Simplified quantification of myocardial flow reserve with flurpiridaz F 18: validation with microspheres in a pig model

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.

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.

Rest/stress myocardial perfusion imaging by positron emission tomography with 18F-Flurpiridaz: A feasibility study in mice

BACKGROUND

Myocardial perfusion imaging by positron emission tomography (PET-MPI) is the current gold standard for quantification of myocardial blood flow. 18F-flurpiridaz was recently introduced as a valid alternative to currently used PET-MPI probes. Nonetheless, optimum scan duration and time interval for image analysis are currently unknown. Further, it is unclear whether rest/stress PET-MPI with 18F-flurpiridaz is feasible in mice.

METHODS

Rest/stress PET-MPI was performed with 18F-flurpiridaz (0.6-3.0 MBq) in 27 mice aged 7-8 months. Regadenoson (0.1 µg/g) was used for induction of vasodilator stress. Kinetic modeling was performed using a metabolite-corrected arterial input function. Image-derived myocardial 18F-flurpiridaz uptake was assessed for different time intervals by placing a volume of interest in the left ventricular myocardium.

RESULTS

Tracer kinetics were best described by a two-tissue compartment model. K1 ranged from 6.7 to 20.0 mL·cm-3·min-1, while myocardial volumes of distribution (VT) were between 34.6 and 83.6 mL·cm-3. Of note, myocardial 18F-flurpiridaz uptake (%ID/g) was significantly correlated with K1 at rest and following pharmacological vasodilation for all time intervals assessed. However, while Spearman's coefficients (rs) ranged between 0.478 and 0.681, R2 values were generally low. In contrast, an excellent correlation of myocardial 18F-flurpiridaz uptake with VT was obtained, particularly when employing the averaged myocardial uptake from 20 to 40 min post tracer injection (R2 ≥ 0.98). Notably, K1 and VT were similarly sensitive to pharmacological vasodilation induction. Further, mean stress-to-rest ratios of K1, VT, and %ID/g 18F-flurpiridaz were virtually identical, suggesting that %ID/g 18F-flurpiridaz can be used to estimate coronary flow reserve (CFR) in mice.

CONCLUSION

Our findings suggest that a simplified assessment of relative myocardial perfusion and CFR, based on image-derived tracer uptake, is feasible with 18F-flurpiridaz in mice, enabling high-throughput mechanistic CFR studies in rodents.

Novel nuclear medical procedures in the detection of microvascular dysfunction

Coronary microvascular dysfunction is present in two-thirds of patients showing symptoms and signs of myocardial ischemia. Their microcirculation has abnormalities due to endothelial and smooth muscle cell dysfunction. Impairment of this mechanism causes a high risk of adverse cardiovascular event. Diagnosing coronary microvascular dysfunction is challenging. Guidelines recommend the use of nuclear medicine procedures in the above-mentioned indications. Myocardial perfusion imaging with positron emission tomography is a novel procedure with high diagnostic accuracy and quality of images. It has short acquisition, low effective radiation dose and prognostic factors. There are still unknowns about this procedure and all its benefits.

Radiopharmaceuticals for PET and SPECT Imaging: A Literature Review over the Last Decade

Positron emission tomography (PET) uses radioactive tracers and enables the functional imaging of several metabolic processes, blood flow measurements, regional chemical composition, and/or chemical absorption. Depending on the targeted processes within the living organism, different tracers are used for various medical conditions, such as cancer, particular brain pathologies, cardiac events, and bone lesions, where the most commonly used tracers are radiolabeled with 18F (e.g., [¹⁸F]-FDG and NA [¹⁸F]). Oxygen-15 isotope is mostly involved in blood flow measurements, whereas a wide array of ¹¹C-based compounds have also been developed for neuronal disorders according to the affected neuroreceptors, prostate cancer, and lung carcinomas. In contrast, the single-photon emission computed tomography (SPECT) technique uses gamma-emitting radioisotopes and can be used to diagnose strokes, seizures, bone illnesses, and infections by gauging the blood flow and radio distribution within tissues and organs. The radioisotopes typically used in SPECT imaging are iodine-123, technetium-99m, xenon-133, thallium-201, and indium-111. This systematic review article aims to clarify and disseminate the available scientific literature focused on PET/SPECT radiotracers and to provide an overview of the conducted research within the past decade, with an additional focus on the novel radiopharmaceuticals developed for medical imaging.

Parathyroid Imaging: Past, Present, and Future

The goal of parathyroid imaging is to identify all sources of excess parathyroid hormone secretion pre-operatively. A variety of imaging approaches have been evaluated and utilized over the years for this purpose. Ultrasound relies solely on structural features and is without radiation, however is limited to superficial evaluation. 4DCT and 4DMRI provide enhancement characteristics in addition to structural features and dynamic enhancement has been investigated as a way to better distinguish parathyroid from adjacent structures. It is important to recognize that 4DCT provides valuable information however results in much higher radiation dose to the thyroid gland than the other available examinations, and therefore the optimal number of phases is an area of controversy. Single-photon scintigraphy with 99mTc-Sestamibi, or dual tracer 99mTc-pertechnetate and 99mTc-sestamibi with or without SPECT or SPECT/CT is part of the standard of care in many centers with availability and expertise in nuclear medicine. This molecular imaging approach detects cellular physiology such as mitochondria content found in parathyroid adenomas. Combining structural imaging such as CT or MRI with molecular imaging in a hybrid approach allows the ability to obtain robust structural and functional information in one examination. Hybrid PET/CT is widely available and provides improved imaging and quantification over SPECT or SPECT/CT. Emerging PET imaging techniques, such as 18F-Fluorocholine, have the exciting potential to reinvent parathyroid imaging. PET/MRI may be particularly well suited to parathyroid imaging, where available, because of the ability to perform dynamic contrast-enhanced imaging and co-registered 18F-Fluorocholine PET imaging simultaneously with low radiation dose to the thyroid. A targeted agent specific for a parathyroid tissue biomarker remains to be identified.

Added value of myocardial blood flow using 18F-flurpiridaz PET to diagnose coronary artery disease: The flurpiridaz 301 trial

BACKGROUND

18F-Flurpiridaz is a promising investigational radiotracer for PET myocardial perfusion imaging with favorable properties for quantification of myocardial blood flow (MBF). We sought to validate the incremental diagnostic value of absolute MBF quantification in a large multicenter trial against quantitative coronary angiography.

METHODS

We retrospectively analyzed a subset of patients (N = 231) from the first phase 3 flurpiridaz trial (NCT01347710). Dynamic PET data at rest and pharmacologic stress were fit to a previously validated 2-tissue-compartment model. Absolute MBF and myocardial flow reserve (MFR) were compared with coronary artery disease severity quantified by invasive coronary angiography on a per-patient and per-vessel basis.

RESULTS

Stress MBF per-vessel accurately identified obstructive disease (c-index 0.79) and progressively declined with increasing stenosis severity (2.35 ± 0.71 in patients without CAD; 1.92 ± 0.49 in non-obstructed territories of CAD patients; and 1.54 ± 0.50 in diseased territories, P < 0.05). MFR similarly declined with increasing stenosis severity (3.03 ± 0.94; 2.69 ± 0.95; and 2.33 ± 0.86, respectively, P < 0.05). In multivariable logistic regression modeling, stress MBF and MFR provided incremental diagnostic value beyond patient characteristics and relative perfusion analysis.

CONCLUSIONS

Clinical myocardial blood flow measurement with 18F-flurpiridaz cardiac PET shows promise for routine application.

Internal validation of myocardial flow reserve PET imaging using stress/rest myocardial activity ratios with Rb-82 and N-13-ammonia

BACKGROUND

Myocardial flow reserve (MFR) measurement provides incremental diagnostic and prognostic information. The objective of the current study was to investigate the application of a simplified model for the estimation of MFR using only the stress/rest myocardial activity ratio (MAR) in patients undergoing rest-stress cardiac PET MPI.

METHODS AND RESULTS

Rest and dipyridamole stress dynamic PET imaging was performed in consecutive patients using 82Rb or 13NH3 (n = 250 each). Reference standard MFR was quantified using a standard one-tissue compartment model. Stress/rest myocardial activity ratio (MAR) was calculated using the LV-mean activity from 2 to 6 minutes post-injection. Simplified estimates of MFR (MFREST) were then calculated using an inverse power function. For 13NH3, there was good correlation between MFR and MFREST values (R = 0.63), with similar results for 82Rb (R = 0.73). There was no bias in the MFREST values with either tracer. The overall diagnostic performance of MFREST for detection of MFR < 2 was good with ROC area under the curve (AUC) = 83.2 ± 1.2% for 13NH3 and AUC = 90.4 ± 0.7% for 82Rb.

CONCLUSION

MFR was estimated with good accuracy using 82Rb and 13NH3 with a simplified method that relies only on stress/rest activity ratios. This novel approach does not require dynamic imaging or tracer kinetic modeling. It may be useful for routine quality assurance of PET MFR measurements, or in scanners where full dynamic imaging and tracer kinetic modeling is not feasible for technical or logistical reasons.

Cardiac perfusion by positron emission tomography

Myocardial perfusion imaging (MPI) with positron emission tomography (PET) is an established tool for evaluation of obstructive coronary artery disease (CAD). The contemporary 3-dimensional scanner technology and the state-of-the-art MPI radionuclide tracers and pharmacological stress agents, as well as the cutting-edge image reconstruction techniques and data analysis software, have all enabled accurate, reliable and reproducible quantification of absolute myocardial blood flow (MBF), and henceforth calculation of myocardial flow reserve (MFR) in several clinical scenarios. In patients with suspected coronary artery disease, both absolute stress MBF and MFR can identify myocardial territories subtended by epicardial coronary arteries with haemodynamically significant stenosis, as defined by invasive coronary fractional flow reserve measurement. In particular, absolute stress MBF and MFR offered incremental prognostic information for predicting adverse cardiac outcome, and hence for better patient risk stratification, over those provided by traditional clinical risk predictors. This article reviews the available evidence to support the translation of the current techniques and technologies into a useful decision-making tool in real-world clinical practice.

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.

PET and SPECT Tracers for Myocardial Perfusion Imaging

Coronary artery disease has been the leading cause of death since the 1960s, which has motivated the research and development of myocardial perfusion imaging (MPI) agents for early diagnosis and to guide treatment. MPI with SPECT has been the clinical workhorse for MPI, but over the past two decades PET MPI is experiencing growth due to enhanced image quality that results in superior diagnostic accuracy over SPECT. Furthermore, dynamic PET imaging of the tracer distribution process from time of tracer administration to tracer accumulation in the myocardium has enabled routine quantification of myocardial blood flow (MBF) and myocardial flow reserve (MFR) in absolute units. MBF and MFR incrementally improve diagnostic and prognostic accuracy over MPI alone. In some cases (eg, rubidium PET imaging with pharmacologic stress) MPI, MBF, and MFR can be acquired simultaneously without incremental cost, radiation exposure, or significant processing time. Nuclear cardiology clinics have been looking to incorporate MBF quantification into clinical routine, but traditional SPECT and MPI tracers are inadequate for this challenge. Cardiac dedicated SPECT scanners can also perform dynamic imaging and have stimulated research into MBF quantification using SPECT tracers. New perfusion tracers must be tailored for emerging clinical needs (including MBF quantification), technical capabilities of imaging instrumentation, market constraints, and supply chain feasibility. Because these conditions have been evolving, tracers previously considered inferior may be reconsidered for future applications and some recently developed tracers may be suboptimal. This article reviews current, clinically-available tracers and those under development showing greatest potential. It discusses for each tracer the rationale for development, physiological mechanism of uptake by the myocardium, published evaluation results and development state. Finally, it gauges the suitability of each tracer for clinical application. The article demonstrates an acceleration in the pace of perfusion radiotracer development due to better understanding of the relevant physiology, better chemistry tools and small animal imaging. Consequently, bad tracers may fail faster and with less wasted investment, and good tracers may translate more efficiently from bench to bedside.

Quantification of PET Myocardial Blood Flow

PURPOSE OF REVIEW

The aim of this review is to provide an update on quantification of myocardial blood flow (MBF) with positron emission tomography (PET) imaging. Technical and clinical aspects of flow quantification with PET are reviewed.

RECENT FINDINGS

The diagnostic and prognostic values of myocardial flow quantification have been established in numerous studies and in various populations. MBF quantification has also shown itself to be particularly useful in the assessment of coronary microvascular dysfunction and in evaluation of cardiac allograft vasculopathy. Overall, myocardial flow reserve (MFR) and hyperemic MBF can lead to improved risk stratification by providing information complementary to that of other markers of disease severity, such as fractional flow reserve. Flow quantification enhances MPI's ability to detect both significant epicardial disease and microvascular dysfunction. With recent technological and methodological advances, flow quantification with PET is no longer restricted to cyclotron-equipped academic centers.

Multi-atlas cardiac PET segmentation

PURPOSE

We propose a multi-atlas based segmentation method for cardiac PET and SPECT images to deal with the high variability of tracer uptake characteristics in myocardium. In addition, we verify its performance by comparing it to the manual segmentation and single-atlas based approach, using dynamic myocardial PET.

METHODS

Twelve left coronary artery ligated SD rats underwent ([¹⁸F]fluoropentyl) triphenylphosphonium salt PET/CT scans. Atlas-based segmentation is based on the spatial normalized template with pre-defined region-of-interest (ROI) for each anatomical or functional structure. To generate multiple left ventricular (LV) atlases, each LV image was segmented manually and divided into angular segments. The segmentation methods performances were compared in regional count information using leave-one-out cross-validation. Additionally, the polar-maps of kinetic parameters were estimated.

RESULTS

In all images, the highest r² template yielded the lowest root-mean-square error (RMSE) between the source image and the best-matching templates ranged between 0.91-0.97 and 0.06-0.11, respectively. The single-atlas and multi-atlas based ROIs yielded remarkably different perfusion distributions: only the multi-atlas based segmentation showed equivalent high correlation results (r² = 0.92) with the manual segmentation compared with the single-atlas based (r² = 0.88). The high perfusion value underestimation was remarkable in single-atlas based segmentation.

CONCLUSIONS

The main advantage of the proposed multi-atlas based cardiac segmentation method is that it does not require any prior information on the tracer distribution to be incorporated into the image segmentation algorithms. Therefore, the same procedure suggested here is applicable to any other cardiac PET or SPECT imaging agents without modification.

Moving into the next era of PET myocardial perfusion imaging: introduction of novel 18F-labeled tracers

The heart failure epidemic continues to rise with coronary artery disease as one of its main causes. Novel concepts for risk stratification to guide the referring cardiologist towards revascularization procedures are of significant value. Myocardial perfusion imaging using single-photon emission computed tomography (SPECT) agents has demonstrated high accuracy for the detection of clinically relevant stenoses. With positron emission tomography (PET) becoming more widely available, mainly due to its diagnostic performance in oncology, perfusion imaging with that modality is more practical than in the past and overcomes existing limitations of SPECT MPI. Advantages of PET include more reliable quantification of absolute myocardial blood flow, the routine use of computed tomography for attenuation correction, a higher spatiotemporal resolution and a higher count sensitivity. Current PET radiotracers such as rubidium-82 (half-life, 76 s), oxygen-15 water (2 min) or nitrogen-13 ammonia (10 min) are labeled with radionuclides with very short half-lives, necessitating that stress imaging is performed under pharmacological vasodilator stress instead of exercise testing. However, with the introduction of novel ¹⁸F-labeled MPI PET radiotracers (half-life, 110 min), the intrinsic advantages of PET can be combined with exercise testing. Additional advantages of those radiotracers include, but are not limited to: potentially improved cost-effectiveness due to the use of pre-existing delivery systems and superior imaging qualities, mainly due to the shortest positron range among available PET MPI probes. In the present review, widely used PET MPI radiotracers will be reviewed and potential novel ¹⁸F-labeled perfusion radiotracers will be discussed.

Hybrid cardiac imaging using PET/MRI: a joint position statement by the European Society of Cardiovascular Radiology (ESCR) and the European Association of Nuclear Medicine (EANM)

Positron emission tomography (PET) and magnetic resonance imaging (MRI) have both been used for decades in cardiovascular imaging. Since 2010, hybrid PET/MRI using sequential and integrated scanner platforms has been available, with hybrid cardiac PET/MR imaging protocols increasingly incorporated into clinical workflows. Given the range of complementary information provided by each method, the use of hybrid PET/MRI may be justified and beneficial in particular clinical settings for the evaluation of different disease entities. In the present joint position statement, we critically review the role and value of integrated PET/MRI in cardiovascular imaging, provide a technical overview of cardiac PET/MRI and practical advice related to the cardiac PET/MRI workflow, identify cardiovascular applications that can potentially benefit from hybrid PET/MRI, and describe the needs for future development and research. In order to encourage its wide dissemination, this article is freely accessible on the European Radiology and European Journal of Hybrid Imaging web sites.

KEY POINTS

• Studies and case-reports indicate that PET/MRI is a feasible and robust technology. • Promising fields of application include a variety of cardiac conditions. • Larger studies are required to demonstrate its incremental and cost-effective value. • The translation of novel radiopharmaceuticals and MR-sequences will provide exciting new opportunities.

Rapid computation of single PET scan rest-stress myocardial blood flow parametric images by table look up

PURPOSE

We have recently reported a method for measuring rest-stress myocardial blood flow (MBF) using a single, relatively short, PET scan session. The method requires two IV tracer injections, one to initiate rest imaging and one at peak stress. We previously validated absolute flow quantitation in ml/min/cc for standard bull's eye, segmental analysis. In this work, we extend the method for fast computation of rest-stress MBF parametric images.

METHODS

We provide an analytic solution to the single-scan rest-stress flow model which is then solved using a two-dimensional table lookup method (LM). Simulations were performed to compare the accuracy and precision of the lookup method with the original nonlinear method (NLM). Then the method was applied to 16 single scan rest/stress measurements made in 12 pigs: seven studied after infarction of the left anterior descending artery (LAD) territory, and nine imaged in the native state. Parametric maps of rest and stress MBF as well as maps of left (f_LV ) and right (f_RV ) ventricular spill-over fractions were generated. Regions of interest (ROIs) for 17 myocardial segments were defined in bull's eye fashion on the parametric maps. The mean of each ROI was then compared to the rest (K_1r ) and stress (K_1s ) MBF estimates obtained from fitting the 17 regional TACs with the NLM.

RESULTS

In simulation, the LM performed as well as the NLM in terms of precision and accuracy. The simulation did not show that bias was introduced by the use of a predefined two-dimensional lookup table. In experimental data, parametric maps demonstrated good statistical quality and the LM was computationally much more efficient than the original NLM. Very good agreement was obtained between the mean MBF calculated on the parametric maps for each of the 17 ROIs and the regional MBF values estimated by the NLM (K_1mapLM  = 1.019 × K_1ROINLM  + 0.019, R²  = 0.986; mean difference = 0.034 ± 0.036 mL/min/cc).

CONCLUSIONS

We developed a table lookup method for fast computation of parametric imaging of rest and stress MBF. Our results show the feasibility of obtaining good quality MBF maps using modest computational resources, thus demonstrating that the method can be applied in a clinical environment to obtain full quantitative MBF information.

Single-scan rest/stress imaging: validation in a porcine model with 18F-Flurpiridaz

PURPOSE

¹⁸F-labeled myocardial flow agents are becoming available for clinical application but the ∼2 hour half-life of ¹⁸F complicates their clinical application for rest-stress measurements. The goal of this work is to evaluate in a pig model a single-scan method which provides quantitative rest-stress blood flow in less than 15 minutes.

METHODS

Single-scan rest-stress measurements were made using ¹⁸F-Flurpiridaz. Nine scans were performed in healthy pigs and seven scans were performed in injured pigs. A two-injection, single-scan protocol was used in which an adenosine infusion was started 4 minutes after the first injection of ¹⁸F-Flurpiridaz and followed either 3 or 6 minutes later by a second radiotracer injection. In two pigs, microsphere flow measurements were made at rest and during stress. Dynamic images were reoriented into the short axis view, and regions of interest (ROIs) for the 17 myocardial segments were defined in bull's eye fashion. PET data were fitted with MGH2, a kinetic model with time varying kinetic parameters, in which blood flow changes abruptly with the introduction of adenosine. Rest and stress myocardial blood flow (MBF) were estimated simultaneously.

RESULTS

The first 12-14 minutes of rest-stress PET data were fitted in detail by the MGH2 model, yielding MBF measurement with a mean precision of 0.035 ml/min/cc. Mean myocardial blood flow across pigs was 0.61 ± 0.11 mL/min/cc at rest and 1.06 ± 0.19 mL/min/cc at stress in healthy pigs and 0.36 ± 0.20 mL/min/cc at rest and 0.62 ± 0.24 mL/min/cc at stress in the ischemic area. Good agreement was obtained with microsphere flow measurement (slope = 1.061 ± 0.017, intercept = 0.051 ± 0.017, mean difference 0.096 ± 0.18 ml/min/cc).

CONCLUSION

Accurate rest and stress blood flow estimation can be obtained in less than 15 min of PET acquisition. The method is practical and easy to implement suggesting the possibility of clinical translation.

Assessment of regional pulmonary blood flow using 68Ga-DOTA PET

Background

In vivo determination of regional pulmonary blood flow (PBF) is a valuable tool for the evaluation of many lung diseases. In this study, the use of ⁶⁸Ga-DOTA PET for the in vivo quantitative determination of regional PBF is proposed. This methodology was implemented and tested in healthy pigs and validated using fluorescent microspheres. The study was performed on young large white pigs (n = 4). To assess the reproducibility and consistency of the method, three PET scans were obtained for each animal. Each radiotracer injection was performed simultaneously to the injection of fluorescent microspheres. PBF images were generated applying a two-compartment exchange model over the dynamic PET images. PET and microspheres values were compared by regression analysis and Bland–Altman plot.

Results

The capability of the proposed technique to produce 3D regional PBF images was demonstrated. The correlation evaluation between ⁶⁸Ga-DOTA PET and microspheres showed a good and significant correlation (r = 0.74, P < 0.001).

Conclusions

Assessment of PBF with the proposed technique allows combining the high quantitative accuracy of PET imaging with the use of ⁶⁸Ga/⁶⁸Ge generators. Thus, ⁶⁸Ga-DOTA PET emerges as a potential inexpensive method for measuring PBF in clinical settings with an extended use.

Electronic supplementary material

The online version of this article (doi:10.1186/s13550-017-0259-2) contains supplementary material, which is available to authorized users.

Recent Advances in Nuclear Cardiology

Nuclear cardiology is one of the major fields of nuclear medicine practice. Myocardial perfusion studies using single-photon emission computed tomography (SPECT) have played a crucial role in the management of coronary artery diseases. Positron emission tomography (PET) has also been considered an important tool for the assessment of myocardial viability and perfusion. However, the recent development of computed tomography (CT)/magnetic resonance imaging (MRI) technologies and growing concerns about the radiation exposure of patients remain serious challenges for nuclear cardiology. In response to these challenges, remarkable achievements and improvements are currently in progress in the field of myocardial perfusion imaging regarding the applicable software and hardware. Additionally, myocardial perfusion positron emission tomography (PET) is receiving increasing attention owing to its unique capability of absolute myocardial blood flow estimation. An F-18-labeled perfusion agent for PET is under clinical trial with promising interim results. The applications of F-18 fluorodeoxyglucose (FDG) and F-18 sodium fluoride (NaF) to cardiovascular diseases have revealed details on the basic pathophysiology of ischemic heart diseases. PET/MRI seems to be particularly promising for nuclear cardiology in the future. Restrictive diseases, such as cardiac sarcoidosis and amyloidosis, are effectively evaluated using a variety of nuclear imaging tools. Considering these advances, the current challenges of nuclear cardiology will become opportunities if more collaborative efforts are devoted to this exciting field of nuclear medicine.

Synthesis and bioevaluation of 4-chloro-2-tert-butyl-5-[2-[[1-[2-[(18) F]fluroethyl]-1H-1,2,3-triazol-4-yl]methyl]phenylmethoxy]-3(2H)-pyridazinone as potential myocardial perfusion imaging agent with PET

This study reports the synthesis and characterization of 4-chloro-2-tert-butyl-5-[2-[[1-[2-[(18) F]fluroethyl]-1H-1,2,3-triazol-4-yl]methyl]phenylmethoxy]-3(2H)-pyridazinone ([(18) F]Fmp2) for myocardial perfusion imaging (MPI). The tosylate precursor and non-radioactive compound [(19) F]Fmp2 were synthesized and characterized by infrared, (1) H-NMR, (13) C-NMR, and mass spectra (MS). The radiotracer [(18) F]Fmp2 was obtained by one-step nucleophilic substitution of tosyl with (18) F, and evaluated as an MPI agent in vitro and in vivo. Starting from [(18) F]KF/K222 solution, the typical decay-corrected radiochemical yield (RCY) was 38 ± 8.8% with high radiochemical purity (>98%). The specific activity was calculated as 10 GBq/µmol at the end of synthesis determined by HPLC analysis. In the mice biodistribution, [(18) F]Fmp2 showed very high initial heart uptake (53.35 ± 5.47 %ID/g at 2 min after injection) and remarkable retention. The heart/liver, heart/lung, and heart/blood ratios were 7.98, 8.20, and 53.13, respectively at 2 min post-injection. In the Positron Emission Tomography (PET) imaging study of Chinese mini-swine, the standardized uptake value of the liver decreased modestly during the 2 h post-injection, while the heart uptake and heart/liver ratios continued to increase with time. [(18) F]Fmp2 exhibited good stability, high heart uptake and low lung uptake in mice and Chinese mini-swine. It may be worthy of further modification to improve liver clearance for MPI in the future.

(18)F-labeled rhodamines as potential myocardial perfusion agents: comparison of pharmacokinetic properties of several rhodamines

INTRODUCTION

We recently reported the development of the [(18)F]fluorodiethylene glycol ester of rhodamine B as a potential positron emission tomography (PET) tracer for myocardial perfusion imaging (MPI). This compound was developed by optimizing the ester moiety on the rhodamine B core, and its pharmacokinetic properties were found to be superior to those of the prototype ethyl ester. The goal of the present study was to optimize the rhodamine core while retaining the fluorodiethyleneglycol ester prosthetic group.

METHODS

A series of different rhodamine cores (rhodamine 6G, rhodamine 101, and tetramethylrhodamine) were labeled with (18)F using the corresponding rhodamine lactones as the precursors and [(18)F]fluorodiethylene glycol ester as the prosthetic group. The compounds were purified by semipreparative HPLC, and their biodistribution was measured in rats. Additionally, the uptake of the compounds was evaluated in isolated rat cardiomyocytes.

RESULTS

As was the case with the different prosthetic groups, we found that the rhodamine core has a significant effect on the in vitro and in vivo properties of this series of compounds. Of the rhodamines evaluated to date, the pharmacologic properties of the (18)F-labeled diethylene glycol ester of rhodamine 6G are superior to those of the (18)F-labeled diethylene glycol esters of rhodamine B, rhodamine 101, and tetramethylrhodamine. As with (18)F-labeled rhodamine B, [(18)F]rhodamine 6G was observed to localize in the mitochondria of isolated rat cardiomyocytes.

CONCLUSIONS

Based on these results, the (18)F-labeled diethylene glycol ester of rhodamine 6G is the most promising potential PET MPI radiopharmaceutical of those that have evaluated to date, and we are now preparing to carry out first-in-human clinical studies with this compound.

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.

Flow-Dependent Uptake of ¹²³I-CMICE-013, a Novel SPECT Perfusion Agent, Compared with Standard Tracers

Rotenone derivatives have shown promise in myocardial perfusion imaging (MPI). CMICE-013 is a novel (123)I-labeled rotenone derivative developed for SPECT MPI. The objective of this study was to assess the image quality of CMICE-013 and compare its uptake with tetrofosmin, sestamibi, and (201)Tl in vivo in a porcine model of stress-induced myocardial ischemia.

METHODS

Microspheres were injected simultaneously with the radiotracer injections at rest and stress to measure blood flow. Mimicking a 1-d tetrofosmin protocol, stress imaging used 3 times as much activity and occurred 1 h after the rest injection. SPECT images were obtained at both rest and stress. After imaging, the heart was sectioned into 44-50 pieces. In each heart sample, the tracer uptake was measured in a γ counter. The images were aligned, and the decay-corrected ratio of the signals at rest and stress was used to separate the well-counter signal into rest and stress components. The uptake at rest and stress was compared with microsphere flow measurements.

RESULTS

The CMICE-013 images showed good contrast between the heart and surrounding organs, with heart-to-liver and heart-to-lung uptake ratios similar to those of the standard tracers. Uptake of CMICE-013 was 1.5% of the injected dose at rest and increased more rapidly with increased blood flow than did the standard SPECT tracers. The percentage injected dose of CMICE-013 taken up by the heart was greater (P < 0.05) than (201)Tl, tetrofosmin, or sestamibi at flows greater than 1.5 mL/min/g.

CONCLUSION

CMICE-013 is a promising new SPECT MPI agent.

The role of PET quantification in cardiovascular imaging

Positron Emission Tomography (PET) has several clinical and research applications in cardiovascular imaging. Myocardial perfusion imaging with PET allows accurate global and regional measurements of myocardial perfusion, myocardial blood flow and function at stress and rest in one exam. Simultaneous assessment of function and perfusion by PET with quantitative software is currently the routine practice. Combination of ejection fraction reserve with perfusion information may improve the identification of severe disease. The myocardial viability can be estimated by quantitative comparison of fluorodeoxyglucose (¹⁸FDG) and rest perfusion imaging. The myocardial blood flow and coronary flow reserve measurements are becoming routinely included in the clinical assessment due to enhanced dynamic imaging capabilities of the latest PET/CT scanners. Absolute flow measurements allow evaluation of the coronary microvascular dysfunction and provide additional prognostic and diagnostic information for coronary disease. Standard quantitative approaches to compute myocardial blood flow from kinetic PET data in automated and rapid fashion have been developed for ¹³N-ammonia, ¹⁵O-water and ⁸²Rb radiotracers. The agreement between software methods available for such analysis is excellent. Relative quantification of ⁸²Rb PET myocardial perfusion, based on comparisons to normal databases, demonstrates high performance for the detection of obstructive coronary disease. New tracers, such as ¹⁸F-flurpiridaz may allow further improvements in the disease detection. Computerized analysis of perfusion at stress and rest reduces the variability of the assessment as compared to visual analysis. PET quantification can be enhanced by precise coregistration with CT angiography. In emerging clinical applications, the potential to identify vulnerable plaques by quantification of atherosclerotic plaque uptake of ¹⁸FDG and ¹⁸F-sodium fluoride tracers in carotids, aorta and coronary arteries has been demonstrated.

Absolute quantitation of myocardial blood flow in human subjects with or without myocardial ischemia using dynamic flurpiridaz F 18 PET

Absolute quantitation of myocardial blood flow (MBF) by PET is an established method of analyzing coronary artery disease (CAD) but subject to the various shortcomings of available radiotracers. Flurpiridaz F 18 is a novel PET radiotracer that exhibits properties of an ideal tracer.

METHODS

A new absolute perfusion quantitation method with flurpiridaz was developed, taking advantage of the early kinetics and high first-pass extraction by the myocardium of this radiotracer, and the first-in-human measurements of MBF performed in 7 healthy subjects and 8 patients with documented CAD. PET images with time-activity curves were acquired at rest and during adenosine stress.

RESULTS

In healthy subjects, regional MBF between coronary artery territories did not differ significantly, leading to a mean global MBF of 0.73 mL/min/g at rest and 2.53 mL/min/g during stress, with a mean global myocardial flow reserve (MFR) of 3.70. CAD vascular territories with <50% stenosis demonstrated a mean MBF of 0.73 at rest and 2.02 during stress, leading to a mean MFR of 2.97. CAD vascular territories with ≥50% stenosis exhibited a mean MBF of 0.86 at rest and 1.43 during stress, leading to a mean MFR of 1.86. Differences in stress MBF and MFR between normal and CAD territories, as well as between <50% and ≥50% stenosis vascular territories, were significant (P < 0.01).

CONCLUSION

Absolute quantitation of MBF in humans with the novel PET radiotracer flurpiridaz is feasible over a wide range of cardiac flow in the presence or absence of stress-inducible myocardial ischemia. The significant decrease in stress MBF and ensuing MFR in CAD territories allows a clear distinction between vascular territories exhibiting stress-inducible myocardial ischemia and those with normal perfusion.

Rat imaging and in vivo stability studies using [11C]-dimethyl-diphenyl ammonium, a candidate agent for PET-myocardial perfusion imaging

BACKGROUND

PET myocardial perfusion imaging (MPI) holds several advantages over SPECT for diagnosing coronary artery disease. The short half-lives of prevailing PET-MPI agents hamper wider clinical application of PET in nuclear cardiology; prompting the development of novel PET-MPI agents. We have previously reported on the potential of radiolabeled ammonium salts, and particularly on that of [(11)C]dimethyl-diphenyl-ammonium ([(11)C]DMDPA), for cardiac PET imaging. This study was designed to improve the radiosynthesis and increase the yield of [(11)C]DMDPA, characterize more meticulously the kinetics of radioactivity distribution after its injection via micro-PET/CT studies, and further explore its potential for PET-MPI.

METHODS

The radiosynthetic procedure of [(11)C]DMDPA was improved with respect to the previously reported one. The kinetics of radioactivity distribution following injection of [(11)C]DMDPA were investigated in juvenile and young adult male SD rats using microPET/CT, and compared to those of [(13)N]NH3. Furthermore, the metabolic fate of [(11)C]DMDPA in vivo was examined after its injection into rats.

RESULTS

Following a radiosynthesis time of 25-27 min, 11.9 ± 1.1 GBq of [(11)C]DMDPA was obtained, with a 43.7% ± 4.3% radiochemical yield (n = 7). Time activity curves calculated after administration of [(11)C]DMDPA indicated rapid, high and sustained radioactivity uptake in hearts of both juvenile and young adult rats, having a two-fold higher cardiac radioactivity uptake compared to [(13)N]NH3. Accordingly, at all time points after injection to both juvenile and young adult rats, image quality of the left ventricle was higher with [(11)C]DMDPA compared to [(13)N]NH3. In vivo stability studies of [(11)C]DMDPA indicate that no radioactive metabolites could be detected in plasma, liver and urine samples of rats up to 20 min after injection, suggesting that [(11)C]DMDPA is metabolically stable in vivo.

CONCLUSIONS

This study further illustrates that [(11)C]DMDPA holds, at least in part, essential qualities required from a PET-MPI probe. Owing to the improved radiosynthetic procedure reported herein, [(11)C]DMDPA can be produced in sufficient amounts for clinical use.

Myocardial perfusion imaging with PET

PET-myocardial perfusion imaging (MPI) allows accurate measurement of myocardial perfusion, absolute myocardial blood flow and function at stress and rest in a single study session performed in approximately 30 min. Various PET tracers are available for MPI, and rubidium-82 or nitrogen-13-ammonia is most commonly used. In addition, a new fluorine-18-based PET-MPI tracer is currently being evaluated. Relative quantification of PET perfusion images shows very high diagnostic accuracy for detection of obstructive coronary artery disease. Dynamic myocardial blood flow analysis has demonstrated additional prognostic value beyond relative perfusion imaging. Patient radiation dose can be reduced and image quality can be improved with latest advances in PET/CT equipment. Simultaneous assessment of both anatomy and perfusion by hybrid PET/CT can result in improved diagnostic accuracy. Compared with SPECT-MPI, PET-MPI provides higher diagnostic accuracy, using lower radiation doses during a shorter examination time period for the detection of coronary artery disease.

Effect of the prosthetic group on the pharmacologic properties of 18F-labeled rhodamine B, a potential myocardial perfusion agent for positron emission tomography (PET)

We recently reported the development of the 2-[(18)F]fluoroethyl ester of rhodamine B as a potential positron emission tomography (PET) tracer for myocardial perfusion imaging. This compound, which was prepared using a [(18)F]fluoroethyl prosthetic group, has significant uptake in the myocardium in rats but also demonstrates relatively high liver uptake and is rapidly hydrolyzed in vivo in mice. We have now prepared (18)F-labeled rhodamine B using three additional prosthetic groups (propyl, diethylene glycol, and triethylene glycol) and found that the prosthetic group has a significant effect on the in vitro and in vivo properties of these compounds. Of the esters prepared to date, the diethylene glycol ester is superior in terms of in vitro stability and pharmacokinetics. These observations suggest that the prosthetic group plays a significant role in determining the pharmacological properties of (18)F-labeled compounds. They also support the value of continued investigation of (18)F-labeled rhodamines as PET radiopharmaceuticals for myocardial perfusion imaging.

PET: Is myocardial flow quantification a clinical reality?

Positron emission tomography (PET) enables quantitative measurements of myocardial blood flow (MBF) and myocardial flow reserve (MFR). Recent developments and improved availability of PET technology have resulted in growing interest in translation of quantitative flow analysis from mainly a research tool to routine clinical practice. Quantitative PET measurements of absolute MBF and MFR have potential to improve accuracy of myocardial perfusion imaging in diagnosis of multivessel coronary artery disease as well as definition of the extent and functional importance of stenoses. This article reviews recent advances and experience in the quantitative myocardial perfusion imaging together with issues that need to be resolved for quantitative analysis to become clinical reality.

Quantitative cardiac positron emission tomography: the time is coming!

In the last 20 years, the use of positron emission tomography (PET) has grown dramatically because of its oncological applications, and PET facilities are now easily accessible. At the same time, various groups have explored the specific advantages of PET in heart disease and demonstrated the major diagnostic and prognostic role of quantitation in cardiac PET. Nowadays, different approaches for the measurement of myocardial blood flow (MBF) have been developed and implemented in user-friendly programs. There is large evidence that MBF at rest and under stress together with the calculation of coronary flow reserve are able to improve the detection and prognostication of coronary artery disease. Moreover, quantitative PET makes possible to assess the presence of microvascular dysfunction, which is involved in various cardiac diseases, including the early stages of coronary atherosclerosis, hypertrophic and dilated cardiomyopathy, and hypertensive heart disease. Therefore, it is probably time to consider the routine use of quantitative cardiac PET and to work for defining its place in the clinical scenario of modern cardiology.