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We measure the velocity distribution of slow fluid flow in model systems using stimulated echoes and velocity phase encoding. We show velocity images of slow coherent flow with velocities of the order of 0.1 mm/s which is slower than can be obtained by the bolus-tracking method. The eddy current errors were compensated by phase correction.
Unusually high image contrast in vivo magnetic resonance imaging of the brain becomes observable at high magnetic fields when the blood oxygenation level is lowered. The cause of the contrast has been attributed to a magnetic susceptibility effect induced by paramagnetic deoxyhemoglobin in red cells. When the cylinder axis of a blood vessel is not parallel to the main magnetic field, the susceptibility difference produces varying local fields around the blood vessel. In gradient-echo images, not in spin-echo images, these local fields cause intravoxel dephasing of the water signal of the surrounding tissue. This description of the contrast enhancement has been confirmed by a series of in vitro blood sample experiments and image simulations. A predicted contrast change has been demonstrated in brain images of a mouse placed at two different orientations in the magnet. From the simulated images, the dependence of the contrast on the field strength has been estimated.
One of the most common sources of distortion in in vivo spectroscopy is the inhomogeneity of the main magnetic field. This effect is particularly problematic when performing spectroscopic imaging, as the shim cannot be simultaneously optimized for all voxels. In this paper we present a technique to measure inhomogeneity rapidly, then show how to use the measurement to improve the analysis of the spectrum. This technique can be applied in conjunction with any spectroscopic localization method and any spectral quantitation algorithm. We present results from spectroscopic imaging of phantoms, then show application to a single-voxel water-suppressed proton brain study. We find that the quantitation of the in vivo spectrum is made immune to inhomogeneous line broadening.
Atherosclerotic disease of the renal artery can lead to reduction in arterial caliber and ultimately to conditions including renovascular hypertension. Renal artery stenosis is conventionally assessed, using angiography, according to the severity of the stenosis. However, the severity of a stenosis is not a reliable indicator of functional significance, or associated differential pressure, of a stenosis. A methodology is proposed for estimation of the renal artery differential pressure (RADP) from MR imaging. Realistic computational fluid dynamics (CFD) models are constructed from MR angiography (MRA) and phase-contrast (PC) MR. The CFD model is constructed in a semiautomated manner from the MR images using the Isosurface Deformable Model (IDM) for surface reconstruction and a Marching Front algorithm for construction of the volumetric CFD mesh. Validation of RADP estimation was performed in a realistic physical flow-through model. Under steady flow, the CFD estimate of the differential pressure across a stenosis in the physical flow-through model differed by an average of 5.5 mmHg from transducer measurements of the pressure differential, for differential pressures less than 60 mmHg. These results demonstrate that accurate estimates of differential pressure at stenoses may be possible based only on structural and flow images.
In this study, the relative endplate position of fibers of rat gastrocnemius caput mediale (GM) muscle was determined by counting numbers of sarcomeres. Isolated fibers were teased from the proximal, intermediate, and distal regions of the muscle. Endplates of distal fibers were located on the proximal third of their lengths. Endplates of intermediate fibers were located at half fiber length, and for proximal fibers, a variable endplate position was obtained: in half of the muscles studied, endplates occurred around the proximal one-third and in the other half near the midpoint of the fiber. Endplate position relative to fiber length was thus found to be dependent on the region of the muscle. Changes in the orientation of endplate zone relative to the muscle belly is likely to take place with changes in muscle length, as shown by a planimetric muscle model. It is argued that architecture of pennate muscles may highly affect characteristics of motor unit potentials.
AIMS: We aim at developing a non-invasive method for grading and diagnosing urinary bladder outlet obstruction, based on noise recording with a perineal contact microphone during voiding. We found that the noise production during voiding depends amongst others on the viscoelastic properties of the urethral wall. To further test our method, we need a realistic biophysical model of the male urethra. METHODS: We made various model urethras with different viscoelastic properties from a 10% aqueous solution of polyvinyl alcohol cryogel. We measured the viscoelastic properties of each model and compared them to those of the male pig urethra. The male pig urethra was used, as it is physiologically comparable to the human male urethra. The viscoelastic properties of both model and pig urethras were measured by applying strain to the urethral wall in a stepwise manner and recording the pressure response. We fitted the step-response of a mechanical model to this pressure response and derived the viscoelastic properties from the coefficients of this response. RESULTS: A uniform model urethra that was freeze-thawed three times, with a Y-shaped flow channel was found to best represent the male pig urethra. CONCLUSIONS: We consider the three times freeze-thawed model urethra with a Y-shaped flow channel the best model of the human male urethra. And we therefore use this model urethra for studying the relation between noise recording during urine flow and the degree of bladder outlet obstruction.
AIMS: Using a lumped parameter theoretical model of bladder outlet function, we previously explored the relationship between the tube law of the urethra and the pressure-flow characteristics during micturition [Mijailovich et al., 2004]. To validate this theoretical model, we constructed a mechanical analog of the male lower urinary tract that incorporated elements simulating all relevant parameters of the theoretical model. In addition, we determined the effect of alterations in compliance of the flow-controlling zone on these relationships. METHODS: In the mechanical analog, the bladder neck and urethra were represented by a thin-walled conduit made of latex rubber and the prostate with a fluid filled cuff of variable compliance encircling the conduit. We measured in the same system steady-state pressure-flow (p-Q) and pressure-area (p-A) relationships of the flow-controlling zone. The effects of bladder outlet obstruction and prostatic compliance on these relationships were simulated by varying cuff pressure and capacitative coupling of the cuff, respectively. RESULTS: We demonstrated two previously described flow regimes-critical for low Q, and subcritical for higher Q. In the critical flow regime, the cross-sectional area of the collapsible conduit downstream of the cuff became narrow up to a site where the area suddenly expanded (elastic jump). Pressure losses across the cuff region decreased with increasing Q as the elastic jump approached the cuff, and the jump vanished when Q became subcritical. By altering prostatic cuff compliance and cuff pressure, we showed that an increase in opening pressures was associated with a steeper p-Q relationship. CONCLUSIONS: Using this mechanical analog, the measurements of p-A and p-Q relationships under various experimental conduit conditions validated our previous theoretical model of the male lower urinary tract. Both the experimental measurements and our previous theoretical model predictions indicate that an increase in opening pressure is associated with an increase in the slope of the p-Q relationship which becomes steeper with decreased cuff compliance. These data are in accordance with urodynamic findings in patients with benign prostatic hypertrophy (BPH) and suggest that a reduction in prostatic compliance exacerbates the severity of obstruction.
Considerable practice is necessary to develop the skills essential to successful and safe performance of flexible bronchoscopy, especially in pediatric patients. We review a number of model systems, both mechanical and living, for learning and maturing bronchoscopic skills. For each of the animal models (canine, feline, leporine, and simian), the relevant anatomy and anesthetic techniques as well as the relative advantages and disadvantages of each model are discussed.
Historically, the study of the prostate anatomy has been characterized by a proliferation of contradictory findings. The major divergent views of prostate anatomy are here reviewed and compared in order to facilitate further study and the ultimate selection of the best anatomical model. The details of Lowsley's original concept of the prostate lobes and the subsequent evolution of this concept into several contradictory hypotheses are traced. Discrepancies between the findings of Lowsley, Franks, and McNeal are explained. Conclusions are drawn which may facilitate the further study of anatomy and disease in both the human animal prostate.
BACKGROUND: Previous studies of the tracheoesophageal sulcus and the sulci of the developing heart have suggested that the catenoidal or saddle-shaped configuration of the sulcus had mechanical properties that were important to developmental processes by causing regional growth limitation. We examined the development of the human perineal region to determine if a similar configuration exists in relation to the urorectal septum. We wished to re-examine the controversial issue of the role of the urorectal sulcus in the partitioning of the cloaca. METHODS: Digitally scanned photomicrographs of serial histologic sections of embryos from Carnegie stages 13, 15, 18, and 22, obtained from the Carnegie Embryological Collection were used. Each image was digitally stacked, aligned, and isolated using image-editing software. Images were compiled using 3-D image-visualization software (T-Vox), into full 3-D voxel-based volume renderings. Similarly, digital models were made of the urogenital sinus, anorectum, cloaca, allantois, mesonephric ducts, ureters, and kidneys by isolating their associated epithelium in each histologic section and compiling the data in T-Vox. Methods were developed to create registration models for determining the exact position and orientation of outlined structures within the embryos. RESULTS: Models were oriented and resectioned to determine the configuration of the urorectal sulcus. The results show that the urorectal sulcus maintains a catenoidal configuration during the developmental period studied and, thus, would be expected to limit caudal growth of the urorectal septum. CONCLUSION: The observations support the concept that the urorectal septum is a passive structure that does not actively divide the cloaca into urogenital and anorectal components.
This report describes a technique for producing plastic casts of the cardiovascular and respiratory systems of the chick embryo at Stage 36 or older. For casts of the cardiovascular system, polymerization compound is injected in the right ventricle filling the heart, venous, and arterial system. A two stage injection produced more detailed casts of the pulmonary vasculature. For respiratory system casts, the compound is injected in the trachea filling the bronchi and air sacs. Casts are prepared by tissue corrosion in 10% potassium hydroxide. This technique is an alternative to serial section reconstruction in the study of developing cardiovascular and respiratory system.
A cell kinetic model of proliferating tissues and its implementation on the massively parallel computer, Connection Machine Model 2 (CM-2), is presented. The model is applicable to all proliferating tissues of the organism and is illustrated on the crypt-villus unit (CU) of the gastrointestinal mucosa. The crypt unit consists of two compartments, a progenitor (P) in which cells proliferate, and a proliferative quiescent (Q) compartment, in which cells are incapable of synthesizing DNA and therefore do not divide. Cells are formed in P, enter Q, traverse it, and die at its outer boundary. The healthy CU maintains a steady state; cell proliferation is controlled by cell elimination. The crypt cylinder is mapped onto a two-dimensional CM-2 matrix such that each processor represents a cell in the unit. The interprocessor links serve as proliferation controls. Using cell counts, the frequency of P cells in each cell location along the crypt column was estimated and various columnar kinetic parameters were calculated. The kinetic analysis shows that cell migration occurs along the crypt column in the direction of the colonic lumen at a velocity averaging 0.244 cell locations per hour. In order to compare model results with experimental kinetic studies, the model was used to stimulate a labeling experiment. The results are in agreement with published kinetic experiments that were done on a normal CU. The model allows three-dimensional visualization of the spatial and temporal events within the crypt.
A 3D cellular anisotropic automata model with modifiable geometry is described. The modeling parameters include grain size, fiber orientation, and free-wall and septal thickness. From this modifiable model, three specific models corresponding to normal heart, left ventricular hypertrophy, and ventricular dilatation were generated. Each model is a conduction and propagation model in which the atria, the major atrial vessel bases, the ventricles, and the specialized conduction system are represented. Muscle tissues are modeled as bundles of fibers with anisotropic conduction speed of the activation wavefronts. Regional variations of conduction, refractory gradients, and regional potential gradients can also be specified before each simulation. Each element has adaptive properties with respect to cycle length and to the prematurity of incoming impulses. Action potentials can be specified for each cell and an equivalent source formulation is carried out to simulate the vectorcardiogram and the corresponding 12-standard-lead electrocardiogram.
In two-dimensional echocardiography the sonographer must synthesize multiple tomographic slices into a mental three-dimensional (3D) model of the heart. Computer graphics and virtual reality environments are ideal to visualize complex 3D spatial relationships. In augmented reality (AR) applications, real and virtual image data are linked, to increase the information content. In the presented AR simulator a 3D surface model of the human heart is linked with echocardiographic volume data sets. The 3D echocardiographic data sets are registered with the heart model to establish spatial and temporal congruence. The heart model, together with an animated ultrasound sector represents a reference scenario, which displays the currently selected two-dimensional echocardiographic cutting plane calculated from the volume data set. Modifications of the cutting plane within the echocardiographic data are transferred and visualized simultaneously and in real time within the reference scenario. The trainee can interactively explore the 3D heart model and the registered 3D echocardiographic data sets by an animated ultrasound probe, whose position is controlled by an electromagnetic tracking system. The tracking system is attached to a dummy transducer and placed on a plastic puppet to give a realistic impression of a two-dimensional echocardiographic examination.
The 3-D ventricle model in this study was reconstructed from a series of MRI torso cross-section data. We used a 3-D voxel array to represent the ventricle. As in cardiac simulations proposed by previous studies, the activation sequence and body surface ECG were simulated in this model. But to reduce the amount of elements in the model, so that the amount of parameters in the model can be handled numerically, we propose another approach to simulate cardiac activity. A mesh model was constructed on the closed surface formed by epicardiac and endocardiac surfaces of the ventricle. We propose a method to simulate the activation sequence on the epicardiac and endocardiac surfaces of the mesh model. As with the uniform double layer theorem, body surface ECG can be estimated in terms of epicardiac and endocardiac surface current source. Consequently, we can also generate ECG waveforms corresponding to this mesh simulation. Both the depolarization sequence and ECG simulated by the mesh model resemble those generated by the 3-D voxel model. However, the mesh model greatly simplified the process of ECG simulation. Both the simulation of depolarization and ECG estimation were expressed in terms of clear and simple mathematical representations. Consequently, we can analytically investigate the effects of the mesh model's parameters on the cardiac activation sequence and ECG. It could be a useful tool to numerically study the relation of ECG waveforms and electrical activity of the heart.
Reconstructing three-dimensional (3-D) shapes of structures like internal organs from tomographic data is an important problem in medical imaging. Various forms of the deformable surface model have been proposed to tackle it, but they are either computationally expensive or limited to tubular shapes. In this paper a 3-D reconstruction mechanism that requires only 2-D deformations is proposed. Advantages of the proposed model include that it is conformable to any 3-D shape, efficient, and highly parallelizable. Most importantly, it requires from the user an initial 2-D contour on only one of the tomograph slices to start with. Experimental results are shown to illustrate the performance of the model.
Sensory afferents in the leech are labeled with both constitutive and developmentally regulated glycosylations (markers) of their cell adhesion molecules (CAMs). Their constitutive mannose marker, recognized by Lan3-2 monoclonal antibody (mAb), mediates the formation of their diffuse central arbors. We show that, at the ultrastructural level, these arbors consist of large, loosely organized axons rich with filopodia and synaptic vesicles. Perturbing the mannose-specific adhesion of this first targeting step leads to a gain in cell-cell contact but a loss of filopodia and synaptic vesicles. During the second targeting step, galactose markers divide afferents into different subsets. We focus on the subset labeled by the marker recognized by Laz2-369 mAb. Initially, the galactose marker appears where afferents contact central neurons. Subsequently it spreads proximally and distally, covering the entire afferent surface. Afferents now gain cell-cell contact, with central neurons and self-similar afferents, but lose filopodia and synaptic vesicles. Extant synaptic vesicles prevail where afferents are apposed to central neurons. These neurons develop postsynaptic densities and en passant synapses are forming. Perturbing the galactose-specific adhesion of this second targeting step causes a loss of cell-cell contact but a gain in filopodia and synaptic vesicles, essentially returning afferents to the first targeting step. The transformation of afferent growth, progressing from mannose- to galactose-specific adhesion, is consistent with a change from cell-matrix to cell-cell adhesion. By performing opposing functions in a temporal sequence, constitutive and developmentally regulated glycosylations of CAMs collaborate in the synaptogenesis of afferents and the consolidation of self-similar afferents.
Regional delivery of suitable drugs by intra-arterial infusion may offer a therapeutic advantage. High concentrations in the tumor are sought with reduced systemic toxicity. Adequate mixing of drug solutions with perfusing blood is essential to provide uniform distribution of drug to tumor-bearing tissue distal to the infusion site. Using a glass model of the iliofemoral and pelvic arteries, we have demonstrated that a streaming phenomenon occurs. Laminar "streamers" of slowly infused drug solution originate at the catheter tip and proceed nonuniformly into distal arterial branches. The intensity of streaming and the pattern of distribution are highly sensitive to catheter tip placement and quite unpredictable. The consequence of regional therapy under streaming conditions is severe maldistribution of drug in the infused tissues with potentially high levels delivered to normal tissues and simultaneous subtherapeutic levels delivered to tumor. Our in vitro model can be used to test appropriate infusion techniques that enhance mixing such as pulsed infusions and novel catheter designs.