Cardiovascular Magnetic Resonance Imaging


Cardiovascular Magnetic Resonance Imaging

Raymond Y. Kwong

With excellent spatial and temporal resolution, unrestricted tomographic fields, and no exposure to ionizing radiation, cardiac magnetic resonance imaging (CMR) provides morphologic and functional information relevant to a broad array of cardiovascular diseases. This chapter reviews the current evidence for use of CMR in diagnosing and treating cardiovacular disease.

Basic Principles of Magnetic Resonance Imaging

The Magnetic Field and the Gradient Coil System

Magnetic resonance imaging (MRI) is based on imaging of protons within the abundant hydrogen atoms in the human body. The hydrogen protons behave like tiny magnets. When a patient is placed inside the CMR scanner within a static magnetic field (called B0), spins either align with or opposite of the main direction of B0. The summation of the aligned and opposing spins forms a net magnetization vector that aligns along the longitudinal axis (z axis) of the magnet at static state before deposition of any radiofrequency (RF) pulse. B0 is designed to have the same strength along each of the three orthogonal directions (designated x, y, and z) inside the CMR bore; thus it is a homogeneous magnetic field. The homogeneous B0 is fine-tuned by the computer-controlled adjustments of currents in small coils mounted within the magnet (known as active shimming). Apart from lining up with B0, spins also precess (wobble about the axis of the B0 field) at a frequency ω0 (the Larmor frequency) proportional to B0 as described by the following equation: ω0 = γB0, where γ is the gyromagnetic ratio (a constant for hydrogen for a given field strength). In order to introduce a system of spatial address of the Larmor frequency, three orthogonal sets of gradient coils are placed so that a slight linear alteration in the strength of B0 can be created in each of the x, y, and z directions. As a result, magnetic spins precess at frequencies according to their locations along each of the three orthogonal axes, and they can be selectively excited by specific radiofrequency pulses.1

Generation of Magnetic Resonance Signal, Signal Contrast, and Image Formation

In order to create a magnetic resonance image, an RF pulse with a frequency matched to the Larmour frequency of the magnetic spins will excite magnetic spins of interest to a higher energy state, which leads to transition of the net magnetization vector from the z axis onto the x-y plane. The extent to which the magnetization vector is tipped away from the direction of B0 (z axis) defines the flip angle, reflects the amount of energy deposition in tissue, and is a function of the strength and duration of the RF pulse. The magnitude of the vector onto the x-y plane will determine the amount of signal generated, which is received by a set of surface coils. For the purpose of imaging a specific slice plane through the body, the magnetic gradient causes a spread of Larmor frequencies perpendicular to a prescribed slice plane. The RF pulse will then excite only the slice plane with magnetic spins precessing at frequencies matching the frequency bandwidth of the RF pulse.

The absorbed electromagnetic energy will be released by two coexisting mechanisms, longitudinal magnetization recovery and transverse magnetization decay. Longitudinal magnetization recovery corresponds to the exponential rate of recovery of the longitudinal component (z-direction) of the magnetization vector, characterized by a time constant, T1, which is defined as the time to recover 63% of the original longitudinal magnetization vector. T1 is a physical characteristic of tissue and is affected by the field strength of the scanner, with values progressively greater (longer times) at higher field strengths (in Tesla units). T1 characterization therefore allows generation of images that reflect the differences of T1 between tissue types. A T1-weighted scan will keep the time between delivery of two successive flip angles (repetition time) short, so tissues with different T1 values will demonstrate different signal intensity as they follow a T1 recovery. The transverse magnetization decay results from interaction between neighboring spins (spin-spin interaction) leading to exponential loss of the transverse component of the net magnetization vector, defined by the time constant T2. T2 also is a tissue-specific parameter and is defined as the time to lose 63% of the transverse magnetization. Unlike T1 values, T2 values are less related to the field strength of the scanner. The choice of signal contrast weighting of the imaging method is dictated in part by the physiologic characteristics of the tissue being studied. For qualitative interpretation, signal enhancement (from T1 effects) is in general preferred over darkening (T2*) (see explanation further on) effects, so most pulse sequences used in CMR are relative T1-weighted signal-enhancing techniques. T2-weighted and T2*-weighted CMR are primarily used for imaging of myocardial edema and iron content, respectively. With the application of magnetic field gradients in any of the three orthogonal directions, the magnetic resonance signal can carry spatial localization information, produced by encoding steps known as slice select, phase encoding, and frequency encoding. All relevant information of the magnetic resonance signal is stored in a data matrix called the k-space, which will undergo two-dimensional inverse Fourier transformation to form an image.

Contrast Agents in Cardiac Magnetic Resonance

Currently only gadolinium-based contrast agents (GBCAs) are used in clinical practice. When injected as an intravenous bolus, a GBCA takes 15 to 30 seconds for transit through the cardiac chambers and blood vessels (first-pass phase) before it diffuses into the extracellular space. At approximately 10 to 15 minutes after injection, a transient equilibrium between contrast washing-in into the extracellular space and washing-out to the blood pool is reached. Myocardial perfusion CMR and most magnetic resonance angiography (MRA) examinations are performed during the first-pass phase, whereas late gadolinium enhancement (LGE) images are obtained during the equilibrium phase. Several GBCAs are commercially available in the United States; however, their use in CMR imaging is considered off-label. Mild reactions from GBCAs occur in approximately 1% of patients receiving these agents, but severe or anaphylactic reactions are very rare. All GBCAs are chelated to make the compounds nontoxic and to allow renal excretion. Exposure to the nonchelated component of GBCAs (Gd3+) has been associated with a rare condition known as nephrogenic systemic fibrosis (NSF), which is an interstitial inflammatory reaction that leads to severe skin induration, contracture of the extremities, fibrosis of internal organs, and even death. Risk factors for development of NSF include high-dose (>0.1 mmol/kg) GBCA regimens with estimated glomerular filtration rate (eGFR) less than 30 mL/min/1.73 m2, need for hemodialysis, an eGFR less than 15 mL/min/1.73 m2, use of gadodiamine (Omniscan, General Electric Healthcare, Chalfont St. Giles, Buckinghamshire, United Kingdom), acute renal failure, and presence of concurrent proinflammatory events. With the use of weight-based dosing and pretest screening, recent data suggest that NSF is now extremely rare. Previously, an incidence of 0.02% in 83,121 patients exposed to GBCAs over 10 years was noted; however, with current eGFR screening guidelines that have been widely practiced since 2006, a near-zero incidence has been reported.2

Technical Aspects of Cardiac Magnetic Resonance Pulse Sequences

CMR uses a range of strategies to overcome technical difficulties caused by cardiac, respiratory, and blood flow motion. Synchronized gating to the electrocardiogram (ECG) is routinely performed. Cardiac gating can be either prospective (triggering by an ECG waveform followed by a fixed period of acquisition during all cardiac cycles) or retrospective (continuous data acquisition with subsequent reconstruction based on ECG timing). For cine imaging, retrospective gating is preferred because it covers the entire cardiac cycle and is less prone to artifacts. To reduce blurring from cardiac motion, many CMR techniques fractionate the data for an image to acquire data only within a narrow window of the cardiac cycle (segmented approach). Currently, patient breath-holding remains the most common method to contain respiratory motion during CMR data acquisition, although navigator-based techniques (tracking of diaphragmatic motion to control respiratory motions) and respiratory motion averaging are options in some pulse sequences. Finally, by rapidly acquiring data of an entire image within a cardiac cycle, single-shot imaging and real-time cine imaging (continuous acquisition of single-shot images) can overcome both respiratory and cardiac motions, but at the expense of reduced temporal and spatial resolution. Table e17-1 image shows a summary of the most common clinical CMR pulse sequence techniques at our center. Minor variations exist in these parameters between centers and vendors. CMR uses bright-blood cine imaging or dark-blood fast spin-echo (FSE) imaging to assess cardiac morphology and structure. Cine CMR is the modality that serves as a reference standard for quantifying ventricular volumes. Among the cine techniques, cine steady-state free precession (SSFP) is the technique of choice. It can acquire a cine movie at a high temporal resolution of 30 to 45 milliseconds during a breath-hold of less than 10 seconds, thereby capturing the whole heart in motion volumetrically in 3 to 5 minutes (Fig. 17-1; Video 17-1image). For dark-blood techniques, T1-weighted FSE is used for assessing morphology of cardiac chambers, vascular structures, and pericardium and for imaging of fat (Fig. 17-2). T2-weighted FSE with fat suppression is used for imaging of myocardial edema occurring as a result of ischemia, infection, or infiltration. Three main techniques have been developed to quantify intramyocardial motion: myocardial grid or line tagging, phase contrast velocity mapping of myocardial motion, and displacement encoding with stimulated echoes (DENSE). Tagging assesses myocardial strain by marking the myocardium with parallel dark lines or a grid so that myocardial deformation can be visualized or quantified. Circumferential and radial strain also can be calculated and displayed with a color-coded scale. Although myocardial tagging is the most widely available, phase contrast velocity mapping and DENSE techniques can be completed at higher spatial resolution.

TABLE e17-1

Summary of Common Clinical Cardiac Magnetic Resonance Pulse Sequence Techniques at Brigham and Women’s Hospital

Cine cardiac structure and ventricular function Cine SSFP*
Real-time cine SSFP
Bright T2W/T1W for cine SSFP and real-time cine SSFP; T1W for FGRE 1.5-2.5 mm/30-45 msec per phase

Yes for ECG-gated cine SSFP and FGRE
No for real-time cine
No Cine SSFP has higher SNR and CNR (between endomyocardium and blood) than FGRE but is sensitive to field inhomogeneity (especially at 3T), giving rise to banding artifact
FGRE has weaker endocardial definition than cine SSFP but is an alternative with severe artifact in cine SSFP
Real-time cine SSFP: use in patients with significant arrhythmia or difficulty breath-holding; it has the lowest spatial and temporal resolutions


Quantitative regional myocardial strain Myocardial tagging (newer but less widely available techniques for regional strain exist, (see text) Bright T1W
Yes No Tissue tracking quantitation of intramyocardial motion
Disadvantages: fading of tag lines near end of cardiac cycle and time-consuming strain analysis (postprocessing)


Structure, morphology, and fat imaging Standard FSE*
Single-shot (SS) FSE (or HASTE)
Dark T1W ± fat suppression 0.8-1.5 mm every cardiac cycle Yes for standard FSE
No for SS FSE
No Standard FSE has better image quality but relatively long scan time
Fat suppression can be achieved by fat saturation pulse (more specific) or by suppressing tissues with short T1 (a technique known as STIR, which is less specific for fat).
SS FSE covers the whole heart quickly and is useful in patients with arrhythmia or limited breath-holding


Myocardial scar by LGE imaging Standard 2D segmented FGRE*
2D SS SSFP technique
3D whole-heart techniques (breath-hold or navigator-guided)
PSIR (phase sensitive image reconstruction)
Bright T1W (10-30 min after 0.1-0.2 mmol/kg GBCA injection) 1.5-2.0 mm/ 150-200 msec (for standard 2D)
Adjust inversion time and time delay after ECG detection to null “normal” myocardium signal and to image in diastole, respectively
Yes for standard 2D technique
No for SS technique
Yes Standard 2D technique has higher spatial and temporal resolutions than the SS technique
2D SS technique covers the whole heart quickly and is useful in patients with arrhythmia or difficulty breath-holding
PSIR is inversion time–insensitive and is more robust in nulling normal myocardium signal
New 3D application using navigator-guidance yields higher SNR than 2D and can achieve spatial resolution of <1 mm without the need for breath-holding
Refer to Table 17-e3 for patterns of LGE in various cardiomyopathies
2D segmented LGE


Myocardial perfusion imaging Saturation prepared gradient-echo–based 2D techniques:

Bright T1W 2.0-3.0 mm
130-180 msec per slice
3-4 locations every cardiac cycle or 6-8 locations every 2 cardiac cycles during vasodilator stress and rest.
0.05-0.1 mmol/kg IV GBCA injected at 4 or 5 mL/sec (qualitative assessment only)
No, but breath-hold is preferable Yes Breath-holding useful to track contrast enhancement in specific segments.
Parallel imaging acceleration and sparse sampling to reduce acquisition time per slice and extend slice coverage of the heart


Myocardial edema imaging T2W FSE*
T1W EGE ratio
T2 prep SSFP
T2 map (SSFP readout)
Dark (FSE-based), Bright (SSFP-based) T2W + fat suppression (for T2W techniques)
T1W (for EGE technique)
In-plane spatial and temporal resolutions similar to standard FSE
Slice thickness 7-10 mm to improve SNR
Algorithm to correct for distance of the heart from the receiver surface coils is required
Yes No
Yes for EGE ratio
Myocardial edema appears as a transmural area of high SI on T2W images
In FSE techniques, beware of artifacts from slow flow, especially adjacent to regional wall motion abnormality or the LV apex, which may mimic edema
Regional myocardial signal variation from phase array coils may mimic edema
In absence of LGE, T2W edema reflects reversible myocardial injury
Using T2W FSE techniques, an SI ratio of myocardium to skeletal muscle >1.9 has been reported to be abnormal in myocarditis
An EGE ratio between myocardium and skeletal muscle of ≥4 or an absolute myocardial SI increase of 45% after contrast is considered abnormal in myocarditis
The bright-blood SSFP-based technique has improved CNR and is less susceptible to slow-flow artifact.
T2 map is insensitive to surface coil–related signal inhomogeneity and slow-flowing blood related artifact


Myocardial iron content imaging T2*W multiple echo times FGRE Bright T2*W 2.0-3.0 mm/~100-150 msec
One short-axis midventricular location
A series of images with 8 echo times that goes from ~4 msec to 35 msec
Axial ungated acquisition of the liver for comparison
Yes No Measurement is most accurate and reproducible in the midseptum
T2* value describes the exponential decay of myocardial SI as the echo time increases
T2* value of <20 msec with LV dysfunction (without other obvious cause) indicates iron overload cardiomyopathy


Cardiac thrombus LGE with long inversion time
EGE imaging
Bright T1W In-plane spatial and temporal resolutions similar to those with LGE imaging
EGE is acquired within the first 5 min after gadolinium injection
Yes Yes LGE imaging with inversion time set at 600 msec or longer or EGE imaging can detect thrombus indicated by an intense “black” regions
Look for thrombus in locations of stagnant flows


Cardiac blood flow Phase contrast imaging cine GE Bright Velocity-related phase shift 1.5-2.5 mm/50 msec per phase
Keep number of lines of k space per cardiac cycle (segments) low to improve temporal resolution during free-breathing studies
No (multiple signal averages used) No Multiple averages can reduce ghosting artifacts from respiratory motion during free breathing.
Should keep velocity encoding strength slightly greater than the highest expected flow velocity, to avoid aliasing while maximizing accuracy


Coronary MRA 3D whole-heart volume using SSFP or FGRE*
Target vessel approach
Bright T2-prepared 3D SSFP or FGRE technique ~ 0.6-1.0 mm in plane
Free-breathing navigator-guided 3D technique is currently most widely used
No, but
Yes for target vessel approach
Yes or No (with SSFP-based technique in 1.5 T) Compared with the target-vessel approach, 3D coronary MRA has higher SNR and provides volumetric whole-heart coverage
T2-prepared SSFP sequence with suppression of the adjacent epicardial fat provides the strong blood vessel contrast
Contrast-enhanced FGRE based technique is used in 3T


Anatomy for electrophysiologic mapping of the pulmonary vein 3D FGRE MRA of the left atrial volume and pulmonary veins Bright T1W FGRE 1.5- to 2.5-mm isotropic volume
Timing bolus is required to achieve proper timing of imaging during first-pass transit of the contrast bolus
Gating is optional but may improve border definition at the expense of prolonging breath-hold
Yes Yes Subtraction mask scan is necessary to enhance the MRA images
Coronal (more common) or axial 3D MRA of the entire left atrium and the pulmonary vein is generated for electrophysiologic mapping; use same parameters as in the subtraction mask scan



* More commonly used option.

CNR = contrast-to-noise ratio; EGE = early gadolinium enhancement; EPI = echoplanar imaging; FGRE = fast gradient-recalled echo; GE = gradient-echo imaging; HASTE = half-Fourier acquisition single-shot turbo spin echo; SE = spin echo imaging; SI = signal intensity; SNR = signal-to-noise ratio; SS = single-shot; T = Tesla; 3D = three-dimensional; 2D = two-dimensional; T1W = T1-weighted; T2*W = T2*-weighted.

(Note: Dark-blood techniques and myocardial iron content determination by T2* imaging should be performed before administration of gadolinium contrast.)

T1-weighted imaging techniques such as LGE imaging can detect accumulation of GBCA into the extracellular compartment of the myocardium secondary to infarction, infiltration, or fibrosis. LGE is detected 5 to 15 minutes after an intravenous injection of GBCA (0.1 to 0.2 mmol/kg) (hence the designation “late”). LGE data can be captured in two- or three-dimensional representation. Several technical improvements in LGE imaging have emerged. Phase-sensitive inversion recovery (PSIR) reference imaging incorporates the phase polarity information that enhances myocardial tissue contrast. Single-shot LGE imaging offers an option to overcome motion when cardiac gating or patient breath-holding is not possible. Navigator-guided LGE eliminates the need for breath-holding and allows three-dimensional acquisition with in-plane resolution below 1 mm (Fig. 17-3, Video 17-2image). CMR perfusion imaging examines the first-pass transit of an intravenous bolus of GBCA as it travels through the coronary circulation. Several perfusion techniques are available, which are fast bright-blood gradient-echo imaging sequences in which three to five short-axis slices of the heart are acquired every cardiac cycle, during the injection of a GBCA bolus. Gadolinium provides strong signal enhancement in well-perfused regions, compared with hypoenhancement (dark regions) in poorly perfused myocardium. At a spatial resolution of approximately 2 mm in plane, CMR perfusion imaging can provide information of myocardial blood flow at the endocardial/epicardial or at a segmental level (Fig. 17-4; Video 17-3image). Dynamic three-dimensional perfusion imaging can provide greater myocardial coverage and improved image quality and has shown promising preliminary clinical results.3 T2-weighted imaging detects myocardial edema, from ischemic injury or inflammation, and it has been shown to have high correlation to the area at risk after acute myocardial infarction (MI). It also complements LGE imaging in determining the chronicity of an MI and allowing for accurate measurement of salvageable myocardium. The pulse sequence options for T2-weighted imaging include black-blood short TI inversion recovery (STIR) FSE and the newer SSFP-type methods,4 and their merits are listed in Table e17-1. T2* is a transverse relaxation parameter sensitive to tissue iron content. T2* imaging is a well-validated method for measuring tissue iron content. A T2* less than 20 milliseconds (value for normal myocardium, approximately 40 to 50 milliseconds) is diagnostic of myocardial iron overload, and a T2* less than 10 milliseconds is evidence of severe iron overload5 (Fig. 17-5; Video 17-4image). Despite challenges posed by small luminal sizes and cardiac and respiratory motions, technical advances in coronary MRA imaging have favored the use of whole-heart three-dimensional acquisition (with or without navigator guidance), with promising preliminary clinical results6 (Fig. 17-6). Similar to Doppler echocardiography (see Chapter 14), phase contrast imaging allows quantitation of velocities of blood flow and myocardial motion and intravascular flow rates. Parallel imaging is a family of techniques that speeds up CMR (k-space) data acquisition by combining information obtained separately from each element of the surface receiver coils. Incorporating parallel imaging can reduce acquisition time, improve temporal resolution, or even eliminate certain artifacts. The main disadvantage of parallel imaging is a reduction in signal-to-noise ratio resulting from undersampling of the k-space data.

Patient Safety in Cardiac Magnetic Resonance

Clinical CMR scanners generate strong magnetic fields. The magnetic field component can be disabled with difficulty by evaporating the cooling liquid helium to the outside environment, but this action carries significant risk and is associated with high restoration costs. Common implants hazardous in CMR scanning include cochlear implants, neurostimulators, hydrocephalus shunts, metal-containing ocular implants, pacing wires, and metallic cerebral aneurysm clips. A full list is available at (the official website for the Institute for Magnetic Resonance Safety, Education, and Research). Sternal wires, mechanical heart valves, annuloplasty rings, coronary stents, nonmetallic catheters, and orthopedic or dental implants are safe. Most claustrophobic patients can be managed with oral sedation alone or the use of a scanner with large bore size.

The risks associated with performing MRI in patients with a pacemaker or an implantable cardioverter-defibrillator (ICD) (see Chapter 36) include generation of an electrical current from the metallic hardware, device movement induced by the magnetic field, inappropriate discharging and sensing, and heating as a result of the “antenna effect.” However, a number of experienced centers have reported safety in performing CMR in a controlled setting in patients who have recent pacemaker models and are not pacemaker-dependent. The first pacemaker designed to allow MRI scanning has been approved by the U.S. Food and Drug Administration (FDA), but imaging over the chest and neck region currently is not recommended.

Cardiac Magnetic Resonance Assessment of Specific Disorders and Conditions

Discussed in this section are clinical applications of CMR. Table e17-2 image summarizes the CMR protocols, by study indications, used at our center. A detailed description of CMR protocols endorsed by the Society of Cardiovascular Magnetic Resonance (SCMR) can be found at In addition, the SCMR has established reporting guidelines to provide a framework for enhancing communication with referring physicians.8

TABLE e17-2

Typical Cardiac Magnetic Resonance Protocols at Brigham and Women’s Hospital

Myocardial viability for benefit from coronary revascularization
Short-axis stack and selected long-axis locations
Myocardial ischemia
Vasodilating stress
Dobutamine stress

Short-axis stack and selected long-axis locations
3 short-axis and 2 or 3 long-axis locations for stress cine

Acute myocardial infarction
Short-axis stack and selected long-axis locations
Detecting ACS or other causes of myocardial injury
Short-axis stack and selected long-axis locations
Assessing the etiology of an undiagnosed cardiomyopathy or a specific cardiomyopathy
Short-axis stack and selected long-axis or axial locations
Pericardial disease
Short-axis stack and selected long-axis or axial/oblique locations
Valvular heart disease
Short-axis stack and selected long-axis or axial/oblique locations
Cardiac mass/thrombus
Short-axis stack and selected long-axis or axial/oblique locations
CMR for left atrial mapping and pulmonary vein ablation
Short-axis stack and selected long-axis
Coronal 3D locations for MRA


ACS = acute coronary syndrome; CMP = cardiomyopathy; EGE = early gadolinium enhancement; FGRE = fast gradient recalled echo; HCM = hypertrophic obstructive cardiomyopathy; 3D = three-dimensional; T1W = T1-weighted; T2W = T2 weighted.

Coronary Artery Disease

Current CMR protocol for coronary artery disease (CAD) integrates cine imaging, T2-weighted edema imaging, myocardial perfusion at rest and stress, and LGE imaging of MI and provides a comprehensive evaluation of myocardial anatomy and physiology. Coronary MRA is done as a part of the examination in more experienced centers. As noted, Table e17-2 summarizes the CMR protocols used in our center; the typical CMR findings are described in Table e17-3image.

Myocardial Infarction

LGE imaging currently is the most accurate noninvasive method for quantifying infarct size and morphology. Infarct size estimated by LGE imaging has been well validated against histologic pattern, and commercial software systems are available to perform infarct size quantitation. With an excellent spatial resolution of 1.5 to 2 mm and a high contrast-to-noise ratio, LGE imaging provides detection of subendocardial infarction beyond either single photon emission computed tomography (SPECT) or positron emission tomography (PET) imaging (see Chapter 16). The robustness of the LGE imaging across MRI models was demonstrated by a double-blinded multicenter randomized clinical trial in which LGE was shown to detect acute and chronic infarcts with a sensitivity of 99% and 94%, respectively.9 In the acute MI setting, when LGE imaging is performed early (within the first 5 minutes) after contrast injection, microvascular obstruction (no-reflow) can be seen as a dense hypoenhanced area surrounded by a bright region representing the infarct (Fig. 17-7). This noninvasive method for quantifying microvascular obstruction has been validated against angiographic parameters of microcirculatory flow. Recent reports have even demonstrated detection of myocardial hemorrhage as a result of reperfusion injury (Fig. 17-8; Video 17-5image). Acute right ventricular (RV) injury also can be detected at high sensitivity (see Fig. 17-7). Some evidence suggests that the high spatial resolution and contrast-to-noise ratio of LGE imaging translate to useful patient prognostic information. For patients with acute MI, presence of either microvascular obstruction or acute RV injury has prognostic implication independent of left ventricular (LV) infarct size and LV ejection fraction (LVEF).10 In the nonacute setting, an infarct identified solely by LGE imaging in patients without a history or ECG evidence of MI or in patients with diabetes, is a strong predictor of adverse events, independent of common clinical risk markers.11 The strength of LGE imaging in detecting clinically unrecognized MI has been extended to the population level. A recent large community-based cohort study demonstrated that LGE imaging detected a very high prevalence of unrecognized (and hence untreated) MI in older persons, which was missed by ECG. This group of patients experienced remarkably increased mortality risk.12

Several pilot studies demonstrated that infarct tissue heterogeneity quantified from LGE images may describe arrhythmogenic substrates that develop as the result of an MI. Schmidt and co-workers reported that monomorphic ventricular tachycardia during electrophysiologic studies was more strongly associated with infarct heterogeneity than with LVEF.13 Roes and colleagues found that infarct heterogeneity was a strong predictor of spontaneous ventricular arrhythmias necessitating appropriate ICD therapy in patients with MI.14 These findings are concordant with the observed association between infarct tissue heterogeneity and patient mortality in yet another study.15

Assessment of Myocardial Viability and Benefit from Coronary Revascularization

CMR allows multifaceted assessment of structure and physiology associated with myocardial viability. End-diastolic wall thickness alone has limited accuracy in predicting recovery of segmental function because the wall tissue may include irreversibly damaged myocardium and a thinned epicardial rim of viable myocardium. From early cine CMR studies, it has been shown that end-diastolic wall thickness of 5.5 mm or greater and dobutamine-induced systolic wall thickening of 2 mm or greater have excellent specificity and sensitivity in the prediction of segmental contractile recovery after revascularization (sensitivity, 89%; specificity, 94%). In a landmark paper by Kim and colleagues, the transmural extent of myocardial scar detected by LGE imaging was shown to accurately predict a progressive stepwise decrease in functional recovery despite successful coronary revascularization.16 This prediction of segmental functional recovery was especially strong in segments with resting akinesia or dyskinesia. Compared with dobutamine cine CMR, LGE imaging is easy to perform and interpret, and a 50% transmurality cut-off point is sensitive in predicting segmental contractile recovery. Even very thin myocardial regions without LGE have the potential for increase in thickness and recovery of function after revascularization.17 On the other hand, the high specific­ity from low-dose dobutamine cine imaging provides a physiologic assessment of the midmyocardial and subepicardial contractile reserve, particularly in segments with subendocardial MI involving less than 50% of the transmural extent. At our center, it appears that LGE imaging alone suffices for answering most questions raised in imaging for myocardial viability. However, low-dose dobutamine cine CMR can be complementary in assessing myocardial viability early after acute MI when tissue edema is prominent or when high test specificity is demanded to justify bypass surgery in patients at high preoperative risk.

Many earlier imaging-based viability studies were limited by retrospective design, lack of treatment assignment, and use of recovery of segmental function as an endpoint that provides little information about long-term patient outcomes. The Surgical Treatment of Ischemic Heart Failure (STICH) trial overcame the limitations of previous studies by prospectively assessing the role of viability imaging in decision-making toward cardiac bypass surgery or aggressive medical therapy in patients with CAD and LVEF below 35%.18 Although detection of myocardial viability was associated with patient survival, SPECT perfusion imaging or dobutamine echocardiography failed to identify patients who will derive the greatest survival benefit from addition of coronary artery bypass grafting (CABG) to aggressive medical therapy. One can postulate that because CMR can interrogate multiple targets of myocardial viability, it may provide a more precise viability assessment than SPECT or dobutamine echocardiography and may be more useful in guiding decision making in patients such as those studied in the STICH trial. Prospective studies of this issue are needed.

Detecting Acute Coronary Syndromes and Differentiating from Noncoronary Causes.

A number of prospective single-center studies combined the diagnostic utility of cine wall motion imaging, myocardial perfusion, and LGE imaging using CMR in assessing acute chest pain syndromes. Collective evidence from these studies indicated that CMR has high sensitivity and specificity for detecting acute coronary syndromes and in risk-stratifying patients presenting with acute chest pain (Fig. 17-9). Adding T2-weighted imaging characterizing the acute area at risk to wall motion and LGE imaging can increase the specificity of diagnosing acute coronary syndrome in patients presenting with chest pain but with negative findings on ECG and serum troponin assays.19 Furthermore, T2-weighted imaging is unique in that the detection of the extent of the salvageable myocardium can be achieved for days after emergent restoration of coronary flow. Finally, CMR captures a range of abnormalities that are useful in differentiating acute coronary syndrome from noncoronary causes of chest pain.20,21

Jun 4, 2016 | Posted by in CARDIOLOGY | Comments Off on Cardiovascular Magnetic Resonance Imaging
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