Yu Ding1, Qi Liu1, Jingyuan Lyu1, Yuan Zheng1, and Jian Xu1
1UIH America, Inc, Houston, TX, United States
Synopsis
We design a novel acentric spiral pattern and combine with partial
Fourier to form a new sampling strategy in 3-D Cartesian acquisition. This new
strategy was implemented and tested in isotropic 3-D single breath hold cardiac
cine imaging with isotropic spatial resolution and whole heart coverage.
Volunteer study shows that this new sampling strategy greatly reduced the
number of samples and have acceptable image quality.
Introduction
Cardiac cine magnetic resonance imaging (MRI) is an
important tool for the assessment of cardiac morphology and function [1].
Recently, the feasibility of highly accelerated 3-D single breath hold cine
imaging has been demonstrated using a 3-D Cartesian spiral phyllotaxis
acquisition pattern [2]. Partial Fourier is a common imaging technique to
reduce MRI scan time by using k-space conjugate symmetry. It would be desirable
to combine Cartesian spiral pattern with partial Fourier technique in pursuit
of further acceleration. However, design of such imaging strategy is nontrivial
because of conflicts between centric spiral patterns and acentric partial
Fourier pattern in k-space. In fact, to the best of our knowledge, there is no
publication attempting to combine these two techniques. In this study, a novel
acentric Cartesian spiral pattern is designed and combined with partial Fourier
to form a new acquisition strategy for single-breath-hold isotropic 3-D cardiac
cine. This new acquisition strategy has been tested in volunteer study. Methods
The proposed method is illustrated in four sub-sections:
Acentric spiral pattern generation:
Recall that traditional Archimedean spiral can be
described by the equation in polar coordinate: $$$r = b + aθ$$$, where $$$r$$$ is the distance
to the center of k-space; $$$θ$$$ is the angle; $$$a$$$ and $$$b$$$ are two parameters, which
controls the curvature and the angular offset, respectively. The acentric
spiral is generated using the following four steps: 1) generate an acentric
grid corresponding to the k-space with partial Fourier; 2) apply an affine
transform to the half space that partial Fourier is used, and move the geometric
center of the grid to the center of k-space; 3) draw the Archimedean spiral described
above on the transformed grid; and 4) apply an inverse affine transform to
squeeze the centric spiral curve to acentric spiral curve. Then the centric
spiral becomes acentric spiral. Figure 1
shows an example of the acentric spiral pattern.
Variable density sampling and k-space down-sampling:
The variable density sampling principle is adopted. The
sampling probability density $$$\rho$$$ of each k-space point is inversely proportional
to the distance to the k-space center $$$r$$$, i.e. $$$ρ = 1/(r + c)$$$, where $$$c$$$ is an
adjustable parameter.
The k-space is partitioned into 25 spiral arms, each
arm is further partitioned into 15 segments. A simple criterion to determine
the boundaries between partitions is used: make the sum of the sampling
probability ρ for all points in each segment identical. After partition, the
down-sampling pattern is generated by random selection of one point in each
segment. The above process is repeated multiple times to generate a series of
pre-determined down-sampling patterns for dynamic imaging.
ECG-gated acquisition and image
reconstruction:
The data acquisition is prospectively ECG-triggered.
In each RR interval, one arm is collected continuously according to the
pre-determined pattern. After data
acquisition, all k-space data is averaged to form a fully sampled center of
k-space. The coil sensitivity map is calculated using the ESPIRiT method [3]. The
raw data is reconstructed using compressed sensing, and the spatial-temporal TV
is used as $$$L1$$$ regularization.
Volunteer study setup:
The proposed technique was implemented with a bSSFP
sequence. All the studies were conducted on a 3.0 T scanner (uMR 790, United
Imaging Healthcare, Shanghai, China) with a 24-channel cardiac coil. The
volunteer was instructed to hold breath during the scan for 25 heart beats. Imaging
parameters are: FA = 45 deg, FOV = 300×280×130 mm , matrix = 144×128×64
(interpolated to 144×134×64, with 75% Partial Fourier in phase-encoding and partition-encoding
direction), bandwidth = 1200Hz/pixel, TR/TE = 2.85/1.31 ms. Results
In the current study setup, 3-D cardiac cine was
acquired in 20 seconds, achieving whole heart coverage with 2.1 mm3
isotropic spatial resolution in the volunteer study without significant visible
artifacts. The effective acceleration rate was more than 17-fold. Figure 2
shows eight mid-short axis slices in systole and diastole of a volunteer. Discussion
We have developed a new sampling strategy in 3-D
Cartesian acquisition using the combination of acentric spiral trajectory and
partial Fourier. The new sampling strategy greatly reduced the total number of
samples. Comparing to centric spiral trajectory without partial Fourier, the
proposed method reduces the total number of samples by more than 40%. Preliminary
volunteer test shows that the proposed method has acceptable image quality,
which shows it is a promising approach for highly accelerated 3-D cardiac cine
imaging with isotropic resolution and whole heart coverage.
In this study, a relatively long breath-hold (25 heart
beats, approximately 20 sec) was used. In the future study, we plan to further
reduce the breath hold to around 15 seconds by increasing acceleration rate to
improve patient comfort and increase compliance. More studies on volunteer and
patients are warranted before it can be applied to clinical practice. Acknowledgements
No acknowledgement found.References
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