Mireille Lortie

Quantification of myocardial blood flow with 82Rb dynamic PET imaging

PurposeThe PET tracer 82Rb is commonly used to evaluate regional perfusion defects for the diagnosis of coronary artery disease. There is limited information on the quantification of myocardial blood flow and flow reserve with this tracer. The goal of this study was to investigate the use of a one-compartment model of 82Rb kinetics for the quantification of myocardial blood flow.MethodsFourteen healthy volunteers underwent rest and dipyridamole stress imaging with both 13N-ammonia and 82Rb within a 2-week interval. Myocardial blood flow was estimated from the time-activity curves measured with 13N-ammonia using a standard two-compartment model. The uptake parameter of the one-compartment model was estimated from the time-activity curves measured with 82Rb. To describe the relationship between myocardial blood flow and the uptake parameter, a nonlinear extraction function was fitted to the data. This function was then used to convert estimates of the uptake parameter to flow estimates. The extraction function was validated with an independent data set obtained from 13 subjects with documented evidence of coronary artery disease (CAD).ResultsThe one-compartment model described 82Rb kinetics very well (median R-square = 0.98). The flow estimates obtained with 82Rb were well correlated with those obtained with 13N-ammonia (r = 0.85), and the best-fit line did not differ significantly from the identity line. Data obtained from the subjects with CAD confirmed the validity of the estimated extraction function.ConclusionIt is possible to obtain accurate estimates of myocardial blood flow and flow reserve with a one-compartment model of 82Rb kinetics and a nonlinear extraction function.

Concomitant treatment with oral L-arginine improves the efficacy of surgical angiogenesis in patients with severe diffuse coronary artery disease: The Endothelial Modulation in Angiogenic Therapy randomized controlled trial

OBJECTIVE Endothelial dysfunction and decreased nitric oxide bioavailability may explain why therapeutic angiogenesis and cell therapy have mostly failed in humans. Building from previous large animal work, the Phase I Endothelial Modulation in Angiogenic Therapy trial tested the hypothesis that L-arginine, a nitric oxide donor, may be safe and effective in potentiating surgical angiogenesis in humans. METHODS Patients with surgical triple-vessel coronary disease and a severely diffusely diseased left anterior descending artery were randomized in 2 x 2 factorial fashion to receive ten 200-microg injections of vascular endothelial growth factor-165 plasmid DNA or placebo in the anterior myocardium along the proximal and mid-left anterior descending arteries, plus oral L-arginine supplementation at a dose of 6 g per day or placebo for 3 months. The distal left anterior descending artery and other coronary arteries were grafted. End points included 3-month changes in myocardial perfusion and contractility of the anterior myocardium, using (13)N-ammonia positron emission tomography and echocardiography. Baseline scans were obtained 3 to 7 days postoperatively to delineate treatment effects from the effects of coronary artery bypass grafting. RESULTS Patient (N = 19) characteristics were equivalent between groups. There was no perioperative or late mortality. Patients who received the combination of vascular endothelial growth factor and L-arginine had improved anterior wall perfusion on positron emission tomography (P = .02), a trend toward smaller perfusion defects (P = .10), and better anterior wall contractility (P = .02, Kruskal-Wallis) at 3 months versus baseline. This was corroborated by a trend toward better disease perception at 3 months versus baseline on the Seattle Angina Questionnaire (score improvement of 47 +/- 35, combination treatment group; P = .1, Kruskal-Wallis). CONCLUSION To our knowledge, this is the first study to examine concomitant substrate modification in patients undergoing new biosurgical therapies by using vascular endothelial growth factor angiogenesis. The results suggest safety and efficacy. Concomitant endothelial modulation with L-arginine not only has the potential to make angiogenesis effective but also may have implications for cell therapy trials.

PET of (R)-11C-Rolipram Binding to Phosphodiesterase-4 Is Reproducible and Sensitive to Increased Norepinephrine in the Rat Heart

Phosphodiesterase-4 (PDE4) plays a critical role in the regulation of β-adrenergic receptor–stimulated cyclic adenosine monophosphate cell signaling in the heart. (R)-rolipram, a PDE4-selective inhibitor, has been studied previously as a radiotracer for the quantification of PDE4 levels. The aim of this study was to characterize (R)-11C-rolipram binding in the rat myocardium in vivo, using small-animal PET. Methods: Male Sprague–Dawley rats (n = 30) were administered (R)-11C-rolipram and imaged for 60 min to evaluate tracer binding and reproducibility, quantified using Logan slope analysis of the distribution volume. Dynamic 13N-ammonia imaging was performed to quantify myocardial blood flow and assist in cardiac regional analysis. Saturation studies evaluated the sensitivity of (R)-11C-rolipram to PDE4 blocking by unlabeled cold (R)-rolipram (0.0001–1.0 mg/kg), for estimation of the median effective dose (ED50) in the heart. (R)-11C-rolipram response to enhanced norepinephrine stimulation of the β-adrenergic receptor with desipramine (20 mg/kg, intravenous) was also studied. Intrarat variability studies (n = 5) were conducted with test–retest imaging at 16 ± 7 d. Results: A reduction of Logan slope was observed with increasing cold mass coadministered with the tracer, with an ED50 of 0.0019 mg/kg (95% confidence interval, 0.0014–0.0052) estimated from the saturation studies. This ED50 predicted less than 10% enzyme occupancy at 0.0002 mg of cold (R)-rolipram per kilogram (mass/body weight). Low-occupancy imaging at 0.00018 ± 0.00002 mg/kg produced a mean Logan slope of 5.5 ± 0.85 mL/cm3. Enzyme saturation of more than 90%, compared with low-occupancy conditions, occurred at more than 0.02 mg/kg, with a complete blocking dose (>1 mg of (R)-rolipram per kilogram) resulting in a Logan slope of 3.3 ± 0.1 mL/cm3, representing a 40% reduction. Compared with baseline, a Logan slope of 6.8 ± 0.7 mL/cm3 in desipramine-challenged animals was observed, representing a 30% increase due to acute norepinephrine stimulation, despite a reduction in myocardial blood flow. Intrarat and intraoperator variability was less than 5% between repeated measures. Conclusion: (R)-11C-rolipram shows the ability to monitor increases and decreases in PDE4 availability in the rat myocardium, with good reproducibility.

Use of a column-switching high-performance liquid chromatography method to assess the presence of specific binding of (R)- and (S)-[11C]rolipram and their labeled metabolites to the phosphodiesterase-4 enzyme in rat plasma and tissues

INTRODUCTION To complement recent studies using the high-affinity (11)C-labeled phosphodiesterase-4 (PDE4) inhibitor (R)-rolipram and the less active enantiomer (S)-[(11)C]rolipram for in vivo quantification of PDE4 levels, we evaluated the presence of radiolabeled metabolites and their potential binding to PDE4 in the rat plasma, brain, heart, pancreas, skeletal muscle and brown adipose tissue. METHODS A reverse-phase capture and analytical HPLC column-switch method was used to detect (R)-[(11)C]rolipram, (S)-[(11)C]rolipram and their radiolabeled metabolites in rat plasma and tissue extracts. The relative proportion of PDE4-specific binding of the radiotracers and their labeled metabolites was analyzed following co-injections with a saturating dose of unlabeled (R)-rolipram at 45 min post-tracer injection in tissue extracts. RESULTS Radiolabeled metabolites were found in the plasma (72-75% of total radioactive signal), and in the heart, skeletal muscle, pancreas and brown adipose tissue (44-52%), but not in the brain. In comparison to polar labeled metabolites, the proportion of unchanged (R)-[(11)C]rolipram was reduced in PDE4-rich organs by co-injection of unlabeled (R)-rolipram. Conversely, no changes were obtained in brown adipose tissue, or with (S)-[(11)C]rolipram, suggesting that radiolabeled metabolites of (R)-[(11)C]rolipram display no specific binding to PDE4. CONCLUSIONS Radiolabeled hydrophilic metabolites are unlikely to compete with (R)-[(11)C]rolipram for PDE4-specific retention. However, due to the high proportion of the radioactive metabolites in the total radioactive signal, any kinetic modeling calculations in the peripheral tissues will need to take into account the presence of labeled metabolites.

Fully Automated Software for Polar-Map Registration and Sampling from PET Images

Left ventricle myocardial activity (LV) from volumetric images is commonly presented in a 2D format called a polar map. Few applications offer complete automation of the registration and sampling process, which may lead to biasing of the results by the user and increased variability. A fully automated process is described which includes two steps: reorientation of the data to a common LV reference frame, and registration of the LV using a deformable spline model. The software implementation of this algorithm consists of a graphical user interface (GUI) that enables the user to verify the process, and to intervene in cases where automation may fail. 135 PET images (consisting of several tracers, in rest and stress, and in humans and dogs) were processed and visually validated. All but one case (99.3%) were successfully handled by the automated process.

PET Measurements of cAMP-Mediated Phosphodiesterase-4 with (R)-[11C]Rolipram

Cyclic adenosine monophosphate (cAMP) is the common second messenger in signal-transduction cascades originating at a number of monoamine receptors involved in neurotransmission, cardiac function and smooth muscle contraction. Altered regulation of cAMP synthesis (at receptors, G-protein subunits or adenylyl cyclase) and breakdown by phosphodiesterase (PDE) enzymes have been implicated in a number of pathologies. The PDE4 inhibitor (R)-rolipram, and the less active (S)- enantiomer, have been labeled with carbon-11 and characterized by in vivo and in vitro experiments for use in the evaluation of altered PDE4 levels in the brain and cardiac tissues. (R)-[11C]Rolipram has been shown to bind selectively to PDE4 over other PDE isozymes, with specific binding reflecting approximately 80 and 40% of the total detected radioactivity in the rat brain and the heart, respectively. Tracer retention in PDE4-rich tissues is increased by cAMP-elevating treatments, as detected by in vivo PET studies and ex vivo biodistribution experiments. In vivo PET imaging studies display strong region-specific signal in the brain and heart, as evaluated in rats, pigs, monkeys and humans. Impaired cAMP-mediated signaling was observed in animal models of aging, obesity, anthracycline-induced cardiotoxicity and myocardial infarction using (R)-[11C]rolipram. Given the critical role of cAMP in multiple hormonal pathways, the good safety profile and well-characterized pharmacokinetics, (R)-[11C]rolipram PET imaging provides a novel tool for serial monitoring of cAMP-mediated signaling at the PDE4 level, yielding insight into pathological progression with potential for directing therapy.

Uniformity and repeatability of normal resting myocardial blood flow in rats using [13N]-ammonia and small animal PET.

ObjectiveThis study aimed to quantitatively evaluate population variability, regional uniformity and repeatability of myocardial blood flow measurements using [13N]-ammonia and small animal PET. MethodsSerial PET scans were conducted on Sprague–Dawley rats using [13N]-ammonia to study relative perfusion and absolute myocardial blood flow (ml/min/g). FlowQuant automated analysis software was used to produce five-segment polar maps to investigate regional myocardial blood flow differences. The interobserver and intraobserver repeatability was assessed quantitatively using Bland–Altman analysis. ResultsAbsolute myocardial blood flow values were 4.3±1.1 ml/min/g, corresponding to a population variability of 25.5%. There were significant age-related increases in resting myocardial blood flow (r2=0.59, P<0.001). The test–retest differences had a coefficient of repeatability of 24.5% of the mean myocardial blood flow. The operator variability was small, relative to the population variability. ConclusionRepeatable myocardial blood flow values are minimally influenced by operator intervention. However, age-related myocardial blood flow increases must be taken into account when comparing measurements between experimental groups.

Quantification of regional myocardial blood flow in a canine model of stunned and infarcted myocardium: comparison of rubidium-82 positron emission tomography with microspheres.

BackgroundMyocardial viability and quantification of regional myocardial blood flow (MBF) are important for the diagnosis of heart disease. Positron emission tomography is the current gold standard for determining myocardial viability, but most positron-emitting perfusion tracers require an on-site cyclotron. Rubidium-82 (82Rb) is a myocardial perfusion tracer that is produced using an on-site generator. This study investigates 82Rb-measured MBF in canine models of stunned and infarcted myocardium compared with selected measurements obtained concurrently using microspheres. MethodsMyocardial stunning and infarction were created in canines by occluding the left anterior descending for 15 min and 2 h, respectively. Stunning was produced in all animals; six animals were reperfused after the 2 h occlusion, whereas the other six animals remained occluded permanently. Regional MBF was measured in each group during rest and dobutamine stress at acute and chronic (8 weeks postinsult) time points using dynamic 82Rb perfusion imaging and radioactively labeled microspheres. ResultsAverage resting MBF with microspheres and 82Rb was 0.68±0.02 versus 0.73±0.01 (P<0.001) in nonischemic tissue, and 0.53±0.03 versus 0.42±0.02 (P<0.001) in the region-at-risk tissue, respectively. Average MBF during stress with microspheres and 82Rb was 2.78±0.15 versus 3.53±0.16 (P<0.05) in the nonischemic tissue, and 1.90±0.20 versus 2.31±0.26 (P = NS) in the region-at-risk tissue, respectively. ConclusionDespite the small significant differences, the dynamic 82Rb measurements provide estimates of MBF in stunned and acutely and chronically infarcted tissue at rest and during hyperemia that correspond with clinical interpretation.

Constant-Activity-Rate Infusions for Myocardial Blood Flow Quantification with 82Rb and 3D PET

Quantification of myocardial blood flow (MBF) can be performed using dynamic 82Rb PET imaging. However, the small dynamic range of some 3D PET systems can prevent accurate measurement of the first-pass activity with standard 82Rb bolus infusions. The purpose of this study was to investigate the use of slow constant-activity-rate infusions to optimize MBF quantification with 3D PET. Methods: Dynamic 3D PET imaging was performed in a normal dog with 150 MBq of 82Rb infused over 15, 30, 60, 120 and 240 s. Left ventricular mean MBF was quantified using a one-compartment model of 82Rb kinetics, and compared to standard values obtained using 13N-ammonia with a two-compartment model. Results: Peak deadtime and coincidence count-rates decreased by almost 50% and 70% respectively with 240 vs. 30 s tracer infusion, but integral true coincidences (prompt - delayed) were maintained. Mean MBF values remained accurate (1.4 vs. 1.3 mL/min/g) using 240 vs. 30 s infusions, and vs. standard 13N-ammonia MBF values (1.3 mL/min/g). Precision of the MBF estimates was also improved by a factor of 3.5. Conclusion: Constant-activity-rate infusions reduce the dynamic range needed for MBF quantification with 82Rb and 3D PET imaging. Using a one-compartment model, the precision of MBF estimates are also improved with slower infusions. This may permit improved MBF quantification using 3D PET, with high integral counts maintained for conventional perfusion imaging.

Analysis of [11C]methyl-candesartan kinetics in the rat kidney for the assessment of angiotensin II type 1 receptor density in vivo with PET ☆

INTRODUCTION Angiotensin II type 1 (AT(1)) receptors play a key role in the regulation of renal and cardiovascular functions and have been implicated in the pathogenesis of many diseases. The aim of this study was to assess binding of the novel radioligand [(11)C]methyl-candesartan to AT(1) receptors in the rat kidney in vivo with PET. METHODS Dynamic PET images were acquired for 60 min at baseline, with coinjection of candesartan (5 mg/kg), and after injection of PD123,319 (5 mg/kg). Volumes of distribution (R(C)∙V(T)) were estimated with a two-compartment model, by Logan analysis, and by taking the tissue-to-plasma activity ratio at 50-60 min post-injection. RESULTS The two-compartment model did not describe the kinetics at baseline adequately and the baseline scans were too short to obtain accurate estimates of R(C)∙V(T) with the Logan approach. Based on the tissue-to-plasma ratios, roughly one-third of V(T) at baseline could be attributed to AT(1) receptor binding. There were no indications of AT(2) receptor binding in the rat kidney. CONCLUSION It may be possible to detect changes in AT(1) receptor density in the rat kidney in vivo with [(11)C]methyl-candesartan and PET. Imaging AT(1) receptors with PET may provide valuable information on the role of these receptors in the pathogenesis of diseases such as hypertension, diabetic nephropathy, ventricular remodeling, and heart failure.