MRI at The Women’s Imaging Center

MRIs generally produce more distinction of close tissue types than CT scans. MRI depends on the relaxation time differences between tissues taking advantage of our body’s high water content. MRI is not optimal though when organs are air-filled or moving. The greater resolution of MRI allows the identification of detail such as a fine tear in the meniscus of the knee while a CT scan can be used to identify subtle differences in contrasting tissues. For example, a thin-section CT scan of the lungs can produce great tissue distinction between the structures of the lung and the air-filled background. CT images are acquired very quickly in comparison to MRI images. Therefore, moving structures such as the lungs and bowel are better evaluated with CT imaging. 

Both MRI and CT sometimes use an injection of a dye to make the different tissues stand out from one another. A very small tumor in the breast may be bright (enhanced) on an MRI as the dye shows the blood vessels coursing through the tumor to distinguish it from the adjacent normal tissue. CT scan may also take advantage of dye to make different processes stand out such as an infection (abscess) in the bowel. The indication for MRIs or CT scans depends on the tissue type or organ system to be evaluated and the question to be answered. One type of scan is not better than another. The MRI imaging may take 25 to 35 minutes depending on whether special imaging sequences are needed while CT scans may take 15 to 20 minutes without contract. The contrast dye may increase the exam time by 2 because imaging is done before and after contrast.