F-18 fluorodeoxyglucose uptake and water-perfusable tissue fraction in assessment of myocardial viability

Improvement in the estimation of perfusable tissue fraction and myocardial flow reserve in the ischemic myocardial lesions using ECG-gated dynamic myocardial PET with 15O-water

OBJECTIVE

Perfusable tissue fraction (PTF) and myocardial flow reserve (MFR) from 15O-water dynamic positron emission tomography (PET) are parameters of myocardial viability. However, myocardial motion causes errors in these values. We aimed to develop accurate estimation of PTF and MFR in ischemic lesions using an electro-cardiogram (ECG)-gated dynamic myocardial PET with 15O-water.

METHODS

Twenty-seven patients with ischemic heart disease were enrolled. All patients underwent percutaneous coronary intervention (PCI) or coronary artery bypass grafting (CABG). List mode 3D PET data and ECG signals were acquired using Philips Gemini TF64 instrument. For each scan, 500 MBq of 15O-water was infused slowly for 2 min, and the dynamic data were scanned for 6 min. Both non-gated dynamic images and ECG-gated diastolic dynamic images were reconstructed. On the myocardial PET images of each patient, the entire myocardial region of interest (ROI) was set and divided into 17 segments. Myocardial blood flow in the resting state (rest MBF), hyperemic state (stress MBF), PTF, and MFR in each segment were estimated from both non-gated and ECG-gated dynamic PET images. Coronary arteriograms were obtained for all patients. In total, 128 normal segments without stenosis and 50 ischemic segments with > 90% stenosis were evaluated.

RESULTS

In the ischemic myocardial segments, the PTF with ECG-gated PET was estimated as significantly lower than that with non-gated PET (0.63 ± 0.09 vs. 0.72 ± 0.08 [mL/mL], p < 0.001). The ECG-gated PET estimated a significantly lower PTF in the ischemic segments than in the normal segments (0.63 ± 0.09 vs. 0.67 ± 0.07 [mL/mL], p < 0.01). In the normal segments, the ECG-gated PET detected no significant difference in MFR compared with those from the non-gated PET (2.15 ± 0.76 vs. 2.24 ± 0.79, p = 0.28). However, in the ischemic myocardial segments, the MFR with ECG-gated PET was estimated as significantly lower than that with the non-gated PET (1.23 ± 0.29 vs. 1.69 ± 0.71, p < 0.001). The ECG-gated PET presented a significantly higher inter-observer reproducibility of PTF and rest MBF than the non-gated PET (p < 0.01). Neither stress MBF nor MFR yielded significant differences in inter-observer reproducibility between the ECG-gated and non-gated PET.

CONCLUSIONS

The ECG-gated dynamic 15O-water PET suppressed the myocardial motion effect and resulted in a lower PTF and MFR in ischemic myocardial lesions than the non-gated PET. The ECG-gated PET seemed to be better than the conventional non-gated dynamic PET for the detection of ischemic myocardial lesion.

Quantitative Perfusion Imaging with Total-Body PET

Recently, PET systems with a long axial field of view have become the current state of the art. Total-body PET scanners enable unique possibilities for scientific research and clinical diagnostics, but this new technology also raises numerous challenges. A key advantage of total-body imaging is that having all the organs in the field of view allows studying biologic interaction of all organs simultaneously. One of the new, promising imaging techniques is total-body quantitative perfusion imaging. Currently, 15O-labeled water provides a feasible option for quantitation of tissue perfusion at the total-body level. This review summarizes the status of the methodology and the analysis and provides examples of preliminary findings on applications of quantitative parametric perfusion images for research and clinical work. We also describe the opportunities and challenges arising from moving from single-organ studies to modeling of a multisystem approach with total-body PET, and we discuss future directions for total-body imaging.

Assessment of myocardial viability with [15O]water PET: A validation study in experimental myocardial infarction

BACKGROUND

Assessment of myocardial viability is often needed in patients with chest pain and reduced ejection fraction. We evaluated the performance of reduced resting MBF, perfusable tissue fraction (PTF), and perfusable tissue index (PTI) in the assessment of myocardial viability in a pig model of myocardial infarction (MI).

METHODS AND RESULTS

Pigs underwent resting [15O]water PET perfusion study 12 weeks after surgical (n = 16) or 2 weeks after catheter-based (n = 4) occlusion of the proximal left anterior descending coronary artery. MBF, PTF, and PTI were compared with volume fraction of MI in matched segments as assessed by triphenyl tetrazolium chloride staining of LV slices. MBF and PTF were lower in infarcted than non-infarcted segments. Segmental analysis of MBF showed similar area under the curve (AUC) of 0.85, 0.86, and 0.90 with relative MBF, PTF, and PTI for the detection of viable myocardium defined as infarct volume fraction of < 75%. Cut-off values of relative MBF of ≥ 67% and PTF of ≥ 66% resulted in accuracies of 90% and 81%, respectively.

CONCLUSIONS

Our results indicate that resting MBF, PTF, and PTI based on [15O]water PET perfusion imaging are useful for the assessment of myocardial viability.

15O-Water PET MPI: Current Status and Future Perspectives

Myocardial perfusion imaging with 15O-water positron emission tomography (PET) is a validated tool for quantitative measurement of myocardial blood flow (MBF) and myocardial flow reserve (MFR). Current scanner and software technology enable quantification of global and regional MBF in clinical PET myocardial perfusion imaging studies. Reduced stress MBF or MFR measured by 15O-water PET accurately detects hemodynamically significant coronary artery stenosis defined by intracoronary fractional flow reserve (FFR) measurement in patients with suspected obstructive coronary artery disease (CAD). Furthermore, MBF and MFR provide prognostic information on mortality and risk of myocardial infarction. Clinical experience in some centers indicates that clinical application of 15O-water PET in evaluation of CAD is feasible and guides management decisions on revascularization. This review discusses basic concepts of measuring MBF with 15O-water PET and reviews clinical studies on its application in evaluation of obstructive CAD.

Systemic Dosing of Thymosin Beta 4 before and after Ischemia Does Not Attenuate Global Myocardial Ischemia-Reperfusion Injury in Pigs

The use of cardiopulmonary bypass (CPB) and aortic cross-clamping causes myocardial ischemia-reperfusion injury (I-RI) and can lead to reduced postoperative cardiac function. We investigated whether this injury could be attenuated by thymosin beta 4 (TB4), a peptide which has showed cardioprotective effects. Pigs received either TB4 or vehicle and underwent CPB and aortic cross-clamping for 60 min with cold intermittent blood-cardioplegia and were then followed for 30 h. Myocardial function and blood flow was studied by cardiac magnetic resonance and PET imaging. Tissue and plasma samples were analyzed to determine the amount of cardiomyocyte necrosis and apoptosis as well as pharmacokinetics of the peptide. In vitro studies were performed to assess its influence on blood coagulation and vasomotor tone. Serum levels of the peptide were increased after administration compared to control samples. TB4 did not decrease the amount of cell death. Cardiac function and global myocardial blood flow was similar between the study groups. At high doses a vasoconstrictor effect on mesentery arteries and a vasodilator effect on coronary arteries was observed and blood clot firmness was reduced when tested in the presence of an antiplatelet agent. Despite promising results in previous trials the cardioprotective effect of TB4 was not demonstrated in this model for global myocardial I-RI.

Radionuclide Imaging in Congestive Heart Failure: Assessment of Viability, Sarcoidosis, and Amyloidosis

Radionuclide imaging provides both established and emerging diagnostic and prognostic tools to assist clinicians in the management of patients with ischemic cardiomyopathy, cardiac sarcoidosis, and cardiac amyloidosis. This review highlights the underlying pathophysiology of each entity and associated diagnostic and clinical challenges, and describes the available radionuclide imaging techniques. Specific protocols, advantages and disadvantages, comparison with other noninvasive imaging modalities, and discussion of the evolving role of hybrid imaging are also included.

(15)O-Water PET Study of the Effect of Imatinib, a Selective Platelet-Derived Growth Factor Receptor Inhibitor, Versus Anakinra, an IL-1R Antagonist, on Water-Perfusable Tissue Fraction in Colorectal Cancer Metastases

High interstitial fluid pressure (IFP) in colorectal cancer metastases may decrease the uptake and, thus, the effects of antitumor drugs. Imatinib, a selective inhibitor of platelet-derived growth factor receptors, and anakinra, an interleukin-1 receptor antagonist, respectively, increase drug uptake or decrease IFP in preclinical models of carcinoma. Drug-induced decrease in IFP in human metastases has not been objectively shown but should be reflected by an increase in water-perfusable tissue fraction (PTF) or tumor blood flow (TBF) using (15)O-water PET/CT and kinetic modeling. Hence, the aim of this study was to assess the effects of imatinib and anakinra on PTF and TBF in colorectal cancer metastases in patients.

METHODS

Nine patients with documented progressive disease despite all established therapy underwent (15)O-water PET/CT at baseline and at 2 d and 6-7 d after the start of oral administration of imatinib (400 mg/d). After a washout period of 1 wk, the protocol was repeated with anakinra (100 mg/d) subcutaneously. Six patients underwent a second baseline scan on the same day to assess reproducibility of PTF and TBF measurements. Volumes of interest were drawn over liver metastases and aorta. PTF and TBF were calculated using the standard single-tissue-compartment model.

RESULTS

Imatinib administration during 6-7 d increased PTF from 0.62 ± 0.12 to 0.69 ± 0.13, compared with baseline and day 2 (P = 0.02, Wilcoxon test). No significant changes were found in TBF. PTF values were no longer significantly different from baseline 1 wk after the last imatinib dosage. Anakinra induced no significant change in PTF or TBF. The repeatability coefficients of PTF and TBF in liver lesions were 22% and 28%, respectively.

CONCLUSION

Imatinib increases PTF of colorectal cancer metastases in patients and hence may increase the delivery of antitumor drugs. (15)O-water PET/CT and kinetic modeling provide insights into the microenvironment of human cancers.

Advanced tracers in PET imaging of cardiovascular disease

Cardiovascular disease is the leading cause of death worldwide. Molecular imaging with targeted tracers by positron emission tomography (PET) allows for the noninvasive detection and characterization of biological changes at the molecular level, leading to earlier disease detection, objective monitoring of therapies, and better prognostication of cardiovascular diseases progression. Here we review, the current role of PET in cardiovascular disease, with emphasize on tracers developed for PET imaging of cardiovascular diseases.

Assessment of myocardial perfusion and viability by positron emission tomography

An important evolution has taken place recently in the field of cardiovascular Positron Emission Tomography (PET) imaging. Being originally a highly versatile research tool that has contributed significantly to advance our understanding of cardiovascular physiology and pathophysiology, PET has gradually been incorporated into the clinical cardiac imaging portfolio contributing to diagnosis and management of patients investigated for coronary artery disease (CAD). PET myocardial perfusion imaging (MPI) has an average sensitivity and specificity around 90% for the detection of angiographically significant CAD and it is also a very accurate technique for prognostication of patients with suspected or known CAD. In clinical practice, Rubidium-82 ((82)Rb) is the most widely used radiopharmaceutical for MPI that affords also accurate and reproducible quantification in absolute terms (ml/min/g) comparable to that obtained by cyclotron produced tracers such as Nitrogen-13 ammonia ((13)N-ammonia) and Oxygen-15 labeled water ((15)O-water). Quantification increases sensitivity for detection of multivessel CAD and it may also be helpful for detection of early stages of atherosclerosis or microvascular dysfunction. PET imaging combining perfusion with myocardial metabolism using (18)F-Fluorodeoxyglucose ((18)F FDG), a glucose analog, is an accurate standard for assessment of myocardial hibernation and risk stratification of patients with left ventricular dysfunction of ischemic etiology. It is helpful for guiding management decisions regarding revascularization or medical treatment and predicting improvement of symptoms, exercise capacity and quality of life post-revascularization. The strengths of PET can be increased further with the introduction of hybrid scanners, which combine PET with computed tomography (PET/CT) or with magnetic resonance imaging (PET/MRI) offering integrated morphological, biological and physiological information and hence, comprehensive evaluation of the consequences of atherosclerosis in the coronary arteries and the myocardium.