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PET-MRI: a review of challenges and solutions in the development of integrated multimodality imaging
Stefaan Vandenberghe, Paul Marsden
TL;DR
PET-MRI must reconcile compact, MRI-compatible hardware with accurate quantitative PET reconstruction and clinically useful integration. This review synthesizes detector and system designs, interference-reduction strategies, attenuation correction, and image-improvement approaches, concluding that fully integrated simultaneous human systems are now available while attenuation correction remains less accurate than in clinical PET-CT.
Problem
Integrating PET and MRI requires compact MRI-compatible hardware, interference control, and reliable attenuation correction, with clinical utility still being established.
Method
The review surveys PET-MRI detector concepts, system configurations, representative integrated systems, interference solutions, attenuation-correction methods, motion compensation, and resolution recovery.
Results
Complete simultaneous fully integrated PET-MRI systems for human imaging have emerged, while attenuation correction remains a limiting factor and is not yet as accurate as in clinical PET-CT.
Takeaways & Limitations
PET-MRI is progressing as a clinical and research modality, with MRI also guiding PET reconstruction and motion compensation to improve image quality.
Takeaways & Limitations
MRI-based attenuation correction lacks a direct relationship between MRI signal and the 511 keV linear attenuation coefficient, requiring tissue segmentation and predefined values.
Abstract
from arXiv · showhide
The integration of positron emission tomography (PET) and magnetic resonance imaging (MRI) has been an ongoing research topic for the last 20 years. This paper gives an overview of the different developments and the technical problems associated with combining PET and MRI in one system. After explaining the different detector concepts for integrating PET-MRI and minimising interference the limitations and advantages of different solutions for the detector and system are described for preclinical and clinical imaging systems. The different integrated PET-MRI systems are described in detail. Besides detector concepts and system integration the challenges and proposed solutions for attenuation correction and the potential for motion correction and resolution recovery are also discussed in this topical review.
1. Introduction and overview
This review examines the major hardware, integration, quantitative-imaging, and clinical challenges in combining PET and MRI, together with approaches developed to address them.
- System integration: PET-MRI integration is substantially more complex than PET-CT because PET components must operate within strong magnetic fields while preserving both modalities’ performance.Configurations range from sequential in-line systems to fully integrated simultaneous scanners.
- System integration: Fully integrated PET-MRI can reduce footprint, support simultaneous imaging protocols, and achieve acquisition times comparable to MRI acquisitions.It requires compact MRI-compatible detectors and minimized interference between PET and MRI components.
- Detector technology: Compact MRI-compatible PET detectors are the central hardware challenge, particularly for scanners operating inside MRI bores with fields typically between 0.5 and 10 T.Clinical simultaneous systems became viable with solid-state photodetectors such as APDs and SiPMs, which are largely insensitive to strong magnetic fields.
- Quantitative imaging: Accurate PET attenuation correction remains unresolved because MRI, emission, and transmission data do not straightforwardly provide linear attenuation coefficients at 511 keV.Although several methods have been developed, attenuation correction remains an active research area.
- Clinical use: Routine clinical benefits of PET-MRI remain uncertain despite potential dose reduction and complementary molecular, anatomical, soft-tissue, functional, and molecular information.The most promising applications are areas such as the brain, breast, and abdomen, where MRI is already preferred anatomically.
2. Technical challenges for combining PET and MRI
Integrating PET and MRI requires coordinated hardware and processing changes to address physical constraints, modality interference, and the demands of simultaneous quantitative imaging. The review also considers clinical and research motivations, including improved registration, flexible MRI contrast, reduced dose, and dynamic multimodality studies.
- Motivations and optimisation: PET-MRI is motivated by MRI’s sequence flexibility, improved contrast, co-registered information for PET quality, and the possibility of substantially reducing delivered dose.The review also identifies research opportunities in functional, dynamic, kinetic, and multimodality-tracer studies.
- Motivations and optimisation: After whole-body systems became available, optimisation extended beyond hardware to co-registered acquisition, reconstruction, visualisation, and disease-specific clinical protocols.Protocol design includes selecting MRI sequences and PET tracers and determining imaging time.
- Detector and system integration: Physical integration is constrained because both whole-body modalities have 60–80 cm inner bores and outer bores above one metre.Combining them in one simultaneous system therefore requires reducing the size of at least one modality.
- Interference and compatibility: MRI-compatible PET development must address interference from static and changing magnetic fields, radiofrequency signals, and conductive components near the MRI hardware.These effects can disrupt PET photomultiplier tubes and electronics, while PET components can also introduce MRI interference and heating.
- Interference and compatibility: Replacing or relocating photomultiplier tubes is necessary because conventional tubes lose gain even in weak static magnetic fields and cannot tolerate several-tesla MRI fields.The review describes field-insensitive alternatives and relocation outside high-field regions as solutions.
- Detector and system integration: Simultaneous PET-MRI requires a compact MRI-compatible PET detector that preserves PET performance while minimising interference inside the MRI scanner.The detector is identified as the key component for constructing simultaneous systems comparable to current PET-CT platforms.
3. MRI compatible PET detectors
MRI-compatible PET detectors adapt scintillator–photodetector designs to operate within magnetic fields while minimizing interference and fitting inside the MRI bore. Development has progressed from long-fiber PMT arrangements toward compact APD and SiPM solid-state solutions, each involving performance and integration trade-offs.
- Detector requirements: MRI-compatible PET detectors generally retain inorganic scintillators coupled through light guides to photodetectors, provided the scintillator does not create MRI susceptibility artifacts.LSO, LYSO, and BGO have susceptibility similar to human tissue, whereas gadolinium-containing GSO and LGSO are unsuitable.
- Detector concepts: Long optical fibers relocate PMTs outside the magnetic field, but light loss, attenuation, and dispersion degrade energy and temporal resolution.These limitations motivated replacing PMTs with solid-state photodetectors.
- System integration: Compact solid-state photodetectors enable annular PET scanners to fit inside essentially unmodified MRI systems, while detector layouts and RF shielding must limit PET–MRI interference.Small detector elements support one-to-one coupling or light-sharing designs, but one-to-one coupling can require many high-power readout channels.
- Representative implementations: MRI-compatible PET development includes APD, analog SiPM, and digital SiPM modules, alongside alternative solid-state detectors whose PET imaging potential remains unclear.The review illustrates these concepts through representative detector and module designs.
- APD-based detectors: APDs provide high quantum efficiency but typically achieve only a few-nanosecond temporal resolution with LSO, limiting their suitability for time-of-flight PET.Their gain is also highly sensitive to temperature and voltage, requiring careful control and nearby low-noise electronics.
4. MRI compatible PET systems
MRI-compatible PET systems use sequential, split-magnet, fibre-based, APD, and SiPM configurations to enable combined imaging while managing magnetic-field and interference constraints. Integrated systems can provide acceptable multimodal performance, but sensitivity, artefacts, registration, and high-field MRI remain important limitations.
- Early systems using light guides and optical fibres: Early fibre-based PET-MRI systems avoided magnetic and conducting components in the MRI field of view, but optical attenuation produced poor timing, energy resolution, and sensitivity.Optical fibres typically reduced scintillation light by a factor of 5–10.
- Sequential and split-magnet systems: Sequential PET-MRI configurations reuse established PET and MRI technology, while split-magnet systems place PET in a gap where photomultiplier tubes experience acceptably low fields.These approaches provide alternatives when detectors cannot operate reliably inside the MRI field.
- Simultaneous small-animal systems: Placing PET detectors inside the MRI field can preserve acceptable performance for standard sequences when components and shielding are carefully selected, although typical small-animal sensitivity remains below 1%.Later systems increased axial field of view to 5–7 cm to improve sensitivity for whole-body mouse imaging.
- SiPM-based systems: SiPM-based systems showed minimal effects on MRI field homogeneity and PET during RF or gradient switching, and detector modules achieved 520 ps temporal resolution in a 3 T magnet.The reported compatibility was attributed primarily to direct SiPM signal digitization and attention to compatibility issues.
- Clinical brain systems: The brainPET insert achieved 7% sensitivity and <3 mm FWHM reconstructed spatial resolution, but correcting ring, out-of-field, and RF-coil artefacts was necessary for accurate quantification.Its APD timing resolution was 4.9 ns, which was not sufficient for time-of-flight imaging.
- Clinical whole-body systems: A clinical time-of-flight PET-MRI system reported 10.5% energy resolution, 390 ps average timing resolution, 4.2 mm spatial resolution, and 22.5 kcps MBq−1 NEMA sensitivity.Its sensitivity was supported by thick detectors, a small detector bore, a long axial field of view, and Compton-scatter recovery.
5. Quantitative image reconstruction in PET-MRI
Quantitative PET-MRI reconstruction depends on robust attenuation correction because attenuation strongly affects image quality and regional quantification. The review compares MRI-, emission-, and transmission-based approaches while highlighting simultaneous acquisition and motion-correlated anatomical imaging as advantages.
- Attenuation-correction requirements: PET-MRI attenuation correction remains a major barrier because integrated systems lack PET-CT’s direct conversion of CT images into attenuation coefficients.The review discusses MRI-based, emission-based, and transmission-based alternatives.
- Attenuation-correction requirements: Human PET-MRI attenuation methods must include all attenuating objects in the field of view, be robust against diagnostic errors, and ideally acquire maps simultaneously with PET.Clinical requirements also include minimal added acquisition time and limited additional patient dose.
- MRI-based attenuation correction: MRI-based correction is difficult because MRI signal has no direct relationship to 511 keV attenuation and standard sequences poorly distinguish bone, lung, and air.These methods therefore segment tissues and assign predefined attenuation coefficients, while MRI field-of-view truncation and variable objects create additional errors.
- Evaluation and alternatives: Template and atlas methods produce good brain-imaging results, while UTE methods further improve accuracy by accounting more accurately for bone density.A template can incorporate structural detail that segmented approaches may miss, but applicability remains limited in populations without anatomical abnormalities.
- Synergistic reconstruction: Simultaneous PET-MRI can acquire anatomical images continuously and in precise spatial and temporal correlation with PET emission data, supporting motion compensation and reduced breathing-phase mismatch.Compared with PET-CT, simultaneous acquisition can minimize artefacts caused by differing respiratory phases.
6. Conclusions
Integrated simultaneous PET-MRI systems have progressed from development to clinical installation, driven by compact solid-state photomultipliers. Remaining work centers on attenuation correction, quantitative operation, motion compensation, and synergistic PET-MRI protocols.
- System development: Compact solid-state photomultipliers enabled complete simultaneous, fully integrated PET-MRI systems for human imaging.These systems are installed in several clinical centres worldwide.
- Remaining challenges: Attenuation correction remains a limiting factor because its accuracy does not yet match that of clinical PET-CT.MRI is also used to guide PET reconstruction and motion compensation to improve image quality.
- Research direction: PET-MRI research is shifting toward operational and quantitative human imaging and combinations of multifunctional MRI sequences with PET tracer protocols.The review identifies these synergistic combinations as important for realizing the added value of simultaneous PET and MRI.