Clinical Solutions for Technical and Physical Challenges of CT Pulmonary Angiography (Based on 16 Slice Scanners)

Introduction:
Pulmonary embolism occurs when a blood clot gets stuck in an artery in the lung, blocking blood flow to part of the lung. Because of its associated risks and because CT pulmonary angiography offers similar or better information, direct pulmonary angiography is now rarely performed, except in the patient who may undergo pulmonary embolectomy. CT pulmonary angiography (CTPA) is known as the gold standard for the diagnosis of pulmonary embolism. Although development of multi-slice scanners with higher speed, lower radiation dose, bolus tracking, and test bolus has increased the quality of images and resolves the many of previous challenges; However, radiology technologists have many challenges for performing this procedure in practice. This challenges refer to patient physiopathology conditions (e.g heart output, kidney disease, breath holding problems and etc.), contrast medium (low/high Iodine concentration, dye temperature), scanner’s speed an equipment with angiographic applications (Blus tracking or test bolus), type of injector (dual or single head), using low- dose protocols, and type of catheter (central or peripheral venous) will be discussed. Firstly,
the fundamental principles of CTPA from the patient educations to the multi-planar reconstructions were discussed in summary. Finally, the clinical solutions were provided to decreasing the dye volume at the lowest, obtaining the best delay time, optimizing the protocol parameters, with considering the patients’ safety based on the recent studies.
Results and Discussions:
Although, the patient weight is recommended for determining the contrast medium (CM) volume for CTPA, however vessels diameter and patient height must be included for calculating the contrast medium volume. Also, using dual head injector and test bolus (instead of bolus tracking) can decrease the total contrast volume to 35 mL. Temperature can increase the contrast medium viscosity and have inverse effect on the choosing the injection rate in patient with unstable veins. Using a test bolus to determine CM arrival time at two locations (i.e., ascending aorta and pulmonary trunk) can be performed to better determine the arrival time of the diagnostic CM in the target vessel of choice, especially in patient with heart failure. Using the bolus
tracking instead of test bolus can be more useful in some patient with urgency condition or for double rule-out study (CTA for Pulmonary Embolus and thoracic aorta). Craniocaudal CT pulmonary angiography is suggested in recent 16-slice scanners with short scan time equal to the patient breath holding (5-10 s). Craniocaudal had a similar degree of respiratory motion artifact to caudocranial scanning, in contrast, cause to better peak contrast enhancement in the distal pulmonary branches. Inserting the region of interest (ROI) in the nearest place to the beginning of scan is necessary, especially in scanners with high diagnostic delay time. In this way, there is not necessary to starting the scan from top of lung. Thus inserting the ROI in 1-2 cm upper than aortic arch helps to decrease the diagnostic delay and decreasing
the patient radiation dose. Although, using lowest rotation time is necessary for tracking the bolus to increasing the accuracy of determining the peak contrast enhancement; however, decreasing the scan speed is more helpful in patient with insufficient heart output (high time of peak enhancement) and with tall lung. In this cases, the contrast medium has enough time to arrival to the distal pulmonary branches. The tube voltage decreasing instead of tube current should be considered in low- dose protocols. Recent studies are shown that using the lower tube voltage (80-100 kVp) can allow the lower contrast medium usage with same diagnostic value for patient with medium size. Maximum intensity reconstruction (MIP) can’t be useful when the pulmonary veins are enhanced because of inappropriate timing.