Fast flair imaging of the brain using the fast spin-echo and gradient spin-echo technique

The purpose of this study was to compare the gradient spin-echo (GRASE) to the fast spin-echo (FSE) implementation of fast fluid-attenuated inversion recovery (FLAIR) sequences for brain imaging. Thirty patients with high signal intensity lesions on T₂-weighted images were examined on a 1.5 T MR system. Scan time-minimized thin-section FLAIR-FSE and FLAIR-GRASE sequences were obtained and compared side by side. Image assessment criteria were lesion conspicuity, contrast between different types of normal tissue, image quality, and artifacts. In addition, contrast ratios and contrast-to-noise ratios were determined. Compared to FSE, the GRASE technique allowed a 17% reduction in scan time but conspicuity of small lesions in particular was significantly lower on FLAIR-GRASE images because of higher image noise and increased artifacts. Gray-white differentiation was slightly worse on FLAIR-GRASE. Physiological ferritin deposition appeared slightly darker on FLAIR-GRASE images and susceptibility artifacts were stronger. Fatty tissue was less bright with FLAIR-GRASE. With current standard hardware equipment, the GRASE technique is not an adequate alternative to FSE for the implementation of fast FLAIR sequences in routine clinical MR brain imaging.     

Conventional high resolution versus fast T(2)-weighted MR imaging of the heart: assessment of reperfusion induced myocardial injury in an animal model

Cardiac image quality in terms of spatial resolution and signal contrast was assessed for conventional and newly developed T(2)-weighted fast spin-echo imaging with high k-space segmentation. The capability in revealing regional myocardial edema and cellular damage was examined by a porcine model using histopathologic correlation. Twelve porcine hearts were excised from slaughtered animals and instantly perfused with 1000 mL cold cardioplegic solution. After 4 h of cold ischemia the hearts were reperfused for one hour using a "Langendorff" perfusion model followed by MR imaging at 1.5 Tesla. Three additional pig hearts served as controls and were studied by MR directly after harvesting. Histopathological analysis of regional tissue changes was performed macro- and microscopically. Short axis T(2)-weighted (3000/45 and 90) high quality fast spin-echo (FSE) images were recorded without cardiac action and signal intensity was correlated with histology. These images also served as gold standard for evaluation of newly developed faster sequences allowing measuring times shorter than 20 s. Fast T(2)-weighted imaging comprised single-slice fast spin echo (moderate echo train length of 23 echoes, FSE(m)), and multi-slice single-shot half-Fourier fast spin-echo (71 echoes, FSE(HASTE)) sequences, supplemented by versions with inversion recovery preparation (FSE(m)IR and FSE(HASTE)IR). Systolic function after reperfusion was restored in 10 porcine hearts. Tissue alterations included myocardial edema and contraction band necrosis which was found to be most severe in myocardium with maximum T(2) SI. Especially FSE(m) and FSE(m)IR sequences allowed differentiation of all categories of tissue damage on a high level of significance. In contrast, single-shot FSE(HASTE) and FSE(HASTE)IR sequences did not provide sufficient image quality to discriminate moderate and severe myocardial damage (p > 0.05). Different degrees of myocardial injury after ischemia and reperfusion can be staged by MR imaging, especially using conventional high resolution T(2)-weighted FSE sequences. The animal study indicates that fast T(2)-weighted FSE(m) and FSE(m)IR sequences lead to superior image quality and diagnostic accuracy compared to FSE(HASTE) and FSE(HASTE)IR imaging.     

Factors influencing contrast in fast spin-echo MR imaging.

Multi-echo pulse sequences for producing T2-weighted images in much reduced imaging times have recently been developed for routine clinical use. A number of recent articles have described the contrast obtained with fast spin-echo (FSE) sequences and have generally indicated that they depict tissues very similarly to conventional spin-echo (SE) imaging. There are, however, some important differences in contrast between some tissues in FSE images. This work presents a detailed study of the contrast obtained with FSE imaging sequences and examines the image sequence and tissue parameters which influence contrast. The use of multiple refocusing pulses produces several subtle effects not seen in conventional SE imaging sequences, and in this study the precise nature and extent of such effects are described. The relative contributions to image contrast of magnetization transfer, the decoupling of J-modulation effects, the production of stimulated echoes and direct saturation effects, of diffusion and of the effects of the differential attenuation of different spatial frequencies, are each quantified. The mechanisms responsible for the brighter fat signal seen in FSE images, as well as the loss of signal from some other tissues, are explained. Computer simulations, phantom experiments, and clinical images are all used to support the conclusions.     

Breath-held MR Cholangiopancreatography (MRCP) using a 3D Dixon fat–water separated balanced steady state free precession sequence

A novel 3D breath-held Dixon fat–water separated balanced steady state free precession (b-SSFP) sequence for MR cholangiopancreatography (MRCP) is described and its potential clinical utility assessed in a series of patients. The main motivation is to develop a robust breath-held alternative to the respiratory gated 3D Fast Spin Echo (FSE) sequence, the current clinical sequence of choice for MRCP. Respiratory gated acquisitions are susceptible to motion artifacts and blurring in patients with significant diaphragmatic drift, erratic respiratory rhythms or sleep apnea. A two point Dixon fat–water separation scheme was developed which eliminates signal loss arising from B₀ inhomogeneity effects and minimizes artifacts from perturbation of the b-SSFP steady state. Preliminary results from qualitative analysis of 49 patients demonstrate robust performance of the 3D Dixon b-SSFP sequence with diagnostic image quality acquired in a 20–24 s breath-hold.     

In vivo multiple spin echoes imaging of trabecular bone on a clinical 1.5 T MR scanner

In vivo multiple spin echoes (MSE) images of bone marrow in trabecular bone were obtained for the first time on a clinical 1.5 T scanner. Despite of a reduced sensitivity of the MSE trabecular bone images with respect to the cerebral matter ones, it is possible to observe some features in the MSE trabecular bone images that may be useful in the diagnosis of osteopenic states. Two different CRAZED-type MSE imaging sequences based on spin-echo and EPI imaging modalities were applied in phantom and in vivo. Preliminary experimental results indicate that EPI imaging readout seems to conceal the MSE contrast correlated with pore dimension in porous media. However it is still possible to detect anisotropy effects related to the bone structure in MSE-EPI images. Some strategies are suggested to optimize the quality of MSE trabecular bone images.     

Evaluation of the inner ear by 3D fast asymmetric spin echo (FASE) MR imaging: phantom and volunteer studies

The 3D fast asymmetric spin echo (FASE) method combines the half-Fourier technique and 3D fast spin echo (FSE) sequence. The advantage of this method is that it maintains the same spatial resolution as FSE while markedly reducing the imaging time. The purpose of the present study was to evaluate the usefulness of the 3D FASE technique in displaying the inner ear structure using phantom and volunteer studies. 3D FSE sequence images were obtained for comparison, and the optimum 3D FASE sequence was investigated on a 1.5T MR scanner. The results of phantom experiments showed increased signal-to-noise ratio (SNR) with prolonging repetition time (TR) on both 3D FASE and 3D FSE sequences. Although the SNR of 3D FASE images was 20-25% lower than that of 3D FSE images with the same TR, the SNR per minute with 3D FASE was about twice that with 3D FSE. On 3D FASE images, a higher spatial resolution was obtained with 2- or 4-shot images than with single-shot images. However, no significant difference was observed between 2-shot and 4-shot images. In the volunteer study, 3D FASE images using a TR of 5000 ms and an effective echo time (TEeff) of 250 ms showed a high SNR and spatial resolution and provided excellent contrast between cerebrospinal fluid and nerves in the internal auditory canal. The highest contrast was achieved in the 2-shot/2 number of excitations sequence. 3D FASE provides the same image quality as 3D FSE with a significant reducing in imaging time, and gives strong T2-weighted images. This method enables detailed visualization of the tiny structures of the inner ear.     

Comparative study of fast MR imaging: quantitative analysis on image quality and efficiency among various time frames and contrast behaviors

The purpose of this study is to quantitatively compare the image quality and efficiency provided by widely available fast MR imaging pulse sequences. A composite phantom with various T1 and T2 values and subjected to periodic motion was imaged at 1.5 T. The fast MRI sequences evaluated included fast spin-echo (FSE), single shot fast spin-echo (SSFSE), echo-planar imaging (EPI), multi-slice gradient recalled (MPGR), fast MPGR (FMPGR), and fast multi-slice spoiled gradient echo (FMPSPGR). T1-weighted (T1WI), T2-weighted (T2WI), proton-density-weighted (PDWI), and T2*-weighted (T2*WI) images were evaluated in breath-hold and non-breath-hold time frames. Analysis included measurement of image signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), nonuniformity, ghosting ratio, SNR per unit time and CNR per unit time. Among fast T2WI sequences, FSE with breath-hold time frame resulted in the highest image quality and in superior SNR and CNR efficiency by a factor of 5 or 6 as compared with conventional spin echo sequence. Among fast T1WI sequences, FMPGR and FMPSPGR both with non-breath-hold time frame produced the highest image quality and SNR and CNR efficiency by a factor of greater than 5 as compared with conventional spin echo. Among fast PDWI and T2*WI sequences, FSE produced the highest SNR and CNR, and was maximally efficient with a factors of greater than 6 as compared with conventional spin echo.     

A Study on Usefulness Comparison of Multiple-Shot FSE PROPELLER and IDEAL-FSE Techniques for Magnetic Susceptibility Artifact

The aim of this study was to visualize multiple-shot fast spin echo (FSE) images using a periodically rotated overlapping parallel line with enhanced reconstruction (PROPELLER) technique. An iterative decomposition of water and fat with echo asymmetry and a least squares estimation (IDEAL) technique were also performed to reduce the image distortion or susceptibility artifact depending on the difference in magnetic susceptibility to the surrounding tissues caused by metal insertion. The utility of this technique was examined quantitatively. A ferromagnetic image was generated from all the metals, but the IDEAL technique caused less image distortion than the PROPELLER technique. The 3-point IDEAL technique, which used the difference in the signal phase of fat and water, required more time for the examination and image reconstruction than the PROPELLER technique, which was based on a rotating blade in k-space. On the other hand, the IDEAL technique was more useful for reducing the susceptibility artifacts. The use of a proper technique in clinical trials based on these results is expected to provide better clinical information for imaging diagnoses.     

Multiecho multimoment refocussing of motion in magnetic resonance imaging: MEM-MO-RE

Gradient moment nulling techniques for refocussing of spin dephasing resulting from movement during application of magnetic resonance imaging gradients have gained widespread application. These techniques offer advantages over conventional imaging gradients by reducing motion artifacts due to intraview motion, and by recovering signal lost from spin dephasing. This paper presents a simple technique for designing multiecho imaging gradient waveforms that refocus dephasing from the interaction of imaging gradients and multiple derivatives of position. Multiple moments will be compensated at each echo. The method described relies on the fact that the calculation of time moments for nulled moment gradient waveforms is independent of the time origin chosen. Therefore, waveforms used to generate the second echo image for multiple echo sequences with echo times given by TEn = TE1 + (n - 1) * (TE2 - TE1) may also be used for generation of the third and additional echo images. All echoes will refocus the same derivatives of position. Multiecho, multimoment refocussing (MEM-MO-RE) images through the liver in a patient with ampullary adenocarcinoma metastatic to the liver demonstrate the application of the method in clinical scanning.     

Flow compensation for the fast spin echo triple-echo Dixon sequence

The fast spin echo (FSE) triple-echo Dixon (FTED) sequence uses bipolar triple-echo readout during each echo-spacing period of FSE to collect all the images necessary for Dixon water and fat separation in a single scan. In comparison to other FSE implementations of the Dixon technique, the triple echo readout used in FTED incurs minimal deadtime in the pulse sequence design and thus greatly enhances the overall scan efficiency. A potential drawback of FTED is that the time dependence of the gradient moment along the frequency encode direction becomes more complicated than in FSE and flow compensation based on the gradient moment (GM) nulling is difficult to achieve. In this work, the first order GM along the frequency encode direction of FTED was examined and two different methods to minimize the GM were proposed. The first method nulls the GM at all the locations of the refocusing radiofrequency pulses so that the Carr-Purcell Meiboom-Gill condition is always maintained. The second method minimizes the GM of the spin echo component of the FSE signal at the echo locations. The efficacy of both methods in reducing the first order GM and flow-related artefacts was demonstrated both in phantom and in images in vivo.     

Flow effects in long-range dipolar field MRI

Incoherent spin motion, such as diffusion, can lead to significant signal loss in multiple spin echoes (MSE) experiments, sometimes to its complete extinction. Coherent spin motion, such as laminar flow, can also modify the magnetization in MSE imaging and yield additional contrast. Our experimental results indicate that MSE is flow-sensitive. Our theoretical analysis and experimental results show how the effect of the distant dipolar field can be annihilated by flow. This effect can be quantified by directly solving the nonlinear Bloch equation, taking into account the deformation of the dipolar field by motion. Unexpected results have been observed, such as a recovery of the dipolar interaction due to flow in the "magic angle" condition.     

The influence of stimulated echoes on contrast in fast spin-echo imaging

Tissue contrast obtained using fast spin-echo (FSE) and conventional spin-echo (SE) sequences is not identical and a number of mechanisms are thought to contribute to these contrast differences. The effect of stimulated echoes has previously been apparently ruled out as a contributing mechanism. Signal-to-noise ratios of single-slice matched FSE and conventional SE sequences were compared in aqueous solutions of CuSO₄, Cr₂(SO₄)₃ and MnSO₄ with various T₁ and T₂ values. Enhancement of the FSE signal was observed in short T₂ solutions and the effect was greater in samples where the T₁ to T₂ ratio was high. Reducing the refocusing pulse flip angle to increase the contribution from stimulated echoes also resulted in slightly increased enhancement. Experimental results were verified by computer simulations. Our results show that stimulated echoes do contribute to the contrast differences between FSE and conventional SE images and may be significant in the imaging of brain hemorrhage.     

Autocalibrated parallel imaging reconstruction with sampling pattern optimization for GRASE: APIR4GRASE

PurposeTo reduce artifacts and scan time of GRASE imaging by selecting an optimal sampling pattern and jointly reconstructing gradient echo and spin echo images.MethodsWe jointly reconstruct images for the different echo types by considering these as additional virtual coil channels in the novel Autocalibrated Parallel Imaging Reconstruction with Sampling Pattern Optimization for GRASE (APIR4GRASE) method. Besides image reconstruction, we identify optimal sampling patterns for the acquisition. The selected optimal patterns were validated on phantom and in-vivo acquisitions. Comparison to the conventional GRASE without acceleration, and to the GRAPPA reconstruction with a single echo type was also performed.ResultsUsing identified optimal sampling patterns, APIR4GRASE eliminated modulation artifacts in both phantom and in-vivo experiments; mean square error (MSE) was reduced by 78% and 94%, respectively, compared to the conventional GRASE with similar scan time. Both artifacts and g-factor were reduced compared to the GRAPPA reconstruction with a single echo type.ConclusionAPIR4GRASE substantially improves the speed and quality of GRASE imaging over the state-of-the-art, and is able to reconstruct both spin echo and gradient echo images.     

Manifestation of magic angle phenomenon: comparative study on effects of varying echo time and tendon orientation among various MR sequences

The purpose of this study was to systematically investigate the effect of varying the echo time (TE) values and angle of the tendon to the main magnetic field (B(o)) upon the signal intensity observed with the magic angle phenomenon in tendons among most commonly used MR pulse sequences, including conventional spin echo (CSE), fast spin echo (FSE) and gradient echo (GRE) sequences. The intact bovine Achilles tendon was imaged using a clinical 1.5-T MR scanner. Magic angle phenomenon occurs in CSE, FSE and GRE sequences with different grade, appearing most severe in CSE, middle in FSE, and weakest in GRE sequence. In addition, the tendon signal changes produced by the magic angle phenomenon could be greatly reduced by increasing the TE to above a certain critical value in all three sequences. These critical TE values were different among CSE (40 msec), FSE (70 msec), and GRE (30 msec) sequences.     

医学MRI系统的FSE实现

介绍了快速自旋回波成像方法（FSE）在医学磁共振成像（MRI）系统上的实现. 提出了一种简单的预扫描方案,可以有效克服FSE成像方法的固有缺陷. 软件系统的预扫描定制方案,使得本系统可以适应于不同的预扫描要求,谱仪的无关性保证了本系统可以很方便地附加在任何谱仪上. 最后给出了FSE方法在DRX谱仪上的实现和在200 mT系统上获取的实验结果.     

Fast T2-weighted MR imaging: impact of variation in pulse sequence parameters on image quality and artifacts

The purpose of this study was to quantitatively evaluate in a phantom model the practical impact of alteration of key imaging parameters on image quality and artifacts for the most commonly used fast T(2)-weighted MR sequences. These include fast spin-echo (FSE), single shot fast spin-echo (SSFSE), and spin-echo echo-planar imaging (EPI) pulse sequences. We developed a composite phantom with different T1 and T2 values, which was evaluated while stationary as well as during periodic motion. Experiments involved controlled variations in key parameters including effective TE, TR, echo spacing (ESP), receive bandwidth (BW), echo train length (ETL), and shot number (SN). Quantitative analysis consisted of signal-to-noise ratio (SNR), image nonuniformity, full-width-at-half-maximum (i.e., blurring or geometric distortion) and ghosting ratio. Among the fast T(2)-weighted sequences, EPI was most sensitive to alterations in imaging parameters. Among imaging parameters that we tested, effective TE, ETL, and shot number most prominently affected image quality and artifacts. Short T(2) objects were more sensitive to alterations in imaging parameters in terms of image quality and artifacts. Optimal clinical application of these fast T(2)-weighted imaging pulse sequences requires careful attention to selection of imaging parameters.     

Quantitative diffusion coefficient maps using fast spin-echo MRI

In this work, we have evaluated the performance of a diffusion-sensitive fast spin-echo (FSE) pulse sequence. The proposed pulse sequence utilises velocity-compensating diffusion-encoding gradients and includes the collection of navigator echoes. Spoiler gradients were inserted in the slice-selecting direction to minimise effects from stimulated echoes. Calculations of the b values showed that cross-terms between imaging gradients and diffusion gradients only led to a marginal increase of b values. Pixel-wise calculation of apparent diffusion coefficient (ADC) maps was performed numerically, considering cross-terms between diffusion-encoding and imaging gradients. The sequences investigated used echo train lengths of 16, 8 and 4 echoes and were encoded in either the slice-, frequency- or phase-encoding direction. In order to allow for higher b values a pulse-sequence version using non-motion compensating diffusion-encoding gradients was written. Phantom measurements were performed and the diffusion coefficients of water and acetone were reasonable. Seven healthy volunteers (age 28–50 years) were examined and apparent diffusion coefficient values agreed well with expected values. Diffusion-weighted images, apparent diffusion coefficient maps and images corresponding to the trace of the diffusion tensor of good quality were retrieved in vivo.     

Imaging patients pre and post deep brain stimulation: Localization of the electrodes and their targets

PurposeDeep brain stimulation (DBS) has become a widely performed surgical procedure for patients with medically refractory movement disorders and mental disorders. It is clinically important to set up a MRI protocol to map the brain targets and electrodes of the patients before and after DBS and to understand the imaging artifacts caused by the electrodes.MethodsFive patients with DBS electrodes implanted in the habenula (Hb), fourteen patients with globus pallidus internus (GPi) targeted DBS, three pre-DBS patients and seven healthy controls were included in the study. The MRI protocol consisted of magnetization prepared rapid acquisition gradient echo T1 (MPRAGE T1W), 3D multi-echo gradient recalled echo (ME-GRE) and 2D fast spin echo T2 (FSE T2W) sequences to map the brain targets and electrodes of the patients. Phantom experiments were also run to determine both the artifacts and the susceptibility of the electrodes. Signal to noise ratio (SNR) on T1W, T2W and GRE datasets were measured. The visibility of the brain structures was scored according to the Rose criterion. A detailed analysis of the characteristics of the electrodes in all three sequence types was performed to confirm the reliability of the postoperative MRI approach. In order to understand the signal behavior, we also simulated the corresponding magnitude data using the same imaging parameters as in the phantom sequences.ResultsThe mean ± inter-subject variability of the SNRs, across the subjects for T1W, T2W, and GRE datasets were 20.1 ± 8.1, 14.9 ± 3.2, and 43.0 ± 7.6, respectively. High resolution MPRAGE T1W and FSE T2W data both showed excellent contrast for the habenula and were complementary to each other. The mean visibility of the habenula in the 25 cases for the MPRAGE T1W data was 5.28 ± 1.11; and the mean visibility in the 20 cases for the FSE T2W data was 5.78 ± 1.30. Quantitative susceptibility mapping (QSM), reconstructed from the ME-GRE sequence, provided sufficient contrast to distinguish the substructures of the globus pallidus. The susceptibilities of the GPi and globus pallidus externa (GPe) were 0.087 ± 0.013 ppm and 0.115 ± 0.015 ppm, respectively. FSE T2W sequences provided the best image quality with smallest image blooming of stimulator leads compared to MPRAGE T1W images and GRE sequence images, the measured diameters of electrodes were 1.91 ± 0.22, 2.77 ± 0.22, and 2.72 ± 0.20 mm, respectively. High resolution, high bandwidth and short TE (TE = 2.6 ms) GRE helped constrain the artifacts to the area of the electrodes and the dipole effect seen in the GRE filtered phase data provided an effective mean to locate the end of the DBS lead.ConclusionThe imaging protocol consisting of MPRAGE T1W, FSE T2W and ME-GRE sequences provided excellent pre- and post-operative visualization of the brain targets and electrodes for patients undergoing DBS treatment. Although the artifacts around the electrodes can be severe, sometimes these same artifacts can be useful in identifying their location.     

Separation of spin populations with gradient echoes as an aid in pulse sequence tuning

Tuning of nuclear magnetic resonance pulse sequences with pulsed "crusher" gradients or phase cycling serves to remove unwanted spin populations from the data acquisition window. Verification that unwanted spin population are not detected is often determined by the absence of obvious artifacts in an image. This approach is unsatisfactory in some instances because signal contamination with unwanted spin populations may not be obvious. This is a particular concern with multiple-spin echo, volume-selective, and other multiple-pulse sequences. A solution to this problem is the separation of spin populations using gradient echoes, allowing the existence of unwanted populations to be easily observed separately. Tuning of a pulse sequence is straightforward when spin populations can be independently observed.     

Venous cavernoma at 8 Tesla MRI

Cavernous angiomas or cavernomas are vascular malformations, which may be associated with risk of bleeding episodes. We present a case report comparing high resolution 8 Tesla gradient echo (GE) imaging with routine fast spin echo (FSE) at 1.5 Tesla in a patient with venous cavernoma. A 55-year-old male with a history of hemorrhagic stroke was studied using high-resolution 8 Tesla magnetic resonance imaging (MRI) system, which revealed venous cavernoma (9 x 8.6 mm) in the left parietal region and visualized adjacent microvascular supply. Signal loss was prominent in the cavernoma region compared to surrounding brain tissue, and signal intensity declined by factor 7.3 +/- 2.4 (679 +/- 62%) on GE images at 8 Tesla. Cavernoma was not apparent on routine T(2)-weighted FSE images at 1.5 Tesla MRI. This case report indicates that GE images at 8 Tesla can be useful for evaluation of vascular pathologies and microvasculature.