Comparison of septal defects in 2D and 3D echocardiography using active contour models.

Three-dimensional ultrasound is emerging as a viable resource for the imaging of internal organs. Quantitative studies correlating ultrasonic volume measurements with MRI data continue to validate this modality as a more efficient alternative for 3D imaging studies. However, the processing required to form 3D images from a set of 2D images may result in a loss of spatial resolution and may give rise to artifacts. This paper examines a method of automatic feature extraction and data quantification in 3D data sets as compared with original 2D data. This work will implement an active contour algorithm to automatically extract the endocardial borders of septal defects in echocardiographic images, and compare the size of the defects in the original 2D images and the 3D data sets.

A computer-based statistical pattern recognition for Doppler spectral waveforms of intracranial blood flow.

A computer-based statistical pattern recognition system has been developed for the analysis of transcranial Doppler (TCD) spectral waveforms of the intracranial middle cerebral artery with varying degrees of increased intracranial pressure. This system extracts multidimensional features from TCD waveforms and performs a cluster analysis of those features. The system can automatically recognize the pattern of spectral waveform and classify it as a normal, abnormal, or borderline subclass of TCD spectral waveform. An optimum decision function was generated based on the Bayes Gaussian classifier. The accuracy of the Bayes Gaussian model the spectral waveforms reaches 100% by estimating posterior probability and using the resubstituting method of estimating misclassification in the training TCD data.

Resolution limitations in intravascular ultrasound imaging.

Interest in the use of ultrasound to characterize the structure and composition of blood vessel walls has risen dramatically as a result of the development of intravascular ultrasonic imaging transducers mounted on the tips of small-diameter catheters. A study of the resolution of these transducers is needed to understand the limitations in the visualization of these structures. Theoretic and experimental studies of the resolution of the two principal designs of intravascular ultrasonic transducers, the mechanically scanned single element and the multielement circular array, were carried out. Comparisons of the two designs reveal that they have similar resolutions. However, the resolutions in two of the three dimensions are shown to decrease linearly with increasing radial distance. Significant errors in image interpretation, particularly in larger diameter vessels, will result if this variation in resolution is not accounted for.

Computationally efficient sound field calculations for a circular array transducer.

A computationally efficient method is presented for calculating field pressure distributions from a circular phased array transducer. This method employs a form of the rectangular radiator approach modified for use with the geometry of a circular array. The curved surface of the elements, radiating either continuous wave or pulsed excitation signals, is divided into incremental rectangular areas small enough so that the Fraunhofer approximation can be applied. Once the directivity of a single element is found, the array beam pattern can be calculated using superposition and suitable coordinate transformations. The validity of this approach is verified through comparisons with experimental data from a circular phased array. The results show that the location and amplitude of the grating lobes and main lobe width can be predicted with reasonable accuracy by using this method.

Sound field calculations for an ultrasonic linear phased array with a spherical liquid lens.

Lenses are often used to provide focusing in the elevation dimension of ultrasonic linear phased-array transducers. The use of a liquid lens in this application adds a variable geometric focusing capability, determined by the radius of curvature of the lens surface and speed of sound in the liquid, to the electronic focusing produced by the linear phased array. An efficient method to calculate the sound field radiated from the linear phased-array transducer through the liquid lens is presented. It treats the lens surface as a secondary source distribution according to Huygens's principle, and employs a modified form of the rectangular radiator method to calculate the field. The appropriate phases for the array elements to focus and steer the beam are calculated by considering the refraction on the lens surface. Comparisons of computer simulations and experimental measurements of the field intensity distribution of a prototype linear array transducer with a liquid lens demonstrate the accuracy of the proposed method.

Ultrasonic phased arrays with variable geometric focusing for hyperthermia applications.

An ultrasonic applicator, which utilizes both electronic and variable geometric focusing, for deep-localized hyperthermia is investigated. The applicator is based around a linear phased array that furnishes its electronic focusing capability. The output of the array radiates through a spherical liquid-lens that provides the applicator a variable geometric focusing capability as well. A lens of this type adds dynamic focusing to the elevation dimension of the linear phased array. By controlling the volume of liquid in the lens (and thus the radius of curvature of its membrane), dynamic control of the geometrical focus can be achieved. Comparisons of computer simulations and experimental measurements of the field intensity distribution of a small-scale prototype applicator are presented. Important design parameters, such as the choice of the liquid for the lens and the size and number of array elements, are examined.

Ultrasonic phased array controller for hyperthermia applications.

Multiple and mechanically scanned ultrasound transducer systems have demonstrated the efficacy of using ultrasound to produce deep localized hyperthermia. The use of ultrasonic phased arrays has been proposed as an alternative to these systems. A phased array offers a more flexible approach to heating tumours in that the size, shape, and position of its focal region can be altered during the course of treatment in order to achieve the desired temperature distribution. This added flexibility comes at the cost of increased complexity of the hardware necessary to drive the transducer because each element requires its own amplifer with both phase and amplitude control. In order for phased arrays with large numbers of elements to be feasible for hyperthermia applications, the complexity of this circuitry must be minimized. This paper describes a circuit design which simplifies the electronics required to control a phased array transducer system for hyperthermia applications. The design is capable of controlling virtually any type of phased array transducer operating at frequencies less than 2 MHz. The system performance was verified through beam profile measurements using a 48-element tapered phased array transducer.

A perfused tissue phantom, for ultrasound hyperthermia.

A perfused tissue phantom, developed as a tool for analyzing the performance of ultrasound hyperthermia applicators, was investigated. The phantom, consisting of a fixed porcine kidney with thermocouples placed throughout the tissue, was perfused with degassed water by a variable flow rate pump. The phantom was insonated by an unfocused multielement ultrasound applicator and the temperatures in the phantom were recorded. The results indicate that for testing protocols where tissue phantoms are needed, the fixed kidney preparation offers an opportunity to use a more realistic phantom than has previously been available to assess the heating performance of ultrasound hyperthermia applicators.

Analysis of a multielement ultrasound hyperthermia applicator.

An unfocused multielement ultrasound applicator was developed for hyperthermia treatment of superficial tumors. The applicator contains sixteen 3.8-cm(2) individually controllable elements on a 15.2-cm(2) piezoelectric ceramic plate. The acoustical power output of each element can be independently applied to facilitate uniform heating throughout the treatment area while minimizing undesired heating in normal tissues. The performance of the applicator was examined by measuring acoustical power output and beam profiles. The results of this analysis indicated that the applicator is capable of producing required therapeutic output levels with excellent localization and control of the power deposition.