Comparison of a 30-channel head array with a birdcage for 23Na MRI at 7 Tesla
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


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.


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-coils2,3, in particular receive array technology4, and higher main field strengths5. 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).6

Materials 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


Q- factors of both RF systems as well as B1-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 B1+ field and strong B1- 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).


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 B1 profiles, which allow image reconstruction with additional input parameter, e.g. as shown for adaptive combination.


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.


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Figure 1: Design of the Tx/Rx birdcage (a) and 30-channel Rx array: Overview (b-d) and receive array with removed cover (e). Next neighbouring coil elements are decoupled by overlap, each element yielding an active and a passive 23Na detuning circuit, and preamplifier decoupling for additional element decoupling, and finally passive 1H traps. For patient comfort, the coil setup is openly designed, including a mirror.

Table 1: Q- factors of both RF systems as well as B1-efficiencies on the head.

Figure 2: B1+ maps of Tx/Rx birdcage and 30-channel coil (left and middle) in units of normalized flip angle (norm. FA) and array sensitivity (right). The sensitivity maps were deduced from the amplitude ratio of birdcage and 30-receiver data corrected for the B1- field of the birdcage. Transversal B1+ homogeneity is displayed (top row, middle). Good coverage of the brain is seen in the 30-channel sensitivity maps. The pulse voltage (Vp) is shown as an indicator of efficiency.

Figure 3: 23Na MRI data from Tx/Rx birdcage (1st column) and 30-channel array data reconstructed using sum-of-squares (SOS) (2nd column), ADC (3rd column) and B1--corrected ADC (4th column). The nominal resolution is 2mm with an acquisition duration of 25min. Image quality is strongly improved for 30-channel reconstructions and anatomical details become clearly visible at locations where the Tx/Rx data show high noise level. The 1H data displays a good field homogeneity (5th column).

Figure 4: SNR Maps for birdcage and 30-channel coil (left) and noise correlation of the latter (right). The SNR is seen to be strongly improved for the 30-channel data. In the center same SNR amplitude is found but towards the outside sensitivity increases and a SNR increase of roughly a factor of 2 can be acknowledged. Mean and maximum correlation are found to be 13% and 50%.

Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)