192 KADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



made if necessary. Ennor and Rosenberg (57) have described a method 

 for estimation of P^'- and Na^'* from a mixture of each in body fluids and 

 tissues. The P^- was extracted as a molybdate complex into isobutanol, 

 and the Na-^ remained in the aqueous phase. 



In Vivo Measurements. The advantages of in vivo measurements have 

 been mentioned in Chap. 1, and an excellent review of this subject has 

 been compiled by Strajman and Pace (58). The locations within a tissue 

 from which radiation will be detected by a device at the tissue surface will 

 be determined primarily by the solid angle subtended by the counter tube, 

 the scattering in the tissue, and the energy of radiation. If dynamic fac- 

 tors are involved, the measurements may be dependent upon such factors 

 as blood perfusion of tissues, mixing of substances with blood, and pen- 

 etration into cells. These complicating considerations have virtually 

 precluded development of absolute quantitative relationships, and reli- 

 ance has been placed on standardization of conditions and comparative 

 measurements. 



It is usually helpful to reduce as much as possible the tissue volume 

 from which radiation is received, in order to increase the resolving power 

 of the instrument. This is accomplished by shielding the counter and 

 especially by providing considerable colHmation. Such assemblies are 

 called directional counters. The limiting factor to the amount of collima- 

 tion that can be employed is the reduction of sensitivity. Gamma-ray- 

 emitting isotopes are primarily employed on account of their penetrating 

 power. However, in some cases P^-, a beta emitter, has been used. As 

 a matter of fact, it is often necessary in this type of measurement to shield 

 out the beta rays so that the surface contribution will not be magnified. 

 Since the sensitivity of response to gamma rays is so important, it is clear 

 that the development of the scintillation counter has been of particular 

 value in this connection. Maclntyre (59), for example, reported that, 

 for measurements of P^^ uptake in human beings, the scintillation counter 

 was 30 to 50 times more sensitive than the Geiger counter and that the 

 use of the former allowed a reduction of the tracer dosage from 100 to 

 about 25 ^c. Allen and Goodwin (60) were able to do thyroid-function 

 studies with a scintillation counter (150 times more sensitive than a Geiger 

 counter) after administration to patients of as little as 1 jjlc P^^ 



The in vivo method has perhaps been most wddely applied using P^Mn 

 the study of thyroid function in human beings. This is due to the selective 

 accumulation of iodine in a gland of small proportions, the information on 

 pathological states which may be gained from thyroid studies, the need for 

 information on which to base therapeutic I^^^ treatment for malignancies, 

 and the possibility of using P^^ uptake as a reliable measure of basal 

 metabolic rate. The general procedure is to administer I^^> and some 

 time later to make an in vivo measurement over the thyroid, usually at a 



