5.3 Forearm Scanning Indications & Technique
Key Takeaways
ISCD recommends forearm DXA when the hip and/or spine cannot be measured or interpreted, in hyperparathyroidism, or when the patient exceeds the DXA table weight limit.
Use the one-third (33%) radius of the non-dominant forearm for adult densitometric classification. A peripheral device requires a validated young-adult reference at this site.
In primary hyperparathyroidism, the cortical-rich one-third radius can show low BMD even when the spine is in the normal range.
Verify the side, image quality and ROI against the specific scanner instructions. A serial comparison needs reproducible placement at the same site.
Decide when the forearm adds a diagnostic site
The adult Official Positions of the International Society for Clinical Densitometry (ISCD) recommend measuring bone mineral density (BMD) at the posterior-anterior lumbar spine and hip. They also identify three circumstances for adding forearm dual-energy X-ray absorptiometry (DXA).
| ISCD indication | How to apply it |
|---|---|
| Hip and/or spine cannot be measured or interpreted | Look for joint replacements, spinal hardware, positioning limits or other factors that invalidate a central site. Keep any valid central result. |
| Hyperparathyroidism | Include the one-third radius because the cortical-rich measurement can show skeletal involvement that is less apparent at the spine or hip. |
| Patient exceeds the DXA table weight limit | Use the limit for that scanner model. If safe forearm acquisition is feasible with the patient seated beside the table, use the model’s protocol. |
For example, a patient with bilateral hip replacements may still have a valid spine measurement. A second patient may have assessable L1 and L3 with structural artifacts at L2 and L4. Two usable vertebrae can supply the spine BMD even when they are nonadjacent. If only L1 is usable, base the diagnosis on another valid site. When access or technical quality limits an acquisition, document why the result was omitted or how its interpretation is limited.
Use the one-third radius for adult densitometric diagnosis
ISCD specifies the one-third (33%) radius of the non-dominant forearm. A peripheral DXA device can provide a diagnostic one-third radius T-score when it uses a validated young-adult reference database. Other forearm regions, including the ultradistal radius and a combined radius-ulna measurement, are excluded from WHO diagnostic classification. A heel quantitative ultrasound score also cannot be substituted for the one-third radius T-score.
The one-third region samples cortical-rich diaphyseal bone. The ultradistal region, closer to the wrist joint, contains more trabecular bone. The composition difference helps explain why two forearm measurements can yield different results. The ISCD diagnostic rule specifies which region to report for classification. It does not depend on a fixed tissue percentage or precision figure.
Worked example in primary hyperparathyroidism
A postmenopausal woman with confirmed primary hyperparathyroidism has a valid lumbar spine T-score of −0.8, total hip T-score of −1.4, and one-third radius T-score of −3.2. The distal one-third radius is an indicated diagnostic site and provides the lowest valid T-score. The overall densitometric classification is osteoporosis. The 2022 Fifth International Workshop includes the one-third radius in the skeletal evaluation of primary hyperparathyroidism. In this case the low BMD also meets a skeletal criterion for considering parathyroid surgery. The DXA result describes skeletal involvement. The diagnosis of hyperparathyroidism comes from the clinical and biochemical evaluation.
Select the side and verify image quality
Record hand dominance, previous fractures, surgery, and any limitation affecting safe or reproducible positioning. Review the non-dominant side first. A fracture deformity or metal in the region of interest (ROI) can make the measurement unreliable. If it cannot be used, consider whether the other side can provide an interpretable result using the facility’s acquisition protocol. Record the side and reason for the choice. For a patient with dialysis access in one arm, account for access protection and possible local effects on BMD when deciding whether and where a scan is appropriate.
Follow the acquisition and analysis instructions for the specific scanner. For example, the Hologic Horizon user guide instructs the operator to measure forearm length in centimeters, enter the length in the analysis software, and anchor the global ROI to the tip of the ulnar styloid. It describes both seated and supine positioning. Check that the radius and ulna appear straight and centered and that the intended anatomy and adequate background are included. The starting point and image orientation may differ by side and model.
A scan with the forearm rotated or the global ROI displaced can sample different bone or include a local artifact. Before accepting a surprising value, check the length entry, landmarks, alignment and bone map. On a follow-up scan, check the original images for side, positioning and region placement. A new side or materially different ROI cannot be treated as a directly comparable serial measurement. If the follow-up is technically comparable, use the site's applicable facility precision and least significant change before reporting a BMD difference.
ISCD Adult Official Positions · Hologic Horizon user guides · Fifth International Workshop on Primary Hyperparathyroidism
A postmenopausal woman with confirmed primary hyperparathyroidism has a valid spine T-score of −0.8 and a valid total hip T-score of −1.4. What is the reason to obtain a one-third radius DXA measurement?
The ultradistal radius has replaced the one-third radius for diagnostic classification.
The forearm result confirms hypercalcemia when spine and hip scores disagree.
The cortical-rich one-third radius can show skeletal involvement that is less apparent at the spine or hip.
Primary hyperparathyroidism is diagnosed by applying the WHO criteria to the radius.
A patient’s follow-up forearm DXA shows a lower one-third radius BMD. Review shows that the global ROI was displaced from the prior landmark and the patient’s position differed. What should the interpreter do first?
Compare the T-scores instead of the BMD values.
Compare the current ultradistal radius with the prior one-third radius.
Report the change as bone loss if the numeric difference exceeds the facility LSC.
Review the acquisition and ROI, and establish a technically comparable measurement before applying the LSC.
According to the Official Positions of the International Society for Clinical Densitometry (ISCD), which region of interest on the forearm is validated and approved for establishing a diagnostic classification of osteoporosis?
The 33% radius (one-third radius) only.
The ultra-distal radius only.
The mid-radius and ultra-distal radius combined.
The total forearm (radius plus ulna composite).
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