Franciszek Hennel

Silent MRI with soft gradient pulses.

A method to reduce the acoustic noise generated by gradient systems in magnetic resonance imaging (MRI) is proposed based on the linear response theory. Since the acoustic frequency response function of typical gradient coils is low in the range below 200 Hz, the noise level can be significantly reduced by using gradient pulse sequences whose spectra are limited to this frequency range. Such “soft,” i.e., band‐limited, pulse shapes can be designed using sinusoidal ramps individually adjusted to available delays. “Silent” versions of three basic MRI sequences [gradient‐echo (GE), spin‐echo (SE), and rapid acquisition with relaxation enhancement (RARE)] were programmed on 2 and 3 T whole‐body scanners. High‐quality images could be acquired at noise levels as low as 40 dBA (GE and SE) and 60 dBA (RARE). Magn Reson Med 42:6–10, 1999.

MRI with zero echo time: Hard versus sweep pulse excitation

Zero echo time can be obtained in MRI by performing radiofrequency (RF) excitation as well as acquisition in the presence of a constant gradient applied for purely frequency‐encoded, radial centre‐out k‐space encoding. In this approach, the spatially nonselective excitation must uniformly cover the full frequency bandwidth spanned by the readout gradient. This can be accomplished either by short, hard RF pulses or by pulses with a frequency sweep as used in the SWIFT (Sweep imaging with Fourier transform) method for improved performance at limited RF amplitudes. In this work, the two options are compared with respect to T2 sensitivity, signal‐to‐noise ratio (SNR), and SNR efficiency. In particular, the SNR implications of sweep excitation and of initial or periodical acquisition gaps required for transmit‐receive switching are investigated. It was found by simulations and experiments that, whereas equivalent in terms of T2 sensitivity, the two techniques differ in SNR performance. With ideal, ungapped simultaneous excitation and acquisition, the sweep approach would yield higher SNR throughout due to larger feasible flip angles. However, acquisition gapping is found to take a significant SNR toll related to a reduced acquisition duty cycle, rendering hard pulse excitation superior for sufficient RF amplitude and also in the short‐T2 limit. Magn Reson Med, 2011.

Sweep MRI with algebraic reconstruction.

In the recently proposed technique Sweep Imaging with Fourier Transform (SWIFT), frequency‐modulated radiofrequency pulses are used in concert with simultaneous acquisition to facilitate MRI of samples with very short transverse relaxation time. In the present work, sweep MRI is reviewed from a reconstruction perspective and several extensions and modifications of the current methodology are proposed. An algorithm for algebraic image reconstruction is derived from a comprehensive description of signal formation, including interleaved radiofrequency transmission and acquisition of arbitrary timing as well as the relevant filtering and decimation steps along the receiver chain. The new reconstruction approach readily permits several measures of optimising the signal sampling strategy as demonstrated in simulations and imaging experiments. Employing a variety of radiofrequency pulse envelopes, water and rubber phantoms as well as bone samples with transverse relaxation time in the order of 500 μsec were imaged at signal bandwidths of up to 96 kHz. Magn Reson Med, 2010.

Radial spectroscopic MRI of hyperpolarized [1‐13C] pyruvate at 7 tesla

The transient and nonrenewable signal from hyperpolarized metabolites necessitates extensive sequence optimization for encoding spatial, spectral, and dynamic information. In this work, we evaluate the utility of radial single‐timepoint and cumulative spectroscopic MRI of hyperpolarized [1‐13C] pyruvate and its metabolic products at 7 Tesla (T).

SENSE reconstruction for multiband EPI including slice-dependent N/2 ghost correction.

Sensitivity encoding (SENSE) reconstruction of multiband echo planar imaging (EPI) may cause artifacts when simultaneously excited slices require different phase correction to remove the EPI‐specific ghost shifted by half of the matrix size (N). We propose a simplified solution of this problem that combines SENSE unfolding with the EPI phase correction in the image domain.

ZTE imaging with enhanced flip angle using modulated excitation

Zero echo time (ZTE) imaging is a fast, robust, and silent three‐dimensional technique for direct MRI of tissues with rapid transverse relaxation. It is conventionally performed with hard, block‐shaped excitation pulses short enough to excite spins uniformly over a large bandwidth. With this approach, the achievable flip angle (FA) is limited by the available B1 amplitude. The purpose of this work is to accomplish ZTE imaging with larger FAs by combined amplitude and frequency modulation of the excitation pulse while keeping the pulse duration short enough to limit acquisition dead time.

A high-performance gradient insert for rapid and short-T2 imaging at full duty cycle

The goal of this study was to devise a gradient system for MRI in humans that reconciles cutting‐edge gradient strength with rapid switching and brings up the duty cycle to 100% at full continuous amplitude. Aiming to advance neuroimaging and short‐T2 techniques, the hardware design focused on the head and the extremities as target anatomies.

MRI with phaseless encoding

Fourier encoded MRI signal is complex and, therefore, sensitive to uncontrolled phase variations caused, e.g., by object motion. An alternative encoding is proposed which leads to phaseless (positive real) signals and allows the phase fluctuations to be removed by simple magnitude calculation before the Fourier transform.

In-plane “superresolution” MRI with phaseless sub-pixel encoding

Acquisition of high‐resolution imaging data using multiple excitations without the sensitivity to fluctuations of the transverse magnetization phase, which is a major problem of multi‐shot MRI.