Aortic stenosis (AS) is a narrowing of the aortic valve that restricts blood flow and causes increased pressure across the valve. It is distinguished from aortic valve sclerosis where the aortic valve is narrowed, but there is no hemodynamically significant transvalvular pressure gradient. AS may be acquired (e.g., fibrocalcific senile AS) or congenital (e.g., bicuspid aortic valve) and has three principal causes: calcification of a normal tricuspid valve, rheumatic disease, and a congenital bicuspid valve with superimposed calcification.
2. Benefits of cardiac MRI use:
Cardiac MRI is well-suited for the examination of aortic stenosis because it can accurately quantify the aortic valve area, the transvalvular pressure gradient, and left ventricular systolic function. It is totally non-invasive and does not require contrast.
3. MRI technique:
Short and long axis SSFP images are obtained to quantify end-diastolic and end-systolic LV volumes from which one can calculate the ejection fraction.
There are two methods used for evaluating aortic stenosis.
- Planimetry – Short axis SSFP images through the aortic valve are acquired.
- Continuity Equation – A contiguous series of Phase Contrast images through the LVOT and aortic valve perpendicular to the aortic stenosis jet are acquired.
Aortic Valve Area (AVA)
- Planimetry – AVA is measured directly using the aortic valve SSFP images. It is imperative to choose the correct slice and phase for AVA measurement.
- Continuity Equation – Using the Phase Contrast images at peak systole, measurements of LVOT area and velocity, and AV velocity are made.
Calculating the aortic transvalvular pressure gradient:
- Peak pressure gradient = 4 x (peak AV velocity)²
- Mean pressure gradient = 2.4 x (AV velocity)²
5. Which imaging findings affect treatment?
The severity of aortic stenosis can be quantified in terms of AVA, transvalvular peak or mean pressure gradients, or peak AV Velocity (Vmax). Surgery is generally reserved for patients with severe aortic stenosis unless they are undergoing cardiac surgery for other reasons. Surgical timing generally depends on symptoms, LV ejection fraction, and LV dimensions. AHA guidelines are shown in detail here.
The criteria for classification of aortic stenosis are as follows:
|AVA||Peak Pressure||Mean Pressure||Peak Velocity|
|Mild AS||1.5 – 2 cm²||<36 mmHg||<25 mmHg||<3 m/sec|
|Moderate AS||1.0 – 1.5 cm²||36-64 mmHg||25-40 mmHg||3-4 m/sec|
|Severe AS||<1.0 cm²||>64 mmHg||>40 mmHg||>4 m/sec|
6. Drawbacks of existing tests:
Echocardiography is routinely used to assess aortic stenosis severity. In patients where the results are uncertain or discordant with the clinical assessment, confirmation is usually obtained by cardiac catheterization. However, this test is invasive and requires passing a catheter across the aortic valve. The risk of embolic stroke increases with the severity of the aortic stenosis. An advantage of MRI is that it is totally noninvasive and provides two independent methods for assessing stenosis severity.