bone densitometers DXA

Question 1

know that there is cortical (outer part) and trabecular (spongy inner) structure of the bone

Question 2

define osteopenia

Answer 2

low bone mass

• lower density than average but not low enough to be considered osteoporotic

Question 3

what is osteoperosis

Answer 3

• microarchitecture of trabecular bone deteriorates and loss bone mass

Question 4

what are the general causes of osteoporosis

Answer 4

iodiopathic (spontaneous)
or
caused by secondary cause

Question 5

what age range has the peak bone mass

Answer 5

30-40

Question 6

what is the difference between type 1 and 2 primary osteoporosis

Answer 6

type 1 = postmenopausal

type 2 = senile (age related_

Question 7

give some examples of secondary causes of osteoperosis

Answer 7

• diseases e.g autoimmune conditions
• meds e.g high dose steroids
• lifestyle e.g low calcium or smoking

Question 8

what are some genetic factors that can cause osteoperosis

Answer 8

ethinicity
gender
early menopause
family history
small frame/ low BMI

Question 9

What should be the average calcium daily intake for adults

Answer 9

700 mg

Question 10

average vitamin D intake daily for adult is

Answer 10

10mg

Question 11

what is Sheffield FRAX

Answer 11

fracture risk assessment

• diagnostic tool used to evaluate the 10-year probability of bone fracture risk.

Question 12

What is DXA, dual energy xray absoptiometery

Answer 12

DXA scans measure bone mineral density

Question 13

what are the most routinely scanned anatomy with DXA

Answer 13

hip and lumbar spine as they are most load bearing and biggest impact if fractured

Question 14

know that dual energy xray absorptiometre uses very low dose

Question 15

what is a VFA examination using DXA

Answer 15

vertebral fracture assessment

• resembles lateral spine xray of L5 up to T6/4
• low dose

Question 16

What is a VFA used for

Answer 16

assess each vertebra for any degree of vertebral fracture but not any other pathology

Question 17

why is DXA used to measure lumbar and hip

Answer 17

• more trabecular bone prevaelnet
• both areas whee ravoidance of fracture is best

Question 18

know that a DXA scan questionnaire bust be done prior to scan to check for osteoporosis risk factors

Question 19

what must be checked prior to scan

Answer 19

• patient ID
• pregnancy status where applicable

Question 20

how does a DXA scan work

Answer 20

• 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.

Question 21

early DXA scans used pencil beam scanners, what are these

Answer 21

• pencil beam has single detector
• uses single radiation detector
• attenuation is just one path through the body measured at any instant

Question 22

what was the scan time and resolution of a pencil beam scanner

Answer 22

time= 5-10 mins
resolution = 1mm

Question 23

what type of scanner do modern DEXA scans use

Answer 23

fan beam scanners

Question 24

how does a fan beam scanner work

Answer 24

• attenuation alongmany paths through body measured simultaneously
• allowing larger FOV to be scanned at one time

Question 25

what is the scan time and resolution of a fan beam scanner

Answer 25

scan time = 1min per site resolution = 0.5mm

Question 26

what are the 2 values received from a DEXA scan + units

Answer 26

- BMC bone mineral content (g/cm) - BMD bone mineral density (g/cm^2)

Question 27

what is used to calculate the mean peak bone mass (in general)

Answer 27

a large reference population

Question 28

what is a t score

Answer 28

standard deviation that shows difference against the mean of the peak of bone mass

Question 29

what is a z score

Answer 29

standard deviation showing difference against averge bone mass in population of their own age group

Question 30

know that the BMC AND BMD values are used to calculate the patients T/Z scores

Question 31

what value of t score would place someone in the category of normal, osteopenia, osteoporotic according to the WHO classification

Answer 31

T score > -1SD = normal T score < -1 SD - 2.5 = osteopenia T score

Question 32

what is a limitation with DEXA

Answer 32

only in pixels containing no bone can soft tissue composition (fat versus lean) be assessed e.g degenerative changes or stent