10.1 The Need for Multislice CT
So far, this book has focused on the detector configuration in which a single row of a detector is irradiated by the x-ray beam. Although helical (spiral) CT has significantly improved volume coverage, many clinical applications demand an even greater volume coverage and thinner slices. A good example is CT angiography. For this application, a rapid volume acquisition is needed during the plateau phase of contrast enhancement. For thoraco-abdominal aorta studies, the volume includes the whole chest and abdomen, which can reach between 45 and 60 cm along the patient (z) axis. A more demanding case is the runoff study of the abdominal aorta and the legs, which covers the area from the celiac artery to the calves, with a typical z coverage between 90 and 120 cm.
To ensure optimal contrast enhancement and minimal patient respiratory motion, an entire study should be completed in less than 20 sec. For instance, to cover 40 cm in 20 sec, the patient table must travel at a speed of 2 cmâsec. With a single-slice scanner rotating at 0.5 sec per revolution, it is difficult to obtain a slice thickness of 5 mm or less at this coverage. For example, consider a likely scanning protocol of 5-mm collimation at a 2:1 helical pitch. As discussed in Chapter 9, the effective slice thickness of a reconstructed image increases quickly with an increase in helical pitch. Consequently, the slice thickness of this protocol is 6.4 mm at FWHM and 11.1 mm at FWTM. This is clearly suboptimal for the visualization of small vascular structures. If we use a thinner collimation or slower helical pitch, the time to cover the same volume increases, which leads to several complications. First, we may lose the optimal timing for contrast enhancement. When the scan time is significantly lengthened, the contrast washout effect becomes significant. Although we can partially compensate for the effect by prolonging the contrast injection time, we often run into limitations such as the total amount of allowable contrast administered to the patient. Prolonged scan time and contrast injection also preclude the examination of organs in their arterial (free of venous) phase.
The second problem with a thinner collimation or slower helical pitch is patient motion. This is particularly important for chest and abdominal studies. A typical patient can hold his or her breath for approximately 20 to 30 sec. For studies that extend beyond this range, multiple breath-holds are required.