DEXA scan

Recommended if 2

-Lateral vertebral assessment LVA for older patients if loss <1 in of height, unexplained back pain, DXA scan gives borderline reading -Athletic or active population if history or signs/symptoms of female triad, whole body composition assessment

Peripheral Scan

-forearm -heel -very low patient dose

Reliability

-how does the measurement vary when done repeatedly?

Radiation exposure

-less than 1/10th dose of standard chest x-ray -less than 1-2 days exposure to natural background radiation -practitioner can be in the room w/o protective draping

Practitioner requirements

-quality control -Exam by ARRT- 60 questions -results reported to the state CO -requires practicing densitometrists to be trained, registered with ARRT and hold current certification by an organization recognized by that state

Why is DEXA better?

-skin calipers: measures only fat under skin, high source of technical error, not good for lean or obese people -Bioelectrical independence: high rate of false positive and false negative results, to many variables to control -underwater weighting: being submerged in water, heavy equipment, time consuming -Bod Pod: only two dimensional pictures, not readily available, very expensive -CT very high dose of radiation and expensive, not readily available for body fat measured.

Risks

-slight cancer risk associated w/x-ray exposure -pregnancy risks same as for plain radiographs

Types of central scanners

-pencil beam -wide angle fan beam -narrow angle fan beam

Test recommended if

-post menopausal women, not taking estrogen, tall or thin -hx hip fx or smoking -man with clinical bone loss -use Rx known to cause bone loss -type 1 diabetes, liver disease, kidney disease or family hx of osteoporosis -high bone turnover -hyperthyroidism -fx after only mild trauma -x-ray evidence of vertebral fx

Benefits

-simple, quick, non-invasive -no anesthesia -low radiation -most accurate method available for osteoporosis diagnosis -considered accurate estimator of fx risk -widely available

Pencil beam

-single x-ray beam to produce image -first and original method -high precision -takes longer -good for less than 15 patients per day

How it works

-x-ray machine sends low-dose x-rays with 2 distinct energy peaks -measure attenuation at both energies & calculate area density at each point by solving 2 simultaneous equations -one peak absorbed by soft tissue -one peak absorbed more by bone -if bone density is low, beam absorbed more easily -if bone density is normal or high, x-ray less abosorbed

Wide angle fan beam

-x-ray tube emissions received by multiple detectors -newer version -less exposure time -less precise

Required education

-associates or bachelors degree in radiology or MRI technology -licensure and/or certification depending on state -this is above and beyond and x-ray technologist

What does it tell us?

-calculates BMD as mass/area -compare to database -WHO -z-score: age & sex matched -if unusually low/high, indicate need for further medical testing -t-score: sex matched and average age 30 yrs old -most valuable diagnostic score -used to estimate risk of fx -usually 2 years to see significant changes

Limitations

-cannot predict who will experience fx, but can provide indications of relative risk -limited use in people w/spinal deformities or previous spinal surgery -central DXA more sensitive than pDXA, by more expensive -pDXA no helpful in following response to treatment

DEXA scan

-dual energy X-ray absorptiometry- DEXA -two x-ray beams with differing energy levels, there is a high energy and a low energy beam -when soft tissue absorption is subtracted out, the BMD can be determined from the absorption of each beam by bone

Central Scan

-full body -spine and hip -low patients dose

Validity

-how does the measure deviate from the true value? -compared against other indirect measures -obvious validity issues w/comparing against anthropoetric measures vs gold standard of chemical measurements of cadavers

Use in athletic or active populations

-minimize concerns -protocols for whole body composition assessment -minimal clothing -fasted w/ no fluid intake on morning of scan -no exercise morning of scan -optimized hydration status -bladder voided -standardized positioning technique w/positioning age -No Ca supplementation 24 hours prior

Uses

Bone mineral density -diagnosis -track measurement effects Fracture risk -age -body weight -hx prior fx -family hx osteoporotic fx -lifestyle Lean mass or fat free soft tissue Fat mass Bone mass

Reliability influences

Technical -software analysis; manual vs automated -technician error- experience -model of scanner- comparisons bx machines -positioning protocol -clothing -scanning mode Biological -food intake -hydration intake -exercise -extremity mass fluctuations

T-score

-1.0 or higher is normal bone density -from -1.0 to -2.4 is low bone mass or osteopenia -from -2.5 or below is the presence of osteoporosis

When discovered?

-1960's at the University of Madison -John Cameron -used it to identify level of osteoporosis in older women

Narrow angle fan beam

-3rd generation -combination of the other two -puts the details together and is quicker