Magnetic resonance imaging (MRI) for the assessment of myocardial viability: an evidence-based analysis.
In July 2009, the Medical Advisory Secretariat (MAS) began work on Non-Invasive Cardiac Imaging Technologies for the Assessment of Myocardial Viability, an evidence-based review of the literature surrounding different cardiac imaging modalities to ensure that appropriate technologies are accessed by patients undergoing viability assessment. This project came about when the Health Services Branch at the Ministry of Health and Long-Term Care asked MAS to provide an evidentiary platform on effectiveness and cost-effectiveness of noninvasive cardiac imaging modalities.After an initial review of the strategy and consultation with experts, MAS identified five key non-invasive cardiac imaging technologies that can be used for the assessment of myocardial viability: positron emission tomography, cardiac magnetic resonance imaging, dobutamine echocardiography, and dobutamine echocardiography with contrast, and single photon emission computed tomography.A 2005 review conducted by MAS determined that positron emission tomography was more sensitivity than dobutamine echocardiography and single photon emission tomography and dominated the other imaging modalities from a cost-effective standpoint. However, there was inadequate evidence to compare positron emission tomography and cardiac magnetic resonance imaging. Thus, this report focuses on this comparison only. For both technologies, an economic analysis was also completed.A summary decision analytic model was then developed to encapsulate the data from each of these reports (available on the OHTAC and MAS website).The Non-Invasive Cardiac Imaging Technologies for the Assessment of Myocardial Viability is made up of the following reports, which can be publicly accessed at the MAS website at: www.health.gov.on.ca/mas or at www.health.gov.on.ca/english/providers/program/mas/mas_about.htmlPOSITRON EMISSION TOMOGRAPHY FOR THE ASSESSMENT OF MYOCARDIAL VIABILITY: An Evidence-Based AnalysisMAGNETIC RESONANCE IMAGING FOR THE ASSESSMENT OF MYOCARDIAL VIABILITY: An Evidence-Based Analysis


OBJECTIVE
The objective of this analysis is to assess the effectiveness and cost-effectiveness of cardiovascular magnetic resonance imaging (cardiac MRI) for the assessment of myocardial viability. To evaluate the effectiveness of cardiac MRI viability imaging, the following outcomes were examined: the diagnostic accuracy in predicting functional recovery and the impact of cardiac MRI viability imaging on prognosis (mortality and other patient outcomes).


OBJECTIVE
CONDITION AND TARGET POPULATION  LEFT VENTRICULAR SYSTOLIC DYSFUNCTION AND HEART FAILURE: Heart failure is a complex syndrome characterized by the heart's inability to maintain adequate blood circulation through the body leading to multiorgan abnormalities and, eventually, death. Patients with heart failure experience poor functional capacity, decreased quality of life, and increased risk of morbidity and mortality. In 2005, more than 71,000 Canadians died from cardiovascular disease, of which, 54% were due to ischemic heart disease. Left ventricular (LV) systolic dysfunction due to coronary artery disease (CAD) () is the primary cause of heart failure accounting for more than 70% of cases. The prevalence of heart failure was estimated at one percent of the Canadian population in 1989. Since then, the increase in the older population has undoubtedly resulted in a substantial increase in cases. Heart failure is associated with a poor prognosis: one-year mortality rates were 32.9% and 31.1% for men and women, respectively in Ontario between 1996 and 1997.
IN GENERAL, THERE ARE THREE OPTIONS FOR THE TREATMENT OF HEART FAILURE: medical treatment, heart transplantation, and revascularization for those with CAD as the underlying cause. Concerning medical treatment, despite recent advances, mortality remains high among treated patients, while, heart transplantation is affected by the limited availability of donor hearts and consequently has long waiting lists. The third option, revascularization, is used to restore the flow of blood to the heart via coronary artery bypass grafting (CABG) or, in some cases, through minimally invasive percutaneous coronary interventions (balloon angioplasty and stenting). Both methods, however, are associated with important perioperative risks including mortality, so it is essential to properly select patients for this procedure.
Left ventricular dysfunction may be permanent, due to the formation of myocardial scar, or it may be reversible after revascularization. Reversible LV dysfunction occurs when the myocardium is viable but dysfunctional (reduced contractility). Since only patients with dysfunctional but viable myocardium benefit from revascularization, the identification and quantification of the extent of myocardial viability is an important part of the work-up of patients with heart failure when determining the most appropriate treatment path. Various non-invasive cardiac imaging modalities can be used to assess patients in whom determination of viability is an important clinical issue, specifically: dobutamine echocardiography (echo),stress echo with contrast,SPECT using either technetium or thallium,cardiac magnetic resonance imaging (cardiac MRI), andpositron emission tomography (PET).
Stress echocardiography can be used to detect viable myocardium. During the infusion of low dose dobutamine (5 - 10 µg/kg/min), an improvement of contractility in hypokinetic and akentic segments is indicative of the presence of viable myocardium. Alternatively, a low-high dose dobutamine protocol can be used in which a biphasic response characterized by improved contractile function during the low-dose infusion followed by a deterioration in contractility due to stress induced ischemia during the high dose dobutamine infusion (dobutamine dose up to 40 ug/kg/min) represents viable tissue. Newer techniques including echocardiography using contrast agents, harmonic imaging, and power doppler imaging may help to improve the diagnostic accuracy of echocardiographic assessment of myocardial viability.
Intravenous contrast agents, which are high molecular weight inert gas microbubbles that act like red blood cells in the vascular space, can be used during echocardiography to assess myocardial viability. These agents allow for the assessment of myocardial blood flow (perfusion) and contractile function (as described above), as well as the simultaneous assessment of perfusion to make it possible to distinguish between stunned and hibernating myocardium.  SPECT: SPECT can be performed using thallium-201 (Tl-201), a potassium analogue, or technetium-99 m labelled tracers. When Tl-201 is injected intravenously into a patient, it is taken up by the myocardial cells through regional perfusion, and Tl-201 is retained in the cell due to sodium/potassium ATPase pumps in the myocyte membrane. The stress-redistribution-reinjection protocol involves three sets of images. The first two image sets (taken immediately after stress and then three to four hours after stress) identify perfusion defects that may represent scar tissue or viable tissue that is severely hypoperfused. The third set of images is taken a few minutes after the re-injection of Tl-201 and after the second set of images is completed. These re-injection images identify viable tissue if the defects exhibit significant fill-in (> 10% increase in tracer uptake) on the re-injection images. The other common Tl-201 viability imaging protocol, rest-redistribution, involves SPECT imaging performed at rest five minutes after Tl-201 is injected and again three to four hours later. Viable tissue is identified if the delayed images exhibit significant fill-in of defects identified in the initial scans (> 10% increase in uptake) or if defects are fixed but the tracer activity is greater than 50%. There are two technetium-99 m tracers: sestamibi (MIBI) and tetrofosmin. The uptake and retention of these tracers is dependent on regional perfusion and the integrity of cellular membranes. Viability is assessed using one set of images at rest and is defined by segments with tracer activity greater than 50%.
Positron emission tomography (PET) is a nuclear medicine technique used to image tissues based on the distinct ways in which normal and abnormal tissues metabolize positron-emitting radionuclides. Radionuclides are radioactive analogs of common physiological substrates such as sugars, amino acids, and free fatty acids that are used by the body. The only licensed radionuclide used in PET imaging for viability assessment is F-18 fluorodeoxyglucose (FDG). During a PET scan, the radionuclides are injected into the body and as they decay, they emit positively charged particles (positrons) that travel several millimetres into tissue and collide with orbiting electrons. This collision results in annihilation where the combined mass of the positron and electron is converted into energy in the form of two 511 keV gamma rays, which are then emitted in opposite directions (180 degrees) and captured by an external array of detector elements in the PET gantry. Computer software is then used to convert the radiation emission into images. The system is set up so that it only detects coincident gamma rays that arrive at the detectors within a predefined temporal window, while single photons arriving without a pair or outside the temporal window do not active the detector. This allows for increased spatial and contrast resolution.
Cardiac magnetic resonance imaging (cardiac MRI) is a non-invasive, x-ray free technique that uses a powerful magnetic field, radio frequency pulses, and a computer to produce detailed images of the structure and function of the heart. (ABSTRACT TRUNCATED)