Key points
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MPI has been extensively studied to examine the effects of interventions or changes using well-validated techniques.
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Variations in imaging protocol, tracer dose, processing, and quality could contribute to variability in the presence, type, and severity of perfusion abnormalities.
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Side-by-side visual comparison and the use of supervised automated methods are helpful in serial testing to study improvement/worsening in regional and global indices of MPI.
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Serial MPI can be helpful in determining the success and or complications of PCI and CABG and in the evaluation of patients with recurrent or new symptoms.
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Serial MPI can be used to compare the effects of a treatment or intervention versus an alternative or placebo.
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Serial MPI can be used to assess the effect of a new treatment in patients with conditions other than CAD, such as alcohol septal ablation in patients with hypertrophic cardiomyopathy.
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Serial MPI studies can be used to evaluate the effectiveness of optimum medical therapy in suppressing ischemia.
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Serial MPI can be used to determine the effectiveness of newer forms of treatment for angina pectoris, such as angiogenesis or stem cell therapy.
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Serial MPI studies can be used to compare an already approved tracer or stress agent with a new and, as yet, unapproved tracer or stress agent.
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Serial gated blood pool imaging for the evaluation of LVEF has been extensively used to monitor patients receiving chemotherapy
Background
Gated SPECT MPI is an established method for the assessment of myocardial perfusion and LVEF. In patient care and research, there is a need for serial testing either because of a change in the clinical presentation, because of a desire to examine the effects of a variety of treatments or interventions, or the introduction of newer imaging tracers or stress agents. The comparison of two or more serial studies is anything but straightforward. The reader needs to decide whether there is an improvement or worsening in extent, severity, location, and type (reversible or fixed) of perfusion abnormality. For example, there could be an improvement in one region and a worsening in another region and yet the global score remains unchanged.
Guidelines by the American Society of Nuclear Cardiology endorse the adoption of a model in which the LV is represented by 17 segments to localize the perfusion defect and define its extent as the number of myocardial segments involved. This is usually combined with the use of a scoring system to assess the activity level in each segment (e.g., 0 = normal and 4 = absent). The summed scores (summed rest score, summed stress score, and summed difference score) therefore represent extent and severity (but not location). Alternatively, polar maps can define the extent, severity, and reversibility of the perfusion abnormality (expressed as percent of LV myocardium or as a percent of the vascular territory). As mentioned in Chapter 2 , these scores could be obtained by visual interpretation or by automated programs. It should be noted that any differences between the two sets of images could be real, due to technical problems, or because of variability (intraindividual and interindividual) in the interpretation of serial images. Therefore, all acquisition and processing parameters should be standardized, image artifacts minimized, and image quality enhanced as much as possible. Image quality is probably the single most important variable that impacts the precision of the comparison.
For clinical applications, serial studies should be compared side-by-side using both visual and automated analysis. For research purposes, side-by-side analysis is as important as long as the readers are blinded to the sequence of the images. Although most of the above discussion focused on the perfusion pattern, attention to gating is also very important to assess improvement or deterioration in LV function.
Here are a few of the many questions that should be answered in comparing two or more sets of images:
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Are the images normal or abnormal?
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Do the images show fixed or reversible defects (or both)?
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If present, what is the number of abnormal segments in each?
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If present, what is the number of reversible segments in each?
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What is the rate of agreement based on summed stress score or summed difference score?
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What is the rate of agreement based on polar maps for total abnormality or reversible abnormality?
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What are the rates of agreement in regional perfusion?
Generally, concordance is highest for simple comparisons (such as normal vs. abnormal) and concordance is highest when most images are normal.
The following cases will address some of the salient features of serial testing:
Case 7-1
Progression of Disease ( Figure 7-1 )
A 71-year-old woman with a history of DM, paroxysmal atrial fibrillation, HTN, and hyperlipidemia presented with atypical chest pains of several weeks’ duration. Due to the presence of rate-dependent left bundle branch block (LBBB) ( Figure 7-1, A ) she underwent adenosine MPI, which showed a normal perfusion pattern and LVEF ( Figure 7-1, B ). She was managed medically with risk factor modification. Approximately 3.5 years later, she sustained a traumatic injury to the left knee and underwent regadenoson MPI before planned major surgery. The images showed a large area of ischemia with normal LVEF ( Figure 7-1, C ). On coronary angiography she had severe stenosis of the proximal LAD, which was treated with a bare metal stent. She underwent total knee replacement 3 months later without incident.
Comments
The comparison of the serial MPI images in this patient is arguably an easy task because the earlier images were normal. Nevertheless, the recent images should always be examined side-by-side with the earlier images and not be based on the report alone. This practice is important in maintaining quality and consistency in interpretation. The earlier study also serves the useful purpose of defining the normal variation in tracer concentration in different myocardial segments, which is very valuable in determining the degree of reversibility in the current study.
Case 7-2
Serial Testing for a Change in Symptoms ( Figure 7-2 )
A 56-year-old woman, known to have mitral valve prolapse, presented to her primary care physician with atypical chest pain of several months duration that are not associated with exertion. Her comorbidities include family history of premature CAD, HTN, and hyperlipidemia. The baseline ECG showed right bundle branch block. She ran on a treadmill for 7.5 minutes, reaching a heart rate of 150 bpm with no ischemic ST changes. The MPI showed normal perfusion and normal LVEF ( Figure 7-2, A ). Five years later, she presented with left-sided chest pain that woke her from sleep and was described as being different from her previous episodes. She was seen in the emergency room 2 hours later but the pain had subsided by this time and biomarkers were normal. She underwent exercise MPI 2 days later and had no chest pain or ECG changes after 6.5 minutes of exercise achieving 80% of maximum predicted heart rate. The gated SPECT MPI again showed essentially normal perfusion (except for apical attenuation) and normal LVEF ( Figure 7-2, B ).
Comments
The evaluation of atypical symptoms can be difficult, especially in women in whom typical angina occurs much less frequently than in men. Considering the change in symptoms, risk factors, and 5 years since the last stress test, a repeat stress MPI was appropriate for this patient. Again, the two sets of images needed to be evaluated side-by-side with special attention to breast location and body habitus. In lean tall women with mitral valve prolapse, attenuation artifacts could be due to the right ventricle (affecting the septum and inferior wall) as well as the breasts (affecting the anterior wall).
Case 7-3
Serial Testing in a Patient With Known CAD and New Symptoms ( Figure 7-3 )
A 48-year-old man with prior anterior myocardial infarction underwent an exercise MPI 3 weeks after failed thrombolysis. Coronary angiography showed occluded LAD, which was collateralized from the RCA. The LCX had 50% diameter stenosis. He had HTN, COPD, hyperlipidemia, and gastroesophageal reflux symptoms. He was an active smoker. He did well with medical therapy. Two years later, he had one episode of prolonged lower substernal pain, which abated after a sublingual nitroglycerin tablet and antacids. He had a repeat stress test. The images from both tests are shown in Figure 7-3, A, B .
Comments
Both sets of images reveal a large scar (fixed defect) in the distribution of the LAD. Myocardial infarct size can be quantified with MPI. The infarct size has prognostic implications since patients with larger infarcts have a high mortality and the size of the infarct is also associated with a lower LVEF and a larger LV end-systolic volume. There is no evidence of ischemia in either the LAD or the LCX, which had a moderate stenosis 2 years earlier. The LV function has remained unchanged.
In the early era of interventional therapy, assessment of infarct size, area at risk, and salvage area was popularized with injection of the tracer before the intervention (and imaging after the intervention such as fibrinolysis, angioplasty, or stenting) and then a repeat rest study before hospital discharge. The perfusion defect on the early image (before treatment when presumably the infarct-related artery is occluded) reflects the area at risk (the area that could potentially undergo necrosis without revascularization), while the perfusion defect on the predischarge study reflects necrosis area, and the difference between the two reflects the salvaged myocardium.
Case 7-4
Serial Testing After Coronary Revascularization ( Figure 7-4 )
A 62-year-old man presented with recurrent angina 2 years after he underwent CABG. After exercising for 10 minutes on a treadmill, he achieved a peak heart rate of 130 bpm (despite being on beta-blocker therapy); he developed chest pain and 1-mm ST depression in leads V 4 -V 6 . The MPI showed an abnormal perfusion pattern ( Figure 7-4, A ). Coronary angiography showed multivessel native CAD, patent LIMA graft to the LAD, and occluded vein graft to the OM1 branch of the LCX artery that was not amenable to percutaneous revascularization. His medical therapy was intensified and he was enrolled in a structured cardiac rehabilitation program. His symptoms improved and an exercise MPI confirmed improved perfusion ( Figure 7-4, B ). Repeat exercise MPI 2 years later showed a stable perfusion pattern.
