Crystal Characteristics

Two basically different types of intraoperative probe are currently available: one based on a scintillation crystal and the other based on a semiconductor crystal. A scintillation detector consists of a scintillation crystal, a light guide, a photo-multiplier tube, and electronics. Thallium-activated sodium iodide [NaI(Tl)] is the most widely used scintillation crystal in gamma cameras and is also used in this type of probe, although mostly cesium iodide (Csl) is used in probes. Semiconductor probes are more common. This type of detector consists of a semiconductor crystal, a preamplifier, and electronics. A cadmium-telluride (CdTe, or sometimes zinc-doped: CdZnTe) crystal is most commonly used in semiconductor probes.

Intrinsic detector efficiency (the ratio between the number of gamma photons entering the probe and the number detected) depends on crystal material, on the dimensions (area and thickness) of the crystal, and on the gamma energy. For CdTe, the intrinsic efficiency decreases with increasing gamma energy and will be 20-30% lower for 99mTc than for 57Co when a 2-mm-thick crystal is used.

Energy resolution is the ability to discriminate between gamma photons with different energies. This depends not only on the crystal that is used in the probe but also on the thickness of the crystal. Semiconductor detectors generally have a better energy resolution than scintillation detectors. An overview of several available probes based on both types of crystals is given in Table 1. One should be aware of the fact that new devices enter the market rapidly and that characteristics may change.

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