# 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,

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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 20-25 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?

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