It’s likely that most of us have come across the term “MRI contrast gadolinium” either in a report, in the news, or when we or someone we know have received an injection for an MRI exam. The vast majority of us don’t have a good grasp on what it is and/or why it is necessary for imaging studies.
First, MRI contrast gadolinium is not iodine-based contrast. This is important to note, because an iodine-based contrast agent used during a CT scan, Intravenous Pyelogram (IVP) or heart catheterization study can cause an allergic reaction. This raises concerns when the term “contrast” is used. The term describes any substance used in an imaging study to reveal abnormal tissue, in opposition to normal tissue, and also opposed to how that tissue presented prior to the administration of a contrast agent.
In MRI, we utilize a specific agent called gadolinium. This clear, viscous liquid is administered intravenously to the patient at a pre-calculated amount, usually during the second half of the study. Unlike the iodine contrast, the gadolinium itself is not visualized, but rather the changes it causes in the tissue. As the gadolinium molecules interact with the body, they change the nuclear precession of the hydrogen molecules in the tissue. (For more information on Nuclear Precession please check out the Technology section on our website). This change in precession causes an augmentation in the signal echo that is read out, causing it to show up hyper-intense (bright) on the images. Because of this interaction, only a few types of sequences are viable for post-contrast imaging. The most widely-used sequences are T1 weighted images, sometimes with the addition of Fat Saturation. The T1 images are done before and after the administration of gadolinium to reveal any abnormalities.
So why does gadolinium only affect abnormal tissue? Though there are some normal tissues that are affected by the gadolinium contrast, it is most often passed through the circulatory system and then dispersed into the interstitial space or filtered out by the kidneys. Because of the hydrophilic nature of gadolinium contrast, it will not have an effect on normal healthy tissue. However, if there is degradation of the blood-brain barrier, the contrast is allowed to leak through and invade the abnormal pathology.
In the images below, you will see that on the pre-contrast images, the brain tissue looks mostly uniform and cohesive in shape, contrast and density. However, after the administration of the gadolinium, we see that there are large tumors within the normal tissue of each of these cases. Based on the characteristics of the post-contrast imaging, we are able to gain valuable information on the pathology involved. With this information, the turn-around of a diagnosis becomes faster and treatment can begin almost immediately in some cases.
Figure 1. "Dixie," a 12-year-old FS Golden Retriever presented for seizures and mentation changes. Axial T1 pre-contrast imaging at the level of the TMJ shows a mass that is hypointense centrally with isointense to normal gray matter.