PET/MR (Simultaneous Hybrid Imaging)

Hybrid modality combining a PET ring with an MRI scanner — simultaneously, not sequentially like PET/CT. The PET detector ring is integrated inside the MRI magnet bore, enabling co-registered molecular + anatomic imaging without patient motion between acquisitions.

Lower ionizing radiation dose than PET/CT (no CT — though there's no substitute for CT attenuation correction; PET/MR uses MR-based attenuation correction, which has known accuracy limitations).

Why PET/MR exists

Pediatric oncology — eliminating CT dose matters. A pediatric patient undergoing serial PET/CT accumulates meaningful lifetime radiation exposure.
Neurology research — fMRI + PET simultaneous acquisition opens research questions (metabolism + connectivity).
Cardiac — PET perfusion + late-gadolinium-enhanced cardiac MR in one session.
Prostate MR + PSMA-PET — high-resolution soft-tissue staging + molecular staging together.

Adoption reality

PET/MR has been clinically available since 2011 but adoption remains niche. Reasons:

Capital cost — 2× PET/CT
MR attenuation correction challenges — PET quantification not as reliable as PET/CT
Complex install + operation — combines MRI safety program with PET nuclear licensing
Patient throughput lower than PET/CT (longer scan times)

Physics

PMT-replacement: Photomultiplier tubes don't work in magnetic fields. PET/MR uses Avalanche Photodiodes (APDs) or silicon photomultipliers (SiPM) — both tolerate magnetic fields.
LSO crystals (similar to LYSO). ToF capability varies by generation.
MR-based attenuation correction — MR sequences (Dixon-based) estimate tissue attenuation for PET reconstruction; cortical bone and lung are challenging (MRI has limited signal from those tissues).

Key specs

Field strength — typically 3 T (Biograph mMR); ViewRay MR-Linac uses 0.35 T for therapy, not PET/MR
PET detector — APD or SiPM
ToF — varies (Biograph mMR non-ToF; some research systems have ToF)
Axial FOV — similar to PET/CT
MR coil ecosystem — needs to accommodate PET ring geometry

Systems

Siemens Biograph mMR — 3 T + APD PET, first commercial PET/MR (2011)
GE Signa PET/MR — 3 T + SiPM (newer ToF-capable)

Regulatory

Combines both parent modalities:

NRC / Agreement State license for radiotracers
MRI safety program (Zone I-IV, 5-gauss line, ferromagnetic screening)
RSO + physicist (often combined roles)
ACR accreditation (separate PET/MR category)

PET/CT (non-MR sibling)
MRI (parent MR modality)
MR-Linac (other MRI hybrid — therapy, not diagnostic)
Biograph mMR