N-13 ammonia as an indicator of myocardial blood flow

Long Axial Field of View PET/CT: Technical Aspects in Cardiovascular Diseases

Positron emission tomography / computed tomography (PET/CT) plays a pivotal role in the assessment of cardiovascular diseases (CVD), particularly in the context of ischemic heart disease. Nevertheless, its application in other forms of CVD, such as infiltrative, infectious, or inflammatory conditions, remains limited. Recently, PET/CT systems with an extended axial field of view (LAFOV) have been developed, offering greater anatomical coverage and significantly enhanced PET sensitivity. These advancements enable head-to-pelvis imaging with a single bed position, and in systems with an axial field of view (FOV) of approximately 2 meters, even total body (TB) imaging is feasible in a single scan session. The application of LAFOV PET/CT in CVD presents a promising opportunity to improve systemic cardiovascular assessments and address the limitations inherent to conventional short axial field of view (SAFOV) devices. However, several technical challenges, including procedural considerations for LAFOV systems in CVD, complexities in data processing, arterial input function extraction, and artefact management, have not been fully explored. This review aims to discuss the technical aspects of LAFOV PET/CT in relation to CVD by highlighting key opportunities and challenges and examining the impact of these factors on the evaluation of most relevant CVD.

Total-body dynamic PET/CT imaging reveals kinetic distribution of [13N]NH3 in normal organs

PURPOSE

To systematically investigate kinetic metrics and metabolic trapping of [13N]NH3 in organs.

METHODS

Eleven participants performed total-body [13N]NH3 dynamic positron emission tomography (PET). Regions of interest were drawn in organs to obtain time-to-activity curves (TACs), which were fitted with an irreversible two-tissue compartment model (2TC) to investigate constant rates K1, k2 and k3, and to calculate Ki. Additionally, one-tissue compartment model using full data (1TCfull) and the first four minutes of data (1TC4min) were fitted to TAC data. K1 and k2 were compared among different models to assess [13N]NH3 trapping in organs.

RESULTS

Kinetic rates of [13N]NH3 varied significantly among organs. The mean K1 ranged from 0.049 mL/cm3/min in the muscle to 2.936 mL/cm3/min in the kidney. The k2 and k3 were lowest in the liver (0.001 min- 1) and in the pituitary (0.009 min- 1), while highest in the kidney (0.587 min- 1) and in the liver (0.800 min- 1), respectively. The Ki was largest in the myocardium (0.601 ± 0.259 mL/cm3/min) while smallest in the bone marrow (0.028 ± 0.022 mL/cm3/min). Three groups of organs with similar kinetic characteristics were revealed: (1) the thyroid, the lung, the spleen, the pancreas, and the kidney; (2) the liver and the muscle; and (3) the cortex, the white matter, the cerebellum, the pituitary, the parotid, the submandibular gland, the myocardium, the bone, and the bone marrow. Obvious k3 was identified in multiple organs, and significant changes of K1 in multiple organs and k2 in most organs were found between 2TC and 1TCfull, but both K1 and k2 were comparable between 2TC and 1TC4min.

CONCLUSION

The kinetic rates of [13N]NH3 differed among organs with some have obvious 13N-anmmonia trapping. The normal distribution of kinetic metrics of 13N-anmmonia in organs can serve as a reference for its potential use in tumor imaging.

Current status and perspectives of nuclear cardiology

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

Radiation Exposure to the Personnel Performing Myocardial Blood Flow Quantification Study Using 13N-ammonia Positron Emission Tomography/Computed Tomography

Purpose

The present study aimed to evaluate radiation exposure to staff performing coronary flow reserve (CFR) measurement using 13N-ammonia.

Materials and Methods

The radiation exposure rate during the administration of 13N-ammonia for the rest and stress part of the study was noted using an ionization chamber-based calibrated survey monitor. The radiation exposure to persons involved in dispensing radioactivity (D1), administering radioactivity (D2) and monitoring the patient during pharmacological stress (D3) were measured using an energy compensated Si-diode personal pocket dosimeter.

Results

The average dose received by individuals with dosimeters D1, D2, and D3 was 1.28 ± 0.79 µSv, 1.56 ± 0.51 µSv, and 0.88 ± 0.97 µSv per injection, respectively, during the rest of study and 1.56 ± 0.96 µSv, 2.64 ± 1.22 µSv, and 2.2 ± 1.7 µSv per injection, respectively, during stress study. The average exposure rate during the administration of 13N-ammonia at 0.5 m and 1.5 m from the injection site was found to be 259 µSv/h and 53.4 µSv/h, respectively, during the rest study and 301 µSv/h and 67.25 µSv/h, respectively, during stress study.

Conclusion

The exposure to the staff performing CFR study with 13N-ammonia was well within prescribed limits by the International Commission on Radiological Protection 103. The CFR measurement with 13N-ammonia positron emission tomography/computed tomography can be included in routine workups of cardiac patients without the fear of radiation exposure.

Transfer learning-based attenuation correction for static and dynamic cardiac PET using a generative adversarial network

PURPOSE

The goal of this work is to demonstrate the feasibility of directly generating attenuation-corrected PET images from non-attenuation-corrected (NAC) PET images for both rest and stress-state static or dynamic [13N]ammonia MP PET based on a generative adversarial network.

METHODS

We recruited 60 subjects for rest-only scans and 14 subjects for rest-stress scans, all of whom underwent [13N]ammonia cardiac PET/CT examinations to acquire static and dynamic frames with both 3D NAC and CT-based AC (CTAC) PET images. We developed a 3D pix2pix deep learning AC (DLAC) framework via a U-net + ResNet-based generator and a convolutional neural network-based discriminator. Paired static or dynamic NAC and CTAC PET images from 60 rest-only subjects were used as network inputs and labels for static (S-DLAC) and dynamic (D-DLAC) training, respectively. The pre-trained S-DLAC network was then fine-tuned by paired dynamic NAC and CTAC PET frames of 60 rest-only subjects to derive an improved D-DLAC-FT for dynamic PET images. The 14 rest-stress subjects were used as an internal testing dataset and separately tested on different network models without training. The proposed methods were evaluated using visual quality and quantitative metrics.

RESULTS

The proposed S-DLAC, D-DLAC, and D-DLAC-FT methods were consistent with clinical CTAC in terms of various images and quantitative metrics. The S-DLAC (slope = 0.9423, R2 = 0.947) showed a higher correlation with the reference static CTAC as compared to static NAC (slope = 0.0992, R2 = 0.654). D-DLAC-FT yielded lower myocardial blood flow (MBF) errors in the whole left ventricular myocardium than D-DLAC, but with no significant difference, both for the 60 rest-state subjects (6.63 ± 5.05% vs. 7.00 ± 6.84%, p = 0.7593) and the 14 stress-state subjects (1.97 ± 2.28% vs. 3.21 ± 3.89%, p = 0.8595).

CONCLUSION

The proposed S-DLAC, D-DLAC, and D-DLAC-FT methods achieve comparable performance with clinical CTAC. Transfer learning shows promising potential for dynamic MP PET.

Changes in myocardial blood flow in a canine model of left sided breast cancer radiotherapy

BACKGROUND

Left-sided breast cancer patients receiving adjuvant radiotherapy are at risk for coronary artery disease, and/or radiation mediated effects on the microvasculature. Previously our laboratory demonstrated in canines with hybrid 18FDG/PET a progressive global inflammatory response during the initial one year following treatment. In this study, the objective is to evaluate corresponding changes in perfusion, in the same cohort, where resting myocardial blood flow (MBF) was quantitatively measured.

METHOD

In five canines, Ammonia PET (13NH3) derived MBF was measured at baseline, 1-week, 1, 3, 6 and 12-months after cardiac external beam irradiation. MBF measurements were correlated with concurrent 18FDG uptake. Simultaneously MBF was measured using the dual bolus MRI method.

RESULTS

MBF was significantly increased at all time points, in comparison to baseline, except at 3-months. This was seen globally throughout the entire myocardium independent of the coronary artery territories. MBF showed a modest significant correlation with 18FDG activity for the entire myocardium (r = 0.51, p = 0.005) including the LAD (r = 0.49, p = 0.008) and LCX (r = 0.47, p = 0.013) coronary artery territories.

CONCLUSION

In this canine model of radiotherapy for left-sided breast cancer, resting MBF increases as early as 1-week and persists for up to one year except at 3-months. This pattern is similar to that of 18FDG uptake. A possible interpretation is that the increase in resting MBF is a response to myocardial inflammation.

Highlighting Fibroblasts Activation in Fibrosis: The State-of-The-Art Fibroblast Activation Protein Inhibitor PET Imaging in Cardiovascular Diseases

Fibrosis is a common healing process that occurs during stress and injury in cardiovascular diseases. The evolution of fibrosis is associated with cardiovascular disease states and causes adverse effects. Fibroblast activation is responsible for the formation and progression of fibrosis. The incipient detection of activated fibroblasts is important for patient management and prognosis. Fibroblast activation protein (FAP), a membrane-bound serine protease, is almost specifically expressed in activated fibroblasts. The development of targeted FAP-inhibitor (FAPI) positron emission tomography (PET) imaging enabled the visualisation of FAP, that is, incipient fibrosis. Recently, research on FAPI PET imaging in cardiovascular diseases increased and is highly sought. Hence, we comprehensively reviewed the application of FAPI PET imaging in cardiovascular diseases based on the state-of-the-art published research. These studies provided some insights into the value of FAPI PET imaging in the early detection of cardiovascular fibrosis, risk stratification, response evaluation, and prediction of the evolution of left ventricular function. Future studies should be conducted with larger populations and multicentre patterns, especially for response evaluation and outcome prediction.

Quantitative Assessment of Myocardial Ischemia With Positron Emission Tomography

Recent advances in positron emission tomography (PET) technology and reconstruction techniques have now made quantitative assessment using cardiac PET readily available in most cardiac PET imaging centers. Multiple PET myocardial perfusion imaging (MPI) radiopharmaceuticals are available for quantitative examination of myocardial ischemia, with each having distinct convenience and accuracy profile. Important properties of these radiopharmaceuticals ( 15 O-water, 13 N-ammonia, 82 Rb, 11 C-acetate, and 18 F-flurpiridaz) including radionuclide half-life, mean positron range in tissue, and the relationship between kinetic parameters and myocardial blood flow (MBF) are presented. Absolute quantification of MBF requires PET MPI to be performed with protocols that allow the generation of dynamic multiframes of reconstructed data. Using a tissue compartment model, the rate constant that governs the rate of PET MPI radiopharmaceutical extraction from the blood plasma to myocardial tissue is calculated. Then, this rate constant ( K1 ) is converted to MBF using an established extraction formula for each radiopharmaceutical. As most of the modern PET scanners acquire the data only in list mode, techniques of processing the list-mode data into dynamic multiframes are also reviewed. Finally, the impact of modern PET technologies such as PET/CT, PET/MR, total-body PET, machine learning/deep learning on comprehensive and quantitative assessment of myocardial ischemia is briefly described in this review.

Development of new copper-64 labeled rhodamine: a potential PET myocardial perfusion imaging agent

Background

Myocardial perfusion imaging (MPI) is one of the most commonly performed investigations in nuclear medicine procedures. Due to the longer half-life of the emerging positron emitter copper-64 and its availability from low energy cyclotron, together with its well-known coordination chemistry, we have synthesized ⁶⁴Cu-labeled NOTA- and ⁶⁴Cu-NOTAM-rhodamine conjugates as potential cardiac imaging agents using PET.

Results

⁶⁴Cu-NOTA- and ⁶⁴Cu-NOTAM-rhodamine conjugates were synthesized using a traightforward and one-step simple reaction. Radiochemical yields were greater than 97% (decay corrected), with a total synthesis time of less than 25 min. Radiochemical purities were always greater than 98% as assessed by TLC and HPLC. These synthetic approaches hold considerable promise as a simple method for ⁶⁴Cu-rhodamine conjugates synthesis, with high radiochemical yield and purity. Biodistribution studies in normal Fischer rats at 60 min post-injection, demonstrated significant heart uptake and a good biodistribution profile for both the radioconjugates. However, the ⁶⁴Cu-NOTAM-rhodamine conjugate has shown more heart uptake (~ 10% ID/g) over the ⁶⁴Cu-NOTA-rhodamine conjugate (5.6% ID/g).

Conclusions

These results demonstrate that these radioconjugates may be useful probes for the PET evaluation of MPI.

A practical fully automated radiosynthesis of [18F]Flurpiridaz on the module modular lab-pharmtracer without external purification

Background

[18F]Flurpiridaz is a promising novel cardiac PET imaging tracer formed by the radiolabeling of pyridaben derivative with fluorine-18. Clinical studies on [18F]Flurpiridaz are currently at the phase III level for the assessment of MPI. Providing high image quality thanks to its relatively long half-life, F-18 is a high-potential radionuclide for the early detection of CAD. In this study, we aimed to develop a fully automated synthesis of [18F]Flurpiridaz without further preparative HPLC purification.

Results

Precursor 6 was obtained by multi-step synthesis starting from mucochloric acid (1) as a sole product with 35% yield and identified by spectroscopic measurement. Manually cold labeling experiments were performed using the stable isotope [19F]F, and TBA-HCO3 PTC provided desirable fluorinated compound with high yield. A fully automated [18F]Flurpiridaz synthesis on the ML-PT device provided 55–65% radiochemical yield with more than 98% radiochemical purity. The final product purification method demonstrated that [18F]Flurpiridaz could be obtained without an external preparative HPLC system as a pharmaceutical quality.

Conclusion

A novel and fascinating strategy was developed for the fully automated synthesis of [18F]Flurpiridaz (7) on ML PT. Organic synthesis of precursor 6 was achieved with a desirable yield and characterized by NMR and HR-MS. A detailed set of cold experiments were completed for optimization conditions before hot trials and TBA-HCO3 increased molar activity with a minimum amount of side products. Radiolabeling showed that our self-designed automated synthesis method enables high radiochemical yield and radiochemical purity for the production of [18F]Flurpiridaz. The desirable radiopharmaceutical quality of the product was obtained without using an additional preparative HPLC system. [18F]Flurpiridaz (7) preserved its stability within 12 h and final specifications were consistent with the acceptance criteria in Ph. Eur. regulations.

Determination of [N-13]-ammonia extraction fraction in patients with coronary artery disease by calibration to invasive coronary and fractional flow reserve

BACKGROUND

This study presents a new extraction fraction (EF) model based on physiological measures of invasive coronary flow reserve (CFR) and fractional flow reserve (FFR) in patients with suspected coronary artery disease (CAD) and normal index microcirculatory resistance (IMR). To ascertain the clinical relevance of the new EFs, flow measurements using the newly patient-determined EFs were compared to flow measurements using traditional animal-determined EFs.

METHODS

39 patients were retrospectively selected that included a total of 91 vascular territories with invasive coronary angiography physiological measures. [N-13]-ammonia dynamic rest/adenosine-stress PET imaging was conducted in all patients and absolute myocardial flow was estimated using four published compartmental models. The extraction fraction during hyperemic flow was iteratively estimated by maximizing the agreement between invasive CFR and FFR with the non-invasive analogs myocardial flow reserve (MFR) and relative flow reserve (RFR) at similar physiological states, respectively.

RESULTS

Using the new patient-determined EFs, agreement between CFR vs MFR for Model 1 and 2 was moderate and poor for Model 3 and 4. All models showed moderate agreement for FFR vs RFR. When using published models of animal-determined EFs, agreement between CFR vs MFR remained moderate for Model 1 and 2, and poor for Model 3 and 4. Similarly, all models showed moderate agreement for FFR vs RFR using animal-determined EF values. None of the observed differences were statistically significant.

CONCLUSIONS

Flow measurements using extraction fraction correction for [N-13]-ammonia based on calibration to invasive intracoronary angiography physiological measures in patients with CAD were not discordant from those reported in the literature. Either patient-determined or traditional animal-determined EF correction, when used with the appropriate flow model, yields moderate agreement with invasive measurements of coronary flow reserve and fractional flow reserve.

Impact of residual subtraction on myocardial blood flow and reserve estimates from rapid dynamic PET protocols

BACKGROUND

13N-ammonia and 18F-flurpiridaz require longer delays between rest and stress studies to allow for decay, lowering clinical throughput. In this study, we investigated the impact of residual subtraction on MBF and MFR estimates, as well as its effects on diagnostic accuracy.

METHODS

We retrospectively analyzed 63 patients who underwent a dynamic ammonia rest/stress study and 231 patients from the flurpiridaz 301 trial. Residual subtraction was performed by subtracting the mean pre-injection activity in each sampled region from that region's time activity curve. Corrected and uncorrected MBF and MFR were analyzed. Diagnostic accuracy was compared to quantitative coronary angiograms (QCA) for the flurpiridaz population.

RESULTS

With delays between injections above 3 half-lives, and a doubled stress dose, residual activity did not meaningfully increase ammonia MBF (< 5%). For shorter injection delays, stress MBF was overestimated by 13.6% ± 5.0% (P < .001). Residual activity had a large effect on flurpiridaz stress MBF, overestimating it by 37.9% ± 23.2% (P < .001). Comparison to QCA showed a significant improvement in AUC with residual subtraction (from 0.748 to 0.831, P = .001). MFR yielded similar results.

CONCLUSIONS

Accounting for residual activity has a marked impact on stress MBF and MFR and improves diagnostic accuracy relative to QCA.

Impact of left bundle branch block on myocardial perfusion and metabolism: A positron emission tomography study

BACKGROUND

Better understanding of pathophysiological changes, induced by left bundle branch block (LBBB), may improve patient selection for cardiac resynchronization therapy (CRT). Therefore, we assessed the effect of LBBB on regional glucose metabolism, 13N-NH3-derived absolute and semiquantitative myocardial blood flow (MBF), and their relation in non-ischemic CRT candidates.

METHODS

Twenty-five consecutive non-ischemic patients with LBBB underwent 18F-FDG and resting dynamic 13N-NH3 PET/CT prior to CRT implantation. Regional 18F-FDG uptake, absolute MBF, and late 13N-NH3 uptake were analyzed and corresponding septal-to-lateral wall ratios (SLR) were calculated. Segmental analysis was performed to evaluate "reverse mismatch," "mismatch," and "match" patterns, based on late 13N-NH3/18F-FDG uptake ratios.

RESULTS

A significantly lower 18F-FDG uptake was observed in the septum compared to the lateral wall (SLR 0.53 ± 0.17). A similar pattern was observed for MBF (SLR 0.68 ± 0.18), whereas late 13N-NH3 uptake showed a homogeneous distribution (SLR 0.96 ± 0.13). 13N-NH3/18F-FDG "mismatch" and "reverse mismatch" segments were predominantly present in the lateral (52%) and septal wall (61%), respectively.

CONCLUSIONS

Non-ischemic CRT candidates with LBBB demonstrate lower glucose uptake and absolute MBF in the septum compared to the lateral wall. However, late static 13N-NH3 uptake showed a homogenous distribution, reflecting a composite measure of altered regional MBF and metabolism, induced by LBBB.

Dynamic cardiac PET motion correction using 3D normalized gradient fields in patients and phantom simulations

This work expands on the implementation of three-dimensional (3D) normalized gradient fields to correct for whole-body motion and cardiac creep in [N-13]-ammonia patient studies and evaluates its accuracy using a dynamic phantom simulation model.

METHODS

A full rigid-body algorithm was developed using 3D normalized gradient fields including a multi-resolution step and sampling off the voxel grid to reduce interpolation artifacts. Optimization was performed using a weighted similarity metric that accounts for opposing gradients between images of blood pool and perfused tissue without the need for segmentation. Forty-three retrospective dynamic [N-13]-ammonia PET/CT rest/adenosine-stress patient studies were motion corrected and the mean motion parameters plotted at each frame time point. Motion correction accuracy was assessed using a comprehensive dynamic XCAT simulation incorporating published physiologic parameters of the heart's trajectory following adenosine infusion as well as corrupted attenuation correction commonly observed in clinical studies. Accuracy of the algorithm was assessed objectively by comparing the errors between isosurfaces and centers of mass of the motion corrected XCAT simulations.

RESULTS

In the patient studies, the overall mean cranial-to-caudal translation was 7 mm at stress over the duration of the adenosine infusion. Noninvasive clinical measures of relative flow reserve and myocardial flow reserve were highly correlated with their invasive analogues. Motion correction accuracy assessed with the XCAT simulations showed an error of <1 mm in late perfusion frames that broadened gradually to <3 mm in earlier frames containing blood pool.

CONCLUSION

This work demonstrates that patients undergoing [N-13]-ammonia dynamic PET/CT exhibit a large cranial-to-caudal translation related to cardiac creep primarily at stress and to a lesser extent at rest, which can be accurately corrected by optimizing their 3D normalized gradient fields. Our approach provides a solution to the challenging condition where the image intensity and its gradients are opposed without the need for segmentation and remains robust in the presence of PET-CT mismatch.

Direct comparisons of left ventricular volume and function by simultaneous cardiac magnetic resonance imaging and gated 13N-ammonia positron emission tomography

OBJECTIVES

Hybrid PET/MRI is useful for the simultaneous evaluation of both -ammonia PET and MRI data. The aim of the current study was to compare the accuracy of gated -ammonia PET with MRI for the measurement of left ventricular volumes and function in patients with coronary artery disease (CAD), using simultaneous acquisitions on a hybrid PET/MRI system.

METHODS

Fifty-one consecutive patients with suspected CAD who underwent -ammonia PET/MRI were enrolled in this study. End-diastolic volume (EDV), end-systolic volume (ESV), and left ventricular ejection fraction (LVEF) were simultaneously evaluated using both gated -ammonia PET and cine MRI. Regional wall motion was visually scored on a 4-point scale using a 17-segment model for both methods.

RESULTS

The correlations between each EDV (R = 0.99, P < 0.001), ESV (R = 0.98, P < 0.001), and LVEF (R = 0.99, P = 0.001) measured by gated -ammmonia and MRI were very high. These high correlations were also observed in postmyocardial infarction patients. Furthermore, the regional wall motion scores determined on gated -ammonia PET and MRI showed an agreement of 89.0% with a kappa value of 0.82 ± 0.02.

CONCLUSION

EDV, ESV, LVEF, and regional wall motion measured by gated -ammonia PET were highly correlated with those measured by MRI.

Hybrid PET/MR imaging in myocardial inflammation post-myocardial infarction

Hybrid PET/MR imaging is an emerging imaging modality combining positron emission tomography (PET) and magnetic resonance imaging (MRI) in the same system. Since the introduction of clinical PET/MRI in 2011, it has had some impact (e.g., imaging the components of inflammation in myocardial infarction), but its role could be much greater. Many opportunities remain unexplored and will be highlighted in this review. The inflammatory process post-myocardial infarction has many facets at a cellular level which may affect the outcome of the patient, specifically the effects on adverse left ventricular remodeling, and ultimately prognosis. The goal of inflammation imaging is to track the process non-invasively and quantitatively to determine the best therapeutic options for intervention and to monitor those therapies. While PET and MRI, acquired separately, can image aspects of inflammation, hybrid PET/MRI has the potential to advance imaging of myocardial inflammation. This review contains a description of hybrid PET/MRI, its application to inflammation imaging in myocardial infarction and the challenges, constraints, and opportunities in designing data collection protocols. Finally, this review explores opportunities in PET/MRI: improved registration, partial volume correction, machine learning, new approaches in the development of PET and MRI pulse sequences, and the use of novel injection strategies.

EANM procedural guidelines for PET/CT quantitative myocardial perfusion imaging

The use of cardiac PET, and in particular of quantitative myocardial perfusion PET, has been growing during the last years, because scanners are becoming widely available and because several studies have convincingly demonstrated the advantages of this imaging approach. Therefore, there is a need of determining the procedural modalities for performing high-quality studies and obtaining from this demanding technique the most in terms of both measurement reliability and clinical data. Although the field is rapidly evolving, with progresses in hardware and software, and the near perspective of new tracers, the EANM Cardiovascular Committee found it reasonable and useful to expose in an updated text the state of the art of quantitative myocardial perfusion PET, in order to establish an effective use of this modality and to help implementing it on a wider basis. Together with the many steps necessary for the correct execution of quantitative measurements, the importance of a multiparametric approach and of a comprehensive and clinically useful report have been stressed.

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.

Preliminary and Comparative Experiment Study Between 18F-Flurpiridaz and 13N-NH3·H2O Myocardial Perfusion Imaging With PET/CT in Miniature Pigs

OBJECTVES

To comparatively explore the differences between 18F-Flurpiridaz and 13N-NH3·H2O PET/CT myocardial perfusion imaging in miniature pigs.

METHODS

Ten Bama minipigs were divided into normal group and myocardial infarction group. The changes of the ratio of left ventricular myocardium to main organs with time were calculated and the best imaging time was confirmed for 18F-Flurpiridaz imaging in normal group. The image quality score, summed rest score(SRS), Extend, total perfusion deficit(TPD) and left ventricle ejection fraction(LVEF) were respectively compared for 18F-Flurpiridaz and 13N-NH3·H2O in infarction group.

RESULTS

18F-Flurpiridaz was rapid distributed in myocardium, and the background counts of cardiac cavity were very low, and no obvious interference extracardiac radioactivity was observed. The radioactive ratio of the left ventricular myocardium to cardiac blood pool and adjacent liver were high. Compared with 13N-NH3·H2O, there were no significant differences in functional parameters, including SRS, Extend, TPD and LVEF.

CONCLUSION

The results preliminaryly show that 18F-FIurpiridaz is a promising positron MPI agent with good image quality, ability of accurately evaluating cardiac function, and also convenience for application.

Assessment of myocardial blood flow and coronary flow reserve with positron emission tomography in ischemic heart disease: current state and future directions

Positron emission tomography (PET) is a versatile imaging technology that allows assessment of myocardial perfusion, both at a spatially relative scale and also in absolute terms, thereby enabling noninvasive evaluation of myocardial blood flow (MBF) and coronary flow reserve (CFR). Assessment of MBF using FDA-approved PET isotopes, such as ⁸²Rb and ¹³N-ammonia, has been well validated, and several software packages are currently available, thereby allowing for MBF evaluation to be incorporated into routine workflow in contemporary nuclear laboratories. Incremental diagnostic and prognostic information provided with the knowledge of MBF has the potential for widespread applications. Improving the ability to identify the true burden of obstructive epicardial coronary stenoses and allowing for noninvasive assessment of coronary micro circulatory function can be achieved with MBF assessment. On the other hand, attenuated CFR has been shown to predict adverse cardiovascular prognosis in a variety of clinical settings and patient subgroups. With expanding applications of MBF, this tool promises to provide unique insight into the integrity of the entire coronary vascular bed beyond what is currently available with relative perfusion assessment. This review intends to provide an in-depth discussion of technical and clinical aspects of MBF assessment with PET as it relates to patients with ischemic heart disease.

Calculation of left ventricular volumes and ejection fraction from dynamic cardiac-gated 15O-water PET/CT: 5D-PET

Background

Quantitative measurement of myocardial blood flow (MBF) is of increasing interest in the clinical assessment of patients with suspected coronary artery disease (CAD). ¹⁵O-water positron emission tomography (PET) is considered the gold standard for non-invasive MBF measurements. However, calculation of left ventricular (LV) volumes and ejection fraction (EF) is not possible from standard ¹⁵O-water uptake images. The purpose of the present work was to investigate the possibility of calculating LV volumes and LVEF from cardiac-gated parametric blood volume (V_B) ¹⁵O-water images and from first pass (FP) images.

Sixteen patients with mitral or aortic regurgitation underwent an eight-gate dynamic cardiac-gated ¹⁵O-water PET/CT scan and cardiac MRI. V_B and FP images were generated for each gate. Calculations of end-systolic volume (ESV), end-diastolic volume (EDV), stroke volume (SV) and LVEF were performed with automatic segmentation of V_B and FP images, using commercially available software. LV volumes and LVEF were calculated with surface-, count-, and volume-based methods, and the results were compared with gold standard MRI.

Results

Using V_B images, high correlations between PET and MRI ESV (r = 0.89, p < 0.001), EDV (r = 0.85, p < 0.001), SV (r = 0.74, p = 0.006) and LVEF (r = 0.72, p = 0.008) were found for the volume-based method. Correlations for FP images were slightly, but not significantly, lower than those for V_B images when compared to MRI. Surface- and count-based methods showed no significant difference compared with the volume-based correlations with MRI. The volume-based method showed the best agreement with MRI with no significant difference on average for EDV and LVEF but with an overestimation of values for ESV (14%, p = 0.005) and SV (18%, p = 0.004) when using V_B images. Using FP images, none of the parameters showed a significant difference from MRI. Inter-operator repeatability was excellent for all parameters (ICC > 0.86, p < 0.001).

Conclusion

Calculation of LV volumes and LVEF from dynamic ¹⁵O-water PET is feasible and shows good correlation with MRI. However, the analysis method is laborious, and future work is needed for more automation to make the method more easily applicable in a clinical setting.

Role of Cardiac PET in Clinical Practice

OPINION STATEMENT

Early identification of atherosclerosis and at-risk lesions plays a critical role in reducing the burden of cardiovascular disease. While invasive coronary angiography serves as the gold standard for diagnosing coronary artery disease, non-invasive imaging techniques provide visualization of both anatomical and functional atherosclerotic processes prior to clinical presentation. The development of cardiac positron emission tomography (PET) has greatly enhanced our capability to diagnose and treat patients with early stages of atherosclerosis. Cardiac PET is a powerful, versatile non-invasive diagnostic tool with utility in the identification of high-risk plaques, myocardial perfusion defects, and viable myocardial tissue. Cardiac PET allows for comparisons of myocardial function both at time of rest and stress, providing accurate assessments of both myocardial perfusion and viability. Furthermore, novel PET techniques with unique radiotracers yield clinically relevant data on high-risk plaques in active progressive atherosclerosis. While PET exercise stress tests were previously difficult to perform given short radiotracer half-life, the development of the novel radiotracer Flurpiridaz F-18 provides a promising future for PET exercise stress imaging. In addition, hybrid imaging with computed tomography angiography (CTA) and cardiac magnetic resonance (CMR) provides integration of cardiac function and structure. In this review article, we discuss the principles of cardiac PET, the clinical applications of PET in diagnosing and prognosticating patients at risk for future cardiovascular events, compare PET with other non-invasive cardiac imaging modalities, and discuss future applications of PET in CVD evaluation and management.

Fully quantitative cardiovascular magnetic resonance myocardial perfusion ready for clinical use: a comparison between cardiovascular magnetic resonance imaging and positron emission tomography

BACKGROUND

Recent studies have shown that quantification of myocardial perfusion (MP) at stress and myocardial perfusion reserve (MPR) offer additional diagnostic and prognostic information compared to qualitative and semi-quantitative assessment of myocardial perfusion distribution in patients with coronary artery disease (CAD). Technical advancements have enabled fully automatic quantification of MP using cardiovascular magnetic resonance (CMR) to be performed in-line in a clinical workflow. The aim of this study was to validate the use of the automated CMR perfusion mapping technique for quantification of MP using 13N-NH3 cardiac positron emission tomography (PET) as the reference method.

METHODS

Twenty-one patients with stable CAD were included in the study. All patients underwent adenosine stress and rest perfusion imaging with 13N-NH3 PET and a dual sequence, single contrast bolus CMR on the same day. Global and regional MP were quantified both at stress and rest using PET and CMR.

RESULTS

There was good agreement between global MP quantified by PET and CMR both at stress (-0.1 ± 0.5 ml/min/g) and at rest (0 ± 0.2 ml/min/g) with a strong correlation (r = 0.92, p < 0.001; y = 0.94× + 0.14). Furthermore, there was strong correlation between CMR and PET with regards to regional MP (r = 0.83, p < 0.001; y = 0.87× + 0.26) with a good agreement (-0.1 ± 0.6 ml/min/g). There was also a significant correlation between CMR and PET with regard to global and regional MPR (r = 0.69, p = 0.001 and r = 0.57, p < 0.001, respectively).

CONCLUSIONS

There is good agreement between MP quantified by 13N-NH3 PET and dual sequence, single contrast bolus CMR in patients with stable CAD. Thus, CMR is viable in clinical practice for quantification of MP.

Noninvasive PET quantitative myocardial blood flow with regadenoson for assessing cardiac allograft vasculopathy in orthotopic heart transplantation patients

BACKGROUND

Risk stratification and early detection of cardiac allograft vasculopathy (CAV) is essential in orthotopic heart transplantation (OHT) patients. This study assesses the changes in myocardial blood flow (MBF) noninvasively in OHT patients using quantitative cardiac PET with regadenoson.

METHODS

Twelve patients (Group 1) (8 males, 4 females, mean age 55 ± 7 years) with no history of post OHT myocardial ischemia were enrolled 5.4 ± 2.0 years after OHT. Fifteen patients (Group 2) (9 males, 6 females, mean age 71 ± 9 years) with intermediate pretest probability but not documented evidence for coronary artery disease (CAD) were also included to serve as control. Global and regional MBFs were assessed using dynamic 13N-NH3 PET at rest and during regadenoson-induced hyperemia. The coronary flow reserve (CFR) was also calculated as the ratio of hyperemic to resting MBF.

RESULTS

Mean regadenoson-induced rate-pressure products were similar in both groups, while there was an increase in resting rate-pressure product in Group 1 patients. Both mean and median values of resting MBF were higher in Group 1 than Group 2 patients (1.33 ± 0.31 and 1.01 ± 0.21 mL/min/g for Groups 1 and 2, respectively, P < .001), while mean hyperemic MBF values were similar in both Groups (2.68 ± 0.84 and 2.64 ± 0.94 mL/min/g, P = NS) but median hyperemic MBF values were lower in Group 1 than Group 2 patients (2.0 vs. 2.60 mL/min/g, P = .018). Both mean and median CFR values demonstrated a significant reduction for Group 1 compared to Group 2 patients (2.07 ± 0.74 vs 2.63 ± 0.48, P = .025).

CONCLUSIONS

This study suggests that the MBF in OHT patients may be abnormal at resting state with diminished CFR. This hints that the epicardial and microvascular coronary subsystem may be exacerbated after OHT leading to the gradual progression of CAV.

Myocardial perfusion imaging with PET

Noninvasive assessment of coronary artery disease remains a challenging task, with a large armamentarium of diagnostic modalities. Myocardial perfusion imaging (MPI) is widely used for this purpose whereby cardiac positron emission tomography (PET) is considered the gold standard. Next to relative radiotracer distribution, PET allows for measurement of absolute myocardial blood flow. This quantification of perfusion improves diagnostic accuracy and prognostic value. Cardiac hybrid imaging relies on the fusion of anatomical and functional imaging using coronary computed tomography angiography and MPI, respectively, and provides incremental value as compared with either stand-alone modality.

Measurement of absolute myocardial blood flow in humans using dynamic cardiac SPECT and 99mTc-tetrofosmin: Method and validation

BACKGROUND

The objective of this study was to measure myocardial blood flow (MBF) in humans using 99mTc-tetrofosmin and dynamic single-photon emission computed tomography (SPECT).

METHODS

Dynamic SPECT using 99mTc-tetrofosmin and dynamic positron emission tomography (PET) was performed on a group of 16 patients. The SPECT data were reconstructed using a 4D-spatiotemporal iterative reconstruction method. The data corresponding to 9 patients were used to determine the flow-extraction curve for 99mTc-tefrofosmin while data from the remaining 7 patients were used for method validation. The nonlinear tracer correction parameters A and B for 99mTc-tefrofosmin were estimated for the 9 patients by fitting the flow-extraction curve [Formula: see text] for K 1 values estimated with 99mTc-tefrofosmin using SPECT and MBF values estimated with 13N-NH3 using PET. These parameters were then used to calculate MBF and coronary flow reserve (CFR) in three coronary territories (LAD, RCA, and LCX) using SPECT for an independent cohort of 7 patients. The results were then compared with that estimated with 13N-NH3 PET. The flow-dependent permeability surface-area product (PS) for 99mTc-tefrofosmin was also estimated.

RESULTS

The estimated flow-extraction parameters for 99mTc-tefrofosmin were found to be A = 0.91 ± 0.11, B = 0.34 ± 0.20 (R 2 = 0.49). The range of MBF in LAD, RCA, and LCX was 0.44-3.81 mL/min/g. The MBF between PET and SPECT in the group of independent cohort of 7 patients showed statistically significant correlation, r = 0.71 (P < .001). However, the corresponding CFR correlation was moderate r = 0.39 yet statistically significant (P = .037). The PS for 99mTc-tefrofosmin was (0.019 ± 0.10)*MBF + (0.32 ± 0.16).

CONCLUSIONS

Dynamic cardiac SPECT using 99mTc-tetrofosmin and a clinical two-headed SPECT/CT scanner can be a useful tool for estimation of MBF.

Clinical Utility and Future Applications of PET/CT and PET/CMR in Cardiology

Over the past several years, there have been major advances in cardiovascular positron emission tomography (PET) in combination with either computed tomography (CT) or, more recently, cardiovascular magnetic resonance (CMR). These multi-modality approaches have significant potential to leverage the strengths of each modality to improve the characterization of a variety of cardiovascular diseases and to predict clinical outcomes. This review will discuss current developments and potential future uses of PET/CT and PET/CMR for cardiovascular applications, which promise to add significant incremental benefits to the data provided by each modality alone.

Assessment of coronary flow reserve using a combination of planar first-pass angiography and myocardial SPECT: Comparison with myocardial (15)O-water PET

Coronary flow reserve (CFR), defined as the ratio of maximum coronary flow increase from baseline resting blood flow, is one of the most sensitive parameters to detect early signs of coronary arteriosclerosis at the microvascular level. Myocardial perfusion PET is a well-established technology for CFR measurement, however, availability is still limited. The aim of this study is to introduce and validate myocardial flow reserve measurement by myocardial perfusion SPECT.

METHODS

Myocardial perfusion SPECT at rest and ATP stress (0.16mg/Kg/min) was performed in 10 patients with known coronary artery disease. Immediately after the injection of Tc-99m sestamibi (MIBI), left ventricular (LV) dynamic planar angiographic data were obtained for 90s. Coronary flow reserve index as measured by MIBI SPECT (CFRMIBI) was calculated as follows: CFRMIBI=CmsSbmb/CmbSbms, where subscripts b, s, Cm, and Sbm indicate baseline, during stress, myocardial counts with MIBI SPECT, and integral of LV counts with first pass angiography, respectively. Additionally, standard stress/rest (15)O-water PET to estimate CFR was performed in all patients as standard of reference.

RESULTS

CFRMIBI increased in conjunction with CFR, but underestimated blood flow at high flow rates. The relationship between CFRMIBI (Y) and CFRPET (X) was well fitted as follows: Y=1.40x(1-exp(1.79/x)) (r=0.84).

CONCLUSIONS

The index of CFRMIBI reflects the CFR by (15)O-water PET but underestimates flow at high flows, maybe as a reflection of pharmacokinetic limitations of MIBI.

The reproducibility of time-of-flight PET and conventional PET for the quantification of myocardial blood flow and coronary flow reserve with (13)N-ammonia

BACKGROUND

This study aimed to validate the reproducibility of quantitative analysis using time-of-flight (TOF) and conventional PET with (13)N-ammonia ((13)N-NH3).

METHODS AND RESULTS

Phantom images were reconstructed with and without TOF, and recovery coefficients (RCs) and the percent contrast of each sphere over the percent background variability were assessed. In the clinical study, 21 subjects underwent dynamic (13)N-NH3 PET scanning under stress and rest conditions. The dynamic acquisition images and intra- and inter-observer reproducibility of myocardial blood flow (MBF) and coronary flow reserve (CFR) were compared between reconstructions (with and without TOF). In the phantom study, RCs and the percent contrast of each sphere over the percent background variability was improved with TOF. In the clinical study, the noise of blood pool and myocardial images with TOF was less than that without TOF. Territorial and global intra- and inter-observer reproducibility of MBF and CFR values was excellent. Although segmental intra- and inter-observer reproducibility was excellent, there were larger variations in apex and the segment near the right ventricle (RV) without TOF. These variations became inconspicuous with TOF.

CONCLUSION

Visual image quality, RCs, and percent contrast over percent background variability with TOF were better than that without TOF. Excellent correlations and good agreements in quantitative values were observed. TOF improved the variation of segmental values.

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.

PET Radiotracers of the Cardiovascular System

Cardiovascular PET provides exquisite measurements of key aspects of the cardiovascular system and as a consequence it plays central role in cardiovascular investigation. Moreover, PET is now playing an ever increasing role in the management of the cardiac patient. Central to the success of PET is the development and use of novel radiotracers that permit measurements of key aspects of cardiovascular health such as myocardial perfusion, metabolism, and neuronal function. Moreover, the development of molecular imaging radiotracers is now permitting the interrogation of cellular and sub cellular processes. This article highlights these various radiotracers and their role in both cardiovascular research and potential clinical applications.

Quantitative Myocardial Perfusion with Dynamic Contrast-Enhanced Imaging in MRI and CT: Theoretical Models and Current Implementation

Technological advances in magnetic resonance imaging (MRI) and computed tomography (CT), including higher spatial and temporal resolution, have made the prospect of performing absolute myocardial perfusion quantification possible, previously only achievable with positron emission tomography (PET). This could facilitate integration of myocardial perfusion biomarkers into the current workup for coronary artery disease (CAD), as MRI and CT systems are more widely available than PET scanners. Cardiac PET scanning remains expensive and is restricted by the requirement of a nearby cyclotron. Clinical evidence is needed to demonstrate that MRI and CT have similar accuracy for myocardial perfusion quantification as PET. However, lack of standardization of acquisition protocols and tracer kinetic model selection complicates comparison between different studies and modalities. The aim of this overview is to provide insight into the different tracer kinetic models for quantitative myocardial perfusion analysis and to address typical implementation issues in MRI and CT. We compare different models based on their theoretical derivations and present the respective consequences for MRI and CT acquisition parameters, highlighting the interplay between tracer kinetic modeling and acquisition settings.

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.

Variance Estimation for Myocardial Blood Flow by Dynamic PET

The estimation of myocardial blood flow (MBF) by (13)N-ammonia or (82)Rb dynamic PET typically relies on an empirically determined generalized Renkin-Crone equation to relate the kinetic parameter K1 to MBF. Because the Renkin-Crone equation defines MBF as an implicit function of K1, the MBF variance cannot be determined using standard error propagation techniques. To overcome this limitation, we derived novel analytical approximations that provide first- and second-order estimates of MBF variance in terms of the mean and variance of K1 and the Renkin-Crone parameters. The accuracy of the analytical expressions was validated by comparison with Monte Carlo simulations, and MBF variance was evaluated in clinical (82)Rb dynamic PET scans. For both (82)Rb and (13)N-ammonia, good agreement was observed between both (first- and second-order) analytical variance expressions and Monte Carlo simulations, with moderately better agreement for second-order estimates. The contribution of the Renkin-Crone relation to overall MBF uncertainty was found to be as high as 68% for (82)Rb and 35% for (13)N-ammonia. For clinical (82)Rb PET data, the conventional practice of neglecting the statistical uncertainty in the Renkin-Crone parameters resulted in underestimation of the coefficient of variation of global MBF and coronary flow reserve by 14-49%. Knowledge of MBF variance is essential for assessing the precision and reliability of MBF estimates. The form and statistical uncertainty in the empirical Renkin-Crone relation can make substantial contributions to the variance of MBF. The novel analytical variance expressions derived in this work enable direct estimation of MBF variance which includes this previously neglected contribution.

Precision and accuracy of clinical quantification of myocardial blood flow by dynamic PET: A technical perspective

A number of exciting advances in PET/CT technology and improvements in methodology have recently converged to enhance the feasibility of routine clinical quantification of myocardial blood flow and flow reserve. Recent promising clinical results are pointing toward an important role for myocardial blood flow in the care of patients. Absolute blood flow quantification can be a powerful clinical tool, but its utility will depend on maintaining precision and accuracy in the face of numerous potential sources of methodological errors. Here we review recent data and highlight the impact of PET instrumentation, image reconstruction, and quantification methods, and we emphasize (82)Rb cardiac PET which currently has the widest clinical application. It will be apparent that more data are needed, particularly in relation to newer PET technologies, as well as clinical standardization of PET protocols and methods. We provide recommendations for the methodological factors considered here. At present, myocardial flow reserve appears to be remarkably robust to various methodological errors; however, with greater attention to and more detailed understanding of these sources of error, the clinical benefits of stress-only blood flow measurement may eventually be more fully realized.

Beyond 18F-FDG: Characterization of PET/CT and PET/MR Scanners for a Comprehensive Set of Positron Emitters of Growing Application--18F, 11C, 89Zr, 124I, 68Ga, and 90Y

This study aimed to investigate image quality for a comprehensive set of isotopes ((18)F, (11)C, (89)Zr, (124)I, (68)Ga, and (90)Y) on 2 clinical scanners: a PET/CT scanner and a PET/MR scanner.

METHODS

Image quality and spatial resolution were tested according to NU 2-2007 of the National Electrical Manufacturers Association. An image-quality phantom was used to measure contrast recovery, residual bias in a cold area, and background variability. Reconstruction methods available on the 2 scanners were compared, including point-spread-function correction for both scanners and time of flight for the PET/CT scanner. Spatial resolution was measured using point sources and filtered backprojection reconstruction.

RESULTS

With the exception of (90)Y, small differences were seen in the hot-sphere contrast recovery of the different isotopes. Cold-sphere contrast recovery was similar across isotopes for all reconstructions, with an improvement seen with time of flight on the PET/CT scanner. The lower-statistic (90)Y scans yielded substantially lower contrast recovery than the other isotopes. When isotopes were compared, there was no difference in measured spatial resolution except for PET/MR axial spatial resolution, which was significantly higher for (124)I and (68)Ga.

CONCLUSION

Overall, both scanners produced good images with (18)F, (11)C, (89)Zr, (124)I, (68)Ga, and (90)Y.

Novel synthesis and initial preclinical evaluation of (18)F-[FDG] labeled rhodamine: a potential PET myocardial perfusion imaging agent

Myocardial perfusion imaging is one of the most commonly performed investigations in nuclear medicine studies. Due to the clinical importance of [(18)F]-fluoro-2-deoxy-D-glucose ([(18)F]-FDG) and its availability in almost every PET center, a new radiofluorinated [(18)F]-FDG-rhodamine conjugate was synthesized using [(18)F]-FDG as a prosthetic group. In a convenient and simple one-step radiosynthesis, [(18)F]-FDG-rhodamine conjugate was prepared in quantitative radiochemical yields, with total synthesis time of nearly 20 min and radiochemical purity of greater than 98%, without the need for HPLC purification, which make these approaches amenable for automation. Biodistribution studies in normal rats at 60 min post-injection demonstrated a high uptake in the heart (>11% ID/g) and favorable pharmacokinetics. Additionally, [(18)F]-FDG-rhodamine showed an extraction value of 27.63%±5.12% in rat hearts. These results demonstrate that [(18)F]-FDG-rhodamine conjugate may be useful as an imaging agent for the positron emission tomography evaluation of myocardial perfusion.

Cardiac PET perfusion tracers: current status and future directions

PET myocardial perfusion imaging (MPI) is increasingly being used for noninvasive detection and evaluation of coronary artery disease. However, the widespread use of PET MPI has been limited by the shortcomings of the current PET perfusion tracers. The availability of these tracers is limited by the need for an onsite ((15)O water and (13)N ammonia) or nearby ((13)N ammonia) cyclotron or commitment to costly generators ((82)Rb). Owing to the short half-lives, such as 76 seconds for (82)Rb, 2.06 minutes for (15)O water, and 9.96 minutes for (13)N ammonia, their use in conjunction with treadmill exercise stress testing is either not possible ((82)Rb and (15)O water) or not practical ((13)N ammonia). Furthermore, the long positron range of (82)Rb makes image resolution suboptimal and its low myocardial extraction limits its defect resolution. In recent years, development of an (18)F-labeled PET perfusion tracer has gathered considerable interest. The longer half-life of (18)F (109 minutes) would make the tracer available as a unit dose from regional cyclotrons and allow use in conjunction with treadmill exercise testing. Furthermore, the short positron range of (18)F would result in better image resolution. Flurpiridaz F 18 is by far the most thoroughly studied in animal models and is the only (18)F-based PET MPI radiotracer currently undergoing clinical evaluation. Preclinical and clinical experience with Flurpiridaz F 18 demonstrated a high myocardial extraction fraction, high image and defect resolution, high myocardial uptake, slow myocardial clearance, and high myocardial-to-background contrast that was stable over time-important properties of an ideal PET MPI radiotracer. Preclinical data from other (18)F-labeled myocardial perfusion tracers are encouraging.

Positron-emitting myocardial blood flow tracers and clinical potential

Positron-emitting myocardial flow radiotracers such as (15)O-water, (13)N-ammonia and (82)Rubidium in conjunction with positron-emission-tomography (PET) are increasingly applied in clinical routine for coronary artery disease (CAD) detection, yielding high diagnostic accuracy, while providing valuable information on cardiovascular (CV) outcome. Owing to a cyclotron dependency of (15)O-water and (13)N-ammonia, their clinical use for PET myocardial perfusion imaging is limited to a few centers. This limitation could be overcome by the increasing use of (82)Rubidium as it can be eluted from a commercially available (82)Strontium generator and, thus, is independent of a nearby cyclotron. Another novel F-18-labeled myocardial flow radiotracer is flurpiridaz which has attracted increasing interest due to its excellent radiotracer characteristics for perfusion and flow imaging with PET. In particular, the relatively long half-life of 109 minutes of flurpiridaz may afford a general application of this radiotracer for PET perfusion imaging comparable to technetium-99m-labeled single-photon emission computed tomography (SPECT). The ability of PET in conjunction with several radiotracers to assess myocardial blood flow (MBF) in ml/g/min at rest and during vasomotor stress has contributed to unravel pathophysiological mechanisms underlying coronary artery disease (CAD), to improve the detection and characterization of CAD burden in multivessel disease, and to provide incremental prognostic information in individuals with subclinical and clinically-manifest CAD. The concurrent evaluation of myocardial perfusion and MBF may lead to a new era of a personalized, image-guided therapy approach that may offer potential to further improve clinical outcome in CV disease patients but needing validation in large-scale clinical trials.

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.