SI joints MRI protocol
The exam for inflammatory back pain. Its answer, active sacroiliitis, is a few millimetres of bright marrow beside a curved joint, so the whole protocol is built on two oblique planes taken from the angle of the sacrum.
By the ScanFlo Editorial Team · Updated
In this section · Protocols
When this study is used
- Inflammatory back pain; suspected axial spondyloarthritis (sacroiliitis).
- Buttock pain with a suspected SI joint infection or insufficiency fracture.
- Follow-up of known sacroiliitis on treatment.
Patient & coil positioning
- Head-first supine, straight on the spine coil, with a pad under the knees.
- Centre the laser on the ASIS, the bony prominence at the front of the pelvis; the SI joints lie at this level.
Localizer
Three-plane localizer, then a sagittal T2 through the sacrum: the oblique planes are planned on that sagittal, so it must show the whole length of the sacrum clearly.
Sequence planning
For each sequence below you get the reference line the slice group follows, how much it covers, and why the sequence is in the protocol at all. Plan against the localizer, not by eye.
Sagittal T2 TSE
SagittalPlanning: On the coronal, parallel to the midline, covering from one SI joint to the other.
Why: Shows the angle of the sacrum, which every oblique plane is planned from; also the lower lumbar discs and the sacral canal.
Coronal oblique T1 TSE
Coronal obliquePlanning: On the sagittal, parallel to the long axis of the sacrum (its posterior border). Cover the whole sacrum from front to back.
Why: Both SI joints along their whole length on one slice: erosions, sclerosis, fat metaplasia and ankylosis.
Axial oblique T2 TSE
Axial obliquePlanning: On the sagittal, perpendicular to the sacrum, from above the joints to below them.
Why: The joints in cross-section, separating the synovial (lower) and ligamentous (upper) parts.
Coronal oblique STIR and T2
Coronal obliquePlanning: Copy the T1 coronal oblique's slices (Copy Parameter → Slices); do not re-plan.
Why: STIR is the key sequence: bone marrow oedema of active sacroiliitis is bright against suppressed fat.
Axial oblique STIR and T1
Axial obliquePlanning: Copy the T2 axial oblique's slices.
Why: Confirms oedema and structural change in the second plane.
In the video
- The video scans a ScanFlo SI Joints case. The angles are that patient's, so follow the landmarks, not the numbers.
- Sagittal T2: 15 slices, 3 mm, TR 3120 / TE 93 ms.
- Coronal oblique T1: 24 slices, 3 mm, TR 514 / TE 25 ms; about 43° from axial on the sagittal, so the console labels it Transverse.
- Axial oblique T2: 28 slices, 3 mm, TR 5260 / TE 97 ms; about 29° on the sagittal.
- The STIR and T2 coronal copy the T1 coronal oblique; the STIR and T1 axial copy the T2 axial oblique (Copy Parameter → Slices).
- The queue takes about 10 minutes; the video shows the scan as a time-lapse.
Representative parameters
Treat these as ranges. Field strength, coil and vendor all move the numbers. What must not move is understanding why each sits where it does.
| Sequence | Field | Slice / FOV | Notes |
|---|---|---|---|
| T2 TSE sagittal | 1.5–3 T | 3 mm / 30–33 cm | Planning reference |
| T1 TSE coronal oblique | 1.5–3 T | 3 mm / 23 cm | Erosions, fat metaplasia |
| STIR coronal oblique | 1.5–3 T | 3 mm / 23 cm | Active inflammation |
| T2 TSE axial oblique | 1.5–3 T | 3 mm / 23 cm | Cross-section of the joint |
| STIR / T1 axial oblique | 1.5–3 T | 3 mm / 23 cm | Second plane |
Artifacts & how to fix them
| Artifact | Typical cause | Mitigation |
|---|---|---|
| Bowel motion | Peristalsis in front of the sacrum | Anterior sat band; phase direction away from the joints |
| Patchy fat suppression | Field inhomogeneity across the pelvis | Use STIR rather than spectral fat-sat; shim on the sacrum |
| Chemical shift at the joint | Fat–water boundary along the joint line | Wider bandwidth; check against the second plane |
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 |
|---|---|---|
| Active sacroiliitis | Coronal oblique STIR | Bright marrow oedema beside the joint, on two or more slices |
| Erosions | Coronal oblique T1 | Dark defects breaking the joint's cortical line |
| Fat metaplasia | Coronal oblique T1 | Bright fat beside the joint: a sign of past inflammation |
| Insufficiency fracture | STIR + T1 | Oedema with a dark fracture line through the sacral ala, often both sides |
Review checklist
- Both SI joints along their whole length on the coronal oblique.
- Coronal oblique parallel to the sacrum; axial oblique perpendicular to it.
- STIR and T2 coronal copied from the T1 coronal oblique; STIR and T1 axial copied from the T2 axial oblique.
- STIR fat suppression uniform across both joints.
Mistakes that cost repeats
- A true coronal instead of the sacral oblique, so each slice crosses the curved joint obliquely and hides oedema.
- Re-planning each row by eye, so the STIR no longer matches the T1 slice for slice.
- Calling a single bright slice sacroiliitis; look for oedema on consecutive slices and in both planes.
Common questions
- Why plan the SI joints oblique?
- The joints lie along the sacrum, which tilts backwards. A coronal oblique parallel to the sacrum shows both joints along their whole length on one slice, and an axial oblique perpendicular to it shows them in cross-section; straight coronal and axial slices cut the joints at an angle.
- Why is the coronal oblique labelled Transverse on the console?
- The console names a plane after the axis it is closest to. When the sacrum is steep, the coronal oblique sits nearer to axial than to coronal, so the console labels it Transverse (T>C). The plane is still the one you planned.
- Why is STIR the key sequence?
- Active sacroiliitis is bone marrow oedema, and STIR suppresses fat uniformly across the pelvis, so oedema stands out as bright beside the joint even where spectral fat-sat would fail.
Try it in ScanFlo: Plan the coronal oblique parallel to the sacrum and scan it, then plan a straight coronal and compare. On the straight coronal each slice catches only part of each joint; on the oblique the whole joint lies in one slice. See ScanFlo MRI →