Resolution enhancement in single-shot imaging using simultaneous acquisition of spatial harmonics (SMASH).

Spatial resolution in single-shot imaging is limited by signal attenuation due to relaxation of transverse magnetization. This effect can be reduced by minimizing acquisition times through the use of short interecho spacings. However, the minimum interecho spacing is constrained by limits on gradient switching rates, radiofrequency (RF) power deposition and RF pulse length. Recently, simultaneous acquisition of spatial harmonics (SMASH) has been introduced as a method to acquire magnetic resonance images at increased speeds using a reduced number of phase-encoding gradient steps by extracting spatial information contained in an RF coil array. In this study, it is shown that SMASH can be used to reduce the effects of relaxation, resulting in single-shot images with increased spatial resolution without increasing imaging time. After a brief theoretical discussion, two strategies to reduce signal attenuation and increase spatial resolution in single-shot imaging are introduced and their performance is evaluated in phantom studies. In vivo single-shot echoplanar imaging (EPI), BURST, and half-Fourier single-shot turbo spin-echo (HASTE) images are then presented demonstrating the practical implementation of these resolution enhancement strategies. Images acquired with SMASH show increased spatial resolution and improved image quality when compared with images obtained with the conventional acquisitions. The general principles presented for imaging with SMASH can also be applied to other partially parallel imaging techniques.     

肝细胞癌射频治疗后的磁共振表现

目的：评价MRI平扫及Gd-DTPA动态增强对射频治疗肝细胞癌疗效随访的价值。材料和方法：回顾性分析了55例RF术前及术后的MRI表现,并与活检及长期随访结果进行对照。结果：RF术前病灶在T1加权上多呈相对低信号、T2加权像上呈相对高信号,RF术后病灶转变成T1加权较高信号、T2加权均匀一致的等低信号,残存的癌结节表现为病灶周围包膜不规整或周边小结节并在动态增强时呈单峰型“快进快出”强化表现,活检提示癌灶残存。结论：MR平扫评价RF的疗效基本可靠,动态增强能进一步提高评价信心及准确率。RF术后定期MR随访具有重要意义。     

Implementation of three‐dimensional wavelet encoding spectroscopic imaging: In vivo application and method comparison

We have recently proposed a two‐dimensional Wavelet Encoding‐Spectroscopic Imaging (WE‐SI) technique as an alternative to Chemical Shift Imaging (CSI), to reduce acquisition time and crossvoxel contamination in magnetic resonance spectroscopic imaging (MRSI). In this article we describe the extension of the WE‐SI technique to three dimensions and its implementation on a clinical 1.5 T General Electric (GE) scanner. Phantom and in vivo studies are carried out to demonstrate the usefulness of this technique for further acquisition time reduction with low voxel contamination. In wavelet encoding, a set of dilated and translated prototype functions called wavelets are used to span a localized space by dividing it into a set of subspaces with predetermined sizes and locations. In spectroscopic imaging, this process is achieved using radiofrequency (RF) pulses with profiles resembling the wavelet shapes. Slice selective excitation and refocusing RF pulses, with single‐band and dual‐band profiles similar to Haar wavelets, are used in a modified PRESS sequence to acquire 3D WE‐SI data. Wavelet dilation and translation are achieved by changing the strength of the localization gradients and frequency shift of the RF pulses, respectively. The desired spatial resolution in each direction sets the corresponding number of dilations (increases in the localization gradients), and consequently, the number of translations (frequency shift) of the Haar wavelets (RF pulses), which are used to collect magnetic resonance (MR) signals from the corresponding subspaces. Data acquisition time is reduced by using the minimum recovery time (TR_min), also called effective time, when successive MR signals from adjacent subspaces are collected. Inverse wavelet transform is performed on the acquired data to produce metabolite maps. The proposed WE‐SI method is compared in terms of acquisition time, pixel bleed, and signal‐to‐noise ratio to the CSI technique. The study outcome shows that 3D WE‐SI provides accurate results while reducing both acquisition time and voxel contamination. Magn Reson Med 61:6–15, 2009. © 2008 Wiley‐Liss, Inc.     

Cardiac magnetic resonance parallel imaging at 3.0 Tesla: technical feasibility and advantages

PURPOSE: To quantify changes in signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), specific absorption rate (SAR), RF power deposition, and imaging time in cardiac magnetic resonance imaging with and without the application of parallel imaging at 1.5 T and 3.0 T. MATERIALS AND METHODS: Phantom and volunteer data were acquired at 1.5 T and 3.0 T with and without parallel imaging. RESULTS: Doubling field strength increased phantom SNR by a factor of 1.83. In volunteer data, SNR and CNR values increased by factors of 1.86 and 1.35, respectively. Parallel imaging (reduction factor = 2) decreased phantom SNR by a factor of 1.84 and 2.07 when compared to the full acquisition at 1.5 T and 3.0 T, respectively. In volunteers, SNR and CNR decreased by factors of 2.65 and 2.05 at 1.5 T and 1.99 and 1.75 at 3.0 T, respectively. Doubling the field strength produces a nine-fold increase in SAR (0.0751 to 0.674 W/kg). Parallel imaging reduced the total RF power deposition by a factor of two at both field strengths. CONCLUSIONS: Parallel imaging decreases total scan time at the expense of SNR and CNR. These losses are compensated at higher field strengths. Parallel imaging is effective at reducing total power deposition by reducing total scan time.     

Phase-offset multiplanar (POMP) volume imaging: a new technique.

Phase-offset multiplanar (POMP) imaging is a technique that excites several sections simultaneously for improved imaging efficiency. The centers of the reconstructed images from each of the POMP sections are offset from each other in the phase-encoding direction by means of view-dependent phase modulation of the radio-frequency (RF) excitation pulses and are placed adjacent to each other in the reconstruction. With a suitable reconstruction matrix size, the images can be made nonoverlapping and stored separately. At constant imaging time, signal-to-noise ratio (S/N), and resolution, POMP imaging produces a factor NP more sections than a conventional sequence but with a reduced field of view. Alternatively, imaging time may be increased by the factor NP to retain the same field of view but with the expected S/N advantage. The average RF power deposited by the 90 degrees composite RF pulse is greater by the factor Np, but the power for the 180 degrees pulse is unchanged. The POMP method is discussed and compared with three-dimensional and Hadamard techniques.     

Simultaneous multislice (SMS) imaging techniques

Simultaneous multislice imaging (SMS) using parallel image reconstruction has rapidly advanced to become a major imaging technique. The primary benefit is an acceleration in data acquisition that is equal to the number of simultaneously excited slices. Unlike in‐plane parallel imaging this can have only a marginal intrinsic signal‐to‐noise ratio penalty, and the full acceleration is attainable at fixed echo time, as is required for many echo planar imaging applications. Furthermore, for some implementations SMS techniques can reduce radiofrequency (RF) power deposition. In this review the current state of the art of SMS imaging is presented. In the Introduction, a historical overview is given of the history of SMS excitation in MRI. The following section on RF pulses gives both the theoretical background and practical application. The section on encoding and reconstruction shows how the collapsed multislice images can be disentangled by means of the transmitter pulse phase, gradient pulses, and most importantly using multichannel receiver coils. The relationship between classic parallel imaging techniques and SMS reconstruction methods is explored. The subsequent section describes the practical implementation, including the acquisition of reference data, and slice cross‐talk. Published applications of SMS imaging are then reviewed, and the article concludes with an outlook and perspective of SMS imaging. Magn Reson Med 75:63–81, 2016. © 2015 The Authors. Magnetic Resonance in Medicine Published by Wiley Periodicals, Inc. on behalf of International Society of Medicine in Resonance.     

Transmit SENSE.

The idea of using parallel imaging to shorten the acquisition time by the simultaneous use of multiple receive coils can be adapted for the parallel transmission of a spatially-selective multidimensional RF pulse. As in data acquisition, a multidimensional RF pulse follows a certain k-space trajectory. Shortening this trajectory shortens the pulse duration. The use of multiple transmit coils, each with its own time-dependent waveform and spatial sensitivity, can compensate for the missing parts of the excitation k-space. This results in a maintained spatial definition of the pulse profile, while its duration is reduced. This work introduces the concept of parallel transmission with arbitrarily shaped transmit coils (termed "Transmit SENSE"). Results of numerical studies demonstrate the theoretical feasibility of the approach. The experimental proof of principle is provided on a commercial MR scanner. The lack of multiple independent transmit channels was addressed by combining the excitation patterns from two separate subexperiments with different transmit setups. Shortening multidimensional RF pulses could be an interesting means of making 3D RF pulses feasible even for fast T(2)(*) relaxing species or strong main field inhomogeneities. Other applications might benefit from the ability of Transmit SENSE to improve the spatial resolution of the pulse profile while maintaining the transmit duration.     

Simulation-based investigation of partially parallel imaging with a linear array at high accelerations.

Partially parallel imaging strategies such as SMASH, SENSE, and PILS rely on the sensitivity distribution of phased array RF coils to reduce MRI imaging time. Using an N-element phased array, these techniques allow maximum accelerations, L, such that L < or = N, with acceleration defined as the factor by which scan time is reduced in comparison to traditional, fully gradient encoded acquisitions. As N increases in modern MRI facilities or using special hardware extensions, its role as the primary limitation in partially parallel imaging will be reduced and other limiting factors will become dominant. Two such factors include available SNR and the variation of sensitivity distributions with imaging depth. Simulations have been conducted to evaluate the impact of slice depth and noise on partially parallel reconstructions for the case of a square linear array of overlapped elements that are parallel to the imaging plane. Results indicate that even when sensitivity distributions are exactly known, the linear surface array can only provide high accelerations over a limited imaging depth due to changing suitability of the sensitivity distributions for partially parallel reconstruction. This work emphasizes the importance of simulations for target-based partially parallel array design.     

Angiographic Imaging with 2D RF Pulses

Magnetic resonance angiography (MRA) was performed by using RF pulses designed to excite a limited spatial extent in two orthogonal directions. The restriction in the second spatial dimension can be used to increase inflow enhancement and to improve small field-of-view imaging. A rectangular excitation was produced with an “echo-planar” k-space trajectory and a sine-modulated RF waveform. In vivo images have demonstrated that vessels are more clearly delineated with the two-dimensional excitation. Aliasing artifacts in small field-of-view imaging are significantly reduced, although in some cases complete elimination is not possible due to the nature of the gradient trajectory.     

彩色多普勒血流显像随访54例射频治疗肝癌的疗效观察

 目的应用彩色多普勒血流显像(Color Dopplerflow imaging,CDFI)评价集束电极射频(简称射频)治疗肝癌6～18月的疗效。方法应用CDFI对射频治疗前及治疗后6、12和18月的肝癌患者肿块及肝动脉的血流动力学情况进行观察。结果射频治疗后6、12、18月肿块血流信号消失或明显减少,血流信号消失率分别为73.5％、51.1％、41.2％;仍能检测到血流信号的肿块最大血流速度(Vmax)及及阻力指数(RI)无明显变化;肝动脉的内径、Vmax及RI降低。结论CDFI对集束电极射频治疗肝癌后的血流动力学变化的疗效判断具有重要临床意义。     

Multishot EPI-SSFP in the heart.

Refocused steady-state free precession (SSFP), or fast imaging with steady precession (FISP or TrueFISP), has recently proven valuable for cardiac imaging because of its high signal-to-noise ratio (SNR) and excellent blood-myocardium contrast. In this study, various implementations of multiecho SSFP or EPI-SSFP for imaging in the heart are presented. EPI-SSFP has higher scan-time efficiency than single-echo SSFP, as two or more phase-encode lines are acquired per repetition time (TR) at the cost of a modest increase in TR. To minimize TR, a noninterleaved phase-encode order in conjunction with a phased-array ghost elimination (PAGE) technique was employed, removing the need for echo time shifting (ETS). The multishot implementation of EPI-SSFP was used to decrease the breath-hold duration for cine acquisitions or to increase the temporal or spatial resolution for a fixed breath-hold duration. The greatest gain in efficiency was obtained with the use of a three-echo acquisition. Image quality for cardiac cine applications using multishot EPI-SSFP was comparable to that of single-echo SSFP in terms of blood-myocardium contrast and contrast-to-noise ratio (CNR). The PAGE method considerably reduced flow artifacts due to both the inherent ghost suppression and the concomitant reduction in phase-encode blip size. The increased TR of multishot EPI-SSFP led to a reduced specific absorption rate (SAR) for a fixed RF flip angle, and allowed the use of a larger flip angle without increasing the SAR above the FDA-approved limits.     

Wide-bore 1.5 Tesla MR imagers for guidance and monitoring of radiofrequency ablation of renal cell carcinoma: initial experience on feasibility

This study was conducted to test and demonstrate the feasibility of magnetic resonance (MR)-guided radiofrequency (RF) ablation of renal cell carcinoma (RCC) using a 1.5 T whole-body scanner equipped with a wide-bore superconductive magnet. Two patients with contrast-enhancing renal masses were treated with multipolar RF ablation (Celon ProSurge). Applicator navigation and near real-time ablation monitoring were performed in a wide-bore 1.5 T scanner using adapted fluoroscopic and diagnostic sequences. In addition to T2-weighted imaging for ablation monitoring, perfusion-weighted images acquired with an arterial spin-labeling technique (FAIR-TrueFISP) were applied. Results were compared to a previous study on 12 patients performed at 0.2 T. Navigation and monitoring of RF ablation using the wide-bore system operating at 1.5 T were clearly improved compared to former experiences on a 0.2 T MR unit. Fluoroscopic and diagnostic images for MR guidance could be acquired with distinctly higher image quality and shorter acquisition time resulting in higher accuracy of applicator placement and shorter treatment time. Spin-labeling perfusion imaging exhibited good image quality, potentially providing additional clinically important information. MR-guided RF ablation of RCC can safely be performed in a 1.5 T wide-bore scanner offering higher image quality, shorter acquisition time, and new monitoring modalities not feasible at 0.2 T.     

Single-point (constant-time) imaging in radiofrequency Fourier transform electron paramagnetic resonance.

This study describes the use of the single-point imaging (SPI) modality, also known as constant-time imaging (CTI), in radiofrequency (RF) Fourier transform (FT) electron paramagnetic resonance (EPR). The SPI technique, commonly used for high-resolution solid-state nuclear magnetic resonance (NMR) imaging, has been successfully applied to 2D and 3D RF-FT-EPR imaging of phantoms containing narrow-line EPR spin probes. The SPI scheme is essentially a phase-encoding technique that operates by acquiring a single data point in the free induction decay (FID) after a fixed delay (phase-encoding time), following the pulsed RF excitation, in the presence of static magnetic field gradients. Since the phase-encoding time remains constant for a given image data set, the spectral information is automatically deconvolved, providing well-resolved pure spatial images. Therefore, images obtained using SPI are artifact-free and the resolution is not significantly limited by the line width, compared to the images obtained using the conventional filtered back-projection (FBP) scheme, suggesting that the SPI modality may have advantages for EPR imaging of large objects. In this work the advantages and limitations of SPI as compared to FBP are investigated by imaging suitable phantom objects. Although SPI takes longer to perform than the FBP method, optimization of the data collection scheme may increase the temporal resolution, rendering this technique suitable for in vivo studies. Spectral information can also be extracted from a series of SPI images that are generated as a function of the delay from the excitation pulse.     

Fast high-resolution T1 mapping of the human brain.

A sequence for the acquisition of high-resolution T1 maps, based on magnetization-prepared multislice fast low-angle shot (FLASH) imaging, is presented. In contrast to similar methods, no saturation pulses are used, resulting in an increased dynamic range of the relaxation process. Furthermore, it is possible to acquire data during all relaxation delays because only slice-selective radiofrequency (RF) pulses are used for inversion and excitation. This allows for a reduction of the total acquisition time, or scanning with a reduced bandwidth, which improves the signal-to-noise ratio (SNR). The method generates quantitative T1 maps with an in-plane resolution of 1 mm, slice thickness of 4 mm, and whole-brain coverage in a clinically acceptable imaging time of about 19 s per slice. It is shown that the use of off-center RF pulses does not result in imperfect inversion or magnetization transfer (MT) effects. In addition, an improved fitting algorithm based on smoothed flip angle maps is presented and tested successfully.     

PROPELLER EPI: an MRI technique suitable for diffusion tensor imaging at high field strength with reduced geometric distortions.

A technique suitable for diffusion tensor imaging (DTI) at high field strengths is presented in this work. The method is based on a periodically rotated overlapping parallel lines with enhanced reconstruction (PROPELLER) k-space trajectory using EPI as the signal readout module, and hence is dubbed PROPELLER EPI. The implementation of PROPELLER EPI included a series of correction schemes to reduce possible errors associated with the intrinsically higher sensitivity of EPI to off-resonance effects. Experimental results on a 3.0 Tesla MR system showed that the PROPELLER EPI images exhibit substantially reduced geometric distortions compared with single-shot EPI, at a much lower RF specific absorption rate (SAR) than the original version of the PROPELLER fast spin-echo (FSE) technique. For DTI, the self-navigated phase-correction capability of the PROPELLER EPI sequence was shown to be effective for in vivo imaging. A higher signal-to-noise ratio (SNR) compared to single-shot EPI at an identical total scan time was achieved, which is advantageous for routine DTI applications in clinical practice.     

Reduced field-of-view MRI with two-dimensional spatially-selective RF excitation and UNFOLD.

When the region of interest (ROI) is smaller than the object, one can increase MRI speed by reducing the imaging field of view (FOV). However, when such an approach is used, features outside the reduced FOV will alias into the reduced-FOV image along the phase-encoding direction. Reduced-FOV methods are designed to correct this aliasing problem. In the present study, we propose a combination of two different approaches to reduce the acquired FOV: 1) two-dimensional (2D) spatially-selective RF excitation, and 2) the unaliasing by Fourier-encoding the overlaps using the temporal dimension (UNFOLD) technique. While 2D spatially-selective RF excitation can restrict the spins excited within a reduced FOV, the UNFOLD technique can help to eliminate any residual aliased signals and thus relaxes the requirement for a long RF excitation pulse. This hybrid method was implemented for MR-based temperature mapping, and resulted in artifact-free images with a fourfold improvement in temporal resolution.     

New FOCI pulses with reduced radiofrequency power requirements

PURPOSE: To propose new frequency offset corrected inversion (FOCI) pulses with significantly reduced radiofrequency (RF) power deposition for spin echo imaging by incorporating the variable-rate selective excitation (VERSE) schemes into the pulse design. MATERIALS AND METHODS: Two schemes are proposed to design the new FOCI pulses with dramatically reduced peak RF power requirements. In scheme A, the time-dilation function is derived from a predefined adiabaticity factor modulation function. In scheme B, the time-dilation function is predefined, while the adiabaticity factor is conserved. RESULTS: The new FOCI pulses are shown to be able to operate at reduced specific absorption rate (SAR), specifically at the same peak RF power as that of a five- or seven-lobe sinc inversion pulse of the same duration. Using the new FOCI pulse, significant gain in sensitivity was observed in in vivo spin-echo echo-planar imaging, which was attributed to the improved refocusing slice profile. CONCLUSION: The new FOCI pulses can replace the 180 degrees five- or seven-lobe sinc pulses in spin-echo imaging with the same peak RF power requirement and significantly improved slice profile.     

Rotating Frame RF Current Density Imaging

RF current density imaging (RF-CDI) is a new MRI technique for imaging the Larmor frequency current density parallel to B₀ in electrolytic media. To extend the use of RF-CDI to biological tissue for generating conductivity contrast, the sensitivity must be increased and the data requirements reduced. A rotating frame approach, in which a large B₁ field is applied simultaneously as a rotary echo with RF current, is proposed to meet these requirements. Rotating frame magnetic fields are encoded in the phase of an MRI image. Trials have now been performed with this sequence in a three-compartment cylindrical phantom containing doped water or mineral oil for detecting displacement, conduction and fringe field currents. In a postmortem rat study, 85.56 MHz RF currents injected by implanted electrodes created tissue dependent contrast because of the electrical properties of tissue. A sensitivity and artifact analysis was also performed. The sensitivity of this method is determined by the maximum RF pulse duration. SAR limits pose an upper bound on this time and B₁, whereas the avoidance of phase artifacts imposes a lower bound on B₁.     

Dual contrast GRASE (gradient-spin echo) imaging using mixed bandwidth

Equal time spacing of RF pulses in the CPMG sequence imposes a constraint of equal signal read periods in spin-echo train imaging. GRASE imaging differs by using multiple read gradients in each π-π time interval, which are not constrained to be equal in number or duration. This additional degree of freedom is developed in dual contrast imaging. Closely spaced read periods are used for the PDW image to reduce T₂ decay effects, while fewer low-bandwidth read periods in each of several π-π intervals are used to raise the signal-to-noise ratio and avoid signal averaging in the T₂-weighted image. Key words: magnetic resonance imaging; gradient-spin echo; fast imaging.     

Parallel imaging of hyperpolarized helium-3 with simultaneous slice excitation.

Hyperpolarized (HP) gas imaging of the lungs is an ideal potential application for parallel imaging. This is due to the fact that there is limited scan time (breath hold of 20 s) and limited non-renewable polarization. Reduced phase encode parallel imaging is demanding on hardware in that it requires multiple receivers. In this work, simultaneous parallel acquisition of hyperpolarized (HP) 3He images from multiple slices was demonstrated in phantoms and in vivo using a simultaneous slice excitation method, at a field strength of 1.5 T. The pulse sequence allows simultaneous acquisition of n slices per RF excitation, thus reducing the number of RF pulses needed to fully cover a given volume with multi-slicing. Unlike conventional parallel imaging, this method does not require prior reference scan information, which would consume some of the finite longitudinal polarization in lung ventilation studies with HP gas.