SXA

What is the energy of x-rays? Maximum energy of emitted x-ray beam depends on tube voltage applied across the x-ray tube; As x-ray tube voltage increases, average x-ray photon energy increases, and total x-ray intensity increases

What happens when x-ray pass through the body?

Does attenuation depend on the material? Basic of x-ray bone densitometry For a given thickness, attenuation increases with density of the material;

Does attenuation depend on energy of x-ray beam? Basics of DXA Attenuation depends on: i) density of material; and ii) Average x-ray photon energy; Used to measure amounts of both bone mineral and soft tissue

How would you do x-ray absorptiometry? X-ray beam passes through the body containing bone; Measures the attenuation; Solve an equation to measure the thickness of the bone mineral in terms of areal density, (g/ cm 2 ). Proportional to thickness; An equation containing 2 UNKNOWN quantities; –Thickness of Bone mineral; –Thickness of soft-tissue;

What types of x-ray absorptiometry are there? Single energy x-ray absorptiometry (SXA); Dual energy x-ray absorptiometry (DXA);

How does single energy x-ray absorptiometry work? Surround part of the body to be measured with water to keep total thickness constant. Restrict to peripheral sites: heel or forearm; Scan beam through the bone and measures attenuation through bone at different points; Calculate areal density (mass/area) of bone mineral at each point; Sum over bone area to give bone mineral content (BMC) in g; Divide BMC by bone area to give average bone mineral density (g cm -2 )

What is the x-ray spectrum for SXA?

SXA

Out put of single energy x-ray absorptiometry (SXA)

DXA

Bone densitometry is a safe, painless x-ray technique that compares bone density to the peak bone density that someone of your same sex and ethnicity should have reached at about age when it is at its highest. Bone densitometry allows your doctor to: Detect a potential problem before fracture occurs, Predict chances of future fractures and Determine your rate of bone loss. All of these factors can then be weighed to determine a course of treatment.

Schematic DXA scanner Patient in position for lumbar spine BMD measurement

Two different average x-ray energy: Method-I Continuously switching high x-ray tube voltage between low- and high- values; Average energies: 90 kev (at 140 kV); 40 keV (at 70 kV) At any time, only ONE x-ray spectrum is present

Two different average x-ray energy: Method-II Use suitable metal filter to create 2 separate energy peaks in x-ray spectrum; Average energies: 70 kev (at 140 kV); 35 keV (at 70 kV) All the time, x-ray spectrum look like this

How does dual energy x-ray absorptiometry work? Use x-ray beam, which has two different average x-ray energies; Measures attenuation at LOW-and HIGH- energy separately; Calculate area density at each point by solving two attenuation equations; Calculate BMC, bone area, average BMD Can measure at both axial sites (spine, hip) and pheripheral sites;

DXA Bone Densitometer

What does the x-ray beam look like?