Pingyu Liu

Photoacoustic ultrasound: Pulse production and detection in 0.5% Liposyn

Theoretical predictions and experimental measurements of photoacoustic pulse production within a 0.5% solution of Liposyn, a highly scattering, optical propagation medium, are reported. A simple model for photoacoustic energetics is developed that predicts photoacoustic signal pressure as a function of depth within a turbid medium following surface irradiation from an infrared source. The model is valid for very short irradiation duration. The model predicts that the acoustic pressure produced at a distance r from the center of a small, highly absorbing sphere of radius R consists of two, opposite polarity pulses, one originating from the near and one from the far side of the sphere. The magnitude of these biphasic pulses is expected to be proportional to the energy fluence (E) incident on the surface of the sphere and to the ratio, R/r. Furthermore, the energy fluence (E) that reaches the sphere is roughly proportional to e-mu effZ, where mu eff is the effective attenuation coefficient of the turbid medium and Z is the depth of the embedded sphere below the irradiated surface. The variation of E with depth within the absorber and biphasic acoustic pulse production have been verified experimentally. Further experiments demonstrate that a small (3-mm diameter), highly absorbing sphere can be detected and localized at a depth of 37.5 mm within a 0.5% solution of Liposyn with a spatial resolution of 1 x 6 mm2, using a biologically safe level of infrared irradiation (lambda = 1064 nm) and a conventional ultrasound transducer (frequency = 2.25 MHz). These results suggest that photoacoustic ultrasound imaging may have application to biologic systems such as the human breast.

Digital Angiography Using a Matched Filter

Both digital subtraction and recursive filtering schemes have been employed successfully for intravenous and intraarterial arteriography. Either processing method results in an image(s), S, which is a linear combination of discrete images Ij acquired during the flow of iodinated contrast material, i.e., S = 1; ¿Nj=0kjlj, j = 0 where kj are the weighting coefficients for the N+1 samples. It is shown that for a given set of images {Ij} there exists a set of weighting coefficients {kj} which maximizes the iodine signal to noise ratio and simultaneously removes stationary background anatomy. The kj are related to the contrast dilution curve measured over an artery of interest, kj = s[j]-s¿, where {s[j]} is the set of measured image variations due to the flow of contrast material, and s¿, is the mean value of the s[j]. This choice of kj defines a matched filter. Compared to subtraction angiography, matched filtering is 4-6 times more dose efficient.

Photoacoustic ultrasound: theory

Localizing optical absorption within biologic tissue is compromised by the ubiquitous scattering of light that takes place within such tissues. As an alternative to purely optical detection schemes, regional absorption of optical radiation can be detected and localized within highly scattering tissues by detecting the acoustic waves that are produced whenever differential absorption of radiation takes place within such tissues. When the source of optical radiation is delivered in pulses of <EQ 1 microsecond(s) ec duration, the acoustic waves that are produced lie in the medical ultrasonic frequency range, and can be localized using conventional ultrasonic transducers and reconstruction methodology. Localizing such acoustic waves is not adversely affected by optical scattering. This paper introduces a simplistic theory of acoustic wave production within turbid media. The relationships among the irradiating optical pulse power, regional absorption, and strength of acoustic wave production are developed. Estimates of contrast and spatial resolution are presented, assuming a conventional, focused ultrasound transducer and translational scanning are used. Initial theoretical work indicates that optical absorption can be localized with millimeter spatial resolution for 10% absorption or less in biologic tissues as thick as 6.0 cm using safe levels of optical radiation.

Semianalytical approach to pulsed light propagation in scattering and absorbing media

A new approach to solve the radiative transport equation for time-resolved spectroscopy is presented. A new phase function that shows much better agreement with Mie theory than Henyey-Greensteins phase function is introduced. Initial laser beams are properly modeled. For every small time increment, precise and analytical solutions are found to satisfy the radiative transport equation for the uniform field, wide beam, and narrow beam. Computer simulations give promising results. Different conditions of initial beams, media, and medium abnormalities are discussed. A strategy for using the semianalytical solutions to reconstruct the regional distribution of the scattering attenuation coefficient and future work are described.

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.

Photoacoustic ultrasound: experimental results

Recent theoretical calculations by our group (2134-14) indicate that regional optical absorption of radiation within highly scattering media, such as biologic tissue, can be localized by detecting photo-acoustic waves that are produced during regional, optical absorption. This paper reports our initial experimental verification that measurable ultrasonic waves are produced when differential optical absorption takes place within turbid media simulating biologic tissue. For these experiments, an aquarium filled with a 0.5% intralipid solution was used to simulate the scattering properties of biologic tissue. Regional, optical absorption was produced by suspending black, latex spheres (3 - 10 mm diameter) within the intralipid bath. A broadband, xenon flash lamp (1 microsecond(s) ec rise time) was used for one set of experiments and a Nd:YAG laser ((lambda) equals 1064 nm, pulse width < 10 ns) was used for another set. A variety of focused, ultrasound transducers (0.5 - 2.5 MHz) were used successfully to detect and localize photo-acoustic waves. Lateral scanning of the transducers was used to localize the position of the absorption cells with a spatial resolution approximately 1 mm.

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.

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.