Diagnostic Performance of Whole-Body PET/MRI for Detecting Malignancies in Cancer Patients: A Meta-Analysis

MRI compatibility study of a prototype radiofrequency penetrable oval PET insert at 3 T

PURPOSE

To perform an MRI compatibility study of an RF field-penetrable oval-shaped PET insert that implements an MRI built-in body RF coil both as a transmitter and a receiver.

METHODS

Twelve electrically floating RF shielded PET detector modules were used to construct the prototype oval PET insert with a major axis of 440 mm, a minor axis of 350 mm, and an axial length of 225 mm. The electric floating of the PET detector modules was accomplished by isolating the cable shield from the detector shield using plastic tape. Studies were conducted on the transmit (B1) RF field, the image signal-to-noise ratio (SNR), and the RF pulse amplitude for a homogeneous cylindrical (diameter: 160 mm and length: 260 mm) phantom (NaCl + NiSO4 solution) in a 3 T clinical MRI system (Verio, Siemens, Erlangen, Germany).

RESULTS

The B1 maps for the oval insert were similar to the MRI-only field responses. Compared to the MRI-only values, SNR reductions of 51%, 45%, and 59% were seen, respectively, for the spin echo (SE), gradient echo (GE), and echo planar (EPI) images for the case of oval PET insert. Moreover, the required RF pulse amplitudes for the SE, GE, and EPI sequences were, respectively, 1.93, 1.85, and 1.36 times larger. However, a 30% reduction in the average RF reception sensitivity was observed for the oval insert.

CONCLUSIONS

The prototype floating PET insert was a safety concern for the clinical MRI system, and this compatibility study provided clearance for developing a large body size floating PET insert for the existing MRI system. Because of the RF shield of the insert, relatively large RF powers compared to the MRI-only case were required. Because of this and also due to low RF sensitivity of the body coil, the SNRs reduced largely.

Study on the radiofrequency transparency of partial-ring oval-shaped prototype PET inserts in a 3 T clinical MRI system

The purpose of this study is to evaluate the RF field responses of partial-ring RF-shielded oval-shaped positron emission tomography (PET) inserts that are used in combination with an MRI body RF coil. Partial-ring PET insert is particularly suitable for interventional investigation (e.g., trimodal PET/MRI/ultrasound imaging) and intraoperative (e.g., robotic surgery) PET/MRI studies. In this study, we used electrically floating Faraday RF shield cages to construct different partial-ring configurations of oval and cylindrical PET inserts and performed experiments on the RF field, spin echo and gradient echo images for a homogeneous phantom in a 3 T clinical MRI system. For each geometry, partial-ring configurations were studied by removing an opposing pair or a single shield cage from different positions of the PET ring. Compared to the MRI-only case, reduction in mean RF homogeneity, flip angle, and SNR for the detector opening in the first and third quadrants was approximately 13%, 15%, and 43%, respectively, whereas the values were 8%, 23%, and 48%, respectively, for the detector openings in the second and fourth quadrants. The RF field distribution also varied for different partial-ring configurations. It can be concluded that the field penetration was high for the detector openings in the first and third quadrants of both the inserts.

Diagnostic performance of whole-body [18F]FDG PET/MR in cancer M staging: A systematic review and meta-analysis

OBJECTIVES

To calculate the pooled diagnostic performances of whole-body [18F]FDG PET/MR in M staging of [18F]FDG-avid cancer entities.

METHODS

A diagnostic meta-analysis was conducted on the [18F]FDG PET/MR in M staging, including studies: (1) evaluated [18F]FDG PET/MR in detecting distant metastasis; (2) compared[ 18F]FDG PET/MR with histopathology, follow-up, or asynchronous multimodality imaging as the reference standard; (3) provided data for the whole-body evaluation; (4) provided adequate data to calculate the meta-analytic performances. Pooled performances were calculated with their confidence interval. In addition, forest plots, SROC curves, and likelihood ratio scatterplots were drawn. All analyses were performed using STATA 16.

RESULTS

From 52 eligible studies, 2289 patients and 2072 metastases were entered in the meta-analysis. The whole-body pooled sensitivities were 0.95 (95%CI: 0.91-0.97) and 0.97 (95%CI: 0.91-0.99) at the patient and lesion levels, respectively. The pooled specificities were 0.99 (95%CI: 0.97-1.00) and 0.97 (95%CI: 0.90-0.99), respectively. Additionally, subgroup analyses were performed. The calculated pooled sensitivities for lung, gastrointestinal, breast, and gynecological cancers were 0.90, 0.93, 1.00, and 0.97, respectively. The pooled specificities were 1.00, 0.98, 0.97, and 1.00, respectively. Furthermore, the pooled sensitivities for non-small cell lung, colorectal, and cervical cancers were 0.92, 0.96, and 0.86, respectively. The pooled specificities were 1.00, 0.95, and 1.00, respectively.

CONCLUSION

[18F]FDG PET/MR was a highly accurate modality in M staging in the reported [18F]FDG-avid malignancies. The results showed high sensitivity and specificity in each reviewed malignancy type. Thus, our findings may help clinicians and patients to be confident about the performance of [18F]FDG PET/MR in the clinic.

CLINICAL RELEVANCE STATEMENT

Although [18F]FDG PET/MR is not a routine imaging technique in current guidelines, mostly due to its availability and logistic issues, our findings might add to the limited evidence regarding its performance, showing a sensitivity of 0.95 and specificity of 0.97.

KEY POINTS

• The whole-body [18F]FDG PET/MR showed high accuracy in detecting distant metastases at both patient and lesion levels. • The pooled sensitivities were 95% and 97% and pooled specificities were 99% and 97% at patient and lesion levels, respectively. • The results suggested that 18F-FDG PET/MR was a strong modality in the exclusion and confirmation of distant metastases.

18F-FDG PET/MRI in Detection of Pulmonary Malignancies: A Systematic Review and Meta-Analysis

Background There have been conflicting results regarding fluorine 18-labeled fluorodeoxyglucose (¹⁸F-FDG) PET/MRI diagnostic performance in lung malignant neoplasms. Purpose To evaluate the diagnostic performance of ¹⁸F-FDG PET/MRI for the detection of pulmonary malignant neoplasms. Materials and Methods A systematic search was conducted within the Scopus, Web of Science, and PubMed databases until December 31, 2021. Published original articles that met the following criteria were considered eligible for meta-analysis: (a) detecting malignant lesions in the lung, (b) comparing ¹⁸F-FDG PET/MRI with a valid reference standard, and (c) providing data for the meta-analytic calculations. A hierarchical method was used to pool the performances. The bivariate model was used to find the summary points and 95% CIs. The hierarchical summary receiver operating characteristic model was used to draw the summary receiver operating characteristic curve and calculate the area under the curve. The Higgins I² statistic and Cochran Q test were used for heterogeneity assessment. Results A total of 43 studies involving 1278 patients met the inclusion criteria and were included in the meta-analysis. ¹⁸F-FDG PET/MRI had a pooled sensitivity and specificity of 96% (95% CI: 84, 99) and 100% (95% CI: 98, 100), respectively. ¹⁸F-FDG PET/CT had a pooled sensitivity and specificity of 99% (95% CI: 61, 100) and 99% (95% CI: 94, 100), respectively, which were comparable with those of ¹⁸F-FDG PET/MRI. At meta-regression, studies in which contrast media (P = .03) and diffusion-weighted imaging (P = .04) were used as a part of a pulmonary ¹⁸F-FDG PET/MRI protocol showed significantly higher sensitivities. Conclusion Fluorine 18-labeled fluorodeoxyglucose (¹⁸F-FDG) PET/MRI was found to be accurate and comparable with ¹⁸F-FDG PET/CT in the detection of malignant pulmonary lesions, with significantly improved sensitivity when advanced acquisition protocols were used. © RSNA, 2023 Supplemental material is available for this article.

Study on the RF transparency of electrically floating and ground PET inserts in a 3T clinical MRI system

PURPOSE

The positron emission tomography (PET) insert for a magnetic resonance imaging (MRI) system that implements the radiofrequency (RF) built-in body coil of the MRI system as a transmitter is designed to be RF-transparent, as the coil resides outside the RF-shielded PET ring. This approach reduces the design complexities (e.g., large PET ring diameter) related to implementing a transmit coil inside the PET ring. However, achieving the required field transmission into the imaging region of interest (ROI) becomes challenging because of the RF shield of the PET insert. In this study, a modularly RF-shielded PET insert is used to investigate the RF transparency considering two electrical configurations of the RF shield, namely the electrical floating and ground configurations. The purpose is to find the differences, advantages and disadvantages of these two configurations.

METHODS

Eight copper-shielded PET detector modules (intermodular gap: 3 mm) were oriented cylindrically with an inner-diameter of 234 mm. Each PET module included four-layer LYSO scintillation crystal blocks and front-end readout electronics. RF-shielded twisted-pair cables were used to connect the front-end electronics with the power sources and PET data acquisition systems located outside the MRI room. In the ground configuration, both the detector and cable shields were connected to the RF ground of the MRI system. In the floating configuration, only the RF shields of the PET modules were isolated from the RF ground. Experiments were conducted using two cylindrical homogeneous phantoms in a 3T clinical MRI system, in which the built-in body RF coil (a cylindrical volume coil of diameter 700 mm and length 540 mm) was implemented as a transceiver.

RESULTS

For both PET configurations, the RF and MR imaging performances were lower than those for the MRI-only case, and the MRI-system provided SAR values that were almost double. The RF homogeneity and field strength, and the SNR of the MR images were mostly higher for the floating PET configuration than they were for the ground PET configuration. However, for a shorter axial FOV of 125 mm, both configurations offered almost the same performance with high RF homogeneities (e.g., 76 ± 10%). Moreover, for both PET configurations, 56 ± 6% larger RF pulse amplitudes were required for MR imaging purposes. The increased power is mostly absorbed in the conductive shields in the form of shielding RF eddy currents; as a result, the SAR values only in the phantoms were estimated to be close to the MRI-only values.

CONCLUSIONS

The floating PET configuration showed higher RF transparency under all experimental setups. For a relatively short axial FOV of 125 mm, the ground configuration also performed well which indicated that an RF-penetrable PET insert with the conventional design (e.g., the ground configuration) might also become possible. However, some design modifications (e.g., a wider intermodular gap and using the RF receiver coil inside the PET insert) should improve the RF performance to the level of the MRI-only case. This article is protected by copyright. All rights reserved.

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.

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.

The Landscape of Clinical Trials Evaluating the Theranostic Role of PET Imaging in Oncology: Insights from an Analysis of ClinicalTrials.gov Database

In the war on cancer marked by personalized medicine, positron emission tomography (PET)-based theranostic strategy is playing an increasingly important role. Well-designed clinical trials are of great significance for validating the PET applications and ensuring evidence-based cancer care. This study aimed to provide a comprehensive landscape of the characteristics of PET clinical trials using the substantial resource of ClinicalTrials.gov database. We identified 25,599 oncology trials registered with ClinicalTrials.gov in the last ten-year period (October 2005-September 2015). They were systematically reviewed to validate classification into 519 PET trials and 25,080 other oncology trials used for comparison. We found that PET trials were predominantly phase 1-2 studies (86.2%) and were more likely to be single-arm (78.9% vs. 57.9%, P <0.001) using non-randomized assignment (90.1% vs. 66.7%, P <0.001) than other oncology trials. Furthermore, PET trials were small in scale, generally enrolling fewer than 100 participants (20.3% vs. 25.7% for other oncology trials, P = 0.014), which might be too small to detect a significant theranostic effect. The funding support from industry or National Institutes of Health shrunk over time (both decreased by about 5%), and PET trials were more likely to be conducted in only one region lacking international collaboration (97.0% vs. 89.3% for other oncology trials, P <0.001). These findings raise concerns that clinical trials evaluating PET imaging in oncology are not receiving the attention or efforts necessary to generate high-quality evidence. Advancing the clinical application of PET imaging will require a concerted effort to improve the quality of trials.