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Body composition scans, read line by line

In the official pages this guide draws on, a body composition scan is a DXA scan: an x-ray technique that compares two images taken at a low and a high x-ray energy to derive measures such as bone mineral content, fat mass and lean soft tissue mass, as ARPANSA describes it. This guide, published by Dotto, reads the scan the way Australia’s health and radiation bodies set it out: what it measures, the radiation it uses, who may run one, and the simpler measures healthdirect uses.

Method: Dual-energy x-ray absorptiometry, written DXA or DEXA
Measures: Fat mass, lean soft tissue mass, bone mineral content and areal bone mineral density, among others
Coverage: A total body scan, for overall body composition
Typical dose: 0.001 to 0.01 mSv, ARPANSA
Licensing: Set by each state and territory

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

One machine, two x-ray energies

ARPANSA describes DXA as an x-ray imaging technique used mainly to work out the mass of one material in the presence of another, from the way each one weakens an x-ray beam at different energies. For body composition, two images are made at a low and a high energy and compared electronically. Victoria’s Better Health Channel describes the same principle for bone: beams of differing energy pick up bone and soft tissue density separately, and the technique can measure bone density in the spine, hip, forearm and total body.

healthdirect and the Better Health Channel both describe DXA as a bone test. healthdirect calls a bone density scan, a DEXA or DXA scan, a medical test used to help diagnose osteoporosis. Assessing overall body composition, ARPANSA says, needs a total body scan.

Radiation

The radiation question

Radiation note

ARPANSA gives the dose in DEXA scans as typically between 0.001 and 0.01 millisieverts (mSv). Its Radiation Health Committee says it is concerned about the scan’s increasing use in the fitness and weight loss industry.

The committee’s position is plain. A DXA scan done to find body fat and lean tissue, with no clinical indication behind it, is in its words “not a justified use of ionising radiation”, because it brings no net benefit. Other ways of measuring lean body mass, which use no ionising radiation, are available, it adds.

DXA and radiation sets out the dose, the committee’s statement and the cautions for pregnancy and for children.

Guides

Five guides, one per region of the report

01
What a scan measures
How DXA separates bone from soft tissue, the measures ARPANSA names, and how bone density results are scored.
02
DXA and radiation
The dose as ARPANSA states it, its committee’s view on scans for body fat, and the caution for pregnancy and children.
03
Who may run a DXA scan
Licences are set by each state and territory. Victoria’s use licence and its body composition condition, and Medicare’s rule.
04
BMI and waist measures
healthdirect’s adult BMI ranges and waist measurements, how to take them, and the limits it names.
05
Before a scan
Referral, what to discuss with a doctor, Medicare’s bone density items, the day itself and the results.

Simple measures

Two measures that need no machine

healthdirect describes two measurements that need no scan: body mass index (BMI), worked out from weight and height, and waist circumference, measured halfway between the lowest rib and the top of the hipbone. Together, it says, they can show whether you are a healthy weight for your height.

It is open about BMI’s limit: BMI cannot tell how much muscle or fat you have. For fat stored inside the body, around the organs, healthdirect calls measuring the waist the main way to see whether there is too much. BMI and waist measures gives healthdirect’s numbers and the groups for whom it says they are less accurate.

Regulators

Who sets the rules

In June 2016, ARPANSA’s statement on DXA said that DXA as a medical procedure, for bone mineral density or other uses, was regulated by its code RPS14, which required appropriate referral, justification and approval by appropriately qualified medical practitioners. ARPANSA has since replaced that code: its Code for Radiation Protection in Medical Exposure (RPS C-5, 2019), together with RPS C-1, supersedes RPS14.

Licences to operate radiation equipment are a state matter, and the requirements vary between states and territories. In Victoria, a use licence is needed to operate a radiation source, and a management licence condition, M1758, requires management licence holders to follow the department’s own requirements for assessing body composition with DXA. Who may run a DXA scan sets out Victoria’s rules as Victoria’s.

This guide is published by Dotto from the pages of ARPANSA, healthdirect, the Victorian Department of Health and its Better Health Channel, MBS Online and the federal Department of Health. It names no clinic, device or scanning service, and every source is listed at the foot of each page.