136 M. BRUCER absorption andscatter is to look at the shape of the spectrum. Figure 6 shows the pulse-height spectrum from an iodine 131 point source. Superimposed over this is the pulse-height spectrum from a patient’s neck in which a thyroid gland was present. These two spectra are ar- Acta Radiolégica Interamericana taining 100 ml of iodine 131 solution The size and shape of these nine standards clo- sely simulate most of the standards that are In current use. The effect of the size and shape of these standards is shown in (Fig 8). When a 5 cc test-tube sample is used as the base line 7 SCALE N@tiTl] CIFFERENTIAL PULSE MEIGHT SPECTAUM tant WE FULL ru + ARTE ETHC SCALE © COUNTE ere 153 m WORMALIZED #O1NT ts ETM ot8H 10 wt Fuh THE DIFFERENCE SPECTRUM HIRE? OF OER! rn . ~ ee ARY GONE SPECTR, 4 MINUS IODINE POINT SOURCE Fes dd. HEIGHT x . . “ . ee - - . p 7 ‘ oot oo - TEST Tenet Smt Using SAMPLE FUEL * ; go Fic. 6. Pulse-height speetrum of a patient's neck super- Fic. 7. The 100 per cent total dose standards used in the survey manikin kit. spectrum. for comparison, there is very little change in major peaks. When the point-source spec- trum is subtracted from the extended source Spectrum, it is possible to illustrate the differences that have cccurred by drawing out what can be called a first-order difference spectrum In (Fig 6) this differente spectrum shows that there is a great excess of low-energy radiation produced in the patient's neck. There is some absorption of the 640 kev peak but, it should be remembered, the differences that might occur in the 640 kev peak disaspear on the graph because the two spectra have been normalized at this point. With the shape of this difference spectrum kept in mind, we can look at what happens when the size and shape of the 100 per cent total dose standards that are commonly used is varied. Figure 7 shows the selection of 100 per cent total dose standards uscd in the survey manikin kit. These range from a very small capsule containing 0.4 ml of iodine 131 sclution, to a paper cup con- ' ; i iI <j PAPER CUR 100 mi ve FULL ood bitrarily normalized at the 640 peak, and the spectrum is smoothed to show only the | . _ atm imposed over the pulse-height spectrum from an iodine 131 point source to give a first-order difference | BEAKER | (OO m4 Stfash o1a@ (3) BEAKER —iqpiwe 13) POINt $ouRcE | PATENT NECK WiTK TRYROS eto eae _ aly its difference spectrum and that of the large and small beakers and the paper cup. However, when the test-tube sample is compa- red with a commonly used gelatin capsule (in two different kinds of phantoms) it is seen that there is a tremendous change in the difference spectra. It is interesting that none of these spectra even closely resemble the spectrum from a patient’s neck. Therefore, when one is using any of these standards in air, or even many of the very carefully constructed neck-shaped phantoms, one is not comparing the same things, and the [00 per cent dose standard is not truly a standard. During the course of our investigation of these spectra, we had almost reached the conclusion that a phantom for the patient's neck could not be designed. Recently, however, a method has been devised that makes it appear that such a phantom can be designed but that the specifications will have to be exceedingly strict. A number of these phantoms have been made for clinical trial by the Thyrcid Uptake Calibration Committee and they now being sold commercially. ;