CT-angiography before TAVI using high-pitch spiral acquisition mode

Introduction:

Preparation for transcatheter aortic valve implantation (TAVI) requires imaging of the aortic root, aorta, and iliac arteries. Contrast agent exposure can be a problem in TAVI candidates who often have impaired renal function. High-pitch spiral CT allows high-resolution imaging of large volumes in a very short time. We evaluated the ability of high-pitch spiral imaging with dual source CT (“Flash Mode”) and a minimized amount of intravenous contrast to evaluate aortic root anatomy and suitability of vascular access for TAVI procedures.

Methods:

25 patients (15 female, 10 male, mean age 82±7 years, mean aortic valve area 0.8±0.2cm², mean log. Euroscore 34±14%) scheduled for TAVI underwent CT angiography of the aorta using a dual source CT system and prospectively ECG-triggered high-pitch spiral acquisition mode (120kV, 320mAs, 0.28s gantry rotation, collimation 2×128×0.6mm, pitch 3.4, triggered at 60% of the R-R interval). 40ml contrast agent (350mg iodine/ml) were injected intravenously at a rate of 4ml/s. Cross-sectional images were reconstructed with 1.0mm slice thickness and 0.8mm increment. We analyzed contrast enhancement by measuring CT attenuation in the aortic root, aortic arch, and external iliac arteries, as well as tissue enhancement next to the respective arteries. Image noise was determined in the aortic root. Contrast to noise ratios were determined by dividing the difference between luminal attenuation and attenuation adjacent to the arteries by the image noise.

Results:

Mean length of the scan range was 586±49mm (range: 465–651mm) and mean duration of image acquisition was 1.37±0.11s (range: 1.18–1.58s). The mean dose length product was 255±92 mGy*cm. Mean CT attenuation was 310±73 HU in the aortic root, 324±71 HU in the aortic arch, and 307±105 HU in the iliac arteries. Contrast-to-noise ratios were 17.1±6.9 in the aortic root, 17.4±6.3 HU in the aortic arch and 16.7±6.9 HU in the external iliac arteries. Based on the CT images, aortic root dimensions and distance of coronary ostia from the annulus (mean left: 12±2mm, right: 14±2mm) could be determined in all cases. Regarding vascular access, 14 patients were considered suitable for a transfemoral approach, 11 were not, in 1 case results were equivocal due to insufficient contrast enhancement. All 14 patients scheduled for the transfemoral approach completed the procedure without vascular complications.

Conclusion:

High-pitch spiral CT allows imaging of the entire aorta and iliac arteries in less than 2 seconds. Due to the short overall scan time, sufficient intraluminal attenuation and contrast-to noise ratio is achieved with low amounts of intravenous contrast agent, which is of critical importance in TAVI candidates with often severe renal comorbidity.