Backscatter directivity and integrated backscatter power of arterial tissue

A 27 MHz transducer, mounted on an ultrasonic microscope, was used to quantify the dependence of backscatter power on the angle of incidence of arterial vessels. Due to variations in the angle of incidence significant variations in backscatter power were found in the intima, the muscular and elastic media, the adventitia and the external elastic lamina. The muscular and the elastic media show anisotropic behaviour in their angle dependence, i.e. the extent of the angle dependence depends on the direction of angle variation. This anisotropic nature is probably caused by the dominant orientation of smooth muscle cells or elastin fibers in these tissue layers. Measurements on 13 specimens of the iliac artery showed that each tissue type of the vessel has its own specific angle dependent behaviour. In the future this property might be used for quantitative tissue characterization.     

Orientation and frequency dependence of backscatter coefficient in normal and pathological breast tissues

The backscatter coefficient was measured on five groups of normal and pathological breast specimens: (1) as a function of frequency (in the range 4–14 MHz) and (2) at a single fixed frequency (10 MHz), as a function of the angle of incidence between the beam and the specimen (approximately 60°). The results of the study are discussed in relation to the content of cells and collagen fibers in breast tissues. The absolute value of backscatter coefficient is larger in tissues with a prevalence of collagenous fibers in comparison to tissues with only cells. In fibrofatty tissue, the inhomogeneity of the specimens is probably responsible for the highest backscatter value. The power law frequency dependence of the backscatter coefficient is of a diffractive nature in tissues characterized by only cells; in tissues with a strong prevalence of collagenous fibers, the power law frequency dependence increases. Periodicities in the angular patterns have been quantified by the autocorrelation functions for each group of specimens. The results of the study suggest a means for assessing tissue structure in normal and pathological breast tissue.     

Ultrasonic signal processing for in vivo attenuation measurement: short time Fourier analysis

Short-time Fourier analysis is well suited for processing tissue echographic signals which are nonstationary. We have investigated the use of short-time Fourier analysis to provide an estimation of the echographic spectral composition as a function of time. It will be shown that the time dependence of the spectral centroid of this representation allows one to deduce easily the frequency-dependent attenuation. A simple correction of the noninvariant filtering effect due to diffraction is used to unbias the attenuation slope estimation. This new signal processing technique was first tested on simulated echographic data from a 1-D tissue model. Experimental results obtained from echo signals on a tissue-like phantom and on in vivo liver tissue show the influence of diffraction and attenuation respectively.     

Imaging tumor response following liver-directed intra-arterial therapy

Liver-directed intra-arterial therapies are palliative treatment options for patients with unresectable liver cancer; their use has also resulted in patients being downstaged leading to curative resection and transplantation. These intra-arterial therapies include transarterial embolization, conventional transarterial chemoembolization (TACE), drug-eluting bead TACE and radioembolization. Assessment of imaging response following these liver-directed intra-arterial therapies is challenging but pivotal for patient management. Size measurements based on computed tomography or magnetic resonance imaging (MRI) have been traditionally used to assess tumor response to therapy. However, these anatomic changes lag behind functional changes and may require months to occur. Further, these intra-arterial therapies cause acute tumor necrosis, which may result in a paradoxical increase in tumor size on early follow-up imaging despite complete cell death or necrosis. This concept is unique comparing to changes seen following systemic chemotherapy. The recent development of functional imaging techniques including diffusion-weighted MRI (DW MRI) and positron emission tomography (PET) allow for early assessment of treatment response and even prediction of overall tumor response to intra-arterial therapies. Although the results of DW MRI and PET studies are promising, the impact of these imaging modalities to assess treatment response has been limited without standardized protocols. The aim of this review article is to delineate the best practice for assessing tumor response in patients with primary or secondary hepatic malignancies undergoing intra-arterial therapies.     

Validation of diffusion spectrum magnetic resonance imaging with manganese-enhanced rat optic tracts and ex vivo phantoms

Diffusion spectrum imaging (DSI) has been demonstrated to resolve crossing axonal fibers by mapping the probability density function of water molecules diffusion at each voxel. However, the accuracy of DSI in defining individual fiber orientation and the validity of Fourier relation under finite gradient pulse widths are not assessed yet. We developed an ex vivo and an in vivo model to evaluate the error of DSI with gradient pulse widths being relatively short and long, respectively. The ex vivo model was a phantom comprising sheets of parallel capillaries filled with water. Sheets were stacked on each other with capillaries crossed at 45 degrees or 90 degrees. High-resolution T2-weighted images (T2WI) of the phantom served as a reference for the orientation of intersecting capillaries. In the in vivo model, manganese ions were infused into rats' optic tracts. The optic tracts were enhanced on T1-weighted images (T1WI) and served as a reference for the tract orientation. By comparing DSI with T2WI, the deviation angles between the primary orientation of diffusion spectrum and the 90 degrees and 45 degrees phantoms were 1.19 degrees +/- 4.82 degrees and -0.71 degrees +/- 4.91 degrees, respectively. By comparing DSI with the T1WI of rat optic tracts, the deviation angle between primary orientation of diffusion spectrum and optic tracts was -0.41 degrees +/- 6.18 degrees. In addition, two sequences of DSI using short and long gradient pulses were performed in a rat brain. The bias of the primary orientation between these two sequences was approximately 10 degrees. In conclusion, DSI can resolve crossing fiber orientation accurately. The effect of finite gradient pulse widths on the primary orientation is not critical.     

Detection of the whole myocardium in 2D-echocardiography for multiple orientations using a geometrically constrained level-set

The segmentation of the myocardium in echocardiographic images is an important task for the diagnosis of heart disease. This task is difficult due to the inherent problems of echographic images (i.e. low contrast, speckle noise, signal dropout, presence of shadows). In this article, we propose a method to segment the whole myocardium (endocardial and epicardial contours) in 2D echographic images. This is achieved using a level-set model constrained by a new shape formulation that allows to model both contours. The novelty of this work also lays in the fact that our framework allows to segment the whole myocardium for the four main views used in clinical routine. The method is validated on a dataset of clinical images and compared with expert segmentation.     

Mathematical model of fiber orientation in anisotropic fascia layers at large displacements

A mathematical model is developed to determine the relationship between stretch and the orientation of fibers in the fascia. The transversely isotropic stress- strain relation for large displacements valid for the human fascia reinforced by the collagen fibers is employed. The relation between the orientation of fibers in the un-deformed and deformed state depending upon the stretch is plotted. It is observed that for greater fiber angle orientation, the fibers are more resistant to reorientation as the fascia is stretched longitudinally. It is also concluded that the reinforced fascia will always be in tension as the stretch is applied. However, we suggest future research to resolve the tension and compression issues in fascia.     

Sonographic Interstitial Syndrome

Objective. Ultrasound lung comets (ULCs) now have an acknowledged correlation with extravascular lung water, but they present in different orders and numbers in different pathologic pulmonary entities. How these artifacts are created is not yet known, and the literature gives discordant hypotheses. Understanding their formation is the first step in understanding lung disease. The purpose of this study was to show the morphologic and genetic variability of interstitial lung disease studied with echography and thus to propose a unitary mechanism for the formation of ULCs. Methods. This study included 3 parts: (1) a retrospective analysis of echographic lung images of patients with interstitial syndrome; (2) an analysis of the literature for definitions of the size of the pulmonary lobule; and (3) an experimental model of different air‐water interfaces scanned with varying ultrasonic frequencies. Results. The retrospective analysis of echographic lung images included 176 patients with diffuse ULCs: 118 patients had acute pulmonary edema; 18 had acute lung injury/acute respiratory distress syndrome; and 40 were premature neonates with respiratory distress syndrome. Experimental models permitted us to discover that ring‐down artifacts are produced only by single and double layers of bubbles in specific structural settings. Conclusions. Reverberation between bubbles with a critical radius seems to be at the origin of ring‐down artifacts. Echographic manifestations of interstitial lung disease, whose genesis lies in the partial air loss of lobes and segments, are acoustic phenomena originating from variations in the tissue‐fluid relationship of the lung. A correlation between anatomopathologic characteristics and structures of sonographic artifacts could allow more rapid and noninvasive diagnoses.     

Effect of the size and shape of a red blood cell on elastic light scattering properties at the single-cell level

We demonstrate the use of a double-beam optical tweezers system to stabilize red blood cell (RBC) orientation in the optical tweezers during measurements of elastic light scattering from the trapped cells in an angle range of 5-30 degrees. Another laser (He-Ne) was used to illuminate the cell and elastic light scattering distribution from the single cell was measured with a goniometer and a photomultiplier tube. Moreover, CCD camera images of RBCs with and without laser illumination are presented as complementary information. Light scattering from a RBC was measured in different fixed orientations. Light scattering from cells was also measured when the length of the cell was changed in two different orientations. Light scattering measurements from spherical and crenate RBCs are described and the results are compared with other cell orientations. Analysis shows that the measured elastic light scattering distributions reveal changes in the RBC's orientation and shape. The effect of stretching on the changes in scattering is larger in the case of face-on incidence of He-Ne laser light than in rim-on incidence. The scattering patterns from RBCs in different orientations as well as from a spherical RBC were compared with numerical results found in literature. Good correlation was found.     

健康志愿者肩关节肱二头肌长头肌腱位置、方向及形态学的MRI测量

背景：以往的研究中仅对肱二头肌长头肌腱在肱二头肌腱沟入口偏离度进行定性描述,未进行定量测量.目的：通过分析健康志愿者肩关节中立位、外旋位和内旋位MRI之轴位图像上肱二头肌长头肌腱位置、方向及形态学改变,探讨肱二头肌长头肌腱MRI形态学特征以利临床评价肱二头肌长头肌腱.方法：纳入35名无症状志愿者,在肩关节中立位、外旋位、内旋位进行MR扫描.2名评价者对MR图像进行评价,排除标准为具有肩袖、喙肱韧带、上盂肱韧带、滑车韧带病灶或退行性改变者.一名测量者在轴位3D WATSc序列的上、中、下3个测量层面上对肱二头肌长头肌腱位置、方向及形态进行测量.结果与结论：健康志愿者肩关节中立位、外旋位和内旋位MR轴位图像上肱二头肌长头肌腱改变结果：①肱二头肌长头肌腱位置：中立位肱二头肌长头肌腱在内外方向的偏离度最大.②肱二头肌长头肌腱方向：肱二头肌长头肌腱的方向角度均为锐角,在中、下测量层面上,肩关节3个体位的肱二头肌长头肌腱角度之间的差异具有显著性意义.③形态：在下测量层面上,肩关节3个体位肱二头肌长头肌腱形态的改变具有显著性意义.结果显示健康志愿者肱二头肌长头肌腱位置、方向、形态与肩关节旋转体位具有潜在关联.     

Identifying acoustic scattering sources in normal renal parenchyma from the anisotropy in acoustic properties

Acoustical and histological properties of dog kidney parenchyma are examined in vitro to determine sources of acoustic scattering in the normal kidney. The speed of sound, attenuation, backscatter, effective scatterer size and scattering strength were measured within the frequency range 1-15 MHz and at eight angles of incidence with respect to the predominant nephron orientation. Significant angular dependence, or anisotropy, was observed in backscatter coefficient and scattering strength estimates; attenuation was found to be weakly anisotropic. All three parameters, each measured at 19 degrees C, exhibited values that were maximum for perpendicular incidence and minimum for parallel incidence. Speed of sound and scatterer size estimates were observed to be independent of scanning angle. Comparisons between these data for renal cortex and histological observations suggest that the glomerulus is the principal scatterer at low frequencies, and renal tubules and blood vessels at high frequencies.     

The effect of hemodynamics, vessel wall compliance and hematocrit on ultrasonic Doppler power: an in vitro study.

Previous in vitro studies in rigid tubes under pulsatile flow conditions have reported a lack of a cyclic variation in blood echogenicity that contradicts in vivo results. To investigate whether or not these variations can be attributed to the compliance of the vessel wall, a series of in vitro experiments with compliant tubes, under pulsatile flow conditions, was performed. Two important factors that may affect the Doppler power were investigated: 1. the dependence on hematocrit and 2. the effect of the vessel wall elasticity. In the present study, it is shown that, at the low beat rates, the peak of the mean Doppler power within the flow cycle depends on the vessel wall compliance. When the vessel becomes more compliant, the peak is shifted from the early to the late systole. Additionally, there is a correlation between the power peak and hematocrit that is more evident in compliant vessels. At a higher pulsation rate of 37 beats/min, a different variation is observed. A drop in the power occurs near peak systole in compliant tube experiments and is more pronounced as the vessel becomes more constricted. The observed power drop agrees with previously reported in vivo results, but is not seen in rigid tube experiments. The results of this study suggest that proper interpretation of cyclic variations in Doppler power requires a knowledge of hemodynamic parameters, such as the modulus of elasticity of the vessel wall, propagation velocity or, possibly, the phase angle of input impedance.     

T₂* mapping and B₀ orientation-dependence at 7 T reveal cyto- and myeloarchitecture organization of the human cortex

Ultra-high field MRI (≥ 7 T) has recently shown great sensitivity to depict patterns of tissue microarchitecture. Moreover, recent studies have demonstrated a dependency between T?* and orientation of white matter fibers with respect to the main magnetic field B?. In this study we probed the potential of T?* mapping at 7 T to provide new markers of cortical architecture. We acquired multi-echo measurements at 7 T and mapped T?* over the entire cortex of eight healthy individuals using surface-based analysis. B? dependence was tested by computing the angle θ(z) between the normal of the surface and the direction of B?, then fitting T?*(θ(z)) using model from the literature. Average T?* in the cortex was 32.20 +/- 1.35 ms. Patterns of lower T?* were detected in the sensorimotor, visual and auditory cortices, likely reflecting higher myelin content. Significantly lower T?* was detected in the left hemisphere of the auditory region (p<0.005), suggesting higher myelin content, in accordance with previous investigations. B? orientation dependence was detected in some areas of the cortex, the strongest being in the primary motor cortex (?R?*=4.10 Hz). This study demonstrates that quantitative T?* measures at 7 T MRI can reveal patterns of cytoarchitectural organization of the human cortex in vivo and that B? orientation dependence can probe the coherency and orientation of gray matter fibers in the cortex, shedding light into the potential use of this type of contrast to characterize cyto-/myeloarchitecture and to understand the pathophysiology of diseases associated with changes in iron and/or myelin concentration.     

Muscle fiber orientation in normal and hypertrophied hearts.

Muscle fiber orientation in the left ventricular myocardial layer was histometrically estimated in normal, concentric and eccentric hypertrophied hearts. The angle of inclination of muscle fibers from coronal section was largest in the innermost and outermost zones and was progressively diminished toward the middle layer in all the hearts. In the inner layer, the inclination was depressed in eccentric hypertrophy, while there was no difference in fiber orientation in the outer layer. Concentric hypertrophy proved to be an intermediate stage between the normal heart and eccentric hypertrophy, and no opposite behavior was found in the arrangement of heart muscle fibers. The results supported the following view on the mechanism in the activity of hypertrophied hearts. Cardiac hypertrophy is a common accommodation process to increased load, regardless of the difference in the macroscopical shape. In hypertrophy, the ventricular cavity is inevitably enlarged. Consequently, the tension exerted on the ventricular wall on account of intraventricular pressure increases, and the contractile force required from a unit thickness of the ventricular wall is enhanced. The muscle fibers meet this increased demand by starting contraction from an over-stretched state and by ceasing its activity after smaller shortening than in the normal heart.     

Morphological study of the healthy human oculomotor nerve by atomic force microscopy

This study concerns an investigation of oculomotor nerves via imaging semithin sections with an atomic force microscope (AFM). The study used 10 oculomotor nerves removed from individuals aged between 40 and 60 years old at time of death. We adopted semithin sections for our AFM studies to extract topographical data of the fibers of this nerve. Both light microscope and electron microscope images of the same samples were employed to augment our capabilities for orientation in the AFM analyses. Transverse sections of the oculomotor nerve, imaged via AFM, show clearly identifiable ring-like nerve fibers with the myelinated axons. The samples were simultaneously characterized by error signal mode and lateral force microscopy. These AFM-related techniques revealed no information concerning friction force and elasticity due to the presence of the embedding material (epoxy resin), but showed the myelinated nerve fibers with ring-like structures corresponding to the myelin sheaths. These are the first AFM images of physiological human oculomotor nerves, containing clear three-dimensional representations of these samples.     

Directional Variability of Stimulation Threshold Measurements in Isolated Guinea Pig Cardiomyocytes: Relationship with Orthogonal Sequential Defibrillating Pulses

BARDOU A.L., ET AL.: Directional Variability of Stimulation Threshold Measurements in Isolated Guinea Pig Cardiomyocytes: Relationship with Orthogonal Sequential Defibrillating Pulses. Reports on delivery of separated orthogonal pulses markedly improving cardiac defibrillation have suggested that the stimulation threshold of heart fibers varies in accordance with their orientation within the electric field. The present work was aimed at investigating the directional variability of stimulation thresholds in isolated guinea pig cardiomyocytes. This variability was measured in 48 single myocytes by rotating each one through a theta (θ) angle between two-fixed parallel electrodes 1.1 cm apart, thus making θ vary between the electric field and the myocyte axis. For θ= 0°, the mean longitudinal current stimulation threshold was 16.92 ± 4.20 mA (n = 48). When θ was increased by increments of 10° up to 90°, the stimulation threshold increased in an exponential way. For θ= 90°, the mean transverse stimulation threshold was 63.23 ± 13.30 mA. These results clearly demonstrate the dependence of isolated cardiomyocyte stimulation thresholds on their orientation within the electric field and may account for the improved efficacy of defibrillation previously observed after delivery of orthogonal pulses.     

Ultrasound beam orientation during standard two-dimensional imaging: assessment by three-dimensional echocardiography.

Standard two-dimensional echocardiographic image planes are defined by anatomic landmarks and assumptions regarding their orientation when these landmarks are visualized. However, variations of anatomy and technique may invalidate these assumptions and thus limit reproducibility and accuracy of cardiac dimensions recorded from these views. To overcome this problem, we have developed a three-dimensional echocardiograph consisting of a real-time scanner, three-dimensional spatial locater, and personal computer. This system displays the line of intersection of a real-time image and an orthogonal reference image and may be used to assess actual image orientation during standardized two-dimensional imaging when the line-of-intersection display is not observed by the operator. Three hundred forty standard images were assessed from 85 examinations by 11 echocardiographers. Twenty-four percent of the unguided standard images were optimally positioned within +/- 5 mm and +/- 15 degrees of the standard. Of the optimal images, two thirds were parasternal long-axis views. A subsequent study with three-dimensional echocardiography and line-of-intersection guidance of image positioning showed 80% of the guided images to be optimally positioned, a threefold improvement (p < 0.001). Two-dimensional echocardiography does not achieve reasonably consistent optimal positioning of standard imaging views, suggesting that measurements taken from these views are likely to be suboptimal. Three-dimensional echocardiography that uses line-of-intersection guidance improves image positioning threefold and should therefore improve the accuracy and reproducibility of quantitative echocardiographic measurements derived from these images.     

New assessment of endothelium-dependent flow-mediated vasodilation to characterize endothelium dysfunction

The vascular endothelium plays an important role in the regulation of vascular tone, cell growth, inflammation, and thrombogenicity. Endothelium dysfunction, then, is considered to promote several disorders that initiate the atherosclerosis process. Vascular tone dysfunction can be determined by high-resolution ultrasonographic imaging of the brachial artery, enabling one to assess endothelium-dependent flow-mediated dilation (FMD). It is based on the principle that an increase in blood flow, specifically in shear stress, provokes the release of nitric oxide and then a vasodilation that can be quantified. In this study, brachial artery diameter evolution was continuously followed during baseline and hyperemia after forearm occlusion using a custom designed software. Some techniques used to measure FMD are limited by operator dependence. We present a new, automated, and versatile method of FMD quantification based on B-mode echographic images and edge detection algorithms. Edges for each image in the acquired sequences are recognized as interfaces based on the grey-level profiles of the averaged pixel values. Within-reading and within-subject FMD% coefficients of variation were 7% and 10%, respectively. This technique largely improves manual measurements and was shown to be appropriate for wide clinical use.     

Mapping the orientation of intravoxel crossing fibers based on the phase information of diffusion circular spectrum

A new method is presented to map the orientation of intravoxel crossing fibers by using the phase of the diffusion circular spectrum harmonics. In a previous study [Zhan, W., Gu, H., Xu, S., Silbersweig, D.A., Stern, E., Yang, Y., 2003. Circular spectrum mapping for intravoxel fiber structures based on high angular resolution apparent diffusion coefficients. Magn. Reson. Med. 49, 1077-1088], we demonstrated that the magnitude of the 4th-order harmonic of the diffusion circular spectrum can be used to identify the existence of fiber crossings. However, the orientation of the intravoxel crossing fibers remained unknown. This study extends the diffusion circular spectrum mapping method so that it is able to identify the orientation of the intravoxel crossing fibers by utilizing the phase information of the circular spectrum. In general, the phase of the circular harmonic determines the rotation of the apparent diffusion coefficient (ADC) profile on the sampling circle that is spanned by the major and medium eigenvector of the diffusion tensor and thus can be used to determine the orientation of the crossing fibers. Simulation results show that the regular tensor-based major eigenvector maps have obvious artifacts in the fiber-crossing area, whereas the estimated crossing fibers by the proposed method are much more consistent with the orientation of the actual intravoxel fibers. Diffusion MRI experiments were performed on five healthy human brains using a 3T scanner. The brain regions with fiber crossings were selected by thresholding the magnitudes of the 4th-order circular spectrum map. Intravoxel crossing fibers were estimated by the phase of the 4th-order harmonic for each voxel within these areas. The estimated intravoxel crossing fibers demonstrated a clear consistency with the orientations of fiber tracks in the surrounding tissues, reducing the fiber orientation discontinuity of the regular major eigenvector map.     

High-resolution real-time ultrasonic scanner.

High spatial resolution is required for echographic exploration of the skin, microvessels or small laboratory animals. With the scanner described here, high resolution is obtained by means of a strongly focused, wide-band 17 MHz center frequency transducer (-6 dB bandwidth: 22 MHz). The movement of this transducer above the skin provides a 6 mm wide and 5 mm deep echographic cross-section with an image rate of 15 images/s. The resolution is about 0.08 mm in axial and 0.2 to 0.3 mm in lateral directions. The device was tested on phantoms in water and in vivo on normal and pathological skin in the Department of Dermatology. With the easy-to-handle probe, explorations were made on psoriasis, basocellular carcinoma, malignant melanoma and sarcoidosis.