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 body-counting 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 x-rays.

• Food or fluid intake have little affect on results.

• DEXA is approved for research in USA.

DEXAEquipment:• 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.