Descending aorta subject-specific one-dimensional model validated against in vivo data

The aorta plays a major role in the cardiovascular system and its function and structure are primarily affected by aging, eating habits, life style and other cardiovascular risk factors, inducing increased stiffness which is associated with cardiovascular and cerebral morbi-mortality. Our objective was to develop and validate a robust subject-specific one-dimensional wave propagation numerical model of the descending aorta. This model with a cross-sectional area, velocity and pressure formulation is built using geometric and hemodynamic data measured on a specific person and is validated against in vivo data acquired on the same subject at three distinct anatomical locations along the thoracic aorta. We studied seven healthy volunteers, who underwent carotid applanation tonometry and aortic cardiovascular magnetic resonance (CMR). Responses of our model in terms of changes in central pressure waveform with arterial alterations were consistent with previously described physiological knowledge. Quantitative validation averaged over the three descending aortic locations and the seven subjects provided low rms errors (given in percentage of the maximal clinical value) between simulated and CMR data, i.e. area: 10±6%, velocity: 11±3%, flow rate: 9±3%. Finally, we also found low rms (5±2%) when comparing simulated pressure in the proximal aortic location against tonometric carotid pressure curves. In conclusion, this simple model performs similar to more complex models of the entire systemic arterial tree at a fraction of the cost, and could be of major usefulness in the non-invasive and local estimation of proximal biomechanical and hemodynamic indices.     

Development and validation of a subject-specific moving-axis tibiofemoral joint model using MRI and EOS imaging during a quasi-static lunge

The aims of this study were to introduce and validate a novel computationally-efficient subject-specific tibiofemoral joint model. Subjects performed a quasi-static lunge while micro-dose radiation bi-planar X-rays (EOS Imaging, Paris, France) were captured at roughly 0°, 20°, 45°, 60°, and 90° of tibiofemoral flexion. Joint translations and rotations were extracted from this experimental data through 2D-to-3D bone reconstructions, using an iterative closest point optimization technique, and employed during model calibration and validation. Subject-specific moving-axis and hinge models for comparisons were constructed in the AnyBody Modeling System (AMS) from Magnetic Resonance Imaging (MRI)-extracted anatomical surfaces and compared against the experimental data. The tibiofemoral axis of the hinge model was defined between the epicondyles while the moving-axis model was defined based on two tibiofemoral flexion angles (0° and 90°) and the articulation modeled such that the tibiofemoral joint axis moved linearly between these two positions as a function of the tibiofemoral flexion. Outside this range, the joint axis was assumed to remain stationary. Overall, the secondary joint kinematics (ML: medial–lateral, AP: anterior-posterior, SI: superior-inferior, IE: internal-external, AA: adduction-abduction) were better approximated by the moving-axis model with mean differences and standard errors of (ML: −1.98 ± 0.37 mm, AP: 6.50 ± 0.82 mm, SI: 0.05 ± 0.20 mm, IE: 0.59 ± 0.36°, AA: 1.90 ± 0.79°) and higher coefficients of determination (R²) for each clinical measure. While the hinge model achieved mean differences and standard errors of (ML: −0.84 ± 0.45 mm, AP: 10.11 ± 0.88 mm, SI: 0.66 ± 0.62 mm, IE: −3.17 ± 0.86°, AA: 11.60 ± 1.51°).     

Fast Keyhole MR Imaging Using Optimized k-Space Data Acquisition

In some dynamic magnetic resonance imaging (MRI) applications, the sample is still, and only the signal intensity changes with time. For such cases, the keyhole imaging principle can be used. In standard keyhole imaging, a low-frequency image signal is acquired, using a limited number of phase-encoding steps, which correspond to the rectangular sampling region in the k-space center. However, such a region practically never coincides with the position of the k-space points, which carry the most relevant low-frequency image information. In this paper we propose an improved keyhole method, which allows dynamic acquisition of a low-frequency image signal from selected most relevant k-space points via fast imaging mechanisms. Dynamic data acquisition is executed in the presence of time-varying magnetic-field (MF) gradients after single sample excitation. Special care has been taken in the design of the gradient sequence to minimize gradient load. This improved keyhole imaging method has been considered theoretically and verified experimentally on a model system.     

Obesity and spinal loads; a combined MR imaging and subject-specific modeling investigation

Epidemiological studies have identified obesity as a possible risk factor for low back disorders. Biomechanical models can help test such hypothesis and shed light on the mechanism involved. A novel subject-specific musculoskeletal-modelling approach is introduced to estimate spinal loads during static activities in five healthy obese (BMI > 30 kg/m²) and five normal-weight (20 < BMI < 25 kg/m²) individuals. Subjects underwent T1 through S1 MR imaging thereby measuring cross-sectional-area (CSA) and moment arms of trunk muscles together with mass and center of mass (CoM) of T1-L5 segments. MR-based subject-specific models estimated spinal loads using a kinematics/optimization-driven approach. Average CSAs of muscles, moment arms of abdominal muscles, mass and sagittal moment arm of CoM of T1-L5 segments were larger in obese individuals (p < 0.05 except for the moment arm of CoMs) but moment arms of their back muscles were similar to those of normal-weight individuals (p > 0.05). Heavier subjects did not necessarily have larger muscle moment arms (e.g., they were larger in 64 kg (BMI = 20.7 kg/m²) subject than 78 kg (BMI = 24.6 kg/m²) subject) or greater T1-L5 trunk weight (e.g., the 97 kg (BMI = 31 kg/m²) subject had similar trunk weight as 109 kg (BMI = 33.3 kg/m²) subject). Obese individuals had in average greater spinal loads than normal-weight ones but heavier subjects did not necessarily have greater spinal loads (117 kg (BMI = 40.0 kg/m²) subject had rather similar L5-S1 compression as 105 kg (BMI = 34.7 kg/m²) subject). Predicted L4-L5 intradiscal pressures for the normal-weight subjects ranged close to the measured values (R² = 0.85–0.92). Obese individuals did not necessarily have greater IDPs than normal-weight ones.     

Reliability and robustness of muscle architecture measurements obtained using diffusion tensor imaging with anatomically constrained tractography

For detailed analyses of muscle adaptation mechanisms during growth, ageing or disease, reliable measurements of muscle architecture are required. Diffusion tensor imaging (DTI) and DTI tractography have been used to reconstruct the architecture of human muscles in vivo. However, muscle architecture measurements reconstructed with conventional DTI techniques are often anatomically implausible because the reconstructed fascicles do not terminate on aponeuroses, as real muscle fascicles are known to do. In this study, we tested the reliability of an anatomically constrained DTI-based method for measuring three-dimensional muscle architecture. Anatomical magnetic resonance images and diffusion tensor images were obtained from the left legs of eight healthy participants on two occasions one week apart. Muscle volumes, fascicle lengths, pennation angles and fascicle curvatures were measured in the medial and lateral gastrocnemius, soleus and the tibialis anterior muscles. Averaged across muscles, the intraclass correlation coefficient was 0.99 for muscle volume, 0.81 for fascicle length, 0.73 for pennation angle and 0.76 for fascicle curvature. Measurements of muscle architecture obtained using conventional DTI tractography were highly sensitive to variations in the stopping criteria for DTI tractography. The application of anatomical constraints reduced this sensitivity significantly. This study demonstrates that anatomically constrained DTI tractography can provide reliable and robust three-dimensional measurements of whole-muscle architecture. The algorithms used to constrain tractography have been made publicly available.     

^H磁共振波谱在脑胶质瘤的应用进展

磁共振波谱（magnetic resonance spectroscopy,MRS）技术的出现使活体检测组织的代谢和生化信息成为可能,随着其技术的不断成熟,其在临床的应用范围日益扩大。脑胶质瘤具有与正常脑组织不同的代谢特征,借助MRS技术一方面可以反映其代谢特征,另外可将其与正常脑组织区分,因此MRS技术特别是^1H-MRS在脑胶质瘤的诊断、鉴别诊断、分级及预后评估中应用日益广泛。本文就相关进展进行综述。     

多功能磁共振造影剂研究进展

磁共振成像技术因对人体无创、任意方向断层扫描三维图像且分辨率较高、提供形态与功能两方面诊断评价等突出优点,成为了临床上用于疾病诊断的重要手段之一。临床上使用磁共振造影剂可以提高成像的分辨率和灵敏度,提高图像质量,增强对比度和可读性。但是,各种成像技术由于实现原理不同,具有各自的优势和缺陷,靠传统单一的诊断模式无法提供疾病的全面信息,因而在对各种复杂疾病进行诊断时会受到一定的限制。因此,将磁共振成像与其他成像技术如CT成像、超声成像等联合起来使用,则可以达到优势互补的效果,能为疾病的临床诊断提供更快捷精确的信息,同时可将磁共振成像与各种治疗方式结合在一起,即开发基于磁共振成像的诊断治疗一体化试剂,以实现对疾病的即时治疗和实时监控。本文主要介绍了磁共振成像造影剂的原理和种类,并且综述了目前国内外在基于磁共振成像的多功能造影剂/诊疗制剂这一领域的研究进展,最后就未来可能的研究方向进行了展望。     

Three-dimensional reconstruction of brain surface anatomy based on magnetic resonance imaging diffusion-weighted imaging: A new approach

Fifty normal noninfarct patients and 12 cases with infarcts of the cerebrum were examined with routine magnetic resonance imaging and echo-planar diffusion-weighted imaging. The diffusion-weighted three-dimensional images were reconstructed with volume-rendering processing on workstation. Precentral gyrus, post-central gyrus, superior parietal lobule, superior frontal gyrus, precentral sulcus, central sulcus, postcentral sulcus, intraparietal sulcus and superior frontal sulcus were best shown of all structures with an arbitrary score of 2.61–2.77. Supramarginal gyrus, middle frontal gyrus, inferior frontal gyrus and lateral sulcus were clearly shown in the majority of the cerebra with average scores of 2.0–2.49; angular gyrus, inferior frontal sulcus and superior temporal gyrus were not demonstrated satisfactorily and their average scores were 1.67–1.89. Middle temporal gyrus, inferior temporal gyrus, superior temporal sulcus and inferior temporal sulcus were difficult to identify, and thus had average scores of 0.87–1.26. Brain surface structures were better displayed in the older group of individuals than in the younger group. The structures in the 12 cases with acute or chronic cerebrum infarcts were also satisfactorily demonstrated with this new technique.     

Magnetic resonance imaging based modeling of microvascular perfusion in patients with peripheral artery disease

Peripheral artery disease (PAD) is associated with an increased risk of adverse cardiovascular events, impaired lower extremity blood flow and microvascular perfusion abnormalities in the calf muscles which can be determined with contrast-enhanced magnetic resonance imaging (CE-MRI). We developed a computational model of the microvascular perfusion in the calf muscles. We included 20 patients (10 PAD, 10 controls) and utilized the geometry, mean signal intensity and arterial input functions from CE-MRI calf muscle perfusion scans. The model included the microvascular pressure (p_v), outflow filtration coefficient (OFC), transfer rate constant (k^t), porosity (φ), and the interstitial permeability (K_tissue). Parameters were fitted and the simulations were compared across PAD patients and controls. Intra-observer reproducibility of the simulated mean signal intensities was excellent (intraclass correlation coefficients >0.995). k^t and K_tissue were higher in PAD patients compared with controls (4.72 interquartile range (IQR) 3.33, 5.56 vs. 2.47 IQR 2.10, 2.85; p = 0.003; and 3.68 IQR 3.18, 4.41 vs. 1.81 IQR 1.81, 1.81; p < 0.001). Conversely, porosity (φ) was lower in PAD patients compared with controls (0.52 IQR 0.49, 0.54 vs. 0.61 IQR 0.58, 0.64; p = 0.016). Porosity (φ) was correlated with the ankle brachial index (r = 0.64, p = 0.011). The proposed computational microvascular model is robust and reproducible, and essential model parameters differ significantly between PAD patients and controls.     

Early ontogeny of the human femoral bicondylar angle

The presence of a femoral bicondylar angle consistently and significantly greater than 0° has been a hallmark of hominid bipedality, but its pattern of development has not been documented. We have therefore compiled cross-sectional data on the development of the articular bicondylar angle for a clinical sample of modern humans and of the metaphyseal bicondylar angle for two Recent human skeletal samples, one predominantly European in origin and the other Amerindian. All three samples exhibit a pattern of a bicondylar angle of 0° at birth and then a steady average increase in the angle from late in the first year postnatal, through infancy, and into the juvenile years. The two skeletal samples reach low adult values by approximately 4 years postnatal, whereas the clinical sample with a lowered activity level appears to attain consistent adult values slightly later (approximately 6 years postnatal). In addition, two modern human individuals, one nonambulatory and the other minimally ambulatory, show no and little development, respectively, of a bicondylar angle. These data, in conjunction with clinical and experimental observations on the potential and form of angular changes during epiphyseal growth, establish a high degree of potential for plasticity in the development of the human bicondylar angle and the direct association of a bipedal locomotion and (especially) posture with the developmental emergence of a human femoral bicondylar angle. © 1994 Wiley-Liss, Inc.     

Patterns of variability in mineralization of the primate femoral diaphysis.

The purpose of this research was to study intra- and interspecific variability in mineral density in the femoral diaphysis of nonhuman primates. Four hundred five sections were taken from five sites along the femoral diaphysis of 34 macaques (Macaca sp.), 24 squirrel monkeys (Saimirisciureus), and 23 tamarins (Saguinus labiatus). The mineral density at eight positions around each section was measured. Analysis of variance indicated significant (p less than 0.05) interactions among species, sex, cross sectional levels, and positions within each level. Both Macaca and S. sciureus showed sexually dimorphic patterns of mineral distribution. S. labiatus exhibited no sexual dimorphism in distribution, but was sexually dimorphic in density magnitude. No significant dimensional differences in density were found among species, though significant differences in pattern were evident. Highly significant differences (p less than 0.01) were found among cross sectional levels and among positions within the levels within each species.     

Balance—FFE序列在胆管磁共振检查中的应用

目的：探讨磁共振平衡式稳态自由进动梯度回波序列（Balance—FFE）在胆管疾病中的应用价值。方法：92例胆管病变患者均进行了冠状面的Balance—FFE序列扫描和磁共振胰胆管造影（MRCP）。将Balance-FFE的图像和3DMRCP像及MRCP原始图像对病变的显示率进行X。检验。结果：胆管系统在Balance—FFE序列中呈明显高信号,并能直接显示胆管结石和胆管狭窄,还能显示胆管外病变和胰腺病变,对周围淋巴结的显示也比较清楚。经x^2检验,Balance—FFE序列和MRCP原始像对胆管系统病变的显示没有统计学意义（P〉0．05）,而3DMRCP像对病变的显示能力不如Balance．FFE序列和MRCP原始像,对胆道病变的显示能力具有统计学意义（P〈0．05）。结论：Balance-FFE序列对胆道病变能清晰显示,成像速度快,图像信噪比高,伪影较少,与MRCP结合能提高对胆管系统病变的诊断率,因此Balance-FFE序列应作为胆管疾病磁共振扫描的常规序列,可作为MRCP序列的有效补充。     

Preparation, characterization and in vivo assessment of Gd-albumin and Gd-dendrimer conjugates as intravascular contrast-enhancing agents for MRI

We report in vivo and in vitro MRI properties of six gadolinium-dendrimer and gadolinium-albumin conjugates of derivatized acyclic diethylenetriamine-N,N′,N′,N″, N″-pentaacetic acid (1B4M) and macrocyclic 1,4,7,10-tetraazacyclododecane-N,N′,N″,N?-tetraacetic acid (C-DOTA). The three albumin-based agents have comparable protein to chelate ratios (1:16-18) as well as molar relaxivity (8.8-10.4 mM^− 1 s^− 1). The three dendrimer based agents have blood clearance half-lives ranging from 17 to 66 min while that of the three albumin-based agents are comparable to one another (40-47 min). The dynamic image obtained from use of the albumin conjugate based on the macrocycle (C-DOTA) showed a higher contrast compared to the remaining two albumin based agents. Our conclusion from all of the results is that the macrocyclic-based (DOTA) agents are more suitable than the acyclic-based (1B4M) agent for in vivo use based on their MRI properties combined with the kinetic inertness property associated with the more stable Gd(III) DOTA complex.     

膝关节挫伤的磁共振影像表现

目的 :探讨磁共振短时的反转恢复序列 (STIR)在膝关节骨挫伤中的临床应用。方法 :通过 32例膝关节外伤病例在常规SE序列、FSE序列和STIR序列中的影像表现 ,分析STIR序列的优越性。结果 :32例共 45个骨挫伤病灶 ,T1W发现 38个 (占 84% ) ,T2W发现 37个 (占 82 % ) ,STIR序列病灶全部显示( 1 0 0 % )。结论 :STIR序列对骨挫伤的敏感性较高 ,能显示微小的骨髓水肿 ,充血及骨小梁的微骨折及其周围的骨软骨、关节囊的细微变化 ,对膝关节外伤具有较高价值。     

Muscle cross-section measurement by magnetic resonance imaging

Muscle cross-section areas were measured by magnetic resonance imaging (MRI) in the thigh of a human cadaver, the results being compared with those obtained by photography of corresponding anatomic macroslices. A close correlation was found between MRI and photographic evaluation, differences between the methods ranging from nil to 9.5%, depending on the scan position and the muscle groups. In vivo MRI measurements were performed on 12 female and 16 male students, the objectivity, the test-retest reliability and the variability of the MRI measurements being studied by fixing the scan position either manually or by coronary scan. The latter method appeared to be more objective and reliable. The coefficients of variation for muscle cross-section areas measured by MRI were in the range of those for the planimetry of given cross-section areas. Allowing for differentiation between several small muscle bundles in a given area, MRI proved to be a suitable method to quantify muscle cross-sections for intra- and interindividual analysis of muscle size.     

Accounting for spatial variation of trabecular anisotropy with subject-specific finite element modeling moderately improves predictions of local subchondral bone stiffness at the proximal tibia

IntroductionPreviously, a finite element (FE) model of the proximal tibia was developed and validated against experimentally measured local subchondral stiffness. This model indicated modest predictions of stiffness (R² = 0.77, normalized root mean squared error (RMSE%) = 16.6%). Trabecular bone though was modeled with isotropic material properties despite its orthotropic anisotropy. The objective of this study was to identify the anisotropic FE modeling approach which best predicted (with largest explained variance and least amount of error) local subchondral bone stiffness at the proximal tibia.MethodsLocal stiffness was measured at the subchondral surface of 13 medial/lateral tibial compartments using in situ macro indentation testing. An FE model of each specimen was generated assuming uniform anisotropy with 14 different combinations of cortical- and tibial-specific density-modulus relationships taken from the literature. Two FE models of each specimen were also generated which accounted for the spatial variation of trabecular bone anisotropy directly from clinical CT images using grey-level structure tensor and Cowin’s fabric-elasticity equations. Stiffness was calculated using FE and compared to measured stiffness in terms of R² and RMSE%.ResultsThe uniform anisotropic FE model explained 53–74% of the measured stiffness variance, with RMSE% ranging from 12.4 to 245.3%. The models which accounted for spatial variation of trabecular bone anisotropy predicted 76–79% of the variance in stiffness with RMSE% being 11.2–11.5%.ConclusionsOf the 16 evaluated finite element models in this study, the combination of Synder and Schneider (for cortical bone) and Cowin’s fabric-elasticity equations (for trabecular bone) best predicted local subchondral bone stiffness.     

磁共振成像对乳腺肿瘤的诊断价值

乳腺癌是危及女性健康的常见恶性肿瘤之一,病死率较高,且发病年龄呈年轻化趋势。目前临床对乳腺疾病的检查方法很多,既往检查主要包括钼靶、超声等,因价格便宜、操作方便,已成为常规的乳腺疾病检查方法,但两者的敏感性和特异性较低并有自身的局限性。CT软组织分辨率较高,但检查过程中的X线剂量较大,并且动态增强时间较长,故作为乳腺钼靶的补充检查手段。这些检查方法对乳腺疾病均有不同的诊断意义,在当前众多诊断乳腺疾病方法中,具有无辐射,较高软组织分辨力及可多方位多层面成像的乳腺磁共振(MRI)成像有其独到的优势,某些方面能弥补超声和钼靶检查的局限性,乳腺磁共振可提供病灶形态学和增强血流动力学表现,可用于常规检查方法不能确诊病灶的鉴别诊断。乳腺肿瘤MRI成像对临床诊断、鉴别诊断及手术方案的选择有着极其重要的作用。本文就乳腺MRI影像技术、MRI影像学表现及其临床应用予以综述,探讨MRI在乳腺肿瘤中的应用。     

超顺磁性造影剂的研究进展

超顺磁性造影剂是一种新型的磁共振造影剂,对肝脾等部位的成像效果显著,已成为国内外研究的热点之一。本文对超顺磁性造影剂的增强原理和制备方法进行了总结及评论。     

Flow patterns in the jugular veins of pulsatile tinnitus patients

Pulsatile Tinnitus (PT) is a pulse-synchronous sound heard in the absence of an external source. PT is often related to abnormal flow in vascular structures near the cochlea. One vascular territory implicated in PT is the internal jugular vein (IJV). Using computational fluid dynamics (CFD) based on patient-specific Magnetic Resonance Imaging (MRI), we investigated the flow within the IJV of seven subjects, four symptomatic and three asymptomatic of PT. We found that there were two extreme anatomic types classified by the shape and position of the jugular bulbs: elevated and rounded. PT patients had elevated jugular bulbs that led to a distinctive helical flow pattern within the proximal internal jugular vein. Asymptomatic subjects generally had rounded jugular bulbs that neatly redirected flow from the sigmoid sinus directly into the jugular vein. These two flow patterns were quantified by calculating the length-averaged streamline curvature of the flow within the proximal jugular vein: 130.3±8.1 m⁻¹ for geometries with rounded bulbs, 260.7±29.4 m⁻¹ for those with elevated bulbs (P<0.005). Our results suggest that variations in the jugular bulb geometry lead to distinct flow patterns that are linked to PT, but further investigation is needed to determine if the vortex pattern is causal to sound generation.     

Functional activation studies of word processing in the recovery from aphasia

Some reviews on theories of recovery in aphasia put an emphasis on neural network models based on empirical data from evoked-potentials in aphasia as an approach to mapping recovery of cognitive function to neural structure. We will focus here on what we call an "anatomical" approach to look at recovery in aphasia. "Anatomical" theories of recovery stated by classical aphasiologists have contributed to the understanding of language representations in the human brain. But many aspects of these theories can only be investigated by using modern techniques of lesion analysis, psychometric assessment and functional imaging. Whereas structure-function relations have been primarily established by looking for the association of deficit symptoms with certain lesions, functional activation methods offer a means to study more directly the functional anatomy of recovered or retained functions in neuropsychological patients. To falsify or build up anatomical theories of recovery we will propose a stepwise approach of inference. The methodological pitfalls of this approach will be discussed by focussing on anatomical hypotheses of semantic word comprehension and its impairment and recovery in aphasia.