Enhanced myocardial 18F-2-fluoro-2-deoxyglucose uptake after orthotopic heart transplantation assessed by positron emission tomography

18F-fluorodeoxyglucose use after cardiac transplant: A comparative study of suppression of physiological myocardial uptake

BACKGROUND

18F-fluorodeoxyglucose (FDG) has been useful in the evaluation of myocardial inflammatory processes. However, it is challenging to identify them due to physiological 18F-FDG uptake. There are no publications demonstrating the application of FDG in post-transplant rejection in humans yet. The aim of this study is to determine the feasibility of suppression of myocardial FDG uptake in post-transplant patients, comparing three different protocols of preparation.

METHODS

Ten patients after heart transplantation were imaged by FDG associated with three endomyocardial biopsies (EMB), scheduled in the first year after the procedure. Before each imaging, patients were randomized to one of three preparations: (1) hyperlipidic-hypoglycemic diet; (2) fasting longer than 12 hours; and (3) fasting associated with intravenous heparin. All patients would undergo the three methods. FDG images were analyzed using visual analysis scores and relative radiotracer cardiac uptake (RRCU).

RESULTS

The suppression rate of radiotracer activity ranged from 55% to 62%. Visual analysis showed that preparation 3 presented less efficacy in the suppression compared to the others. However, RRCU did not show difference between the preparations.

CONCLUSIONS

Suppression of physiological myocardial FDG uptake after cardiac transplantation is feasible. The usefulness of heparin in the suppression is unclear.

Emerging imaging techniques after cardiac transplantation

Improvements in survival after cardiac transplantation have in part been driven by improved graft surveillance. Graft surveillance relies mainly on 3 techniques: coronary angiography, endomyocardial biopsy and echocardiography. Developments in invasive and non-invasive imaging technology have revolutionized assessment of the heart in both health and disease, offering new insights into tissue composition and myocardial metabolism. Herein we aim to review the strengths and weaknesses of these techniques, and summarize the evidence in the following 5 fields of cardiac imaging after transplantation: cardiovascular magnetic resonance; computed tomography; positron emission tomography; single-photon emission computed tomography; and optical coherence tomography and molecular imaging techniques.

Cardiac applications of PET

Routine use of cardiac positron emission tomography (PET) applications has been increasing but has not replaced cardiac single-photon emission computerized tomography (SPECT) studies yet. The majority of cardiac PET tracers, with the exception of fluorine-18 fluorodeoxyglucose (18F-FDG), are not widely available, as they require either an onsite cyclotron or a costly generator for their production. 18F-FDG PET imaging has high sensitivity for the detection of hibernating/viable myocardium and has replaced Tl-201 SPECT imaging in centers equipped with a PET/CT camera. PET myocardial perfusion imaging with various tracers such as Rb-82, N-13 ammonia, and O-15 H2O has higher sensitivity and specificity than myocardial perfusion SPECT for the detection of coronary artery disease (CAD). In particular, quantitative PET measurements of myocardial perfusion help identify subclinical coronary stenosis, better define the extent and severity of CAD, and detect ischemia when there is balanced reduction in myocardial perfusion due to three-vessel or main stem CAD. Fusion images of PET perfusion and CT coronary artery calcium scoring or CT coronary angiography provide additional complementary information and improve the detection of CAD. PET studies with novel 18F-labeled perfusion tracers such as 18F-flurpiridaz and 18F-FBnTP have yielded high sensitivity and specificity in the diagnosis of CAD. These tracers are still being tested in humans, and, if approved for clinical use, they will be commercially and widely available. In addition to viability studies, 18F-FDG PET can also be utilized to detect inflammation/infection in various conditions such as endocarditis, sarcoidosis, and atherosclerosis. Some recent series have obtained encouraging results for the detection of endocarditis in patients with intracardiac devices and prosthetic valves. PET tracers for cardiac neuronal imaging, such as C-11 HED, help assess the severity of heart failure and post-transplant cardiac reinnervation, and understand the pathogenesis of arrhytmias. The other uncommon applications of cardiac PET include NaF imaging to identify calcium deposition in atherosclerotic plaques and β-amyloid imaging to diagnose cardiac amyloid involvement. 18F-FDG imaging with a novel PET/MR camera has been reported to be very sensitive and specific for the differentiation between malignant and nonmalignant cardiac masses. The other potential applications of PET/MR are cardiac infectious/inflammatory conditions such as endocarditis.

Use of [18F]FDG Positron Emission Tomography to Monitor the Development of Cardiac Allograft Rejection

BACKGROUND

Positron emission tomography (PET) has the potential to be a specific, sensitive and quantitative diagnostic test for transplant rejection. To test this hypothesis, we evaluated F-labeled fluorodeoxyglucose ([F]FDG) and N-labeled ammonia ([N]NH3) small animal PET imaging in a well-established murine cardiac rejection model.

METHODS

Heterotopic transplants were performed using minor major histocompatibility complex-mismatched B6.C-H2 donor hearts in C57BL/6(H-2) recipients. C57BL/6 donor hearts into C57BL/6 recipients served as isograft controls. [F]FDG PET imaging was performed weekly between posttransplant days 7 and 42, and the percent injected dose was computed for each graft. [N]NH3 imaging was performed to evaluate myocardial perfusion.

RESULTS

There was a significant increase in [F]FDG uptake in allografts from day 14 to day 21 (1.6% to 5.2%; P < 0.001) and uptake in allografts was significantly increased on posttransplant days 21 (5.2% vs 0.9%; P = 0.005) and 28 (4.8% vs 0.9%; P = 0.006) compared to isograft controls. Furthermore, [F]FDG uptake correlated with an increase in rejection grade within allografts between days 14 and 28 after transplantation. Finally, the uptake of [N]NH3 was significantly lower relative to the native heart in allografts with chronic vasculopathy compared to isograft controls on day 28 (P = 0.01).

CONCLUSIONS

PET imaging with [F]FDG can be used after transplantation to monitor the evolution of rejection. Decreased uptake of [N]NH3 in rejecting allografts may be reflective of decreased myocardial blood flow. These data suggest that combined [F]FDG and [N]NH3 PET imaging could be used as a noninvasive, quantitative technique for serial monitoring of allograft rejection and has potential application in human transplant recipients.

Functional contribution of α1D-adrenoceptors in the renal vasculature of left ventricular hypertrophy induced with isoprenaline and caffeine in Wistar-Kyoto rats

This study investigated the role of α1D-adrenoceptor in the modulation of renal haemodynamics in rats with left ventricular hypertrophy (LVH). LVH was established in Wistar-Kyoto (WKY) rats with isoprenaline (5.0 mg · (kg body mass)(-1), by subcutaneous injection every 72 h) and caffeine (62 mg · L(-1) in drinking water, daily for 14 days). Renal vasoconstrictor responses were measured for noradrenaline (NA), phenylephrine (PE), and methoxamine (ME) before and immediately after low or high dose intrarenal infusions of BMY 7378, a selective α1D-adrenoceptor blocker. The rats with LVH had higher mean arterial blood pressure and circulating NA levels, but lower renal cortical blood perfusion compared with the control group (all P < 0.05). In the LVH group, the magnitude of the renal vasoconstrictor response to ME was blunted, but not the response to NA or PE (P < 0.05), compared with the control group (LVH vs. C, 38% vs. 50%). The magnitude of the drop in the vasoconstrictor responses to NA, PE, and ME in the presence of a higher dose of BMY 7378 was significantly greater in the LVH group compared with the control group (LVH vs. C, 45% vs. 25% for NA, 52% vs. 33% for PE, 66% vs. 53% for ME, all P < 0.05). These findings indicate an impaired renal vasoconstrictor response to adrenergic agonists during LVH. In addition, the α1D-adrenoceptor subtype plays a key role in the modulation of vascular responses in this diseased state.

Extraction of an input function from dynamic micro-PET images using wavelet packet based sub-band decomposition independent component analysis

Positron emission tomography (PET) can be used to quantify physiological parameters. However to perform quantification requires that an input function is measured, namely a plasma time activity curve (TAC). Image-derived input functions (IDIFs) are attractive because they are noninvasive and nearly no blood loss is involved. However, the spatial resolution and the signal to noise ratio (SNR) of PET images are low, which degrades the accuracy of IDIFs. The objective of this study was to extract accurate input functions from microPET images with zero or one plasma sample using wavelet packet based sub-band decomposition independent component analysis (WP SDICA). Two approaches were used in this study. The first was the use of simulated dynamic rat images with different spatial resolutions and SNRs, and the second was the use of dynamic images of eight Sprague-Dawley rats. We also used a population-based input function and a fuzzy c-means clustering approach and compared their results with those obtained by our method using normalized root mean square errors, area under curve errors, and correlation coefficients. Our results showed that the accuracy of the one-sample WP SDICA approach was better than the other approaches using both simulated and realistic comparisons. The errors in the metabolic rate, as estimated by one-sample WP SDICA, were also the smallest using our approach.

Value of radionuclide studies in cardiac transplantation

Effective noninvasive evaluation of acute and chronic allograft rejection remains an important challenge in patients with cardiac transplantation. Radionuclide studies have demonstrated utility because of their ease of use, giving relevant information about the pathophysiology of the transplanted heart, along with valuable diagnostic and prognostic indicators. This article focuses on reviewing the pathophysiological changes of the transplanted heart and implications for radionuclide studies.

Measurement of input functions in rodents: challenges and solutions

INTRODUCTION

Tracer kinetic modeling used in conjunction with positron emission tomography (PET) is an excellent tool for the noninvasive quantification of physiological, biological and molecular processes and their alterations due to disease. Currently, complex multi-compartment modeling approaches are being applied in a variety of clinical studies to determine myocardial perfusion, viability and glucose utilization as well as fatty acid metabolism and oxidation in the normal and diseased heart. These kinetic models require two key measurements of tracer activity over time, tracer activity in arterial blood (input function) and its corresponding activity in the organ of interest. The alteration in the time course of tracer activity as it travels from blood to the organ of interest describes the kinetics of the tracer. To be able to implement these approaches in rodent models of disease using small-animal PET (microPET), it is imperative that the input function is measured accurately.

METHODS

The blood input functions in rodent experiments were obtained by (1) direct blood sampling, (2) direct measurement of blood activity by a beta-detecting probe that counts the activity in the blood, (3) an arterial-venous bypass (A/V shunt), (4) factor analysis of dynamic structures from dynamic PET images and (5) measurement from region-of-interest (ROI) analysis of dynamic PET images. Direct blood sampling was used as the reference standard to which the results of the other techniques were compared.

RESULTS

Beta probes are difficult to operate and may not provide accurate blood input functions unless they are used intravenously, which requires complicated microsurgery. A similar limitation applies to the A/V shunt. Factor analysis successfully extracts the blood input function for mice and rats. The ROI-based method is less accurate due to limited image resolution of the PET system, which results in severe partial volume effect and spillover from myocardium.

CONCLUSION

The current reference standard, direct blood sampling, is more invasive and has limited temporal resolution. With current imaging technology, image-based extraction of blood input functions is possible by factor analysis, while forthcoming technological developments are likely to allow extraction of input function directly from the images. These techniques will reduce the level of complexity and invasiveness for animal experiments and are likely to be used more widely in the future.

Evolving role of positron emission tomography in the management of patients with inflammatory and other benign disorders

Fluoro-2-deoxyglucose (FDG) positron emission tomography (PET) has evolved from a research imaging modality assessing brain function in physiologic and pathologic states to a pure clinical necessity. It has been successfully used for diagnosing, staging, and monitoring a variety of malignancies. FDG-PET imaging also is evolving into a powerful imaging modality that can be effectively used for the diagnosis and monitoring of a certain nononcological diseases. PET has been shown to be very useful in the diagnosis of osteomyelitis, painful prostheses, sarcoidosis, fever of unknown etiology, and acquired immunodeficiency syndrome. Based on recent observations, several other disorders, such as environment-induced lung diseases, atherosclerosis, vasculitis, back pain, transplantation, and blood clot, can be successfully assessed with this technique. With the development and the introduction of several new PET radiotracers, it is expected that PET will secure a major role in the management of patients with inflammatory and other benign disorders.

Myocardial efficiency and sympathetic reinnervation after orthotopic heart transplantation: a noninvasive study with positron emission tomography

BACKGROUND

The lack of cardiac catecholamine uptake and storage caused by sympathetic denervation may influence performance of the transplanted heart. Reinnervation, occurring late after transplantation, may partially resolve these effects. In this study, oxidative metabolism and its relation to cardiac work were compared in allografts and normal and failing hearts, and the effects of sympathetic reinnervation were evaluated.

METHODS AND RESULTS

Twenty-seven nonrejecting, symptom-free transplant recipients, 11 healthy control subjects, and 10 patients with severe dilated cardiomyopathy underwent PET with (11)C acetate for assessment of oxidative metabolism by the clearance constant k(mono) and radionuclide angiography or MRI for measurement of ventricular function, geometry, and work. Efficiency was estimated noninvasively by a work-metabolic index [WMI=(stroke volumexheart ratexsystolic pressure)/k(mono)]. In 14 of 27 transplants, presence of regional reinnervation was identified with PET and the catecholamine analogue (11)C hydroxyephedrine (extent, 24+/-14% of left ventricle). The WMI was comparable in normal subjects and reinnervated and denervated transplants (6.2+/-2.3 versus 4.9+/-2.0 versus 4.9+/-1.2. 10(6) mm Hg. mL; P=NS) and significantly lower in cardiomyopathy patients (3.0+/-1.3. 10(6) mm Hg. mL; P<0.001). For normal subjects and transplant recipients, the WMI was significantly correlated with afterload (peripheral vascular resistance; r=-0.65, P<0.01), preload (end-diastolic volume; r=0.78, P<0.01), and stroke volume (r=0.81, P<0.01) but not with hydroxyephedrine retention (transplants only; r=0.09, P=NS).

CONCLUSIONS

After transplantation, cardiac efficiency is improved compared with failing hearts and comparable to normal hearts. Differences between denervated and reinnervated allografts were not surveyed. Additionally, the dependency on loading conditions and contractility was preserved, suggesting that normal regulatory interactions for efficiency are intact and that sympathetic tone does not play a role under resting conditions.

Magnetic resonance-based assessment of global coronary flow and flow reserve and its relation to left ventricular functional parameters: a comparison with positron emission tomography

BACKGROUND

Measurement of coronary sinus blood flow (CSF) by phase-contrast magnetic resonance (PC-MR) imaging at rest and during hyperemia may allow noninvasive assessment of global coronary hemodynamics.

METHODS AND RESULTS

Sixteen healthy volunteers (age, 22 to 32 years) were examined with MR and PET in random order within 1 to 2 days. At rest and during hyperemia (dipyridamole 0.56 mg/kg), CSF was measured by a cine PC-MR technique (temporal resolution, 40 ms; spatial resolution, 1.25x0.8 mm(2)), and myocardial blood flow (MBF) was measured by [(13)N]NH(3) PET. PET and MR agreed closely for coronary flow reserve (CFR; mean difference, 2.2+/-14.7%; Bland-Altman method). CSF divided by either total left ventricular mass or an estimate of drained myocardium (LVM(drain)) correlated highly with PET flow data (r=0.93 and 0.95, respectively) and with measures of oxygen demand, ie, heart rate, afterload-corrected fiber shortening, and peak systolic stress determined by MR (overall correlation coefficients, 0.81 and 0.87, respectively, multivariate analysis). CSF/LVM(drain) did not differ significantly from PET-derived MBF (difference, 3.6+/-16.6%). In orthotopic heart transplant recipients (n=9), CFR was reduced and blood supply-demand relationships at rest were shifted toward higher flows (P<0.0001).

CONCLUSIONS

This integrated MR approach allows comprehensive assessment of autoregulated and hyperemic coronary flow and is suitable for serial measurements in patients. In transplanted hearts, elevated resting flow is the major cause of reduced CFR.

Oxidative metabolism of the transplanted human heart assessed by positron emission tomography using C-11 acetate

For investigation of the metabolic profile of the transplanted human heart, positron emission tomography with C-11 acetate was performed to compare global and regional oxidative metabolism in 14 transplant patients with that in 10 healthy volunteers. Because no difference between transplants and normals could be observed, the results suggest that oxidative metabolism remains stable after transplantation, whereas cardiac work remains the major determinant.