MR Artifacts

1. MR Artifacts Susceptibility Gradient Field RF K-Space Motion Chemical Shift Gibbs (Ringing, Truncation) Artifacts Aliasing (Wraparound) Partial Volume High Speed Imaging Effect of Field Strength

2. Spike (Herringbone) • Bad data point/noise spike in k-space • Either very high / low intensity compared w/ rest of image • Spike is convolved with all other image info during FT • Since each image pixel is a weighted sum of all individual points in k-space • Results in dark stripes overlaid on image • Occurs with high duty cycle gradients sequences • Loose connection/breakdown of connections in RF coil

3. Uses • Can be used in cardiac imaging • Prep. Pulses applied before imaging sequence • Forms echoes in different parts of k-space • FT produces tags in grid-like pattern • Tags applied at start of each cardiac phase • Images acquired at multiple phases of cardiac cycle • Follow changes in tag position during cycle assess cardiac motion

4. Spike Artifact Image space K-space

5. Herringbone Artifact

6. Zipper Artifact • Caused by leakage of e-m energy into magnet room • Results in region of increased noise • Width of 1-2 pixels extends in frequency encode direction • Through entire series • Room shielded from outside e-m signals • Signals from equipment brought into room • OR RF shield compromised

7. Zipper

8. Motion-related • Patient Motion, either: • Voluntary • non periodic • Eye movement • Swallowing •  Smearing of image • Involuntary • Periodic • Respiratory • Cardiac • Pulsatile movement of vessels & CSF • Bowel motion •  Coherent ghosts formed • Blurring/ghosting in phase-encode direction • Time difference in adjacent points in PE direction relatively long • = TR • Introduces phase difference between adjacent k-space lines

9. Phase Mis-mapping • PEG has different amplitude every TR, unlike FEG/SSG • As anatomy moves, misplaced in PE direction as PEG changes • Anatomy given different phase values depending on its position along gradient • Time delay between PE & readout • anatomy may have moved between PEG & FEG when placed into k space

10. Swap PE & FE Directions • Artifact occurs only in PE direction • Change axis/direction • Pick axis Produce least interference w/ ROI • Example: Sagittal C Spine • Usually FE performed in z-axis (head to foot) • Longest axis • PE would then be AP (Y axis) • However: Artifacts in AP direction • Swallowing • Carotids pulsatile motion • Ghosting over spinal cord • Swap PE & FE axes • Y gradient (AP) performs FE • Z gradient performs PE • Artifacts now harmless in head to foot direction

11. Use Pre-Sat Pulses • Placing pre-sat volumes over areas producing artifacts • Nullifies signal • Reduces artifact • Example: Sagittal C Spine • Pre sat pulse over throat • reduces swallowing artifact • Reduces artifact from flowing nuclei in blood vessels

12. Respiratory MotionREMEDIES • Breath hold • Patient cooperation req’d • May take multiple breath holds • Respiratory gating • Image acquisition only at certain phases in resp. cycle • Acquisition time ↑ • Respiratory compensation/phase reordering • ROPE (Resp. Ordered Phase Encoding) • PE steps ordered on basis phase in resp. cycle • Difficult if resp. not regular • Real-time navigator echo gating • Echo from diaphragm determines its position • Navigator echo interleaved with actual imaging sequence • Real-time monitoring • data only acquired during specific range of diaphragmatic motion

13. Patient Motion Without breath-holding With breath-holding; With Cardiac pulsation artifacts

14. Resp. Compensation & K Space

15. Respiratory Motion Compensation Without compensation With compensation

16. Navigator Placement Graph shows diaphragmatic movement (white wave & green line). Aqua overlay shows navigator section from which displacement info obtained Yellow boxes: best time to image

17. Cardiac Pulsation • ECG gating • Time acquisition to occur @ same phase of each cardiac cycle • Coordinate excitation pulse w/ R wave of systole • Further artifact suppression if breath hold

18. Segmented K-Space • Echo-planar imaging • Single RF pulse/excitation • Continuous reversal of echoes using gradient pulses • Acquire all lines in k-space to form a single image • Alternate lines in k-space read in opposite direction • Prior to FT lines must be reversed • Introduces phase errors in alternate k-space lines • Errors from: • Nonlinear gradient reversal • Eddy currents • Poor shimming • Results in image ghosts

19. EPI

20. SS- vs. MS EPI Single Shot EPI Multi-Shot EPI # ghost images ↑ As # of discontinuities in k-space ↑ Errors  # echoes / shot & # of segments in k-space Minimize artifact May be necessary to obtain additional navigator echo • Single additional ghost image • Reduced intensity • Shifted by ½ FOV • ½ k-space lines are different from other half • “N/2 Ghost” • Reduce artifact • Minimize phase errors

21. Fast Spin Echo • Segmented k-space artifacts can also occur • Minor timing errors in sequence • Between multiple RF pulses • Between data collection windows • Eddy currents • Call Service Support

22. Ghost Artifacts Single Shot EPI N/2 ghost multi Shot EPI Initial Image FSE

23. Flow Artifacts • Flowing blood source of artifacts • Ghosting in phase-encoding direction • SE sequences not as susceptible • Flow appears dark (no signal) • Blood exposed to 90° excitation pulse flows out of imaging section before 180° refocusing pulse • Blood that moved into imaging section never exposed to 90° pulse • GRE Sequences susceptible • In-flow effect  bright blood

24. Reducing Flow Artifacts Pre-saturation Pulse • Attenuate signals upstream of imaging volume • Reduces intensity of fluid flowing into FOV • Apply saturation band adjacent to imaging section • 90° pulse • All spins tilted towards axial plane • Spoiled with gradient crusher pulses before image acquisition • Saturated spins exhibit no signal when moving into imaging volume

25. Reducing Flow Artifacts Cardiac Pulsation Flow-Related Artifact Suppression Cardiac pulsation artifact After saturation band applied

26. Reducing Flow Artifacts Flow Compensation/Gradient Moment Nulling • Flowing spins not in phase with static spins when echo forms • Flowing spins brought back into phase by motion-compensating gradient pulses • No effect on static spins • Penalty is increased echo time

27. Gradient Moment Nulling

28. Cardiac Pulsation Cardiac pulsation artifact After Motion Compensation

29. Flow Artifact Flow artifact in right to left direction (phase encoding) from the popliteal vessel, seen as a small bright artifact along the middle of the femoral bone.

30. Susceptibility Artifact • Tissues placed in magnetic field become temp. magnetized • Slightly alters local magnetic field • Difference in susceptibility between tissues • Field inhomogeneity at tissue boundaries field gradient • Spins dephase faster • Signal loss • Low signal intensity • Signal loss worst for bone-soft tissue & air-tissue boundaries • Air, bone much lower magnetic susceptibility than most tissues • Geometric distortions introduced

31. SE vs. EPI • SE sequences less affected • 180° refocusing pulse cancels susceptibility gradients • EPI more severely affected • Echoes are refocused by using gradients over long time period • Minimize by orienting PE gradient along same axis as susceptibly gradients • Reduce artifact by: • Use SE sequences • Reduce echo time • Increase acquisition matrix • Proper shimming over VOI to improve local field inhomogeneity

32. Reorienting PE Gradient Anterior-Posterior PE (same axis as susceptibility gradients) Left-Right PE direction

33. SE vs. GRE Susceptibility Spin echo Gradient echo

34. Metal Implants • Most severe susceptibility artifact • Metal > magnetic susceptibility than tissues • Typically areas of complete signal loss • Minimize effect: • Large receiver bandwidth • Decreased echo time • Fast SE with high bandwidth • Watch heating of adjacent tissue

35. Dental Fillings

36. Chemical Shift • Molecular protons surrounded by clouds of e- • In external magnetic field electric current induced • This current will induce a magnetic moment • Antiparallel to external field • Reduce local magnetic field felt by proton • “electronic shielding” • Protons in water vs. protons in fat • Significantly different chemical environments • Resonance frequencies different • Precess @ different frequencies • Chemical Shift • Larmor frequency shift between water protons & fat protons

37. Chemical Shift • Artifacts in frequency encode direction • Slight mis-registration of fat content • slight shift in frequency of fat protons • Amount of shift depends on: • # samples in FE direction • Receiver bandwidth Gx x

38. Chemical Shift

39. Reduce Chemical Shift • Less important in FSE imaging • Higher bandwidth receiver window used • EPI very susceptible • Long duration of sampling affects/shifts any off-resonance signal (fat) • Minimized by: • Applying frequency-selective RF pulse to nullify fat signal before imaging sequence • Successful fat sat only if magnetic field homogeneous throughout ROI • Proper shimming2 distinct signal for fat & water • STIR sequences can also be used • Fat T1 short • Can be used to suppress fat signal with inversion recovery sequence • TI needed to null signal from any tissue = 0.7 x T1

40. EPI & Chemical Shift Reduced chemical shift artifact; fat saturation Severe chemical shift artifact; Insufficient fat suppression

41. Fat Saturation vs. Inversion

42. Chemical Shift Artifact

43. Applies to GE sequences • Since H2O precesses faster than fat it gets 360° ahead of fat in short time period • Thus times when fat & H2O are totally in phase, times when totally out of phase • Dark boundaries when out of phase • Does not only appear in FE direction as CS Artifact of 1st kind Chemical Shift of the 2nd Kind In-phase @ points A, C, E Out-of-phase @ points B, D

44. In- vs. Out-Of-Phase 3-cm left adrenal mass In Phase Mean signal intensity in mass = 115 Out of Phase Mean signal intensity in mass = 66 • Fat present in lesion • Benign adrenal adenoma

45. In- vs. Out-Of-Phase metastatic non-small cell and squamous cell carcinoma of the lung demonstrate a left adrenal mass In Phase Out of Phase • No differences in lesion signal between 2 images • No Fat present in lesion • Metastases generally do not contain fat

46. Aliasing (Wraparound) • Overlapping opposite side of image of signals outside FOV • Spatial encoding of objects outside FOV cannot be distinguished from inside FOV • Imaging FOV smaller than anatomy being imaged • PE direction • FOV = distance along gradient to complete 1 cycle • Gradient will move from -180° to +180° across FOV • If RF transmission coil sensitivity extends beyond FOV • Spins outside FOV excited • Will be part of next cycle in PE direction (i.e. -360° to +360° etc. )

47. Phase Equivalence • Phase angles of spins outside FOV essentially equivalent to spins within FOV • But on opposite sides of image • For FT: • Spins at x° = x+360° • Results in overlap of signal outside FOV with signal within FOV

48. Phase-Encoding PE: Modifying phase of spins in a direction of slice plane Step 1: Phase shifts range from -180° to +180° Step 2: Shifts increased by multiple of 180 (360, 540 Etc.) . . . Step N Meaningful phase range

49. Phase shift Outside FOV Phase = 200° Inside FOV: Phase shifts range from -180° to +180° Outside FOV: Phase shifts < -180° or > +180° Mismapped to equivalent phase inside image. Equivalent phase = -160°

50. FOV FOV ≥ imaged anatomy Wrap around artifacts FOV < imaged anatomy