A Novel Low-dose-contrast Cardiac Computed Tomography Method for Pre-Procedural Guidance in Transcatheter Aortic Valve Replacement

A. INTENT AND PURPOSE

Background:

Optimizing procedural and patient outcomes relies heavily on tomographic imaging data guidance for patients being evaluated for trans-catheter aortic valve replacement or implantation (TAVR) by providing accurate information of the aortic valve apparatus (aortic annulus and its pertinent neighboring structures).

The aortic valve annulus is a dynamic complex elliptical or ovoid 3D entity that is also subject to constant shape deformation across the cardiac cycle in addition to being influenced by the thoracic and intra-thoracic anatomy or variations. Given this inherent nature of the aortic valve annulus, electrocardiographic gated cardiac computed tomographic angiography (CCTA), a robust dynamic 3-dimensional imaging modality, is now widely accepted as the "gold-standard" for evaluating the aortic valve apparatus (AVA). To maximize the information obtained by CT, imaging needs to be performed with intravenous contrast injection.2 CCTA requires administration of iodinated contrast. The volume of iodinated contrast medium is of concern in many patients because candidates for TAVR frequently have impaired renal function.2 Contrast reduction and adherence to protocols for prevention of contrast-induced nephropathy is recommended.2 For access planning via the ilio-femoral route, some groups have reported direct aortic injection with extremely low volumes of contrast.

Reduction of contrast volumes for CCTA of the AVA can be achieved by using lower flow rates than for coronary CT angiography. Although 5 mL/s is typically recommended for coronary imaging, 3 mL/s may sometimes be sufficient for imaging patients in the workup for TAVR.2 Though suggested, these CCTA lower flow rates have not been systematically performed and validated in a population being evaluated for candidacy for TAVR. And as recommended by guidelines, a standard bolus of 80 mL to 120 mL of low-osmolar iodinated contrast is usually necessary for optimal TAVR CCTA scanning.1 In these patients, reducing the dose of iodinated contrast at the time of CCTA acquisition in an attempt to minimize renal injury, and at the same time being able to accurately assess the aortic annulus apparatus (AVA) will be of paramount importance. Some reduction of contrast nephropathy can be accomplished by minimizing contrast volume and the use of iso-osmolar or low-osmolar contrast agents. It has been suggested that the contrast volume threshold can be estimated by using the ratio of contrast volume to creatinine clearance. Nephrotoxicity is more likely when the contrast volume/creatinine clearance ratio exceeds 3.7:1.12 And using this, 3.7 x eGFR (estimated glomerular filtration rate) can be set as the maximal contrast dose for an imaging test or invasive procedure.12 14 CCTA acquisition methods Images were acquired with a Brilliance 64-slice CT scanner (Philips Healthcare, Andover, Massachusetts, USA). As per the recommendations on radiation protection in cardiovascular CT by the Society of Cardiovascular Computed Tomography (SCCT), a tube potential of 100 kV will be considered for patients weighing <90 kg or with a body mass index (BMI) <30; whereas a tube potential of 120 kV will be considered for patients weighing >90 kg and with a BMI >30. It will be adjusted, based on each individual patient's size, to the lowest setting that guarantees acceptable image noise for a suitable signal-to-noise ratio (SNR) to perform analysis.15 The patients' eGFR was first calculated using the MDRD (modification of disease in renal diet) GFR formula. The maximal allowable contrast dose for the CCTA was calculated using 3.7 x [(patient's BSA/1.73) x MDRD-eGFR].14 Next, the dose of contrast chosen for the CCTA was < 50% of the maximal allowable contrast dose. The MDRD formula can underestimate the GFR by about 6.2% in patients with chronic kidney disease and by about 29% in healthy individuals, making this an extremely suitable conservative approach for calculating the contrast dosing, with a tendency only to underdose and never to overdose the contrast. With our extensive experience in doing this, we have decreased the total contrast dose for CCTA of the AVA to 45 ml with satisfactory image quality and SNR for post-processing.