Resources/Protocols
Routine knee
A puzzle of triangles and ligaments where geometry is everything. A flat, un-angled knee scan hides the ACL; the right angle opens it up. Master the planning lines and the knee stops being difficult.
When this study is used
- Pain, locking or giving-way after trauma.
- Suspected meniscal tear, ACL/PCL rupture or collateral ligament injury.
- Cartilage assessment or unexplained effusion.
Patient & coil positioning
- Feet-first supine in a dedicated multi-channel knee coil, joint space centred in the coil.
- Rotate the knee ~10–15° externally — this aligns the ACL for the sagittal view; a straight knee buckles the ligament and mimics a tear.
- Pad the contralateral leg and between the knees to stop motion transfer.
- Strap the thigh and ankle of the examined leg — a settled joint is a still joint.
Localizer
Three-plane localizer. The axial must show the femoral condyles, patella and tibial plateau; the coronal must span the full medial–lateral extent. Everything that follows is angled off these images.
Sequence planning
Each sequence below lists how the slice group is positioned — the reference line it follows, its coverage, and why it earns its place in the protocol. Plan against the localizer, not by eye.
Sagittal PD fat-sat
SagittalPlanning — Angle parallel to the 'Roman arch' — the inner wall of the lateral femoral condyle — on the axial localizer. This single step aligns the slices with the ACL and is the most critical decision in the exam. Cover MCL to LCL.
Why — Menisci as dark bow-ties, the ACL as a continuous ribbon, and bright fluid marking tears.
Coronal PD fat-sat / STIR
CoronalPlanning — Parallel to the femoral shaft on the sagittal — naturally perpendicular to the tibial plateau. Cover posterior condyles to patellar tendon.
Why — Collateral ligaments, meniscal bodies and bone-marrow oedema (bright on STIR).
Axial PD fat-sat
AxialPlanning — Parallel to the joint line on the sagittal; patella to tibial tuberosity.
Why — Patellofemoral cartilage, retinacula and joint fluid.
Sagittal T1
SagittalPlanning — Same coverage as the PD; higher resolution for anatomy.
Why — Bucket-handle fragments, marrow signal and fracture lines.
Representative parameters
Ranges, not commandments — field strength, coil and vendor move the numbers. What must not move is understanding why each sits where it does.
| Sequence | Field | Matrix / FOV | Notes |
|---|---|---|---|
| PD fat-sat | 1.5–3 T | 320–384 / 14–16 cm | Workhorse for menisci and cartilage |
| STIR | 1.5–3 T | 256–320 / 14–16 cm | Marrow oedema; robust fat suppression |
| T1 SE | 1.5–3 T | 320 / 14–16 cm | Anatomy, fracture lines |
| 3D GRE fat-sat | 1.5–3 T | isotropic 0.5 mm | Cartilage mapping, loose bodies |
Artifacts & how to fix them
| Artifact | Typical cause | Mitigation |
|---|---|---|
| Inhomogeneous fat-sat | Shim centred on the whole leg, not the joint | Re-shim on the joint; switch to STIR; check rotation |
| Poor meniscal detail | Voxel too large or oblique slices | Higher matrix, smaller FOV, slices strictly perpendicular |
| Popliteal pulsation ghost | Artery pulsation across the meniscus | Phase-encode head-to-foot to shoot ghosts away |
| Metal blooming (TKR) | Susceptibility from prosthesis | Very high bandwidth, spin echo only, MARS if available |
What am I looking at?
You are not reporting the study, but recognising pathology helps you keep it in the field of view and know when to add a sequence or contrast. How the common findings read on the console:
| Finding | Sequence to check | How it reads |
|---|---|---|
| Meniscal tear | Sagittal PD FS | Bright signal cutting the dark triangle and reaching the articular surface |
| ACL rupture | Sagittal oblique | 'Mop-end' discontinuity, abnormal slope, oedema in the notch |
| Bone contusion | Coronal STIR | Bright marrow signal at the point of impact |
| Cartilage loss | 3D GRE FS | Focal thinning of the intermediate-grey cartilage against dark bone |
Review checklist
- ACL visualised continuously on several consecutive slices.
- Both menisci shown as clean bow-ties, anterior and posterior horns.
- Fat suppression uniform across the joint.
- Collateral ligaments and bone marrow covered on the coronal.
Mistakes that cost repeats
- A straight sagittal that leaves the ACL looking torn — re-check the Roman-arch angle.
- Fat-sat shimmed on the leg rather than the joint, ruining the dark background.
- FOV so large the meniscus turns blocky.
- Phase direction left AP, letting popliteal ghosts land on the menisci.
Common questions
- Why rotate the knee externally for MRI?
- About 10–15° of external rotation aligns the ACL with the sagittal plane so it appears as one continuous ribbon. A neutral or internally rotated knee buckles the ligament and can make a normal ACL look torn.
- What is the 'Roman arch' in knee planning?
- The inner wall of the lateral femoral condyle. Angling the sagittal slices parallel to it aligns them with the ACL fibres — the single most important geometric step in a knee exam.
- Why PD fat-sat rather than plain T2 for menisci?
- Proton-density fat-sat gives high signal-to-noise with bright fluid against dark menisci, so a tear reaching the surface stands out while fine meniscal anatomy stays sharp.
Try it in ScanFlo: Plan a sagittal stack aligned to the Roman arch, then a second cut straight, and scan both. Watch the ACL appear continuous in one and 'torn' in the other — that is why the angle is non-negotiable.