Purpose

The purpose of this discussion is to give you a basic understanding of signal contrasts and acquisition and how they might be used in an imaging study.

Objectives

1. Be able to define "contrast."
2. Understand the difference between endogenous and exogenous contrasts and their uses.
3. Be able to describe the different types of contrasts used in neuroimaging research.
4. Be able to select the correct contrast to answer an experimental question.
5. Be able to describe common image acquisition techniques.

Contrast

What is Contrast?

• The degree to which different colors in an image are visually distinct.
• Images with low contrast have reduced color differences (and often appear greyed out or foggy).
• Images with high contrast have striking differences between colors.

Why is it important?

• Contrast determines how well an image depicts differences/changes.
• In MRI, this contrast selection determines what an image can actually show or which tissues it is sensitive too.

Types of Contrast

• Static contrasts – use differences in tissue composition to construct an image.
• Motion contrasts – use tissue motion to construct an image.
• Chemical contrasts – target specific atomic nuclei for imaging, somewhat like NMR spectroscopy.

Static Contrasts

• T₁
• T₂
• T₂*

T ₁ contrast

• Used to assess gross anatomy and construct anatomical models.
• Contrast formed using known differences in T1 recovery.
• Shorter recovery times lead to brighter intensities.
• Long TR and short TE.

T ₂ contrast

• Also used assess anatomy.
• Contrast formed using known differences in T₂ decay.
• Relies on differences in spin-spin interactions across tissues.
• Long TR and intermdiate TE.

T ₂* contrast

• Primary contrast of fMRI.
• Formed by measuring the effect of local inhomogeneities in the magnetic field on precession frequencies.
• Sensitive to differences in deoxygenated hemoglobin.
• Long TR and short TE.

Motion Contrasts

• Diffusion weighted
• Perfusion weighted contrast

Diffusion weighted contrast

• Used to map white matter tracts.
• Water diffusion patterns are measured.
• Diffusion in tracts is anisotropic.

Acquisition

Acquisition

• Refers to the method used to traverse imaging space and measure signal at each voxel.
• EPI (echo planar imaging) – scanner utilizes a fast protocol and a grid like pattern, measuring signal line by line (like an old dot matrix printer).
• Spiral imaging – scanner starts in the center of the target and extends in a spiral like pattern.
• Multi-band imaging – requires a large coil array and magnetic gradient, information gathered from multiple slices at once.