Artefacts in MRI

Artefacts in MRI
From />artefacteneng.htm
Artefacts in NMR are useless pixels in a picture wich do not contribute
to the picture itself. It is caused by the patient or external factors.
Patient-related artefacts are static (constant during measurement) or
dynamic. Static artefacs are generated by  " chemical shift" differences
(see chemical shift) and, or different susceptibilities. Dynamic artefacts
are caused by breathing, heartbeat, bloodflow or any other movement
made by the patient. External artefacts are hardware related or caused
by external factors

And more from />gpl_page#RF


Chemical shift artefact
Chemical shift artefact: In spine, abdomen and eyes were tissue is
surrounded with fat and after Gadolineum (because fat precesses almost at
the same frequency as pathology tissue). In the frequentie-encoding direction
the spatial resolution of the signal is determined by the frequency of this
signal. In the same organ water is slighty different in frequency from fat (3.5
ppm) An MR-scanner will mismap this frequency difference. Fat is being
shifted in the frequency encoding direction. In other words: Pixels are placed
in the wrong voxel) It is mainly at high field strenghts chemical shift plays an
important role. Fatsuppression (STIR/prepuls) will eliminate the artefact.
Shimming the magnetic field is essential.
Chemical shift contours is seen in Gradient echo sequence only. At 1.0 T fat
and water spins are in and out of phase every 3,4 ms after the alpha pulse.
So TE is 3,6 the sequence is IN phase, TE is 7,2, the sequence is OUT of
phase. 

Susceptibility artefact:
Susceptibility artefact: Caused
by microscopic differences in
susceptibility for the magnetic
field. Bone/air or metal/tissue
combinations give this artefact.
Defasing of spins and frequency
shift of surrounding tissue  is the
result. You can recognize the
artefact by the dark and white
disturbances of the tissue The
more defasing (long TE) the more
susceptibility. Gradient echo´s do
not use a 180° refasing pulse, so
there´s a lot of defasing there.
Here´s a gradient echo picture of
the knee with a screw in the tibia


Aliasing/Wrap around

Aliasing/Wrap around: If the FOV is smaller than the object The
anatomy outside the FOV will be misplaced inside the picture
because it gives a signal in the receiver coil. These data will be
encoded but will be"undersampled" and misplace IN the FOV.
"Oversampling" is this problems solution. In the frequency encoding
direction  the signal is sampled twice as fast.  In the fase encoding
direction more fase encodings will be generated. (scantime is rising).
Changing the frequency and fase encoding direction is sometimes a
solution too.

Flow artefacs
Flow artefacs: Caused by blood/liqor. Important are: Flow direction,
flowspeed and the sequence used. One can eliminate these artefacs
by using  prepulses to saturate these moving protons.