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Medical Imaging > Dual Energy X-Ray Absorptiometry > Flashcards

Dual Energy X-Ray Absorptiometry Flashcards

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USMLE® Step 1 flashcards
1
Q

What is DXA?

A

Non-invasive technology that is used to measure bone mass for an accurate & precise skeletal assessment

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2
Q

What can bone mineral density measurements be used for?

A

To establish or confirm a diagnosis of osteoporosis, predict future fracture risk & monitor changes

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3
Q

What does bone loss result from?

A

Imbalance between rates of formation and resorption

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4
Q

What 2 types of bone make up the human skeleton?

A

20% Trabecular bone
80% Cortical bone

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5
Q

What % of the adult skeleton is remodeled each year?

A

10%

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6
Q

What does bone turnover prevent?

A

Fatigue damage & important in maintaining calcium homeostasis

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7
Q

What is bone mineral content?

A

Measurement of bone mineral found in a specific area (g)

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8
Q

What is bone mineral density?

A

Integral mass of bone mineral in measurement region divided by projected area (thickness)

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9
Q

What does DXA rely on?

A

Differential absorption of 2 distinct photon energies - high & low

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10
Q

How do you decipher between bone & soft tissue?

A

Lower energy photon is significantly attenuated by bone compared to high energy photon

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11
Q

How is BMD then measured?

A

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

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12
Q

What are 6 advantages of DXA?

A

Gold standard
measures central or axial skeletal site & total body
correlation with bone strength in-vitro
validated in many clinical trails
widely available

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13
Q

What are 2 sources of variability within the patient?

A

ROI must be consistent (within 2%)
If ROI is not placed correctly - no valid comparison can be made

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14
Q

What is the precision of the technique?

A

Ability of the technique to reproduce the same result

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15
Q

What is the accuracy of the technique?

A

Differences between true value and the measured value

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16
Q

What are 2 factors that affect precision?

A

Instrument induced errors
operator induced errors

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17
Q

What are 5 things to assure equipment quality assurance & control?

A

Daily calibration
QA
Reference data
Calibrate & QA after service
Maintain service & error logs

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18
Q

What is a major benefit of instigating a QC protocol involving the regular scanning of a phantom?

A

May allow early identification of changes in instrument performance prior to instrument malfunction

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19
Q

What does rational do?

A

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

20
Q

What are 2 skeletal sites to measure?

A

Spine and HIP

21
Q

When is foreman BMD measured?

A

Hip and/or spine cannot be measure or interpreted
Hyperparathyroidism
Very obese patients
mobility issues

22
Q

Why is the lumbar spine the most frequently studied skeletal site for DXA?

A

Because of its sensitivity to ageing, menopause & secondary causes of bone loss

23
Q

What is the total mass of bone in the spine and what amount is the lumbar region of this?

A

130g in total
Lumbar is 45g of this

24
Q

Which bone type of the spine is the fastest to lose BMD & fastest to respond to treatment?

A

Trabecular bone

25
Q

What are 3 signs of osteoporotic spine?

A

Kyphosis
Loss of height
Protruding abdomen

26
Q

What are 3 things that happen after a hip fracture?

A

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

27
Q

What is hip measurement predictive of?

A

Cervical & trochanteric fractures

28
Q

What is the integrity of trabecular structure in the proximal femur important for?

A

Factor in bone strength

29
Q

What % of bone does femoral neck contain?

A

75% cortical bone
25% trabecular bone

30
Q

What types of bone does the radius contain?

A

75% cortical bone
25% trabecular bone

31
Q

What does vertebral fracture assessment identify?

A

Loss of height due to vertebral fracture/vertebral deformity
reduction must be 20% to make a confirmation of vertebral collapse

32
Q

What does a total body scan look at?

A

obesity - anorexia - CF
wasting syndrome caused by AIDs
develop complete physical fitness profiles
to maximize performance of atheletes

33
Q

What 3 things does DXA scan to look at total body composition?

A

BMD
lean tissue mass
fat tissue mass

34
Q

What is sarcopenia?

A

Loss of muscle mass & strength with age

35
Q

What are 6 symptoms of sarcopenia?

A

physical disability
falls
loss of independence
reduced QOL
difficulty performing ADLs
Mortality

36
Q

What is the consequence of radionuclides & radioplaque agents in DXA?

A

Affect BMD result as they are agents with high inherent density

37
Q

What is patient prep?

A

fasting state
clothing
time of day
physical activity
empty bladder

38
Q

What does the scan mode depend on?

A

Thickness of region being scanned

39
Q

What does the scan mode either do?

A

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

40
Q

What are the 3 VFA analysis?

A

5 - 20% = mineral compression - no fracture
20 - 25% = anterior wedging - mild fracture
>25% = anterior wedging - severe fracture

41
Q

Which vertebrae should be used for BMD measurement?

A

L1-L4
exclude vertebrae that are affected by local structural change or artifact

42
Q

What regions should be used for HIP?

A

Total hip
neck of femur
DO NOT USED WARD’S REGION

43
Q

What is the T score?

A

Indicates difference between patients BMD & ideal peak bone mass achieved by a young adult

measured BMD - young adult mean BMD/ young adult SD

44
Q

What is a Z score?

A

Difference of patients BMD to mean BMD of their peers

measured BMD - age matched mean BMD/ age matched Sd

45
Q

What are safety measures for DXA?

A

Distance 1-2m
screens/aprons
extra wall shielding

Medical Imaging (10 decks)

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