Bone Mineral Densitometry (BMD) Flashcards
what is osteoporosis?
small passages or pores through bone
- generalized skeletal disease
- low bone mass
- micro architectural deterioration of bone tissue
- increase in bone fragility and risk of fracture
when do fractures occur with osteoporosis?
late in the disease process when skeletal integrity is compromised
- try to identify high risk individuals
bone biology
made if minerals - primarily calcium and phosphate
2 types of bone
- cortical - dense - 80%
- trabecular - cancellous - 20%
What is trabecular bone?
- delicate lattice structure
- adds strength without excessive weight
- supports compressive loading and flexibility
- produces red bone marrow
what are osteoclasts?
break down bone - resorption
what are osteoblasts?
bone building - formation
what is bone remodelling?
the rate of resorption compare to formation determines bone mass increase or decrease
- osteoclast activity>osteoblast activity = net loss of bone mass
when is your bone mass at its peak?
between 20 and 30
- remains stable in middle age
When does bone mass start decreasing?
50 in women and 65 in men
- becomes pronounced at menopause
what are the 2 factors contributing to the bone mass later in life?
- peak bone mass attained in early adult life
- rate of age-related bone loss
What is BMC and BMD?
bone mineral content
bone mineral density
what are values used for in BMD?
- assess bone strength
- assist with diagnosis of diseases
- monitor the effectiveness of treatment
- predict risk of future fractures
what is the radiographic appearance of osteoporosis?
- increased radiolucency
- verticilization of trabecular (loss of horizontal/secondary trabeculae)
- increased radiodensity of cortical rim
where are changes most prominent in the axial skeleton?
changes are most prominent in the axial skeleton (spine) and in the ends of long bones in the appendicular skeleton (proximal femur, distal radius, calcaneus)
what are the risk/complications associated with osteoporosis?
- may result in decreased quality of life due to pain, deformity and disability
- increased risk of morbidity and mortality, especially from hip fractures
what are the risk factors for osteoporosis?
- female
- increased age
- estrogen deficiency
- caucasian
- low body weight
- low BMI
- family hx of osteoporosis/#s
- Hx of prior #s as an adult
- smoking tobacco
Osteoporosis fractures
- most osteoporotic fractures are caused by falls
- vertebral fractures are the most common
- one vertebral fracture significantly increases the risk of future fractures and progressive curvature of the spine
what is the treatment for osteoporosis?
- Several meds can slow down/stop bone loss or increase bone mass
- Estrogen replacement therapy is problematic
- Bisphosphonates - slow down osteoclast action
- Calcitonins - ↓ Osteoclast activity
- Parathyroid hormone, Vitamin D, and Ca2+
Advantages of dual energy x-ray absorptiometry (DEXA or DXA) over other methods for osteoporosis?
- low radiation dose
- widely available
- easy to use
- short scan times (< 30 seconds)
- good precision
- stable calibration
What is the main use for DEXA? What else is it used for?
diagnosis of osteoporosis
- predict risk of fractures
- determine if treatment is required
- monitor response to treatment
How is dual energy achieved?
kVp switching - generator emits alternating radiation of high and low kVp while moving across the body
Filtering - Generator emits a constant beam while a rare-earth filter separates high energy from low energy photons
What is a Hologic system?
synchronously switching between 100kVp and 140kVp results in effective photon energies of 40 and 80 keV
What are the GE and Norland systems?
- 2 peaks produced by x-ray beam passing through Cerium or Samarium filters
- Cerium filter is used with a tube voltage of 100kV to give effective photon energies of 40 & 70 keV
What is the DXA subtraction technique?
- Patient scanned at two different x-ray photon energies
- Soft tissue is subtracted out
- The difference in attenuation of x-rays at different energies in bone is used to generate 2D measurements of bone mineral content
What are the principles of DEXA?
- low and high energy x-rays are attenuated differently, producing a unique attenuation pattern
- computations are performed to subtract the soft tissue signals, producing a profile of the bone
how is the profile image of the bone created?
- As the source and detector move linearly across the scanned area, a bone profile is generated pixel by pixel
- Software identifies pixels that represent where the bone edge begins and ends
- This allows for a two-dimensional area to be calculated
- The software sums the number of pixels containing bone to calculate the bone area (BA) that was scanned
- The bone density is then calculated across the bone profile
how is bone mineral density calculated?
BMD = BMC/BA
- This can be used to determine if a change in BMD is due to BMC, BA or both
- A decrease in BMC results in a decrease in BMD
- A decrease in BA results in an increase in BMD
Which of the BMC or BA would most likely change? why?
BMC -due to medications, aging and diet
BA should always be the same or at least very similar
What is the pencil beam system?
- Narrow stream of x-ray photons received by a single detector
- X-rays and detector move in a rectilinear pattern
- Long scan times (3 -10 min)
What is the array beam system?
- Fan beam
-Wide slit collimation & multi-cell detector - Scanning motion reduced to one direction
- Greatly reduced time (down to 30s)
how to prepare for DEXA?
To properly plan the study, detailed patient information is required:
- Bone diseases such as spondylarthrosis or ankylosing spondylitis
- Previous fracture or joint replacements
- Contraindications to DEXA include pregnancy and recent oral administration of contrast (less than 5 days)
- No special patient prep
Areas of study for DXA?
- usually includes lumbar spine and proximal femur
- forearm is used if hip or spine cannot be measured
- scan of lumbar spine includes vertebral bodies of L1 - L4
- at least 2 evaluable vertebrae are required
- the study of the femur can be preformed on either left or right hip
incorrect positioning is one of the main causes of errors
patient positioning for the L-spine?
patient lies supine with legs flexed over a support pad
patient positioning for the hip?
patient lies supine with the legs slightly abducted and internally rotated - to reduce foreshortening of the femoral necks
patient positioning of forearm?
patient is seated beside table, arm resting on the tabletop with the hand pronated
Image analysis
- once the image is acquired, different regions of interest are selected
- inadequate placement of the ROIs is another important source of error
- system automatically proposes specific areas
DXA imaging FOV
must require 1-2 cm above and below the area to be imaged
ROI for the hip?
must be placed on the femoral neck
parameters evaluated in bone densitometry?
BMC is the quantity of calcium estimated by the energy absorbed in a specific region
- BMD is the mean quantity of mineral per unit area
Variation in BMD?
- BMD values vary among manufacturers for the same patient
- measurement is most useful when it is compared with an appropriate sex-matched reference population
- all reference databases are separated by gender and provide BMD mean and standard deviation at each age
T score
- Number of standard deviations the patient’s BMD is above or below the mean for the young, normal, sex-matched individuals with peak bone mass
- Used to assess fracture risk, diagnose osteoporosis or osteopenia when determining if therapy is recommended
T score ranges
- Above -1SD is considered normal
- Between -1 and -2.5 is classified as osteopenia (low bone mass)
- Below -2.5 is defined as osteoporosis
Z score
- Number of standard deviations the patient’s BMD is above or below the mean reference population of the patient’s respective age and sex group
- Used to determine if the measured BMD is reasonable and if an evaluation for secondary osteoporosis is warranted
T score formula?
T score = measured BMD - young adult mean BMD/young adult SD
Z score formula?
Z score = measured BMD - age matched mean BMD/age matched SD
radiation dose?
Effective radiation dose in Sv for DXA scans is low compared to conventional radiography and is similar to background radiation
- Distance is the best form of protection for the technologist
- Additional lead shielding should not be used to prevent artifacts
technologist distance for DXA?
- 1m from x-ray source for pencil beam scanners
- 3m for array beam scanners
Limitations of DXA?
- Cannot predict who will experience a fracture
- Compression fractures or osteoarthritis may interfere with accuracy of test
- central DXA machines are more sensitive than pDXA machine but more expensive
- baseline required
if a patient has compression fractures or osteoarthritis what would you use instead?
CT
are spine or hip measurements better?
- The best way of predicting fracture risk at any given site is to measure BMD at that site—for example, DXA of the proximal femur predicts femoral fracture better than measurements at other sites
- The spine is the best site for follow-up measurements because treatment changes are usually the largest at this site
What are the two different types of BMD beams that are used? What are the pros and cons of each?
pencil beam and array beam
- pencil beam takes longer than the array
- scanning motion only in one direction with array
- array has more beam divergence
What ares do we image in BMD? Are there any limitations?
L spine or hip - forearm last resort if we have to
- wouldn’t use spine or hip if there has been a hip replacement, previous hip fracture, previous lumbar fracture - need at least 2
