32-channel phased-array receive with asymmetric birdcage transmit coil for hyperpolarized xenon-129 lung imaging

Abstract

Hyperpolarized xenon-129 has the potential to become a noninvasive contrast agent for lung MRI. In addition to its utility for imaging of ventilated airspaces, the property of xenon to dissolve in lung tissue and blood upon inhalation provides the opportunity to study gas exchange. Implementations of imaging protocols for obtaining regional parameters that exploit the dissolved phase are limited by the available signal-to-noise ratio, excitation homogeneity, and length of acquisition times. To address these challenges, a 32-channel receive-array coil complemented by an asymmetric birdcage transmit coil tuned to the hyperpolarized xenon-129 resonance at 3 T was developed. First results of spin-density imaging in healthy subjects and subjects with obstructive lung disease demonstrated the improvements in image quality by high-resolution ventilation images with high signal-to-noise ratio. Parallel imaging performance of the phased-array coil was demonstrated by acceleration factors up to three in 2D acquisitions and up to six in 3D acquisitions. Transmit-field maps showed a regional variation of only 8% across the whole lung. The newly developed phased-array receive coil with the birdcage transmit coil will lead to an improvement in existing imaging protocols, but moreover enable the development of new, functional lung imaging protocols based on the improvements in excitation homogeneity, signal-to-noise ratio, and acquisition speed.

Keywords: asymmetric-birdcage; hyperpolarized; lung MRI; phased-array; xenon-129.

Figure 1

Design of the asymmetric birdcage: a) cross sectional view; b) overall view. Drawings show the mechanical shape of the asymmetric birdcage with the bottom half of the receive-array in place. Also shown is the horizontal plane split of the birdcage together with the latches to open and close the top half of the birdcage coil for easy subject access. The box inside which the patient's head lies is used to mount cables and also contains the T/R switch.

Figure 2

Human volunteer inside the finished combined chest coil. The coil is placed on the patient table. The top parts of birdcage and array coils can be opened for easy subject access (a). In the closed position the coil is ready to be moved inside the magnet bore (b).

Figure 3

32-channel phased-array receive coil. The array layout contains 15 elements on the top chest plate and 17 elements on the bottom plate (a). The array elements are placed in critical overlap with each neighbor in a hexagonal tiling pattern. The circuit diagram of the individual element incorporates tuning and matching capacitors, a passive detuning trap circuit, a preamplifier decoupling parallel resonant circuit, and an active detuning bias line with chokes to separate RF from DC lines (b). The top array is pictured at an early manufacturing stage illustrating the element layout with solder pads for components (c). The finished array is shown with preamplifiers directly mounted above surface coil receivers (d).

Figure 4

Flip-angle (FA) map for the birdcage transmit coil (a). The map shows a homogeneous FA distribution within each coronal image slice, but a slight gradient along the anterior-posterior direction (b). The whole-lung flip-angle mean (standard deviation) is 5.2° (0.4°). The total anterior-posterior difference was ~ 10 %, with higher flip angles towards the posterior image slices.

Figure 5

Noise correlation coefficient matrix. The rows and columns correspond to the receiver channel number (only 31 channels are shown since one channel was disconnected at the time of the measurement). The mean (maximum) correlation coefficient of all 31 channels was 0.25 (0.73).

Figure 6

Ventilation scans from a healthy volunteer using the xenon chest coil (a) and a COPD subject (b). Image acquisition was a 2D FLASH sequence, with resolution 2.1 × 2.1 × 10 mm³ (3.3 × 3.3 × 15 mm³) and acceleration factor 2 (3) for the healthy volunteer (and the COPD patient). White arrows in b indicate poor ventilation mainly in the right upper lobe for the COPD patient. These images are from subjects shown in Figure 1 of reference [35].

Figure 7

3D-TrueFISP spin-density acquisition fully sampled (a) and accelerated in 2D by R=3x2 (b). The acceleration reduced the total 3D acquisition time from 8 s to 2 s. Also shown is a spiral acquisition of an asthmatic subject with a fully sampled reconstruction(c) and an undersampled reconstruction from simulated parallel imaging using only 4 out of 11 spiral interleaves (d). These images are from the subject with asthma shown in Figure 1 of reference [35].