Lipid elimination with an echo-shifting N/2-ghost acquisition (LEENA) MRI.

PURPOSE: The Dixon techniques provide uniform water-fat separation but require multiple image sets, which extend the overall acquisition time. Here, an alternative rapid single acquisition method, lipid elimination with an echo-shifting N/2-ghost acquisition (LEENA), was introduced. METHODS: The LEENA method utilized a fast imaging with steady-state free precession sequence to obtain a single k-space dataset in which successive k-space lines are acquired to allow the fat magnetization to precess 180°. The LEENA data were then unghosted using either image-domain (LEENA-S) or k-space domain (LEENA-G) parallel imaging techniques to reconstruct water-only and fat-only images. An off-resonance correction technique was incorporated to improve the uniformity of the water-fat separation. RESULTS: Uniform water-fat separation was achieved for both the LEENA-S and LEENA-G methods for phantom and human body and leg imaging applications at 1.5T and 3T. The resultant water and fat images were qualitatively similar to conventional 2-point Dixon and fat-suppressed images. CONCLUSION: The LEENA-S and LEENA-G methods provide uniform water and fat images from a single MRI acquisition. These straightforward methods can be adapted to 1.5T and 3T clinical MRI scanners and provide comparable fat/water separation with conventional 2-point Dixon and fat-suppression techniques.

Device localization and dynamic scan plane selection using a wireless magnetic resonance imaging detector array.

PURPOSE: A prototype wireless guidance device using single sideband amplitude modulation (SSB) is presented for a 1.5T magnetic resonance imaging system. METHODS: The device contained three fiducial markers each mounted to an independent receiver coil equipped with wireless SSB technology. Acquiring orthogonal projections of these markers determined the position and orientation of the device, which was used to define the scan plane for a subsequent image acquisition. Device localization and scan plane update required approximately 30 ms, so it could be interleaved with high temporal resolution imaging. Since the wireless device is used for localization and does not require full imaging capability, the design of the SSB wireless system was simplified by allowing an asynchronous clock between the transmitter and receiver. RESULTS: When coupled to a high readout bandwidth, the error caused by the lack of a shared frequency reference was quantified to be less than one pixel (0.78 mm) in the projection acquisitions. Image guidance with the prototype was demonstrated with a phantom where a needle was successfully guided to a target and contrast was delivered. CONCLUSION: The feasibility of active tracking with a wireless detector array is demonstrated. Wireless arrays could be incorporated into devices to assist in image-guided procedures.

Magnetic resonance fingerprinting.

Magnetic resonance is an exceptionally powerful and versatile measurement technique. The basic structure of a magnetic resonance experiment has remained largely unchanged for almost 50 years, being mainly restricted to the qualitative probing of only a limited set of the properties that can in principle be accessed by this technique. Here we introduce an approach to data acquisition, post-processing and visualization--which we term 'magnetic resonance fingerprinting' (MRF)--that permits the simultaneous non-invasive quantification of multiple important properties of a material or tissue. MRF thus provides an alternative way to quantitatively detect and analyse complex changes that can represent physical alterations of a substance or early indicators of disease. MRF can also be used to identify the presence of a specific target material or tissue, which will increase the sensitivity, specificity and speed of a magnetic resonance study, and potentially lead to new diagnostic testing methodologies. When paired with an appropriate pattern-recognition algorithm, MRF inherently suppresses measurement errors and can thus improve measurement accuracy.

Parallel excitation for B-field insensitive fat-saturation preparation.

Multichannel transmission has the potential to improve many aspects of MRI through a new paradigm in excitation. In this study, multichannel transmission is used to address the effects that variations in B(0) homogeneity have on fat-saturation preparation through the use of the frequency, phase, and amplitude degrees of freedom afforded by independent transmission channels. B(1) homogeneity is intrinsically included via use of coil sensitivities in calculations. A new method, parallel excitation for B-field insensitive fat-saturation preparation, can achieve fat saturation in 89% of voxels with M(z) ≤ 0.1 in the presence of ± 4 ppm B(0) variation, where traditional CHESS methods achieve only 40% in the same conditions. While there has been much progress to apply multichannel transmission at high field strengths, particular focus is given here to application of these methods at 1.5 T.

Resolution enhanced T1-insensitive steady-state imaging.

Resolution enhanced T(1)-insensitive steady-state imaging (RE-TOSSI) is a new MRI pulse sequence for the generation of rapid T(2) contrast with high spatial resolution. TOSSI provides T(2) contrast by using nonequally spaced inversion pulses throughout a balanced steady-state free precession (SSFP) acquisition. In RE-TOSSI, these energy and time intensive adiabatic inversion pulses and associated magnetization preparation are removed from TOSSI after acquisition of the data around the center of k-space. Magnetization evolution simulations demonstrate T(2) contrast in TOSSI as well as reduction in the widening of the point spread function width (by up to a factor of 4) to a near ideal case for RE-TOSSI. Phantom experimentation is used to characterize and compare the contrast and spatial resolution properties of TOSSI, RE-TOSSI, balanced SSFP, Half-Fourier Acquisition Single-Shot Turbo Spin Echo (HASTE), and turbo spin echo and to optimize the fraction of k-space acquired using TOSSI. Comparison images in the abdomen and brain demonstrate similar contrast and improved spatial resolution in RE-TOSSI compared with TOSSI; comparison balanced SSFP, HASTE, and turbo spin echo images are provided. RE-TOSSI is capable of providing high spatial resolution T(2)-weighted images in 1 s or less per image.

Improved temporal resolution in cardiac imaging using through-time spiral GRAPPA.

Previous work has shown that the use of radial GRAPPA for the reconstruction of undersampled real-time free-breathing cardiac data allows for frame rates of up to 30 images/s. It is well known that the spiral trajectory offers a higher scan efficiency compared to radial trajectories. For this reason, we have developed a novel through-time spiral GRAPPA method and demonstrate its application to real-time cardiac imaging. By moving from the radial trajectory to the spiral trajectory, the temporal resolution can be further improved at lower acceleration factors compared to radial GRAPPA. In addition, the image quality is improved compared to those generated using the radial trajectory due to the lower acceleration factor. Here, we show that 2D frame rates of up to 56 images/s can be achieved using this parallel imaging method with the spiral trajectory.

In vivo assessment of oxygen consumption via Deuterium Magnetic Resonance.

We present a novel approach to simultaneously measure, in vivo, noninvasively, glucose and oxygen consumption via Deuterium Magnetic Resonance (DMR). Mice are administered deuteriated glucose by intravenous injection. The rate of formation of nascent (deuteriated) mitochondrial water is then measured via DMR. The rate of glucose metabolism and oxygen utilization is assessed by tracking their separate peaks in DMR spectra during dynamic scanning. Further studies will aim to validate these results by comparison with in vivo (17)O-MRI (mitochondrial function), (13)C-MRI and (19)FDG-PET (glucose metabolism) and ex vivo 1H- and 2H-MR, as well as mass spectrometry.

Improvements in multislice parallel imaging using radial CAIPIRINHA.

Multislice parallel imaging involves the simultaneous sampling of multiple parallel slices which are subsequently separated using parallel imaging reconstruction. The CAIPIRINHA technique improves this reconstruction by manipulating the phase of the RF excitation pulses to shift the aliasing pattern such that there is less aliasing energy to be reconstructed. In this work, it is shown that combining the phase manipulation used in CAIPIRINHA with a non-Cartesian (radial) sampling scheme further decreases the aliasing energy for the parallel imaging algorithm to reconstruct, thereby further increasing the degree to which a multi-channel receiver array can be utilized for parallel imaging acceleration. In radial CAIPIRINHA, individual bands (slices) in a multislice excitation are modulated with view-dependent phase, causing a destructive interference of entire slices. This destructive interference leads to a reduction in aliasing compared to the coherent shifts one observes when using this same technique with a Cartesian trajectory. Recovery of each individual slice is possible because the applied phase pattern is known, and a conjugate-gradient reconstruction algorithm minimizes the contributions from other slices. Results are presented with a standard 12-channel head coil with acceleration factors up to 14, where radial CAIPIRINHA produces an improved reconstruction when compared with Cartesian CAIPIRINHA.

Whole animal imaging.

Translational research plays a vital role in understanding the underlying pathophysiology of human diseases, and hence development of new diagnostic and therapeutic options for their management. After creating an animal disease model, pathophysiologic changes and effects of a therapeutic intervention on them are often evaluated on the animals using immunohistologic or imaging techniques. In contrast to the immunohistologic techniques, the imaging techniques are noninvasive and hence can be used to investigate the whole animal, oftentimes in a single exam which provides opportunities to perform longitudinal studies and dynamic imaging of the same subject, and hence minimizes the experimental variability, requirement for the number of animals, and the time to perform a given experiment. Whole animal imaging can be performed by a number of techniques including x-ray computed tomography, magnetic resonance imaging, ultrasound imaging, positron emission tomography, single photon emission computed tomography, fluorescence imaging, and bioluminescence imaging, among others. Individual imaging techniques provide different kinds of information regarding the structure, metabolism, and physiology of the animal. Each technique has its own strengths and weaknesses, and none serves every purpose of image acquisition from all regions of an animal. In this review, a broad overview of basic principles, available contrast mechanisms, applications, challenges, and future prospects of many imaging techniques employed for whole animal imaging is provided. Our main goal is to briefly describe the current state of art to researchers and advanced students with a strong background in the field of animal research.

Characterization and reduction of saturation banding in multiplanar coherent and incoherent steady-state imaging.

Hypointense band artifacts occur at intersections of nonparallel imaging planes in rapidly acquired MR images; quantitative or numerical analysis of these bands and strategies to mitigate their appearance have largely gone unexplored. The magnetization evolution in the different regions of multiplanar images was simulated for three common rapid steady-state techniques (spoiled gradient echo, steady state free precession, balanced steady state free precession). Saturation banding was found to be highly dependent on the pulse sequence, acquisition time, and phase-encoding order. Encoding the center of k-space at the end of the acquisition of each slice (i.e., reverse centric phase encoding) is demonstrated to be a simple and robust method for significantly reducing the relative saturation in all imaging planes. View ordering and resolution dependence were confirmed in multiplanar abdominal images. The added importance of reducing the artifact in accelerated acquisition techniques (e.g., parallel imaging) is particularly notable in multiplanar balanced steady state free precession images in the brain.

MR monitoring of NaCl-enhanced radiofrequency ablations: observations on low- and high-field-strength MR images with pathologic correlation.

PURPOSE: To test the hypothesis that magnetic resonance (MR) imaging can be used to monitor both intraparenchymal injection of NaCl solution and subsequent radiofrequency ablation (RFA) within tissues pretreated with NaCl, report the low- and high-field-strength MR appearance of NaCl-enhanced RFAs, and compare MR findings with pathologic findings. MATERIALS AND METHODS: Ten ex vivo calf liver specimens were injected with saturated NaCl (seven were mixed with methylene blue during MR fluoroscopic monitoring) and reexamined with fast imaging with steady-state progression (FISP), true FISP, reversed FISP (PSIF), and fast spin-echo T2-weighted MR sequences. The NaCl-to-liver contrast-to-noise ratio (CNR) was calculated for various sequences, and CNRs were compared with the Student t test. Distribution on MR images was compared with the results of pathologic analysis. Forty additional in vivo monopolar RFAs were performed in paraspinal muscles of seven minipigs after animal care committee approval (10 standard control ablations, 30 were preceded by direct injection of saturated NaCl at various volumes [3-9 mL] and rates [1 or 6mL/min]). Postablation low-field-strength (n = 20) and high-field-strength (n = 20) MR examinations consisted of T2-weighted imaging, short inversion time inversion-recovery (STIR) imaging, and contrast material-enhanced T1-weighted imaging. Ablation shape, conspicuity, volume, and signal intensity were compared between the two groups and with the results of pathologic analysis. The difference in volumes with and without NaCl injection was evaluated by using two-way analysis of variance. RESULTS: Mean CNR was highest on fast spin-echo T2-weighted images and was significantly higher for PSIF than for FISP (P < .0001) or true FISP (P = .003). NaCl distribution on MR images corresponded with the results of pathologic analysis in ex vivo livers. Interactive in vivo monitoring of NaCl injection and electrode placement was feasible. NaCl-enhanced ablations had irregular shapes, a higher CNR, and significantly larger volumes (F = 22.0; df = 1, 90; P < .00001). All ablations had intermediate or low signal intensity with high-signal-intensity rims on all images. Fluid signals overlaid NaCl-enhanced ablations on fast spin-echo T2-weighted and STIR images, particularly on high-field-strength MR images. CONCLUSION: MR imaging can be used to reliably monitor the distribution of injected NaCl solution in tissues. Interventional MR imaging techniques can be used to guide and monitor RFAs within NaCl pretreated tissues, with good correlation with pathologic results.

Multiple overlapping k-space junctions for investigating translating objects (MOJITO).

It is a well-known property in Fourier transform magnetic resonance imaging (MRI) that rigid body translational motion in image space results in linear phase accumulation in k -space. This work describes Multiple Overlapping k-space Junctions for Investigating Translating Objects (MOJITO), a correction scheme based on phase differences at trajectory intersections caused by 2-D alterations in the position of an object during MR imaging. The algorithm allows both detection and correction of motion artifacts caused by 2-D rigid body translational motion. Although similar in concept to navigator echoes, MOJITO does not require a repeating path through k-space, uses k-space data from a broader region of k -space, and uses the repeated data in image reconstruction; this provides the potential for a highly efficient self-navigating motion correction method. Here, the concept and theoretical basis of MOJITO is demonstrated using the continuous sampling BOWTIE trajectory in simulation and MR experiments. This particular trajectory is selected since it is well suited for such an algorithm due to numerous trajectory intersections. Specifically, the validity of the technique in the presence of noise and off-resonance effects is demonstrated through simulation.

Effect of contrast media on single-shot echo planar imaging: implications for abdominal diffusion imaging.

PURPOSE: The goal of this study was to determine the effect of contrast media on the signal behavior of single-shot echo planar imaging (ssEPI) used for abdominal diffusion imaging. MATERIALS AND METHODS: The signal of an ssEPI spin echo sequence in a water phantom with varying concentrations of gadolinium was modeled with Bloch equations and the predicted behavior validated on a phantom at 1.5T. Six volunteers were given gadolinium contrast and signal intensity (SI) time courses for regions of interest (ROIs) in the liver, pancreas, spleen, renal cortex, and medulla were analyzed. Student's t-test was used to compare precontrast SI to 0, 1, 4, 5, 10, and 13 minutes following contrast. RESULTS: The results show that following contrast ssEPI SI goes through a nadir, recovering differently for each organ. Maximal contrast-related signal losses relative to precontrast signal are 20%, 20%, 53%, and 67% for the liver, pancreas, renal cortex, and medulla, respectively. The SIs remain statistically below the precontrast values for 5, 4, and 1 minute for the pancreas, liver, and spleen, and for all times measured for the renal cortex and medulla. CONCLUSION: Abdominal diffusion imaging should be performed prior to contrast due to adverse effects on the signal in ssEPI.

Halting the effects of flow enhancement with effective intermittent zeugmatographic encoding (HEFEWEIZEN) in SSFP.

PURPOSE: To describe a new method for performing dark blood (DB) magnetization preparation in TrueFISP (bSSFP) and apply the technique to high-resolution carotid artery imaging. MATERIALS AND METHODS: The developed method (HEFEWEIZEN) provides directional flow suppression, while preserving bSSFP contrast, by periodically applying spatial saturation in short repetition time (TR) TrueFISP. Steady-state free precession (SSFP) conditions are maintained throughout the acquisition for the imaging slice magnetization. HEFEWEIZEN was implemented on a 1.5 T scanner with standard receiver coils. Studies were performed in phantoms, eight asymptomatic volunteers, and two patients with low- and high-grade carotid artery stenosis. RESULTS: Average flow suppression was 88% +/- 4% (arterial) and 85% +/- 3% (venous) in a multislice study. Stationary signal, contrast, and fine details were maintained with only slight signal suppression (11% +/- 11%). Comparison to diffusion-prepared SSFP in the common carotid artery demonstrated significant improvement in wall-lumen contrast-to-noise ratio efficiency (P = 0.024). DB contrast was achieved with only 13% increased acquisition time (14.3 sec). Further acceleration was possible by confining the DB preparation to the central 60% of k-space. CONCLUSION: A fast, short TR, DB TrueFISP pulse sequence was developed and tested in the carotid arteries of asymptomatic volunteers and patients.

Coronary MR imaging: navigator echo biofeedback increases navigator efficiency--initial experience.

RATIONALE AND OBJECTIVES: The aim of this study was to investigate whether a respiratory biofeedback system could increase navigator efficiency and maintain image quality compared to conventional respiratory-gated magnetic resonance coronary angiography (MRCA). MATERIALS AND METHODS: Eighteen healthy volunteers underwent MRCA using three different respiratory-gating protocols. A conventional expiratory free-breathing (FB) sequence was compared to two approaches using navigator echo biofeedback (NEB), a midinspiratory approach (NEBin) and an expiratory approach (NEBex). Navigator data reflecting the position of the diaphragm relative to a 3-mm gating window were made available to the subject using a video projector in combination with a Plexiglas screen and mirror goggles. Image quality was graded by two radiologists in consensus using a visual score ranging from 1 (not visible) to 4 (excellent vessel depiction). RESULTS: The NEB approaches improved navigator efficiency (71.1% with NEBex and 68.0% with NEBin vs 42.2% with FB), thus reducing total imaging time. This difference was statistically significant (P(NEBin)=.007; P(NEBex)=.001). Image quality in the NEBex group was comparable to that in the FB group (median score, 2.44 vs 2.52), but it proved to be significantly lower (median score, 1.94 vs 2.52) for the right coronary artery and the left anterior descending coronary artery in the NEBin group. CONCLUSION: NEB maintains image quality and significantly increases navigator efficiency, thereby decreasing total imaging time by about 40% compared to a conventional FB acquisition strategy.

A technique for MRI-guided transrectal deep pelvic abscess drainage.

OBJECTIVE: The purpose of this article is to introduce a technique for transrectal drainage of deep pelvic abscesses performed under interactive MRI guidance. CONCLUSION: A new method for triorthogonal image plane MRI guidance was developed and used to interactively monitor the puncture needle on continuously updated sets of adjustable three-plane images. The merits and limitations of the technique are highlighted and the patient population that is likely to benefit from this approach is suggested.

Zigzag sampling for improved parallel imaging.

Conventional Cartesian parallel MRI methods are limited to the sensitivity variations provided by the underlying receiver coil array in the dimension in which the data reduction is carried out, namely, the phase-encoding directions. However, in this work an acquisition strategy is presented that takes advantage of sensitivity variations in the readout direction, thus improving the parallel imaging reconstruction process. This is achieved by employing rapidly oscillating phase-encoding gradients during the actual readout. The benefit of this approach is demonstrated in vivo using various zigzag-shaped gradient trajectory designs. It is shown that zigzag type sampling, in analogy to CAIPIRINHA, modifies the appearance of aliasing in 2D and 3D imaging, thereby utilizing additional sensitivity variations in the readout direction directly resulting in improved parallel imaging reconstruction performance.

Dynamics of MRI-Guided thermal ablation of VX2 tumor in paraspinal muscle of rabbits.

This study combines fast magnetic resonance imaging (MRI) and model simulation of tissue thermal ablation for monitoring and predicting the dynamics of lesion size for tumor destruction. In vivo experiments were conducted using radiofrequency (RF) thermal ablation in paraspinal muscle of rabbit with a VX2 tumor. Before ablation, turbo-spin echo (TSE) images visualized the 3-D tumor (necrotic core and tumor periphery) and surrounding normal tissue. MR gradient-recalled echo (GRE) phase and magnitude images were acquired repeatedly in 3.3 s at 30-s intervals during and after thermal ablation to follow tissue temperature distribution dynamics and lesion development in tumor and surrounding normal tissue. Final lesion sizes estimated from GRE magnitude, post-ablation TSE, and stained histologic images were compared. Model simulations of temperature distribution and lesion development dynamics closely corresponded to the experimental data from MR images in tumor and normal tissue. The combined use of MR image monitoring and model simulation has the potential for improving pretreatment planning and real-time prediction of lesion-size dynamics for guidance of thermal ablation of tumors.

Pulse sequences and system interfaces for interventional and real-time MRI.

The use of MRI for intervention and real-time imaging has seen many changes since its inception in the late 1980s. Initial interventional MRI researchers made great strides in building this new specialty, creating devices, sequences, and applications to push the field forward. More recently, researchers have gained more access to the systems themselves, and have taken advantage of this situation to create truly interactive interventional systems. Techniques such as fully interactive scan adjustments and device tracking can be accomplished in real time due to increased transparency between vendors and researchers. Additionally, pulse sequences have undergone an evolution as well, with the constant emergence of novel acquisition schemes to generate image contrast quickly, increase temporal resolution and cover k-space with nonrectilinear trajectories. We will look at both emerging system interface concepts and novel pulse sequences that we believe will continue to push innovation in the field of interventional MRI.

High-field magnetic resonance imaging of the response of human prostate cancer to Pc 4-based photodynamic therapy in an animal model.

INTRODUCTION: High-field magnetic resonance imaging (MRI) is an emerging technique that provides a powerful, non-invasive tool for in vivo studies of cancer therapy in animal models. Photodynamic therapy (PDT) is a relatively new treatment modality for prostate cancer, the second leading cause of cancer mortality in American males. The goal of this study was to evaluate the response of human prostate tumor cells growing as xenografts in athymic nude mice to Pc 4-sensitized PDT. MATERIALS AND METHODS: PC-3, a cell line derived from a human prostate malignant tumor, was injected intradermally on the back flanks of athymic nude mice. Two tumors were initiated on each mouse. One was treated and the other served as the control. A second-generation photosensitizing drug Pc 4 (0.6 mg/kg body weight) was delivered to each animal by tail vein injection 48 hours before laser illumination (672 nm, 100 mW/cm(2), 150 J/cm(2)). A dedicated high-field (9.4 T) small-animal MR scanner was used for image acquisitions. A multi-slice multi-echo (MSME) technique, permitting noninvasive in vivo assessment of potential therapeutic effects, was used to measure the T2 values and tumor volumes. Animals were scanned immediately before and after PDT and 24 hours after PDT. T2 values were computed and analyzed for the tumor regions. RESULTS: For the treated tumors, the T2 values significantly increased (P<0.002) 24 hours after PDT (68.2+/- 8.5 milliseconds), compared to the pre-PDT values (55.8+/-6.6 milliseconds). For the control tumors, there was no significant difference (P = 0.53) between the pre-PDT (52.5+/-6.1 milliseconds) and 24-hour post-PDT (54.3+/-6.4 milliseconds) values. Histologic analysis showed that PDT-treated tumors demonstrated necrosis and inflammation that was not seen in the control. DISCUSSION: Changes in tumor T2 values measured by multi-slice multi-echo MR imaging provide an assay that could be useful for clinical monitoring of photodynamic therapy of prostate tumors.