Y-90 SPECT ML image reconstruction with a new model for tissue-dependent bremsstrahlung production using CT information: a proof-of-concept study

Abstract

While the yield of positrons used in Y-90 PET is independent of tissue media, Y-90 SPECT imaging is complicated by the tissue dependence of bremsstrahlung photon generation. The probability of bremsstrahlung production is proportional to the square of the atomic number of the medium. Hence, the same amount of activity in different tissue regions of the body will produce different numbers of bremsstrahlung photons. Existing reconstruction methods disregard this tissue-dependency, potentially impacting both qualitative and quantitative imaging of heterogeneous regions of the body such as bone with marrow cavities. In this proof-of-concept study, we propose a new maximum-likelihood method that incorporates bremsstrahlung generation probabilities into the system matrix, enabling images of the desired Y-90 distribution to be reconstructed instead of the 'bremsstrahlung distribution' that is obtained with existing methods. The tissue-dependent probabilities are generated by Monte Carlo simulation while bone volume fractions for each SPECT voxel are obtained from co-registered CT. First, we demonstrate the tissue dependency in a SPECT/CT imaging experiment with Y-90 in bone equivalent solution and water. Visually, the proposed reconstruction approach better matched the true image and the Y-90 PET image than the standard bremsstrahlung reconstruction approach. An XCAT phantom simulation including bone and marrow regions also demonstrated better agreement with the true image using the proposed reconstruction method. Quantitatively, compared with the standard reconstruction, the new method improved estimation of the liquid bone:water activity concentration ratio by 40% in the SPECT measurement and the cortical bone:marrow activity concentration ratio by 58% in the XCAT simulation.

Energy window optimization in bremsstrahlung imaging after Yttrium-90 microsphere therapy

“…According to the visual evaluations and ANOVA/ Tukey test results, it was concluded that energy window of 90-110 keV is the optimal energy window while 60-400 keV energy ranges show the lowest image quality for Y-90 bremsstrahlung imaging. These ranges are consistent with other investigations related energy window optimization with medium energy collimator [6,8,12,13]. Namely, due to the Y-90 Bremsstrahlung energy spectrum has large numbers of low-energy photons, it is concluded that 50-100 keV energy window is the most proper energy window when using the medium energy collimator [23].…”

“…For example, it is reported that the fraction of detected photons undergoing septal penetration for a 400-450 keV window is much larger than for a 100-150 keV window [24]. It is reported that energy window conditions with wider ranges of energy width (60-400 keV) cause higher FWHM, as a result of more unexpected photons [8]. As a summary of the above, effect of energy region and energy-window width on the image quality parameters found in this study can be seen from figure 11.…”

“…Using bremsstrahlung photons allows for more accurate localization and identification of the spheres' intrahepatic and possibly extrahepatic distribution [7]. However, for therapies such as Y-90 transarterial radioembolization, the absence of monoenergetic gamma photons makes the Y imaging more complex [8]. In 90 Y bremsstrahlung imaging, tissue attenuation, varying count rates of bremsstrahlung photons, low spatial resolution, continuous and wide spectrum acquisition, and the type of collimator used to affect the image quality [9,10].…”

ICRU REPORT 96, Dosimetry-Guided Radiopharmaceutical Therapy

“…Another consideration is the degradation in spatial resolution because of the finite distance between the location of the 90 Y decay and the location where the external bremsstrahlung photons are generated, akin to positron range effects in PET. A further complication is that the bremsstrahlung yield and energy spectrum is tissue dependent (Lim et al , 2018c), but this effect is not significant when imaging homogenous regions such as the liver.…”

Quantitative Imaging of Alpha-Emitting Therapeutic Radiopharmaceuticals

“…Quantitative imaging of bremsstrahlung radiation requires a good combination of radionuclide collimator and gamma cameras [17][18][19][20][21][22]. For quantitative imaging of gammas, combined SPECT with xray computed tomography (SPECT/CT) will add great benefits for quantification [23]. For Tb-149, PET/CT will be a quantitative imaging tool by the same token with its position emission, a rarity for alpha-emitters [24].…”