138 M. Brucer Well over 95 per cent of patients presenting themselves to a physician for a thyroid uptake measurement will meet this requirement. In those patients where the thyroid tissue is spread over an area greater than 4” by 4”, the standard intercalibration technique will not be accurate. Almost all the methods of doing thyroid uptake measurements use some kind offiltration. Two very different kinds have been used. These have been separated as follows: The term “‘A-filter” is used when a piece of material is purposely interposed between the patient and the detector but is kept in a position very close to the detector. The term “B-filter” (or thyroid eclipsing shield, or a filter in the B position) is used when a piece of material is interposed between the patient and the detector but is put very close to the patient’s neck. Every method of doing thyroid uptake uses a detector, and this detector must be consi- dered in two parts. First of all is the detector itself, and this can be any kind of GeigerMuller tube, or inonization chamber, or electroscope, of gamma-tay-sensitive crystal. The detector is usually placed within some sort of shield and collimator arrangement. Some kind of counting signal is always attached to the detector. In some laboratories this may be a very complex spectrometer or it may be a simple scaling device. In some laboratories very simple rate meters are in use and are giving very acurate results. In some laborato- fies very complex electronic systems have been devised and are giving very inaccurate results. Every method of doing uptake includes some kind of measurementof a standard. In a few laboratories this standard is measured once for a particular instrument and the instrumentis said to be calibrated for measuring thyroid uptake directly, Usually these methods have ben highly inaccurate, but this is because the instruments themselvesare in- accurate and not because the methodis necessarily a poor one. Acta Radiolégica Interamericana All the methods interpose a distance be- tween the neck of the patient and the detector is placed on the surface of the neck it may be called zero distance, but there is still space between the tube and the source of activity. Measurement of distance is a most important part of the thyroid uptake technique. All the persons doing thyroid uptake measurements apply somesort of formula for calculating the final result. In order to simplify the survey and for teaching thyroid uptake techniques, we have adopted a standard formulary to describe thyroid measurements. The following system of symbols is used: P =the measurement taken over the patient’s neck with no interposed filters, Pa =the measurement over the patient's neck with an “A-filter’ placed very close to the detector. Pr=the measurement taken over the pa- tient’s neck with a thyroid eclipsing shield placed very close to the patient’s neck. S =the measurementtaken over the standard in or out of the phantom. Such items as the distance, the type of scaler, the type of detector, are listed as footnotes to the formula. Many different kinds of formulas were found in use during the thyroid uptake calibration survey. In fact, no laboratory was found that was using exactly the same method as any other laboratory. A simplified generalization, however could be made for four specific kinds of formula. These are as follows: The O formula: P— RB TU = SRB In this “no-filter” formula the thyroid upta- ke was measured without any filtration, but a room background measurement was made