The Best Parameters for Imaging Agent Injection and Scanning Methods in Computed Tomography Angiography

Patients All cases with suspicion of PE or AD referred for CTA in our hospital between March 2018 and February 2020 had been reviewed. Those cases with known allergic history, poor renal function (creatinine level>1.5 mg/dl), pregnant women or age less than twenty years old were excluded from the study. The investigators finally enrolled and analyzed 101 cases of CTA. The collected data included age, sex, blood pressure, body mass index (BMI), Hounsfield unit (HU) at bifurcation of PA, HU at the descending aorta with the same level of PA bifurcation, time to peak enhancement, and radiation dose. This study had been approved by the institutional review board in our hospital. All patients signed informed consent.

In addition, forty cases with suspicion of PE or AD performing traditional two-time CT scans were reviewed retrospectively. For controlling the dose of contrast media, the two-time CT scans in our hospital were performed with single 80ml contrast media injection at 3ml/sec, followed by 20ml normal saline injection at 3ml/sec. The CT PA angiography was performed when the density of the PA achieved baseline PA density plus 150 HU during tracking scans, followed immediately by the CT aortogram. The investigators compared the experimental groups and traditional groups.

Experimental designs of contrast media injection After the contrast injection, the contrast media goes to the right heart via peripheral veins and then the contrast is pumped out to the PA. The interval from contrast injection to the peak enhancement of PA is defined as P seconds. After pulmonary circulation, the contrast media travels through the pulmonary veins, the left atrium, and the left ventricle and contracts into the aorta. The interval from contrast injection to the peak enhancement of aorta is defined as A seconds. Afterward, the contrast media starts a new cycle of systemic circulation.

Based on the published data, for the pulmonary artery, adequate attenuation is known to be greater than 180 HU, and for the aorta, sufficient attenuation is known to be greater than 200 HU. However, the requirement of vascular enhancement for diagnosing PE is high because the size of the PA is significantly tapering and the thrombus in the distal PA branches will be vague if the enhancement of the PA is not vivid. Therefore, insufficient enhancement of the PA restricts the ability to diagnose PE. The investigators design a biphasic contrast media injection protocol, including a low rate(2ml/s) first phase injection and high rate(3ml/s) second phase injection, followed by high rate(3ml/s) normal saline injection. The first phase injection is kept for (A - P) seconds and the second phase injection is kept until the rest contrast media is completely injected. Theoretically, when the start time of the scan is at the A seconds, the first phase contrast media is about to significantly enhance the aorta, though the peak time and peak enhancement will not be optimal because the time intervals are calculated on the basis of high rate (3ml/s) injection. On the other hand, the second phase contrast media will maximally enhance the PA at the same time because the second phase contrast media has been injected for P seconds [A-(A-P)=P].

The pre-diagnostic axial CT images were set at the level of the PA bifurcation using 10ml test bolus contrast media injection at the 3ml/s rate. Radiology technologists in our team placed circular region of interest (ROI) within the areas of PA bifurcation and the descending aorta at the same level. The CT densities were collected and demonstrated the time-enhancement data. The peak enhancement time for the PA was chosen as P seconds and that for the aorta as A seconds.