Introduction

Cine imaging using SSFP and velocity mapping sequences are the cornerstone of CMR valvular assessment. Breath-hold high-resolution FSE can be useful for valve morphology and developments such as moving slice velocity mapping and real-time imaging will improve evaluation in the near future. CMR reporting in valvular heart disease generally includes:

Further reporting requirements are highlighted following discussion of the specific valvular lesions.

Aortic stenosis

AS can occur at the valvular, subvalvular or supravalvular level. Valvular AS most commonly occurs with degeneration of a normal three-cusp aortic valve architecture. Bicuspid valves degenerate far earlier than tricuspid valves and are the second most common cause of AS in the adult, with rheumatic heart disease being the third. Subvalvular AS is caused by a fibrous or fibromuscular membrane which encircles the LVOT causing outflow tract obstruction. These patients typically present in adulthood with recurrence of a previously resected subvalvular membrane. Subvalvular stenosis can also be caused by asymmetric septal hypertrophy, abnormal MV or papillary muscle insertion, or posterior displacement of the infundibular septum. Supravalvular AS is uncommon, producing narrowing usually at the sino-tubular junction. It occurs in patients with Williams syndrome (60%), and there are familial forms and sporadic idiopathic cases.

AS has a long asymptomatic period after which clinical symptoms such as angina, exertional dyspnoea and effort syncope may result. Since symptoms develop late in the pathophysiology of AS, clinical deterioration soon follows if valve replacement is not undertaken. Obstruction to the LVOT leads to elevated LV pressures and compensatory LVH which initially maintains cardiac output and reduces LV wall stress, but ultimately decreases LV compliance and increases end-diastolic pressure. Increased myocardial oxygen demand causes myocardial ischaemia and later LV failure.

CMR is a robust method of calculating the severity of AS and correlates well with continuous wave Doppler echocardiography. However, it is far superior to echocardiography in identifying the cardiovascular functional and morphological sequelae of AS, such as post-stenotic dilatation of the ascending aorta, degree of LVH, and myocardial viability (Figures 4.1 and 4.2). These are important factors in the assessment of patients prior to valve replacement surgery, which often includes CABG and aortic root replacement. CMR also provides an excellent modality for serial surveillance of parameters such as LV function, volumes and mass in patients under follow-up and post operatively. This is done using the pre-contrast part of the protocol outlined in Chapter 1. Cines can demonstrate thickening and bulging of the aortic cusps and turbulent flow in the aorta. Imaging in an oblique sagittal plane parallel to the valve may demonstrate restricted opening and closure of the valve, and is frequently satisfactory for direct planimetry of the valve orifice and evaluation of bicuspid valve morphology. Direct planimetry can be performed using cine imaging or velocity mapping sequences (Figure 4.3).

Figure 4.1 SSFP cine images illustrating part of the CMR assessment in a 66-year-old man with mild AS. The first LVOT view (a) shows signal loss across the AV indicative of turbulent blood flow (solid white arrow) and no evidence of LVH. The second LVOT view (b) reveals mild aortic root dilatation at the sinuses of Valsalva (dashed white arrow). The AV is trileaflet (c; black arrow) and the remainder of the ascending aorta is mildly dilated (d; Ao). Further quantification of LV parameters with a ventricular short-axis stack of cines demonstrated mild LV dilatation with preserved function.

Figure 4.2 SSFP cine CMR views from a 34-year-old man with severe AS.

The LVOT views (a, b) show marked signal loss across the AV (solid white arrows) and LVH (solid black arrows). The AV is bileaflet (c; dashed black arrow) and there is an associated coarctation of the aorta (d; dashed white arrow).

Figure 4.3 The SSFP cine CMR LVOT view (a) allows positioning of an oblique sagittal plane just beyond the AV (white line) which then corresponds to the cine views obtained in diastole (b) and systole (c) showing bicuspid valve morphology with a fused commissure (solid white arrows). Velocity mapping in this plane (d) and these latter two images allow direct valve planimetry. However, velocity mapping is mainly used to determine the peak AV velocity. To do this, a measurement cursor is placed at several points within the valve orifice during systole (dashed white arrow) and the peak value recorded in Venc optimized images.

Velocity mapping is primarily used, however, to calculate the pressure gradient across the AV using the modified Bernoulli equation;

where ΔP (mmHg) is the pressure drop across the stenosis, and V_max is the peak velocity (m/s) determined by velocity mapping 1 to 1.5 cm above and parallel to the valve plane (Figure 4.3d). The typical Venc setting for the LVOT is 2 m/s, and 2.5 to 4 m/s for the aorta. An initial Venc of 2.5 m/s is often used and adjusted upwards if there is velocity aliasing (Figure 1.3). The velocity of blood flow through the AV is greater than that through the MV.

Velocity mapping, SSFP cines and FSE imaging are combined to determine the level of obstruction in AS. It is important to use information from all these sequences since a thin subaortic shelf is not reliably excluded even with high-resolution FSE CMR techniques. The initial planes and sequence required are the LVOT cines with subsequent imaging targeted by the pattern of turbulence demonstrated on these views.

Severity of AS is graded as: mild >1.5 cm², moderate 1.0–1.5 cm², severe <1.0 cm², and critical <0.8 cm². In severe AS, the peak transvalvular gradient is usually >50 mmHg.

Specific CMR reporting features for AS include:

Aortic regurgitation

AR can be caused by valve leaflet abnormalities, dilatation of the aorta or distortion of the aortic root. Valves that are stenotic generally also have some regurgitation. Leaflet abnormalities include degeneration of a normal valve, bicuspid AV, infective endocarditis, rheumatic disease, sequelae of balloon dilatation or valvotomy for congenital AS, and thickening of the aortic cusps from the jet of subvalvular obstruction. The ascending aorta may dilate secondary to hypertension, aortitis (Reiter syndrome, ankylosing spondylitis, giant cell arteritis, relapsing polychondritis) or annuloaortic ectasia (Marfan syndrome, osteogenesis imperfecta, Ehlers–Danlos syndrome and pseudoxanthoma elasticum). Additionally, there is an aortopathy associated with bicuspid AV which causes dilatation of the ascending aorta. Distortion of the aortic root is seen with dissection, trauma and sinus of Valsalva aneurysm.

AR results in volume overload of the LV causing increased end-diastolic pressure. This is counteracted by compensatory ventricular dilatation and an increase in LV compliance which accommodates the regurgitant volume and preserves cardiac output. Patients may remain asymptomatic in this compensated phase for many years until diastolic filling pressures continue to rise and LV dysfunction or myocardial ischaemia ensue. Early detection of decompensation is critical to the timing of definitive surgical repair. Factors which influence surgical timing include severity of AR, New York Heart Association functional class, LVEF, LVESV and LVEDV, and degree of dilatation of the aortic root. In particular, LVEF and LVESV are powerful predictors of outcome in AR, and the reproducibility of these measurements as well as the reproducibility of regurgitant volume with velocity mapping CMR is excellent.

Cines in the LVOT plane demonstrate the flow void due to the regurgitant jet which can be qualitatively assessed by the size of the jet, after taking into account the effects of echo time (Figure 4.4a). Velocity mapping is performed in a plane perpendicular to the AV positioned just above the valve but below the coronary ostia (Figures 4.4b–c). This plane optimally demonstrates forward and regurgitant flow and can further characterize the morphology of the valve. The regurgitant fraction is then calculated by graphing the flow volume curve using dedicated software (Figures 4.4d–e). The area under the curve below zero in diastole represents the regurgitant volume and the regurgitant fraction is calculated by dividing this into the forward SV in systole (Figure 4.5). An alternative method is to calculate the aortic flow and pulmonary flow at a level just above the respective valve planes. The difference in forward flow provides an estimate of the degree of AR in single-valve disease, as does the quantitative difference between RV and LVSV. The degree of ventricular dilatation also helps to qualitatively assess the severity of regurgitation. In the presence of dilatation, LGE imaging is performed to look for myocardial fibrosis, and this is especially useful in the preoperative setting.

Figure 4.4 SSFP cine CMR LVOT view (a) showing a central jet of AR (solid white arrow) and dilated proximal aortic root (dashed white arrow). SSFP cines in the subsequent oblique sagittal plane (b, c) show a normal trileaflet valve and confirm one central jet (black arrows) which appears black due to turbulence. This latter plane is used for the velocity mapping sequence, which in this case shows forward flow as black and regurgitant flow as white (d, e). The region of interest is delineated throughout the cardiac cycle, in this case the AV area through systole (black circle) and diastole (white circle), and software analysis is performed to generate a curve from which the regurgitant fraction is calculated.

Figure 4.5 Flow velocity time curve from the case illustrated in Figure 4.4.

Using forward SV in systole (dark red) and the regurgitant volume in diastole (light red), the regurgitant fraction (regurgitant volume/SV) is automatically calculated by a dedicated software analysis tool. The value here was 20%, consistent with mild AR.

Severity of AR by regurgitant fraction is graded as: mild <30%, mild to moderate 30–39%, moderate to severe 40–49%, and severe >50%.

Specific CMR reporting points for AR include:

Mitral stenosis

Mitral stenosis (MS) is usually secondary to rheumatic heart disease. Unusual causes of MS include congenital valvular, subvalvular or supravalvular stenosis, leaflet deposits from amyloid or carcinoid, extensive mitral annular calcification and left atrial myxoma. The stenotic valve leads to a pressure gradient across the MV, causing left atrial pressures to rise. This leads to pulmonary venous hypertension and eventually to pulmonary arterial hypertension and RV failure.

CMR complements echocardiographic evaluation of MS, especially in patients with poor acoustic windows. Cines taken in the four-chamber, two-chamber and LVOT views demonstrate the thickened, hypokinetic and domed MV leaflets and a signal void arising from the MV in diastole extending into the LV (Figure 4.6). The LA is usually enlarged in moderate to severe stenosis and this is easily visualized. Additionally, the pulmonary trunk may be enlarged and tricuspid regurgitation (TR) may be present in cases with pulmonary arterial hypertension. This can be assessed in more detail by reviewing the four-chamber cine and acquiring an RVOT cine view, while a preliminary appraisal of the pulmonary arteries is readily made with the transaxial FSE acquisition. SSFP cines or velocity mapping performed in a plane parallel to the MV can be performed for direct planimetry of the stenotic valve orifice, although these can be limited by motion of the valve through the fixed imaging plane (Figure 4.7). Calculation of the valve area by direct planimetry correlates well with echocardiographic and cardiac catheterization data, but may slightly overestimate measurements obtained by these modalities.

Figure 4.6 SSFP cine images from a patient with MS secondary to rheumatic heart disease. The MV is thickened, there is turbulent inflow with limited valvular excursion (a; black arrows), and a dilated LA. The AV is also thickened and there is associated AR (b; white arrow).

Figure 4.7 SSFP cine image performed in a plane parallel to the MV (white arrow) from the case illustrated in Figure 4.6. Direct planimetry revealed a valve area of 1.8 cm³.

Velocity mapping is used to quantify the transvalvular pressure gradient using the modified Bernoulli equation, with V_max calculated using in-plane or preferably through-plane velocity mapping in the LV just distal to the MV. This method can underestimate the gradient. The Venc is set at a much lower value than in AS, for example 1.5 m/s.

Severity of MS is graded as: mild ≥1.5 cm², moderate 1.0–1.5 cm², and severe <1.0 cm².

Specific CMR reporting points for MS include:

Mitral regurgitation

MR can be caused by abnormalities of the MV leaflets (myxomatous degeneration (Figure 4.8), rheumatic disease, endocarditis, parachute MV), chordae tendineae (myxomatous or traumatic chordal rupture, endocarditis), papillary muscle (acute infarction) or annulus (annular dilatation, periannular calcification). Chronic MR produces LA and LV dilatation due to increased SV. Pulmonary venous hypertension may be present but is usually less severe than with MS, and pulmonary arterial hypertension is also less common. Acute MR causes pulmonary venous hypertension which may be asymmetrical (right upper lobe) without atrial or ventricular enlargement.

Figure 4.8 Four-chamber SSFP cine CMR from a 40-year-old woman with familial MV prolapse (MVP). MVP is thought to be caused by myxomatous degeneration of the valve leaflets and chordae tendineae. The middle of the valve bows beyond the annulus during systole; however, the tips of the leaflets do not pass beyond this point (black arrows). Mild associated MR is noted in this case (white arrow) but is not always present.

CMR quantification of the severity of MR is useful in evaluating the signs of decompensation, monitoring response to treatment and planning MV repair or replacement. Important information required for preoperative assessment includes an EF less than 50–60% and an end-systolic diameter greater than 5.0–5.5 cm, both readily measurable with CMR. In the presence of LV dilatation or impairment, myocardial assessment with LGE is advisable.

Cines in the four-chamber, two-chamber and LVOT views demonstrate the signal void from the regurgitant jet. The jet may be central in annular dilatation or eccentric in valve leaflet, chordae tendineae or papillary muscle dysfunction. Multiple or complex jets may be present (Figure 4.9). Qualitative assessment of the severity of regurgitation can be performed by the size of the jet (accounting for the echo time) and the degree of LA dilatation. Quantification of regurgitant fraction can be performed in several ways along lines similar to that already described with AR. MR secondary to central, uncomplicated jets can be quantified by prescribing a region of interest with velocity mapping CMR using a plane placed en-face and just distal to the valve. This plane can also be used to measure effective regurgitant orifice area. A method in single regurgitant lesions is via determination of the difference in LVSV with either flow in the proximal ascending aorta or MPA, or the RVSV if no right-sided regurgitation is present.

Figure 4.9 (a) Four-chamber, (b) modified two-chamber, (c) LVOT and (d) basal short-axis cines from a 42-year-old woman with multiple, eccentric jets of MR (black arrows). LVEF was 47% and regurgitant fraction calculated by comparison of LV and RV SV was 60%, indicating severe regurgitation. LGE demonstrated no enhancement.

Severity of MR by regurgitant fraction is graded as: mild <30%, mild to moderate 30–39%, moderate to severe 40–49%, and severe >49%.

Additional CMR reporting points for MR are:

Other valve lesions

Mixed valvular disease is often present, particular in rheumatic heart disease which principally involves the MV and less commonly the AV. In late disease with pulmonary arterial hypertension, TR is also present. CMR quantification of regurgitant fractions can become complicated in this setting but is possible using velocity mapping at the individual valve planes and generating several flow velocity time curves for evaluation in comparison with LVSV and RVSV. Valve planimetry of the mitral and aortic valves can also be undertaken to yield values for an effective regurgitant orifice (Table 4.1). As with echocardiography, all available indicators of severity must be assessed collectively prior to documenting final conclusions on the clinical report.

Table 4.1 Grading of severity by effective regurgitant orifice (mm²)

CMR is also useful in quantifying the degree of TR, pulmonary regurgitation (PR) and pulmonary stenosis (PS). Its multiplanar capability is useful in the assessment of global RV function which may be limited using echocardiography.

Metallic prosthetic valves are safely imaged with CMR. Although assessment of paravalvular abscesses or quantification of regurgitant lesions can be performed, results may be markedly hindered by metal artefact and conclusions should therefore be interpreted within this context (Figure 4.10). FSE sequences are less affected in the presence of metal than SSFP cine or velocity mapping CMR.

Figure 4.10 LVOT views by (a) SSFP cine imaging and (b) velocity mapping CMR in a 27-year-old woman who had undergone aortic root enlargement and AV replacement (21-mm Medtronic Hall tilting disc prosthesis) 12 years previously.

Signal void from the metal valve is clearly seen using these two sequences (black arrows), but the artefact does not preclude assessment of the valve function.

Indications

CMR can:

CMR cannot: