2417 - Extravasation Imaging and Quantitative Analysis of Activity Infiltration Using a Portable Handheld Gamma Camera

Purpose/Objective(s): To evaluate the ability of a handheld gamma camera to provide real-time imaging and post-injection quantitative determination of activity uptake in patients receiving Lu-177 based radiopharmaceutical treatments.

Materials/Methods: Using a new handheld gamma camera, RAVIN (M3D Inc., USA), images and gamma energy spectra (in units of count-per-second (CPS)) of a typical Lu-177 radiopharmaceutical source were acquired at a source-to-camera distance (SCD) of 100 cm and used to create a calibration factor, CF(CPS/mCi), for the prominent Lu-177 emission peaks at 113 keV and 208 keV. Additionally, images and spectra were obtained for SCD = 50, 75, 125, and 150 cm, and at depths of 0, 1, 2, 5, and 10 cm within a tissue-equivalent plastic phantom (used to simulate scatter and attenuation from the patient). Finally, gamma spectra and images of uptake and distribution were acquired from a single patient during radiopharmaceutical injection and again approximately 15 minutes post-injection. Using the CF derived for the RAVIN camera and the measured spectra, the Lu-177 activity within the patient was estimated and is referred to here as the patient activity (PA).

Results: The CF (CPS/mCi) for both the 113 keV and 208 keV gamma emission peaks from Lu-117 decreased as a function of the SCD and decreased exponentially as a function of depth within the tissue-equivalent phantom. PA was calculated for any distance or phantom depth using the relation: PA [mCi] = CPS[measured]*CF[calibration]*DF[SCDmeasurement]*AF[measurement], where DF is a distance factor (DF = CPS[SCDcalibration]/CPS[SCDmeasurement]) and AF is the attenuation correction factor (AF = CPS[depth = 0 cm]/CPS[measurement depth]). Distribution of Lu-177 from the injection site, up the arm and through the torso was visualized during and immediately after the injection of the treatment dose. Images acquired approximately 15 minutes after administration showed uptake within the patient abdomen. Using the 208 keV gamma emission data and the measured CF, DF, and AF (measured at an empirically determined 2.5 cm depth) factors for the RAVIN, the imaged activity in the patient was found to agree with the injected activity to within ~2.0% for the first 7 patients studied (11 activity measurements).

Conclusion: The RAVIN camera was able to provide real-time images to verify proper administration of the radiopharmaceutical injection. When properly calibrated and corrected for distance and depth, the camera was also able to estimate the activity of Lu-177 within the patient.