压缩感知磁共振成像脉冲序列的设计与实现

为了进一步减少数据扫描时间,提高磁共振成像速度,我们对压缩感知MRI脉冲序列进行了研究。采用相位编码方向变密度欠采集的方式,设计了基于自旋回波的压缩感知序列;并采用PPL语言编程,在永磁和超导MR扫描仪上实现了该序列,对压缩感知欠采集与自旋回波全采集的数据进行了图象重建和分析。结果表明:该序列达到了压缩感知的要求,采集的头部和膝盖数据的最佳欠采样加速因子分别为2和4,大大节省了数据采集时间。该序列的实现使得低场强MR扫描仪在不提高梯度性能的情况下,便可实现快速成像,为提高我国在MR扫描仪上的研发水平奠定了基础。     

永磁型磁共振胰胆管水成像技术及其临床应用

目的 用MR水成像(water imaging)技术探讨磁共振胰胆管成像(MRCP)在胰胆管病变中的诊断价值.方法 检查病人54例,其中正常志愿者12例,肝内外胆管结石27例,恶性胆管梗阻7例,胆总管囊肿2例,胆囊术后胆管扩张3例,胆管炎性狭窄2例,慢性胰腺炎1例.MRCP选择性应用单次激发超快速自旋回波序列(SSFSE-XL)(二维).结果 采用单次激发超快速自旋回波序列 (SSFSE-XL)序列所得MRCP图像清晰显示正常胆管树,胆道梗阻患者均能显示梗阻部位及程度.结论 利用开放式低场强磁共振进行MRCP检查具有无创、安全、快速、简单易行等特点,能够明确有无梗阻及梗阻程度,明确梗阻部位及判断梗阻原因.SSFSE-MRCP检查成功率高,操作简单快速,对胰胆管病变的定位诊断准确,结合常规MRI平扫、MRI增强扫描使用,对肝外胆管结石和恶性胆管梗阻及其他胰胆管病变的定性诊断有较高准确性及诊断价值.     

胎儿神经系统MRI成像技术应用讨论

胎儿神经系统磁共振成像技术从1983年Smith等[1]第一次运用常规MRI序列扫描，发展到用快速MRI序列，使胎儿MRI技术逐渐为诊断胎儿神经系统疾病的重要手段。现就其技术发展及在神经系统疾病的临床应用综述如下。1胎儿神经系统MRI成像技术发展和神经系统疾病诊断范围运用
　　快速MRI常用1．5T超导磁共振。成像序列很多，包括平面回波、梯度回波、快速自旋回波技术，如单次激发快速自旋回波序列(single-shot fast spin-echo，SS-FSE)，又称单次激发弛豫增强快速采集(single-shot rapid acquisition with relaxation en-hancement，single-shot RARE)、半傅立叶采集单次激发快速自旋回波(half-Flourier acquisition single-shot turbo spin-echo，HASTE)，其次使用的是真实稳态进动快速成像(true fast imaging with steady-state precession， true FISP)或者快速稳态进动采集序列(fast-imaging employing steady-state acquisition，FI-ESTA)。最常用SSFSE或HASTE序列，它们是快速T2W扫描序列，每层扫描时间达到数秒，一个扫描序列只需几秒、十几秒即可完成，大大缩短成像时间，明显减少胎动伪影[2,3]。 T1W应用较少，一般用于怀疑有出血的患者[4]。患者仰卧位或者左侧卧位，Yuh等[5]认为左侧卧位可减少孕妇腹主动脉搏动伪影，也可减少孕妇长时间仰卧后产生不适。足先进防止幽闭恐怖症。盆腔线圈或体线圈。     

基于不同pH值的磁共振T2加权成像

目的:应用磁共振成像技术来探讨无损伤地使样品不同pH值成像的可能性.方法:通过制作不同pH值的样品模型,在高场下(9.4 T)应用磁共振频谱来测量样品的pH值,再用临床磁共振仪(1.5 T)对不同pH值的样品进行成像,成像采用的序列是自旋回波和快速自旋回波序列,选取这两种序列下获得的pH值成像的感兴趣区进行信号强度的测量,并探讨pH值与横向弛豫时间T2的关系.结果:获得了样品模型在高场下的磁共振频谱图,证实了pH值依赖于咪唑环上氢的化学位移,据此可以测量pH值.磁共振pH值成像的结果表明,不同pH值的样品模型所成的T2加权像的信号强度是有差别的.结论:pH值低的样品,T2加权像信号强度较高,这使磁共振pH值成像成为可能.     

一种消除MRI拉链伪影的脉冲序列

背景:磁共振成像会产生各种类型的伪影,经常出现的有运动伪影、卷褶伪影、金属伪影、部分容积效应伪影、化学位移伪影、相位伪影和拉链伪影等,但由自旋回波序列成像过程中形成拉链伪影的控制和校正方法还未见报道。目的:通过分析自旋回波序列成像过程中拉链伪影的产生机制,探索出一种新的消除拉链伪影的脉冲序列。方法:编写标准SE序列得到标准图,编写伪影序列得到直线干扰图像和拉链伪影图,编写改进序列得到改进后的除拉链伪影图。结果与结论:5°-瞬间梯度-120°回波序列可得到直线干扰图像,90°-瞬间梯度-120°回波序列可得到拉链伪影图,而改进后序列90°-120°-瞬间梯度回波可彻底消除拉链伪影。结果提示,改进后的脉冲序列显示的图形效果彻底消除了拉链伪影。     

磁共振场强及序列选择对金属植入物伪影大小影响探讨

目的比较不同场强不同序列伪影的差异。方法制作钛合金水模,分别在1.5 T和3.0 T MRI扫描仪上采用矢状位快速自旋回波(TSE)-T1、TSE-T2、TSE的短反转时间的反转恢复(STIR)的脂肪抑制、TSE的频率选择饱和法(FS)抑制、梯度回波序列(GRE)/快速小角度激发(FLASH)、扩散加权成像(DWI)等常用序列扫描。分析扫描得到图像伪影特点及进行伪影大小测量。结果GRE/FLASH序列图像和DWI图像伪影最大,FS抑制序列得到图像伪影比STIR序列图像伪影大,各序列伪影形状也有差别。在不同场强下相同类型序列扫描伪影也不同,高场强伪影明显大于低场强伪影。同一场强不同序列伪影大小不同,同一类型序列3.0 T MRI图像伪影比1.5 T MRI图像伪影要大,差异有统计学意义(P0.05)。结论可以通过场强选择、序列的选择来减少金属植入物磁共振扫描带来的伪影。     

基于深度学习技术的磁共振成像研究进展

磁共振（MR）成像是当前应用于临床医学诊断的重要医学成像手段之一。如何缩短扫描时间进行快速成像一直以来都是MR成像领域中的热门研究问题。近年来，随着深度学习的兴起和快速发展，深度学习被广泛应用于医学图像处理领域中。目前基于深度学习的MR成像方法作为MR成像的新兴方向，相应的研究已取得了一系列进展。本文对几种常见的基于深度学习的MR成像方法进行归纳和简要分析，并对其研究前景进行了展望。     

软、硬质钴铬合金及金合金对磁共振成像伪影的影响

背景:磁共振成像具有高对比度、无骨伪影、任意方位断层等优点,被广泛应用于头颈外科、神经外科和口腔颌面外科疾病的诊断。但某些金属材料在磁共振成像中形成的伪影严重影响了图像质量,这些材料的使用限制了磁共振成像在头颈部的应用和准确诊断。目的:观察硬质钴铬铸造合金、软质钴铬铸造合金、金合金3种金属材料在5种不同头颈部扫描序列中的伪影。方法:采用5种头颈常规及快速成像序列,分别对放置于圆柱型水模中心的3种金属试样进行轴位扫描,测量伪影,并进行统计分析。结果与结论:3种金属在磁共振成像相同成像序列中,金合金产生的伪影最小,硬质钴铬形成的伪影最大,各组间比较差异有显著性意义(P均0.05);相同金属材料,自旋回波序列产生伪影最小,梯度回波序列产生伪影较大,平面回波序列产生伪影最大并使图像变形。结果提示,与非贵金属口腔修复材料相比,以贵金属作为口腔修复材料,在头颈部磁共振成像过程中产生的伪影最小,因此通过合理选择口腔金属修复材料及成像技术,可得到最佳磁共振成像。     

2D-SSh-MRCP序列与基于压缩感知技术的CS-3D-MRCP序列在胰胆管成像中的对比研究

目的比较屏气条件下采集的单次激发快速自旋回波2D-SSh-MRCP序列与基于压缩感知技术的呼吸触发3D快速自旋回波容积扫描CS-3D-MRCP序列在胰胆管成像中的应用价值,以期指导临床实际扫描。方法分析首都医科大学附属北京天坛医院50例同时行2D-SSh-MRCP序列和CS-3DMRCP序列扫描的患者的临床和影像资料,由2名经验丰富的诊断医师对肝内外胆道成像、胰管成像、胆囊形态以及结石的诊出率进行分析和评分,采用SPSS软件检验诊断医师之间的一致性,采用t检验分析上述指标的组间差异。结果两名诊断医师的评分一致性较强(Kappa08);2D-SSh-MRCP序列与CS-3D-MRCP序列在肝内外胆管系统的显示评分方面无统计学差异(P005);CS-3D-MRCP序列在胆囊形态和胰管显示评分方面显著高于2D-SSh-MRCP序列(P005);2D-SSh-MRCP序列对胆囊内结石和胆管内结石检出率(909%,950%)高于CS-3D-MRCP序列(773%,850%)。结论 2D-SSh-MRCP序列对胆囊内结石和胆管内结石检出率较高;CS-3D-MRCP序列在胆囊结构及胰管显示方面较好;建议在CS-3D-MRCP序列之后加扫2D-SSh-MRCP序列,二者配合更利于胰胆管结构的检查。     

MRI检查在适合保留生育能力手术的早期宫颈癌患者的选择中的作用:初步试验

据统计 ,近 30 %的宫颈癌患者年龄小于 35岁 ,生育能力的保留对这些患者意义重大。外科开展的宫颈切除术既能根治性切除肿瘤的目的又能保存完整的子宫体即保留生育能力。宫颈癌直接蔓延超过内口深入子宫肌层的患者不适于进行宫颈切除术。本文评价了术前 MRI检查判断患者能否行宫颈切除术的价值。1996 .0 1～ 1997.10月入院的 30例早期宫颈癌患者经病理组织活检确诊后进行 MRI检查。采用 1.5 T扫描仪 ,扫描参数如下 :横断面、矢状面快速自旋回波序列 T2 WI( 5 0 0 0～ 6 0 0 0 / 119,TR/ TE) ,脂肪饱和自旋回波序列 T2 WI( 5 0 0 /11,…     

Optimization of multiple spin-echo sequences for 3D polymer gel dosimetry

The overall performance of polymer gel dosimeters for three-dimensional radiation dosimetry is determined by the temporal and spatial stability of the gels, dose sensitivity and image quality with respect to both systematic and stochastic deviations. The dose resolution (D(p)delta) is determined by the dose sensitivity and the signal-to-noise ratio (SNR) in the dose images. The dose sensitivity can be altered by changing the chemical composition of the polymer gel. The SNR is determined by the scanner and the imaging sequence. In the dose verification of conformal radiotherapy treatments the chosen number of slices may reach a number of 10-20. For these experiments, to obtain a sufficient SNR within a reasonable measurement time using a certain MR scanner, the imaging sequence should be optimized. A few other studies have emphasized the importance of optimizing the imaging sequence with respect to dose resolution (D(p)delta) or SNR but do not give quantitative values for the optimal sequence parameters for scanning a polymer gel dosimeter in three dimensions. In this paper, it is proved that a multiple spin-echo sequence is preferable to a single spin-echo sequence. It is also shown that when using a multiple spin-echo sequence it is not the inter-echo time that should be optimized but the number of echoes. An algebraical expression is derived for the dose resolution in terms of sequence parameters. A mathematical formalism and look-up tables are provided that can be used to optimize both a single and a slice-selective multiple spin-echo sequence to acquire a set of dose images at various locations. The use of the optimization protocol is illustrated by some examples. The optimization protocol enables the user to derive the optimal sequence parameters to acquire a set of dose maps obtained by quantitative T2 imaging for each polymer gel dosimeter within the shortest time possible.     

快速液体衰减反转回复序列技术在脑部疾病中的应用

对76例脑部疾病患者进行快速液体衰减反转恢复序列(fluid attenuated inversion recovery，FLAIR)序列及常规快速自旋回波(fast spin echo，FSE)序列扫描，比较两种序列对病灶的显示情况。结果发现。在76例脑部疾病中，FLAIR共显示病灶172个，而B加权像(T2WI)只显示123个。FAST FLAIR显示病灶较T2WI清楚明确。在FAST FLAIR上病灶与正常脑组织的对比度更高。FLAIR技术对颅脑病变的显示优于FSE序列T2加权像，特别是对脑表面或脑室周围病灶的显示，FLAIR有很高的临床使用价值，可作为颅脑MR检查常规序列的重要补充。     

An improved nuclear magnetic resonance diffusion coefficient imaging method using an optimized pulse sequence

Two-dimensional diffusion coefficient maps (images) of a carefully controlled diffusion phantom have been measured by a new diffusion imaging sequence using a 0.6-T whole-body nuclear magnetic resonance (NMR) scanner having a gradient field strength of 2.5 mT/m. The free induction decay (FID) data for the diffusion coefficient images were collected by varying the duration of the readout gradient in the conventional two-dimensional Fourier imaging sequence. The experimental results obtained by the proposed NMR diffusion measurement technique indicate a close agreement with other previous measurements. The selection of optimum spin-echo time for maximum signal-to-noise ratio (SNR) in diffusion imaging is studied and also experimentally confirmed. Finally, a preclinical study with human volunteers has been performed and results are presented.     

Comparison of high-resolution echo-planar spectroscopic imaging with conventional MR imaging of prostate tumors in mice

High spectral and spatial resolution (HiSS) MRI of rodent tumors has previously been performed using conventional spectroscopic imaging to obtain images with improved contrast and anatomic detail. The work described here evaluates the use of much faster echo-planar spectroscopic imaging (EPSI) to acquire HiSS data from rodent tumor models of prostate cancer. A high-resolution EPSI pulse sequence was implemented on a 4.7 T Bruker scanner. Three-dimensional EPSI data were Fourier-transformed along the k-space and temporal (free-induction decay) axes to produce detailed water and fat spectra associated with each small image voxel. The data were used to generate images of spectral parameters, e.g. peak-height images for each small voxel. Two variants of EPSI were performed; gradient-echo or spin-echo excitation with EPSI readout. These imaging methods were tested in commonly used rodent prostate cancers, including seven mice implanted with non-metastatic AT2.1 (n=3) and metastatic AT3.1 (n=4) prostate tumors on the hind leg, and 10 mice implanted with LNCaP prostate cancers in situ. The peak-height images derived from EPSI datasets provide more detailed tumor anatomy, improved signal-to-noise and contrast-to-noise ratios compared with the gradient-echo or spin-echo images at all echo times. The results suggest that HiSS MRI data from small animal models of prostate cancer can be acquired using EPSI, and that this approach improves imaging of heterogeneous tissue and vascular environments inside the tumors compared with conventional MR techniques.     

Applications of sliding window reconstruction with cartesian sampling for dynamic contrast enhanced MRI

Applications of dynamic contrast enhanced MR imaging are increasing and require both high spatial resolution and high temporal resolution. Perfusion studies using susceptibility contrast in particular require very high temporal resolution. The sliding window reconstruction is a technique for increasing temporal resolution. It has previously been applied to radial and spiral sampling, but these schemes require extensive correction and interpolation during image reconstruction. Fourier raw data can be reconstructed simply and quickly using the fast fourier transform (FFT). This paper presents a new Fourier-based sampling scheme and sliding window reconstruction that facilitates fast scanning without needing correction or interpolation. This technique can be used on virtually any MR scanner since it requires no specialized hardware. It is implemented here as a dual gradient echo sequence providing simultaneous T(1)- and T(2)*-weighted images with a time resolution of 1.1 s.     

Artefacts in multi-echo T2 imaging for high-precision gel dosimetry: I. Analysis and compensation of eddy currents

In BANG gel dosimetry, the spin-spin relaxation rate, R2 = I/T2, is related to the radiation dose that has been delivered to the gel phantom. R2 is calculated by fitting the pixel intensities of a set of differently T2-weighted base images. In gel dosimetry for radiotherapy, an accuracy of 5% in dose and 3 mm spatially, whichever is lower, is the objective. Therefore, possible sources of artefacts must be considered and dealt with. To obtain a set of base images a multiple spin-echo sequence is used. However, in a conventional MR scanner eddy currents will be provoked by switching the imaging gradients. As the eddy currents change in the course of the sequence, the net magnetization will be affected accordingly. Hence, eddy currents may have a significant influence on the quantitative R2 images themselves as well as on their slice position. In this study, we report an analysis of the eddy currents as they appear in the multiple spin-echo sequence. Eddy currents are measured using a frequency shift method resulting in eddy current field maps. The related geometrical displacements are obtained by use of a pyramidal phantom. The R2 versus dose relation is determined in the three main directions of the magnet, revealing a dependence of the measured R2 on slice orientation. The time course of eddy currents is then used in a computer simulation to estimate the effects they produce in the recorded R2 images. A compensation method for eddy current effects in multi-echo T2 mapping is described.     

Basic principles of magnetic resonance imaging

With rapid progress in MR imaging techniques, it has now become one of the important clinical imaging techniques. In this paper, the basic principles of NMR and MR imaging techniques are briefly described. Furthermore, the technical background of principal MR imaging techniques such as the spin-echo, gradient-echo, fast spin-echo and ultra-fast MR imaging technique is reviewed. The principles of proton density-, T1- and T2-weighted contrast images are also described. Finally, the technical background of functional MR imaging is briefly reviewed and the typical fMR image obtained by activation of motor cortex with finger tapping is presented.     

Non-invasive temperature imaging of muscles with magnetic resonance imaging using spin-echo sequences

The application of spin-echo magnetic resonance imaging sequences on non-invasive temperature imaging for temperature mapping of human limbs is investigated. In an in vitro expriment performed on a meat sample, the equilibrium magnetisation P and the spin-lattice relaxation time T₁ are calculated from the values for the repetition time TR and the signal intensities obtained by a spin-echo sequence at different tissue temperatures tures as measured by a fibre-optic probe. T₁ is linearly correlated to the tissue temperature, and P is linearly correlated to the reciprocal value of the absolute temperature. Both effects, taken together, lead to a non-linear dependency of the signal intensity on temperature. Therefore a TR leading to maximum temperature dependency of the signal intensity is calculated and used in the futher experiments. In the in vivo experiments, the lower legs of two volunteers are cooled from outside. Images are acquired with a spin-echo sequence (1.5T, TR=1200 ms, TE=10 ms). A rise in signal intensity in the muscle with falling skin temperature is observed, particularly in more peripheral muscle layers. This study shows that spin-echo sequences can be used to monitor temperature changes and temperature differences in living muscle tissue.     

不同材料金属烤瓷冠磁共振成像伪影的比较

背景：口腔内有金属烤瓷冠的患者在进行磁共振成像检查时会形成伪影,但是在临床方面鲜见有关不同材料金属烤瓷冠在磁共振成像形成伪影的定量报道。 目的：研究不同材料金属烤瓷冠在FSE T2WI序列中磁共振成像伪影的大小。 方法：在核磁室收集需做磁共振成像检查且有右下颌第一磨牙烤瓷冠修复的患者48例,分为镍铬合金烤瓷冠组、钴铬合金烤瓷冠组及纯钛烤瓷冠组。采用1.5T MRI扫描仪对所有患者进行FSE T2WI扫描,观察同一序列不同材料金属烤瓷冠患者成像伪影的大小。 结果与结论：选择金属修复体边界出现清晰的弧形高信号环带且可进行测量的样本共45例,每种材料各15例。不同材料的金属烤瓷冠患者在同一序列产生的伪影大小不同,镍铬合金烤瓷冠组、钴铬合金烤瓷冠组和纯钛烤瓷冠组的伪影分别为(321.67±33.29),(263.53±34.95),(143.67±31.13) mm²,3组间两两比较差异均有显著性意义(P < 0.05)。表明3种临床常用的金属烤瓷冠修复材料中,纯钛烤瓷冠形成的伪影最小,镍铬合金烤瓷冠伪影最大。中国组织工程研究杂志出版内容重点：生物材料;骨生物材料; 口腔生物材料; 纳米材料; 缓释材料; 材料相容性;组织工程全文链接：     

Image distortion in MRI-based polymer gel dosimetry of gamma knife stereotactic radiosurgery systems

We have studied effects of MR (magnetic resonance) image distortion on polymer gel dosimetry of Gamma Knife stereotactic radiosurgery systems. MR images of BANG polymer gel phantoms were acquired by using a Hahn spin-echo sequence and a fast 3D imaging GRASS sequence. Image artifacts were studied by varying the directions of frequency encoding and the receiver bandwidth. The phantoms were also CT (computed tomography) scanned. The studies showed that the measured dose distributions are shifted by 1.8+/-0.5 mm (2 pixels) in the frequency encoding direction. The magnitude of the shift is inversely proportional to the receiver bandwidth in agreement with theory. Comparison of MRI with CT showed the same image shift. We concluded that the discrepancy is caused by MR image distortion due to a difference in susceptibility effects between the phantom and the fiducial markers of the Leksell localization box.