A proposal for PET/MRI attenuation correction with μ-values measured using a fixed-position radiation source and MRI segmentation

Several MRI-based attenuation correction methods have been reported for PET/MRI; these methods are expected to make efficient use of high-quality anatomical MRIs and reduce the radiation dose for PET/MRI scanning. The accuracy of the attenuation map (μ-map) from an MRI depends on the accuracy of tissue segmentation and the attenuation coefficients to be assigned (μ-values). In this study, we proposed an MRI-based μ-value estimation method with a non-rotational radiation source to construct a suitable μ-map for PET/MRI. The proposed method uses an accurately segmented tissue map, the partial path length of each tissue, and detected intensities of attenuated radiation from a fixed-position (rather than a rotating) radiation source to obtain the μ-map. We estimated the partial path length from a virtual blank scan of fixed-point radiation with the same scanner geometry using the known tissue map from MRI. The μ-values of every tissue were estimated by inverting a linear relationship involving the partial path lengths and measured radioactivity intensity. Validation of the proposed method was performed by calculating a fixed- point data set based upon real a real transmission scan. The root-mean-square error between the μ-values derived from a conventional transmission scan and those obtained with our proposed method were 2.4±1.4%, 17.4±9.1% and 6.6±4.3% for brain, bone and soft tissue other than brain, respectively. Although the error estimates for bone and soft tissue are not insignificant, the method we propose is able to estimate the brain μ-value accurately and it is this factor that most strongly affects the quantitative value of PET images because of the large volumetric ratio of the brain.