Dark Band Artifact
A dark band artifact in MRI can occur due to a number of reasons, but most commonly it is caused by magnetic field inhomogeneity. MRI scanners use strong magnetic fields to generate images, and any imperfections in the magnetic field can cause variations in signal intensity and result in artifacts. The dark band artifact appears as a straight or curved band of reduced signal intensity that runs parallel to the main magnetic field.
The most common cause of dark band artifacts is magnetic susceptibility differences between tissues or materials within the body. These differences can be caused by the presence of metal implants, air-filled cavities, or bone. Metal objects in particular can cause large magnetic field distortions and create dark band artifacts that extend beyond the area of the metal itself.
Other causes of dark band artifacts include gradient coil eddy currents, which can create magnetic field fluctuations, and radiofrequency (RF) interference, which can cause signal loss and produce dark bands in the image.
Here are some strategies to minimize or avoid dark band artifact :
Increasing the bandwidth: Increasing the bandwidth in MRI can potentially reduce dark band artifacts by reducing the effects of magnetic field inhomogeneity.
Reducing the voxel size: Reducing voxel size helps minimize geometric distortions in susceptibility artifacts by improving spatial resolution. However, it may also increase signal dropout in regions with strong magnetic field inhomogeneities due to higher sensitivity to local susceptibility differences.
Changing the imaging plane orientation: The orientation of the imaging plane relative to the main magnetic field can affect the appearance of the artifact. By changing the imaging plane orientation, the artifact may be moved to a less critical region of the image or eliminated altogether.
Increasing the echo train length (ETL): This can reduce the amount of signal loss due to T2 decay, which is the primary cause of dark band artifacts in FSE/TSE sequences.
Using Shorter TE values: Shorter TE helps by minimizing the time available for phase shifts to accumulate, thereby reducing the susceptibility artifact.
Using spin-echo sequences instead of gradient-echo sequences : Spin-echo sequences reduce susceptibility artifacts because the 180° refocusing pulse corrects dephasing, while gradient-echo sequences lack this correction, leading to more signal loss and distortion.
