MRI Artifacts

By,Dr. Shravani

Introduction :Artifacts are parts of reconstructed images that are not present in the true anatomy.• Artifacts are dependent on a variety of factors from patientmovement to magnetic field in homogeneities.• Artifacts can lead to misdiagnosis if they are not recognized and/or removed.• Ideally, we want all image artifacts to be below the level of user's perception.

•Hardware Issues e.g. calibration, power stability

•Software problems e.g. programming errors

•Physiological phenomena e.g. blood flow

•Physics limitations e.g. Gibbs and susceptibility

Sources :

•Chemical Shift Artifact•Aliasing•Truncation Artifact•Zipper Artifact•Motion Artifacts (Phase direction)•Field in homogeneity•Slice-overlap Artifact•RF Overflow Artifact•Eddy Current Artifacts

Types of Artifacts

•Artifact produced when anatomy that exists outside the field of vision is mapped inside the field of vision. •Anatomy outside selected FOV, still produces a signal if it is in close proximity to the receiver coil.•Data from the signal must be encoded , i.e. it must be allocated a pixel position.•If the data is under sampled, the signal is mismapped into pixels within the FOV rather than outside.•Also called WRAP AROUND, as the anatomy outside the FOV is folded into the selected FOV.•Occurs along both the frequency and phase axis.

Aliasing (or) Wrap around

Aliasing (or)Wrap-around

Correction:

•Increase the FOV (decreases resolution).•Oversampling the data in the frequency direction (standard) and increasing phase steps in the phase-encoded direction – phase compensation (time or SNR penalty).

•Swapping phase and frequency direction so phase is in the narrower direction.

•Use surface coil so no signal detected outside of FOV.

•Protons in fat and water inherently precess at different frequencies in an applied magnetic field, and the separation between their resonance frequencies increases with increasing field strength.

•Since the resonance frequencies of fat and water are used to encode their spatial locations, the chemical shift differences lead to spatial misregistration of the MR signal.

•Chemical shift artifacts on clinical MR images manifest most prominently at fat-water interfaces such as those in the bladder and the periorbital regions .

Chemical Misregistration Artifact

Remedy :

•By Scanning at lower field strengths and by keeping FOV to a minimum.•Using widest receive bandwidth in keeping with good SNR and the smallest FOV possible.

Coronal T1 weighted gradient echo images of the posterior abdomen acquired on 1.5 T system.Left acquired with TE of 2.8msRight acquired with TE of 4.2msArrow shows Chemical misregistration artifact.

Chemical shift artifact resulting from the presence of subcutaneous scalp fat.(a) Axial T1-weighted image of the brain shows the propagation of chemical shift artifact(arrows) along the frequency encoding axis, in the anteroposterior direction. (b) Axial T1- weighted image obtained with use of a higher bandwidth sampling rate and with acquisition time increased to allow a relatively constant SNR shows reduction of the chemical shift artifact.

•Axial T2 weighted images of the abdomen •Using a receive bandwidth of 32000 Hz (above)•Using a receive bandwidth of 8000 Hz (below)•The arrow shows the chemical shift artifact at the border of the left kidney

Truncation Artifact•Results from under sampling of data.

•Interfaces of high and low signals are incorrectly represented on the Image.

•Common site for this kind of artifact is T1 sagittal imaging of Cervical spine.

•Due to Low signal of CSF and high signal from Spinal cord. (Gibbs artifact)

•Occurs only in Phase direction only.

•Produces a low intensity band running through a high intensity area.

Remedy•Under sampling of data must be avoided.•Increase the number of phase encoding steps•Using 256 X 256 matrix instead of 256 x 128.

Magnetic Susceptibility Artifact

•Ability of a substance to be magnetized.

•Some tissues magnetize to different degrees than others, resulting in difference in precessional frequency and phase, Causes dephasing at the interface of these tissues and a signal loss.

•Due to Metal and Iron content of hemorrhage as these magnetize to much greater than surrounding tissues.

•More prominent in gradient echo sequences as the gradient reversal cannot compensate for the phase difference at the interface.

Sagittal gradient echo sequence of the knee.Magnetic susceptibility artifact from screws degrades the image.

Zipper Artifact•Appears as a dense line on the image at a specific point.

•Caused by extraneous RF entering the room at a certain frequency and interfering with the inherently weak signal coming from the patient.

•Caused by a leak in the RF shielding of the room.

Remedy

•To locate the leak and repair it.

Zipper Artifacts

Shading Artifact•Shading is an artifact which produces a loss of signal intensity in one part of image.•Due to uneven excitation of nuclei with in the patient due to RF pulses applied at flip angles other than 90 and 180 degrees.•Caused by abnormal loading on the coil or by coupling of the coil at one point.•Occurs with large patient, who touches one side of the body coil and couples it at that point.

Remedy•Ensure coil is loaded correctly.•Patient not touching the coil at any point.

Ghosting and SmearingGhosting and smearing, common artifacts produced by voluntary or involuntary motion of the patient, have long plagued clinical MR imaging.

motion-related artifacts may result from •esophageal contraction and vascular pulsation during head and neck imaging.• respiration and cardiac activity during thoracic.•from bowel peristalsis during abdominal and pelvic imaging.

Axial T1-weighted MR imageswithout (a) and with (b) a significant ghosting artifact due to patient motion.Motion-related artifacts typically are propagated in the phase encoding direction—in b, along the horizontal axis.

Comparison of axial T2-weighted spin-echo (a) and fast spin echo (b) MR images of the brain at the level of the pons shows higher spatial resolution in b, which was obtained with an impressive reduction in acquisition time (2 minutes 22seconds, compared with 12 minutes48 seconds for a). The higher signal intensity of per orbital fat in b is an effect of J coupling and other factors.

Axial images of the Chest Without ECG gating (above)With ECG gating (below)Anatomical detail of heart is demonstrated clearly on gated image

Axial T1 weighted images of the abdomenWith out respiratory compensation (above)With respiratory compensation (below)The ghosting arrow has been reduced on the lower image.

Slice-overlap (cross-slice) Artifacts

• Loss of signal seen in an image from a multi-angle, multi-slice acquisition.

• Example: Two groups of non-parallel slices in the same sequence, e.g., L4-5 and L5-S1. The level acquired second will include spins that have already been saturated.

Slice-overlap (cross-slice) Artifacts

Slice-overlap Artifacts

Slice-overlap Artifacts

• Correction:• Avoid steep change in angle between slice

groups.• Use separate acquisitions.• Use small flip angle, i.e. GE sequence

Partial Volume Effect

• Partial volume occurs if slice thickness > thickness of tissue of interest• If small structure is entirely contained within the slice thickness along with other tissue of differing signal intensities then the resulting signal displayed on the image is a combination of these two intensities. This reduces contrast ofthe small structure.• If the slice is the same thickness or thinner than the small structure, only that structures signal intensity is displayed on the image.• Typically would use 3mm slices for cranial nerves and 5-10mm slices for liver.

Metallic Artifacts

• Similar to susceptibility artifacts.

• Metals have much higher susceptibility than tissue.

• Large Bo in homogeneities around object causing signal loss and distortion.

• Implants absorb RF energy, so local field varies.

• RF problems affect SE sequences as well as GE.

Examples of artifacts due to presence of stainless steel screws in healthy 37-year-old man.A and B, In gradient-echo image with ± 62.5 kHz receive bandwidth (A) and spin-echo image with ± 16 kHz receive bandwidth (B), solid arrows show signal loss that can be due to dephasing or from signal being shifted away from region. Dotted arrow in B shows geometric distortion of femoral condoyle, and dashed arrows show signal pile-up, which can be combination of in-plane and through-slice displacement of signal from multiple locations to one location.

Common Artifacts in MRI Due to Presence of Metallic Implants

Cross-talk Artifact

• Result of imperfect slice excitation, i.e. non-rectangular, of adjacent slices causing reduction in signal over entire image.

• May be reduced by using gap, interleaving slices and optimized (but longer) rf pulses.

Cross-talk Artifact

Field inhomogeneity

• Types:• Main magnetic field• RF coil inhomogeneity• Dielectric effects – worst at 3T+

• May cause variation in intensity across image• May cause non-uniform fat suppression

Field inhomogeneity – Bo

Field inhomogeneity- Dielectric

Field inhomogeneity

• Coil – Use volume vs. surface coil, allow space between coil and body.

• Dielectric – use phased array coils, software compensation

RF Overflow Artifacts (Clipping)

• Causes a nonuniform, washed-out appearance to an image.

• Occurs when the signal received from the amplifier exceeds the dynamic range the analog-to-digital converter causing clipping.

• Autoprescanning usually adjusts the receiver gain to prevent this from occurring.

RF Overflow Artifacts

Eddy Current Artifacts

• Varying magnetic field from gradients can induce electrical currents in conductors such as the cryostat causing distortion of the gradient waveforms.

• Particularly a problem with echo-planar imaging that uses strong, rapidly changing gradients.

Eddy Current Artifacts

Image courtesy of http://www.mr-tip.com/

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