Artifact:
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.

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.

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.

GOOD EFFECTS:

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.

Causes are:

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

2. Shimming     

3. To avoid ADC overflow.

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.

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

Corrective Measures

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.

¢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.

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