Photoacoustics for molecular imaging and therapy

Sound waves generated by light are the basis of a sensitive medical imaging technique with applications to cancer diagnosis and treatment.

Display pixel-based synthetic aperture focusing method for intravascular ultrasound imaging

An intravascular ultrasound image reconstruction technique that combines synthetic aperture focusing and display pixel-based focusing methods is presented. Although the synthetic aperture focusing method can improve intravascular ultrasound image quality, the final displayed images are usually blurry in the angular direction due to the limitations of the digital scan converter. The display pixel-based focusing method can eliminate blurring effects caused by the digital scan converter. Therefore, the image quality can be further improved by applying the display pixel-based focusing method to the synthetic aperture focusing method, especially for intravascular ultrasound images. The experimental studies were performed to evaluate display pixel-based synthetic aperture focusing method. The computational complexity of the display pixel-based synthetic aperture focusing method was discussed in comparison with that of the synthetic aperture focusing method.

Pulsed magneto-acoustic imaging

Nanoparticles are attracting considerable interest as contrast agents for many different imaging modalities. Moreover, imaging the events at the cellular and molecular level is possible by using nanoparticles that have the desired targeting moiety. Unfortunately, ultrasound imaging cannot visualize the nano-structures directly due to its limited spatial resolution and contrast. We present a new technique, pulsed magneto-acoustic imaging, capable of imaging magnetic nanoparticles indirectly. In this method, a high-strength pulsed magnetic field is used to induce motion within the magnetically labeled tissue and ultrasound is used to detect internal tissue motion. Experiments on tissue-mimicking phantoms and ex-vivo animal tissues demonstrated a clear contrast between normal and iron-laden samples labeled with 5 nm magnetic nanoparticles. In addition, the sensitivity of this new imaging technique was investigated for different concentrations of magnetic agents. The results of the study suggest that magnetic nanoparticles can be used as contrast agents in pulsed magneto-acoustic imaging. Furthermore, PMA imaging could become an imaging tool capable of visualizing the cellular and molecular composition of deep-lying structures.

Ultrasound guidance and monitoring of laser-based fat removal

BACKGROUND AND OBJECTIVES

We report on a study to investigate feasibility of utilizing ultrasound imaging to guide laser removal of subcutaneous fat. Ultrasound imaging can be used to identify the tissue composition and to monitor the temperature increase in response to laser irradiation.

STUDY DESIGN/MATERIALS AND METHODS

Laser heating was performed on ex vivo porcine subcutaneous fat through the overlying skin using a continuous wave laser operating at 1,210 nm optical wavelength. Ultrasound images were recorded using a 10 MHz linear array-based ultrasound imaging system.

RESULTS

Ultrasound imaging was utilized to differentiate between water-based and lipid-based regions within the porcine tissue and to identify the dermis-fat junction. Temperature maps during the laser exposure in the skin and fatty tissue layers were computed.

CONCLUSIONS

Results of our study demonstrate the potential of using ultrasound imaging to guide laser fat removal.

Combined ultrasound and photoacoustic imaging to detect and stage deep vein thrombosis: phantom and ex vivo studies

Treatment of deep venous thrombosis (DVT)--a primary cause of potentially fatal pulmonary embolism (PE)--depends on the age of the thrombus. The existing clinical imaging methods are capable of visualizing a thrombus but cannot determine the age of the blood clot. Therefore, there is a need for an imaging technique to reliably diagnose and adequately stage DVT. To stage DVT (i.e., to determine the age of the thrombus, and therefore, to differentiate acute from chronic DVT), we explored photoacoustic imaging, a technique capable of noninvasive measurements of the optical absorption in tissue. Indeed, optical absorption of the blood clot changes with age, since maturation of DVT is associated with significant cellular and molecular reorganization. The ultrasound and photoacoustic imaging studies were performed using DVT-mimicking phantoms and phantoms with embedded acute and chronic thrombi obtained from an animal model of DVT. The location and structure of the clots were visualized using ultrasound imaging, while the composition, and therefore age, of thrombi were related to the magnitude and spatiotemporal characteristics of the photoacoustic signal. Overall, the results of our study suggest that combined ultrasound and photoacoustic imaging of thrombi may be capable of simultaneous detection and staging of DVT.

Ultrasound measurements of cavitation bubble radius for femtosecond laser-induced breakdown in water

A recently developed ultrasound technique is evaluated by measuring the behavior of a cavitation bubble that is induced in water by a femtosecond laser pulse. The passive acoustic emission during optical breakdown is used to estimate the location of the cavitation bubble's origin. In turn, the position of the bubble wall is defined based on the active ultrasonic pulse-echo signal. The results suggest that the developed ultrasound technique can be used for quantitative measurements of femtosecond laser-induced microbubbles.

Adaptive beamforming for photoacoustic imaging

An adaptive photoacoustic image reconstruction technique that combines coherence factor (CF) weighting and the minimum variance (MV) method is introduced. The backprojection method is widely used to reconstruct photoacoustic tomography images. Owing to the scattering of light, the quality of the photoacoustic imaging can be degraded. CF, an adaptive weighting technique, is known to improve the lateral resolution of photoacoustic images. In addition, an MV adaptive beamforming method can further improve the image quality by suppressing signals from off-axis directions. Experimental studies are performed to quantify the spatial resolution and contrast of the adaptive photoacoustic beamforming methods.

Photoacoustic imaging and temperature measurement for photothermal cancer therapy

Photothermal therapy is a noninvasive, targeted, laser-based technique for cancer treatment. During photothermal therapy, light energy is converted to heat by tumor-specific photoabsorbers. The corresponding temperature rise causes localized cancer destruction. For effective treatment, however, the presence of photoabsorbers in the tumor must be ascertained before therapy and thermal imaging must be performed during therapy. This study investigates the feasibility of guiding photothermal therapy by using photoacoustic imaging to detect photoabsorbers and to monitor temperature elevation. Photothermal therapy is carried out by utilizing a continuous wave laser and metal nanocomposites broadly absorbing in the near-infrared optical range. A linear array-based ultrasound imaging system is interfaced with a nanosecond pulsed laser to image tissue-mimicking phantoms and ex-vivo animal tissue before and during photothermal therapy. Before commencing therapy, photoacoustic imaging identifies the presence and spatial location of nanoparticles. Thermal maps are computed by monitoring temperature-induced changes in the photoacoustic signal during the therapeutic procedure and are compared with temperature estimates obtained from ultrasound imaging. The results of our study suggest that photoacoustic imaging, augmented by ultrasound imaging, is a viable candidate to guide photoabsorber-enhanced photothermal therapy.

Quantitative ultrasound method to detect and monitor laser-induced cavitation bubbles

An ultrasound technique to measure the spatial and temporal behavior of the laser-induced cavitation bubble is introduced. The cavitation bubbles were formed in water and in gels using a nanosecond pulsed Nd:YAG laser operating at 532 nm. A focused, single-element, 25-MHz ultrasound transducer was employed both to detect the acoustic emission generated by plasma expansion and to acoustically probe the bubble at different stages of its evolution. The arrival time of the passive acoustic emission was used to estimate the location of the cavitation bubble's origin and the time of flight of the ultrasound pulse-echo signal was used to define its spatial extent. The results of ultrasound estimations of the bubble size were compared and found to be in agreement with both the direct optical measurements of the stationary bubble and the theoretical estimates of bubble dynamics derived from the well-known Rayleigh model of a cavity collapse. The results of this study indicate that the proposed quantitative ultrasound technique, capable of detecting and accurately measuring laser-induced cavitation bubbles in water and in a tissue-like medium, could be used in various biomedical and clinical applications.

Photoacoustic imaging and temperature measurement for photothermal cancer therapy

Photothermal therapy is a noninvasive, targeted, laser-based technique for cancer treatment. During photothermal therapy, light energy is converted to heat by tumor-specific photoabsorbers. The corresponding temperature rise causes localized cancer destruction. For effective treatment, however, the presence of photoabsorbers in the tumor must be ascertained before therapy and thermal imaging must be performed during therapy. This study investigates the feasibility of guiding photothermal therapy by using photoacoustic imaging to detect photoabsorbers and to monitor temperature elevation. Photothermal therapy is carried out by utilizing a continuous wave laser and metal nanocomposites broadly absorbing in the near-infrared optical range. A linear array-based ultrasound imaging system is interfaced with a nanosecond pulsed laser to image tissue-mimicking phantoms and ex-vivo animal tissue before and during photothermal therapy. Before commencing therapy, photoacoustic imaging identifies the presence and spatial location of nanoparticles. Thermal maps are computed by monitoring temperature-induced changes in the photoacoustic signal during the therapeutic procedure and are compared with temperature estimates obtained from ultrasound imaging. The results of our study suggest that photoacoustic imaging, augmented by ultrasound imaging, is a viable candidate to guide photoabsorber-enhanced photothermal therapy.

Ultrasound imaging to monitor photothermal therapy - feasibility study

This study investigates the feasibility of ultrasound imaging to monitor temperature changes during photothermal treatment. Experiments were performed on tissue-mimicking phantoms and ex-vivo animal tissue samples. Gold nanoparticles were utilized as photoabsorbers. Prior to laser irradiation, structural features of the phantoms and tissue were visualized by ultrasound imaging. Ultrasound thermal imaging, performed during laser heating, showed that the temperature elevation was localized to the region of embedded or injected nanoparticles. The results of our study suggest that ultrasound imaging is a candidate approach to remotely guide photothermal therapy.

Spectroscopic intravascular photoacoustic imaging to differentiate atherosclerotic plaques

The potential of intravascular photoacoustic (IVPA) imaging to detect atherosclerosis was previously demonstrated using a 532 nm nanosecond pulsed laser and an intravascular ultrasound (IVUS) imaging catheter. However, to differentiate vulnerable plaques, the composition of plaques needs to be imaged. Therefore, we introduce a multi-wavelength photoacoustic imaging method to distinguish various types of plaques. Multi-spectral IVPA imaging of ex vivo samples of normal and atherosclerotic rabbit aorta was performed at several wavelengths within 680-900 nm range. The spectral variation of photoacoustic response was extracted and a spectroscopic analysis was performed. The results of our preliminary study suggest that the spectroscopic intravascular photoacoustic imaging technique can be used to differentiate fibrous and lipid components of the atherosclerotic plaques.

Remote temperature estimation in intravascular photoacoustic imaging

Intravascular photoacoustic (IVPA) imaging is based on the detection of laser-induced acoustic waves generated within the arterial tissue under pulsed laser irradiation. In general, laser radiant energy levels are kept low (20 mJ/cm(2)) during photoacoustic imaging to conform to general standards for safe use of lasers on biological tissues. However, safety standards in intravascular photoacoustic imaging are not yet fully established. Consequently, monitoring spatio-temporal temperature changes associated with laser-tissue interaction is important to address thermal safety of IVPA imaging. In this study we utilize the IVUS-based strain measurements to estimate the laser-induced temperature increase. Temporal changes in temperature were estimated in a phantom modeling a vessel with an inclusion. A cross-correlation-based time delay estimator was used to assess temperature-induced strains produced by different laser radiant energies. The IVUS-based remote measurements revealed temperature increases of 0.7+/-0.3 degrees C, 2.9+/-0.2 degrees C and 5.0+/-0.2 degrees C, for the laser radiant energies of 30 mJ/cm(2), 60 mJ/cm(2) and 85 mJ/cm(2), respectively. The technique was then used in imaging of ex vivo samples of a normal rabbit aorta. For arterial tissues, a temperature elevation of 1.1 degrees C was observed for a laser fluence of 60 mJ/cm(2) and lesser than 1 degrees C for lower energy levels normally associated with IVPA imaging. Therefore, the developed ultrasound technique can be used to monitor temperature during IVPA imaging. Furthermore, the analysis based on the Arrhenius thermal damage model indicates no thermal injury in the arterial tissue, suggesting the safety of IVPA imaging.

Ex vivo Characterization of Atherosclerosis using Intravascular Photoacoustic Imaging

The imaging of plaque composition represents one of the important steps in the interventional management of atherosclerosis. Intravascular photoacoustic (IVPA) imaging has the potential to play a major role in the detection and differentiation of an atherosclerotic lesion. The difference in the optical properties of the arterial wall and plaque constituents could be utilized to obtain high resolution photoacoustic images. In this work, through ex vivo imaging studies using a rabbit model of atherosclerosis, we evaluate the ability of IVPA imaging to detect and characterize the plaque. Specifically, the difference in the magnitude of the photoacoustic signals from the free lipids, macrophage foam cells, blood and the rest of the arterial wall were helpful in providing the contrast and detecting the fibro-cellular inflammatory plaque. The constituents identified in the IVPA images were confirmed by the results from histology.

Elasticity imaging using conventional and high-frame rate ultrasound imaging: experimental study

High-frame rate ultrasound imaging is necessary to track fast deformation in ultrasound elasticity imaging, but the image quality may be degraded. Previously, we investigated the performance of strain imaging using numerical models of conventional and ultrafast ultrasound imaging techniques. In this paper, we performed experimental studies to quantitatively evaluate the strain images and elasticity maps obtained using conventional and high frame rate ultrasound imaging methods. The experiments were carried out using point target and tissue mimicking phantoms. The experimental results were compared with the results of numerical simulation. Our experimental studies confirm that the signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), and axial/lateral resolution of the displacement and strain images acquired using high-frame rate ultrasound imaging are slightly lower but comparable with those obtained using conventional imaging. Furthermore, the quality of elasticity images also exhibits similar trends. Thus, high-frame rate ultrasound imaging can be used reliably for static elasticity imaging to capture the internal tissue motion if the frame rate is critical.

Motion of a solid sphere in a viscoelastic medium in response to applied acoustic radiation force: Theoretical analysis and experimental verification

The motion of a rigid sphere in a viscoelastic medium in response to an acoustic radiation force of short duration was investigated. Theoretical and numerical studies were carried out first. To verify the developed model, experiments were performed using rigid spheres of various diameters and densities embedded into tissue-like, gel-based phantoms of varying mechanical properties. A 1.5 MHz, single-element, focused transducer was used to apply the desired radiation force. Another single-element, focused transducer operating at 25 MHz was used to track the displacements of the sphere. The results of this study demonstrate good agreement between theoretical predictions and experimental measurements. The developed theoretical model accurately describes the displacement of the solid spheres in a viscoelastic medium in response to the acoustic radiation force.

Intravascular photoacoustic imaging using an IVUS imaging catheter

Catheter-based imaging of atherosclerosis with high resolution, albeit invasive, is extremely important for screening and characterization of vulnerable plaques. Currently, there is a need for an imaging technique capable of providing comprehensive morphological and functional information of plaques. In this paper, we present an intravascular photoacoustic imaging technique to characterize vulnerable plaques by using optical absorption contrast between normal tissue and atherosclerotic lesions. Specifically, we investigate the feasibility of obtaining intravascular photoacoustic (IVPA) images using a high-frequency intravascular ultrasound (IVUS) imaging catheter. Indeed, the combination of IVPA imaging with clinically available IVUS imaging may provide desired functional and morphological assessment of the plaque. The imaging studies were performed with tissue-mimicking arterial vessel phantoms and excised samples of rabbit artery. The results of our study suggest that catheter-based intravascular photoacoustic imaging is possible, and the combination of IVPA with IVUS has the potential to detect and differentiate atherosclerosis based on both the structure and composition of the plaque.

Strain imaging using conventional and ultrafast ultrasound imaging: numerical analysis

In elasticity imaging, the ultrasound frames acquired during tissue deformation are analyzed to estimate the internal displacements and strains. If the deformation rate is high, high-frame-rate imaging techniques are required to avoid the severe decorrelation between the neighboring ultrasound images. In these high-frame-rate techniques, however, the broader and less focused ultrasound beam is transmitted and, hence, the image quality is degraded. We quantitatively compared strain images obtained using conventional and ultrafast ultrasound imaging methods. The performance of the elasticity imaging was evaluated using custom-designed, numerical simulations. Our results demonstrate that signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR) and spatial resolutions in displacement and strain images acquired using conventional and ultrafast ultrasound imaging are comparable. This study suggests that the high-frame-rate ultrasound imaging can be reliably used in elasticity imaging if frame rate is critical.

Sonographic elasticity imaging of acute and chronic deep venous thrombosis in humans

OBJECTIVE

The purpose of this study was to assess the ability of sonographic elasticity imaging to distinguish acute from chronic deep venous thrombosis (DVT).

METHODS

Fifty-four patients, 26 with acute DVT and 28 with chronic DVT, were studied, and we analyzed the data in 46 patients, 23 with acute (mean age, 5.7 days) and 23 with chronic (>8 months) DVT. Scanning was performed with a 5-MHz linear array transducer during continuous freehand external deformation of each thrombus using the ultrasound scan head. The strains in the thrombi were normalized to the average strain between the skin surface and the back wall of the vein. Relative thrombus echogenicity was measured by comparing the echogenicity of the thrombus with that of the adjacent arterial lumen. Statistical analyses were performed with the Mann-Whitney U test and receiver operating characteristic analysis.

RESULTS

The median normalized strain magnitude for the acute cases was 2.75, with an interquartile range of 2.4 to 3.71, whereas the median normalized strain magnitude for the chronic cases was 0.94, with interquartile range of 0.48 to 1.36. The difference was highly significant (P < 10(-7)). The area under the receiver operating characteristic curve (A(z)) was 0.97 +/- 0.02 (SE). The echogenicity difference between the populations was highly significant (P < 10(-5)), with A(z) of 0.92 +/- 0.04. The difference between the A(z) values was not significant (P > .05).

CONCLUSIONS

In this population, sonographic elasticity imaging performs at least as well as thrombus echogenicity. Thrombus aging using elasticity imaging would be particularly helpful in evaluating symptoms in patients with post-thrombotic syndrome.

Correspondence of ultrasound elasticity imaging to direct mechanical measurement in aging DVT in rats

Previous ultrasound elasticity imaging experiments supported a generally accepted concept that the hardness of deep venous thrombi increases with thrombus aging. Results also showed that this noninvasive imaging technique can accurately predict thrombus age through strain estimates, in a well-controlled animal study. In the present study, as an alternative means to characterize elastic properties of thrombi, we used a direct mechanical measurement system to estimate Young's modulus of ex vivo thrombi. Unlike conventional indentation tests, the device uses a specific compression geometry for cylindrical tissue specimens. We also proposed an approximation scheme to retrieve Young's modulus from force-displacement measurements made using the device. Finite element simulations and calibrations on tissue-mimicking phantoms validated the system. Then, using two groups of rats with surgically-induced thrombi, we further investigated the correlation between Young's modulus measured ex vivo and elasticity images reconstructed in vivo. This comparison was accomplished by converting the intrathrombus strains measured in the in vivo studies into Young's modulus estimates using a model-based approach. Good agreement between time-dependent Young's modulus estimates observed in vivo and direct measurements of Young's modulus using the mechanical device helps to confirm the ability of elasticity imaging to age deep venous thrombi for efficient treatment.

Gas bubble and solid sphere motion in elastic media in response to acoustic radiation force

The general approach to estimate the displacement of rounded objects (specifically, gas bubbles and solid spheres) in elastic incompressible media in response to applied acoustic radiation force is presented. In this study, both static displacement and transient motion are analyzed using the linear approximation. To evaluate the static displacement of the spherical inclusion, equations coupling the applied force, displacement, and shear modulus of the elastic medium are derived. Analytical expressions to estimate the static displacement of solid spheres and gas bubbles are presented. Under a continuously applied static force, both the solid sphere and the initially spherical gas bubble are displaced, and the bubble is deformed. The transient responses of the inclusions are described using motion equations. The displacements of the inclusion in elastic incompressible lossless media are analyzed using both frequency-domain and time-domain formalism, and the equations of motion are derived for both a solid sphere and a gas bubble. For a short pulsed force, an analytical solution for the equations of motion is presented. Finally, transient displacement of the gas bubble in viscoelastic media is considered.

Staging deep venous thrombosis using ultrasound elasticity imaging: animal model

Deep venous thrombi undergo progressive hardening with age. However, the evolution rate remains poorly characterized by both invasive and noninvasive techniques. In a previous study (Emelianov et al. 2002), we demonstrated the potential of ultrasound elasticity imaging to noninvasively detect and age thrombus using a rat-based model. Knowing that thrombi harden over time is useful, but the value of the technique relies on whether the age of a thrombus can be predicted from strain estimates, and how accurate these predictions are. The objective of the present study is to answer these two questions. In the previous study, thrombus elasticity changes were monitored only on day 3, 6 and 9 after surgically induced formation of thrombosis in rat inferior vena cavas. In this study, ultrasound elasticity imaging was performed on two independent groups of rats (16 in total) starting from day 3 through day 10 with more temporal samples through the thrombus maturation process. For each rat, thrombus hardness was quantified at each scan interval by measures of normalized strains and reconstructed relative Young's moduli. In both groups, strain magnitudes exhibit progressive decrease as clots age. The relationship between the normalized strain and the clot age was developed from the first group and evaluated by the second group. Statistical analysis showed that the age estimation accuracy is within 0.8 day. If further research can successfully transfer the animal clot-hardening model to human patients, we believe that elasticity imaging will become a key component of venous compression ultrasound for effective diagnosis and treatment of deep venous thrombosis.

Feasibility of applying ultrasound strain imaging to detect renal transplant chronic allograft nephropathy

Chronic renal transplantation fibrosis, often termed Chronic Allograft Nephropathy, may progress undetected. Since renal fibrosis may be accompanied by a change in measurable elastic tissue properties, ultrasound strain management may be useful in it detection. Ultrasound strain imaging was performed for two subjects with renal transplants; one with normal renal function and one with mild renal insufficiency and biopsy demonstrated fibrosis. Subjects underwent ultrasound examination with application of a controlled deformation using phase-sensitive, two-dimensional speckle tracking to evaluate internal tissue motion to measure tissue displacement and strain. Measurements over multiple beams for an equivalent deformational stress showed there was a threefold difference in renal cortical strain between the two subjects. These data suggest that ultrasound elasticity imaging may prove useful in measuring mechanical changes related to fibrosis with the transplant kidney.

Nonlinear elasticity imaging: theory and phantom study

In tissue the Young's modulus cannot be assumed constant over a wide deformation range. For example, direct mechanical measurements on human prostate show up to a threefold increase in Young's modulus over a 10% deformation. In conventional elasticity imaging, these effects produce strain-dependent elastic contrast. Ignoring these effects generally leads to suboptimal contrast (stiffer tissues at lower strain are contrasted against softer tissues at higher strain), but measuring the nonlinear behavior results in enhanced tissue differentiation. To demonstrate the methods extracting nonlinear elastic properties, both simulations and measurements were performed on an agar-gelatin phantom. Multiple frames of phase-sensitive ultrasound data are acquired as the phantom is deformed by 12%. All interframe displacement data are brought back to the geometry of the first frame to form a three-dimensional (3-D) data set (depth, lateral, and preload dimensions). Data are fit to a 3-D second order polynomial model for each pixel that adjusts for deformation irregularities. For the phantom geometry and elastic properties considered in this paper, reconstructed frame-to-frame strain images using this model result in improved contrast to noise ratios (CNR) at all preload levels, without any sacrifice in spatial resolution. From the same model, strain hardening at all preload levels can be extracted. This is an independent contrast mechanism. Its maximum CNR occurs at 5.13% preload, and it is a 54% improvement over the best case (preload 10.6%) CNR for frame-to-frame strain reconstruction. Actual phantom measurements confirm the essential features of the simulation. Results show that modeling of the nonlinear elastic behavior has the potential to both increase detectability in elasticity imaging and provide a new independent mechanism for tissue differentiation.

Model-based reconstructive elasticity imaging of deep venous thrombosis

Deep venous thrombosis (DVT) and its sequela, pulmonary embolism, is a significant clinical problem. Once detected, DVT treatment is based on the age of the clot. There are no good noninvasive methods, however, to determine clot age. Previously, we demonstrated that imaging internal mechanical strains can identify and possibly age thrombus in a deep vein. In this study the deformation geometry for DVT elasticity imaging and its effect on Young's modulus estimates is addressed. A model-based reconstruction method is presented to estimate elasticity in which the clot-containing vessel is modeled as a layered cylinder. Compared to an unconstrained approach in reconstructive elasticity imaging, the proposed model-based approach has several advantages: only one component of the strain tensor is used; the minimization procedure is very fast; the method is highly efficient because an analytic solution of the forward elastic problem is used; and the method is not very sensitive to the details of the external load pattern--a characteristic that is important for free-hand, external, surface-applied deformation. The approach was tested theoretically using a numerical model, and experimentally on both tissue-like phantoms and an animal model of DVT. Results suggest that elasticity reconstruction may prove to be a practical adjunct to triplex scanning to detect, diagnose, and stage DVT.