Mahesh Bharath Keerthivasan1, Sagar Mandava1, Kevin Johnson2, Diego R Martin3, Ali Bilgin1,3,4, and Maria I Altbach3
1Electrical and Computer Engineering, University of Arizona, Tucson, AZ, United States, 2Siemens Healthcare, Tucson, AZ, United States, 3Medical Imaging, University of Arizona, Tucson, AZ, United States, 4Biomedical Engineering, University of Arizona, Tucson, AZ, United States
Synopsis
A technique to increase slice coverage in dark blood fast spin echo sequences by a multi-band excitation is presented. The proposed technique can acquire multiple slices at the exact null point of blood. The radial version of the single slice sequence can generate black blood images, TE images and T2 maps within a single breath-hold. In this work we present a model based reconstruction to generate TE images and T2 maps for upto 4 slices in a single breath-hold.
Purpose
Black-blood cardiac
MRI can be done either via the use of the outflow effect or with a
double inversion recovery (DBIR) preparation. The former allows
whole heart coverage with single-shot 2D acquisitions but come at the
cost of poor flow suppression and low spatial resolution. A DBIR fast
spin-echo (FSE) sequence can generate black-blood images with good
flow suppression and high spatial resolution but is limited to being
a single slice technique due to the non-selective inversion pulse
used in the DBIR module. Techniques to increase slice coverage in
DBIR-FSE tradeoff SNR by blood nulling quality [1,2,3]. Multi-band
(MB) excitation allows better SNR efficiency (as the slices are
excited and imaged simultaneously) and the slices are acquired at the
same inversion time [4]. In this work we present a technique that
supports multi-slice coverage in cardiac triggered breath-held
black-blood acquisitions with MB excitation and present a framework
to obtain accurate T2 maps from all the slices.
Methods
MB
pulses for 4 slices were generated by a summing design. The pulses
were designed to spatially encode the signal in the slice dimension
according to the following matrix. $$\begin{bmatrix}-1& 1& 1&
1 \\1& -1& 1& 1 \\1& 1& -1& 1 \\1& 1&
1& -1 \\\end{bmatrix} $$ Four slices are excited simultaneously
(covering a 27.5 mm volume with 5 mm slices) and a slab selective
refocusing pulse was used to refocus a 33 mm slab. The selective
inversion pulse in the DBIR module inverts the entire 33 mm slab. The
MB excite pulses were used with a radial DBIR-FSE sequence as shown
in Fig. 1. The k-space data from a single acquisition can be
separated into individual TEs which are reconstructed to generate
high-resolution TE images and T2 maps. Imperfections in the
refocusing pulses leads to indirect echoes which cause a deviation of
the signal decay from a single exponential and this problem is
amplified in MB excitation. Figure 2a shows the individual
slice profiles that are simultaneously excited by the MB excitation
pulse along with the refocusing pulse profile. It can be noticed that
with non-rectangular refocusing profiles the edge slices are more
contaminated by indirect echoes such as stimulated echoes. This
results in a signal evolution that is different from the two slices
in the center of the slab Fig 2b. Using a simple mono-exponential
signal model to estimate the T2s will result in estimation errors.
Recently,
a slice-resolved extended phase graph (SEPG) fitting algorithm was
proposed [6] to accurately estimate T2s from data contaminated by
indirect echoes and this model was incorporated into an iterative
algorithm (CURLIE) to reconstruct TE images from under-sampled radial
FSE data [5] by solving the following optimization problem
$$argmin_{I_0, T2, B1} \sum_{j=1}^{n} || FT[C_j(I_0, T2, B1,
\alpha_0,\ldots,\alpha_j)] - K_j ||_2^2 $$ In the above
equation FT is the forward Fourier Transform, is the undersampled
k-space data at the jth TE, and $$$C_j$$$ is the SEPG model expressed
as a function of $$$I_0$$$, T2, B1 and the profiles of the excitation
and refocusing RF pulses. The non-linear SEPG signal model is
linearized by using a principal component approach and solved
using a conjugate gradient algorithm [5].
To
use the SEPG model, the excitation profile of the individual slices
is used along with the refocusing slice profile to generate the
training SEPG signal decays used to generate the principal
components. The T2 maps for each slice are then estimated by fitting
the TE decay curves per pixel using the SEPG model.
Short
axis data were acquired at 3T (Skyra, Siemens) with the radial
MB-DBIR-FSE pulse sequence on normal volunteers. A total of 96 views
were acquired for each MB encode with ETL=16, echo spacing=8.1 ms,
bandwidth=501 Hz/pixel and TR=1 R-R. The acquisition interleaves the
MB pulses across TR’s.
Results
Data
acquired with the MB-DBIR-FSE pulse sequence were reconstructed to
yield anatomical black-blood images, 16 TE images (reconstructed from
6 radial lines per TE) and T2 maps for each of the four slices. The
64 images and corresponding T2 maps are obtained from data acquired
in a single breath hold. The 4 slice black-blood images and the
corresponding T2 maps are shown in Fig. 3.
Conclusion
A technique that can
generate several black-blood images and the associated T2 maps in the
myocardium is presented. Due to the high acceleration factors and the
substantial deviation of the refocussing pulse from ideal behavior, a
model based scheme which accounts for this non-ideal behavior was
used in conjunction with sparsity constrained reconstructions to
yield T2 maps from all the slices.
Acknowledgements
No acknowledgement found.References
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