Review ArticleMyocardial perfusion imaging in the acute care setting: Does it still have a role?
Introduction
Patients with possible acute coronary syndrome (ACS) are a common Emergency Department (ED) problem, accounting for approximately 10% of all ED visits.1 Initial risk stratification hinges on the initial ECG, clinical presentation, and a set of cardiac biomarkers that includes troponin.1 In patients who are hemodynamically stable, the presence of ischemic ECG changes, either elevation or depression, identifies a high-risk group who require admission and early aggressive care with anti-platelet and anti-thrombotic treatment, with the addition of early reperfusion with those who have ST elevation. In the absence of ischemic ECG changes, patients with a history of coronary disease represent an intermittent risk group.2 The group of patients who remain, who have a non-ischemic ECG and no history of coronary disease, make up the majority of all ED chest pain patients. Although risk of an ACS is relatively low, it is not negligible. Many of the risk stratification protocols and strategies that have been developed are targeted to accurately identify those few high-risk patients in this group.
To accelerate the evaluation process, numerous studies have incorporated various types of imaging into chest pain evaluation protocols. Imaging has various potential advantages when applied to this population. By accurately identifying high-risk patients, initiation of appropriate pharmacotherapy and admission is accelerated. In low-risk patients, the frequency of unnecessary admissions is reduced. By accurately differentiating between high- and low-risk patients, the probability of inappropriate disposition is decreased. As a result, costs should be reduced through increased overall efficiency.
Utilization of acute imaging for identifying ED patients who have ACS is not a new concept. In the 1970s, Wackers et al3 used thallium-201 imaging to evaluate 203 patients who presented with presumed ACS. All patients who had myocardial infarction (MI) had abnormal MPI, whereas the majority of those who had atypical chest pain or stable angina had normal MPI, with a minority having equivocal MPI; none had rest MPI interpreted as abnormal.
Despite this3 and other promising studies,4 the use of acute thallium-201 MPI for evaluating chest pain patients was not adapted widely for various reasons. First, acute imaging with thallium-201 is logistically difficult. Thallium-201 begins to redistribute shortly after myocardial uptake; therefore, imaging must be performed shortly after injection to obtain images reflecting the acute presentation. The isotope characteristics of thallium-201 in combination with the lower sensitivity of planar imaging have limited image quality.
In the 1990s, the development of the 99mTc isotopes, sestamibi, and tetrofosmin led to a renewed interest in using imaging for acute evaluation of chest pain patients. Technetium isotopes have the advantage in that they do not redistribute.5 Therefore, patients can be injected during symptoms and imaged later after stabilization. The images that are obtained at that time reflect the ischemic state and coronary blood flow at the time of isotope injection. The favorable energy and dosimetry of 99mTc also allows gated acquisition and reconstruction of dynamic functional images,6 thereby providing simultaneous assessment of both regional and global ventricular functions that can be correlated with perfusion defects.7 By using computer algorithms, ejection fraction can be quantitatively obtained.8
The value of incorporating gating into acute MPI interpretation has been under-recognized. Kontos et al9 analyzed outcomes from 2,826 consecutive patients admitted after undergoing acute rest MPI. Overall images were interpreted as abnormal in 40% and normal in 32%. Patients who had perfusion defects but with normal wall motion (27%) were considered negative for ACS and classified as non-acute MPI. Event rates were similar and not significantly different for those with non-acute images compared to those who had normal images using a variety of end points, including CK-MB MI, troponin MI, revascularization alone, or the combination of revascularization and MI (Figure 1). Patients who had either non-acute or normal images had a significantly lower event rate than those who had acute images. In another study, regional wall motion on SPECT had significantly more incremental diagnostic value than did perfusion in 163 patients undergoing an ED evaluation for possible myocardial ischemia.10
Section snippets
Clinical Studies
Preliminary studies in the early 1990s indicated the feasibility of acute imaging.11,12 One of the first studies to evaluate ED chest pain patients was performed by Hilton et al13 in which 102 ED patients who presented with a non-ischemic ECG underwent acute MPI. Patients were classified as low, intermediate, or high risk based on clinical characteristics. Patients who had abnormal MPI had a significantly higher cardiac event rate (71%) compared to those with either equivocal (13%) or normal
Limitations of Acute MPI
Using acute MPI has a number of limitations. First, a perfusion defect can result from acute ischemia, acute infarction, or old infarction. Differentiating between new and old infarctions can be performed by measuring serial cardiac markers. Differentiating between acute ischemia and old infarction can be more difficult, and is often based on the patient’s presenting symptoms and history. If further differentiation is necessary, repeat imaging in a symptom-free state can be performed. If the
Comparison with Computed Tomographic Angiography
Recently, computed tomographic angiography (CTA) has emerged as a potential useful test for evaluating ED chest pain patients (387). Table 2 compares the two imaging techniques. Both techniques are likely to be available at large volume centers. Although imaging time is significantly shorter for CTA (s) compared to MPI (min), when taking into account for patient preparation time, the differences are likely to be small. As use of newer solid-state imaging cameras becomes more common (imaging
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Adapted from a presentation at American Society of Nuclear Cardiology, Philadelphia, PA, September 2010.