DEXA Terminology

What are some of the Terminology associated with DEXA Scans?

 

Dual Energy X-Ray Absorptiometry

A technology that uses both high and low energy x-rays to determine the areal mass of tissue which is comprised of bone, fat & lean mass

Densitometer

Machine that measures the density of tissue by determining the amount of high & low energies that
are absorbed

Body Composition

Process of dividing the body weight into compartments typically the bone, lean tissue and fat tissue. This is reported as a percentage or in grams. It is based on the patient’s R-value. Body composition is sometimes referred to as tissue quantization.

R Value

The attenuation ratio of high-energy and low-energy photons in soft tissue and bone. The ratio is used to distinguish soft tissue from bone, to determine bone density, and to distinguish fat from lean soft tissue.

Bone Mineral Density

Measurement of bone mineral found in the region of interest. BMD is measured in grams per centimeter squared. BMD is derived using BMC divided by area, where BMC is measured in grams and area is measured in centimeters squared.

QA: Quality Assurance

A series of tests that calibrate the system and monitor its ability to meet performance specifications.

ROI

The anatomical area imaged and analyzed, and for which bone mineral density values are calculated.

SD: Standard Deviation

An estimate of the variability of sample values from the sample mean.

T-score

The number of standard deviations above (+) or below (-) the mean peak density.
The difference between the patient’s BMD and the mean young adult value of the reference population, divided by the reference standard deviation (SD). A T-Score of -2 means the patient is 2 SDs below the reference population.

Young Adult

Compares an individual’s BMD (Bone Mineral Density) with the mean value for young normal’s; reference population of subjects 20-39 years old and the same sex as the patient. Your T-score is your bone density compared with what is normally expected in a healthy young adult at peak bone density. It is expressed in units called standard deviation which shows if your bone density is above or below the average. The more standard deviations below 0, indicated as negative numbers, the lower your BMD and the higher your risk of fracture

Z-score

The number of standard deviations above (+) or below (-) the mean density for an individual of that age and sex. The difference between the patient’s BMD and the mean age-matched value of the reference population, divided by the reference population standard deviation (SD). A Z-score of -2 means the patient is 2 SDs below the reference population.

Age Matched

Average tissue fat percentage for a person’s age. Also, your Z-score is your bone density compared with what is normally expected in someone of the same age group. It is expressed in units called standard deviation which shows if your bone density is above or below the average. If your Z-score is -2 or lower, it may suggest a secondary cause of bone loss, something other than aging is causing bone loss.

Significant Change

This value is calculated by multiplying the precision error by 2.8 for a 95% confidence level. For a BMD change to be considered statistically significant, it must equal or exceed the calculated significant change.

Fracture Risk

High fracture risk is 10-year absolute fracture risk >20%; moderate fracture risk is 10-year absolute fracture risk in the range of 10% to 20%; low fracture risk is 10-year absolute fracture risk <10%.

LSC

Least significant change = amount by which one BMD value must differ from another in order for the difference to be statistically significant at a 95% level of confidence.

Cortical Bone

Dense, hard bone with microscopic spaces. Cortical bone is typically found in long bones such as the femur, forearm, and tibia, and in the outer region of vertebrae.

Trabecular Bone

Porous bone composed of an intricate, latticed network of fibrous, calcified mineral. It is typically found at points of compression, such as lumbar vertebrae.

Three Compartment Model

DEXA differentiates the body into three sub-types of tissue: Bone, lean and fat mass. Most methods of body composition analysis such as skin-fold measurements, bioelectric impedance analysis (BIA) & hydrostatic weighing use a two compartment model that only classifies the body as having fat mass and fat-free

Android

Apple shaped body (excess fat around abdomen, trunk and inside the abdominal cavity can increase one’s risk for diabetes & heart disease.

Gynoid

Pear shaped body types (excess fat around hips, thigh & buttocks) are less prone to harmful illness

A/G Ratio

Android (waist) to Gynoid (hip) Ratio – Carrying too much fat in the wrong areas can increase your chance of serious health problems. Men with waist to hip ratio of > 1 & women with ratio > 0.8 have increased risk of cardiovascular illness, diabetes, stroke, cancer etc

TISSUE % FAT

fat(g) / fat(g) + lean(g) x 100
REGION % FAT:  fat(g) / tissue(g) + BMC x 100

FAT(g)

Includes all the fatty tissue in the body i.e. fatty tissue found within the organs of the body & also the subcutaneous fat found under the skin.

LEAN(g)

Sum of all muscle and soft organ tissue

BMC

Bone Mineral Content – Sum of all skeletal tissue within the body measured by the densitometer

Total Mass(kg)

Sum of Fat + Lean + BMC

BMD

Bone Mineral Density expressed in g/cm2 – Amount of bone mineral content within a certain area measured by the densitometer

World Health Organizations Definitions (WHO)

      • Normal:  T-score -1 and above

 

      • Osteopenia (Low Bone Mass) T-score between -1 and -2.5

 

      • Osteoporosis T-score -2.5 and below

 

      • Severe Osteoporosis T-score -2.5  and below with fragility fractures

 

    • Fragility Fracture:  Low trauma fracture with minimal trauma i.e. trip or a fall from a standing height

 

Radiation Dose

      • DEXA Scan:   0.1-10 uSv

 

      • Daily Natural Background Radiation:  5-8uSv per day

 

    • Los Angeles to New York Return Flight:  50uSv