Relaxation and Contrast Part 2

This is the second part of the previous lecture, and it explores the technical and clinical nuances of MRI signal encoding and advanced contrast mechanisms. The lecture begins with a focus on spatial encoding, explaining how magnetic field gradients are used to define slices and determine the position of signals within those slices, which also accounts for why MRI scans are time-consuming and noisy. A significant portion of the lecture is dedicated to the distinction between Spin Echo and Gradient Echo sequences; while Spin Echo provides "true" T2 tissue contrast, Gradient Echo is sensitive to T2* effects, making it invaluable for detecting magnetic field disturbances caused by metal fragments or iron in blood (such as microbleeds and veins). Furthermore, the lecture details perfusion imaging, where rapid scanning tracks the "first pass" of a Gadolinium contrast agent to map cerebral blood volume—a critical tool for staging tumors and identifying blood flow defects. The lecture concludes by walking through a typical clinical neuro-oncology protocol, demonstrating how a combination of FLAIR, diffusion-weighted, and pre- and post-contrast T1/T2 sequences is layered to provide a comprehensive diagnostic picture.

Learning objectives

By the end of this lecture, students will be able to:

• Identify the primary safety hazards associated with the strong static magnetic field in MRI.
• Explain how magnetic field gradients are used for slice selection and spatial encoding (frequency and phase encoding).
• Contrast the physical differences between Spin Echo and Gradient Echo sequences, specifically regarding T2 and T2* weighting.
• Recognize the clinical utility of T2* imaging for detecting microbleeds and the "central vein sign" in Multiple Sclerosis.
• Summarize the process of dynamic perfusion imaging (CBV mapping) using contrast agent transit.