Very fast MR imaging by field echoes and small angle excitation

An MR imaging technique has been developed producing head and body images of diagnostic quality in only a few seconds acquisition time. The Fourier type imaging technique uses excitation with relatively small excitation angels, echoes produced by gradient inversion, and extremely fast profile repetition. A typical result at 0.5 T is an artifact-free head image of 128 x 128 resolution, 10 mm slice thickness in an acquisition time of 2 seconds.     

Two-dimensional imaging with a single-sided NMR probe

A new low field unilateral NMR sensor equipped with a two-dimensional gradient coil system was built. A new NMR-MOUSE concept using a simple bar magnet instead of the classical U-shaped geometry was used to produce magnetic field profiles comparatively homogeneous in extended lateral planes defining a suitable field of view for 2D spatial localization. Slice selection along the depth direction is obtained by means of the highly constant static magnetic field gradient produced by this magnet geometry. Implementing a two-dimensional phase-encoding imaging method 2D cross sections of objects were obtained with high spatial resolution. By retuning the probe it was possible to change the depth of the selected slice obtaining a 3D imaging method. The details of the construction of the new device are presented together with imaging tests to show the quality of space encoding.     

Volumetric q-space imaging by 3D diffusion-weighted MRI

High b-value diffusion magnetic resonance imaging (MRI) enables us to detect far smaller architectures, by using q-space analysis, than the resolution in conventional MRI. Average displacement, one of the q-space parameters, quantitatively reflects architecture size and is very useful in observing small changes in microstructures in vivo (e.g., neurodegeneration, tumor heterogeneity, and others). Diffusion-weighted imaging (DWI) is performed by a two-dimensional (2D) multislice method; however, due to finite slice thickness and slice gap, there is a partial-volume effect that makes it difficult to detect the net q-space signal. On the other hand, three-dimensional (3D) MRI, having the advantages of very thin slice thickness and no slice gap (contiguous slices), allows volumetric evaluation acquired in a small isotropic voxel, as compared to 2D multislice imaging. Little is known about the isotropic high-resolution 3D DWI application to q-space analysis. In this study, we have developed and implemented a high b-value 3D DWI sequence, applied q-space analysis to study the reliability of high b-value 3D DWI and obtained a microscopic analytical map with isotropic high resolution and less contamination.     

Biochemical EPR imaging of skin

EPR Imaging (EPRI) of spin labels is a powerful method for investigating skin and can give information about biochemical processes which are involved in numerous skin diseases. Furthermore it enables the non invasive investigation of the liberation, penetration and distribution of spin labelled drugs. The basis of these measurements is spectral spatial EPR imaging employing modulated field gradients and simultaneous field scans (MOSS). A skin region (?=6 mm) was treated with a 10 μl spin label solution (1 mM). EPR spectra of 128 points were recorded in 128 spatial planes resulting in a 128×128 image matrix. A spatial resolution of better than 10 μm can be obtained for a spectral line width of 0.1 mT and a gradient of 4 Tm^?1.In vivo imaging on mammalian skin can be performed by employing surface coils at S-band frequencies, 3 GHz.     

Spectroscopic imaging display and analysis.

A system for display of magnetic resonance (MR) spectroscopic imaging (SI) data is described which provides for efficient review and analysis of the multidimensional spectroscopic and spatial data format of this technique. Features include the rapid display of spectra from selected image voxels, formation of spectroscopic images, spectral and image data processing operations, methods for correlating spectroscopic image data with high resolution 1H MR images, and hardcopy facilities. Examples are shown for 31P and 1H spectroscopic imaging studies obtained in human and rat brain.     

Single sided imaging sensor

A portable single sided sensor to render depth profiles of MR signal non-invasively is presented. The device utilizes a tailored permanent magnet array producing flat sensitive volumes up to 40 mm from the mobile inspection head. Automated slice selection is performed using a set of switched capacitors in the tuning and receiver circuits. A single channel spectrometer drives the multi-slice acquisition. The RF coil and magnet array geometries have been tailored to house gradient sets to further extend the image rendering capabilities for in-plane spatial resolution (2D and 3D MRI). The probe has been tested acquiring moisture profiles in concrete and porous rocks and measuring diffusion parameters on a phantom sample.     

FLASH imaging: rapid NMR imaging using low flip-angle pulses. 1986

A new method for rapid NMR imaging dubbed FLASH (fast low-angle shot) imaging is described which, for example, allows measuring times of the order of 1 s (64 × 128 pixel resolution) or 6 s (256 × 256 pixels). The technique takes advantage of excitation pukes with small hip angles eliminating the need of waiting periods in between successive experiments. It is based on the acquisition of the free induction decay in the form of a gradient echo generated by reversal of the read gradient. The entire imaging time is only given by the number of projections desired times the duration of slice selection and data acquisition. The method results in about a 100-fold reduction in measuring time without sacrificing spatial resolution. Further advantages are an optimized signal-to-noise ratio, the applicability of commercial gradient systems, and the deposition of extremely low rf power. FLASH imaging is demonstrated on phantoms, animals, and human extremities using a 2.3 T 40 cm bore magnet system. ¹H NMR images are obtained with variable relaxation time contrasts and without motional artifacts.     

MRI inter-slice reconstruction using super-resolution

MRI reconstruction using super-resolution is presented and shown to improve spatial resolution in cases when spatially-selective RF pulses are used for localization. In 2-D multislice MRI, the resolution in the slice direction is often lower than the in-plane resolution. For certain diagnostic imaging applications, isotropic resolution is necessary but true 3-D acquisition methods are not practical. In this case, if the imaging volume is acquired two or more times, with small spatial shifts between acquisitions, combination of the data sets using an iterative super-resolution algorithm gives improved resolution and better edge definition in the slice-select direction. Resolution augmentation in MRI is important for visualization and early diagnosis. The method also improves the signal-to-noise efficiency of the data acquisition.     

Single-shot line scan imaging using stimulated echoes

A new high-speed MRI method is described for single-shot line scan imaging (LSI) based on stimulated echoes (STE). To allow for multislice imaging, the technique comprises a series of slice-selective preparation pulses (each corresponding to the first RF pulse of a STE sequence), a slab-selective refocusing pulse (second RF pulse), and multiple line-selective read pulses (third RF pulses). An alternative version employs packages of two slice-selective pulses followed by multiple line-selective read pulses. Experimental applications deal with human brain imaging on a clinical MRI system at 2.0 T. The technique offers user-selectable trade-offs between volume coverage (1-15 sections) and in-plane spatial resolution (1-5 mm linear pixel dimension) within total acquisition times of less than 500 ms. Although LSI yields a lower signal-to-noise ratio than Fourier imaging, single-shot LSI with STEs is free from resonance offset effects (e.g., magnetic field inhomogeneities and susceptibility differences) that are typical for echo-planar imaging. Moreover, the technique exhibits considerable robustness against motion and provides access to arbitrary fields-of-view, i.e., localized imaging of inner volumes without aliasing artifacts due to phase wrapping.     

Investigation of insect morphology by MRI: assessment of spatial and temporal resolution

Classically, the investigation of the internal morphology of insects relies on histologic methods, e.g., the preparation of thin tissue sections. However, the preparation of serial sections is time consuming and means the irreversible loss of the animal. In the present investigation, we have analyzed the potential of NMR imaging as a tool for the morphologic classification of insects with sufficient spatial resolution. With a 512 matrix, 15 mm FOV, 200 microm slice thickness, images with an in-plane spatial resolution of 30 microm are obtained with a signal-to-noise ratio of 70. These conditions require only seven averages, resulting in an experimental time of only 50 min. Such image quality already permits the differentiation of fine structural and morphologic details such as e.g., intestinal tracts and copulation organ in a beetle. Also, wing controlling dorsal muscle groups as well as leg structures and joints are clearly distinguishable. We conclude that the spatial resolution and contrast condition of MR imaging are quite promising for the new approach of zoological insect classification using NMR imaging. Further principally available technical enhancement of sensitivity and spatial resolution will provide an attractive alternative to invasive techniques for the classification of, sometimes, rare and precious insect specimen.     

AOTF成像光谱仪光学系统的最优方案选择

基于声光可调谐滤波器（AOTF）的成像光谱仪是一种新型的成像光谱仪,它除了具有一般成像光谱仪的二维空间信息与一维光谱信息外,还能获得目标的偏振信息。在实际应用中AOTF可接收的光束孔径角一般不大于5～6,因而受到所采用声光晶体可接收角度的限制,AOTF光谱成像仪的光学系统不能同时兼顾大孔径与大视场。要求AOTF成像光谱仪视场满足128128像元,TeO2晶体尺寸为10 mm10 mm,根据声光可调谐滤波器成像光谱仪光学系统的特点,分析比较了2种光学系统总体方案,最终将系统的孔径光栏放置在晶体上,TeO2晶体尺寸限制了孔径光栏的尺寸,晶体可接收的角度决定了系统的视场角,提高了能量利用率,获得较高的图像信噪比。     

Gradient-echo line scan imaging using 2D-selective RF excitation

A gradient-echo line scan imaging technique was developed which employs two-dimensional spatially selective radiofrequency (2DRF) pulses for consecutively exciting individual columns of transverse magnetization, i.e., image lines. Although a variety of trajectories are possible for 2DRF excitation, the current implementation involved a blipped-planar trajectory in conjunction with additional saturation RF pulses to suppress side excitations above and below the desired image section, i.e., along the blip direction of the 2DRF pulse. Human brain imaging at 2.0 T (Siemens Vision, Erlangen, Germany) resulted in measuring times of 5.2 s for a 5-mm section at 1.0 x 1.0 mm in-plane resolution. Functional neuroimaging of the motor cortex at 1.2 s temporal resolution and 0.78 x 1.56 mm in-plane resolution exploited the capability of imaging inner volumes (here a 25-mm strip) without signal aliasing.     

Functional neuroimaging of inner fields-of-view with 2D-selective RF excitations

Echo-planar imaging is widely used in functional neuroimaging but suffers from its pronounced sensitivity to field inhomogeneities that cause geometric distortions and image blurring which both limit the effective in-plane resolution achievable. In this work, it is shown how inner-field-of-view techniques based on 2D-selective RF excitations (2DRF) can be applied to reduce the field-of-view in the phase-encoding direction without aliasing and increase the in-plane resolution accordingly. Free-induction-decay (FID) EPI and echo-train-shifted (T2*-weighted) and standard (T2-weighted) spin-echo (SE) EPI with in-plane resolutions of up to 0.5×1.0 mm² (slice thickness 5 mm) were acquired at 3 T. Unwanted signal contributions of 2DRF side excitations were shifted out of the object (FID-EPI) or of the refocusing plane by tilting the excitation plane (SE-EPI). Brain activation in healthy volunteers was investigated with checkerboard and finger-tapping block-design paradigms. Brain activation could be detected with all sequences and contrasts, most reliably with FID-EPI due to its higher signal amplitude and the longer 2DRF excitation that are more sensitive to magnetic field inhomogeneities. In conclusion, inner-FOV EPI based on 2DRF excitations could help to improve the spatial resolution of fMRI of focal target regions, e.g. for applications in the spinal cord.     

MR protocols for imaging the guinea pig knee

Magnetic resonance images of the femorotibial joints of male Dunkin-Hartley guinea pigs were obtained in two and three dimensions at 2.35 T using a wide range of T₁- and T₂-weighted imaging sequences. The effect of slice position on visualisation of articular cartilage, bone and periarticular tissues in sagittal and coronal sections was investigated along with the resolution and signal/noise ratio achievable. Based on that survey, a two-dimensional spin echo sequence (repetition time = 1500 ms, echo time = 40 ms) was found to give optimum visualisation of the normal joint anatomy with in-plane resolution of 75 × 150 μm and a 1 mm slice thickness in an imaging time of 25 min. This protocol was also found to be highly effective in distinguishing many features of the spontaneous, osteoarthritic-like pathology found in the joints of older animals compared to juveniles and therefore provides a means of monitoring disease progression longitudinally. Three-dimensional spin echo imaging methods demonstrated focal changes in signal intensity in the articular cartilage of the medial tibial plateau in older animals. The resulting imaging times of several hours, however, precludes their routine use in vivo.     

Event-related functional MR imaging of visual cortex stimulation at high temporal resolution using a standard 1.5 T imager

The authors report the technical feasibility of measuring event-related changes in blood oxygenation for studying brain function in humans at high temporal resolution. Measurements were performed on a conventional wholebody 1.5 T clinical scanner with a nonactive-shielded standard gradient system of 1 ms rise time for a maximum gradient strength of 10 mT/m. The radiofrequency (RF) transmitting and receiving MR unit consists of a commercially available circular polarized head coil. Magnet shimming with all first-order coils was performed to the volunteer's head resulting in a magnetic field homogeneity of about 0.1–0.2 ppm. The measuring sequence used was a modified 3D, first-order flow rephased, FLASH sequence with reduced bandwidth = 40 Hz/pixel, TR = 80 ms, TE = 56 ms, flip angle = 40–50°, matrix = 64 × 128, field-of-view = 200–250 mm², slice thickness = 4 mm, NEX = 1, 128 partitions, and a total single scan time of about 10 min. In this sequence the 3D gradient table was removed and the 3D partition-loop acts as a time-loop for sequential measurement of 128 or 32 consecutive images at the same slice position. This means that event-related functional MRI could be performed with an interscan delay of 80 ms for a series of 128 sequential images or with an interscan delay of 320 ms for a simultaneous measurement of four slices with a series of 32 sequential images for each slice. We used a TTL signal given by the gradient board at the beginning of every line-loop in the measuring sequence and a self-made “TTL-Divider-Box” for the event triggering. This box was used to count and scale down the TTL signals by a factor of 128 and to trigger after every 128th TTL signal a single white flash-light, which was seen by the volunteer in the dark room of the scanner with a period of 10.24 s. As a consequence, the resulting event-related scan data coincide at each line of the series of 128 sequential images, which were repeated in 128 × 80 ms or 32 × 320 ms for the single- or four-slice experiment, respectively. Visual cortex response magnitude measured was about 5–7% with an approximate Gaussian shape and a rise time from stimulus onset to maximum of about 3–4 s, and a fall time to the baseline of about 5–6 s after end of stimulus. The reported data demonstrate the feasibility of functional MRI studies at high temporal resolution (up to 80 ms) using conventional MR equipment and measuring sequence.     

A system for low field imaging of laser-polarized noble gas

We describe a device for performing MRI with laser-polarized noble gas at low magnetic fields (<50 G). The system is robust, portable, inexpensive, and provides gas-phase imaging resolution comparable to that of high field clinical instruments. At 20.6 G, we have imaged laser-polarized (3)He (Larmor frequency of 67 kHz) in both sealed glass cells and excised rat lungs, using approximately 0.1 G/cm gradients to achieve approximately 1 mm(2) resolution. In addition, we measured (3)He T(2)(*) times greater than 100 ms in excised rat lungs, which is roughly 20 times longer than typical values observed at high ( approximately 2 T) fields. We include a discussion of the practical considerations for working at low magnetic fields and conclude with evidence of radiation damping in this system.     

Compensation of fat layer effects in ultrasound imaging using an inclined-fat-layer model derived from magnetic resonance images

When cutaneous fat layers are in the ultrasound imaging region, the phase aberration caused by the fat layers induce image distortion as well as spatial resolution degradation. The phase aberration may complicate clinical procedures particularly when ultrasound imaging is employed for spatial positioning of medical devices like a biopsy needle or HIFU. To compensate the fat layer effects more precisely in beamforming, an inclined-fat-layer model has been established from the magnetic resonance images of the same imaging region as in the ultrasound scanning. We have verified utility of the fat layer model by taking images of a metal needle put into an inclined-fat-layer mimicking phantom. The ultrasound images taken with a 128-element linear phase array operating at 6 MHz have shown better resolution and less distortion when receive beamforming was performed with the phase delay data derived from the inclined-fat-layer model.     

Improved functional mapping of the human amygdala using a standard functional magnetic resonance imaging sequence with simple modifications

As the amygdala is involved in various aspects of emotional processing, its characterization using neuroimaging modalities, such as functional magnetic resonance imaging (fMRI), is of great interest. However, in fMRI, the amygdala region suffers from susceptibility artifacts that are composed of signal dropouts and image distortions. Various technically demanding approaches to reduce these artifacts have been proposed, and most require alterations beyond a mere change of the acquisition parameters and cannot be easily implemented by the user without changing the MR sequence code. In the present study, we therefore evaluated the impact of simple alterations of the acquisition parameters of a standard gradient-echo echo-planar imaging technique at 3 T composed of echo times (TEs) of 27 and 36 ms as well as section thicknesses of 2 and 4 mm while retaining a section orientation parallel to the intercommissural plane and an in-plane resolution of 2x2 mm(2). In contrast to previous studies, we based our evaluation on the resulting activation maps using an emotional stimulation paradigm rather than on MR raw image quality only. Furthermore, we tested the effects of spatial smoothing of the functional raw data in the course of postprocessing using spatial filters of 4 and 8 mm. Regarding MR raw image quality, a TE of 27 ms and 2-mm sections resulted in the least susceptibility artifacts in the anteromedial aspect of the temporal lobe. The emotional stimulation paradigm resulted in robust bilateral amygdala activation for the approaches with 2-mm sections only -- but with larger activation volumes for a TE of 36 ms as compared with that of 27 ms. Moderate smoothing with a 4-mm spatial filter represented a good compromise between increased sensitivity and preserved specificity. In summary, we showed that rather than applying advanced modifications of the MR sequence, a simple increase in spatial resolution (i.e., the reduction of section thickness) is sufficient to improve the detectability of amygdala activation.     

High-resolution deconvolved beamforming for three-dimensional imaging sonars

Three-dimensional(3D) imaging sonars based on the conventional beamforming(CBF) suffers from relatively wide main-lobes and high sidelobe level.To improve the spatial resolution of 3D imaging sonars,a deconvolved beamforming method is proposed with the iterative Richardson-Lucy algorithm in this paper.At first,each distance slice can be processed to obtain images by the CBF.For the near field,the Fresnel approximation is used.Then,the deconvolution technique is applied to the CBF outputs to obtain high-resolution images and suppress the sidelobe level.The simulations demonstrate that the proposed algorithm is able to improve the spatial resolution significantly and suppress the sidelobes for 3D imaging sonars.Meanwhile,the algorithm shows similar robustness with the CBF in the case of wideband and sparse array.The priority of the proposed algorithm is also validated by the tank experiment data.The presented results indicate that the spatial resolution is increased by one time and the average sidelobe level is reduced by 20 dB.     

Three-dimensional fluid pressure mapping in porous media using magnetic resonance imaging with gas-filled liposomes

This paper presents and demonstrates a method for using magnetic resonance imaging to measure local pressure of a fluid saturating a porous medium. The method is tested both in a static system of packed silica gel and in saturated sintered glass cylinders experiencing fluid flow. The fluid used contains 3% gas in the form of 3-mum average diameter gas filled 1,2-distearoyl-sn-glycero-3-phosphocholine (C18:0, MW: 790.16) liposomes suspended in 5% glycerol and 0.5% Methyl cellulose with water. Preliminary studies at 2.35 T demonstrate relative magnetic resonance signal changes of 20% per bar in bulk fluid for an echo time T(E)=40 ms, and 6-10% in consolidated porous media for T(E)=10 ms, over the range 0.8-1.8 bar for a spatial resolution of 0.1 mm(3) and a temporal resolution of 30 s. The stability of this solution with relation to applied pressure and methods for improving sensitivity are discussed.