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Distortion Artifacts in MRI and their correction
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Distortion Artifacts in MRI and their correction

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  • Spin echo <br />
  • Happens If the wavelength of the RF field is in the same order of magnitude as the geometric dimensions of the imaged object <br />
  • This is because the spatial position in MRI is encoded using varying Larmor frequencies induced by the applied magnetic gradient pulses. <br />
  • 1.5 T machines had high ANR and Gibbs was suppressed because of high noise. More in 3T <br />
  • Filter smooths the image and reduces the signal at the edges of K-space <br />

Distortion Artifacts in MRI and their correction Distortion Artifacts in MRI and their correction Presentation Transcript

  • Distortion artifacts in MRI and their correction Vibha Chaswal, Ph.D.
  • MRI Gradient fields • The gradient fields are superimposed over the static magnetic field. For transverse images: • Z gradient–selects the slice, and is applied whilst the 90° RF pulse is on • X gradient–creates a frequency change in emitted signal producing lines (or columns) of data, and is applied when the echo RF is being measured • Y gradient–produces a change in the phase of rotation in different voxels, and is applied between the 90° and 180° RF pulses at a different strength during each cycle. The number of phase steps (e.g. 256) affects scan time. • “Frequency encoding” and “Phase encoding” directions (X and Y) are interchangeable.
  • Gradient spatial encoding Pulse sequencing diagram
  • MRI distortion Artifacts A whole range…….. Equipment-related • Non-uniformity of signal • Wraparound (phase wrap, aliasing) • Peripheral signal artifacts • Slice overlap (cross-excitation) • Noise and RF interference • Truncation (ringing, Gibbs artifact) • Phase smearing Patient-related • Signal void due to freezing • Partial volume averaging • Chemical shift • Motion artifacts (blurring, ghosting and pulsatility) • Flow artifacts • Susceptibility and image distortion • Magic angle
  • B0 inhomogeneity and susceptibility • Extremely homogeneous static magnetic field is required around isocenter of the magnet for MRI. • B0 homogeneity of the empty magnet: typical value of 1ppm (0.001%) in a 50 cm diameter spherical volume. • susceptibility = local changes in Bo due to a substance’s own magnetic properties • Artifacts occur at interfaces between substances of different susceptibilities, e.g. air/bone/metal/ calcification/hemorrhage; implants and previous surgery, tooth fillings, mascara, ear-rings, hair-slides – worse at higher field strengths
  • B0 inhomogeneity • Decreased frequency-selective fat-saturation efficacy of pulses • Effect more pronounced the further the image slice is positioned from the isocenter of the magnet (a) off-center acquisition with insufficient fat suppression (arrows) (b) Identical acquisition at isocenter Reduction: improve field shimming Optimal positioning of the patient in z-direction Dietrich et. al., Artifacts in 3-T MRI: Physical background and reduction strategies, EJR, 65, 2008
  • B0 inhomogeneity • Relatively small frequency shifts lead to substantial reduction of the transversal steady-state magnetization and, thus, to band-shaped signal losses in the image Reduction: Move the artifact out of the region of interest (apply frequency offset to nominal resonant frequency) Dietrich et. al., Artifacts in 3-T MRI: Physical background and reduction strategies, EJR, 65, 2008
  • Susceptibility: Metal artifacts • An important manifestation of a susceptibility-related artifact is the signal loss in gradient-echo images around metallic implants or surgical clips Right hip prosthesis is seen as region of signal drop out (closed arrows) Figure from: Mechlin et al, AJR 143, December 1984 STIR axial MRI image of a patient, arrows B point to artifacts due to right hip prosthesis. Figure from: Dr. Geller et al, Malignant Peripheral Nerve Sheath Tumors (MPNST), ESUN, june 2006, 3(3) http://sarcomahelp.org/learning_center/mpnst.html Reduction: Decrease Echo time, TE, of pulse sequence Increase receiver band-width
  • Susceptibility: Geometric distortion artifact • • Usually in echo-planar imaging (EPI) At interfaces between soft-tissue and bone/air e.g., in the base of the skull or in the head-and-neck region Reduction: a) decrease the echo-spacing of the read out train e.g., by increasing the receiver bandwidth or by parallel imaging techniques b) Use fast-spin echo technique (b) EPI acquisition without parallel imaging, severe distortion artifacts are visible (arrows) and (c) EPI acquisition with acceleration factor 2, distortion artifacts are still present but considerably reduced (arrows). Dietrich et. al., Artifacts in 3-T MRI: Physical background and reduction strategies, EJR, 65, 2008
  • B1 inhomogeneity • Spatial inhomogeneity of the B1 field results in flip-angle deviations depending on the spatial position • Reduced signal intensity in these areas or to altered contrast particularly in FLASH sequences whose T1-weighting depends on the flip angle
  • Dielectric resonance effects/ RF interference/ standing wave effect • Constructive or destructive interferences of the transmitted RF field may be observed resulting in either regional (e.g., central) brightening or regional signal loss, respectively. Signal loss due to wavelength effect Reduction: a) Effect can be mitigated by positioning a dielectric cushion close to the ROI, or b) Manually modify the RF-transmitter amplitude in order to reduce B1-inhomogeneityinduced signal loss
  • Chemical Shift Artifact • For MRI in vivo, two important groups of molecules : protons in water and protons in fat tissue. • Larmor frequencies difference of these protons ~ 3.5 ppm. • chemical shift leads to a slight geometric shift of the relative position of water and fat protons in readout direction.
  • Chemical Shift Artifact Reduction image-compromising superposition of fat tissue onto other tissues Reduction: a)manually increase the RF-transmitter amplitude and apply image post-processing filters to obtain more uniform image intensities. b)Increase (~double) receiver bandwidth c)Based on anatomy switch readout and phase encoding directions
  • Blood Flow related artifact • Magnetohydrodynamic effect: The flow of blood ions perpendicular to the strong static magnetic field, B0, gives rise to induced voltages and currents; “(a) Signal loss in the pulmonary vessels in non-gated 3-T single-shot fast-spin-echo (FSE) acquisition possibly related to the magnetohydrodynamic effect. (b) Non-gated 1.5-T single-shot FSE acquisition demonstrating visual signal in pulmonary vessels. (c) ECG-gated 3-T single-shot FSE acquisition of the same volunteer as in (a) with data acquisition during diastole; the vessel signal is restored.” …….Deitrich et al, EJR 65(2008)
  • SNR related artifacts • Artifacts to Noise ratio (ANR) • Artifacts are masked by increased statistical noise. • Gibbs Ringing: caused by data clipping at the edges of K-space (raw data readings are still over noise level at the borders of acquired K-space)
  • Gibbs artifact Reduction: Increase spatial resolution or, Apply reconstruction filters (Hanning filter)
  • Resources Many more Artifacts but time restriction ⇒Resources for MR Artifacts ad their reduction ⇒MR-TIP: online database (open forum) from University of Illinois, Excellent resource ⇒ Review article by Dietrich et.al,”Artifacts in 3-T MRI: Physical Background and reduction strategies” EJR, Vol 65, 2008
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