A magnetic resonance imaging (MRI) scan produces detailed images of organs and soft tissues. Sometimes, specialized agents are required to enhance the clarity and visibility of certain structures. Before a contrast-enhanced MRI, blood work is required as a standard safety practice. This pre-screening verifies that the procedure can be performed safely and effectively.
Understanding MRI Contrast Agents
MRI contrast agents are typically Gadolinium-Based Contrast Agents (GBCAs), which are injected intravenously to improve image quality. The contrast agent works by altering the magnetic properties of water molecules in the body, creating a brighter signal on the resulting images.
This enhancement is necessary to clearly delineate blood vessels, identify inflammation, or better characterize tumors that might be difficult to see on a non-contrast scan. The substance is administered into a vein, circulates through the bloodstream, and briefly accumulates in the tissues of interest. GBCAs must be processed and eliminated from the body efficiently to ensure patient safety.
The kidneys are responsible for filtering the contrast agent from the blood and passing it out of the body through urine. Most of the agent is cleared within 24 hours of the injection. The blood work ensures that the body’s waste removal system can handle this task effectively.
The Crucial Check: Assessing Kidney Function
The blood work ordered before an MRI with contrast is specifically designed to assess how well the kidneys are functioning. This test measures the level of a waste product called Creatinine in the blood. Creatinine is a byproduct of muscle metabolism normally filtered out of the blood by the kidneys.
A high level of Creatinine suggests inefficient waste removal. This value is used to calculate the Estimated Glomerular Filtration Rate (eGFR), a more accurate indicator of kidney performance. The eGFR estimates the volume of blood the kidneys filter per minute and is the definitive measure used to assess risk.
The medical rationale for this check is the risk of a severe complication called Nephrogenic Systemic Fibrosis (NSF). NSF is a rare but serious progressive condition that involves the hardening and thickening of the skin and sometimes internal organs. The risk of developing NSF is concentrated almost exclusively in patients who have severe kidney impairment, typically those with an eGFR below 30 mL/min/1.73m².
When kidney function is significantly impaired, the contrast agent remains in the body for a prolonged period. This extended retention increases the chance that the gadolinium may detach from its protective chemical cage, leading to the deposition of the toxic free metal in tissues. The pre-scan blood test confirms the patient is not experiencing acute or chronic kidney disease that would place them in this high-risk category.
Adjusting Imaging Plans Based on Results
The results of the kidney function blood test directly inform the clinical decision-making process. If the eGFR is within the normal range, the patient can proceed with the contrast-enhanced MRI using the contrast agent. No special precautions are required for patients with an eGFR above 60 mL/min/1.73m².
If the blood work reveals an eGFR that is too low, indicating impaired kidney function, the imaging team must adjust the plan. For patients with severely reduced function, the radiologist may decide to proceed with a non-contrast MRI, which avoids the use of the agent entirely, if the diagnostic information can still be obtained. In cases where contrast is deemed absolutely necessary for a diagnosis, the team will almost exclusively use a newer, highly stable type of contrast agent known as macrocyclic GBCAs, which have a much lower association with NSF.
The dose of the contrast agent may also be minimized to reduce the body’s exposure. In some situations, the scan may be delayed until a nephrologist, a kidney specialist, can be consulted to assess the patient’s individual risk and determine the safest course of action. The blood test serves as a preventative measure, allowing the medical team to tailor the imaging approach to the patient’s specific health status and minimize risks.