Basic evaluation of a single probe system for lymph node detection with F-18 FDG

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The gamma probe system developed consists of a NaI(T1) detector 9mm in diameter, a data acquisition unit ,central processing unit and data display unit. Two different kinds of tungsten collimators were constructed for Tc-99m: One is a straight bore of 40mm in length, 9mm in diameter and 3.5mm in wall thickness. The distance between the tip of the collimator and the detector is variable from 0 to 20mm. The other is a one with tapered tip, the diameter decreasing from 8mm to 3mm within 5mm distance. The distance between the tip of the collimator and the surface of the detector is variable from 5mm to 20mm. Two different types of tungsten collimator for F-18 were constructed: One is a straight bore type of 9mm in diameter, 33mm in length and 10mm in thickness. The other is one with tapering tip; 3mm in the tip diameter, 9mm in diameter at the 5mm from the tip, 23mm in length and 10mm in thickness. The counting rate performance of the system was studied with Tc-99m and F-18. The response curve of each collimator in water was obtained. Minimum differentiation of two point sources was determined in each collimator. The side penetration of 511keV photon was checked in the detector system for F-18.There was linear relationship between activities and counts up to 35kcps for both nuclides. The sensitivity in water was rapidly decreased with the straight bore collimator: 70% at the depth of 20mm. With the tapering collimator the sensitivity loss in water was gradual: 80% at 40mm. The minimum differentiation capability was 20mm with the tapering collimator and 180mm with straight bore collimator. The side penetration of 511keV photon was less than 10%.These results indicate that this system may be a useful aid for sentinel node detection or lymph node metastasis detection with F-18 FDG.