Bone Densitometry

David Rawlings

Regional Medical Physics Department

Newcastle General Hospital

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

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

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

Osteoporosis …

• a multi-factorial disease

• characterised by increased fracture risk

• may be amenable to treatment (e.g. HRT, bisphosphonate, calcium supplementation)

“A selective case finding strategy is recommended to target those at high

absolute risk of fracture” (National Osteoporosis Society, 1999)

Therefore NOT population screening!!!

How do we diagnose osteoporosis…?

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

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.

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

Peripheral DEXA (forearm or heel) may

be available within the primary care sector

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

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”

DEXA reports bone density (g/cm2) at each region of interest

(ROI) imaged

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

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

Risk of future fracture increases by factor of between 1.4 and 2.6 for every

1SD decrease in BMD Marshall et al 1996

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

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

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

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

DEXA works by measuring a narrow beam of x rays transmitted through bone

I0

X Rays in

I= < I0

X Rays out

Narrow x-ray beams obey a well defined exponential law of absorption

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

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

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

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

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)

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

The indications for DEXAbased 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

New patient clinical requests 2000-2001

0 200 400 600 800

Steroids 21%

Low Trauma Fx 11%

Early Menopause 9%

Vert Deformity 9%

Osteopenia 8%

HRT Critical 4%

Transplant 4%

Hypogonadism 4%

Malabsorption 3%

Amenorrhoea 3%

Liver Disease 2%

Eating Disorders 1%

Others

Guideline is around 1000 new patient requests per year based on

300 000 population

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?

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

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)

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.

How to find out more…

• National Osteoporosis Society

• www.NOS.org.uk