A Projection-Domain Low-Count Quantitative SPECT Method for α-Particle-Emitting Radiopharmaceutical Therapy

Single-photon emission-computed tomography (SPECT) provides a mechanism to estimate regional isotope uptake in lesions and at-risk organs after administration of α-particle-emitting radiopharmaceutical therapies (α-RPTs). However, this estimation task is challenging due to the complex emission spectra, the very low number of detected counts (~20 times lower than in conventional SPECT), the impact of stray-radiation-related noise at these low counts, and the multiple image-degrading processes in SPECT. The conventional reconstruction-based quantification methods are observed to be erroneous for α-RPT SPECT. To address these challenges, we developed a low-count quantitative SPECT (LC-QSPECT) method that directly estimates the regional activity uptake from the projection data (obviating the reconstruction step), compensates for stray-radiation-related noise, and accounts for the radioisotope and SPECT physics, including the isotope spectra, scatter, attenuation, and collimator-detector response, using a Monte Carlo-based approach. The method was validated in the context of 3-D SPECT with 223Ra, a commonly used radionuclide for α-RPT. Validation was performed using both realistic simulation studies, including a virtual clinical trial, and synthetic and 3-D-printed anthropomorphic physical-phantom studies. Across all studies, the LC-QSPECT method yielded reliable regional-uptake estimates and outperformed the conventional ordered subset expectation-maximization (OSEM)-based reconstruction and geometric transfer matrix (GTM)-based post-reconstruction partial-volume compensation methods. Furthermore, the method yielded reliable uptake across different lesion sizes, contrasts, and different levels of intralesion heterogeneity. Additionally, the variance of the estimated uptake approached the Cramér-Rao bound-defined theoretical limit. In conclusion, the proposed LC-QSPECT method demonstrated the ability to perform reliable quantification for α-RPT SPECT.

[Experimental evaluation of inflammatory process in the prostate caused by transurethral transmission of uropathogen in low counts]

BACKGROUND

According to the literature, bacterial count of uropathogens isolated from expressed prostate secretion and urine which is sufficient for a diagnosis of bacterial prostatitis I and II categories, remains contradictory. Undoubtedly, the identification of microorganisms from affected organ in high titers indicates the presence of a relevant infectious-inflammatory process. In turn, there is no consensus on the development of bacterial prostatitis at lower titers of uropathogens. Thus, the aim of our study was to identify and compare the potential features of the development and occurrence of an infectious inflammatory process in the prostate during the reproduction of bacterial prostatitis in an animal model using a low titer of causative uropathogens.

MATERIALS AND METHODS

A total of 16 "New Zealand" mature male rabbits aged 24+/-2 weeks old with weight of 3.5+/-0.3 kg were examined. Inoculation was performed via transurethral route, according to the developed experimental technique. E. coli was used as bacterial agent with a count of 1 x 103 CFU/ml, 1 x 105 CFU/ml and 1 x 107 CFU/ml. All animals were randomized into 4 groups of 4 individuals depending on the titer of the inoculated microorganisms (groups 1-3, respectively), group 4 - control (with inoculation by Sol. NaCl 0.9%). Sacrification and vivisection were performed on days 1, 3, 7 and 14 of the control days. Biopsy specimens from the lower urinary tract and internal genital organs of laboratory animals (bladder, urethra, prostatic complex - 6 biopsies #1A-1D, 2A, 2B) were evaluated morphologically and bacteriologically. Analytical evaluation of the experimental data was presented using descriptive statistical methods.

RESULTS

In experimental groups (Groups 1-3), bacteriological examination of prostatic complex biopsies showed growth of microflora in all samples in titers of 101-107 CFU / ml. In group 1, the maximum concentration of uropathogen was observed on day 7, compared to day 1 in both groups 2 and 3. In all observed cases, the highest degree of bacterial contamination was noted in the biopsy specimens from paraprostatic tissues and distal part of the prostate, which was 4.0+/-1.7 lg CFU/ml and 3.5+/-1.9 lg CFU/ml, respectively, and the smallest in proximal prostatic loci (1C) and bladder neck (2B) - 3.0+/-1.2 lg COE / ml and 3.0+/-1.7 lg COE / ml, respectively. According to the morphological study, a relevant progression of the suppurative and destructive inflammation (with foci of colliquation necrosis) was identified in group 1 in the biopsies from the prostate with a maximum degree of changes on day 7 with subsequent formation of loose connective tissue proliferation areas by 14 days. This indicates the conversion of the inflammatory process to the chronic stage. These changes corresponded with the results of histopathological studies in groups 2 and 3 where higher titers of bacterial agent were used. In group 4 (control) the commensal flora was bacteriologically determined in the biopsies, but there were no signs of inflammation, according to the results of the morphological study.

CONCLUSION

In experimental model, we found that E. coli 103 CFU / ml induces the development of a phasic inflammatory process in the structures of the prostatic complex. These processes resulted in the formation of irreversible proliferative changes. As a consequence, it shold be recommended to consider these signs of contamination when evaluating the results of bacteriological examination of expressed prostate secretion/urine samples during planning treatment strategy.