Assessment of attenuation correction for myocardial PET imaging using combined PET/MRI

What Role Does PET/MRI Play in Musculoskeletal Disorders?

Musculoskeletal disorders of nononcological origin are one of the most frequent reasons for consultation. Patients suffering from musculoskeletal disorders also consult more than once for the same reason. This results in multiple clinical follow-ups after several radiological and serum examinations, the main ones including X-rays targeting the painful anatomical region and inflammatory serum parameters. As part of their work up, patients suffering from musculoskeletal disorders often require multisequence, multi-parameter MRI. PET/MRI is a promising imaging modality for their diagnosis, with the added advantage of being able to be performed in a single visit. PET/MRI is particularly useful for diagnosing osteomyelitis, spondylodiscitis, arthritis, many pediatric pathologies, and a wide range of other musculoskeletal pathologies. PET/MRI is already used to diagnose malignant bone tumors such as osteosarcoma. However, current knowledge of the indications for PET/MRI in nononcological musculoskeletal disorders is based on studies involving only a few patients. This review focuses on the usefulness of PET/MRI for diagnosing nononcological musculoskeletal disorders.

Deep learning applications for quantitative and qualitative PET in PET/MR: technical and clinical unmet needs

We aim to provide an overview of technical and clinical unmet needs in deep learning (DL) applications for quantitative and qualitative PET in PET/MR, with a focus on attenuation correction, image enhancement, motion correction, kinetic modeling, and simulated data generation. (1) DL-based attenuation correction (DLAC) remains an area of limited exploration for pediatric whole-body PET/MR and lung-specific DLAC due to data shortages and technical limitations. (2) DL-based image enhancement approximating MR-guided regularized reconstruction with a high-resolution MR prior has shown promise in enhancing PET image quality. However, its clinical value has not been thoroughly evaluated across various radiotracers, and applications outside the head may pose challenges due to motion artifacts. (3) Robust training for DL-based motion correction requires pairs of motion-corrupted and motion-corrected PET/MR data. However, these pairs are rare. (4) DL-based approaches can address the limitations of dynamic PET, such as long scan durations that may cause patient discomfort and motion, providing new research opportunities. (5) Monte-Carlo simulations using anthropomorphic digital phantoms can provide extensive datasets to address the shortage of clinical data. This summary of technical/clinical challenges and potential solutions may provide research opportunities for the research community towards the clinical translation of DL solutions.

Reproducibility of Left Ventricular Function Derived From Cardiac Magnetic Resonance and Gated 13N-Ammonia Positron Emission Tomography Myocardial Perfusion Imaging: A Head-to-Head Comparison Using Hybrid Positron Emission Tomography/Magnetic Resonance

RATIONALE AND OBJECTIVES

Cardiac magnetic resonance (CMR) and gated 13N-ammonia positron emission tomography myocardial perfusion imaging (PET-MPI) offer accurate and highly comparable global left ventricular ejection fraction (LVEF) measurements. In addition to accuracy, however, reproducibility is crucial to avoid variations in LVEF assessment potentially negatively impacting treatment decisions. We performed a head-to-head comparison of the reproducibility of LVEF measurements derived from simultaneously acquired CMR and PET-MPI using different state-of-the-art commercially available software.

MATERIALS AND METHODS

93 patients undergoing hybrid PET/MR were retrospectively included. LVEF was derived from CMR and PET-MPI at two separate core labs, using two state-of-the-art software packages for CMR (cvi42 and Medis Suite MR) and PET (QPET and CardIQ Physio). Intra- and inter-reader agreement was assessed using correlation and Bland-Altman (BA) analyses.

RESULTS

While intra- and inter-reader reproducibility of LVEF was high among both modalities and all software packages (r ≥ 0.87 and ICC≥0.91, all significant at p < 0.0001), LVEF derived from PET-MPI and analyzed with QPET outperformed all other analyses (intra-reader reproducibility: r = 0.99, ICC=0.99; inter-reader reproducibility: r = 0.98, ICC=1.00; Pearson correlations significantly higher than all others at p ≤ 0.0001). BA analyses showed smaller biases for LVEF derived from PET-MPI (-0.1% and +0.9% for intra-reader, -0.4% and -0.8% for inter-reader agreement) than those derived from CMR (+0.7% and +2.8% for intra-reader, -0.9% and -2.2% for inter-reader agreement) with similar results for BA limits of agreement.

CONCLUSION

Gated 13N-ammonia PET-MPI provides equivalent reproducibility of LVEF compared to CMR. It may offer a valid alternative to CMR for patients requiring LV functional assessment.

Supplemental Transmission Aided Attenuation Correction for Quantitative Cardiac PET

Quantitative PET attenuation correction (AC) for cardiac PET/CT and PET/MR is a challenging problem. We propose and evaluate an AC approach that uses coincidences from a relatively weak and physically fixed sparse external source, in combination with that from the patient, to reconstruct μ -maps based on physics principles alone. The low 30 cm3 volume of the source makes it easy to fill and place, and the method does not use prior image data or attenuation map assumptions. Our supplemental transmission aided maximum likelihood reconstruction of attenuation and activity (sTX-MLAA) algorithm contains an attenuation map update that maximizes the likelihood of terms representing coincidences originating from tracer in the patient and a weighted expression of counts segmented from the external source alone. Both external source and patient scatter and randoms are fully corrected. We evaluated performance of sTX-MLAA compared to reference standard CT-based AC with FDG PET/CT phantom studies; including modeling a patient with myocardial inflammation. Through an ROI analysis we measured ≤ 5 % bias in activity concentrations for PET images generated with sTX-MLAA and a TX source strength ≥ 12.7 MBq, relative to CT-AC. PET background variability (from noise and sparse sampling) was substantially reduced with sTX-MLAA compared to using counts segmented from the transmission source alone for AC. Results suggest that sTX-MLAA will enable quantitative PET during cardiac PET/CT and PET/MR of human patients.

Recent advances in combined Positron Emission Tomography and Magnetic Resonance Imaging

Hybrid imaging modalities combine two or more medical imaging techniques offering exciting new possibilities to image the structure, function and biochemistry of the human body in far greater detail than has previously been possible to improve patient diagnosis. In this context, simultaneous Positron Emission Tomography and Magnetic Resonance (PET/MR) imaging offers great complementary information, but it also poses challenges from the point of view of hardware and software compatibility. The PET signal may interfere with the MR magnetic field and vice-versa, posing several challenges and constrains in the PET instrumentation for PET/MR systems. Additionally, anatomical maps are needed to properly apply attenuation and scatter corrections to the resulting reconstructed PET images, as well motion estimates to minimize the effects of movement throughout the acquisition. In this review, we summarize the instrumentation implemented in modern PET scanners to overcome these limitations, describing the historical development of hybrid PET/MR scanners. We pay special attention to the methods used in PET to achieve attenuation, scatter and motion correction when it is combined with MR, and how both imaging modalities may be combined in PET image reconstruction algorithms.

Recent Advances in Positron Emission Tomography/Magnetic Resonance Imaging Technology

More than a decade has passed since the clinical deployment of the first commercial whole-body hybrid PET/MR scanner in the clinic. The major advantages and limitations of this technology have been investigated from technical and medical perspectives. Despite the remarkable advantages associated with hybrid PET/MR imaging, such as reduced radiation dose and fully simultaneous functional and structural imaging, this technology faced major challenges in terms of mutual interference between MRI and PET components, in addition to the complexity of achieving quantitative imaging owing to the intricate MRI-guided attenuation correction in PET/MRI. In this review, the latest technical developments in PET/MRI technology as well as the state-of-the-art solutions to the major challenges of quantitative PET/MR imaging are discussed.

Hybrid PET/MR in Cardiac Imaging

In the last few years, technological advances in MR imaging, PET detectors, and attenuation correction algorithms have allowed the creation of truly integrated PET/MR imaging systems, for both clinical and research applications. These machines allow a comprehensive investigation of cardiovascular diseases, by offering a wide variety of detailed anatomical and functional data in combination. Despite significant pathophysiologic mechanisms being clarified by this new data, its clinical relevance and prognostic significance have not been demonstrated yet.

International EANM-SNMMI-ISMRM consensus recommendation for PET/MRI in oncology

PREAMBLE

The Society of Nuclear Medicine and Molecular Imaging (SNMMI) is an international scientific and professional organization founded in 1954 to promote the science, technology, and practical application of nuclear medicine. The European Association of Nuclear Medicine (EANM) is a professional non-profit medical association that facilitates communication worldwide between individuals pursuing clinical and research excellence in nuclear medicine. The EANM was founded in 1985. The merged International Society for Magnetic Resonance in Medicine (ISMRM) is an international, nonprofit, scientific association whose purpose is to promote communication, research, development, and applications in the field of magnetic resonance in medicine and biology and other related topics and to develop and provide channels and facilities for continuing education in the field.The ISMRM was founded in 1994 through the merger of the Society of Magnetic Resonance in Medicine and the Society of Magnetic Resonance Imaging. SNMMI, ISMRM, and EANM members are physicians, technologists, and scientists specializing in the research and practice of nuclear medicine and/or magnetic resonance imaging. The SNMMI, ISMRM, and EANM will periodically define new guidelines for nuclear medicine practice to help advance the science of nuclear medicine and/or magnetic resonance imaging and to improve the quality of service to patients throughout the world. Existing practice guidelines will be reviewed for revision or renewal, as appropriate, on their fifth anniversary or sooner, if indicated. Each practice guideline, representing a policy statement by the SNMMI/EANM/ISMRM, has undergone a thorough consensus process in which it has been subjected to extensive review. The SNMMI, ISMRM, and EANM recognize that the safe and effective use of diagnostic nuclear medicine imaging and magnetic resonance imaging requires specific training, skills, and techniques, as described in each document. Reproduction or modification of the published practice guideline by those entities not providing these services is not authorized. These guidelines are an educational tool designed to assist practitioners in providing appropriate care for patients. They are not inflexible rules or requirements of practice and are not intended, nor should they be used, to establish a legal standard of care. For these reasons and those set forth below, the SNMMI, the ISMRM, and the EANM caution against the use of these guidelines in litigation in which the clinical decisions of a practitioner are called into question. The ultimate judgment regarding the propriety of any specific procedure or course of action must be made by the physician or medical physicist in light of all the circumstances presented. Thus, there is no implication that an approach differing from the guidelines, standing alone, is below the standard of care. To the contrary, a conscientious practitioner may responsibly adopt a course of action different from that set forth in the guidelines when, in the reasonable judgment of the practitioner, such course of action is indicated by the condition of the patient, limitations of available resources, or advances in knowledge or technology subsequent to publication of the guidelines. The practice of medicine includes both the art and the science of the prevention, diagnosis, alleviation, and treatment of disease. The variety and complexity of human conditions make it impossible to always reach the most appropriate diagnosis or to predict with certainty a particular response to treatment. Therefore, it should be recognized that adherence to these guidelines will not ensure an accurate diagnosis or a successful outcome. All that should be expected is that the practitioner will follow a reasonable course of action based on current knowledge, available resources, and the needs of the patient to deliver effective and safe medical care. The sole purpose of these guidelines is to assist practitioners in achieving this objective.

A review of PET attenuation correction methods for PET-MR

Despite being thirteen years since the installation of the first PET-MR system, the scanners constitute a very small proportion of the total hybrid PET systems installed. This is in stark contrast to the rapid expansion of the PET-CT scanner, which quickly established its importance in patient diagnosis within a similar timeframe. One of the main hurdles is the development of an accurate, reproducible and easy-to-use method for attenuation correction. Quantitative discrepancies in PET images between the manufacturer-provided MR methods and the more established CT- or transmission-based attenuation correction methods have led the scientific community in a continuous effort to develop a robust and accurate alternative. These can be divided into four broad categories: (i) MR-based, (ii) emission-based, (iii) atlas-based and the (iv) machine learning-based attenuation correction, which is rapidly gaining momentum. The first is based on segmenting the MR images in various tissues and allocating a predefined attenuation coefficient for each tissue. Emission-based attenuation correction methods aim in utilising the PET emission data by simultaneously reconstructing the radioactivity distribution and the attenuation image. Atlas-based attenuation correction methods aim to predict a CT or transmission image given an MR image of a new patient, by using databases containing CT or transmission images from the general population. Finally, in machine learning methods, a model that could predict the required image given the acquired MR or non-attenuation-corrected PET image is developed by exploiting the underlying features of the images. Deep learning methods are the dominant approach in this category. Compared to the more traditional machine learning, which uses structured data for building a model, deep learning makes direct use of the acquired images to identify underlying features. This up-to-date review goes through the literature of attenuation correction approaches in PET-MR after categorising them. The various approaches in each category are described and discussed. After exploring each category separately, a general overview is given of the current status and potential future approaches along with a comparison of the four outlined categories.

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.

PET-MRI of Coronary Artery Disease

Simultaneous positron emission tomography and magnetic resonance imaging (PET-MRI) combines the anatomical detail and tissue characterization of MRI with the functional information from PET. Within the coronary arteries, this hybrid technique can be used to identify biological activity combined with anatomically high-risk plaque features to better understand the processes underlying coronary atherosclerosis. Furthermore, the downstream effects of coronary artery disease on the myocardium can be characterized by providing information on myocardial perfusion, viability, and function. This review will describe the current capabilities of PET-MRI in coronary artery disease and discuss the limitations and future directions of this emerging technique. LEVEL OF EVIDENCE: 5 TECHNICAL EFFICACY: Stage 3.

Left ventricular function and volumes from gated [13N]-ammonia positron emission tomography myocardial perfusion imaging: A prospective head-to-head comparison against CMR using a hybrid PET/MR device

BACKGROUND

Positron emission tomography (PET) myocardial perfusion imaging (MPI) can be used to evaluate left ventricular (LV) volumes and function. We performed a head-to-head comparison of LV function and volumes obtained simultaneously using [13N]-ammonia-PET and cardiac magnetic resonance (CMR), with the latter serving as the reference standard.

METHODS AND RESULTS

In this prospective study, 51 patients underwent [13N]-ammonia-PET MPI and CMR using a hybrid PET/MR device. Left ventricular end-systolic volumes (LVESV), end-diastolic volumes (LVEDV), stroke volumes (LVSV), ejection fractions (LVEF), and segmental wall motion were analyzed for both methods and were compared using correlational and Bland-Altman (BA) analysis; segmental wall motion was compared using ANOVA. The agreement between [13N]-ammonia-PET and CMR for LVEF was good, with minimal bias (- .6%) and narrow BA limits of agreement (- 7.9% to 6.8%), but [13N]-ammonia-PET systematically underestimated LV volumes, with high bias in LVESV (- 11.2 ml), LVEDV (- 28.9 ml), and LVSV (- 17.5 ml). Mean segmental wall motion in [13N]-ammonia-PET differed significantly among the corresponding normokinetic (6.6 ± 2 mm), hypokinetic (5.1 ± 2 mm), and akinetic (3.3 ± 2 mm) segments in CMR (P < .01).

CONCLUSION

LVEF and LV wall motion can be accurately assessed using [13N]-ammonia-PET MPI, although LV volumes are significantly underestimated compared to CMR.

DeTransUnet: attenuation correction of gated cardiac images without structural information

Objective. Myocardial perfusion imaging (MPI) with positron emission tomography (PET) is a non-invasive imaging method, and it is of great significance to the diagnosis and prognosis of coronary heart disease. Attenuation correction (AC) for PET images is a necessary step for further quantitative analysis. In order not to use magnetic resonance (MR) or computed tomography (CT) images for AC, this work proposes DeTransUnet to obtain AC PET images directly from no-attenuation corrected (NAC) PET images. Approach. The proposed DeTransUnet is a 3D structure which combines the multi-scale deformable transformer layers and the 3D convolutional neural network (CNN). And it integrates the advantages of transformer with long-range dependence and CNN suitable for image calculation. The AC images using CT images for AC and scatter correction (SC) and are considered as training labels, while the NAC images are reconstructed without AC and SC. Standard uptake value (SUV) values are calculated for both NAC and AC images to exclude the influence of weight and injection dose. With NAC SUV images as the inputs of the DeTransUnet, the outputs of DeTransUnet are AC SUV images. Main results. The proposed DeTransUnet was performed on an MPI gated-PET dataset, and the results were compared with Unet2D and Unet2.5D. The metrics of the whole image and the left ventricular myocardium show that the proposed method has advantages over other deep learning methods. Significance. The proposed DeTransUnet is a novel AC framework that does not require CT or MR images. It can be used as an independent AC method on PET/MR instrument. In addition, when CT images contain defects or cannot be registered with PET images on PET/CT instrument, DeTransUnet is able to repair the defects and keep consistent with the NAC images.

A two-stage cardiac PET and late gadolinium enhancement MRI co-registration method for improved assessment of non-ischemic cardiomyopathies using integrated PET/MR

PURPOSE

Respiratory motion causes mismatches between PET images of the myocardium and the corresponding cardiac MR images in cardiac integrated PET/MR. The mismatch may affect the attenuation correction and the diagnosis of non-ischemic cardiomyopathies. In this study, we present a two-stage cardiac PET and MR late gadolinium enhancement (LGE) co-registration method, which seeks to improve diagnostic accuracy of non-ischemic cardiomyopathies via better image co-registration using an integrated whole-body PET/MR system.

METHODS

The proposed PET and LGE two-stage co-registration method was evaluated through comparison with one-stage direct co-registration and no-registration. One hundred and ninety-one slices of LGE and forty lesions were studied. Two trained nuclear medicine physicians independently assessed the displacement between LGE and PET to qualitatively evaluate the co-registration quality. The changes of the mean SUV in the normal myocardium and the LGE-enhanced lesions before and after image co-registration were measured to quantitatively evaluate the accuracy and value of image co-registration.

RESULTS

The two-stage method had an improved image registration score (4.93 ± 0.89) compared with the no-registration method (3.49 ± 0.84, p value < 0.001) and the single-stage method (4.23 ± 0.81, p value < 0.001). Furthermore, the two-stage method led to increased SUV value in the myocardium (3.87 ± 2.56) compared with the no-registration method (3.14 ± 1.92, p value < 0.001) and the single-stage method (3.32 ± 2.16, p value < 0.001). The mean SUV in the LGE lesion significantly increased from 2.51 ± 2.09 to 2.85 ± 2.35 (p value < 0.001) after the two-stage co-registration.

CONCLUSION

The proposed two-stage registration method significantly improved the co-registration between PET and LGE in integrated PET/MR imaging. The technique may improve diagnostic accuracy of non-ischemic cardiomyopathies via better image co-registration.

REGISTERED NO

DF-2020-085,2020.04.30.

Splenic switch-off as a novel marker for adenosine response in nitrogen-13 ammonia PET myocardial perfusion imaging: Cross-validation against CMR using a hybrid PET/MR device

BACKGROUND

No methodology is available to distinguish truly reduced myocardial flow reserve (MFR) in positron emission tomography myocardial perfusion imaging (PET MPI) from seemingly impaired MFR due to inadequate adenosine response. The adenosine-induced splenic switch-off (SSO) sign has been proposed as a potential marker for adequate adenosine response in cardiac magnetic resonance (CMR). We assessed the feasibility of detecting SSO in nitrogen-13 ammonia PET MPI using SSO in CMR as the standard of reference.

METHODS AND RESULTS

Fifty patients underwent simultaneous CMR and PET MPI on a hybrid PET/MR device with co-injection of a gadolinium-based contrast agent and nitrogen-13 ammonia during rest and adenosine-induced stress. In CMR, SSO was assessed visually (positive vs negative SSO) and quantitatively by calculating the ratio of the peak signal intensity of the spleen during stress over rest (SIR). In PET MPI, the splenic signal activity ratio (SAR) was calculated as the maximal standard uptake value of the spleen during stress over rest. The median SIR was significantly lower in patients with positive versus negative SSO in CMR (0.57 [IQR 0.49 to 0.62] vs 0.89 [IQR 0.76 to 0.98]; P < .001). Similarly, median SAR in PET MPI was significantly lower in patients with positive versus negative SSO (0.40 [IQR 0.32 to 0.45] vs 0.80 [IQR 0.47 to 0.98]; P < .001).

CONCLUSION

Similarly to CMR, SSO can be detected in nitrogen-13 ammonia PET MPI. This might help distinguish adenosine non-responders from patients with truly impaired MFR due to microvascular dysfunction or multivessel coronary artery disease.

PET/MR Technology: Advancement and Challenges

When this article was written, it coincided with the 11th anniversary of the installation of our PET/MR device in Munich. In fact, this was the first fully integrated device to be in clinical use. During this time, we have observed many interesting behaviors, to put it kindly. However, it is more critical that in this process, our understanding of the system also improved - including the advantages and limitations from a technical, logistical, and medical perspective. The last decade of PET/MRI research has certainly been characterized by most sites looking for a "key application." There were many ideas in this context and before and after the devices became available, some of which were based on the earlier work with integrating data from single devices. These involved validating classical PET methods with MRI (eg, perfusion or oncology diagnostics). More important, however, were the scenarios where intermodal synergies could be expected. In this review, we look back on this decade-long journey, at the challenges overcome and those still to come.

Assessing the qualitative and quantitative impacts of simple two-class vs multiple tissue-class MR-based attenuation correction for cardiac PET/MR

BACKGROUND

Hybrid PET/MR imaging has significant potential in cardiology due to its combination of molecular PET imaging and cardiac MR. Multi-tissue-class MR-based attenuation correction (MRAC) is necessary for accurate PET quantification. Moreover, for thoracic PET imaging, respiration is known to lead to misalignments of MRAC and PET data that result in PET artifacts. These factors can be addressed by using multi-echo MR for tissue segmentation and motion-robust or motion-gated acquisitions. However, the combination of these strategies is not routinely available and can be prone to errors. In this study, we examine the qualitative and quantitative impacts of multi-class MRAC compared to a more widely available simple two-class MRAC for cardiac PET/MR.

METHODS AND RESULTS

In a cohort of patients with cardiac sarcoidosis, we acquired MRAC data using multi-echo radial gradient-echo MR imaging. Water-fat separation was used to produce attenuation maps with up to 4 tissue classes including water-based soft tissue, fat, lung, and background air. Simultaneously acquired 18F-fluorodeoxyglucose PET data were subsequently reconstructed using each attenuation map separately. PET uptake values were measured in the myocardium and compared between different PET images. The inclusion of lung and subcutaneous fat in the MRAC maps significantly affected the quantification of 18F-fluorodeoxyglucose activity in the myocardium but only moderately altered the appearance of the PET image without introduction of image artifacts.

CONCLUSION

Optimal MRAC for cardiac PET/MR applications should include segmentation of all tissues in combination with compensation for the respiratory-related motion of the heart. Simple two-class MRAC is adequate for qualitative clinical assessment.

Review of cardiovascular imaging in the Journal of Nuclear Cardiology 2020: positron emission tomography, computed tomography, and magnetic resonance

Although the year 2020 was different from other years in many respects, the Journal of Nuclear Cardiology published excellent articles pertaining to imaging in patients with cardiovascular disease due to the dedication of the investigators in our field all over the world. In this review, we will summarize a selection of these articles to provide a concise review of the main advancements that have recently occurred in the field and provide the reader with an opportunity to review a wide selection of articles. We will focus on publications dealing with positron emission tomography, computed tomography, and magnetic resonance and hope that you will find this review helpful.

Cardiovascular 18F-fluoride positron emission tomography-magnetic resonance imaging: A comparison study

BACKGROUND

18F-Fluoride uptake denotes calcification activity in aortic stenosis and atherosclerosis. While PET/MR has several advantages over PET/CT, attenuation correction of PET/MR data is challenging, limiting cardiovascular application. We compared PET/MR and PET/CT assessments of 18F-fluoride uptake in the aortic valve and coronary arteries.

METHODS AND RESULTS

18 patients with aortic stenosis or recent myocardial infarction underwent 18F-fluoride PET/CT followed immediately by PET/MR. Valve and coronary 18F-fluoride uptake were evaluated independently. Both standard (Dixon) and novel radial GRE) MR attenuation correction (AC) maps were validated against PET/CT with results expressed as tissue-to-background ratios (TBRs). Visually, aortic valve 18F-fluoride uptake was similar on PET/CT and PET/MR. TBRMAX values were comparable with radial GRE AC (PET/CT 1.55±0.33 vs. PET/MR 1.58 ± 0.34, P = 0.66; 95% limits of agreement - 27% to + 25%) but performed less well with Dixon AC (1.38 ± 0.44, P = 0.06; bias (-)14%; 95% limits of agreement - 25% to + 53%). In native coronaries, 18F-fluoride uptake was similar on PET/MR to PET/CT regardless of AC approach. PET/MR identified 28/29 plaques identified on PET/CT; however, stents caused artifact on PET/MR making assessment of 18F-fluoride uptake challenging.

CONCLUSION

Cardiovascular PET/MR demonstrates good visual and quantitative agreement with PET/CT. However, PET/MR is hampered by stent-related artifacts currently limiting clinical application.

Splenic switch-off as a predictor for coronary adenosine response: validation against 13N-ammonia during co-injection myocardial perfusion imaging on a hybrid PET/CMR scanner

BACKGROUND

Inadequate coronary adenosine response is a potential cause for false negative ischemia testing. Recently, the splenic switch-off (SSO) sign has been identified as a promising tool to ascertain the efficacy of adenosine during vasodilator stress cardiovascular magnetic resonance imaging (CMR). We assessed the value of SSO to predict adenosine response, defined as an increase in myocardial blood flow (MBF) during quantitative stress myocardial perfusion 13 N-ammonia positron emission tomography (PET).

METHODS

We prospectively enrolled 64 patients who underwent simultaneous CMR and PET myocardial perfusion imaging on a hybrid PET/CMR scanner with co-injection of gadolinium based contrast agent (GBCA) and 13N-ammonia during rest and adenosine-induced stress. A myocardial flow reserve (MFR) of  > 1.5 or ischemia as assessed by PET were defined as markers for adequate coronary adenosine response. The presence or absence of SSO was visually assessed. The stress-to-rest intensity ratio (SIR) was calculated as the ratio of stress over rest peak signal intensity for splenic tissue. Additionally, the spleen-to-myocardium ratio, defined as the relative change of spleen to myocardial signal, was calculated for stress (SMRstress) and rest.

RESULTS

Sixty-one (95%) patients were coronary adenosine responders, but SSO was absent in 18 (28%) patients. SIR and SMRstress were significantly lower in patients with SSO (SIR: 0.56 ± 0.13 vs. 0.93 ± 0.23; p < 0.001 and SMRstress: 1.09 ± 0.47 vs. 1.68 ± 0.62; p < 0.001). Mean hyperemic and rest MBF were 2.12 ± 0.68 ml/min/g and 0.78 ± 0.26 ml/min/g, respectively. MFR was significantly higher in patients with vs. patients without presence of SSO (3.07 ± 1.03 vs. 2.48 ± 0.96; p = 0.038), but there was only a weak inverse correlation between SMRstress and MFR (R = -0.378; p = 0.02) as well as between SIR and MFR (R = -0.356; p = 0.004).

CONCLUSIONS

The presence of SSO implies adequate coronary adenosine-induced MBF response. Its absence, however, is not a reliable indicator for failed adenosine-induced coronary vasodilatation.

Attenuation correction for human PET/MRI studies

Attenuation correction has been one of the main methodological challenges in the integrated positron emission tomography and magnetic resonance imaging (PET/MRI) field. As standard transmission or computed tomography approaches are not available in integrated PET/MRI scanners, MR-based attenuation correction approaches had to be developed. Aspects that have to be considered for implementing accurate methods include the need to account for attenuation in bone tissue, normal and pathological lung and the MR hardware present in the PET field-of-view, to reduce the impact of subject motion, to minimize truncation and susceptibility artifacts, and to address issues related to the data acquisition and processing both on the PET and MRI sides. The standard MR-based attenuation correction techniques implemented by the PET/MRI equipment manufacturers and their impact on clinical and research PET data interpretation and quantification are first discussed. Next, the more advanced methods, including the latest generation deep learning-based approaches that have been proposed for further minimizing the attenuation correction related bias are described. Finally, a future perspective focused on the needed developments in the field is given.

Procedural recommendations of cardiac PET/CT imaging: standardization in inflammatory-, infective-, infiltrative-, and innervation (4Is)-related cardiovascular diseases: a joint collaboration of the EACVI and the EANM

With this document, we provide a standard for PET/(diagnostic) CT imaging procedures in cardiovascular diseases that are inflammatory, infective, infiltrative, or associated with dysfunctional innervation (4Is). This standard should be applied in clinical practice and integrated in clinical (multicenter) trials for optimal procedural standardization. A major focus is put on procedures using [¹⁸F]FDG, but 4Is PET radiopharmaceuticals beyond [¹⁸F]FDG are also described in this document. Whilst these novel tracers are currently mainly applied in early clinical trials, some multicenter trials are underway and we foresee in the near future their use in clinical care and inclusion in the clinical guidelines. Finally, PET/MR applications in 4Is cardiovascular diseases are also briefly described. Diagnosis and management of 4Is-related cardiovascular diseases are generally complex and often require a multidisciplinary approach by a team of experts. The new standards described herein should be applied when using PET/CT and PET/MR, within a multimodality imaging framework both in clinical practice and in clinical trials for 4Is cardiovascular indications.

[A review on cardiac positron emission tomography/magnetic resonance imaging in diagnosis of cardivascular diseases]

There are various examination methods for cardiovascular diseases. Non-invasive diagnosis and prognostic information acquisition are the current research hotspots of related imaging examinations. Positron emission tomography (PET)/magnetic resonance imaging (MRI) is a new advanced fusion imaging technology that combines the molecular imaging of PET with the soft tissue contrast function of MRI to achieve their complementary advantages. This article briefly introduces several major aspects of cardiac PET/MRI in the diagnosis of cardiovascular disease, including atherosclerosis, ischemic cardiomyopathy, nodular heart disease, and myocardial amyloidosis, in order to promote cardiac PET/MRI to be more widely used in precision medicine in this field.

Myocardial creep-induced misalignment artifacts in PET/MR myocardial perfusion imaging

Purpose

Misalignment between positron emission tomography (PET) datasets and attenuation correction (AC) maps is a potential source of artifacts in myocardial perfusion imaging (MPI). We assessed the impact of adenosine on the alignment of AC maps derived from magnetic resonance (MR) and PET datasets during MPI on a hybrid PET/MR scanner.

Methods

Twenty-eight volunteers underwent adenosine stress and rest 13N-ammonia MPI on a PET/MR. We acquired Dixon sequences for the creation of MRAC maps. After reconstruction of the original non-shifted PET images, we examined MRAC and PET datasets for cardiac spatial misalignment and, if necessary, reconstructed a second set of shifted PET images after manually adjusting co-registration. Summed rest, stress, and difference scores (SRS, SSS, and SDS) were compared between shifted and non-shifted PET images. Additionally, we measured the amount of cranial movement of the heart (i.e., myocardial creep) after termination of adenosine infusion.

Results

Realignment was necessary for 25 (89.3%) stress and 12 (42.9%) rest PET datasets. Median SRS, SSS, and SDS of the non-shifted images were 6 (IQR = 4–7), 12 (IQR = 7–18), and 8 (IQR = 2–11), respectively, and of the shifted images 2 (IQR = 1–6), 4 (IQR = 7–18), and 1 (IQR = 0–2), respectively. All three scores were significantly higher in non-shifted versus shifted images (all p < 0.05). The difference in SDS correlated moderately but significantly with the amount of myocardial creep (r = 0.541, p = 0.005).

Conclusion

Misalignment of MRAC and PET datasets commonly occurs during adenosine stress MPI on a hybrid PET/MR device, potentially leading to an increase in false-positive findings. Our results suggest that myocardial creep may substantially account for this and prompt for a careful review and correction of PET/MRAC data.

Review of cardiovascular imaging in the Journal of Nuclear Cardiology 2019: Positron emission tomography, computed tomography and magnetic resonance

In 2019, the Journal of Nuclear Cardiology published excellent articles pertaining to imaging in patients with cardiovascular disease. In this review we will summarize a selection of these articles to provide a concise review of the main advancements that have recently occurred in the field and provide the reader with an opportunity to review a wide selection of articles. In this first article of this 2-part series we will focus on publications dealing with positron emission tomography, computed tomography and magnetic resonance. We will specifically discuss imaging as it relates to coronary artery disease, atherosclerosis and inflammation, coronary artery calcification, cardiomyopathies, cardiac implantable electronic devices, prosthetic valves, and left ventricular assist devices. The second part of this review will place emphasis on myocardial perfusion imaging using single-photon emission computed tomography.

Importance of attenuation correction in PET/MR image quantification: Methods and applications

The generation of accurate attenuation correction (AC) maps is a basic step to allow for quantitative PET/MR imaging. However, generating MR-based AC maps is a challenge because there is no direct relationship between the PET attenuation coefficients (μ) and the intensity of the MR signal, contrary to what happens with the intensity of CT images. In fact, ignoring the bone causes a distorted and biased distribution of the calculated SUV values. To solve this problem, several MR-based AC methods have been proposed in the literature. In this paper we describe how these methods work, and the challenge they faced to translate into full body applications. Currently, in research environments, the accuracy of AC methods is no longer a limiting factor to solve in order to carry out quantitative in vivo molecular imaging studies. However, many of these methods present a series of limitations for their real implementation in the clinical practice due to insufficient clinical validation and the difficulty of their implementation in a real environment (as described in the examples of clinical applications). Thus, we need the PET/MR community to work on the standardization of the use and assessment of different AC methods. In this scenario, the opening and access by vendors to the implementation of new AC methods in their PET/MR scanners plays a crucial role.

Cardiac PET/MRI: Current Clinical Status and Future Perspectives

Combined PET/MRI has now been in clinical routine for almost 10 years. Since then, it has not only had to face validation, comparison and research questions, it has also been increasingly used in clinical routine. A number of cardiovascular applications have become established here, whereby viability imaging and assessment of inflammatory and infiltrative processes in the heart are to be emphasized. However, further interesting applications are expected in the near future. This review summarizes the most important clinical applications on the one hand and mentions interesting areas of application in research on the other.

Quantitative PET/MR imaging of lung cancer in the presence of artifacts in the MR-based attenuation correction maps

Background

Positron emission tomography (PET)/magnetic resonance (MR) imaging may become increasingly important for assessing tumor therapy response. A prerequisite for quantitative PET/MR imaging is reliable and repeatable MR-based attenuation correction (AC).

Purpose

To investigate the frequency and test–retest reproducibility of artifacts in MR-AC maps in a lung cancer patient cohort and to study the impact of artifact corrections on PET-based tumor quantification.

Material and Methods

Twenty-five lung cancer patients underwent single-day, test–retest, 18F-fluorodeoxyglucose (FDG) PET/MR imaging. The acquired MR-AC maps were inspected for truncation, susceptibility, and tissue inversion artifacts. An anatomy-based bone template and a PET-based estimation of truncated arms were employed, while susceptibility artifacts were corrected manually. We report the frequencies of artifacts and the relative difference (RD) on standardized uptake value (SUV) based quantification in PET images reconstructed with the corrected AC maps.

Results

Truncation artifacts were found in all 50 acquisitions (100%), while susceptibility and tissue inversion artifacts were observed in six (12%) and 26 (52%) of the scans, respectively. The RD in lung tumor SUV was < 5% from bone and truncation corrections, while up to 20% RD was introduced after susceptibility artifact correction, with large inconsistencies between test–retest scans.

Conclusion

The absence of bone and truncation artifacts have limited effect on the PET quantification of lung lesions. In contrast, susceptibility artifacts caused significant and inconsistent underestimations of the lung tumor SUVs, between test–retest scans. This may have clinical implications for patients undergoing serial imaging for tumor therapy response assessment.

Simultaneous 13N-Ammonia and gadolinium first-pass myocardial perfusion with quantitative hybrid PET-MR imaging: a phantom and clinical feasibility study

BACKGROUND

Positron emission tomography (PET) is the non-invasive reference standard for myocardial blood flow (MBF) quantification. Hybrid PET-MR allows simultaneous PET and cardiac magnetic resonance (CMR) acquisition under identical experimental and physiological conditions. This study aimed to determine feasibility of simultaneous 13N-Ammonia PET and dynamic contrast-enhanced CMR MBF quantification in phantoms and healthy volunteers.

METHODS

Images were acquired using a 3T hybrid PET-MR scanner. Phantom study: MBF was simulated at different physiological perfusion rates and a protocol for simultaneous PET-MR perfusion imaging was developed. Volunteer study: five healthy volunteers underwent adenosine stress. 13N-Ammonia and gadolinium were administered simultaneously. PET list mode data was reconstructed using ordered subset expectation maximisation. CMR MBF was quantified using Fermi function-constrained deconvolution of arterial input function and myocardial signal. PET MBF was obtained using a one-tissue compartment model and image-derived input function.

RESULTS

Phantom study: PET and CMR MBF measurements demonstrated high repeatability with intraclass coefficients 0.98 and 0.99, respectively. There was high correlation between PET and CMR MBF (r = 0.98, p < 0.001) and good agreement (bias - 0.85 mL/g/min; 95% limits of agreement 0.29 to - 1.98). Volunteer study: Mean global stress MBF for CMR and PET were 2.58 ± 0.11 and 2.60 ± 0.47 mL/g/min respectively. On a per territory basis, there was moderate correlation (r = 0.63, p = 0.03) and agreement (bias - 0.34 mL/g/min; 95% limits of agreement 0.49 to - 1.18).

CONCLUSION

Simultaneous MBF quantification using hybrid PET-MR imaging is feasible with high test repeatability and good to moderate agreement between PET and CMR. Future studies in coronary artery disease patients may allow cross-validation of techniques.

Gating Approaches in Cardiac PET Imaging

Cardiac PET provides high sensitivity and high negative predictive value in the diagnosis of coronary artery disease and cardiomyopathies. Cardiac, respiratory as well as bulk patient motion have detrimental effects on thoracic PET imaging, in particular on cardiovascular PET imaging where the motion can affect the PET images quantitatively as well as qualitatively. Gating can ameliorate the unfavorable impact of motion additionally enabling evaluation of left ventricular systolic function. In this article, the authors review the recent advances in gating approaches and highlight the advances in data-driven approaches, which hold promise in motion detection without the need for complex hardware setup.

Impact of improved attenuation correction on 18F-FDG PET/MR hybrid imaging of the heart

PURPOSE

The aim of this study was to evaluate and quantify the effect of improved attenuation correction (AC) including bone segmentation and truncation correction on 18F-Fluordesoxyglucose cardiac positron emission tomography/magnetic resonance (PET/MR) imaging.

METHODS

PET data of 32 cardiac PET/MR datasets were reconstructed with three different AC-maps (1. Dixon-VIBE only, 2. HUGE truncation correction and bone segmentation, 3. MLAA). The Dixon-VIBE AC-maps served as reference of reconstructed PET data. 17-segment short-axis polar plots of the left ventricle were analyzed regarding the impact of each of the three AC methods on PET quantification in cardiac PET/MR imaging. Non-AC PET images were segmented to specify the amount of truncation in the Dixon-VIBE AC-map serving as a reference. All AC-maps were evaluated for artifacts.

RESULTS

Using HUGE + bone AC results in a homogeneous gain of ca. 6% and for MLAA 8% of PET signal distribution across the myocardium of the left ventricle over all patients compared to Dixon-VIBE AC only. Maximal relative differences up to 18% were observed in segment 17 (apex). The body volume truncation of -12.7 ± 7.1% compared to the segmented non-AC PET images using the Dixon-VIBE AC method was reduced to -1.9 ± 3.9% using HUGE and 7.8 ± 8.3% using MLAA. In each patient, a systematic overestimation in AC-map volume was observed when applying MLAA. Quantitative impact of artifacts showed regional differences up to 6% within single segments of the myocardium.

CONCLUSIONS

Improved AC including bone segmentation and truncation correction in cardiac PET/MR imaging is important to ensure best possible diagnostic quality and PET quantification. The results exhibited an overestimation of AC-map volume using MLAA, while HUGE resulted in a more realistic body contouring. Incorporation of bone segmentation into the Dixon-VIBE AC-map resulted in homogeneous gain in PET signal distribution across the myocardium. The majority of observed AC-map artifacts did not significantly affect the quantitative assessment of the myocardium.

Cardiac PET/MRI-an update

It is now about 8 years since the first whole-body integrated PET/MRI has been installed. First, reports on technical characteristics and system performance were published. Early after, reports on the first use of PET/MRI in oncological patients were released. Interestingly, the first article on the application in cardiology was a review article, which was published before the first original article was put out. Since then, researchers have gained a lot experience with the PET/MRI in various cardiovascular diseases and an increasing number on auspicious indications is appearing. In this review article, we give an overview on technical updates within these last years with potential impact on cardiac imaging and summarize those scenarios where PET/MRI plays a pivotal role in cardiovascular medicine.

Motion-corrected whole-heart PET-MR for the simultaneous visualisation of coronary artery integrity and myocardial viability: an initial clinical validation

PURPOSE

Cardiac PET-MR has shown potential for the comprehensive assessment of coronary heart disease. However, image degradation due to physiological motion remains a challenge that could hinder the adoption of this technology in clinical practice. The purpose of this study was to validate a recently proposed respiratory motion-corrected PET-MR framework for the simultaneous visualisation of myocardial viability (18F-FDG PET) and coronary artery anatomy (coronary MR angiography, CMRA) in patients with chronic total occlusion (CTO).

METHODS

A cohort of 14 patients was scanned with the proposed PET-CMRA framework. PET and CMRA images were reconstructed with and without the proposed motion correction approach for comparison purposes. Metrics of image quality including visible vessel length and sharpness were obtained for CMRA for both the right and left anterior descending coronary arteries (RCA, LAD), and relative increase in 18F-FDG PET signal after motion correction for standard 17-segment polar maps was computed. Resulting coronary anatomy by CMRA and myocardial integrity by PET were visually compared against X-ray angiography and conventional Late Gadolinium Enhancement (LGE) MRI, respectively.

RESULTS

Motion correction increased CMRA visible vessel length by 49.9% and 32.6% (RCA, LAD) and vessel sharpness by 12.3% and 18.9% (RCA, LAD) on average compared to uncorrected images. Coronary lumen delineation on motion-corrected CMRA images was in good agreement with X-ray angiography findings. For PET, motion correction resulted in an average 8% increase in 18F-FDG signal in the inferior and inferolateral segments of the myocardial wall. An improved delineation of myocardial viability defects and reduced noise in the 18F-FDG PET images was observed, improving correspondence to subendocardial LGE-MRI findings compared to uncorrected images.

CONCLUSION

The feasibility of the PET-CMRA framework for simultaneous cardiac PET-MR imaging in a short and predictable scan time (~11 min) has been demonstrated in 14 patients with CTO. Motion correction increased visible length and sharpness of the coronary arteries by CMRA, and improved delineation of the myocardium by 18F-FDG PET, resulting in good agreement with X-ray angiography and LGE-MRI.

Hybrid positron emission tomography-magnetic resonance of the heart: current state of the art and future applications

Hybrid positron emission tomography-magnetic resonance (PET-MR) imaging is a novel imaging modality with emerging applications for cardiovascular disease. PET-MR aims to combine the high-spatial resolution morphological and functional assessment afforded by magnetic resonance imaging (MRI) with the ability of positron emission tomography (PET) for quantification of metabolism, perfusion, and inflammation. The fusion of these two modalities into a single imaging platform not only represents an opportunity to acquire complementary information from a single scan, but also allows motion correction for PET with reduction in ionising radiation. This article presents a brief overview of PET-MR technology followed by a review of the published literature on the clinical cardio-vascular applications of PET and MRI performed separately and with hybrid PET-MR.

Technical aspects of cardiac PET/MRI

PET/MRI is a novel modality that enables to combine PET and MR images, and has significant potential to evaluate various cardiac diseases through the combination of PET molecular imaging and MRI functional imaging. Precise management of technical issues, however, is necessary for cardiac PET/MRI. This article describes several technical points, including patient preparation, MR attenuation correction, parallel acquisition of PET with MRI, clinical aspects, and image quality control.