Artefacts in MRI

Static artefacs are generated by " chemical shift" differences see chemical shift and, or different susceptibilities.. Chemical shift artefactChemical shift artefact: In spine, abdomen a

Artefacts in MRI

From http://members.lycos.nl/mri/Nieuw/

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 http://www.hull.ac.uk/mri/lectures/Gpl%20web%20page/ 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