4D-MRI for Precision Medicine

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Radiotherapy for cancer has been a forerunner of personalized medicine, developing individualized treatments based on patient-specific anatomical information. Despite many advances in radiotherapy over the past decade, which have effectively enhanced local or loco-regional tumor control for many pat...

Radiotherapy for cancer has been a forerunner of personalized medicine, developing individualized treatments based on patient-specific anatomical information. Despite many advances in radiotherapy over the past decade, which have effectively enhanced local or loco-regional tumor control for many patients, there remains substantial room for improvement. The challenges for radiotherapy to further widen the therapeutic window in the era of precision medicine are mainly two-fold: (a) further improve radiation dose conformity to the defined target volume, and (b) adapt novel biological strategies for personalized treatment. Four-dimensional (4D) imaging and deformable image registration (DIR) are key tools in modern radiotherapy, playing critical roles in many recent advances, including 4D radiotherapy, adaptive radiotherapy, and treatment assessment. However, current 4D imaging and DIR technologies are facing significant challenges as the requirement for precision increases. The current standard of 4D imaging in radiotherapy is 4D-CT. However, it has two major limitations preventing it from precision radiotherapy applications: (a) low soft-tissue contrast. 4D-CT is therefore not ideal for abdominal applications; (b) motion artifacts caused by irregular breathing. 4D-CT motion artifacts have been shown to cause errors in various radiotherapy applications, including motion measurement, target volume delineation, dose calculation, DIR, and lung ventilation calculation. 4D-MRI is an emerging 4D imaging technology for radiotherapy. It has superior soft-tissue contrast to 4D-CT and is therefore superb for abdominal imaging. Despite many recent advances in 4D-MRI, current 4D-MRI implementations have inadequate image quality for precision radiotherapy application due to at least one of the following deficiencies: low temporal and/or spatial resolutions, long image acquisition time, and suboptimal contrast in the lungs. Resulting 4D-MRI images lack sufficient anatomical details for clinical applications, which can adversely affect the performance of DIR. Current DIR techniques focus on morphological similarity but not on the physiological plausibility of the deformation. Studies have shown that an increased morphological similarity of the aligned data does not always imply increased registration accuracy. Therefore, more sophisticated approaches are desirable. The investigators will take a systematic approach to address the aforementioned limitations of 4D imaging and deformable image registration (DIR) based on the development and cross-fertilization of two major techniques: ultra-quality 4D-MRI and physiological-based hybrid DIR. There are two parts of this research, comprising three main objectives: Part 1. Technical development in healthy subjects: The investigators will extend their existing pulse sequence strategy for ultra-quality 3D MRI to enable ultra-quality 4D-MRI. Compared to 4D-CT and current 4D-MRI techniques, the proposed ultra-quality 4D-MRI technique offers the following advantages: (a) high spatial resolution (1.5 mm isotropic) with rich image features (e.g. vessel trees) in the whole torso; (b) high temporal pseudo-resolution (>20 phases/cycle); and (c) (nearly) free of motion artifacts. • Objective 1: Develop an MRI pulse sequence and image reconstruction pipeline that generates images meeting these three design goals. Part 2. Evaluation of 4D-MRI in a patient study: 4D-MRI will be compared with existing DIR and 4D-CT methods. There will be two classes of comparisons, each formulated as a separate objective: Objective 2: Compare motion modelling based on 4D-MRI with deformable image registration (DIR) in healthy volunteers and cancer patients. An improved motion modeling method will be developed that is tailored for the ultra-quality 4D-MRI applications. The investigators hypothesize that a new motion modeling method based on 4D-MRI will outperform current DIR algorithms for respiratory motion estimation. This hypothesis will be tested by comparing the new method to five DIR algorithms which include a mix of commercial software and publicly available algorithms. Objective 3: Compare 4D-MRI with 4D-CT in lung and liver cancer patients. The overall hypothesis of this objective is that the ultra-quality 4D-MRI provides better image quality than 4D-CT for motion management of radiotherapy in the lungs and the liver, especially in patients with irregular breathing.

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