Abstract
371
Objectives An MR/PET scanner with simultaneous MR and PET imaging requires attenuation correction for the MR components (surface coils and patient table) during PET image reconstruction. It is possible to calculate the attenuation properties of materials from molecular composition; however, the MR components consist of composites whose exact composition is difficult to determine. Attenuation maps obtained with CT have metal artifacts and quantitative inaccuracies due to beam hardening. The objective was to develop a method for estimating the attenuation coefficients of the MR components without employing a transmission source or a scan with a clinical CT.
Methods The CAD drawings of the MR components were segmented into parts having the same material composition. The MR/PET system (Siemens Biograph mMR) scanned the uniformity phantom offered with the system both with and without the attenuating objects. The attenuation map was then obtained by iteratively minimizing the difference between the emission sinograms with and without the MR components, as represented by the segmented CAD drawings. The minimization procedure optimized the attenuation values assigned to the segmented MR parts (plastic foam, circuit boards, etc). The method required registration before minimization of the CAD representations of the MR components with their positions during the emission measurement.
Results The quantitative accuracy of the reconstructed emission values verified the attenuation map accuracy. The estimated attenuation coefficients from our method were within 15% of the attenuation coefficients measured by CT. Moreover, closer agreement was observed between the estimates of this iterative method and the true attenuation coefficients than between the attenuation coefficients measured with CT and the true attenuation coefficients.
Conclusions These results indicate that this method can be used to generate complete attenuation maps for the MR PET hardware that falls in to PET field of view