Fig. 4.1
(a) A basal short axis slice showing the half open leaflets of the mitral valve, with the P1, P2 and P3 scallops of the posterior leaflet. (b) A contiguous stack of oblique slices of 5 mm thickness is aligned orthogonal to the central part of the line of coaptation, covering the extent of the valve. (c–e) Three of the slices are shown which show the A1-P1, A2-P2 and A3-P3 regions of the valve, respectively
Acquire contiguous stack of oblique slices, 5 mm thickness, aligned orthogonal to the central part of the line of coaptation (Fig. 4.1b). Start from the superior (i.e., anterolateral) commissure adjacent to A1-P1 and progress towards the inferior (i.e., posteromedial) commissure adjacent to A3-P3. Slices should be 5 mm in thickness with no gap. Typically 8–10 slices cover the length of the valve. Three are illustrated (Fig. 4.1c–e).
Acquire further pair of oblique slices orthogonal to the oblique line of leaflet coaptation at each end of the valve adjacent to the commissures (across A1-P1 and A3-P3)
Aortic and Pulmonary Flow Study
Phase contrast through-plane velocity mapping of aortic and main pulmonary artery flow is performed. The velocity encoding (VENC) should be adapted to actual velocity using the lowest velocity without aliasing. The flow velocity and volume should be measured perpendicular to the vessel distal to valve leaflet tips. A useful anatomical landmark for the aortic flow is to measure the flow just above the sinotubular junction at end systole. The recorded net forward volumes are used to calculate regurgitant volume and regurgitant fraction (see below). Because coronary flow does not pass though this slice, the aortic flow measured is typically about 5 % less than main pulmonary artery flow in the absence of a shunt.
In-plane/Through-Plane Fast Low Angle Shot (FLASH) of Mitral Valve (Optional)
FLASH cine acquisitions can have higher sensitivity than bSSFP for the visualisation of the regurgitant jets, depending on their echo time and other aspects of sequence design. A specific FLASH sequence may therefore be found useful for identification of the number, origin, and direction of the regurgitation jet (s), with possible qualitative assessment of the severity of regurgitation.
Phase Velocity Mapping of MR (Optional)
Through-plane phase velocity mapping of the mitral inflow gives a measurement of the anterograde velocity of the mitral inflow which is equal to mitral E and A velocities on pulsed wave Doppler echocardiography. Similar to echo, the measurement should be performed just distal to the tips of the mitral leaflets in diastole. The velocity encoding (VENC) should be adapted to actual velocity using the lowest velocity without aliasing. In addition, as an alternative to the FLASH approach, through-plane breath hold velocity mapping can be used, placed immediately on the atrial side of the closed MV, orthogonal to the jet(s), to map the locations and number of regurgitant lesions. Although MR jet velocity is expected to be higher, a VENC of 250 cm/s is usually adequate because of partial volume averaging, and can result in more effective visualisation than too high a VENC.
Viability
Assessment of myocardial viability and any scar or fibrosis is based on the late enhancement of the myocardium following gadolinium contrast injection. Gadolinium contrast is injected intravenously at a dose of 0.1–0.2 mmol/kg. After 8–10 min wait, the late gadolinium enhanced (LGE) images are acquired. Same views as for cine imaging (except for the mitral valve stack images) should be acquired. Slice thickness is the same as for cine imaging. In-plane resolution is about 1.4–1.8 mm. The acquisition duration per R-R interval should be below 200 ms but should be less in the setting of tachycardia to avoid image blurring. Inversion time is set to null myocardium. We recommend routine use of phase sensitive inversion recovery (PSIR) sequence in addition to magnitude images. The PSIR sequence requires less frequent adjustment of the inversion time (TI) and is particularly useful when the TI used to acquire the magnitude images was not optimal. Read out is usually every other heartbeat. It should be modified to every heartbeat in bradycardic patients and to every third heartbeat in tachycardic patients.
Analysis of the Images
Assess Mitral Valve Structure
Mitral leaflets and the mitral apparatus should be assessed on the 2-chamber, 3-chamber (i.e., LVOT view), 4-chamber, basal short axis, and MV stack cine images for any evidence of thickening, calcification, prolapse, restriction or tethering of the leaflets. The mitral valve stack images help to localise the pathology to mitral leaflet scallops (A1-A3, P1-P3) according to Carpentier’s nomenclature. In functional mitral regurgitation, the leaflets appear structurally normal but the mitral annulus is often dilated. Annular dilatation is present when the anteroposterior (AP) diameter is more than 35 mm or the AP diameter/anterior leaflet length ratio is more than 1.3.
The following measurements can be readily performed on CMR (similar to echocardiography) and help to quantify the extent of LV remodelling and the severity of altered geometry of the mitral valve, as well as the risk of postoperative failure of mitral valve repair.
In LVOT view:
- (a)
The length of the anterior and posterior leaflets and the anterior-posterior (AP) diameter of the mitral annulus (Fig. 4.2a). Severe annular dilatation (annular diameter >50 mm) is a predictor of operation failure.
Fig. 4.2
The panels each show a frame of 3-chamber cines in which functionally relevant measurements can be made. (a) The lengths of the anterior and posterior leaflets of the mitral annulus and its anterior-posterior diameter. (b) The thickness of the basal septum at end diastole. (c) The shortest distance between the basal septal bulge and the coaptation point. The aorto-mitral angle, being the angle between the mitral annular plane and the aortic annular plane, can also be measured in systole and diastole. When the aorto-mitral angle is more than 120°, the risk of SAM and LVOT obstruction following repair would be low. The more acute the aorto-mitral angle, the higher the risk of SAM and LVOT obstruction following repair
- (b)
Basal anterior septum in diastole (Fig. 4.2b). A septal bulge of more than 15 mm in thickness is a predictor of SAM and LVOT obstruction after repair.
- (c)
C-Sept: C-Sept is the shortest distance between the basal septal bulge and the coaptation point in systole (Fig. 4.2c). Coaptation point is the point where the mitral leaflets coapt in systole. C-sept <2.5 cm is a predictor of SAM and LVOT obstruction after mitral valve repair. This is because the mitral valve repair procedure usually includes ring annuloplasty. The implantation of the ring moves the whole mitral valve anteriorly towards the LVOT and increases the risk of SAM and LVOT obstruction.
- (d)
Aorto-mitral angle in systole and diastole. The aorto-mitral angle is the angle between the mitral annular plane and the aortic annular plane. When the aorto-mitral angle is more than 120°, the risk of SAM and LVOT obstruction following repair would be low. The sharper the aorto-mitral angle, the higher the risk of SAM and LVOT obstruction following repair.
- (a)
In 4 chamber view:
- (a)
Tenting area: This is the area between the mitral annular plane and the mitral leaflets in mid systole in the 4 chamber view (Fig. 4.3a). A tenting area of more than 2.5 cm2 is a predictor of unsuccessful repair. This is because a large tenting area implies that the leaflets are pulled down severely by the papillary muscles which itself means the LV is severely dilated and remodelled, and if the remodelling continues after the operation the patient will develop severe MR again.
Fig. 4.3
In a four chamber view (a), the tenting area is the area between the mitral annular plane and the mitral leaflets in mid systole. The coaptation distance is the longest distance between the coaptation point and the mitral annular plane in systole. The posterior leaflet angle (b) is the angle which is the largest angle between the mitral annular plane and the posterior leaflet in mid-systole
- (b)
Coaptation distance: This is the longest distance between the coaptation point and the mitral annular plane in systole. A coaptation distance more than 1 cm predicts unsuccessful surgery and post-operative MR (Fig. 4.3b)
- (c)
Posterior leaflet angle: The posterior leaflet angle which is the largest angle between the mitral annular plane and the posterior leaflet in mid-systole is another indicator of LV remodelling and displacement of the papillary muscles. A posterior leaflet angle of more than 45° predicts unsuccessful operation (Fig. 4.3b).
- (a)
In short axis view:
The intercommissural diameter of the mitral annulus can be measured from the basal LV short axis view of the mitral valve (Fig. 4.4).
Fig. 4.4
The intercommissural diameter of the mitral annulus as measured from the basal LV short axis view of the mitral valve
There are two papillary muscles in the left ventricle: the anterolateral papillary muscle and the posteromedial papillary muscle. Both papillary muscles should be assessed for rupture, infarction, fibrotic elongation, and displacement. Due to the limited spatial resolution of MRI, chordae tendineae are usually not seen unless they are thickened.
Confirm the Presence and Mechanism of MR
The MR jet can be seen readily on the bSSFP and FLASH cine sequences.
Functional MR in DCM
In functional MR secondary to DCM, both leaflets are symmetrically tethered which leads to a symmetrical tenting pattern of the mitral valve in systole (Fig. 4.3c). The same pattern can be seen in patients with ischaemic cardiomyopathy due to both anterior and inferoposterior infarction (see below). In dilated cardiomyopathy, functional MR is a consequence of:
- (a)
Apical displacement of the papillary muscles. This leads to tethering of both mitral leaflets towards the apex (Carpentier type IIIb).
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