ISCD CBDT flashcards

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

Areal bone mineral density

BMD reported by DXA in g/cm², calculated as bone mineral content divided by the projected two-dimensional bone area. It underestimates density in large bones and overestimates it in small bones.

T-score formula

Patient BMD minus the young adult reference mean, divided by the young adult reference standard deviation. A value of −2.5 or lower defines osteoporosis by WHO criteria.

Z-score formula

Patient BMD minus the age-matched reference mean, divided by the age-matched standard deviation. A Z-score of −2.0 or lower is below the expected range for age.

WHO osteoporosis threshold

A T-score at or below −2.5 at the lumbar spine, femoral neck, total hip, or 33% radius. Values between −1.0 and −2.5 are classified as low bone mass or osteopenia.

Least significant change

The smallest BMD difference between two measurements that is statistically real at 95% confidence, calculated as 2.77 times the precision error. Changes smaller than this are indistinguishable from noise.

Precision versus accuracy

Precision is reproducibility of repeated measurements, expressed as a coefficient of variation. Accuracy is closeness to the true value and depends on scanner calibration rather than technologist technique.

Phantom quality control

A spine phantom scanned daily before patients, with the measured BMD plotted on a control chart. Results outside the manufacturer's limits, commonly about ±1.5%, require correction before clinical scanning.

Invalid vertebra rule

A lumbar vertebra is excluded from analysis if its T-score differs by more than 1.0 unit from an adjacent evaluable vertebra, or if it has fracture, focal lesion, or hardware.

Diagnostic hip regions

Only the femoral neck and total hip are used for diagnosis and T-score classification. Ward's triangle and the greater trochanter are not diagnostic sites and are not used to define osteoporosis.

33% radius site

The distal one-third forearm site, scanned on the non-dominant arm, used when spine and hip cannot be evaluated or measured. It is the preferred site in primary hyperparathyroidism.

Vertebral fracture assessment

A lateral DXA spine image used to detect morphometric vertebral deformity. A reduction of 20% or more in vertebral height defines a prevalent deformity.

FRAX tool

A calculator estimating 10-year probability of major osteoporotic fracture and hip fracture from clinical risk factors, optionally with femoral neck BMD. Treatment is often considered at 20% and 3% thresholds.

DXA beam principle

Two X-ray photon energies are used, and the ratio of low- to high-energy attenuation separates bone mineral from soft tissue. This eliminates the need for a water bath.

Pencil versus fan beam

Pencil-beam DXA uses one detector and a narrow beam, scanning slowly. Fan-beam DXA uses a detector array, scans faster but delivers slightly more dose and demands precise centering.

Cross-calibration

A phantom-based regression that converts BMD values when a patient moves between scanners or after major hardware changes. T-scores are never cross-calibrated, only BMD values.

QCT versus DXA

Quantitative CT measures true volumetric BMD in mg/cm³ and is less affected by degenerative artifact, but uses a higher dose and lacks a WHO T-score framework.

Quantitative ultrasound

Calcaneal QUS measures speed of sound and broadband ultrasound attenuation. It predicts fracture risk but cannot diagnose osteoporosis by WHO criteria or be used for monitoring.

DXA effective dose

Roughly 1–10 µSv per site, comparable to hours to a day of background radiation and far below a chest radiograph. Occupational shielding is not routinely required.

Spine positioning

Supine with hips and knees flexed over a block to flatten lumbar lordosis, mid-sagittal line centered, scanning from about T12 through L5. Correct positioning prevents falsely elevated BMD.

Femur internal rotation

The hip is scanned with the femur internally rotated 15–25 degrees so the femoral neck lies parallel to the table, giving the longest, most reproducible neck profile.

Artifacts raising spine BMD

Aortic calcification, osteophytes, facet sclerosis, and overlying metal increase measured aBMD. Laminectomy and surgical resection lower it. These can mask or mimic osteoporosis.

Body composition model

DXA partitions the field into bone mineral, fat mass, and lean soft tissue. The three-compartment model assumes constant lean-tissue hydration, so edema and dehydration bias fat and lean estimates.

Fragility fracture

A fracture resulting from a fall from standing height or less. A prior hip or spine fragility fracture raises future fracture risk independently of BMD.

Secondary osteoporosis causes

Conditions including glucocorticoid excess, hyperparathyroidism, hyperthyroidism, vitamin D deficiency, malabsorption, hypogonadism, and multiple myeloma. Basic screening includes calcium, vitamin D, PTH, and TSH.

Glucocorticoid-induced osteoporosis

Caused by prednisone at 5 mg/day or more for 3 months or longer. Fracture risk rises early, and BMD monitoring is recommended within 6 months of starting and then annually.

Denosumab discontinuation risk

Stopping denosumab causes rapid rebound bone loss and vertebral fracture risk. Therapy requires continued dosing or transition to another agent to preserve gains.

Anabolic agents

Teriparatide and abaloparatide are PTH-receptor anabolics that build bone; romosozumab is a sclerostin inhibitor with anabolic and antiresorptive effects. They are used for severe or refractory osteoporosis.

Bisphosphonate drug holiday

A planned pause in oral bisphosphonate therapy after roughly 3–5 years in patients whose fracture risk has fallen. It limits rare risks such as atypical femoral fracture and jaw osteonecrosis.

Monitoring interval logic

Follow-up intervals follow the least significant change and expected rate of loss. With 1% spine precision, a 3% annual loss becomes detectable in about 1–2 years; stable patients need 2 years or more.

Root-mean-square precision

A precision estimate pooling the standard deviations of repeated scans across multiple subjects, preferred over a single-subject SD because it better reflects true reproducibility error.

Control chart rules

Quality control charts flag drift when points exceed limits, for example two consecutive phantom measurements beyond 2 SD. Drift triggers recalibration or service before clinical scanning.

Bilateral hip baseline

Both hips are scanned at baseline because side-to-side BMD differences can exceed the least significant change. Follow-up scans use the same hip and same region of interest.