Undergoing a knee MRI often raises questions about the logistics of the procedure, particularly how much of the body must enter the machine. Magnetic Resonance Imaging (MRI) is a non-invasive tool that uses strong magnetic fields and radio waves to create detailed images of soft tissues, such as the ligaments and cartilage in the knee. Understanding the physical setup and positioning can help alleviate concerns about the patient experience and the feeling of being confined.
Understanding MRI Machine Types
The extent to which a patient enters the machine depends on the specific design of the scanner being used. The most common type is the closed-bore system, which resembles a long, tube-like tunnel, typically with a diameter of about 60 centimeters. These machines feature high magnetic field strengths (1.5 to 3.0 Tesla), which allows them to produce high-resolution images quickly. While the confined nature of the closed-bore design can cause anxiety, wide-bore variants now offer a larger opening, usually around 70 centimeters, to improve comfort.
Alternatively, open MRI machines are designed with two flat magnets positioned above and below the patient, leaving the sides completely open. This configuration significantly reduces the feeling of being enclosed and is often preferred by individuals with claustrophobia or those with larger body types. However, this open design results in a much lower magnetic field strength, commonly around 0.3 Tesla, which can lead to lower image quality and may require longer scan times. A third option is the extremity MRI scanner, a smaller, dedicated machine specifically for limbs like the knee. This requires only the targeted body part to be placed inside a small opening while the patient sits comfortably next to it.
Positioning the Knee for Scanning
For a knee scan in a standard closed-bore MRI, the patient is almost always positioned on the sliding table feet-first while lying on their back. The goal is to align the knee precisely within the center of the machine’s magnetic field, known as the isocenter. This is where the magnetic field is most uniform and strongest for optimal image acquisition. This feet-first positioning means that the patient’s head and upper torso remain entirely outside the tube. Typically, only the lower body, from the feet up to the thigh, is within the bore of the machine.
The knee is placed within a specialized radiofrequency (RF) coil, often a hard plastic shell, which acts as a sensitive receiver for the radio signals generated during the scan. This dedicated knee coil is placed directly around the joint to maximize the signal-to-noise ratio and produce clear, focused images of the ligaments, tendons, and cartilage. Immobilization pads or cushions are used around the knee and ankle to ensure the joint remains perfectly still throughout the procedure. Even slight movement can distort the resulting images.
Managing Claustrophobia and Noise
The sensory experience inside an MRI machine involves dealing with the physical confines and the intense acoustic output generated during the procedure. The loud, repetitive knocking, tapping, and buzzing sounds are produced by the rapid switching of electrical currents in the gradient coils of the magnet. Depending on the magnetic field strength, the noise level can reach up to 130 decibels, which exceeds safe exposure limits without protection. Patients are always provided with hearing protection, such as foam earplugs or specialized MRI-safe headphones, which can attenuate the noise by up to 30 decibels or more.
For individuals who experience anxiety or claustrophobia, several strategies can be implemented to improve comfort. The technologist typically maintains constant communication with the patient via an intercom system. The patient is also given a squeeze-ball or panic button to signal for the scan to be stopped at any time. Some facilities offer mirror setups that allow the patient to see outside the bore, or they may provide music or video entertainment through the headphones as a distraction. In cases of severe anxiety, a physician may prescribe a mild sedative to be taken before the scan, though the patient must arrange for transportation home afterward.
Pre-Scan Preparation Checklist
Preparation before an MRI focuses on safety and ensuring no metal objects interfere with the powerful magnetic field. Patients must complete a thorough screening questionnaire to disclose any medical implants, such as pacemakers, aneurysm clips, or shrapnel, as these may be contraindicated due to the magnetic forces. All external metal must be removed, including jewelry, watches, hairpins, hearing aids, and credit cards. The technologist will ask the patient to change into a metal-free hospital gown to ensure no hidden metal compromises the scan quality or patient safety.
The technologist will emphasize the necessity of remaining still once the scan is underway. Even small movements can cause motion artifacts that blur the images, potentially requiring portions of the scan to be repeated. If a contrast agent, typically containing gadolinium, is required to enhance image detail, an intravenous line will be inserted before the patient enters the scanner. Patients with a history of kidney issues must inform the staff, as the body’s ability to clear the contrast agent may be affected.