Vol. VII, No 3-4 RADIOIODINE UPTAKE MEASUREMENT To illustrate in summary form the effects of many of these variables in thyroid uptake measurement Fig. 10 shows the combination of all the factors. One kind of instrument was used, the Mediac, to measure two different manikins. One was a high-thyroid, low bodybackground manikin; and the other was a low-thyroid, high body-background manikin. In the measurement of thyroid uptake the four standard formulas were used; the O for- mula (without any filtration), the A formula (with an “A” filter only), the B formula (with a thyroid eclipsing shield only), and the AB formula. Three different kinds of phantoms were used. One was the moot shallow possible kind of phantom; and one, the standard phantom, was a phantom in which the spectrum of emission from the source matched the spectrum of emission from the small errorr. It was found that the actual amount of error was highly related to the thyroid uptake, and therefore the error is given in percentage points of uptake rather than in per cent error. This 3 percentage points of accuracy, however, is possible only TUC-RANK ORDER IMPORTANCE A B WITH SPECTROMETER CONTROL WITHOUT SPECTROMETER CONTROL The spectrum The phantom centimeters from the surface of the patient’s neck to the surface of the detector). It can be seen from this figure that when a very shallow phantom is used, there is a tendency for a very wide range of answers. When the too-deep phantom is used in the hig-thyroid type of patient, all the answers are far too high. When a standard phantom is used, 80 % 20 & The formula The phantom The formula 10 % The standard 50 % 25 Ye The standard 20 % The filter area* (for absolute values only} 2% The collimation® The distance® * Te reasonahie 2 % 1% The filter thickness? 1 % pMCLPonsicow | Goan” PRA POM patient’s neck. The measurements were made at five different distances (from 20 to 60 14} errs. PHAN POM PHANTOM, |g aa 20 30, 400 $00 Cem | 2c 20 3 4G AD GOce Hi-thyraid, lo-baadybackground patient AND FORMULA HAVE BEEN STANDARDIZED DISTANCE, PHANTOM, STANDARD, FILTERS except where there is no filtration, usually a reasonably good answeris given. The results in the low-thyroid, high bodybackground patient illustrate the value of the thyroid eclipsing shield or “B-filter’”. Even where the phantom is improperly designed, if a thyroid eclipsing shield is used, the result is within reason. An important question is, how accurate can a thyroid uptake measurement be under ideal conditions? In running through a very large series of measurements on many different instruments and many different manikins, it was found that with exceptionally stable equipment it is possible to measure the thyroid uptake within about 3 percentages point of accuracy. If the thyroid uptake is close to 90 per cent, then this is a very eo 30 «4630 oem 20 30 0 80 Sem 80 SOS ES Ee Lo-Thyroid, Hi-Bodybackgronnd patient Fic. 10. Summary of the effects of many variables in thyroid uptake measurement. under ideal conditions of artifical manikins and easily controlled geometry. When: pa- tients are used, this accuracy cannot be achieved. It would appear however, that with any instruments and with any kind of pa- tient it is possible to achieve well within 10 percentage points of accuracy and probably, if care is taken, 5 percentage points of ac-