Absent myocardial I-123 BMIPP uptake in a family

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.

CD36 and atherosclerosis

CD36 has been associated with diverse normal and pathologic processes. These include scavenger receptor functions (uptake of apoptotic cells and modified lipid), lipid metabolism and fatty acid transport, adhesion, angiogenesis, modulation of inflammation, transforming growth factor-beta activation, atherosclerosis, diabetes and cardiomyopathy. Although CD36 was identified more than 25 years ago, it is only with the advent of recent genetic technology that in-vivo evidence has emerged for its physiologic and pathologic relevance. As these in-vivo studies are expanded, we will gain further insight into the mechanism(s) by which CD36 transmits a cellular signal, and this will allow the design of specific therapeutics that impact on a particular function of CD36.

Scintigraphic evidence for a specific long-chain fatty acid transporting system deficit and the genetic background in a patient with hypertrophic cardiomyopathy

The mechanism of cardiac uptake of long-chain free fatty acids has not been fully determined. We encountered a hypertrophic cardiomyopathy patient who showed a lack of cardiac uptake of 2 different types of long-chain fatty acid analogues on the scintigraphic images. Flow cytometric analysis revealed no platelet or monocyte CD36 molecule expression (type I CD36 deficiency) and his CD36 gene showed homozygous mutation for 478C to T substitution, leading to an abnormal CD36 amino acid sequence. These findings strongly suggest that a specific transporting system rather than a simple diffusion is commonly involved in the cardiac uptake of long-chain free fatty acids in humans, and that the CD36 protein is the most likely candidate for the specific transporter and to explain scintigraphic defects on fatty acid imaging.

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.

Myocardial metabolic abnormalities in hypertrophic cardiomyopathy assessed by iodine-123-labeled beta-methyl-branched fatty acid myocardial scintigraphy and its relation to exercise-induced ischemia

Reversible thallium-201 (201Tl) abnormalities during exercise stress have been used as markers of myocardial ischemia in hypertrophic cardiomyopathy (HCM) and are most likely to identify relatively underperfused myocardium. Although metabolic abnormalities in HCM were reported, the relationship between impaired energy metabolism and exercise-induced ischemia has not been fully elucidated as yet. To assess the relationship between myocardial perfusion abnormalities and fatty acid metabolic abnormalities, 28 patients with HCM underwent exercise 201Tl and rest 123I-15-(p-iodophenyl)-3-methyl pentadecanoic acid (BMIPP) scintigraphy. Perfusion abnormalities were observed by exercise 201Tl in 19/28 patients with HCM. 123I-BMIPP uptake was decreased compared with delayed 201Tl in 106/364 (29%) of the total myocardial segments (p<0.01, McNemar symmetry test). Such disparity between 123I-BMIPP and 201Tl was observed more often in the 49/75 (65%) segments with reversible exercise 201Tl defects (p<0.001). Our results indicate that exercise-induced myocardial ischemia exists in HCM, resulting in metabolic abnormalities. The combination of 123I BMIPP and 201Tl suggests that myocardial ischemia may play an important role in metabolic abnormalities in HCM.

Absent myocardial accumulation of two different radioiodinated pentadecanoic acids

This article presents two cases with preserved myocardial 201Tl uptake and absent uptake of two kinds of radioiodinated fatty acids: iodine-123-labeled 15-(p-iodophenyl)-3-(R,S)-methylpentadecanoic acid (BMIPP) and iodine-123-labeled 15-(p-iodophenyl)-9-(R,S)-methylpentadecanoic acid (9MPA). Although coronary angiography showed no stenotic lesion and left ventriculography revealed no wall motion abnormality, no myocardial uptake of BMIPP and 9MPA was observed in the first case. In the second case, no myocardial accumulation was recognized even in the initial phase of dynamic SPECT acquired soon after the injection of 9MPA. The results suggest that the non-visualized myocardium was not specific for BMIPP imaging and that rather than the early back diffusion of the tracers from the myocardium, abnormality of the myocardial cell membrane was a possible mechanism accounting for the phenomenon.