Original ArticleA second look with prone SPECT myocardial perfusion imaging reduces the need for angiography in patients at low risk for cardiac death or MI
Introduction
Mortality due to coronary artery disease is declining, in part because of improved detection and treatment.1 Single-photon emission computed tomography myocardial perfusion imaging (SPECT-MPI) is useful for identification of patients with obstructive coronary stenoses, for whom treatment may offer relief of symptoms or prolongation of life.2
In the setting of carefully controlled prospective studies, SPECT-MPI can yield diagnostic sensitivities and specificities of ~85% for properly selected patients with suspected coronary artery disease.3, 4, 5, 6 SPECT-MPI may also be used to stratify patients according to risk for cardiac death.7, 8, 9 Few data are available, however, to evaluate the performance of SPECT-MPI as applied in broad clinical practice. Pooled data indicate that the majority of patients who undergo elective invasive coronary angiography, which is usually preceded by non-invasive testing, do not have physiologically important coronary artery stenoses.10 The findings suggest an opportunity to reduce exposure to the risks and expense of invasive coronary angiography by improving the diagnostic accuracy of SPECT-MPI in broad clinical practice.
Signal attenuation by non-cardiac soft tissue can produce artifactual appearance of perfusion abnormalities in SPECT-MPI. A number of techniques have been introduced to minimize soft tissue artifacts.11 Integration of signal data acquired during exposure to an external radiation source identifies attenuation inhomogeneities, correction for which can improve diagnostic specificity.12,13 x-ray computed tomography has been employed for the same purpose, and produces improved diagnostic performance in some,14 but not all settings.12,15 Image acquisition in the prone position, which can be considered a “second look”, in addition to supine acquisition, can reduce the number of false-positive SPECT-MPI studies13,16, 17, 18, 19, 20 without reducing the prognostic accuracy for cardiac death.21 It is difficult to determine, however, the degree to which any of those techniques influence clinical decision-making, specifically the pivotal decision to proceed with invasive coronary angiography.
Attenuation correction techniques are not widely utilized for SPECT-MPI. A survey of centers in Europe found that less than one quarter routinely apply any method of attenuation correction for SPECT-MPI.22 Utilization rates for attenuation correction are probably lower in the United States.23,24 Fewer than 20% of instruments which utilize newer cadmium-zinc-telluride (CZT) scintillation detectors are purchased with a proprietary option that allows registration of x-ray computed tomography image data for the purpose of attenuation correction. (Personal communication, Christopher Porter, GE Health Care, Waukesha, WI.)
In early 2012, our center replaced existing equipment with a CZT SPECT system. At that time, we implemented a protocol which acquires stress images in the supine position, and also acquires a “second look” stress image set in the prone position for all patients. The aim of this study was to determine the relationship between SPECT-MPI findings and clinical outcomes in the first 748 of those patients.
Section snippets
Methods
All studies were approved by the Institutional Review Board at the University of Iowa. For the purposes of this retrospective study, the requirement for written informed consent was waived.
Results
Baseline patient characteristics are shown in Table 1. The small subset of patients who were lost to clinical follow-up were similar, but less likely to have diabetes than the study group as a whole (Supplemental Table S1). The primary indication for SPECT-MPI was chest pain in 58%, dyspnea in 10%, pre-operative risk stratification prior to non-cardiac surgery in 28%, and for other or undetermined indications in 4%.
Discussion
The most important finding of this study is that patients who had apparent perfusion abnormalities during supine stress SPECT-MPI, which resolved during second look prone imaging, were at very low risk for cardiac death or myocardial infarction during medium term follow-up. Critically, patients in the Normal-Prone group had utilization rates for invasive coronary angiography and coronary revascularization that were comparable to patients with Normal SPECT-MPI. Our findings address a fundamental
New Knowledge Gained
Patients with apparent perfusion abnormalities found on supine stress SPECT-MPI, which resolved during second look prone imaging, were at very low risk for cardiac death or myocardial infarction during medium term follow-up, comparable to those with no perfusion abnormalities. In addition, the Normal-Prone group had utilization rates for invasive coronary angiography and coronary revascularization that were comparable to patients with Normal SPECT-MPI.
Limitations
Our findings reflect both the disadvantages and the advantages of a retrospective study. Our study commenced after all SPECT-MPI interpretations, clinical decisions, and specified endpoints had occurred. Clinical outcomes, particularly the decision to refer patients for invasive coronary angiography, were thus subject to individual clinician preference and expertise, without protocol guidance. The SPECT-MPI results reported here are the same results that were available to referring physicians.
Conclusions
Patients with apparent perfusion abnormalities during stress supine SPECT-MPI, which resolve during second look prone imaging, have rates of cardiac death or myocardial infarction which are comparable to patients with no apparent perfusion abnormalities, during medium term follow-up. Excellent prognosis in those patients is achieved with a very low rate of utilization of invasive coronary angiography after SPECT-MPI. Application of second look prone imaging for the purpose of attenuation
Disclosures
Dr Worden, Dr Lindower, Dr Burns, Dr Chatterjee, and Dr Weiss declare no conflict of interest.
Funding
none.
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