Open and Extremity MRI
Specialty MRI configurations that serve populations and use cases that whole-body closed-bore 1.5 T / 3 T MRI doesn't:
- Open MRI — lower field strength (typically 0.3–0.7 T), open magnet geometry (C-shape, vertical gap, "pancake" style). Primary use case: claustrophobic patients, bariatric patients, pediatric with parent-in-room, interventional MRI-guided procedures.
- Extremity MRI — dedicated small-bore scanners for the extremities (wrist, ankle, knee, elbow). Low field (0.2–0.4 T typical). Office-based — no helium, limited RF shielding, much lower capital cost.
- Weight-bearing MRI — tilting-magnet scanners that allow imaging in loaded / standing position. Distinctive for spine, knee, hip biomechanics.
Open MRI
Why open MRI exists
Claustrophobia affects ~10-15% of MRI patients, of whom a meaningful fraction cannot complete a closed-bore scan even with sedation. Open MRI sacrifices field strength (hence image quality) for patient acceptance. Also important for:
- Pediatric patients where a parent needs to be present
- Bariatric patients who don't fit a 60 cm or even 70 cm bore
- Interventional procedures (real-time image guidance) — open geometry allows physician access
Field strength trade-off
At 1.5 T, SNR scales favorably with scan time — modern techniques produce excellent images. At 0.3 T (typical open MRI field), SNR is ~1/5 that of 1.5 T. Sequence times lengthen; certain applications (diffusion, spectroscopy, perfusion) are difficult or impossible. Routine anatomy is still diagnostic-quality, just not premium.
Systems
- Hitachi Airis II (0.3 T, now under Fujifilm)
- Philips Panorama HFO (0.7 T open — higher-field open, rarer)
- Paramed MrJ (open MRI for Europe / emerging markets)
Extremity MRI
Clinical niche
Dedicated extremity MRI fits in an orthopedic office or outpatient suite without the infrastructure of whole-body MRI:
- No helium (low-field permanent magnet or resistive coils instead of superconducting)
- Minimal RF shielding (smaller scan volume)
- Standard clinical power (no 3-phase 480 V)
- Lower floor load
- Lower capital cost — often ~1/3 to 1/5 of a whole-body 1.5 T
Systems
- Esaote O-scan — dedicated extremity workhorse
Image quality
At 0.3 T with dedicated coils tuned to small FOV, extremity MRI produces clinically adequate images for knee internal derangement, ankle ligament, wrist TFCC + carpal bones, elbow. Not equivalent to 3 T whole-body for cartilage-detail work, but adequate for most clinical indications.
Weight-bearing MRI
Esaote G-scan Brio + similar tilt-magnet scanners acquire images with the patient in a loaded / standing / sitting position. Relevant for:
- Spine biomechanics (disc herniation visible only in loaded position)
- Knee patellofemoral tracking
- Hip impingement under load
Niche clinical application; limited adoption but distinctive capability.
Systems
- Hitachi Airis II (open)
- Esaote O-scan (extremity)
Regulatory
Lower-field MRI has fewer safety considerations than 3 T:
- Reduced 5-gauss line footprint
- No quench (permanent / resistive magnets don't quench)
- Reduced RF concerns
- Still requires ACR accreditation pathway for reimbursement