samantha’j.’holdsworth - stanford...
TRANSCRIPT
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Fluid a(enua+ng inversion recovery (FLAIR) imaging with readout-‐
segmented (RS)-‐EPI Samantha J. Holdsworth
Lucas Center for Imaging, Department of Radiology, Stanford University, Palo Alto, CA, U.S.A
Fluid a(enua+ng inversion recovery (FLAIR)
• FLAIR imaging nulls fluids • Typically coupled with Fast Spin
Echo (FSE) readout trajectory • Highly sensi+ve sequence for
lesion detec+on – It is used in brain imaging to
suppress CSF in the image, so as to bring out the periventricular hyperintense lesions, such as MS plaques, edema, chronic microvascular ischemia
Aim
• To implement a fast FLAIR method with RS-‐EPI – as a poten+al fast-‐alterna+ve to FSE FLAIR – and a less distorted alterna+ve to EPI FLAIR
IntroducCon: FSE vs. EPI
Subsampled EPI (parallel
imaging)
R = 3
EPI (collect all lines at once) FSE (Not distorted)
kx
ky
FSE is a spin-echo refocusing approach whereby a few lines of k-space are read-out in one TR (slow).
EPI is a fast imaging approach but images are distorted
Can reduce distortion in EPI with parallel imaging, but images still remain distorted.
Readout-‐segmented (RS)-‐EPI pulse sequence
k-space
RS-EPI1-2 (with parallel imaging)
Porter DA, Heidemann RM MRM 62:468–475 (2009), [2] Holdsworth et al. EJR 65:36–46 (2008)
ky
kx
Segments are gridded together to form final k-space
Image distortion can be reduced using RS-EPI, while still scanning quickly (scan time: EPI<RS-EPI<FSE)
IntroducCon: PI-‐accelerated EPI versus RS-‐EPI
RS-EPI with PI
EPI with PI
• Target res. = 288 x 288 • Accelera+on factor = 3 • Two b = 0 s/mm2 • 15 diffusion direc+ons (b = 1000 s/
mm2) • RS-‐EPI = 7 blinds, width = 64 • EPI = 7 repe++ons
S.J. Holdsworth et al. EJR 65:36–46 (2008)
RS-EPI gives images with reduced distortion compared with EPI
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RS-‐EPI is also compaCble with retrospecCve 2D moCon correcCon
uncorrected Motion corrected
S.J. Holdsworth, et al. MRM 62:1629-1640, 2009
It is also possible to correct for motion for RS-EPI techniques, by using the navigator segment to extract the realignment parameters
Methods: FLAIR RS-‐EPI sequence
ky
kx
Methods • 3T GE system and 8-‐channel head coil
• 1 volunteer, 5 pediatric pa+ents • FLAIR RS-‐EPI:
– FOV = 22cm-‐24cm, matrix size = 2522 – segment width = 64, 5 segments, TR =10s, TE = 98ms, TI=2.2s, R = 2,
signal averages = 2, 32 slices, scan +me = 1:45min ky
kx
Results: FLAIR RS-‐EPI on paCents
8 year old girl 5 year old boy 4 year old boy 12 year old girl
Results: FLAIR technique comparison (whole brain)
FLAIR FSE FLAIR RS-EPI FLAIR EPI
6 year old patient (3T)
2:45min 1:45min 24seconds
The RS-EPI image more closely matches the undistorted FSE image. RS-EPI was acquired with 2 signal averages, but we can probably get away with 1 average, putting the scan time down to under a minute
Results: FLAIR FSE vs. RS-‐EPI
FLAIR FSE FLAIR RS-EPI 2:45min 1:45min
17 year old boy with vasogenic edema (3T) (scan times are for whole brain)
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Results: FLAIR RS-‐EPI (volunteer)
1:45min NEX = 2
52sec NEX=1
LimitaCons of FLAIR RS-‐EPI
• Distor+on is s+ll an issue.
RS-EPI scan time = 52 seconds
Future work Add Navigator: (motion correction) Partial Fourier (POCS):
(further scan reduction)
kx
ky
Summary
• RS-‐EPI is a poten+al fast alterna+ve trajectory to FSE or EPI for FLAIR-‐based methods
Future work: • Test clinical u+lity of FLAIR RS-‐EPI • Test further scan +me reduc+on • Test mo+on-‐correc+on capability in pa+ents
Acknowledgements: Stefan Skare, Kristen Yeom, Michael Moseley
Funding: The Lucas Foundation, Center of Advanced MR Technology at Stanford (P41-09784)