Chapter 6. Cardiac Magnetic Resonance Imaging in Ischemic Heart Disease

This chapter will focus on acute and chronic ischemic heart disease (IHD); the underlying pathophysiology; and the assessment of myocardial function, perfusion, infarct size, and myocardium at risk, all by cardiovascular magnetic resonance (CMR).

Understanding of CMR imaging in both acute and chronic IHD necessitates understanding of the sequence of pathophysiologic events that occur during ischemia. IHD is a dominating cause of death in Western countries and an emerging problem in developing countries. The development of IHD is related to both hereditary factors and lifestyle factors including smoking, diet, and lack of exercise. This section provides an overview of pathophysiology of ischemia with a focus on the mechanisms relevant to the etiology, diagnosis, and treatment of acute myocardial ischemia, stress-induced ischemia, myocardial stunning, hibernation, and infarction. These mechanisms are discussed in relation to the ischemic cascade, findings of myocardial perfusion and function at rest and stress, and the assessment of myocardial viability and the need for coronary revascularization.

The temporal sequence of events referred to as the ischemic cascade1 is illustrated in Fig. 6–1. Ischemia can be described as an imbalance between myocardial oxygen supply and demand. Ischemia can thus be conceptualized in two different ways. Reduced supply in the resting condition is exemplified by myocardial ischemia in the setting of plaque rupture, thrombus formation, and acute coronary occlusion. By comparison, a person may have adequate myocardial perfusion at rest despite having a stenosis in a coronary artery. Upon physical exertion, demand is increased, and myocardial ischemia may ensue despite unchanged supply, or rather, inability to increase supply in order to meet increased demand. In terms of treatment, a situation where myocardial oxygen supply is insufficient in relation to demand (ischemia) can be alleviated by increasing supply (ie, increased perfusion following surgical or percutaneous revascularization), but also by reducing demand (ie, β-blocker therapy). If possible, both the supply and the demand sides of the ischemic imbalance can be therapeutically targeted.

Acute Coronary Syndrome

Acute coronary syndrome2 (ACS) refers to clinical symptoms related to acute myocardial ischemia, sometimes called unstable coronary artery disease. If myocardium is affected by severe enough ischemia for a sufficient duration of time, the ultimate consequence is irreversible cell death. This is represented by infarction, seen at the top extreme of the ischemic cascade (see Fig. 6–1). During the healing process, the dead myocytes are replaced by connective tissue, forming a fibrotic scar, and myocardial contraction in the affected region cannot be restored. Furthermore, patients undergoing myocardial infarction are often subject to repeated infarction and, dependent on infarct size and location, negative left ventricular remodeling with possible dilatation and heart failure. For restoration of blood supply in ACS, either medical thrombolysis or percutaneous coronary intervention (PCI) may be used. However, the restoration of epicardial flow by PCI is not necessarily associated with restored myocardial microvascular flow, and thus, other means for restoring microvascular flow may be needed.

Major determinants of final infarct size include determinants of the severity of ischemia, which depends on the degree of collateralization, preconditioning, size of the myocardium at risk, duration of ischemia, and metabolic demand during ischemia including core body temperature. The most common cause of myocardial infarction is rupture of an atherosclerotic plaque with formation of a thrombus leading to occlusion of a coronary artery.3 The rupture of a plaque results in exposure of collagen, lipids, smooth muscle cells, and tissue factors to the blood, leading to activation of platelets and the coagulation system.4-6 Glycoprotein (GP) IIb/IIIa receptors on the surface of the platelets enable aggregation of platelets through cross-bridges of fibrinogen, and several vasoactive and procoagulative mediators are released. Treatment resulting in inhibition of these receptors is therefore of interest. A GPIIb/IIIa inhibitor is often used as adjuvant therapy in PCI because it may prevent obstruction of microvascular vessels, thus increasing microvascular blood flow after the restoration of epicardial flow.7 The same agent has previously been proven to attenuate circulating inflammatory markers,8 which are likely to take part in formation of the scar.

In ACS, the occlusion of a coronary artery results in a sudden imbalance in myocardial supply and demand. However, a more gradual development of occlusive atherosclerotic disease, as seen in many patients, may lead to recruitment of functional collateral vessels. These functional collaterals provide perfusion to myocardium that otherwise would be at risk of infarction during coronary occlusion and thus alleviate the ischemic burden on the myocardium supplied by the affected coronary artery during acute occlusion.9 In addition, the myocardium may be protected by preconditioning caused by repeated episodes of ischemia due to spontaneous closing and opening of the occluded artery during ischemia. Both collateral flow and preconditioning attenuate the ischemic injury and reduce final infarct size. Notably, the presence of native collaterals differs between species,10 which is one explanation for the diverse results from studies of the impact of duration of ischemia on final infarct size in different animal models.

Myocardium at Risk

Apart from collateralization and preconditioning, a major determinant of final infarct size is the size of the myocardium subjected to ischemia. This is called myocardium at risk and depends on where in the coronary arterial bed the occlusion occurs. Myocardium at risk represents the maximum amount of myocardium that might be infarcted as a result of the ischemia.

A myocardial salvage index can be calculated to estimate how much of the jeopardized myocardium (myocardium at risk) has been salvaged due to reperfusion therapy. This is calculated as follows: (myocardium at risk – final infarct size)/myocardium at risk.

Regarding salvage of jeopardized myocardium (ie, myocardium at risk), animal data have shown that the duration of ischemia needed to cause 50% of the myocardium to develop into infarction is approximately 40 minutes in pig and rat and 3 hours in dog. For humans, the corresponding time to reach 50% infarction is just under 5 hours (Fig. 6–2).11 Thus, when studying the efficiency of treatment with regard to infarct evolution, conclusions drawn from animal studies may not be directly translatable to humans, at least not with regard to time scale. This is especially important to consider when planning phase III studies, where animal research is commonly used as a basis for justifying the expected effects of a treatment in humans.

Infarct Evolution

Reimer et al12 showed in 1977 that a myocardial infarct evolves gradually during ischemia as a "wave front" that originates at the endocardium and spreads toward the epicardium as a function of time. Final infarct size is thus also related to the duration of ischemia, defined as the time from onset of occlusion to restoration of perfusion of the ischemic myocardium. The time course of infarct evolution has been well described for several species.10,13-20 In man, however, estimation of the time course of infarct evolution during occlusion of a coronary artery has been adapted from studies with physiologically diverse inclusion criteria that have been compensated for by the inclusion of large numbers of patients. The Gruppo Italiano per lo Studio della Streptochinasi nell'Infarto Miocardico 1 study (GISSI-1)21 and Second International Study of Infarct Survival (ISIS-2)22 showed that fibrinolytic therapy within 1 hour from the onset of symptoms results in a more than 50% reduction in mortality. Results from previous studies also indicate that treatment within 6 hours from onset of symptoms results in a mortality reduction twice that of treatment 6 to 12 hours after onset of pain.23 Overall, this shows that, similarly to the results from animal models, early reperfusion is of great value in patients. Recently, the infarct evolution in man was directly studied in a group of patients with strict inclusion criteria that closely mimicked an experimental setting.11 Two representative cases for early and late reperfusion in man are shown in Fig. 6–3, relating the final infarct size determined by late gadolinium enhancement CMR imaging to the myocardium at risk determined by myocardial perfusion single photon emission computed tomography (SPECT) imaging. Results from this study demonstrate that infarct evolution in man is considerably slower than in pigs, rats, or dogs11 (Fig. 6–4).

The aerobic metabolism in ischemic myocardium ceases within 10 seconds of acute coronary occlusion. This initially manifests itself as impaired myocardial relaxation, otherwise known as diastolic dysfunction. Relaxation is the energy-dependent phase of the contractile cycle in the cardiac myocyte, and it is limited by the supply of energy from adenosine triphosphate (ATP). ATP is necessary during relaxation in order to pump Ca2+ from the cytosol back into the sarcoplasmatic reticulum through ATP-dependent Ca2+ channels.24 Hence, following reduction in the supply of ATP, relaxation is the first process to be influenced. Within minutes, as ischemia persists, reduced amounts of creatine phosphate25 and the free energy hydrolysis of ATP26 both contribute to the continued reduction in the amount of ATP. Thus, the accumulated ATP debt further impacts on the ATP-dependent transport of Ca2+, and contraction is impaired. Systolic dysfunction is the result of this compromised contraction in addition to compromised relaxation. In acute occlusion, anaerobic metabolism is quickly inhibited as a result of accumulation of metabolites.27 In combination with accumulation of metabolites, the myocyte cell membrane eventually ruptures, thereby defining the necrosis that is the hallmark of infarction. Rupture of cell membrane leads to inability to exclude small molecules, which is an excellent marker for cell death.28

Myocardial infarction is first seen in the endocardial layers of the myocardium. The endocardium is most sensitive to ischemia because myocardial depolarization and contraction originate in the endocardium and spread toward the epicardium. In contrast, repolarization and relaxation a short time later originate in the epicardium and spread toward the endocardium. The endocardium thereby has a shorter time span for myocardial perfusion during diastole compared with the epicardium. Furthermore, the largest-caliber coronary arteries lie on the epicardial surface of the myocardium, and the most fine-caliber arterioles are localized in the endocardium. The endocardium is preferentially subjected to the highest pressures of systolic ejection due to the ensuing pressure gradient from endocardium to epicardium. Taken together, the endocardium is the region of the myocardium that is most susceptible to the imbalances of supply and demand in ischemia and is the first to succumb to infarction. Whereas the endocardial lateral borders of infarction are established early, the transmural extent of infarction extends in a wave-front manner throughout the myocardial wall. The time course of this wave front relates to the duration of ischemia.12,16 The endocardial lateral borders of the final infarct are therefore roughly equivalent to the lateral borders of myocardium at risk. Importantly, this is not the case at earlier stages of occlusion or when infarction has been aborted.29

Chronic Ischemia

In the setting of chronic IHD due to slow accumulation of atherosclerotic plaque over years, a more subtle progression through the ischemic cascade may occur compared with the setting of acute coronary occlusion. Patients may present in several different physiologic states of myocardial dysfunction that have not yet led to infarction or scar in the myocardium.

The term nonviable myocardium will here be used to describe myocardium that has been irreversibly damaged by infarction. The terms viability and viable myocardium have been used extensively in the literature to selectively describe reversibly dysfunctional myocardium while excluding normally functioning myocardium, which also is alive.30 However, the term viable will be used to describe all myocardium—normal and dysfunctional—that is not irreversibly damaged.31 Consequently, the term dysfunctional but viable myocardium will instead be used to selectively describe myocardium with intact cell membranes and that is in a state of potentially reversible dysfunction. Dysfunction in this setting refers to reduced systolic function.

The first event in the ischemic cascade may often be observed as compromised perfusion. Reduced perfusion may be caused by progressive stenosis due to the development of atherosclerotic plaque.32 Diastolic dysfunction is usually thought to precede systolic function. The notion that diastolic dysfunction can exist in isolation, however, is a subject of debate.33,34 Because a reduced amount of ATP impairs relaxation and causes diastolic dysfunction, although it has not been studied, it is reasonable to assume that increasing the deficit of ATP will affect contraction as well. Studies using sensitive measures of systolic function, such as tissue Doppler imaging by echocardiography, have identified previously undetectable reduced systolic function in patients who exhibit diastolic dysfunction.33,35 Furthermore, it has also been proposed that diastolic filling is facilitated by the kinetic energy of the blood that enters from the pulmonary veins during ventricular systole.36,37 This would imply that diastolic dysfunction could be augmented by reduced function during systole.

Following the reduction of myocardial function, further ATP debt affects the ATP-dependent Na+/K+ pump in the cell membrane.38 Disturbances in depolarization and repolarization and, ultimately, failure to sustain the membrane potential can be observed as changes in the electrocardiogram (ECG). The accumulation of metabolites eventually leads to the development of chest pain, possibly due to the accumulation of adenosine.39 Finally, if ischemia persists long enough, the result is irreversible cell rupture and infarction.

In summary, the ischemic cascade (see Fig. 6–1) describes a sequence of physiologic observations that are involved in the development of dysfunctional but viable myocardium. However, subtypes of dysfunctional but viable myocardium can readily be identified based on unique combinations of physiologic characteristics and their appropriate treatment. Notably, accurate classification by necessity includes assessment of perfusion, function, and viability. By definition, it is not possible to accurately classify dysfunctional but viable myocardium from a pathophysiologic perspective if any one of these three physiologic characteristics is left out of an assessment. Dysfunctional but viable myocardium can be categorized into subtypes based on important differences with regard to presence or absence of compromised perfusion, function, cellular integrity, and need for revascularization. The characteristics of different categories of ischemically compromised myocardium are summarized in Table 6–1.

Stress-Induced Ischemia

Stress-induced ischemia is characterized by viable myocardium that has normal perfusion and function at rest, but where perfusion and function are compromised at stress. Stress leads to an increased oxygen demand that typically can be achieved by, for example, physical exertion, increased sympathetic discharge leading to increased heart rate, increased blood pressure, and increased wall tension.40 Stress can also be induced pharmacologically. Patients exhibiting stress-induced ischemia will benefit from revascularization, particularly those with a large extent of stress-induced ischemia.41

One should be aware that this definition of stress-induced ischemia represents a simplified definition that is presented for purposes of comparison. It should be acknowledged that situations may occur where the duration of stress-induced ischemia is short enough to only induce a reduction in perfusion, but not sufficient enough to induce a reduction in contractile function. Likewise, situations may occur where the duration or severity of ischemia due to stress may be sufficient to induce a prolonged but ultimately reversible reduction in function. The occurrence of prolonged resting dysfunction following the reversal of ischemia is called stunned myocardium.

Stunned Myocardium

Stunned myocardium is characterized by a prolonged post-ischemic reduction in function in the presence of normal perfusion and the absence of infarction.42 The classic definition of stunning stems from observations of a prolonged but reversible reduction in systolic function following successful restoration of perfusion in the setting of experimental occlusion and reperfusion43 and, later, acute occlusion and reperfusion in the clinical setting of ST-segment elevation myocardial infarction.44,45 In these settings, rest and stress perfusion will both have been restored to normal, and the observed reduction in function will spontaneously resolve with time. Therefore, it is not necessary to revascularize stunned myocardium according to this classical definition.

The cellular mechanisms governing stunning have not been completely elucidated. Dominant views include the influence of oxidant stress from reactive oxygen species, as well as disturbances in calcium homeostasis.46,47 There are, however, situations where stunning occurs as a result of stress-induced ischemia, and thus, these patients would require revascularization. Such scenarios can be referred to as repetitive stunning.

Repetitive Stunning

Reduced function at rest is an important part of the definition of dysfunctional but viable myocardium, including stunning. As discussed earlier, stress-induced ischemia of sufficient severity and duration may induce a reduction in function that persists until the next ischemic episode despite the return of normal resting perfusion between ischemic episodes. This has been observed in both experimental animals48 and patients with chest pain upon exertion.49 These patients have stress-induced ischemia, and if they are not revascularized, the risk of stunning is present whenever these patients are subjected to sufficient stress. Furthermore, such repetitive stunning cumulatively induces a greater reduction in postischemic resting function than one episode alone.50,51 In summary, myocardium exhibiting repetitive stunning is repeatedly stunned by stress-induced ischemia and should therefore be revascularized.

Hibernating Myocardium

Hibernating myocardium52 is defined as "a state of persistently impaired myocardial and left ventricular function at rest due to reduced coronary blood flow that can be partially or completely restored to normal if the myocardial oxygen supply/demand relationship is favorably altered, either by improving blood flow and/or by reducing demand."53 Thus, hibernating myocardium is characterized by a chronic reduction in resting function as an adaptation to reduced resting perfusion in the absence of infarction. Clinical studies have shown that myocardium with reduced function and perfusion at rest may regain function following improvement in resting perfusion by revascularization.54-58

The ultrastructural and histologic morphology of hibernating myocardium has been studied using transmural needle biopsies taken at the time of open heart surgery. In summary, the features include signs of atrophy, most notably in the contractile myofibrils, and signs of degeneration and possibly dedifferentiation, most notably in the interstitial space.59 The severity of interstitial fibrosis has been shown to increase with increased duration of the symptomatic ischemia.60 Also, the amount of myocytes with excess glycogen is exponentially related to the time required for functional recovery following revascularization.61 Importantly though, noninvasive assessment of changes in perfusion and metabolism cannot currently distinguish between myocardium of mild or severe histologic degeneration.62

Controversy exists regarding whether hibernating myocardium with reduction in both resting function and perfusion readily exists or whether the more common mechanism for resting dysfunction is repeated episodes of stress-induced ischemia due to a reduced coronary flow reserve leading to stunning in myocardium with otherwise normal resting perfusion.59

An experimental model of hibernation in swine sustained for 1 month has shown normal resting perfusion in one third of the volume of dysfunctional but viable myocardium.63 However, studies of patients with chronic IHD have shown normal resting perfusion in approximately 90% of regions of the left ventricle identified as having dysfunctional but viable myocardium.64,65 These clinical data support the notion that dysfunctional but viable myocardium is predominantly comprised of repetitive stunning. By comparison, a review of 26 studies comprising 372 patients undergoing quantitative assessment of resting myocardial blood flow in dysfunctional but viable myocardium showed that 49% of the patients were found to have significantly reduced resting perfusion in dysfunctional but viable myocardium compared with normal myocardium.59 Furthermore, others have identified hibernating myocardium in >20% of the left ventricle in as many as 30% to 40% of patients with IHD and a left ventricular ejection fraction ≤30%.66,67 It is possible that these differences in findings regarding the prevalence of hibernating myocardium reflect differences in patient selection criteria in these reports.

The notion that dysfunctional but viable myocardium with reduced resting perfusion does exist is supported by findings of serially assessed improvement in resting perfusion and function following revascularization.54-58 These data favor the concept that hibernating myocardium with some degree of reduced resting perfusion is a considerable component of dysfunctional but viable myocardium.

Of importance for the clinician, reduced perfusion at stress is prevalent regardless of whether the dysfunctional but viable myocardium is due to repetitive stunning or hibernation. Both situations merit revascularization. Reduced resting perfusion appears to be less prevalent in dysfunctional but viable myocardium but can nonetheless exist. Furthermore, it is likely that both hibernation and repetitive stunning may coexist in the same patient and even the same region of myocardium.68 This implies a downregulation in function as an adaptation to reduced resting perfusion, but also an exacerbation of the compromise in function and perfusion at stress. Hence, the exact differentiation between hibernation and repetitive stunning may be of limited clinical importance. The identification of dysfunctional but viable myocardium in need of revascularization, regardless of subtype, is of paramount importance.69

CMR Contrast Agents

Paramagnetic Contrast Agents

Despite the excellent soft tissue contrast shown by CMR, some situations may benefit from increased tissue contrast. The paramagnetic CMR contrast agents predominantly affect the image contrast by shortening the longitudinal relaxation time, T1, which is measured in milliseconds. The effects induced by magnetic resonance (MR) contrast agents are typically discussed in terms of 1/T1, called R1, the relaxation rate (ms–1), which is increased in the presence of paramagnetic contrast agents.70,71 Importantly, the contrast agent itself is not visualized by CMR. Rather, CMR visualizes the relaxation-altering effect that the paramagnetic agent exerts on the hydrogen protons in its immediate vicinity. This effect depends on the number of protons available to affect, the distance to these protons, and the rotational tumbling frequency of the water-particle complex72 and is related to the contrast agent concentration and its relaxivity (ie, how "good" the contrast agent is at affecting the protons) (Fig. 6–5). The relaxivity in vitro for the agents most often used today is approximately 4 s–1 mM–1 at 20 MHz and 37°C. The contrast agent concentration in a certain tissue depends on the pharmacokinetics of the contrast agent and tissue architecture. In vitro, the contrast agent concentration is considered to be linearly related to relaxivity. In vivo, however, this is limited by additional relaxation effects.73 The signal intensity in the image is therefore not necessarily linearly related to the relaxation rate. Contrast agent concentration in a tissue, however, is proportional to the change in R1 (ΔR1), defined as the difference between R1 before and after contrast administration. Thus, R1 and ΔR1 can be quantified by using a Look-Locker sequence74 or modified Look-Locker sequence for pixel mapping of R1.75 A Look-Locker sequence uses an inversion pulse followed by multiple small flip angle excitation pulses. Thereby the longitudinal relaxation rate of the tissue can be estimated. This may be used in order to determine contrast agent concentrations in different regions within the myocardium, such as infarcted and normal myocardium.

The most common paramagnetic agent used today is gadolinium (Gd), which has seven unpaired electrons and thus high relaxivity.72 Because Gd is toxic, it is chelated to a ligand, such as diethylene triamine pentaacetic acid (DTPA), 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA), or DTPA bismethyl amide (DTPA-BMA), in order to reduce toxicity.76 Gd bound to ligands such as those mentioned distributes in the extracellular space77 in the same way as inulin78 and acts mainly on R1.

Nephrogenic Systemic Fibrosis

Recently, some Gd-based contrast agents have been suggested to be related to a rare but severe disease called nephrogenic systemic fibrosis (NSF).79 The disease is typically characterized by fibrosis of the skin and connective tissues. It is recommended that estimated glomerular filtration rate (eGFR) be assessed in patients above age 60 and patients with hypertension, diabetes, or hepatorenal disease. In cases of eGFR <30 mL/min, it is recommended that the examination using Gd-based contrast agents be cancelled. For patients with eGFR <60 mL/min, it is recommended that the dose be reduced to 0.1 mmol/kg. Patients with hepatorenal syndrome and patients with reduced renal function who have had or are awaiting liver transplantation should be considered at risk of NSF if eGFR is <60 mL/min. It is proposed that the value of the examination should be considered and that contrast agent may be administered, even in patients with low eGFR. Finally, there are macrocyclic contrast agents that have not been associated with NSF, and one might argue that these agents should be used preferentially. The numbers of proven cases of NSF are low in total, and the incidence may be decreased further by using adequate dose and choice of agent to be administered and using postexamination dialysis in certain cases.80,81

Intravascular MR Contrast Agents

Intravascular MR contrast agents have been thought to overcome some of the limitations of extravascular contrast agents (eg, in angiography where the contrast stays in the vessel without extravasation). These agents have been tested for coronary artery MR imaging (MRI),82,83 infarct imaging,84-86 and determination of microvascular obstruction.87 The most promising applications are for angiography, but their clinical potential has not yet been fully explored.

Myocardium at Risk

T2-Weighted MRI

Both clinical and experimental studies of treatment for acute myocardial infarction are facilitated by accurate measurement of myocardial infarct size expressed as percentage of the myocardium at risk.88,89 The number of patients needed for a certain power is significantly lower when infarct size is normalized to myocardium at risk. The reference standard in clinical research for quantifying myocardium at risk is myocardial perfusion SPECT (MPS) (see Chapter 3). Quantifying myocardium at risk by MPS, however, requires that a radioactive tracer is injected prior to opening the occluded coronary artery. The tracers technetium 99m (99mTc) -sestamibi and 99mTc-tetrofosmin both distribute in the myocardium in proportion to blood flow90 and are taken up by the viable myocytes, probably by a potential-driven transport of the lipophilic cation91 and binding to the mitochondria. Very little change in myocardial distribution is seen over time. The tracer does not redistribute and has a half-life of 6 hours. Imaging can typically be performed up to 4 hours after injection.92 Importantly, because the myocardial uptake is fast and redistribution is minimal, image acquisition will represent the perfusion of the myocardium at the time of tracer injection. Thus, if the tracer is injected during occlusion, image acquisition undertaken following reperfusion will still represent the perfusion of the myocardium as it was during occlusion.93,94 However, there are issues with using MPS to quantify myocardium at risk, such as the logistics of radiotracer availability, need for administration before opening of the occluded coronary artery, and possible interference with post-PCI care when acquiring images. Considering these limitations with MPS imaging, other clinical methods to quantify myocardium at risk are warranted.

T2-weighted CMR has been demonstrated to show myocardium at risk in experimental studies95-97 (Fig. 6–6). Similar findings in human studies have suggested that T2-weighted imaging may be related to myocardium at risk.98-101 Recently, T2-weighted short tau inversion recovery (STIR) CMR imaging was directly validated in humans using MPS imaging as reference method and demonstrated that myocardium at risk can be quantified in humans at 1 day and at 1 week after the acute event.102 T2-STIR MRI is thus a promising tool for measuring myocardium at risk simultaneously with final infarct size 1 week after the acute event (Fig. 6–7).

The brightness in T2 images is believed to be mainly related to increased free-water content caused by edema, which prolongs T2 relaxation.103 This prolongation has been suggested to be related to the duration of ischemia.104 The increase in T2 following acute ischemia is relatively small. Thus, images may only show subtle signal intensity changes. As a result, surface coil intensity correction and through-plane motion need to be compensated for. Recent enhancements include black-blood T2-STIR,102,105 single-shot, bright-blood, T2-prepared steady-state free precession (SSFP) during free breathing,99 and hybrid T2 bright-blood turbo spin echo (TSE) SSFP techniques with high signal-to-noise ratio and contrast-to-noise ratio97 and single breath-hold T2 mapping.106 The bright-blood sequences may help solve the issue of difficulties differentiating slow-flowing blood from edema of the myocardial wall, whereas T2 mapping provides the ability for absolute quantification of tissue T2.

Contrast-Enhanced Ssfp MRI

In a recent study, quantification of myocardium at risk with contrast-enhanced SSFP MRI was demonstrated and validated using MPS imaging as reference method107 (Fig. 6–8).

Endocardial Extent of Infarction

It has been argued that the endocardial extent of infarction can be used as a surrogate measure of myocardium at risk.108,109 However, there are clinical situations with early reperfusion and a high degree of salvage or aborted infarction when the endocardial extent of infarction cannot be measured or is inaccurate.29 Thus, in these situations, salvage should not be calculated based on the endocardial extent of infarction because it is not a reliable quantitative measure of myocardium at risk (Fig. 6–9).

Viability and Infarct Imaging

Late Gd Enhancement MRI

In vivo and ex vivo experiments have demonstrated that infarct size can be accurately assessed using late Gd-enhanced (LGE) CMR (Figs. 6–10 and 6–11).110-112 Depiction of myocardial infarction relies on the pharmacokinetics and biodistribution of Gd chelate (ie, Gd-DTPA, Gd-DOTA, or Gd-DTPA-BMA). Importantly, the Gd chelates are small enough to readily pass across the vessel wall into the interstitial space but large enough that they will not pass across the cell wall. Thus, these Gd chelates are referred to as extracellular contrast agents. Healthy cells have intact cell membranes and exclude the tracers. Thus, extracellular MR contrast media distributes in proportion to the extracellular space.77 Animal experiments using echoplanar MR with isotope validation indicate that the extracellular space is approximately 20% in normal myocardium, 30% in myocardium at risk or salvaged myocardium, and 90% in necrosis where cells with ruptured sarcolemmas can no longer exclude the contrast agent15 (Fig. 6–12).

The concept of using paramagnetic agents for enhancement of image contrast was illustrated already by Bloch in 1946.70 The use of Gd-DTPA as a nonspecific contrast agent for CMR was discussed in 1984 by Weinmann et al,71 and the basics for using segmented inversion-recovery gradient-recalled echo imaging was described in 1990 by Edelman et al113 for enhanced image contrast originally for liver imaging. This was further developed by Simonetti et al114 who introduced the technique for imaging of myocardial infarction. Image contrast between the infarcted region and normal myocardium was further accentuated by both using an extracellular contrast agent and adjusting the image acquisition so that the inversion time (TI) was set to null the signal from normal myocardium. To reduce motion artifacts, the acquisition is synchronized to diastasis during the latter part of diastole in order to minimize the effect of cardiac movement on image quality. General motion artifacts are also reduced by adding gradient-moment refocusing. The normal myocardium shows up as black in a magnitude-based inversion-recovery image, because the TI is set to null the signal from normal myocardium. Because normal myocardium has a distribution volume of approximately 20%,77 the infarcted region with a distribution volume of approximately 90% will show as bright due to the increased relaxation rate related to the relatively higher contrast agent concentration in this region (Fig. 6–13). It should be noted that the contrast agents most often used distribute passively into the extracellular space and do not accumulate in or bind to the injured myocardium.115 Furthermore, in cases of acute occlusion, the ischemic region may still appear bright despite absence of contrast agent116 (Fig. 6–14). This is related to the use of magnitude images where a region lacking contrast agent will display an image intensity that is similar for reperfused infarction and nonreperfused ischemia. However, this ischemic region does not necessarily represent infarction because if this ischemic region is reperfused in a timely manner, a high degree of salvage can be expected. This potential pitfall may be avoided when imaging with a phase-sensitive inversion-recovery sequence, as described below.

Extracellular MR Contrast Dynamics

The enhancement of an infarcted region by LGE CMR is initially (during approximately the first 5 minutes after injection) related primarily to blood flow.117 After approximately 10 minutes, a pseudo–steady-state is reached (Fig. 6–15), and contrast agent concentration throughout the body declines with renal clearance and is fully cleared after approximately 24 hours in a person with normal renal function. At this pseudo–steady-state, extracellular MR contrast agents are distributed in body tissues according to their fractional tissue distribution volume (the fraction of the tissue volume in which the contrast agent can distribute).77 In the case of extracellular contrast agents, the fractional distribution volume corresponds to the extracellular space available in the compartment. For instance, normal myocardium has approximately 20% extracellular space (Fig. 6–16), whereas the blood pool has approximately 50% extracellular space (1-hematocrit). Acutely infarcted myocardium has undergone sarcolemmal rupture, and thus, the formerly intracellular space is exposed to and has become contiguous with the extracellular space, totaling approximately 90%.77 Moreover, the tissue subjected to ischemia and reperfusion is swollen due to edema for hours to days.

First-Pass Dynamics

The first pass of MR contrast agent is usually used to acquire short axis images for assessment of myocardial perfusion at rest and stress to determine presence of stress-induced ischemia (see later section "Myocardial Perfusion MRI").

An MR contrast agent that has been injected into a peripheral vein, usually the brachial vein, enters the right atrium from the superior vena cava, continues sequentially to the right ventricle and the lungs, and then appears in the left atrium, left ventricle, aorta, the coronary arteries, and the myocardium. If structures like thrombi or tumors are present anywhere in the great vessels or cardiac chambers, these exclude the MR contrast during the first pass and therefore appear dark. If they stay dark after the myocardium has enhanced, they are likely thrombi or other structures with low vascularization. If they enhance intermediately, they may be sparsely vascularized tumors like lipomas or fibromas. If they enhance to the same extent as myocardium or more, they may represent highly vascularized tumors such as sarcomas. First-pass images acquired in the four-chamber view may therefore be used to diagnose these conditions.

Extravasation of an extracellular MR contrast medium during the first pass through the capillary bed is high, up to 40%, which challenges imaging of vessels and direct quantification of myocardial perfusion (see earlier section "Intravascular MR Contrast Agents").

Early Images and Microvascular Obstruction

A dark core of microvascular obstruction may be seen in larger infarcts at first-pass perfusion, early Gd enhancement (EGE), and LGE MR imaging118 (Fig. 6–17). This core is visualized with a much slower initial rise of enhancement and is typically located at the subendocardial level. This hypoenhanced core usually persists for several minutes but starts to decrease in size after approximately 5 minutes. The optimal time to detect and quantify microvascular obstruction is therefore suggested to be approximately 2 to 4 minutes after contrast injection.119 The term microvascular obstruction comes from the severe capillary damage due to microemboli obstructing the microvasculature, endothelial damage, and myocardial inflammation. Administration of a GPIIb/IIIa inhibitor seems to increase blood flow on the microvascular level,7 and microvascular obstruction is seen to a lesser extent in patients receiving this treatment early after pain onset.

A CMR report should include information on the amount of microvascular obstruction because this determines outcome, predicts event-free survival,120 and is more common in larger infarcts.121 The region of microvascular obstruction is resorbed and heals to thin scar over time.122

Late Gd Enhancement

LGE CMR images are acquired for visualization and quantification of myocardial necrosis, microvascular obstruction, healed myocardial infarction (scar), and other types of fibrosis due to cardiomyopathies or myocarditis.

LGE CMR has emerged as a powerful tool for accurate and high-resolution assessment of myocardial viability.123 Validation studies have demonstrated the ability of LGE CMR to differentiate between viable and nonviable myocardium independent of wall motion, reperfusion status, or infarct age.110,124,125 Studies in humans have shown infarct transmurality to be predictive of recovery of regional function following acute infarction,126,127 elective revascularization,128-132 and β-blocker therapy.133 Infarct size by LGE CMR is prognostically significant independent of measures of systolic function.134 It has been shown that myocardial infarction by LGE CMR exceeding approximately one quarter of the left ventricle in the acute setting will predict negative remodeling (increased end-diastolic volume and reduced ejection fraction) over time.135

LGE CMR images are typically acquired 10 to 30 minutes after Gd-based contrast agent administration.136 The size of measured infarct on LGE CMR images does not change if images are acquired between 10 and 30 minutes after contrast agent injection (see "Extracellular MR Contrast Dynamics" earlier).136 Myocardial infarct size overestimation has been suggested when imaging is performed too early after Gd administration (<10 minutes), and therefore, this should be taken into consideration.

LGE CMR performed within the first 24 hours after an acute infarction may result in overestimation of infarct size in experimental137,138 and human135,139-142 studies. This might be ascribed to edema within the core of the infarction but may also be attributed to edema of the peri-infarction zone. The core of the infarction is known to shrink over the first 24 hours. Infarct size continues to decline over the first year142 (Fig. 6–18). The time line for when infarct size is no longer overestimated has not been fully evaluated, and the pathophysiologic background of enhancement and its overestimation of myocardial infarction size during the first week after infarction are not completely understood. Notably, LGE CMR accurately depicts infarct size in the setting of chronic fibrotic scar.110 Because LGE may be used both for acute and chronic infarction, a T2-weighted sequence sensitive for edema may be added to differentiate one from the other.98

Importance of Finding the Optimal Inversion Time

Usually, infarct size by LGE CMR is considered adequate for clinical applications if imaging is undertaken using the optimal TI for nulling the signal from normal myocardium. The optimal value of TI for nulling the signal from normal myocardium depends on the scanner manufacturer and sequence timing parameters, as well as contrast agent dose, time after contrast agent administration, and renal clearance. To facilitate finding the optimal TI for nulling signal from normal myocardium, a Look-Locker sequence may be applied74 (Fig. 6–19). In this sequence, the images are acquired as described earlier for determination of relaxation rate, and each image acquired is associated with the time after the inversion pulse. Thus, one can choose the TI when the signal from the normal myocardium is nulled in the image. To facilitate finding the optimal TI using a Look-Locker sequence, one needs to acquire representative images while choosing imaging parameters similar to those that are to be used for the subsequent viability sequence.

Phase-Sensitive Lge

The inversion-recovery sequence adapted by Simonetti et al114 was further developed by Kellman et al.143-145 They introduced a reconstructed image considering not only the magnitude, but also the phase of the magnetization during recovery. The phase-sensitive inversion-recovery (PSIR) sequence is therefore not so dependent on selecting the optimal TI for nulling the signal from normal myocardium (Fig. 6–20). The normal myocardium is presented as black in the image, and contrast/brightness can be adjusted for enhancing the image contrast further. The phase-sensitive reconstructed image is typically the image that is most useful for diagnosis. In cases with artifactual nulling of the signal from structures other than normal myocardium in the phase-sensitive reconstructed image, however, the standard magnitude image acquired simultaneously may be used for diagnosis. In such cases, this image is still dependent on the optimal TI for nulling the signal from normal myocardium. Therefore, the aim should be to select an appropriate TI to facilitate diagnosis, even when using a PSIR sequence.

In patients with arrhythmias, the standard magnitude or PSIR images may be difficult to interpret due to different timings of acquisition in the cardiac cycle resulting in motion artifacts. In the case of arrhythmias, it is also of value to image every other heart beat to allow recovery of the signal before the next inversion pulse.

In these situations, a single-shot SSFP sequence may also be applied.146 This yields higher image contrast due to decreased blurriness and less motion artifacts (Fig. 6–21). Moreover, this sequence can be acquired during free breathing with respiratory gating or with a very short breath-hold (1 second) and may therefore also be applied in patients who are not able to hold their breath for the duration of a typical breath-hold sequence (15 to 25 seconds).147 The downside of the single-shot SSFP sequence is a decreased sensitivity to detect small areas of fibrosis/infarct.

Relation of Lge CMR to Other Methods for Viability Assessment

An important challenge in the identification of dysfunctional but viable myocardium is the determination of myocardial viability, particularly in the setting of chronic IHD. Several noninvasive imaging techniques offer this possibility, and they include thallium-201 and 99mTc tracers used for MPS, 18-fluorodeoxyglucose (18-FDG) positron emission tomography (PET), dobutamine stress echo-cardiography, dobutamine stress CMR, and LGE CMR.

A comprehensive review of non-CMR techniques for viability assessment was undertaken for 105 studies of just over 3000 patients. In these studies, regional functional improvement following revascularization occurred in 53% of the evaluated segments that were identified as dysfunctional but viable prior to intervention. The overall mean weighted sensitivity and specificity of the techniques was 84% and 69%, respectively, for identification of regional functional improvement after revascularization.148 Furthermore, pooled data have highlighted the prognostic importance of viability testing by showing that patients with significant viability who were revascularized showed a greater than 50% reduction in cardiac event rate compared with both patients who were treated medically and patients without viability who were treated either medically or with revascularization.149

Human LGE CMR studies have demonstrated excellent repeatability137 and good agreement compared with 18-FDG PET,129,150,151 MPS,152-154 and myocardial contrast echocardiography.155 The assessment of infarct size by MPS, however, can both overestimate153 and underestimate152 infarct size by LGE CMR. Importantly, one study showed that 47% of subendocardial infarcts identified by LGE CMR were missed by MPS152; this is likely attributable to the limited resolution in MPS. In addition, LGE CMR can be used to definitively assess viability in patients with suspected attenuation artifacts by MPS.156 The spatial resolution in LGE CMR is much greater than in MPS and can resolve small periprocedural infarctions not detected by other techniques.157,158

Low-dose dobutamine stress CMR imaging involves an infusion of a low dose of dobutamine in order to assess whether or not an improvement can be seen in regional contractile function, as a measure of dysfunctional but viable myocardium. Improvement in regional function assessed by low-dose dobutamine stress CMR has shown close agreement with viability by PET159 and a sensitivity of 89% and specificity of 94% for predicting improvement in function following revascularization.160 Moreover, assessment using grid tagging161 and dobutamine has shown a similarly high sensitivity of 89% and specificity of 93% for predicting functional improvement after revascularization.162

Studies comparing low-dose dobutamine stress and LGE CMR have shown conflicting results. Two studies have shown similar or better results with LGE CMR compared with low-dose dobutamine when it comes to predicting functional recovery following revascularization.163,164 However, other studies have shown that contractile reserve with dobutamine is superior to LGE CMR for predicting functional recovery165 and may add information particularly regarding the potential for functional recovery of segments with intermediate infarct transmurality.166 Thus, the relative contributions of contractile reserve and viability by LGE CMR to functional recovery and prognostic benefit following revascularization are worthy of further study.167

In summary, LGE CMR has developed as the reference standard for assessment of transmurality and extent of infarction. It offers the advantages of increased spatial resolution over alternative techniques. In particular, LGE CMR is an imaging technique whereby nonviable infarction is imaged directly through an increased tissue distribution volume of contrast agent in nonviable tissue, as opposed to relying on the absence of signal or analysis of regional function as with nuclear and echocardiographic techniques, respectively.

Myocardial Perfusion MRI

The importance of assessing stress-induced ischemia in the detection of coronary artery disease is paramount. The data on stress perfusion testing are extensive in the setting of MPS, which is the clinically most prevalent imaging modality for myocardial ischemia stress testing. Numerous studies using MPS have shown that the risk of cardiac events increases with the extent and severity of stress perfusion defects assessed by MPS.41 Furthermore, the number of lives saved due to treatment with revascularization compared with medical treatment increases with the amount of ischemic myocardium identified by MPS.168

Pooled analysis of 79 studies (8964 patients) using MPS to detect coronary artery disease by invasive angiography showed a weighted mean sensitivity of 86% and specificity of 74%.31 Of patients with normal MPS studies, only those with a high suspicion for coronary artery disease are referred for invasive coronary angiography. Therefore, the relatively low specificity for MPS may reflect such a referral bias. The percentage of normal MPS studies in a population with a low likelihood of coronary artery disease is called the normalcy rate. Thus, normalcy is a better descriptor of the performance of a test in a population without disease. The normalcy rate for MPS in 10 studies (543 patients) was 89%.31

For CMR, stress by external work has limitations because it is awkward to attempt physical exertion when lying supine in an MR scanner. Importantly, this inevitably leads to motion artifacts from both breathing and bulk motion. Thus, image interpretation becomes difficult, and sensitivity and specificity of the examination during external work are low.

Pharmacologic stress can be undertaken using either a vasodilator agent, such as adenosine or dipyridamole, for assessment of perfusion or a β-agonist, such as dobutamine with or without the addition of atropine, for assessment of regional contractile function. In CMR, these agents are used in the same fashion as they are used in MPS and stress echocardiography.

Stress Perfusion with Adenosine or Dipyridamole

CMR has an emerging role in the assessment of perfusion. Quantitative analysis of first-pass contrast kinetics has been used to identify stenosis by invasive coronary angiography and has been shown to correspond to perfusion by PET169 and perfusion by microspheres.170 Stress perfusion CMR is hampered by a lack of standardization with regard to the most optimal sequence parameters for image acquisition. There is continued development in the field, with the ultimate goal of achieving a method that can provide quantitative pixel maps of the absolute blood flow in the myocardium in units of mL/min/g tissue, analogous to PET.171,172 In the absence of that level of methodologic maturity, CMR perfusion assessment is performed by visual analysis, which yields sensitivity in excess of 90% and specificity of approximately 70%.

Adenosine and dipyridamole have almost identical effects. Adenosine acts directly on vascular smooth muscle, causing nearly maximal vasodilatation, whereas dipyridamole prevents breakdown of adenosine that is produced naturally in tissue. The basic principle of vasodilator stress perfusion CMR is that the vasodilator causes an up to five-fold increase in blood flow to healthy myocardium, whereas myocardium subtended by a stenotic artery will have an impeded ability to upregulate blood flow and thus exhibits a relatively delayed arrival of the first pass of an intravenously administered bolus of a contrast agent. This delayed arrival is visualized as a hypointense region corresponding to the distribution of a coronary artery (Fig. 6–22).

A multicenter multivendor trial comparing MPS and CMR stress perfusion found that CMR outperformed MPS with regard to overall diagnostic accuracy.169 However, that study had several limitations in design. Most notably, gated MPS was not used in the MPS analysis, despite it being known that gated MPS adds significantly to the diagnostic accuracy of MPS.173-175 Despite the challenges with CMR stress perfusion imaging, prognostic studies have begun to be published that show similar results to MPS. Importantly, a positive CMR stress perfusion examination performed on patients being evaluated for unclear chest pain in the emergency department is independently associated with poor prognosis.176 By comparison, a normal CMR stress perfusion study is associated with a greater than 99% probability of 3-year event-free survival.176-179 In addition, the presence of a reversible perfusion defect by CMR is prognostically poor in a way that is complementary to the prognostic value of LGE CMR for identifying myocardial infarction.180

Adenosine infusion is safe.181 During the stress study, patients should be monitored regarding heart rate and rhythm, blood pressure, and symptoms. Adenosine/dipyridamole should not be used in patients with poor pulmonary function or atrioventricular block IIa or higher due to the risk of adenosine-induced complete atrioventricular block.

Stress Function with High-Dose Dobutamine

The results from dobutamine stress CMR are reproducible, and high levels of specificity and sensitivity have been shown (~80%-90%).182 Imaging is undertaken at rest and during increased stress levels. Because dobutamine and atropine are used for increasing contractility, they are primarily useful for assessment of abnormalities of myocardial contraction177,182,183 (Fig. 6–23).

The administration of dobutamine usually follows a scheme with an increased body weight–adjusted infusion rate over time. High-dose dobutamine increases myocardial oxygen demand, and thus, myocardial regions supplied by a stenotic coronary artery typically develop abnormalities in myocardial contractility due to inadequate ability to upregulate the supply of oxygenated blood. The high-dose regimen is needed to increase sensitivity and specificity for visualizing wall motion abnormalities.

Adverse effects are rare184; however, patients should be monitored regarding heart rate and rhythm, blood pressure, wall motion abnormalities, and symptoms. Because wall motion abnormalities precede ST-segment changes in the ischemic cascade, and wall motion may be visualized during scanning, a standard single-lead ECG, as available at the scanner, is adequate for rhythm monitoring. However, the dobutamine regimen should not be used in patients with severe hypertension, significant aortic stenosis, unstable angina pectoris, or other debilitating systemic disease.

Images are acquired during rest and increased pharmacologic stress. To image the contractility defects, fast imaging is required. Either SSFP or real-time images are used, often as a short breath-hold sequence of approximately 5 seconds acquiring approximately 30 to 40 images per heartbeat with a spatial resolution of 1.5 × 1.5 mm. When using real-time imaging, the wall motion abnormalities can be visualized during the examination and thus can be used for monitoring the patient.185 However, the real-time images have lower spatial and temporal resolution than breath-hold images, and thus breath-hold images are preferred. Improved parallel real-time imaging may enhance image quality despite faster acquisition and thus facilitate evaluation of wall motion, while also shortening the scan time.186 Similar to CMR stress perfusion, a normal CMR high-dose dobutamine stress study is associated with a greater than 99% probability of 3-year event-free survival.177

Sensitivity of the examination may be increased by adding tagging prepulses to the acquisition.187 The tagging prepulses are applied perpendicular to the imaging plane and appear in the images as dark lines. Thus, local tissue deformation and contractility may be visualized and assessed. Recent developments include other tissue tracking methods such as displacement encoding with stimulated echoes (DENSE),188 harmonic phase imaging (HARP),189 and strain-encoded CMR.190 The dysfunctional region by strain-encoded CMR has been shown to correspond to region of infarction by LGE CMR.191 Furthermore, phase-contrast strain measurements can be performed for assessment of regional function.192 However, these acquisitions are currently too time consuming to be used for monitoring the patient during stress imaging.

To evaluate the images for diagnosis, the cine loops of the left ventricle are usually analyzed in 17 segments according to the American Heart Association.193 Because these segments are based on a standard coronary anatomy, one should be aware that the segments shown may be supplied by other coronary arteries than expected. In particular, there is considerable overlap between the perfusion territories subtended by the left circumflex and right coronary arteries.194 Collateral vessels also may contribute to variations from typical patterns. Thus, it is not possible to define which coronary artery has a stenosis or occlusion based on wall motion alone. The images in each segment are usually first graded by quality as good, acceptable, or poor. The wall motion is graded as normal, hypokinetic, akinetic, or dyskinetic. The wall motion score is thereafter divided by the number of diagnostic segments. The findings are considered to be diagnostic for ischemia either if wall motion is not increasing during increased stress or if systolic wall thickening is lacking. A reduction in wall motion or wall thickening is also considered to be indicative of ischemia.

Noninvasive Coronary Angiography

CMR can be used to perform noninvasive coronary angiography analogously to computed tomography (CT). A recent comprehensive meta-analysis confirmed that CT with 16-slice detectors or more shows a significantly better sensitivity and specificity for significant stenosis compared with CMR, although only five head-to-head studies exist.195 One head-to-head study showed that CT had a greater number of evaluable coronary artery segments compared with CMR.196 Interestingly, CMR was able to provide an accurate diagnosis in two thirds of the cases in which coronary segments were unevaluable by CT due to extensive calcification. CMR coronary angiography has also been shown to be able to quantify an increased coronary artery wall thickness in patients with nonsignificant coronary artery disease compared with controls.197

A whole-heart SSFP coronary artery MR angiography was described in 2003 that enables coronary imaging in three dimensions without the use of MR contrast agent198 (Fig 6–24). This sequence has been used in clinical trials with promising results.199,200

Advantages with CMR include the lack of radiation exposure, but drawbacks include longer examination time and lower spatial resolution, in addition to the lower diagnostic accuracy. Evaluation of suspected anomalous coronary artery anatomy is possibly the strongest indication for CMR coronary angiography.201

CMR is complementary to and often provides incremental value over other imaging modalities for assessment of IHD. Of specific and proven value is LGE CMR for determination of viability and scar. In combination with stress perfusion imaging and cine imaging for function, CMR is the only modality that can integrate assessment of function, perfusion, and viability in one session, which is necessary for a comprehensive evaluation of viable but dysfunctional myocardium. The recent introduction of assessments for myocardium at risk adds to CMR's versatility. CMR is usually less observer dependent compared with other competing modalities and does not use radiation, and most applications are noninvasive. CMR is as close to a one-stop shop for ischemic heart disease as a modality can get.