Mitral Regurgitation Imaging

Heart valves permit forward flow of blood through the cardiac chambers when open and prevent backward leakage when closed. Mitral regurgitation is characterized by abnormal backflow of blood through the mitral valve during the systolic period of the cardiac cycle. Mitral regurgitation is the most common type of moderate or severe heart valve disease in adults older than 55 years. The left ventricle (LV) must pump additional volume to compensate for the amount regurgitated. As mitral regurgitation (MR) becomes severe, the continued hemodynamic burden may lead to ventricular dysfunction, heart failure, and sudden death. ^[1]  

Identification of the mechanism and etiology of MR is most commonly achieved by transthoracic echocardiography (TTE). Mitral valve morphology is assessed by B-mode imaging, and color flow Doppler imaging is used to localize the origin of MR jets. If the image quality is poor with TTE, transesophageal echocardiography (TEE) may be used to more accurately define anatomy and function. TEE may identify lesions that are not detected by TTE,  such as vegetations or flail segments. Severity of MR is most commonly assessed using computational fluid dynamics during TTE or TEE. ^{[2, 3, 4, 5, 6, 7]}

Echocardiography is the diagnostic imaging modality of choice because t can provide vivid images of the LV and the mitral valve, and it may provide clues to the mitral valve abnormalities responsible for the regurgitation. The Doppler echocardiographic technique is excellent for determining the severity of mitral regurgitation.

(See the images below depicting mitral regurgitation.)

Severe mitral regurgitation as depicted with color Doppler echocardiography.

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Mitral regurgitation as seen with left ventricular (LV) cineangiography.

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Mitral regurgitation may be acute or chronic. Common causes of severe acute mitral regurgitation include ruptured chordae tendineae, ischemic papillary muscle dysfunction or rupture, and infective endocarditis. Chronic severe mitral regurgitation is commonly caused by myxomatous degeneration of the valve, rheumatic heart disease, or mitral annular calcification.

Acute mitral regurgitation often requires prompt surgical correction. However, symptomatic patients with chronic mitral regurgitation may be initially treated with digitalis, afterload reduction, and diuretics. After the LV function begins to deteriorate, clinical and echocardiographic parameters may be used to determine the timing for surgical reconstruction or replacement of the mitral valve. ^{[8, 9, 10]}

Echocardiography

Echocardiography is the preferred examination. It demonstrates the extent of LA and LV enlargement, as well as the presence and severity of mitral regurgitation. Two-dimensional (2D) echocardiography or transesophageal echocardiography (TEE) with Doppler echocardiography and color flow Doppler imaging enables detailed assessment of the structure and function of the mitral valve. ^{[11, 12, 13, 14, 15, 16, 17, 18]}

In the past, the use of echocardiography resulted in overestimations of the prevalence of mitral valve prolapse (MVP). Early studies suggested a prevalence of as high as 21% in healthy young women. In early studies, normal bowing of the mitral valve was interpreted as representing an MVP.

The limitations are minimal. Color-flow Doppler echocardiography of the valve helps determine the severity of regurgitation, but because this technique measures flow velocity rather than actual flow, it is subject to error in interpretation.

The Doppler technique helps exclude mitral regurgitation and differentiate between mild and severe degrees of the condition. However, color-flow Doppler examination may not be sufficient for more exact quantification of mitral regurgitation or for determining whether the lesion is severe enough to cause LV dysfunction.

Electrocardiography

The electrocardiogram (ECG) may exhibit an LA abnormality, LVH, and, in some patients, atrial fibrillation. ECG evidence of LV enlargement occurs in about one third of patients with severe mitral regurgitation. In approximately 15% of patients, the ECG shows evidence of RV hypertrophy. ^[19]

Chest radiography

Although the heart may not be enlarged in patients with acute mitral regurgitation, severe pulmonary edema is frequently present as a result of left-sided cardiac failure. In cases involving chronic mitral regurgitation, the LA and the LV border appears enlarged, and it may be massive because of volume overload and increased pressure. When the LA is enlarged, it may extend toward the right side, and it may appear as a double shadow along the right atrial border. Coexistent pulmonary arterial hypertension or tricuspid regurgitation may cause dilation of the right atrium and ventricle, as well as enlargement of the pulmonary arteries.

Magnetic resonance imaging

MRI can demonstrate the abnormality of the valve apparatus, and it may be useful in evaluating the amount of regurgitant flow with velocity encoding (VENC) and with model-independent measurements of stroke volumes of RV and LV. ^{[20, 13]}

Computed tomography

CT can show the cardiac silhouette and chamber sizes with clarity. However, because of the widespread use of echocardiography, CT is rarely performed in the evaluation of mitral regurgitation. The degree of confidence is moderate in mitral regurgitation. False findings are rare in mitral regurgitation.

Cardiac catheterization

Cardiac catheterization is often needed. In patients older than 40 years or in those with symptoms suggestive of coronary disease, cardiac catheterization should include coronary arteriography. The main indications for catheterization include (1) the need to evaluate a discrepancy between echocardiographic findings and the clinical presentation, (2) the need to detect other associated valvular lesions and to assess the severity of those lesions, and (3) the need to determine whether coronary artery disease is present and, if so, to assess the extent of disease.

Guidelines

American College of Cardiology (ACC) guidelines emphasize the severity of the MR in deciding whether patients are eligible for mitral valve surgery. Thus, accurately quantifying the severity of mitral regurgitation and being able to differentiate nonsevere from severe mitral regurgitation is the most important question in the clinical evaluation of patients with mitral regurgitation. ^[21]  

According to  appropriate use criteria based on the ACC guidelines, cardiac magnetic resonance imaging (CMR) is appropriate in chronic primary MR to assess ventricular volumes and function or MR severity, especially when the assessment of MR on transesophageal echocardiography (TTE) is not definitive. In addition, for chronic secondary MR, CMR is indicated to establish and/or assess myocardial viability, which in turn might influence the management of functional MR. ^[22]

The ACC guidelines for MR include the following ^[21] :

• The identification of MR mechanism and etiology is most commonly achieved by transthoracic echocardiography (TTE). 
• When uncertainty exists regarding MR severity (ie, mild to moderate or moderate to severe), consider further testing. Further testing may include right and left heart catheterization, stress echo, transesophageal echocardiography, or cardiovascular MRI in select circumstances.
• Color flow Doppler imaging can often overestimate MR. This is most common with high blood pressure, high LV systolic pressure in aortic stenosis or hypertrophic obstructive cardiomyopathy, peak velocities ≥6.0 m/s, or single frame measurements (proximal isovelocity surface area, vena contracta width, vena contracta area) in nonholosystolic MR.
• Adjunctive criteria that support mild MR include a soft or incomplete MR jet on CW Doppler, normal PA systolic pressure, or MR duration < 30% of systole.
• Adjunctive criteria that support severe MR include a dense triangular color-wave Doppler profile, a well-aligned color-wave MR jet velocity < 4.5 m/s indicating high left atrial pressure, dilated LA or LV with no other cause, and pulmonary artery systolic pressure >50 mm Hg with no other cause.

Cardiomegaly

The chest radiograph usually shows an enlarged cardiac silhouette in patients with MR. The heart is increased in size secondary to enlargement of both the LA and the LV. The absence of cardiomegaly indicates that the mitral regurgitation is either mild or acute.

The enlarged atrial appendage may be seen along the middle portion of the left cardiac border. Also, a double shadow may be present on the right cardiac border, indicating an increase in the size of the LA.

In patients with combined mitral stenosis and mitral regurgitation, overall cardiac enlargement and particularly LA dilatation are prominent findings. Relatively mild cardiomegaly and clinically significant changes in the lung fields suggest predominant mitral stenosis. In comparison, predominant mitral regurgitation is most likely when the heart is greatly enlarged and when the changes in the lungs are relatively inconspicuous.

Calcification

Calcification of the mitral annulus, an important cause of mitral regurgitation in the elderly, is most prominent in the posterior third of the cardiac silhouette. The lesion is best visualized on chest radiographs in the lateral or right anterior oblique projections, on which it appears as a dense, coarse, C-shaped opacity.

Pulmonary edema

Pulmonary interstitial edema with Kerley B lines is usually seen in patients with progressive LV failure and chronic mitral regurgitation.

In acute mitral regurgitation, the chest radiograph reveals pulmonary edema, moderate or no enlargement of the LA, and little if any cardiac enlargement.

Accurate measurement of the degree of mitral regurgitation is critical. Doppler echocardiography provides an estimate of the area and depth of the regurgitant jet, but the finding is only semiquantitative. A direct quantitative, noninvasive measurement of the mitral regurgitant volume can be precisely determined by using MRI. ^{[15, 18]}

MRI is the most accurate technique for measuring regurgitant flow, and it provides measurements that are well correlated with those of quantitative Doppler imaging. MRI is also the most accurate noninvasive technique that enables the measurement of ventricular end diastolic volume (EDV), end systolic volume (ESV), and mass. ^[23]

Errors may occur when cine gradient-echo (GRE) imaging is used to measure the area of the mitral annulus and the mitral flow velocity, because of  the constant motion of the atrioventricular valves during each cardiac cycle

Spin-echo MRI

Spin-echo (SE) images show structural consequences of mitral regurgitation, such as an enlarged LV and LA.

Cine gradient-echo MRI

Cine gradient-echo (GRE) images may be used to assess the severity of mitral regurgitation by calculating the regurgitant fraction. The procedure involves mapping the area of the signal void starting from the mitral valve and extending into the LA. The signal void of mitral regurgitation is best seen on the 4-chamber and the coronal oblique views.

Another way to calculate regurgitant fraction involves estimation of the ventricular volumes. This method is usable only if the mitral valve is the sole regurgitant valve.

VENC MRI

VENC (encoding velocity) MRI is another method for determining the severity of mitral regurgitation. In this method, the diastolic inflow across the mitral annulus is compared with systolic outflow across the ascending aorta. In cases of mitral regurgitation, the LV inflow is increased. As an alternative, regurgitant volume may be determined by measuring flows in the ascending aorta and pulmonary artery. In fact, the best way to quantify the mitral regurgitant volume is to combine the ventricular volume calculations obtained by using cine GRE sequences with the estimated forward flow in aorta obtained with VENC MRI.

Two-dimensional echocardiography

In patients with severe mitral regurgitation, 2D echocardiography shows enlargement of the LA and LV. The cause of mitral regurgitation may often be apparent on the transthoracic echocardiogram. Potential causes include rupture of the chordae tendineae; MVP; a flail leaflet; vegetations; and LV dilatation. Calcification of the mitral annulus, which appears as a band of dense echoes, may be seen between the mitral valve and the posterior wall of the heart.

Three-dimensional echocardiography

Three-dimensional (3D) transthoracic echocardiography (TTE) and 3D color Doppler imaging may be helpful in elucidating the mechanism of mitral regurgitation. The imaging of the mitral valve is excellent on transesophageal echocardiography (TEE), and the images offer clues to the mitral valve abnormalities responsible for the regurgitation (see the image below). ^[16]

Four-chamber apical view of a 2-dimensional transthoracic echocardiogram demonstrates mitral valve prolapse (MVP), a common cause of mitral regurgitation.

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Color-flow Doppler imaging

Color-flow Doppler imaging of the valve helps in semiquantitatively determining the severity of regurgitation (see the images below). This technique measures flow velocity rather than actual flow; therefore, it is sometimes subject to error. Color-flow Doppler examination may not be sufficient for exactly quantifying mitral regurgitation or for determining whether the severity of the lesion is sufficient to cause eventual LV dysfunction.

Severe mitral regurgitation as depicted with color Doppler echocardiography.

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Mitral regurgitation as seen with pulsed Doppler echocardiography.

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Mitral regurgitation as seen with color Doppler echocardiography.

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Left, Color Doppler apical 4-chamber view shows severe mitral regurgitation with the regurgitant jet hitting the distant wall of the left atrium and encircling it, as well as traversing back into the pulmonary veins. Right, Pulsed Doppler images with a high-velocity jet in the systolic phase of the cardiac cycle; this represents severe mitral regurgitation.

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Mitral regurgitation in a patient with mitral valve prolapse (MVP). The regurgitant is pointed posteriorly as a result of a prolapsing posterior mitral leaflet.

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Acute severe mitral regurgitation in a patient with trauma. The mitral-regurgitation jet is wide, a finding that semiquantitatively indicates severity. The normal size of the left atrium (LA) should be noted, because the LA has not had sufficient time to adapt and enlarge, owing to the acuteness of the regurgitation.

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The severity of mitral regurgitation is directly proportional to the size of the regurgitant jet within the left atrium. The size of the jet is typically indexed to the size of the left atrium. Jets that are peripheral impinge on a wall, rather than the center, causing predictable problems with assessment of severity. Because of the Coanda effect, a regurgitant jet impinging on a wall results in a color-flow area smaller than an equivalent central regurgitant volume. A jet impinging on a wall leads to underestimations of the regurgitant volume by approximately 40%.

In cases of moderate and severe mitral regurgitation, flow in the pulmonary veins may reverse direction in systole. A variation on this finding is attenuation of normal forward flow in the pulmonary vein during ventricular systole. Three-dimensional reconstruction of mitral regurgitation jets has been shown to be feasible. ^{[24, 25, 26, 27, 28, 29, 30, 15, 17]}

Doppler echocardiography

In cases of mitral regurgitation, Doppler echocardiography characteristically reveals a high-velocity jet in the LA during systole. The severity of the regurgitation is a function of the distance from the valve at which the jet can be detected and the size of the LA.

In estimating the severity of mitral regurgitation, both color-flow Doppler and pulsed Doppler results are used. Color Doppler imaging involves measurement of the area of the mitral jet. If the area of the jet is greater than 8 cm², the mitral regurgitation is considered severe. Findings that indicate that mitral regurgitation is severe include reversal of flow in the pulmonary veins during systole and a high peak mitral inflow velocity. ^[18]

Transesophageal echocardiography

TEE is better than TTE in imaging the regurgitant mitral valve. Angiographic assessments of mitral regurgitation are well correlated with the color-flow mapping obtained by using TEE rather than TTE.

Mitral valve prolapse

Echocardiography is useful in diagnosing MVP, in determining the severity of associated mitral regurgitation, and in showing the pathologic anatomy of the mitral valve. An extreme form of MVP involves myxomatous degeneration of the valves with leaflet thickening (>3-5 mm), marked symmetrical bowing of the valve behind the annular plane, and/or highly asymmetric buckling of one or both leaflets into the LA associated with mitral regurgitation. Because of the eccentric leaflet buckling, the mitral regurgitation jet may be eccentric rather than central.

Acute mitral regurgitation after acute myocardial infarction

Two complications of myocardial infarction that produce confusing clinical signs are mitral insufficiency caused by rupture of an infarcted papillary muscle and a ventricular septal defect that occurs after infarction and necrosis of the septum. These are easily identified by noting the intracardiac flow patterns seen on cine GRE images.

Radionuclide angiography may be useful in assessing mitral regurgitation, and gated blood-pool nuclear imaging or first-pass angiography may reveal an increased EDV.

The regurgitant fraction may be estimated from the ratio of LV stroke volume to RV stroke volume. In patients with mitral regurgitation and impaired LV function, the ejection fraction fails to rise normally during exercise.

Radionuclide angiograms are useful for interval follow-up, and progressive increases in ventricular EDV and/or ESV often suggest that surgical treatment is necessary.

Radionuclide angiography is good for assessing LV function. False findings are unusual in mitral regurgitation.

LV angiography may be performed to evaluate mitral regurgitation.

The immediate appearance of contrast material in the LA after its injection into the LV indicates mitral regurgitation. The regurgitant volume may be determined from the difference between the total LV stroke volume, which is estimated by using angiocardiography, and the simultaneous measurement of the effective forward stroke volume, which is determined by using the Fick method.

In patients with severe mitral regurgitation, the regurgitant volume may approach the effective forward stroke volume; in rare instances, it may even exceed this volume. Qualitative but clinically useful estimates of the severity of mitral regurgitation may be made by means of cineangiographic observation of the degree of opacification of the LA and the pulmonary veins after the injection of contrast material into the LV (see the image below).

Mitral regurgitation as seen with left ventricular (LV) cineangiography.

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The cause of the regurgitation (eg, MVP) and a flail leaflet may often be distinguished by using angiography. Mitral regurgitation secondary to rheumatic heart disease is angiographically characterized by a central regurgitant jet and by thickened leaflets that have reduced motion. In regurgitation resulting from other conditions, particularly dilatation or calcification of the mitral annulus or ruptured chordae tendineae and papillary muscles, the systolic jet may be eccentric; in such cases, the valves consist of thin filaments that display excessive motion.