History:
55 year old male with an incidental left renal mass seen on
ultrasound, and previous allergic reaction to iodinated intravenous
contrast. The patient was referred to Main Street Radiology for a
renal MRI examination.
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Figure
A - T2 weighted
Blue arrow: simple right renal cyst
Yellow arrow: "hyperintense" left renal cyst |
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Figure
B- T1 weighted pre-contrast |
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Figure
C- T1 post-contrast |
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Figure
D- Subtraction image |
Findings:
A simple cyst is seen within the right kidney, with typical signal
characteristics: high signal on T2-weighted image (Figure A), low
signal on T1-weighted image (Figure B), and no enhancement on the
post-contrast T1-weighted image (Figure C).
At the posterior aspect of the left
kidney, an atypical lesion is seen, demonstrating low signal on T2 and
high signal on T1. The presence of contrast enhancement cannot be
established on the post-contrast T1 image, due to the pre-existing
high signal nature of the lesion. Subtraction image (Figure D) clearly
demonstrates lack of contrast enhancement. These findings are
compatible with a "hyperintense" cyst, a benign finding.
Discussion:
In characterizing renal lesions, it is extremely important to
determine the presence or absence of contrast enhancement, to
distinguish benign cysts from neoplasms.
Traditionally, CT of the kidneys
performed before and after the administration of intravenous contrast
has been considered the gold standard for renal mass characterization.
Accurate quantitative analysis can be performed by measuring the
density (Hounsfield unit) of the lesion before and after the
administration of contrast. Typically, greater than 10 Hounsfied unit
increase in density after contrast enhancement is only seen in
neoplastic processes.
Renal MRI does not have a comparable
quantitative analysis to determine contrast enhancement, and
radiologists had to rely on traditionally less accurate qualitative
analysis. However a new technique involving "subtraction"
imaging has increased the sensitivity of qualitative MRI analysis to
99% (Radiology 2004;232:373-378).
Subtraction imaging is a
post-processing technique, where the pre-contrast images are digitally
"subtracted" from the post-contrast images, resulting in
images than accentuate contrast enhancement.
At Main Street Radiology, we routinely
perform subtraction imaging for all renal MRI's. Only the most
sophisticated MRI machines with latest software are capable of
performing subtraction imaging.
Renal MRI is indicated in patients who
need a contrast-enhanced renal CT, but has a history of renal
insufficiency or allergy to iodinated contrast
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