Transcript Breast Cancer Trends 1973-1999

In vivo
MRI of Fast Relaxing Spins
Using a Swept Radiofrequency
Djaudat Idiyatullin, Curt Corum, Jang-Yeon Park, Michael Garwood
Center for Magnetic Resonance Research,
Cancer Center, and Department of Radiology, University of Minnesota
What means the “fast relaxing spins”?
10-3 – 10-5 s
Range of relaxation times (T2):
Dark zone for regular imaging sequences (gradient echo)
Imaging of fast relaxing spins
T2 ~ t 90
Echo , slice selection
→
Excitation → acquisition (FID)
impractical
Imaging of fast relaxing spins
1
BLAST
G
Sensitivity, contrast: low
acq
Problems: first points in k-space,
distortion the excitation profile in image space
(Back-projection Low Angle Shot Technique)
Imaging of fast relaxing spins
1
BLAST
G
Sensitivity, contrast: low
acq
Problems: first points in k-space,
distortion the excitation profile in image space
1
RF-c
G
(Back-projection Low Angle Shot Technique)
Amplitude and frequency modulated pulses
Hyperbolic secant (HSn) pulses
low peak power to excite a large bandwidth,
flat excitation profile
Imaging of fast relaxing spins
1
BLAST
G
Sensitivity, contrast: low
acq
Problems: first points in k-space,
distortion the excitation profile in image space
1
RF-c
G
acq
(Back-projection Low Angle Shot Technique)
Amplitude and frequency modulated pulses
Hyperbolic secant (HSn) pulses
low peak power to excite a large bandwidth,
flat excitation profile
Interleaved excitation and sampling
Sensitive to spins with a very short T2
energy of the signal distributed in time
Problem: it is not a regular FID
spins + sweep excitation (< 90o) → linear system
Correlation method for linear system
Excitation, x(t)
Spin system, h(t)
Response

r (t ) 
 x( )h(t   )d

FT
X ( )
FT
R( )  X ( ) H ( )
Conjugate
multiplication
System spectrum H ( )
Correlation method for linear system
Excitation, x(t)
M0
Spin system, h(t)
Response

r (t ) 

x( )h(t   )d
Rx

FT
X ( )
FT
Xx
R( )  X ( ) H ( )
Conjugate
multiplication
System spectrum H ( )
Hx
Hy
-50
-25
0
/2
25
(kHz)
Simulated data for HS4 pulse
50
a
SWeep
Imaging with
Fourier
Transform
(SWIFT)
p
x
dw
(c)
x
Tp
TR
(b)
1
. . .
RF-c
HSn pulses
Flip angle < 90 degree
T R ~ Tp
Bw=sw=2πN/Tp
Projection reconstruction
acq
Gx
Gy
Gz
(a)
First in vivo SWIFT 3D image
Slices of 3D image
of the feet
sw = 20kHz
128 x 64 x64
4T
Sensitivity to short T2
MIP of 3D images
of empty 16-element
TEM head coil
sw = 32kHz
128x128 x 64
4T
Sensitivity to short T2
MIP of 3D images
of plastic toy in the
breast coil
sw = 39kHz
128x128 x 128
D=25cm
4T
The breast coil’s
building material
has T2 ~ 0.3 ms.
Selected slices of 3D images of a normal mandible
and surrounding areas in a 48-year-old man (4T).
Gradient-echo
sw = 80 kHz, TE = 3ms
256 x 256 x 64
SWIFT
sw = 62 kHz,
256 x 128 x 64
Direct MRI of the teeth
demineralization
plaque
pulp
dentin
cementum
root
3D MRI of decayed molar tooth obtained with SWIFT (10 min)
sw = 62 kHz, 4.7T
Conclusions
(a) fast; The method avoids the delays and gradient switching, and also time for
an excitation pulse (it’s combined with the acquisition period).
(b) sensitive to short T2 ; any T2 > 1/sw.
(c) reduced motion artifacts; Because the SWIFT method has no “echo time” it
is expected to be less sensitive to motion and flow artifacts than conventional
MRI methods.
(d) reduced signal dynamic range; The different frequencies are excited
sequentially the resulting signal is distributed in time, leading to a decreased
amplitude of the acquired signal. This allows more effective utilization of the
dynamic range of the digitizer.
(e) quiet. The SWIFT method uses a small step when changing gradients between
projections, and the fast gradient switching that creates loud noise can be
avoided.
Fast and quiet MRI using a swept radiofrequency
Djaudat Idiyatullin, Curt Corum, Jang-Yeon Park, Michael Garwood
Journal of Magnetic Resonance 181 (2006), available online
Acknowledgments. This research was supported by NIH grants RR008079 and CA92004. The authors
would like to thank Dr. Ivan Tkac for helping with reconstruction software implementation and Dr.
Jutta Ellermann for assistance in conducting the experiments