Direct radiofrequency phase control in MRI by digital waveform playback at the Larmor frequency.

The goal of this work was to increase the level of acceleration using multiband excitation pulses as described in Refs. (1) and (2). These pulses reduce the time needed to acquire a single volumetric gradient-recalled echo-planar image of the brain. Multichannel coils are used to separate overlapping slices as described in Refs. (3–5). In an abstract in 2009 (1), we showed that the number of receivers can be reduced when the relative phases of excited slices are set properly. In addition, it was shown that a single receiver channel is sufficient for 2-fold acceleration when the relative phase of slices is set to 90°. The thrust of this paper is improved formation of multiband excitation pulses, which has not been explicitly considered in previous publications in the field. The standard gradient-echo (GE) eight-channel receive-only high-resolution brain array coil was used for publications (1) and (2). Accelerations greater than two were reported in Ref. (1), but separated images were of lower quality. In this work, the presence of ghost slices of low amplitude for spectral separations larger than 50 kHz was observed. Ghosts were visible on images of a ball-shaped phantom due to different diameters of cross sections. It was concluded that the limit of the eight-channel system had been reached. After updating the acquisition system to 32 channels, these experiments were repeated. Two different 32-channel receive-only coils were evaluated. To our surprise, at 4-fold acceleration the separated images with 32 and eight channels showed similar poor quality. Ghost slices remained visible. Development of a different method of radiofrequency (RF) pulse creation rather than the Fourier transform approximation was considered (6). First, however, RF pulses were produced and observed with a pickup loop placed in the magnet, which immediately revealed the source of the problem. Figure 1a shows a spectral profile of a pulse created by the standard modulator of the GE Signa EXCITE 3T scanner. The two main ghost slices on either side of four prescribed slices have amplitudes of 36%. These cause significant interference in the separated images because reference slices that are used in the separation algorithm do not include these ghosts. There are other ghosts of lower amplitudes in the spectrum, which also disrupt the separation. The spectrum of the same pulse played on the GE MR750 3T scanner is shown in Figure 1b. It is better but two main ghosts exhibit amplitudes of 9% and several lower amplitude ghosts are also visible. FIG. 1 Four-fold multiband spectral profiles created by: (a) the GE 3-T Signa EXCITE original modulator; (b) the GE 3-T MRI750 original modulator; and (c) the Pentek 78621 16-bit digital waveform synthesizer. The algorithm used to create tailored pulses produces two digital arrays, one with amplitude data (14 bits) and one with phase (12 bits) data, which are loaded to a transceiver processor and storage exciter in 2-μs increments in the GE Signa EXCITE system. A pulse simulation program showed that even when applying high nonlinearities to the amplitude and phase, distortions as seen in Figure 1a cannot be obtained. However, when phase data were shifted by 3 μs against the amplitude data, the simulator replicated the pattern of Figure 1a. Correction of this behavior requires a shift of digital phase data by 3 μs on hardware with 2-μs update time only. The Fourier transform shift theorem was implemented in the magnetic resonance imaging (MRI) sequence and artifacts were minimized when using a 3-μs shift. The same sequence when played on the MR750 system showed the best result at a 0-μs shift, which leads to the conclusion that although the MR750 is improved, a flaw remains in the system. In spite of the fact that the spectrum is better than the corrected one on the EXCITE system, as seen in Figure 1b, it was still judged to be inadequate. To improve the spectral quality of RF pulses, a Pentek Waveform Playback PCIe card model 78621 (Upper Saddle River, NJ), which is based on the Texas Instrument DAC5688 chip, was evaluated. This card is controlled by a Virtex-6 FPGA (field-programmable gate array) and is mainly intended to produce radar beams. This card can be synchronized with others of the same type. It has absolute phase accuracy and timing required for varying radar beams without the need for antenna rotation. In the interpolate mode, the D/A converter of this card creates RF pulses with a 2-ns sampling time as well as smooth, stair-step-less modulation of the I and Q channels at a 16-bit resolution. As a result, the spectral quality of an RF pulse created with the same algorithm as pulses in Figure 1a is better as shown in Figure 1c. The same power transmitter and pickup loop were used to acquire RF pulses for all images. Figure 2 shows several excerpts of RF pulses produced by different modulators and acquired by an Agilent digital oscilloscope DOS6104A. Figure 2a shows the central 1.2-ms region of a 6.4-ms RF pulse. Three other frames: b, c, and d show the short part of the pulse in frame 1 marked by green color. Stair steps of RF modulation are clearly visible for both 3-T Signa EXCITE and MR750 scanners. There are no such steps in the pulse created by the Pentek 78621 card. The spectrum of the pulse created by the DAC5688 chip as seen in Figure 1c has at least two orders of magnitude lower out-of-slice excitations compared with the spectrum of Figure 1a. Ghost slices are not evident. For comparison, the simulation of an ideal stair-step modulation showed harmonics far away from the central spectrum, which do not excite protons inside the imaging object. It is hypothesized that the rounding of steps, as seen in Figure 2d, is the source of the ghost artifacts seen in Figure 1b. FIG. 2 Oscillograms of RF pulses obtained by different modulators: (a) the central region of a 6.4-ms long RF pulse at a resolution of 200 μs/cm. The 12-μs segment marked by green is expanded in other windows at a resolution of 2 μs/cm; ... The unprecedented spectral phase quality and time-course stability of pulses obtained using the DAC5688 chip have another benefit. The 90° RF pulses used in standard two-dimensional MRI methods were programmed to achieve interslice phase coherency that is usually lost because of frequency offsets from the central Larmor frequency. The benefit of this technology becomes more important with increase in the magnetic field of whole-body MRI scanners where phase images of the brain (7,8) carry more information than amplitude images. With phase alignment between all two-dimensional slices, consistent phase analysis in three dimensions can be carried out without the need for additional (and rather long) three-dimensional acquisition. Moreover, in the realm of echo-planar imaging, especially at high resolution, phase contrast in an arbitrary oblique plane can be obtained by postprocessing the full set of phase coherent slices. Another benefit of the DAC5688 chip was discovered. The phase stability is so high that it is no longer necessary to use a phase reference in the receiver. This was an incidental finding that is not central to the paper. Images with and without a reference were compared.