Mithunan Modchalingam

Prior to the administration of radiation therapy, imaging must be performed to allow for the delineation of target volumes and the creation of a treatment plan. The current standard for imaging is CT which provides anatomical information but lacks the superior soft tissue contrast of MRI. With the use of MRI images target volumes can be delineated with an increased accuracy thereby reducing unnecessary dose to healthy tissue. Unlike CT images however, MRI images have intrinsic distortions which can cause in-plane and through-plane shifts leading to reduced spatial precision. As a result, this translates to a reduced accuracy of the treatment. The degree of these distortions depend on different sequence parameters however and steps can be taken to reduce them. Along with image distortions, MRI also has longer scan times than those of CT. In head and neck cases a conventional T2 FLAIR sequence can take up to 10 minutes which can lead to clinical inconvenience and patient discomfort. A multi-echo gradient echo sequence can reduce these scan times while ideally providing T2*-weighted images acceptable for radiation therapy planning.
A grid phantom is being constructed in order to determine the distortion field of our 3 T Ingenia MRI. Upon completion the distortion field can then be quantified through the use of two separate methods. First a non-linear registration can be performed where the MRI image acts as the moving image and the CT the reference image. This registration provides a pixel by pixel displacement map, which through the knowledge of the pixel dimensions, can be inferred to represent the distortion field. Secondly the grid intersection points of the phantom can be identified and assigned Cartesian coordinates. These points on the CT and MRI images can then be compared thereby giving the distortion at each grid point. Upon quantification of the distortion field, a multi-echo gradient echo pulse sequence can be optimized in order to reduce these geometric distortions while aiming to reduce the scan times below that of conventional sequences.

Gerald Clavet Fellowship (2016-2017)
McGill Graduate Excellence Fellowship (2015)
Ryerson Physics Award (2014)
NSERC USRA (2014)
Ryerson Physics Faculty Scholarship (2013)