GENERAL PROCEDURES FOR RADIOASSAY 195 



stainless-steel tube was held rigidly in a lead shield and mounted flush 

 against the crystal of a scintillation counter. A volume of 0.6 ml was 

 thereby exposed to the counter at any one time. A concentration of 

 0.01 fjLc/m\ of I'^' in the circulating blood gave a reading of 1000 to 1500 

 counts/min. The injection was made on the venous side of the external 

 loop, so that the first concentration recorded was diluted by cardiac 

 mixing. For work with beta emitters, a thin-mica-window tube was sub- 

 stituted for the scintillation counter, and a thin rubber tubing was used in 

 place of the stainless steel. This method eliminated errors due to timing 

 of samples, withdrawal of samples, or geometric variations. 



There are studies that require needle-type detection devices so that 

 they may be inserted into a tissue or vein. Such Geiger counters have 

 been made but usually suffer from poor operating characteristics. Kohl 

 (69) has compared a Geiger probe with a scintillation probe for localiza- 

 tion of brain tumors. The Geiger needle was 2 mm in diameter and 10 cm 

 long, whereas the scintillation needle was 3 mm in diameter and 8 cm long. 

 The scintillation device was judged the most suitable for the purpose on 

 the basis of stability when subjected to physical shock or change of tem- 

 perature. Jonas (70) used a scintillation crystal of about 42-mm- detec- 

 tion area to study the pattern of uptake of K"*- from sunflower plants in 

 nutrient solution. 



In general, commercial Geiger or scintillation counters are satisfactory 

 for most in vivo studies. It is pointed out that adequate supports should 

 be available, since otherwise a heavily shielded counter tube is awkward 

 to handle. Special apparatus such as automatic scanning devices or 

 multicounter arrangements still have to be assembled by the investigator. 



EXPRESSION OF DATA 



Data in the literature from radioisotope studies have been reported in 

 various ways. In most cases the reported values do serve the purpose of 

 permitting interpretation. Sometimes, however, the expressed results 

 have not included all the factors reciuired for complete comparisons. In 

 any event, intercomparisons between laboratories often become an exer- 

 cise in the juggling of units. It cannot be hoped that a standard usage 

 will be generally adopted. However, it w411 be most helpful if the results 

 are presented unambiguously with sufficient information to permit com- 

 parisons. A review of conventions in radiotracer studies has been com- 

 piled by Schulman and Falkenheim (71). 



The fundamental property involved is the amount of radioactivity. 

 This can be expressed in various ways, such as millicuries, microcuries, 

 counts per minute, counts per second, or percentage of dose. It is always 

 understood that radioactivity when expressed as counts refers to some 



