DEXA Scan

Today we are going over the basics of DEXA. This will help patients understand the test and what is being measured. It will also help clinical providers understand this test better. Let's dive in!

The Basics

DEXA stands for Dual Energy X-Ray Absorptiometry

-              Xray machine used for analyzing body composition

-              Great at telling the difference between bone, fat, and lean tissue (mostly muscle)

-              Uses 2 xray beams of different energies for this, hence Dual (via subtracting attenuation coefficients….or something like that)

-              Most studied and widely used tool for looking at bone density

-              USPSTF recommends all women over age 65 get one

-              But 65 is way to late to get your first DEXA. The Cliff of old age is already luming at that point.

-              I recommend getting one in your 20s or 30s to get a baseline and to help plan for the future. This will give you decades to improve things like bone density or get more muscled.

Image of a patient getting a DEXA Scan

Health Risks

Contains Ionizing Radiation (xray) – But at a very low dose (though some older machines are much higher)

•       daily life = 10 microSieverts

•       DEXA scan = 5 microSieverts

•       plain Xray = 50 microSieverts

•       whole body CT scan = 12,000 microSieverts

Data from the International Atomic Energy Agency.

MicroSieverts = Unit intended to represent the health risk from ionizing radiation

In spite of being ridiculously low pregnant woman are still excluded from this study.

Measurements Provided By DEXA

-              Bone Density

-              Lateral Spine and Abdominal Aorta Calcification

-              Body Fat

-              Visceral Fat

-              Muscle – Appendicular Lean Mass Index

-              More capabilities continue to be added

Bone Density

This is a measure of how dense or mineralized the bones are. It accounts for minerals like calcium, but not proteinaceous connective tissue like collagen. So good but not perfect.

-              Bone density is usually measured on both hips and the lumbar spine. Can be measured at other bony sites as well (e.g. wrist)

-              Reported as a T and Z Score

-              T Score: how dense one’s bones are relative to a healthy sex matched 30-year-old defined as 0. Each -1 or +1 move is a standard deviation from the average, with negative numbers representing less dense bone.

-              T Scores of <= -1 are defined as osteopenia, Score <= -2.5 are Osteoporosis

-              Z scores are measured the same way. But the 0 value is set as someone that is sex and aged matched to the individual being tested

-              Osteoporosis and Osteopenia are characterized by reduced bone density and increased fracture risk – bad deal.

Lateral Spine and Abdominal Aorta Calcification

Modern DEXAs can take beautiful pictures of the spine. Stress fractures, disk problems, and even spinal degeneration can be seen. Additionally, if there is calcified plaques on the abdominal aorta, these can be visualized and even scored with a DEXA Scan

Body Fat

Great way to look at total body fat. Reported as a percent of total body mass. This measurement includes visceral and subcutaneous fat. Subcutaneous fat (the stuff you can squeeze on the outside of the body) is not considered harmful to health and provides energy reserves and padding. Though most of us do not like too much of it.

Visceral Fat

This is the fat around the internal abdominal organs. Increased levels of visceral fat are very metabolically unhealthy and increase the risk for metabolic syndrome, diabetes, heart attacks, strokes, and many types of cancer tremendously.  DEXA scans can measure this type of fat accurately. If your waste line is large, you probably have too much visceral fat. Most often measured in grams.

Men’s waistline > 40 inches = bad

Women’s waistline > 35 inches = bad

Muscle

DEXA looks at lean tissue and this mostly consists of muscle. So how muscled are you?

Appendicular Lean Mass (ALM) = Muscle on both arms + Muscle on both legs

Appendicular Lean Mass Index (ALMI) = ALM / Height

ALMI

A great way to look at how muscled a person is normalized to height (in meters2)

Normal Values

Men > 7 kg/m2

Women > 5.5 kg/m2

Though, I tell my patients to shoot for a bit more than this.

Notes for Testing Day

-              If tracking changes over time, try and use the same machine or atleast the same model

-              Most insurance paid DEXA scans just look at bone density, but there is so much more.

-              Don’t wear any metal on your testing day

-              If looking at body composition, try and fast for 4 to 6 hours before your exam

-              If you have a hip replacement or lumbar spine hardware, bone density cannot be measured in these areas.

Thank you so much

Bill Brandenburg, MD

References

-              International Atomic Energy Agency

-              Hologic Horizon DEXA Scan Protocols and Software

-              Experience Performing 100s of scans