Structural and Functional Imaging

• This is a Functional MRI Image !?

Structural and Functional Imaging

• This is a structural MRI image (an “anatomical” image)

Structural and Functional Imaging

• What you really want is both images co-registered

Structural and Functional Imaging

• What you really want is both images co-registered

• Why? What’s wrong with the functional image alone?

Structural and Functional Imaging

• Functional images tend to be lower resolution and fail to convey spatial information

Pixels

Structural and Functional Imaging

• Structural images have finer (smaller) pixels

Pixels

Structural and Functional Imaging

• Brain scan images (CAT, PET, MRI, fMRI) are all made up of pixels (stands for picture elements)

Pixels

Structural and Functional Imaging

• “Slices” are assembled into “volumes”

Pixels

Structural and Functional Imaging

• Volumes are composed of “volume elements” or voxels

Voxels

Structural and Functional Imaging

• Another thing you want: the ability to tell other people where something is– “the activity was centered on voxel #653”

will not work in a scientific journal

Structural and Functional Imaging

• MRI anatomical spaces– Talairach Space:

• Based on detailed analysis of one elderly woman• Talairach & Tournoux (1988)

– Montreal Neurological Institute Template (MNI)• based on average of 152 different brains, each normalized to Talairach

space• advantage: gyri and sulci are more representative• disadvantage: it’s blurry

– MNI “Representative Brain”• the one brain from the 152 in the MNI Template set that is most like the

average• advantage: it’s not blurry• disadvantage: it’s still just one person’s brain

Structural and Functional Imaging

• Reasons for normalizing to standard stereotaxic space (templates)– two levels: within-subject and between-

subjects

Structural and Functional Imaging

• Within-Subject Reasons:

1. structural and functional volumes may not be coregistered due to• movement• distortion

2. results can be described in standard coordinates

3. data across sessions can be averaged

Structural and Functional Imaging

• Between-Subject Reasons:

1. Volumes will not match because of variability across individuals

2. results can be described in standard coordinates

3. data across participants can be averaged

Preprocessing of Structural and Functional Images

• Normalizing images to fit a standard template (e.g. Talairach)

1. Define Coordinate System using easily recognizable landmarks

• Origin in the Anterior Commissure• y-axis connects AC and PC • x-axis perpendicular interhemispheric

plane and through AC • z-axis perpendicular to x and y

The Talairach Coordinate System

ACPC

The Talairach Coordinate System

AC - PC line defines y-axis

+y

-y

The Talairach Coordinate System

x-axis perpendicular to interhemispheric plane

+y

-y

-x

+x

The Talairach Coordinate System

z-axis perpendicular to x-y plane

+y

-y

-x

+x

+z

-z

Structural and Functional Imaging

• Cortical Flattening– Software such as

BrainVoyager can “inflate” the cortex like a balloon so that sulci and gyri are “flattened”

– functional data can be transformed with the same complex function

– functional and structural data can be overlaid so that distribution on cortical sheet can be visualized