223Ra is an alpha emitter with an half life of 11.43 days and a reduced emission of betas and gammas (about 5%) per decay, due to the decay of its daughters nuclides. Its chemical affinity with calcium and the high dose delivered locally make 223Ra dichloride (Xofigo®) a relevant radiopharmaceutical for targeting areas of increased bone turnover (standard injected activity 50 kBq/kg). This work studied acquisition parameters aimed to optimize 223Ra imaging after treatment.
A Siemens e.cam camera was used to acquire spectrum and LSF (Line Spread Function) of a 223Ra source. The photon peaks used for the acquisition were 81 and 84 keV (acquired in the same window at 82 keV), 154 keV and 269 keV with a window width respectively of 10%, 10% and 8%. Images were acquired using Low Energy High Resolution (LEHR), Medium Energy Low Penetration (MELP) and High Energy (HE) collimators, with a pixel matrix of 1024X1024.
LSF was measured by imaging a capillary with a diameter of 1.3 mm, filled with 190 kBq of 223Ra and placed at a distance of 10 cm from the collimator, along the axial axis, for a total time of 120 min. Activity was carefully measured using a Mecmurphil MP-DC activity meter, specifically calibrated for the nuclide of interest.
Images were analyzed selecting in each image a single ROI with dimensions of 210X180 pixel. The profile of the LSF obtained in this way was fitted with a Gaussian function and the fit was used to calculate FWHM and FWTM.
Visual inspection on the three images showed that only MELP collimators produce an acceptable image quality. In fact, as expected, LEHR collimators produce evident artifacts due to the penetration of high energy photons and the contribution of scattered photons while images produced with HE collimators show an improved quality, but are affected by the typical artifacts due to the size of collimator holes. Therefore, a quantitative analysis was conducted on MELP and HE collimator images only.
For MELP and HE collimators, the FWHMs were 12 mm and 13 mm and the FWTMs were 26 mm and 28 mm. Quantitative evaluation of MTF was possible only for MELP (90% and 50% at 0.12 cm-1 and 0.36 cm-1), since in the case of the HE collimator the image was affected by evident artifacts.
The sensitivity measured with the three collimators were of 2.30 cps/kBq, 0.35 cps/kBq and 0.24 cps/kBq with LEHR, MELP and HE collimators respectively.
Image quality obtained with 223Ra is quite poor, due to the low number of photon emitted in each decay (about 2%) and the limited activity that is usually injected for therapy.
Nevertheless, the quality of image than can be obtained is sufficient to acquire images adequate to confirm the biodistribution of the radiopharmaceutical.
The study showed that this image quality can be achieved using a setup with MELP collimators, three acquisition windows centered on 82 keV, 154 keV and 269 keV and a width of 8-10%.