DXA

Question 1

What is the reason for using two different energy x-ray beams in DXA scans for measuring BMD?

Answer 1

To remove soft tissue attenutation from bones, so you just see the BMD

Question 2

Define Z score

Answer 2

Compares BMD of individual to age-matched peers

Question 3

Define T score

Answer 3

Compares BMD to peak bone mass for young normals

Question 4

True or False: Z scores of -2 or above are considered normal in children and premenopausal women

Answer 4

True

Question 5

What is a fragility fracture

Answer 5

is any fall from a standing height or less, that results in a fracture

Question 6

Why does your risk of osteoporosis increase post-menopause?

Answer 6

Menopause: increase in bone turnover = rapid reduction in bone density.

Question 7

Describe trabecular bone

Answer 7

Trabecular – honeycomb bone (end of long bones) 5-7X more metabolically active

Question 8

Describe cortical bone

Answer 8

Cortical- more dense bone

Question 9

What are compression fractures

Answer 9

vertebral fractures (trabecular bone type), where the vertebral bodies compress and crush.

Question 10

What are 2 strengths of DXA

Answer 10

• low radiation dose
• multiple assessment sites possible
• strongest predictor of hip fracture

Question 11

what are two weaknessess of DXA

Answer 11

• Cost
• Size of equipment

Question 12

How does qualitative ultrasound work for bone density measurement

Answer 12

Uses ultrasound pulses to measure bone ‘equality’ so attenuation through bone and speed of sound

Question 13

Give a brief overview of how DXA works

Answer 13

• DXA utilises X-ray tube (underneath) & Detectors (above)
• Emits a narrowly collimated beam of X-rays at 2 different energies (reasonably accurate: subtract soft-tissue from bone density)
• When these pass through the subject, intensity of beam registered by detector

Question 14

What is the attenuation coefficient dependent on

Answer 14

Attenuation coefficient dependent upon atomic number and photon energy

Question 15

Why does DXA use two types of energy

Answer 15

To distinguish between soft tissue and bone better

Question 16

What are the two types of DXA energy

Answer 16

• Hologic – uses different part of the AC sin wave
• GE Lunar – alternate filtering with cerium (metal) filters.
  
  • Flips in an out of the x-ray beam very quickly to minimise movement artefacts

Question 17

What does this graph imply

Answer 17

As you increase, the photon energy of the tissue vs bone separates.

Question 18

Why is the bone coloured red?

Answer 18

• The soft tissue is subtracted from the area next to the ROI (regions of interest)
• Coloured bone as red, soft tissue as black
• This provides an areal BMD, which is 2-dimensional and therefore not totally corrected for bone size.

Question 19

Why does the low detail of images not matter

Answer 19

DXA is more for measurement than pathology identification- so doesn’t have to be as clear

Question 20

These scans are of the same spine- why are the BMD measurements so different

Answer 20

The spine is osteoarthritic- this transiently increases BMD, so has to be considered in the calculations

Question 21

You can’t make a diagnosis of osteoporosis from one vertebra. Therefore, how is osteoporosis confirmed

Answer 21

Would need to measure hip etc. to confirm a diagnosis of osteoporosis, affecting multiple sites

Question 22

How does osteoarthritis appear on a DXA scan?

Answer 22

Bright white spots

Question 23

This is a calcified aorta.

True or False: DXA can’t determine calcification of vasculature vs normal calcium in the bone.

Answer 23

True

Question 24

What does this graph suggest

Answer 24

Depending on the angle of hip rotation, it can greatly influence the boen density measurement score. Important to be correctly rotated for an accurate measurement

Question 25

True or false: Higher BMI = Higher BMD ?

Answer 25

True

protected from osteoporosis- force through bones and hormones stored in adipose tissue

But higher fracture risk than expected, due to higher impact of falling

Question 26

How much BMD do adults loose a year?

Answer 26

~1%

Question 27

Why are our BMD scores compared to our younger reference range?

Answer 27

quantifies difference between patient’s BMD and peak bone mass for young normals

We loose bone as we get older- so compare this to our younger reference range too.

Bone architecture changes- older more likely to fracture, even with same BMD as someone younger.

Question 28

Why are the heel and spine related so strongly

Answer 28

Heel related to spine- virtually identical bone pattern to the spine

So similar pattern of ageing.

Question 29

What are 3 risk factors for osteoporosis

Answer 29

• history of atraumatic fracture
• x-ray osteopenia
• medical condition or use of medications known to affect bone metabolism (corticosteroids)
• premature menopause before the age of 45 years or history of amenorrhea of longer than 6 months duration
• Family history of osteoporosis
• low body mass index
• current smoking habit

Question 30

This patient (indicated by X) has a bone density that is 1SD below age-matched normals and 2.5 below the mean bone density of young normals

What condition does she have

Answer 30

osteoporosis

Question 31

Who should the WHO criteria be used for

Answer 31

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

Question 32

How should vertebral fractures be identified using DXA

Answer 32

Measure lateral spine to look for vertebral fractures- lots clinically undiagnosed