Coil overview

The ¹H/²³Na bilateral breast coil consists of two unilateral units, one for each breast. The two units and the associated circuitry are integrated into a tailored plastic housing that includes two dome cavities (posterior diameter = 14.5 cm, depth = 10 cm and volume ~2 L) to accommodate the breasts (Figure 1a).

(a) The ¹H/²³Na bilateral breast coil in its tailored housing. (b) Left and right units were separated by two RF shields (in blue) which reduced coupling enabling bilateral operation. (c) Each unit of the coil is constituted by three layers. From the outermost to the innermost: a three-turns series solenoid for ²³Na transmission, a three-turns series solenoid for ¹H excitation and reception and a four-channel ²³Na receive array. For sake of simplicity the series connection among the loops as well as the lumped elements are not reported.

Because of proximity, coupling between the two unilateral units caused peak resonance splitting. We addressed this problem by surrounding each unit by a passive conductive radiofrequency (RF) shield (diameter = 20.5 cm, height = 11.5 and 13 cm in the medial and in the lateral regions, respectively) to enable bilateral operation (Figure 1b). The conductive paths were broken up into sections that were bridged by high value capacitors in order to reduce gradient-induced eddy currents while maintaining RF transparency. Scattering (S) parameters were measured with the network analyzer to assess decoupling between left and right solenoids (both proton and sodium).

Each unit is composed by three nested layers (Figure 1c): (1) the outermost layer is a three-loop solenoid for sodium excitation, (2) the intermediate layer is a three-loop solenoid for proton excitation and reception and (3) innermost layer, the closest to the breast, is a four-channel receive array for sodium signal reception.

Detailed circuit schematics of the left unit of the ¹H transmit/receive coil and ²³Na transmit coil and receive array are presented respectively in Figures 2 and ​and3.3. The ²³Na transmit solenoid and the ¹H transmit/receive solenoid have similar interfaces. In both cases, a two-way power divider equally splits the waveform from the RF power amplifiers in order to feed left and right units.

Circuit schematic of the ¹H transmit/receive coil and interface. For simplicity, only the schematic of the left unit is illustrated. The left and right units were fed through a two ways power divider. The status of the DC bias is forward in transmit mode and reverse in receive mode. Cable traps tuned to 297.2 MHz and 78.6 MHz were implemented to reject common mode currents. The three loops of the ¹H solenoid were connected in series. RFC: radiofrequency choke; RFS: radiofrequency short.

Circuit schematic of the ²³Na transmit coil and ²³Na receive array with interface. (a) The ²³Na transmit solenoid. The interface of the ²³Na transmit solenoid has a similar setup as the one implemented for the ¹H transmit/receive solenoid. T/R switches were implemented in order to enable reception with the ²³Na solenoids to facilitate debugging; the array is used for signal reception during normal operation. The three loops of the ²³Na solenoid were connected in series. Series diodes D in two locations were used to detune the solenoids when receiving with the array. (b) ²³Na receive array. Cable traps tuned to the two frequencies of interest were implemented on the interface along with bazooka baluns on coaxial cables longer than a quarter-wavelength between the coil and the interface. Each ²³Na coil included three proton traps to suppress ¹H currents, an active detuning circuit and a fuse F. RFC: radiofrequency choke; RFS: radiofrequency short.

The ²³Na transmit solenoid interface included a transmit/receive switch to enable signal reception. SNR and B₁⁺ measurements were performed before implementing the ²³Na receive array. This data was used as a baseline to assess the SNR advantage of the sodium array over the volume coils and the interaction between the array and the volume coils. All imaging experiments were performed on Siemens 7 T whole-body scanner (MAGNETOM, Siemens Healthineers, Erlangen, Germany).