When a physician orders a magnetic resonance imaging (MRI) scan, the request often specifies “with and without contrast.” This instruction means the procedure involves two distinct sets of images taken during the same session. The first set of images is acquired without special medication, establishing a baseline picture of internal structures. Following this initial scan, a contrast agent is injected intravenously, and a second set of images is immediately taken to highlight specific tissues and processes. This combined approach provides radiologists with the necessary information to distinguish between medical conditions and determine their activity level.
The Baseline Scan Without Contrast
The MRI scan performed without contrast is the fundamental part of the examination, relying solely on the body’s natural magnetic properties to create detailed images. This baseline scan is highly effective for visualizing basic anatomy, fluid-filled structures, and tissue composition, providing a clear picture of structural abnormalities like torn ligaments, herniated spinal discs, or bone marrow issues.
This initial scan establishes the natural background signal intensity of the tissues before any enhancing agent is introduced. This allows the radiologist to assess the size, shape, and location of any existing abnormalities, such as chronic scar tissue. For many common orthopedic and neurological issues, this non-contrast scan alone provides sufficient diagnostic information.
How Contrast Improves Imaging
The addition of a contrast agent enhances the visibility of specific biological processes that might otherwise be indistinguishable from surrounding healthy tissue. The agent works by shortening the T1 relaxation time of nearby water molecules, causing the target tissues to appear much brighter on the resulting image. This effect is useful for identifying areas with increased blood flow or compromised biological barriers.
Active disease processes like inflammation, acute infection, and tumor growth are often associated with leaky blood vessels. The contrast agent escapes these compromised vessels and accumulates in the abnormal tissue, making the area “light up” on the scan. This capability allows doctors to detect small tumors, identify active lesions in conditions like multiple sclerosis, or pinpoint the location of an abscess, which is crucial for treatment planning.
Why Doctors Order Both Types of Scans
Ordering the scan “with and without contrast” is necessary for accurate diagnostic interpretation by the radiologist. The non-contrast image serves as a direct reference point, showing any natural brightness or pre-existing structural issues. Certain tissues, like fat or blood products, can appear naturally bright on a non-contrast MRI, a phenomenon known as T1 shortening.
Radiologists must compare the two sets of images side-by-side to determine if the increased brightness seen on the second scan is due to the injected contrast agent or if it was already present on the baseline scan. This comparison prevents misdiagnosis, as a naturally bright spot could be mistaken for active disease if only the contrast-enhanced image were reviewed. By performing this subtraction, the radiologist can precisely localize and confirm the presence of active processes, ensuring that treatment is directed appropriately.
Understanding the Contrast Agent
The contrast substance used in MRI is a Gadolinium-Based Contrast Agent (GBCA), which contains the rare-earth metal gadolinium. Gadolinium is a paramagnetic metal that interacts with the MRI scanner’s magnetic field to create signal enhancement. The agent is administered through an intravenous (IV) injection, usually into a vein in the arm, just before the second part of the scan begins.
While generally considered safe, the agent carries associated risks. In patients with severe kidney impairment, there is a rare risk of developing Nephrogenic Systemic Fibrosis (NSF). For this reason, kidney function is often checked before a GBCA is administered. The contrast agents are designed to be efficiently cleared from the body through the kidneys.