Patient specific and accurate pseudo CT are needed for the adoption of MR-only in the radiation therapy workflow. Zero TE (ZTE) acquisition has proven to be very robust and reliable for bone segmentation, with the additional advantage of showing a reproducible inverse linear correlation with corresponding CT HU for ZTE intensity values in the bone range. Here we specifically investigate the impact on the dose accuracy of continuous versus single HU value assignment for bones and the strength of the Zero TE inverse linear correlation to CT values for accurate pseudo CT conversion.
Four patient data-sets from the study evaluated in1,2,3 were analyzed, where a 3.0T GE SIGNA PET/MR scanner (GE Healthcare, Chicago, IL) and a head array coil were used for ZTE data acquisition. A CT scan was also provided for each patient. A CT bone mask was generated for values larger than 150 HU. The DL method was used to generate the pseudo CT bone mask for pseudo CT values larger than 150 HU. The pseudo CT bone mask was rigidly registered to its corresponding reference CT bone mask. The following steps were applied to the original CT data:
1) a single bulk density value of 700 HU was applied to all voxels from the CT bone mask, corresponding to the mean CT bone density value of all patients considered.
2) A single soft tissue value of 42 HU was assigned to all voxels from the CT bone mask.
3) The pseudo CT bone mask was embedded into the data-set generated in step 2) and the inverse linear scaling of (-2000*(ZTEBONE-1) + 42) was applied to the ZTE values of the pseudo CT bone mask voxels.
4) The DL pseudo CT values were applied to the pseudo CT bone mask voxels embedded in the CT data-set from step 2).
The datasets were then imported into a Radiation Therapy Planning software (RayStation, RaySearch, Stockholm, Sweden). Dose distributions for all evaluated plans were analyzed and exported from the treatment planning system into the image analysis software MICE (Medical Interactive Creative Environment)5 to perform a gamma analysis over the full volume as defined by the CT scan.
1. Wiesinger F, Bylund M, Yang J, et al. “Zero TE-based pseudo-CT image conversion in the head and its application in PET/MR attenuation correction and MR-guided radiation therapy planning”. Magn. Reson. Med. 2018. doi: 10.1002/mrm.27134
2. Cozzini C, Bylund M, Jonsson J H, et al. “Feasibility of Zero TE MR based Radiation Therapy Planning for Head application”, ISMRM 2017
3. Kaushik S, Cozzini C, Bylund M, et al. “Deep Learning based pseudo-CT computation and its application for PET/MR attenuation correction and MR-guided radiation therapy planning”, ISMRM 2018
4. Edmund, Jens M., and Tufve Nyholm. "A review of substitute CT generation for MRI-only radiation therapy." Radiation Oncology12.1 (2017): 28.
5. T. Nyholm, et al, 3rd ESTRO Forum, 2015 [6] Wiesinger F, Kaushik S, Engstrom M, et al. “MR to pseudo CT conversion combining Deep-Learning and Analytical Image Processing” ISMRM 2019, submitted