C. Robert Appledorn

Simulation of photoacoustic signal production in human breast phantoms at 1064 nm

Photoacoustic signals generated by breasts irradiated with short microwave, infrared or optical pulses could be used to detect breast cancer. Since radiation at this spectrum is non-ionizing, the photoacoustic approach provides a special safety feature. The purpose of the paper is to present a means to predict photoacoustic pressure signals for different breast phantoms and irradiation conditions. The photoacoustic wave equation was derived for linear, non-viscous liquid media. The equation was solved assuming uniform acoustic properties in an infinite medium. Compressed breast phantoms were used as the objects of simulation. The spatial dependence of electromagnetic energy absorption was given by another research paper of this conference. The time dependence of the absorption was assumed to be either uniform or bell- shaped. Photoacoustic pressure signals received by transducers at different locations were calculated numerically.

Energy deposition patterns in the breast at 1064 nm for photoacoustic ultrasound

The simulation of energy deposition within the compressed human breast following its illumination with a short duration pulse of near-infrared light is examined. Different scattering and absorption conditions are studied: homogeneous scattering with homogeneous absorption, homogeneous scattering with heterogeneous absorption (i.e., the introduction of an abnormality), and heterogeneous scattering with homogeneous absorption. Some of the results were used in a companion paper for the simulation of photoacoustic ultrasonic waves resulting from the quick absorption of energy by a region exhibiting increased differential absorption over that of immediately adjacent areas. A method for simulating heterogenous scattering properties is introduced. It is observed that changes in the scattering coefficient within a region do not influence the absorption patterns of the region.

Statistical Methods in Pattern Recognition

When measurements group together and begin to form clusters in some measurement feature space, one tends to remark that a pattern is developing. Furthermore, the size and shape of the pattern can be provided a statistical description. In a variety of applications, one is faced with the problem of using these statistical descriptions to classify a particular measurement to a specific cluster; that is, to make a decision regarding which pattern group generated the measurement.

Microwave applicators for photoacoustic ultrasonagraphy

This investigation describes the design and performance of two, water-immersed, microwave applicators (433 MHz) for use with photoacoustic ultrasonography (PAUS). A cylindrical, open-aperture waveguide was chosen because it could be integrated easily into our PAUS instrumentation. The microwave flux distributions for two microwave applicators were measured using a specially-constructed temperature probe, consisting of an optical fiber with a temperature-sensitive phosphor that was placed inside a thin, polyethylene tube filled with 0.5 M saline. Using this instrumentation, we mapped the microwave flux distribution for each applicator. The physical characteristics of these applicators are discussed.

Simulation of photon paths through isotropically scattering media

A method is presented for simulating the paths traced by photons traveling through a homogenous isotropically scattering medium. An initial photon path is defined with a string of characters that represent the most direct path between a source pixel and a detection pixel. This string then is expanded systematically in unit increments using well-defined rules. In two dimensions using a hexagonal lattice, these rules assume that only six possible directions of scatter are allowed. In three dimensions using a Cartesian lattice, there also are only six directions. The method is tractable and lends itself to computational implementation. Because the method is deterministic, it is more efficient than Monte Carlo methods when investigating paths between specific source and detection pixels. When boundaries are imposed on an object, it is possible to investigate millions of photon paths of a given specified length and to determine probabilistically the pixels visited by the photons within the object.

Time-resolved optical diffusion tomography

A mathematical model is proposed describing time-resolved output measurements obtained on the surface of a diffusely scattering body due to an input pulse of near-IR light at a different location also on the surface. Such measurements can be obtained using a pulsed near-IR laser coupled with a CCD streak camera. The scattering body is assumed to exhibit homogenous scattering and spatially varying absorption. Using this model, an iterative algorithm is derived using maximum likelihood methods that allows the reconstruction of the spatial absorption pattern from a set of time-resolved tomographic measurements. The methodology places no restrictions upon the time-of-arrival of the detected photons, thus permitting the entire time-resolved signal to be used in the reconstruction process. The reconstruction algorithm is easily initialized and preliminary results indicate that stable reconstructions can be performed.

Initial experience with a radiology imaging network to newborn and intensive care units

A digital image network has been installed in the James Whitcomb Riley Hospital for Children on the Indiana University Medical Center to create a limited all digital imaging system. The total system is composed of commercial components, Philips/AT&T CommView system, and connects an existing Philips Computed Radiology (PCR) system to viewing workstations located in the intensive care unit and the new born nursery. The purpose and design of the system is to input the portable chest images from the PCR system, and to display these images at the remote workstations on high resolution monitors for direct viewing by referring clinicians, thus eliminating some of their visits to the radiology department three floors away. The design criteria includes the ability to centrally control all image management functions on the remote workstations to relieve the clinicians from any image management tasks except for recalling patient images. The principal components of the system are the Philips PCR system, the acquisition module (AM), and the PCR interface to the data management module (DMM). Connected to the DMM are a display workstation (DW), a optical disk drive, and a fiber optic to ethernet gateway. The ethernet link is the network connection to the two results viewing stations (RVS) located approximately 100 meters from the DMM. The DMM acts as an image file server and image archive device. The DMM manages the image database and can load images to both the DWs and the two RVSs. The system has met the initial design specifications and can successfully capture images from the PCR and direct them to the RVSs. Additional studies are beginning to determine the optimal image management procedures such as when to archive and purge images from the DMM.

Error-Free Data Compression Methods Applicable to PACS

Digital imaging procedures are characterized by the large volumes of data that can be produced. With routine nuclear medicine studies, a single patient procedure will quickly exceed a one megabyte (Mbyte) storage requirement. Even with the introduction of economical large capacity disk drives allowing for on-line storage, these data continue to pose a significant impediment to the development of PACS (picture archiving and communication) systems. This is because the required storage (space) becomes too large for the available storage and, more significantly, the transmission rate (time) between locations can become too slow to be acceptable in a clinical environment.