Jing Yuan

Combining Two-Dimensional Spatially Selective RF Excitation, Parallel Imaging, and UNFOLD for Accelerated MR Thermometry Imaging

MR thermometry can be a very challenging application, as good resolution may be needed along spatial, temporal, and temperature axes. Given that the heated foci produced during thermal therapies are typically much smaller than the anatomy being imaged, much of the imaged field‐of‐view is not actually being heated and may not require temperature monitoring. In this work, many‐fold improvements were obtained in terms of temporal resolution and/or 3D spatial coverage by sacrificing some of the in‐plane spatial coverage. To do so, three fast‐imaging approaches were jointly implemented with a spoiled gradient echo sequence: (1) two‐dimensional spatially selective RF excitation, (2) unaliasing by Fourier encoding the overlaps using the temporal dimension (UNFOLD), and (3) parallel imaging. The sequence was tested during experiments with focused ultrasound heating in ex vivo tissue and a tissue‐mimicking phantom. Temperature maps were estimated from phase‐difference images based on the water proton resonance frequency shift. Results were compared to those obtained from a spoiled gradient echo sequence sequence, using a t‐test. Temporal resolution was increased by 24‐fold, with temperature uncertainty less than 1°C, while maintaining accurate temperature measurements (mean difference between measurements, as observed in gel = 0.1°C ± 0.6; R = 0.98; P > 0.05). Magn Reson Med 66:112–122, 2011.

Multipathway sequences for MR thermometry

MR‐based thermometry is a valuable adjunct to thermal ablation therapies as it helps to determine when lethal doses are reached at the target and whether surrounding tissues are safe from damage. When the targeted lesion is mobile, MR data can further be used for motion‐tracking purposes. The present work introduces pulse sequence modifications that enable significant improvements in terms of both temperature‐to‐noise‐ratio properties and target‐tracking abilities. Instead of sampling a single magnetization pathway as in typical MR thermometry sequences, the pulse‐sequence design introduced here involves sampling at least one additional pathway. Image reconstruction changes associated with the proposed sampling scheme are also described. The method was implemented on two commonly used MR thermometry sequences: the gradient‐echo and the interleaved echo‐planar imaging sequences. Data from the extra pathway enabled temperature‐to‐noise‐ratio improvements by up to 35%, without increasing scan time. Potentially of greater significance is that the sampled pathways featured very different contrast for blood vessels, facilitating their detection and use as internal landmarks for tracking purposes. Through improved temperature‐to‐noise‐ratio and lesion‐tracking abilities, the proposed pulse‐sequence design may facilitate the use of MR‐monitored thermal ablations as an effective treatment option even in mobile organs such as the liver and kidneys. Magn Reson Med, 2011.

Quality factor of Bi(2223) high-temperature superconductor tape coils at radio frequency

Bi(2223) high-temperature superconductor (HTS) tapes have been chosen for radio-frequency (rf) coils in magnetic resonance imaging (MRI) applications recently. This is because they are easier to fabricate and adjust, and have lower costs compared to HTS films. We study the quality factor (Q) of Bi(2223) HTS rf coils in the rf range. Several rf coils with different sizes and resonant frequencies have been fabricated and the values of Q measured at 77 K. A theoretical model is developed to investigate the relationship of Q with coil size, solder joint and resonant frequency. The model shows that Q increases with larger size and higher frequency. The value of Q of a five-inch HTS tape coil is about six times higher than a copper coil with the same size, and is expected to have 2.5 times signal-to-noise ratio improvement in MRI.

Reduced field-of-view single-shot fast spin echo imaging using two-dimensional spatially selective radiofrequency pulses

To demonstrate reduced field‐of‐view (RFOV) single‐shot fast spin echo (SS‐FSE) imaging based on the use of two‐dimensional spatially selective radiofrequency (2DRF) pulses.

A 4-Channel Coil Array Interconnection by Analog Direct Modulation Optical Link for 1.5-T MRI

Optical glass fiber shows great advantages over coaxial cables in terms of electromagnetic interference, thus, it should be considered a potential alternative for magnetic resonance imaging (MRI) receive coil interconnection, especially for a large number coil array at high field. In this paper, we propose a 4-channel analog direct modulation optical link for a 1.5-T MRI coil array interconnection. First, a general direct modulated optical link is compared to an external modulated optical link. And then the link performances of the proposed direct modulated optical link, including power gain, frequency response, and dynamic range, are analyzed and measured. Phantom and in vivo head images obtained using this optical link are demonstrated for comparison with those obtained by cable connections. The signal-to-noise (SNR) analysis shows that the optical link achieves 6%-8% SNR a improvement over coaxial cables by elimination of electrical interference between cables during MR signal transmission.

Fast fat-suppressed reduced field-of-view temperature mapping using 2DRF excitation pulses

The purpose of this study is to develop a fast and accurate temperature mapping method capable of both fat suppression and reduced field-of-view (rFOV) imaging, using a two-dimensional spatially-selective RF (2DRF) pulse. Temperature measurement errors caused by fat signals were assessed, through simulations. An 11×1140μs echo-planar 2DRF pulse was developed and incorporated into a gradient-echo sequence. Temperature measurements were obtained during focused ultrasound (FUS) heating of a fat-water phantom. Experiments both with and without the use of a 2DRF pulse were performed at 3T, and the accuracy of the resulting temperature measurements were compared over a range of TE values. Significant inconsistencies in terms of measured temperature values were observed when using a regular slice-selective RF excitation pulse. In contrast, the proposed 2DRF excitation pulse suppressed fat signals by more than 90%, allowing good temperature consistency regardless of TE settings. Temporal resolution was also improved, from 12 frames per minute (fpm) with the regular pulse to 28 frames per minute with the rFOV excitation. This technique appears promising toward the MR monitoring of temperature in moving adipose organs, during thermal therapies.

Fat–water selective excitation in balanced steady-state free precession using short spatial–spectral RF pulses

Fat suppression is important but challenging in balanced steady-state free precession (bSSFP) acquisitions, for a number of clinical applications. In the present work, the practicality of performing fat-water selective excitations using spatial-spectral (SPSP) RF pulses in bSSFP sequence is examined. With careful pulse design, the overall duration of these SPSP pulses was kept short to minimize detrimental effects on TR, scan time and banding artifact content. Fat-water selective excitation using SPSP pulses was demonstrated in both phantom and human bSSFP imaging at 3T, and compared to results obtained using a two-point Dixon method. The sequence with SPSP pulses performed better than the two-point Dixon method, in terms of scan time and suppression performance. Overall, it is concluded here that SPSP RF pulses do represent a viable option for fat-suppressed bSSFP imaging.

Concatenated and parallel optimization for the estimation of T1 map in FLASH MRI with multiple flip angles

Most traditional methods for T1 map estimation in MRI with fast low‐angle‐shot sequences are aimed at high efficiency by compromising the fitting accuracy. In this paper, the fundamental problem of parameter estimation in fast low‐angle‐shot MRI was re‐examined, and an accurate and fast optimization approach, named concatenated optimization for parameter estimation, was proposed for the regression of data points acquired with multiple flip angles. The initial estimation of T1 was obtained from the linear regression, followed by the constrained nonlinear regression based on the initial estimates. This heterogeneous initialization strategy improves the fitting accuracy and reduces the computational time. A computationally efficient implementation of concatenated optimization for parameter estimation was achieved based on the graphic processing unit, named as concatenated optimization for parameter estimation graphic processing unit. In experimental comparison with Frams method and the Fitter Tool in Jim, the proposed methods are capable of achieving significantly higher efficiency and more accurate estimations. Magn Reson Med 63:1431–1436, 2010.

Spatially varying fat-water excitation using short 2DRF pulses

Conventional spatial‐spectral radiofrequency pulses excite the water or the fat spins in a whole slice or slab. While such pulses prove useful in a number of applications, their applicability is severely limited in sequences with short pulse repetition time due to the relatively long duration of the pulses. In the present work, we demonstrate that, by manipulating the parameters of a two‐dimensional spartially‐selective (2DRF) pulse designed to excite a two‐dimensional spatial profile, the chemical‐shift sensitivity of the pulse can be exploited to obtain potentially useful spatially varying fat‐water excitation patterns. Magn Reson Med 63:1092–1097, 2010.

Multiresolution MRI temperature monitoring in a reduced field of view

PURPOSEnThe purpose was to develop a new magnetic resonance imaging technique for fast temperature monitoring with extended volumexa0coverage.nnnMATERIALS AND METHODSnThe Multiple Resolutions Along Phase-Encode and Slice-Select Dimensions (MURPS) method was implemented in both a two-dimensional (2D) spoiled gradient echo (SPGR) sequence and a multishot echo-planar imaging (EPI) sequence. Both modified sequences were used to acquire image data from three slices with variable phase-encode resolution and slice thickness. In the SPGR sequence, a 2D resonant frequency pulse was also implemented to enable imaging within a reduced field of view, and this was used to monitor (at 1.5 T) the temperature changes in a live rabbit and in gel phantoms heated by focused ultrasound. A modified EPI sequence was tested during heating of a phantom undergoing motion.nnnRESULTSnThe in vivo experiments demonstrated that temperature changes in unexpected locations away from the focal plane, such as near bone structures, could be detected due to the extra volume coverage afforded by the MURPS method. Temperature changes in a moving phantom were resolved using the MURPS EPI sequence with an acquisition rate of three slices every 300 ms.nnnCONCLUSIONnThe MURPS method enables temperature monitoring over multiple slices without loss of temporal resolution compared with single-slice imaging and, if combined with multishot EPI, enables volume temperature monitoring in moving organs.