Residency · Residency · Nuclear Medicine
PET/MRI: Principles and Clinical Applications
Introduction
PET/MRI integrates the molecular sensitivity of positron emission tomography with the superior soft tissue contrast and functional capabilities of magnetic resonance imaging. Unlike PET/CT, PET/MRI offers reduced radiation dose by eliminating the CT component and provides simultaneous multiparametric imaging. Clinical applications are expanding in neuroimaging, pediatric oncology, musculoskeletal imaging, and select body oncology indications.
Technical Principles
System Architectures
Simultaneous (integrated) PET/MRI systems place a PET detector ring within the MRI bore, enabling truly simultaneous acquisition of PET and MRI data. This design requires MR-compatible PET detectors such as silicon photomultipliers (SiPMs). Examples include the Siemens Biograph mMR and GE SIGNA PET/MR. Sequential systems use separate PET and MRI gantries with a shared patient table, analogous to PET/CT workflow but substituting MRI for CT. Sequential designs introduce the possibility of temporal misregistration. Simultaneous systems are now the dominant configuration.
MR-Compatible PET Detectors
Conventional photomultiplier tubes are incompatible with strong magnetic fields. First-generation MR-compatible detectors used avalanche photodiodes (APDs). Current systems employ silicon photomultipliers (SiPMs), which offer faster timing resolution and higher sensitivity. Shielding and radiofrequency compatibility engineering are required to minimize interference between the PET and MRI subsystems.
MR-Based Attenuation Correction (MRAC)
In PET/CT, CT provides direct electron-density maps for attenuation correction. MRI signal intensity, however, does not directly correlate with photon attenuation, making MRAC the primary technical challenge of PET/MRI. Several approaches have been developed: Dixon-based segmentation classifies tissue as air, lung, fat, or soft tissue; atlas-based methods use anatomic atlases to estimate bone attenuation; deep learning approaches generate pseudo-CT maps from MRI data; and ultrashort echo time (UTE/ZTE) sequences detect bone signal for improved head MRAC. Despite progress, MRAC remains challenging for bone and lung attenuation estimation.
Advantages Over PET/CT
Radiation Dose Reduction
Eliminating the CT component reduces radiation by 50-80% depending on the protocol. This is particularly advantageous for pediatric patients, young adults, and anyone requiring repeated imaging for longitudinal monitoring. MRI contrast agents (gadolinium) contribute no radiation.
Superior Soft Tissue Contrast
MRI provides unparalleled soft tissue characterization through multiparametric sequences including T1, T2, FLAIR, diffusion-weighted imaging, and dynamic contrast enhancement. For brain, liver, pelvis, and musculoskeletal imaging, MRI is clearly superior to CT. Simultaneous functional MRI data complement metabolic PET data.
Simultaneous Multiparametric Imaging
Temporally matched PET and MRI data eliminate misregistration artifacts. MR navigator sequences can be used for motion correction to improve PET image quality. Pharmacokinetic modeling benefits from simultaneous tracer and contrast dynamics, and total examination time may be reduced compared to sequential PET and MRI as separate studies.
Clinical Applications
Neuroimaging
PET/MRI enables FDG PET with structural MRI and volumetric analysis in a single session for dementia evaluation. For epilepsy, interictal FDG PET is coregistered with high-resolution MRI for surgical planning. Brain tumor assessment combines amino acid PET tracers (F-18 FET, C-11 methionine) with MR perfusion and spectroscopy. Emerging tracers for neuroinflammation are paired with advanced MRI sequences.
Pediatric Oncology
The radiation dose reduction is a major advantage in children. FDG PET/MRI has been shown to be non-inferior to PET/CT for lymphoma staging. For sarcomas, MRI provides superior soft tissue characterization of the primary tumor. Neuroblastoma assessment combines MIBG or DOTATATE PET with whole-body MRI.
Body Oncology
Prostate cancer benefits from PSMA PET combined with multiparametric MRI for detection and staging. Rectal cancer staging pairs local MRI assessment with FDG metabolic evaluation. Gynecologic malignancies (cervical and endometrial cancer) benefit from MRI's superior pelvic soft tissue contrast. Hepatic lesions can be evaluated with gadoxetate-enhanced MRI combined with FDG or Ga-68 DOTATATE PET.
Musculoskeletal Applications
PET/MRI enables assessment of inflammatory arthritis by combining metabolic and structural data, characterization of bone and soft tissue tumors, differentiation of infection from inflammation, and sports medicine applications combining morphologic and metabolic assessment.
Limitations and Challenges
Technical Limitations
MRAC artifacts occur near bone, metal implants, and air-tissue interfaces. MRI-incompatible implants and devices preclude scanning in some patients. Acquisition times are longer than PET/CT (30-60 minutes versus 15-20 minutes), and capital equipment costs are higher.
| Feature | PET/CT | PET/MRI |
|---|---|---|
| Radiation dose | Higher (CT component) | 50–80% lower (no CT) |
| Soft tissue contrast | Moderate | Superior |
| Lung nodule detection | Excellent | Inferior |
| Bone cortex | Well seen | Poorly seen |
| Attenuation correction | Direct (CT Hounsfield units) | Indirect (MRAC; challenging) |
| Scan time | 15–20 min | 30–60 min |
| Availability | Widespread | Limited (<200 systems worldwide) |
| Best applications | Lung, general oncology | Brain, pelvis, pediatrics, MSK |
Clinical Limitations
Lung nodule detection is inferior to CT because MRI has limited sensitivity for pulmonary parenchyma. Bone cortex visualization is also inferior to CT. Fewer than 200 systems exist worldwide, limiting availability. Workflow complexity requires expertise in both PET and MRI interpretation.
Practical Considerations
Claustrophobia is more common with the enclosed bore. Gadolinium contrast is contraindicated in severe renal insufficiency. MRI safety screening is required for all patients, and longer patient preparation and scanning time affect throughput.
Emerging Directions
Total-body PET/MRI concepts are under development. Deep learning is improving MRAC and image reconstruction. Radiomics and multiparametric feature extraction are being explored for outcome prediction. Hybrid PET/MRI-guided interventional procedures and theranostic applications combining diagnostic PET/MRI with treatment planning are active areas of research.
Clinical Pearls
MR-based attenuation correction remains the primary technical challenge of PET/MRI, as MRI signal does not directly reflect photon attenuation; deep learning and ultrashort echo time sequences are steadily improving accuracy. PET/MRI offers a 50-80% radiation dose reduction compared to PET/CT, making it particularly advantageous for pediatric patients and those requiring serial longitudinal imaging. PET/MRI is non-inferior to PET/CT for lymphoma staging and superior for lesion characterization in the brain, liver, pelvis, and musculoskeletal system. The primary clinical limitations are inferior lung nodule detection compared to CT and limited global availability of systems.
References
- Catana C. "Principles of Simultaneous PET/MR Imaging." Magn Reson Imaging Clin N Am. 2017;25(2):231-243.
- Vontobel J, et al. "PET/MRI in Oncology: A Clinical Review." Eur J Nucl Med Mol Imaging. 2015;42(6):942-954.
- Drzezga A, et al. "First Clinical Experience with Integrated Whole-Body PET/MR: Comparison to PET/CT in Patients with Oncologic Diagnoses." J Nucl Med. 2012;53(6):845-855.
- Beiderwellen K, et al. "Simultaneous FDG PET/MRI: A New Approach for Pediatric Oncology." Pediatr Radiol. 2014;44(11):1361-1369.