Determination of gamma camera calibration factors for quantitation of therapeutic radioisotopes

Abstract

BackgroundCamera calibration, which translates reconstructed count map into absolute activity map, is a prerequisite procedure for quantitative SPECT imaging. Both planar and tomographic scans using different phantom geometries have been proposed for the determination of the camera calibration factor (CF). However, there is no consensus on which approach is the best. The aim of this study is to evaluate all these calibration methods, compare their performance, and propose a practical and accurate calibration method for SPECT quantitation of therapeutic radioisotopes. Twenty-one phantom experiments (Siemens Symbia SPECT/CT) and 12 Monte Carlo simulations (GATE v6.1) using three therapy isotopes (131I, 177Lu, and 188Re) have been performed. The following phantom geometries were used: (1) planar scans of point source in air (PS), (2) tomographic scans of insert(s) filled with activity placed in non-radioactive water (HS + CB), (3) tomographic scans of hot insert(s) in radioactive water (HS + WB), and (4) tomographic scans of cylinders uniformly filled with activity (HC). Tomographic data were reconstructed using OSEM with CT-based attenuation correction and triple energy window (TEW) scatter correction, and CF was determined using total counts in the reconstructed image, while for planar scans, the photopeak counts, corrected for scatter and background with TEW, were used. Additionally, for simulated data, CF obtained from primary photons only was analyzed.ResultsFor phantom experiments, CF obtained from PS and HS + WB agreed to within 6% (below 3% if experiments performed on the same day are considered). However, CF from HS + CB exceeded those from PS by 4–12%. Similar trend was found in simulation studies. Analysis of CFs from primary photons helped us to understand this discrepancy. It was due to underestimation of scatter by the TEW method, further enhanced by attenuation correction. This effect becomes less important when the source is distributed over the entire phantom volume (HS + WB and HC).ConclusionsCamera CF could be determined using planar scans of a point source, provided that the scatter and background contributions are removed, for example using the clinically available TEW method. This approach is simple and yet provides CF with sufficient accuracy (~ 5%) to be used in clinics for radiotracer quantification.

Comprehensive SPECT/CT system characterization and calibration for 177Lu quantitative SPECT (QSPECT) with dead-time correction

“…Part of this activity (18,535 MBq) was diluted in 6.1 L of water and filled the main compartment of the phantom, with an excess of a chelating agent (diethylenetriamine pentaacetate, DTPA) to avoid precipitation of activity on the phantom walls [19]. The remaining 574 MBq were diluted in 31.8 mL of water and used to fill the spheres, resulting in a concentration ratio of 6:1 between the spheres and the cylinder (18 and 3 MBq/mL, respectively) [20].…”

“…Initially, R Po of the reconstructed SPECT images was computed using the entire volume of the phantom. However, particularly in the presence of a large volume of nonradioactive attenuating material, such as when using the CTDI phantoms, excess scattered counts may be ineffectively eliminated with DEW or TEW scatter correction techniques [20,26]. In an attempt to compensate for this, the following segmentation techniques were applied to compute R Po :…”

“…Several methods have been proposed to evaluate these parameters, some more demanding than others in terms of acquisition time, decay period, and image processing [8,9,[16][17][18][19][20][21]. Planar imaging-based calibration is faster and more convenient to execute and is expected to yield accurate CF [9,20] relative to fully tomographic calibration as the reference method [9,21,22]. The objectives of this study were to perform a comprehensive characterization of our SPECT/CT system's response (combined effects of CF and τ) over its full range of quantifiable 177 Lu activities and to compare various acquisition and analysis methods for camera calibration for QSPECT imaging, with the aim to simplify this process.…”

Comparison of commercial dosimetric software platforms in patients treated with 177Lu‐DOTATATE for peptide receptor radionuclide therapy

“…Our sensitivity results, despite some differences in experimental settings, are of the same order of magnitude as those presented by other authors. 61,62 Yet, as the dosimetric studies presented here are based on SPECT/CT images, we decided (contrary to GE DTK recommendations) to use a SPECTbased calibration factor. The comparison of planar vs SPECT-based calibration factors is not strikingly different, and our choice seems more consistent with a global (calibration + analysis) dosimetric workflow.…”

Clinical Implementation of PLANET®Dose for Dosimetric Assessment after 177Lu-DOTATATE: Comparison with Dosimetry Toolkit® and OLINDA/EXM® V1.0

“…In a clinical dosimetry study, the preliminary step of calibration is crucial to obtain accurate activity quanti cation (40,41). According to the manufacturer's recommendations, a 2D CF is preconized for DTK, whereas the calibration procedure is left to the physicist's discretion in PLANET®Dose.…”