Accelerated MRI at 9.4 T with electronically modulated time‐varying receive sensitivities

Abstract

Purpose To investigate how electronically modulated time‐varying receive sensitivities can improve parallel imaging reconstruction at ultra‐high field. Methods Receive sensitivity modulation was achieved by introducing PIN diodes in the receive loops, which allow rapid switching of capacitances in both arms of each loop coil and by that alter B1− profiles, resulting in two distinct receive sensitivity configurations. A prototype 8‐channel reconfigurable receive coil for human head imaging at 9.4T was built, and MR measurements were performed in both phantom and human subject. A modified SENSE reconstruction for time‐varying sensitivities was formulated, and g‐factor calculations were performed to investigate how modulation of receive sensitivity profiles during image encoding can improve parallel imaging reconstruction. The optimized modulation pattern was realized experimentally, and reconstructions with the time‐varying sensitivities were compared with conventional static SENSE reconstructions. Results The g‐factor calculations showed that fast modulation of receive sensitivities in the order of the ADC dwell time during k‐space acquisition can improve parallel imaging performance, as this effectively makes spatial information of both configurations simultaneously available for image encoding. This was confirmed by in vivo measurements, for which lower reconstruction errors (SSIM = 0.81 for acceleration R = 4) and g‐factors (max g = 2.4; R = 4) were observed for the case of rapidly switched sensitivities compared to conventional reconstruction with static sensitivities (SSIM = 0.74 and max g = 3.2; R = 4). As the method relies on the short RF wavelength at ultra‐high field, it does not yield significant benefits at 3T and below. Conclusions Time‐varying receive sensitivities can be achieved by inserting PIN diodes in the receive loop coils, which allow modulation of B1− patterns. This offers an additional degree of freedom for image encoding, with the potential for improved parallel imaging performance at ultra‐high field.

Reconfigurable dipole receive array for dynamic parallel imaging at ultra‐high magnetic field

“…is the encoding operator consisting of the coil sensitivities S γ of the γ-th coil at spatial location r 𝜌 and a discrete Fourier operator according to the applied k-space trajectory k at time point t k . 2 As shown previously, 10 this encoding operator can be extended to allow for time-varying receive sensitivities as…”

“…( 1) decomposes into multiple small sub-blocks, such that the full encoding operator E can be replaced by sensitivity matrices S containing only the coil sensitivity values of aliased voxel groups from all channels. 2,10 We found previously 10 that time-varying sensitivities bring the largest g-factor improvement if sensitivities are switched rapidly for every analog-to-digital converter (ADC) sample of a two-fold oversampled acquisition (i.e., during a k-space readout line). In this case, g-factors are identical to the hypothetical case of having all sensitivities of the dynamic configurations statically active as "virtual receive channels" at the same time.…”

“…9 Recently, we proposed a novel type of parallel imaging acquisition based on the dynamic modulation of the sensitivity profiles of local RF receive coils at ultra-high field strength. 10 Conventional parallel imaging relies on static sensitivity patterns of local receive coils, which is then used to accelerate image acquisition. This principle can be fundamentally expanded if these static sensitivity profiles are rapidly modulated during signal acquisition.…”

“…This shifts the current distribution along the loop length and leads to two distinct receive sensitivity patterns within a single receive element during signal reception. 10 The spatial difference between the two distinct configurations within a single coil element decreases the g-factor compared to a single and static receive sensitivity profile. However, our previous design allows to alter the receive sensitivity profile only in the transverse plane, but not along the z-axis, which is allowed in the new design based on reconfigurable dipoles.…”

“…Dynamic modulation virtually increases the number of array elements without decreasing their size, and therefore, improves parallel imaging and preserves elements' SNR. In previous works, 10 we presented a novel approach based on dynamically altering the receive sensitivity profile of surface loop elements using fast‐switching positive‐intrinsic‐negative (PIN)‐diodes or varactor diodes during acquisition. This shifts the current distribution along the loop length and leads to two distinct receive sensitivity patterns within a single receive element during signal reception 10 .…”