Qualitative and Quantitative Assessment of Isotropic Ankle Magnetic Resonance Imaging: Three-Dimensional Isotropic Intermediate-Weighted Turbo Spin Echo versus Three-Dimensional Isotropic Fast Field Echo Sequences

Objective

To compare the image quality of volume isotropic turbo spin echo acquisition (VISTA) imaging method with that of the three-dimensional (3D) isotropic fast field echo (FFE) imaging method applied for ankle joint imaging.

Materials and Methods

MR imaging of the ankles of 10 healthy volunteers was performed with VISTA and 3D FFE sequences by using a 3.0 T machine. Two radiologists retrospectively assessed the tissue contrast between fluid and cartilage (F-C), and fluid and the Achilles tendon (F-T) with use of a 4-point scale. For a quantitative analysis, signal-to-noise ratio (SNR) was obtained by imaging phantom, and the contrast ratios (CRs) were calculated between F-T and F-C. Statistical analyses for differences in grades of tissue contrast and CRs were performed.

Results

VISTA had significantly superior grades in tissue contrast of F-T (p = 0.001). Results of 3D FFE had superior grades in tissue contrast of F-C, but these result were not statistically significant (p = 0.157). VISTA had significantly superior CRs in F-T (p = 0.002), and 3D FFE had superior CRs in F-C (p = 0.003). The SNR of VISTA was higher than that of 3D FFE (49.24 vs. 15.94).

Conclusion

VISTA demonstrates superior tissue contrast between fluid and the Achiles tendon in terms of quantitative and qualitative analysis, while 3D FFE shows superior tissue contrast between fluid and cartilage in terms of quantitative analysis.     

Fast three dimensional sodium imaging

An efficient scheme for fast three dimensional acquisition of sodium MR images is described. This scheme relies on the use of three dimensional k-space trajectories with constant sample density to achieve significant (60–70%) reductions in total data acquisition time over conventional projection imaging schemes. The performance of this data acquisition scheme is demonstrated with acquisition of sodium data sets on phantoms and normal human volunteers at 1.5 and 3.0 Tesla. The experimental results demonstrate that high quality three dimensional sodium images (0.2 cc voxel size, 10:1 signal-to-noise ratio) can be acquired at clinical field strengths (1.5 Tesla) in under 10 min.     

Parallel Imaging Artifacts in Body Magnetic Resonance Imaging

Objective

To familiarize the reader with the fundamental concepts of partial parallel imaging (PPI); to review the technical aspects of PPI including calibration scan, coil geometry, and field of view (FOV); and to illustrate artifacts related to parallel imaging and describe solutions to minimize their negative impact.

Results

PPI has led to a significant advance in body magnetic resonance imaging by reducing the time required to generate an image without loss of spatial resolution. Although PPI can improve image quality, it is not free of artifacts, which can result in significant image degradation. Knowledge of these artifacts and how to minimize their effect is important to optimize the use of parallel imaging for specific body magnetic resonance imaging applications.

Conclusions

The reader will be introduced to the fundamental principles of PPI. Common imaging characteristics of PPI artifacts will be displayed with an emphasis on those seen with image-based methods, the principles behind their generation presented, and measures to minimize their negative impact will be proposed.     

Frequency resolved single-shot MR imaging using stochastic k-space trajectories

A new single-shot stochastic imaging technique with a random k-space path that provides very selective filtering with respect to chemical shift or off-resonance signals of the investigated tissue is proposed. It is demonstrated that in stochastic imaging only on-resonance compartments are visible whereas frequency shifted compartments cancel to noise that is distributed over the whole image. This method can be used as a single-shot chemical shift selective imaging technique and allows to calculate frequency resolved spectra for each spatial position of the image based on a single signal aquisition. The single-shot stochastic imaging sequence makes high demands on the gradient system and the theoretical k-space trajectory is distorted by imperfect gradient performance. Therefore an additional k-space guided imaging technique that uses the true, measured k-space trajectory to correct artifacts generated by eddy currents and delay times of the rapid switched gradients is presented. In vitro and in vivo measurements demonstrate the successful implementation of single-shot stochastic imaging on a conventional MR scanner with unshielded gradient systems.     

Projection-reconstruction methods: Fast imaging sequences and data processing

New, rapid two- and three-dimensional imaging sequences based on steady-state gradient echoes and projection-reconstruction (PR) techniques are proposed. Quantitative studies show that fast PR sequences and classical, fast gradient-echo Fourier transform sequences lead to identical contrasts. In order to minimize inhomogeneity effects, a particular focus has been placed on echo-time reduction. The use of a weighting window permits one to acquire severely truncated echoes; partial k-space scanning may be considered.     

Reduction of motion artifacts in cine MRI using variable-density spiral trajectories

Dynamic cardiac imaging in MRI is a very challenging task. To obtain high spatial resolution, temporal resolution, and signalto-noise ratio (SNR), single-shot imaging is not sufficient Use of multishot techniques resolves this problem but can cause motion artifacts because of data inconsistencies between views. Motion artifacts can be reduced by signal averaging at some cost in increased scan time. However, for the same increase in scan time, other techniques can be more effective than simple averaging in reducing the artifacts. If most of the energy of the inconsistencies is limited to a certain region of k-space, increased sampling density (oversampling) in this region can be especially effective in reducing motion artifacts. In this work, several variable-density spiral trajectories are designed and tested. Their efficiencies for artifact reduction are evaluated in computer simulations and in scans of normal volunteers. The SNR compromise of these trajectories is also investigated. The authors conclude that variable-density spiral trajectories can effectively reduce motion artifacts with a small loss in SNR as compared with a uniform density counterpart.     

Block Regional Interpolation Scheme for k-Space (BRISK): A Rapid Cardiac Imaging Technique

We introduce an acquisition method, “block regional interpolation scheme for k-space” (BRISK), to reduce the acquisition time for cardiac imaging. The method exploits the high degree of correlation that exists between time-resolved cardiac images. For representative k-space data sets, Fourier analysis was applied along the cardiac phase dimension to reveal that different regions of k-space can be effectively sampled at different rates. A reduced sampling strategy was implemented, and unsampled points were generated by Fourier interpolation. Time savings of up to 75% are quite feasible and 25% BRISK scans compare well with 100% scans. Simulations and acquisitions using a normal volunteer and patients are presented.     

Fast Fluid Attenuated Inversion Recovery (FLAIR) imaging and associated artefacts in Magnetic Resonance Imaging (MRI)

MRI Fast Fluid Attenuated Inversion Recovery (FLAIR) sequences have become established in a wide range of central nervous system diseases. FLAIR images demonstrate excellent lesion conspicuity in a variety of disease processes. The fast FLAIR technique although giving heavily T2 weighted images combined with suppressed CSF resulting in high contrast-to-noise ratios for long T2 lesions within the brain parenchyma is susceptible to several potential imaging artefacts, including CSF related artefacts degrading the diagnostic value of the scans.     

Undersampling k-space using fast progressive 3D trajectories.

In 3D MRI, sampling k-space with traditional trajectories can be excessively time-consuming. Fast imaging trajectories are used in an attempt to efficiently cover the k-space and reduce the scan time without significantly affecting the image quality. In many applications, further reductions in scan time can be achieved via undersampling of the k-space; however, no clearly optimal method exists. In most 3D trajectories the k-space is divided into regions that are sampled with shots that share a common geometry (e.g., spirals). A different approach is to design trajectories that gradually but uniformly cover the k-space. In the current work, successive shots progressively add sampled regions to the 3D frequency space. By cutting the sequence short, a natural undersampled method is obtained. This can be particularly efficient because in these types of trajectories the contribution of new information by later shots is less significant. In this work the performance of progressive trajectories for different degrees of undersampling is assessed with trajectories based on missile guidance (MG) ideas. The results show that the approach can be efficient in terms of reducing the scan time, and performs better than the stack of spirals (SOS) technique, particularly under nonideal conditions.     

Susceptibility Magnetic Resonance Imaging Using Spectral Decomposition

Susceptibility differences of materials in magnetic resonance imaging (MRI) usually lead to the intravoxel spin phase variations. Subsequently, the phase variation in the voxel results in a reduction of the signal intensity. This signal intensity reduction is known as the susceptibility effect in MRI and has been studied extensively. In this paper, a new spectral decomposition technique is proposed with which the signal change due to the susceptibility effect can be analyzed. Further, an NMR pulse sequence for the spectral decomposition of the susceptibility was developed and applied to susceptibility imaging of venous blood possessing paramagnetic properties. The computer simulations of the spectral decomposition method and their corresponding experimental results obtained using both a phantom and human volunteers are reported.     

Real-Time Functional Magnetic Resonance Imaging

A recursive algorithm suitable for functional magnetic resonance imaging (FMRI) calculations is presented. The correlation coefficient of a time course of images with a reference time series, with the mean and any linear trend projected out, may be computed with 22 operations per voxel, per image; the storage overhead is four numbers per voxel. A statistical model for the FMRI signal is presented, and thresholds for the correlation coefficient are derived from it. Selected images from the first real-time functional neuroimaging experiment (at 3 Tesla) are presented. Using a 50-MHz workstation equipped with a 14-bit analog-to-digital converter, each echo planar image was acquired, reconstructed, correlated, thresh-olded, and displayed in pseudocolor (highlighting active regions in the brain) within 500 ms of the RF pulse.     

Radiologic and Pathological Correlation of Staging of Rectal Cancer with 3 Tesla Magnetic Resonance Imaging

Purpose

To assess the sensitivity and specificity of 3 Tesla magnetic resonance imaging (MRI) in the prediction of extramural spread and metastatic adenopathy in rectal carcinoma.

Materials and Methods

This was a prospective cohort study that included forty consecutive patients with rectal carcinoma from the Department of Colorectal Surgery. Three Tesla (3T) MRI was performed on these patients after a 4-hour fast and cleansing water enema. TI-weighted and T2-weighted images were obtained with high-resolution images T2-weighted sequences through the pelvis. Sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) of 3T MRI for prediction of metastatic adenopathy and extramural spread were calculated. The TNM staging based on MRI was compared with histopathology of the resected specimen (taken as the criterion standard).

Results

In our study, sensitivity, specificity, PPV, and NPV of 3T MRI for prediction of metastatic adenopathy were 100%, 78.3%, 77.3%, and 100%, respectively. Sensitivity, specificity, PPV, and NPV of 3T MRI for prediction of extramural tumour spread were 100% and 20%, 89.7% and 100%, respectively (ie, prediction of stages T3 and above).

Conclusion

MRI allows accurate measurement of the depth of extramural tumour spread. In the assessment of metastatic adenopathy, however, MRI has a low specificity. This study shows that MRI is unlikely to miss any significant parameter in staging of rectal carcinoma. However, it has a tendency to overstage extramural spread of tumour.     

骨挫伤的MRI诊断价值

目的探讨骨挫伤的MRI影像表现及MRI在骨挫伤影像诊断中的价值.方法回顾性分析34例骨挫伤和3例隐匿性骨折病例的X线平片、CT及MRI影像资料,分析骨挫伤的MRI影像表现,评估X线平片、CT及MRI在骨挫伤影像诊断中的作用.结果全部病例的X线平片均未见异常.CT能有效显示3例隐匿性骨折的部位和形态,但34例骨挫伤病例CT均未能予以诊断.3例CT诊断为隐匿性骨折的病例MRI诊断为骨挫伤而未能显示骨折的存在.MRI能有效显示不同部位骨挫伤的病变部位、范围及形态,在SE序列上骨挫伤的典型MRI表现为不规则片状T1WI低信号、T2WI高信号的异常信号改变,骨皮质及骨轮廓不发生变化.结论在X线平片、CT及MRI三种影像检查方法中,MRI是唯一能有效诊断骨挫伤的检查方法,但MRI不能明确区别骨挫伤和未分离、移位的隐匿性骨折.     

Magnetic Resonance Imaging of Placenta Accreta Spectrum: A Step-by-Step Approach

Placenta accreta spectrum (PAS) is an abnormal placental adherence or invasion of the myometrium or extrauterine structures. As PAS is primarily staged and managed surgically, imaging can only guide and facilitate diagnosis. But, imaging can aid in preparations for surgical complexity in some cases of PAS. Ultrasound remains the imaging modality of choice; however, magnetic resonance imaging (MRI) is required for evaluation of areas difficult to visualize on ultrasound, and the assessment of the extent of placenta accreta. Numerous MRI features of PAS have been described, including dark intraplacental bands, placental bulge, and placental heterogeneity. Failure to diagnose PAS carries a risk of massive hemorrhage and surgical complications. This article describes a comprehensive, step-by-step approach to diagnostic imaging and its potential pitfalls.     

胎儿羊膜带综合征所致四肢畸形的MRI诊断

目的:探讨快速MRI对胎儿羊膜带综合征所致四肢先天畸形的诊断价值。方法:搜集本院行产前超声检查(US)和MRI检查的68例孕妇中发现并经引产后胎儿尸检证实的胎儿四肢畸形病例资料12例。这12例孕妇年龄22~34岁,平均27.3岁;其12例胎儿,胎龄20~38周,平均26.8周。US检查后3天内行胎儿肢体轴面、冠状面、矢状面MRI扫描。MRI扫描采用单次激发快速自旋回波(SSFSE)序列。观察胎儿四肢骨骼及软组织的形态、结构,并与US及尸检结果进行比较。结果:快速MRI视野大,运动伪影少,组织分辨力高,能清楚显示四肢骨骼、软组织等解剖结构。引产后尸检发现12例四肢畸形(左前臂缺失3例,右前臂残端畸形4例,左侧手指缺失1例,下肢狭窄环2例,马蹄内翻足畸形2例);US诊断10例,误诊、漏诊各1例;MRI诊断11例,漏诊1例。比较两者正确诊断数及误漏诊数,采用四格表的确切概率法检验,P>0.05,差异无统计学意义。结论:MRI对胎儿四肢先天畸形具有较高的诊断价值,是胎儿肢体先天畸形影像学诊断的重要手段之一。     

脊髓动静脉畸形的MRI表现

本文报道5例脊髓动静脉畸形的MRI表现，畸形血管位于颈段1例，胸段1例，胸腰段3例，4例畸形血管位于脊髓背侧，1例位于脊髓腹侧，背侧及髓内，脊髓动静脉畸形的MRI特征性表现为：髓内或脊髓表面点，条状无信号影，与传统影像学方法相比，MRI在诊断脊髓动静脉畸形上具有无创伤，显示病变更全面，直观，便于随访观察的优点。     

儿童脑白质营养不良的磁共振 成像（MRI）表现

目的探讨儿童脑白质营养不良较特征的MRI表现,以期为临床诊断提供一定的帮助。方法对50例脑白质营养不良患儿进行MRI检查,将获得的图像按病变累及脑叶的情况和累及脑内特殊结构的情况进行分析。结果1.ALD以顶枕叶白质受累为主,MLD除累及顶枕叶外,额叶受累的机会也较多,而CD和PMD则累及全脑叶,AD以先累及额叶为主。ALD以中央白质受累为主,MLD仅累及中央白质而周边白质未受累,CD、PMD和AD中央及周边白质均受累。ALD和CD较易累及小脑白质。2.ALD主要累及胼胝体压部,MLD则常累及胼胝体全部,而CD、PMD、GLD和AD一般均不累及胼胝体;ALD以累及内囊后肢后部为主,MLD则内囊后肢后部和前部均累及,PMD较CD有更多的机会累及内囊全部;对于外囊,ALD以累及其后部为主,而CD和PMD均累及外囊全部;累及脑干皮质脊髓束是ALD的一个显著特征,除1例晚发型GLD外,其余脑白质病变均未累及脑干皮质脊髓束。另外,ALD也较易累及中脑外侧丘系,ALD、MLD和PMD累及丘脑的机会也较多。结论MRI为一种无创、安全和敏感的发现早期脑白质病变的影像学方法,不同脑白质营养不良的MRI表现有一定的特征性,对临床诊断可提供一定的帮助。     

Fast three-dimensional k-space trajectory design using missile guidance ideas.

Three-dimensional (3D) k-space trajectories are needed to acquire volumetric images in MRI. While scan time is determined by the trajectory efficiency, image quality and distortions depend on the shape of the trajectories. There are several 3D trajectory strategies for sampling the k-space using rectilinear or curve schemes. Since there is no evidence about their optimality in terms of image quality and acquisition time, a new design method based on missile guidance ideas is explored. Since air-to-air missile guidance shares similar goals and constraints with the problem of k-space trajectory design, a control approach for missiles is used to design a 3D trajectory. The k-space is divided into small cubes, and each one is treated as a target to be sampled. The main goal is to cover the entire space as quickly and efficiently as possible, with good performance under different conditions. This novel design method is compared to other trajectories using simulated and real data. As an example, a trajectory that requires 0.11 times the number of shots needed by the cylindrical 3DFT acquisition was designed. This trajectory requires more shots (1.66 times) than the stack of spirals, but behaves better under nonideal conditions, such as off-resonance and motion.