bone densitometers DXA Flashcards
know that there is cortical (outer part) and trabecular (spongy inner) structure of the bone
define osteopenia
low bone mass
- lower density than average but not low enough to be considered osteoporotic
what is osteoperosis
- microarchitecture of trabecular bone deteriorates and loss bone mass
what are the general causes of osteoporosis
iodiopathic (spontaneous)
or
caused by secondary cause
what age range has the peak bone mass
30-40
what is the difference between type 1 and 2 primary osteoporosis
type 1 = postmenopausal
type 2 = senile (age related_
give some examples of secondary causes of osteoperosis
- diseases e.g autoimmune conditions
- meds e.g high dose steroids
- lifestyle e.g low calcium or smoking
what are some genetic factors that can cause osteoperosis
ethinicity
gender
early menopause
family history
small frame/ low BMI
What should be the average calcium daily intake for adults
700 mg
average vitamin D intake daily for adult is
10mg
what is Sheffield FRAX
fracture risk assessment
- diagnostic tool used to evaluate the 10-year probability of bone fracture risk.
What is DXA, dual energy xray absoptiometery
DXA scans measure bone mineral density
what are the most routinely scanned anatomy with DXA
hip and lumbar spine as they are most load bearing and biggest impact if fractured
know that dual energy xray absorptiometre uses very low dose
what is a VFA examination using DXA
vertebral fracture assessment
- resembles lateral spine xray of L5 up to T6/4
- low dose
What is a VFA used for
assess each vertebra for any degree of vertebral fracture but not any other pathology
why is DXA used to measure lumbar and hip
- more trabecular bone prevaelnet
- both areas whee ravoidance of fracture is best
know that a DXA scan questionnaire bust be done prior to scan to check for osteoporosis risk factors
what must be checked prior to scan
- patient ID
- pregnancy status where applicable
how does a DXA scan work
- tissue is exposed to 2 difference energy source
- allowing contraction of 2 simultaneous attention equations
- measures attenuation at high and low energies
two distinct energy peaks through the bones being examined. One peak is absorbed mainly by soft tissue and the other by bone. The soft tissue amount can be subtracted from the total and what remains is a patient’s bone mineral density.
early DXA scans used pencil beam scanners, what are these
- pencil beam has single detector
- uses single radiation detector
- attenuation is just one path through the body measured at any instant
what was the scan time and resolution of a pencil beam scanner
time= 5-10 mins
resolution = 1mm
what type of scanner do modern DEXA scans use
fan beam scanners
how does a fan beam scanner work
- attenuation alongmany paths through body measured simultaneously
- allowing larger FOV to be scanned at one time
what is the scan time and resolution of a fan beam scanner
scan time = 1min per site
resolution = 0.5mm
what are the 2 values received from a DEXA scan + units
- BMC bone mineral content (g/cm)
- BMD bone mineral density (g/cm^2)
what is used to calculate the mean peak bone mass (in general)
a large reference population
what is a t score
standard deviation that shows difference against the mean of the peak of bone mass
what is a z score
standard deviation showing difference against averge bone mass in population of their own age group
know that the BMC AND BMD values are used to calculate the patients T/Z scores
what value of t score would place someone in the category of normal, osteopenia, osteoporotic according to the WHO classification
T score > -1SD = normal
T score < -1 SD - 2.5 = osteopenia
T score </= -2.5 SD =osteoperotic
what is a limitation with DEXA
only in pixels containing no bone can soft tissue composition (fat versus lean) be assessed
e.g degenerative changes or stent
