An Investigation into Some of the Factors Influencing Scan Quality*
Authors: Scott B. W. 
Published: March 2008

Although scanning is performed for purposes other than the detection of cold tumours, it was felt that this was a logical basis for a method of investigating and comparing the image‐producing properties of various radioisotopes. The method involves the measurement of the contrast for a cold sphere in a tank of radioisotope. Contrast is denned as the percentage change in the counting rate of the tank when the sphere is placed in it, in the field of view of the detector. It depends, to a varying extent, on a number of factors:

The size of the sphere

the depth of the sphere below the surface

the depth of the tank

the spatial resolution of the collimator at the position occupied by the sphere

the collimator‐tank distance

the amount of scattered radiation included in the counting rate

the energy of the primary gamma rays.

The contrast for a cold sphere in the surface of a tank, at the required distance from a particular collimator, is measured directly, but its value at other depths is most easily and accurately found by an indirect method. The variation of contrast with depth is due to the variation with depth of the response of the sphere of activity displaced by the cold sphere. This response is obtained from the response of a hot sphere in air–which will be the same for all radioisotopes for which septum penetration is negligible–by correcting for attenuation in water. The latter corrections are easily obtained from the response of a plane source containing the radioisotope being studied. The contrast for a sphere at any depth is then obtained by multiplying its value in the surface by the ratio of the response for a hot sphere at the required depth to that in the surface.

This method was used to determine the contrast for a 3 cm. sphere under various conditions using three broad‐focus medium‐energy collimators with essentially the same resolution (1–5 cm.). Radioisotopes emitting primary photons in the range 77 to 364 KeV. were used. It was shown that the contrast for the sphere in the surface was greater for a radioisotope emitting low‐energy photons but for positions deep in the phantom it was greater for one emitting photons of higher energy, while at some intermediate depth it was the same for both. The result is that, in practice, over a wide range of depths there is no significant difference between the contrast for the radioisotopes emitting photons in the energy range studied.

Due to the variations in the spatial resolution with depth of all collimators, contrast is very dependent on collimator‐organ distance. It has been shown in one instance that the change in contrast due to a change of 2 cm. in this distance produces a greater variation in the contrast than would be obtained by substituting 131I for 197Hg. In practice the distance chosen will depend on the thickness of the organ and the depth at which maximum contrast and detail are required. When scanning a large organ for maximum detail, this effect will necessitate frequent adjustment, during scanning, of the height of the detector above the patient.

Since, for geometrical reasons, the spatial resolution varies more widely for a collimator used with a 5 in. detector than for one used with a 3 in. detector, the variation of contrast with collimator‐organ distance varies more widely with the former, especially when the sphere is situated between the collimator and the focus.

In spite of the very considerable contribution of scattered photons to the counting rate when low energies (77 KeV.) are used, the results of contrast measurements, using the methods described, have shown that there should be no significant difference in the quality of scans obtained using radioisotopes emitting gamma rays in the range considered. These conclusions have been confirmed by phantom studies. In cases where the dose to the patient depends primarily on gamma‐ray absorption, emitters of photons with energy as low as 77 KeV. May therefore be used, with reduced dose to the patient, without significantly prejudicing scan quality.