Transcript Advanced Techniques

Complex Techniques
• Hydrometry
• Whole-Body Counting and Neutron
Activation Analysis
• Dual Energy X-Ray Absorptiometry
• Imaging Techniques
Hydrometry
• Water is the most abundant of the
constituents of the body.
• It makes up 70-75% of weight at birth, but
less than 40% in obese adults.
Hydrometry
Dilution Principle
• States that the volume of the compartment
is equal to the amount of tracer added to the
compartment divided by the concentration
of the tracer in that compartment.
Hydrometry
• With this method, the concentration of
either hydrogen or oxygen isotopes in
biological fluids (e.g., saliva, plasma, and
urine) after equilibration is measured and
used to estimate total body water.
Hydrometry
• The tracers used in the in-vivo dilution do
not behave in an ideal manner.
• Thus, measurement of TBW in-vivo
requires careful attention to these deviations
from the basic assumptions underlying the
dilution principle.
Hydrometry
Precision of total body water
• The precision of the total body water
measurement is dependent on the analytical
method as well as the dose of tracer
administered to the subject.
Hydrometry
• In general, mass spectrometric methods
have been the most precise.
– Using two isotopes the precision is 1 - 2%.
– Using other analytical methods the precision is
between 2 - 4%.
Hydrometry
• Procedural variations such as the
physiological fluid selected for sampling,
equilibration time, the estimated correction
factor for the isotropic dilution space, and
the method selected for assaying the labeled
water may all contribute to an increased
technical error.
Whole-Body Counting
• Scintillation detectors were developed in the
early 1950’s.
• They measure the body’s natural potassium
as well as other radioactivity in the body.
Whole-Body Counting
• In 1958, Kulwich, Feinstein, and Anderson
correlated natural potassium concentration
with fat free mass.
Whole-Body Counting
• There are an estimated 75 counters in the
US.
• There are more than 180 whole-body
counters worldwide.
• Two-thirds of these perform body
potassium measurements in humans.
Whole-Body Counting
• Potassium is naturally distributed in three
isotopic states.
• The isotope 40K is radioactive.
Whole-Body Counting
• Gamma rays from 40K are high-energy
gammas, many of which exit the body and
can be easily detected by external counting
• The smaller the subject, the lower the 40K
content and thus the weaker gamma signal.
Whole-Body Counting
• Factors such as age, fitness, or restricted
mobility due to surgery or illness do not
tend to affect the precision of total body
potassium measurements.
• The 40K signal is natural and continuous,
therefore the measurement can be
interrupted as necessary, until counting is
completed.
Whole-Body Counting
The three requirements for 40K whole bodycounting include:
• Efficient gamma-ray detectors that can be
placed close to the subject.
• Shielding for these detectors to reduce the
natural background radiation levels (cosmic
rays, radioactive contaminants in
construction materials)
Whole-Body Counting
• Computer-based instruments that enable
identification of the unique gamma rays.
Whole-Body Counting
Precision:
• For whole-body counters, precision is in the
range of 2-5% for adults.
• In infants and very young children, accurate
measurements have been difficult to obtain.
The precision is only 8-12% for 40 minute
sample times.
Whole-Body Counting
• Cost depends on age range of the population
being examined.
• Total cost for an adult whole-body counter
is $10,000-15,000.
• Cost of a special shielded room starts at
$80,000.
• These are just start-up costs.
Neutron Activation Analysis
• IVNAA measures 11 elements from nuclear
reactions.
• Protein, mineral, and fat can be estimated
from these elements:
– Carbon = Lipid
– Nitrogen = Protein
– Calcium = Bone
Neutron Activation Analysis
• IVNNA use a whole-body counter.
• It delivers a moderate beam of fast neutrons
to the subject.
• Atoms of target elements capture these
neutrons.
• This creates an unstable isotope.
Neutron Activation Analysis
• Unstable isotopes produce gamma rays
when returning to a stable state.
• Gamma rays are measured:
– energy level identifies the element
– the activity indicates its abundance.
Prompt-Gamma Activation
Analysis
• The isotope gets very excited with the
added neurons.
• This lasts only a fraction of a nanosecond
before it returns to a stable state.
• It is therefore measured simultaneously as
the neutrons are exposed to the isotope.
Neutron Activation Analysis
Disadvantages:
• Radiation exposure.
• Must be performed by medical personnel
• Cost $30,000 - $300,000.
Ultrasound
• Uses high frequency sound waves that are
produced by a piezoelectric crystal in a
transducer.
Ultrasound
• A common source of error is the occurrence
of multiple echoes from connective tissue
layers within the SAT, particularly over the
abdomen.
Ultrasound
• The disadvantages of ultrasound, in
comparison with calipers, are its greater
expense and lesser portability.
• Careful site location is also crucial.
Ultrasound
• Its advantages over the measurement of
skinfold thickness in the obese are now
recognized more widely.
Dual Energy X-Ray
Absorptiometry (DEXA)
• DEXA was originally developed to estimate
bone mineral content.
• DEXA estimation of body fat is based on
the attenuation of two energy x-rays
through the body.
DEXA
• From each x-ray, DEXA software provides
an estimation of the ratio of fat and lean
tissue in each computerized pixel.
DEXA
Femoral Neck
Lumbar Spine
DEXA
• To perform a DEXA reading, subjects lie in
a prone position for approximately 12
minutes for an x-ray scan from head to toe.
DEXA
• This technology is attractive because it can
be used to assess regional body composition
as well.
• Additionally, this method requires virtually
no effort on the part of the participant and
does not depend on technician skill.
DEXA
Applicability:
• DEXA is applicable through low radiation
exposure.
– Radiation levels are 100 times less than
common x-ray.
DEXA
Disadvantages:
• Can not be used during pregnancy.
• Special software required for infants.
DEXA
Advantages:
• All ages are included.
• Low radiation as compared to common xrays.
• Food or fluid intake have little affect on
results.
• DEXA is approved for research in USA.
DEXA
Equipment:
• Hologic QDR (1988)
– Uses alternately pulsed x-ray beams.
• Lunar DPX (1990)
– Sends rays from beneath person and measures
energy sent through them.
• Norland XR (1993)
– Uses dynamic filtration which adjusts to patient
thickness.
DEXA
Summary
• There is general agreement that some of the
new DEXA software programs offer precise
measurement of body composition.
• With additional studies, it is very possible
that DEXA may become the new “gold”
standard of body fat assessment.
DEXA
Summary
• However, more validation studies are
needed to determine the affect of various
conditions such as hydration levels and
disease state.
Imaging Techniques
• New technologies have allowed researchers
to measure body composition in ways that
were once impossible:
– Computer assisted tomography (CAT scan)
– Magnetic resonance imaging (MRI)
Computer Assisted Tomography
• A radiological technique that is commonly
used for diagnostic purposes.
• Cross sectional areas of adipose tissue,
muscle and bone can be measured with CT
scans at any body site.
Computer Assisted Tomography
Basic Principles:
• An x-ray tube and detectors are located at
opposite poles of a large ring.
• Tube rotates around the table
• Subject lies with arm stretched above their
head.
• Computer generates image from attenuated
beams to recognize tissues.
Computer Assisted Tomography
Precision:
• Adipose tissue from CT correlated high (r =
0.98) with chemically extracted lipid mass
in rats.
• Difference between measurements of total
AT volume is about 0.6%.
Computed Tomography and BC
• CT is an accurate technique for the
assessment of body composition, and it does
not rely on assumptions such as the
constancy of either the density or the water
content of fat free mass.
Computed Tomography and BC
• The most important application of CT in the
field of body composition has been to the
measurement of abdominal VAT.
Magnetic Resonance Imaging
• Based on the interaction between nuclei
(proton) of hydrogen atoms and the
magnetic fields generated and controlled by
the instrumentation of the MRI system.
MRI
• The MRI field strength is 10,000 times
stronger than that of earth.
• Protons tend to align with the magnetic
field.
• A pulsed radio frequency (RF) field is
applied to body tissues causing protons to
absorb energy.
MRI
• RF field is turned off, protons return to
original position and release energy
absorbed.
MRI
• MRI data acquisition is programmed to take
advantage of specific proton density and
relaxation times (rate at which absorbed
energy is released) to contrast between AT
and skeletal muscle.
MRI
Limitations:
• Time required to obtain quality images
approximately 8 minutes for the abdominal
region.
• Motion caused by respiration and cardiac
function decrease image quality.
MRI
Accuracy
• Only one report compared MRI
measurements of AT with those derived by
dissection of human cadavers.
• Found mean of 6% difference in VAT of 3
cadavers.
MRI and BC
• Because multiple images can be obtained
without any known health risks to the
subject, MRI is well suited for assessment
of whole body AT distribution.
MRI and BC
• There have been several groups using a
multi-slice protocol to evaluate whole body
AT distribution.
MRI and BC
• MRI is useful for discrimination between
VAT and SAT accumulation.
MRI and Whole-Body Lean
Tissue
• Whole body lean tissue from MRI in obese
men and women can be predicted with an
accuracy of 3.6% and 6.5% respectively.
Summary
• Need for further validation of CT and MRI.
• Tools of choice for precise measurements of
SAT and VAT
• VAT assessment is critical in assessment of
health risks.