title: 131I activity quantification of gamma camera planar images.
abstract: A procedure to estimate the activity in target tissues in patients during the therapeutic administration of131I radiopharmaceutical treatment for thyroid conditions (hyperthyroidism and differentiated thyroid cancer) using a gamma camera (GC) with a high energy (HE) collimator, is proposed. Planar images are acquired for lesions of different sizes r, and at different distances d, in two HE GC systems. Defining a region of interest (ROI) on the image of size r, total counts ngare measured. Sensitivity S (cps MBq-1) in each acquisition is estimated as the product of the geometric G and the intrinsic efficiency η0. The mean fluence of 364 keV photons arriving at the ROI per disintegration G, is calculated with the MCNPX code, simulating the entire GC and the HE collimator. Intrinsic efficiency η0is estimated from a calibration measurement of a plane reference source of131I in air. Values of G and S for two GC systems-Philips Skylight and Siemens e-cam-are calculated. The total range of possible sensitivity values in thyroidal imaging in the e-cam and skylight GC measure from 7 cps MBq-1to 35 cps MBq-1, and from 6 cps MBq-1to 29 cps MBq-1, respectively. These sensitivity values have been verified with the SIMIND code, with good agreement between them. The results have been validated with experimental measurements in air, and in a medium with scatter and attenuation. The counts in the ROI can be produced by direct, scatter and penetration photons. The fluence value for direct photons is constant for any r and d values, but scatter and penetration photons show different values related to specific r and d values, resulting in the large sensitivity differences found. The sensitivity in thyroidal GC planar imaging is strongly dependent on uptake size, and distance from the GC. An individual value for the acquisition sensitivity of each lesion can significantly alleviate the level of uncertainty in the measurement of thyroid uptake activity for each patient.
authors.affiliation: Medical Physics, Hospital Clínico Universitario de Valladolid, Avda. Ramón y Cajal 3, 47005 Valladolid, Spain.
authors.forename: Raquel
authors.initials: R
authors.lastname: Barquero
authors.forename: Hugo P
authors.initials: HP
authors.lastname: Garcia
authors.forename: Monica G
authors.initials: MG
authors.lastname: Incio
authors.forename: Pablo
authors.initials: P
authors.lastname: Minguez
authors.forename: Alexander
authors.initials: A
authors.lastname: Cardenas
authors.forename: Daniel
authors.initials: D
authors.lastname: Martínez
authors.forename: Michael
authors.initials: M
authors.lastname: Lassmann
mesh_terms: D015902:Gamma Cameras; D006801:Humans; D007457:Iodine Radioisotopes / Q000627:therapeutic use*; D017785:Photons; D049268:Positron-Emission Tomography / Q000295:instrumentation / Q000379:methods*; D011829:Radiation Dosage; D019275:Radiopharmaceuticals / Q000627:therapeutic use*; D061089:Radiotherapy, Image-Guided / Q000295:instrumentation / Q000379:methods*; D013964:Thyroid Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy
chemical_list: D007457:Iodine Radioisotopes; D019275:Radiopharmaceuticals