High-speed interlaced spin-echo magnetic resonance imaging.

A new method is introduced for increasing the efficiency in multislice single spin-echo MRI. The method interlaces the excitation and measurement of different slices, resulting in an effective use of the echo delay time between RF excitation and reception. Under certain conditions, the method allows for scan time reduction compared to standard single spin-echo MRI, in particular for long echo times. The technique is demonstrated in examples of brain scans, indicating that a substantial increase is scan speed can be achieved without loss in image signal-to-noise ratio or contrast. Potential applications include perfusion imaging using T(2)-contrast agents, as well as BOLD-based functional imaging. Magn Reson Med 43:905-908, 2000. Published 2000 Wiley-Liss, Inc.     

Multiple spin-echo magnetic resonance imaging

Spin-echo magnetic resonance (MR) imaging detects a variety of pathologic states with great sensitivity. A technique for producing multiple spin-echo images in multisection operation is presented. This method of intensity-image acquisition is compared with retrospective intensity-image synthesis from routine data sets. Both yield long echo time (TE) images with similar image contrast and comparable and often increased diagnostic utility. Technical and clinical considerations are addressed, including signal-to-noise levels, flow effects, and patient throughput.     

Efficient biphasic spin-echo magnetic resonance imaging

The concept of temporal echo multiplexing is defined and used to develop rapid biphasic spin-echo sequences for imaging the heart. Three imaging sequences, based on four-echo and two-echo multiplexing and rapid single echo (i.e., conventional spin-echo imaging), are compared. Preliminary results indicate that two-echo multiplexing yields a significantly reduced acquisition time window with image quality that is only slightly inferior to single-echo imaging. Single-echo biphasic imaging results in the most consistent image quality.     

The effects of finite sampling in spin-echo or field-echo magnetic resonance imaging

The artifacts associated with finite sampling in magnetic resonance imaging are shown to be significant when the ratio of half the sampling time, Ts/2, to transverse relaxation time, T2, is on the order of unity or greater. For both symmetric and asymmetric sampling, these artifacts include enhanced high spatial frequency image intensity (bright edges), induced systematic noise, loss of resolution, and changes in phase. Simple filtering of the raw data does not remove the problem in tissues with position-dependent T2 values. An exact deconvolution method is described to remove these problems associated with finite position-dependent T2 values.     

Isotropic diffusion weighting in radial fast spin-echo magnetic resonance imaging.

Radial fast spin-echo (radial-FSE) methods enable multishot diffusion-weighted MRI (DWMRI) to be carried out without significant artifacts due to motion and/or susceptibility and can be used to generate DWMRI images with high spatial resolution. In this work, a novel method that allows isotropic diffusion weighting to be obtained in a single radial k-space data set is presented. This is accomplished by altering the direction of diffusion weighting gradients between groups of TR periods, which yield sets of radial lines that possess diffusion weighting sensitive to motion in different directions. By altering the diffusion weighting directions and controlling the view ordering appropriately within the sequence, an effectively isotropic diffusion-weighted image can be obtained within one radial-FSE scan. The order in which radial lines are acquired can also be controlled to yield data sets without significant artifacts due to motion, T(2) decay, and/or diffusion anisotropy.     

Lymphatic imaging in experimental filariasis using magnetic resonance.

RATIONALE AND OBJECTIVES. To evaluate acquired lymphatic abnormalities caused by filariasis, the authors examined the peripheral lymphatic system in normal ferrets and those chronically infected with Brugia malayi using magnetic resonance imaging (MRI). The findings were compared with previously obtained lymphangioscintigraphic (LAS) images in ferrets both with and without experimental filariasis. METHODS. Fifteen ferrets (11 infected with B. malayi and four noninfected controls) underwent whole body coronal MRI using a quadrature transmission-receive head coil at 0.5 Tesla operating at a resonant frequency of 21.5 mHz for protons with a 25-cm field of view. RESULTS. In contrast to normal animals, infected ferrets showed dilated hindlimb dermal lymphatic collaterals, enlarged high-signal intensity groin lymph nodes with punctate low-signal intensity centers and separate low-signal intensity spots with irregular thin channels, suggestive of nests of viable adult nematodes within tortuous lymphatics and nodes. MRI correlated with the LAS findings, and the interpretations were supported by light, scanning electron, and video microscopy. CONCLUSIONS. T2-weighted MRI in conjunction with LAS accurately depicts the peripheral lymphatic system in filarial-infected ferrets. These two modalities are useful complementary techniques to examine disorders characterized by lymphatic insufficiency.     

Evaluation of bone contusions with fat-saturated fast spin-echo proton-density magnetic resonance imaging.

OBJECTIVE: To evaluate the efficacy of fast spin-echo proton-density magnetic resonance imaging (MRI) with fat saturation sequences in the evaluation of bone contusions at the knee. METHODS: Analysis of 46 consecutive knee MRI examinations performed on patients referred from a sports medicine clinic after knee trauma. All examinations included coronal fast spin-echo proton-density fat saturation, fast spin-echo proton-density and fast spin-echo T2-weighted sequences. All 3 coronal sequences were blindly reviewed independently of each other by 3 experienced musculoskeletal radiologists to identify and grade bone contusions. RESULTS: Thirty-five bone contusions were identified in 24 patients. All bone contusions were identified on fast spin-echo proton-density fat saturation sequences, which was significantly greater than the percentage identified on either fast spin-echo T2-weighted sequences (21/35, 60%, p < 0.001) or fast spin-echo proton-density sequences (10/35, 29%, p < 0.001). Fourteen (40%) of the contusions were identified only on the fast spin-echo proton-density fat saturation sequences. The average grade of contusion for all 35 examinations was also significantly higher on the fast spin-echo proton-density fat saturation sequences than on the fast spin-echo proton-density and fast spin-echo T2-weighted sequences (p < 0.05). CONCLUSION: Fast spin-echo proton-density fat saturation sequences are more sensitive in the detection of bone contusions than fast spin-echo proton-density and fast spin-echo T2-weighted sequences. Assessment of other structures in the knee with fast spin-echo proton-density fat saturation MRI provides good spatial resolution and adequate T2-weighted information. It may have advantages over the more heavily T2-weighted fast spin-echo T2 fat saturation and inversion recovery sequences.     

Clinical and experimental sodium magnetic resonance imaging

In conclusion, sodium MR imaging has potential for providing physiologic information relevant to cell mitosis, cell energy state, rCBV, and seizures. Considerable technical and experimental development is necessary, however, before sodium MRI becomes a routine examination in the clinical setting.     

Dynamically shimmed multivoxel 1H magnetic resonance spectroscopy and multislice magnetic resonance spectroscopic imaging of the human brain

In vivo multivoxel Magnetic Resonance Spectroscopy (MRS) and multislice Magnetic Resonance Spectroscopic Imaging (MRSI) are extremely susceptible to poor homogeneity of the static magnetic field. Existing room-temperature (RT) shim technology can adequately optimize the B(0) homogeneity of local volumes, such as single voxels. However, the widespread global homogeneity required for in vivo spectral acquisitions from multiple volumes in the human brain cannot be attained with a single RT shim setting. Dynamic shim updating (DSU) allows for use of local RT shim B(0) homogeneity compensation capabilities in a global fashion. Here, by updating first- and second-order shims on a voxel- and slice-specific basis using a pre-emphasized DSU system, we present multivoxel MRS and multislice MRSI of the human brain. These results demonstrate that DSU can increase multivoxel MRS acquisition capabilities and significantly improve the quality of multislice MRSI data.     

Three-dimensional high-resolution fast spin-echo coronary magnetic resonance angiography.

Due to SNR constraints, current "bright-blood" 3D coronary MRA approaches still suffer from limited spatial resolution when compared to conventional x-ray coronary angiography. Recent 2D fast spin-echo black-blood techniques maximize signal for coronary MRA at no loss in image spatial resolution. This suggests that the extension of black-blood coronary MRA with a 3D imaging technique would allow for a further signal increase, which may be traded for an improved spatial resolution. Therefore, a dual-inversion 3D fast spin-echo imaging sequence and real-time navigator technology were combined for high-resolution free-breathing black-blood coronary MRA. In-plane image resolution below 400 microm was obtained. Magn Reson Med 45:206-211, 2001.     

Fast spin-echo for multiple mouse magnetic resonance phenotyping.

High-resolution magnetic resonance imaging is emerging as a powerful tool for phenotyping mice in biologic studies of genetic expression, development, and disease progression. In several applications, notably random mutagenesis trials, large cohorts of mice must be examined for abnormalities that may occur in any part of the body. In the aim of establishing a protocol for imaging multiple mice simultaneously in a standardized high-throughput fashion, this study investigates variations of a three-dimensional fast spin-echo sequence that implements driven equilibrium, modified refocusing, and partial excitation pulses. Sequence variations are compared by simulated and experimental measurements in phantoms and mice. Results indicate that when using a short repetition time (TR相似文献   

Comparison of spin-echo echo-planar imaging magnetic resonance elastography with gradient-recalled echo magnetic resonance elastography and their correlation with transient elastography

PURPOSEThis study aimed to assess the agreement between liver stiffness (LS) values obtained by the gradient-recalled echo (GRE) magnetic resonance elastography (MRE) and spin-echo echo-planar imaging (SE-EPI) MRE with those of transient elastography (TE), respectively.METHODSWe retrospectively included 48 participants who underwent liver MRE with both GRE and SE-EPI sequences in the same session and also TE within 1 year. We obtained LS values for MRE by drawing free-hand region of interest, and TE was performed using a FibroScan device. We assessed the relationship between the mean LS values obtained by each MRE sequence and TE using the correlation coefficients and Bland–Altman plots, respectively. We also compared LS values and technical failure rates of measured values from MRE between SE-EPI and GRE sequences using the paired t-test and McNemar’s test. The MRE failure was defined as the absence of pixel value with a confidence index above 95%.RESULTSThe LS values from SE-EPI and GRE sequences strongly correlated with those from TE (GRE; r = 0.73, P < .001 vs. SE-EPI; r = 0.79, P < .001). In addition, the LS values from the 2 MRE sequences showed excellent relationship (intraclass correlation coefficient, 0.94 [0.89-0.97], P < .001). The LS values from SE-EPI and GRE MRE were not significantly different (4.14 kPa vs. 3.88 kPa, P = .19). Furthermore, the technical success rate of SE-EPI MRE was superior to that of GRE (100% vs. 83.8%, P = .031).CONCLUSIONThe measured LS values obtained using TE correlated strongly with those obtained using GRE and SE-EPI MRE techniques, even though SE-EPI-MRE resulted a higher technical success rate than GRE-MRE. Therefore, we believe that TE, GRE, and SE-EPI MR elastography techniques may complement each other according to the appropriate individual situation.

Main points

• The liver stiffness (LS) values measured by transient elastography (TE) and those of gradient-recalled echo (GRE) and spin-echo echo-planar imaging (SE-EPI) MRE techniques are strongly correlated with one another.
• The technical success rate of SE-EPI MRE was superior to that of GRE MRE without a significant difference in LS values.
• Therefore, TE, GRE, and SE-EPI MRE techniques may complement each other in appropriate individual situations.

Currently, hepatic fibrosis is considered to have the potential to be reversed with treatment, especially during the early stages, but can progress to cirrhosis if left untreated.^1-3 Thus, the identification and staging of fibrosis prior to the development of liver cirrhosis (LC) are important when managing chronic liver disease (CLD),⁴ which includes chronic hepatitis, alcoholic liver disease, and non-alcoholic fatty liver disease (NAFLD).Variable non-invasive techniques for evaluating liver fibrosis including transient elastography (TE), ultrasound shear wave elastography, and magnetic resonance elastography (MRE) have been developed.^5-7 Among these techniques, TE, which measures the velocity of acoustic shear waves traveling the liver using ultrasound,⁸ is the most validated and highly reproducible technique for diagnosing liver fibrosis.^9-11 However, high failure rates and ­unreliable measurements in patients with obesity or ascites are known to be major weaknesses of TE.^12,13Meanwhile, MRE is the non-invasive imaging modality available with the highest diagnostic accuracy in evaluating liver elasticity.^14-16 Based on magnetic resonance imaging (MRI), the propagating shear waves through the liver are imaged and processed using an algorithm to generate cross-sectional images displaying the magnitude of the complex shear modulus.¹⁷ Compared to ultrasound-based techniques, MRE can provide more comprehensive liver imaging examinations and larger coverage.^17,18Until recently, the most common pulse sequence used for MRE has been based on gradient-recalled echo (GRE) sequence, after being well-validated for liver stiffness (LS) evaluation by many previous studies.^17,19,20 However, GRE MRE is known to be more sensitive to T2* decay, which results in a high technical failure rate in the iron-overloaded liver.²⁰ On the other hand, the spin-echo echo-planar imaging (SE-EPI) MRE sequence, a relative newcomer, is less affected by transverse relaxation signal decay^21,22 and has resulted in a higher overall technical success rate than GRE MRE with shorter acquisition time and no significant difference in LS values.^17,20,23In the context of developing the appropriate clinical approach for non-invasive assessment of liver fibrosis, there is intense interest in the correlation between alternative techniques. In several studies comparing TE and GRE MRE, MRE has been found to be generally superior to TE in diagnosing hepatic fibrosis.^16,24,25 However, there are only a few studies comparing SE-EPI MRE and TE,²⁶ and there is no published study so far as we know that compares the LS values obtained by GRE MRE, SE-EPI MRE, and TE in the same study participants.Accordingly, the purpose of this study is to assess the agreement between the LS values obtained by GRE MRE and SE-EPI MRE with those obtained by TE, respectively. In addition, we aimed to compare the LS values and technical success rates obtained by GRE MRE and SE-EPI MRE in the same setting.     

Flow phenomena, classification, and clinical aspects of magnetic resonance angiography

Both time-of-flight and phase-contrast techniques are currently being employed in producing angiographic MR images of arteries and veins. Both two-dimensional and three-dimensional acquisition methods can be used to display projective vascular images with the maximum intensity projection algorithm, eliminating the need for image subtraction. MR angiography has expanded the armamentarium of MR imaging, especially in the head and neck, and should in some organ systems reduce the need for conventional x-ray catheter angiography.     

Analysis of spin-echo rephasing with pulsatile flow in 2D FT magnetic resonance imaging

The effects of pulsatile flow on spin phases in spin-echo magnetic resonance imaging are considered. General expressions for the spin phases of the first four echoes are derived in terms of the Fourier coefficients of flow. These expressions are valid for any time-dependent acceleration and, hence, are not restricted to constant acceleration. The derived expressions are then theoretically evaluated for aortic flow and examined at different points in the cardiac cycle. Our results show that rephasing may occur at certain points in the cardiac cycle for either even or odd echoes depending upon the particular Fourier coefficients of the velocity function and the spin-echo delay time. However, even-echo rephasing is not always necessarily valid. Furthermore, the possibility of determining the flow velocities in the body with an appropriate series of imaging studies is also discussed.     

实验性自身免疫性脑脊髓炎的MRI研究进展

实验性自身免疫性脑脊髓炎(EAE)是公认的研究人类多发性硬化(MS)的动物模型。MRI能提供大鼠中枢神经系统的高分辨率图像,在活体上动态监测EAE病变的演变过程。随着MR设备及MRI新技术的不断研发及应用,EAE大鼠的MRI研究将更好的诠释EAE病灶的病理生理演变过程,为探索MS病因、发病机制和评估疗效提供了一种重要的研究手段。     

Present clinical status of magnetic resonance imaging

Magnetic resonance imaging has achieved clinical importance and has become the primary tool in the investigation of head, spinal cord, pelvis, cancellous bone, pericardium (with gating), gallbladder, lymph node masses, and very soon, in the evaluation of joints. In many areas, particularly with respiratory and ECG gating, this method promises to replace all other imaging modalities with the precision of information obtained. Today the exceptions are the alimentary tube and its mesentery, between the esophagus and rectum, and the peripheral lung.