5.1 Lumbar Spine Acquisition & ROI Rules
Key Takeaways
Use the posterior–anterior lumbar acquisition for spine BMD. Center the patient and use a positioning block as feasible to reduce lordosis.
The iliac crests, sacrum, ribs and vertebral shape help locate L1–L4. Use several landmarks because transitional segments and rib variants can mislead a single-landmark count.
Check coverage, rotation, motion, vertebral labels and boundaries before accepting the analysis or comparing BMD with an earlier exam.
Acquire a centered PA lumbar image
The standard lumbar spine acquisition for bone mineral density (BMD) is posterior–anterior (PA). Older material and some scanners may call the acquisition AP. In the usual supine geometry the beam enters posteriorly. The two-dimensional image includes the vertebral bodies and posterior elements in projection, so the measurement is not an isolated sample of vertebral trabecular bone. Superimposed mineralized structures and external radiopaque objects can affect the result.
Review the patient's prior scan when available. Ask about spinal surgery and identify external objects that could obscure anatomy. Remove radiopaque clothing or objects from the scan field. Use the correct scan mode and centering for the scanner. The field must show enough anatomy to label L1–L4 and identify artifacts. A fixed laser start distance is not transferable to every scanner and acquisition mode.
- Position the patient supine with the trunk centered, the spine as straight as the patient can tolerate, and the arms outside the scan field.
- Use the lumbar positioning block to support the legs. Flex the hips and knees to approximately 90 degrees when feasible. This reduces lumbar lordosis and helps display the intervertebral spaces.
- Check the image for centering, motion, rotation and sufficient coverage. If pain, deformity or limited mobility requires a different position, record the limitation and assess whether the image supports interpretation.
- Review vertebral numbering, global region of interest (ROI), intervertebral boundaries and bone detection before accepting the result. For follow-up, retrieve the previous image and verify that the same levels and comparable analysis were used.
Locate the vertebral levels
Use several landmarks together. The ISCD DXA Atlas shows a bottom-up approach using the iliac crests to locate the usual L4–L5 interspace and the first body above the sacrum as an initial L5 landmark. Ribs and vertebral shape can corroborate the count. Labeling a different vertebral level changes both the sampled BMD and the normative comparison.
| Landmark | What it helps locate | What can cause an error |
|---|---|---|
| Tops of the iliac crests | The usual region of the L4–L5 interspace | Rotation, limited coverage or a numbering variant |
| Sacrum and adjacent lowest mobile body | The lower end of the lumbar sequence | A lumbosacral transitional segment can make that assignment uncertain |
| Lowest visible ribs | Often T12, useful as a check against the pelvic landmarks | A missing or short rib or an additional lumbar rib can mislead a rib-only count |
| Vertebral shape and disc boundaries | Distinguish adjacent levels and place analysis boundaries | Scoliosis, fractures and degenerative change can alter the expected profile |
A transitional vertebra can have features of both the lumbar spine and sacrum. When a rib count and the pelvic landmarks disagree, review the scan coverage and available prior imaging. Do not shift all labels based only on the lowest rib. If needed, seek corroboration from suitable spine imaging. Document any recognized variant and the convention used to analyze the DXA. Check that the same anatomy was followed on earlier studies before interpreting a difference.
Worked localization problem
A follow-up study has an automated L1 label immediately below the lowest visible rib. When you compare it with the original scan, the same body is labeled T12 on the previous image. The current global ROI also does not show the top of both iliac crests. The images may represent the same anatomy with shifted labels, or the current field may omit a landmark. First verify the anatomy using the sacrum, the visible iliac crest, and prior imaging. If the current acquisition cannot show the required levels, correct the acquisition when appropriate. If the anatomy can be matched, review both analyses before making a quantitative comparison. A matching label alone is insufficient when the underlying image differs.
Hand off the image for interpretation
Describe limitations at the affected levels and distinguish acquisition issues from a structural finding. Motion or poor centering may call for technical correction. Focal sclerosis, hardware or a compressed vertebra can require exclusion during interpretation even when acquisition is technically adequate. Review the lumbar spine interpretation rules for deciding which levels can contribute to the reported measurement.
ISCD DXA Atlas spinal level identification · ISCD transitional vertebra case · International Working Group DXA practice guideline
What is the purpose of supporting the legs on a positioning block for the standard PA lumbar spine DXA?
Flexing the hips and knees reduces lumbar lordosis and helps display the intervertebral spaces.
The block changes the atomic number of the soft tissue.
The block moves the twelfth ribs away from all vertebrae.
Hip flexion is used to profile the lesser trochanter.
On a follow-up lumbar scan, the lowest visible rib and the iliac crest suggest different vertebral labels. The previous DXA used yet another label for the same body. What is the best first step?
Always assign the body just below the last rib as L1.
Use the label that yields the highest BMD.
Review coverage, pelvic and sacral landmarks, and prior images to match the anatomy before making a comparison.
Apply the prior LSC directly because the scanner has not changed.
Sections you finish are checked off in the contents.