What Is Susceptibility Artifacts On Mri

What are Susceptibility Artifacts on MRI? A Comprehensive Guide

Magnetic Resonance Imaging (MRI) is a powerful diagnostic tool providing detailed anatomical images of the body. However, the process is susceptible to various artifacts that can compromise image quality and diagnostic accuracy. One significant source of artifact is magnetic susceptibility, which refers to the ability of a material to become magnetized in an external magnetic field. This article delves into the intricacies of susceptibility artifacts in MRI, exploring their causes, manifestations, and strategies for mitigation.

Understanding Magnetic Susceptibility

Before delving into artifacts, it's crucial to grasp the concept of magnetic susceptibility. Different tissues and materials within the body possess varying degrees of susceptibility. Paramagnetic substances, like air and some metals, have a positive susceptibility, meaning they are attracted to the magnetic field. Diamagnetic substances, including water and fat, have a negative susceptibility, slightly repelling the magnetic field. This difference in susceptibility creates local magnetic field inhomogeneities within the imaged object, leading to signal distortions and the appearance of artifacts.

The Role of the Main Magnetic Field

The MRI scanner generates a powerful, uniform main magnetic field (B0). Ideally, this field remains consistent throughout the imaging volume. However, the presence of materials with differing susceptibilities disrupts this uniformity. The variations in magnetic field strength caused by these susceptibility differences result in phase shifts of the MRI signal. These phase shifts, if significant enough, translate into visible artifacts on the final image.

Types of Susceptibility Artifacts

Susceptibility artifacts manifest in various ways, depending on the location, type, and size of the susceptibility-inducing material. Some common types include:

1. Geometric Distortion

This is a frequently encountered artifact, characterized by a distortion of the shape and size of anatomical structures near the interface between tissues with differing susceptibility. For example, air-tissue interfaces, such as those found in the lungs, sinuses, and around the teeth, often exhibit significant geometric distortion. The image appears stretched or compressed near these boundaries, making accurate measurements difficult.

2. Signal Loss/Voiding

In areas where there's a large difference in susceptibility, such as at the interface between air and soft tissue, the signal can be significantly reduced or even completely lost. This results in a "void" or dark area on the image. This is particularly common near metallic implants or foreign bodies. The strong magnetic field gradients created at these interfaces cause rapid dephasing of the protons, leading to signal dropout.

3. Magnetic Susceptibility Induced Shift (MSIS)

This artifact appears as a shift or displacement of anatomical structures. It's often seen near paramagnetic materials, and the magnitude of the shift depends on the strength of the magnetic field and the susceptibility difference between the materials. This artifact can lead to misinterpretations of the anatomical relationships between structures.

4. Blooming Artifacts

Blooming, or edge enhancement, is characterized by the apparent broadening of interfaces between tissues with different magnetic susceptibilities. The affected region seems to "bloom" outwards from the interface. This is often seen around the edges of structures such as blood vessels or metallic implants.

5. Zipper Artifacts

While not directly caused by susceptibility itself, zipper artifacts can be exacerbated by susceptibility differences. These vertical lines appear on the image and are often caused by RF interference or gradient coil malfunctions. However, the presence of susceptibility-inducing materials can worsen these artifacts, making them more prominent and difficult to ignore.

Sources of Susceptibility Artifacts

Various factors contribute to the generation of susceptibility artifacts:

• Air-Tissue Interfaces: The significant difference in susceptibility between air and soft tissues is a major source of artifacts, particularly in the lungs, sinuses, and around teeth. This is why images of these regions frequently show geometric distortion and signal voiding.

• Metallic Implants: Surgical implants, such as screws, plates, or stents, are highly paramagnetic and create substantial magnetic field inhomogeneities. These inhomogeneities result in severe artifacts, including signal loss, geometric distortion, and blooming.

• Dental Materials: Metallic dental fillings and braces also contribute to susceptibility artifacts in the maxillofacial region. The size and composition of these materials directly influence the severity of the artifacts.

• Calcifications: Calcified tissues, such as those found in atherosclerotic plaques, can also induce susceptibility artifacts, albeit generally less severe than metallic implants.

• Hemorrhage: The presence of blood products, particularly deoxyhemoglobin, alters the local magnetic susceptibility, potentially leading to artifacts, especially in the case of acute hemorrhages.

• Contrast Agents: Certain contrast agents used in MRI can affect susceptibility, although this is often less of a concern compared to the other sources mentioned above.

Minimizing Susceptibility Artifacts

Several strategies can be employed to reduce or mitigate susceptibility artifacts:

1. Image Acquisition Techniques

• Gradient Echo Sequences: These sequences are more sensitive to susceptibility artifacts than spin-echo sequences. Careful choice of imaging parameters, such as echo time (TE) and bandwidth, can help minimize their impact. Shorter TE sequences generally produce less susceptibility artifacts.

• Spin Echo Sequences: These sequences are less susceptible to artifacts compared to gradient echo sequences. They provide more robust signal in regions affected by susceptibility differences.

• Specialized Sequences: Some specialized sequences, designed to reduce susceptibility-related artifacts, are available on modern MRI scanners. These sequences employ various techniques to minimize the effects of magnetic field inhomogeneities.

• Higher Field Strength: Higher field strength magnets increase signal-to-noise ratio, improving image quality. However, they can also exacerbate susceptibility artifacts, requiring careful adjustment of imaging parameters.

2. Post-Processing Techniques

Various post-processing techniques can help to improve the appearance of images affected by susceptibility artifacts. These include:

• Image Filtering: Filtering techniques can help to reduce noise and smooth out some artifacts. However, these techniques can also blur fine details, potentially compromising diagnostic information.

• Artifact Correction Algorithms: Specialized software algorithms are being developed to automatically detect and correct susceptibility artifacts. These algorithms utilize advanced signal processing techniques to compensate for the effects of magnetic field inhomogeneities.

3. Patient Positioning and Preparation

• Careful Patient Positioning: Avoiding the placement of susceptibility-inducing materials close to the region of interest during the scan can reduce the impact of artifacts.

• Metal Removal: If possible, removing metal objects from the vicinity of the imaging area can minimize artifact generation.

Impact on Diagnosis

The presence of susceptibility artifacts can significantly impact diagnostic accuracy. They can obscure important anatomical details, leading to misinterpretations of the images. This is particularly concerning in areas where subtle findings are crucial for diagnosis, such as in neuroimaging. Radiologists need to be aware of the appearance of these artifacts and understand their potential impact on image interpretation.

Conclusion

Susceptibility artifacts are an inherent limitation of MRI, stemming from the interaction of the magnetic field with materials of varying magnetic susceptibility. Understanding the causes, manifestations, and mitigation strategies of these artifacts is essential for radiologists and MRI technologists. The use of appropriate imaging techniques, coupled with careful patient preparation and post-processing methods, can minimize their impact and enhance the diagnostic value of MRI scans. Continuous advancements in MRI technology and software are continually being developed to further reduce the effects of these artifacts, improving the quality and reliability of MRI scans. Further research in this field is vital for improving the accuracy and efficiency of MRI diagnostics. The interplay between hardware, software, and careful image acquisition protocols is fundamental for overcoming this fundamental challenge in MRI.