An Introduction to MR Sequences Used in Oncology with Application to MR-Linac

Magnetic resonance imaging is a powerful technique with several advantages. First and foremost, it offers the ability to image tissues using multiple sequences, each of which can highlight different tissue characteristics, dramatically improving diagnostic ability. In radiology, it is not uncommon for CT or ultrasound imaging to recommend MRI follow-up for definitive diagnosis. MRI is also able to provide excellent soft tissue contrast, elucidating fine differences in anatomy. Also, unlike CT, this is done without any radiation. This chapter begins with the basics of MR physics, which provides a building block to understand how sequences are generated. Next, it includes a discussion of MR sequences, including how they are generated and what they are used for. This includes an introduction to T1- and T2-weighted imaging, spin echo and gradient echo, proton density, diffusion-weighted imaging, and various fat reduction techniques (chemical shift, inversion recovery, fat saturation). Information relevant to Elekta Unity and ViewRay MRIdian is presented. Diagrams highlighting fundamental MRI principles are provided, as well as real-world patient examples. An understanding of these principals is essential for the application of a new technology such as MR-Linac. The use of gadolinium-based contrast agents is discussed, along with their safety considerations. There is also a discussion of MR artifacts, which can both impede and aid in diagnosis. Finally, there is a section on general MRI safety concerns.