Magnetic Resonance Imaging of Fast Relaxing Spins: Acquisition during Adiabatic Excitation November 14, 2005, CMRR : Djaudat Idiyatullin.

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Presentation transcript:

Magnetic Resonance Imaging of Fast Relaxing Spins: Acquisition during Adiabatic Excitation November 14, 2005, CMRR : Djaudat Idiyatullin

Mike’s crazy idea is working

Interleaved excitation and sampling during a frequency-swept pulse 1.Steady state 2.Sensitive to spins with a very short T2 3.It is not FID but the signal predictable by Bloch simulation. BIR4 … …

How to extract information from this weird sampling during swept excitation? Least square method Monte Carlo simulation Wavelet transform

How to extract information from this weird sampling during swept excitation? Solution: 1.Away from adiabatic condition 2. Linear system- Correlation method

Linear system Output r(t) Input x(t) System h(t) A system is linear if: 1.Linearity : Output = C * Input 2.Shift invariant : delaying of Input → same delaying of Output 0 τ t x(τ) → x(τ)h(t- τ) Convolution Fourier theorem:

Evolution of the isochromats during HS8 pulse (dw=10mks, (~ 30 degree), R2=500Hz)

Linear system

Correlation method for linear system Response r(t) Excitation x(t) Spin system h(t) FT System spectrum *

Simulated data HS4 pulse 100 isochromats from -12.5kHz step 250Hz dw=10mks R1=500Hz

SWeep Imaging with Fourier Transform (SWIFT) HSn pulses Flip angle < 90 degree Tr ~ Tp Bw=sw=2πN/Tp Back-projection reconstruction

SWIFT, characteristics Signal intensity depends only on T 1 and spin density (M 0 ) : Maximum signal intensity Ernst angle: Maximum T 1 contrast: Spin density contrast: Sensitive to short T 2 :

SWIFT, hardware problems “Dead time” after pulse 4.7T, 7T : ~ 3μs : sw < 130kHz 4T : ~ 20μs : sw < 40kHz FIFO underflow happens if: Tr < 5ms for 128 sampling Tr < 10ms for 256 sampling sw ~ kHz

MIP of 3D image sw=32kHz 128x128 x 64 4T Empty “16”-element TEM head coil

3D image of thermoplastic T2~0.3ms sw=100kHz 128x128 x T Sensitivity to short T2

MIP of 3D image plastic toy in breast coil sw=39kHz 128x128 x 128 D=25cm 4T Sensitivity to short T2

Slices of 3D image of feet sw=20kHz 4T First in vivo SWIFT 3D images

Slices of 3D image raspberry in vivo sw=100kHz 128x128x128 D=3cm 4.7T Sensitivity to raspberry

AdvantagesDisadvantages fastToo fast for VARIAN FIFO underflow Sensitive to short T2Sensitive to coil material Reduced motion artifacts (zero echo time, back projection reconstruction) Problems with slice selection Reduced signal dynamic range ? quietToo quiet

Another Mike’s crazy idea

Breast MR scanner

Thanks to: Ivan Tkac Gregor Adriany Peter Andersen Tommy Vaughan Xiaoliang Zhang Carl Snyder Brian Hanna John Strupp Janis Zeltins Patrick Bolan Lance DelaBarre Ute Goerke all CMMR Fast & Quiet MRI by Sweeping Radiofrequency Djaudat Idiyatullin, Curt Corum, Jang-Yeon Park, Michael Garwood Macros, C programming Hardware Software Yellow pages of CMRR Discussion