Multidetector Computed Tomography Imaging Before Aortic Valve Implantation
acquisitions: a cardiac assessment of the aortic valve immediately followed by abdominal and pelvic CT angiography, which is essential for assessing the retrograde route of the device. Other authors suggest performing only one acquisition.5
Review of the Technique
At our institution, MDCT is performed on a Discovery 750 HD 64-row MDCT scanner (GE Healthcare, Milwaukee, WI, US) and the axial data sets of images are sent to the Advantage Workstation version 4.5 (GE Healthcare, Milwaukee, WI), which is equipped with commercial software for cardiac and iliofemoral analysis. CT angiography of the aorta and iliofemoral arteries is almost always feasible even in tired, elderly patients. The cardiac examination can be more difficult to perform for some patient-related reasons (difficulties with holding the breath, deafness, tachycardia, or tachyarrhythmia).
A good MDCT examination requires high-quality vein access. Renal failure is frequent in this population, but the risk posed to the kidneys by the use of iodinated contrast agent needs to be balanced against the natural history of the aortic stenosis if left untreated. Estimating and following up the creatinine clearance using the modification of the diet in renal disease (MDRD) equation is essential. At our institution, we try to space the injections and limit the volume and concentration of contrast agents. Patient hydration and administration of bicarbonates are well-known renal-protection techniques that can be implemented. Radiation dose control is not the first priority in this elderly population with high-risk disease. However, the rules of good practice are always applied (collimation, minimal time and zone exposure), and adaptive statistical iterative reconstruction (ASiR) is routinely used to lower the dose while maintaining equivalent image quality.
A comfortable position on the table, clear and controlled explanations, and apnea tests prior to acquisition are other factors that help to guarantee the quality of the examination. The patients never receive beta-blockers because of concerns regarding the critical aortic stenosis. Pre-operative explorations are confronted with and validated by clinical data from other imaging modalities during a multidisciplinary meeting with interventionalists, radiologists, and surgeons.
Cardiac Exploration A collimation of 64 x 0.625 mm and a rotation of 35 ms are used. The tube voltage is usually set at 120 kVp because of calcium accumulations, except in patients with a low body mass index (BMI), in whom 100 kVp is used. Patients are scanned with retrospective gating with a similar field of view (FOV) to that used for a coronary artery examination; however, the FOV needs to be extended when mammary bypass grafts are present. As the entire systolic–diastolic motion of the aortic valve needs to be analyzed, electocardiogram (ECG) dose modulation is avoided. The tube current is manually set at between 450 and 550 mA. About 70–80 ml of iodine contrast material is administered, then flushed by saline. The flow rate is typically set at 5 ml/s.
Peak enhancement detection of the ascending aorta is essential because of varying hemodynamic parameters and different degrees of aortic stenosis among these patients. Data acquisition is
US RADIOLOGY
Figure 1: The Edwards SAPIEN Transcatheter Heart Valve has been Emphasized on the Reformat View Using a VR 3D Preset Dedicated to Metal Implant Analysis
Figure 2: Aortic Valve Calcification Grade AB
CD
A: Grade 1—no calcification; B: Grade 2—mild calcification (small isolated spots); C: Grade 3—moderate calcification (multiple large spots); D: Grade 4—heavy calcification.
performed during an inspiratory breath-hold. Reconstruction covers at least 10 phases of the cardiac cycle (RR interval), allowing analysis of the entire systolic–diastolic set and assessment of the left ventricular ejection fraction as well as mitral motion. Filters adapted for highly attenuating structures are recommended, as is a high-resolution reconstruction.
CT evaluation of the aortic root begins with an assessment of the aortic valve calcification: intensity, topography, and extension to the interventricular septum and aortic valve. At our institution, we routinely use the classification suggested by Willmann et al.,6
as follows 33
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