18F-FDG PET/CT and PET/MRI Perform Equally Well in Cancer: Evidence from Studies on More Than 2,300 Patients

Utility of computed tomography and 18 F-fluorodeoxyglucose with positron emission tomography/computed tomography for distinguishing appendiceal mucocele caused by mucinous adenocarcinoma from other pathologies

AIM

Differentiating appendiceal mucocele with mucinous adenocarcinoma from other pathologies before surgery is difficult. The objective of this study was to evaluate the utility of CT and ¹⁸ F-fluorodeoxyglucose (FDG) with positron emission tomography (PET)/CT for differentiating mucinous adenocarcinoma of appendiceal mucocele from other pathologies.

METHOD

The study included 25 patients who underwent surgery for clinically diagnosed appendiceal mucoceles detected on CT at the University of Tokyo Hospital. Among these patients, 19 underwent FDG-PET/CT preoperatively. We compared features of the CT imaging findings and maximum standard uptake values (SUV_max ) detected by FDG-PET/CT between mucocele with mucinous adenocarcinoma and other pathologies.

RESULTS

A total of 13 men (52%) and 12 women (48%) were included in this study, with a median age of 65 years (range 34-83). There were six patients (24%) with pathologically confirmed mucinous adenocarcinoma, 15 patients (60%) with appendiceal mucinous neoplasm and four patients (16%) with simple mucocele caused by chronic inflammation. On the CT findings, wall irregularity was the only significant feature for the two groups in this study (83.3% vs 0.0%, P < 0.01). There was a significant difference in the SUV_max levels on PET/CT between the two groups (100.0% vs 20.0%, P < 0.01).

CONCLUSION

Distinguishing between mucocele with mucinous adenocarcinoma and other pathologies using imaging modalities is challenging. Our results suggest that wall irregularity on CT and elevated SUV_max on PET/CT are useful factors that can be employed for such discrimination.

PET/MRI of the hepatobiliary system: Review of techniques and applications

Simultaneous positron emission tomography and MRI (PET/MRI) is an emerging technology that offers the benefits of MRI, including excellent soft tissue contrast, lack of ionizing radiation, and functional MRI techniques, with the physiologic information provided by PET. Although most PET/MRI systems are currently installed in tertiary care centers, PET/MRI technology is becoming increasingly widespread. The usefulness of PET/MRI varies by tumor type and organ system and has been shown to have utility in evaluation of primary and secondary hepatic neoplasms. Understanding the appropriate applications, techniques and relevant imaging findings is important for practicing radiologists considering or currently utilizing PET/MR for the evaluation of primary liver neoplasms, including hepatocellular carcinoma (HCC), as well as staging of biliary neoplasms including cholangiocarcinoma and gallbladder cancer, identification of liver metastases, and staging of neuroendocrine tumor.

The Labeling, Visualization, and Quantification of Hyaluronan Distribution in Tumor-Bearing Mouse Using PET and MR Imaging

PURPOSE

Hyaluronan (HA) based biomaterials are widely used as tissue scaffolds, drug formulations, as well as targeting ligands and imaging probes for diagnosis and drug delivery. However, because of the presence of abundant endogenous HA presented in various tissues in vivo, the pharmacokinetic behavior and biodistribution patterns of exogenously administered HAs have not been well characterized.

METHODS

The HA backbone was modified with Diethylenetriamine (DTPA) to enable the chelation of gadolinium (Gd) and aluminum (Al) ions. Series of PET and MR imaging were taken after the injection of HA-DTPA-Gd and HA-DTPA-Al¹⁸F while using¹⁸F-FDG and Magnevist(DTPA-Gd) as controls. The Tomographic images were analyzed and quantified to reveal the distribution and locations of HA in tumor-bearing mice.

RESULTS

The labeled HAs had good stability in plasma. They retained binding affinity towards CD44s on tumor cell surface. The injected HAs distributed widely in various organs, but were found to be cleared quickly except inside tumor tissues where the signals were higher and persisted longer.

CONCLUSION

Medical imaging tools, including MR and PET, can be highly valuable for examining biomaterial distribution non-invasively. The HA tumor accumulation properties may be explored for the development of active targeting drug carriers and molecular probes.

Prospective evaluation of whole-body MRI and 18F-FDG PET/MRI in N and M staging of primary breast cancer patients

OBJECTIVES

To evaluate and compare the diagnostic potential of whole-body MRI and whole-body 18F-FDG PET/MRI for N and M staging in newly diagnosed, histopathologically proven breast cancer.

MATERIAL AND METHODS

A total of 104 patients (age 53.4 ± 12.5) with newly diagnosed, histopathologically proven breast cancer were enrolled in this study prospectively. All patients underwent a whole-body 18F-FDG PET/MRI. MRI and 18F-FDG PET/MRI datasets were evaluated separately regarding lesion count, lesion localization, and lesion characterization (malignant/benign) as well as the diagnostic confidence (5-point ordinal scale, 1-5). The N and M stages were assessed according to the eighth edition of the American Joint Committee on Cancer staging manual in MRI datasets alone and in 18F-FDG PET/MRI datasets, respectively. In the majority of lesions histopathology served as the reference standard. The remaining lesions were followed-up by imaging and clinical examination. Separately for nodal-positive and nodal-negative women, a McNemar chi2 test was performed to compare sensitivity and specificity of the N and M stages between 18F-FDG PET/MRI and MRI. Differences in diagnostic confidence scores were assessed by Wilcoxon signed rank test.

RESULTS

MRI determined the N stage correctly in 78 of 104 (75%) patients with a sensitivity of 62.3% (95% CI: 0.48-0.75), a specificity of 88.2% (95% CI: 0.76-0.96), a PPV (positive predictive value) of 84.6% % (95% CI: 69.5-0.94), and a NPV (negative predictive value) of 69.2% (95% CI: 0.57-0.8). Corresponding results for 18F-FDG PET/MRI were 87/104 (83.7%), 75.5% (95% CI: 0.62-0.86), 92.2% (0.81-0.98), 90% (0.78-0.97), and 78.3% (0.66-0.88), showing a significantly better sensitivity of 18F-FDG PET/MRI determining malignant lymph nodes (p = 0.008). The M stage was identified correctly in MRI and 18F-FDG PET/MRI in 100 of 104 patients (96.2%). Both modalities correctly staged all 7 patients with distant metastases, leading to false-positive findings in 4 patients in each modality (3.8%). In a lesion-based analysis, 18F-FDG PET/MRI showed a significantly better performance in correctly determining malignant lesions (85.8% vs. 67.1%, difference 18.7% (95% CI: 0.13-0.26), p < 0.0001) and offered a superior diagnostic confidence compared with MRI alone (4.1 ± 0.7 vs. 3.4 ± 0.7, p < 0.0001).

CONCLUSION

18F-FDG PET/MRI has a better diagnostic accuracy for N staging in primary breast cancer patients and provides a significantly higher diagnostic confidence in lesion characterization than MRI alone. But both modalities bear the risk to overestimate the M stage.

Diagnostic Value of FDG PET/MRI in Females With Pelvic Malignancy-A Systematic Review of the Literature

Hybrid imaging with F-18 fludeoxyglucose positron emission tomography/magnetic resonance imaging (FDG PET/MRI) has increasing clinical applications supplementing conventional ultrasound, CT, and MRI imaging as well as hybrid PET/CT imaging in assessing cervical, endometrial, and ovarian cancer. This article summarizes the existing literature and discusses the emerging role of hybrid PET/MRI in gynecologic malignancies. Thus, far, the published literature on the applications of FDG PET/MRI shows that it can have a significant impact on patient management by improving the staging of the cancers compared with PET/CT, influencing clinical decision and treatment strategy. For disease restaging, current literature indicates that PET/MRI performs equivalently to PET/CT. There appears to be a mild-moderate inverse correlation between standard-uptake-value (SUV) and apparent-diffusion-coefficient (ADC) values, which could be used to predict tumor grading and risk stratification. It remains to be seen as to whether multi-parametric PET/MRI imaging could prove valuable for prognostication and outcome. PET/MRI provides the opportunity for reduced radiation exposure, which is particularly relevant for a young female in need of multiple scans for treatment monitoring and follow-up. Fast acquisition protocols and optimized methods for attenuation correction are still evolving. Major limitations of PET/MRI remains such as suboptimal detection of small pulmonary nodules and lack of utility for radiation treatment planning, which pose an impediment in making PET/MRI a viable one-stop-shop imaging option to compete with PET/CT.

18F-FDG PET/MRI in adult sarcomas

Aim

This mini-review aims to summarize the main findings on PET/MRI in patients with sarcomas.

Materials and methods

A literature search was carried out on MEDLINE databases, such as PubMed, Scopus, Web of Science and Google Scholar, using the search terms “PET”, “positron emission tomography”, “PET/MRI”, “positron emission tomography/magnetic resonance imaging”, and “sarcoma”. No restrictions (i.e. language, age, type of manuscript, or the like) were applied to the search strategy. Abstracts, reviews, letters to editors, and editorials were excluded.

Results

Eight studies were ultimately included. From 2013 to 2019, a total of 154 adult patients with sarcomas underwent 18F-fluorodeoxyglucose (18F-FDG) PET/MRI. Of these patients, 129 had soft tissue sarcomas (STS), 5 had bone sarcomas, and 20 had cardiac masses. Thirty-two patients had PET/MRI for early-stage disease, 45 for recurrences, and 57 for the assessment of response to treatment. For staging purposes, the studies suggest that PET/MRI can significantly improve the delineation of surgical margins. At restaging, PET/MRI can also detect sarcoma recurrences more accurately than conventional imaging.

Conclusions

18F-FDG PET/MRI has promising indications in patients with sarcomas, from disease staging to the assessment of response to treatment. Further studies are warranted to confirm these results, especially in subgroups with specific histopathological features.

Clinical PET/MR

Oncologic imaging has been a major focus of clinical research on PET/MR over the last 10 years. Studies so far have shown that PET/MR with ¹⁸F-Fluorodeoxyglucose (FDG) overall provides a similar accuracy for tumor staging as FDG PET/CT. The effective radiation dose of whole-body FDG PET/MR is more than 50% lower than for FDG PET/CT, making PET/MR particularly attractive for imaging of children. However, the longer acquisition times and higher costs have so far limited broader clinical use of PET/MR technology for whole-body staging. With the currently available technology, PET/MR appears more promising for locoregional staging of diseases for which MR is the anatomical imaging modality of choice. These include brain tumors, head and neck cancers, gynecologic malignancies, and prostate cancer. For instance, PET imaging with ligands of prostate-specific membrane antigen, combined with multi-parametric MR, appears promising for detection of prostate cancer and differentiation from benign prostate pathologies as well as for detection of local recurrences. The combination of functional parameters from MR, such as apparent diffusion coefficients, and molecular parameters from PET, such as receptor densities or metabolic rates, is feasible in clinical studies, but clinical applications for this multimodal and multi-parametric imaging approach still need to be defined.

Hybrid positron emission tomography imaging for initial staging of sinonasal tumors: Total lesion glycolysis as prognosticator of treatment response

BACKGROUND

To assess hybrid positron emission tomography (PET) imaging in the initial staging and outcome prediction of sinonasal malignancies.

METHODS

Retrospective study on patients with sinonasal malignancies undergoing hybrid PET imaging for initial staging.

RESULTS

Complete remission (CR) was achieved in 45 of 65 patients (69.2%). Overall sensitivity for detection of primaries using 18F-fluoro-deoxy-d-glucose PET (FDG-PET) was 95.4%, for lymph node metastases 100% and distant metastases (DM) 100%. On univariate analysis, PET parameter total lesion glycolysis (TLG) was associated with achieving CR after primary treatment (176.8 ± 157.2 vs 83.7 ± 110.8, P = .03). Multivariate logistic regression demonstrated that TLG adjusted for the T classification best predicted achievement of CR.

CONCLUSIONS

Hybrid PET imaging yields an excellent sensitivity in detecting primary tumors, lymph node metastases and DM in sinonasal malignancies. TLG of the primary tumor is an independent prognostic factor for achieving CR after initial treatment.

Prostate Cancer: Prostate-specific Membrane Antigen Positron-emission Tomography/Computed Tomography or Positron-emission Tomography/Magnetic Resonance Imaging for Staging

Positron-emission tomography (PET) with prostate-specific membrane antigen (PSMA) has been increasingly used to image prostate cancer in the last decade. In the staging setting several studies have already been published suggesting PSMA PET can be a valuable tool. They, however, did not translate into recommendations by guidelines. Both PSMA PET/computed tomography (CT) and PET/magnetic resonance imaging have been investigated in the staging setting, showing higher detection rate of prostate cancer lesions over the conventional imaging work-up and some studies already showed an impact on disease management. The aim of this review is to provide an overview of the existing published data regarding PSMA PET for staging prostate cancer, with emphasis on PET/magnetic resonance imaging. Despite the fact that PSMA is a relatively new tool and not officially recommended for staging yet, there are >50 original studies in the literature assessing PSMA PET performance in the staging setting of prostate cancer, and some meta-analyses.

PET/MR in Head and Neck Cancer - An Update

In academic centers, PET/MR has taken the road to clinical nuclear medicine in the past 6 years since the last review on its applications in head and neck cancer patients in this journal. Meanwhile, older sequential PET + MR machines have largely vanished from clinical sites, being replaced by integrated simultaneous PET/MR scanners. Evidence from several studies suggests that PET/MR overall performs equally well as PET/CT in the staging and restaging of head and neck cancer and in radiation therapy planning. PET/MR appears to offer advantages in the characterization and prognostication of head and neck malignancies through multiparametric imaging, which demands an exact preparation and validation of imaging modalities, however. The majority of available clinical PET/MR studies today covers FDG imaging of squamous cell carcinoma arising from a broad spectrum of locations in the upper aerodigestive tract. In the future, specific PET/MR studies are desired that address specific histopathological tumor entities, nonepithelial malignancies, such as major salivary gland tumors, squamous cell carcinomas arising in specific locations, and malignancies imaged with non-FDG radiotracers. With the advent of digital PET/CT scanners, PET/MR is expected to partake in future technical developments, such as novel iterative reconstruction techniques and deviceless motion correction for respiration and gross movement in the head and neck region. Owing to the still comparably high costs of PET/MR scanners and facility requirements on the one hand, and the concentration of multidisciplinary head and neck cancer treatment mainly at academic centers on the other hand, a more widespread use of this imaging modality outside major hospitals is currently limited.

Improving PET Imaging Acquisition and Analysis With Machine Learning: A Narrative Review With Focus on Alzheimer's Disease and Oncology

Machine learning (ML) algorithms have found increasing utility in the medical imaging field and numerous applications in the analysis of digital biomarkers within positron emission tomography (PET) imaging have emerged. Interest in the use of artificial intelligence in PET imaging for the study of neurodegenerative diseases and oncology stems from the potential for such techniques to streamline decision support for physicians providing early and accurate diagnosis and allowing personalized treatment regimens. In this review, the use of ML to improve PET image acquisition and reconstruction is presented, along with an overview of its applications in the analysis of PET images for the study of Alzheimer's disease and oncology.

Diagnostic Accuracy and Confidence of [18F] FDG PET/MRI in comparison with PET or MRI alone in Head and Neck Cancer

The usefulness of PET/MRI in head and neck malignancy has not been fully elucidated. The purpose of our study was to evaluate the diagnostic accuracy and confidence of PET/MRI in comparison with PET or MRI alone. This study included 73 consecutive patients who underwent [18F] FDG PET/MRI in head and neck under the suspicion of malignancy. A neuroradiologist and a nuclear medicine specialist reviewed MRI and PET images, respectively and independently, followed by a consensus review of PET/MRI one month later. For 134 lesions, accuracy and confidence were compared among PET, MRI, and PET/MRI. For lesion base, PET/MRI had a sensitivity of 85.7%, a specificity of 89.1%, a PPV of 89.6%, a negative predictive value of 85.1%, and an accuracy of 87.3%. AUCs of PET/MRI per lesion (0.926) and per patient (0.934) for diagnosing malignancy were higher than PET (0.847 and 0.747, respectively) or MRI (0.836 and 0.798, respectively) alone (P < 0.05). More than 80% of the cases (111/134) showed diagnostic concordance between PET and MRI. PPV of PET/MRI was higher in malignant concordant cases (93.2%, 55/59) than in discordant cases (62.5%, 5/8) (p = 0.040). Confident scoring rate in malignant concordant cases was higher on PET/MRI (96.6%, 57/59) than on MRI (76.3%, 45/59) (p = 0.003). In conclusion, compared with PET or MRI alone, PET/MRI presents better diagnostic performance in accuracy and confidence for diagnosis of malignancy. PET/MRI is useful in patients with head and neck cancer.

Diagnostic performance of zero-TE lung MR imaging in FDG PET/MRI for pulmonary malignancies

OBJECTIVES

This study aimed to evaluate the diagnostic performance of the lung zero-echo time (ZTE) sequence in FDG PET/MRI for detection and differentiation of lung lesions in oncologic patients in comparison with conventional two-point Dixon-based MR imaging.

METHODS

In this single-institution retrospective study approved by the institutional review board, 209 patients with malignancies (97 men and 112 women; age range, 17-89 years; mean age, 66.5 ± 12.9 years) underwent ¹⁸F-FDG PET/MRI between August 2017 and August 2018, with diagnostic Dixon and ZTE under respiratory gating acquired simultaneously with PET. Image analysis was performed for PET/Dixon and PET/ZTE fused images by two readers to assess the detectability and differentiation of lung lesions. The reference standard was pathological findings and/or the data from a chest CT. The detection and differentiation abilities were evaluated for all lesions and subgroups divided by lesion size and maximum standardized uptake value (SUVmax).

RESULTS

Based on the reference standard, 227 lung lesions were identified in 113 patients. The detectability of PET/ZTE was significantly better than that of PET/Dixon for overall lesions, lesions with a SUVmax less than 3.0 and lesions smaller than 4 mm (p < 0.01). The diagnostic performance of PET/ZTE was significantly better than that of PET/Dixon for overall lesions and lesions smaller than 4 mm (p < 0.01).

CONCLUSIONS

ZTE can improve diagnostic performance in the detection and differentiation of both FDG-avid and non-FDG-avid lung lesions smaller than 4 mm in size, yielding a promising tool to enhance the utility of FDG PET/MRI in oncology patients with lung lesions.

KEY POINTS

• The detection rate of PET/ZTE for lesions with a SUVmax of less than 1.0 was significantly better than that of PET/Dixon. • The performance for differentiation of PET/ZTE for lesions that were even smaller than 4 mm in size were significantly better than that of PET/Dixon. • Inter-rater agreement of PET/ZTE for the differentiation of lesions less than 4 mm in size was substantial and better than that of PET/Dixon.

Adapt and conquer: Metabolic flexibility in cancer growth, invasion and evasion

BACKGROUND

It has been known for close to a century that, on average, tumors have a metabolism that is different from those found in healthy tissues. Typically, tumors show a biosynthetic metabolism that distinguishes itself by engaging in large scale aerobic glycolysis, heightened flux through the pentose phosphate pathway, and increased glutaminolysis among other means. However, it is becoming equally clear that non tumorous tissues at times can engage in similar metabolism, while tumors show a high degree of metabolic flexibility reacting to cues, and stresses in their local environment.

SCOPE OF THE REVIEW

In this review, we want to scrutinize historic and recent research on metabolism, comparing and contrasting oncogenic and physiological metabolic states. This will allow us to better define states of bona fide tumor metabolism. We will further contextualize the stress response and the metabolic evolutionary trajectory seen in tumors, and how these contribute to tumor progression. Lastly, we will analyze the implications of these characteristics with respect to therapy response.

MAJOR CONCLUSIONS

In our review, we argue that there is not one single oncogenic state, but rather a diverse set of oncogenic states. These are grounded on a physiological proliferative/wound healing program but distinguish themselves due to their large scale of proliferation, mutations, and transcriptional changes in key metabolic pathways, and the adaptations to widespread stress signals within tumors. We find evidence for the necessity of metabolic flexibility and stress responses in tumor progression and how these responses in turn shape oncogenic progression. Lastly, we find evidence for the notion that the metabolic adaptability of tumors frequently frustrates therapeutic interventions.

Discrepancies between positron emission tomography/magnetic resonance imaging and positron emission tomography/computed tomography in a cohort of oncological patients

INTRODUCTION

This study aims to evaluate discrepant findings between positron emission tomography/magnetic resonance imaging (PET/MRI) and positron emission tomography/computed tomography (PET/CT) in a cohort of oncological patients and to undertake a phantom study to assess the potential for extended PET acquisitions to lead to false-positive findings on PET/MRI.

METHODS

Discrepant findings from a series of 106 patients undergoing same-day ¹⁸ F-fluorodeoxyglucose (FDG)-PET/CT and PET/MRI were reviewed. Phantom studies explored the potential for PET acquisition time to contribute to discrepancy.

RESULTS

There were 14 discrepant cases, 5 (35.7%) of which related to PET/MRI acquisitions that had been extended to 10 min. Three of these five cases proved to be falsely positive. Phantom studies showed greater contrast recovery and signal to noise ratio for 10-min PET/MRI acquisitions compared to 2-min acquisitions using PET/CT. There were no discrepancies when PET/CT showed disseminated disease (P = 0.036).

CONCLUSIONS

Extended PET/MRI acquisitions used to accommodate multiple MRI sequences may be associated with false-positive findings compared to PET/CT. PET/MRI is more likely to have incremental value when the prior probability for disseminated disease is low.

Performing clinical 18F-FDG-PET/MRI of the mediastinum optimising a dedicated, patient-friendly protocol

OBJECTIVE

To construct a mediastinal-specific fluorine-18-fluorodeoxyglucose (F-FDG)-PET/MR protocol with high-quality MRI of minimal acquisition-time and comparable diagnostic value to F-FDG-PET/computed tomography (CT).

MATERIALS AND METHODS

Fifteen healthy participants received PET/MRI and 10 patients with mediastinal tumours (eight non-small-cell lung, two oesophageal cancer) received F-FDG-PET/MRI immediately after F-FDG-PET/CT. Sequences volume interpolated breath-hold examination (T1-VIBE) and Half-Fourier acquisition single-shot turbo spin echo (T2-HASTE) were optimised by varying the parameters: breath-hold (BH, end-expiration), fat suppression (spectral adiabatic inversion recovery), and ECG-triggering (ECG, end-diastole). Image quality (IQ) of each sequence-variation was qualitatively scored by medical experts and quantitatively assessed by calculating signal-to-noise ratios, contrast relative to muscle, standardized-uptake-value, and tumour-to-blood ratios. Patient comfort was evaluated on patients' experience. Diagnostic accuracy of F-FDG-PET/MRI was compared to F-FDG-PET/CT, in reference to histopathology/cytopathology.

RESULTS

ECG-triggered T1-VIBE images showed the highest signal-to-noise ratio (P < 0.01) and the largest contrast between mediastinal soft-tissues, regardless of BH or free-breathing acquisition. IQ of ECG-triggered T1-VIBE scans in BH were scored qualitatively highest with good reader agreement (κ = 0.62). IQ of T2-HASTE was not significantly affected by BH acquisition (P > 0.9). Qualitative IQ of T1-VIBE and T2-HASTE declined after spectral adiabatic inversion recovery fat-suppression. All patients could maintain BH at end-expiration and reported no discomfort. Diagnostic performance of F-FDG-PET/MR was not significantly different from F-FDG-PET/CT with comparable staging, standardized-uptake-values, and tumour-to-blood ratios. However, T-status was more often over-staged on F-FDG-PET/CT, while N-status was more frequently under-staged on F-FDG-PET/MR.

CONCLUSION

ECG-triggered T1-VIBE sequences acquired during short, multiple BHs are recommended for mediastinal imaging using F-FDG-PET/MR. With dedicated protocols, F-FDG-PET/MRI will be useful in thoracic oncology and aid in diagnostic evaluation and tailored treatment decision-making.

Microstrip Transmission Line RF Coil for a PET/MRI Insert

Purpose:

We proposed and developed a new microstrip transmission line radiofrequency (RF) coil for a positron emission tomography (PET) insert for MRI, which has low electrical interactions with PET shield boxes. We performed imaging experiments using a single-channel and a four-channel proposed RF coils for proof-of-concept.

Methods:

A conventional microstrip coil consists of a microstrip conductor, a ground conductor, and a dielectric between the two conductors. We proposed a microstrip coil for the PET insert that replaced the conventional single-layer ground conductor with the RF shield of the PET insert. A dielectric material, which could otherwise attenuate gamma photons radiated from the PET imaging tracer, was not used. As proof-of-concept, we compared conventional and the proposed single-channel coils. To study multichannel performance, we further developed a four-channel proposed RF coil. Since the MRI system had a single-channel transmission port, an interfacing four-way RF power division circuit was designed. The coils were implemented as both RF transmitters and receivers in a cylindrical frame of diameter 150 mm. Coil bench performances were tested with a network analyzer (Rohde & Schwarz, Germany), and a homogeneous phantom study was conducted for gradient echo imaging and RF field (B₁) mapping in a 3T clinical MRI system (Verio, Siemens, Erlangen, Germany).

Results:

For all coils, the power reflection coefficient was below −30 dB, and the transmission coefficients in the four-channel configuration were near or below −20 dB. The comparative single-channel coil study showed good similarity between the conventional and proposed coils. The gradient echo image of the four-channel coil showed expected flashing image intensity near the coils and no phase distortion was visible. Transmit B₁ field map resembled the image performance.

Conclusion:

The proposed PET-microstrip coil performed similarly to the conventional microstrip transmission line coil and is promising for the development of a compact coil-PET system capable of simultaneous PET/MRI analysis with an existing MRI system.

Sequential PET/diffusion-weighted imaging in the evaluation of myocardial perfusion and viability in coronary artery disease: a preliminary study

OBJECTIVES

To evaluate the utility of sequential F-18 fluorodeoxyglucose PET/diffusion-weighted imaging in assessing myocardial perfusion and viability in coronary artery disease.

METHODS

Fourteen coronary artery disease patients and five non-coronary artery disease subjects underwent sequential cardiac F-18 fluorodeoxyglucose PET/diffusion-weighted imaging using a trimodality PET/computed tomography-MRI system. The perfusion data were acquired by measuring low b-values apparent diffusion coefficient using diffusion-weighted imaging. Regional myocardial viability was determined by perfusion/metabolism patterns. The perfusion/metabolism patterns obtained by low b-values apparent diffusion coefficient/fluorodeoxyglucose uptake were analyzed and compared with the results from the combination of rest methoxyisobutylisonitrile (Tc-MIBI) myocardial perfusion single-photon emission computed tomography with F-18 fluorodeoxyglucose PET/computed tomography.

RESULTS

Ten coronary artery disease patients and five non-coronary artery disease subjects were included in the final analysis. Low b-values apparent diffusion coefficient defects involved with 25 myocardial regions were demonstrated in nine coronary artery disease patients, while Tc-MIBI defects involved with 21 myocardial regions were shown in the same patients. The agreement between low b-values apparent diffusion coefficient and MIBI uptake was good in coronary artery disease patients (κ = 0.627, P < 0.001) and was better still in the whole subjects (κ = 0.733, P < 0.001). Low b-values apparent diffusion coefficient/fluorodeoxyglucose uptake demonstrated mismatch patterns in six coronary artery disease patients and MIBI/fluorodeoxyglucose uptake revealed mismatch patterns in seven coronary artery disease patients. Agreement in the evaluation of regional myocardial viability between low b-values apparent diffusion coefficient/fluorodeoxyglucose uptake and MIBI/fluorodeoxyglucose uptake was high in coronary artery disease patients (κ = 0.627, P < 0.001) and all subjects (κ = 0.728, P < 0.001).

CONCLUSIONS

Low b-values apparent diffusion coefficient/fluorodeoxyglucose uptake is comparable to MIBI/fluorodeoxyglucose uptake in assessing perfusion/metabolism patterns, indicating that microperfusion might dominate the diffusion signal at low b-values and sequential PET/diffusion-weighted imaging might be useful to evaluate myocardial viability in coronary artery disease patients.

Preliminary clinical results for PET/MR compared with PET/CT in patients with nasopharyngeal carcinoma

The present study aimed to assess the performance of positron emission tomography-magnetic resonance imaging (PET/MR) for the visualization and characterization of lesions. In addition, the present study investigated whether the apparent diffusion coefficient (ADC) and intravoxel incoherent motion parameters exhibited any significant correlation with standardized uptake values (SUV) in patients with nasopharyngeal carcinoma (NPC). A total of 35 patients with NPC underwent whole body PET-computed tomography (CT) and head and neck MR imaging (MRI) scans using the PET/CT-MRI system. Image quality, lesion conspicuity and the diagnostic confidence of PET/CT, T1 weighted (T1w) PET/MR and T2w PET/MR imaging were assessed. The true diffusion coefficient (D), the pseudo-diffusion coefficient or diffusion within the microcirculation (D*), and the perfusion fraction or the contribution of water moving in the capillaries (f), and ADC, were calculated. The correlation between the ADC, D*, D and f values and the SUV were analyzed using Pearson's correlation analysis. Similar image quality was obtained using PET/CT, T1w PET/MR and T2w PET/MR imaging. However, the T1w PET/MR and T2w PET/MR imaging were more effective than PET/CT in analyzing the lesion conspicuity of the primary tumors and lymph nodes. In addition, T2w PET/MR imaging was more efficient than T1w PET/MR imaging in analyzing primary tumors and lymph nodes. Pearson's correlation analysis showed no significant correlation between the SUV and ADC, and D*, D and f values in NPC. The present results suggested that the application of PET/MR is feasible and could serve as a reliable alternative to PET/CT, while SUV and ADC, D*, D and f values were identified as independent biomarkers in NPC.

Comparison of 18F-FDG PET/MRI, MRI, and 18F-FDG PET/CT for the detection of synchronous cancers and distant metastases in patients with oropharyngeal and hypopharyngeal squamous cell carcinoma

PURPOSE

In this prospective study, we sought to compare the clinical utility of fluorodeoxyglucose PET/MRI, MRI, and PET/CT in the detection of synchronous cancers and distant metastases in patients with oropharyngeal and hypopharyngeal squamous cell carcinoma (OHSCC).

METHODS

We examined 198 consecutive patients with biopsy-proven OHSCC who agreed to receive chemoradiation. All patients underwent pretreatment PET/MRI and PET/CT on the same day. Patients were followed-up for a minimum of 12 months or until death. The McNemar's test and receiver-operating characteristic (ROC) curves were used to compare sensitivity/specificity and the diagnostic capabilities of PET/MRI, MRI, and PET/CT, respectively.

RESULTS

We identified 55 patients (27.7%) who had synchronous cancers and/or distant metastases (number of involved sites: 83). The results of site-based analysis revealed that the sensitivity of PET/MRI was 15.7% higher than that of MRI (73.5% versus 57.8%, p < 0.001) and 3.6% higher compared with PET/CT (73.5% versus 69.9%, p = 0.083), whereas the sensitivity of PET/CT was 12.1% higher than that of MRI (69.9% versus 57.8%, p = 0.012). On a patient-basis, ROC curve analysis demonstrated that PET/MRI yielded a greater area under curve than MRI (0.930 versus 0.905, p = 0.023). There were no significant differences in terms of diagnostic capability between MRI and PET/CT (0.905 versus 0.917, p = 0.469) and between PET/MRI and PET/CT (0.930 versus 0.917, p = 0.062).

CONCLUSIONS

In our cohort, PET/MRI showed a significantly higher diagnostic capability than MRI and no significant difference compared with PET/CT for the detection of synchronous cancers or distant metastases in patients with OHSCC.

Task Group 174 Report: Utilization of [18 F]Fluorodeoxyglucose Positron Emission Tomography ([18 F]FDG-PET) in Radiation Therapy

The use of positron emission tomography (PET) in radiation therapy (RT) is rapidly increasing in the areas of staging, segmentation, treatment planning, and response assessment. The most common radiotracer is ¹⁸ F-fluorodeoxyglucose ([¹⁸ F]FDG), a glucose analog with demonstrated efficacy in cancer diagnosis and staging. However, diagnosis and RT planning are different endeavors with unique requirements, and very little literature is available for guiding physicists and clinicians in the utilization of [¹⁸ F]FDG-PET in RT. The two goals of this report are to educate and provide recommendations. The report provides background and education on current PET imaging systems, PET tracers, intensity quantification, and current utilization in RT (staging, segmentation, image registration, treatment planning, and therapy response assessment). Recommendations are provided on acceptance testing, annual and monthly quality assurance, scanning protocols to ensure consistency between interpatient scans and intrapatient longitudinal scans, reporting of patient and scan parameters in literature, requirements for incorporation of [¹⁸ F]FDG-PET in treatment planning systems, and image registration. The recommendations provided here are minimum requirements and are not meant to cover all aspects of the use of [¹⁸ F]FDG-PET for RT.

Molecular Imaging in Oncology Using Positron Emission Tomography

BACKGROUND

Anatomical and molecular data can be acquired simultaneously through the use of positron emission tomography (PET) in combination with computed tomography (CT) or magnetic resonance imaging (MRI) as a hybrid technique. A variety of radiopharmaceuticals can be used to characterize various metabolic processes or to visualize the expression of receptors, enzymes, and other molecular target structures.

METHODS

This review is based on pertinent publications retrieved by a selective search in PubMed, as well as on guidelines from Germany and abroad and on systematic reviews and meta-analyses.

RESULTS

Established radiopharmaceuticals for PET, such as 2-[18F]fluoro-2- deoxyglucose ([18F]FDG), enable the visualization of physiological processes on the molecular level and can provide vital information for clinical decision-making. For example, PET can be used to evaluate pulmonary nodules for malignancy with 95% sensitivity and 82% specificity. It can be used both for initial staging and for the guidance of further treatment. Alongside the PET radiopharmaceuticals that have already been well studied and evaluated, newer ones are increasingly becoming available for the noninvasive phenotyping of tumor diseases, e.g., for analyzing the expression of prostate-specific membrane antigen (PSMA), of somatostatin receptors, or of chemokine receptors on tumor cells.

CONCLUSION

PET is an important component of diagnostic algorithms in oncology. It can help make diagnosis more precise and treatment more individualized. An increasing number of PET radiopharmaceuticals are now expanding the available options for imaging. Many radiopharmaceuticals can be used not only for noninvasive analysis of the expression of therapeutically relevant target structures, but also for the ensuing, target-directed treatment with radionuclides.

The Contribution of Multiparametric Pelvic and Whole-Body MRI to Interpretation of 18F-Fluoromethylcholine or 68Ga-HBED-CC PSMA-11 PET/CT in Patients with Biochemical Failure After Radical Prostatectomy

Our purpose was to assess whether the addition of data from multiparametric pelvic MRI (mpMR) and whole-body MRI (wbMR) to the interpretation of 18F-fluoromethylcholine (18F-FCH) or 68Ga-HBED-CC PSMA-11 (68Ga-PSMA) PET/CT (=PET) improves the detection of local tumor recurrence or of nodal and distant metastases in patients after radical prostatectomy with biochemical failure. Methods: The current analysis was performed as part of a prospective, multicenter trial on 18F-FCH or 68Ga-PSMA PET, mpMR, and wbMR. Eligible men had an elevated level of prostate-specific antigen (PSA) (>0.2 ng/mL) and high-risk features (Gleason score > 7, PSA doubling time < 10 mo, or PSA > 1.0 ng/mL) with negative or equivocal conventional imaging results. PET was interpreted with mpMR and wbMR in consensus by 2 radiologists and compared with prospective interpretation of PET or MRI alone. Performance measures of each modality (PET, MRI, and PET/mpMR-wbMR) were compared for each radiotracer and each individual patient (for 18F-FCH, or 68Ga-PSMA for patients who had 68Ga-PSMA PET) and to a composite reference standard. Results: There were 86 patients with PET (18F-FCH [n = 76] and/or 68Ga-PSMA [n = 26]) who had mpMR and wbMR. Local tumor recurrence was detected in 20 of 76 (26.3%) on 18F-FCH PET/mpMR, versus 11 of 76 (14.5%) on 18F-FCH PET (P = 0.039), and in 11 of 26 (42.3%) on 68Ga-PSMA PET/mpMR, versus 6 of 26 (23.1%) on 68Ga-PSMA PET (P = 0.074). Per patient, PET/mpMR was more often positive for local tumor recurrence than PET (P = 0.039) or mpMR (P = 0.019). There were 20 of 86 patients (23.3%) with regional nodal metastases on both PET/wbMR and PET (P = 1.0) but only 12 of 86 (14%) on wbMR (P = 0.061). Similarly, there were more nonregional metastases detected on PET/wbMR than on PET (P = 0.683) or wbMR (P = 0.074), but these differences did not reach significance. Compared with the composite reference standard for the detection of disease beyond the prostatic fossa, PET/wbMR, PET, and wbMR had sensitivity of 50%, 50%, and 8.3%, respectively, and specificity of 97.1%, 97.1%, and 94.1%, respectively. Conclusion: Interpretation of PET/mpMR resulted in a higher detection rate for local tumor recurrence in the prostatic bed in men with biochemical failure after radical prostatectomy. However, the addition of wbMR to 18F-FCH or 68Ga-PSMA PET did not improve detection of regional or distant metastases.

Attenuation correction of a flat table top for radiation therapy in hybrid PET/MR using CT- and 68Ge/68Ga transmission scan-based μ-maps

Hybrid PET/MR offers new opportunities in radiation oncology for tissue/tumour characterisation and response assessment. Attenuation correction (AC) is an important issue especially in the presence of immobilization devices and flat table tops (FTT). The goal of this study was to compare two methods of AC using CT- and ⁶⁸Ge/⁶⁸Ga transmission scan-based attenuation maps (μ-maps) for a custom-designed FTT. Measurements were performed in the mMR PET/MR and TrueV PET/CT Biograph Siemens scanners with three different phantoms, namely the Siemens MR-QA, a cubic canister and the NEMA IEC body phantom. The study revealed that the MR image quality is not hampered by the presence of the FTT. For cubic canister applying the scanner's inherent AC alone resulted in inaccuracies in PET images, with up to -4.0% underestimation of the activity. The mean NEMA sphere activity measurements without FTT, agreed within 3.5% with the respective inserted activity. Placing the FTT in the PET/MR scanner resulted in a difference to the injected activity of 4.5% when the table was not corrected for. By introducing the μ-maps the discrepancy between the used activity and the measurements decreased down to 2.6%. To improve the AC of the FTT the creation of a dedicated μ-map was necessary while the CT-based μ-map performed equally good as the source transmission scan-based one.

Chest Magnetic Resonance Imaging Decreases Inter-observer Variability of Gross Target Volume for Lung Tumors

Purpose: PET/CT is a standard medical imaging used in the delineation of gross tumor volume (GTV) in case of radiation therapy for lung tumors. However, PET/CT could present some limitations such as resolution and standardized uptake value threshold. Moreover, chest MRI has shown good potential in diagnosis for thoracic oncology. Therefore, we investigated the influence of chest MRI on inter-observer variability of GTV delineation.

Methods and Materials: Five observers contoured the GTV on CT for 14 poorly defined lung tumors during three contouring phases based on true daily clinical routine and acquisition: CT phase, with only CT images; PET phase, with PET/CT; and MRI phase, with both PET/CT and MRI. Observers waited at least 1 week between each phases to decrease memory bias. Contours were compared using descriptive statistics of volume, coefficient of variation (COV), and Dice similarity coefficient (DSC).

Results: MRI phase volumes (median 4.8 cm³) were significantly smaller than PET phase volumes (median 6.4 cm³, p = 0.015), but not different from CT phase volumes (median 5.7 cm³, p = 0.30). The mean COV was improved for the MRI phase (0.38) compared to the CT (0.58, p = 0.024) and PET (0.53, p = 0.060) phases. The mean DSC of the MRI phase (0.67) was superior to those of the CT and PET phases (0.56 and 0.60, respectively; p < 0.001 for both).

Conclusions: The addition of chest MRI seems to decrease inter-observer variability of GTV delineation for poorly defined lung tumors compared to PET/CT alone and should be explored in further prospective studies.

Image quality and detectability in Siemens Biograph PET/MRI and PET/CT systems-a phantom study

BACKGROUND

The technology of modern positron emission tomography (PET) systems continuously improving, and with it the possibility to detect smaller lesions. Since first introduced in 2010, the number of hybrid PET/magnetic resonance imaging (MRI) systems worldwide is constantly increasing. It is therefore important to assess and compare the image quality, in terms of detectability, between the PET/MRI and the well-established PET/computed tomography (CT) systems. For this purpose, a PET image quality phantom (Esser) with hot spheres, ranging from 4 to 20 mm in diameter, was prepared with fluorodeoxyglucose and sphere-to-background activity concentrations of 8:1 and 4:1, to mimic clinical conditions. The phantom was scanned on a PET/MRI and a PET/CT system for both concentrations to obtain contrast recovery coefficients (CRCs) and contrast-to-noise ratios (CNRs), for a range of reconstruction settings. The detectability of the spheres was scored by three human observers for both systems and concentrations and all reconstructions. Furthermore, the impact of acquisition time on CNR and observer detectability was investigated.

RESULTS

Reconstructions applying point-spread-function modeling (and time-of-flight for the PET/CT) yielded the highest CRC and CNR in general, and PET/CT demonstrated slightly higher values than PET/MRI for most sphere sizes. CNR was dependent on reconstruction settings and was maximized for 2 iterations, a pixel size of less than 2 mm and a 4 mm Gaussian filter. Acquisition times of 97 s (PET/MRI) and 150 s (PET/CT) resulted in similar total net true counts. For these acquisition times, the smallest detected spheres by the human observers in the 8:1 activity concentration was the 6-mm sphere with PET/MRI (CNR = 5.6) and the 5-mm sphere with PET/CT (CNR = 5.5). With an acquisition time of 180 s, the 5-mm sphere was also detected with PET/MRI (CNR = 5.8). The 8-mm sphere was the smallest detected sphere in the 4:1 activity concentration for both systems.

CONCLUSION

In this experimental study, similar detectability was found for the PET/MRI and the PET/CT, although for an increased acquisition time for the PET/MRI.

Feasibility of Quantitative Magnetic Resonance Fingerprinting in Ovarian Tumors for T1 and T2 Mapping in a PET/MR Setting

Multiparametric magnetic resonance imaging (MRI) can be used to characterize many cancer subtypes including ovarian cancer. Quantitative mapping of MRI relaxation values, such as T 1 and T 2 mapping, is promising for improving tumor assessment beyond conventional qualitative T 1- and T 2-weighted images. However, quantitative MRI relaxation mapping methods often involve long scan times due to sequentially measuring many parameters. Magnetic resonance fingerprinting (MRF) is a new method that enables fast quantitative MRI by exploiting the transient signals caused by the variation of pseudorandom sequence parameters. These transient signals are then matched to a simulated dictionary of T 1 and T 2 values to create quantitative maps. The ability of MRF to simultaneously measure multiple parameters, could represent a new approach to characterizing cancer and assessing treatment response. This feasibility study investigates MRF for simultaneous T 1, T 2, and relative proton density (rPD) mapping using ovarian cancer as a model system.

Interchangeability of radiomic features between [18F]-FDG PET/CT and [18F]-FDG PET/MR

PURPOSE

Radiomics is a promising tool for identification of new prognostic biomarkers. However, image reconstruction settings and test-retest variability may influence the absolute values of radiomic features. Unstable radiomic features cannot be used as reliable biomarkers. PET/MR is becoming increasingly available and often replaces PET/CT for different indications. The aim of this study was to quantify to what extend [18F]-FDG PET/CT radiomics models can be transferred to [18F]-FDG PET/MR and thereby to investigate the feasibility of combined PET/CT-PET/MR models. For this purpose, we compared PET radiomic features calculated on PET/MR and PET/CT and on a 4D-gated PET/MR dataset to select radiomic features that are robust to attenuation correction differences and test-retest variability, respectively.

METHODS

Two cohorts of patients with lung lesions were studied. In the first cohort (n = 10), inhale and exhale phases of a 4D [18F]-FDG PET/MR (4DPETMR) scan were used as a surrogate for a test-retest dataset. In the second cohort (n = 9), patients underwent first an [18F]-FDG PET/MR scan (SIGNA PET/MR, GE Healthcare, Waukesha) followed by an [18F]-FDG PET/CT scan (Discovery 690, GE Healthcare) with a delay of 33 ± 5 min (PETCT-PETMR). Lesions were segmented on inhale and exhale 4D-PET phases and on the individual PET scans from PET/CT and PET/MR with two semi-automated methods (gradient-based and threshold-based). The scan resolution was 2.73 × 2.73 × 3.27 mm and 2.34 × 2.34 × 2.78 mm for the PET/CT and PET/MR, respectively. In total, 1355 radiomic features were calculated, i.e., shape (n = 18), intensity (n = 17), texture (n = 136), and wavelet (n = 1184). The intraclass correlation coefficient (ICC) was calculated to compare the radiomic features of the 4DPETMR (ICC(1,1)) and PETCT-PETMR (ICC(3,1)) datasets. An ICC > 0.9 was considered stable among both types of PET scans.

RESULTS AND CONCLUSION

The 4DPETMR showed highest stability for shape, intensity, and texture (>80%) and lower stability for wavelet features (40%). Gradient-based method showed higher stability compared to threshold-based method except from shape features. In PETCT-PETMR, more than 61% of shape and intensity features were stable for both segmentation methods. However, a reduced stability was observed for texture (50%) and wavelet (<30%) features. More wavelet features were robust in the smoothed images (low-pass filtering) compared to images with emphasized heterogeneity (high-pass filtering). Comparing stable features of both investigations, highest agreement was found for intensity and lower agreement for shape, texture, and wavelet features. Only 53.6% of stable texture features in 4DPETMR were also stable in PETCT-PETMR, and even less in case of wavelet features (40.4%). Approximately 16.9% (texture) and 43.2% (wavelet) of stable PETCT-PETMR features are unstable in 4DPETMR. To conclude, shape and intensity features were robust when comparing two types of [18F]-FDG PET scans (PET/CT and PET/MR). Reduced stability was observed for texture and wavelet features. We identified multiple origins of instability of radiomic features, such as attenuation correction differences, different uptake times, and spatial resolution. This needs to be considered when models based on PET/CT are transferred PET/MR models or when combined models are used.

Applying Amide Proton Transfer MR Imaging to Hybrid Brain PET/MR: Concordance with Gadolinium Enhancement and Added Value to [18F]FDG PET

PURPOSE

The purpose of this study is to evaluate the diagnostic concordance and metric correlations of amide proton transfer (APT) imaging with gadolinium-enhanced magnetic resonance imaging (MRI) and 2-deoxy-2-[¹⁸F-]fluoro-D-glucose ([¹⁸F]FDG) positron emission tomography (PET), using hybrid brain PET/MRI.

PROCEDURES

Twenty-one subjects underwent brain gadolinium-enhanced [¹⁸F]FDG PET/MRI prospectively. Imaging accuracy was compared between unenhanced MRI, MRI with enhancement, APT-weighted (APTW) images, and PET based on six diagnostic criteria. Among tumors, the McNemar test was further used for concordance assessment between gadolinium-enhanced imaging, APT imaging, and [¹⁸F]FDG PET. As well, the relation of metrics between APT imaging and PET was analyzed by the Pearson correlation analysis.

RESULTS

APT imaging and gadolinium-enhanced MRI showed superior and similar diagnostic accuracy. APTW signal intensity and gadolinium enhancement were concordant in 19 tumors (100 %), while high [¹⁸F]FDG avidity was shown in only 12 (63.2 %). For the metrics from APT imaging and PET, there was significant correlation for 13 hypermetabolic tumors (P < 0.05) and no correlation for the remaining six [¹⁸F]FDG-avid tumors.

CONCLUSIONS

APT imaging can be used to increase diagnostic accuracy with no need to administer gadolinium chelates. APT imaging may provide an added value to [¹⁸F]FDG PET in the evaluation of tumor metabolic activity during brain PET/MR studies.

Deep Learning Based Attenuation Correction of PET/MRI in Pediatric Brain Tumor Patients: Evaluation in a Clinical Setting

Aim: Positron emission tomography (PET) imaging is a useful tool for assisting in correct differentiation of tumor progression from reactive changes. O-(2-18F-fluoroethyl)-L-tyrosine (FET)-PET in combination with MRI can add valuable information for clinical decision making. Acquiring FET-PET/MRI simultaneously allows for a one-stop-shop that limits the need for a second sedation or anesthesia as with PET and MRI in sequence. PET/MRI is challenged by lack of a direct measure of photon attenuation. Accepted solutions for attenuation correction (AC) might not be applicable to pediatrics. The aim of this study was to evaluate the use of the subject-specific MR-derived AC method RESOLUTE, modified to a pediatric cohort, against the performance of an MR-AC technique based on deep learning in a pediatric brain tumor cohort.

Methods: The modifications to RESOLUTE and the implementation of a deep learning method were performed using 79 pediatric patient examinations. We analyzed the 36 of these with active brain tumor area above 1 mL. We measured background (B), tumor mean and maximal activity (T_MEAN, T_MAX), biological tumor volume (BTV), and calculated the clinical metrics T_MEAN/B and T_MAX/B.

Results: Overall, we found both RESOLUTE and our DeepUTE methodologies to accurately reproduce the CT-AC clinical metrics. Regardless of age, both methods were able to obtain AC maps similar to the CT-AC, albeit with DeepUTE producing the most similar based on both quantitative metrics and visual inspection. In the patient-by-patient analysis DeepUTE was the only technique with all patients inside the predefined acceptable clinical limits. It also had a higher precision with relative %-difference to the reference CT-AC (T_MAX/B mean: -0.1%, CI: [-0.8%, 0.5%], p = 0.54) compared to RESOLUTE (T_MAX/B mean: 0.3%, CI: [-0.6%, 1.2%], p = 0.67) and DIXON-AC (T_MAX/B mean: 5.9%, CI: [4.5%, 7.4%], p < 0.0001).

Conclusion: Overall, we found DeepUTE to be the AC method that most robustly reproduced the CT-AC clinical metrics per se, closely followed by RESOLUTE modified to pediatric cohorts. The added accuracy due to better noise handling of DeepUTE, ease of use, as well as the improved runtime makes DeepUTE the method of choice for PET/MRI attenuation correction.

Quantitative accuracy of positron emission tomography/magnetic resonance and positron emission tomography/computed tomography for cervical cancer

With the spread of positron emission tomography/magnetic resonance (PET/MR), the question of comparability of studies becomes important. We aim to determine whether PET/MR and PET/computed tomography (PET/CT) are comparable for the case of cervical cancer. Fifteen cervical cancer patients identified by either a radiation oncologist or an oncologic surgeon had both PET/MR and PET/CT performed for initial staging within 3 weeks. We then compared the results both quantitatively (measuring standardized uptake values [SUVs] on visible lesions) as well as qualitatively (having radiologists and nuclear medicine physicians interprets the results). While interpretations between PET/MR and PET/CT varied in many cases, SUVs of primary lesions were similar to within 25% in all but one case, and correlation coefficient was 0.92. Maximum SUV ranged between 4.9 and 25.2 for PET-MR and between 5.8 and 30.4 for PET-CT for primary tumors and between 1.5 and 18.8 for PET-MR and between 1.8 and 20.8 for PET-CT for nodes. However, clinical reads often varied significantly between PET/MR and PET/CT. This suggests that SUV is similar on PET/MR and PET/CT although the differing anatomic modalities available for correlation may make the difference in terms of qualitative interpretation.

PET/MRI in Infection and Inflammation

Hybrid positron emission tomography/magnetic resonance imaging (PET/MR) systems are now more and more available for clinical use. PET/MR combines the unique features of MR including excellent soft tissue contrast, diffusion-weighted imaging, dynamic contrast-enhanced imaging, fMRI and other specialized sequences as well as MR spectroscopy with the quantitative physiologic information that is provided by PET. Most of the evidence of the potential clinical utility of PET/MRI is available for neuroimaging. Other areas, where PET/MR can play a larger role include head and neck, upper abdominal, and pelvic tumours. Although the role of PET/MR in infection and inflammation of the cardiovascular system and in musculoskeletal applications are promising, these areas of clinical investigation are still in the early phase and it may be a little longer before these areas reach their full potential in clinical practice. In this review, we outline the potential of hybrid PET/MR for imaging infection and inflammation. A background to the main radiopharmaceuticals and some technical considerations are also included.

Feasibility of Deep Learning-Based PET/MR Attenuation Correction in the Pelvis Using Only Diagnostic MR Images

This study evaluated the feasibility of using only diagnostically relevant magnetic resonance (MR) images together with deep learning for positron emission tomography (PET)/MR attenuation correction (deepMRAC) in the pelvis. Such an approach could eliminate dedicated MRAC sequences that have limited diagnostic utility but can substantially lengthen acquisition times for multibed position scans. We used axial T2 and T1 LAVA Flex magnetic resonance imaging images that were acquired for diagnostic purposes as inputs to a 3D deep convolutional neural network. The network was trained to produce a discretized (air, water, fat, and bone) substitute computed tomography (CT) (CTsub). Discretized (CTref-discrete) and continuously valued (CTref) reference CT images were created to serve as ground truth for network training and attenuation correction, respectively. Training was performed with data from 12 subjects. CTsub, CTref, and the system MRAC were used for PET/MR attenuation correction, and quantitative PET values of the resulting images were compared in 6 test subjects. Overall, the network produced CTsub with Dice coefficients of 0.79 ± 0.03 for cortical bone, 0.98 ± 0.01 for soft tissue (fat: 0.94 ± 0.0; water: 0.88 ± 0.02), and 0.49 ± 0.17 for bowel gas when compared with CTref-discrete. The root mean square error of the whole PET image was 4.9% by using deepMRAC and 11.6% by using the system MRAC. In evaluating 16 soft tissue lesions, the distribution of errors for maximum standardized uptake value was significantly narrower using deepMRAC (-1.0% ± 1.3%) than using system MRAC method (0.0% ± 6.4%) according to the Brown-Forsy the test (P < .05). These results indicate that improved PET/MR attenuation correction can be achieved in the pelvis using only diagnostically relevant MR images.

Geometry optimization of electrically floating PET inserts for improved RF penetration for a 3 T MRI system

PURPOSE

An electrically floating radio frequency (RF) shielded PET insert with individual PET detectors shielded by separate Faraday cages enables the MRI built-in body RF coil to be used at least as an RF transmitter, in which the RF field penetrates the imaging region inside the PET ring through the narrow gaps between the shielded PET detector modules. Because the shielded PET ring blocks more than 90% of the imaging region for the transmit field from the body RF coil, it is very challenging to obtain the required RF field inside a full-ring floating PET insert. In this study, experiments were performed on the dependence of RF penetrability on different geometric aspects of the shielded PET modules and PET rings to optimize the design parameters to obtain the required RF field inside the PET ring.

METHODS

We developed several prototype cylindrical full-ring PET inserts using completely enclosed empty RF shield boxes (considered as dummy PET modules). Considering the RF shield box, we conducted studies for different axial lengths (240 and 120 mm) and heights (30 and 45 mm) of the shield boxes. On the other hand, considering the PET ring geometry, we also performed studies on three different categories of PET rings: a long-ring insert (longer than the MRI phantom), a short-ring insert (shorter than the MRI phantom), and a two-ring insert that combined two short-rings. In each ring category, two different inter-shield box gaps (1 and 3 mm) were considered. In the case of the two-ring insert, three different ring-gaps (5, 10, and 20 mm) were studied. In total, 21 PET inserts were studied with an inner diameter (i.d.) of 210 mm. To study the effect of ring diameter, another long-ring insert was studied for the 270 mm i.d. Experiments were conducted for the transmit RF (B₁ ) fields and signal-to-noise ratios of spin-echo and gradient-echo images using a homogeneous phantom in a 700 mm bore-diameter 3 T clinical MRI system. RF pulse amplitudes generated automatically by the MRI system were recorded for comparison.

RESULTS

A PET insert with a 3 mm inter-box gap was found to perform the best, at a level which is acceptable for PET imaging. In the case of an insert of multiple short-rings instead of one long-ring insert, the 5 and 10 mm ring-gaps provided higher RF field penetration. Increasing the inter-box gap improved the RF field penetration, whereas a ring-gap that was too wide concentrated the field near the ring-gap region. Relatively reduced RF power was required for wider inter-box gap or ring-gap or larger shield box height. Moreover, the rectangular shield box outperformed the trapezoidal shield box. On the other hand, when we changed the inner or outer diameter of the PET ring by keeping the same transaxial width of the shield boxes, we did not see any noticeable variation.

CONCLUSIONS

Our study results provide comprehensive guidance on the geometrical design aspects of RF-penetrable PET inserts for efficient RF penetration inside the PET ring. By choosing proper geometric design parameters, we could get the RF field that was similar to the MRI-only case.

Prospective comparison of 18F-FDG PET/MRI and 18F-FDG PET/CT for thoracic staging of non-small cell lung cancer

OBJECTIVES

To compare the diagnostic performance of ¹⁸F-FDG PET/MRI and ¹⁸F-FDG PET/CT for primary and locoregional lymph node staging in non-small cell lung cancer (NSCLC).

METHODS

In this prospective study, a total of 84 patients (51 men, 33 women, mean age 62.5 ± 9.1 years) with histopathologically confirmed NSCLC underwent ¹⁸F-FDG PET/CT followed by ¹⁸F-FDG PET/MRI in a single injection protocol. Two readers independently assessed T and N staging in separate sessions according to the seventh edition of the American Joint Committee on Cancer staging manual for ¹⁸F-FDG PET/CT and ¹⁸F-FDG PET/MRI, respectively. Histopathology as a reference standard was available for N staging in all 84 patients and for T staging in 39 patients. Differences in staging accuracy were assessed by McNemars chi² test. The maximum standardized uptake value (SUV_max) and longitudinal diameters of primary tumors were correlated using Pearson's coefficients.

RESULTS

T stage was categorized concordantly in ¹⁸F-FDG PET/MRI and ¹⁸F-FDG PET/CT in 38 of 39 (97.4%) patients. Herein, ¹⁸F-FDG PET/CT and ¹⁸F-FDG PET/MRI correctly determined the T stage in 92.3 and 89.7% of patients, respectively. N stage was categorized concordantly in 83 of 84 patients (98.8%). ¹⁸F-FDG PET/CT correctly determined the N stage in 78 of 84 patients (92.9%), while ¹⁸F-FDG PET/MRI correctly determined the N stage in 77 of 84 patients (91.7%). Differences between ¹⁸F-FDG PET/CT and ¹⁸F-FDG PET/MRI in T and N staging accuracy were not statistically significant (p > 0.5, each). Tumor size and SUV_max measurements derived from both imaging modalities exhibited excellent correlation (r = 0.963 and r = 0.901, respectively).

CONCLUSION

¹⁸F-FDG PET/MRI and ¹⁸F-FDG PET/CT show an equivalently high diagnostic performance for T and N staging in patients suffering from NSCLC.

Early Response Assessment in Pancreatic Ductal Adenocarcinoma Through Integrated PET/MRI

OBJECTIVE

The purpose of this study is to investigate early changes in ¹⁸F-FDG PET/MRI metrics after treatment in patients with advanced pancreatic ductal adenocarcinoma (PDAC) and to correlate those changes with eventual tumor response at standard-of-care CT.

SUBJECTS AND METHODS

Thirteen patients with advanced PDAC underwent integrated FDG PET/MRI before and 4 weeks after treatment initiation. Patients were classified as responders or nonresponders according to Response Evaluation Criteria in Solid Tumors (RECIST) 1.1 at subsequent CT performed 8-12 weeks after treatment initiation. Changes in the primary tumor's maximum standardized uptake value (SUV_max), metabolic tumor volume (MTV), and total lesion glycolysis (TLG) determined at PET and apparent diffusion coefficient (ADC) determined at DWI at 4 weeks were compared between responders and nonresponders.

RESULTS

Seven patients had a partial response according to RECIST, and six did not. Responders displayed significantly greater decreases in MTV (p = 0.003) and TLG (p = 0.006) in the primary pancreatic tumor at 4 weeks. Responders also displayed a greater increase in the mean (p = 0.004) and minimum (p = 0.024) ADC of the primary tumors. Tumor size change at 4 weeks was not significantly different between responders and nonresponders (p = 0.11). PET responders enjoyed longer progression-free survival (PFS) (p = 0.0004) and overall survival (OS) (p = 0.013) than did nonresponders, using either a 60% reduction in MTV or 65% reduction in TLG as a threshold. MRI responders had significantly longer PFS (p = 0.0002) and OS (p = 0.027) than did nonresponders, using a 20% increase in either mean or minimum ADC as a threshold.

CONCLUSION

Integrated PET/MRI can provide early response assessment in patients with advanced PDAC, thus potentially allowing early treatment adaptation in nonresponders.

Clinical PET/MRI: 2018 Update

OBJECTIVE

The purpose of this article is to provide an update on clinical PET/MRI, including current and developing clinical indications and technical developments.

CONCLUSION

PET/MRI is evolving rapidly, transitioning from a predominant research focus to exciting clinical practice. Key technical obstacles have been overcome, and further technical advances promise to herald significant advancements in image quality. Further optimization of protocols to address challenges posed by this hybrid modality will ensure the long-term success of PET/MRI.

A comparison of 18F-FDG PET/MR with PET/CT in pulmonary tuberculosis

PURPOSE

PET/computed tomography (CT) has been shown to detect lesions in patients with pulmonary tuberculosis (PTB) and may be useful for assessing PTB disease in clinical research studies. However, radiation dose is of concern for clinical research in individuals with an underlying curable disease. This study aimed to determine whether PET/MR is equivalent to PET/CT in PTB.

MATERIALS AND METHODS

Ten patients with microbiologically confirmed PTB were recruited. Patients received 129.0±4.1 MBq of fluorine-18-fluorodeoxyglucose. Five of the 10 patients underwent a PET/MR scan, followed by PET/CT. The remaining five were first imaged on the PET/CT, followed by the PET/MRI. PET acquisition began at 66.7±14.4 min (mean±SD) after injection when performing PET/MR first (PET/CT: 117.2±5.6 min) and 92.4±7.6 min when patients were imaged on PET/MR second (PET/CT: 61.1±3.9 min). PET data were reconstructed iteratively with Ordinary-Poisson Ordered-Subset Expectation-Maximization and reconstruction parameters were matched across the two scanners. A visual lesion detection task and a standardized uptake value (SUV) analysis were carried out. The CT Hounsfield unit values of PTB lesions were also compared with MR-based attenuation correction mu-map tissue classes.

RESULTS

A total of 108 PTB lesions were detected on PET/MR and 112 on PET/CT. SUV analysis was carried out on 50 of these lesions that were observed with both modalities. Mean standardized uptake value (SUVmean) and maximum standardized uptake value (SUVmax) were significantly lower on PET/MR (SUVmean: 2.6±1.4; SUVmax: 4.3±2.5) than PET/CT (SUVmean: 3.5±1.5; SUVmax: 5.3±2.4).

CONCLUSION

PET/MR visual performance was shown to be comparable to PET/CT in terms of the number of PTB lesions detected. SUVs were significantly lower on PET/MR. Dixon-based attenuation correction underestimates the linear attenuation coefficient of PTB lesions, resulting in lower SUVs compared with PET/CT. However, the use of PET/MR to measure the response of lung lesions to assess response to treatment in research studies is unlikely to be affected by these differences in quantification.

Clinical utility of simultaneous whole-body 18F-FDG PET/MRI as a single-step imaging modality in the staging of primary nasopharyngeal carcinoma

PURPOSE

Both head and neck magnetic resonance imaging (MRI) and 18F-fluorodeoxyglucose (18F-FDG) positron emission tomography (PET)/computed tomography (CT) play a crucial role in the staging of primary nasopharyngeal carcinoma (NPC). In this study, we sought to prospectively investigate the clinical utility of simultaneous whole-body 18F-FDG PET/MRI for primary staging of NPC patients.

METHODS

We examined 113 patients with histologically confirmed NPC who underwent pretreatment, simultaneous whole-body PET/MRI and PET/CT for primary tumor staging. The images obtained with the different imaging modalities were interpreted independently and compared with each other.

RESULTS

PET/MRI increased the accuracy of head and neck MRI for assessment of primary tumor extent in four patients via addition of FDG uptake information to increase the conspicuity of morphologically subtle lesions. PET/MR images were more discernible than PET/CT images for mapping tumor extension, especially intracranial invasion. Regarding the N staging assessment, the sensitivity of PET/MRI (99.5%) was higher than that of head and neck MRI (94.2%) and PET/CT (90.9%). PET/MRI was particularly useful for distinguishing retropharyngeal nodal metastasis from adjacent nasopharyngeal tumors. For distant metastasis evaluation, PET/MRI exhibited a similar sensitivity (90% vs. 86.7% vs. 83.3%), but higher positive predictive value (93.1% vs. 78.8% vs. 83.3%) than whole-body MRI and PET/CT, respectively.

CONCLUSIONS

For tumor staging of NPC, simultaneous whole-body PET/MRI was more accurate than head and neck MRI and PET/CT, and may serve as a single-step staging modality.

Whole-body PET/MRI of Pediatric Patients: The Details That Matter

Integrated PET/MRI is a hybrid imaging technique enabling clinicians to acquire diagnostic images for tumor assessment and treatment monitoring with both high soft tissue contrast and added metabolic information. Integrated PET/MRI has shown to be valuable in the clinical setting and has many promising future applications. The protocol presented here will provide step-by-step instructions for the acquisition of whole-body 2-deoxy-2-(¹⁸F)fluoro-D-glucose (¹⁸F-FDG) PET/MRI data in children with cancer. It also provides instructions on how to combine a whole-body staging scan with a local tumor scan for evaluation of the primary tumor. The focus of this protocol is to be both comprehensive and time-efficient, which are two ubiquitous needs for clinical applications. This protocol was originally developed for children above 6 years, or old enough to comply with breath-hold instructions, but can also be applied to patients under general anesthesia. Similarly, this protocol can be modified to fit institutional preferences in terms of choice of MRI pulse sequences for both the whole-body scan and local tumor assessment.

Integration of PET/MR Hybrid Imaging into Radiation Therapy Treatment

Hybrid PET/MR imaging is in early development for treatment planning. This article briefly reviews research and clinical applications of PET/MR imaging in radiation oncology. With improvements in workflow, more specific tracers, and fast and robust acquisition protocols, PET/MR imaging will play an increasingly important role in better target delineation for treatment planning and have clear advantages in the evaluation of tumor response and in a better understanding of tumor heterogeneity. With advances in treatment delivery and the potential of integrating PET/MR imaging with research on radiomics for radiation oncology, quantitative and physiologic information could lead to more precise and personalized RT.