18F-FDG PET imaging of myocardial viability in an experienced center with access to 18F-FDG and integration with clinical management teams: the Ottawa-FIVE substudy of the PARR 2 trial

Feasibility of shortening scan duration of 18F-FDG myocardial metabolism imaging using a total-body PET/CT scanner

PURPOSE

To evaluate 18F-FDG myocardial metabolism imaging (MMI) using a total-body PET/CT scanner and explore the feasible scan duration to guide the clinical practice.

METHODS

A retrospective analysis was conducted on 41 patients who underwent myocardial perfusion-metabolism imaging to assess myocardial viability. The patients underwent 18F-FDG MMI with a total-body PET/CT scanner using a list-mode for 600 s. PET data were trimmed and reconstructed to simulate images of 600-s, 300-s, 120-s, 60-s, and 30-s acquisition time (G600-G30). Images among different groups were subjectively evaluated using a 5-point Likert scale. Semi-quantitative evaluation was performed using standardized uptake value (SUV), myocardial to background activity ratio (M/B), signal to noise ratio (SNR), contrast to noise ratio (CNR), contrast ratio (CR), and coefficient of variation (CV). Myocardial viability analysis included indexes of Mismatch and Scar. G600 served as the reference.

RESULTS

Subjective visual evaluation indicated a decline in the scores of image quality with shortening scan duration. All the G600, G300, and G120 images were clinically acceptable (score ≥ 3), and their image quality scores were 4.9 ± 0.3, 4.8 ± 0.4, and 4.5 ± 0.8, respectively (P > 0.05). Moreover, as the scan duration reduced, the semi-quantitative parameters M/B, SNR, CNR, and CR decreased, while SUV and CV increased, and significant difference was observed in G300-G30 groups when comparing to G600 group (P < 0.05). For myocardial viability analysis of left ventricular and coronary segments, the Mismatch and Scar values of G300-G30 groups were almost identical to G600 group (ICC: 0.968-1.0, P < 0.001).

CONCLUSION

Sufficient image quality for clinical diagnosis could be achieved at G120 for MMI using a total-body PET/CT scanner, while the image quality of G30 was acceptable for myocardial viability analysis.

A Novel Approach to Identifying Hibernating Myocardium Using Radiomics-Based Machine Learning

Background To assess the feasibility of a machine learning (ML) approach using radiomics features of perfusion defects on rest myocardial perfusion imaging (MPI) to detect the presence of hibernating myocardium. Methodology Data of patients who underwent 99mTc-sestamibi MPI and 18F-FDG PET/CT for myocardial viability assessment were retrieved. Rest MPI data were processed on ECToolbox, and polar maps were saved using the NFile PMap tool. The reference standard for defining hibernating myocardium was the presence of mismatched perfusion-metabolism defect with impaired myocardial contractility at rest. Perfusion defects on the polar maps were delineated with regions of interest (ROIs) after spatial resampling and intensity discretization. Replicable random sampling allocated 80% (257) of the perfusion defects of the patients from January 2017 to September 2022 to the training set and the remaining 20% (64) to the validation set. An independent dataset of perfusion defects from 29 consecutive patients from October 2022 to January 2023 was used as the testing set for model evaluation. One hundred ten first and second-order texture features were extracted for each ROI. After feature normalization and imputation, 14 best-ranked features were selected using a multistep feature selection process including the Logistic Regression and Fast Correlation-Based Filter. Thirteen supervised ML algorithms were trained with stratified five-fold cross-validation on the training set and validated on the validation set. The ML algorithms with a Log Loss of <0.688 and <0.672 in the cross-validation and validation steps were evaluated on the testing set. Performance matrices of the algorithms assessed included area under the curve (AUC), classification accuracy (CA), F1 score, precision, recall, and specificity. To provide transparency and interpretability, SHapley Additive exPlanations (SHAP) values were assessed and depicted as beeswarm plots. Results Two hundred thirty-nine patients (214 males; mean age 56 ± 11 years) were enrolled in the study. There were 371 perfusion defects (321 in the training and validation sets; 50 in the testing set). Based on the reference standard, 168 perfusion defects had hibernating myocardium (139 in the training and validation sets; 29 in the testing set). On cross-validation, six ML algorithms with Log Loss <0.688 had AUC >0.800. On validation, 10 ML algorithms had a Log Loss value <0.672, among which six had AUC >0.800. On model evaluation of the selected models on the unseen testing set, nine ML models had AUC >0.800 with Gradient Boosting Random Forest (xgboost) [GB RF (xgboost)] achieving the highest AUC of 0.860 and could detect the presence of hibernating myocardium in 21/29 (72.4%) perfusion defects with a precision of 87.5% (21/24), specificity 85.7% (18/21), CA 78.0% (39/50) and F1 Score 0.792. Four models depicted a clear pattern of model interpretability based on the beeswarm SHAP plots. These were GB RF (xgboost), GB (scikit-learn), GB (xgboost), and Random Forest. Conclusion Our study demonstrates the potential of ML in detecting hibernating myocardium using radiomics features extracted from perfusion defects on rest MPI images. This proof-of-concept underscores the notion that radiomics features capture nuanced information beyond what is perceptible to the human eye, offering promising avenues for improved myocardial viability assessment.

Evaluation of Clinical Variables Affecting Myocardial Glucose Uptake in Cardiac FDG PET

PURPOSE

Cardiac 2-deoxy-2-[F-18]fluoro-D-glucose positron emission tomography (FDG PET) is widely used to assess myocardial viability in patients with ischemic heart disease. While sufficient glucose uptake is a prerequisite for accurate interpretation of cardiac viability, there are a lack of data on which clinical variables have the most significant impact on myocardial glucose metabolism. Therefore, this study was designed to evaluate several clinical variables that could affect myocardial glucose metabolism.

METHODS

A total of 214 consecutive cases were retrospectively enrolled in this study. All subjects received 250 mg of acipimox and underwent glucose loading as preparation for cardiac FDG PET/CT. Three-dimensional regions of interest (ROIs) were drawn on PET/CT fusion images. Myocardial glucose uptake ratio (MGUR = SUVmax of LV myocardium/SUVmean of liver) was then calculated. Multiple clinical variables including body mass index (BMI), blood glucose levels at different times, administered insulin dosage, lipid profiles, and ejection fraction were measured and analyzed for correlation with myocardial glucose uptake. After dichotomizing the subjects based on a BMI of 25, each group's MGUR was compared.

RESULTS

Myocardial uptake showed significant correlations with BMI (r = -0.162, p = 0.018), HbA1c (r = -0.150, p = 0.030), and triglyceride levels (r = -0.137, p = 0.046). No other clinical variables showed a significant correlation with myocardial glucose uptake. After multiple linear regression analysis, BMI (p = 0.032) and HbA1c (p = 0.050) showed a correlation with MGUR. In group analysis, after dividing patients based on BMI, the obese group showed significantly lower myocardial uptake than the non-obese group (3.8 ± 1.9 vs. 4.4 ± 2.1, p = 0.031).

CONCLUSIONS

Among several clinical variables, BMI and HbA1c levels were related to myocardial glucose uptake. A prospective study would be needed to examine whether a protocol that additionally considers BMI and HbA1c levels is necessary for the current cardiac FDG PET protocol.

Noninvasive Quantification of Glucose Metabolism in Mice Myocardium Using the Spline Reconstruction Technique

The spline reconstruction technique (SRT) is a fast algorithm based on a novel numerical implementation of an analytic representation of the inverse Radon transform. The purpose of this study was to compare the SRT, filtered back-projection (FBP), and the Tera-Tomo 3D algorithm for various iteration numbers, using small-animal dynamic PET data obtained from a Mediso nanoScan® PET/CT scanner. For this purpose, Patlak graphical kinetic analysis was employed to noninvasively quantify the myocardial metabolic rate of glucose (MRGlu) in seven male C57BL/6 mice (n=7). All analytic reconstructions were performed via software for tomographic image reconstruction. The analysis of all PET-reconstructed images was conducted with PMOD software (version 3.506, PMOD Technologies LLC, Fällanden, Switzerland) using the inferior vena cava as the image-derived input function. Statistical significance was determined by employing the one-way analysis of variance test. The results revealed that the differences between the values of MRGlu obtained via SRT versus FBP, and the variants of he Tera-Tomo 3D algorithm were not statistically significant (p > 0.05). Overall, the SRT appears to perform similarly to the other algorithms investigated, providing a valid alternative analytic method for preclinical dynamic PET studies.

Myocardial viability on trial

The concept of myocardial viability is usually referred to areas of the myocardium, which show contractile dysfunction at rest and in which contractility is expected to improve after revascularization. The traditional paradigm states that an improvement in function after revascularization leads to improved health outcomes and that assessment of myocardial viability in patients with ischaemic left ventricular dysfunction (ILVD) is a prerequisite for clinical decisions regarding treatment. A range of retrospective observational studies supported this 'viability hypothesis'. However, data from prospective trials have diverged from earlier retrospective studies and challenge this hypothesis. Traditional binary viability assessment may oversimplify ILVD's complexity and the nuances of revascularization benefits. A conceptual shift from the traditional paradigm centred on the assessment of viability as a dichotomous variable to a more comprehensive approach encompassing a thorough understanding of ILVD's complex pathophysiology and the salutary effect of revascularization in the prevention of myocardial infarction and ventricular arrhythmias is required.

Hot spot imaging in cardiovascular diseases: an information statement from SNMMI, ASNC, and EANM

This information statement from the Society of Nuclear Medicine and Molecular Imaging, American Society of Nuclear Cardiology, and European Association of Nuclear Medicine describes the performance, interpretation, and reporting of hot spot imaging in nuclear cardiology. The field of nuclear cardiology has historically focused on cold spot imaging for the interpretation of myocardial ischemia and infarction. Hot spot imaging has been an important part of nuclear medicine, particularly for oncology or infection indications, and the use of hot spot imaging in nuclear cardiology continues to expand. This document focuses on image acquisition and processing, methods of quantification, indications, protocols, and reporting of hot spot imaging. Indications discussed include myocardial viability, myocardial inflammation, device or valve infection, large vessel vasculitis, valve calcification and vulnerable plaques, and cardiac amyloidosis. This document contextualizes the foundations of image quantification and highlights reporting in each indication for the cardiac nuclear imager.

Myocardial Revascularization in Patients With Ischemic Cardiomyopathy: For Whom and How

Background Myocardial revascularization has been advocated to improve myocardial function and prognosis in ischemic cardiomyopathy (ICM). We discuss the evidence for revascularization in patients with ICM and the role of ischemia and viability detection in guiding treatment. Methods and Results We searched for randomized controlled trials evaluating the prognostic impact of revascularization in ICM and the value of viability imaging for patient management. Out of 1397 publications, 4 randomized controlled trials were included, enrolling 2480 patients. Three trials (HEART [Heart Failure Revascularisation Trial], STICH [Surgical Treatment for Ischemic Heart Failure], and REVIVED [REVascularization for Ischemic VEntricular Dysfunction]-BCIS2) randomized patients to revascularization or optimal medical therapy. HEART was stopped prematurely without showing any significant difference between treatment strategies. STICH showed a 16% lower mortality with bypass surgery compared with optimal medical therapy at a median follow-up of 9.8 years. However, neither the presence/extent of left ventricle viability nor ischemia interacted with treatment outcomes. REVIVED-BCIS2 showed no difference in the primary end point between percutaneous revascularization or optimal medical therapy. PARR-2 (Positron Emission Tomography and Recovery Following Revascularization) randomized patients to imaging-guided revascularization versus standard care, with neutral results overall. Information regarding the consistency of patient management with viability testing results was available in ≈65% of patients (n=1623). No difference in survival was revealed according to adherence or no adherence to viability imaging. Conclusions In ICM, the largest randomized controlled trial, STICH, suggests that surgical revascularization improves patients' prognosis at long-term follow-up, whereas evidence supports no benefit of percutaneous coronary intervention. Data from randomized controlled trials do not support myocardial ischemia or viability testing for treatment guidance. We propose an algorithm for the workup of patients with ICM considering clinical presentation, imaging results, and surgical risk.

Cardiovascular positron emission tomography: established and emerging role in cardiovascular diseases

Cardiac positron emission tomography (PET) imaging has established themselves firmly as excellent and reliable functional imaging modalities in assessment of the spectrum of coronary artery disease. With the explosion of technology advances and the dream of flow quantification now a reality, the value of PET is now well realized. Cardiac PET has proved itself as precise imaging modality that provides functional imaging of the heart in addition to anatomical imaging. It has established itself as one of the best available techniques for evaluation of myocardial viability. Hybrid PET/computed tomography provides simultaneous integration of coronary anatomy and function with myocardial perfusion and metabolism, thereby improving characterization of the dysfunctional area and chronic coronary artery disease. The availability of quantitative myocardial blood flow evaluation with PET provides additional prognostic information and increases diagnostic accuracy in the management of patients with coronary artery disease. Hybrid imaging seems to hold immense potential in optimizing management of cardiovascular diseases and furthering clinical research.

Myocardial Viability Testing in the Management of Ischemic Heart Failure

Although major advances have occurred lately in medical therapy, ischemic heart failure remains an important cause of death and disability. Viable myocardium represents a cause of reversible ischemic left ventricular dysfunction. Coronary revascularization may improve left ventricular function and prognosis in patients with viable myocardium. Although patients with impaired left ventricular function and multi-vessel coronary artery disease benefit the most from revascularization, they are at high risk of complications related to revascularization procedure. An important element in selecting the patients for myocardial revascularization is the presence of the viable myocardium. Multiple imaging modalities can assess myocardial viability and predict functional improvement after revascularization, with dobutamine stress echocardiography, nuclear imaging tests and magnetic resonance imaging being the most frequently used. However, the role of myocardial viability testing in the management of patients with ischemic heart failure is still controversial due to the failure of randomized controlled trials of revascularization to reveal clear benefits of viability testing. This review summarizes the current knowledge regarding the concept of viable myocardium, depicts the role and tools for viability testing, discusses the research involving this topic and the controversies related to the utility of myocardial viability testing and provides a patient-centered approach for clinical practice.

Cardiac functional imaging

During the last 20 years, cardiac imaging has drastically evolved. Positron emission tomography (PET), fast three-dimensional (3D) imaging with the latest generations of echocardiography & multi-detector computed tomography (CT), stress perfusion assessed by magnetic resonance imaging (MRI), blood flow analysis using four-dimensional (4D) flow MRI, all these techniques offer new trends for optimal noninvasive functional cardiac imaging. Dynamic functional imaging is obtained by acquiring images of the heart at different phases of the cardiac cycle, allowing assessment of cardiac motion, function, and perfusion. Between CT and Cardiac MRI (CMR), CMR has the best temporal resolution, which is suitable for functional imaging while cardiac CT provides higher spatial resolution with isotropic data that have an identical resolution in the three dimensions of the space. The latest generations of CT scanners enable whole heart assessment in one beat, offering also an acceptable temporal resolution with the possibility to display the images in a dynamic mode. Another rapidly growing technique using functional and molecular imaging for the assessment of biological and metabolic pathways is the PET using radio-labeled tracers. Meanwhile, the oldest cardiac imaging tool with doppler ultrasound technology has never stopped evolving. Echocardiography today performs 3D imaging, stress perfusion, and myocardial strain assessment, with high temporal resolution. It still is the first line and more accessible exam for the patient. These different modalities are complementary and may be even combined into PET-CT or PET-MRI. The ability to combine the functional/molecular data with anatomical images may implement a new dimension to our diagnostic tools.

Positron Emission Tomography in Coronary Heart Disease

With advances in scanner technology, postprocessing techniques, and the development of novel positron emission tomography (PET) tracers, the applications of PET for the study of coronary heart disease have been gaining momentum in the last few years. Depending on the tracer and acquisition protocol, cardiac PET can be used to evaluate the atherosclerotic lesion (plaque imaging) and to assess its potential consequences—ischemic versus nonischemic (perfusion imaging) and viable versus scarred (viability imaging) myocardium. The scope of this review is to summarize the role of PET in coronary heart disease.

Total-Body PET Kinetic Modeling and Potential Opportunities Using Deep Learning

The uEXPLORER total-body PET/CT system provides a very high level of detection sensitivity and simultaneous coverage of the entire body for dynamic imaging for quantification of tracer kinetics. This article describes the fundamentals and potential benefits of total-body kinetic modeling and parametric imaging focusing on the noninvasive derivation of blood input function, multiparametric imaging, and high-temporal resolution kinetic modeling. Along with its attractive properties, total-body kinetic modeling also brings significant challenges, such as the large scale of total-body dynamic PET data, the need for organ and tissue appropriate input functions and kinetic models, and total-body motion correction. These challenges, and the opportunities using deep learning, are discussed.

Myocardial Viability Assessment Before Surgical Revascularization in Ischemic Cardiomyopathy: JACC Review Topic of the Week

Ischemic cardiomyopathy results from the combination of scar with fibrosis replacement and areas of dysfunctional but viable myocardium that may improve contractile function with revascularization. Observational studies reported that only patients with substantial amounts of myocardial viability had better outcomes following surgical revascularization. Accordingly, dedicated noninvasive techniques have evolved to quantify viable myocardium with the objective of selecting patients for this form of therapeutic intervention. However, prospective trials have not confirmed the interaction between myocardial viability and the treatment effect of revascularization. Furthermore, recent observations indicate that recovery of left ventricular function is not the principal mechanism by which surgical revascularization improves prognosis. In this paper, the authors describe a more contemporary application of viability testing that is founded on the alternative concept that the main goal of surgical revascularization is to prevent further damage by protecting the residual viable myocardium from subsequent acute coronary events.

PET Cardiac Imaging (Perfusion, Viability, Sarcoidosis, and Infection)

Cardiovascular disease is the leading cause of death worldwide. Given the increased availability of radiopharmaceuticals, improved positron emission tomography (PET) camera systems and proven higher diagnostic accuracy, PET is increasingly utilized in the management of various cardiovascular diseases. PET has high temporal and spatial resolution, when compared to Single Photon Emission Computed Tomography. In clinical practice, hybrid imaging with sequential PET and Computed Tomography acquisitions (PET/CT) or concurrent PET and Magnetic Resonance Imaging are standard. This article will review applications of cardiovascular PET/CT including myocardial perfusion, viability, cardiac sarcoidosis/inflammation, and infection.

Myocardial Perfusion and Viability Imaging in Coronary Artery Disease: Clinical Value in Diagnosis, Prognosis, and Therapeutic Guidance

Coronary artery disease is a leading cause of morbidity and mortality worldwide. Noninvasive imaging tests play a significant role in diagnosing coronary artery disease, as well as risk stratification and guidance for revascularization. Myocardial perfusion imaging, including single photon emission computed tomography and positron emission tomography, has been widely employed. In this review, we will review test accuracy and clinical significance of these methods for diagnosing and managing coronary artery disease. We will further discuss the comparative usefulness of other noninvasive tests-stress echocardiography, coronary computed tomography angiography, and cardiac magnetic resonance imaging-in the evaluation of ischemia and myocardial viability.

Multiparametric cardiac18F-FDG PET in humans: pilot comparison of FDG delivery rate with82Rb myocardial blood flow

Myocardial blood flow (MBF) and flow reserve are usually quantified in the clinic with positron emission tomography (PET) using a perfusion-specific radiotracer (e.g.⁸²Rb-chloride). However, the clinical accessibility of existing perfusion tracers remains limited. Meanwhile,¹⁸F-fluorodeoxyglucose (FDG) is a commonly used radiotracer for PET metabolic imaging without similar limitations. In this paper, we explore the potential of¹⁸F-FDG for myocardial perfusion imaging by comparing the myocardial FDG delivery rateK₁with MBF as determined by dynamic⁸²Rb PET in fourteen human subjects with heart disease. Two sets of FDGK₁were derived from one-hour dynamic FDG scans. One was the original FDGK₁estimates and the other was the correspondingK₁values that were linearly normalized for blood glucose levels. A generalized Renkin-Crone model was used to fit FDGK₁with Rb MBF, which then allowed for a nonlinear extraction fraction correction for converting FDGK₁to MBF. The linear correlation between FDG-derived MBF and Rb MBF was moderate (r= 0.79) before the glucose normalization and became much improved (r> 0.9) after glucose normalization. The extraction fraction of FDG was also similar to that of Rb-chloride in the myocardium. The results from this pilot study suggest that dynamic cardiac FDG-PET with tracer kinetic modeling has the potential to provide MBF in addition to its conventional use for metabolic imaging.

Acquisition, Processing, and Interpretation of PET 18F-FDG Viability and Inflammation Studies

PURPOSE OF REVIEW

This article reviews the acquisition protocols and image interpretation for ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) imaging with positron emission tomography (PET) applied to the evaluation of myocardial viability and inflammation.

RECENT FINDINGS

Cardiac PET with ¹⁸F-FDG provides essential information for the assessment of myocardial viability and inflammation and is usually combined with PET perfusion imaging using ⁸²Rb or ¹³N-ammonia. Viable myocardium maintains glucose metabolism which can be detected via the uptake of ¹⁸F-FDG by PET imaging. The patient is prepared for viability imaging by shifting the metabolism of the heart to maximize the uptake of glucose and hence of ¹⁸F-FDG. Comparison of the ¹⁸F-FDG and myocardial perfusion images allows distinction between regions of the myocardium that are hibernating and thus may recover function with intervention, from those that are infarcted. Increased glucose utilization in the inflammatory cells also makes ¹⁸F-FDG a useful imaging technique in conditions such as cardiac sarcoidosis. Here, suppression of normal myocardial uptake is essential for accurate image interpretation. ¹⁸F-FDG PET broadens the scope of information potentially available through a cardiac PET study. With careful patient preparation, it provides valuable insights into myocardial viability and inflammatory processes such as sarcoidosis.

Nuclear cardiology: state of the art

Radionuclide imaging remains an essential component of modern cardiology. There is overlap with the information from other imaging techniques, but no technique is static and new developments have expanded its role. This review focuses on ischaemic heart disease, heart failure, infection and inflammation. Radiopharmaceutical development includes the wider availability of positron emission tomography (PET) tracers such as rubidium-82, which allows myocardial perfusion to be quantified in absolute terms. Compared with alternative techniques, myocardial perfusion scintigraphy PET and single photon emission computed tomography (SPECT) have the advantages of being widely applicable using exercise or pharmacological stress, full coverage of the myocardium and a measure of ischaemic burden, which helps to triage patients between medical therapy and revascularisation. Disadvantages include the availability of expertise in some cardiac centres and the lack of simple SPECT quantification, meaning that global abnormalities can be underestimated. In patients with heart failure, despite the findings of the STICH (Surgical Treatment for Ischemic Heart Failure) trial, there are still data to support the assessment of myocardial hibernation in predicting when abolition of ischaemia might lead to improvement in ventricular function. Imaging of sympathetic innervation is well validated, but simpler markers of prognosis mean that it has not been widely adopted. There are insufficient data to support its use in predicting the need for implanted devices, but non-randomised studies are promising. Other areas where radionuclide imaging is uniquely valuable are detection and monitoring of endocarditis, device infection, myocardial inflammation in sarcoidosis, myocarditis and so on, and reliable detection of deposition in suspected transthyretin-related amyloidosis.

Quantification of myocardial oxygen extraction fraction: A proof-of-concept study

PURPOSE

To demonstrate a proof of concept for the measurement of myocardial oxygen extraction fraction (mOEF) by a cardiovascular magnetic resonance technique.

METHODS

The mOEF measurement was performed using an electrocardiogram-triggered double-echo asymmetric spin-echo sequence with EPI readout. Seven healthy volunteers (22-37 years old, 5 females) were recruited and underwent the same imaging scans at rest on 2 different days for reproducibility assessment. Another 5 subjects (23-37 years old, 4 females) underwent cardiovascular magnetic resonance studies at rest and during a handgrip isometric exercise with a 25% of maximal voluntary contraction. Both mOEF and myocardial blood volume values were obtained in septal regions from respective maps.

RESULTS

The reproducibility was excellent for the measurements of mOEF in septal myocardium (coefficient of variation: 3.37%) and moderate for myocardial blood volume (coefficient of variation: 19.7%). The average mOEF and myocardial blood volume of 7 subjects at rest were 0.61 ± 0.05 and 11.0 ± 4.3%, respectively. The mOEF agreed well with literature values that were measured by PET in healthy volunteers. In the exercise study, there was no significant change in mOEF (0.61 ± 0.06 vs 0.62 ± 0.07) or myocardial blood volume (12 ± 6% vs 13 ± 4%) from rest to exercise, as expected.

CONCLUSION

The implemented cardiovascular magnetic resonance method shows potential for the quantitative assessment of mOEF in vivo. Future technical work is needed to improve image quality and to further validate mOEF measurements.

Multimodality Imaging of Myocardial Viability

PURPOSE OF REVIEW

Myocardial viability is an important pathophysiologic concept which may have significant clinical impact in patients with left ventricular dysfunction due to ischemic heart disease. Understanding the imaging modalities used to assess viability, and the clinical implication of their findings, is critical for clinical decision-making in this population.

RECENT FINDINGS

The ability of dobutamine echocardiography, single-photon emission computed tomography, positron emission tomography, and cardiac magnetic resonance imaging to predict functional recovery following revascularization is well-established. Despite different advantages and disadvantages for each imaging modality, each modality has demonstrated reasonable performance characteristics in identifying viable myocardium. Recent data, however, has called into question whether this functional recovery leads to improved clinical outcomes. Although the assessment of viability can be used to aid in clinical decision-making prior to revascularization, its broad application to all patients is limited by a lack of data confirming improvement in clinical outcomes. Thus, viability assessments may be best applied to select patients (such as those with increased surgical risk) and integrated with clinical, laboratory, and imaging data to guide clinical care. Future research efforts should be aimed at establishing the impact of viability on clinical outcomes.

PET Parametric Imaging: Past, Present, and Future

Positron emission tomography (PET) is actively used in a diverse range of applications in oncology, cardiology, and neurology. The use of PET in the clinical setting focuses on static (single time frame) imaging at a specific time-point post radiotracer injection and is typically considered as semi-quantitative; e.g. standardized uptake value (SUV) measures. In contrast, dynamic PET imaging requires increased acquisition times but has the advantage that it measures the full spatiotemporal distribution of a radiotracer and, in combination with tracer kinetic modeling, enables the generation of multiparametric images that more directly quantify underlying biological parameters of interest, such as blood flow, glucose metabolism, and receptor binding. Parametric images have the potential for improved detection and for more accurate and earlier therapeutic response assessment. Parametric imaging with dynamic PET has witnessed extensive research in the past four decades. In this paper, we provide an overview of past and present activities and discuss emerging opportunities in the field of parametric imaging for the future.

Multiparametric Cardiac 18F-FDG PET in Humans: Kinetic Model Selection and Identifiability Analysis

Cardiac ¹⁸F-FDG PET has been used in clinics to assess myocardial glucose metabolism. Its ability for imaging myocardial glucose transport, however, has rarely been exploited in clinics. Using the dynamic FDG-PET scans of ten patients with coronary artery disease, we investigate in this paper appropriate dynamic scan and kinetic modeling protocols for efficient quantification of myocardial glucose transport. Three kinetic models and the effect of scan duration were evaluated by using statistical fit quality, assessing the impact on kinetic quantification, and analyzing the practical identifiability. The results show that the kinetic model selection depends on the scan duration. The reversible two-tissue model was needed for a one-hour dynamic scan. The irreversible two-tissue model was optimal for a scan duration of around 10-15 minutes. If the scan duration was shortened to 2-3 minutes, a one-tissue model was the most appropriate. For global quantification of myocardial glucose transport, we demonstrated that an early dynamic scan with a duration of 10-15 minutes and irreversible kinetic modeling was comparable to the full one-hour scan with reversible kinetic modeling. Myocardial glucose transport quantification provides an additional physiological parameter on top of the existing assessment of glucose metabolism and has the potential to enable single tracer multiparametric imaging in the myocardium.

Myocardial Viability Testing by Positron Emission Tomography: Basic Concepts, Mini-Review of the Literature and Experience From a Tertiary PET Center

Ischemic heart disease ranges in severity from slightly reduced myocardial perfusion with preserved contractile function to chronic occlusion of coronary arteries with myocardial cells replaced by acontractile scar tissue-ischemic heart failure (iHF). Progression towards scar tissue is thought to involve a period in which the myocardial cells are acontractile but still viable despite severely reduced perfusion. This state of reduced myocardial function that can be reversed by revascularization is termed "hibernation." The concept of hibernating myocardium in iHF has prompted an increasing amount of requests for preoperative patient workup, but while the concept of viability is widely agreed upon, no consensus on clinical testing of hibernation has been established. Therefore, a variety of imaging methods have been used to assess hibernation including morphology based (MRI and ultrasound), perfusion based (MRI, SPECT, or PET) and/or methods to assess myocardial metabolism (PET). Regrettably, the heterogeneous body of literature on the subject has resulted in few robust prospective clinical trials designed to assess the impact of preoperative viability testing prior to revascularization. However, the PARR-2 trial and sub-studies has indicated that >5% hibernating myocardium favors revascularization over optimized medical therapy. In this paper, we review the basic concepts and current evidence for using PET to assess myocardial hibernation and discuss the various methodologies used to process the perfusion/metabolism PET images. Finally, we present our experience in conducting PET viability testing in a tertiary referral center.

Assessment of Myocardial Viability in Ischemic Heart Disease by PET/MRI: Comparison of Left Ventricular Perfusion, Hibernation, and Scar Burden

RATIONALE AND OBJECTIVES

Hybrid positron emission tomography-magnetic resonance (PET-MR) is a novel imaging technology that enables a comprehensive assessment of myocardial viability. The aim of this study was to intra-individually compare simultaneously acquired viability parameters from MRI and PET to determine complementary and redundant information.

MATERIALS AND METHODS

Thirty-nine patients with ischemic heart disease (IHD) underwent cardiac PET-MR for myocardial viability assessment. Cardiac magnetic resonance (CMR), including late gadolinium enhancement (LGE), and PET, including a dynamic dual-tracer acquisition of [¹³N]ammonia ([¹³N]NH₃)/[¹⁸F]fluorodeoxyglucose ([¹⁸F]FDG), were performed simultaneously. Allocation, extent, and transmural degree of left ventricular (LV) scars were measured from LGE. Perfusion, viability, and hibernation were assessed by PET.

RESULTS

A comparison of scar location revealed six more areas of infarction on MR than on PET. Mean LV scarring by CMR was 14% (range, 2% to 42%) and 14% (range, 1% to 46%) by PET (CMR vs. PET: p = 0.9). An intra-individual comparison of scarring showed a good inter-method correlation (r = 0.7), which was also evident in the subgroup with low ejection fraction (EF) (r = 0.6). Hibernation and transmural degree of scars showed a moderate to weak correlation (r = 0.4), which was even worse in the low EF group (r = 0.1).

CONCLUSIONS

In patients with IHD, there was a good correlation between PET and CMR for the LV scar extent using hybrid cardiac PET-MR. The degree of transmural scarring by CMR showed no correlation to PET hibernation. Therefore, cardiac PET-MR might be a suitable tool for a comprehensive assessment of myocardial viability if used to predict response to cardiac reperfusion strategies.

Nuclear investigative techniques and their interpretation in the heart and vascular disease

Over the last several decades, myocardial perfusion imaging with single photon emission tomography and positron emission tomography has been a mainstay for the evaluation of patients with known or suspected coronary artery disease non-invasively. Technical advances in imaging modalities and radiopharmaceutical have revolutionaries the understanding of pathogenesis and management of various diseases. In this article, we shall discuss the various available imaging nuclear medicine techniques, radiopharmaceutical, and common indications. In the era of “precision medicine,” imaging has to be patient centered. We will briefly review the upcoming areas of nuclear medicine imaging apart from perfusion imaging, such as advances in myocardial blood flow quantitation and molecular imaging.

Molecular Imaging of the Heart

Multimodality cardiovascular imaging is routinely used to assess cardiac function, structure, and physiological parameters to facilitate the diagnosis, characterization, and phenotyping of numerous cardiovascular diseases (CVD), as well as allows for risk stratification and guidance in medical therapy decision-making. Although useful, these imaging strategies are unable to assess the underlying cellular and molecular processes that modulate pathophysiological changes. Over the last decade, there have been great advancements in imaging instrumentation and technology that have been paralleled by breakthroughs in probe development and image analysis. These advancements have been merged with discoveries in cellular/molecular cardiovascular biology to burgeon the field of cardiovascular molecular imaging. Cardiovascular molecular imaging aims to noninvasively detect and characterize underlying disease processes to facilitate early diagnosis, improve prognostication, and guide targeted therapy across the continuum of CVD. The most-widely used approaches for preclinical and clinical molecular imaging include radiotracers that allow for high-sensitivity in vivo detection and quantification of molecular processes with single photon emission computed tomography and positron emission tomography. This review will describe multimodality molecular imaging instrumentation along with established and novel molecular imaging targets and probes. We will highlight how molecular imaging has provided valuable insights in determining the underlying fundamental biology of a wide variety of CVDs, including: myocardial infarction, cardiac arrhythmias, and nonischemic and ischemic heart failure with reduced and preserved ejection fraction. In addition, the potential of molecular imaging to assist in the characterization and risk stratification of systemic diseases, such as amyloidosis and sarcoidosis will be discussed. © 2019 American Physiological Society. Compr Physiol 9:477-533, 2019.

CABG in patients with left ventricular dysfunction: indications, techniques and outcomes

Ischemic chronic heart failure (CHF) represents one of the cardiovascular diseases with the worst degree of morbidity and mortality in the western world, and with the highest health care costs. Despite several studies demonstrated that surgical revascularization (CABG), especially in the presence of viable myocardium, improve heart function, and therefore, survival, the matter remains unclear and controversial. In the late 1970s, the Coronary Artery Surgery Study showed that a subgroup of patients with coronary artery disease, angina, and reduce LV function had a significant survival benefit after CABG compared to those treated medically. The key concept behind this observation was the presence of viable myocardium, which can resume function following revascularization. In contrary, the surgical treatment for ischemic heart failure (STICH) trial, which randomized patients with CAD and LV dysfunction to evidence-based medical therapy or CABG plus medical therapy, failed to demonstrate at a median follow-up of 56 months a significant difference between the CABG group and the medical therapy group in the rate of death from any cause. However, the results of the STICH extension study (STICHES) at 10 years follow-up demonstrated that CABG is associated with a significant reduction in all-cause mortality, cardiovascular mortality, and readmission for heart compared to optimal medical therapy (OMT) in patients with severe ischemic LV dysfunction. Therefore, this review discusses the available evidences in literature, from observational studies to randomized trials, including operative techniques and controversial issues, in order to better clarify the role of CABG in the current management of ischemic patients with LVD.

Viability testing to guide myocardial revascularisation in patients with heart failure

Myocardial revascularisation has the potential to restore ventricular function and improve survival in patients with heart failure due to underlying coronary artery disease. Viability testing is routinely used to identify which patients are likely to benefit, given that revascularisation may entail substantial procedural risk. However, while the concept of viability testing and revascularisation of patients with ‘hibernating myocardium’ is strongly supported by observational series, randomised studies have failed to demonstrate clear benefit. This divergence in the evidence base is reflected in current European and US guidelines, in which viability testing has a class II recommendation. In this article, we review the current evidence for routine viability testing prior to revascularisation of patients with heart failure, outline its use in clinical practice and discuss ongoing trials in the field.

Accuracy of myocardial viability imaging by cardiac MRI and PET depending on left ventricular function

AIM

To compare myocardial viability assessment accuracy of cardiac magnetic resonance imaging (CMR) compared to [¹⁸F]-fluorodeoxyglucose (FDG)- positron emission tomography (PET) depending on left ventricular (LV) function.

METHODS

One-hundred-five patients with known obstructive coronary artery disease (CAD) and anticipated coronary revascularization were included in the study and examined by CMR on a 1.5T scanner. The CMR protocol consisted of cine-sequences for function analysis and late gadolinium enhancement (LGE) imaging for viability assessment in 8 mm long and contiguous short axis slices. All patients underwent PET using [¹⁸F]-FDG. Myocardial scars were rated in both CMR and PET on a segmental basis by a 4-point-scale: Score 1 = no LGE, normal FDG-uptake; score 2 = LGE enhancement < 50% of wall thickness, reduced FDG-uptake ( ≥ 50% of maximum); score 3 = LGE ≥ 50%, reduced FDG-uptake (< 50% of maximum); score 4 = transmural LGE, no FDG-uptake. Segments with score 1 and 2 were categorized “viable”, scores 3 and 4 were categorized as “non-viable”. Patients were divided into three groups based on LV function as determined by CMR: Ejection fraction (EF), < 30%: n = 45; EF: 30%-50%: n = 44; EF > 50%: n = 16). On a segmental basis, the accuracy of CMR in detecting myocardial scar was compared to PET in the total collective and in the three different patient groups.

RESULTS

CMR and PET data of all 105 patients were sufficient for evaluation and 5508 segments were compared in total. In all patients, CMR detected significantly more scars (score 2-4) than PET: 45% vs 40% of all segments (P < 0.0001). In the different LV function groups, CMR found more scar segments than PET in subjects with EF< 30% (55% vs 46%; P < 0.0001) and EF 30%-50% (44% vs 40%; P < 0.005). However, CMR revealed less scars than PET in patients with EF > 50% (15% vs 23%; P < 0.0001). In terms of functional improvement estimation, i.e., expected improvement after revascularization, CMR identified “viable” segments (score 1 and 2) in 72% of segments across all groups, PET in 80% (P < 0.0001). Also in all LV function subgroups, CMR judged less segments viable than PET: EF < 30%, 66% vs 75%; EF = 30%-50%, 72% vs 80%; EF > 50%, 91% vs 94%.

CONCLUSION

CMR and PET reveal different diagnostic accuracy in myocardial viability assessment depending on LV function state. CMR, in general, is less optimistic in functional recovery prediction.

Ischemic Mitral Regurgitation: Abnormal Strain Overestimates Nonviable Myocardium

BACKGROUND

Therapy for moderate ischemic mitral regurgitation remains unclear. Determination of myocardial viability, a necessary prerequisite for an improvement in regional contractility, is a likely key factor in determining response to revascularization alone. Myocardial strain has been proposed as a viability measure but has not been compared with late gadolinium enhancement (LGE) cardiac magnetic resonance imaging. We hypothesized that abnormal strain overestimates nonviable left ventricular (LV) segments measured with LGE and that ischemia and mechanical tethering by adjacent transmural myocardial infarction (TMI) also decreases strain in viable segments.

METHODS

Sixteen patients with mild or greater ischemic mitral regurgitation and 7 healthy volunteers underwent cardiac magnetic resonance imaging with noninvasive tags (complementary spatial modulation of magnetization [CSPAMM]), LGE, and stress perfusion. CSPAMM images were post-processed with harmonic phase and circumferential and longitudinal strains were calculated. Viability was defined as the absence of TMI on LGE (hyperenhancement >50% of wall thickness). The borderzone was defined as any segment bordering TMI. Abnormal strain thresholds (±1 to 2.5 SDs from normal mean) were compared with TMI, ischemia, and borderzone.

RESULTS

7.4% of LV segments had TMI on LGE, and more than 14.5% of LV segments were nonviable by strain thresholds (p < 0.005). In viable segments, ischemia impaired longitudinal strain (least perfused one-third of LV segments: -0.18 ± 0.08 versus most perfused: -0.22 ± 0.1, p = 0.01) and circumferential strain (-0.12 ± 0.1 versus -0.16 ± 0.08, p < 0.05). In addition, infarct proximity impaired longitudinal strain (-0.16 ± 0.11 borderzone versus -0.18 ± 0.09 remote, p = 0.05).

CONCLUSIONS

Impaired LV strain overestimates nonviable myocardium compared with TMI on LGE. Ischemia and infarct proximity also decrease strain in viable segments.