Recent advances in nuclear cardiology in the study of coronary artery disease

Ischaemic memory imaging using metabolic radiopharmaceuticals: overview of clinical settings and ongoing investigations

"Ischaemic memory" is defined as a prolonged functional and/or biochemical alteration remaining after a particular episode of severe myocardial ischaemia. The biochemical alteration has been reported as metabolic stunning. Metabolic imaging has been used to detect the footprint left by previous ischaemic episodes evident due to delayed recovery of myocardial metabolism (persistent dominant glucose utilization with suppression of fatty acid oxidation). β-Methyl-p-[(123)I]iodophenylpentadecanoic acid (BMIPP) is a single-photon emission computed tomography (SPECT) radiotracer widely used for metabolic imaging in clinical settings in Japan. In patients with suspected coronary artery disease but no previous myocardial infarction, BMIPP has shown acceptable diagnostic accuracy. In particular, BMIPP plays an important role in the identification of prior ischaemic insult in patients arriving at emergency departments with acute chest pain syndrome. Recent data also show the usefulness of (123)I-BMIPP SPECT for predicting cardiovascular events in patients undergoing haemodialysis. Similarly, SPECT or PET imaging with (18)F-FDG injected during peak exercise or after exercise under fasting conditions shows an increase in FDG uptake in postischaemic areas. This article will overview the roles of ischaemic memory imaging both under established indications and in ongoing investigations.

Metabolic imaging using SPECT

INTRODUCTION

In normal condition, the heart obtains more than two-thirds of its energy from the oxidative metabolism of long chain fatty acids, although a wide variety of substrates such as glucose, lactate, ketone bodies and amino acids are also utilised. In ischaemic myocardium, on the other hand, oxidative metabolism of free fatty acid is suppressed and anaerobic glucose metabolism plays a major role in residual oxidative metabolism. Therefore, metabolic imaging can be an important technique for the assessment of various cardiac diseases and conditions.

MATERIALS AND METHODS

In SPECT, several iodinated fatty acid traces have been introduced and studied. Of these, (123)I-labelled 15-(p-iodophenyl)3-R, S-methylpentadecanoic acid (BMIPP) has been the most commonly used tracer in clinical studies, especially in some of the European countries and Japan.

RESULTS AND DISCUSSION

In this review article, several fatty acid tracers for SPECT are characterised, and the mechanism of uptake and clinical utility of BMIPP are discussed in detail.

Significance of 123I-MIBG scintigraphy as a pathophysiological indicator in the assessment of Parkinson's disease and related disorders: it can be a specific marker for Lewy body disease

Recently, reliable and clear evidence for the usefulness of 123I-MIBG scintigraphy in the diagnosis of Parkinson's disease (PD) has been accumulated and it has become increasingly popular as one of the most accurate means of diagnosing the disease. PD, one of the most common neurodegenerative disorders, is characterized by resting tremor, rigidity, bradykinesia or akinesia, and postural instability. The disease is characterized pathologically by distinctive neuronal inclusions called Lewy bodies in many surviving cells of dopaminergic neurons of the substantia nigra pars compacta and other specific brain regions. Furthermore Lewy body type degeneration in the cardiac plexus has been observed in PD. In PD, cardiac MIBG uptake is reduced markedly even in the early disease stages; therefore, MIBG imaging can be used as an indicator of the presence of PD rather than disease severity. Other parkinsonian syndromes such as multiple system atrophy, progressive supranuclear palsy, and corticobasal degeneration demonstrate normal cardiac MIBG uptake or only mild reduction of MIBG uptake, indicating that MIBG imaging is a powerful method to differentiate PD from other parkinsonian syndromes. Dementia with Lewy bodies (DLB) also shows severe reduction of MIBG uptake, whereas Alzheimer's disease (AD) demonstrates normal MIBG uptake, permitting differentiation of DLB from AD using MIBG scintigraphy. In pure autonomic failure, which shares similar pathological findings with PD and is thought to be associated with diffuse loss of sympathetic terminal innervation, cardiac MIBG uptake also decreases markedly. Considering all the data together, marked reduction of cardiac MIBG uptake seems to be a specific marker of Lewy body disease and thus extremely useful in the differentiation from other diseases with similar symptoms without Lewy bodies.

Prognostic value of ECG-gated thallium-201 single-photon emission tomography in patients with coronary artery disease

BACKGROUND

The phenomenon of reversible impairment in LV function has been well described and is known as myocardial stunning.

OBJECTIVE

Thallium-201 myocardial perfusion gated SPECT was used to evaluate myocardial stunning and its incremental prognostic value in patients with coronary artery disease.

PATIENTS AND METHODS

Fifty-six patients (aged 63+/-11 years) with coronary artery disease were included in this study. All subjects underwent exercise thallium scintigraphy. ECG-gated SPECT was obtained both at post-stress (10 minutes after the injection of 111 MBq of thallium at the time of peak exercise) and at rest (180 minutes). The left ventricular ejection fraction (LVEF) and end-systolic and end-diastolic volume (ESV, EDV) were determined by a quantitative gated SPECT (QGS) program.

RESULTS

Follow-up was complete in all patients (mean 569 days). The magnitude of the depression of post-stress LVEF relative to the rest LVEF was correlated with the severity of ischemia (p < 0.05). The group with a median LVEF of more than 45% had a significantly higher event-free rate (p < 0.01).

CONCLUSION

Assessment of post-stress left ventricular function by gated-SPECT provides incremental prognostic information and is useful in predicting cardiac events in patients with suspected or definite coronary artery disease.

Ischemic "memory image" in acute myocardial infarction of 123I-BMIPP after reperfusion therapy: a comparison with 99mTc-pyrophosphate and 201Tl dual-isotope SPECT

Ischemic "memory image" is a phenomenon of 123I-15-(p-iodophenyl)-3-(R,S)-methylpentadecanoic acid (BMIPP) in which an area at risk of acute myocardial infarction (AMI), could be detected as a defect in a couple of weeks even after successful reperfusion therapy. The purpose of this study was to clarify the incidence of the ischemic "memory image" of 123I-BMIPP in patients with AMI by comparing 99mTc-PYP and 201Tl dual-isotope SPECT. Materials consisted of 14 patients with successfully reperfused AMI and 20 patients with old myocardial infarction (OMI). All AMI patients underwent PYP/Tl dual-isotope SPECT within 1 week after the onset of AMI, and BMIPP SPECT was performed within 1 week after the PYP/Tl dual-isotope SPECT. The extent and severity of the defect of BMIPP and Tl were visually scored into four grades: 0 = no defect to 3 = large or severe defect. These scores were compared. PYP positive AMI lesions were concordant with BMIPP defects (13/14). In AMI, both the extent and severity scores of BMIPP were higher than 201Tl (p < 0.001). Differences (BMIPP - Tl) of extent and severity scores were greater in AMI than in OMI (p < 0.001). In conclusion, the ischemic "memory image" obtained by means of the BMIPP is a common phenomenon (13/14) in AMI, and helpful in evaluating the area at risk.

Assessment of transient left ventricular dilatation on rest and exercise on Tc-99m tetrofosmin myocardial SPECT

In myocardial perfusion imaging, multiple-vessel involvement of coronary artery disease (CAD) sometimes makes diagnosis difficult. Transient left ventricular (LV) dilatation on Tl-201 myocardial SPECT is a useful finding that enables the clinician to identify patients with multiple-vessel disease. The aim of this study was to confirm the utility of measuring transient LV dilatation for the detection of multiple-vessel CAD in exercise Tc-99m tetrofosmin myocardial SPECT. The participants were 55 CAD patients and 20 controls who underwent Tc-99m tetrofosmin myocardial SPECT exercise and resting imaging. During exercise, 370 MBq (10 mCi) Tc-99m tetrofosmin was injected. Exercise images were obtained 30 minutes after injection. At 210 minutes after injection, 740 MBq (20 mCi) Tc-99m tetrofosmin was administered intravenously. The rest SPECT images were acquired 30 minutes later. Thirty-six radii at every 10 degrees were generated from the center of short-axis images. An area surrounded by 36 maximal points of the myocardial Tc-99m tetrofosmin counts on each radius was calculated for exercise and rest images. The area surrounded by the 36 maximal points in the same slice of the exercise and rest images was assigned the variables A (Ex) and A(R), respectively. The transient dilatation index (TDI) of the left ventricle was calculated using the formula mean A (Ex)/A(R) in the apical, middle, and basal myocardial short-axis images. In the controls, the TDI was 0.970 +/- 0.021. In patients with CAD, the TDIs of one-vessel disease, two-vessel disease, and three-vessel disease were 1.034 +/- 0.032, 1.093 +/- 0.046, and 1.131 +/- 0.076, respectively. The TDIs were significantly greater in patients who had more occluded coronary arteries (P < 0.01). If the mean + 2SD of the TDI (1.012) in controls were assumed to be the normal upper limit, the sensitivity, specificity, and accuracy of this method in detecting two-vessel or three-vessel disease would be 91.4%, 76.9%, and 84%, respectively. The TDI is a useful index for evaluating subendocardial ischemia non-invasively and detecting multiple-vessel disease clinically.

Changes in perfusion and fatty acid metabolism of rat heart with autoimmune myocarditis

To elucidate the change in perfusion and aerobic metabolism in myocarditis, tissue counting and dual tracer ex vivo autoradiography with Tl-201 and a free fatty acid analog, I-123- or I-125-labeled (p-iodophenyl)-methyl-pentadecanoic acid (BMIPP), were performed in rats with myocarditis induced by immunization with cardiac myosin. Inflammatory damage was classified histologically. At the acute stage (2-4 weeks after the antigen-injection), total heart uptakes of Tl and BMIPP and the ratio (BMIPP/Tl) were significantly reduced in myocarditis rats (N = 15) compared with the controls (N = 12). Myocardial distribution of Tl and BMIPP was not homogeneous. Relative uptake of Tl and BMIPP (N = 9, 128 regions) was gradually decreased with the extent of inflammation, and the regional BMIPP/Tl was smaller than the control. At the subacute stage (7 weeks after the antigen-injection), total Tl uptake in myocarditis rats (N = 5) recovered to the control level (N = 4), but that of BMIPP was still significantly lower than the control. BMIPP/Tl was still significantly lower in myocarditis. Myocardial distribution of Tl and BMIPP recovered to be more homogeneous. Relative uptake of Tl and BMIPP (N = 6, 78 regions) still gradually but significantly decreased with the extent of inflammation. Regional BMIPP/Tl was still depressed in myocarditis. These results indicate that myocardial perfusion and aerobic metabolism were discrepant and heterogeneously suppressed with severe inflammation during the acute stages, but the difference decreases with time. Examination with Tl-201 and BMIPP may provide information about the severity of myocarditis.

Future aspects of BMIPP

Iodinated fatty acid compounds have an important role in early detection of myocardial abnormalities and provide insights into pathological states in the heart. Among them, 15-(p-iodophenyl)-3R,S-methyl pentadecanoic acid (BMIPP) has been most widely used providing excellent images of the left ventricular myocardium due to high myocardial uptake and long retention. The previous chapters have focused on the basic characters and clinical applications of this compound. However, the precise mechanisms of myocardial kinetics should be further investigated under various conditions. Most of the studies showed reduced BMIPP uptake relative to perfusion in a variety of myocardial disorders, whereas an increase in BMIPP uptake relative to perfusion is often reported. The potential mechanisms of such conflicting results are discussed, but basic studies should be performed to clarify such results in detail. There are a number of clinical values of this compound. Since alteration of fatty acid is observed in the repetitive ischemia, BMIPP can be used for detecting severe ischemic episodes. The concept of 'ischemic memory' imaging can be applied for patients with unstable or vasospastic angina at rest and for those with acute myocardial infarction with successful revascularization to identify the risk area. The discordant decrease in BMIPP uptake relative to perfusion is often seen in ischemic but viable myocardium, and therefore, the combined imaging of BMIPP and perfusion can be used for assessment of tissue viability. Furthermore, abnormal BMIPP uptake is most often observed in hypertrophic cardiomyopathy, and thus, this compound can be used for an early detection and differential diagnosis of the cardiomyopathy. Although BMIPP imaging seems to be quite promising in many fields, the number of patient data remain limited. In this respect, a multicenter study with a vast majority of patients is warranted to confirm these important values of BMIPP. In addition, this attractive tracer should be available all over the world to confirm its clinical value in the near future.