The sinuses and mastoid air cells normally contain nothing but air, so they should be devoid of signal. If signal is present, it indicates inflammation or the presence of something which doesn't belong there. So, a signal void there is good.

You may also read references to signal void in your MRI scans of the brain; specifically, in regard to the blood vessels which comprise the Circle of Willis. Flowing blood should give no signal in these arteries (on non-contrast sequences), so you may see a comment like "vessels exhibit normal flow void."

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