what does DEXA stand for?
- Dual energy X-ray absorptiometry
what is osteoporosis
- a health condition that weakens the bones- more susceptible to fractures
what is DEXA
- bone is isolated from soft tissue attenuation by scanning with 2 different x-ray energies
- data analysis includes a t-score and a Z- score
t-scores in DEXA
indication number of the number of SD between the measurement and standard data
Z-score DEXA
- compares individual measurement to a data mean for those similar in age
how does DEXA work?
- x-ray photons of 2 different energies from 2 different sources in an alternating fashion
- pencil beam and fan beam technology require the linear displacement of the detector which moves backward and forward under the patient’s body C-arm.
- PB uses small, angled x-ray beams
- FB generators use a wider beam that reduces scan times, but increases patient dose
- the difference in attenuation of the x-rays generates 2-D measurements of bone mineral content
- a low-resolution image is built up which is measured for bone density changes
- the signal degree is related to the amount of bone mass present
- the greater the bone mineral density, the smaller the signal picked up by photon counter
Dual energy- 2 energy beams
pencil beam
fan beam
limitations and issues with DEXA
- Object to image receptor distance affects size distortion - algorithms are based on average-sized adults, therefore those above and below average can create interpretation difficulties
- very low resolution
- Shows what is wrong, but not the reason why
- values are only based on average caucasian women
- 2D images lack volumetric data and evaluation
advantages of DEXA
- has a small radiation dose
- stable calibration
- sensitive and accurate in the measurement of subtle changes in BMD
- can predict the risk of future fractures
- inexpensive
- non-invasive
DEXA technique
- a region of interest is drawn around the bone
- graph is plotted (age on the x-axis), (BMD on y-axis)
- graph plots the area the patient lies in, in relation to age and BMD (normal, osteopenic, osteoporotic)
concerns in regards to DEXA
Magnification
radiation dose
DEXA magnification
- occurs especially in FB scanners
- object to image receptor distance can affect size distortion on the image
- since the algorithms are based on average-sized adults, larger adults and small paeds create interpretation difficulties
DEXA radiation dose
- relatively low dose to accurately measure BMD - microsieverts
- overall lifetime risks are the same as smoking 1/10th of a cigarette
things to consider
Ethnicity gender paediatric estimations clinical history positioning artefacts anatomical variance e.g. L6
which patients are likely to suffer from loss of Bone Density (adolescents)
those with:
- chronic illness
- poor diet
- injuries/illnesses causing mobilisation
- cancers
- genetic or hormonal disorders
which patients are likely to suffer from loss of Bone Density (adults)
those with
- chronic illness
- steroid/corticosteroid use
- hormonal imbalance
- poor diet/eating disorder
- smoking and drinking
osteoporosis is
a defined vakue of loss of bone density
osteopenia
a decreased bone density
disadvantages of data when applied to patient managemet
- data aqcusition is based on caucasian post-menopausal women (50+) - therefore reliability decreases due to:
Ethnicity gender paediatric estimations clinical history positioning artefacts anatomical variance e.g. L6
difference between pencil beam and fan beam
PB uses small angled x-ray beams and a single detector that move moves across the patient in a linear direction. scans take longer, but dose is lower
FB generators use a wider beam and has multiple detectors that reduces scan times but increases radiation to patient
