Dual-isotope SPECT using simultaneous acquisition of Tc-99m and I-123 radioisotopes: A double-injection technique for peri-ictal functional neuroimaging

The acquisition of multiple radiotracer studies at different time points du ring a neurological event permits the study of different functional activat ion states in humans. Peri-ictal SPECT is a promising technique for localiz ing the epileptogenic zone and would be enhanced by the ability to acquire sequentially coregistered ictal and postictal SPECT images of a single seiz ure, This study was designed to develop and validate an accurate method for the simultaneous acquisition of (TC)-T-99m and I-123 SPECT images of the b rain. Methods: A multicompartment, transaxial Hoffman brain-slice phantom w as filled with Tc-99m, I-123 Or a 3:1 mixture of the two isotopes. Planar a nd SPECT images were acquired by a dual-head gamma camera system equipped w ith parallel and fanbeam collimators, respectively. Thirty-two energy windo ws (2 keV width) were acquired over the energy range 120-184 keV. From the planar data, the signal-to-noise characteristics and crosstalk were measure d for each energy window and used to devise an energy window acquisition st rategy that was then applied to the SPECT data. Three summed energy windows were created: a primary Tc-99m image (130-146 keV), a primary I-123 image (152-168 keV) and a secondary Tc-99m crosstalk image (134-140 keV), A fract ion (0.041) of the Tc-99m crosstalk image was subtracted from the I-123 ima ge. No crosstalk correction was performed on the primary Tc-99m image. Resu lts: (a) Planar images: results showed 1.3% crosstalk in the I-123 image co mpared with 19.7% for a 10% asymmetric energy window alone. I-123 crosstalk into the Tc-99m window was 2.79% and was relatively constant with changes in the location of the Tc-99m energy window. (b) Tomographic images: result s showed 1.51% Tc-99m crosstalk in the I-123 image compared with 12.44% for the uncorrected image and 3.70% I-123 crosstalk in the Tc-99m image. Concl usion: An effective technique for the simultaneous acquisition of Tc-99m an d I-123 radiotracer distributions in the brain has been developed and valid ated in a phantom model and should have clinical application in peri-ictal functional activation studies of the brain.