Clinical and research applications of simultaneous positron emission tomography and MRI

The utility of PET imaging in depression

This educational review article aims to discuss growing evidence from PET studies in the diagnosis and treatment of depression. PET has been used in depression to explore the neurotransmitters involved, the alterations in neuroreceptors, non-neuroreceptor targets (e.g., microglia and astrocytes), the severity and duration of the disease, the pharmacodynamics of various antidepressants, and neurobiological mechanisms of non-pharmacological therapies like psychotherapy, electroconvulsive therapy, and deep brain stimulation therapy, by showing changes in brain metabolism and receptor and non-receptor targets. Studies have revealed alterations in neurotransmitter systems such as serotonin, dopamine, GABA, and glutamate, which are linked to the pathophysiology of depression. Overall, PET imaging has furthered the neurobiological understanding of depression. Despite these advancements, PET findings have not yet led to significant changes in evidence-based practices. Addressing the reasons behind inconsistencies in PET imaging results, conducting large sample size studies with a more standardized methodological approach, and investigating further the genetic and neurobiological aspects of depression may better leverage PET imaging in future studies.

Brain tumor segmentation of MRI images: A comprehensive review on the application of artificial intelligence tools

BACKGROUND

Brain cancer is a destructive and life-threatening disease that imposes immense negative effects on patients' lives. Therefore, the detection of brain tumors at an early stage improves the impact of treatments and increases the patients survival rates. However, detecting brain tumors in their initial stages is a demanding task and an unmet need.

METHODS

The present study presents a comprehensive review of the recent Artificial Intelligence (AI) methods of diagnosing brain tumors using MRI images. These AI techniques can be divided into Supervised, Unsupervised, and Deep Learning (DL) methods.

RESULTS

Diagnosing and segmenting brain tumors usually begin with Magnetic Resonance Imaging (MRI) on the brain since MRI is a noninvasive imaging technique. Another existing challenge is that the growth of technology is faster than the rate of increase in the number of medical staff who can employ these technologies. It has resulted in an increased risk of diagnostic misinterpretation. Therefore, developing robust automated brain tumor detection techniques has been studied widely over the past years.

CONCLUSION

The current review provides an analysis of the performance of modern methods in this area. Moreover, various image segmentation methods in addition to the recent efforts of researchers are summarized. Finally, the paper discusses open questions and suggests directions for future research.

Clinical usefulness of PET/MRI in differentiating anterior mediastinal masses

OBJECTIVES

To investigate the clinical usefulness of 18F-fluorodeoxyglucose (FDG) PET/MRI in differentiating anterior mediastinal lesions, including small ones.

MATERIAL AND METHODS

Among 96 patients who underwent 18F-FDG PET/MRI screening for anterior mediastinal lesions, we retrospectively reviewed images of 42 patients with histologically or clinically diagnosed thymic carcinomas, thymomas or anterior mediastinal cysts. MRI findings and maximum standardized uptake value (SUVmax) were compared among the three categories. In addition, small lesions measuring <3.0 cm which did not show very high signal intensity (isointense to water) on T2 weighted images (T2WI) were sub-analyzed.

RESULTS

Significant differences in SUVmax were observed among anterior mediastinal cysts (P < 0.001, vs. thymomas and thymic carcinomas), thymomas (P = 0.032, vs. thymic carcinomas) and thymic carcinomas. Regarding the MRI findings, anterior mediastinal cysts showed higher T2WI signal intensity (P = 0.004 vs. thymomas and P = 0.042 vs. thymic carcinomas) and thymic carcinomas tended to show ill-defined contours (P = 0.024 vs. anterior mediastinal cysts and P = 0.036 vs. thymomas). SUVmax was also significantly higher in small thymic tumors than small anterior mediastinal cysts without very high T2WI signal intensity (P = 0.003).

CONCLUSION

18F-FDG PET/MRI is clinically useful in differentiating anterior mediastinal lesions, including those smaller than 3 cm.

Imaging and treatment of brain tumors through molecular targeting: Recent clinical advances

Molecular imaging techniques have rapidly progressed over recent decades providing unprecedented in vivo characterization of metabolic pathways and molecular biomarkers. Many of these new techniques have been successfully applied in the field of neuro-oncological imaging to probe tumor biology. Targeting specific signaling or metabolic pathways could help to address several unmet clinical needs that hamper the management of patients with brain tumors. This review aims to provide an overview of the recent advances in brain tumor imaging using molecular targeting with positron emission tomography and magnetic resonance imaging, as well as the role in patient management and possible therapeutic implications.

Feasibility Study of a Time-of-Flight Brain Positron Emission Tomography Employing Individual Channel Readout Electronics

The purpose of this study was to investigate the feasibility of a time-of-flight (TOF) brain positron emission tomography (PET) providing high-quality images. It consisted of 30 detector blocks arranged in a ring with a diameter of 257 mm and an axial field of view of 52.2 mm. Each detector block was composed of two detector modules and two application-specific integrated circuit (ASIC) chips. The detector module was composed of an 8 × 8 array of 3 × 3 mm² multi-pixel photon counters and an 8 × 8 array of 3.11 × 3.11 × 15 mm³ lutetium yttrium oxyorthosilicate scintillators. The 64-channel individual readout ASIC was used to acquire the position, energy, and time information of a detected gamma ray. A coincidence timing resolution of 187 ps full width at half maximum (FWHM) was achieved using a pair of channels of two detector modules. The energy resolution and spatial resolution were 6.6 ± 0.6% FWHM (without energy nonlinearity correction) and 2.5 mm FWHM, respectively. The results of this study demonstrate that the developed TOF brain PET could provide excellent performance, allowing for a reduction in radiation dose or scanning time for brain imaging due to improved sensitivity and signal-to-noise ratio.

Multiple Myeloma: Role of Imaging in Diagnosis, Staging, and Treatment Response Assessment

Multiple myeloma is a common hematologic malignancy of plasma cells. Differentiating multiple myeloma from the precursor stages of monoclonal gammopathy of undetermined significance and smoldering multiple myeloma is very important because the treatment approach is different for each. The diagnosis is mainly clinical, while the role of imaging is confined to the staging process, assessing response to therapy, and monitoring for disease progression. In this article, we examine the role of different imaging modalities in patients with multiple myeloma.

Comparison between simultaneously acquired arterial spin labeling and 18F-FDG PET in mesial temporal lobe epilepsy assisted by a PET/MR system and SEEG

Objective

In the detection of seizure onset zones, arterial spin labeling (ASL) can overcome the limitations of positron emission tomography (PET) with ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG), which is invasive, expensive, and radioactive. PET/magnetic resonance (MR) systems have been introduced that allow simultaneous performance of ASL and PET, but comparisons of these techniques with stereoelectroencephalography (SEEG) and comparisons among the treatment outcomes of these techniques are still lacking. Here, we investigate the effectiveness of ASL compared with that of SEEG and their outcomes in localizing mesial temporal lobe epilepsy (MTLE) and assess the correlation between simultaneously acquired PET and ASL.

Methods

Between October 2016 and August 2017, we retrospectively studied 12 patients diagnosed with pure unilateral MTLE. We extracted and quantitatively computed values for ASL and PET in the bilateral hippocampus. SEEG findings and outcome were considered the gold standard of lateralization. Finally, the bilateral asymmetry index (AI) was calculated to assess the correlation between PET and ASL.

Results

Our results showed that hypoperfusion in the hippocampus detected using ASL matched the SEEG-defined epileptogenic zone in this series of patients. The mean normalized voxel value of ASL in the contralateral hippocampus was 0.97 ± 0.19, while in the ipsilateral hippocampus, it was 0.84 ± 0.14. Meanwhile, significantly decreased perfusion and metabolism were observed in these patients (Wilcoxon, p < 0.05), with a significant positive correlation between the AI values derived from PET and ASL (Pearson's correlation, r = 0.74, p < 0.05).

Significance

In our SEEG- and outcome-defined patients with MTLE, ASL could provide significant information during presurgical evaluation, with the hypoperfusion detected with ASL reliably lateralizing MTLE. This non-invasive technique may be used as an alternative diagnostic tool for MTLE lateralization.

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ASL has been increasingly used in presurgical evaluations in epilepsy recent years.

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Comparisons of ASL and PET with a PET/MR system using SEEG and treatment outcomes as gold-standard are still lacking.

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Decreased perfusion consistent with hypometabolism and SEEG was observed with ASL.

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ASL offers an effective non-invasive alternative to PET in evaluation of MTLE.

Current concepts in tumor imaging with whole-body MRI with diffusion imaging (WB-MRI-DWI) in multiple myeloma and lymphoma

Whole-body MRI (WB-MRI) with diffusion-weighted imaging (DWI) can now be used to stage and restage multiple myeloma (MM) and lymphoma. Magnetic resonance imaging (MRI) is the standard tool to detect BM involvement (BMI). The 2016 diagnostic criteria of the International Myeloma Working Group identify WB-MRI and fluorodeoxyglucose positron emission tomography/computed tomography (FDG-PET/CT) as the most sensitive imaging techniques for detecting skeletal and extra-skeletal MM invasion, respectively. Preliminary findings have also shown that WB-MRI is better than CT and equal to PET/CT in staging aggressive lymphoma and Hodgkin lymphoma, whereas MRI is better for diagnosing BMI in patients with low-grade lymphoma. Signal intensity (SI) and the apparent diffusion coefficient (ADC) are useful metrics to quantify the chemotherapy response in WB-MRI.

Recent advances in the management of multiple myeloma: clinical impact based on resource-stratification. Consensus statement of the Asian Myeloma Network at the 16th international myeloma workshop

Predicated on our improved understanding of the disease biology, we have seen remarkable advances in the management of multiple myeloma over the past few years. Recently approved drugs have radically transformed the treatment paradigm and improved survivals of myeloma patients. The progress has necessitated revision of the diagnostic criteria, risk-stratification and response definition. The huge disparities in economy, healthcare infrastructure and access to novel drugs among different Asian countries will hinder the delivery of optimum myeloma care to patients managed in resource-constrained environments. In the light of the tremendous recent changes and evolution in myeloma management, it is timely that the resource-stratified guidelines from the Asian Myeloma Network be revised to provide updated recommendations for Asia physicians practicing under various healthcare reimbursement systems. This review will highlight the most recent advances and our recommendations on how they could be integrated in both resource-abundant and resource-constrained facilities.

International Myeloma Working Group consensus criteria for response and minimal residual disease assessment in multiple myeloma

Treatment of multiple myeloma has substantially changed over the past decade with the introduction of several classes of new effective drugs that have greatly improved the rates and depth of response. Response criteria in multiple myeloma were developed to use serum and urine assessment of monoclonal proteins and bone marrow assessment (which is relatively insensitive). Given the high rates of complete response seen in patients with multiple myeloma with new treatment approaches, new response categories need to be defined that can identify responses that are deeper than those conventionally defined as complete response. Recent attempts have focused on the identification of residual tumour cells in the bone marrow using flow cytometry or gene sequencing. Furthermore, sensitive imaging techniques can be used to detect the presence of residual disease outside of the bone marrow. Combining these new methods, the International Myeloma Working Group has defined new response categories of minimal residual disease negativity, with or without imaging-based absence of extramedullary disease, to allow uniform reporting within and outside clinical trials. In this Review, we clarify several aspects of disease response assessment, along with endpoints for clinical trials, and highlight future directions for disease response assessments.

A 32 mm × 32 mm × 22 mm monolithic LYSO:Ce detector with dual-sided digital photon counter readout for ultrahigh-performance TOF-PET and TOF-PET/MRI

New applications for positron emission tomography (PET) and combined PET/magnetic resonance imaging (MRI) are currently emerging, for example in the fields of neurological, breast, and pediatric imaging. Such applications require improved image quality, reduced dose, shorter scanning times, and more precise quantification. This can be achieved by means of dedicated scanners based on ultrahigh-performance detectors, which should provide excellent spatial resolution, precise depth-of-interaction (DOI) estimation, outstanding time-of-flight (TOF) capability, and high detection efficiency. Here, we introduce such an ultrahigh-performance TOF/DOI PET detector, based on a 32 mm  ×  32 mm  ×  22 mm monolithic LYSO:Ce crystal. The 32 mm  ×  32 mm front and back faces of the crystal are coupled to a digital photon counter (DPC) array, in so-called dual-sided readout (DSR) configuration. The fully digital detector offers a spatial resolution of ~1.1 mm full width at half maximum (FWHM)/~1.2 mm mean absolute error, together with a DOI resolution of ~2.4 mm FWHM, an energy resolution of 10.2% FWHM, and a coincidence resolving time of 147 ps FWHM. The time resolution closely approaches the best results (135 ps FWHM) obtained to date with small crystals made from the same material coupled to the same DPC arrays, illustrating the excellent correction for optical and electronic transit time spreads that can be achieved in monolithic scintillators using maximum-likelihood techniques for estimating the time of interaction. The performance barely degrades for events with missing data (up to 6 out of 32 DPC dies missing), permitting the use of almost all events registered under realistic acquisition conditions. Moreover, the calibration procedures and computational methods used for position and time estimation follow recently made improvements that make them fast and practical, opening up realistic perspectives for using DSR monolithic scintillator detectors in TOF-PET and TOF-PET/MRI systems.

Cerebral metabolism and perfusion in MR-negative individuals with refractory focal epilepsy assessed by simultaneous acquisition of (18)F-FDG PET and arterial spin labeling

The major challenge in pre-surgical epileptic patient evaluation is the correct identification of the seizure onset area, especially in MR-negative patients. In this study, we aimed to: (1) assess the concordance between perfusion, from ASL, and metabolism, from ¹⁸F-FDG, acquired simultaneously on PET/MR; (2) verify the utility of a statistical approach as supportive diagnostic tool for clinical readers. Secondarily, we compared ¹⁸F-FDG PET data from the hybrid PET/MR system with those acquired with PET/CT, with the purpose of validate the reliability of ¹⁸F-FDG PET/MR data.

Twenty patients with refractory focal epilepsy, negative MR and a defined electro-clinical diagnosis underwent PET/MR, immediately followed by PET/CT. Standardized uptake value ratio (SUVr) and cerebral blood flow (CBF) maps were calculated for PET/CT-PET/MR and ASL, respectively. For all techniques, z-score of the asymmetry index (z_AI) was applied for depicting significant Right/Left differences. SUVr and CBF images were firstly visually assessed by two neuroimaging readers, who then re-assessed them considering z_AI for reaching a final diagnosis.

High agreement between ¹⁸F-FDG PET/MR and ASL was found, showing hypometabolism and hypoperfusion in the same hemisphere in 18/20 patients, while the remaining were normal. They were completely concordant in 14/18, concordant in at least one lobe in the remaining. z_AI maps improved readers' confidence in 12/20 and 15/20 patients for ¹⁸F-FDG PET/MR and ASL, respectively. ¹⁸F-FDG PET/CT-PET/MR showed high agreement, especially when z_AI was considered.

The simultaneous metabolism-perfusion acquisition provides excellent concordance on focus lateralisation and good concordance on localisation, determining useful complementary information.

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Simultaneous PET/MR to evaluate cerebral perfusion and glucose metabolism in MR-negative refractory focal epilepsy patients.

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ASL and ¹⁸F-FDG PET/MR showed excellent concordance on lateralisation and good concordance on localisation of focus.

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ASL and ¹⁸F-FDG PET/MR can provide complementary information for focus localisation.

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An individually-tailored z-score approach can allow a better identification of the epileptic focus.

Comparison of (18)F-FDG PET/CT and PET/MRI in patients with multiple myeloma

PET/MRI represents a promising hybrid imaging modality with several potential clinical applications. Although PET/MRI seems highly attractive in the diagnostic approach of multiple myeloma (MM), its role has not yet been evaluated. The aims of this prospective study are to evaluate the feasibility of (18)F-FDG PET/MRI in detection of MM lesions, and to investigate the reproducibility of bone marrow lesions detection and quantitative data of (18)F-FDG uptake between the functional (PET) component of PET/CT and PET/MRI in MM patients. The study includes 30 MM patients. All patients initially underwent (18)F-FDG PET/CT (60 min p.i.), followed by PET/MRI (120 min p.i.). PET/CT and PET/MRI data were assessed and compared based on qualitative (lesion detection) and quantitative (SUV) evaluation. The hybrid PET/MRI system provided good image quality in all cases without artefacts. PET/MRI identified 65 of the 69 lesions, which were detectable with PET/CT (94.2%). Quantitative PET evaluations showed the following mean values in MM lesions: SUVaverage=5.5 and SUVmax=7.9 for PET/CT; SUVaverage=3.9 and SUVmax=5.8 for PET/MRI. Both SUVaverage and SUVmax were significantly higher on PET/CT than on PET/MRI. Spearman correlation analysis demonstrated a strong correlation between both lesional SUVaverage (r=0.744) and lesional SUVmax (r=0.855) values derived from PET/CT and PET/MRI. Regarding detection of myeloma skeletal lesions, PET/MRI exhibited equivalent performance to PET/CT. In terms of tracer uptake quantitation, a significant correlation between the two techniques was demonstrated, despite the statistically significant differences in lesional SUVs between PET/CT and PET/MRI.

Role of Magnetic Resonance Imaging in the Management of Patients With Multiple Myeloma: A Consensus Statement

Purpose

The aim of International Myeloma Working Group was to develop practical recommendations for the use of magnetic resonance imaging (MRI) in multiple myeloma (MM).

Methods

An interdisciplinary panel of clinical experts on MM and myeloma bone disease developed recommendations for the value of MRI based on data published through March 2014.

Recommendations

MRI has high sensitivity for the early detection of marrow infiltration by myeloma cells compared with other radiographic methods. Thus, MRI detects bone involvement in patients with myeloma much earlier than the myeloma-related bone destruction, with no radiation exposure. It is the gold standard for the imaging of axial skeleton, for the evaluation of painful lesions, and for distinguishing benign versus malignant osteoporotic vertebral fractures. MRI has the ability to detect spinal cord or nerve compression and presence of soft tissue masses, and it is recommended for the workup of solitary bone plasmacytoma. Regarding smoldering or asymptomatic myeloma, all patients should undergo whole-body MRI (WB-MRI; or spine and pelvic MRI if WB-MRI is not available), and if they have > one focal lesion of a diameter > 5 mm, they should be considered to have symptomatic disease that requires therapy. In cases of equivocal small lesions, a second MRI should be performed after 3 to 6 months, and if there is progression on MRI, the patient should be treated as having symptomatic myeloma. MRI at diagnosis of symptomatic patients and after treatment (mainly after autologous stem-cell transplantation) provides prognostic information; however, to date, this does not change treatment selection.

Real-time handheld emission spot allocator (rthESA) for simultaneous fusion imaging with ultrasound

OBJECTIVES

First, to report on initial experiences and technical parameters of a newly developed real-time handheld emission spot allocator (rthESA), and second, to report on the simultaneous acquisition of rthESA and US data as rthESA/US fusion images.

METHODS

The rthESA consisted of five semiconductor-detectors arranged in alternate position in two rows. This design allowed the examination of focal activities in the same plane as US. The signals were interpreted by an ad hoc software and the real-time allocation of spot radiation sources within air- and water phantoms was investigated for (99m)Tc, 131I, and 18F. A compact US probe was fixed in plane with the rthESA and connected to a standard US equipment. Experiments with a liver phantom were performed to verify the integration of (99m)Tc-rthESA data and US images.

RESULTS

The allocation proved to be successful for all radionuclides. The system showed a noticeable performance latency, most pronounced for positions far from the detector (1 cm distance: 0.7 ± 0.5 s; 4 cm distance: 6.1 ± 3.2 s). Within the liver phantom, the rthESA enabled the correct allocation of a spot radiation source within a live US image.

CONCLUSIONS

The rthESA allowed an exact localization of spot radiation sources in single plane, with additional consideration of the distance from the detector, leading to real-time allocation and simultaneous overlay with US images. In spite of clear technical limitations in need of further development, this proof-of-concept study shows that this hybrid detector has the potential to provide integrated simultaneous nuclear medicine and US images.

The use of molecular imaging combined with genomic techniques to understand the heterogeneity in cancer metastasis

Tumour heterogeneity has, in recent times, come to play a vital role in how we understand and treat cancers; however, the clinical translation of this has lagged behind advances in research. Although significant advancements in oncological management have been made, personalized care remains an elusive goal. Inter- and intratumour heterogeneity, particularly in the clinical setting, has been difficult to quantify and therefore to treat. The histological quantification of heterogeneity of tumours can be a logistical and clinical challenge. The ability to examine not just the whole tumour but also all the molecular variations of metastatic disease in a patient is obviously difficult with current histological techniques. Advances in imaging techniques and novel applications, alongside our understanding of tumour heterogeneity, have opened up a plethora of non-invasive biomarker potential to examine tumours, their heterogeneity and the clinical translation. This review will focus on how various imaging methods that allow for quantification of metastatic tumour heterogeneity, along with the potential of developing imaging, integrated with other in vitro diagnostic approaches such as genomics and exosome analyses, have the potential role as a non-invasive biomarker for guiding the treatment algorithm.