Imaging description

The ability to identify pulmonary emboli depends on the quality of the CT scan. Factors that diminish image quality include low contrast enhancement level, noise, motion artifact, and beam hardening artifact from high-density contrast in the SVC. The level of contrast enhancement achieved depends on the rate of contrast infusion and how the scan is timed. Contrast infusion is currently performed with infusion rates of 4–5 ml/second. Volume can be reduced to 100 ml for an average-sized patient, but for larger patients a higher volume may be necessary to ensure adequate contrast-to-noise ratio.

Optimally, the acquisition timing is tailored to the patient's hemodynamics, often performed with the bolus tracking technique. In this method, a region of interest (ROI) positioned on the main pulmonary artery samples the density as contrast is infused. A preset trigger point is applied, and when the contrast level reaches this density, the scan is initiated.

Performing a well-timed exam requires knowledge of the speed of the scanner being used. The new scanners that can perform studies in a few seconds enable the trigger timing to be delayed, so that the very peak of contrast enhancement is captured. For example, using a 64-slice scanner the study is triggered at 120–150 HU. This usually results in a high level of enhancement in the pulmonary arteries (Figure 42.1), prior to aortic enhancement. With a 128-slice dual-source scanner, the trigger point is delayed to 200 HU (Figure 42.2), and very high-quality imaging can result from high pulmonary artery enhancement and absence of motion artifact.

Despite an understanding of how scanner speed dictates scan timing for bolus tracking timing, pitfalls can occur when bolus tracking is used. One pitfall is malpositioning of the ROI cursor due to patient breathing or movement (Figure 42.3). In some cases, although the ROI cursor is well positioned and scan timed properly, the contrast enhancement level is suboptimal (Figure 42.4).