Artefacts in MRI

Artefacts in MRI From http://members.lycos.nl/mri/Nieuw/ artefacteneng.htm Artefacts in NMR are useless pixels in a picture wich 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 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  ... 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... 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. .. 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