Magnetic Resonance Imaging (MRI) is a diagnostic tool that provides highly detailed pictures of structures inside the body without using ionizing radiation. The technology relies on a powerful magnetic field and radio waves to generate signals from the body’s water molecules, which a computer then translates into cross-sectional images. For the knee, an MRI is effective for visualizing soft tissues, including the meniscal cartilage, ligaments like the ACL and PCL, and the surrounding tendons and muscles. This capability allows healthcare providers to assess injuries, degenerative conditions, and various pathologies within the joint.
Pre-Scan Preparation and Safety Screening
The initial phase of a knee MRI focuses on patient safety and administrative readiness due to the extreme power of the magnetic field. Before entering the scanning room, a thorough screening process is conducted to identify any metal objects inside or outside the body. Patients must disclose all medical implants, including pacemakers, defibrillators, cochlear implants, certain aneurysm clips, or shrapnel, as these items can be affected by the magnet or cause serious injury.
To ensure no unintentional metal enters the magnetic field, patients are typically asked to change into a hospital gown, removing all personal clothing. This step eliminates the risk posed by hidden metallic components such as zippers, snaps, underwire bras, or metallic threads in athletic wear. All external metal items, including jewelry, watches, hearing aids, hairpins, and keys, must be left outside the controlled environment.
If the imaging requires a contrast agent, most commonly a Gadolinium-based formulation, additional screening is necessary. This substance is injected intravenously to enhance the visibility of specific tissues, such as areas of inflammation or blood vessels. Because the kidneys clear the contrast material, patients over 70, those with diabetes, or individuals with a history of kidney issues require a recent blood test to check kidney function before administration.
Inside the Machine: The Scanning Procedure
Once safety screening is complete, the physical procedure begins with the patient lying on a padded table, typically on their back. The knee being scanned is carefully positioned within a specialized plastic device called a radiofrequency coil or cradle. This coil is designed to fit around the knee and serves to transmit and receive the radio signals necessary to capture the highest possible image quality.
For a knee MRI, the table will slide into the bore of the machine feet-first, meaning the patient’s head and upper body often remain outside the main tube. During the scan, the machine generates loud, repetitive knocking, thumping, or whirring sounds. These noises are produced as electrical currents are rapidly switched on and off in the gradient coils to create the precise magnetic field variations needed for image acquisition. To mitigate the intense sound, patients are provided with earplugs or noise-canceling headphones.
Throughout the entire procedure, which usually lasts between 30 and 60 minutes, remaining completely motionless is necessary. Any movement can blur the images and necessitate repeating the sequence, extending the overall scan time. The technologist operates the machine from an adjacent room but maintains constant visual and verbal communication with the patient through an intercom system. If a contrast agent is part of the ordered study, it is administered at a specific point during the scan sequence, usually through a small intravenous line placed in the hand or arm.
What Happens After the MRI
Following the successful completion of the imaging sequences, the table slides out of the machine, and the technologist removes the knee coil. Unless a sedative was administered prior to the procedure, the patient is free to get off the table, change back into their clothes, and immediately resume all normal daily activities.
The digital images captured during the scan are then sent to a physician specializing in medical image interpretation, known as a radiologist. This specialist analyzes the hundreds of cross-sectional images, searching for specific signs of injury or disease in the ligaments, cartilage, and bone marrow. The radiologist then synthesizes their findings into a detailed report, which is transmitted to the ordering healthcare provider.
For non-urgent cases, the processing and interpretation of the images take between one and two weeks before the final report is ready. In situations requiring immediate attention, the radiologist can prioritize the study, and a preliminary report may be available within 24 to 48 hours. The ordering physician is responsible for reviewing the radiologist’s report, correlating the findings with the patient’s symptoms, and communicating the final results and next steps to the patient.