高场超导MRI系统铁磁屏蔽的几何优化

磁共振成像（MRI）系统是一种重要的医学影像诊断设备,它根据核磁共振原理对处于静磁场中的人体器官进行成像,具有清晰度高和任意层面成像等优点,在医学检查和诊断方面有着重要的作用,与低场MRI相比,高场MRI系统可提高质子的磁化率,增加图像的信噪比,缩短MRI信号采集时间,从而使脑功能成像的信号变化更为明显;但是较高的背景...     

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.     

Parallel MRI at microtesla fields

Parallel imaging techniques have been widely used in high-field magnetic resonance imaging (MRI). Multiple receiver coils have been shown to improve image quality and allow accelerated image acquisition. Magnetic resonance imaging at ultra-low fields (ULF MRI) is a new imaging approach that uses SQUID (superconducting quantum interference device) sensors to measure the spatially encoded precession of pre-polarized nuclear spin populations at microtesla-range measurement fields. In this work, parallel imaging at microtesla fields is systematically studied for the first time. A seven-channel SQUID system, designed for both ULF MRI and magnetoencephalography (MEG), is used to acquire 3D images of a human hand, as well as 2D images of a large water phantom. The imaging is performed at 46 mu T measurement field with pre-polarization at 40 mT. It is shown how the use of seven channels increases imaging field of view and improves signal-to-noise ratio for the hand images. A simple procedure for approximate correction of concomitant gradient artifacts is described. Noise propagation is analyzed experimentally, and the main source of correlated noise is identified. Accelerated imaging based on one-dimensional undersampling and 1D SENSE (sensitivity encoding) image reconstruction is studied in the case of the 2D phantom. Actual threefold imaging acceleration in comparison to single-average fully encoded Fourier imaging is demonstrated. These results show that parallel imaging methods are efficient in ULF MRI, and that imaging performance of SQUID-based instruments improves substantially as the number of channels is increased.     

Signal-to-noise ratio comparison of phased-array vs. implantable coil for rat spinal cord MRI

The signal-to-noise ratio (SNR) performance and practicality issues of a four-element phased-array coil and an implantable coil system were compared for rat spinal cord magnetic resonance imaging (MRI) at 7 T. MRI scans of the rat spinal cord at T10 were acquired from eight rats over a 3 week period using both coil systems, with and without laminectomy. The results demonstrate that both the phased array and the implantable coil systems are feasible options for rat spinal cord imaging at 7 T, with both systems providing adequate SNR for 100-mum spatial resolution at reasonable imaging times. The implantable coils provided significantly higher SNR, as compared to the phased array (average SNR gain of 5.3x between the laminectomy groups and 2.5x between the nonlaminectomy groups). The implantable coil system should be used if maximal SNR is critical, whereas the phased array is a good choice for its ease of use and lesser invasiveness.     

Accurate and sensitive measurements of magnetic susceptibility using echo planar imaging

Susceptibility differences are common causes for artifacts in magnetic resonance (MR); therefore, it is important to choose phantom materials in a way that these artifacts are kept at a minimum. In this study, a previously proposed MR imaging (MRI) method [Beuf O, Briguet A, Lissac M, Davis R. Magnetic resonance imaging for the determination of magnetic susceptibility of materials. J Magn Reson 1996; Series B(112):111-118] was improved to facilitate sensitive in-house measurements of different phantom materials so that such artifacts can more easily be minimized. Using standard MRI protocols and distilled water as reference, we measured magnetic volume susceptibility differences with a clinical MR system. Two imaging techniques, echo planar imaging (EPI) and spin echo, were compared using liquid samples whose susceptibilities were verified by MR spectroscopy. The EPI sequence has a very narrow bandwidth in the phase-encoding direction, which gives an increased sensitivity to magnetic field inhomogeneities. All MRI measurements were evaluated in two ways: (1) manual image analysis and (2) model fitting. The narrow bandwidth of the EPI made it possible to detect very small susceptibility differences (equivalent susceptibility difference, Deltachi(e)> or =0.02 ppm), and even plastics could be measured. Model fitting yielded high accuracy and high sensitivity and was less sensitive to other image artifacts as compared with manual image analysis.     

19F Magnetic resonance imaging of perfluorooctanoic acid encapsulated in liposome for biodistribution measurement

The tissue distribution of perfluorooctanoic acid (PFOA), which is known to show unique biological responses, has been visualized in female mice by (19)F magnetic resonance imaging (MRI) incorporated with the recent advances in microimaging technique. The chemical shift selected fast spin-echo method was applied to acquire in vivo (19)F MR images of PFOA. The in vivo T(1) and T(2) relaxation times of PFOA were proven to be extremely short, which were 140 (+/- 20) ms and 6.3 (+/- 2.2) ms, respectively. To acquire the in vivo (19)F MR images of PFOA, it was necessary to optimize the parameters of signal selection and echo train length. The chemical shift selection was effectively performed by using the (19)F NMR signal of CF(3) group of PFOA without the signal overlapping because the chemical shift difference between the CF(3) and neighbor signals reaches to 14 kHz. The most optimal echo train length to obtain (19)F images efficiently was determined so that the maximum echo time (TE) value in the fast spin-echo sequence was comparable to the in vivo T(2) value. By optimizing these parameters, the in vivo (19)F MR image of PFOA was enabled to obtain efficiently in 12 minutes. As a result, the time course of the accumulation of PFOA into the mouse liver was clearly pursued in the (19)F MR images. Thus, it was concluded that the (19)F MRI becomes the effective method toward the future pharmacological and toxicological studies of perfluorocarboxilic acids.     

0.7 and 3 T MRI and Sap Flow in Intact Trees: Xylem and Phloem in Action

Dedicated magnetic resonance imaging (MRI) hardware is described that allows imaging of sap flow in intact trees with a maximal trunk diameter of 4 cm and height of several meters. This setup is used to investigate xylem and phloem flow in an intact tree quantitatively. Due to the fragile gradients in pressure present in both xylem and phloem, methods to study xylem and phloem transport must be minimally invasive. MRI flow imaging by means of this hardware certainly fulfils this condition. Flow is quantified in terms of (averaged) velocity, volume flow (flux) and flow conducting area, either in imaging mode or as a nonspatially resolved total. Results obtained for one tree, imaged at two different field strengths (0.7 and 3 T), are compared. An overall shortening of observed T ₂ values is manifest going from 0.7 to 3 T. Although some susceptibility artefacts may be present at 3 T, the results are still reliable and the gain in sensitivity results in shorter measurement time (or higher signal-to-noise ratio) with respect to the 0.7 T system. The results demonstrate that by use of dedicated hardware, xylem and phloem flow and its mutual interaction, can be studied in intact trees in relation to the water balance and in response to environmental (stress) conditions. Authors' address: Henk Van As, Laboratory of Biophysics, Wageningen University, Drejenlaan 3, 6703 HA Wageningen, The Netherlands     

An optimized solenoidal head radiofrequency coil for low-field magnetic resonance imaging

Applications of low-field magnetic resonance imaging (MRI) systems (<0.3 T) are limited due to the signal-to-noise ratio (SNR) being lower than that provided by systems based on superconductive magnets (≥1.5 T). Therefore, the design of radiofrequency (RF) coils for low-field MRI requires careful consideration as significant gains in SNR can be achieved with the proper design of the RF coil. This article describes an analytical method for the optimization of solenoidal coils. Coil and sample losses are analyzed to provide maximum SNR and optimum B₁ field homogeneity. The calculations are performed for solenoidal coils optimized for the human head at 0.2 T, but the method could also be applied to any solenoidal coil for imaging other anatomical regions at low field. Several coils were constructed to compare experimental and theoretical results. A head magnetic resonance image obtained at 0.2 T with the optimum design is presented.     

Quantitative evaluation of optimal imaging parameters for single-cell detection in MRI using simulation

Super-paramagnetic iron oxide (SPIO) nanoparticles are actively investigated to enhance disease detection through molecular imaging using magnetic resonance imaging (MRI). Detection of the cells labeled by SPIO depends on the MRI protocols and pulse sequence parameters that can be optimized. To evaluate the sensitivity and specificity of the image acquisition methods and to obtain optimal imaging parameters for single-cell detection, we further developed an MRI simulator. The simulator models an object (tissue) at a microscopic level to evaluate effects of spatial distribution and concentration of nanoparticles on the resulting image. In this study, the simulator was used to evaluate and compare imaging of the labeled cells by the gradient-echo (GE), true-FISP [fast imaging employing steady-state acquisition (FIESTA)] and echo-planar imaging (EPI) pulse sequences. Effects of the imaging and object parameters, such as field strength, imaging protocol and pulse sequence parameters, imaging resolution, cell iron load, position of SPIO within the voxel and cell division within the voxel, were investigated in the work. The results suggest that true-FISP has the highest sensitivity for single-cell detection by MRI.     

Directly detected Mn MRI: Application to phantoms for human hyperpolarized C MRI development

In this work we demonstrate for the first time directly detected manganese-55 (⁵⁵Mn) magnetic resonance imaging (MRI) using a clinical 3 T MRI scanner designed for human hyperpolarized ¹³C clinical studies with no additional hardware modifications. Due to the similar frequency of the ⁵⁵Mn and ¹³C resonances, the use of aqueous permanganate for large, signal-dense, and cost-effective “¹³C” MRI phantoms was investigated, addressing the clear need for new phantoms for these studies. Due to 100% natural abundance, higher intrinsic sensitivity, and favorable relaxation properties, ⁵⁵Mn MRI of aqueous permanganate demonstrates dramatically increased sensitivity over typical ¹³C phantom MRI, at greatly reduced cost as compared with large ¹³C-enriched phantoms. A large sensitivity advantage (22-fold) was demonstrated. A cylindrical phantom (d = 8 cm) containing concentrated aqueous sodium permanganate (2.7 M) was scanned rapidly by ⁵⁵Mn MRI in a human head coil tuned for ¹³C, using a balanced steady state free precession acquisition. The requisite penetration of radiofrequency magnetic fields into concentrated permanganate was investigated by experiments and high frequency electromagnetic simulations, and found to be sufficient for ⁵⁵Mn MRI with reasonably sized phantoms. A sub-second slice-selective acquisition yielded mean image signal-to-noise ratio of ~ 60 at 0.5 cm³ spatial resolution, distributed with minimum central signal ~ 40% of the maximum edge signal. We anticipate that permanganate phantoms will be very useful for testing HP ¹³C coils and methods designed for human studies.     

Advantages of using multiple-echo image combination and asymmetric triangular phase masking in magnetic resonance venography at 3 T

The present work explores the possibility of localizing veins with magnetic resonance venography using susceptibility weighted imaging. It also seeks new approaches, directed by the spatial specificity of activated brain regions, that have sufficient precision for practical use in functional MRI studies. A 3D flow compensated multiple gradient echo sequence, featuring optimized T2* weighting within a reasonable time of acquisition (11 min) and a small voxel size (0.5x0.5x1 mm3), was used to acquire MR images at 3 T. Post-processing consisted of homodyne filtering, linear phase scaling and magnitude masking prior to minimum intensity projection (mIP). The multiple echo approach provided a satisfactory (48+/-7%) increase in signal-to-noise ratio with respect to conventional methods. Specific features of the blood oxygenation level-dependent phase effect were simulated and used for designing and exploring different phase masking methods in relation to vessel morphology and MRI voxel geometry. As with simulations, the best results were obtained with an asymmetric triangular phase masking, featuring an improved venographic contrast without any increase in the full-width at half-maximum. The multiple echo approach provided satisfactory vessel localization capacity by using asymmetric triangular phase masking and a 4-mm-thick mIP. The venographic contrast obtained enabled the detection of vessels with diameter down to approximately 500 microm, suggesting the applicability of the proposed method as an additional technique in fMRI studies.     

MR phase imaging: sensitive and contrast-enhancing visualization in cellular imaging

The successful translation of stem-cell therapies requires a detailed understanding of the fate of transplanted cells. Magnetic resonance imaging (MRI) has provided a noninvasive means of imaging cell dynamics in vivo by prelabeling cell with T(2) shortening iron oxide particles. However, this approach suffers from a gradual loss of sensitivity since active cell mitosis could decrease the cellular contrast agent (CA) concentration below detection level. In addition, the interpretation of images may be confounded by hypointensities induced by factors other than this CA susceptibility effect (CASE). We therefore examined the feasibility of exploiting the phase information in MRI to increase the sensitivity of cellular imaging and to differentiate the CASE from endogenous image hypointensity. Phase aliasing and the B(0) field inhomogeneity effect were removed by applying a reliable unwrapping algorithm and a high-pass filter, respectively, thus delineating phase variations originating from high spatial frequencies due to the CASE. We found that the filtered phase map detects labeled cells with high sensitivity and can readily differentiate the cell migration track from the white matter, both of which are hypointense in T(2)-weighted magnitude images. Furthermore, an approximate fivefold contrast-to-noise ratio enhancement can be achieved with an MRI phase map over conventional T(2)-weighted magnitude images.     

Linear-fitting-based similarity coefficient map for tissue dissimilarity analysis in T_2~*-w magnetic resonance imaging

Similarity coefficient mapping(SCM) aims to improve the morphological evaluation of T*2weighted magnetic resonance imaging(T*2-w MRI). However, how to interpret the generated SCM map is still pending. Moreover, is it probable to extract tissue dissimilarity messages based on the theory behind SCM? The primary purpose of this paper is to address these two questions. First, the theory of SCM was interpreted from the perspective of linear fitting. Then, a term was embedded for tissue dissimilarity information. Finally, our method was validated with sixteen human brain image series from multiecho T*2-w MRI. Generated maps were investigated from signal-to-noise ratio(SNR) and perceived visual quality, and then interpreted from intra- and inter-tissue intensity. Experimental results show that both perceptibility of anatomical structures and tissue contrast are improved. More importantly, tissue similarity or dissimilarity can be quantified and cross-validated from pixel intensity analysis. This method benefits image enhancement, tissue classification, malformation detection and morphological evaluation.     

A combined analysis and magnetic resonance imaging technique for computerised automatic measurement of cartilage thickness in the distal interphalangeal joint

It is well known that magnetic resonance imaging (MRI) contrast can be controlled, albeit sometimes at the expense of image resolution and signal-to-noise ratio, and most studies of articular joints have used a single MRI protocol, which is optimised for subjective image analysis. Inevitably that single protocol frequently compromises the detection of one or another of the boundaries between which any measurement must be made. This paper describes an alternative approach in which the criteria for computerised edge detection necessary for fully automated measurement of cartilage thickness are used to define the MRI acquisition parameters. This necessitates the combined use of two MRI sequences, one optimised for the cartilage-bone boundary, and the other for cartilage-synovial fluid. This provides a highly effective combination and its efficacy is demonstrated for the distal interphalangeal joint of a range of asymptomatic adults.     

Hemorrhagic cerebral metastases from thyroid cancer on T2*-weighted gradient echo MRI

We report a case of multiple hemorrhagic cerebral metastases from papillary thyroid cancer, with reference to T(2)*-weighted gradient echo (GRE) magnetic resonance imaging (MRI). Small metastatic nodules were recognized as round nodules with signal loss on T(2)*-weighted GRE MRI, and were more pronounced compared with other sequences. Lesions were later confirmed as hemorrhagic on T(1)- and T(2)-weighted MRI. T(2)*-weighted GRE MRI was a sensitive tool for early detection of metastases displaying hemorrhagic changes.     

Diffusion-weighted imaging of the brain at 7 T with echo-planar and turbo spin echo sequences: preliminary results

Ultra-high-field clinical MRI scanners (e.g., 7 T and above) are becoming increasingly prevalent and can potentially enhance diagnostic ability through higher contrast, resolution and/or sensitivity. Diffusion-weighted MRI is a highly valued component in today's radiological exam and may benefit from the enhanced signal-to-noise ratio provided by high field with the appropriate imaging strategy. The most common diffusion pulse sequence readout (echo-planar imaging (EPI)) has been widely employed for in vivo human 7 T diffusion tensor imaging (DTI). In this article, we present results of brain DTI at 7 T with two diffusion-weighted imaging pulse sequence readouts: echo-planar imaging (EPI-DTI) and turbo spin echo (TSE-DTI). Results indicate that analogous coverage, quality and resolution typical of lower field (2 mm) can be obtained by properly processed EPI-DTI at 7 T, and, with some reduction in efficiency and sharpness, TSE-DTI at 7 T. Furthermore, 7 T TSE-DTI shows promise in obtaining higher-resolution results in targeted acquisitions of specific brain areas.     

Evaluation of Enhancement Effects as a Function of the Molarity of Gd-Based Contrast Media at 3.0 and 1.5 T: Based on the T1 Effective Pulse Sequence Parameter

The purpose of this study was to evaluate the alterations of diluted molarity of contrast media to emit the maximum signal intensity by changing the parameters of pulse sequences. The phantom was developed by diluting the magnetic resonance imaging (MRI) T1 contrast medium. The phantom images were obtained by 1.5 and 3.0 T MRI systems. We conducted Pearson’s analysis to reveal the correlation of the signal-to-noise ratio (SNR)_90%, the change of the concentration range of the contrast media which shows over 90% SNR, with changing the parameters of T1 effect pulse sequences in both 1.5 and 3.0 T imaging. As the flip angle increased, the SNR increased for all contrast media in magnetization-prepared rapid gradient echo and two-dimensional fast low angle shot pulse sequences at 1.5 and 3.0 T. Although the SNR increased until 30°, the SNR was almost the same over 30° in volumetric interpolated breath-hold examination at 1.5 and 3.0 T. The minimum contrast molarity of the representing SNR_90% was decreased according to the increasing time to repeat in spin echo. The present study revealed that the high concentration technique of contrast media on three pulse sequences (VIBE, MPRAGE, and 2D FLASH) could be useful to obtain images with better SNR.     

A realization of digital wireless transmission for MRI signals based on 802.11b

In this paper, a digital wireless transmission system based on 802.11b standard for magnetic resonance imaging (MRI) application is designed and built for the first time to eliminate the interference aroused by coil array cables. The analysis shows that the wireless receiver has a very high sensitivity to detect MRI signals. The modulation technique of differential quadrature phase shift keyed (DQPSK) can be applied to MRI data transmission with rate of 2 Mbps and bandwidth of 2 MHz. The bench test verifies that this wireless link has a dynamic range over 86 dB supporting up to 3 T MRI system data transmission. The 2D spin echo imaging of phantom is performed and the SNR of the image obtained by the wireless transmission can be comparable with that got by the coaxial cables.     

Off-resonance saturation MRI of superparamagnetic nanoprobes: theoretical models and experimental validations

Off-resonance saturation (ORS) is a new magnetic resonance imaging (MRI) method that has shown greatly improved contrast sensitivity for the detection of cancer-specific biomarkers by superparamagnetic nanoprobes in vivo. However, quantitative understanding of the ORS contrast mechanism and its dependence on the structural parameters of superparamagnetic nanoprobes are still lacking. Here we propose a quantitative model of ORS contrast and its experimental validation by superparamagnetic polymeric micelles (SPPM) with precisely controlled structural properties. Size selected, monodisperse Fe?O? nanoparticles (6.1 ± 0.2 nm) were used to form a series of SPPM nanoprobes with specifically controlled corona thickness using 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-methoxypoly(ethylene glycol) (DSPE-PEG) with different PEG molecular weights. Transmission electron microscopy and dynamic light scattering showed that SPPM were uniform in size. The average hydrodynamic diameters of SPPM with PEG lengths of 0.55, 1, 2, and 5 kD were 16.6 ± 2.8, 18.4 ± 2.9, 24.1 ± 3.4, and 28.9 ± 3.4 nm, respectively. MRI experiments at 7 T determined that r? values of SPPM with 0.55, 1, 2, and 5 kD PEG as corona were 201 ± 3, 136 ± 8, 107 ± 5, and 108 ± 8 FemM?1s?1, respectively. ORS intensity from Z-spectra of SPPM showed a significant correlation with the inverse of T? relaxation rates (1/T?, s?1) of the SPPM nanoprobes regardless of the PEG corona thickness. These data provide the fundamental understanding of the structure-property relationships between the SPPM nanostructures and ORS sensitivity, which offers useful mechanistic insights for the future improvement of SPPM nanoprobes in cancer molecular imaging applications.     

Ferumoxide-enhanced MRI in patients with colorectal cancer and rising CEA: surgical correlation in early recurrence

The purpose of this study was to evaluate the value of Ferumoxide-enhanced magnetic resonance (MR) imaging in the detection of hepatic metastases in high-risk patients treated for colorectal cancer that have rising CEA. We used 19 patients treated previously for colorectal cancer with rising CEA levels underwent an unenhanced T(1)-weighted (T1W), T(2)-weighted (T2W), STIR, and Ferumoxide-enhanced hepatic MRI. Following these studies, a laparotomy was performed and the liver was evaluated by palpation and intraoperative ultrasound. Two observers who were blinded to surgical results evaluated each MR sequence separately. The number of lesions considered highly suspicious for metastatic lesions were determined for each sequence and were compared to the results of surgery. The McNemar test was used to compare the outcomes of the different sequences. MR Imaging was unable to detect small (<5 mm) metastases discovered at surgery. The best non-contrast sequences for detecting metastases were the STIR with 42% sensitivity, 83% specificity and an overall accuracy of 56% and the T1W sequence (sensitivity 38%, specificity 100%, accuracy 57%), which were not significantly different (p 0.4). The noncontrast T2W sequence had a sensitivity of 29% and a specificity of 77% with an overall accuracy of 46%. When all pre contrast scans were grouped together the common sensitivity was 42%, specificity was 77% and accuracy was 54%. The post-ferumoxide T(2)W scans had a sensitivity of 42%, specificity of 85%, and accuracy of 57%, but did not detect any additional lesions. There was no statistical difference between the pre- and post-contrast studies with regard to identifying patients with metastatic disease (p 0.1). In conclusion, we found small hepatic metastases in patients with early signs of recurrent colorectal cancer are difficult to detect on MRI. Ferumoxide-enhanced MRI was unable to detect additional hepatic metastases and performed no better than unenhanced MRI in detecting small hepatic metastasis.