with slice-by-slice imaging (Fig. 11-8).

Transporting the patient too quickly leads to image degradation caused by motion artifacts or may cause the patient to feel motion sickness. It is therefore important that the patient moves at a constant speed. In general, patients are moved at a table speed of about 10 millimeters (mm) per second during a continuous 1-second scanning. If a 24-second scan is taken, then the anatomic volume scanned is 240 mm (24 cm). Scanning times differ by manufacturer but average about 32 seconds. In addition the slice thickness may range from 1 to 10 cm.

Several problems can result from data acquisition with spiral geometry:

These problems can be solved through the use of special postprocessing techniques. One such method involves a “dedicated reconstruction algorithm that synthesizes raw data representing a perfectly planar slice from the original spiral data by interpolation” (Kalender, 1995). Interpolation is a mathematical technique whereby an unknown value can be estimated given two known values on either side (see Chapter 6). Interpolation and extrapolation are illustrated in Figure 11-11.

Image Reconstruction

Inconsistent data that are obtained from 360-degree spiral/helical scan rotation are used directly in the image reconstruction process. Motion artifacts are apparent as shown in Figure 11-12, A, for phantom images and in Figure 11-13, A, for patient images.

To eliminate these motion artifacts arising from the continuous movement of the patient during scanning, two steps are needed:

A number of interpolation algorithms are used to produce the planar data set, and linear interpolation (LI) represents the “simplest approach” (Kalender, 1995). Two interpolation algorithms for single-slice spiral/helical CT are the 360-degree LI algorithm and the 180-degree LI algorithm.

Equipment Components

A block diagram of the major equipment components of a spiral/helical CT scanner is shown in Figure 11-19. The most noteworthy feature is the use of slip rings to connect the stationary and rotating parts of the scanner.

The rotating part of the system consists of the x-ray tube, high-voltage generator, detectors, and detector electronics (DAS). The stationary part consists of the front-end memory and computer and the first-stage high-voltage component.

The x-ray tube and detectors rotate continuously during data collection because the cable wraparound problem has been eliminated by slip-ring technology. Because large amounts of projection data are collected very quickly, increased storage is needed. This is accommodated by the front-end memory, fast solid state, and magnetic disk storage.

In spiral/helical CT scanners, the x-ray tube is energized for longer periods of time compared with conventional CT tubes. This characteristic requires x-ray tubes that are physically larger than conventional x-ray tubes and have heat storage capacities greater than 3 million heat units (MHU) and anode cooling rates of 1 MHU per minute (Bushong, 2004).

X-ray detectors for single-slice spiral/helical CT scanning are 1D arrays and should be solid state because their overall efficiency is greater than gas ionization detectors.

The high-voltage generator for spiral/helical CT scanners is a high-frequency generator with a high power output. The high-voltage generator is mounted on the rotating frame of the CT gantry and positioned close to the x-ray tube (see Fig. 11-19). X-ray tubes operate at high voltages (about 80 to 140 peak kilovolts) to produce x-rays with the intensity needed for CT scanning. At such high voltages, arcing between the brushes and rings of the gantry may occur during scanning. To solve this problem, one approach is to divide the power supply into a first stage on the stationary part of the scanner, where the voltage is increased to an intermediate level, and a second stage on the rotating part of the scanner, where the voltage is increased to the required high voltages needed for x-ray production and finally rectified to direct current potential (see Fig. 11-19) (Napel, 1995). Another approach passes a low voltage across the brushes to the slip rings, the high-voltage generator, and then the x-ray tube. In both designs, only a low to intermediate voltage is applied to the brush/slip-ring interface, thus decreasing the chances of arcing.

Slip-Ring Technology

One of the major technical factors that contribute to the success of spiral/helical CT scanning is slip-ring technology (see Fig. 11-19). The purpose of the slip ring is to allow the x-ray tube and detectors (in third-generation CT systems, see Fig. 11-19) to rotate continuously so that a volume of the patient, rather than one slice, can be scanned very quickly in a single breath hold. The slip rings also eliminate the long, high-tension cables to the x-ray tube used in conventional start-stop CT scanners. As the x-ray tube rotates continuously, the patient also moves continuously through the gantry aperture so that data can be acquired from a volume of tissue.

The technical aspects of using slip rings for data acquisition are described in Chapter 5. These include the design and power supply to the rings and a comparison of low-voltage and high-voltage slip-ring CT scanners in terms of the high voltage supplied to the x-ray tube.

Pitch

The pitch is a term associated with a fastener, and it is actually the distance between the turns on the fastener. In spiral/helical CT, the pitch is defined as the distance (in mm) that the CT table moves during one revolution of the x-ray tube (Fig. 11-20). The pitch is used to calculate the pitch ratio, which is a ratio of the pitch to the slice thickness or beam collimation. The pitch ratio is as follows:

When the distance the table travels during one complete revolution of the x-ray tube equals the slice thickness or beam collimation, the pitch ratio (pitch) is 1:1, or simply 1. A pitch of 1 results in the best image quality in spiral/helical CT scanning. The pitch can be increased to increase volume coverage and speed up the scanning process (Fig. 11-21). Pitch is of particular significance because it affects image quality and patient dose and also plays a role in the overall outcome of the clinical examination.

Volume Coverage

The volume coverage is given by the following relationship:

for fixed scan time and fixed slice thickness (Bushong, 2004).

Another factor that influences the volume coverage is gantry rotation time. The volume coverage in this case becomes:

Collimation and Table Speed

Collimation determines the slice thickness and in most cases is equal to the table increment (a pitch of 1). Collimation selection also depends on the type of tissue to be examined. Smaller structures usually require narrow collimation, whereas larger structures are imaged with wider collimation.

Another parameter of importance is the table increment (mm per second), also referred to as the table feed or table speed. As the table increment increases with respect to the collimation, pitch increases, resulting in a loss of image quality. To cover a certain volume, it is important to keep in mind two parameters that influence image quality: collimation and table incrementation. The goal is to use the smallest possible collimation at a pitch of 2 (with 180-degree interpolation algorithm). Hence, for a volume length of 30 cm, either a 5-mm or 8-mm collimation can be used with a table speed of 10 mm.

Scan Time

Another operator-defined parameter in spiral/helical CT that affects the outcome of the examination is the scan time, which refers to the duration of the scan. The choice of scan time depends on the patient’s respiratory condition. Most current scanners feature various spiral/helical scan modes (Fig. 11-22) for patients who have difficulty holding the breath. For example, in a multistep spiral/helical scan mode, multiple scans are obtained with a pause between scans to enable patients to breathe so that the required length of tissue can be imaged successfully.

Reconstruction Increment

Another parameter unique to spiral/helical CT is the reconstruction increment (RI), also referred to as the reconstruction interval or reconstruction spacing. The RI determines the degree of sectional overlap to improve image quality. As RI decreases, image quality generally improves. Urban et al (1993) found that for the detection of small lesions in the liver, a 50% overlap (Fig. 11-23) resulted in better visualization of these lesions by 10%. In a set of simulations conducted by Brink et al (1994) to find whether an increased degree of overlap would provide better results, the researchers recommended “reconstructing one or two sections per table increment for routine diagnosis and at least three sections per table increment for multidimensional imaging.”

Spatial and Contrast Resolution

For the same image reconstruction parameters, contrast and spatial resolution should be about the same for both spiral/helical CT and conventional CT. For example, measurements of the scan plane (image plane) spatial resolution conducted by Kalender (1995) with a standard poly(methyl methacrylate (Perspex) hole pattern phantom demonstrate this similarity between spatial resolution for conventional CT and spiral/helical CT. The visual evidence is shown in Figure 11-24. However, as noted by Kalender (1995), the contrast for small objects and the spatial resolution along the z-axis (longitudinal direction) may be somewhat different. For example, in simulations, phantom experiments, and specimen studies that compared contrast and spatial resolution along the z-axis for both conventional and spiral/helical CT, Kalender et al (1994) found that spiral/helical CT provides “significantly better” contrast and spatial separation of 5-mm spheres imaged with a 5-mm slice thickness.

Noise

Noise in spiral/helical CT is affected by a range of parameters, including beam intensity, beam quality (energy), slice thickness, and matrix size (pixel size). Additionally, noise in spiral/helical CT is also affected by the interpolation algorithm. Compared with conventional CT (all other factors held constant), the 360-degree LI algorithm produces less noise; however, when the 360-degree LI and the 180-degree LI algorithms were compared, it was found that the 180-degree LI algorithm produced more noise and thus degraded image quality (Kalender, 1994b). However, this problem has little effect on the image quality of bony structures because the subject contrast of bone is greater than its surrounding soft tissues (Fig. 11-25).

Slice Sensitivity Profile

The SSP “describes how thick a section is imaged and to what extent details within the section contribute to the signal” (Kalender, 1994b). For conventional CT, the shape of the SSP is rectangular (Fig. 11-26, A). The section thickness is measured in the middle of the rectangle. This point is referred to as the full width at half maximum (FWHM). For spiral/helical CT, the SSP also depends on the pitch and the interpolation algorithm. Figure 11-26, B, shows the SSPs for a pitch of 1 and 2. Figure 11-26, C, shows SSPs for a 180-degree LI algorithm and a 360-degree algorithm. The SSP degrades with a pitch of 2 and a 360-degree LI algorithm. (The FWHM does not approximate closely the actual slice thickness selected.) As the SSP widens, image quality degrades.

Dose Comparison

During the early development and use of spiral/helical CT, it was thought that the dose would be higher simply from volume data acquisition. Comparison studies show that the dose in conventional CT is about equal to that of spiral/helical CT.

There are several reasons why patient dose is less in SSCT than in conventional CT, as follows:

The effective doses from spiral/helical CT for the head, chest, abdomen and pelvis are 1.1, 6.7, 4.3, and 2.7 millisieverts (mSv), respectively (Kalender, 1994b). The effective dose allows the quantification of the risk from partial-body exposure on the basis of that received from an equal whole-body dose.

Huda et al (1997), for example, reported effective dose range from 1.5 to 5.3 mSv for abdominal examinations in pediatric and adult patients. To place all this in perspective, the effective dose from natural background radiation is about 2.4 mSv per year (Kalender, 1994b).