Why put a wedge under the knees for a lumbar MRI?

The lumbar spine curves forward and lifts away from the table coil. Flexing the hips over a knee wedge flattens the spine against the coil, which raises signal-to-noise and opens the disc spaces for clearer imaging.

How do you avoid operating on the wrong level?

Count from a fixed landmark — down from C2 on a whole-spine scout or up from the sacrum — and document any transitional vertebra (e.g. LSTV) with your counting reference, so the surgeon and radiologist agree on numbering.

Why phase-encode head-to-foot on lumbar sagittals?

With anterior–posterior phase, pulsation from the aorta ghosts directly over the spinal canal. Head-to-foot phase moves that motion artefact along the spine instead of across the pathology.

Resources/Protocols

Routine lumbar spine

The most common spine exam you will run. The questions are specific — a herniated disc causing sciatica, stenosis squeezing the cauda equina, a fracture — and the answers hinge on flattening the spine and counting levels correctly.

Protocols/Spine

When this study is used

Back pain, sciatica or radiculopathy.
Suspected disc herniation, spinal stenosis or cauda equina syndrome.
Suspected fracture, metastasis or spondylolysis.

Patient & coil positioning

Head-first supine with a firm wedge under the knees — flexing the hips flattens the lordosis against the coil, lifting signal and opening the disc spaces.
Arms crossed on the chest, strapped — arms at the sides enter the field of view and wrap over the canal.
Spine array coil centred at L3–L4, roughly the iliac crest.

Localizer

Three-plane localizer. The sagittal must show the conus medullaris (usually T12–L1) at the top and the sacrum below — miss the conus and you miss tumours.

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 T2

Sagittal

Planning — Parallel to the canal on the coronal, phase head-to-foot so aortic ghosts miss the spine. Foramen to foramen.

Why — Myelogram effect — bright CSF, dark discs and nerves — best for compression.

Sagittal T1

Sagittal

Planning — Same alignment and coverage.

Why — Anatomical map; marrow signal for cancer or fracture.

Axial T2 (angled stacks)

Axial

Planning — Angle a stack to each disc L1–L2 through L5–S1; the L5–S1 disc is steep and needs a marked tilt. 3–4 slices per disc, centred on the nucleus.

Why — Disc, canal and nerve-root assessment at each level.

Sagittal STIR

Sagittal

Planning — Match sagittal coverage; shim on the spine, excluding bowel gas.

Why — Marrow oedema — acute fracture, metastasis, ligament injury.

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
T2 TSE sagittal	1.5–3 T	320 / 28–32 cm	Phase H–F to avoid aortic ghost
T1 TSE sagittal	1.5–3 T	320 / 28–32 cm	Marrow, anatomy
T2 axial	1.5–3 T	256–320 / 18–20 cm	Angled per disc; thin for roots
STIR sagittal	1.5–3 T	256 / 28–32 cm	Fracture, metastasis

Artifacts & how to fix them

Artifact	Typical cause	Mitigation
CSF pulsation ghost	Turbulent CSF, aortic pulsation	Anterior sat band over the aorta; flow compensation
Inhomogeneous STIR	Large FOV and bowel gas	Shim on the spine only, excluding the belly
Blurry nerve roots	Partial-volume on thick axials	Drop to 3 mm; raise matrix
Phase wrap	Arms in the FOV	Arms crossed on chest; oversampling

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
Stenosis	Axial T2	Round thecal sac squeezed to a trefoil or slit
Cauda equina syndrome	Sagittal/axial T2	Canal filled with grey disc, no bright fluid — surgical emergency, flag it
Pars defect	Sagittal / parasagittal T1	Dark break in the neck of the bone between the facets
Metastasis / fracture	Sagittal STIR + T1	Bright oedema on STIR; dark marrow replacement or fracture line on T1

Review checklist

Conus medullaris and the whole sacrum included on the sagittal.
Axial stacks angled per disc, L5–S1 tilted to match its slope.
STIR fat suppression uniform along the spine.
Any cauda-equina compression escalated immediately.

Mistakes that cost repeats

Flat positioning leaving the lordosis lifted off the coil, dropping signal.
A straight axial block cutting the steep L5–S1 disc obliquely.
Miscounting levels in transitional anatomy — document the counting reference.
AP phase direction letting aortic ghosts streak over the canal.

Common questions

Why put a wedge under the knees for a lumbar MRI?: The lumbar spine curves forward and lifts away from the table coil. Flexing the hips over a knee wedge flattens the spine against the coil, which raises signal-to-noise and opens the disc spaces for clearer imaging.
How do you avoid operating on the wrong level?: Count from a fixed landmark — down from C2 on a whole-spine scout or up from the sacrum — and document any transitional vertebra (e.g. LSTV) with your counting reference, so the surgeon and radiologist agree on numbering.
Why phase-encode head-to-foot on lumbar sagittals?: With anterior–posterior phase, pulsation from the aorta ghosts directly over the spinal canal. Head-to-foot phase moves that motion artefact along the spine instead of across the pathology.

Try it in ScanFlo: Plan a lumbar sagittal with the knees flat, then re-plan after adding a knee wedge, and compare the disc spaces and signal. The flattened lordosis is the difference between a grainy scan and a crisp one.

← All spine studies