A nested phosphorus and proton coil array for brain magnetic resonance imaging and spectroscopy

Abstract

A dual-nuclei radiofrequency coil array was constructed for phosphorus and proton magnetic resonance imaging and spectroscopy of the human brain at 7T. An eight-channel transceive degenerate birdcage phosphorus module was implemented to provide whole-brain coverage and significant sensitivity improvement over a standard dual-tuned loop coil. A nested eight-channel proton module provided adequate sensitivity for anatomical localization without substantially sacrificing performance on the phosphorus module. The developed array enabled phosphorus spectroscopy, a saturation transfer technique to calculate the global creatine kinase forward reaction rate, and single-metabolite whole-brain imaging with 1.4cm nominal isotropic resolution in 15min (2.3cm actual resolution), while additionally enabling 1mm isotropic proton imaging. This study demonstrates that a multi-channel array can be utilized for phosphorus and proton applications with improved coverage and/or sensitivity over traditional single-channel coils. The efficient multi-channel coil array, time-efficient pulse sequences, and the enhanced signal strength available at ultra-high fields can be combined to allow volumetric assessment of the brain and could provide new insights into the underlying energy metabolism impairment in several neurodegenerative conditions, such as Alzheimer's and Parkinson's diseases, as well as mental disorders such as schizophrenia.

Keywords: Creatine kinase reaction; Degenerate birdcage; Dual-tuned coil; FLORET pulse sequence; Transceive phased array.

Figure 1

Illustration of various single and dual tuned coil arrangements. The corresponding Q values are listed in Table 2.

Figure 2

Photograph of the ³¹P/¹H 16-channel array and interface with the protective cover removed. Overlays outline the cable trap and interface boards that accommodate the power dividers, transmit/receive switches, and preamplifiers.

Figure 3

Flattened two-dimensional schematic diagram of the ³¹P/¹H coil array and interface. For simplicity one ³¹P and one ¹H coil of the 16-channel nested array are highlighted in black while neighboring elements are displayed in light gray. The status of the DC bias is forward in transmit mode and reverse in receive mode. In receive mode, components D₁ and iso provide isolation between the RF amplifier and preamplifiers in receive mode, while the iso’ components minimize insertion loss between the coils and preamplifiers. The λ/4, 50Ω circuit help balance the transmit path and improve isolation. Typical component values are; C₁ = 40 pF, C₂ = 56 pF, C₃ = 27 pF, C₄ = 75 pF, C₅ = 5.3 pF, C₆ = 30 pF, C₇ = 36 pF, L₁ = 24 nH, RFC: radiofrequency choke (1 to 10 uH), and RFS: radiofrequency short (330 to 1000 pF).

Figure 4

Photographs of the five coils examined in this study: (a) the developed ³¹P/¹H 16-channel array, (b) ³¹P/¹H knee birdcage coil manufactured by Rapid Biomedical, (c) ¹H 24-channel Duyn array by Nova Medical, (d) ¹H head birdcage coil by Invivo Corp, and (e) ³¹P/¹H single-channel loop by Quality Electrodynamics.

Figure 5

³¹P SNR (a) and B₁⁺ maps (b) in the 42mM Pi head and knee phantoms in the central sagittal (left column) and transverse planes (right column) for the developed array (top row) and two commercially available coils. B₁⁺ maps were smoothed using a two dimensional median filter with a 3×3 kernel. The results are summarized in Table 4. The maps were masked to remove regions with low SNR or B₁⁺.

Figure 6

¹H SNR (a) and B₁⁺ maps (b) in the 42mM Pi head and knee phantoms in the central sagittal (left column) and transverse planes (right column) for the developed array (top row) and four commercially available coils. The results are summarized in Table 4. The maps were masked to remove regions with low SNR or B₁⁺.

Figure 7

³¹P/¹H data set: a) anatomical ¹H MP-RAGE in the sagittal plane, b) typical ³¹P-CSI spectrum from a 4.66 mL voxel whose location is outlined in (a), spectroscopic images of (c) PCr and (d) γ-ATP in the sagittal plane derived from the 3D–CSI data, e) global ³¹P spectra (4-averages, TR = 12 s) acquired in the absence (t_sat = 0, left) and presence (t_sat = 6.8 s, right) of γ-ATP saturation. Complete saturation of γ-ATP is observed despite the use of non-adiabatic saturation pulses. Eight spectra were acquired by varying t_sat in order to estimate the global CK forward reaction rate.

Figure 8

¹H MP-RAGE and single metabolite ³¹P images (2.74 mL voxel, 15 min acquisition per metabolite) using a spectrally selective 3D–non uniform FLORET sequence: a) sagittal ¹H MP-RAGE (left) and coregistered images of PCr (middle), and γ-ATP (right). b) axial ¹H MP-RAGE and co-registered PCr and γ-ATP images.