Dual Energy X-Ray Absorptiometry Flashcards
What is DXA?
Non-invasive technology that is used to measure bone mass for an accurate & precise skeletal assessment
What can bone mineral density measurements be used for?
To establish or confirm a diagnosis of osteoporosis, predict future fracture risk & monitor changes
What does bone loss result from?
Imbalance between rates of formation and resorption
What 2 types of bone make up the human skeleton?
20% Trabecular bone
80% Cortical bone
What % of the adult skeleton is remodeled each year?
10%
What does bone turnover prevent?
Fatigue damage & important in maintaining calcium homeostasis
What is bone mineral content?
Measurement of bone mineral found in a specific area (g)
What is bone mineral density?
Integral mass of bone mineral in measurement region divided by projected area (thickness)
What does DXA rely on?
Differential absorption of 2 distinct photon energies - high & low
How do you decipher between bone & soft tissue?
Lower energy photon is significantly attenuated by bone compared to high energy photon
How is BMD then measured?
Difference in attenuation with the mass attenuation coefficient for each energy and tissue type (bone or soft tissue) to determine mass per unit area in path of beam by solving simultaneous equations for each tissue type point by point
What are 6 advantages of DXA?
Gold standard
measures central or axial skeletal site & total body
correlation with bone strength in-vitro
validated in many clinical trails
widely available
What are 2 sources of variability within the patient?
ROI must be consistent (within 2%)
If ROI is not placed correctly - no valid comparison can be made
What is the precision of the technique?
Ability of the technique to reproduce the same result
What is the accuracy of the technique?
Differences between true value and the measured value
What are 2 factors that affect precision?
Instrument induced errors
operator induced errors
What are 5 things to assure equipment quality assurance & control?
Daily calibration
QA
Reference data
Calibrate & QA after service
Maintain service & error logs
What is a major benefit of instigating a QC protocol involving the regular scanning of a phantom?
May allow early identification of changes in instrument performance prior to instrument malfunction
What does rational do?
Allow for precise & accurate positioning
Allows for precise & accurate re-positioning
maintains consistency in technique
improves quality of follow up scans
increases reliability of serial scan measurements
What are 2 skeletal sites to measure?
Spine and HIP
When is foreman BMD measured?
Hip and/or spine cannot be measure or interpreted
Hyperparathyroidism
Very obese patients
mobility issues
Why is the lumbar spine the most frequently studied skeletal site for DXA?
Because of its sensitivity to ageing, menopause & secondary causes of bone loss
What is the total mass of bone in the spine and what amount is the lumbar region of this?
130g in total
Lumbar is 45g of this
Which bone type of the spine is the fastest to lose BMD & fastest to respond to treatment?
Trabecular bone
What are 3 signs of osteoporotic spine?
Kyphosis
Loss of height
Protruding abdomen
What are 3 things that happen after a hip fracture?
Result in permanent disability in more than 30% of patients
12-20% die within one year
More than 50% of survivors are unable to return to independent living
What is hip measurement predictive of?
Cervical & trochanteric fractures
What is the integrity of trabecular structure in the proximal femur important for?
Factor in bone strength
What % of bone does femoral neck contain?
75% cortical bone
25% trabecular bone
What types of bone does the radius contain?
75% cortical bone
25% trabecular bone
What does vertebral fracture assessment identify?
Loss of height due to vertebral fracture/vertebral deformity
reduction must be 20% to make a confirmation of vertebral collapse
What does a total body scan look at?
obesity - anorexia - CF
wasting syndrome caused by AIDs
develop complete physical fitness profiles
to maximize performance of atheletes
What 3 things does DXA scan to look at total body composition?
BMD
lean tissue mass
fat tissue mass
What is sarcopenia?
Loss of muscle mass & strength with age
What are 6 symptoms of sarcopenia?
physical disability
falls
loss of independence
reduced QOL
difficulty performing ADLs
Mortality
What is the consequence of radionuclides & radioplaque agents in DXA?
Affect BMD result as they are agents with high inherent density
What is patient prep?
fasting state
clothing
time of day
physical activity
empty bladder
What does the scan mode depend on?
Thickness of region being scanned
What does the scan mode either do?
Decreases number of x-ray photons by decreasing tube current and/or increasing scan speed
Increasing no. of x -ray photons by increasing current and/or decreasing scan speed
What are the 3 VFA analysis?
5 - 20% = mineral compression - no fracture
20 - 25% = anterior wedging - mild fracture
>25% = anterior wedging - severe fracture
Which vertebrae should be used for BMD measurement?
L1-L4
exclude vertebrae that are affected by local structural change or artifact
What regions should be used for HIP?
Total hip
neck of femur
DO NOT USED WARD’S REGION
What is the T score?
Indicates difference between patients BMD & ideal peak bone mass achieved by a young adult
measured BMD - young adult mean BMD/ young adult SD
What is a Z score?
Difference of patients BMD to mean BMD of their peers
measured BMD - age matched mean BMD/ age matched Sd
What are safety measures for DXA?
Distance 1-2m
screens/aprons
extra wall shielding
