question and answer
When to use gadolinium contrast for MRI
July 2005
ANNE WALLACE, MD, of Toronto, ON, wants to know, "What extra information does the positive contrast agent gadolinium add to a regular magnetic resonance imaging (MRI) scan of the brain?"
In computed tomography (CT), intravenous iodinated contrast is used to enhance the image with information on the vascularity and characteristics of organs and pathologic lesions. The iodine density blocks the passage of the x-ray photons, causing the contrast to appear denser (white) on the CT images. Since MRI is based on magnetization, not ionizing radiation, the intravenous contrast material must have magnetic properties different from that of the surrounding tissue. The most common agent used is gadolinium diethylenetriamine pentaacetic acid (Gd-DTPA) — a paramagnetic agent that allows positive contrast enhancement, i.e. it shows up as bright on T1-weighted sequences. This provides a new array of diagnostic information, including differentiation of tumour from edema and inflammation and soft tissue from scar tissue (as in recurrent lumbar discs). It also facilitates the dynamic assessment of lesion types (breast cancer vs benign fibroadenoma; hepatic hemangiomas vs hepatocellular carcinoma). In the brain, Gd-enhancement reveals areas where the blood-brain barrier has been breached, providing vital information on patients with tumours and other pathologic processes. For multiple sclerosis, Gd-enhancement helps distinguish active demyelination plaques from chronic quiescent ones. Other valuable uses for Gd in brain MRIs include the assessment of early ischemia, parenchymal brain infections and meningeal lesions.

It's important for referring physicians to know that this contrast agent is very well tolerated by patients, with no clinically relevant side effects and an extremely low chance of allergic reaction, due to the inert nature of the base substance. MM

Reference:
Radiology: Imaging Diagnosis Intervention. Vol. 3, Ch. 59, p. 1-26, 2005.
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