Anne Slawig1, Tobias Wech1, Valentin Ratz1, Johannes Tran-Gia1,2, Henning Neubauer1, Thorsten Bley1, and Herbert Köstler1
1Departement for Diagnostic and Interventional Radiology, University of Würzburg, Würzburg, Germany, 2Department of Nuclear Medicine, Würzburg, Germany
Synopsis
Banding artefacts in images
acquired by bSSFP are a big challenge in fast MRI as they can considerably
reduce image quality and deteriorate the diagnostic value. As the steady state
tolerates small shifts in frequency, it is possible to acquire a frequency-modulated
bSSFP. Unfortunately a simple gridding reconstruction of such a measurement
suffers from signal loss. Our study proposes a multi-frequency reconstruction
and demonstrates its capability of reconstructing banding-free 3D images while
retaining the high signal levels of
standard bSSFP.Purpose
Balanced steady state free precession
(bSSFP) sequences are frequently used in fast MR imaging due to short scan
times, high signal yield and excellent contrast. Unfortunately, these advantages
are often outweighed by serious banding artefacts, which can considerably
reduce image quality. In clinical routine, this is typically resolved by acquiring multiple
images using different offset-frequencies1. In
2002, Foxall et al. demonstrated that the steady state of bSSFP does tolerate a
slow change in frequency2. Such an acquisition with dynamic
frequency-sweep proved suitable to create banding-free images even in the
presence of high field inhomogeneities3. The purpose of this study
was to present an optimized reconstruction algorithm for a frequency-modulated stack-of-stars
bSSFP acquisition and to apply it
to
in vivo imaging of the inner ear.
Materials and Methods
The bSSFP measurements of a healthy volunteer were performed on a 3T MR system (Siemens MAGNETOM
Prisma) using a 32-channel head coil. The acquisition was performed with and
without frequency-modulation, where the latter featured a shift in the offset
frequency for each projection, covering one cycle in the off-resonance profile
off bSSFP. All datasets were acquired using a stack of stars trajectory with 64
partitions, each comprising 680 projections. Other imaging parameters were: TE
2.6ms TR 5.2ms, flip angle = 50°, resolution 0.6x0.6x0.6 mm3, total
acquisition time: 5min 9s .
All
images were reconstructed using gridding,
a 3D Fourier transform and a square-root of the sum-of-squares combination of
all coils.
Additionally, all acquisitions featuring
the shift in frequency were reconstructed for 30 different off-resonance
frequencies. The procedure first compensates for the linear drift in signal
phase caused by the acquisition pattern and leads to a binary distribution of
phase with one 180° phase shift per cycle of the off-resonance profile. The
algorithm then eliminates this phase jump for different assumed frequencies,
and therefore positions of the jump, causing all measured lines to combine
constructively.
Reconstructing all assumed off-resonances generates
an image stack, whose maximum intensity projection provided the final result.
Results
Measurements show severe banding artefacts for standard bSSFP imaging. All frequency-modulated
acquisitions removed bandings successfully, but suffered from a loss in signal
intensity with respect to the standard reconstruction. The proposed multi-frequency
reconstruction allows retaining the high signal levels of standard bSSFP while
still removing banding artefacts.
Discussion
In our study we were able to demonstrate
that a multi-frequency reconstruction is capable of reconstructing banding-free
3D images of the inner ear. Additionally it provides an increased signal level
compared to a simple gridding reconstruction. Main advantages of bSSFP, namely
acquisition speed and image contrast, could be preserved by the multi-frequency
reconstruction of frequency-modulated stack-of-stars bSSFP.
Acknowledgements
No acknowledgement found.References
1. Bangerter
et.al., Analysis of multiple-acquisition SSFP. MRM. 2004;51(5): 1038-47
2. Foxall D.L. Frequency-Modulated
Steady-State Free Precession Imaging. MRM.
2002;48:502-508
3. Benkert et.al. Fast isotropic banding-free bSSFP imaging
using 3D dynamically phase-cycled radial bSSFP (3D DYPR-SSFP). Z Med Phys. 2015
4. Man LC1,
Pauly JM, Macovski A. Multifrequency interpolation for fast off-resonance
correction. MRM. 1997;37(5):785-92.