Cardiac morphology and function

Breath-hold SE imaging with multislice transaxial FSE followed by multislice SSFP in all three orthogonal planes (transaxial, coronal and sagittal) is useful for optimal planning of subsequent planes. Adequate anatomical coverage is preserved despite the temporal limitation imposed by breath-holding using staggered acquisition of contiguous slices and allowing breathing in-between, or additional software which speeds up the scan further at the expense of resolution. In the presence of significant metallic artefact FSE acquisitions are degraded to a lesser degree than GE sequences.

Subsequent cines are acquired in planes determined by the underlying diagnosis and provide higher resolution anatomical and functional information on parameters including biventricular size, function and mass. Assessment of the RV and pulmonary vasculature is of particular importance in ACHD along with extensive flow velocity mapping for measurement of peak velocities, regurgitant fractions and shunt performance (Figure 7.2). The Venc is set to account for low velocities in venous systems and higher velocities within shunts. Information from both the cines and velocity mapping is crucial for interpretation of the patterns of blood flow. Cine images also direct optimal positioning of velocity mapping sequences.

Figure 7.2 Preoperative CMR assessment of a 73-year-old woman with PS.

(a) Modified sagittal SSFP cine view showing the stenotic pulmonary valve (PV; black arrow), an aneurysmal LPA (maximal diameter 4.2 cm) and right ventricular hypertrophy (RVH). Turbulent blood flow above the valve is noted as signal loss within the otherwise high intensity blood signal. A perpendicular imaging plane was then acquired just above the valve, within the narrow part of the jet (black line). (b) Resulting SSFP cine showing the PV with restricted opening (solid white arrow). (c) Subsequent velocity mapping CMR in the same plane. Peak velocity was measured at 2.8 m/s within the area indicated (dashed white arrow).

Contrast use

MRA with gadolinium is useful for the assessment of pulmonary branch artery, aortic and major arterial anatomy. The LGE technique is being increasingly used to identify ischaemic or post-surgical scarring of ventricular myocardium.

Ventricular septal defect

VSDs are the most common congenital heart defects. They range from small haemodynamically insignificant and isolated lesions to large defects associated with multiple cardiac anomalies such as in TOF. CMR is complementary to echocardiography in the assessment of simple and complex VSDs. It allows multiplanar visualization of the spatial relationship of the defect to surrounding structures, can diagnose concurrent cardiac pathology and their functional sequelae, and quantify shunt size and effect on the RV and PAs. SE imaging en-face and parallel to the VSD provides anatomical information. SSFP cines show signal loss in the RV due to blood flow through the VSD and are useful in assessing the shape and dimensions of the defect (Figure 7.3). Shunt ratio can be calculated in two ways: firstly, comparison of pulmonary to systemic flow (Qp:Qs) obtained by velocity mapping CMR, and secondly, from the difference in ventricular SV obtained from the ventricular short-axis stack of cines (Figure 7.4).

Figure 7.3 Four-chamber SSFP GE cines from a 65-year-old man with a suspected Gerbode defect (VSD communicating directly between LV and RA).

(a) Small, basal, probably muscular VSD is identified (dashed white arrow). (b) Flow through this defect is into the RV (solid white arrow) and not the RA. Associated TR (black arrow) is in fact responsible for the dilated RA. Of note, the apparent interatrial defect seen on these images is a consequence of the imaging plane and not an ASD, and there is RVH.

Figure 7.4 Calculation of Qp:Qs for the VSD case presented in Figure 7.3 using velocity mapping CMR.

(a, b) Oblique sagittal views of the ascending aorta in cross section by (a) magnitude and (b) phase-subtracted components of velocity mapping CMR. This plane is obtained from the SSFP cine LVOT view by imaging perpendicularly and just distal to the AV. (c, d) Oblique transaxial views of the proximal MPA in cross section by (c) magnitude and (d) phase-subtracted components of velocity mapping CMR. This plane is obtained from the SSFP GE cine RVOT view by imaging perpendicularly and just distal to the PV. (b, d) Flow within the aorta (white circle) and MPA (black circle) is calculated throughout the cardiac cycle and was 5.0 and 9.2 L/min, respectively, giving a Qp:Qs of 1.8:1. Calculations of the Qp:Qs by comparison of SV will be overestimated in the presence of severe TR.

Specific reporting features in the CMR assessment of VSD are therefore:

Atrial septal defect

There are three types of ASDs, all of which can present initially in adulthood. The commonest defect is the ostium secundum defect (75%) in the area of the foramen ovale. Echocardiography is first line for the diagnosis of ASD. CMR is used where echocardiography is suboptimal, identifying the defect through visualization of low signal on the right side of the atrial septum on SSFP cines. CMR quantifies shunt size, determines the anatomical and functional effects of the shunt, and accurately identifies anomalous pulmonary venous drainage (APVD; Figure 7.5).

Figure 7.5 Secundum ASD with Qp:Qs of 2.2:1 calculated by velocity mapping at the aorta and MPA as described with the VSD in Figure 7.4.

(a) Atrial transaxial SSFP cine used to position the subsequent velocity map. (b) Velocity mapping CMR shows flow from the LA to the RA as signal loss (black arrow). (c) MIP reconstruction of CE-MRA data of the anatomy of the pulmonary vasculature showing no evidence of APVD (white arrow)—there are two right-sided pulmonary veins and three left-sided pulmonary veins.

Specific reporting features for ASD are:

Partial anomalous pulmonary venous drainage and major aortopulmonary collateral arteries

CMR has a useful role in the detection and serial evaluation of extracardiac venous and arterial vascular anomalies in ACHD, such as partial anomalous pulmonary venous drainage (PAPVD) and major aortopulmonary collateral arteries (MAPCAs). CMR imaging protocols in PAPVD and MAPCAs incorporate MRA for optimal detection of vessels and this often provides additional information to findings at cardiac catheterization as well as confirming the diagnosis (Figure 7.6). Velocity mapping quantifies shunt sizes, and SE techniques can surmount image artefact from endovascular coils.

Figure 7.6 Two MIP reconstructions of CE-MRA data from a 63-year-old woman showing a CMR diagnosis of PAPVD with the left upper pulmonary vein draining via a vertical vein into a dilated left brachiocephalic vein (white arrows).

Qp:Qs in this case was 1.4:1 and there was no evidence of RV dysfunction or pulmonary hypertension.

CMR reports include:

Pulmonary stenosis, right ventricular outflow tract and conduit obstruction

In contrast to the assessment of ASD and VSD, the RVOT and conduits from the RV to the PA are often poorly visualized by echocardiography. CMR provides detailed imaging of the level and anatomy of RVOT obstruction with SE and SSFP cine imaging in orthogonal planes. Serial, contiguous cines in the transaxial plane are useful for functional assessment of extracardiac ventriculopulmonary shunts, such as the Rastelli shunt. Velocity mapping quantifies severity of stenosis or regurgitation and may be used at valve level, within the MPA, or separately within the right and left PAs (Figure 7.7). CE-MRA is an excellent noninvasive method of assessing branch pulmonary stenoses and identification of MAPCAs.

Figure 7.7 SSFP cines from a 28-year-old man with an underlying diagnosis of truncus arteriosus.

The patient had undergone the Rastelli procedure (a valved conduit from the anterior wall of the RV to the PA with VSD closure) aged 3. The conduit had been replaced 4 years prior to this CMR study. (a) RVOT view showing the homograft in the pulmonary position to be functioning well without significant stenosis or regurgitation (white arrow). Metallic artefact from sternal wiring is noted (double-headed white arrows). (b) Transaxial view of the MPA and bifurcation into the LPA and RPA shows narrowing at the origin of the latter with relative distal hypoplasia (solid black arrow). Signal loss within the proximal RPA indicates turbulent blood flow and velocity mapping was therefore performed in a plane perpendicular and just distally (dashed black line). Peak velocity was 2.4 m/s, confirming significant proximal RPA stenosis.

Patent ductus arteriosus and aortopulmonary window

PDA describes persistent arterial communication between the proximal LPA and descending thoracic aorta distal to the origin of the left subclavian artery. It is usually an isolated lesion in adults. Flow through a PDA is detectable on cines, which show turbulence in the anterior portion of the PA proximal to its bifurcation (Figure 7.8). CMR can quantify shunt size and evaluate signs of pulmonary hypertension prior to decisions regarding device closure.

Figure 7.8 Oblique sagittal SSFP cine view from a 71-year-old woman with Ehlers– Danlos syndrome and PDA (demonstrated by marked signal loss from the aorta into the PA; white arrow).

Aortopulmonary window is a connection between the ascending aorta and PA which presents in a fashion similar to that of PDA and can be differentiated by cines.

Ebstein anomaly

The diagnosis of Ebstein’s anomaly is usually made on echocardiography but CMR has a role in defining anatomy and identifying those requiring and suitable for surgical correction (Figure 7.9). Apical displacement of the septal and mural leaflets of the tricuspid valve (TV) is identified on long-axis cines. Serial short-axis cines assess enlargement of the anterior leaflet and degree of TV dysplasia, and are later analysed to provide volumetric and functional data on the RA and RV. Velocity mapping CMR is usually performed in order to quantify the degree of TR, but is occasionally also used to quantify the severity of TS. Both SSFP cines and velocity mapping are used to evaluate the presence of abnormal atrial communication.

Figure 7.9 Pre- and postoperative four-chamber SSFP cines from a CMR study performed on a 16-year-old girl with severe Ebstein anomaly.

(a) There is 4 cm apical displacement of the septal leaflet of the TV (black arrow). The RA, atrialized part of the RV, and RV are dilated. RVEF was 45% in the presence of severe TR, regurgitant fraction 49% by comparison of LV and RVSV. (b) Satisfactory early (day 8) postoperative findings at CMR. Surgery comprised repair of the TV, plication of the RV, cryoablation of the RA, and ASD closure with a fenestrated patch and one-way Gortex flap to allow right-to-left shunting in the presence of elevated RA pressures. The reconstructed TV was noted to be functioning well without TR and only mild tricuspid stenosis (TS), orifice area 3 cm² and peak recorded diastolic velocity of 1.2 m/s.

Tetralogy of Fallot

TOF comprises VSD, RVOT obstruction, RVH and an overriding aorta. Definitive repair includes VSD patch closure and resection of RVOT obstruction, with older patients often having prior palliative shunt procedures such as the Blalock–Taussig (subclavian to PA anastomosis) shunt. Reconstruction of the RVOT leads to important PR and serial assessments of regurgitation and the RV are crucial following repair. Other postoperative complications, such as aneurysm formation in the RVOT and dilatation of the aortic root, as well as co-existing congenital anomalies, including right-sided aortic arch and aberrant coronary anatomy, can often be identified at the initial orthogonal multislice FSE sequences and are then further characterized with higher resolution SSFP sequences.

Serial, contiguous ventricular short-axis cines enable calculation of standard biventricular parameters and also highlight residual septal defects, confirmed using velocity mapping CMR. Comparison of RV and LVSV or velocity mapping above the pulmonary valve (PV) provides PV regurgitant fraction, with a value of 40% representing free PR. Cines and velocity mapping at the RVOT, LPA and RPA evaluate stenoses at these sites. Branch PS can arise due to distortion from palliative arterial shunts, particularly the Pott (descending aorta to LPA) or Waterson (ascending aorta to MPA or RPA) shunts, and may provoke worse PR. Patency of residual shunts should be assessed due to the risk of pulmonary hypertension through augmented pulmonary blood flow.

LGE in the RV and LV is common following TOF repair and is related to adverse clinical markers including ventricular dysfunction. In particular, RV enhancement predicts arrhythmia which has potential prognostic implications (Figure 7.10).

Figure 7.10 (a) RVOT SSFP cine view and (b) equivalent LGE view from a 27-year-old man with repaired TOF highlighting areas of enhancement at the free wall and septal surface of the RV (white arrows).

Serial CMR assessments without contrast can be used to monitor drug treatment and determine timing of repeat intervention to the PV.

Specific comments in TOF CMR reports include:

Corrected transposition of the great arteries

In transposition of the great arteries (TGA) the aorta arises from the RV and the PA arises from the LV. Almost all surviving adults with TGA have undergone operative correction in childhood, either removal of the atrial septum and insertion of a baffle (Mustard operation) or folding of the atrial wall (Senning operation), to redirect atrial flow but leaving the transposed ventriculoarterial connections. The primary aim in postoperative imaging is assessment of pulmonary and systemic venous flow pathways (Figure 7.11). CMR readily identifies stenoses and baffle leaks with subsequent multiplanar and multislice evaluation allowing anatomical clarification and accurate quantification. SSFP cines are acquired to visualize each pathway in order to identify narrowing or signal loss due to turbulence. Through-plane velocity mapping is then used to identify flow velocities above 1 m/s, indicative of flow obstruction at atrial level. Dilatation of the azygous and hemi-azygous veins is a useful pointer to significant systemic venous pathway obstruction.

Figure 7.11 Part of the CMR evaluation in a 30-year-old man who had undergone a Mustard operation for TGA aged 2 months.

(a) SSFP cine views of the systemic venous pathway shows a narrowed superior vena cava channel (SVC; dashed white arrow) and unobstructed IVC channel. Peak velocities of the two channels were subsequently measured at 1.2 and 1 m/s respectively by velocity mapping CMR. (b) SSFP cine view of the pulmonary venous pathways shows them to be unobstructed (dashed black arrows). (c) SSFP cine view of the outflow tracts reveals RVH and PR (solid black arrow). (d) LGE is noted at the RV free wall (solid white arrow).

Cine assessment of systemic RV function is very important in these patients since they are at risk of RV failure and TR and AR should be quantified when present.

Late enhancement of the systemic RV is also common in adults after the Mustard or Senning procedures, and the extent correlates with ventricular dysfunction and arrhythmia.

Patients who have undergone corrective arterial switch operations require assessment of supravalvular PS or AS and branch PA stenoses. Such patients are also at risk of ostial coronary artery stenoses at the site of coronary artery reimplantation, and these can also be evaluated by CMR.

In congenitally corrected transposition of the great arteries (ccTGA), the atria have normal position and venous return but there is atrioventricular discordance together with TGA. CMR identifies the morphological features of each atrium and ventricle, cardiac malposition, and associated congenital anomalies such as VSD, PS or subvalvular PS, and Ebstein-like TV. Accurate functional assessment of the systemic RV, which is susceptible to failure, is required prior to consideration for double switch repairs (Senning and arterial switch).

CMR reports therefore include:

Univentricular hearts and Fontan repair

In patients with one effective ventricle, the Fontan operation routes systemic venous return directly into the PAs leaving the ventricle to pump oxygenated blood to the systemic circulation. Pulmonary blood flow is dependent on elevated pressure within the systemic veins and any obstruction to flow, such as with thrombus, makes the circuit fail. Contemporary repair involves total cavopulmonary connection, usually by an atrial tunnel or extracardiac conduit. CMR is invaluable in the assessment of such shunts since echocardiography is often limited by the retrosternal position of the connection. Targeted SSFP cines of the connection identify obstruction as signal loss from turbulent blood flow. These acquisitions are also early indicators of RA thrombus formation, which should later be confirmed or refuted using EGE (Figure 7.12). Cine imaging is extended to the pulmonary veins to exclude post-operative compression, with through-plane flow velocities above 1 m/s coinciding with atrial systole being suggestive of obstruction. Serial, contiguous short-axis SSFP cines allow determination of univentricular function and since only one SV value is available, quantification of atrioventricular valve regurgitation requires velocity mapping at the aorta and MPA.

Figure 7.12 Part of the SSFP GE cine evaluation of a 22-year-old man with a Fontan circuit.

(a) Assessment of the caval pathways in a near sagittal plane reveals a patent SVC and IVC. The RA is markedly dilated and contains regions of slow flow characterized by signal loss and one area of probable thrombus (black arrow). (b) Assessment of the atriopulmonary connection in a near coronal plane shows a patent conduit from the RA to the MPA (white arrow). Slow flow is once again demonstrated by signal loss within the RA.

Specific CMR reporting comments for the Fontan circuit are on: