6.5 Whole Body DXA, Body Composition, and Pediatric Scanning

6.5 Whole Body DXA, Body Composition, and Pediatric Scanning

Quick Answer: Whole body DXA extends the same two-energy physics to the entire body, resolving bone mineral, fat mass, and lean soft tissue overall and by region. In children and adolescents aged 5–19, the standard measurement is total body less head (TBLH) together with the PA lumbar spine, results are expressed as Z-scores, and a Z-score of −2.0 or lower is reported as low bone mineral content or density for chronological age. Whole body DXA and pediatric scanning are named topics in the ARRT specifications effective January 1, 2027.

Whole Body DXA and Body Composition

What It Measures

A whole body acquisition sweeps the entire patient in a single pass, typically taking a few minutes. Because the two-energy measurement distinguishes bone from soft tissue, and because the soft tissue itself can be further resolved into fat and lean components by its attenuation ratio, a whole body scan produces a three-compartment model:

Results are also reported regionally — head, arms, legs, trunk, and android and gynoid compartments — which is what makes the study useful beyond a single number.

Clinical Applications

• Pediatric bone health assessment, where TBLH is a standard site.
• Body composition monitoring in obesity management, bariatric surgery follow-up, athletic assessment, and nutritional intervention.
• Sarcopenia assessment through appendicular lean mass indexed to height.
• Monitoring in chronic disease such as HIV, cystic fibrosis, inflammatory bowel disease, and chronic kidney disease, where both fat and lean compartments change.

Acquisition Requirements

Whole body scanning has its own positioning discipline:

• The patient must fit entirely within the scan field. Tall patients may require a manufacturer-specified offset or partial-scan protocol; wide patients may exceed the field laterally, and a documented, reproducible approach must be used.
• The patient lies supine and centered, arms at the sides with palms down and separated from the trunk, so the software can resolve arms from trunk.
• Legs are straight and internally rotated slightly, with feet secured by strap or tape to prevent drift.
• All metal is removed — the whole body is the field, so jewelry, hair clips, belts, and clothing hardware all matter.
• Quiet breathing and complete stillness for the whole acquisition.
• Non-removable items such as prostheses, implanted devices, and casts must be documented, and regional results affected by them interpreted with that knowledge.

Why Total Body Less Head

The skull is dense and contributes a disproportionately large fraction of total body bone mineral — a much larger fraction in a child than an adult. It also responds very little to disease, nutrition, growth, or therapy. Including it therefore dilutes exactly the signal a clinician wants to see and makes serial change harder to detect. Total body less head excludes the cranium and is the standard for pediatric assessment, and is also commonly preferred for monitoring in adults.

Pediatric and Adolescent Scanning

Children are not small adults, and pediatric densitometry has its own rules because a growing skeleton changes in size as well as in density.

The Core Problem: Areal Density and Body Size

DXA reports areal BMD — mineral per unit of projected area — and therefore carries no information about bone depth. A larger bone contains more mineral along the beam path and reports a higher areal BMD even when true volumetric density is identical. In an adult population this confound is modest. In a pediatric population spanning ages 5 to 19, body size varies enormously, and two children of the same age can differ by a factor of three in body mass.

The consequence is that raw areal BMD systematically under-reports bone status in small children and over-reports it in large ones. Adjustments for height, height-age, or bone mineral apparent density are used to compensate, and interpretation belongs to a clinician familiar with pediatric bone disease.

Site Selection

Reporting Rules

The terminology rules are strict and are examined directly:

• Use Z-scores, never T-scores. A young skeleton has not reached peak bone mass, so comparing a 10-year-old against a young-adult reference is meaningless.
• Z-score ≤ −2.0 is reported as "low bone mineral content" or "low bone mineral density for chronological age."
• Do not use the word "osteoporosis" on the basis of densitometry alone. Diagnosing osteoporosis in a child requires low bone mass together with a clinically significant fracture history — conventionally a long-bone fracture of the lower extremity, a vertebral compression fracture, or two or more long-bone fractures of the upper extremity.
• Do not use the word "osteopenia" in pediatric reporting.
• Reference databases must be appropriate to the child's age, sex, and where applicable ancestry, and the database used should be stated.

Practical Technique with Children

• Dose is a heightened concern. Children have longer remaining lifespans and more radiosensitive tissue, so ALARA applies with additional force: acquire the minimum number of sites ordered, and avoid repeats caused by preventable motion.
• Prevent motion rather than correcting it. Explain in age-appropriate language, demonstrate on a doll or on yourself, use the fast acquisition mode if available, and allow a parent to remain in the room where policy permits.
• A parent remaining in the room requires the same radiation-protection consideration as any other non-occupational person, and a pregnant caregiver should not be the one to stay.
• Secure, do not restrain. Velcro straps and positioning sponges hold position; struggling against restraint produces more motion than it prevents.
• Serial comparison in a growing child must account for growth. Absolute BMD is expected to rise simply because the child is getting bigger, so a stable Z-score during growth means bone accrual is tracking normally.

Pediatric Scanning and the ARRT Clinical Log

Performing and analyzing DXA scans on pediatric patients aged 5 to 19 is one of the elective clinical experience activities BD candidates may use, at two occurrences. Candidates who work in a facility that scans children should consider taking that elective, because the pediatric rules above are examined regardless of whether the candidate has performed the scans.