Resonant cavity perturbation: an alternative method to bioimpedance analysis for the assessment of total body water

TY - CONF

T1 - Resonant cavity perturbation

T2 - The use of bioimpedance in patients with kidney disease

AU - Robinson, Martin Paul

AU - Oldroyd, B

AU - Rhodes, L

AU - Hind, K

AU - Thurlow, S

PY - 2015/7/3

Y1 - 2015/7/3

N2 - Background. Measurement of fluids is important in the treatment of kidney disease but is difficult to do accurately. Bioimpedance analysis (BIA) measurements tend to be dominated by the impedance of the limbs rather than the trunk [1, 2] while other methods are indirect and involve ionising radiation (dual X-ray absorptiometry) or are too slow for clinical practice (deuterium dilution). The resonant cavity perturbation (RCP) method [3, 4, 5] is a promising alternative. Methods. The RCP method relies on the dielectric properties of body tissues, i.e. permittivity and conductivity [6], being well correlated with their water content. Thus placing a human subject in a resonant cavity perturbs its resonant frequencies, and this enables total body water (TBW) to be calculated. Our practical implementation uses a screened room, 2.94m wide, 2.32m high and 4.60m long, with the subject lying on a polystyrene ‘bed’ at the centre. Two pairs of antennas with a ‘top hat’ design give improved performance from the earlier monopoles, and monitor two cavity modes with vertical electric field (60MHz) and horizontal field (72MHz) – the presence of a subject lowers these frequencies by around 0.5MHz. RCP measurements of TBW have been included in a number of body-composition studies involving both adult and child subjects. Results. Recent studies have shown the precision of the RCP measurements of TBW, due to the uncertainty involved in re-positioning of the subject, is 1.5% in adults (n=51) and 2.4% in children (n=36). Various predictive equations have been established using linear regression and other methods. For the same group of 36 children, the best predictive equation had an r-squared of 0.93 and related the TBW to the shifts in both the vertical and horizontal modes, with a correction for body mass index (BMI). Measurement of body water by RCP has shown very good agreement with alternative methods including BIA and isotope dilution. Discussion & Conclusion. RCP is fast, with multiple measurements taking a few minutes, and comfortable for the subject as the screened room is spacious and there is no need for electrodes. The radiation is non-ionising, and RF power levels are low to keep the exposure (i.e. the specific absorption rate, SAR) well within international guidelines [7]. At tens of MHz, field penetration is sufficient that the resonant frequency shift responds to water anywhere in the body. The main disadvantage is that the cavity needs to be large in comparison to body volume, thus taking up space. Accurate predictive equations are possible if a correction is made for body size and shape – we are currently investigating waist circumference, abdominal depth and width, and gluteal hip dimensions. RCP is a promising technique with many advantages for assessment of body water. References. 1. Woodrow G, Oldroyd B, Turney JH, Smith MA. Segmental bioelectrical impedance in patients with chronic renal failure. Clin. Nutr. 1996; 15:275-279. 2. Fuller NJ, Fewtrell MS, Dewitt O, Elia M, Wells JC. Segmental bioelectrical impedance analysis in children aged 8-12y: 1. The assessment of whole body composition Int. J. Obes. Relat. Disord. 2002; 26:684-691. 3. Robinson MP, Clegg J, Stone DA. A novel method of studying total body water content using a resonant cavity: experiments and numerical stimulation. Phys. Med. Biol. 2003; 48:113-125. 4. Stone DA, Robinson MP. Total body water measurements using resonant cavity perturbation techniques. Phys. Med. Biol. 2004; 49: 1773-1788. 5. Robinson MP, Stone DA, Marvin AC, Zhen Z. Improved accuracy of resonant cavity perturbation determination of total body water by using a dual mode measurement. Proc. XXVIIIth Triennial General Assembly of the International Union of Radio Science, New Delhi, India, 23-29 Oct. 2005. 6. Gabriel S, Lau RW, Gabriel C. The dielectric properties of biological tissues. Parametric models for the dielectric spectrum of tissues. Phys. Med. Biol. 1996; 41: 2271-2293 7. International Commission on Non-Ionising Radiation Protection (ICNRP). Guidelines for limiting exposure to time-varying electric, magnetic and electromagnetic fields (up to 300 GHz). Health Phys. 1998; 44:1630–1639

AB - Background. Measurement of fluids is important in the treatment of kidney disease but is difficult to do accurately. Bioimpedance analysis (BIA) measurements tend to be dominated by the impedance of the limbs rather than the trunk [1, 2] while other methods are indirect and involve ionising radiation (dual X-ray absorptiometry) or are too slow for clinical practice (deuterium dilution). The resonant cavity perturbation (RCP) method [3, 4, 5] is a promising alternative. Methods. The RCP method relies on the dielectric properties of body tissues, i.e. permittivity and conductivity [6], being well correlated with their water content. Thus placing a human subject in a resonant cavity perturbs its resonant frequencies, and this enables total body water (TBW) to be calculated. Our practical implementation uses a screened room, 2.94m wide, 2.32m high and 4.60m long, with the subject lying on a polystyrene ‘bed’ at the centre. Two pairs of antennas with a ‘top hat’ design give improved performance from the earlier monopoles, and monitor two cavity modes with vertical electric field (60MHz) and horizontal field (72MHz) – the presence of a subject lowers these frequencies by around 0.5MHz. RCP measurements of TBW have been included in a number of body-composition studies involving both adult and child subjects. Results. Recent studies have shown the precision of the RCP measurements of TBW, due to the uncertainty involved in re-positioning of the subject, is 1.5% in adults (n=51) and 2.4% in children (n=36). Various predictive equations have been established using linear regression and other methods. For the same group of 36 children, the best predictive equation had an r-squared of 0.93 and related the TBW to the shifts in both the vertical and horizontal modes, with a correction for body mass index (BMI). Measurement of body water by RCP has shown very good agreement with alternative methods including BIA and isotope dilution. Discussion & Conclusion. RCP is fast, with multiple measurements taking a few minutes, and comfortable for the subject as the screened room is spacious and there is no need for electrodes. The radiation is non-ionising, and RF power levels are low to keep the exposure (i.e. the specific absorption rate, SAR) well within international guidelines [7]. At tens of MHz, field penetration is sufficient that the resonant frequency shift responds to water anywhere in the body. The main disadvantage is that the cavity needs to be large in comparison to body volume, thus taking up space. Accurate predictive equations are possible if a correction is made for body size and shape – we are currently investigating waist circumference, abdominal depth and width, and gluteal hip dimensions. RCP is a promising technique with many advantages for assessment of body water. References. 1. Woodrow G, Oldroyd B, Turney JH, Smith MA. Segmental bioelectrical impedance in patients with chronic renal failure. Clin. Nutr. 1996; 15:275-279. 2. Fuller NJ, Fewtrell MS, Dewitt O, Elia M, Wells JC. Segmental bioelectrical impedance analysis in children aged 8-12y: 1. The assessment of whole body composition Int. J. Obes. Relat. Disord. 2002; 26:684-691. 3. Robinson MP, Clegg J, Stone DA. A novel method of studying total body water content using a resonant cavity: experiments and numerical stimulation. Phys. Med. Biol. 2003; 48:113-125. 4. Stone DA, Robinson MP. Total body water measurements using resonant cavity perturbation techniques. Phys. Med. Biol. 2004; 49: 1773-1788. 5. Robinson MP, Stone DA, Marvin AC, Zhen Z. Improved accuracy of resonant cavity perturbation determination of total body water by using a dual mode measurement. Proc. XXVIIIth Triennial General Assembly of the International Union of Radio Science, New Delhi, India, 23-29 Oct. 2005. 6. Gabriel S, Lau RW, Gabriel C. The dielectric properties of biological tissues. Parametric models for the dielectric spectrum of tissues. Phys. Med. Biol. 1996; 41: 2271-2293 7. International Commission on Non-Ionising Radiation Protection (ICNRP). Guidelines for limiting exposure to time-varying electric, magnetic and electromagnetic fields (up to 300 GHz). Health Phys. 1998; 44:1630–1639

KW - bioimpedance, physiological measurement, medical

M3 - Abstract

Y2 - 3 July 2015

ER -