130 Acta Radioldgica Interamericana M. BRUCER By 1950 the National Bureau of Standards in the United States in cooperation with many other national bureaus had been able to calibrate a sample of iodine with a high degree of precision. This intercalibration was on the basis of purely physical techniques and was a laboratory procedure. The com- are arbitrarily defined as anything under 100 kev; the low energies are defined as those up to 250 kev; the medium energies are those up to 500 kev; and the high energies are the rest of the spectrum. The high-energy peak shown in the spectrum consists of two gamma photons that are not very important BABBITT METAL SHIELDS DIFFERENCE BETWEEN “TOO THIN" AND "TOO _—— . ; . No F (TH) ENERGY SPECTRA of TODINE 131 “. on reain in glam AND 10.541 ta.) sire Na HT) ENERGY SPECTRA ". FQOINE 131 ae: — MOCK. IODINE on resin in bobbirt #* e = > “ONERGIER : . ‘HIGH - ENERGIES LecceeeROD kee. BS a ker,8thChore Fig. 1. Comparison of the sodium ivdide speetra of iodine 131 and mock-iodine. mittee decided in 1954 that one of the reasons for the wide variation in results was the lack of a similar intercalibration stan- dard that met clinical requirements. It was felt that a clinical intercalibration procedure, which would be adaptable to all the various techniques in use, was necessary. Such a cli- of Totat Area ~ . o Ole . 0,3 xeRay LOW ENERGIES | ENERGIES 100 kaw MEGIUM ENERGIES 250 kev ‘hy 500 kev sce ENERGIES $5th Channel Fig. 2. The results of interposing succesive layers of babbit metal between raw mock-iodine and the detecting crystal. in thyroid uptake measurements because they contribute so little to the total number of counts recorded by most instruments. The medium-energy peaks are the most promi- to simulate the gamma-ray spectrum from nent in the iodine spectrum. The low energy peak consists mostly of Compton scattered radiation. The peaks in the range arbitrarily called X-rays energies consist both of photons emitted by the iodine and scatter from interactions in both the instruments and the mock-iodine are compared. In this figure it is seen that when radioactive iodine is “view- trum matches this iodine spectrum remarkably well. The high-energy peak is slightly higher but this will not be picked up by any instruments except complex spectrometers, which are seldom used for thyroid ra- nical intercalibration demanded the use of a long-lived isotope. The short half life of radioactive todine was one of the main deterrents to clinical intercalibration. A jong-lived gamma emitter can be made iodine 131. This material is now called mockiodine and consists of the proper mixture of barium 133 and cesium 137. In Fig. 1 the sodium iodide spectra of iodine 131 and ed” by a sodium iodide crystal, it emits a number of gamma photonsof different energies. An arbitrary division has been made on the spectrum to divide the energies emitted into four classifications, The X-ray energies surroounding media. The mock-iodine spec- dioiodine uptake measurements. The medium- energy peaks show the major difference between 10dine and mock-iodine. There is too much of the 280 kev radiation and too little of the 360 kev radiation in mock-iodine.