Design and Evaluation of a Focused Gamma Probe with Custom Scintillation Detector

Introduction: Precise intraoperative localization in cancer surgery is crucial for complete excision of occult lesions and identification of sentinel lymph nodes. Radioguided surgery (RGS) is a common surgical guidance technique in oncology that uses a hand-held gamma probe to localize tissue that is preoperatively labeled with a radiotracer. With advancements in radiopharmaceuticals, especially receptor targeted radiolabels, RGS is employed in a growing number of applications. As such, there is a need for gamma probes that offer real-time, high-resolution detection to facilitate tumour margin assessment and resolve radiolabeled tissues in close proximity. The objective of this work is to design and evaluate a new type of focused gamma probe that offers real-time spot detection of radioactive sources in a remote focal zone.

Methods: The proposed focused gamma probe is a hand-held device that comprises a highly convergent tungsten collimator and custom scintillation detector. The 3D-printed tungsten collimator features tapered, close-packed, hexagonal holes that are focused to a point 35 mm below the tip of the collimator. The scintillation detector consists of a thallium-doped cesium iodide (CsI(Tl)) crystal (20x20x20 mm) coupled to a 12x12 mm silicon photomultiplier (SiPM) with a built-in bias generator and preamplifier, encased in a 25x25x85 mm aluminum housing. The collimator, detector, and a 2 mm thick lead side/back shield are assembled within a lightweight 3D-printed plastic housing. The signal from the gamma probe is read-out by a miniature multichannel analyzer (MCA), the Topaz-SiPM, which is powered and controlled by a Raspberry Pi Zero W2 microprocessor (Figure 1).

The performance of the focused gamma probe system was evaluated using 37 kBq sealed radioactive sources. The sensitivity and spatial resolution were assessed by moving the detector laterally across a ⁵⁷Co (122 keV) source, located 35 mm below the gamma probe in the nominal focal zone, and recording photon counts for 100 seconds at each position. The energy resolution was measured by acquiring an energy spectrum from each of a ⁵⁷Co and ¹³⁷Cs sealed source.

Results: In its current configuration, the focused gamma probe easily fits in the palm of the hand and only weighs 220 grams. The focused gamma probe also demonstrated strong performance, with a FWHM spatial resolution of 3.6 mm and a sensitivity of 4.6 cps/kBq in open-air with a 122 keV source located 35 mm below the tip of the probe. The energy resolution was found to be 8%, 15%, and 31% at 662 keV, 122 keV, and 35 keV, respectively. Additionally, it was found during testing that this system could acquire 2048-channel energy spectra at a rate of 25 Hz, which is sufficient for real-time monitoring.

Conclusions: The focused gamma probe demonstrated strong spatial resolution, while simultaneously providing a sensitivity that is optimal for real-time gamma detection. The spatial resolution of the focused gamma probe is up to 10 times better than existing probes when a radioactive source is located a few centimeters below the tip of the probe. Further, the use of compact, state-of-the-art electronics ensures that the focused gamma probe is suitable for hand-held use without compromising on performance. The reported energy resolution is comparable to existing systems, thereby allowing energy discrimination and detection of clinically relevant radionuclides, such as ¹²⁵I and ^99mTc. Overall, with a spatial resolution of only 3.6 mm, the focused gamma probe could enable in vivo tumour margin assessment and localization of close radiolabeled structures, such as those in complex anatomical areas.

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