RADIOIODINE UPTAKE MEASUREMENT Vol. VII, N° 3-4 and was subtracted from both the measure- ment over the patient and the measuremnt over the standard. . The A formula: _ Py — RB, TU = —RB, In this formula an “A-filter”, usually a thin piece of lead, was interposed between the patient’s neck and the detector but was placed very close to the detector. The B formula: TU = —— 139 method is so similar to the 0 formula method. it was dropped from any further consideration. In many laboratories, very complex formulas interposed factors (or even fudge fac- tors) that sometimes resulted in very accurate thyroid uptake measurements. In most instances these fudge factors were valid only for certain kinds of patients, usually the very high thyroid-uptake, very low bodybackground patients. In a number of instances very complex formulas were intimately associated with very complex and unusual designs of instruments. Since these instruments In this formula a thyroid eclipsing shield was used. These may have been of manydifferent sizes and shapes but were always pla- were available to very few laboratories (and most persons doing thyroid uptake measure- ly at a distance from the detector. Here a ped from further consideration. With the four type-formulas applied to almost every kind of instrumentation under almost every kind of thyroid uptake circumstance and in many different kinds of patients, it was found that a generalized statement of error could be made. Among those persons who used instruments in which there was a high degree of control of the spectrum (and this includes ced very close to the patient’s neck and usual- measurement was taken over the patient with- out a filter and another measurement was taken with the filter in place. The measurement with thefilter was substracted from the measurement without the filter. This results in a kind of body-background correction. The AB formula: Py — Pas TU ==" Ss — Sap This is the formula that was devised after extensive study. It is a method that has been used in very few laboratories but is the formula that showed the best answers underall Another very common formula was used to measure thyroid uptake. It was a formula that involved the subtraction of a thig measurement or of some other kind of measurement to correct for body background. The results from laboratories using the thigh correction and similar results with this kind of a body-backgroundcorrection in the ORINS laboratory were so exceedingly poor that this method has been dropped from all further consideration. Another method that was very commonly used was a very simple one in which no correction was made for body background or for room background. Since this ment could not even dream of obtaining them), these complex formulas were drop- all the spectrometers and also those instruments that had good discriminator control) the degree fo control over the spectrum accounted for the lion’s share of the variation in thyroid uptake measurements under any one circumstance. Where the spectrometer control excluded all scattered radiation, the results of the measurement were usually very good. Where the spectrometer control included scattered radiation, the results of the measurement were invariably very poor. About three quarters of the error could be accounted for on the basis of the control of the spectrum seen by the detector. Even with very good spectrometer control it is possible to make significant errors in the thyroid uptake. The most significant factor is the adoption of an improper phantom. One of the primary things that a phantom