Assuntos
Absorciometria de Fóton/instrumentação , Composição Corporal , Acrilatos , Tecido Adiposo/anatomia & histologia , Alumínio , Peso Corporal , Densidade Óssea , Osso e Ossos/anatomia & histologia , Calibragem , Desenho de Equipamento , Feminino , Humanos , Masculino , Imagens de Fantasmas , Análise de RegressãoRESUMO
The objective of the study was to determine the precision of total- and regional-body composition measurements from a total-body scan using dual-energy X-ray absorptiometry (DXA). This is critical information necessary to determine the smallest change from baseline that could be detected with statistical significance when conducting longitudinal measurements of body composition variables in an individual. Twenty volunteers were scanned once each day for 4 consecutive days using a Lunar DPX-L densitometer and manufacturer-supplied software (version 1.3z). Coefficients of variation (CV, %) derived from data using the (preferred) extended research mode of analysis were 0.62, 1.89, 0.63, 2.0, 1.11, 1.10, and 1.09% for total-body bone mineral density (BMD), total percentage fat, total body tissue mass, fat mass, lean mass, bone mineral content (BMC), and total bone calcium, respectively. Regional measurements (arm, leg, trunk, pelvis, and spine) were less precise than total body measurements, with CVs in the range of 1% to 3% (but fat mass for arms was 4.26%, trunk 3.08%, BMC 3.65%). Small but statistically significant differences in mean values for most body composition variables were found when data were compared between extended and standard modes of analysis. Inconsistent use of analysis mode in a cohort or when following a patient longitudinally may negatively affect precision. We conclude that the measurement precision of total and regional body composition variables was generally comparable to the precision limits typically associated with lumbar spine and proximal femur BMD data.
Assuntos
Absorciometria de Fóton/instrumentação , Composição Corporal , Adulto , Fatores Etários , Idoso , Análise de Variância , Feminino , Humanos , Masculino , Pessoa de Meia-Idade , Fatores Sexuais , SoftwareRESUMO
Experimental results with DXA body composition are partly attributable to the DXA technology itself (X-ray physics) and partly to the particular implementation (Lunar DPX, Hologic QDR, etc). This paper identifies a number of advantages and disadvantages of DXA body composition measurement, distinguishing between those which are common to all DXA and those which are specific to particular instruments.
Assuntos
Absorciometria de Fóton/instrumentação , Composição Corporal , Absorciometria de Fóton/métodos , Adulto , Constituição Corporal , Água Corporal/fisiologia , Desenho de Equipamento , Humanos , Reprodutibilidade dos Testes , Sensibilidade e EspecificidadeRESUMO
This study examines how well a Lunar DPX can compensate for certain aging changes in its X-ray source. Total body scans were done on a phantom having variable soft tissue composition. Aging of the X-ray source was simulated by placing a thin aluminum foil over the beam exit window, and by deliberately misadjusting the high voltage power supply. The % fat measurement was found to change by up to 12%, but returned to within about 2% afer recalibration.
Assuntos
Absorciometria de Fóton/instrumentação , Composição Corporal , Imagens de Fantasmas , Absorciometria de Fóton/métodos , Tecido Adiposo/anatomia & histologia , Calibragem , Humanos , Plásticos , Reprodutibilidade dos Testes , EsqueletoRESUMO
This paper begins with a fundamental description of the dual-energy X-ray absorptiometry (DXA) technique for measurement of bone mineral. It describes how, in extending the technique to do accurate assessment of body fat and lean, it is important that material standards for fat and lean exist, and that a suitable model for fat distribution in the body be developed. The computational steps employed in DXA and in the familiar underwater weighing (UWW) technique are compared and contrasted. Experimental data on over 350 human subjects shows that the percent fat results of DXA and UWW do not agree. However when both methods are used to determine body tissue density, there is good agreement. The authors suggest that the discrepancy may lie with the equations that are used in UWW to compute % fat from body density.
RESUMO
This paper discusses a number of strengths and weaknesses of two methods for determination of body fat, Dual-energy X-ray Absorptiometry (DXA) and underwater weighing (UWW). Several error sources are theoretically quantified. One source of error in the UWW method, variation in bone mass fraction, is examined using data gathered on 219 human subjects who were measured by both methods. The experimental data show the expected linear form but do not exactly match the theoretical curves, indicating that all error sources are not completely understood. The data suggest a possible error in the Brozek equation which is commonly used to compute % fat in UWW.
RESUMO
One source of error in the underwater weighing (UWW) method of body fat determination is variability in bone mass as a fraction of total nonfat mass. We examined this error theoretically and experimentally using data gathered on 219 human subjects who were measured by UWW and also by the newer technique of dualenergy X-ray absorptiometry (DXA). The experimental data suggest an error was made in the assumed bone mass fraction used in the derivation of the Brozek equation, the standard means of converting the body density values obtained in UWW to body fat percent. Using this new experimental data, a new equation (set) is proposed for use in UWW measurements.
