Jonathan M. Lommen1, Frank Resmer2, Nicolas G.R. Behl1, Michael Sauer2, Nadia Benkhedah1, Andreas K. Bitz1, Reiner Umathum1, Mark E. Ladd1, Titus Lanz2, and Armin M. Nagel1,3
1Medical Physics in Radiology, German Cancer Research Center (DKFZ), Heidelberg, Germany, 2Rapid Biomed, Rimpar, Germany, 3Diagnostic and Interventional Radiology, University Medical Center Ulm, Ulm, Germany
Synopsis
A 30 channel Rx array for 23Na MRI at 7 Tesla is compared with a reference Tx/Rx birdcage. Strong SNR improvement by the array technology is acknowledged and good B1 homogeneity could be found. Images were reconstructed using der adaptice combine algorithm which provides a near perfect channel combination.Introduction
The physiological information about
tissue viability contained in the sodium (
23Na) MRI signal recently
triggered a number of clinical studies.
1 However,
23Na MRI is
still challenging due to the inherently low signal-to-noise ratio (SNR). SNR
efficient acquisition and sophisticated reconstruction schemes make best use of
the accessible
23Na MRI signal. On the other hand, the SNR can be
increased by hardware improvements of RF-coils
2,3, in particular receive array
technology
4, and higher main field strengths
5.
In this work, the performance of a
30-channel head coil array was compared with a single channel quadrature
birdcage coil. To exploit the capabilities of the 30-channel array,
reconstruction was performed using adaptive combination (ADC).
6Materials and Methods
A multichannel 23Na/1H RF
coil system for a Siemens Magnetom 7T MR system is presented. It is composed of
a dual tuned quadrature 23Na/1H Tx/Rx birdcage and a 30-channel 23Na Rx array
for detection. For optimum 23Na performance the 23Na birdcage is equipped with
PIN diodes in order to work in Tx mode while the array is used for detection.
Rings (∅≈30cm) and rungs (length 24cm) of the birdcage are
made from printed circuit boards (thickness 1.5mm). A
capacitive coupling scheme is used, making use of dual tuned cable traps for
efficient and independent operation at both frequencies (79MHz and
297MHz). Both frequencies are
quadrature driven in Tx and Rx, using quadrature hybrids. The Rx array is made
in a volumetric helmet design and yields 30 equivalent Rx 23Na channels (cf.
Figure 1). This assembly was compared to a dual tuned 23Na/1H Tx/Rx birdcage.
In-vivo measurements were performed on
a healthy volunteer employing both coil setups. Image and noise data were
acquired for ADC channel combination. The signal was sampled using a
density-adapted 3D projection scheme7. Acquisition parameters: TE/TR=0.35/30ms, TRO=10ms, α=53°, Nproj=25000, nominal isotropic
spatial resolution of 2 mm and 2 averages yielding a total acquisition duration
of 25min. Noise data: Nproj=1000, other parameters identical,
acquisition duration 30s. B1 mapping8 was conducted to evaluate the RF-field
characteristics and for correction of the receive profile. Sequence parameters:
TE/TR=0.5/150ms, α=180°/90°, Nproj=3000 and total acquisition
time of 15min. The nominal resolution of 4 mm was interpolated by a factor of
2 to match the image data. Sensitivity profiles were determined through the
signal ratio of the two different receivers of the 30-channel coil corrected
for the receive field of the birdcage by the B1 maps.
30-channel data were reconstructed employing the
ADC algorithm (optimized block size: 16, interpolation factor: 4).6,9 SNR maps were
calculated employing the pseudo multiple-replica approach.10,11
Results
Q- factors of both RF systems as well as
B
1-efficiencies on the head were measured and are summarised in Table 1. The
23Na Tx efficiency of the 30-channel coil is reduced over that of the reference birdcage
while the
1H Tx efficiency is the same (cf. Figure 2 for pulse voltages). The
23Na noise correlation of the 30 array
channels shows a maximum of 50% and a mean of 13% (cf. Figure 4).
A good
transversal homogeneity of the B
1+ field and strong B
1-
sensitivity to the outer boundary were found (cf. Figure 2).
The
advantage of the array coil is clearly displayed in the in-vivo images by the
enhanced image quality in outer brain regions (cf. Figure 3). Single-channel
birdcage reconstruction is compared to the array data (standard sum-of-squares
and ADC). The SNR maps show a strongly increased SNR.
In the outer region the SNR increase is about a factor of 2,
whereas in the center comparable SNR was measured (cf. Figure 4).
Discussion
A 30 channel
23Na/
1H RF coil system consisting of a
dual tuned
23Na/
1H birdcage and a 30 channel
23Na Rx array for 7T MR
on the head was presented and compared to a reference Tx/Tx birdcage. The coil setup was evaluated on the
workbench and in the MR system. The reduced Tx efficiency might be caused by the shielding of the Rx array. However, SNR in
23Na MRI can be strongly
increased when employing array receiver coils due to the higher sensitivity of
the single channels. Additional benefit for image reconstruction is
gained from noise statistics and B
1 profiles, which allow image
reconstruction with additional input parameter, e.g. as shown for adaptive
combination.
Acknowledgements
This work was funded in part by the Helmholtz Alliance ICEMED - Imaging
and Curing Environmental Metabolic Diseases, through the Initiative and
Networking Fund of the Helmholtz Association.References
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