System/Imaging Imperfections

Transcription

1 System/Imaging Imperfections B0 variations: Shim, Susceptibility B1 variations: Transmit, Receive Gradient Imperfections: Non-linearities Delays and Eddy currents Concomitant terms 1

2 B0 Variations - Off-Resonance Imperfections in the B0 field (~1ppm) Coil design Imperfections in coil placement, currents Susceptibility variations (~1-5ppm, normal body) Chemical shift (-3.5ppm for fat, most common) 2

3 Susceptibility Tendency to be magnetized H field is continuous Η = χβ B0 is convolution of χ with a dipole field 3

4 Susceptibility Air-tissue interfaces and complex shapes Field can be improved by shimming Measured B0 may still vary considerably (500Hz here) 4! Image Mask B0 Shimmed B0

5 Chemical Shift Refers to the frequency shift due to electron shielding Reduces the resonance frequency Most common: Fat-Water: -3.5ppm (220Hz at 1.5T / 440Hz at 3T) Actually multiple peaks in fat (more complicated) Many Others ~ Spectroscopy F W (Frequency) ppm 5

6 Shimming Passive Shimming: Add small materials to correct field Active Shims: Coils with adjustable currents to correct field Linear: Small current usually added to imaging gradient High-Order: z 2 -(x 2 +y 2 )/2, 3zx, 3zy, 3(x 2 -y 2 ), 6xy 6

7 Minimizing Effects of B0 Variations Spin-echoes: Minimize T2 * dephasing High bandwidths / short readouts: Minimize imaging artifacts Post-processing corrections Fat suppression (eliminate species w/ Chem Shift) 7

8 B1 Transmit (B1 + ) Variations Coil Inhomogeneities (minimal with Birdcage) Dielectric effects (worse at high fields - shorter wavelengths, standing waves) RF amplifier non-linearity (small, harmonics) 3.0T 4.0T 7.0T 3T Images courtesy G. Glover, Stanford Univ 4T Images courtesy C. Charles, Duke Univ. 9/2000 7T Images courtesy T. Vaughan, M. Garwood, Univ. Minn. 6/2000 8

9 Minimizing Effects of B1 + Variations Better coil profiles B1 shimming: Adjust amplitude/phase of transmit currents Measure B1 and compensate signal, measurement Parallel transmit (multiple B1 + fields) B1-insensitive pulses (adiabatic, BIR4,...) 9

10 B1 - Receive Primarily coil sensitivity variations Somewhat fixable in reconstruction Measure sensitivities (SENSE) Surface-coil intensity correction Coil 1 Coil 2 Coil 3 Coil 4 Coil 5 Coil 6 Coil 7 Coil 8 All Coils 10

11 Gradient nonlinearity Ideally, linear mapping of position to Bz Must end somewhere(!) db/dt limited too Distortion of image Loss of resolution Aliasing 11

12 Gradient Non-linearity Correction Apply grad-warp warping to correct distortion Note image boundaries are curved 12 Marc Alley

13 Gradient Non-linearity and B0 Variation B(z) = B Gz (r)+ B 0 (r) Axial If B(r1) = B(r2), positions are indistinguishable Aliasing or Annefact (if RF coil sensitive in region) Applies to slice selection too: Can correct slice location Warps slice (harder to fix) Saggital 13 Marc Alley

14 Example: B0 and Gradient Nonlinearity Where does this point alias to? 14

15 RF / Gradient Delays Amplifier delays, circuit delays Can vary between scanners but also calibrated Cartesian imaging insensitive, but other methods much more affected Gradient delays can be axis-dependent RF transmit and receive delays can vary 15

16 Delay Questions How do these delays affect standard imaging: Slice select gradient delay (general)? Dephasing/signal loss (refocusing gradient not aligned) Readout gradient delay of 2 samples? 2 cycles of linear phase in readout direction Phase-encode gradient delay of 2 readout samples? Not noticeable (possibly affects 2 outer kx-space samples) 16

17 Eddy Currents Generated by gradient switching, subject independent Linear system models (dg/dt): B 0 (! r,t)= X h i (! r,t) G i (t) i=x,y,z Spatially-independent terms (global phase/rotation) Linear terms ~ gradient errors (self, cross axis) Higher-order terms - hardest to correct 17

18 Concomitant Gradients B c (x, y, z) = 1 2B 0 G 2 xz 2 + G 2 yz 2 + G 2 z x 2 + y 2 4 G x G z xz G y G z yz Maxwell terms - impossible to create Bz variation without some Bx and By variation Bigger problem at lower field strengths Some correction schemes (From Bernstein, MRM 39:300 (1998) 18

19 Summary: System Imperfections B0 variations: Shim, Susceptibility B1 variations: Transmit, Receive Gradient Imperfections: Non-linearities Delays and Eddy currents Concomitant terms 19