Computed tomography (CT) is gaining popularity as a minimally invasive diagnostic modality in veterinary science. The use of contrast agents is well described and used with increasing frequency, having marked benefits over the invasive angiographic procedures used previously. Methods to perform CT angiographic (CTA) studies include bolus tracking, test bolus and empirical scan delay techniques. In human medicine, empirical scan delays have been extensively investigated but due to the marked patient variability encountered in veterinary medicine, this technique cannot, at this stage, be confidently used. This then poses the question that if both techniques can be used, is one significantly better than the other one when performing, in particular thoracic CTA? CTA studies were performed on 6 adult beagles, using the bolus tracking (BT) technique and the test bolus (TB) technique on two separate occasions, at least 2 weeks apart. Each beagle acted as its own control. The patients were placed under general anaesthesia. Two ml/kg of 300mgI/ml iodinated contrast agent was injected through a 20 gauge catheter place in the cephalic vein for the BT technique. Scans were initiated when the contrast in the aorta reached 150 Hounsfield units (HU). For the TB technique, the dogs received a test dose of 15% of 2ml/kg of 300mgI/ml iodinated contrast agent injected manually into the cephalic catheter, followed by a series of low dose sequential scans. Time attenuation curves were generated using dynamic evaluation software programs (DYNAEVA®). The full dose of 2ml/kg of the 300mgI/ml iodinated contrast agent was then administered and the scans were conducted at optimal times as identified from the time attenuation curves. The full dose of contrast administration was administered using a pressure injector operated at 3ml/sec and was followed by a manual saline flush for all studies. Mean attenuation in HU was taken at three consecutive levels in the aorta (Ao), caudal vena cava (CVC) and right caudal pulmonary artery (rCPA) by placing a region of interest (ROI) in the vessel of interest. These observations were done for both arterial and venous phases using the BT and TB studies in five of the six dogs. Additional observations included the visualisation of the smaller thoracic vessels, in particular the arteries, the study duration, milliampere (mA), computed tomography dose index volume (CTDI(vol)), dose length product (DLP) and the pressure and the duration of the contrast injection. These observations were done in all six dogs. Statistical analysis included the comparison of the attenuation achieved in the major vessels (Ao, CVC and rCPA) for the two techniques. The test bolus technique was deemed to be the gold standard, as this is adapted for each individual patient. In one beagle the study was considered non diagnostic with a later time to peak enhancement noted in the Ao than the CVC. No statistical analysis was done on the dynamic and attenuation data obtained in this dog. In all the other studies, the attenuation achieved in the vessels evaluated was deemed to be of diagnostic quality except for the attenuation achieved in the CVC during the arterial phase. The attenuation in the Ao during the arterial phase for the BT technique was not significantly different (660.52 HU ± 138.49) than the TB technique (469.82 HU ± 199.52)(p = 0.13). The attenuation in the Ao during the venous phase for the BT and TB techniques was also not significantly different (BT = 190.6 HU ± 28.29 and TB = 188.8 HU ± 21.9, p = 0.92). The attenuation in the CVC during the arterial phase and venous phases for the BT techniques were not significantly different (arterial phase BT = 37.84 HU ± 20.8, arterial phase TB = 91.48 HU ± 66.54, p = 0.069; venous phase BT = 171.3 HU ± 32.36, venous phase TB = 191.08 HU ± 19.59, p = 0.087). The attenuation in the rCPA during the arterial phase was not statistically different between the two techniques (BT= 606.34 HU ± 143.37 and TB = 413.72 HU ± 174.99, p = 0.28), nor was the attenuation in the rCPA during the venous phase (BT = 174.34 HU ± 27.25 and TB = 164.46 HU ± 18.51, p = 0.51). The mean mA for the BT technique (3538.5 ± 171.27) was significantly lower than the TB technique (3929.6 ± 312.3) with p = 0.024. The mean CTDI(vol) for the BT technique (24.42 ± 11.89) was significantly lower than the TB CTDI(vol) (45.32 ± 0.94) with a p-value of 0.013. The mean DLP did not differ significantly between the two techniques (BT = 139.1 ± 7.65, TB = 162.8 ± 33.1) (p = 0.12). The BT technique resulted in a significantly shorter procedural duration and utilised less contrast material than the TB technique. The injection duration and injection pressures did not differ significantly between the two techniques (p = 0.23 and p = 0.62 respectively). This study identifies that there is no preference for either technique when evaluating the Ao, CVC or rCPA, however, the BT technique is shown to be shorter in procedural duration, utilises less contrast material and results in less radiation dose to the patient when compared to the TB technique. Copyright
Dissertation (MMedVet)--University of Pretoria, 2011.