SoHyun Han1
1Korea Basic Science Institute, Korea, Republic of
Synopsis
Keywords: Contrast mechanisms: fMRI
This lecture will describe the data acquisition for functional MRI focusing on 2D echo planar imaging (EPI) sequence. Sequence parameters (TE, TR, echo spacing, voxel size, acceleration factors, etc.) will be introduced. Parallel imaging (SENSE, GRAPPA, SMS) methods and typical EPI related artifacts (ghosting, geometric distortions, signal loss) will also be discussed.
Introduction
Blood
oxygenation level-dependent (BOLD) functional MRI measures signal changes
driven by the magnetic susceptibility of blood [1]. To observe these changes
effectively, the contrast by magnetic susceptibility should be sensitive enough
to be detected, and temporal resolution should be fast enough to measure its
changes. The most popular pulse sequence for BOLD fMRI is the 2D gradient-echo
(GE) echo planar imaging (EPI) due to its high sensitivity and time efficiency [2].
This lecture will focus on practical considerations in designing protocols for
BOLD fMRI acquisition using 2D GE-EPI sequence [3].Important parameters in functional MRI acqusition
To detect BOLD
fMRI, spatial and temporal properties, and signal-to-noise ratio (SNR) are
important parameters to consider. Such properties including SNR, temporal SNR
(tSNR), and contrast-to-noise ratio (CNR) are closely related with the imaging
parameters TE, TR, voxel size, echo spacing, acceleration factor and so on. The
BOLD contrast is based on T2*-weighted contrast which is dependent on the echo
time (TE). BOLD contrast can be maximized when TE matches the T2* of the tissue
of interest [4]. However, we need to consider that long TE leads to signal drop
due to magnetic field inhomogeneities and results in increase of TR. The
repetition time (TR) corresponds to acquisition temporal resolution in EPI
sequence. To increase temporal resolution, spatial resolution or volume
coverage should be decreased. In terms of spatial resolution, high spatial
resolution is beneficial to resolve finer structures [5], but it can cause
severe image artifact and reduce SNR because of the increase in spatial
encoding time. On the other side, lowering spatial resolution induces partial
volume effect.Echo Planar Imaging (EPI)
Echo planar
imaging (EPI) acquisition enable 2D k-space sampling in one shot by continuously
switching readout and phase encoding gradients. To cover multiple slices, this
process is repeated for each slice, resulting in TR as a sampling rate within a
few seconds.Parallel Imaging & Simultaneous multislice
EPI acquisition
can be accelerated in both in-plane by reducing the number of spatial encoding
steps and the slice direction by exciting multiple slices simultaneously.
In-plane acceleration [6,7] can reduce image artifacts by reducing the number
of spatial encoding steps, but the image SNR decreases by the square root of
the acceleration factor. Acceleration in slice direction can increase the
volume coverage without increasing TR, but it leads to an increase in power
deposition and decrease image SNR by the poor condition of the image
reconstruction problem and shortened T1 recovery [8-11].EPI artifacts
The most common
artifacts in 2D GE-EPI images are FOV/2 ghosts [12], geometric distortion [13],
and signal dropout. FOV/2 ghosts are caused by phase errors between the odd and
even readout lines of the k-space trajectory due to the system imperfections
such as eddy currents. Geometric distortion is caused by the prolonged echo
train length in field inhomogeneous areas (air/tissue interfaces). Phase errors
are accumulated over time in phase encoding direction. Signal dropout is
induced by field inhomogeneities within an imaging voxel. The detail of those artifacts will be explained and approaches to relieve them will be also discussed.Acknowledgements
No acknowledgement found.References
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