Overview | Aortic Valve on Gated CT | Aortic Stenosis | Aortic Regurgitation | Protocols | Videos

4D Imaging of the Aortic Valve: Gated Cardiac Imaging of the Aortic Valve on 64 Slice MDCT: Overview

Introduction:

Computed tomography (CT) coronary angiography is feasible with multidetector row CT (MDCT) scanners and involves simultaneous acquisition of scan data and an EKG tracing. This allows for retrospective image reconstruction during any phase of the cardiac cycle. For coronary artery imaging, reconstruction is typically performed during late diastole in order to minimize cardiac motion artifact. However, images can be generated from all points through the cardiac cycle for 4D images to visualize myocardial contractility and valve motion. This exhibit demonstrates the feasibility of using gated MDCT to assess the aortic valve.

Imaging Method

The scan is performed from the carina through the heart with retrospective EKG gating, 120 kVp, 850 effective mAs, 0.6 mm detector collimation, 0.33-0.37 second gantry rotation time, 0.75 mm thick slices at 0.4 mm intervals using a 64 slice MDCT scanner. The scan delay is determined by injecting a test bolus of 20 cc contrast followed by 40 cc saline and measuring the time to peak attenuation in the ascending aorta and adding 6 seconds to this delay. 80 cc nonionic contrast is injected at 4 cc/second followed by a 40 cc flush of saline. 10 sets of images are reconstructed through the cardiac cycle at 10% intervals from 0-90% of the R-R interval.

For assessment of the aortic valve, all 10 image sets are simultaneously loaded for processing on a commercially available workstation. An axial oblique volume rendered view of the valve is created such that the aortic root had a circular shape and all three cusps can be seen simultaneously. The reverse ramp function is used to decrease the density of the contrast filled blood around the valves. The 4D function is then used to visualize valve motion through the cardiac cycle.

Image review:

The number of valve leaflets (3 or 2), leaflet thickness, opening and closing of the leaflets, and presence of valvular calcification can be determined. The normal aortic valve has three leaflets, is paper thin in width, is noncalcified, and functions to prevent regurgitation while allowing forward flow of blood in systole without resistance. Complete apposition of the valve leaflets is noted during diastole and the aortic valve orifice opens widely during systole.  Abnormal valves can result in aortic regurgitation or stenosis.

The aortic valve is well seen on CT during systole and diastole. Since all the data is already available from the CT coronary angiogram, no additional imaging is necessary for the patient. The ability to demonstrate the valve may be helpful in patients with ascending aortic aneurysms, Type A dissections, and coronary artery disease.

Alternative Imaging Techniques:

The aortic valve is typically evaluated by transthoracic echocardiography (TTE) which relies mainly on Doppler imaging to detect a regurgitant jet or an increased flow velocity for aortic stenosis. The aortic valve orifice area is indirectly estimated from the velocity of the blood. However, this can lead to errors as flow can be affected by left ventricular contractility, preload and afterload. The angle of insonation and adequacy of the acoustic window can also affect the results of the study.

Similar to echocardiography, cardiac catheterization also computes the aortic valve area indirectly from the cardiac output, aortic valve gradient and heart rate, which are not constant factors.

Multiplanar transesophageal echocardiography (TEE) and magnetic resonance imaging (MRI) image the aortic valve directly and measure the orifice area. However, TEE is invasive and visualization of the aortic orifice can be difficult if there is heavy valvular calcification. MRI is an alternative and planimetric measurement of the aortic valve area on MRI correlates well with TEE and has good reproducibility. Signal void due to valve calcification and turbulent flow can be sources of error in estimating the edge of the valve leaflets on MRI. Similar to echocardiography, the regurgitant jet is also seen on MRI.