Clinical optoacoustic imaging combined with ultrasound for coregistered functional and anatomical mapping of breast tumors.

Optoacoustic imaging, based on the differences in optical contrast of blood hemoglobin and oxyhemoglobin, is uniquely suited for the detection of breast vasculature and tumor microvasculature with the inherent capability to differentiate hypoxic from the normally oxygenated tissue. We describe technological details of the clinical ultrasound (US) system with optoacoustic (OA) imaging capabilities developed specifically for diagnostic imaging of breast cancer. The combined OA/US system provides co-registered and fused images of breast morphology based upon gray scale US with the functional parameters of total hemoglobin and blood oxygen saturation in the tumor angiogenesis related microvasculature based upon OA images. The system component that enabled clinical utility of functional OA imaging is the hand-held probe that utilizes a linear array of ultrasonic transducers sensitive within an ultrawide-band of acoustic frequencies from 0.1 MHz to 12 MHz when loaded to the high-impedance input of the low-noise analog preamplifier. The fiberoptic light delivery system integrated into a dual modality probe through a patented design allowed acquisition of OA images while minimizing typical artefacts associated with pulsed laser illumination of skin and the probe components in the US detection path. We report technical advances of the OA/US imaging system that enabled its demonstrated clinical viability. The prototype system performance was validated in well-defined tissue phantoms. Then a commercial prototype system named Imagio™ was produced and tested in a multicenter clinical trial termed PIONEER. We present examples of clinical images which demonstrate that the spatio-temporal co-registration of functional and anatomical images permit radiological assessment of the vascular pattern around tumors, microvascular density of tumors as well as the relative values of the total hemoglobin [tHb] and blood oxygen saturation [sO2] in tumors relative to adjacent normal breast tissues. The co-registration technology enables increased accuracy of radiologist assessment of malignancy by confirming, upgrading and/or downgrading US categorization of breast tumors according to Breast Imaging Reporting And Data System (BI-RADS). Microscopic histologic examinations on the biopsied tissue of the imaged tumors served as a gold standard in verifying the functional and anatomic interpretations of the OA/US image feature analysis.

In vivo optoacoustic temperature imaging for image-guided cryotherapy of prostate cancer.

The objective of this study is to demonstrate in vivo the feasibility of optoacoustic temperature imaging during cryotherapy of prostate cancer. We developed a preclinical prototype optoacoustic temperature imager that included pulsed optical excitation at a wavelength of 805 nm, a modified clinical transrectal ultrasound probe, a parallel data acquisition system, image processing and visualization software. Cryotherapy of a canine prostate was performed in vivo using a commercial clinical system, Cryocare® CS, with an integrated ultrasound imaging. The universal temperature-dependent optoacoustic response of blood was employed to convert reconstructed optoacoustic images to temperature maps. Optoacoustic imaging of temperature during prostate cryotherapy was performed in the longitudinal view over a region of 30 mm (long) × 10 mm (deep) that covered the rectum, the Denonvilliers fascia, and the posterior portion of the treated gland. The transrectal optoacoustic images showed high-contrast vascularized regions, which were used for quantitative estimation of local temperature profiles. The constructed temperature maps and their temporal dynamics were consistent with the arrangement of the cryoprobe and readouts of the thermal needle sensors. The temporal profiles of the readouts from the thermal needle sensors and the temporal profile estimated from the normalized optoacoustic intensity of the selected vascularized region showed significant resemblance, except for the initial overshoot, that may be explained as a result of the physiological thermoregulatory compensation. The temperature was mapped with errors not exceeding ±2 °C (standard deviation) consistent with the clinical requirements for monitoring cryotherapy of the prostate. In vivo results showed that the optoacoustic temperature imaging is a promising non-invasive technique for real-time imaging of tissue temperature during cryotherapy of prostate cancer, which can be combined with transrectal ultrasound-the current standard for guiding clinical cryotherapy procedure.

Three-Dimensional Optoacoustic and Laser-Induced Ultrasound Tomography System for Preclinical Research in Mice: Design and Phantom Validation.

In this work, we introduce a novel three-dimensional imaging system for in vivo high-resolution anatomical and functional whole-body visualization of small animal models developed for preclinical and other type of biomedical research. The system (LOUIS-3DM) combines a multiwavelength optoacoustic tomography (OAT) and laser-induced ultrasound tomography (LUT) to obtain coregistered maps of tissue optical absorption and speed of sound, displayed within the skin outline of the studied animal. The most promising applications of the LOUIS-3DM include 3D angiography, cancer research, and longitudinal studies of biological distributions of optoacoustic contrast agents.

Measurement of tissue optical properties by time-resolved detection of laser-induced transient stress.

We report on a technique utilizing time-resolved detection of laser-induced stress transients for the measurement of optical properties in turbid media specifically suitable for biological tissues. The method was tested initially in nonscattering absorbing media so that it could be compared with spectrophotometry. The basis of this method is provided by the conditions of temporal stress confinement in the irradiated volume where the pressure generated in tissues heated instantly by laser pulses is proportional to the absorbed laser energy density, and the exponential profile of the initial stress distribution in the irradiated volume corresponds to the z-axial distribution of the absorbed laser fluence. Planar thermoelastic waves can propagate in water-containing media with minimal distortion, and their axial profiles can be detected by an acoustic transducer with sufficient temporal resolution. The acoustic waves induced by 14-ns laser pulses in nonscattering media, turbid gels, and tissues were measured by a piezoelectric transducer with a 3-ns response time. Temporal profiles of stress transients yielded z-axial distributions of the absorbed laser energy in turbid and opaque media, provided that the speed of sound in these media was known. The absorption and effective scattering coefficients of beef liver, dog prostate, and human aortic atheroma at three wavelengths, 1064 nm (in near infrared), 532 nm (visible), and 355 nm (near UV), were deduced from laser-induced stress profiles with additional measurements of total diffuse reflectance.

Pulsed laser ablation of soft tissues, gels, and aqueous solutions at temperatures below 100 degrees C.

BACKGROUND AND OBJECTIVE: It is desirable for laser microsurgical procedures to remove tissue accurately and with minimal thermal and mechanical damage to adjacent non-irradiated tissues. Pulsed laser ablation can potentially remove biological tissue with microprecision if appropriate irradiation conditions are applied. The major goal of this study was to determine whether laser ablation is possible at temperatures below 100 degrees C. Another aim was to test thermoelastic and recoil stress magnitudes and to estimate their effects on phantom and biological tissue. STUDY DESIGN/MATERIALS AND METHODS: Pulsed laser ablation of water (aqueous solution of potassium chromate) and water containing soft tissues (collagen gel and pig liver) irradiated under confined stress conditions was studied. The ablation mechanism and stages of the ablation process were determined based on time-resolved measurements of laser-induced acoustic waves with simultaneous imaging of the ablation process by laser-flash photography. RESULTS: This study reveals the important role of tensile thermoelastic stress, which produces efficient cavitation that drives material ejection at temperatures substantially below 100 degrees C. Ablation thresholds for the aqueous solution, collagen gel, and liver were 20, 38, and 55 J/cm3, respectively, which correspond to temperature jumps of 5, 10, and 15 degrees C. Two distinct stages of material ejection were observed: (1) initial removal of small volumes of material due to the rupture of single subsurface bubbles, (2) bulk material ablation in the form of jets produced by intense hydrodynamic motions formed upon collapse of large bubbles after coalescence of smaller bubbles. The duration of material ejection upon short-pulse ablation may vary from microseconds to submilliseconds, and depended on the mechanical properties of materials and the incident laser fluence. CONCLUSION: Nanosecond laser ablation of water, gels, and soft tissue under confined-stress conditions of irradiation may occur at temperatures below 100 degrees C. This ablation regime minimizes thermal injury to adjacent tissues and involves thermoelastic stress and recoil pressure magnitudes, which may be tolerated by tissues adjacent to an ablated crater.

XeCl laser ablation of atherosclerotic aorta: luminescence spectroscopy of ablation products.

XeCl laser ablation of atherosclerotic aorta tissue was investigated. Luminescence spectra of ablation products were measured for soft and hard arterial tissues. A pronounced difference observed between plume luminescence for various plaques and normal vessel wall correlates with the chemical composition of the tissue. The mechanism of plume luminescence appeared to be thermochemical excitation ablation products (particles, atoms, molecules, etc.) in the air. The process of soft tissue ablation was delayed with respect to the exciting laser pulse at relatively low laser fluences close to the ablation threshold. The kinetics of the ablation process as a function of laser-pulse energy fluence is reported. The data indicate that tissue ejection mechanism, which involves vapor bubbles formation, expansion and explosion, is suitable for the description of the XeCl excimer laser ablation of soft tissues.

Studies of acoustical and shock waves in the pulsed laser ablation of biotissue.

Quantitative studies are conducted into the absolute pressure values of the acoustical and shock waves generated and propagating in a biotissue under pulsed (tau p = 50 ns) UV (lambda = 308 nm) laser irradiation (below and above the ablation threshold). Powerful (several hundreds of bars in pressure) high-frequency (f approximately 10(7) Hz) acoustic compression and rarefaction pulses are found to be generated in the biotissue. The amplitudes and profiles of the acoustic pulses developing in atherosclerotic human aorta tissues and an aqueous CuCl2 solution under laser irradiation are investigated as a function of the laser pulse energy fluence. The results obtained point to the absence of the cold spallation of the objects of study by rarefaction waves. Based on experimental data, the rise rates, pressure gradients, and propagation velocities of shock waves in the biotissue are calculated. The experimental data are found to agree well with the theoretical estimates.

XeCl laser-induced fluorescence of atherosclerotic arteries. Spectral similarities between lipid-rich lesions and peroxidized lipoproteins.

Autofluorescence spectroscopy of arterial surfaces provides information about the distribution and composition of atherosclerotic plaques. The aim of the study was to determine whether accumulation of peroxidized lipoproteins in arterial walls, a process postulated to play a role in initiating atherosclerotic changes, can be demonstrated by fluorescence spectroscopy. XeCl excimer laser (308 nm)-induced fluorescence of human aortas containing early lipid-rich noncollagenous lesions exhibited marked red shifts and broadening of the fluorescence spectra compared with spectra from nonatherosclerotic aortas. Similar profiles were observed in spectra obtained from oxidatively modified low density lipoprotein but not native low density lipoprotein. In hypercholesterolemic rabbits with early foam cell lesions, spectral shifts resembled those of oxidized beta-very low density lipoprotein, the major lipoprotein accumulating in arteries of rabbits fed cholesterol. XeCl laser-fluorescence spectroscopy of arterial surfaces may be useful for the identification of arteries accumulating modified lipoproteins (oxidized low density lipoprotein), a chemical change indicative of atherosclerosis in its early and probably reversible stages.

XeCl laser ablation of atherosclerotic aorta: optical properties and energy pathways.

The energetics of 308-nm excimer laser irradiation of human aorta were studied. The heat generation that occurred during laser irradiation of atherosclerotic aorta equaled the absorbed laser energy minus the fraction of energy for escaping fluorescence (0.8-1.6%) and photochemical decomposition (2%). The absorbed laser energy is equal to the total delivered light energy minus the energy lost as specular reflectance (2.4%, air/tissue) and diffuse reflectance (11.5-15.5%). Overall, about 79-83.5% of the delivered light energy was converted to heat. We conclude that the mechanism of XeCl laser ablation of soft tissue involves thermal overheating of the irradiated volume with subsequent explosive vaporization. The optical properties of normal wall of human aorta and fibrous plaque, both native and denatured were determined. The light scattering was significant and sufficient to cause a subsurface fluence (J/cm2) in native aorta that equaled 1.8 times the broad-beam radiant exposure, phi o (2.7 phi o for denatured aorta). An optical fiber must have a diameter of at least 800 microns to achieve a maximum light penetration (approximately 200 microns for phi o/e) in the aorta along the central axis of the beam.

Laser ablation of atherosclerotic blood vessel tissue under various irradiation conditions.

A quantitative analysis is presented of the destruction of normal wall and atherosclerotic plaque areas of blood vessels by laser radiation. Threshold laser radiant exposure values were measured experimentally in vitro, along with the ablation efficiency for various laser wavelengths and irradiation conditions. Correlations were found between the ablation efficiency and fluence thresholds on the one hand and the optical properties of the blood vessel tissues on the other. Fibrous plaque was demonstrated to be selectively destroyed by the second-harmonic output from a pulsed Nd:YAG laser at lambda = 532 nm.

UV laser induced RNA-protein crosslinks and RNA chain breaks in tobacco mosaic virus RNA in situ.

The efficiency of RNA-protein crosslink and RNA chain break formation under nanosecond or picosecond UV-laser pulse irradiation of tobacco mosaic virus was determined. It was found that on high-intensity UV-laser irradiation the quantum yields of both reactions increase considerably as compared to the usual (low-intensity) UV-irradiation. The RNA-protein crosslink quantum yield was found to be 1.8 x 10(-5) and 1.2 x 10(-4) and that of RNA chain breaks 1.7 x 10(-4) and 8.9 x 10(-4) for nanosecond and picosecond irradiation, respectively.