Showing papers by "Septimiu E. Salcudean published in 2007"

Real-Time Vessel Segmentation and Tracking for Ultrasound Imaging Applications

Abstract: A method for vessel segmentation and tracking in ultrasound images using Kalman filters is presented. A modified Star-Kalman algorithm is used to determine vessel contours and ellipse parameters using an extended Kalman filter with an elliptical model. The parameters can be used to easily calculate the transverse vessel area which is of clinical use. A temporal Kalman filter is used for tracking the vessel center over several frames, using location measurements from a handheld sensorized ultrasound probe. The segmentation and tracking have been implemented in real-time and validated using simulated ultrasound data with known features and real data, for which expert segmentation was performed. Results indicate that mean errors between segmented contours and expert tracings are on the order of 1%-2% of the maximum feature dimension, and that the transverse cross-sectional vessel area as computed from estimated ellipse parameters a, b as determined by our algorithm is within 10% of that determined by experts. The location of the vessel center was tracked accurately for a range of speeds from 1.4 to 11.2 mm/s.

Needle Insertion Point and Orientation Optimization in Non-linear Tissue with Application to Brachytherapy

Abstract: This paper presents a new method of optimizing the needle insertion point, heading and depth for needle insertion into deformable tissue. The goal is to minimize the distance between a number of specified targets and the needle. Assuming a rigid needle and a deformable tissue described by a finite element model, an iterative optimization method is proposed that uses the needle insertion simulation. At each iteration, the best fitted 3D line to the targets in the simulated deformed configuration is used as a candidate for the new insertion line in the next iteration. This method has been implemented in a prostate brachytherapy simulator to minimize seed misplacement errors. The targets are designed to lie on a straight line in the undeformed configuration inside the prostate. To increase the accuracy while simulating the prostate rotation, a non-linear model is used. The neo-Hookean material model is exploited to determine the effects of geometric and mechanical nonlinearities and compressibility effects. It is shown that the optimization algorithm converges in few iterations and decreases the targeting error effectively.

Modeling of needle-tissue interaction using ultrasound-based motion estimation

Abstract: A needle-tissue interaction model is an essential part of every needle insertion simulator. In this paper, a new experimental method for the modeling of needle-tissue interaction is presented. The method consists of measuring needle and tissue displacements with ultrasound, measuring needle base forces, and using a deformation simulation model to identify the parameters of a needle-tissue interaction model. The feasibility of this non-invasive approach was demonstrated in an experiment in which a brachytherapy needle was inserted into a prostate phantom. Ultrasound radio-frequency data and the time-domain cross-correlation method, often used in ultrasound elastography, were used to generate the tissue displacement field during needle insertion. A three-parameter force density model was assumed for the needle-tissue interaction. With the needle displacement, tissue displacement and needle base forces as input data, finite element simulations were carried out to adjust the model parameters to achieve a good fit between simulated and measured data.

Tissue strain imaging using a wavelet transform-based peak search algorithm

Abstract: A new method is proposed to estimate the motion and relative local compression between two successive ultrasound RF signals under different compression states. The algorithm uses the continuous wavelet transform to locate the peaks in the RF signals. The estimated peaks in the pre- and post-compression signals are assigned to each other by a peak matching technique with the goal of minimizing the number of false matches. The method allows local shifts of the tissue to be estimated. The method has been tested in one-dimensional simulations and phantom experiments. The signal-to-noise ratio and the rms error are shown to be better than for the standard cross-correlation method (CC). The new estimator remains unbiased for up to 10% strain which is a larger range than that of CC. The maximum signal-to-noise ratio is 3 times as high as that of the CC method, showing higher sensitivity as well. The method is computationally efficient, achieving 0.7 msec/RF line on a standard personal computer.

Parameter Identification for a Needle-Tissue Interaction Model

Abstract: In this work, needle-tissue interaction forces are modeled by a three parameter force distribution composed of two step functions with variable amplitudes and spacing. A finite element based simulation is used to adjust the parameters and fit the simulation results to the experimental data. In experiments, needle displacements and needle base forces were measured along with tissue displacements. A real-time version of the time-domain cross-correlation method was employed in this study to estimate the tissue displacements from ultrasound radio-frequency data, as done in elastography. In addition to the force model parameters, the elastic parameters of the tissue were adjusted to match the simulated and measured displacements.

P6D-5 Enhancement of Bone Surface Visualization Using Ultrasound Radio-Frequency Signals

Abstract: Detection of bone surfaces in ultrasound images would be useful for ultrasound guided orthopedic surgery, biopsy and brachytherapy. However, bones are often poorly visualized with conventional B-mode ultrasound due to speckle, shadowing, reverberation and other artifacts in tissue. In this paper, we investigate two new techniques for the enhancement of bone surface visualization using ultrasound radio frequency (RF) signals, instead of using conventional B-mode images. The first approach uses strain imaging or elastography, and the second method directly monitors the reflected power of the RF signal. The potential of the proposed methods is demonstrated through phantom and in vivo experiments. Experimental results show that the two methods produce satisfactory contrast between bone surfaces and soft tissue, and are suitable for real-time applications. The good performance of these approaches suggests that they have promise in a clinical setting.

Simulation of Ultrasound Radio-Frequency Signals in Deformed Tissue for Validation of 2D Motion Estimation with Sub-Sample Accuracy

Abstract: Motion estimation in sequences of ultrasound echo signals is essential for a wide range of applications. In time domain cross correlation, which is a common motion estimation technique, the displacements are typically not integral multiples of the sampling period. Therefore, to estimate the motion with sub-sample accuracy, 1D and 2D interpolation methods such as parabolic, cosine, and ellipsoid fitting have been introduced in the literature. In this paper, a simulation framework is presented in order to compare the performance of currently available techniques. First, the tissue deformation is modeled using the finite element method (FEM) and then the corresponding pre-/post-deformation radio-frequency (RF) signals are generated using Field II ultrasound simulation software. Using these simulated RF data of deformation, both axial and lateral tissue motion are estimated with sub-sample accuracy. The estimated displacements are then evaluated by comparing them to the known displacements computed by the FEM. This simulation approach was used to evaluate three different lateral motion estimation techniques employing (i) two separate 1D sub-sampling, (ii) two consecutive 1D sub-sampling, and (Hi) 2D joint sub-sampling estimators. The estimation errors during two different tissue compression tests are presented with and without spatial filtering. Results show that RF signal processing methods involving tissue deformation can be evaluated using the proposed simulation technique, which employs accurate models.

Method and apparatus for processing ultrasound image signals

Abstract: A method and apparatus for processing ultrasonic image signals is disclosed. The method involves receiving a plurality of input sample values representing reflected sound waves in a ultrasonic system, exponentiating each input sample to produce a plurality of respective exponentiated sample values, conditioning the exponentiated sample values to produce conditioned exponentiated sample values for receipt by an imageformer, and causing the imageformer to produce an image representing the ultrasonic sound waves in response to the conditioned exponentiated sample values.

Fast B-Mode Ultrasound Image Simulation of Deformed Tissue

Abstract: This paper presents a fast image synthesis procedure inside elastic volumes under deformation simulated by the finite element method (FEM). Given the node displacements of a mesh and the 3D image voxel data of a volume prior to deformation, the method maps the image pixels, to be synthesized, from the deformed configuration back to the nominal pre-deformed configuration, where the pixel intensities are obtained easily through interpolation in the regular-grid structure of the voxel volume. This mapping requires the identification of the mesh element enclosing each image pixel, in order to use its corresponding shape function for smooth interpolation. To accelerate this point location operation, a fast method of marking the projection of the deformed mesh on the image pixels at every frame is introduced. In order to evaluate our method, a deformable tissue phantom was constructed and its 3D ultrasound volume was acquired in its nominal state. B-mode images of the phantom were then synthesized under the simulated deformation of an ultrasound probe. Results show that realistic B-mode images can be simulated in real-time with the proposed technique, even under large deformations. The technique is also implemented on a real-time system for ultrasound exploration with deformation.

Power strain imaging based on vibro-elastography techniques

Abstract: This paper describes a new ultrasound elastography technique, power strain imaging, based on vibro-elastography (VE) techniques. With this method, tissue is compressed by a vibrating actuator driven by low-pass or band-pass filtered white noise, typically in the 0-20 Hz range. Tissue displacements at different spatial locations are estimated by correlation-based approaches on the raw ultrasound radio frequency signals and recorded in time sequences. The power spectra of these time sequences are computed by Fourier spectral analysis techniques. As the average of the power spectrum is proportional to the squared amplitude of the tissue motion, the square root of the average power over the range of excitation frequencies is used as a measure of the tissue displacement. Then tissue strain is determined by the least squares estimation of the gradient of the displacement field. The computation of the power spectra of the time sequences can be implemented efficiently by using Welch's periodogram method with moving windows or with accumulative windows with a forgetting factor. Compared to the transfer function estimation originally used in VE, the computation of cross spectral densities is not needed, which saves both the memory and computational times. Phantom experiments demonstrate that the proposed method produces stable and operator-independent strain images with high signal-to-noise ratio in real time. This approach has been also tested on a few patient data of the prostate region, and the results are encouraging.

Ultrasound-only dosimetry for prostate brachytherapy: preliminary phantom results

Abstract: Accurate and fast seed localization plays a key role in computing dosimetry for prostate brachytherapy. Because transrectal ultrasound is the primary imaging modality providing the guidance for prostate brachytherapy, an ultrasound-only approach for dosimetry would offer many benefits. In this paper, we propose an ultrasound only dosimetry solution, in which the brachytherapy seeds are located in reflected power images computed from ultrasonic radio frequency signals and the boundary of the prostate is delineated from B-mode TRUS and vibroelastography images as the prostate is stiffer than the surrounding tissue. The location of the implanted seeds relative to the prostate boundary is thus obtained. As only one imaging modality, ultrasound, is used, image registration is easy to implement. A prostate phantom with seeds embedded within it was built to evaluate the proposed approach. To measure the seed localization accuracy in the reflected power images, the phantom was scanned by CT as well. Experimental results show that the implanted seeds can be successfully located in the reflected power images with high contrast and accuracy, and that the contour of the "prostate" can be detected in the ultrasound vibro-elastography images outside the shadow of the seeds.

Real-time synthesis of image slices in deformed tissue from nominal volume images

Abstract: This paper presents a fast image synthesis procedure for elastic volumes under deformation. Given the node displacements of a mesh and the 3D image voxel data of an undeformed volume, the method maps the image plane pixels to be synthesized from the deformed configuration back to the nominal pre-deformed configuration, where the pixel intensities are obtained easily through interpolation in the regular-grid structure of the voxel volume. For smooth interpolation, this mapping requires the identification of the mesh element enclosing each image pixel. To accelerate this point location procedure, a fast method of marking the image pixels is employed by finding the intersection of the mesh and the image, and marking this intersection on the image pixels using Bresenham's line drawing algorithm. A deformable tissue phantom was constructed, it was modeled using the finite element method, and its 3D ultrasound volume was acquired in its undeformed state. Actual B-mode images of the phantom under deformation by the ultrasound probe were then compared with the corresponding synthesized images simulated for the same deformations. Results show that realistic images can be synthesized in real-time using the proposed technique.