I'm in the radiology field (MD), however, as usual, this is not a
substitute for medical advice or direct medical evaluation.
A CT I-, as suggested in your questions and the posted comments, is a
CT performed without iodinated IV contrast media. You may also
sometimes see I+/-, which means with and without IV contrast. This is
a very common shorthand used by both people in radiology and
clinicians who frequently order CT scans.
With regards to meningiomas, benign (meaning they don't generally
spread to other areas) tumors of the meninges of the brain, they can
be seen on non-contrast CT, but are best imaged with IV contrast.
They tend to enhance strongly and often heterogeneously. They tend to
cause symptoms by slowly growing and causing a gradual mass effect on
adjacent structures, which can result in focal neurological symptoms,
prompting a (usually noncontrast) screening CT of the brain.
Here is a quote from a good eMedicine article on meningiomas with
regard to imaging findings:
"Contrast-enhanced CT displays moderate-to-strong homogeneous
enhancement in most tumors (see Images 7-8). Steinhoff et al observed
a nodular blush in 97%, a mixed inhomogeneous blush in 0.5%, and a
ring blush in 1.5%. In a study by Naidich of 136 patients, tumor blush
was nodular and nearly homogeneous in 70% of patients, inhomogeneous
in 24% of patients, and ringlike in 2% of patients (see Images 2-4)."
85% of these tumors demonstrate marked enhancement with IV contrast on
CT. Another useful modality for imaging these tumors is MRI, on which
they show strong enhancement with IV Gadolinium (you may also see Gad,
Gd, Gd+, Gd+/-, IV MRI contrast). The above article also discusses
MRI imaging findings.
If you're interested, you can find an imaging (CT and MRI) database of
meningiomas at this site:
meningioma CT contrast
I hope this information was useful. Feel free to request any clarification.