A 16-Channel 13C Array Coil for Magnetic Resonance Spectroscopy of the Breast at 7T

Abstract

Objective: Considering the reported elevation of ω-6/ω-3 fatty acid ratios in breast neoplasms, one particularly important application of ¹³C MRS could be in more fully understanding the breast lipidome's relationship to breast cancer incidence. However, the low natural abundance and gyromagnetic ratio of the ¹³C isotope lead to detection sensitivity challenges. Previous ¹³C MRS studies have relied on the use of small surface coils with limited field-of-view and shallow penetration depths to achieve adequate signal-to-noise ratio (SNR), and the use of receive array coils is still mostly unexplored.

Methods: This work presents a unilateral breast 16-channel ¹³C array coil and interfacing hardware designed to retain the surface sensitivity of a single small loop coil while improving penetration depth and extending the field-of-view over the entire breast at 7T. The coil was characterized through bench measurements and phantom ¹³C spectroscopy experiments.

Results: Bench measurements showed receive coil matching better than -17 dB and average preamplifier decoupling of 16.2 dB with no evident peak splitting. Phantom MRS studies show better than a three-fold increase in average SNR over the entirety of the breast region compared to volume coil reception alone as well as an ability for individual array elements to be used for coarse metabolite localization without the use of single-voxel or spectroscopic imaging methods.

Conclusion: Our current study has shown the benefits of the array. Future in vivo lipidomics studies can be pursued.

Significance: Development of the 16-channel breast array coil opens possibilities of in vivo lipidomics studies to elucidate the link between breast cancer incidence and lipid metabolics.

Fig. 1.

Dimensioned rendering of coil system. The ¹³C transmit coil system consisted of Helmholtz (green) and saddle pair (red) coils. The ¹H Helmholtz coil (blue) used slightly wider loop spacing to encompass the ¹³C Helmholtz coil. Orientation of receive array elements (yellow) relative to the main magnetic field direction are shown with large elements labeled 1–10 and small elements labeled 11–16 (top right).

Fig. 2.

Circuit diagrams and images for single elements of the ¹³C receive (left) and transmit (right) coils.Insets shown in each image show detailed views of the labeled networks of each circuit diagram. All receive elements used similar layouts. The saddle pair transmit element used a similar layout to the ¹³C Helmholtz element shown pictured but with the LCC decoupling traps replaced by additional segmenting capacitors.

Figure 3:

A Helmholtz coil utilizing the FCE condition and a typical L matching network was employed for transmission/reception of the ¹H signal.

Fig. 4.

Completed hardware for 16-channel ¹³C data acquisition including (from left to right) previously-constructed frequency-translation unit, coil system, and interface box. The coil is shown with acrylic paneling removed to allow better visibility of the array coil.

Fig. 5.

B₁ field maps obtained from ¹³C transmit coil with the array removed (top) and inserted while actively-detuned (middle). The field maps indicate fairly good field homogeneity throughout the coil volume. A large decrease in field intensity is seen after insertion of the detuned array as characterized by the ratio of the fields before and after array insertion (bottom). This loss was attributed to shielding effects from the array elements and their respective circuitry.

Fig. 6.

Bulk spectroscopy experimental results. A greater than three times increase in SNR from the array coil (left) compared a similar acquisition using the volume coil in T/R mode (middle) was seen, indicating the array’s ability to improve SNR over the entire breast volume. The noise data from the scan shows little inter-element coupling as expected from bench measurements with the highest coupling of 13.7% shown between elements 15 and 16 and all other elements showing significantly lower values.

Fig. 7.

Spectra from example receive elements and the combined array spectra using the localization phantom. A bicarbonate spectrum from each row of receive elements is shown with spectral intensity levels generally decreasing for receive elements more distal to the phantom. Note that scaling was kept consistent between all spectra for easy comparison. The results indicate that spatial sensitivity patterns of individual receive elements may be used for coarse signal localization even without using localized sequences.