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Bone Densitometry David Rawlings Regional Medical Physics Department Newcastle General Hospital.
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Transcript of Bone Densitometry David Rawlings Regional Medical Physics Department Newcastle General Hospital.
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Bone Densitometry
David Rawlings
Regional Medical Physics Department
Newcastle General Hospital
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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
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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
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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
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Osteoporosis …
• a multi-factorial disease
• characterised by increased fracture risk
• may be amenable to treatment (e.g. HRT, bisphosphonate, calcium supplementation)
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“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…?
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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
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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.
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DXA at the hip, lumbar spine and whole body is a routine out-patient procedure
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Peripheral DEXA (forearm or heel) may
be available within the primary care sector
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DXA uses x-rays but differs from radiography because: 1) It scans 2) It uses two x-ay energies
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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”
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DEXA reports bone density (g/cm2) at each region of interest
(ROI) imaged
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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
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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
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Risk of future fracture increases by factor of between 1.4 and 2.6 for every
1SD decrease in BMD Marshall et al 1996
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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)
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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
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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
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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
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Guideline is around 1000 new patient requests per year based on
300 000 population
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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?
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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
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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)
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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.
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How to find out more…
• National Osteoporosis Society
• www.NOS.org.uk