The CAT scan is generally used pretty universally for the sinuses.

It shows all sinuses.

You might use an MRI in the specail case where you suspect cancer or fungal
sinusitis, but otherwise it is inferior.

http://adam.about.com/reports/000062_5.htm

"Magnetic Resonance Imaging. MRI is not as effective as CT in defining the
paranasal sinsus anatomy and therefore is not typically used to image the
sinuses for suspected sinusitis. MRI is also more expensive than CT. However,
it can help rule out fungal sinusitis and may help differentiate between
inflammatory disease, malignant tumors, and complications within the skull."

The CT scan is the way to go. Make sure they do a full sinus CT scan and
not a 'head CT' or a 'limited sinus CT'.

One thing to do is to get any sinus CTs you have on a CD for your records.
Most places do this for free and include a viewer with the CT scans so you
can take it to another doctor if you get a second opinion. Most ENT
doctors will want to look at the actual CT scans not the written radiology
report. For me, all I had to do was call up the radiology department 2 or 3
days before pickup, request the CT scans on CD, and fill out and sign a
form when I picked the CD up.