Aliasing or wrap around artifacts


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Aliasing or wrap around artifacts

  1. 1. WRAP AROUND OR ALIASING ARTEFACT<br />Wrap or aliasing produces an image where anatomy that exists outside the FOV is folded on to the top of the anatomy inside the FOV.<br />
  2. 2. CAUSE<br />Aliasing is cause when anatomy that exists outside the FOV is mismapped inside the FOV. <br />Anatomy outside the FOV still produces a signal if it is in close proximity to the receiver coil. <br />
  3. 3. CAUSE<br />Data from this signals must be encoded, i.e. allocated a pixel position. If the data are under sampled, the signal is mismapped into pixels within the FOV rather than outside.<br />This can occur in both frequency encoding and phase encoding axis<br />
  4. 4. FREQUENCY WRAP<br /><ul><li>Aliasing along the frequency encoding axis is known as frequency wrap.
  5. 5. Caused by under sampling the frequencies that are present in the echo regardless from where it originates.
  6. 6. If the Nyquisttheorm is not obeyed and frequencies are not sampled enough, the signal from the anatomy outside the FOV in the frequency encoding direction is mismapped into the FOV.</li></li></ul><li>PHASE WRAP<br /><ul><li>Aliasing along the phase axis of the image is known as phase wrap
  7. 7. Caused by under sampling along the phase axis
  8. 8. When a phase encoding is done it changes the phase from 0 to 3600 inside the FOV along the phase encoding direction
  9. 9. The same phase curve is repeated outside the FOV.</li></li></ul><li>PHASE WRAP<br />Since the signal coming from outside the FOV is having the same phase value which as inside, it is allocated a space were signal from inside the FOV is kept.<br />
  10. 10. REDUCTION IN GENERAL<br /><ul><li>Enlarging the FOV so that all the anatomy creating signal is within the FOV eliminates this artefact
  11. 11. Applying saturation bands will also null signal from this area.</li></li></ul><li>FREQUENCY WRAP REDUCTION<br /><ul><li>Increasing the sampling frequency so that all the frequencies are digitised sufficiently will eliminate the artifact
  12. 12. The above method increase the noise.
  13. 13. In order to avoid that a frequency filter is used to filter out frequencies that occur outside the selected FOV.
  14. 14. Most of the systems apply this automatically.</li></li></ul><li>PHASE WRAP REDUCTION<br /><ul><li>This is termed as no phase wrap or anti-fold over.
  15. 15. It increases the number of phase encoding performed (achieved by enlarging the FOV)
  16. 16. Increasing the FOV will avoid duplication of phase values so that each and every point will have a different phase than another.</li></li></ul><li>PHASE WRAP REDUCTION<br />THE ABOVE METHOD DECREASES THE SPATIAL RESOLUTION(phase encoding is increased to compensate this)<br />INCREASING THE PHASE ENCODING INCREASES SCAN TIME (system automatically reduce the NEX in order to compensate for this)<br />REDUCING NEX WILL INCREASE MOTION ARTEFACT<br />