annulus.

　　Multi-detector Computed Tomography (MDCT) is evolving as a powerful complementary test for the pre-procedural planning of TAVR. It is hoped that improvements in prosthesis sizing will reduce complications during implantation， including paravalvular leak. Major indications for MDCT prior to TAVR include evaluation of the aortic annulus， aortic root， valve structure， calcification and access route. The main drawback of MDCT is exposure to potentially nephrotoxic iodinated contrast agents. Radiation exposure is of less concern given the advanced age of patients undergoing TAVR.

　　The technical aspects of acquiring measurements for TAVR with MDCT， as well as the standard measurements obtained， are well documented in the SCCT expert consensus document. Images of the valve should be acquired with EKG gating using a slice thickness of <1.0 mm. Contrast bolus should be timed for the ascending aorta similar to coronary CTA.

　　For the appropriate sizing of TAVR prosthesis， it is important to assess the aortic annulus size， the size of the sinuses of Valsalva， the coronary ostia distance from the annulus， the size of the aorta at the sino-tubular junction and 40 mm above the annulus. Correct annular sizing is critical for the pre-procedural planning of TAVR as under-sizing of the prosthesis can contribute to paravalvular aortic regurgitation as well as valve embolization. Over- sizing of the prosthesis to the aortic annulus has been associated with aortic root rupture. In addition， the correct positioning of the valve is also important because incorrect positioning can lead to increased paravalvular aortic regurgitation or place the coronary ostia at risk for occlusion.

　　There are several challenges to correctly assessing the aortic annulus. The aortic annulus is known to be an oval shaped， virtual ring formed by the joining of basal attachments of the aortic valvular leaflets. MDCT is much better suited for measuring the elliptical structure of the annulus compared to TTE because TTE systematically underestimates the major dimension of the annulus. The annulus should also be measured during systole because it undergoes conformational， pulsatile changes during the cardiac cycle. Utilizing diastolic measurements risks under-sizing of the prosthesis.

　　In order to measure the annulus on MDCT， the images should be lined up in two orthogonal planes and capture all 3 valve insertion points. Measurements should be performed in ventricular systole (20%～45% of the RR interval). Annular measurements are completed using three primary methods: ①Measurement of the average of the major and minor annulus diameters. D=(Dmax+Dmin)/2; ②Measurement of the perimeter with assumption of circularity of annulus. D=Perimeter/pi; and ③Planimetry of the area with assumption of circularity of annulus [D=2*sqrt(A/pi)]. However， with the use of multimodality imaging， there may be a need to “adjust the sizing of the prosthesis” because annular assessment by TTE is on average 1mm smaller than TEE， which is approximately 1～1.5 smaller than MDCT.

　　In terms of vascular access， MDCT easily delineates unfavorable ilio-femoral anatomy for TAVR such as a minimal luminal diameter of the common iliac， external iliac， or common femoral artery of less than 8 mm; presence of greater than 60% circumferential calcification at the external-internal iliac bifurcation; and severe angulation (<90 degrees) between the common and external iliac arteries. It is also important to obtain similar information regarding the subclavian arteries， ascending aorta， aortic arch， and descending aorta depending on the route of access planned. Additional aortic considerations include aneurysm and dissection formation.

　　There are exciting developments in pre-procedural planning with TAVR. At our institution， there is active development of an automated 3D model of the aortic root and aortic valvular calcification maps， which hold great promise in TAVR prosthesis sizing.