Novel iodinated tracers, MIBG and BMIPP, for nuclear cardiology

Practice recommendation for measuring washout rates in 123I-BMIPP fatty acid images

The purpose of this practice recommendation is to specifically identify the critical steps involved in performing and interpreting 123I-β-methyl-iodophenyl-pentadecanoic acid (BMIPP) single-photon emission computed tomography (SPECT) and measurement of washout rate (WR) from the heart. This document will cover backgrounds, patient preparation, testing procedure, visual image interpretation, quantitation methods using planar and SPECT studies, and reporting of WR. The pitfall and some tips for the calculation of 123I-BMIPP WR are also included. The targets of global and regional WR calculation include ischemic heart disease, cardiomyopathy, heart failure, and triglyceride deposit cardiomyovasculopathy, an emerging rare heart disease.

Current status and perspectives of nuclear cardiology

Nuclear cardiology has long been used to identify myocardial ischemia for appropriate treatment strategies for stable coronary artery disease (CAD). After the Ischemia Trial, it is time to reevaluate the significance of ischemia assessment. Functional imaging continues to play pivotal role in detecting microcirculatory disturbances. PET provides a clear image of blood flow distribution and is useful for the quantitative evaluation of myocardial flow reserve (MFR), which plays an important role in predicting treatment strategies and improving prognosis in CAD. Heart failure has become a major area of focus in cardiovascular medicine. Radionuclide imaging has been widely applied in this field. FDG PET is useful in identifying cardiac sarcoidosis and active inflammation. Clinical values of I-123 MIBG and BMIPP SPECT have been reported worldwide from Japan. Additionally, clinical experiences of Tc-99m pyrophosphate imaging have recently gained attention for assessing cardiac amyloidosis. Cardiac PET/CT and PET/MR imaging permit combined assessment of metabolic/functional/structural analyses of various cardiac diseases. While other non-invasive imaging modalities have rapidly been developed, the roles of radionuclide imaging remain to be valuable for early and accurate diagnosis and patient management in most cases of chronic CAD and various cardiovascular diseases.

Practice Recommendation for Measuring Washout Rates in 123I-BMIPP Fatty Acid Images

The purpose of this practice recommendation is to specifically identify the critical steps involved in performing and interpreting 123I-β-methyl-iodophenyl-pentadecanoic acid (BMIPP) single-photon emission computed tomography (SPECT) and measurement of washout rate (WR) from the heart. This document will cover backgrounds, patient preparation, testing procedure, visual image interpretation, quantitation methods using planar and SPECT studies, and reporting of WR. The pitfall and some tips for the calculation of 123I-BMIPP WR are also included. The targets of global and regional WR calculation include ischemic heart disease, cardiomyopathy, heart failure, and triglyceride deposit cardiomyovasculopathy, an emerging rare heart disease.

Evaluation and Management of Cardiac Sarcoidosis with Advanced Imaging

A high clinical suspicion in the setting of appropriate history, physical exam, laboratory, and imaging parameters is often required to set the groundwork for diagnosis and management. Echocardiography may show septal thinning, evidence of systolic and diastolic dysfunction, along with impaired global longitudinal strain. Cardiac MRI reveals late gadolinium enhancement along with evidence of myocardial edema and inflammation on T2 weighted imaging and parametric mapping. 18F-FDG PET detects the presence of active inflammation and the presence of scar. Involvement of the right ventricle on MRI or PET confers a high risk for adverse cardiac events and mortality.

Assessment of Cardiac Sarcoidosis: FDG PET and BMIPP SPECT

PURPOSE OF REVIEW

Cardiac sarcoidosis (CS) is an inflammatory disease of unknown etiology that can lead to life-threatening arrhythmias, heart failure, and death. Advanced cardiac imaging modalities have improved the clinician's ability to detect this disease. The purpose of this review is to discuss the recent evidence of cardiac metabolic imaging as assessed by [¹⁸F]FDG PET and [¹²³I]BMIPP SPECT in the evaluation of CS patients.

RECENT FINDINGS

[¹⁸F]FDG PET is the gold standard to identify myocardial inflammation. [¹²³I]BMIPP SPECT can uncover early myocardial damage as well as advanced stages of CS when fibrosis prevails. In presence of inflammation, myocardial [¹⁸F]FDG uptake is increased, but in contrast, BMIPP myocardial uptake is reduced or even suppressed. Thus, a complementary role of cardiac metabolic imaging by [¹⁸F]FDG PET and BMIPP SPECT has been proposed to detect the whole spectrum of CS. [¹⁸F]FDG PET is considered an important tool to improve the diagnosis and optimize the management of CS. The role of [¹²³I]BMIPP SPECT in diagnosing CS is still under investigation. Further studies are needed to evaluate the clinical utility of combined cardiac metabolic imaging in the diagnosis, prognosis, and for selecting treatments in CS patients.

Synthesis and Preliminary Evaluation of an 18F-labeled Oleate Analog to Image Fatty Acid Beta-Oxidation in the Absence of Metabolic Defluorination

PURPOSE

Fatty acid oxidation (FAO) is a key parameter for evaluating cardiovascular, oncologic, neurologic, and other metabolic diseases. Several single-photon emission computed tomography and positron emission tomography (PET) tracers have been developed to measure FAO. Among these, 18-[¹⁸F]fluoro-4-thia-oleate ([¹⁸F]FTO), first developed by DeGrado et al., is well characterized. Here, we synthesized several analogs of [¹⁸F]FTO to improve the metabolic stability of the C-¹⁸F bond, and preliminarily evaluated their performance in monkey PET studies.

PROCEDURES

Several secondary ¹⁸F-fluorinated analogs, 17-[¹⁸F]fluoro-4-thia-oleate (17-[¹⁸F]FTO), 15-[¹⁸F]fluoro-4-thia-oleate (15-[¹⁸F]FTO), 12-[¹⁸F]fluoro-4-thia-oleate (12-[¹⁸F]FTO), 7-[¹⁸F]fluoro-4-thia-oleate, (7-[¹⁸F]FTO, [¹⁸F]AS3504073-00), and 6-[¹⁸F]fluoro-4-thia-oleate (6-[¹⁸F]FTO), were synthesized from tosylate or bromide precursors using similar procedures. Nucleophilic ¹⁸F fluorination on each precursor was performed using [¹⁸F]tetrabutylammonium fluoride/tetrabutylammonium hydrocarbonate, followed by hydrolysis of methylester. All synthesized ¹⁸F-labeled compounds were administered to cynomolgus monkeys, and PET measurements were performed. From the monkey PET studies, 7-[¹⁸F]FTO was selected as the best tracer and used to perform preliminary evaluations in mice.

RESULTS

All five compounds had sufficient quality and stability for animal experiments. In monkey PET studies, 12-, 7-, and 6-[¹⁸F]FTO showed greater accumulation in the heart than [¹⁸F]FTO, but not 17- and 15-[¹⁸F]FTO. Only 7-[¹⁸F]FTO did not show significant accumulation in the bone. The standardized uptake values (SUVs) for 12-[¹⁸F]FTO, 7-[¹⁸F]FTO, and 6-[¹⁸F]FTO were 9.77, 9.26, and 7.25 in the heart, and 3.17, n.d., and 1.96 in the bone 1 h after administration, respectively. In mouse distribution studies, SUVs 1 h after administration of 7-[¹⁸F]FTO and [¹⁸F]FTO were 10.4 and 10.0 in the heart, and 0.37 and 3.48 in the femur, respectively. Administration of etomoxir, a carnitine palmitoyltransferase inhibitor, reduced SUVs of 7-[¹⁸F]FTO and [¹⁸F]FTO in the heart by 91% and 87%, respectively.

CONCLUSIONS

We developed a novel PET tracer 7-[¹⁸F]FTO/[¹⁸F]AS3504073-00 for FAO imaging. 7-[¹⁸F]FTO had an excellent PET tracer profile, suggesting it may be a useful tracer for FAO imaging. Further evaluations of the tracer are ongoing.

123I-BMIPP Scintigraphy Shows That CNT-01 (Tricaprin) Improves Myocardial Lipolysis in Patients with Idiopathic Triglyceride Deposit Cardiomyovasculopathy: First Randomized Controlled, Exploratory Trial for TGCV

Background: Triglyceride deposit cardiomyovasculopathy (TGCV) is a rare intractable cardiovascular disorder (Orphanet ORPHAcode: 565612) in which defective intracellular lipolysis results in heart failure and coronary artery disease. Myocardial scintigraphy with ¹²³I-β-methyl-p-iodophenylpentadecanoic acid (BMIPP) is useful to evaluate myocardial TG metabolism; its washout rate (WR) reflects myocardial lipolysis. This study reports the effects of CNT-01 (tricaprin), a developing orphan drug to facilitate lipolysis, on BMIPP-WR in patients with TGCV. Methods: An investigator-initiated, multicenter, randomized, double-blind exploratory, trial (Phase IIa) was conducted (UMIN000035403). Seventeen patients with idiopathic TGCV were orally administered 1.5 g/day of CNT-01 or placebo for 8 weeks. Endpoints included delta BMIPP-WR and clinical parameters such as 6-minwalk distance and TGCV severity score. Results: During the protocol, delta BMIPP-WRs were -0.26±3.28 and 7.08±3.28% (95% confidence intervals, -7.36 to 6.84 and -0.01 to 14.18) in the placebo and CNT-01 groups, respectively. The baseline-adjusted difference of delta BMIPP-WR between the two groups was significant (p=0.035) after one patient was excluded from the placebo group because of pseudonormalization of BMIPP-WR related to coronary bypass graft stenosis. Clinical parameters did not show significant changes. Conclusions: This study proved the mechanism of CNT-01 to improve myocardial lipolysis in TGCV, as demonstrated by BMIPP scintigraphy.

Prediction of Serious Adverse Events of Patients with Hypertrophic Cardiomyopathy by Magnetic Resonance

Although it is well known that patients with hypertrophic cardiomyopathy (HCM) have serious adverse events, such as life-threatening arrhythmia and heart failure, the prediction of such evens is still difficult. Recently, it has been reported that one of the causes of these serious adverse events is microvascular dysfunction, which can be noninvasively evaluated by employing cardiac magnetic resonance (CMR) imaging.We analyzed 32 consecutive HCM patients via CMR imaging and myocardial scintigraphy and divided them into two groups: ventricular tachycardia (VT) group and non-VT group. Myocardial perfusion studies were conducted quantitatively using the QMass^® software, and each slice image was divided into six segments. The time-intensity curve derived from the perfusion image by CMR imaging was evaluated, and the time to 50% of the peak intensity (time 50% max) was automatically calculated for each segment.Although no difference was observed in various parameters of myocardial scintigraphy between the two groups, the VT group exhibited a higher mean of time 50% max and wider standard deviation (SD) of time 50% max in each segment than the non-VT group. The cutoff values were obtained by the receiver operating characteristic curves derived from the mean of time 50% max and SD of time 50% max. The two groups divided by the cutoff values exhibited significant differences in the occurrence of serious adverse events.CMR imaging may be useful for predicting serious adverse events of patients with HCM.

Effects of feeding condition on the myocardial and hepatic accumulation of radioiodine-labeled BMIPP in mice

OBJECTIVE

123I-15-(p-iodophenyl)-3(R,S)-methylpentadecanoic acid ([123I]BMIPP), a fatty acid analog, is widely used for the diagnosis of cardiac diseases. Feeding condition is one of the important factors in the myocardial fatty acid uptake, which may also affect myocardial accumulation of [123I]BMIPP and image quality of [123I]BMIPP scintigraphy. However, the relationship between the myocardial accumulation of [123I]BMIPP and the feeding condition is not entirely clear. Therefore, we determined the myocardial accumulation of [125I]BMIPP in mice at various metabolic statuses induced by fasting in comparison with the hepatic accumulation.

METHODS

Fed or fasted (6-, 12-, and 24-h fasted) mice were intravenously injected with [125I]BMIPP (35.2-75.0 kBq, 4 nmol). Radioactivities in the heart and liver were measured at 1, 5, 10, 30, 60, and 120 min after the injection (n = 5-15/time point for each group), and then, the heart-to-liver (H/L) ratios were calculated.

RESULTS

The myocardial accumulation level of [125I]BMIPP in the fed group was almost the same as that in the 6-h-fasted group at each time point, although it was decreased by 12- and 24-h fasting. The H/L ratios of [125I]BMIPP accumulation level were significantly decreased by fasting (1.92 ± 0.22, 1.45 ± 0.13, 1.12 ± 0.13, and 0.91 ± 0.15 at 10 min, and 3.30 ± 0.62, 2.09 ± 0.35, 1.79 ± 0.34, and 1.27 ± 0.06 at 30 min after the injection, respectively, for the fed group and the 6-, 12-, and 24-h-fasted groups; p < 0.0001), largely owing to the increase in the hepatic accumulation level in the fasting groups.

CONCLUSION

Although short-period (6 h) fasting did not affect the myocardial accumulation level of [125I]BMIPP, the hepatic accumulation level was increased. The present results indicate that the fed condition may provide higher-contrast images in myocardial [123I]BMIPP scintigraphy.

Imaging Metabolically Active Fat: A Literature Review and Mechanistic Insights

Currently, obesity is one of the leading causes death in the world. Shortly before 2000, researchers began describing metabolically active adipose tissue on cancer-surveillance ¹⁸F-fluorodeoxyglucose (FDG) positron emission tomography/computed tomography (PET/CT) in adult humans. This tissue generates heat through mitochondrial uncoupling and functions similar to classical brown and beige adipose tissue in mice. Despite extensive research, human brown/beige fat's role in resistance to obesity in humans has not yet been fully delineated. FDG uptake is the de facto gold standard imaging technique when studying brown adipose tissue, although it has not been rigorously compared to other techniques. We, therefore, present a concise review of established and emerging methods to image brown adipose tissue activity in humans. Reviewed modalities include anatomic imaging with CT and magnetic resonance imaging (MRI); molecular imaging with FDG, fatty acids, and acetate; and emerging techniques. FDG-PET/CT is the most commonly used modality because of its widespread use in cancer imaging, but there are mechanistic reasons to believe other radiotracers may be more sensitive and accurate at detecting brown adipose tissue activity. Radiation-free modalities may help the longitudinal study of brown adipose tissue activity in the future.

Current status of nuclear cardiology in Japan: Ongoing efforts to improve clinical standards and to establish evidence

Nuclear cardiology imaging tests are widely performed in Japan as clinical practice. The Japanese nuclear cardiology community has developed new diagnostic imaging tests using (123)I-beta-methyl-p-iodophenyl-pentadecanoic acid, (123)I-metaiodobenzylguanidine, and (18)F-fluorodeoxyglucose PET for detecting cardiac involvement in sarcoidosis. These tests have become popular worldwide. The Japanese Circulation Society and the Japanese Society of Nuclear Cardiology have published clinical imaging guidelines showing indications and standards for the new imaging tests. JSNC is currently striving to improve the standard of clinical practice and is promoting research activities.

Prone-position acquisition of myocardial (123)I-metaiodobenzylguanidine (MIBG) SPECT reveals regional uptake similar to that found using (11)C-hydroxyephedrine PET/CT

OBJECTIVES

(123)I-metaiodobenzylguanidine (MIBG) has been used to estimate cardiac sympathetic nervous innervation. Heterogeneous MIBG distribution is mainly associated with high physiological MIBG uptakes in the liver. We postulate that prone position acquisition might be especially effective for MIBG, providing for separation from high liver uptake similar to that provided by perfusion single-photon emission computed tomography (SPECT). We investigated whether prone-position acquisition improved MIBG image quality by comparing our results to those acquired using supine MIBG and high-quality (11)C-hydroxyephedrine (HED) positron emission tomography/computed tomography PET/CT.

METHODS

Ten male volunteers (body mass index (BMI) 22.7 ± 3.4) underwent prone and supine MIBG and HED PET. Relative regional tracer uptake was estimated in early MIBG and HED. Acquired images were divided into 17 segments and were grouped into 4 regions: anterior, inferior, septum, and lateral. For each patient, the inferior/anterior ratio was calculated.

RESULTS

The quality of images acquired using prone MIBG was better than that using supine MIBG (p < 0.05). Inferior and septum relative MIBG uptake was reduced in comparison with anterior or lateral MIBG uptake in the supine position (inferior vs. anterior: 69.0 ± 5.6 vs. 82.3 ± 4.6 %, p < 0.01; septum vs. lateral: 66.2 ± 5.1 vs. 81.9 ± 5.4 %, p < 0.01). Prone MIBG showed a significantly higher inferior/anterior uptake ratio in comparison with supine MIBG (n = 24, seg: 92.2 ± 7.2 vs. 83.6 ± 5.7 %, p < 0.05). However, intergroup differences in uptake ratio were demonstrated among prone and supine MIBG and HED. HED PET/CT still showed a higher uptake ratio in comparison with prone MIBG SPECT (103.9 ± 8.0 vs. 92.2 ± 7.2 %, p < 0.05).

CONCLUSION

Even in normal male subjects, standard supine MIBG imaging showed reduced inferior and septum uptake. Uptake with prone MIBG imaging showed a significant improvement over that with supine imaging and was closer to uptake for HED PET/CT. This improvement may be the result of preventing intense uptake by the liver. Prone data acquisition may be a viable alternative in evaluating regional abnormalities using MIBG SPECT in men.

Molecular imaging of brown adipose tissue in health and disease

PURPOSE

Brown adipose tissue (BAT) has transformed from an interfering tissue in oncological (18)F-fluorodeoxyglucose (FDG) positron emission tomography (PET) to an independent imaging research field. This review takes the perspective from the imaging methodology on which human BAT research has come to rely on heavily.

METHODS

This review analyses relevant PubMed-indexed publications that discuss molecular imaging methods of BAT. In addition, reported links between BAT and human diseases such as obesity are discussed, and the possibilities for imaging in these fields are highlighted. Radiopharmaceuticals aiming at several different biological mechanisms of BAT are discussed and evaluated.

RESULTS

Prospective, dedicated studies allow visualization of BAT function in a high percentage of human subjects. BAT dysfunction has been implicated in obesity, linked with diabetes and associated with cachexia and atherosclerosis. Presently, (18)F-FDG PET/CT is the most useful tool for evaluating therapies aiming at BAT activity. In addition to (18)F-FDG, other radiopharmaceuticals such as (99m)Tc-sestamibi, (123)I-metaiodobenzylguanidine (MIBG), (18)F-fluorodopa and (18)F-14(R,S)-[(18)F]fluoro-6-thia-heptadecanoic acid (FTHA) may have a potential for visualizing other aspects of BAT activity. MRI methods are under continuous development and provide the prospect of functional imaging without ionizing radiation.

CONCLUSION

Molecular imaging of BAT can be used to quantitatively assess different aspects of BAT metabolic activity.

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.

Iodine-123-metaiodobenzylguanidine scintigraphy in risk stratification of sudden death in heart failure

Metaiodobenzylguanidine (MIBG) is a false neurotransmitter noradrenaline analogue that is taken up by the 'uptake 1' transporter mechanism in the cell membrane of presynaptic adrenergic neurons and accumulates in catecholamine storage vesicles. Since it is practically unmetabolized, it can be labeled with a radioisotope (iodine-123) in scintigraphic exams to noninvasively assess the functional status of the sympathetic innervation of organs with a significant adrenergic component, including the heart. Studies of its application in nuclear cardiology appear to confirm its value in the assessment of conditions such as coronary artery disease, heart failure, arrhythmias and sudden death. Heart failure is a global problem, with an estimated prevalence of 2% in developed countries. Sudden cardiac death is the main cause of its high mortality. The autonomic nervous system dysfunction, including sympathetic hyperactivity, that accompanies chronic heart failure is associated with progressive myocardial remodeling, declining left ventricular function and worsening symptoms, and contributes to the development of ventricular arrhythmias and sudden death. Since 123I-MIBG cardiac scintigraphy can detect changes in the cardiac adrenergic system, there is considerable interest in its role in obtaining diagnostic and prognostic information in patients with heart failure. In this article we present a literature review on the use of 123I-MIBG scintigraphy for risk stratification of sudden death in patients with heart failure.

Imaging strategies for acute chest pain in the emergency department

OBJECTIVE

Echocardiography, radionuclide myocardial perfusion imaging (MPI), and coronary CT angiography (CTA) are the three main imaging techniques used in the emergency department for the diagnosis of acute coronary syndrome (ACS). The purpose of this article is to quantitatively examine existing evidence about the diagnostic performance of these imaging tests in this setting.

CONCLUSION

Our systematic search of the medical literature showed no significant difference between the modalities for the detection of ACS in the emergency department. There was a slight, positive trend favoring coronary CTA. Given the absence of large differences in diagnostic performance, practical aspects such as local practice, expertise, medical facilities, and individual patient characteristics may be more important.

Myocardial perfusion scintigraphy: past, present and future

During the last two decades, radionuclide myocardial perfusion scintigraphy (MPS) has become established as the main functional cardiac imaging technique for the assessment of ischaemic heart disease (IHD). Despite a growing number of alternative functional imaging techniques, MPS still remains the most widely used technique, with a wealth of literature supporting its usefulness in assessing IHD and predicting prognosis. The technique itself has evolved, making it more reliable and robust, with additional ventricular functional information that further defines the prognosis in these patients. With the advent of hybrid single photon emission with CT and positron emission tomography with CT cameras together with the development of new camera technology that enables faster images with less radiation and better resolution, MPS will remain an essential part of IHD investigation. There are new promising radiopharmacological developments and applications such as radiolabelled fatty acids and meta-iodobenzylguanidine. These will widen the scope of nuclear medicine imaging to include patients with cardiac failure and acute chest pain presenting to accident and emergency departments. Nuclear medicine cardiac investigations will continue to have an essential role in the diagnosis, stratification and prognosis of patients with cardiac disease, complementing the new developing cardiac modalities such as CT coronary angiography and MRI.

Newer applications of nuclear cardiology in systolic heart failure: detecting coronary artery disease and guiding device therapy

Radionuclide-based imaging techniques can be applied to the heart failure population to derive clinically useful information. This review discusses the specific role of myocardial perfusion imaging for determining heart failure etiology, and the potential application of radionuclide-based imaging techniques for the optimal selection of patients with heart failure for device therapy.

Validation of automated quantitation of myocardial perfusion and fatty acid metabolism abnormalities on SPECT images

BACKGROUND

Myocardial perfusion and fatty acid imaging have played important roles in the risk stratification of patients with coronary artery disease (CAD). However, visual image assessment requires considerable experience and training. Therefore, an automated program has been developed that can quantify perfusion and fatty acid uptake on myocardial single emission computed tomography (SPECT). The present study aimed to validate the automated quantitative program.

METHODS AND RESULTS

A total of 50 patients were studied with known or suspected CAD who underwent stress ²⁰¹Thallium (²⁰¹Tl) and resting ¹²³I-labelled β-methyl iodophenyl pentadecanoic acid (BMIPP) SPECT. The SPECT images were quantified in 17 segments visually and using our Heart Score View software. Values were compared with those in a normal Japanese database and calculated summed stress (SSS), summed rest (SRS), summed difference (SDS), and summed BMIPP scores for each modality. Summed scores obtained using standard visual analysis and Heart Score View significantly correlated (²⁰¹Tl: SSS: r=0.934; SRS: r=0.827; SDS: r=0.743 summed BMIPP score: r=0.913) (each P<0.001) and Bland-Altman analysis revealed good agreement between the 2 approaches.

CONCLUSIONS

Correlations between scores determined using Heart Score View software and standard visual interpretation were linear for both perfusion and fatty acid images. Thus, our new automated program might be useful for the risk stratification of patients with CAD in the clinical setting.