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Large field of view Sodium-23 and Carbon-13 imaging at 3T using a dedicated multinuclear birdcage body coil

Joshua D Kaggie¹, Titus Lanz², Mary McLean¹, Frank Riemer³, Rolf F Schulte⁴, Martin J Graves¹, Fiona J Gilbert¹, and Ferdia A Gallagher¹
¹Radiology, University of Cambridge, Cambridge, United Kingdom, ²Rapid Biomedical GmbH, Rimpar, Germany, ³MMIV, Radiology, Haukeland University Hospital, Bergen, Sweden, ⁴GE Healthcare, Munich, Germany

Synopsis

There has been a significant increase in carbon-13 and sodium-23 MRI research over the last decade, both in the areas of technical development and clinical applications. However, the lack of whole-body transmit systems for these nuclei limits large field of view imaging and uniform excitation. Here we demonstrate the imaging of natural abundance sodium and carbon signal from the abdomen using a 50 cm long 4-rung birdcage (40 cm inner bore) for abdominal ²³Na/¹³C MRI. This whole body ²³Na/¹³C coil has the potential to be used for a wide range of clinical applications.

Introduction

Sodium-23 (²³Na) and carbon-13 (¹³C) magnetic resonance imaging (MRI) have been shown to have a wide range of clinical applications. There has been recent increased interest in multinuclear imaging with the advent of hyperpolarized technologies, such as ¹³C-MRI as a marker of metabolic activity(1). ²³Na-MRI also indicates Na+K+ATPase activity in cancer and stroke(2-4), glycosaminoglycan changes in skin and cartilage (5-9), and absorption mechanisms in renal(10,11) and myocardial function(12). While multinuclear MRI has been investigated for several decades(13,14), its clinical potential remains unmet(15) due to the lack of appropriate whole-body radiofrequency (RF) coils.

Heteronuclear MRI requires custom transmit RF coils, which are not integrated into clinical MRI systems. While there are several RF coil configurations use ²³Na and ¹³C transmit/receive arrays, these often lack either transmit field uniformity or have a limited patient size that can be imaged. Here we demonstrate a 4-rung asymmetric birdcage for large field of view x-nuclear MRI, and perform ²³Na and ¹³C imaging with the same hardware.

Methods

A custom transmit/receive 4-rung, 50cm long birdcage coil was developed (Figures 1&2; Rapid Biomedical GmbH, Rimpar, Germany), with rungs sited 30cm apart (equivalent to the clam-shell coil(16)). The coil was tuned and matched to 33.8MHz, which is the ²³Na resonant frequency at 3T. The coil was adjusted to the table shape along the lower portion, with a rounded top that matched the MRI bore. The inner diameter of the end rings was 40cm diagonally; the coil outer diameter was 56cm at its maximal left-to-right length. The asymmetric shape required additional ring capacitors to balance the coil tuning. No proton traps were included into the coil for this developmental coil to simplify coil evaluation. A standard T/R switch with hybrid splitter was used for quadrature transmit/receive.

Imaging was performed on a 3 T MRI system (MR750, GE Healthcare, Waukesha, WI). Sodium B1-maps were obtained with double angle mapping with a 60mM NaCl, 32x22x22cm³ phantom.

3D cones sodium images of a normal volunteer were also obtained with informed consent and ethical approval as follows: TR: 100ms, TE: 0.7ms, flip angle: 70°, voxel size: 4*4*8mm³, field-of-view: 48cm³, averages: 5, interleaves: 1402, bandwidth: 166kHz, total scan time: 11.7min. Matched 2D spiral images (5 slices, 6cm thick, 5mm spacing) were obtained of 13C and 23Na using the same 32-point k-space trajectory. The ¹³C was acquired with spectral-spatial excitation at the frequency of a natural abundance fat peak. We imaged a volunteer with a proton gradient echo sequence using a low flip angle to minimise patient heating, i.e., specific absorption rate (SAR).

Results

The |S11| of the coil at either I or Q port was 34dB and |S21| between ports was 24-26dB. The unloaded Q varied between 730-860 depending on port, which decreased to 23-27 when loaded with a large saline phantom.

Figure 3 shows ²³Na B1 maps, demonstrating that the B₁ coefficient-of-variation was 16% across the entire phantom, other than near the rungs, where the B₁ was nearly twice that of the nominal values.

In vivo sodium MRI show the kidneys, liver, spleen, spine, and aorta (Figure 4). Figure 5 shows imaging of both ¹³C fat and ²³Na. The small number of rungs enabled the possibility of imaging a large volunteer (Figure 1B). The 50cm length of the coil allowed ²³Na imaging between the pelvis and heart, while still obtaining a 90° flip angle within a 1ms pulse width using an 8 kW power amplifier.

Discussion

We demonstrated ²³Na and ¹³C body imaging with a large FOV, with a coil that can accommodate large patients. Future developments of the coil will enable a removeable top for easier patient positioning.
This coil was built for ²³Na, which is easier to test than ¹³C due to its higher single-peak in vivo signal. The coil was operable with sufficient SNR to obtain ¹³C fat images, as we have previously demonstrated in the prostate(17). The Q-ratio was very high, which occurs with large, low-frequency coils that are body-noise dominated. The high Q-ratio suggests that proton decoupling could be added without significant loss of efficiency, which would improve the safety for clinical ¹H sequences. Our ¹³C results suggest that the coil could similarly be used at the ¹²⁹Xe frequency without modification.

Despite these challenges, we achieved a 90˚ flip angle with a hard pulse width of 1 ms and <8 kW power. Transmit B₁ power increases inversely with frequency, raising the difficulty of short pulses. The short T₁ of ²³Na (20-50ms) requires near 90˚ pulses for optimal SNR with TRs longer than 100ms, which increases SAR. The long pulse width results in a 4-fold linear increase of ²³Na-SAR when compared with an equivalent ¹H pulse with a 250µs pulse width, which allowed us to compare SAR with a ¹H sequence. A birdcage such as this has quadrature transmission, which results in 40% decreased SAR when compared with a linear surface coil(18).

Conclusion

Here we demonstrate a 50cm long 4-rung ²³Na/¹³C body coil with a very high field uniformity. This birdcage coil enabled imaging of the abdomen and has potential use with larger patients than is achievable with previous systems.

Acknowledgements

This work has been supported by funding from GlaxoSmithKline, Cancer Research UK, European Union's Horizon 2020 research and innovation programme under grant agreement no. 761214, the National Institute of Health Research (NIHR) Cambridge Biomedical Research Centre and Addenbrooke’s Charitable Trust.

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Figures

Figure 1. A) 33 MHz 4-rung birdcage coil and T/R switch. B) A subject is shown inside the system, with a flexible ¹³C receive array, as a physical test of the system flexibility. While this setup was not used, it shows the possibility using the birdcage coil with a dedicated receive array in larger subjects.

Figure 2. A view of the capacitors for tuning and matching, where C is the capacitance for a symmetric birdcage tuned to 33.8MHz, and C1, C2, and C3 have been added to enable electrical symmetry. RF coupling occurs at the RF1 and RF2 ports.

Figure 3. A-C) Sodium B₁ maps from dual angle mapping using a 32x22x22cm³ saline phantom. D) A histogram of the flip angles throughout the phantom, showing a <16% coefficient-of-variation.

Figure 4. A ²³Na image of a single slice in the A) coronal signal and B) axial directions. This 11.7 minute scan obtained a resolution of 4*4*8 mm³ with a 48 cm³ field-of-view.

Figure 5. Coronal views of the abdomen showing A) total sodium concentrations (TSC) and B) fat obtained with ¹³C with the same CSI pulse sequence and the birdcage coil, without moving the volunteer. Higher SNR is visible in the fat immediately next to a coil rung. Related C) water and D) fat images acquired with the ¹H body coil, with the birdcage in place, as acquired with a Dixon scan. E) A fusion image showing ²³Na overlaid onto the ¹³C fat images, which is used here to evaluate both nuclei. The ²³Na image is also useful for ¹³C image localization.

Proc. Intl. Soc. Mag. Reson. Med. 28 (2020)

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