M R I Physics Course. Jerry Allison Ph.D. Chris Wright B.S. Tom Lavin M.S.M.P. Department of Radiology Medical College of Georgia

Transcription

1 M R I Physics Course Jerry Allison Ph.D. Chris Wright B.S. Tom Lavin M.S.M.P. Department of Radiology Medical College of Georgia

2 M R I Physics Course chapter 12 Artifacts and Suppression Techniques Artifacts / Undesirable Contrast Chemical Shift artifact Truncation (Gibbs) Artifact Errors in data Aliasing (wrap around) Zippers / Stars (Zero line artifact) Flow artifact Metallic artifact (magnetic field perturbations)

3 M R I Physics Course chapter 12 Artifacts and Suppression Techniques Motion Magnetic Susceptibility artifact Surface Coil artifact N/2 Ghosts in EPI

4 Chemical shift artifacts -At 1.5T the Larmor frequency of fat is approximately 220 Hz lower than water. MRI images can be considered to be images of free water with a superimposed spatially misregistered fat image. -Chemical shift artifacts produce high intensity and low intensity bands near fat/water interfaces, primarily in the frequency encode direction. -Dark boundaries between regions are sometimes called contour artifacts. 4

5 Kidney example: from the website ( by Ray Ballinger) 5

6 Copyright Radiological Society of North America, 1999 Williams, R. L. et al. Radiology 1999;212:

7 Chemical shift at 7T: oil and water

8 Truncation (Gibbs) artifact -Truncation artifact or Gibbs phenomena appear as a periodic ringing at high contrast interfaces. -Truncation artifact is caused by underestimation or overestimation of the signal at high contrast boundaries. 8

9 Truncation (Gibbs) artifact -The step function resulting from a high contrast interface cannot be accurately depicted by the finite number of sine and cosine waves prescribed by a typical MR image acquisition matrix. -Truncation artifacts can be diminished by a decrease in pixel size or raw data filtering. It is important to remember that raw data filtering affects all image data not just the artifactual portions. -Pixel size can be decreased by increasing the imaging matrix or decreasing the field of view. 9

10 Partial Volume Artifacts -Caused by an imaging voxel containing two different tissues and therefore possessing a signal average of both tissues. 10

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12 Errors in data -Errors in data can cause striped images, washed out images and zipper artifacts. -Failure of the array processor can produce a variety of stripes and patterns (i.e. corduroy artifacts) on reconstructed images. 12

13 -Leaking RF shield: The exam room is RF shielded to attenuate RF energy from radio, television, etc. Failure of the shield can produce zipper artifacts. To demonstrate a leaking RF shield, scan with the door open. A simple test of the RF shield is to see if a transistor radio receives signal in the exam room with the door closed. 13

14 RF receiver gain -If the RF receiver gain is set such that the voltage level of the amplified signal exceeds the voltage range of the ADC, washed out images result. -The RF receiver gain settings are generally adjusted by auto prescan to avoid this artifact. 14

15 Aliasing (wrap-around) -Aliasing or wrap-around artifacts can occur when the anatomy exceeds the field of view. Aliasing can occur in the frequency encode direction or the phase encode direction. 15

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17 Aliasing (wrap-around) -Solution 1: Sampling faster may reduce or eliminate aliasing but results in an increased receiver bandwidth with decreased SNR. 17

18 Aliasing (wrap-around) Solution 2:-Use of a larger FOV than actually desired and display of the central non-aliased portion of the image. This is the essence of No Phase Wrap, No Frequency Wrap and Oversampling. Aliasing in the frequency encode direction is generally prevented on modern MRI systems by oversampling. 18

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20 Aliasing (wrap-around) -Siemens: Oversampling in the frequency encode direction doubles the number of points sampled. -GE: Oversampling in the frequency encode direction is always utilized. -Oversampling in the frequency encode direction does not change image acquisition time (or number of slices or echo time). 20

21 Aliasing (wrap-around) -Aliasing also occurs in the phase encode direction. This can be cured by use of No Phase Wrap:GE or Oversampling:Siemens. -Siemens: Oversampling in the phase encode direction increases the number of phase encodes by a user selectable percentage. -GE: No Phase Wrap doubles the number of phase encode samples (and halves the number of excitations). 21

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23 Aliasing (wrap-around) -Removal of phase encode aliasing can require additional scan time. -In gradient recalled echo imaging, aliasing may produce a pronounced zebra stripe artifact. This artifact can be eliminated by oversampling ( No Phase Wrap ) or increasing the FOV. 23

24 Aliasing (wrap-around) -Aliasing can occur in the slice encoding direction in a 3D data set. GE routinely discards the four outermost slices in a 3D data set (acquire 128 slices; displays 124 slices: acquire 64 slices; display 60 slices: acquire 32 slices; display 28 slices). Aliasing or wrap-around in the slice encoding direction is often still visible in the outermost slices. -The phase encode axis is usually chosen as the narrower aspect of anatomy in order to minimize phase encoding aliasing. 24

25 Aliasing (wrap-around) -Solution 3: SAT uration bands can be applied outside the FOV to reduce aliasing artifacts. 25

26 Zippers/Stars (Zero line artifact) -Bands having a dashed appearance through the center of image. -Stars consist of a bright dot in the center of the image with dashed line tails along the frequency and phase encode directions. -The signal for each point in an MRI image is actually a sinc function ([sin x] / x) in both the frequency encode and the phase encode direction. 26

27 Zippers/Stars (Zero line artifact) -The sinc function signal actually contributes to several pixels in an image but ordinarily the signal side lobes is 27 small and is not recognized in an image.

28 Zippers/Stars (Zero line artifact) -A large noise signal can cause zipper or star artifacts. If the noise signal has large amplitude the signal from side lobes is large enough to show up as a dashed pattern in the reconstructed image. 28

29 Flow artifacts -Flow artifacts occur due to in-flow/out-flow time of flight phenomena and due to phase accumulation in a gradient magnetic field. 29

30 Flow artifacts -Flow compensation (gradient motion rephasing) can recover some phase dispersion attributable to flow. 1st order gradient moment nulling: compensation for constant velocity 30

31 Flow artifacts -Depends on -Flow velocity -Flow direction -Slice thickness -TR / TE -Direction of artifacts can be reversed by swapping the phase and frequency encode directions. 31

32 Metallic artifacts (magnetic field perturbations) -Metals can cause distortions in the static magnetic field. -Ferromagnetic metals can dramatically distort the static field due to the alignment of many magnetic domains. -Non-ferromagnetic metals can distort the static magnetic field due to smaller magnetic susceptibility effects. -Metal artifacts are generally worse with gradient recalled echo sequences than with spin echo 32 sequences.

33 Metallic artifacts -During imaging, eddy currents may be induced in some metal objects resulting in induced magnetism. -The effect of metallic objects is signal loss and image distortion: -Ferromagnetic metal -Cosmetics (Fe, Co) -Dental amalgams -Implants -Non-ferromagnetic metal -Shrapnel 33

34 Artifacts and Suppression Techniques Signal voids near metal implants (e.g., fillings, as in this gradient-echo picture) are due to magnetic susceptibility differences. MRI artifact website, Vancouver, BC Copyright 1996, Wayne Patola and Bruce Coulter 34

35 Motion -Patient motion produces ghost images; primarily in the phase encode direction. -In the frequency encode direction, motion produces little displacement in the 4-8 msec interval when data are collected. 35

36 Motion Artifacts and Suppression Techniques -In the phase encode direction, motion can produce large displacements (and associated large phase errors) since it can be as much as a few seconds between collection of adjacent lines of k-space. -Solutions for motion artifacts include: -Restraints -Physiological triggering to capture anatomy in the same aspect of the respiratory or cardiac cycle for each line of k-space. -Cardiac triggering is used frequently. -Peripheral gating is available. -Respiratory triggering is available but impractical. 36

37 Solutions for motion artifacts include: -Respiratory-Ordered Phase Encoding (ROPE) also known as respiratory compensation or retrospective cardiac gating. Each acquisition is assigned to k-space based upon the respiratory phase or cardiac phase when it was acquired. Retrospective cardiac gating is similar to a first pass MUGA scan in Nuclear Medicine. -Fat suppression to eliminate bright ghosting from moving fat. -ChemSAT -STIR -SAT bands 37

38 Magnetic susceptibility artifacts -The magnetic susceptibility of material present in an MRI image varies. Various soft tissues have various magnetic susceptibilities. Air has a very low magnetic susceptibility. Iron deposits from hemorrhage, liver iron overload, etc. have very high magnetic susceptibility. 38

39 Magnetic susceptibility artifacts -Perturbations in the static magnetic field due to magnetic susceptibility can cause geometric distortion and changes in signal intensity. Geometric distortions occur primarily in the frequency encode direction. Signal intensity can be increased or decreased by magnetic susceptibility. The artifacts caused by magnetic susceptibility can sometimes be observed in slices adjacent to the inhomogeneity. These artifacts may be more prominent on gradient recalled echo images or long TE spin echo images. Magnetic susceptibility may be useful contrast for imaging of calcification and hemorrhage. 39

40 Phantom Susceptibility in a T2* Weighted Sequence

41 Susceptibility Artifact in Orthogonal T2* (EPI) images

42 Surface coil artifact -The sensitivity of surface coils falls off dramatically with distance from the coil. Tissues close to the surface coil will have higher intensity than tissues farther from the coil. This artifact is very apparent in T 1 weighted imaging of the spine where subcutaneous fat produces very intense signal due to it s short T 1 value and proximity to the coil. -GE Signa has an option Image Intensity Correction that is designed to reduce surface coil artifacts (not used). 42

43 Surface Coil Artifact -Notice how the SNR varies across the FOV for an 8 channel (parallel) brain coil. 43

44 N / 2 Ghosts in EPI -EPI is very sensitive to ghosting artifacts. Because the MRI signal is sampled under alternating gradients, it is necessary to reverse (invert) every other line. Any imperfection in the entire MRI imaging chain can modulate the MRI signal at half the Nyquist frequency. The result can be an N / 2 ghost in the phase encode direction. - N / 2 ghosts can be minimized by: -calibration scans (with no phase encodes) -image filtering 44

45 GHOSTS! 45

46 Challenge: Dielectric Resonance Larmor frequency increases for stronger magnets RF wavelength approaches body dimensions and FOV dimensions λ air = 4.7m at 1.5T λ air = 2.35m at 3T λ tissue = 0.3m at 3T!!!!!!!! (high dielectric constant) Can produce dielectric resonances which may reduce RF penetration Standing waves Image shading Worse in body imaging than head Worse in large patients (obese) Dielectric resonance effects occur at all field strengths but become apparent at 3T 46

47 Challenge: Dielectric Resonance UGA (Torso Array) 47

48 Challenge: Dielectric Resonance MCG (Torso Array) T2 T2 STIR 48

49 2006 GE Healthcare

50 2006 GE Healthcare

51 2006 GE Healthcare

52 2006 GE Healthcare