Magnetic resonance imaging (MRI) is a powerful diagnostic tool that generates detailed images of the body’s soft tissues, such as organs, muscles, and ligaments. However, the presence of metal implants, like joint replacements or surgical hardware, significantly disrupts the magnetic field necessary for image creation. Metal suppression MRI techniques were developed to overcome this limitation, allowing doctors to obtain clear images of the tissue immediately surrounding metal objects. These specialized methods alter the way the MRI scanner collects and processes data, effectively reducing the severe image distortions that would otherwise obscure the diagnosis.
Understanding Metal Artifacts in Standard MRI
The interference caused by metal in a standard MRI scan stems from a phenomenon called magnetic susceptibility. Metal objects, even non-magnetic ones like titanium, create a large difference in magnetic properties between the implant and the surrounding tissue. This disparity severely distorts the local magnetic field, which is supposed to be uniform across the scanned area.
The disruption manifests in the image as two primary types of artifacts: signal voids and geometric distortion. Signal voids appear as large, dark areas where no signal can be detected because the magnetic field is too inhomogeneous for the protons to properly generate a readable signal. Geometric distortion causes the surrounding tissue to appear warped or stretched, and a bright “signal pile-up” can occur at the edges of the dark void. Artifacts are often more pronounced on 3.0 Tesla scanners than on 1.5 Tesla machines because the severity of this effect increases with the strength of the main magnetic field.
Specialized Techniques for Metal Suppression
Metal suppression techniques address artifacts by modifying the standard MRI pulse sequence. A fundamental strategy involves using a fast spin-echo (FSE) sequence with short echo spacing. This sequence uses rapid 180° refocusing pulses to counteract the dephasing effect of the inhomogeneous magnetic field near the metal. Additionally, the scanner uses a high receiver bandwidth, which reduces the spatial displacement of signals by collecting data over a broader range of frequencies.
In-Plane Correction
To correct for the geometric warping that remains, advanced methods like View Angle Tilting (VAT) are employed to specifically address distortions that occur within the image plane (in-plane artifacts). VAT works by applying an additional magnetic gradient during the readout phase that compensates for the pixel shifts caused by the metal-induced field distortion. This technique is effective for in-plane correction, but it can introduce a slight blurring effect.
Through-Plane Correction
For the more challenging distortions that occur perpendicular to the image plane (through-plane artifacts), two primary three-dimensional techniques are used: Slice Encoding for Metal Artifact Correction (SEMAC) and Multi-Acquisition Variable-Resonance Image Combination (MAVRIC). SEMAC adds extra phase-encoding steps in the slice-selection direction, allowing the scanner to more accurately map the signal back to its correct location. MAVRIC uses multiple narrow band frequency-selective excitations to acquire data from different frequency ranges near the metal. These advanced approaches prioritize reducing the size of the signal void and recovering the image signal immediately adjacent to the metal implant.
Clinical Uses of Artifact Reduction Methods
Metal suppression MRI has expanded the diagnostic utility of the technology for patients with metallic implants. These specialized sequences are routinely used to assess the soft tissues surrounding large orthopedic hardware, such as total hip and knee replacements. Doctors can now accurately look for complications like periprosthetic joint infection, characterized by fluid collections and inflammation, or mechanical loosening of the implant.
For patients with spinal hardware, metal suppression techniques are indispensable for evaluating the spinal cord and nerve roots adjacent to pedicle screws or rods. This imaging allows clinicians to check for nerve compression, scar tissue formation, or abscesses, which would be completely obscured by artifacts on a standard MRI. The ability to visualize the anatomy near the metal also extends to other implants, including evaluating soft tissue damage around dental implants, plates, or surgical clips.
Preparing for a Metal Suppression MRI
Preparing for a metal suppression MRI involves communicating detailed information about any metallic hardware to the healthcare team well in advance of the scan. Providing specifics about the exact type of implant, the manufacturer, and the serial number allows the radiologist to select the most effective artifact-reducing sequence and ensure patient safety. The precise nature of the metal influences the severity of the artifacts.
Patients should be aware that these highly specialized scans often require a longer acquisition time than a standard MRI examination. The advanced techniques, particularly those like SEMAC and MAVRIC, can extend the scan duration beyond the typical 30-to-60-minute window. Remaining completely still during this extended period is important for image quality, and patients are typically positioned comfortably with earplugs to minimize noise from the scanner.