Influence of spill-over for 99mTc images and the effect of scatter correction for dual-isotope simultaneous acquisition with 99mTc and 18F using small-animal SPECT-PET/CT system

A dual-isotope simultaneous acquisition (DISA) of 99mTc and 18F affects the image quality of 99mTc by crosstalk and spill-over from 18F. We demonstrated the influence of spill-over and crosstalk on image quality and its correction effect for DISA SPECT with 99mTc and 18F. A fillable cylindrical chamber of 30 mm with NEMA-NU4 image quality phantom was filled with 99mTc only or a mixed 99mTc and 18F solution (C100). Two small-region chambers were filled with 99mTc only or a mixed 99mTc and 18F solution made at half the radioactivity concentration of C100 (C50) and non-radioactive water (C0). The 18F/99mTc ratio for DISA was set at approximately 0.4-12. Two types of 99mTc transverse images with and without scatter correction (SC and nonSC) were created. The 99mTc images of single-isotope acquisition (SIA) were created as a reference. The DISA/SIA ratio and contrast of 99mTc were compared between SIA and DISA. Although the DISA/SIA ratios with nonSC of C100, C50 and C0 gradually increased with increasing 18F/99mTc ratio, it was nearly constant by SC. The contrasts of C100 and C50 were similar to a reference value for both nonSC and SC. In conclusion, DISA images showed lower image quality as the 18F/99mTc ratio increased. The image quality in hot-spot regions such as C100 and C50 was improved by SC, whereas cold-spot regions such as C0 could not completely remove the influence of spill-over even with SC.

Evaluation of image quality with four positron emitters and three preclinical PET/CT systems

Background

We investigated the image quality of ¹¹C, ⁶⁸Ga, ¹⁸F and ⁸⁹Zr, which have different positron fractions, physical half-lifes and positron ranges. Three small animal positron emission tomography/computed tomography (PET/CT) systems were used in the evaluation, including the Siemens Inveon, RAYCAN X5 and Molecubes β-cube. The evaluation was performed on a single scanner level using the national electrical manufacturers association (NEMA) image quality phantom and analysis protocol. Acquisitions were performed with the standard NEMA protocol for ¹⁸F and using a radionuclide-specific acquisition time for ¹¹C, ⁶⁸Ga and ⁸⁹Zr. Images were assessed using percent recovery coefficient (%RC), percentage standard deviation (%STD), image uniformity (%SD), spill-over ratio (SOR) and evaluation of image quantification.

Results

⁶⁸Ga had the lowest %RC (< 62%) across all systems. ¹⁸F had the highest maximum %RC (> 85%) and lowest %STD for the 5 mm rod across all systems. For ¹¹C and ⁸⁹Zr, the maximum %RC was close (> 76%) to the %RC with ¹⁸F. A larger SOR were measured in water with ¹¹C and ⁶⁸Ga compared to ¹⁸F on all systems. SOR in air reflected image reconstruction and data correction performance. Large variation in image quantification was observed, with maximal errors of 22.73% (⁸⁹Zr, Inveon), 17.54% (⁸⁹Zr, RAYCAN) and − 14.87% (⁶⁸Ga, Molecubes).

Conclusions

The systems performed most optimal in terms of NEMA image quality parameters when using ¹⁸F, where ¹¹C and ⁸⁹Zr performed slightly worse than ¹⁸F. The performance was least optimal when using ⁶⁸Ga, due to large positron range. The large quantification differences prompt optimization not only by terms of image quality but also quantification. Further investigation should be performed to find an appropriate calibration and harmonization protocol and the evaluation should be conducted on a multi-scanner and multi-center level.

Resolution-recovery-embedded image reconstruction for a high-resolution animal SPECT system

The small-animal High-Resolution SPECT (HiReSPECT) is a dedicated dual-head gamma camera recently designed and developed in our laboratory for imaging of murine models. Each detector is composed of an array of 1.2 × 1.2 mm(2) (pitch) pixelated CsI(Na) crystals. Two position-sensitive photomultiplier tubes (H8500) are coupled to each head's crystal. In this paper, we report on a resolution-recovery-embedded image reconstruction code applicable to the system and present the experimental results achieved using different phantoms and mouse scans. Collimator-detector response functions (CDRFs) were measured via a pixel-driven method using capillary sources at finite distances from the head within the field of view (FOV). CDRFs were then fitted by independent Gaussian functions. Thereafter, linear interpolations were applied to the standard deviation (σ) values of the fitted Gaussians, yielding a continuous map of CDRF at varying distances from the head. A rotation-based maximum-likelihood expectation maximization (MLEM) method was used for reconstruction. A fast rotation algorithm was developed to rotate the image matrix according to the desired angle by means of pre-generated rotation maps. The experiments demonstrated improved resolution utilizing our resolution-recovery-embedded image reconstruction. While the full-width at half-maximum (FWHM) radial and tangential resolution measurements of the system were over 2 mm in nearly all positions within the FOV without resolution recovery, reaching around 2.5 mm in some locations, they fell below 1.8 mm everywhere within the FOV using the resolution-recovery algorithm. The noise performance of the system was also acceptable; the standard deviation of the average counts per voxel in the reconstructed images was 6.6% and 8.3% without and with resolution recovery, respectively.