You can add or remove blue pixels from the tissue to artificially enlarge or decrease the defined tissue. Adding or removing blue pixels lets you cover tissue borders that have different pixel values than the highlight window due to partial volume effects.
From the Tissue Definition toolbar, select one of the following options:
To add pixels to the tissue border, click Expand 
or click the Tissue Definition Parameters icon 
and use the Tissue Parameters dialog.
To Remove pixels from the tissue border, click Collapse 
.
NOTE: When you remove bone, we recommend that you use an expand value of 2.