Node Localization Spatial Resolution Sensitivity At A Distance

Spatial resolution describes the ability of the probe to distinguish between two sources that are close together, or to locate the precise position of a source from a distance. The latter is important if we want to locate a sentinel node through the intact skin before the incision is made. Figure 2 shows the relative spatial resolution scanning in a straight line over a source with a closest distance of 26 mm with respect to the front of the probe or collimator for different probe configurations. If a probe is scanned at a fixed distance around the source (and

Figure 2 Relative spatial resolution for different probes and collimators at a distance of 26 mm in air. A57Co source was used. For each configuration, data are normalized to the value for a lateral distance of 0 cm. •: Neoprobe, no collimator;^: Neoprobe, cone collimator;!: Neoprobe shield;*: C-Trak probe, collimator;X: C-Trak probe, no collimator.

Figure 2 Relative spatial resolution for different probes and collimators at a distance of 26 mm in air. A57Co source was used. For each configuration, data are normalized to the value for a lateral distance of 0 cm. •: Neoprobe, no collimator;^: Neoprobe, cone collimator;!: Neoprobe shield;*: C-Trak probe, collimator;X: C-Trak probe, no collimator.

pointing at the source) the term ''angular resolution'' would be more appropriate. The spatial resolution of a probe at this distance can be improved considerably by the use of additional collimation. The C-Trak probe has a strong built-in colli-mation: Without additional collimation, the spatial resolution is comparable to that of Neoprobe with the add-on collimator. Scanning at larger distances reduces the spatial resolution, as is illustrated in Fig. 3 for collimated probes. In Fig. 4, it is shown that the collimator no longer improves spatial resolution at a depth of 5 cm in water: The spatial resolution with and without the collimator is nearly identical.

Adding a collimator to a probe also has disadvantages. As has been mentioned earlier, the effective area of the detector becomes smaller and the minimal distance to the detector is increased so that the maximum sensitivity decreases. Moreover, because the field of view is narrowed, searching for a node the location of which is not precisely known becomes more time-consuming. In general, it is important to use as little collimation as possible, unless it is certain that the

Ganglions Lymphatiques Localisation
Figure 3 Relative spatial resolution at different distances for collimated probes. A 57Co source was used. Values are normalized to the value for air at a distance of 1 cm. ♦: 1 cm air;B: 1 cm water;A: 2 cm water;®: 4 cm water.
Figure 4 Relative spatial resolution at a depth of 5 cm in water for a probe with a shield (no collimation) and a collimator. A57Co source was used. For each configuration, data are normalized to the value for a lateral distance of 0 cm. collimator; ■: shield.

activity in the target is sufficient (e.g., in the case of a 1-day sentinel protocol). Probes that are supplied with different collimators enable the user to adapt the instrument to their own specific requirements and to the needs of the situation in individual patients.

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