Nuclear Cardiology Snapshot
Hossein Jadvar, MD, PhD, MPH, MBA
Cardiovascular applications of nuclear medicine have always been an important part of the Society of Nuclear Medicine (SNM) annual meeting. In this article, we summarize and highlight some of the scientific sessions and poster exhibits that were presented on this topic at the 54th annual meeting of the Society of Nuclear Medicine in Washington, DC.
That Harvard and Yale investigators studied the absolute quantification of myocardial blood flow (MBF) with dynamic rubidium-82 (Rb-82) positron emission tomography (PET) for assessing the extent and severity of suspected coronary artery disease. The validation of the Rb-82 kinetic model was accomplished by comparing rest/stress regional Rb-82 MBF to standard of reference provided by MBF measured via microspheres in dogs with critical coronary stenoses (50% occlusion of the proximal left anterior descending (LAD) after the first diagonal).[1] The coronary flow reserve (CFR) was computed using a 2-compartment kinetic model. Good agreement was found between dynamic PET-derived CFR and microsphere reference values for both normal and stenosed coronaries. The authors concluded that absolute quantification of MBF could be achieved with Rb-82 PET, which can be important in some clinical conditions such as in small vessel disease, balanced ischemia, and cardiac transplant angiopathy.
The researchers from the University of Virginia used fluorodeoxyglucose (FDG) PET to assess glucose metabolism and cardiac function in a mouse model of myocardial hypertrophy.[2] Pressure-overload hypertrophy was induced by transverse aortic constriction. MicroPET imaging was performed after tail vein injection of 300-500 uCi FDG in 3 groups of mice: control, untreated hypertrophied myocardium, and hypertrophied myocardium treated pharmacologically. The myocardial standardized uptake values (SUV) for these groups were 5.0 ± 2.3, 19.2 ± 2.9, and 16.7 ± 1.8, respectively. Therefore, there was an increased utilization of glucose in hypertrophied myocardium with some reduction noted in response to treatment. Myocardial hypertrophy also increased left ventricular volume and decreased left ventricular ejection fraction, which were reversed partially with treatment.
The investigators from Munich, Germany, used multimodality imaging with PET and magnetic resonance imaging (MRI) to follow the fate and viability of progenitor cells transplanted into the myocardium.[3] Human endothelial progenitor cells were transduced with the human sodium/iodine symporter using a retroviral vector. The progenitor cells were also labeled with superparamagnetic iron oxides. T2*-weighted MRI and I-124-labeled reporter PET were performed for localization of the transplanted cells and histopathology served as the standard of reference. The authors noted that multimodality imaging with PET and MRI using magnetic and genetic cell labeling allows for simultaneous assessment of cell localization and viability, although the MRI signal lost specificity for viable cells in about 1 week after transplantation probably related to cell death and/or phagocytosis. In another similar study in humans, the intramyocardial distribution of bone marrow stem cells was assessed in patients with acute or chronic myocardial infarction (MI).[4] Autologous bone marrow cells were labeled with Tc-99m exametazime (HMPAO) and delivered into the LAD coronary artery. Single-photon emission computed tomography (SPECT) was performed 2 hours after intracoronary stem cell delivery. Myocardial perfusion and metabolism were also assessed by Tc-99m methoxyisobutylisonitrile (MIBI) and FDG PET, respectively. It was demonstrated that marrow stem cells localize to at least partially viable myocardium, although in 1 patient with acute MI, no change in ejection fraction was observed despite adequate marrow stem cell distribution to the site of infarct-related hypokinesia. These studies demonstrate the importance of molecular imaging in translational stem cell research.
The Italian investigators used N-13 ammonia (NH3) PET to assess the changes in myocardial perfusion after intracoronary bone marrow stem cells injection in patients with acute myocardial infraction.[5] An improvement in myocardial perfusion was seen in the infarcted zone in patients who had received autologous bone marrow stem cells whereas some decease in myocardial perfusion was noted in patients who had received stem cells mobilized from peripheral blood and in control subjects who were treated only with standard medical care. A relatively similar study was reported by the Chinese researchers from Shanghai who observed reverse redistribution in Tl-201 myocardial perfusion imaging during long-term follow-up after stem cell implantation that was considered to represent good response in view of the correlative improvement in clinical symptoms.[6] Despite this interesting observation, however, multivariate analysis would be necessary to determine if reverse redistribution is an independent predictor of stem cell viability and to establish the exact biologic basis for the reverse redistribution in this setting.
Chinese researchers from Beijing evaluated the diagnostic utility of coronary CT angiography (CTA) in detecting hemodynamically relevant stenosis as assessed with Tc-99m MIBI SPECT myocardial perfusion scintigraphy.[7] The sensitivity, specificity, and positive and negative predictive values for detection of significant coronary stenosis were 100%, 79%, 18%, and 100%, for conventional coronary angiography and 70%, 77%, 13%, and 98%, for CTA, respectively. The authors concluded that coronary CTA has low positive predictive value for predicting myocardial ischemia and therefore myocardial perfusion imaging would be needed to assess for the functional relevance of coronary stenosis. However, in a similar investigation, the Swiss researchers who compared the ability of 64-slice CT angiography to conventional coronary angiography concluded that CTA is a reliable tool for predicting the functional relevance of coronary artery lesions.[8] In this study, the sensitivity, specificity, and positive and negative predictive values for CTA for detecting reversible myocardial perfusion defects were, respectively, 88%, 78%, 32%, and 98%, on an artery basis, and 95%, 54%, 41%, and 97%, on a patient basis. There was a good agreement between CTA and conventional coronary angiography (93%, kappa = 0.84 on artery basis, 95%, kappa = 0.89 on patient basis).
The relationship between coronary calcium score and myocardial perfusion was studied by the Johns Hopkins group.[9] In this investigation, rest and dipyridamole stress Rb-82 myocardial perfusion PET images were obtained and compared to coronary calcium scores (CCS) obtained using a 16-slice PET-CT scanner. The mean CCS was 194 ± 468 (range 0-2122) and was higher with increasing age, in diabetics, and in patients with hypertension. In 32% of patients, abnormal myocardial perfusion was noted despite a CCS of zero. The opposite scenario was also observed with many patients demonstrating high CCS scores but normal myocardial perfusion. The authors concluded that both CCS and myocardial perfusion imaging provide complementary rather than competitive diagnostic information.
The relatively new hybrid SPECT/64-slice CT imaging system was evaluated for concurrent assessment of myocardial perfusion and coronary anatomy.[10] Hybrid imaging with Tc-99m MIBI was performed in 35 patients with chest pain. This initial clinical experience demonstrated the technical feasibility of the hybrid imaging system in assessing for hemodynamically relevant lesions that can facilitate planning for subsequent interventional therapies.