techniques and their assumptions Flashcards
what are the assumptions of skinfold techniques
1) The sum of several skinfolds can be used to estimate total body fat
2) The distribution of subcutaneous fat is similar for all individuals within a biological sex
3) There is a relationship between the sum of skinfolds and the body density
assumptions of CT
1) In automatic segmentation:
Tissue has radiodensity equivalent to published range
2)For QCT
Relationship between HU and density assumed
3)In extrapolation to total body:
Site measured is representative of the rest of the body
CT sources of error
Technical
Drift of anode current in X-ray tube- quality control needed
Participant movement
Participant positioning
Slice selection
Errors in assumptions
Tissue HU may differ from assumed values and this could lead to segmentation errors
Variation in relationship between HU and density could result in error in estimation of density
Local measurements may not be representative of total body
advantages and disadvantages of CT
(imaging)
Advantages
As HU are relative to air and water, all scanners provide comparable data
Cross-sectional imaging allows examination of deep structures and reconstruction of 3D images
Provides quantification of a range of structures: adipose tissue, skeletal muscle, bone, particular organs, etc
Imaging allows differentiation of chemically similar but anatomically distinct structures (e.g. different muscle groups, VAT and SAT)
Little co-operation required
Good sensitivity to detect local changes in e.g. muscle CSA
Disadvantages
X-ray exposure (substantially greater than DXA)
Only limited body areas would be scanned
Restricted use particularly in children and people of reproductive age
Expensive
Limited availability
Principles of CT
Detectors determine attenuation coefficients
Ln (Io/I) = mass x m
m = Ln(Io/I)/mass
Where
Io= initial intensity
I=transmitted intensity
m= linear attenuation coefficient
Principles of DXA
attenuation
Attenuation of X-ray calculated for each pixel (>20,000 depending on body size)
Attenuation at two energies can be used to calculate mass of two components using simultaneous equations
For non-bone pixels, proportion fat and lean calculated
Total mass of bone/non bone lean/fat in whole body or regions calculated
What body composition model does DXA give us?
3C Tissue level
Assumptions of DXA
Attenuation coefficients constant for fat, bone free lean, bone
BUT Bone free lean may be affected by hydration status
Soft tissue composition over bone matches soft tissue composition of other pixels in region
Regions with large proportion bone pixels under-represented
Types of DXA scanners
Pencil beam, Fan beam, Narrow angle fan beam
Sources of error for DXA
Technical
Body size
Limited scan area
Beam hardening at high tissue thicknesses
Greater attenuation of lower energy X-ray
Clothing
Metal causes artefact; light clothing (e.g. vest and shorts) recommended
Food and drink consumption
12h fast recommended, with no exercise
Differences between instruments
Drift within instrument
Daily calibration and quality control needed
Errors in assumptions, e.g.
Lean soft tissue has constant attenuation
Hydration may influence R
hydration changes up to 5% may influence fat content by 1% (Pietrobelli 1998)
Population differences
Pixels overlying bone have same composition as non-bone pixels
Advantages and disadvantages of DXA for body comp assessment
Advantages:
Allows assessment of bone density, sarcopenia and estimation of fat distribution
Good precision and reasonable accuracy whilst being easy for participant
Disadvantages:
Expensive and involves some radiation exposure
Body composition of Male cadaver by dissection
5% skin, 13% bone, 28% fat, 40% muscle and 14% other.
body composition of female cadaver by dissection
5% skin, 12% bone, 43% fat, 28% muscle and 12% other.
Types of adipose tissue
Subcutaneous and visceral
what is SAT
AT stored beneath the skin
Can be assessed by measuring skinfold thicknesses
what is VAT
Visceral Adipose Tissue (VAT)
AT within and around the organs in the thoracic and abdominal cavities
Can be assessed using scanning technologies e.g. MRI
What can we use to measure density
Air displacement plethysmography and hydrodensitometry (UWW)
What can we use to measure bone mineral
DXA
what can we use to measure total body water (TBW)
Hydrometry
what are the main models of body composition
6C atomic level, 2C molecular level, 3c molecular level, 3c cellular level, 3c tissue level, 4c molecular level
what are the densities of the various body components in g/cm3
Water: 0.99, protein 1.34, mineral 3.04, fat 0.90
what is the method of hydrometry
Measurement of total body water
Ingest known volume of a tracer e.g. deuterium oxide (2H2O)
Wait for tracer to equilibrate with body fluids
Take a sample of fluid e.g. saliva
Measure deuterium oxide concentration (e.g. mass spectrometer)
What are the advantages and disadvantages of BMI
Advantages:
Quick and easy calculation. Found to be correlated with risk of cardiovascular disease.
Disadvantages:
Doesn’t assess body composition,
Doesn’t take individual into account.
Hydrometry principles
The volume of a component is equal to the amount of tracer added divided by the concentration of tracer in component.
TBW =tracer mass tracer concentration
TBW makes up a fixed proportion (e.g. 73.2% of FFM (Pace & Rathburn 1945)
FFM=TBW/0.732
What tracers can be used in hydrometry
Small molecules
Urea
Ethanol
Isotopes
Stable (need mass spectrometry)
Deuterium 2H usually as deuterium oxide 2H2O
Oxygen 18 18O usually as H218O
Radioactive (need radiation counter)
Tritium 3H usually as tritiated water 3H2O
what are the assumptions of hydrometry
The tracer is distributed only in the exchangeable pool (i.e. water)
It is equally distributed within this pool
It is not metabolised during the equilibration time
Equilibration is achieved within the equilibration time
what sources of error are there in hydrometry
Incomplete consumption of tracer dose
Incomplete bladder emptying
Fractionation (heavy isotope slightly less likely to evaporate)
Variation of hydration of FFM
Between individuals
Higher hydration in children and those with high muscle mass
Within individuals
Dehydration
advantages and disadvantages of hydrometry
Advantages
Good reproducibility (precision error 1-2%) and accuracy if hydration status controlled
Can be used in multi-compartment models to account for variation in hydration
Moderate participant demand
Disadvantages
Isotopes and equipment are expensive
Errors (in 2C model) from variation in hydration of FFM
