Segmentation of the skull in MRI volumes using deformable model and taking the partial volume effect into account

Segmentation of the skull in medical imagery is an important stage in applications that require the construction of realistic models of the head. Such models are used, for example, to simulate the behavior of electro-magnetic fields in the head and to model the electrical activity of the cortex in EEG and MEG data. In this paper, we present a new approach for segmenting regions of bone in MRI volumes using deformable models. Our method takes into account the partial volume effects that occur with MRI data, thus permitting a precise segmentation of these bone regions. At each iteration of the propagation of the model, partial volume is estimated in a narrow band around the deformable model. Our segmentation method begins with a pre-segmentation stage, in which a preliminary segmentation of the skull is constructed using a region-growing method. The surface that bounds the pre-segmented skull region offers an automatic 3D initialization of the deformable model. This surface is then propagated (in 3D) in the direction of its normal. This propagation is achieved using level set method, thus permitting changes to occur in the topology of the surface as it evolves, an essential capability for our problem. The speed at which the surface evolves is a function of the estimated partial volume. This provides a sub-voxel accuracy in the resulting segmentation.     

Segmentation of MRI brain scans using non-uniform partial volume densities

We present an algorithm that provides a partial volume segmentation of a T1-weighted image of the brain into gray matter, white matter and cerebrospinal fluid. The algorithm incorporates a non-uniform partial volume density that takes the curved nature of the cortex into account. The pure gray and white matter intensities are estimated from the image, using scanner noise and cortical partial volume effects. Expected tissue fractions are subsequently computed in each voxel. The algorithm has been tested for reliability, correct estimation of the pure tissue intensities on both real (repeated) MRI data and on simulated (brain) images. Intra-class correlation coefficients (ICCs) were above 0.93 for all volumes of the three tissue types for repeated scans from the same scanner, as well as for scans with different voxel sizes from different scanners with different field strengths. The implementation of our non-uniform partial volume density provided more reliable volumes and tissue fractions, compared to a uniform partial volume density. Applying the algorithm to simulated images showed that the pure tissue intensities were estimated accurately. Variations in cortical thickness did not influence the accuracy of the volume estimates, which is a valuable property when studying (possible) group differences. In conclusion, we have presented a new partial volume segmentation algorithm that allows for comparisons over scanners and voxel sizes.     

Fast and robust parameter estimation for statistical partial volume models in brain MRI

Due to the finite spatial resolution of imaging devices, a single voxel in a medical image may be composed of mixture of tissue types, an effect known as partial volume effect (PVE). Partial volume estimation, that is, the estimation of the amount of each tissue type within each voxel, has received considerable interest in recent years. Much of this work has been focused on the mixel model, a statistical model of PVE. We propose a novel trimmed minimum covariance determinant (TMCD) method for the estimation of the parameters of the mixel PVE model. In this method, each voxel is first labeled according to the most dominant tissue type. Voxels that are prone to PVE are removed from this labeled set, following which robust location estimators with high breakdown points are used to estimate the mean and the covariance of each tissue class. Comparisons between different methods for parameter estimation based on classified images as well as expectation--maximization-like (EM-like) procedure for simultaneous parameter and partial volume estimation are reported. The robust estimators based on a pruned classification as presented here are shown to perform well even if the initial classification is of poor quality. The results obtained are comparable to those obtained using the EM-like procedure, but require considerably less computation time. Segmentation results of real data based on partial volume estimation are also reported. In addition to considering the parameter estimation problem, we discuss differences between different approximations to the complete mixel model. In summary, the proposed TMCD method allows for the accurate, robust, and efficient estimation of partial volume model parameters, which is crucial to a variety of brain MRI data analysis procedures such as the accurate estimation of tissue volumes and the accurate delineation of the cortical surface.     

PET成像中部分容积效应的成因及影响因素

部分容积效应(Partial volume effect,PVE)是影响医学影像设备图像分析的重要因素.PET成像系统有限的分辨率使PVE更加明显,造成大病灶图像边缘模糊,小病灶图像模糊暗淡;在定量分析上使标准化摄取值(SUV)降低,影响对病灶性质的判定及放化疗等治疗效果的评估.本文对PVE的成因、影响因素、实际影响进行初步的探讨. 1 PVE的成因 PVE是由两个效应共同作用下形成的,即点源扩展及组织分数效应.点源扩展是PET分辨率较低造成的[1],组织分数效应主要和PET采集图像信号的方式有关[2],下面进行详细的叙述.     

腕部MRI体积定量分析评价类风湿关节炎疗效

目的探讨腕部MRI体积定量分析对于评价类风湿关节炎(RA)疗效的价值。方法收集30例活动期RA并接受规律治疗1年患者,应用MRI体积定量分析软件测量治疗前后同一腕关节滑膜炎、骨髓水肿体积,并获取同期患者红细胞沉降率(ESR)和C反应蛋白(CRP),计算28关节疾病活动度(DAS28)。分析治疗前后滑膜炎、骨髓水肿、CRP、ESR、DAS28变化,并将滑膜炎、骨髓水肿变化量与CRP、ESR、DAS28变化量进行相关性分析。结果治疗后滑膜炎体积、骨髓水肿范围、CRP、ESR、DAS28均显著降低(P均<0.05)。滑膜炎与骨髓水肿变化量呈正相关(r=0.61,P<0.01)。滑膜炎、骨髓水肿变化量与ESR、CRP变化量均无明显相关(P均>0.05),滑膜炎、骨髓水肿变化量均与DAS28变化量呈正相关(r=0.50、0.56,P=0.01、<0.01)。结论 MRI定量分析可作为量化指标,准确客观监测RA病情和评价疗效。     

Investigation of spatial resolution, partial volume effects and smoothing in functional MRI using artificial 3D time series

This work addresses the balance between temporal signal-to-noise ratio (tSNR) and partial volume effects (PVE) in functional magnetic resonance imaging (fMRI) and investigates the impact of the choice of spatial resolution and smoothing. In fMRI, since physiological time courses are monitored, tSNR is of greater importance than image SNR. Improving SNR by an increase in voxel volume may be of negligible benefit when physiological fluctuations dominate the noise. Furthermore, at large voxel volumes, PVE are more pronounced, leading to an overall loss in performance. Artificial fMRI time series, based on high-resolution anatomical data, were used to simulate BOLD activation in a controlled manner. The performance was subsequently quantified as a measure of how well the resulted activation matched the simulated activation. The performance was highly dependent on the spatial resolution. At high contrast-to-noise ratio (CNR), the optimal voxel volume was small, i.e. in the region of 2(3) mm(3). It was also shown that using a substantially larger voxel volume in this case could potentially negate the CNR benefits. The optimal smoothing kernel width was dependent on the CNR, being larger at poor CNR. At CNR >1, little or no smoothing proved advantageous. The use of artificial time series gave an opportunity to quantitatively investigate the effects of partial volume and smoothing in single subject fMRI. It was shown that a proper choice of spatial resolution and smoothing kernel width is important for fMRI performance.     

The effect of perfluorocarbon vapour on the measurement of respiratory tidal volume during partial liquid ventilation

During partial liquid ventilation perfluorocarbon vapour is present in the exhaled gases. The volumes of these gases are measured by pneumotachometers. Error in measuring tidal volumes will give erroneous measurement of lung compliance during partial liquid ventilation. We aim to compare measured tidal volumes with and without perfluorocarbon vapour using tidal volumes suitable for use in neonates. Tidal volumes were produced with a 100 ml calibration syringe from 20 to 100 ml and with a calibrated Harvard rodent ventilator from 2.5 to 20 ml. Control tidal volumes were drawn from a humidifier chamber containing water vapour and the PFC tidal volumes were drawn from a humidifier chamber containing water and perfluorocarbon (FC-77) vapour. Tidal volumes were measured by a fixed orifice, target, differential pressure flowmeter (VenTrak) or a hot-wire anenometer (Bear Cub) placed between the calibration syringe or ventilator and the humidifier chamber. All tidal volumes measured with perfluorocarbon vapour were increased compared with control (ANOVA p < 0.001 and post t-test p < 0.0001). Measured tidal volume increased from 7 to 16% with the fixed orifice type flow-meter, and from 35 to 41% with the hot-wire type. In conclusion, perfluorocarbon vapour flowing through pneumotachometers gives falsely high tidal volume measurements. Calculation of lung compliance must take into account the effect of perfluorocarbon vapour on the measurement of tidal volume.     

A framework to analyze partial volume effect on gray matter mean diffusivity measurements

Analyzing gray matter diffusion properties can be challenging due to possible measurement biases originating from averaging of gray matter (GM) and cerebrospinal fluid (CSF) signals. Therefore, a better characterization of CSF contamination effects in different cortical regions is required in order to disentangle actual changes in microstructure of GM itself from changes due to other effects such as macroscopic morphological changes. We propose a localized analysis framework for the CSF contamination effect on GM mean diffusivity measurement and applied this framework to measurements on 15 subjects. Our proposed modeling framework was compared to fluid-attenuated inversion recovery (FLAIR) DTI technique from the same subjects. The results of our studies suggest that GM mean diffusivity value was significantly biased by the CSF contamination effect, and that the amount of contamination strongly depended on the local morphology of the peripheral brain. Expected biases had their maxima in the motor and the somatosensory association cortex, and their minima in mid and inferior temporal areas of the brain where the cortical thicknesses are particularly pronounced. We conclude from our studies that regional differences in tissue compounding ratio must be taken into account when assessing localized GF diffusivity differences.     

MRI液平显像对囊性前庭神经鞘瘤手术效果的评价

目的 评价MRI液平显像对囊性前庭神经鞘瘤手术效果的影响.方法 选择45例囊性前庭神经鞘瘤患者,根据MRI影像学特点分为液平组24例,非液平组21例,对两组瘤周粘连率、全切率及面神经功能情况进行回顾性分析.结果 术中发现液平组和非液平组的瘤周粘连率分别为70.8％（17/24）和28.6％（6/21）,组间比较差异有统计学意义（x2=8.010,P＜0.05）;肿瘤全切率分别为45.8％（11/24）和76.2％（16/21）,组间比较差异有统计学意义（x2=4.300,P＜0.05）.出院及术后1年时进行House-Brackmann分级,Ⅰ～Ⅲ级者在液平组分别占62.5％ （15/24）和50.0％（8/16）,而在非液平组分别占81.0％（17/21）和83.3％（15/18）;出院时组间比较差异无统计学意义（x2=1.86,P＞0.05）,术后1年时组间比较差异有统计学意义（P=0.015）.结论 具有液平显像的囊性前庭神经鞘瘤与周围神经血管结构粘连的发生率更高,手术效果也较差,该发现有助于预测手术效果及手术策略的选择.     

MRI测量中国健康成人脑杏仁核体积

目的 采用MR1测量不同年龄中国健康成人杏仁核体积.方法 于中国健康成人MRI脑数据库中选取100名志愿者,按照年龄分为18?30岁、31?40岁、41?50岁、51?60岁和61?70岁组共5组,每组男、女性各10名.采集头部三维磁化准备双快速采集梯度回波序列图像,获得双侧杏仁核体积及大脑容积,再以颅腔容积进行校正;...     

MRI评价肩关节冈上肌腱部分撕裂

目的 评价肩关节MRI对冈上肌腱部分撕裂的诊断价值.方法 由两名影像诊断医师独立回顾分析264例肩关节MR图像,将分析结果与肩关节镜和(或)手术结果相比较.计算肩关节MRI评价冈上肌腱部分撕裂的诊断敏感度,以及与肩关节镜分型的符合率.采用Kappa一致性检验计算两名影像诊断医师评价的一致性.结果 肩关节镜手术证实冈上肌腱部分撕裂81例(下表面撕裂23例,上表面撕裂47例,腱内撕裂11例).肩关节MRI评估冈上肌腱部分撕裂的敏感度两名医师分别为77.78%(63/81)和79.01%(64/81),K=0.627.肩关节MRI与肩关节镜对于冈上肌腱部分撕裂的分型符合率两名医师分别为70.37%(57/81)和76.54%(62/81).结论 肩关节MRI是评价冈上肌腱部分撕裂比较可靠的方法.     

Estimation of the human liver volume and configuration using three-dimensional ultrasonography: effect of a high-caloric liquid meal

The aim of this study was to investigate whether or not a magnetic position sensing system for free-hand acquisition of 3-D ultrasound images could be used to estimate liver volumes, and to study the effect of a high-caloric meal on these volumes in healthy subjects. In vitro accuracy was evaluated by scanning porcine and rabbit livers. Ten healthy subjects were examined fasting and 30 min after ingesting a high-caloric liquid meal. Portal and hepatic vein blood flow were measured by 2-D duplex sonography. The 3-D system yielded a strong correlation (r = 0.99) between true and estimated volumes in vitro. No significant increase in liver volume in response to the meal was seen. However, portal and hepatic vein flow volume increased significantly. Experience in human subjects suggests that a complete 3-D study of liver volumes can be obtained from multiple acoustic windows. In healthy subjects, no significant increase in liver volume was seen in response to ingestion of a high-caloric liquid meal.     

Characterisation of partial volume effect and region-based correction in small animal positron emission tomography (PET) of the rat brain

Accurate quantification of PET imaging data is required for a useful interpretation of the measured radioactive tracer concentrations. The partial volume effect (PVE) describes signal dilution and mixing due to spatial resolution and sampling limitations, which introduces bias in quantitative results. In the present study we investigated the magnitude of PVE for volumes of interest (VOIs) in the rat brain and the effect of positron range. In simulated (11)C-raclopride studies we examined the influence of PVE on time activity curves in striatal and cerebellar VOIs and binding potential estimation. The performance of partial volume correction (PVC) was studied using the region-based geometric transfer matrix (GTM) method including the question of whether a spatially variant point spread function (PSF) is necessary for PVC of a rat brain close to the centre of the field of view. Furthermore, we determined the effect of spillover from activity outside the brain. The results confirmed that PVE is significant in rat brain PET and showed that positron range is an important factor that needs to be included in the PSF. There was considerable bias in time activity curves for the simulated (11)C-raclopride studies and significant underestimation of binding potential even for very small centred VOIs. Good activity recovery was achieved with the GTM PVC using a spatially invariant simulated PSF when no activity was present outside the brain. PVC using a simple Gaussian fit point spread function was not sufficiently accurate. Spillover from regions outside the brain had a significant impact on measured activity concentrations and reduced the accuracy of PVC with the GTM method using rat brain regions alone, except for the smallest VOI size but at the cost of increased noise. Voxel-based partial volume correction methods which inherently compensate for spillover from outside the brain might be a more suitable choice.     

MRI定量评估胎儿脑体积

目的观察MRI定量脑体积评估胎儿大脑发育的价值。方法对60例排除胎儿中枢神经系统异常或发育不良、孕周(GA)21～37周的单胎妊娠孕妇行胎儿MR检查,采用单次激发快速自旋回波(SSTSE)序列采集胎儿MRI,经后处理后手动分割大脑,测量胎儿三维脑体积指标颅腔内体积(ICV)、脑总容积(TBV)和脑脊液容积(CFV),计算大脑二维径线指标,包括大脑双顶径(BPD)、骨性双顶径(SBD)、枕额径(SOD)及头围(HC)。分析二维径线指标及三维脑体积指标与GA的相关性,并进行回归分析;观察三维脑体积指标与二维径线指标间的关系。结果 TBV(r=0.98)、ICV(r=0.97)、CFV(r=0.89)、BPD(r=0.96)、SBD(r=0.94)、SOD(r=0.96)、HC(r=0.96)均与GA呈高度正相关(P均<0.01)。TBV及ICV与二维径线各指标间均高度正相关(P均<0.01),HC与CFV间亦呈高度相关(P<0.01)。结论 MRI三维定量脑体积可较好评估GA 21～37周胎儿颅脑生长发育,有望为产前诊断胎儿脑疾病及研究其发病机制提供新的影像学手段。     

高分辨MRI测量中国健康成人内嗅皮层体积

目的 获得中国健康成人内嗅皮层体积的正常参考值,分析内嗅皮层体积与年龄、性别的关系.方法 自大型中国成人脑MRI数据库中抽取100份数据,按年龄18?30岁、31?40岁、41?50岁、51?60岁、61～70岁分为5组,每组20名,男、女各10名.手动勾画内嗅皮层边界,应用三维体积分析软件测量其体积,比较不同年龄组、...     

阿尔茨海默病脑灰质体积异常的MRI研究

目的研究阿尔茨海默病(AD)患者MRI图像脑灰质萎缩的模式。方法对19例AD患者和20例正常老年对照者行MRI扫描,通过优化的基于体素的形态学(optimizedvoxelbasedmophometry,OVBM)分析方法,分析AD患者全脑和局部脑灰质的体积异常。结果AD患者全脑灰质体积相对于正常对照者明显降低(P=0.0048),双侧海马、双侧丘脑、双侧颞上回和左侧颞中回体积明显减少(多重校正后,相当于P<0.05)。结论AD患者的全脑灰质体积明显降低,局部区域明显萎缩。     

基于MRI图像的羊水体积的测量

目的 探讨基于MRI图像,通过算法计算测量羊水量的可行性。方法 利用MatLab图像处理技术,对胎儿磁共振图像进行分割,提取目标区域并计算目标区域的面积,乘以层厚得出一层的体积,再将每层体积相加计算得到羊水总体积。结果 通过该算法最终计算得出羊水的体积为495.10 ml。定性分析显示,图像羊水分割边缘与原图目标区域的边缘拟合较好。定量分析显示,手动金标准分割的体积为458.20 ml,本研究算法与手动分割结果的误差率为8.06%。脂肪抑制序列图像分割效果定性评价亦显示羊水分割边缘与原图目标区域的边缘拟合较好;定量分析显示,金标准手动计算得到的羊水量为557.34 ml,本研究算法计算得到的羊水量为604.50 ml,与手动分割的误差率为8.46%。结论 采用本研究算法测量羊水量切实可行。     

MRI study of the tissue volume progression of lower limbs in hemiplegic subjects]

OBJECTIVES: The aim of this paper was to present the first results of a study about lower limbs' soft tissues (fat and muscle) evolution using magnetic resonance images. MATERIALS AND METHODS: Magnetic resonance images method (MRI) was chosen instead of X-ray scanner for its non-invasive and non-radiating properties. Three hemiplegics volunteers were followed during the first 6 months of their therapy. Lower limbs' MRI captures were made at the beginning and in the end of this period. Computerized processing of the data allows automatic recognition of the main lower limb's tissues. Volumes were determined from images and classified in 4 categories (i.e. muscle, fat, spongy bone, cortical bone). RESULTS: Data analysing showed a different evolution of soft tissues in healthy lower limb compared to pathological lower limb. DISCUSSION-CONCLUSION: This results should lead us to a better understanding of therapeutic efficiency.     

回归分析胎儿MRI肺容积预期值

目的 建立大样本量、孕周分布均衡的中国胎儿肺容积（FLV）预期值回归方程。方法 采集457胎胎龄18~39周胸部正常胎儿产前MRI,在单次激发快速自旋回波序列轴位图像上测量FLV。对采集到的样本数据进行回归分析,根据拟合度及检验结果确定回归模型,并得出基于孕周的FLV预期值回归方程。结果 457胎胎儿平均孕周为（29.44±4.61）周,其中孕周 ≤ 28周胎儿占43.11%（197/457）。基于MRI测量FLV值为9.47~131.48 ml,平均（50.78±23.76） ml。回归分析所得方程为FLV预期值=0.004g^2.76（g为孕周,R²=0.845,P<0.001）。结论 本研究所得的FLV预期值回归方程简便实用,适用于大多数产前检查,有助于国内开展胎儿肺产前评估相关研究。     

前列腺癌磁共振成像诊断要点与临床治疗方案选择

MR已经被广泛用于前列腺癌诊断和评估,对于临床诊断和治疗前列腺癌具有重要的指导意义。本文总结多种磁共振成像技术在前列腺癌诊断方面的价值,并对磁共振成像对前列腺癌临床治疗方案选择的影响进行简要介绍,旨在进一步提高磁共振成像在前列腺癌应用方面的认识。