Quantitative NumART2* mapping in functional MRI studies at 1.5 T

Quantitative mapping of the effective transverse relaxation time, T₂* and proton density was performed in a motor activation functional MRI (fMRI) study using multi-echo, echo planar imaging (EPI) and NumART₂* (Numerical Algorithm for Real time T₂*). Comparisons between NumART₂* and conventional single echo EPI with an echo time of 64 ms were performed for five healthy participants examined twice. Simulations were also performed to address specific issues associated with the two techniques, such as echo time-dependent signal variation. While the single echo contrast varied with the baseline T₂* value, relative changes in T₂* remained unaffected. Statistical analysis of the T₂* maps yielded fMRI activation patterns with an improved statistical detection relative to conventional EPI but with less activated voxels, suggesting that NumART₂* has superior spatial specificity. Two effects, inflow and dephasing, that may explain this finding were investigated. Particularly, a statistically significant increase in proton density was found in a brain area that was detected as activated by conventional EPI but not by NumART₂* while no such changes were observed in brain areas that showed stimulus correlated signal changes on T₂* maps.     

用于SAR估计的基于U-Net网络的快速膝关节模型重建

膝关节高场磁共振成像（MRI）时，射频功率沉积（SAR）是一个关键的安全指标.目前对于局部SAR的准确估计只能通过电磁仿真实现，这就要求得到每一个个体的膝关节模型.本文提出一种针对低场磁共振图像的基于卷积神经网络的分割方法，以实现膝关节磁共振图像的快速重建.数据集来自于矢位T₁加权自旋回波图像，将膝关节组织按照"肌肉-脂肪-骨骼"模型进行简化，除脂肪与骨骼之外的其他组织归类为肌肉.采用一种全卷积的神经网络，即U-Net进行逐层的图像分割，卷积层数为4，训练采用交叉熵函数.本文对图像的自动分割结果与手动标注结果进行了定量的比较.此外，采用3 T正交鸟笼线圈进行了SAR仿真，结果验证了组织简化对于SAR估计的可行性，并且所提方法构建的模型可以得到较为精准的局部SAR分布.     

High-resolution segmented EPI in a motor task fMRI study

A high-resolution gradient echo, multi-slice segmented echo planar imaging method was used for functional MRI (fMRI) using a motor task at 1.5 Tesla. Functional images with an in-plane resolution of 1 mm and slice thickness of 4 mm were obtained with good white-gray matter contrast. The multi-shot approach, combined with a short total readout period of 82 ms, limits blurring effects for short T(2)(*) tissues (such as gray matter), assuring truly high-resolution images. In all subjects, motor functions were clearly depicted in the contralateral central sulcus over several slices and sometimes activation was detected in the supplementary motor area and/or ipsilateral central sulcus. The average signal change of 11+/-3% was much higher than in standard low-resolution fMRI EPI experiments, as a result of larger relative blood fractions.     

Comparison of gadolinium chelates with manganese-DPDP for liver lesion detection and characterization: Preliminary results

Nonspecific extracellular gadolinium chelate (NEGd) was prospectively compared with manganese (Mn)-DPDP (Mn) for the detection and characterization of focal liver lesions of various histology. Seventeen patients with known or suspected focal liver lesions underwent NEGd and Mn-enhanced studies at 1.5 T. Study findings were correlated with histology (five patients), computed tomography (CT) examinations (17 patients), and 4- to 13-month imaging follow-up by CT and/or MR (five patients). NEGd studies were performed as serial postcontrast spoiled gradient echo (SGE) sequences, and Mn studies were performed as SGE sequences 15 and 30 min postcontrast and T₁-weighted, fat-suppressed spin echo at 16 min. NEGd and Mn images were prospectively interpreted in a separate blinded fashion. Lesion detection and characterization were determined. NEGd and Mn-enhanced images demonstrated 61 and 49 lesions, respectively (p = .1, NS). A total of 60 and 33 lesions were characterized on NEGd and Mn images, respectively, which was significantly different (p = .008). No differences were observed for the detection and characterization of liver metastases; whereas there was a trend for superior detection and characterization for hepatocellular carcinoma with NEGA.     

In vivo relaxation of N-acetyl-aspartate, creatine plus phosphocreatine, and choline containing compounds during the course of brain infarction: a proton MRS study.

Localized water suppressed proton spectroscopy has opened up a new field of pathophysiological studies of severe brain ischemia. The signals obtained with the pulse sequences used so far are both T₁ and T₂ weighted. In order to evaluate the extent to which changes in metabolite signals during the course of infarction can be explained by changes in T₁ and T₂ relaxation times, eight patients with acute stroke were studied. STEAM sequences with varying echo delay times and repetition times were used to measure T₁ and T₂ of N-acetyl-aspartate (NAA), creatine plus phosphocreatine (Cr+PCr) and choline containing compounds (CHO) in a 27-ml voxel located in the affected area of the brain. Ten healthy volunteers served as controls. We found no difference in T₁ or T₂ of the metabolites between the patients and the normal controls. The T₂ of CHO was longer than that of NAA and Cr+PCr. Our results indicate that spectra obtained in brain infarcts and normal tissue with the same acquisition parameters are directly comparable with respect to relative signal intensities as well as signals scaled with internal and external standards.     

Functional MRI of the motor cortex using a conventional gradient system: comparison of FLASH and EPI techniques

Gradient echo (GE) and echo planar imaging (EPI) techniques are two different approaches to functional MRI (fMRI). In contrast to GE sequences, the ultra short EPI technique facilitates fMRI experiments with high spatial and temporal resolution or mapping of the whole brain. Although it has become the method of choice for fMRI, EPI is generally restricted to modern scanners with a strong gradient system. The aim of our study was to evaluate the applicability of EPI for fMRI of the motor cortex using a 1.5 T scanner with a conventional gradient system of 10 mT/m (rise time: 1 ms). Therefore, EPI was compared with a well-established high resolution fast low angle shot (FLASH) technique (matrix size 128²). The FLASH technique was applied additionally with a 64² matrix size to exclude influences caused by different spatial resolution, because the EPI sequence was restricted to a 64² matrix size. A total of 35 healthy volunteers were included in this study. The task consisted of clenching and spreading of the right hand. FLASH and EPI techniques were compared regarding geometric distortions as well as qualitative and quantitative fMRI criteria: Mean signal increase between activation and rest and the area of activation were measured within the contralateral, ipsilateral, and supplementary motor cortex. The quality of subtraction images between activation and rest, as well as the quality of z-maps and time course within activated regions of interest, was evaluated visually. EPI revealed significant distortions of the anterior and postior brain margins; lateral distortions (relevant for the motor cortex) could be neglected in most cases. The mean signal increase was significantly higher using FLASH 128² compared to FLASH 64² and EPI 64², whereas the activated areas proved to be smaller in FLASH 128² functional images. Both results can be explained by well-documented partial volume effects, caused by different voxel size. Similar quality of the subtraction images and of the time courses in different regions of interest were found for all techniques under investigation, but slightly reduced quality of z-map in FLASH 128². Within the limits of reproducibility and measurement accuracy, the location of contralateral activation was similar using FLASH and EPI sequences. In conclusion, EPI proved to be a reliable technique for fMRI of the motor cortex, even on an MR scanner with a conventional gradient system.     

基于AI+MRI的影像诊断的样本增广与批量标注方法

训练样本是所有领域人工智能（AI）研发的关键因素.目前，基于人工智能+磁共振成像（AI+MRI）的影像诊断存在着训练样本的有效标注数量和类型无法满足研发需求的瓶颈问题.本文利用临床MRI设备对志愿者或阳性病例进行正常或重点病灶区的定量扫描，获取高分辨率各向同性的纵向弛豫时间（T₁）、横向弛豫时间（T₂）、质子密度（Pd）和表观扩散系数（ADC）等物理信息的多维数据矩阵，作为原始数据.开发虚拟MRI技术平台，对原始数据（相当于数字人体样本）进行虚拟扫描，实现不同序列不同参数下的多种类磁共振图像输出.选择感兴趣组织具有最好边界区分度的图像种类，经有经验的影像医生对其进行手动勾画并轨迹跟踪形成三维MASK标注矩阵，作为其他种类图像的图像勾画标注模板，从而实现低成本、高效率的MRI样本增广和批量标注.该平台以临床少量阳性病例作为输入，进行样本增广和标注，极大地减少AI对实际扫描样本的要求，降低了影像医生的精力和时间投入，极大地节省了成本，并输出了数量足够的磁共振图像，为基于AI+MRI的影像诊断研发提供低成本的训练数据解决方案.     

MRI angiography of the carotid artery

Spin echo images of the carotid arteries in longitudinal view have been obtained by selection of oblique imaging planes. Blood flow within the lumen in the region of the carotid bifurcation has been visualized through the use of cardiac gating during end diastole. Using a surface coil placed about the mandible, high resolution images [(0.75 mm)² per pixel) were obtained with scan times typically equal to 9 min and image data matrix equal to 256 × 256. Images obtained with this technique of MRI carotid angiography demonstrate blood flow phenomenon as well as vascular anatomy.     

Field-map correction in read-out segmented echo planar imaging for reduced spatial distortion in prostate DWI for MRI-guided radiotherapy applications

Diffusion-weighted echo planar imaging (DW-EPI) suffers from geometric distortion due to low phase-encoding bandwidth. Read-out segmented echo planar imaging (RS-EPI) reduces distortion but residual distortion remains in extreme cases. Additional corrections need to be applied, especially for radiotherapy applications where a high degree of accuracy is needed. In this study the use of magnetic field map corrections are assessed in DW-EPI and RS-EPI, to reduce geometric uncertainty for MRI-guided radiotherapy applications. Magnetic field maps were calculated from gradient echo images and distortion corrections were applied to RS-EPI images. Distortions were assessed in a prostate phantom by comparing to the known geometry, and in vivo using a modified Hausdorff distance metric using a T2-weighted spin echo as ground truth. Across 10 patients, field map-corrected RS-EPI reduced maximum distortion by 5 mm on average compared to DW-EPI (σ = 1.9 mm). Geometric distortions were also reduced significantly using field mapping with RS-EPI, compared to RS-EPI alone (p ≤ 0.05). The increased geometric accuracy of these techniques can potentially allow diffusion-weighted images to be fused with other MR or CT images for radiotherapy treatment purposes.     

EEG quality during simultaneous functional MRI of interictal epileptiform discharges

This article concerns the evaluation of the quality of interictal epileptiform EEG discharges recorded throughout simultaneous echo planar imaging (EPI). BOLD (blood oxygen level dependent) functional MRI (fMRI) images were acquired continuously on a patient with intractable epilepsy. EEG was sampled simultaneously, during and after imaging, with removal of pulse and imaging artifacts by subtraction of channel-specific running averages. Contiguous EEG epochs recorded with and without fMRI (fMRI+ve vs. fMRI−ve) were next randomized and presented to two blinded observers. Epileptiform discharges were identified retrospectively, and comparison was made in terms of the number of identified events, their amplitude, and spatiotemporal distribution. A spectral analysis was also performed on the EEG. In the randomized comparison of EEG segments, 80 (fMRI+ve) vs. 69 (fMRI−ve) discharges were noted with good interobserver agreement (69%). There were no significant differences in amplitude or spatio-temporal distribution. Comparison of the events detected and measured by two expert observers demonstrated that the Interictal Epileptiform Discharge (IED) characteristics were indistinguishable with and without scanning. We review briefly the existing literature on EEG recording quality for combined EEG/fMRI.     

1.5 T磁共振化学交换饱和转移成像的影响因素分析

本文探讨1.5 T磁共振化学交换饱和转移（Chemical Exchange Saturation Transfer,CEST）成像的影响因素.通过试管模型和临床病例,采用GE Signa HDe 1.5 T磁共振成像（Magnetic Resonance Imaging,MRI）扫描仪分别进行不同矩阵、激励次数、翻转角、磁化传递翻转角的CEST成像对比分析,以及不同激励次数、磁化传递翻转角的Z谱分析,并从成像组织、成像设备、成像技术等方面对原始图信号、酰胺质子转移（Amide Proton Transfer,APT）信号及Z谱进行分析研究.实验结果表明1.5 T MRI扫描仪的CEST图像信噪比相对较低,且磁场稳定性及均匀度影响了CEST成像的效果.在其他参数不变的情况下,降低采集矩阵和增加激励次数与翻转角可以增加原始图像信噪比.磁化传递翻转角为105°时,CEST成像效果最好.激励次数为2、磁化传递翻转角为105°时,所得数据符合组织Z谱情况.模型Z谱在磁化传递频率为-294~-194 Hz范围可显示30%谷氨酸（Glu）、碘剂（I₃₂₀）、纯水（H₂O）、肌酸（Cr）的信号差异,与H₂O差异最大处在-244~-214 Hz.原始图像信号30% I₃₂₀明显高于Glu、H₂O、Cr,Cr略低于Glu,APT图Cr略低于Glu.25例脑肿瘤的APT图呈高信号、12例脑梗塞的APT图呈低信号,CEST原始图像均可区分病变区域.有12例因采集时间、患者配合情况、环境及室温等影响导致CEST成像的失败.由此得出1.5 T场强下,CEST技术受到成像组织、设备、技术等因素的影响,需要进行多方面优化.在保证磁场稳定性及均匀度的情况下,优化参数的CEST成像和Z谱成像可以区分代谢物及其浓度.     

颅脑梯度回波成像:呼吸伪影和导航回波矫正

梯度回波序列是磁共振成像中常用的脉冲序列,然而梯度回波对主磁场波动非常敏感,呼吸等生理运动引起的信号波动会导致图像伪影.该文报道了采用导航回波技术获取呼吸运动导致的局部磁场波动,用以矫正图像回波中随时间变化的相位波动,并将该技术应用于三维多回波梯度回波成像和T2*定量图研究.研究结果显示:矫正前,相位波动幅度随回波时间增长而增大,模图和T2*定量图在相位编码方向有明显伪影,并且男女呼吸伪影水平有显著性差异;矫正后,相位波动幅度大幅下降,图像伪影水平有显著性下降.     

Geometric distortion correction for echo planar images using nonrigid registration with spatially varying scale

One method used to correct geometric and intensity distortions in echo planar images is to register them to undistorted images via nonrigid deformations. However, some areas in the echo planar images are more distorted than others, thus suggesting the use of deformations whose characteristics are adapted spatially. In this article, we incorporate into our previously developed registration algorithm a spatially varying scale mechanism, which adapts the local scale properties of the transformation by means of a scale map. To compute the scale map, a technique is proposed that relies on an estimate of the expected deformation field. This estimate is generated using knowledge of the physical processes that induce distortions in echo planar images. We evaluate the method of spatially varying scale on both simulated and real data. We find that, in comparison with our earlier method using fixed scale, our new method finds deformation fields that are smoother and finds them faster without sacrificing accuracy.     

Effects of echo time variation on perfusion assessment using dynamic susceptibility contrast MR imaging at 3 tesla

The implications of changing the echo time of a gradient-echo echo planar imaging sequence applied to dynamic susceptibility contrast magnetic resonance imaging (DSC-MRI) for perfusion imaging at 3T were investigated. Four echo times in the range of 21 to 45 ms were examined in a total of 17 patients who received a dose of 0.1 mmol/kg bodyweight Gadobutrol (Gadovist, 1.0 mmol/ml). As the primary optimization parameter, the concentration-to-noise ratio (SNRc) was selected as it takes effects of variations in baseline as well as in signal drop into account. In an analysis of gray matter, white matter and arterial regions of interest, SNRc showed the highest values for the shortest applied echo time in all cases. Maps of regional cerebral blood volume (rCBV) and blood flow (rCBF) were calculated using deconvolution based on singular value decomposition. The quality of rCBF and rCBV images was judged to be good or excellent in all cases, independent of the echo time. Calculated gray matter/white matter ratios of rCBF and rCBV displayed no significant dependence on the applied echo time. Considering the better SNRc and arterial signal saturation aspects, we found that the shortest investigated echo time was the superior one. We thus suggest that short echo times should be applied, taking technical limitations and clinical demands into consideration.     

Tissue characterization by image processing subtraction: windowing of specific T1 values.

A method for windowing specific T₁ values is presented. A 1.0 T imager with two routine pulse sequences was employed: A T₁-weighted spin echo (SE) sequence and a short tau inversion recovery STIR sequence (fat-suppressed IR). A T₁ window for fat was obtained by subtracting the STIR image from the SE image. Negative values were coded black. The method was tested on a normal human thigh, on a human liver with confirmed fatty infiltration, and on the livers of four live burbots. The fat-containing tissues of the two human volunteers were well depicted. The differences in fat concentration among the burbot livers were also clearly shown. The fat intensity seen in the images correlated well with the chemically measured fat concentration. This subtraction method for windowing T₁ values proved feasible for fat. The method could be used for tissues with other short T₁ values as well.     

1D spectroscopic imaging with rf echo planar (SIRFEN) methods

A recently developed rf echo planar imaging method has been modified to rapidly generate spectroscopic information along one in-plane axis and spatial information along the other. The method allows the production of one-dimensional chemical shift images (1D CSIs) in acquisition times of 18 sec or less. A specific phase-encode-reordering algorithm provides convenient manipulation of T₂ weighting, yielding partial suppression of short T₂ species like muscle water. The method is demonstrated in phantoms and in vivo with 1D CSIs of human brain and limbs. Abnormal fat distribution is demonstrated in the calf of a patient with aggressive fibromatosis. The advantages of short acquisition times obtainable with SIRFEN are offset by limited spectral resolution, suggesting that primary applications will be confined to rapid spatial mapping of major spectral components.     

基于AFI的快速发射场B1+成像方法

该文提出了一种基于实际翻转角成像(Actual Flip-angle Imaging,AFI)的快速发射场测量方法(Fast AFI,FAFI),将多次激发平面回波成像(Multi-shot Echo Planar Imaging,Multi-shot EPI)的采集方式运用于AFI发射场(B_1~+)测量中,充分利用AFI序列中采集的等待时间,高倍数加速了水模和人体头部、腹部及盆腔的发射场测量.该文在水模和人体(n=16)实验中,验证了采用FAFI序列得到的B_1~+测量结果与AFI结果的一致性.FAFI序列大幅加速了发射场测量,为实现动态B_1~+匀场(B_1~+shimming)和快速局部激发提供了高效的发射场测量方法.     

Macroscopic tumor volume of malignant glioma determined by contrast-enhanced magnetic resonance imaging with and without magnetization transfer contrast

The purposes of this study were to compare the conspicuity and lesion volume of contrast-enhancing macroscopic malignant glioma determined by postcontrast magnetic resonance (MR) imaging with and without magnetization transfer (MT) saturation, and to discuss possible implications for radiotherapy planning. Nineteen patients (age 24–60 years) with histologically proven malignant glioma were prospectively examined by MR imaging. After the administration of gadolinium dimeglumine (0.1 mmol/kg body weight), the lesions were imaged with an MT-weighted FLASH (fast, low-angle shot) pulse sequence and with a conventional T₁-weighted spin-echo (SE) sequence without MT saturation. The mean tumor volumes of gliomas measured on MT-weighted FLASH images were significantly (p < .01) larger than those obtained from T₁-weighted SE images (45 ± 15 cm³ vs. 33 ± 10 cm³). The mean contrast-to-noise ratio of enhancing lesions on MT-weighted FLASH was 48 ± 14 compared with 30 ± 14 on SE images, representing a significant (p < .01) improvement. We conclude that the volume of contrast enhancement of malignant glioma identified on MT-weighted FLASH images represents the area of disrupted blood-brain barrier. If this volume of subtle contrast enhancement is caused by tumor infiltration and represents the boost target volume for stereotactic radiosurgery or brachytherapy, MT-weighted FLASH images would be better than T₁-weighted SE images to define these volumes. These improved delineation of areas at highest risk for recurrence following radiation therapy should enhance the efficacy of treatment planning for high-boost therapy.     

NMR实验参数对IBP-血浆相互作用个性化差异研究的影响

药物与血浆蛋白相互作用强弱是影响药物分布代谢与药效的关键因素之一. 本研究小组已报道用扩散加权谱、弛豫加权谱结合主成分分析（PCA）方法研究布洛芬（IBP）与血浆蛋白相互作用的个体差异性. 该文则研究核磁共振实验参数的设置对血浆与药物相互作用个体差异性研究的影响. 以对照血浆样品组与加入布洛芬血浆组为模型,改变扩散时间、梯度强度、回波时间这3种实验参数,采集了27套不同实验设置的扩散加权谱与10套不同回波时间的弛豫加权谱. 结果表明,扩散时间为0.1 s~0.14 s且梯度强度为1.52×10^-3 T/cm~1.90×10^-3 T/cm时采集的扩散加权谱或回波时间为70 ms~110 ms时采集的弛豫加权谱更适合用来研究血浆与布洛芬相互作用的个性化差异.     

利用核磁共振资料定量评价页岩孔隙结构

页岩气储层的孔隙结构复杂且非均质性较强,导致储层表征及有效性评价面临极大挑战.为了建立页岩气储层孔隙结构的定量评价方法,本文选取了鄂西宜昌地区陡山沱组二段20块岩心,采用0.069 ms的回波间隔开展饱含盐水状态下的核磁共振（NMR）实验.在此基础上,对T₂谱进行了多重分形特征分析,提取了对页岩气储层孔隙结构较敏感的参数,并建立了基于最小与最大广义分维数差值（D_min-D_max）和谱宽（Δα）划分页岩气储层类型的方法及标准.该方法对于有效提高页岩气储层的预测精度、指导开发选层等具有重要意义.