Imaging and Sensing of pH and Chemical State With Cascade Photons of ¹¹¹In Using Ring-Type Compton Camera

Molecular imaging techniques, such as positron emission tomography (PET) and single-photon emission computed tomography (SPECT), provide the activity distribution of radioisotopes. 111In is a radioisotope commonly used in SPECT, which consecutively emits two cascade gamma rays: 171 keV, followed by 245 keV. The anisotropy of the emission directions of cascade gamma rays can provide more information in addition to activity distribution. It contains information regarding the chemical state of the molecule surrounding the 111In atom. A previous mechanical collimator SPECT study indicated the possibility of imaging and obtaining information on the local chemical state through anisotropy measurements. This study utilized a ring-structured Compton camera for anisotropy measurement of 111In cascade gamma rays with potential use in imaging PET and SPECT tracers. Two point sources with different pH values (1.6 and 13.9) were measured separately and simultaneously. The efficiency of events was in the order of <inline-formula> <tex-math notation="LaTeX">$\mathbf {1}\mathbf {0}^{\mathbf {- 6}}$ </tex-math></inline-formula>, which was two orders higher than the collimated detector system used in the previous study. The spatial resolution on the image at the center of the field of view was obtained as <inline-formula> <tex-math notation="LaTeX">$13.7\mathbf {\pm }0$ </tex-math></inline-formula>.16 mm, which is worse than collimated SPECT in previous studies. In the experiment, the measured anisotropy coefficients (ACs) of pH 1.6 and 13.9 were <inline-formula> <tex-math notation="LaTeX">$- 0.18\mathbf {\pm }0.05$ </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">$- 0.01\mathbf {\pm }0.05$ </tex-math></inline-formula>, respectively, when measured separately. Furthermore, those of pH 1.6 and 13.9 were <inline-formula> <tex-math notation="LaTeX">$- 0.16\mathbf {\pm }0.03$ </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">$0.01\mathbf {\pm }0.04$ </tex-math></inline-formula>, respectively, when measured simultaneously. The anisotropy measurements conducted at source positions derived from reconstructed images exhibited differences of more than two standard errors at different pHs. This proof of concept demonstrates the feasibility of AC measurement and discusses the limitations and the areas for improvement for further work.