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/ cm2).• 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?