Total potassium content in the human body may be obtained by measuring naturally occurring radioactive potassium-40, in a whole-body counter. The basic problem in calibrating the whole-body counter is discussed, as well as a phantom that is used to solve this problem. The normal potassium levels for men and for women were determined. Two additional parameters - lean body mass and total body water - derived from the potassium content, were also calculated

Iturralde, M.

Lotter, M.G.

Minnaar, P.C.

Van Reenen, O.R.

English

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2482 S.-A. MEDIESE TYDSKRIF 7 Desember 1974Calibration and Measurement of Potassium-40with a Whole-Body CounterO. R. VANREENEN, M. G. LoITER, P. C. MINNAAR, M.ITURRALDESUMMARYTotal potassium content in the human body may be obtainedby measuring naturally occurring radioactive potassium-40,in a whole-body counter. The basic problem in calibratingthe whole-body counter is discussed, as well as a phantomthat is used to solve this problem. The normal potassiumlevels for men and for women were determined. Twoadditional parameters - lean body mass and total bodywater - derived from the potassium content, were alsocalculated.S. Air. Med. l., 48, 2482 (1974).Potassium, one of the electrolytes in the human body,participates in the propagation of nerve impulses, is anecessary biocatalyst and is involved in muscular con-traction. About 98% of the body's potassium is storedintracellularly. The serum level, although extracellular,is a very important and critically controlled quantity.Normal levels range from 3,5 to 5 mmolflitre, whereasnormal cells contain ± 140 mmol/litre.The upper level in serum is controlled by excretion ofpotassium by the kidneys. The lower level is controlledby extraction of potassium from the body pool. There issufficient evidence' that no correlation exists betweenserum levels and the total body content, basically becausethey are controlled by different mechanisms. The renalregulation of serum potassium is designed for the removalof excess potassium rather than for its conservation,whereas the body content is replenished by food intakeonly. The turnover of potassium is 2 - 3 g per day.Two methods for measuring the total potassium contentof the human body are available: (I) an isotope dilutiontechnique using 42K as a tracer for the exchangeablepotassium pool; and (il) counting of the naturally occurr-ing radio-isotope "K.Owing to its very long half-life of 1,3 x 109 years, .oKis uniformly distributed throughout, comprises 0,0118%of all potassium and emits gamma rays with an energy of1 460 keY. The average human body containing 100 g ofpotassium emits approximately 330 gamma rays perDepartement of Biophysics, University of the Orange FreeState and National Hospital, BloemfonteinO. R. VAN REENENM. G. LbITERP. C. MINNAARDepartment of Internal Medicine, University of the OrangeFree State and National Hospital, BloemfonteinM. ITURRALDEDate receil-ed: 28 August 1974.second. These gamma rays can be detected by means ofthe scintillation detectors in the whole-body counter.One of the basic problems of this method is thecalibration of the whole-body counter. Since the activityand exact distribution of the radioactive potassium in thebody are unknown, some standard of known activity andgeometry had to be used to convert the gamma counts tograms of potassium. Once the potassium content had beencalculated, normal potassium levels could be determined.Anderson and Langham' showed that potassium con-centration, expressed in grams potassium per kg grossbody mass, was a good index to normal levels. They foundthe potassium levels to be functions of both sex and age.Our first task was therefore to construct a phantom, orstandard, for calibration purposes and also to obtainvalues by doing counts on normal individuals of bothsexes and different age groups.SUBJECTS AND METHODSEquipmentA Nuclear Enterprise Whole-Body Counter Model NE8102 equipped with two opposing scanning detectors wasused. The detectors are NaI (Tb) crystals of 10,2 x 12,7cm, which are connected to a single-channel analysersystem with window settings to count at the desired levels.The detection system is housed in a 15-cm thick steel-wallroom of 2 X 2 X 2,5 m where the patient lies on astretcher couch.Description of PhantomThe phantom must have the following characteristics:(i) a simple geometrical shape approximating the bodycontour and dimensions as far as the major forms areconcerned; (ii) enough flexibility to represent various bodymasses and lengths; (iii) the same isotope distribution asthe human body.Tbe whole-body phantom proposed by Snyder et al.3(Fig. 1) consists of three principal sections: (i) an ellipticalcylinder representing both head and neck; (ii) an ellipticalcylinder representing the arms, torso, and hips; (iii) atruncated elliptical cone representing both legs and feet.The dimensions are representative of a man weighing 70kg and 174 cm in height.In order to comply with such a theoretical phantom, amethod was devised to arrange one-litre saline plasticbags to simulate this geometrical configuration. These bags,7 December 1974 S.A. MEDICAL JOURNAL 2483TABLE I. PHANTOM SCALED VERSION PRINT-OUTTABLE 11. POTASSIUM CONTENT OF ADULT BODY ORGANSCalculation of Potassium ContentCounting ProcedureHeight = 174 cm0,0 kg1 - 5 kg6 - 18 kg19 - 30 kg31 - 43 kg44 - 48 kg49 - 58 kg59 - 64,5 kg65 - 76,5 kg68 - 69 kg0,0 kgPotassium (g/100 g)0,360,110,330,220,290,25X M X 2,14 = potassium content in grams(P~B)(F-B)where P = patient's count rate; B = background countrate; F = phantom count rate and M = mass of thepatient (= mass of phantom).The potassium content was calculated using the followingformula:The patient, wearing a gown, is placed on the couch inthe whole-body counter and scanned from the front andback for a total time of 2 160 seconds. Counts are recordedusing the full photopeak of <OK. On the same day, thebackground count and the counts from the patients'phantoms are determined.Mass = 70 kg4 cmo - 20 cm20 - 40 cm40 - 60 cm60 - 80 cm80- 90 cm90 - 110 cm110 - 130 cm130 - 150 cm150 - 170 cm4 cmOrganMuscleSkinBrainLiverIntestineHeartWhen packed and placed on a couch these phantomsrepresented the various body shapes. The advantages ofusing this system for constructing a phantom are: (i) it isflexible-masses of 25 - 100 kg and heights of 150 - 195cm can be simulated; (iI) it is easy to handle and construct;(iii) in addition to the potassium studies, a phantom may,for instance, have its liver replaced by a liver phantomloaded with "Co-vitamin B"" permitting correct geometry,scattering and background values to be obtained; (iv) it isvery cheap and easy to replace.Fig. 1. Adult human phantom of 70 kg mass and 174 cmheight (originally proposed by Snyder et al.').measuring 20 X 10 X 5 cm, with a mass of 1 kg, areideal for this purpose. The bags were all numbered.A computer programme was written whereby, given themass and height of a patient, it calculated the exactdistribution of the bags to simulate the patient's bodyaccording to a scaled version of the specified phantom.Table I is an example of the print-out. From a study ofthe potassium distribution in various parts of the body(Table 11), it is clear that potassium is uniformly distributedthroughout the body. By loading each saline bag with2,14 g of potassium, the saline phantom geometricallysimulated the human body as well as the potassiumdistribution. A 70-kg phantom therefore contains 150 gof po'tassium, equal to the content in an average male.2484 S.-A. MEDIESE TYDSKRIF 7 Desember 1974RESULTS TABLE V. LEAN BODY MASS AND TOTAL BODY WATERDERIVED FROM POTASSIUM CONTENTRange42 - 7530 - 5544 - 70As mentioned, Anderson and Langham' found a relation-ship between the potassium concentration, sex and age ofan individual. Therefore, to obtain normal values, indi-viduals of both sexes and different age groups werestudied. The groups consisted of 45 trainee nurses, 18years of age, and 37 male military trainees in the sameage group.The potassium concentrations obtained are listed inTable Ill. They were compared with normal values(Table IV), published by various authors using basicallythe same technique.TABLE Ill. RESULTS OF POTASSIUM CONCENTRATION ANDCONTENTMen (18 years old) Mean SDLean body mass (kg) ... 57,2 7,6Total body water (kg) 41,8 5,6%Body water (by mass) 59,2 6,6Normal % body water:' men 17 - 34 years - 61,1 %(range 53,3 - 70,3)Women (18 years old)Lean body mass (kg) 40,1 6,4 30 - 55Total body water (kg) 29,3 4,7 21 - 40%Body water (by mass) 48,0. 4,5 38 - 60Normal % body water:' women 20 - 31 years - 51,2%(range 46,5 - 59,9)Mean SD* Range DISCUSSION."Table III shows that a wide range of body shapes wasincluded in the two groups. As can be seen in Fig. 2,there seems to be a linear correlation between potassiumcounts and body mass, or phantom mass. When thepotassium concentration was calculated according to theabove formula, no correlation was found between potas-sium concentration and body mass or body height (Fig. 3).Similar graphs were obtained for the 18-year-old womenand the potassium concentrations compare very well withthe normal values listed in Table N. The conclusionis that the phantom used for the calibration compensatedvery well for the variations in body structure. Althoughcalibration errors may still be present, in that the calculatedvalues are not absolutely correct in terms of grams ofpotassium, such errors can only be shown by the useof <OK.+++ +4 +t ~t....4 ,.+'"...c~oU1,6Men (18 years old)Body mass (kg) 71,4 11,4 52 - 105Body height (cm) 176,9 8,5 160 - 193Potassium content (mmol) 3906,0 518,0 2860-5085Potassium concentration(mmol/kg) 55,2 6,1 41,4 - 64,6Women (18 years old)Body mass (kg) 61,4 8,7 47 - 86Body height (cm) ... 164,0 6,7 150 - 181Potassium content (mmol) 2548,0 "09,0 1855-3510Potassium concentration(mmoljkg) 41,7 3,8 32,7 - 51,8'It Standard deviation.Related ParametersTABLE IV. REPORTED POTASSIUM CONCENTRATIONSPot. cone.Author Number Country (mmol/kg)Men (18 years)McNeill and Green' 3Q Canada 54,2Anderson and Langham' 21 USA 55,0Alien et al. 'o (using 42K) 37 USA 54,2Wayne et al." (using '4K) 7 Scotland 50,6Women (25 - 49 years)MacGillivray et ai' 32 Scotland 41,0(using 4'K)Alien et al. 'o 176 USA 41,7The relationship between the total body potassiumcontent, lean body mass and total body water is alreadywell established..·• The lean body mass and total bodywater (Table V) were calculated as follows:total body' potassium content (mmol)Lean body mass68,1and total body water = lean body mass x 0,732.... PhantomD.I ~21;---.....c....-""i.48:'---'----Ei.---'---;I;ID'----'----;1':';DD'--Mass (kg)Fig. ~. The count rates of phantom (e) and patient (+) asfunction of body mass (phantom mass). The regressionline was calculated on pbantom data only: counts/sec =0,0130 X phantom mass + 0,3915 correlation coefficient0,882.7 December 1974 S.A. MEDICAL JOURNALCONCLUSION2485Fig. 3. Potassium concentration in mmolfkg as a functionof body mass.In Table Ill, the difference in the potassium concen-tration in the two sexes is very well demonstrated andcorrelates with the values obtained by Anderson andLangham.'To confirm the validity of the total body potassiumvalues, the lean body mass and total body water basedon the total potassium content, were calculated. Forbesand Lewis; by direct analysis of cadavers obtained avalue of 68,1 mmol potassium per kilogram of leanbody mass. Although this value may not be universallyapplicable, it was used to calculate the lean body mass.The total body water is stated to be 73,2% of the leanbody mass.' The results are listed in Table V.The normal values listed were obtg.ined by using heavywater. The values listed in Table V, determined fromthe total potassium content of the individual, comparewell with those' of the heavy water method. The conclu-sion is that the estimate of the total potassium content isfairly accurate.E.,Cl>Cl>'"-Qa.60__________________________________ ~ M~~u_50I40 ----L---c6fo.'O,-------'-----.iS'\,-O--~----,lr.D"O--M ass (I<g)A mathematically calculated phantom was constructed tosimulate various body shapes. Results indicate that thephantom correctly compensated for the various bodyshapes used in this study. It is difficult to estimate theexact limits for the phantom and great care should betaken in applying the phantom to masses below 40 kgand above 100 kg. Anderson and Langham' countingthe potassium of 1 590 subjects with a 4'iT liquid scintilla-tion counter, obtained good correlation between thepotassium concentration and age of the individual butfound a difference for the two sexes. Our results show asimilar difference and compare well with the publishedvalues. The age-potassium concentration relation in thelocal population will also be determined as soon as suffi-cient data have been collected.Future projects include determination of potassiumconcentrations in other age groups and normal valuesfor the Black population.We wish to thank. Matron Locbner of the National Hospi-tal, Bloemfontein, and Colonel Dippenaar, Medical Officer ofthe SA Defence Force, for their co-operation; and the SouthAfrican Atomic Energy Board for financial support.REFERE TCESI. Burke, S. R. (1972): The Composition and Function of Body Fluids,p. 43. St Louis: C. V. Mosby.2. Anderson. E. C. and Langham, W. H. (1959): Science, 130, 713.3. Snyder,. W. S., Fischer, H. L jun., Ford, M. R. and Warner, G. G.(J969): 1. Nucl. Med. (MIRD Publication suppl. 3, pampWet 5).4. Woodward. K. T., Trujillo, T. T., Schuch, R. L. and Anderson,E. C. (1956): Nature (Lond.). 178, 97.5. Belcher, E. H. and Yeller. H. (1971): Radio-isolopes in MedicalDiaf?nosis. p_ 285. London: Butterworths.6. Forbes, G. B. and Lewis, A. M. (1956): J. Clin. Invest., 35. 596.7. Documenta Geigy (1962): Scientific Tables, 6th ed., p. 53 Basle:J. R. Geigy.8. McNeill, K. G. and Green, R. M. (1959): Canad. J. Phys., 37, 683.9. MacGiIlivray. I.. Buchanan, T. I. and BiIlewics, W. Z. (1960):Clin. Sei., 19. 17.10. Alien. T. H., Anderson, E. C. and Langham, W. H. (1960):J. Gerontol.. 15, 348.I!. Wayne, E. J., Bluhm, M. M. and Crooks. J. (1958): Studies ofbody composition in normal and pathological states using isotopedilution techniques. United Nations Second Conference on thePeaceful Uses of Atomic Energy. Geneva Papen A/Conf 15/P279.Vienna: IAEA.

Calibration and Measurement of Potassium-40 with a Whole-Body Counter

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Calibration and Measurement of Potassium-40 with a Whole-Body Counter

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