Region 01 · Measuresbodycomposition.com.au

What a body composition scan measures

A DXA scan compares two x-ray images, made at a low and a high energy, to derive measures that, ARPANSA says, include fat mass and lean soft tissue mass as well as bone mineral content and areal bone mineral density. For overall body composition, it adds, the scan has to cover the whole body.

Named measures: Fat mass and lean soft tissue mass; bone mineral content and areal bone mineral density
Bone result: A value in g/cm², with a T-score or Z-score
Bone density sites: Spine, hip, forearm and total body

General information Not medical advice. Call Triple Zero (000) in an emergency, or go to the emergency department of your nearest hospital. For health information, see healthdirect; for radiation safety, see ARPANSA: Code for Radiation Protection in Medical Exposure (RPS C-5).

Method

How two energies separate tissue

ARPANSA describes DXA as an x-ray imaging technique used mainly to work out the mass of one material when another is also present. It does this from what is known about how each material weakens, or attenuates, an x-ray beam at different energies. For body composition, the two images, one at a low and one at a high average energy, are compared electronically to give the measures.

Victoria’s Better Health Channel explains it in terms of bone. In a DEXA scan, x-ray beams of differing energy pick up the density of bone and of soft tissue separately, and the technique can measure bone density in the spine, the hip, the forearm and the total body. A related method, single energy x-ray absorptiometry, uses one beam and measures bone at sites such as the forearm and heel.

An illustration, not a real scan, and it shows no values: two images at different x-ray energies, compared to tell bone from soft tissue, as ARPANSA and the Better Health Channel describe it.

Measures

The measures ARPANSA names

ARPANSA’s statement lists the body tissue composition measures a DXA comparison can derive. It introduces them with “such as”, so they are examples, not a complete list.

The four measures ARPANSA names, in its order, with what the other official pages add.
Measure	What the official pages add
Bone mineral content	Named by ARPANSA among the measures.
Areal bone mineral density	Bone density is reported as a value in g/cm², and as a T-score or Z-score, the Better Health Channel says.
Fat mass	ARPANSA’s committee says a scan for it, with no clinical indication behind it, does not justify the radiation.
Lean soft tissue mass	The same caution applies, and ARPANSA notes that other ways of measuring lean body mass, without ionising radiation, are available.

DXA and radiation sets out the committee’s statement and the dose.

Bone density

How a bone density result is scored

healthdirect describes a bone density scan as a medical test used to help diagnose osteoporosis, a condition in which the building blocks of bone have weakened and bone density is lower. Bones with low density, it says, are more fragile and more likely to break.

The scan usually measures bone density in the lower back (the lumbar spine) and the hip, and sometimes the forearm, healthdirect says. The Better Health Channel says the result is reported as a value in g/cm², and as a T-score or Z-score, which compare your measurement with a reference group of other people. For the T-score that group is young adults of the same sex as the patient; for the Z-score it is people of the same age and sex.

healthdirect says the result is usually presented as a T-score, and reads it in three bands:

healthdirect’s three T-score bands, complete and in its own boundaries. The Better Health Channel also says osteoporosis is diagnosed when the T-score is −2.5 or less.
T-score	healthdirect’s reading
−1 or above	Normal.
Between −1 and −2.5	Lower than normal bone density (osteopenia), which means you may develop osteoporosis in the future.
−2.5 or below	You already have osteoporosis.

The T-score bands healthdirect gives, drawn to scale. The boundaries are its own: −1 or above, between −1 and −2.5, and −2.5 or below.

The results are sent to your doctor, healthdirect says. The Better Health Channel adds a point about ordinary x-rays: they can only detect osteoporosis once around a third of bone mass has already gone, so they cannot find it in its earlier stages.

Body fat

Body fat on the official pages

Most fat is stored under the skin, it says, and is known as subcutaneous fat: the fat you can see and feel. Excess internal fat, which healthdirect also calls visceral fat, sits on and around the vital organs, and healthdirect lists high blood pressure, heart disease, diabetes and some cancers among the health issues it can cause.

The measure healthdirect points to for internal fat is the waist. Measuring around it is the main way to see whether you have too much internal body fat, it says, and it can predict how much fat lies deep in the belly and around the organs. For BMI, healthdirect names a limit: it cannot tell how much muscle or fat you have. BMI and waist measures sets out both measures and their numbers.

If you are worried that you may be carrying an unhealthy amount of internal body fat, healthdirect’s advice is to talk to your doctor.

Repeat tests

Comparing results over time

Someone with osteoporosis, for example, may need repeat bone density tests, the Better Health Channel says, and it suggests having every future test at the same testing facility, on the same machine. Before a DXA scan covers the referral, the day and the follow-up appointment.