MRI ARTIFACTS

MRI artifacts are present as positive or negative intensities that donot represent the imaged anatomy, which can limit the diagnostic potential.Knowledge about impact of acquisition protocols, etiology of artifactproduction will enhance the goal of achieving good diagnostic images.Artifacts may be machine dependent, patient related, and signal processing. It can be classified as (i) susceptibility artifacts, (ii) gradient field artifacts, (iii) motion artifacts, (iv) chemical shift artifacts, (v) wrap around artifacts, (vi) RF artifacts, (vii) K-space errors, (viii) ringing artifacts, and (ix) partial volume artifacts.

Susceptibility Artifacts

Susceptibility artifacts are due to change in magnetic susceptibility of the tissues, which will distort the magnetic field, cause signal loss, due to rapid de-phasing (T2*) at the tissue-air interface, e.g. lungs, and sinuses. It is helpful in diagnosis, e.g. age of hemorrhage, since hemoglobin contain iron, which is ferromagnetic. This artifact is more in GRE than Spin echo sequence.

Gradient Field Artifacts

Gradient field artifacts arise from gradient field nonuniformity or gradientfailure. The field strength of the periphery is less than the ideal, which cause artificial compression of the anatomy, resulting in barrel distortion of image.

Motion Artifacts

Motion artifacts are due to long scan time, cyclic heart motion or breathing,and blood flow. Motion artifacts cause blurring, reduce contrast, produce ghost images and appear in the phase encoding (slow event) direction. Motion artifact suppression technique such as cardiac and respiratory

gating is used.

Chemical Shift Artifacts

Chemical shift refers changes in resonance frequency and appear as displacement in the frequency encoding direction. Stronger themagnetic field, higher the chemical shift. In the case of water andfat, water is placed few pixels higher in thegradient, e.g. fat around optic nerve, vertebral bodies.

Wrap Around Artifacts

Wraparound artifacts are mismapping of anatomy that lies outside of the FOV but within the slice volume. The RF coil pick up signal from outside FOV and allot space in the matrix. Anatomy is displaced to the opposite side of the image, in the phase encoding direction. The causes are nonlinear gradients or under sampling of the frequencies in the returned signal. The remedial measures includes, circuits to

suppress aliasing in FEG (low pass filter) direction, doubling the number of phase encoding steps, doubling the FOV in PEG, reducing the number of excitations, moving the ROI to the center or displaying only central half of the image and use of surface coil that matches FOV.

RF Artifacts

Radiofrequency artifacts arise from surface coil’s variation in uniformity due to RF attenuation, mismatching, sensitivity fall off with distance,

resulting in shading and loss of image brightness. Stray RF signals (TV, radio, motor, fluoro light, computer) reaching the MRI antenna can also create artifacts. The type of RF artifacts includes (i) center point artifact: due to RF amplifiers (two) imbalance, resulting in a bright spot in the centre of

the image, (ii) zipper artifacts: due to narrow band RF noise, and (iii) herringbone artifacts: due to broad band noise (diffused and contrast reduced image). Non-rectangular RF pulses received from adjacent slices excite and saturate protons in adjacent slices. It degrades the SNR on T2 weighted image and reduces image contrast on T1 weighted image. Hence, slice interleaving technique is used.

K-space Errors

Error in K space encoding affect the reconstructed image, auses artificial superimposition of wave patterns across the FOV. This is called K space error artifact and the remedial measure is identification of bad pixels, and averaging the signals in adjacent pixels.

Ringing Artifacts

Truncation or ringing artifacts appear as multiple, regularly spaced parallel bands of alternating bright and dark signal that slowly fades with distance. It is described as a diminishing hyper and hypointensenear sharp boundaries and high contrast interfaces, e.g. skull/brain interface, fat and muscle. This is caused by insufficient sampling of high frequencies and likely for smaller matrix sizes, in PEG direction. The remedy is increased matrix size or reduced FOV.

Partial Volume Artifacts

Partial volume artifact arises from finite size of the voxel over the averaged signal and it results in loss of detail and spatial resolution. Use of smaller pixel size or smaller slice thickness will reduce partial volume artifact. However, it will reduce SNR, which can be maintained with longer imaging time.

signal oscillation from transition, known as Gibbs phenomenon. It occurs