The Histogram Tool can be launched by pressing the button, 
,
or by pressing "f" in an image. Clicking with the mouse in the image
while the histogram tool is active will take a histogram starting at that point
in the image. Similarly, when "f" is pressed, the histogram will be
taken starting at the current mouse location. The histogram is constrained by
outlines, so clicking inside an outline will show a histogram of the voxels
inside that outline. The histogram window looks like this:
The horizontal axis is intensity and the vertical axis is number of voxels.
Clicking with the left mouse button will cause an iso-intensity contour to be drawn at that intensity.
The green and red lines shown here demonstrate the "mid-peak" method of choosing an intensity contour. Clicking with the middle mouse button on one peak (a green line) and dragging to another peak causes the intensity halfway between these two peaks to be used to create an iso-intensity contour (the red line).
Dragging the rightmost mouse button sets the intensity range that is shown in the window to be the points at which the mouse was first clicked and then released. The button "Set Full Intensity Range" changes the range being show back to the full range.
The "clear region" button removes the white overlay that appears when the histogram is first taken. This overlay shows which voxels were included in the histogram. The "Minimum" and "Maximum" displays indicate the intensity range of the histogram. The letters in parenthesis are the keys that can be used instead of the buttons when this window has the keyboard focus.