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MRI evaluation of aortic stenosis: anatomical study

Author: Francisco Alpendurada


 

Cardiovascular magnetic resonance (CMR) is a reliable technique to evaluate the morphology and function of the aortic valve. High quality images can be acquired in any plane or phase of the cardiac cycle, permitting perfect alignment and accurate measurements of the aortic root and valve. Moreover, CMR is relatively insensitive to calcium artefacts, and hence useful to evaluate the aortic valve even when there is significant calcification.
Of the methods employed by CMR to quantify aortic stenosis, direct planimetry is arguably the most validated and established one. This overview describes how to image the aortic valve and how to estimate the aortic valve area by direct planimetry. The key steps involved are detailed below.

1. Planning

Imaging of the aortic valve is planned from the left ventricular outflow tract (LVOT) view and the LVOT cross-cut view (perpendicular to the LVOT view). To study the morphology of the aortic valve, a single cine image aligned with the tips of the cusps usually suffices (figure 1), but to assess the aortic valve area, a stack of cines covering the aortic valve is generally required (figure 2).

 

Figure 1: LVOT view in systole (top left panel) and corresponding cross-cut view (top right panel). The aortic valve view (bottom panel) is obtained from an orthogonal plane to the LVOT views aligned with the tips of the aortic valve (as represented by the solid gray lines).

2. Acquisition

Most studies use a stack of cine images with a reduced thickness of 5mm (for better spatial resolution), and a position interval of 2-3 mm (for precise coverage of the aortic vavle orifice). The cine stack is usually aligned with the aortic valve orifice and perpendicular to the stenotic jet, and should cover the entire aortic valve. Usually the most convenient way to perform a stack of aortic valve cines is to start close to the aortic annulus, and then move upwards along the longitudinal axis of the aortic root (figure 2).

 

Figure 2: From the LVOT and LVOT cross cut systolic frames (top left panel and top right panel, respectively), several planes are laid out perpendicular to the stenotic jet along the aortic root (represented by the sold gray lines). The resulting images are displayed in the bottom panel. In this example, the slice thickness is 5mm and the slice interval is 3mm. These very small increments mean that there will be an overlap between images, allowing for correct positioning over the valve tips for planimetry.(Click on the bottom picture to enlarge in a new window)

3. Selection

Figure 3 displays a stack of images of the aortic root from the annulus to the sinotubular junction, ensuring complete coverage of the aortic valve. The still frame cine images are chosen at the time of the cardiac cycle where the cusps are at their maximal opening, i.e. in early/mid-systole. Images at different levels are then compared for planimetry, bearing in mind that the appropriate position will be at the tips of the valve.

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Figure 3: Stack of images of the aortic root from the aortic annulus to the sinotubular junction. The position interval between images is 3mm, enabling complete coverage of the aortic valve, and appropriate selection of images for planimetry. Images where the flow is seen (12 and 15mm above annulus) are too high and should be excluded from planimetry. Conversely, images where the cusps are not seen (at annulus level and 3mm above annulus) are too low and should also be discarded.(Click on the picture to enlarge in a new window)

4. Measurement

After selection of the most appropriate image for planimetry, a line is drawn over the inner edges of the cusps, resulting in an area that is used to estimate the aortic valve orifice (figure 4).

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Figure 4: In this example, two images (rather than one image) showed good visualization of the aortic cusps (6mm and 9mm above the annulus). Planimetry was performed by drawing a continuous line over the inner edges of the tips of the cusps. Usually the image with the smallest orifice is used to provides the final value (0.8cm2). Alternatively, the average area of the best two images is a reasonable option when both images are too similar to decide the most appropriate slice position.(Click on the picture to enlarge in a new window)