The purpose of this chapter is to outline the relevant physics of gamma ray detection probes. More extensive information can be found in other publications [1,2]. A gamma detection device for surgical use consists of two main parts: a handheld sensor which contains the gamma-sensitive crystal with a preamplifier, and a reading unit. Both the sensor itself and the whole device are often referred to as a ''probe.'' In this chapter, however, we use the word ''probe'' to indicate the sensor only. Because the radionuclide technetium-99m (99mTc) is almost exclusively used as a label for tracers in lymphatic mapping, discussion of the physical characteristics of the probe concerns the performance with 99mTc unless explicitly stated otherwise. Measurements on probes are often performed using a cobalt-57 (57Co) source. Because the energy of the gamma photons emitted by 57Co is only slightly lower than that of 99mTc (122 versus 140 keV), these measurements are, in general, representative for the performance with 99mTc. However, the transmission through shielding material must be measured using 99mTc because using 57Co the correction factor will strongly depend on the thickness of the shielding.

Table 1 Probes ond Crystols



Detector type




Care-Wise, U.S.A.



Neoprobe Corp, U.S.A.



Radiation Monitoring Devices, U.S.A.


CdTe, scintillation

Eurorad, France

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