ARRT Bone Densitometry flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

K-edge filter

A rare-earth filter, such as cerium or samarium, that splits the X-ray spectrum into distinct low- and high-energy peaks, allowing DXA to subtract soft tissue and isolate bone mineral.

Pencil-beam DXA

DXA geometry using a single collimated detector that sweeps in rectilinear passes; it is slower but has high precision, negligible magnification, and lower radiation output.

Fan-beam DXA

DXA geometry using a linear detector array and a wide fan-shaped beam; scanning is faster with better spatial resolution, but magnification and dose are slightly increased.

Areal bone mineral density

DXA reports BMD in g/cm², a two-dimensional projected value that depends on bone size and thickness, unlike the volumetric density in mg/cm³ reported by QCT.

DXA effective dose

A lumbar spine or hip DXA delivers roughly 1 to 10 microsieverts, comparable to a few hours of natural background radiation and well below a chest radiograph.

Anthropomorphic spine phantom

An epoxy block containing hydroxyapatite-equivalent vertebrae that is scanned on a fixed schedule; its measured BMD is plotted on a control chart to detect scanner drift.

Dual-energy X-ray absorptiometry

An imaging method that acquires two X-ray spectra and uses differential attenuation between bone and soft tissue to quantify bone mineral content and areal BMD.

Quantitative ultrasound (QUS)

Measures speed of sound and broadband attenuation at the calcaneus; it predicts fracture but is not interchangeable with DXA and should not be used for monitoring therapy.

T-score definition

Calculated as patient BMD minus the young-adult mean, divided by the young-adult standard deviation; it is the basis of WHO diagnostic categories in postmenopausal women and men aged 50 and older.

Z-score definition

Calculated as patient BMD minus the age-matched mean, divided by the age-matched standard deviation; a value of minus 2.0 or lower is termed below the expected range for age.

WHO osteoporosis threshold

A T-score of minus 2.5 or lower at the lumbar spine, femoral neck, or total hip defines osteoporosis; between minus 1.0 and minus 2.5 is low bone mass, and minus 1.0 or above is normal.

Who receives Z-scores

Premenopausal women, men younger than 50, and children are reported with Z-scores rather than T-scores, because young-adult reference data are inappropriate for these groups.

Pregnancy and DXA

Pregnancy is a relative contraindication to DXA; the examination is generally deferred unless clearly justified, since fetal exposure, although very small, is avoidable.

Artifacts to remove

Metal zippers, buttons, coins, keys, and undergarment hardware must be removed before scanning, and recent barium or contrast studies can invalidate lumbar spine analysis.

Elevating the legs

Placing the supine patient's legs on a foam block flattens lumbar lordosis, opens the intervertebral disc spaces, and improves segmentation of individual vertebrae during spine acquisition.

Height and weight documentation

Standing height and weight measured at each visit are required to derive Z-scores, support FRAX calculation, and detect height loss suggesting vertebral fracture.

Standard spine ROI

L1 through L4 are analyzed by default; a vertebra is excluded when it is fractured, contains focal artifact or hardware, or shows severe degenerative or laminectomy changes.

Two-vertebra rule

At least two evaluable lumbar vertebrae are needed for a valid spine result; if only one vertebra remains, the spine is not diagnostic and the hip is used.

Hip regions of interest

Femoral neck, total hip, and trochanter are reported regions; Ward's area is displayed on printouts but is not used for diagnosis because of poor reproducibility.

Femoral internal rotation

Internally rotating the femur about 15 to 25 degrees with the foot in the positioning device brings the femoral neck parallel to the table so its full length is measured.

Selecting the lower hip

When right and left hip measurements disagree, the lower BMD and lower T-score at the femoral neck or total hip are used for diagnosis and for monitoring change.

33% radius site

The distal one-third radius of the non-dominant forearm, a predominantly cortical site, is scanned when the spine and hip are unavailable, invalid, or uninterpretable.

Vertebral labeling landmarks

The iliac crest aligns with the L4-L5 interspace and the lowest ribs align with T12; correct numbering prevents comparing different vertebrae on serial examinations.

Trabecular bone score

A gray-level texture index computed from the lumbar spine DXA image that reflects trabecular microarchitecture; low values add fracture risk information beyond BMD alone.

Coefficient of variation

CV equals the standard deviation divided by the mean, multiplied by 100 and expressed as a percentage; it quantifies measurement precision for a technologist or scanner.

Least significant change

LSC equals 2.77 times the precision CV at 95 percent confidence; a follow-up BMD difference smaller than the LSC cannot be distinguished from measurement error.

Precision assessment protocol

ISCD guidelines call for duplicate scans in 30 patients, or 15 patients repositioned between scans, to establish in vivo precision for each site and technologist.

Daily phantom scan

Scanning the spine phantom each working day and plotting its BMD detects drift; values outside 1.5 percent of baseline or 3 standard deviations prompt investigation and service.

NHANES III reference database

The femoral neck T-score in the United States is derived from NHANES III data for non-Hispanic white women aged 20 to 29; consistent database use is required for serial comparisons.

Cross-calibration

When a scanner is replaced or software is upgraded, scanning the same patients or a phantom on both systems establishes a conversion before serial BMD results are compared.

Monitoring interval

Repeat DXA is generally not performed sooner than two years unless therapy changes or clinical findings warrant it, because typical annual change falls below the least significant change.

FRAX input data

The FRAX tool estimates 10-year major osteoporotic and hip fracture probability from femoral neck BMD, age, sex, BMI, prior fracture, parental hip fracture, smoking, glucocorticoids, rheumatoid arthritis, and alcohol use.