E. Wong, BE278, UCSD Winter 2013 Bioengineering 278 Magnetic Resonance Imaging Winter 2013 Lecture 4 • Fast Imaging – Echo Planar Imaging – Fast Spin Echo – Balanced SSFP E. Wong, BE278, UCSD Winter 2013 Echoplanar imaging G X (t) G Y (t) RF G Z (t) •Multiple lines of K-space with one excitation •Allows for snapshotimaging •K-space coverage limited by gradient hardware and T 2 * •Main Applications: •BOLD fMRI •Diffusion •Freezing motion TE k y k x
Transcript
E. Wong, BE278, UCSD Winter 2013!
Bioengineering 278 "Magnetic Resonance Imaging"
"Winter 2013 "
Lecture 4!
• Fast Imaging!– Echo Planar Imaging!– Fast Spin Echo!– Balanced SSFP!
E. Wong, BE278, UCSD Winter 2013!
Echoplanar imaging!
GX(t)!
GY(t)!
RF!
GZ(t)!
• Multiple lines of K-space with one excitation!• Allows for �snapshot� imaging!• K-space coverage limited by gradient hardware and T2*!• Main Applications:!• BOLD fMRI!• Diffusion!• Freezing motion!
TE!
ky!
kx!
E. Wong, BE278, UCSD Winter 2013!
The EPI Family!ky!
kx! kx!
ky!
Cartesian EPI! Spiral EPI!
E. Wong, BE278, UCSD Winter 2013!
NYQUIST GHOST!
FT(rows)!
Magnitude! Phase!
E. Wong, BE278, UCSD Winter 2013!
Magnitude! Phase!
FT (columns)!
NYQUIST GHOST!
E. Wong, BE278, UCSD Winter 2013!
NYQUIST GHOST!
Phase!
REAL!
IMAG.!
REAL!
IMAG.!
REAL!
IMAG.!
E. Wong, BE278, UCSD Winter 2013!
TIMING ERRORS!EPI!
SPIRAL!
E. Wong, BE278, UCSD Winter 2013!
Off Resonance Behavior : EPI!
0.5ms!
40ms!
1 / 40ms = 25 Hz / pixel!
1 / 0.5ms = 2000 Hz / pixel!
E. Wong, BE278, UCSD Winter 2013!
SHIM PATTERNS!
PERFECT SHIM! X! Y!
X2! X Y! FAT!
E. Wong, BE278, UCSD Winter 2013!
SPIRAL!
1ms!
30ms!
E. Wong, BE278, UCSD Winter 2013!
Spiral Off Resonance – 2π across acquisition!
Original Image Point Spread Function Blurred Image !
K-Space Data Modulation (Real Part) Original and Blurred Profiles !
E. Wong, BE278, UCSD Winter 2013!
CONVENTIONAL IMAGING!
0ms!
8ms!1 / 0ms = ∞ Hz / pixel!
1 / 8ms = 125 Hz / pixel!
E. Wong, BE278, UCSD Winter 2013!
Fast Spin Echo!
GX(t)!
GY(t)!
RF!
GZ(t)!
…!
…!…!…!
E. Wong, BE278, UCSD Winter 2013!
Fully refocused gradient echo!
GX(t)!
GY(t)!
RF!
GZ(t)!
• FISP, FIESTA, balanced SSFP!• RF phase alternates!• No net gradient per TR!• No dephasing -> no echoes!• VERY frequency sensitive!
• Zeros separated by 1/TR!• Large signal!• Signal depends on α, TE, TR, T1, T2!TR!
