Fifty Shades of Scandium: Comparative Study of PET Capabilities Using Sc-43 and Sc-44 with Respect to Conventional Clinical Radionuclides

Abstract

Scandium-44 has been proposed as a valuable radionuclide for Positron Emission Tomography (PET). Recently, scandium-43 was introduced as a more favorable option, as it does not emit high-energy γ-radiation; however, its currently employed production method results in a mixture of scandium-43 and scandium-44. The interest in new radionuclides for diagnostic nuclear medicine critically depends on the option for image-based quantification. We aimed to evaluate and compare the quantitative capabilities of scandium-43/scandium-44 in a commercial PET/CT device with respect to more conventional clinical radionuclides (fluorine-18 and gallium-68). With this purpose, we characterized and compared quantitative PET data from a mixture of scandium-43/scandium-44 (~68% scandium-43), scandium-44, fluorine-18 and gallium-68, respectively. A NEMA image-quality phantom was filled with the different radionuclides using clinical-relevant lesion-to-background activity concentration ratios; images were acquired in a Siemens Biograph Vision PET/CT. Quantitative accuracy with scandium-43/scandium-44 in the phantom’s background was within 9%, which is in agreement with fluorine-18-based PET standards. Coefficient of variance (COV) was 6.32% and signal recovery in the lesions provided RCmax (recovery coefficient) values of 0.66, 0.90, 1.03, 1.04, 1.12 and 1.11 for lesions of 10-, 13-, 17-, 22-, 28- and 37-mm diameter, respectively. These results are in agreement with EARL reference values for fluorine-18 PET. The results in this work showed that accurate quantitative scandium-43/44 PET/CT is achievable in commercial devices. This may promote the future introduction of scandium-43/44-labelled radiopharmaceuticals into clinical use.

New Radionuclides and Technological Advances in SPECT and PET Scanners

“…These, and similar devices, have also enabled imaging using high magnetic fields in PET/MRI, which is not possible with standard photomultipliers [ 4 ]. These improvements in both detector sensitivity plus time-of-flight and reduced energy windowing have had a high impact on the quantification of non-pure positron emitters [ 24 ]. Figure 6 shows the improvement in quantitative accuracy between two generations of PET/CT devices for Sc-44, which has a high energy prompt gamma that influences the scatter correction of the system.…”

“…Scandium has attracted the attention of researchers and nuclear physicians alike, due to the existence of matched radioisotopes which could have a possible theragnostic application [ 44 , 46 , 47 , 48 , 49 , 50 ]. Sc-43 and Sc-44 are promising for PET imaging, with image quality comparable to the more common clinically used radionuclides, such as F-18 and Ga-68 [ 24 , 51 ]. Sc-47 is a β – emitter suitable for therapeutic purposes and also produces γ-ray emissions that are useful for SPECT imaging.…”

“…Figure 6 shows the improvement in quantitative accuracy between two generations of PET/CT devices for Sc-44, which has a high energy prompt gamma that influences the scatter correction of the system. [24], for two generations of devices. The main difference is the improvement in the quantification observed between devices due to higher sensitivity and reduced energy windowing, as well as the superior time-of-flight on the newer-generation device.…”

“…This was more evident in SPECT, with the introduction of specific clinical designs targeting dedicated clinical questions, such as the development of cardiac-specific devices as described before; however, the general-purpose device continues to have the wellknown design with a gantry and multiple detector heads. [24], for two generations of devices. The main difference is the improvement in the quantification observed between devices due to higher sensitivity and reduced energy windowing, as well as the superior time-of-flight on the newer-generation device.…”

“…Figure 6. Comparison of the impact of different scatter corrections on background calibration, as presented by Lima et al[24], for two generations of devices. The main difference is the improvement in the quantification observed between devices due to higher sensitivity and reduced energy windowing, as well as the superior time-of-flight on the newer-generation device.…”