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CARDIAC RADIOLOGY: SYMPOSIUM Table of Contents   
Year : 2007  |  Volume : 17  |  Issue : 2  |  Page : 120-132
Assessment of valvular heart disease with cardiovascular magnetic resonance


Cardiovascular Division, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, MA, USA

Click here for correspondence address and email
 

   Abstract 

Valvular heart disease is an important clinical diagnosis throughout the world and prompt recognition of moderate and severe valvular abnormalities is an important public health goal. Cardiovascular magnetic resonance (CMR) imaging is a well-established noninvasive technique that allows for a comprehensive assessment of valvular heart disease. In this article, the basics of CMR application in valvular heart disease are reviewed. Essential CMR techniques, including black-blood imaging, cine steady-state free precession and flow velocity mapping are discussed, as applied to specific valvular lesions. In addition, concrete protocols for imaging in various clinical situations are suggested.

Keywords: Cardiovascular magnetic resonance, regurgitation, stenosis, valvular heart disease

How to cite this article:
Gelfand EV, Manning WJ. Assessment of valvular heart disease with cardiovascular magnetic resonance. Indian J Radiol Imaging 2007;17:120-32

How to cite this URL:
Gelfand EV, Manning WJ. Assessment of valvular heart disease with cardiovascular magnetic resonance. Indian J Radiol Imaging [serial online] 2007 [cited 2020 Aug 15];17:120-32. Available from: http://www.ijri.org/text.asp?2007/17/2/120/33621

   Introduction Top


Valvular heart disease is a common and important clinical diagnosis throughout the world. Advances in public health and preventative cardiac care have led to a decline in the incidence of rheumatic heart disease, but have indirectly led to a rise in the prevalence of degenerative valvular disease and congenital valvular abnormalities in an increasingly older population. [1] Population-based data show that survival among subjects with valvular disease is significantly lower than that expected in the comparable general population. Thus, prompt recognition of moderate and severe valvular abnormalities is an important public health goal.

A variety of imaging techniques is available to diagnose and characterize valvular disease and assess concomitant anatomy, with echocardiography (echo) being the clinical "workhorse". Cardiovascular magnetic resonance (CMR) imaging is a well-established noninvasive technique that does not employ potentially nephrotoxic contrast or ionizing radiation and allows for a comprehensive assessment of valvular heart disease. The lack of ionizing radiation is particularly important for valvular disease patients, who frequently undergo serial examinations over the course of decades.

In this review , we will assess the basics of CMR application in valvular heart disease and suggest concrete protocols for imaging in specific clinical situations. It should be noted that CMR imaging of valvular pathology as a component of complex congenital heart disease will not be presented here and the readers are referred to other reviews on this topic. [2],[3],[4],[5],[6],[7],[8],[9]


   Clinical Goals of Valvular Imaging Top


Following an initial diagnosis of a valvular abnormality, the following questions typically need to be answered:

1) What is the etiology of the abnormality?

Whether the valve itself is structurally abnormal (e.g. mitral regurgitation in mitral valve prolapse) or has the lesion arisen as a consequence of a structural deformity of the surrounding structure (e.g. annular dilation leading to mitral regurgitation in dilated cardiomyopathy) has important implications on therapy and surgical planning.

2) What is the severity of the abnormality?

While clinicians have traditionally used qualitative measures of regurgitant severity, such as mild, moderate or severe, this was primarily related to the inability of the techniques to quantify the lesion precisely. As will be shown below, CMR readily provides quantitative data for both stenotic and regurgitant lesions.

3) Are there associated cardiac abnormalities?

Is the valvular abnormality associated with any alteration in anatomic structure of the ventricles and/or the great vessels (e.g. aortic coarctation with a bicuspid aortic valve, left ventricular hypertrophy in aortic stenosis) and are there any other associated anatomic abnormalities?

4) What is the underlying/intrinsic left and right ventricular systolic function?

Ultimately, any imaging test should only be obtained if the results would potentially alter therapy or provide prognostic data. Valve imaging represents no exception. Since medications are currently thought to be of limited value in effecting either stabilization or regression of valvular disease, results of CMR imaging will primarily have impact on the timing of referral for surgical correction and on antibiotic prophylaxis for infective endocarditis.


   General Aspects Of CMR Imaging For Valvular Disease Top


Imaging Sequences

Valvular imaging with CMR generally relies on three types of sequences: anatomic black-blood imaging, functional steady-state free precession (SSFP) imaging and assessment of blood flow with phase-contrast imaging [Table - 1]. Additional sequences, such as gadolinium (Gd)-contrast CMR and MR angiography (MRA) or coronary MRI are used depending on the clinical question.

Black blood protocols utilize a fast spin-echo double inversion recovery sequence. This sequence first uses a nonselective inversion pulse, followed by a slice-selective reinversion pulse. After an inversion time is allowed to pass, the image is acquired, by which time the vascular signal is nulled by blood arriving from outside the imaging plane. At our institution, black blood imaging is done with T1 weighting and without the use of Gd contrast [Table - 2]. We typically obtain axial T1W 10 mm-thick slices spanning the thorax, from the diaphragm to the thoracic outlet.

With cine SSFP imaging, the blood pool exhibits high signal intensity, with flow acceleration/turbulence attenuating the signal intensity through spin dephasing. The in-plane spatial resolution of CMR with current SSFP cine protocols is 1-2 mm, with temporal resolution as high as 20-50 ms. Thus, the spatial resolution is generally superior to that of transthoracic echo and temporal resolution is sufficient to assess both myocardial and valve motion. Temporal resolution with SSFP depends on the number of phase-encoding lines obtained with each R-R interval and increases with the decreasing number of lines, inevitably leading to longer scan and breath-hold times. Thus, these parameters need to be balanced to achieve diagnostic temporal resolution with acceptable scan times. The typical SSFP imaging protocol at our institution is outlined in [Table - 2].

Phase-velocity mapping (PVM, also known as velocity-encoded cine CMR) imaging is the primary modality for evaluating blood flow with CMR. The technique utilizes the property of moving spins to acquire a shift in their phase or rotation, as compared to the stationary spin. The magnitude of the shift is proportional to their velocity. To obtain phase-contrast images, a bipolar gradient is first used to ensure that the spins of stationary tissue are zero. Then, a repeat set of images are acquired with an inverted bipolar gradient, eliminating phase shifts due to other parameters. A subtraction of the two datasets yields tissue and blood pool velocity [Table - 2]. This method provides consistent, repeatable blood flow measurements and is considered the CMR gold standard to quantify blood flow. [10]

ECG gating

Because acquisition times for high-resolution SSFP images are relatively long, blurring would occur in the absence of cardiac synchronization. Therefore, ECG gating is routinely used to compensate for rapid heart motion during systole and diastole. Detailed discussion of the technical aspects of ECG triggering is beyond the scope of this review. At our institution we use retrospective ECG gating in most patients, but prospective gating is used occasionally, primarily for patients with atrial fibrillation and a highly irregular ventricular response.


   Advantages and Disadvantages of CMR Imaging For Valvular Disease Top


The primary strength of CMR imaging is in offering a comprehensive examination, which may include assessment of valvular morphology, associated structures, ventricular volumes, mass and systolic function, as well as coronary artery anatomy. Unlike echocardiography, CMR imaging is not dependent on the presence of adequate acoustic windows and is less dependent on operator experience for consistently obtaining interpretable images. CMR offers excellent spatial resolution and, with ECG gating, has acceptable temporal resolution, which approaches two-dimensional echocardiography. Importantly, CMR uses no ionizing radiation, allowing for safe serial examinations - an important consideration in following patients with dynamic valvular pathology over many decades.

The disadvantages of CMR evaluation include its current relative inability to be used in patients with certain types of metallic implants, most importantly - pacemakers and internal cardioverters-defibrillators. As discussed below, all prosthetic valves are safe for imaging, however they produce local artifact. Hence, the ability of CMR to assess the detailed structure of a prosthetic valve is currently limited. The initial setup of a CMR laboratory requires considerable expense and interpretation of CMR images is relatively more complex, compared to echocardiography or invasive ventriculography. Finally, similar to any "young" imaging modality, data regarding prognostic significance of CMR in valvular disease lags behind that of echocardiography.


   Assessment of Valvular Morphology, Stenosis and Regurgitation Top


Valve leaflet morphology

Valve leaflets may be visualized with both black blood and SSFP sequences. Early work by Arai and colleagues demonstrated the feasibility of visualizing all three of the aortic valve leaflets on black blood images in 85% of their consecutive subjects. [11] Since that time, cine SSFP imaging has largely supplanted black blood sequences for this indication. Both anterior and posterior leaflets of the mitral valve are visualized in the 2-chamber and 4-chamber views and all six mitral scallops can be discerned in the short axis view [Figure - 1], when a slice is obtained at the level of the atrioventricular groove. Similarly, the complete morphology of the tricuspid valve can be assessed in long-axis and short axis views. For imaging the aortic valve, we obtain a cine SSFP acquisition at the valve plane, as defined in the left ventricular outflow tract (LVOT) view. Assessment of the valve as trileaflet or bicuspid can then be made [Figure - 2]. Aortic valve area can be calculated by direct planimetry. [12] Likewise, if detailed imaging of the pulmonic valve is desired, a double-oblique view of the right ventricular outflow tract (RVOT) is used as a guide for defining the plane of the valve. The RVOT view itself can also helpful in identifying subvalvular obstruction or supravalvular/proximal pulmonary artery (PA) stenosis.

Valvular Stenosis

Clues that a valvular stenosis is present include restriction of leaflet motion and flow turbulence in the distal downstream chamber/vessel on SSFP images [Figure - 3]A. Severity of stenosis is then usually assessed with direct planimetry on SSFP images. Data, comparing this method with echo and invasive hemodynamics will be discussed later.

Valvular Regurgitation

Qualitative evaluation of regurgitation severity was first demonstrated using older gradient echo sequences. [13],[14],[15] Using these methods, a discrete signal void in the receiving chamber is seen and the size of its area correlates well with the severity of regurgitation as assessed by Doppler echo. Newer SSFP sequences have improved contrast between myocardium and blood pool. However, these sequences are less flow-dependent due to shorter echo times and thus the signal void phenomenon in valve regurgitation is lessened. Indeed, a study by Krombach et al . showed that while the sensitivity and specificity for detecting valve regurgitation were similar between gradient-echo and SSFP sequences, in approximately one-quarter of patients, the regurgitant jet was significantly less pronounced with SSFP. [16]

In most cases of valvular regurgitation, it is clinically important to know its exact magnitude. Such quantitative assessment can readily be achieved with CMR, using volumetric or phase-velocity methods or combining the two. In cases of a single regurgitant valve, the amount of regurgitation will be accounted by the comparison of LV and RV stroke volumes . These are obtained in the standard fashion, using a short-axis stack of SSFP images. For example, with 30 ml/beat of aortic regurgitation (AR) or mitral regurgitation (MR), the LV stroke volume will be 30 ml higher than the RV stroke volume. Phase velocity mapping can be successfully used to directly quantify the severity of aortic and pulmonic regurgitation. For the aortic valve, phase velocity images are obtained in the ascending aorta, at the level of the pulmonary artery (PA) bifurcation. Computer-assisted planimetry of the vessel across all phases yields an aortic flow curve. AR volume is then directly calculated from the aortic flow curve by integrating diastolic reverse flow [Figure - 4]. This method has been shown to correlate well with Doppler echo, allowing for establishment of clinically meaningful severity grades [Table - 3]. [17] Analogous technique is used to assess pulmonic regurgitation (PR), in which case the phase velocity slice is positioned in the proximal PA.

Application of the phase velocity method to isolated MR is also feasible, by positioning the slice at the mitral leaflet tips. The volume of MR is then calculated as the difference of the LV inflow volume and aortic flow volume [18] or directly as the MR volume. The latter approach is limited by the need to accurately position the slice to measure MR. A more practical approach involves combining the phase velocity method with assessment of LV volumes to quantify MR severity or to assess multiple regurgitant valves. Using this technique, the volume of MR is calculated as the difference between the LV stroke volume (obtained from SSFP images) and the forward aortic flow volume (obtained from phase velocity images). MR fraction is calculated as the ratio of MR volume and LV stroke volume. Although inherently prone to more error because it combines two independent techniques, this method for quantification of MR severity has been shown to correlate well with Doppler echo. [17] Analogous measurements on the right side of the heart yield values for PR and tricuspid regurgitation (TR) volumes and fractions, although data on their correlation with other imaging modalities are currently lacking.

Infective endocarditis

Using fast gradient-echo sequences, valvular vegetations due to infective endocarditis (IE) are visible as areas of low signal, adjacent to the leaflets. [19],[20],[21],[22] Case reports suggest that CMR may be a useful adjunct to echocardiography for the evaluation of complications of IE, such as paravalvular abscesses or fistulae, [23],[24],[25] but no large series demonstrating sensitivity, specificity or accuracy of CMR for endocarditis are available. Thus, CMR is not used routinely for this indication.

Valve tumors

Data on using CMR for assessment of valve tumors are also limited to case reports. For example, Wintersperger and colleagues reported on aortic valve fibroelastoma and mitral valve lymphangioma, diagnosed with CMR. [26]

Assessment of Specific Valvular Lesions

Most valvular lesions are initially identified by echocardiography, with patients referred for CMR to quantify lesion severity or to further clarify the related issues of ventricular structure and function and/or great vessel anatomy. Alternatively, patients are referred for CMR assessment of non-valvular conditions, with valvular dysfunction identified during CMR examination.


   Left-sided Valvular Lession Top


Aortic stenosis

Assessment of aortic stenosis with CMR provides answers to 1) etiology of stenosis - from calcific degeneration to congenitally bicuspid valve, 2) degree of stenosis, 3) magnitude of adaptive LV hypertrophy, 4) LV systolic function and 5) presence or absence other valvular lesions, particularly - mitral regurgitation.

The initial clue to the presence of aortic stenosis is often loss of signal on functional images due to spin dephasing in the proximal ascending aorta, distal to the valve [Figure - 3]A. [27] The SSFP technique allows for direct visualization of the aortic valve in any tomographic plane. Direct planimetry at the leaflet tips yields an aortic valve area (AVA) [Figure - 3]B. Measurement of AVA by CMR has been validated against the more commonly-employed modalities, including planimetry by transthoracic (TTE) [28] and transesophageal echocardiography (TEE) [12],[29],[30] and calculation using Gorlin equation during an invasive hemodynamic study. [12],[28],[30],[31] The correlation is generally best between CMR planimetry and TEE planimetry and less robust between CMR and other modalities. This is likely explained by the fact that both CMR and TEE measure the true "anatomic" stenotic valve area, whereas other techniques are more dependent on jet velocity and direction and LV systolic function. During measurement of AVA by TTE using the continuity equation, significant errors are introduced by imprecise measurement of LVOT diameter and by assuming that LVOT in cross-section is a circular structure. In fact, a CMR study demonstrated that LVOT area is significantly underestimated by TTE. [32] In principle, a planimetry-based technique, such as CMR, could underestimate AVA in patients with severely impaired LV function, where a small stroke volume is insufficient to displace the leaflets. A TEE-based study by Tardif and colleagues, however, suggests that an change in flow conditions does not significantly change the planimetered AVA. [33]

Aortic stenosis can also be quantified using phase velocity-encoded cine CMR and application of the continuity equation. [29],[34],[35],[36],[37],[38] Images are obtained in the cross-sectional orientation through the LVOT and the proximal ascending aorta, just beyond the valve. The velocity-time integrals for LVOT and aortic valve are calculated from the peak flow velocity vs. time curves. After measuring LVOT diameter on SSFP images, the continuity equation is used to calculate AVA. Valve areas obtained by this method correlate well with those obtained by catheterization and echocardiography. Because of high velocities in the ascending aorta with hemodynamically significant AS, aliasing invariably occurs at standard values of encoding velocity (V enc ). Therefore, V enc must be increased to exceed the expected peak flow velocity (typically, 300-500 cm/s). Normal AVA depends on body size and thus must be indexed. In a study of 100 healthy individuals (mean age 39, 50% female) normal AVA was 3.6±0.4 cm 2 and 4.4±0.6 cm 2 for women and men, respectively, with a strong correlation to body height. [39]

Pressure overload imposed by aortic stenosis on the LV leads to concentric myocardial hypertrophy, which can be reliably assessed by CMR. [40] Such pathologic hypertrophy is associated with abnormalities in cardiac rotation and relaxation and can be measured using specialized CMR-based techniques, such as magnetic myocardial tagging. [41],[42] In approximately 1/3 of AS patients, primarily those with severe hypertrophy (wall thickness >18 mm), patchy areas of late enhancement are seen, following administration of Gd contrast. [43],[44] These are most prominent in the mid-ventricular layer, do not correspond to a coronary artery distribution [43] and may represent microscopic areas of myocyte degeneration and replacement fibrosis, previously demonstrated by histology in patients with aortic stenosis. [45] Following aortic valve replacement, there is frequent regression in LV hypertrophy and volumetric CMR methods are more accurate and reproducible than TTE in quantifying the degree of mass reduction. [46]

To image a stenotic aortic valve, we obtain cine SSFP images of the LVOT in the oblique transverse and coronal views. The valve is then visualized en face in the cross-sectional view and AVA is obtained from planimetry [Table - 4]. Resulting AVA is indexed to body surface area. Careful evaluation of aortic valve annulus area, LV mass and systolic function, as well as of the proximal aortic size and degree of aortic and mitral regurgitation (see below) is important for future surgical planning and these values should also be reported. Aortic size is reported from the T1W black blood (T1BB) images. Bicuspid aortic valve and aortic coarctation often co-exist and if the latter is suspected from axial T1BB images, additional sagittal views confirm the diagnosis.

Aortic Regurgotation

Clinically useful CMR assessment of patients with AR must provide answers to the likely etiology of the lesion, severity of regurgitation, LV size and systolic function and size of the thoracic aorta. The latter is particularly important in terms of surgical planning.

Early attempts at estimating AR severity using LV vs RV stroke volume ratios (see above) provided proof of feasibility, but were relatively imprecise. [47] With the advent of blood-flow sensitive cine gradient echo sequences, several groups used AR jet area as a measure of severity and demonstrated moderate correlation (r-values, 0.60-0.88) with Doppler echocardiography. [13],[48],[49],[50],[51],[52],[53] These sequences gave way to the more contemporary SSFP protocols, which have much improved spatial resolution. However, these sequences are less flow-sensitive and the dephasing signal in the LV is not reliably visualized, making AR jet area determination by CMR obsolete.

Current approach to AR quantification relies on phase velocity mapping. [54],[55],[56],[57] Typically an ECG-triggered phase contrast velocity sequence is used to acquire data at the cross-section of the aorta. Volume of AR is calculated directly from the aortic flow curve by integrating diastolic reverse flow [Figure - 4]. This technique has excellent interstudy reproducibility [10],[58] and has been shown to have 100% concordance with Doppler echocardiography to within 1 degree of severity [Table - 4]. [17] Data from imaging of an aortic model suggest that the exact location of the slice may be important, with values from between the valve and the coronary ostia being most accurate. [55]

To image a regurgitant aortic valve, we obtain standard functional cine LV images and quantify aortic flow at the level of PA bifurcation with phase-velocity mapping. Volume of AR and its fraction relative to total aortic flow are then reported, along with measures of LV volumes and systolic function. In severe AR, an additional phase-velocity sequence through the descending thoracic aorta can be useful to demonstrate holodiastolic flow reversal [Figure - 4]B. Size of the thoracic aorta is reported from axial T1BB images [Table - 4] or contrast-enhanced aortic MRA.


   Mitral Stenosis Top


Diagnosis of mitral stenosis (MS) is suspected on CMR, when thickened, restrained mitral leaflets and a diastolic LV signal void are seen on functional cine images [Figure - 5]A. [59],[60] Thickening commonly leads the anterior leaflet to assume a "hockey stick-like" appearance in diastole [Figure - 5]A. Determination of MS severity relies on determination of mitral valve area (MVA). Additional findings of clinical importance are left atrial (LA) size, presence or absence of LA thrombus and RV size and systolic function. Since MS is usually seen as a sequela of rheumatic carditis, co-existing valvular pathologies (most commonly, MR and AR) are easily assessed by CMR [Figure - 5]B.

Determination of MVA is done through measurement of peak post-stenotic valvular velocity with phase velocity mapping. ECG-triggered series' are obtained in the short-axis orientation approximately 1.5 cm above the mitral valve leaflets. [61] Peak E and A waves, analogous to those obtained by the practical gold standard of Doppler echocardiography are defined from the flow velocity curve. The data are then used to quantify pressure half-time (PHT) and valve area is calculated as [MVA = 220/PHT]. Peak and mean transvalvular gradients are obtained from the same dataset. In a study of 17 patients with variable severity of MS, good correlation (r = 0.86) was seen between PHT obtained by CMR and echo. [61] Peak and mean transmitral flow velocities and gradients are also comparable between CMR, echo and invasive hemodynamics. [36],[62] With SSFP sequences, the stenotic mitral valve orifice is easily visualized. Recent studies demonstrated the feasibility of direct, CMR-based planimetry for assessment of MS severity. Several contiguous image slices through the mitral valve leaflets are obtained in an orientation, perpendicular to the jet origin in the LV. [63] The smallest orifice is then planimetered to yield MVA. This method has good correlation with both echo and catheter angiography (r = 0.81 and 0.89, respectively), with CMR slightly overestimating the valve area. [63],[64] In the adult, a CMR threshold of MVA<1.65 cm 2 was found to be diagnostic of MS with a sensitivity of 89% and specificity of 75%. [63]

CMR also has the potential to be the ideal comprehensive imaging modality before and after percutaneous balloon mitral valvuloplasty (PBMV). Because of chronic changes in left atrial (LA) compliance, traditional Doppler echo-based methods for MS assessment are unreliable in the short-term following PBMV, [65] but CMR has been successfully used to overcome these limitations in quantifying MS severity. [59],[64],[66] Presence of LA thrombus in patients with MS typically precludes PBMV, out of concern for systemic embolization. Several reports indicate that CMR methods are able to visualize LA thrombi, [67],[68] but transesophageal echocardiography remains the clinical standard.

To image a stenotic mitral valve, we obtain standard functional cine LV and RV images and planimeter the small mitral orifice in diastole. Aortic flow is obtained in the standard fashion. We then report MVA, severity of associated AR and MR if present, as well as biventricular systolic function [Table - 4].

Mitral Regurgitation

Clinical goals of cardiac imaging in MR include: 1) elucidation of MR etiology, 2) measurement of regurgitation severity, 3) assessment of LV volumes and systolic function, 4) quantification of right ventricular function. In achieving these goals reproducibly, echo has significant limitations. Indeed, MR jets are often eccentric and may be shadowed by adjacent calcified structures or implanted devices, making quantitative methods such as PISA (proximal isovelocity surface area) inadequate. Two-dimensional echocardiographic assessment of RV volumes, mass and systolic function is inferior to volumetric methods. [69] Most importantly, MR results in the systolic unloading of the LV, which can mask significant intrinsic contractile dysfunction and lead to substantial delays in surgical referrals. These and other considerations make CMR an ideal non-invasive imaging modality for comprehensive diagnosis and follow-up of patients with mitral regurgitation.

The etiology of MR is readily evaluated with CMR. Mitral valve prolapse is seen best in the oblique coronal cine SSFP images of the LVOT or in the 4-chamber view [Figure - 6]. Both a short axis slice through the valve plane and a 4-chamber slice stack spanning the valve define the scallop(s) involved in prolapse. Dilated cardiomyopathy with stretching of the mitral annulus is also seen on the standard cine SSFP series. CMR allows for a detailed study of the mitral annular geometry [70],[71],[72] and may substantially aid the surgeon in planning valve repair/replacement.

Evolution of CMR methods for MR quantification paralleled that of AR - from a volumetric method in isolated MR. [47] through measurement of the LA signal dephasing area, [13],[48],[73],[74] to incorporation of the newer flow velocity mapping techniques. [18] The volumetric method does not account for presence of other valvular regurgitation. The correlation between CMR and other methods (Doppler echo and catheter angiography) for the jet area method is good (r = 0.71-0.87), [73],[74],[75] but the spatial resolution of the spoiled gradient-echo sequences makes them less desirable for comprehensive cardiac assessment. Flow velocity mapping also correlates well with Doppler echo (r = 0.87), [18] but does not provide any information on LV anatomy. Therefore, the contemporary approach combines quantification of flow using flow velocity mapping, with volumetric CMR using cine SSFP [Table - 4]. Such a combined approach has been tested against a volumetric method in a small study by Kon and colleagues and was found to maximize intra- and inter-observer agreement, while allowing correction for AR when present. [76] Recently, we found that there is >95% concordance to within one severity grade of MR between the combined method and Doppler echocardiography and we have suggested specific MR fraction cutoffs to define severity by CMR [Table - 3]. [17]

In addition to evaluating etiology and severity of regurgitation, CMR provides insight into associated cardiac structural changes. Volumetric CMR remains the gold standard for LV volumes, mass and systolic function, [40] providing superb reproducibility and interobserver variability. Using CMR, one group demonstrated significant alterations in RV and LV geometry with chronic MR. [77] Both LV dilatation and adaptive eccentric hypertrophy (relatively normal wall thickness with increased mass) are common in longstanding severe MR and can be readily demonstrated. A novel parameter called effective forward ejection fraction (EffEF) is a measure of LV function, which accounts for MR. It is calculated as EffEF = forward aortic stroke volume / LVEDV, where LVEDV is the LV end-diastolic volume. Intuitively, effective LVEF represents the fraction of LV blood volume at end-diastole, which is delivered to the systemic circulation during the following systole. Another group was able to show reverse remodeling of LA and LV following correction of ischemic MR. [78] Given the aforementioned limitations of echocardiography, the ultimate goal of CMR would be to provide prognostic information in chronic MR and guide timing of referral to surgery.

To image a regurgitant mitral valve, we obtain standard functional cine LV images. If MVP is seen, extending the short-axis slice stack approximately 2 cm beyond the atrioventricular groove into the LA demonstrates the involved mitral scallop. Aortic flow is quantified in the standard fashion and volume of mitral regurgitation is reported as V MR = LVSV - V Ao , where LVSV is LV stroke volume and V Ao is total aortic flow. Fraction of MR is determined as Fr MR = V MR /LVEDV and qualitative severity grade is given. Finally, LV EffEF is reported [Table - 4].

Right-sided valvular lesions

Compared to left-sided valvular lesions, primary right-sided valvular pathology is relatively rare. Outside of the centers with primarily congenital heart disease referrals, most of the tricuspid and pulmonic valve disease is secondary in etiology to pulmonary hypertension and/or RV dilation.

Tricuspid valve disease

Imaging of the tricuspid valve is somewhat analogous to that of the mitral valve [Table - 4]. The leaflet morphology can be discerned from standard cine SSFP views and primary anatomic abnormalities, such as Ebstein's anomaly, are easily visualized. [79],[80],[81]

TR is seen as a systolic signal void in the right atrium, but its severity should not be estimated on SSFP images solely on the basis of the jet area. The quantitative approach to TR severity estimation is also analogous to that of the mitral valve. During volumetric planimetry, special attention must be paid to distinguish the RV cavity from both, the RA and the proximal PA, as small errors in planimetry produce larger errors in the calculated volume and severity of TR.

Tricuspid inflow velocity has been advocated by some as a measure of RV diastolic function [82],[83] and is also reduced in patients with dilated cardiomyopathy. [84]

Finally, CMR may emerge as a useful modality in imaging of carcinoid heart disease [85] and tricuspid stenosis, [86] though experience is limited.

Pulmonic valve disease

Pulmonic valve leaflets are commonly seen on cine SSFP images in long axis, when the aortic valve is imaged en face . If primary pulmonic valve pathology is suspected, an orthogonal slice is prescribed in order to visualize all three pulmonic leaflets. [87]

The "aortic valve" SSFP images are also useful in demonstrating PR, which appears as a diastolic signal void in the RV outflow tract. Estimation of PR severity is done routinely by integrating the flow curve through the proximal PA and is particularly important in following patients with several repaired congenital cardiac defects, including tetralogy of Fallot. [88] Both PR and pulmonic stenosis are also seen in carcinoid heart disease, where CMR can provide answers as to the severity of both lesions, as well as RV function. [85]

Imaging of prosthetic cardiac valves

Several studies have conclusively demonstrated that CMR examinations at 1.5T are safe in patients with most, if not all, cardiac valve prostheses. [89],[90],[91] At that field strength, prosthetic valves experience minimal or no magnetic field interaction and heating of ≤0.7˚C. For evidence-based information about safety of a specific prosthetic valve (or other hardware) with MR imaging, the readers are referred to http://www.mrisafety.com.

While imaging mechanical prosthetic valves, local artifacts are frequently seen [Figure - 7]A and smaller artifacts are present with bioprosthetic valves [Figure - 7]B. [90] Mitral annuloplasty bands typically produce little artifact and motion of the native valve leaflet can be assessed in a standard fashion [Figure - 8].

CMR has successfully been used in evaluating prosthetic valve complications, including thrombosis [92] and paravalvular regurgitation. [93] While with the contemporary bileaflet mechanical prostheses there is significant artifact, leaflet motion can still be assessed [Figure - 9]. Thus CMR is emerging as a useful non-invasive modality in the assessment of a wide range of patients with valve prostheses.


   Summary Top


CMR is a safe and noninvasive imaging modality. Combination of black blood anatomic imaging, cine SSFP functional imaging and flow velocity mapping provides for a comprehensive diagnosis and quantification of all clinically-significant valvular pathology, including assessment of valve lesion etiology, as well as its sequelae [Figure - 10]. Though quantitative CMR measures can readily be obtained, further prognostic data are needed to firmly establish the clinical role of CMR in patients with valvular heart disease.

 
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Correspondence Address:
Eli V Gelfand
BIDMC - Cardiology, E/RW-453, 330 Brookline Avenue, Boston, MA 02215
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DOI: 10.4103/0971-3026.33621

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