1. Bone Densitometry David Rawlings Regional Medical Physics Department
Newcastle General Hospital

2. This lecture aims to promote...
awareness of the role of bone densitometry in osteoporosis management
understanding of the physical principles associated with bone densitometry
appreciation of limitations in relation to monitoring

3. This lecture will enable you..
to relate osteoporosis and fracture
to list the clinical indications
to describe principles of measurement
to list important quantities and terms
to describe monitoring regimens

4. Osteoporotic fracture (e.g Colles, hip, vertebra)….
is a low trauma event
may occur after a fall from standing height
affects 40% of white women at 50+
affects 13% of white men at 50+
can occur at any age
is associated with morbidity
causes increased mortality

5. Osteoporosis … a multi-factorial disease
characterised by increased fracture risk
may be amenable to treatment (e.g. HRT, bisphosphonate, calcium supplementation)

6. How do we diagnose osteoporosis…?
“A selective case finding strategy is recommended to target those at high absolute risk of fracture” (National Osteoporosis Society, 1999)
Therefore NOT population screening!!!

7. Some clinical predicators of osteoporosis
Family history
High dose/long term steroids
Excessive alcohol intake
Low calcium intake
Early menopause
Late menarche
Low body weight
Prolonged amenorrhea
Height loss

8. Quantitative indicators of osteoporosis
Bone densitometry using dual x-ray absorptiometry techniques (DXA)
Quantitative ultrasound
Specialised quantitative CT procedures
Biochemical markers
Ordinary x-ray images can suggest osteoporosis but do not give a reliable measure.

9. DXA at the hip, lumbar spine and whole body is a routine out-patient procedure

10. Peripheral DEXA (forearm or heel) may be available within the primary care sector

11. DXA uses x-rays but differs from radiography because: 1) It scans 2) It uses two x-ay energies

12. Why is DXA useful in the management of osteoporosis?
Sensitive indicator of fracture risk
Non invasive
Pre-treatment assessment
Precise – can be used for monitoring
Regarded as “Gold standard”

13. DEXA reports bone density (g/cm2) at each region of interest (ROI) imaged

14. Results reported against sex matched normative data for given ROI
Mean +/-2 standard deviations (SD) shown
Z score is number of SD (+/-) from age match
T score is number of SD (+/-) from young adult
Here Z=-2.58, T=-2.98

15. Information for Clinicians…
Numerical data given as T and Z scores i.e number of SD above or below young or age matched norms. Large negative T or Z indicate increased fracture risk

16. Risk of future fracture increases by factor of between 1. 4 and 2
Risk of future fracture increases by factor of between 1.4 and 2.6 for every 1SD decrease in BMD Marshall et al 1996

17. DEXA can be used to diagnose osteoporosis
Osteoporosis is diagnosed in adults where T=-2.5 or less at the lumbar spine or hip (WHO criteria 1994)
This may not necessarily represent a treatment threshold as a full clinical assessment is indicated prior to treatment

18. Other methods of osteoporosis assessment
CT of lumbar vertebra or extremity
CT signal compared with bone standards
High cost per scan
High radiation dose
Less reliable for monitoring

19. Other methods of osteoporosis assessment
Broad beam ultrasound
Transducers on os calcis
Speed of sound (SOS)
Attenuation (BUA)
Indicates ‘bone quality’
Reflects risk of hip fracture (relative risk of 2 for 1 sd decrease)
Monitoring less reliable

20. Other methods of osteoporosis assessment
Biochemical markers
Serum or urinary markers of bone formation or bone resorption
My be able to assess response to therapy early (~24 weeks)
Relationship between marker change and fracture risk unknown

21. Narrow x-ray beams obey a well defined exponential law of absorption
DEXA works by measuring a narrow beam of x rays transmitted through bone
I= < I0
X Rays out I0
X Rays in
Narrow x-ray beams obey a well defined exponential law of absorption

22. For narrow beam x-rays passing through a bone sample …
absorption depends upon the bone mineral density (BMD) (g/cm2) which varies with the patient
also depends upon absorption coefficient of bone (cm2/g) which varies with the x-ray energy but is well documented
Thus all we need for BMD is one energy

23. For x-rays passing through a tissue sample …
absorption depends upon the tissue density (g/cm2) which varies with the patient thickness
also depends upon absorption coefficient of tissue (cm2/g) which depends on patient fat content
Thus we need two energies to get tissue density

24. For x-rays passing through bone and tissue together …
the absorption coefficient of bone is known beforehand
the absorption coefficient of tissue is unique to the patient
the tissue density varies across the ROI
Thus we need two energies and a tissue baseline to get BMD

25. Schematic of Lumbar spine scan showing operation of tissue baseline compensation
High energy signal
Low energy signal
Scaled high energy signal
Residual signal to determine BMD

26. Specifications of hip and spine DEXA
Long term precision about 2-3% in vivo
Scan 1-2 minutes per region approx
Patient appointment time 20 mins
Patient throughput 4500 patients/year
Radiation dose 8 microSievert (hip +spine)

27. Monitoring using DEXA Least significant change =2√2(Precision) or around 5-8%
Typical changes due to treatment 5-6% at 1y
Monitoring at 1 year may not be diagnostic
Monitoring at 2 years recommended

28. The indications for DEXA based upon NOS ‘Local Provision for Osteoporosis’ and AGO report
Early Menopause
Prolonged Amenorrhoea
HRT Critical
Vertebral Deformity
Low Trauma Fractures
Osteopenia on X-ray
Long term/high dose steroids
Eating disorders
Chronic Liver disease
Alcohol abuse
Kidney dialysis
Hyperparathyroidism
PBC
Hypogonadism
Malabsorption Syndrome
Transplant Assessment
Growth Hormone
JCA
Thyroid Dysfunction
Follow up/previous abnormal DEXA
Other indication / trial patient

29. New patient clinical requests 2000-2001

30. Guideline is around 1000 new patient requests per year based on 300 000 population

31. Upon receipt of a request...
has all information been provided?
is the referrer known?
has at least one indication been checked?
is the patient pregnant?
any contra-indications (e.g. recent contrast)?
non-standard exam?
special patient needs?

32. Scheduling Bone Densitometry after Contrast or Nuclear Medicine Investigations
Tc-99m: no influence (up to 1GBq at 1hour)
Other isotopes may influence BM result
IV contrast 24hrs
Oral contrast 1 week
Barium 1 week
MR contrast 1 day

33. Example of patient pathway
Consultant request (through GP referral?)
DXA
Normal : no further action
Osteopenia (T=-1 to T=-2.5): advice on management
Osteoporosis (T<-2.5): bone clinic investigation
Identify cause and treat
Source: JN Fordham (2000)

34. What we print on our reports…
Osteoporosis is diagnosed in adults where T=-2.5 or less at the lumbar spine or hip (WHO Criteria)
This may not necessarily represent a treatment threshold as current guidelines recommend a full clinical assessment prior to treatment.

35. How to find out more…
National Osteoporosis Society