Subtraction computed tomographic angiography of calcified arteries: preliminary phantom and clinical studies

The technique of subtraction computed tomographic angiography (sCTA) has been proposed for the evaluation of atherosclerotic disease to address limitations in CTA in highly calcified arteries. However, sCTA has not gained acceptance in clinical practice, in part, due to image artifacts caused by patient motion that occur between the acquisition of the two component images. The purpose of this study was to evaluate the effectiveness of computational image co-registration to obtain sCTA.

The study was conducted using a semi-automated implementation of the mutual information (MI) registration algorithm. The results of sCTA were evaluated quantitatively in a phantom representing a calcified artery. Technical success of sCTA was evaluated in 14 calcified arterial segments in two patients. An observer study was carried out to determine interobserver agreement in the interpretation of sCTA. Qualitative observations were made between sCTA and CTA.

Computation time for performing the co-registration for each 2-cm calcification is less than 1 second. The necessary user interaction required minimal expertise. Measurements of the degree of stenosis in the calcified artery phantom agreed to within 8 +/- 4% of gold-standard measurements. Technical success was demonstrated in all calcifications. Strong interobserver agreement was obtained for the detection of hemodynamically significant stenoses (kappa = 0.86). Several apparent pitfalls in the interpretation of CTA in calcified arteries were noted that could potentially be obviated by sCTA.

The study supports the use of a straight-forward implementation of the MI algorithm and provides preliminary evidence validating the use of sCTA in the setting of atherosclerotic disease of the lower extremities.