Test-Bolus versus Bolus-tracking Techniques for CT Angiographic Timing

Abstract

Editor: I have enjoyed reading recent published works on contrast material enhancement by Dr Cademartiri and colleagues. As an advocate for the bolus-tracking technique at computed tomographic (CT) angiography, I welcome the finding in their article in the December 2004 issue of Radiology (1) that bolus tracking provides more homogeneous enhancement than does the test-bolus technique. However, I am concerned that the conclusion statement in this article is overly simplified and thus may be misinterpreted and misused in practice. I believe that their valuable work will be correctly used and implemented if we clarify and address key factors that were not described in the conclusion but were crucial in determining the quality of contrast enhancement between the two groups in their study. The main reason in their study why bolus tracking provides more homogeneous enhancement than the test-bolus technique is that the scan delay was longer with the former than with the latter. I strongly agree with their statement in the discussion section (page 821) that the geometry of contrast enhancement differed in the two groups mainly because the calculated scan delay was 6 seconds later in the bolustracking group than in the test-bolus group. The bolus-tracking method “fortuitously” resulted in a longer and more desirable scan delay than the test-bolus method because there was an intrinsic built-in delay in the bolus-tracking method between the trigger and the start of the diagnostic scan. It is highly likely that, if an additional delay of 6 seconds had been included in the test-bolus group or if the bolus-tracking technique had been triggered earlier or much later, the test-bolus method would have provided better contrast enhancement than the bolus-tracking method. I need to emphasize that the main determinant of contrast enhancement was an additional delay, not the test-bolus versus the bolus-tracking methods. The time determined by the test-bolus or bolus-tracking techniques simply represents the time of contrast material arrival or bolus transit of the contrast material (2). This time should not be lackadaisically assumed to serve as the scan delay, but rather as a means of individualizing the scan delay relative to it by including an “additional delay” (2,3). This requirement of an additional delay for optimizing the scan delay for fast CT was discussed and reported previously (3). The magnitude of the additional delay depends on the injection duration, scan duration, and location of the target organ (2). To achieve adequate contrast enhancement throughout the scan duration, we have to consider the injection duration in the computation of the additional delay (4). Time to peak aortic enhancement is determined by the relative contributions of injection duration and contrast medium traveling time. In the current study, the injection duration was 25 seconds. The time to peak aortic enhancement, which is shortly after the completion of injection (4,5), would be around 30 seconds. Although it was not specified in the article, I estimate from the given CT scan parameters and scan range that the scan duration is approximately 15–20 seconds. By using a common strategy of setting the center of the scan to coincide with the time to peak aortic enhancement, the scan delay is calculated as 20.0–22.5 seconds (30 seconds minus a half of 15–20 seconds). This scan delay matches more closely with the scan delay estimated by the bolus-tracking method (mean, 20.6 seconds) than that with the test-bolus method (mean, 14.6 seconds) in the study. As a result, more favorable contrast enhancement was achieved with the bolus-tracking method. However, we have to keep in mind that when we use different injection and scan protocols, the scan delay should be adjusted accordingly. For a longer injection, a longer scan delay would be preferred. With a faster CT scan, such as when 64–detector row CT is used, the scan duration is shorter, and thus, a longer scan delay is desirable to scan during maximal contrast enhancement. Other minor points include that, although the test-bolus technique used in their study referred to the approach of setting the CT scan delay at the time to peak test-bolus contrast enhancement (6,7), there were other variations of determining the scan delay from the time to peak enhancement in the test-bolus technique (8,9). On page 818, 180gauge should be 18-gauge. In summary, the time of contrast material arrival can be estimated either with a test-bolus or bolus-tracking method, although the bolus-tracking method is preferred because of its efficiency and practicality. It is critical to include an appropriate additional delay, which is related to the scanning speed and injection duration, to determine an optimal scan delay from the estimated time of contrast material arrival. This consideration is particularly important with faster CT. An inadvertently applied bolus-tracking approach may fare no better than a properly designed test-bolus approach.