Florian Maier1, Manuela Rösler1,2, Armin M. Nagel1,3, and Reiner Umathum1
1German Cancer Research Center (DKFZ), Heidelberg, Germany, 2Institute for Biomedical Engineering, University of Zurich and ETH Zurich, Zurich, Switzerland, 3Diagnostic and Interventional Radiology, University Medical Center Ulm, Ulm, Germany
Synopsis
Simultaneous excitation
and acquisition is a promising approach to acquire MR images using very low
transmission power. This concept is also well-suited for imaging tissues and
materials with ultrashort T2* relaxation times since no echo time exists. In
this work, a cSWIFT hardware setup, pulse sequence, and reconstruction were
designed for and implemented on a whole-body 7T system. Spectroscopy and
imaging of phantoms and in vivo imaging
of the human forearm were successfully performed. Feasibility of cSWIFT with
the implemented setup was demonstrated.Purpose
Simultaneous excitation
and acquisition is a promising approach to acquire MR images using very low
transmission power. Due to the instantaneous acquisition of the spin signal
during excitation, no echo times exist (TE = 0). Therefore, this concept is also
well-suited for imaging tissues and materials with ultrashort T2* relaxation
times. Additionally, pulse sequences with low acoustic noise can be designed.
Two simultaneous
excitation and acquisition methods were proposed recently: Continuous Swept
Imaging with Fourier Transform1 (cSWIFT) and Concurrent Excitation
and Acquisition2 (CEA).
In this work, a cSWIFT
hardware setup, pulse sequence, and reconstruction were designed for and
implemented on a whole-body 7T system. To demonstrate the feasibility of cSWIFT
with the presented setup, spectroscopy and imaging of phantoms and in vivo imaging of the human forearm were
performed.
Methods
All experiments were
performed on a whole-body 7T MR system (Magnetom 7T, Siemens Healthcare,
Erlangen, Germany).
Hardware. Fig. 1 shows the
schematic of the setup. From the low-power TX output of the scanner system, a
low-noise amplifier supplies about 65 mW to a small birdcage coil (∅shield = 140 mm; ∅coil = 100 mm; lcoil = 100 mm)
via a carefully adjusted quadrature hybrid (QH). To minimize the unwanted leakage power, the
coil input is coarsely adjusted by trimmer capacitors and finely adjusted by the
reverse voltages of varactor diodes. A sample-and-hold circuit (S/H) keeps these
voltages constant and isolates the coil during the sweeps. A high-dynamic-range,
low-noise amplifier (NF = 0.6 dB; P-1dB = 100 mW; gain = 22 dB) amplifies the
spin signal RX1, while RX2 represents leakage and RX3 enables registration of
the exciting sweep. These RX-signals are processed by the scanner receiver. All
RF hardware was built in-house.
Software. The implemented pulse sequence (Fig. 2) used a swept
RF pulse with variable sweep span (for spectroscopy: 10 kHz; for imaging: 100 kHz) and
duration (for spectroscopy: 100 ms;
for imaging: 10 ms). TR was 10ms longer than the sweep to allow for
slow gradient switching and an additional trigger signal to reload the S/H of
the varactor diodes. To keep the excitation pulse bandwidth small, the pulse
amplitude was ramped up and down slowly within 20% of the sweep time. Every
second TR, a leakage measurement was performed using an additional gradient
pulse (amplitude: 10.0 mT/m; direction: left-right) during the sweep to avoid
resonant spins in the range of the swept pulse. For imaging, 16384 spokes were
acquired in 11 min using a radial imaging scheme based on golden ratios3.
During the acquisition of each spoke, an imaging gradient of 3.76 mT/m was
applied, corresponding to a spherical FoV with a diameter of 50 cm. Images were
reconstructed with 2×2×2 mm3 nominal resolution similar to a recently
published algorithm1 using Python (Python Software Foundation).
Experiments. A spectrum was acquired
using a phantom filled with Vodka (40 % ethanol C2H5OH).
Imaging was performed on a latex phantom and a human forearm of a healthy volunteer.
Results
In the cSWIFT spectrum of
Vodka (Fig. 3) the three resonances are clearly distinguishable at the expected
positions. Signal was received from the latex structure (Fig. 4), albeit
with an actual resolution that was lower than the nominal resolution. Fig. 5
shows
in vivo images of a human
forearm in which the ulna and radius are visible.
Discussion
and Conclusion
A setup to perform simultaneous
excitation and acquisition on a 7T whole-body system was implemented. The
initial results show the feasibility of the concept. An in vivo image using the cSWIFT approach was acquired. However,
image quality and resolution needs to be improved. In particular, a method to
reconstruct images containing tissues with short T2* relaxation times (< 1 ms)
in the presence of tissues with long relaxation times (> 1 ms) needs to be
implemented. Due to the direct measurement of the leakage signal, this approach
is robust against RF pulse inaccuracies.
In conclusion, the
feasibility to acquire cSWIFT images on a 7T whole-body MR system was demonstrated.
Acknowledgements
The authors thank Dr.
Richard R. Bouchard for proofreading and Barbara Dillenberger and Christian
Kindtner for their support.References
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Idiyatullin D et al., J. Magn. Reson. (2012) 220:
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Med. (2009) 61(2): 354-363.