Aortic stenosis is when the aortic valve that keeps blood flowing from the heart’s left chamber to the aorta narrows, disrupting blood flow. The condition – which ranges from mild to severe – is more common in older adults, and is the third most common cardiovascular disease.
Cardiac magnetic resonance imaging (MRI), echocardiography (echo), and cardiac computed tomography (CT) are three imaging modalities which are essential to help diagnose and manage aortic stenosis. In this article, we will compare these three modalities' accuracy, advantages, and limitations in assessing aortic stenosis.
What is aortic stenosis?
Aortic stenosis is a serious valve disease that involves the narrowing of the aortic valve opening, restricting blood flow from the heart’s left ventricle to the aorta, and potentially affecting pressure in the left atrium.
The condition can occur with age, as scarring or calcium damages the valve, restricting the volume of blood that flows through. The bicuspid aortic valve – a congenital heart defect – can also cause aortic stenosis. Aortic stenosis reduces blood flow from the heart around the rest of the body and can produce symptoms such as chest pain, heart murmur, shortness of breath (particularly when exercising), feeling faint (particularly when exercising), fatigue (particularly when exercising), and heart palpitations.
Aortic stenosis affects 20% of people over the age of 65. If the condition is age-related, it usually starts after age 60, but symptoms often don’t appear until after age 70.
Imaging modalities for aortic stenosis
There are many different modalities to help manage and diagnose aortic stenosis, however, there are three cardiac imaging modalities in particular that are regarded as the best for diagnosing and managing the condition.
Echo is recognized as the first-line modality for screening and serial follow-up in aortic stenosis, with transthoracic echocardiography (TTE) regarded by some as the preferred technique, being the gold standard for grading the severity of the condition. Imaging modalities like MRI and CT are also very beneficial in managing and diagnosing patients with AS.
Let’s look at these imaging modalities for aortic stenosis in more detail.
Cardiac MRI for aortic stenosis
Cardiac MRI is not yet accepted as a widespread diagnostic tool for aortic stenosis, mainly owing to disadvantages such as high cost and long scan times. The modality also has limitations such as an inability to identify calcification accurately, signal voids from flow turbulence, and imaging artifacts caused by implantable devices such as pacemakers.
However, MRI does offer the opportunity to perform anatomic and hemodynamic measurements and obtain comprehensive 3-dimensional information. Phase-contrast MRI has emerged as a reliable method of measuring non-invasive flow and velocity measurements, having been validated in multiple patient groups. 4-flow MRI is an alternative to 2D phase-contrast MRI for measuring blood flow velocities with the provision of dynamic quantification in the heart and great vessels, offering good temporal and spatial resolutions. The modality is also regarded as the preferred imaging modality for evaluating the repercussions of aortic stenosis on the left ventricular.
MRI can be a good alternative to invasive techniques such as TTE or cardiac catheterization in aortic stenosis when there are substandard echocardiographic windows or results are ambiguous. Compared to echo, MRI offers a more accurate estimation of the LV mass in the presence of asymmetrical hypertrophy. However, compared to CT, MRI is inferior for assessing aortic valve calcifications, which it is unable to measure.
Echocardiography for aortic stenosis
Echo is the first-line modality for aortic stenosis diagnosis, and TTE is the preferred technique for non-invasive evaluation of aortic stenosis, due to its accessibility, wealth of available research data, and ability to assess flow hemodynamics. TTE is able to grade aortic stenosis severity with measurements using three hemodynamic parameters, including:
- Aortic valve area
- Mean pressure gradient across the aortic valve
- Maximal jet velocity
This modality is key to the diagnosis, assessment, and management of people with aortic stenosis. The benefits of using echo techniques such as TTE include their non-invasive nature, accessibility, low cost, and moderate spatial resolution. Doppler echo can provide both flow and anatomy, but doesn’t provide pressure directly and requires left ventricular outflow tract (LVOT) measurements and good imaging windows.
Compared to MRI, echo provides a less accurate estimation of the LV mass when asymmetrical hypertrophy is present. And while doppler echocardiographic assessment helps to diagnose aortic stenosis in a significant share of patients, diagnostic inconsistencies can be caused by confounding factors such as aortic compliance, ventricular function, and hypertension. As such, MRI and CT can play an important role in accurately assessing aortic stenosis severity and preoperative planning.
Cardiac CT for aortic stenosis
Cardiac CT can help to assess aortic stenosis in several ways. It provides the highest resolution anatomic data of the aortic valve in calcific aortic stenosis and is the best modality for assessing calcification on the valve leaflets and annulus. Calcium scoring can provide a reliable, quantitative, and flow-independent method for assessing aortic stenosis severity. Aside from this ability to enable calcium scoring, CT has the potential to play a larger role in preoperative planning ahead of transcatheter aortic valve replacement (TAVR) procedures, being used for valve sizing and assessing the potential for paravalvular leakage.
Benefits of CT include superior special resolution and provision of 3D anatomy, although these advantages should be weighed up against limitations such as no hemodynamic data (unlike MRI), and radiation exposure (unlike echo or MRI). At the time of writing, clinical guidelines do not recommend CT scans for aortic stenosis diagnosis.
Echo remains a low-cost, accessible method that is considered the first-line modality in grading aortic stenosis severity. Calcium scoring with non-contrast CT has been validated against echo for aortic stenosis assessment and is recommended for assessing disease severity when echocardiography is equivocal. MRI offers the opportunity to obtain hemodynamic measurements and 3D information and is a non-invasive alternative to TTE or cardiac catheterization when there are poor echocardiographic windows.
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