Artifact may be defined as the false futures of images produced during image processing.
Any irregularity noted in an MR image which is related to the imaging process rather than to an anatomical or physiological abnormality.
Artifacts can cause significant image degradation and can lead to misinterpretation.
It is impossible to eliminate all artifacts though they can be reduced to acceptable level.
TYPES OF ARTIFACTS
- Ghost or Motion artifacts.
- Aliasing or wraparound artifacts.
- Chemical shift related artifacts.
- Truncation artifacts.
- Magnetic susceptibility artifacts.
- Zipper artifacts.
- Shading artifacts.
- Cross excitation artifact.
- Parallel imaging artifacts.
- Point artifact Slice overlap or Cross Talk artifacts
GHOST OR MOTION ARTIFACTS
Ghosts are replica of something in the image. Ghosts are produced by anatomy moving along a gradient during pulse sequence resulting into phase mismapping.
Ghosts can originate from any structure that moves during the acquisition of data. Periodic movements such as respiratory, cardiac and vessel pulsation causes coherent ghosts while non periodic movements causes a smearing of the image.
Axis : ghosts almost always seen along phase encoding axis.
Corrective measures :
2.ECG gating for cardaic motion.
3. Respiratory compensation.
4. Phase encoding axis wrap.
ALIASING OR WARAPOUND ARTIFACTS
In aliasing, anatomy that exists outside the FOV appear in an image.
Anatomy outside the selected FOV produces signal if it is in close proximity to the receiver coil. During signal encoding, signals from this outside FOV structures are also allocated pixel positions.
Axis: Aliasing can occur along any axis.
Corrective measures: Increasing FOV and using surface coils.
CHEMICAL SHIFT RELATED ARTIFACTS
It is a common finding on some MRI sequences and used in MRS.
This artifact occurs in the frequency and phase encoding direction and is due to spatial misregistration of fat and water molecules.
The difference in precessional frequencies of protons in water and fat is called chemical shift. It is expressed in ppm.
The frequency of water protons is about 3.5 ppm greater than that of fat protons.
The chemical shift of 3.5 ppm causes water protons to precess at a frequency 220 Hz higher than that of fat proton at 1.5 tesla.
There are 2 types of artifacts related to chemical shift.
1. Chemical shift misregistration :
Axis: along frequency encoding axis.
Corrective measures: fat suppression and increase bandwidth.
2. Interference from chemical shift :
Axis: along phase encoding axis.
Corrective measures: using spin echo sequences and selecting proper TE.
TRUNCATION OR EDGE OR GIBBS OR RINGING ARTIFACTS
Truncation artifacts produce low intensity band running through high intensity area.
These are dark and bright lines that are parallel to adjacent boarders of abrupt intensity change as many seen at spinal cord, CSF, fat and Muscle.
these artefacts commonly seen in phase encoding direction. these are corrected by
- increasing matrix
- change the direction of phase and frequency encoding direction
- using filters.
MAGNETIC SUSCEPTIBILITY ARTIFACTS
Magnetic susceptibility is the ability of a substance to become magnetized.
Some tissues magnetize to different degree than other, resulting into differences in processional frequency and phase.
This causes dephasing at the interface of these tissues and signal loss.
EX- 1. Metal and iron content of hemorrhage.
2.Bindi(small sticker) routinely applied on forehead by indian women.
Axis – frequency and phase encoding gradients.
Corrective measures – 1.Use of SE sequences and Remove all metals.
2.In cases of metallic hardware, susceptibility artifacts can be reduced to certain extent by increasing bandwidth, reduction of TE using thin slices and higher matrix.
3.Use of parallel imaging and use of sequences with radial K space sampling like BLADE/PROPELLER.
1.Used to diagnose hemorrhage, hemosiderin deposits and calcification.
2.Forms the basis of post contrast T2* weighted MR perfusion studies.
3.Used to quantify myocardial and liver iron over load.
This artifacts caused by external RF entering at certain frequency and interfering with inherently weak signal coming from the patient.
These artifacts controlled easily by close the doors during image acquisition that prevents entering of RF from radio stations.
Axis : along frequency encoding axis.
Image has uneven contrast with loss of signal intensity in one part of the image.
1.RF pulses applied at flip angles other than 90 and 180 degree.
2.Abnormal loading of coil.
3.Inhomogenity of magnetic field,
4.Overflow of analog to digital converter.
Axis – Frequency and phase encoding
Corrective measures –
1. Load the coil correctly
3. To avoid ADC overflow.
CROSS EXCITATION ARTIFACTS
The loss of signal within a slice due to pre-excitation from RF pulse meant for an adjacent slice is called cross excitation artifact.
The frequency profile of the RF pulse is imperfect; this means that during slice selection there is some degree of excitation of the adjacent slices as well.
If that adjacent slice is imaged during the same TR in multi-slice imaging and this is commonly seen in Inverse recovery sequences.
CROSS TALK OR SLICE OVERLAP ARTIFACTS
The loss of signal seen in an image from a multi-angle, multi-slice acquisition, as is obtained commonly in the lumbar spine.
It should not be confused with cross excitation which although similar in causation, is not due to angled images.
If the slices obtained at different disk spaces are not parallel, they may overlap. If two levels are acquired at the same time, E.g. L4-L5 and L5-S1.
Axis – Slice selection gradient
Cross-talk cannot be corrected.
To minimise the cross-excitation:
1. Increase interslice gap.
2. Interleaved slices scanning. First slice numbers 1, 3, 5, 7 are excited and then slices 2, 4, 6, 8 are excited.
So that nuclei have time to relax.
PARELLEL IMAGING ARTIFACTS
¢Graininess in the center of the image can be caused by high acceleration factor used in parallel imaging or using small FOV with parallel imaging.
This artifact can be reduced by using lower acceleration factor and increasing FOV.
¢It is seen as a bright spot of increased signal intensity in the center of image.
¢This is caused due to constant offset of DC voltage in the receiver coil which after fourier transformation appear as a bright spot in centre of the image.
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