Magnetic Resonance and Computed Tomographic Imaging of Myocardial Function

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CHAPTER 55 Magnetic Resonance and Computed Tomographic Imaging of Myocardial Function

The work-up of ischemic heart disease necessitates an accurate and reproducible method of assessing cardiac function. The importance of cardiac function analysis in the evaluation of coronary artery disease (CAD) lies in the diagnostic and prognostic implications and, consequently, its usefulness in guiding medical therapy. Being the reference standard technique, MRI can quantify global systolic function and detect regional abnormalities in wall motion and help establish the diagnosis of CAD. In addition, pharmacologic stress imaging protocols using function or perfusion MRI have been shown to provide a sensitive detection of CAD.1 CT seems promising in at least some of these applications,2 although it is currently limited by the temporal resolution and scarce clinical validation data. Exercise stress imaging using MRI has been performed, although its feasibility in assessing patients with or suspected to have ischemic heart disease has been shown only in select, highly experienced sites.3

Prognosis after myocardial infarction (MI) has been correlated with the extent of myocardial necrosis and the degree of contractile dysfunction of the left ventricle.4 In patients with MI and chronic left ventricular (LV) dysfunction, LV ejection fraction and LV volumes are important prognostic parameters.5 The extent of late myocardial hyperenhancement on MRI and regional contractile improvement in response to low-dose dobutamine stress MRI can accurately predict recovery of function after MI, identify myocardial viability, and effectively guide coronary revascularization therapy.69

Although still experimental, growing evidence suggests that assessment of diastolic functional parameters is possible using current CT and MRI techniques. A time/volume curve per heart cycle is calculated on multidetector CT, from which determination of the peak filling rate (PFR) and the ratio of time to peak filling rate to R–R interval (tPFR/RR) have been evaluated as parameters of LV diastolic function. The early-to-late diastolic filling ratio (E/A) on Doppler echocardiography has been found to correlate positively with PFR (r = 0.54) and negatively with tPFR/RR (r = −0.57) on multidetector CT.10 Using MRI, a time/volume curve can likewise be derived, and diastolic function is expressed as PFR, early and active PFR (PFRE and PFRA), and their ratios. Diastolic parameters using echocardiography and MRI have been shown to vary by gender and age. Aging results in a decrease in LV distensibility that increases the early diastolic filling time, allowing the ventricle more time to fill, and increases the contribution of the atrial kick to LV filling. Although Doppler echocardiography provides peak velocities, MRI provides absolute peak filling rates from the time/volume curves. These data are also obtainable from radionuclide ventriculography; however, MRI has the advantage of significantly higher spatial resolution.11

Another more advanced MRI technique that has been studied in the evaluation of diastolic function involves the use of a high temporal resolution velocity-mapping technique that is able to identify previously undetectable myocardial motion patterns. This method has been shown to be comparable to tissue Doppler imaging—a well-established method that allows noninvasive assessment of diastolic function. Tissue Doppler imaging is limited, however, by limitations in acoustic window, low velocity resolution, lack of spatial information, and effect of the angle of insonation on myocardial velocities. Other established MRI methods for quantifying wall motion, such as tagging, phase contrast velocity mapping (tissue phase mapping), and displacement encoding with stimulated echoes (DENSE), can also be used to extract parameters such as strain or radial and circumferential velocity components, which are measures of diastolic function.12

Various imaging modalities are available to assess ventricular systolic function. Direct left ventriculography may be used, but it is rarely employed for the sole purpose of determining LV function because of its invasive nature. Among noninvasive imaging modalities, echocardiography is the most widely used because of its ease, relatively low cost, and widespread availability. Nuclear scintigraphy including gated single photon emission computed tomography (SPECT) and positron emission tomography can assess global and regional LV function, although their accuracies in this regard have been challenged by low spatial and temporal resolution. Radionuclide ventriculography provides accurate and reproducible assessment of global LV function. More recently, multidetector CT has been used in the evaluation of global and regional systolic LV function.13,14 Among all noninvasive imaging modalities, MRI has been shown to be the most accurate and reproducible modality for the evaluation of global and regional systolic LV function.15

INDICATIONS AND APPLICATIONS OF MAGNETIC RESONANCE IMAGING AND COMPUTED TOMOGRAPHY IN THE EVALUATION OF ISCHEMIC HEART DISEASE

Magnetic Resonance Imaging

Improvements in MRI have introduced new methods for evaluating CAD and its consequences. MRI often provides additional data that may be unavailable from other noninvasive imaging modalities, such as echocardiography, nuclear scintigraphy, and CT. Table 55-1 lists indications for MRI in CAD based on the European Consensus Panel report. Not included in this list are less common techniques, such as T2* measurements and T2-weighted edema imaging, which are being increasingly used in more experienced centers for assessment of cardiomyopathy of unknown etiology.

TABLE 55-1 Indications for Magnetic Resonance Imaging in Coronary Artery Disease

Indication Class*
Assessment of Global Ventricular (Left and Right) Function and Mass I
Detection of CAD  
Regional LV function at rest and during dobutamine stress II
Assessment of myocardial perfusion II
Coronary MR angiography (CAD) III
Coronary MR angiography (anomalies) I
Coronary MR angiography of bypass graft patency II
MRI flow measurements in coronary arteries Inv
Arterial wall imaging Inv
Acute and Chronic MI  
Detection and assessment I
Myocardial viability I
Ventricular septal defect III
Mitral regurgitation (acute MI) III
Ventricular thrombus II
Acute coronary syndromes Inv

* Indication class: I, provides clinically relevant information and is usually appropriate; may be used as first-line imaging technique; usually supported by substantial literature; II, provides clinically relevant information and is frequently useful; other techniques may provide similar information; supported by limited literature; III, provides clinically relevant information, but is infrequently used because information from other imaging techniques is usually adequate; Inv, potentially useful, but still investigational.

From Pennell DJ, et al. Clinical indications for cardiovascular magnetic resonance (CMR): consensus panel report. Eur Heart J 2004; 25:1940-1965.

Computed Tomography

CT is increasingly being used in diagnosis of CAD. Because of its prognostic implications, the quantification of coronary artery calcification is a growing clinical application. Arad and colleagues17 were the first to report attempts to predict cardiac events with coronary calcium as detected by electron-beam CT, showing increased risk of cardiovascular events with higher Agatston scores. Coronary calcium scoring is discussed in Chapter 32.

Contrast-enhanced CT angiography of the coronary arteries is another application of multidetector CT used in the delineation of coronary anatomy and the detection of vascular stenosis. This technique has been used in the detection of CAD in patients at risk, and has been found to correlate with findings on invasive angiography.18

Multidetector CT was initially primarily used for detection of coronary artery stenosis and assessment of cardiac morphology, but more recently has been shown to be useful for cardiac function.10,13,14 Considering the need for administration of contrast material, radiation exposure, and limited temporal resolution (80 to 250 ms), use of multidetector CT exclusively for analysis of cardiac function parameters does not seem clinically prudent at present. If the data are already obtained across all phases of the cardiac cycle during a standard coronary CT angiography evaluation, however, the combination of noninvasive coronary artery imaging and assessment of cardiac function with multidetector CT is a suitable approach because it enables a more comprehensive cardiac work-up in patients with suspected CAD.

Quantification of cardiac function using multidetector CT has been shown to be in good agreement with echocardiography, invasive cine ventriculography, SPECT, and MRI as the reference standard.19 In addition, CT provides a useful alternative to MRI in the evaluation of ischemic heart disease in patients in whom MRI is contraindicated (e.g., patient has a periorbital metallic foreign body or metallic device; see Chapter 19 on MR safety for more information). Table 55-2 compares the advantages and disadvantages of the use of CT versus MRI in the evaluation of CAD.

TABLE 55-2 Advantages and Disadvantages of Magnetic Resonance Imaging and Computed Tomography

  MRI Multidetector CT
Advantages No ionizing radiation Widely available
  No exposure to iodinated contrast media Ease and rapidity of performance
  Better temporal and spatial resolution Not contraindicated in patients with cardiac devices
  Better validation in clinical assessment of the following: Single breath-hold acquisition eliminates motion artifact
 

Better quality of images in the assessment of coronary artery anatomy   Can be used clinically to assess the following (although with less validation studies compared with MRI):   Ventricular mass, volumes and global systolic function   Regional myocardial function   MI Disadvantages

Valvular function Diastolic function Stress testing and assessment of myocardial perfusion-ischemia possible, but clinical experience is scant   Radiation exposure   Risk of contrast medium allergy   Risk of worsening renal function from iodinated contrast dye exposure

ASSESSMENT OF CARDIAC FUNCTION

Global Systolic Function

Magnetic Resonance Imaging

Anatomic Planning and Standard Views

To produce high accuracy and reproducibility, standard imaging planes need to be used in quantifying ventricular function. The axes of the heart are defined using transverse, coronal, and sagittal views obtained as three-plane or individual scout images or through the use of real-time imaging techniques. We present one of many approaches to localize quickly and obtain the standard imaging planes for this purpose using cine MRI. Coronal images are used to define the transverse mid-ventricular slices (i.e., oblique axial images or “quasi” four-chamber view*). The short-axis views of the left ventricle can be defined by prescribing views perpendicular to the line between the center of the mitral valve and apex on the “quasi” four-chamber view. Slices are obtained from the base of the left ventricle at the level of the mitral annulus to the apex, allowing for full LV coverage for proper calculation of LV volumes, ejection fraction, and mass.

The four-chamber view (also known as horizontal long-axis view) is planned from a mid-ventricular short-axis view of the left ventricle in which a line is drawn across the center of the left ventricle to the acute margin of the right ventricle (defined by the junction between the right ventricular (RV) free wall and the diaphragmatic surface of the right ventricle). The two-chamber view (also known as vertical long-axis view) can also be planned on the mid-ventricular short-axis view, and is defined by a line through the center of the left ventricle that is perpendicular to the anterior and inferior wall. This view should include the apex, so it is recommended that the four-chamber view be used as a second localizer. The three-chamber view is planned on the basal short-axis view at the level where the mitral valve opening and LV outflow tract is visualized. The slice is defined by a line drawn through the center of the left ventricle and the LV outflow tract (and aortic root), preferably cutting through the LV apex. These views (short-axis, two-chamber, three-chamber, and four-chamber) are similar to the views achieved in transthoracic echocardiography,

RV function is typically evaluated using the previously described LV views to limit total imaging time. A more detailed evaluation of RV structure and function, of particular importance in the evaluation of arrhythmogenic RV dysplasia, requires the acquisition of additional views. Standard transverse views from cranial to caudal slices are roughly perpendicular to the RV outflow tract, RV free wall, and RV inflow. To visualize the diaphragmatic segment of the right ventricle, a slice planned on a coronal scout, generating an oblique sagittal view through the long axis of the RV outflow tract, should be acquired.20

Sequences

Evaluation of cardiac function involves the use of multiphase, or cine, MRI that employs fast gradient-echo imaging with k-space segmentation to acquire images during end-inspiration breath-holding, which minimizes respiratory motion artifact.21 It is generally accepted that a temporal resolution of 50 ms is required to “freeze” end-systolic motion accurately to quantify ventricular volumes and calculate ejection fraction accurately. Segmented k-space breath-hold cine imaging has an in-plane spatial resolution that typically ranges from 1.5 to 2 mm2, slice thickness of 5 to 10 mm, temporal resolution of 30 to 50 ms, and scan times of 8 to 16 s/slice. The k-space segmentation allows for shorter acquisition times and enables the acquisition of a complete set of multiphase images per slice within a breath-hold. Generally, blood appears bright and myocardium appears gray in these cine gradient-echo images; however, the specific appearance of blood, myocardium, and blood flow depends on whether spoiled gradient-recalled-echo or steady-state free precession (SSFP) is used for signal generation within the segmented k-space structure.15

The current standard for quantification of ventricular volumes and function involves the use of SSFP gradient-echo sequences. The SSFP technique has assumed different names from different MRI scanner vendors: true fast imaging with steady-state precession (TrueFISP), fast imaging employing steady-state acquisition (FIESTA), and balanced fast field echo (FFE). Before the development of high-performance gradient systems, earlier methods involved the use of gradient-echo (e.g., fast low-angle shot [FLASH], fast gradient-echo, FFE). Using these methods, longitudinal magnetization is converted to transverse magnetization by means of radiofrequency (RF) excitation and is sampled. This is followed by spoiling of the residual transverse magnetization using any of the following methods: long repetition time (TR), RF spoiling, or application of spoiler gradients.

The use of spoiler gradients is the most commonly employed method, and involves extending the duration of the readout gradient beyond the sampling of the echo and before the next RF pulse. The prolonged application of the gradient results in accelerated dephasing of the residual transverse magnetization.22 Because residual magnetization is spoiled, signal intensity is not maximized. To improve image quality, SSFP imaging of the heart was later developed, which offers specific advantages to cardiac imaging. This method results in improved signal-to-noise and contrast-to-noise ratios.23 With SSFP, residual transverse magnetization is refocused, not spoiled, before the next RF pulse, and signal intensity is maximized. With this method, magnetization is in a steady state, so the RF pulses are applied throughout the acquisition.

SSFP was not practical before the development of high-performance gradient systems because it is prone to dark-band artifacts caused by off-resonance effects when the repetition time is longer than 4 to 5 ms. With the introduction of high-performance gradient systems, 1.5 T MRI systems can now routinely achieve repetition times at less than 2 ms, however, and acquire artifact-free SSFP motion imaging of the heart. Although SSFP sequences have shorter acquisition times, improved contrast, and improved signal-to-noise properties, it has some limitations. Because SSFP generally uses shorter echo times (i.e., TE) and is to some degree inherently motion compensated, this technique is less sensitive to show the signal loss (also known as intravoxel dephasing) from turbulent blood flow abnormalities owing to valvular dysfunction compared with older cine techniques such as fast gradient-echo pulse sequences.

SSFP is extremely sensitive to field inhomogeneities, of particular importance at 3.0 T, where repeated local shimming or frequency manipulation may be necessary to improve image quality. Traditional fast gradient-echo pulse sequences should be used for the detection of subtle flow abnormalities in valvular heart disease, or when field inhomogeneities result in significant artifacts with SSFP.22 The energy deposition associated with continuous application of RF pulses to maintain steady-state magnetization is higher, and specific absorption rate limits may be reached during SSFP, particularly with higher performance gradient scanners.

Myocardial contractility and regional function can be evaluated further through the use of cine MRI with tagging in conjunction with segmented k-space cine gradient-echo imaging, also known as spatial modulation of magnetization. To evaluate diastolic function, the tagging pulse may be applied in late systole, with images acquired throughout diastole. Quantification of wall motion and myocardial strain can be determined based on displacement of the tagged regions.22 To overcome the time-consuming aspect of tag analysis, harmonic phase analysis and DENSE techniques have been developed to improve the postprocessing duration.2426

Image Analysis

Quantification of myocardial mass, ventricular volumes and dimensions, stroke volume, and ejection fraction is currently done using volumetric data computed from tracings of contiguous short-axis images obtained using cine SSFP pulse sequences. It is important to identify correctly the most basal extent of the ventricle because this determines the LV volumes and the LV ejection fraction.27 Manual planimetry of LV endocardial and epicardial borders is performed during end-systole and end-diastole on each of the short-axis slices (Fig. 55-1). Because of the time required to do manual planimetry, automatic and semiautomatic methods have been developed. In studies comparing manual, automatic, and semiautomatic techniques in animals that were sacrificed for ex vivo measurements, it was determined that manual contours were closer to ex vivo measurements than automatic contours, however, which overestimates true volume.28,29 After border detection, cardiac volumes are calculated by multiplying the blood pool area (defined by the endocardial border) by the slice thickness and summing all the slices. Myocardial mass is computed from the difference between the volumes determined from the endocardial and epicardial borders multiplied by the specific gravity of the myocardium (1.05 g/cm3).15

image

image FIGURE 55-1 LV global systolic function is quantified by tracing the epicardial and endocardial borders on short-axis images.

(From Kwong R. Cardiovascular Magnetic Resonance Imaging. Totowa, NJ, Humana Press, 2007.)

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