DXA Flashcards
What is the reason for using two different energy x-ray beams in DXA scans for measuring BMD?
To remove soft tissue attenutation from bones, so you just see the BMD
Define Z score
Compares BMD of individual to age-matched peers
Define T score
Compares BMD to peak bone mass for young normals
True or False: Z scores of -2 or above are considered normal in children and premenopausal women
True
What is a fragility fracture
is any fall from a standing height or less, that results in a fracture
Why does your risk of osteoporosis increase post-menopause?
Menopause: increase in bone turnover = rapid reduction in bone density.
Describe trabecular bone
Trabecular – honeycomb bone (end of long bones) 5-7X more metabolically active
Describe cortical bone
Cortical- more dense bone
What are compression fractures
vertebral fractures (trabecular bone type), where the vertebral bodies compress and crush.
What are 2 strengths of DXA
- low radiation dose
- multiple assessment sites possible
- strongest predictor of hip fracture
what are two weaknessess of DXA
- Cost
- Size of equipment
How does qualitative ultrasound work for bone density measurement
Uses ultrasound pulses to measure bone ‘equality’ so attenuation through bone and speed of sound
Give a brief overview of how DXA works
- 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
What is the attenuation coefficient dependent on
Attenuation coefficient dependent upon atomic number and photon energy
Why does DXA use two types of energy
To distinguish between soft tissue and bone better
What are the two types of DXA energy
- 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
What does this graph imply
As you increase, the photon energy of the tissue vs bone separates.
Why is the bone coloured red?
- 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.
Why does the low detail of images not matter
DXA is more for measurement than pathology identification- so doesn’t have to be as clear
These scans are of the same spine- why are the BMD measurements so different
The spine is osteoarthritic- this transiently increases BMD, so has to be considered in the calculations
You can’t make a diagnosis of osteoporosis from one vertebra. Therefore, how is osteoporosis confirmed
Would need to measure hip etc. to confirm a diagnosis of osteoporosis, affecting multiple sites
How does osteoarthritis appear on a DXA scan?
Bright white spots
This is a calcified aorta.
True or False: DXA can’t determine calcification of vasculature vs normal calcium in the bone.
True
What does this graph suggest
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
True or false: Higher BMI = Higher BMD ?
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
How much BMD do adults loose a year?
~1%
Why are our BMD scores compared to our younger reference range?
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.
Why are the heel and spine related so strongly
Heel related to spine- virtually identical bone pattern to the spine
So similar pattern of ageing.
What are 3 risk factors for osteoporosis
- 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
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
osteoporosis
Who should the WHO criteria be used for
- 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
How should vertebral fractures be identified using DXA
Measure lateral spine to look for vertebral fractures- lots clinically undiagnosed
