MRI-based treatment planning with pseudo CT generated through atlas registration

Abstract

Purpose: To evaluate the feasibility and accuracy of magnetic resonance imaging (MRI)-based treatment planning using pseudo CTs generated through atlas registration. Methods: A pseudo CT, providing electron density information for dose calculation, was generated by deforming atlas CT images previously acquired on other patients. The authors tested 4 schemes of synthesizing a pseudo CT from single or multiple deformed atlas images: use of a single arbitrarily selected atlas, arithmetic mean process using 6 atlases, and pattern recognition with Gaussian process (PRGP) using 6 or 12 atlases. The required deformation for atlas CT images was derived from a nonlinear registration of conjugated atlas MR images to that of the patient of interest. The contrasts of atlas MR images were adjusted by histogram matching to reduce the effect of different sets of acquisition parameters. For comparison, the authors also tested a simple scheme assigning the Hounsfield unit of water to the entire patient volume. All pseudo CT generating schemes were applied to 14 patients with common pediatric brain tumors. The image similarity of real patient-specific CT and pseudo CTs constructed by different schemes was compared. Differences in computation times were also calculated. The real CT in the treatment planning system was replaced with the pseudo CT, and the dose distribution was recalculated to determine the difference. Results: The atlas approach generally performed better than assigning a bulk CT number to the entire patient volume. Comparing atlas-based schemes, those using multiple atlases outperformed the single atlas scheme. For multiple atlas schemes, the pseudo CTs were similar to the real CTs (correlation coefficient, 0.787-0.819). The calculated dose distribution was in close agreement with the original dose. Nearly the entire patient volume (98.3%-98.7%) satisfied the criteria of chi-evaluation (<2% maximum dose and 2 mm range). The dose to 95% of the volume and the percentage of volume receiving at least 95% of the prescription dose in the planning target volume differed from the original values by less than 2% of the prescription dose (root-mean-square, RMS < 1%). The PRGP scheme did not perform better than the arithmetic mean process with the same number of atlases. Increasing the number of atlases from 6 to 12 often resulted in improvements, but statistical significance was not always found. Conclusions: MRI-based treatment planning with pseudo CTs generated through atlas registration is feasible for pediatric brain tumor patients. The doses calculated from pseudo CTs agreed well with those from real CTs, showing dosimetric accuracy within 2% for the PTV when multiple atlases were used. The arithmetic mean process may be a reasonable choice over PRGP for the synthesis scheme considering performance and computational costs.

MR image‐based synthetic CT for IMRT prostate treatment planning and CBCT image‐guided localization

“…A previous study (14) has shown that using multi‐atlas‐based DIR in S‐CT production would produce ‘smooth’ effect in the S‐CT image. The effect could reduce the gray scale contrast and lead to less attenuation for radiation beam.…”

“…While providing promising results, they are not practical since these special sequences are not generally available in clinic. DIR‐based methods have also been explored to generate S‐CT 12 , 13 , 14 , 15 , 16 . However, using DIR algorithms directly 12 , 13 to map HU from the CT images in the atlas library to an S‐CT image suffers from inherent registration errors due to large anatomical differences between different patients.…”

“…However, using DIR algorithms directly 12 , 13 to map HU from the CT images in the atlas library to an S‐CT image suffers from inherent registration errors due to large anatomical differences between different patients. It is possible to reduce registration uncertainty, either by improving the accuracy of DIR algorithm or by combining with a “CT voxel intensity selection algorithm” 14 , 16 . Dosimetric aspect of using MR image‐derived S‐CT image in radiotherapy treatment planning has been well investigated.…”

Do we need a new CT scan for retreatment of intracranial SRS patients?

“…The most relevant comparison is the use of synthetic CTs to calculate dose for treatment of primary brain tumors. Doses calculated with synthetic CT for conventional treatment of primary brain tumors showed gamma rates of 98%12 and 97% (2D analysis through isocenter plane)9 at 2 mm/2% criteria and 80% at 1 mm/1% criteria (2D analysis through isocenter plane with a range of 56%–100%) 9…”

“…In this case, localization was typically provided by stereotactic head frame. Efforts have been made to create “synthetic” or “pseudo” CTs from MR images that can be used for dose calculation and localization 7, 8, 9, 10, 11, 12, 13, 14. These methods show promise in terms of geometric and dosimetric accuracy, but are not in widespread clinical use at this time.…”