13 Valvular Heart Disease Andrew M. Taylor

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13 Valvular Heart Disease

Andrew M. Taylor and Jan Bogaert

A. M. Taylor MD, MRCP, FRCR

Cardiothoracic Unit, Institute of Child Health and Great Ormond Street Hospital for Children, London WC1N 3JH, UK

J. Bogaert MD, PhD

Department of Radiology, Gasthuisberg University Hospital, Catholic University of Leuven, Herestraat 49, 3000 Leuven, Belgium

CONTENTS

13.1 Introduction 353

13.2 Conventional Imaging Modalities 354 13.2.1 Transthoracic Echocardiography 354 13.2.2 Transesophageal Echocardiography 354 13.2.3 X-ray Angiography 354

13.2.4 Radionuclide Angiography 354 13.3 MRI of Cardiac Valves 355 13.3.1 Morphology 355

13.3.1.1 Valve Leaflets 355 13.3.1.2 Thrombus 355 13.3.1.3 Endocarditis 356

13.3.2 Valvular Regurgitation 356 13.3.2.1 Qualitative Assessment with Cine Gradient-Echo Imaging 357 13.3.2.2 Quantitative Evaluation of

Ventricular Volume Measurement 358 13.3.2.3 Quantitative Evaluation with

Velocity Mapping 358

13.3.2.4 Issues Related to MRI Functional Measurements 359

13.3.3 Valvular Stenosis 360 13.3.3.1 Qualitative Assessment with Cine Gradient-Echo Imaging 360

13.3.3.2 Quantitative Evaluation with Velocity Mapping 361 13.3.4 Aortic Valve 361

13.3.4.1 Aortic Regurgitation 361 13.3.4.2 Aortic Stenosis 364 13.3.5 Mitral Valve 366 13.3.5.1 Mitral Regurgitation 366 13.3.5.2 Mitral Stenosis 368 13.3.6 Pulmonary Valve 368

13.3.6.1 Clinical Importance of Pulmonary Regurgitation 370

13.3.7 Tricuspid Valve 370 13.3.7.1 Ebstein’s Anomaly 370 13.3.8 Mixed Valvular Disease 371 13.3.9 Prosthetic Valves 372

13.3.9.1 Ventricular Response to Valve Replacement 373 13.3.9.2 Velocity Mapping in the Aorta after

Aortic Valve Replacement 373 13.3.10 Magnetic Resonance Spectroscopy 374

13.1

Introduction

Over the last 5 years, the principles of valvular heart disease assessment with magnetic resonance imaging (MRI) have not changed significantly. By measuring both flow in the great vessels and right and left ventricular volumes by cardiovascular MRI, valvular function can be accurately and reproducibly quantified. However, what has changed is the speed with which a complete assessment of valvular function and associated pathology can be performed, with improved MR hardware and software that enables rapid acquisition of MR data. With these improvements, assessment of valvular function on a routine basis has become a reality.

When investigating cardiac valves, the information required can be divided into four categories:

• Clarification of the affected valve after ausculta- tion of the heart

• Definition of the valvular anatomy (valve leaflet number, leaflet thickness, presence of infective endocarditis)

• Assessment of valvular function (degree of valvular stenosis or regurgitation)

• Definition of the effect of the valvular dysfunction on other cardiac structures and function (ventricular size, function and mass, pulmonary artery pressure, great vessel anatomy)

These questions can be addressed by combining echocardiography with X-ray angiography. However, cardiovascular MRI can now provide much of the required information in a single investigation that is safe, noninvasive, without exposure to X-rays.

In this chapter, we will present an overview of the MRI techniques that are currently used in the assess-

13.4 Future Directions 375 13.5 Conclusion 375 13.6 Key Points 375 References 376

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ment of valvular heart disease, provide protocols for the cardiovascular MRI assessment of valvular heart disease, and identify new areas of development in this field. We will also discuss the advantages and limitations of cardiovascular MRI with reference to more conventional imaging modalities for investigating valvular heart disease.

13.2

Conventional Imaging Modalities

13.2.1

Transthoracic Echocardiography

Transthoracic echocardiography remains the most important and easily accessible investigation for the assessment of valvular heart disease (Carabello and Crawford 1997). The technique is noninvasive, safe, portable and accurate at localizing the diseased valve. Quantification of valvular stenosis and valve area can be easily performed (Currie et al. 1985);

however, echocardiography is less good at quantifying valvular regurgitation (Hatle and Angelson 1992; Smith and Xie 1998). A semiquantitative assessment can be achieved by measuring the regurgitant jet length and width, but there is poor correlation to X-ray angiographic measurements (Abbasi et al. 1980; Quinones et al. 1980; Bolger et al. 1988). Several echocardiography methods have been developed in an attempt to quantify valvular regurgitation, but all have their limitations. The regurgitant fraction can be calculated, but this is time-consuming; only patients with isolated valvular regurgitation can be readily assessed (Rockey et al. 1987); and echocardiography only provides an estimate of ventricular function (Boehrer et al.

1992). Imaging of the proximal isovelocity surface area has been proposed. However, this technique relies on the assumption that the regurgitant jet orifice is both flat and circular, which is not the case for most patients (Utsunomiya et al. 1991). A final limitation of transthoracic echocardiography is that the imaging plane may be restricted by the lack of good acoustic windows.

13.2.2

Transesophageal Echocardiography

Transesophageal echocardiography is valuable in the assessment of left atrial thrombus (Manning

et al. 1993), atrial septal wall defects (Hausmann et al. 1992), and aortic dissection (Tice and Kisslo 1993; Laissy et al. 1995). In the assessment of valvular heart disease, the technique may clarify valvular anatomy in patients with infective endocarditis (Pedersen et al. 1991) or in patients with poor acoustic windows. With regard to functional assessment, it is less accurate than transthoracic echocardiography because of difficulties with Doppler alignment and has all the limitations of transthoracic echocardiography in the assessment of valvular regurgitation, in addition to being an invasive procedure. A unique use for transesophageal echocardiography is in the intraoperative assessment of valvular function for monitoring and evaluating surgical and percutaneous interventions (Fix et al.

1993; Bryan et al. 1995).

13.2.3

X-ray Angiography

X-ray angiography has been regarded as the gold standard investigation of valvular heart disease, against which other imaging modalities should be compared (Sandler et al. 1963; Sellers et al. 1964;

Carroll 1993). Valvular stenosis can be quantified by calculating the transvalvular gradients and valve areas using the Gorlin formula (Gorlin and Gorlin 1951). However, the grading system used for the assessment of valvular regurgitation is both imprecise and inaccurate. Although generally regarded as safe, the technique is associated with a mortality of approximately 0.1%, and other compli- cations occur such as myocardial infarction, arterial embolization, thrombosis, and dissection (Davis et al. 1979). Also the use of ionizing radiation during the procedure is not ideal (Cohen 1991). In general, X-ray angiography should only be regarded as necessary if the pulmonary artery pressure needs accurate measurement, if the coronary artery anatomy needs defining, or if there is a discrepancy between the clinical symptoms and the noninvasive inves- tigations.

13.2.4

Radionuclide Angiography

The main use of radionuclide angiography is in the monitoring of ventricular function in order to assess when interventions should take place (Rigo et al. 1979; Sorenson et al. 1980). The technique can

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also be used to calculate the regurgitant fraction, but again only when regurgitation in a single valve is present. As with X-ray angiography, the use of ionizing radiation is not ideal, especially as serial measurements of ventricular function are necessary.

13.3

MRI of Cardiac Valves

MRI can provide good functional information about both valvular stenosis and regurgitation, and allows accurate assessment of ventricular function and relevant cardiac and vascular anatomy.

13.3.1 Morphology

13.3.1.1 Valve Leaflets

Normal valves are fast-moving, low proton-density, fine structures that are difficult to visualize with con-

ventional spin-echo (SE) “black-blood” imaging, due to respiratory motion and averaging. However, newer fast SE sequences allow for the acquisition of data at defined points in the cardiac cycle during a single breath-hold (Fig. 13.1). When thickened and immo- bile valve leaflet identification is more common.

Using spoiled gradient-echo (GE) or balanced steady-state free precession (b-SSFP) cine imaging, the moving leaflets are easily seen, because blood flowing over the valve tips leads to signal loss, secondary to eddies and mild turbulence (Kivelitz et al. 2003) (Fig. 13.2).

Despite these improvements in defining valve leaflet morphology, echocardiography remains the investigation of choice for imaging valve leaflet anatomy.

13.3.1.2 Thrombus

It is important to identify the presence of thrombus in the atria, particularly in the presence of mitral stenosis or valvular disease with atrial fibrillation.

SE imaging can identify atrial thrombus, but care must be taken to distinguish between slow-moving

Fig. 13.1a-d. T1-weighted fast spin- echo (FSE) images of the cardiac valves. a Oblique coronal view of the aortic valve (arrowhead). b Oblique sagittal view of the pul- monary valve (white arrow). c Left ventricle inflow/outflow view of the mitral valve (arrow) and aortic valve (arrowhead). d Axial view (systole) of the tricuspid valve (star)

a b

d c

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blood that may appear as increased signal on these images (Dooms and Higgins 1986). Cine MRI and velocity mapping should thus be used to confirm the presence and size of any apparent mass (Fig. 13.3).

Thrombus can also be distinguished from other atrial masses using gadolinium (Gd)-based contrast agents (Weinmann et al. 1984). Recently, we have shown that late-enhancement MRI that has been developed for myocardial infarct imaging (see Chap. 8) can be utilized to image thrombus (Bogaert et al.

2004). Images are acquired using inversion-recovery contrast-enhanced MRI (3D T1-weighted turbo-field- echo technique) performed 10-20 min after intrave- nous injection of Gd-diethylene triamine penta- acetic acid (DTPA) (0.2 mmol/kg body weight). The inversion time is adjusted for optimal suppression of normal myocardial signal (inversion time approx.

200–300 ms). Enhancement of the blood-pool is ex- cellent to depict abnormal intraluminal structures, such as thrombi (Fig. 13.3b).

13.3.1.3 Endocarditis

Only very large vegetations can be demonstrated by SE MRI. Fast GE imaging has been shown to be

helpful in clarifying echocardiographic findings in aortic valve vegetations (Caduff et al. 1996). MRI can be invaluable in the identification of abscesses (Jeang et al. 1986) or if infection has spread outside the heart (Fig. 13.4). Furthermore, sinus of Valsalva rupture, secondary to endocarditis, can be seen on MR images

13.3.2

Valvular Regurgitation

Currently none of the conventional imaging techniques can accurately define valvular regurgitation and it is here that MRI has particular value.

Cardiovascular MR can image the regurgitant jet in any plane, and thus a 3D appreciation of the jet can be acquired. Furthermore, MRI can quantify the regurgitant volume, either as an absolute value or as the regurgitant fraction. Such a noninvasive quantification of the degree of valvular regurgitation, in combination with information about ventricular function, is of particular clinical relevance for the timing of valve replacement.

MR assessment of valvular regurgitation severity can be evaluated by using the following techniques:

Fig. 13.2a-d. Balanced steady-state free precession (b-SSFP) images of the valves. a Oblique coronal view of the aortic valve (arrowhead). b Oblique sagittal view of the pul- monary valve (arrow). c Vertical long-axis view of the mitral valve.

The arrows indicate the insertion points of the chordae tendinae into the tips of the valve leaflets. d Four-chamber view (systole) of the tricuspid (star) and mitral valves (arrow)

a b

d c

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• Qualitative assessment of signal loss on cine MR images

• Quantitative assessment by measurement of ventricular volumes

• Quantitative assessment by phase-contrast velocity mapping

13.3.2.1

Qualitative Assessment with Cine Gradient-Echo Imaging

In cine MRI using the spoiled-GE technique, dephasing of the proton spins, secondary to turbulent flow, leads to signal loss (Evans et al. 1988).

Imaging over multiple frames (20–40) enables accurate assessment of the turbulent flow, throughout the cardiac cycle. For regurgitant lesions the signal loss can be graded in a similar way to X-ray angiography: grade 1 = signal loss close to the valve;

grade 2 = signal loss extending into the proximal chamber; grade 3 = signal loss filling the whole of the proximal chamber; grade 4 = signal loss in the receiving chamber throughout the relevant half of the cardiac cycle (Fig. 13.5) (Underwood et al. 1987a; Sechtem et al. 1987). This qualitative method has been validated, but is unable to separate turbulent volumes when dual valve disease exists (e.g., aortic regurgitation and mitral stenosis), and there remains poor reproducibility of the technique between centers (Sechtem et al.

1988; Wagner et al. 1989; Globits et al. 1991).

In addition, signal loss is very dependent on MR parameters such as echo time (TE), and jet size is easily underestimated when it impinges on the myocardial wall, as has been known from echocardiography for some years.

With the increasing use of b-SSFP cine imaging in cardiovascular MRI (Zur et al. 1990), qualitative assessment of signal loss has become less useful. Though b-SSFP white-blood images are much quicker to acquire with better endocardial/blood pool definition than conventional GE imaging (Pereles et al. 2001; Barkhausen et al. 2001), the sequence is designed to be relatively f low-in- sensitive. This results in reduced visualization of f low disturbance secondary to valvular regurgitation, in particular when regurgitation is mild.

Thus, in subjects who are being imaged for other reasons, incidental detection of mild valvular regurgitation may not be apparent on b-SSFP cine imaging.

Fig. 13.3. a Vertical long-axis b- SSFP image showing thrombus in the left atrial appendage (arrow- head). b Vertical long-axis con- trast-enhanced MR image, using T1-weighted 3D fast-field-echo technique (TR 4.3 ms, TE 1.3 ms, inversion time 240 ms), obtained 8 min after injection of 0.2 mmol/

kg of Gd-DTPA, showing throm- bus in left atrial appendage (ar- rows)

Fig. 13.4. Aortic root pseudoaneurysm following aortic valve replacement for endocarditis. The pseudoaneurysm sur- rounds the aorta (star and white arrowheads), and displaces the left main stem coronary artery (white arrows). Black arrow shows communication between LV and pseudoaneu- rysm, which fills during each cardiac contraction

a b

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13.3.2.2

Quantitative Evaluation of Ventricular Volume Measurement

MRI can now be regarded as the best available in vivo technique for the measurement of ventricular volumes (see Chap. 6) (Sakuma et al. 1993;

Bogaert et al. 1995; Grothues et al. 2002). In particular, b-SSFP imaging enables good blood- pool/endocardial contrast images to be acquired in a single breath-hold (Moon et al. 2002; Alfakih et al. 2003). Using a set of short-axis cuts covering the length of the ventricles, in combination with Simpson’s rule, the stroke volumes of both the right and the left ventricle can be measured (Dulce et al. 1993) (see Chap. 6).

In normal individuals there is a 1:1 relationship between these stroke volumes. Any discrepancy between the ventricular volumes in a patient with regurgitation will identify the regurgitant volume.

The main limitation of this technique, when used alone, is that only patients with a single regurgitant valve can be assessed. Overall imaging times are now significantly shorter than 5 years ago. In general, one or two short-axis images can be acquired in a single breath-hold, and newer sequence imple- mentations enable the acquisition of a complete short-axis stack in a single breath-hold (Lee et al.

2002; Hori et al. 2003; Taylor et al. 2004). Thus, a complete MRI assessment of ventricular volumes can be performed in approximately 40 min (20 min acquisition time, 20 min analysis time (Bellenger et al. 2000).

13.3.2.3

Quantitative Evaluation with Velocity Mapping For velocity mapping, phase information and not magnitude information is displayed. The application of short-lived magnetic gradients allows each point in the imaging plane to be encoded with a phase shift that is directly proportional to the velocity at that point. Because phase shifts can arise from other factors, a second velocity-compensated phase image is acquired, and subtraction yields the actual phase relationship of the protons (Nayler et al. 1986; Underwood et al. 1987b). Velocity encoding can be applied in any direction (through-plane, left to right, up and down), though for flow quantification through-plane imaging is used, and the size of the velocity window defined for increased sensitivity.

Stationary material is represented as mid-gray, while increasing velocities in either direction are shown in increasing grades of black or white (Fig. 13.6). It is important to define the velocity encode window as close to the peak velocity as possible, to reduce aliasing of peak velocities, while maintaining sensitivity for flow measurements.

Measurement of the spatial mean velocity for all pixels in a region of interest of known area enables the calculation of the instantaneous flow volume at any point in the cardiac cycle. Calculation of the flow volume per heart beat can be made by integrating the instantaneous flow volumes for all frames throughout the cardiac cycle. This technique has been validated in vitro and in vivo, and is extremely accurate and reproducible (Firmin et al. 1987; Meier et al. 1988; Bogren et al. 1989a). It now represents the

Fig. 13.5a-c. Balanced-SSFP images. a Oblique coronal left ventricular outflow tract view. Grade 1–2 aortic regurgitation (black arrowhead) in a patient with aortic root dilatation due to Marfan syndrome. b Oblique axial left ventricle inflow/

outflow view. Grade 3 aortic regurgitation (white arrows). c Vertical long-axis view. Grade 4 mitral regurgitation (black ar- rows); note the massively dilated left atrium. (Image c: Courtesy of F. Melonas, R. Truter, and G. Wagner, SCP MRI Center, Cape Town, South Africa)

a b c

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best available in vivo technique for flow measurements. Quantification of the right ventricular (RV) and left ventricular (LV) outputs can be compared using through-plane flow measurements from the proximal ascending aorta and pulmonary trunk (1:1 ratio in normal individuals) (Kondo et al. 1991), thus allowing quantification of pulmonary to systemic shunting (and vice versa) (see Chap. 15.3).

The severity of regurgitation can be defined as follows (Sechtem et al. 1988):

• Mild: regurgitant fraction 15–20%

• Moderate: regurgitant fraction 20–40%

• Severe: regurgitant fraction >40%

Conventionally, phase-contrast GE images are acquired during shallow respiration over several minutes. Faster imaging can be performed, and it is now possible to acquire real-time phase-contrast data (Korperich et al. 2004).

13.3.2.4

Issues Related to MRI Functional Measurements There are several technical issues that must be addressed when performing phase-contrast velocity

mapping. Firstly, phase-contrast imaging should be performed as close to the center of the main mag- net field (B₀) as possible. This ensures phase errors introduced by eddy currents and Maxwell terms are kept to a minimum.

Secondly, the imaging plane should be perpendicular to the vessel. This ensures that the velocity vectors of the majority of the voxels are perpendicular to the imaging plane. This can be achieved by planning the through-plane imaging position from two perpendicular views (e.g., LV outflow tract plane and LV inflow/outflow plane for the aortic valve) (see Figs. 5.5–5.8).

Thirdly, the imaging plane should not be at the level of the valve, but just proximal or distal to the valve annulus. This ensures that artifacts secondary to eddy currents and the complex motion of the valve annulus are kept to a minimum. One possible method to compensate for motion of the valve annulus through the imaging plane is to use a moving slice velocity- mapping (or slice tracking) technique (Kozerke et al. 1999). This experimental method enables the imaging slice to follow the valve annulus during the cardiac cycle, reducing velocity offsets and demonstrating discrepancies in conventional measurements

Fig. 13.6a-d. Phase-contrast veloc- ity map, TE 5 ms, velocity-encoding window ±1.5 m/s. a, b Magnitude image at level of aortic root (white arrow) during a systole and b di- astole. The opening and closing of the three valve leaflets can be clearly seen. c, d Velocity image in the same imaging plane during c systole and d diastole. Stationary material is represented as mid- gray, while flow toward the head in the aortic root is represented by white pixels (black arrow in c, and flow toward the feet in the de- scending aorta (arrowhead in c) is represented by black pixels. There is little or no flow in the aortic root during diastole (star) during valve closure

a b

d c

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of aortic and mitral regurgitation measurements by cardiovascular MR (Kozerke et al. 2001a).

And finally, it is important to have a degree of qual- ity control with regard to ventricular volumes and flow measurements, and it is worthwhile performing two validation experiments on one’s own scanner. The first validation is to use a flow phantom and compare MR measured flows with those acquired using a bucket and stopwatch method (Fig. 13.7). Such an experiment will only be possible in centers with access to a flow phantom, but will help confirm the accuracy of your own scanner. The second validation can be performed in any center and involves assessing ventricular volumes and flow measurements in normal healthy volunteers. In this group of subjects, the stroke volume of the ventricles calculated from the ventricular volume assessment should equal the net forward flow measured by phase-contrast velocity mapping. Thus, the normal subjects act as their own control for all the measurements acquired.

13.3.3

Valvular Stenosis

The presence of valvular stenosis can be identified by signal loss seen in cine MR images. Velocity map-

ping may then be used to establish an accurate peak velocity across the valve to quantify the severity of the stenosis. The use of the mean velocity across the caval veins and mitral valve can be used to describe the inflow curves for the atrioventricular valves (Mohiaddin et al. 1990). Concomitant changes in myocardial mass, due to increase in afterload, can be precisely assessed with cine MRI.

13.3.3.1

Qualitative Assessment with Cine Gradient-Echo Imaging

For stenotic lesions, the degree of signal loss is dependent on the degree of stenosis and the echo time used. Thus, for shorter echo times less proton spin dephasing can take place and more signal is recovered (DeRoos et al. 1989; Kilner et al. 1991).

Figure 13.8 demonstrates the relationship between the echo time and the peak velocity across a stenosis. Thus, in more severe stenosis, a lower echo time must be used to prevent signal loss in the images.

As with the qualitative assessment of valvular regurgitating, signal loss is less marked on the in- creasingly used b-SSFP cine imaging, due to the de- sign and short TE of this sequence. It is thus much more difficult to get a feel for the peak velocity with

Fig. 13.7a-f. MR flow phantom. a Schematic layout of the flow phantom. A programmable pump is used to create pulsatile flow for the range of flows seen in the aorta and pulmonary trunk. b, c Compliant tubing set in methylcellulose gel is used to mimic the compliant vessel. d, e MR phase-contrast imaging is performed to measure flow, through-plane, in the com- pliant tubing (dotted line) – d magnitude image, e velocity map. f The flowing fluid (water with dilute gadolinium to give the same T1 as blood) is collected over 1 min to give a flow in milliliters per minute, which is then correlated to the MR flow calculations

a

b

d c

e

f

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b-SSFP sequences than with conventional spoiled- GE cine imaging.

Valve area can also be measured on cine MR images, and this measurement has been shown to correlate well with Doppler echocardiography and catheterization (Sondergaard et al. 1993a; Friedrich et al. 2002).

13.3.3.2

Quantitative Evaluation with Velocity Mapping Direct measurement from the phase-contrast velocity map enables measurement of the peak velocity across the valve, and application of the modified Bernoulli equation enables an estimate of the gradient across the valve:

∆P = 4V² (1)

where P is the pressure drop across the stenosis (in millimeters of mercury) and V is velocity (in meters per second). The technique is comparable with Doppler echocardiography valvular stenosis measurements and has an in vitro accuracy of 4%

(Simpson et al. 1993). The main advantage of the technique over echocardiography is that the velocity jet can be aligned easily in any direction without the limitation of acoustic windows.

Imaging can be performed through-plane (velocity jet perpendicular to the imaging plane) or in-plane (velocity jet parallel to the imaging plane).

Both strategies have their advantages and disadvan-

tages. For the in-plane method, the entire jet can be visualized, and the point within the jet of peak velocity easily identified. However, not all jets are easily aligned in a single 2D plane, and, for tight narrow jets, there may be partial volume averaging and motion within the imaging slice, and the peak velocity may not be accurately depicted. For through-plane imaging, the jet will always pass through the imaging plane, but, as only part of the stenotic jet is sam- pled, the peak velocity may not be measured. It is thus best to use a combination of the two strategies with initial definition of the jet in-plane and quantification with through-plane imaging at the site of maximum velocity on the in-plane image.

Selection of the correct velocity-encode gradient is essential to maintain sensitivity and accuracy of measurements, while avoiding aliasing. Most scanners now have a fast phase-contrast velocity-mapping sequence that can be acquired in approximately 15–20 s. This enables an estimate of the peak velocity to be made before progressing to the more time-consuming conventional phase-contrast velocity-mapping sequence.

Using data acquired from phase-contrast flow curves, the continuity question can be used to calculate valve area. For the aortic valve, velocity is measured below the valve in the left ventricular outflow tract (LVOT) and above the valve in the aorta (Ao).

The velocity time integral (VTI) is measured for systolic forward flow in both planes (Caruthers et al.

2003) (Fig. 13.9). The aortic valve area A_Ao (centimeters squared) is given by:

A_Ao= A_LVOT(VTI_LVOT/VTI_Ao) (2) where A_LVOT is LVOT area (centimeters squared), VTI_LVOT is LVOT VTI, and VTI_Ao is Ao VTI].

13.3.4 Aortic Valve

13.3.4.1

Aortic Regurgitation

A protocol for imaging patients with aortic regurgitation is shown in Fig. 13.10. Regurgitant aortic jets are best visualized in the coronal or oblique coronal plane. Estimation of the signal void volume is possible (Aurigemma et al. 1991; Nishimura 1992; Ohnishi et al. 1992), but care must be taken to ensure that all the jet has been visualized. The most accurate method for quantifying the regurgitant fraction is TE 3·6

TE 6

TE 14

ﬂ ow rate 0 1 2 3 4 5 6 7 8 9 l/min

jet vel 0 1 2 3 4 5 6 m/s

Fig. 13.8. In vitro jet velocity mapping. Velocity maps ob- tained with MRI of flow – increased from left to right – through the test stenosis, showing the significance of shortening the TE from 14 ms (bottom) to 6 ms (center) and 3.6 ms (top). Only the 3.6-ms TE sequence allows mapping of high-velocity jets, up to a maximum tested velocity of 6.0 m/

s. (Reprinted with permission from Kilner et al. 1991)

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Fig. 13.9a-g. Calculation of the functional aortic valve area using the velocity time integral (VTI) continuity equation for aortic stenosis. a Imaging planes for the aortic (solid line) and left ventricular outflow tract (LVOT, dotted line) mea- surements, prescribed from a left ventricular inflow/outflow view. b-d Aortic plane – magnitude image (b), velocity map (c), and peak velocity versus time curve (d). e-g LVOT plane – magnitude image (e), velocity map (f), and peak velocity vs. time curve (g). The summation of the area under the curve (black shading) during systole is the VTI. See Eq. 2

Regurgitant fraction (%) =

Aortic retrograde flow (mL/beat)×100

Aortic forward flow (mL/beat)] (3) A good correlation has been demonstrated between ventricular volume measurements and velocity mapping in the ascending aorta, for the calculation of aortic reguritant fractions (Sondergaard et al.

1993b). Interstudy reproducibility has been demonstrated to be high and thus the technique is ideal for long-term patient follow-up (Dulce et al. 1992).

Some debate still remains as to the positioning of the plane across the aorta. Current in vivo experi- ence suggests that a position between the coronary ostia and the aortic valve may be most accurate (Chatzimavroudis et al. 1997). Positions above the coronary ostia lead to inaccuracies secondary with phase-contrast velocity-encoding in the slice

direction, in an oblique axial plane above the aortic valve. In order to ensure that the velocity vectors for the majority flow are through-plane in this image, it is best to use two perpendicular images through the ascending aorta and prescribe the oblique axial plane perpendicular to both (see also Fig. 5.5).

A through-plane velocity window of ±1.5 m/s should be set if there is isolated aortic regurgitation.

If significant aortic stenosis is present, a dual velocity window, with a high systolic setting of ±5 m/s, changing to ±1.5 m/s for the diastolic frames, may be necessary. Aortic regurgitation volume is the amount of retrograde diastolic flow and is measured in milliliters per beat or liters per minute (milliliters per beat × heart rate) (Fig. 13.11). The aortic regurgitant fraction is given by:

a b c

d

e f

g

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Fig. 13.10a-f. Aortic regurgitation imaging protocol. a Basal short-axis b-SSFP cine frame. b A plane prescribed through the mitral valve and aortic valve on the basal short-axis view yields the left ventricular inflow/outflow view, which shows grade 2 aortic regurgitation. c A perpendicular plane through the jet of regurgitation yields an oblique coronal plane through the aortic valve and LV outflow tract. d, e An oblique axial plane in the aortic root, just above the aortic valve at the level of the coronary artery openings, is then defined from both b and c for phase-contrast velocity mapping. d Magnitude image. e Velocity map in early diastole shows a narrow jet of aortic regurgitation due to lack of coalition of the valve leaflets centrally (black arrows). f Flow versus time plot for the ascending aorta. Antegrade flow calculated at 120 ml/beat, retrograde flow 30 ml/beat, and aortic regurgitant fraction 25%

Fig. 13.11. Plots of flow volume versus time frames per car- diac cycle. Measurements were made in the ascending aorta for a normal subject and patients with increasing severity of aortic regurgitation (AR). Each point on the graphs rep- resents the blood flow in the aorta for each image in the cardiac cycle. Negative values represent retrograde flow during diastole. Integration of the area under the curve for antegrade and retrograde flow enables calculation of the regurgitant volume per cardiac cycle (milliliters per beat)

to coronary flow and aortic compliance. Further in vivo studies need to be performed to confirm these findings. The commercial application of moving- slice velocity mapping would be a potential aid for slice positioning (Kozerke et al. 2001a).

Aortic regurgitation can be caused by abnormali- ties of the aortic leaflets or annulus or be secondary to dilatation of the aortic root. The etiologies of aortic regurgitation are given in Table 13.1.

In acute aortic regurgitation (bacterial endocarditis, aortic dissection, and trauma), ventricular ad- aptation does not occur and there is a rapid increase in LV filling pressures, reduced cardiac output, increased left atrial pressure, pulmonary edema, and ultimately shock. In chronic aortic regurgitation, the initial response is LV hypertrophy, as compensation for increased wall stress secondary to volume overload. However, over time, the main hemodynamic response to aortic regurgitation is dilatation of the left ventricle and reduction in ventricular function.

a b c

d f e

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Though medical treatments can be used to deal with the symptomatic effects of aortic regurgitation, surgical valvular replacement remains the treatment option for moderate/severe aortic regurgitation.

The timing of surgery is important and is a balance between operating too soon (operative risks, insertion of a valve that will not last for ever, lifelong an- ticoagulation) and operating too late (irreversible left ventricular failure). Currently, operative timing depends on symptoms, chest X-ray appearance, echocardiographic findings, and the longitudinal changes in these parameters (Bonow et al. 1998).

With improving surgical techniques, noninvasive methods of investigation and enhanced knowledge of the natural history and prognosis, there has been progressively earlier application of surgery for both severe aortic and mitral regurgitation (Borer and Bonow 2003) (Table 13.2).

MRI protocols, as outlined above, by providing accurate, quantifiable, and reproducible measurements of regurgitant volume may contribute to the development of more specific criteria for the timing

of surgery. As yet, no large-scale cardiovascular MRI studies addressing these issue have been published, though velocity mapping in the ascending aorta has now been performed to analyze the therapeu- tic effects of ACE inhibition in patients with aortic regurgitation (Globits et al. 1996). MRI has demonstrated the beneficial effects of the therapy and is able to identify those patients with aortic regurgitation who respond favorably to angiotensin-convert- ing enzyme (ACE) inhibition.

13.3.4.2 Aortic Stenosis

Aortic stenosis can be divided into sub-valvular stenosis, valvular stenosis, and supra-valvular stenosis.

The hemodynamic consequence of all forms of aortic stenosis is concentric LV hypertrophy. The causes of aortic stenosis are outlined in Table 13.3. Supra- valvular aortic stenosis is discussed in the chapter about congenital heart disease (see Chap. 15.4.3).

Sub-valvular aortic stenosis can be congenital or, more commonly in adults, occurs secondary to hypertrophic cardiomyopathy (see Chap. 9.4.1). The remainder of this section will concentrate on valvular aortic stenosis.

MRI of aortic stenosis is outlined in Fig. 13.12.

The alignment of the jet and definition of the veloc-

Table 13.1 Causes of aortic regurgitation Valvular causes

Congenital bicuspid aortic valve Rheumatic heart disease Bacterial endocarditis

Myxomatous valve associated with cystic medial necrosis Aortic valve prolapse

Secondary to dilatation of the aortic annulus Marfan syndrome

Syphilitic aortitis Ankylosing spondylitis Reiter disease

Rheumatoid arthritis Ehlers–Danlos syndrome Secondary to aortic dissection Deceleration trauma Hypertension

Table 13.2 Schema for selecting asymptomatic patients with aortic regurgitation for aortic valve replacement (AVR). (CHF, congestive heart failure; LVEF_rest, left ventricular ejection fraction at rest; FS_rest, fractional shortening at rest; LVID_s, left ventricular systolic dimension; ∆, change from rest to exercise; ESS, end-systolic stress; LVIDd, left ventricular diastolic dimension. All dimensions from echocardiography). (Adapted from Borer and Bonow 2003)

Group AVR replacement Criteria Progression to CHF or death (%/year)

I Definite Subnormal LVEF_rest and/or FS_rest 25

II Strongly considered;

definite if ≥2 criteria met LVID_s≥55 mm, ≥25 mm/m²

∆LVEF > –5%

∆LVEF – ∆ESS Index ≥ –17%

10-20

III Consider;

definite if ≥1 group II criteria also met LVID_d≥80 mm

“Rapid” ↓ LVEFrest, FS_rest

“Rapid” ↑ LVIDs, LVID_d

7-10 Table 13.3 Causes of aortic stenosis Acquired

Degenerative (fibrocalcific senile aortic stenosis) Rheumatic heart disease

Congenital Subvalvular

Valvular – bicuspid aortic valve Supravalvular

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ity-encode window are important variables. The jet core appears as high signal sandwiched between two jets of low signal. For most scanners, the shortest TE sequence will now be used and manual selection of the TE is less common. However, if TE selection is still necessary, the TE and velocity window should be set to aim for greatest signal-to-noise and velocity sensitivity, though, if the shortest TE and largest velocity window are defined, most clinically significant jets will be accounted for.

Good in vivo agreement has been demonstrated for a wide range of pressure gradients across the aortic valve (3–148 mmHg) between MR velocity mapping and Doppler echocardiography and X-ray angiography (Eichenberger et al. 1993). In our own practice, we have found it easiest to interpret velocity maps parallel to the jet, as this reveals the jet length in relationship to pre- and poststenotic flows (Kilner et al. 1993a).

Valve area can also be assessed, using either direct visualization of the valve area on cine images (Friedrich et al. 2002; John et al. 2003) (Fig. 13.13) or the application of the continuity equation to MR data (Caruthers et al. 2003) (Fig. 13.9). For direct measurement of valve area, the area of flow through-plane at the level of the origin of the flow jet is performed. The results of this technique to date have been conflicting. Friedrich et al. (2002) have demonstrated that direct MR measurement of aortic valve area agree well with aortic area calculated at cardiac catheterization (r=0.78), and less well with that calculated at echocardiography (r=0.52); while John et al. (2003) have demonstrated good agreement between MR measurements and echocardiog- raphy (r=0.96), but poor agreement between MR and cardiac catheterization (r=0.44). When MR assess- ment of velocity profiles in the LVOT and aorta was performed and the continuity equation applied (see

Fig. 13.12a-c. Balanced-SSFP images of severe aortic stenosis. a Oblique coronal plane through the aortic valve and LVOT in systole shows a tight jet of signal loss (turbulence) in the ascending aorta (white arrows). b Left ventricular inflow/outflow view through the aortic valve and LVOT in diastole shows marked thickening of the aortic valve (black arrows). c Through- plane image in the aortic root above the aortic valve in systole. The narrow jet of turbulent flow can be clearly seen (white arrowheads)

Fig. 13.13a,b. Bicuspid aortic valve (arrow).

a Magnitude image in systole – note the “fish mouth” appearance of the aortic valve aper- ture, when compared with normal valve opening as shown in Fig. 13.6.

b Velocity map in systole – forward flow is shown as white

a b c

a b

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Eq. 2 above), there was good agreement between the MR method and similar data acquired with echocar- diography (r=0.83) (Caruthers et al. 2003).

When imaging aortic stenosis it is also important to assess LV mass and aortic root dimensions. LV mass is increased in aortic stenosis and can be used as a guide to the significance of the aortic stenosis with a reduction in LV hypertrophy seen after aortic valve replacement (Rajappan et al. 2000; Sandstede et al. 2000;

Lamb et al. 2002; Sensky et al. 2003). The aortic root may be dilated secondary to aortic stenosis, and this is well documented in patients with bicuspid aortic valve (Alegret et al. 2003; Novaro et al. 2003; Nkomo et al.

2003). In patients with angina and aortic stenosis, visualization of the coronary arteries is often requested to exclude ischemic heart disease as a contributing factor to the symptoms. At present, this is best performed by direct X-ray cardiac catheterization.

LV contraction can also be assessed using myocardial tagging. In patients with pressure-overloaded hypertrophied ventricles, there is an increase in apical rotation that leads to increased torsion of the left ventricle (Nagel et al. 2000; Sandstede et al. 2002), and delay and prolongation of diastolic untwisting (Nagel et al. 2000). Not only do such studies help improve our understanding of cardiac physiology in aortic stenosis, but also they may help clinically to distinguish pathological causes of LV hypertrophy from physiological changes that can occur in the

“athlete” heart (see Chap. 9.3). Furthermore, 1 year after aortic valve replacement for aortic stenosis, there is normalization of LV torsion, and monitoring such parameters in patients over time may aide in defining the optimal time for operative interven- tion in aortic stenosis (Sandstede et al. 2002).

13.3.5 Mitral Valve

13.3.5.1

Mitral Regurgitation

There is good correlation between the signal loss caused by mitral regurgitation on spoiled-GE cine MR images and Doppler and X-ray angiographic grading (Aurigemma et al. 1990) (Fig. 13.14). The regurgitant jet is best visualized in either the horizontal or the vertical long-axis plane. MR imaging has the advantage of being able to define lesions in subjects with poor acoustic windows. For alignment of the optimal imaging planes for the mitral valve, see Fig. 5.6.

There are three methods for quantifying mitral regurgitation with cardiovascular MR:

For isolated mitral regurgitation, ventricular volume measurements provide an accurate method for quantifying mitral regurgitation (Hundley et al.

1995):

Regurgitant volume (ml/beat) =

LVSV (ml/beat) – RVSV (ml/beat) (4) Regurgitant fraction (%) = [Regurgitant

volume (ml/beat)×100] / [LVSV (ml/beat)] (5) where LVSV is LV stroke volume and RVSV is RV stroke volume.

Secondly, when other valvular disease is present, the mitral regurgitant volume can be calculated from the LV stroke volume and phase-contrast velocity-mapping flow measurements in the aorta:

Fig. 13.14a-c. Balanced-SSFP images of mitral incompetence a, b Horizontal long-axis views. Grade 2 mitral incompetence (black arrows). c Vertical long-axis view of grade 3 mitral incompetence (white arrows) in a patient with mitral annulus dilatation secondary to ischemic heart disease. (Image b: Courtesy of F. Melonas, R. Truter and G. Wagner, SCP MRI Center, Cape Town, South Africa)

a b c

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Regurgitant volume (ml/beat) = LVSV

(ml/beat) – aortic forward flow (ml/beat) (6) A third proposed method for quantifying mitral regurgitation involves calculation of the difference between ventricular outflow and ventricular inflow (Fujita et al. 1994). Velocity mapping in the ascending aorta can be used to assess ventricular outflow, while velocity mapping at the mitral valve annulus during diastole can be used to assess inflow (Fig. 13.15). Thus:

Regurgitant volume (ml/beat) = LV inflow

(ml/beat) – aortic forward flow (ml/beat) (7) Using this method, MRI is able to demonstrate different mitral regurgitant volumes for groups with mild, moderate, and severe mitral regurgitation, defined at echocardiography. There is also good correlation between the calculated regurgitant fraction and the grading of mitral regurgitation severity at Doppler echocardiography (Fujita et al. 1994).

When assessing mitral regurgitation with MRI, it is important to image the left atrium to assess the left atrial size and exclude the presence of thrombi.

As with aortic regurgitation, mitral regurgitation may be acute or chronic (see Table 13.4). The main

hemodynamic consequence of chronic mitral regurgitation is gradual LV dilatation and ultimately LV failure. Importantly, long-term survival is less good if surgical repair is performed when the LV ejection fraction is less than 60% (Enriquez-Sarano et al.

1994). Thus, cardiovascular MRI is well placed as an imaging tool that can accurately quantify and reproducibly monitor the severity of mitral regurgitation and the hemodynamic consequences on LV function. Again, no large-scale clinical studies have assessed the use of MRI for defining the timing of surgical repair of mitral regurgitation.

Fig. 13.15a-d. Measurement of mitral regurgitation by using velocity-encoded MR to measure the difference between outflow across the proximal ascending aorta (a) and in- flow across the mitral annulus (b). Phase images acquired at the level of the proximal part of ascending aorta (arrow in a) and the mitral annulus (arrowhead in b). The graphs show the flow volume versus time frame for mitral in-flow (black curve) and aortic out- flow (red curve) in a normal subject (c) and in a patient with mitral regurgitation (d).

The difference in the areas under the two curves is the volume of mitral regurgitation Table 13.4 Causes of aortic mitral regurgitation

Acute

Bacterial endocarditis

Myocardial infarction with involvement of the papillary muscle

Chronic

Rheumatic heart disease Mitral valve prolapse syndrome Marfan syndrome

Congenital

Idiopathic hypertrophic subaortic stenosis

Persistent ostium primum ASD with cleft mitral valve Functional – secondary to dilatation of the mitral annulus in LV dilatation

a b

d c

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13.3.5.2 Mitral Stenosis

Restrictive opening of the mitral valve in mitral stenosis is most commonly secondary to rheumatic heart disease (Table 13.5). Elevation of left atrial outflow resistance occurs when the mitral valve area is below 2.5 cm². This leads to an increase in the diastolic transmitral gradient, which can be assessed by measuring the peak velocity across the mitral valve.

Table 13.5 Causes of mitral stenosis Congenital

Rheumatic heart disease Obstruction

Thrombus Myxoma Tumor

Peak velocity measurements at MR across the mitral valve (short-axis view below the mitral valve) correlate well with Doppler measurements at echocardiography (Heidenreich et al. 1995). Because of the complicated shape of the stenotic jet in most patients, through-plane measurements of velocity are more accurate than in-plane measurements (Fig. 13.16). Valve area can also be measured.

Velocity-mapping flow curves show loss of the normal dual peaks, with high velocities throughout diastole. The flow curves can be used to determine the mitral pressure half-time (the time taken for the pressure gradient to fall to half its peak value), a use-

ful echocardiographic indicator of mitral stenosis severity:

Valve area (cm²) = 220 Pressure half-time (ms) (8) The severity of mitral stenosis can also be estimated by measuring pulmonary vein flow. In severe mitral stenosis this flow is reversed (Mohiaddin et al.

1991a).

The consequences of mitral stenosis, left atrial enlargement, pulmonary edema and RV dysfunction and pulmonary regurgitation, secondary to pulmonary hypertension, can all be assessed. In particular, the presence of atrial thrombus should be evaluated in all patients in both atria (secondary to atrial fibrillation).

13.3.6

Pulmonary Valve

Difficulties with the alignment of the acoustic window for Doppler echocardiographic investigation has made MR velocity mapping a useful tool in the investigation of the pulmonary valve and pulmonary arteries (Bogren et al. 1989b; Mohiaddin et al. 1991b; Rebergen et al. 1993). The imaging protocols for aortic stenosis and regurgitation apply for investigation of the pulmonary stenosis (Fig. 13.17) and pulmonary regurgitation (Fig. 13.18).

For pulmonary stenosis, measurement and visualization of the pulmonary valve is best performed in a sagittal or oblique sagittal plane. For the quantification of pulmonary regurgitation and RV out-

Fig. 13.16a-c. Mitral stenosis. a Axial GE image in mid-diastole. Turbulent flow is seen through the mitral valve (arrow).

Dotted white lines represent the image plane for the short-axis image. b Short-axis GE image in mid-diastole. Flow through the mitral valve is shown as bright pixels (arrow). The valve orifice area can be easily measured. LA, Left atrium; LV, left ven- tricle; RV, right ventricle. c b-SSFP basal short-axis view of a rare example of a double mitral valve orifice (arrowheads)

a b c

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Fig. 13.17a-d. Severe pulmonary ste- nosis. a FSE “black-blood” oblique axial view through the pulmonary trunk and pulmonary bifurcation.

The thickened, dysplastic pulmonary valve is shown (white arrow). b b-SSFP systolic image in the same plane as a, showing the narrowing at the level of the pulmonary valve (white arrow), and the reduced signal of turbulent blood flow at the pulmonary bifur- cation. c Oblique coronal magnitude image through the distal pulmonary trunk, aligned from (b, dotted line).

Slit-like jet of flow identified (white arrowheads). d Oblique coronal phase- contrast velocity map showing slit-like flow, peak velocity of 4 m/s, which equates to a gradient of approximately 64 mmHg

Fig. 13.18a-e. Moderate/severe pulmonary regurgitation, in a patient, 25 years after total repair of tetralogy of Fallot with a transannular patch. a Oblique sagittal right ventricular outflow tract view, diastolic frame, showing signal loss in the right ventricle secondary to pulmonary regurgitation (arrows). b Magnitude image through the proximal pulmonary trunk, aligned along the dotted line shown in a. Phase-contrast velocity mapping in systole (c) and in diastole (d) – white signal signifies forward flow during systole and black signifies reverse flow during diastole (black arrowheads). e Flow curve for pulmonary regurgitation; regurgitant fraction is 30%

a b

d c

a b c

d e

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flow curves, imaging the main pulmonary trunk in cross-section in a coronal or oblique axial plane is most accurate. These images should be prescribed from two perpendicular RVOT views to ensure that the majority of blood flow is perpendicular to the imaging plane (see Fig. 5.8) (Kivelitz et al. 2003).

13.3.6.1

Clinical Importance of Pulmonary Regurgitation Pulmonary regurgitation is an important sequelae of surgical repair of the RV outflow tract in many congenital heart disease conditions, in particular after complete repair for tetralogy of Fallot.

Cardiovascular MR imaging has been used to quantify these changes, demonstrating elevated RV end- diastolic and end-systolic volumes and reduced RV ejection fraction in patients with severe pulmonary regurgitation (Helbing et al. 1996; Singh et al.

1998; Davlouros et al. 2002; Tulevski et al. 2003).

Chronic RV volume overload has long been regarded as a rather benign consequence of pulmonary regurgitation in this clinical setting. However, there is increasing evidence that RV function may be ir- reversibly compromised by such long-term changes (Therrien et al. 2000). This is exemplified by three findings that have been demonstrated by cardiovascular MRI. First, RV ejection fraction has been shown to be significantly lower in patients with both RV pressure and volume overload as compared to RV pressure-overload alone (Tulevski et al. 2003).

Second, an abnormal RV response to stress [either physiological (Roest et al. 2002) or pharmacological (Tulevski et al. 2003)] has been demonstrated in patients with pulmonary regurgitation and tetralogy of Fallot. In normal subjects, RV ejection fraction increases during stress, while, in patients with tetralogy of Fallot, RV function remains remains unchanged or reduced during stress. And thirdly, there appears to be no improvement in RV function following pulmonary valve replacement (Therrien et al. 2000; Vliegen et al. 2002; Razavi et al. 2004) (see Sect. 13.3.9.1).

The accurate quantification of pulmonary incompetence and its effects on the right ventricle with cardiovascular MRI should help define the natural history of this condition and the response to treatment. Ultimately this may enable the definition of parameters that can be used to optimize treatment interventions.

13.3.7

Tricuspid Valve

Examination of the tricuspid valve is as for the mitral valve. The regurgitant jet of tricuspid regurgitation is best visualized in the transverse plane (Fig. 13.19). Velocity mapping in the superior vena cava can demonstrate tricuspid regurgitation, the causes of which are similar to mitral regurgitation.

Tricuspid regurgitation of varying degrees is most commonly seen secondary to dilatation of the right ventricle, often due to mitral valve pathology and/

or pulmonary hypertension. An important cause of tricuspid regurgitation that should be considered when there is thickening of the valve leaflet is carcinoid syndrome (Fig. 13.20). Tricuspid regurgitation can also occur after blunt trauma. It should be noted that trivial tricuspid regurgitation is a common finding in normal subjects (Waggoner et al.

1981). For alignment of the optimal imaging planes for the tricuspid valve, see Fig. 5.7.

As with echocardiography, quantification of the tricuspid regurgitation is possible by measuring the peak velocity within the regurgitant flow jet. If there is no pulmonary valvular or artery stenosis, the addition of an estimate of the right atrial pressure (RA_Pa) to the calculated pressure difference between the right atrium and right ventricle (∆P across the tricuspid valve, calculated using the modified Bernoulli equation), can give an estimate of the RV systolic pressure (RVS_Pa) and pulmonary artery pressure (PA_Pa).

Thus, tricuspid ∆P = 4(Vmax)² (9) and,

RVS_Pa (mmHg) = PA_Pa (mmHg) =

RA_Pa(mmHg) + ∆P (mmHg) (10) where RA_Pa, the right atrial pressure, is estimated at 10 mmHg.

The tricuspid valve orifice is larger than the mitral orifice. Tricuspid stenosis is uncommon but can be easily diagnosed on GE cine images. Tricuspid atresia is easily identified on SE images.

13.3.7.1

Ebstein’s Anomaly

Ebstein’s anomaly is a congenital abnormality of the tricuspid valve. The septal and mural leaflets are more apically placed than normal, resulting in a malfunctioning, regurgitant tricuspid valve and atrialization of the right ventricle (Anderson et al.

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Fig. 13.19a-d. Balanced-SSFP 4-chamber images of tricuspid regurgitation (arrowheads) throughout systole. Time from the R wave: a 0 ms, b 50 ms, c 100 ms, d 200 ms

1979). The consequence is gross right atrial enlargement and raised right atrial pressure. The anomaly is usually associated with an atrial septal defect (ASD) and there is thus right-to-left shunting at the atrial level and subsequent cyanosis. Ultimately, Ebstein’s anomaly results in gross enlargement of the cardiac contour on the plain chest radiograph.

Treatment is problematic, though expert surgical repair of the tricuspid valve is possible in some centers.

MRI can be used to assess valve morphology, quantify ventricular function, and size right atrial enlargement (Fig. 13.21).

13.3.8

Mixed Valvular Disease

The measurement of both RV and LV stroke volumes with cine MRI, in combination with velocity-map-

a b

d c

Fig. 13.20a,b. Carcinoid syndrome of the heart. a, b b-SSFP 4-chamber images at two different levels, showing thickened, stiff tricuspid valve (black arrows in a), and subvalvular apparatus (white arrows in b). The right ventri- cle is enlarged with flattening of the septum (arrowheads in a). This leads to nonclosure of tricuspid valve at end-systole, with important tricuspid regurgitation

a b

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ping measurement of pulmonary and aortic flow, permits the quantification of valvular regurgitation in all four valves, even if aortic, mitral, pulmonary, and tricuspid regurgitation are all present. Aortic and pulmonary regurgitation are calculated from the velocity maps and diastolic reverse flow, and subtraction of the systolic forward flow from the LVSV and RVSV yields the amount of mitral and tricuspid regurgitation, respectively.

13.3.9

Prosthetic Valves

For nonbiological prosthetic valves and stented biological valves, the applied magnetic field is distorted by differences in the local magnetic fields between the prosthesis and the biological tissue, and by eddy currents induced in the valve. These phenomena lead to signal loss around the prosthesis (Fig. 13.22).

These artifacts can be very severe on GE images and

may degrade the image significantly. This makes imaging turbulent jets in the vicinity of the prosthesis difficult; however, velocity mapping distal to the image artifact can still be accurately performed.

Homografts, autografts, and stentless porcine valve replacements do not cause signal artifact and can be imaged normally.

All valvular prostheses can be safely imaged, at current field strengths, except for Starr-Edwards mitral pre-6000 series valves used from 1960 to 1964, at field strengths of greater than 0.35 T (Randall et al.

1988; Shellock 1988; Shellock et al. 1993). Very few of the aforementioned prosthetic valves still re- main in situ.

MRI offers an ideal method for the noninvasive follow-up of patients after valvular surgery (Deutsch et al. 1992). In vitro, the accuracy of MR velocity measurements distal to a wide variety of valvular prostheses has been confirmed by la- ser Doppler anemometry (Fontaine et al. 1996).

The assessment of prosthetic valve function at low

Fig. 13.21a-c. Ebstein’s anomaly. Coronal long-axis view through the right atrium (RA) and right ventricle (RV), showing the apical displacement of the mural tricuspid valve leaflet (asterisk), and the normal sited antero-superior leaflet (ar- rowheads) in diastole (a) and systole (b). c Axial plane. In all three images the black arrow identifies “atrialization” of the right ventricle

Fig. 13.22a,b. Aortic prosthetic valve with associated mitral in- competence. a b-SSFP left ven- tricular inflow/outflow view showing artifact related to pros- thetic valve (white arrow), and mild jet of turbulent flow in the aortic root (white arrowhead). b b-SSFP image in a parallel imag- ing plane to (a), but in plane with the mitral valve, reveals grade 2/3 mitral regurgitation (black arrows)

a b c

a b