View
238
Download
0
Category
Preview:
DESCRIPTION
What happens when x-ray pass through the body?
Citation preview
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?
Recommended