DXA

Question 1

what is DXA?

Answer 1

Dual X-ray absorptiometry is a diatonic modalities used to measure bone mineral density

Question 2

Whereis DXA used?

Answer 2

1. to measure BMD
2. diagnosis of osteoporosis
3. fracture risk
4. monitor disease progression
5. monitor therapeutic intervention

Question 3

what are the advantages and limitations of DXA?

Answer 3

Advantages
1. low radiation dose
2. multiple assessment sites possible (including spine and hip)
3. strongest predictor of hip fracture  
Limitations 
1. cost - expensive 
2. size - huge

Question 4

what is scan time for DXA?

Answer 4

1/2 - 6mins

Question 5

what are the principles of DXA?

Answer 5

1. utilises dual X-ray tube
2. emits narrow collimated beam of x-rays at 2 different energies
3. consists of solid state detectors (scintillators)

Question 6

what are two usual energies used in DXA?

Answer 6

1. hologic - 70kVP and 140 kVP

2. GE lunar - 70-90 kVP and 40-60kVP

Question 7

deine hologic and GE lunar

Answer 7

1. hologic - uses different parts of AC sin wave

2. GE lunar - alternate filtering with cerium (metal) filters

Question 8

what is the unit of bone mineral density?

Answer 8

g/cm2

Question 9

Define Z and T score?

Answer 9

1. Z score = normalises a patients BMD by assessing the amount of bone loss compared to the expected bone loss for age-matched peers
2. T score - quantifies difference between patients BMD and peak bone mass for young normals

Question 10

what is the T score for the BMD of the following

1. Normal
2. Osteopenic
3. Osteoporotic

Answer 10

1. Normal T > -1 SD
2. Osteopenic T > -1 SD to < -2.5 SD
3. Osteoporotic T < -2.5 SD

Question 11

what is the relationship b/w SD and BMD fracture risk?

Answer 11

for every SD decrease in BMD fracture risk increases by x2 times

Question 12

what are the risk factors for osteoporosis?

Answer 12

1. low body mass index
2. current smoking habit
3. family history of osteoporosis
4. medical condition or. use of medication known to affect bone metabolism
5. x-ray osteopenia
6. history of atraumatic fracture
7. pre-mature menopause (before the. age of 45 years) or history amenorrhea (longer than 6 months_

Question 13

what are the WHO criteria for osteoporosis?

Answer 13

WHO for

1. postmenopausal women only
2. T-scores should NOT be used in children, most pre-menopausal women.
3. Z-scores of -2 or above considered normal in children and premenopausal women
4. Risk factors should be included in fracture risk