DXA

Question 1

What is osteoporosis ?

Answer 1

• A disease which leads to bone fragility
• High risk of fracture
• Treatable via bisphosphonates

T score of -2.5 or less at the hip

Increase morbidity and mortality

Question 2

Describe DXA

Answer 2

A scanning technique using a small x-ray beam with two distinct x-ray energies each of restricted energy range can compare the attenuation of the two beams to determine BMD

Measured at hip, lumbar spine and whole body

Question 3

Define BMD

Answer 3

Bone Mineral density (g/cm2)

Question 4

Assessing BMD

Answer 4

BMD is gaussian distribution for a healthy population

Assess via Z and T scores

Z number of std (-/+) from age match
T number of std (-/+) from young adult

Question 5

Define Osteopenia

Answer 5

• Intermediate bone condition
• T=-1.5 to T=2.5 at hip or lumbar spine
• Causes pain and limits mobility
• Can treat with regular exercise and balanced nutrition

Question 6

Independent risk factors for fragility

Answer 6

old age, low body mass index, female gender, previous fragility fracture, parental hip fracture, current smoking, previous steroid use, rheumatoid arthritis, alcohol use (3+ units/day), secondary causes, low BMD

Question 7

Why is BMD useful?

Answer 7

BMD is sensitive: risk of future fracture increases by factor of between 1.4 and 2.6 for every 1SD decrease in BMD

-> risk of fracture is high if BMD low

Question 8

How does DXA work?

Answer 8

If we know the absorption coefficient of bone we can work out BMD by measuring the x rays transmitted

Need two energies as tissue and bone density vary across the scan range

Question 9

Describe the DXA calculation process

Answer 9

Estimate the attenuation ratio of the high and low energy scan lines for soft tissue

Estimate the amount of bone at all points in the scan

Use a threshold to separate the tissue and bone pixels

Recalculate the ratio for soft tissue

Iterate until the bone and soft tissue pixels are completely separated

Smooth the bone edges

Operator adjusts the edges and places ROIs

Determine area of each ROI by counting bone pixels

Calc BMD by averaging Mb at all points

Calculate bone mineral content as BMD x area

Question 10

Describe the two methods of dual energy imaging

Answer 10

Lunar - Simultaneous counting by reviewing data from two parts of the same k-edge spectrum

Hologic - voltage switching every half cycle (10ms) and recorded signals consecutively at each pixel

Question 11

Issues with each DXA system

Answer 11

Lunar - pulse counting causes pile up must select beam current dep on patient size. Correct by subtracting a portion of high level sig from lower

Hologic - Attenuation is non exponential due to broad spectra. Minimise by using internal ref wheel chopping the beam to add known bone and tissue pixel correction factors.

Question 12

Specs of BMD

Answer 12

Accuracy 1-15%
Scan is 30s long
Patient appointment 20mins
throughput 6000 people/year
Dose 10-20uSv (hip +spine)

adult loses 0.5% bmd per year
pharmaceuticals add several percent

Question 13

Imaging specs

Answer 13

• resolution upto a 1mm
• need to see end plates and inter-veterbral spaces

latest gen - 2linepair / mm

can visual spinal damage = crushing

Question 14

DXA alternativea

Answer 14

CT - resolution not good enough to see micro-architecture
MRI - res not good enough on routine mri (approx 1mm)
uCT - possible but in-vitro only atm

Question 15

DXA referral criteria

Answer 15

• Clinical indication of osteoporosis
• Secondary osteoporosis (endocrine/thyroid disorders)
• Developmental disease influencing the skeleton
• Monitoring treatment
• Pre-op for transplace/hip replacement

Question 16

Clinical scientist role in dexa

Answer 16

Runs the service
Interprets scans
Audits
Planning 
Reports difficult scans
Provides rad safety support

Question 17

Implications of DXA source geometry

Answer 17

X-ray source is nearer to the patient

This causes increase geometric unsharpness