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DEXA Scan (Bone Densitometry): Positioning, T-Scores & Patient Prep

"Bone density scan" — the exam most patients know by name but few technologists get formal training in. DEXA (dual-energy X-ray absorptiometry, also written DXA) is the gold standard for diagnosing osteoporosis and the fastest-growing post-primary credential a radiologic technologist can add. It is quiet, fast, low-dose, and completely different from routine radiography in one crucial way: the patient is not the image. The numbers are.

This guide covers how DEXA works, how to position the three routine sites — PA lumbar spine, hip, and forearm — how T-scores and Z-scores are interpreted, patient preparation, quality control, and the high-yield registry concepts. Scan parameters and positioning aids are manufacturer- and facility-specific (Hologic, GE Lunar, and others each publish their own); this guide is educational and follows ISCD (International Society for Clinical Densitometry) standards, not a universal recipe.

Why DEXA Is Different From Plain Radiography

In conventional radiography you produce an image for a radiologist to interpret. In DEXA you produce numbers: bone mineral density (BMD) expressed as grams per square centimeter (g/cm²) — an areal density, not a true volumetric density like QCT's g/cm³.

Feature	DEXA	QCT (quantitative CT)
Measurement	Areal BMD (g/cm²)	Volumetric BMD (g/cm³)
Energy	Two distinct X-ray photon energies	Single spectrum CT acquisition
Primary sites	PA L1–L4 spine, hip, forearm	Spine (trabecular-rich)
Dose	Very low — comparable to background radiation	Higher than DEXA
Best for	Routine osteoporosis screening and monitoring	Obesity (BMI > 35), extensive spinal degeneration, extremes of height

DEXA uses a C-arm with the X-ray source below the supine patient, emitting photons at two distinct energy levels — one optimized for soft tissue, one for bone. A collimator minimizes scatter. The attenuation of each energy is detected above the patient, and the two measurements are combined mathematically so that soft tissue "cancels out," leaving the bone mineral signal. That is the whole trick of dual-energy: two readings, one bone.

Indications: Who Gets Scanned

The ISCD 2023 Official Positions and the U.S. Preventive Services Task Force agree on the screening targets:

• Women aged 65 and older — routine screening
• Men aged 70 and older — routine screening
• Postmenopausal women under 65 with risk factors: low body weight, prior fracture, high-risk medication use, or a condition associated with bone loss
• Men under 70 with the same risk factors
• Any adult with a fragility fracture (a fracture from minimal or no trauma)
• Anyone being considered for pharmacologic therapy for osteoporosis, or already being treated — to monitor effect
• Adults with diseases or medications associated with bone loss (glucocorticoids ≥ 5 mg prednisone equivalent daily for ≥ 3 months, aromatase inhibitors, androgen deprivation therapy, hyperparathyroidism, chronic renal failure, malabsorption, and others)

The Three Routine Scan Sites

The ISCD standard is to measure both the PA lumbar spine and the hip in all patients. The forearm (33% radius) is added when the hip or spine cannot be measured or interpreted, in hyperparathyroidism, and in very obese patients who exceed the table weight limit.

Site	Region of interest	When routine
PA lumbar spine	L1–L4 (all evaluable vertebrae)	Always
Hip	Femoral neck or total hip — whichever is lowest	Always
Forearm	33% (one-third) radius, nondominant	Hip/spine not measurable, hyperparathyroidism, over table weight limit

PA Lumbar Spine Positioning

• Patient supine, straddling the center line of the table so the spine is centered; arms at sides
• Positioning block or bolster under the legs, with the femurs at approximately 45–90° to the tabletop (varies by manufacturer) — this flattens the lumbar lordosis
• Have the patient raise and lower the hips slightly to flatten the spine
• Align the laser transversely down the midsagittal plane; locate the ASIS for vertical centering
• Place the laser approximately 1–2 inches below the ASIS — the scan starts around the middle of L5
• The scan should end at the top of the rib attachment (T12)
• Evaluation: spine straight in the field, iliac crests evenly displayed in the lower corners, even soft tissue on both sides, all of L1–L4 included

Analysis rules (ISCD): use all evaluable vertebrae from L1–L4. Exclude a vertebra only if it is clearly abnormal and non-assessable, or if it differs by more than 1.0 T-score from adjacent vertebrae (e.g., a compressed or heavily degenerated vertebra). Use three vertebrae if four cannot be used, two if three cannot — and if only one evaluable vertebra remains, diagnosis must be based on a different site.

Hip (Proximal Femur) Positioning

• Patient supine, centered, pelvis level; leg extended and slightly abducted; hands on chest; shoes removed
• Place the positioning device between the feet and strap the foot with Velcro
• Internally rotate the leg approximately 15° (15–20° depending on device) using the positioning wedge
  • Why: internal rotation brings the femoral neck parallel to the tabletop, giving the best sampling of bone
  • It prevents the lesser trochanter from rotating into the femoral neck and shaft — which would falsely elevate BMD and lengthen the femoral neck
• Align the femoral shaft to the long axis of the table; center the laser longitudinally to the center of the femoral shaft (or at the bend of the hip, 1–2 inches inferior)
• Start below the ischium, not at the hip joint
• Evaluation: femoral shaft parallel to the tabletop, proper rotation with the lesser trochanter barely visible, soft tissue visible below the lesser trochanter/ischium and above the acetabular rim, bone medial to the acetabulum, no gaps in the bone map

Diagnosis rule (ISCD): use the femoral neck or total hip, whichever is lowest. Ward's area and the greater trochanter must not be used for diagnosis (they are no longer diagnostic sites per ISCD 2023 — software may still display them, but they do not drive the diagnosis).

Forearm (33% Radius) Positioning

• Use the nondominant arm (6–9% bone mineral content variation exists between arms) with no prior fracture history
• Patient seated beside the table — use the same chair every time: no wheels, no armrests, sitting flat
• Forearm prone (palm down) on the table, long axis parallel to the long axis of the table, hand supported
• Laser start: left forearm starts in the mid-forearm (first row of carpal bones within 15 cm of the start); right forearm starts at the first row of carpal bones
• Evaluation: forearm straight and centered, long axis parallel to the scan field, at least one row of carpal bones included, enough air on the ulnar side to equal the shaft width of the ulna
• The 33% radius (one-third radius) is the diagnostic ROI; other forearm ROIs are not recommended (ISCD)

Clinical pearl: some systems require a physical ulna measurement — elbow on a flat surface, forearm vertical with the fifth digit toward the face, measuring to the ulnar styloid in centimeters. Record it; the software needs it for analysis.

Patient Preparation

Most of DEXA's prep is about removing things that fake the numbers:

• No calcium supplements for 24 hours before the exam (calcium tablets are radio-opaque and elevate BMD)
• No metal: zippers, snaps, underwire, metal jewelry, or clothing with metal; the patient changes into a gown or scrubs
• Remove all radio-opaque objects from the scan area — even a bra clasp outside the field can corrupt the analysis
• Recent contrast precludes the exam: barium or gadolinium from another study requires a 2-week wait; defer if the patient recently had a nuclear medicine tracer
• Pregnancy screening: premenopausal patients are asked about the possibility of pregnancy; DEXA is avoided during pregnancy
• Patients should be able to tolerate lying supine for up to 10 minutes; weight limit is typically ~300 lb (check the specific model)
• If a prior DEXA exists, the patient should bring the results — follow-up should be on the same machine
• Body composition studies: morning appointment after a 12-hour overnight fast

T-Scores, Z-Scores, and the WHO Classification

The T-score is the number of standard deviations between the patient's BMD and the young-adult reference mean (peak bone mass). The Z-score is the number of standard deviations from the age-matched mean. T-scores drive the diagnosis in postmenopausal women and men 50 and older; Z-scores are used in premenopausal women and men under 50.

Category	T-score	Notes
Normal	≥ −1.0	—
Low bone mass (osteopenia)	−1.0 to −2.5	ISCD prefers "low bone mass" over "osteopenia"
Osteoporosis	≤ −2.5	At lumbar spine, total hip, or femoral neck
Severe (established) osteoporosis	≤ −2.5 + fragility fracture	—

• The WHO reference standard for the osteoporosis diagnosis is a T-score of −2.5 or less at the femoral neck (calculated from the NHANES III female, white, age 20–29 database)
• Osteoporosis may be diagnosed in postmenopausal women and men ≥ 50 when the lumbar spine, total hip, or femoral neck T-score is ≤ −2.5
• Z-score ≤ −2.0 is "below the expected range for age" and flags possible secondary causes of bone loss (hyperparathyroidism, malabsorption, glucocorticoids, etc.)
• Osteoporosis cannot be diagnosed in men under 50 on BMD alone — that is a registry-favorite statement

FRAX (Fracture Risk Assessment Tool, WHO 2008) estimates the 10-year probability of a major osteoporotic fracture using clinical risk factors — age, sex, height, weight, prior fracture, glucocorticoid use, rheumatoid arthritis, secondary causes, smoking, and alcohol (≥ 3 drinks/day). In the U.S., the National Osteoporosis Foundation recommends FRAX for patients with T-scores between −1.0 and −2.5.

Quality Control and the Least Significant Change

DEXA's value depends on reproducibility, so QC is a real part of the technologist's job:

• Phantom scans at least once per week as an independent check of system calibration; plot and review the data, verify the phantom mean BMD after any service, and keep service logs
• Precision assessment: each facility must determine its own precision error — never use the manufacturer's supplied number. The method: measure 15 patients 3 times, or 30 patients 2 times, repositioning the patient between each scan, then calculate the root mean square standard deviation (RMS-SD) and the least significant change (LSC) at the 95% confidence interval
• Minimum acceptable precision for an individual technologist (ISCD): lumbar spine 1.9% (LSC 5.3%), total hip 1.8% (LSC 5.0%), femoral neck 2.5% (LSC 6.9%). Worse than these → retraining
• Every technologist should complete one precision assessment after basic training (~100 patient scans), and again after a new system is installed or skills change
• LSC in practice: a follow-up BMD change must exceed the LSC to be called a real change. If the spine LSC is 5.3%, a 2% drop is noise, not bone loss

Follow-up rule: patients should return to the same DXA machine used for their prior study. Never apply an LSC or report BMD change between instruments that have not been cross-calibrated.

Contraindications and Limitations

There are no absolute contraindications to DEXA, but several situations require modification or rescheduling:

• Recent gastrointestinal contrast or radionuclides (2-week wait for barium/gadolinium)
• Pregnancy
• Scoliosis — may affect spine measurements
• Severe degenerative changes or fracture deformity in the measurement area
• Implants, hardware, or foreign material in the measurement area — metal in the ROI falsely elevates BMD
• Inability to attain the position or remain still
• Extremes of BMI — very high or very low body habitus can degrade accuracy; QCT is the preferred alternative for BMI > 35, extensive spinal degeneration, and extremes of height

DEXA also cannot tell primary from secondary osteoporosis on its own — laboratory work-up (calcium, PTH, vitamin D, thyroid studies) is frequently needed to find the cause behind a low BMD.

The ARRT Bone Densitometry Credential

Registered technologists can add post-primary certification in Bone Densitometry (R.T.(BD)) through the ARRT. It is one of the smaller, more attainable post-primary credentials — structured education (typically 16 credits) plus clinical experience in bone densitometry, then the exam. For a technologist working in outpatient imaging, bone density screening volume makes this a practical career move; see the radiologic technologist salary guide for how post-primary credentials affect earning potential.

Internal Linking — Build Your Knowledge

This DEXA guide connects to the rest of the imaging library:

• Hip X-Ray Positioning — the same internal-rotation principle for the femoral neck in conventional radiography
• Thoracic and Lumbar Spine Positioning — spine anatomy and the lordosis-flattening trick DEXA borrows
• Radiation Safety for Radiologic Technologists — where DEXA's low dose fits in the overall protection picture
• CT Scan Physics — how QCT measures volumetric BMD as an alternative
• X-Ray Modality Overview — the physics foundation for all X-ray-based imaging
• CT Scan Modality Overview — QCT and opportunistic BMD screening

Test Your Knowledge — Educational Practice Questions

Quick Reference — DEXA Positioning Checklist

• ✅ Confirm screening indication: women ≥ 65, men ≥ 70, or risk factors
• ✅ Patient prep: no calcium 24 hours, no metal, no recent contrast (2 weeks), pregnancy screened
• ✅ PA L1–L4 spine: supine, legs on bolster (45–90°), laser 1–2" below ASIS (start mid L5), end at T12
• ✅ Hip: supine, leg internally rotated ~15°, femoral shaft parallel to table, start below ischium, lesser trochanter barely visible
• ✅ Forearm (when needed): nondominant arm, prone, same chair every time, 33% radius ROI
• ✅ Use femoral neck or total hip — whichever is lowest; never Ward's area for diagnosis
• ✅ T-score ≤ −2.5 (spine, total hip, or femoral neck) = osteoporosis (postmenopausal women, men ≥ 50)
• ✅ Z-score ≤ −2.0 = "below expected range for age" (premenopausal women, men < 50)
• ✅ Same machine for follow-up; changes must exceed the LSC
• ✅ Weekly phantom scans and documented precision assessment

Authoritative Sources

• ISCD 2023 Official Positions (Adult) — indications, PA L1–L4 and hip ROI rules, T-score/Z-score reporting, precision (1.9/1.8/2.5% with LSC 5.3/5.0/6.9%), phantom scanning, follow-up, Ward's area exclusion (accessed 2026-08-10).
• Krugh M, Langaker M. Dual-Energy X-Ray Absorptiometry. StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2026 — physics (two photon energies, g/cm²), positioning (spine, hip, forearm), indications, patient preparation, WHO T-score classification, FRAX, contraindications (NBK519042).
• AHEC Bone Densitometry Initial Training, Lecture 6: Common Areas Scanned for a DXA Scan — PA lumbar spine, proximal femur, and forearm positioning steps, laser centering, rotation rationale, image evaluation criteria, precision considerations (aheconline.com training PDF).
• RTstudents.com DEXA Basics — primary skeletal sites, protocol summaries (hip internal rotation 15–20°, forearm 33% distal to ulnar styloid), T/Z-score thresholds, LSC definition.
• IAEA Radiation Protection of Patients: DXA Bone Mineral Densitometry — dual-photon-energy technique, preferred regions, low-dose profile, "DXA" terminology (accessed 2026-08-10).
• ARRT: Bone Densitometry post-primary certification — credential pathway for registered technologists (accessed 2026-08-10).
• Clark's Pocket Handbook for Radiographers — AP femur/hip internal rotation principle (15–20°) referenced for the DEXA hip positioning concept; Clark's does not cover bone densitometry as a procedure.