Sacroiliac joint protocol (MRI)

The MRI sacroiliac joint protocol encompasses a set of MRI sequences for the routine assessment of sacroiliac joints.

Note: This article aims to frame a general concept of an MRI protocol for the assessment of the sacroiliac joint. Protocol specifics will vary depending on MRI scanner type, specific hardware and software, radiologist and perhaps referrer preference, patient factors e.g. implants, specific indications and time constraints.

Indications

The classical indication is suspected sacroiliitis.  The protocol can be adjusted for the assessment of sacral or presacral tumors.

1.5 vs 3 tesla

An MRI examination of the sacroiliac joints is generally performed on both 1.5 and 3 tesla.

Patient positioning

An MRI of the sacroiliac joints is conducted with the patient in the supine position.

Technical parameters

Coil

Multi-phased array coils are recommended.

• anterior surface coil
• posterior coil

Scan geometry

• in-plane spatial resolution: ≤0.4 x 0.4 mm
• field of view (FOV):  240-300 mm, as small as possible
• slice thickness: ≤3 mm, interslice gap of 0.3 mm

Planning

• coronal oblique images:             
  • angulation: along the axis of the posterior surface of the S2 vertebral body 
  • volume: covers the whole sacrum
  • slice thickness: ≤3 mm
• axial oblique images:
  • angulation: perpendicular to the coronal oblique slice
  • volume:  including the whole sacroiliac joint, from disc space L5/S1 to S5
  • slice thickness: ≤3 mm
• sagittal images*: (optional)
  • angulation: strictly sagittal to the body axis 
  • volume:  depends on the pathology
  • slice thickness: ≤3 mm

Sequences

Sequences can be adjusted to scanner specifications, and image quality in particular sequences. As with other musculoskeletal imaging water-sensitive sequences are an integral component, such as STIR, T2 weighted fat-saturated images or intermediate weighted images.

For the assessment of the sacroiliac joint at least one T1-weighted sequence should be included.

In the setting of suspected sacroiliitis, an MRI of the sacroiliac joints benefits from contrast media for the assessment of synovitis, capsulitis and/or enthesitis. According to the 2015 EULAR recommendations, it is, however, sufficient to detect bone marrow edema ³.

Standard sequences

A typical MRI of the sacroiliac joints might look as follows ¹:

• T2-weighted (fat-saturated) or STIR or intermediate (fat-saturated)
  • purpose: bone and/or soft-tissue characterization, for the detection of bone marrow edema
  • technique: T2 FS fast spin echo / STIR / IM FS fast spin echo
  • planes: coronal oblique, axial oblique
• T1-weighted
  • purpose: bone and/or soft-tissue characterization and the detection of fatty infiltration and bone erosions including backfill
  • technique:  T1 fast spin echo
  • planes: coronal oblique

Optional sequences

• T2-weighted
  • purpose: bone and soft tissue characterization, tumors
  • technique: T2 fast spin echo, T2 Dixon
  • planes: sagittal * (option for sacral or presacral tumors)
• T1-weighted C+ (fat-saturated)
  • purpose: for the assessment of synovitis, capsulitis and enthesitis
  • technique:  T1 fast spin echo
  • planes: coronal oblique, axial oblique

(*) indicates optional planes

Practical points

• a typical protocol will consist of 3-5 sequences, depending on if contrast is given or not
• an alternative to the STIR or T2 weighted or intermediate weighted (fat-saturated) axial stack would be a T2 weighted or proton density-weighted Dixon variant, which includes a fluid sensitive fat-saturated and a non-fat-saturated image stack
• likewise, the protocol can be supplemented with sagittal sequences for the assessment of sacral tumors