GENERAL PROCEDURES FOR RADIOASSAY 193 



distance of 17 to 20 cm so that the tube will "see" all the gland. A back- 

 ground reading is taken over the thigh which is used to compensate for 

 any contribution to the thyroid measurement which was not due to the 

 gland itself. Comparisons are made with phantoms, most often with 

 I'^' solutions in bottles measured at the same distance. Uncertainties 

 in this method may amount to ±20 per cent (61). Freedberg et al. (61) 

 proposed a method which reduced the error to ±5 per cent and which 

 consisted of using four Geiger tubes arranged horizontally in a circle 

 around the patient's neck. With this arrangement the effects of absorp- 

 tion, scatter, and geometry caused the values to be about 7 per cent high, 

 and this correction factor was applied. The thyroid uptake was calcu- 

 lated by relating the in vivo measurement to the measurement of an ali- 

 quot from the solution given to the patient. Fields and LeRoy (62), 

 using a single Geiger tube, obtained reliable values for the amount of I^^^ 

 in the gland by making measurements at three known distances and 

 eliminating backscatter by a lead absorber of at least 1.5 g/cm^. Calcu- 

 lations were made on the basis of the inverse-square law. 



It should be pointed out that scintillation counters especially, by virtue 

 of their great efficiency, tend to give high results on account of scattered 

 gamma rays of reduced energy. This error can be corrected by one of the 

 following methods: (a) use of a lead absorber in front of the detector, 

 (6) use of a comparison standard that consists of an P''^ dose in a volumet- 

 ric flask submerged in a suitable water phantom, or (c) use of an electronic 

 circuit that will discriminate against the lower-energy gamma rays. 



Cassen et al. (63) and Bauer et al. (64) have described an instrument 

 that produces a visual delineation of the thyroid gland. The device con- 

 sists of a scintillation counter, a visual recorder, and an automatic scan- 

 ner. The pictures obtained are called scintigrams. This automatic 

 method provides valuable information as to the size of the gland and local- 

 ization of pathologic tissue. This type of apparatus has been successfully 

 used to estimate the weight of the human gland (60, 65). Anger (66) has 

 described a scanning device employing 10 scintillation counters by means 

 of which human subjects can be scanned from head to foot in about 45 

 min, and laboratory animals in a shorter time. This procedure, in com- 

 bination with roentgenograms, was most helpful in the location of thyroid 

 metastases. 



There has been considerable interest in the determination of P^' uptake 

 in the thyroid of livestock because of the importance of thyroid function 

 in animal growth and production. The procedures that are suitable for 

 human subjects are not practical for domestic animals, which cannot 

 cooperate. It is desirable to use only the restraint to which the animals 

 are accustomed and to avoid exciting the animal. It is impractical 

 to make measurements at a fixed distance from the gland because of 



