Two-layer heterogeneous breast phantom for photoacoustic imaging.

Photoacoustic tomography (PAT) is emerging as a potentially important aid for breast cancer detection. Well-validated tissue-simulating phantoms are needed for objective, quantitative, and physically realistic testing for system development. Prior reported PAT phantoms with homogenous structures do not incorporate the irregular layered structure of breast tissue. To assess the impact of this simplification, we design and construct two-layer breast phantoms incorporating vessel-simulating inclusions and realistic undulations at the fat/fibroglandular tissue interface. The phantoms are composed of custom poly(vinyl chloride) plastisol formulations mimicking the acoustic properties of two breast tissue types and tissue-relevant similar optical properties. Resulting PAT images demonstrate that in tissue with acoustic heterogeneity, lateral size of imaging targets is sensitive to the choice of sound speed in image reconstruction. The undulating boundary can further degrade a target's lateral size due to sound speed variation in tissue and refraction of sound waves at the interface. The extent of this degradation is also influenced by the geometric relationship between an absorber and the boundary. Results indicate that homogeneous phantom matrixes may underestimate the degradation of PAT image quality in breast tissue, whereas heterogeneous phantoms can provide more realistic testing through improved reproduction of spatial variations in physical properties.

Phantom-based image quality test methods for photoacoustic imaging systems.

As photoacoustic imaging (PAI) technologies advance and applications arise, there is increasing need for standardized approaches to provide objective, quantitative performance assessment at various stages of the product development and clinical translation process. We have developed a set of performance test methods for PAI systems based on breast-mimicking tissue phantoms containing embedded inclusions. Performance standards for mature imaging modalities [magnetic resonance imaging (MRI), computed tomography (CT), and ultrasound] were used to guide selection of critical PAI image quality characteristics and experimental methods. Specifically, the tests were designed to address axial, lateral, and elevational spatial resolution, signal uniformity, penetration depth, sensitivity, spatial measurement accuracy, and PAI-ultrasound coregistration. As an initial demonstration of the utility of these test methods, we characterized the performance of a modular, bimodal PAI-ultrasound system using four clinical ultrasound transducers with varying design specifications. Results helped to inform optimization of acquisition and data processing procedures while providing quantitative elucidation of transducer-dependent differences in image quality. Comparison of solid, tissue-mimicking polymer phantoms with those based on Intralipid indicated the superiority of the former approach in simulating real-world conditions for PAI. This work provides a critical foundation for the establishment of well-validated test methods that will facilitate the maturation of PAI as a medical imaging technology.

Biologically relevant photoacoustic imaging phantoms with tunable optical and acoustic properties.

Established medical imaging technologies such as magnetic resonance imaging and computed tomography rely on well-validated tissue-simulating phantoms for standardized testing of device image quality. The availability of high-quality phantoms for optical-acoustic diagnostics such as photoacoustic tomography (PAT) will facilitate standardization and clinical translation of these emerging approaches. Materials used in prior PAT phantoms do not provide a suitable combination of long-term stability and realistic acoustic and optical properties. Therefore, we have investigated the use of custom polyvinyl chloride plastisol (PVCP) formulations for imaging phantoms and identified a dual-plasticizer approach that provides biologically relevant ranges of relevant properties. Speed of sound and acoustic attenuation were determined over a frequency range of 4 to 9 MHz and optical absorption and scattering over a wavelength range of 400 to 1100 nm. We present characterization of several PVCP formulations, including one designed to mimic breast tissue. This material is used to construct a phantom comprised of an array of cylindrical, hemoglobin-filled inclusions for evaluation of penetration depth. Measurements with a custom near-infrared PAT imager provide quantitative and qualitative comparisons of phantom and tissue images. Results indicate that our PVCP material is uniquely suitable for PAT system image quality evaluation and may provide a practical tool for device validation and intercomparison.

Comparison of windowing effects on elastography images: Simulation, phantom and in vivo studies.

In this paper, we have evaluated the use of smooth windows for ultrasound elastography. In ultrasound elastography, local tissue strain is estimated using operations such as cross-correlation on local segments of RF data. In this process, local data segments are selected by multiplying the RF data by a rectangular window. Such data truncation causes non-ideal spectral behavior, which can be mitigated by using smooth windows. Accordingly, we hypothesize that the use of smooth windows may improve the elastographic signal-to-noise ratio (SNRe) and contrast-to-noise ratio (CNRe) of strain images. The effects of using smooth windows have not been fully characterized for time-domain strain estimators. Thus, we have compared the elastographic performance of rectangular, Hanning, Gaussian, and Chebyshev windows used in conjunction with cross-correlation based algorithm and adaptive stretching algorithm using finite element method (FEM) simulation, experimental phantom, and in vivo data. Smooth windows are found to improve the SNRe by up to 3.94 for FEM data and by up to 1.76 for phantom data which represent 76% and 60.52% improvements, respectively. CNRe improves by up to 12.23 for FEM simulated data and by up to 4.28 for phantom data which represent 213.07% and 248.2% improvements, respectively. Mean structural similarity (MSSIM) was used for assessing the image perceptual quality and smooth windows improved it by up to 0.22 (85.98% improvement) for simulated data. We have evaluated these parameters at 1-6% applied strains for the experimental phantom and at 1%, 2%, 4%, 6%, 8%, and 12% applied strains for FEM simulation. We observed a maximum deterioration in axial resolution of 0.375 mm (which is on the order of the wavelength, 0.3mm) due to smooth windows. "Salt-and-pepper" noise from false-peak errors has also been reduced. Smooth windows increased the lesion-to-background contrast (by increasing the CNRe by 213.07%) of a low contrast lesion (10-dB). For the in vivo cases, use of smooth windows resulted in better depiction of lesions, which is important for lesion classification. In this work, we have used an ATL Ultramark 9 scanner with an L10-5 (7.5 MHz) probe for the phantom experiment and a Sonix SP500 scanner with an L14-5/38 probe (10 MHz) for in vivo data collection.

Can molecular imaging enable personalized diagnostics? An example using magnetomotive photoacoustic imaging.

The advantages of photoacoustic (PA) imaging, including low cost, non-ionizing operation, and sub-mm spatial resolution at centimeters depth, make it a promising modality to probe nanoparticle-targeted abnormalities in real time at cellular and molecular levels. However, detecting rare cell types in a heterogeneous background with strong optical scattering and absorption remains a big challenge. For example, differentiating circulating tumor cells in vivo (typically fewer than 10 cells/mL for an active tumor) among billions of erythrocytes in the blood is nearly impossible. In this paper, a newly developed technique, magnetomotive photoacoustic (mmPA) imaging, which can greatly increase the sensitivity and specificity of sensing targeted cells or molecular interactions, is reviewed. Its primary advantage is suppression of background signals through magnetic enrichment/manipulation with simultaneous PA detection of magnetic contrast agent targeted objects. Results from phantom and in vitro studies demonstrate the capability of mmPA imaging to differentiate regions targeted with magnetic nanoparticles from the background, and to trap and sensitively detect targeted cells at a concentration of a single cell per milliliter in a flow system mimicking a human peripheral artery. This technique provides an example of the ways in which molecular imaging can potentially enable robust molecular diagnosis and treatment, and accelerate the translation of molecular medicine into the clinic.

Magnetomotive photoacoustic imaging: in vitro studies of magnetic trapping with simultaneous photoacoustic detection of rare circulating tumor cells.

Photoacoustic (PA) imaging has been demonstrated to be a promising modality in molecular imaging for detection of nanoparticle-targeted diseased cells or tissues. However, intrinsic absorbers, such as blood, produce strong PA background signals that severely degrade the detection sensitivity and specificity of targeted objects. Magnetomotive photoacoustic (mmPA) imaging, a newly developed molecular imaging modality, introduced dynamic manipulation into traditional PA imaging. Unlike conventional PA imaging, magnetomotive manipulation with simultaneous ultrasound/PA imaging of agents incorporating magnetic nanoparticles enables direct visualization of the signal generating object and can dramatically reduce background signals from strong optical absorbers. This paper briefly reviews recent developments in mmPA imaging, including uses of composite contrast agent, design of magnet system, and data processing for motion filtering. The use of mmPA imaging in detecting rare circulating tumor cells in blood vessels, which remains a big challenge for real-time in vivo examination using current methodologies, was also addressed.

Comparison of 2-D speckle tracking and tissue Doppler imaging in an isolated rabbit heart model.

Ultrasound strain imaging has been proposed to quantitatively assess myocardial contractility. Cross-correlation-based 2-D speckle tracking (ST) and auto-correlation-based tissue Doppler imaging (TDI) [often called Doppler tissue imaging (DTI)] are competitive ultrasound techniques for this application. Compared with 2-D ST, TDI, as a 1-D method, is sensitive to beam angle and suffers from low strain signal-to-noise ratio because a high pulse repetition frequency is required to avoid aliasing in velocity estimation. In addition, ST and TDI are fundamentally different in the way that physical parameters such as the mechanical strain are derived, resulting in different estimation accuracy and interpretation. In this study, we directly compared the accuracy of TDI and 2-D ST estimates of instantaneous axial normal strain and accumulated axial normal strain using a simulated heart. We then used an isolated rabbit heart model of acute ischemia produced by left descending anterior artery ligation to evaluate the performance of the two methods in detecting abnormal motion. Results showed that instantaneous axial normal strains derived using TDI (0.36% error) were less accurate with larger variance than those derived from 2-D ST (0.08% error) given the same spatial resolution. In addition to poorer accuracy, accumulated axial normal strain estimates derived using TDI suffered from bias, because the accumulation method for TDI cannot trace along the actual tissue displacement path. Finally, we demonstrated the advantage 2-D ST has over TDI to reduce dependency on beam angle for lesion detection by estimating strains based on the principal stretches and their corresponding principal axes.

Multifunctional nanoparticles as coupled contrast agents.

Engineering compact imaging probes with highly integrated modalities is a key focus in bionanotechnology and will have profound impact on molecular diagnostics, imaging and therapeutics. However, combining multiple components on a nanometre scale to create new imaging modalities unavailable from individual components has proven to be challenging. In this paper, we demonstrate iron oxide and gold-coupled core-shell nanoparticles (NPs) with well-defined structural characteristics (for example, size, shell thickness and core-shell separation) and physical properties (for example, electronic, magnetic, optical, thermal and acoustic). The resulting multifunctional nanoprobes not only offer contrast for electron microscopy, magnetic resonance imaging and scattering-based imaging but, more importantly, enable a new imaging mode, magnetomotive photoacoustic imaging, with remarkable contrast enhancement compared with photoacoustic images using conventional NP contrast agents.

Differential-absorption photoacoustic imaging.

We present differential-absorption photoacoustic imaging, which detects the difference between transient and ground-state absorption, for contrast enhancement based on suppressing undesired objects. Two tubes were imaged. One contains a Pt(II) octaethylporphine (PtOEP) dye solution and serves as an object of interest, while the other contains an IR-783 (from Sigma-Aldrich) dye solution and serves as an object to suppress. Although the IR-783 tube dominates the conventional photoacoustic image, it is suppressed by 43 dB and consequently significantly overwhelmed by the PtOEP tube in the differential-absorption photoacoustic image. Imaging depth in this mode is also discussed.

Two-dimensional strain imaging of controlled rabbit hearts.

Ultrasound strain imaging using 2-D speckle tracking has been proposed to quantitatively assess changes in myocardial contractility caused by ischemia. Its performance must be demonstrated in a controlled model system as a step toward routine clinical application. In this study, a well-controlled 2-D cardiac elasticity imaging technique was developed using two coplanar and orthogonal linear probes simultaneously imaging an isolated retroperfused rabbit heart. Acute ischemia was generated by left anterior descending (LAD) artery ligation. An excitation-contraction decoupler, 2,3-butanedione monoxime, was applied at a 4-mM concentration to reversibly reduce myocardial contractility. Results using a single probe demonstrate that directional changes in the in-plane principal deformation axes can help locate the bulging area as a result of LAD ligation, which matched well with corresponding Evans Blue staining, and strains or strain magnitude, based on principal stretches, can characterize heart muscle contractility. These two findings using asymmetric displacement accuracy (i.e., normal single-probe measurements with good axial but poor lateral estimates) were further validated using symmetric displacement accuracy (i.e., dual-probe measurements using only accurate axial tracking estimates from each). However, the accuracy of 2-D cardiac strain imaging using a single probe depends on the probe's orientation because of the large variance in lateral displacement estimates.

Phase rotation methods in filtering correlation coefficients for ultrasound speckle tracking.

In speckle-tracking-based myocardial strain imaging, large interframe/volume peak-systolic strains cause peak hopping artifacts separating the highest correlation coefficient peak from the true peak. A correlation coefficient filter was previously designed to minimize peak hopping artifacts. For large strains, however, the correlation coefficient filter must follow the strain distribution to remove peak hopping effectively. This processing usually means interpolation and high computational load. To reduce the computational burden, a narrow band approximation using phase rotation is developed in this paper to facilitate correlation coefficient filtering. Correlation coefficients are first phase rotated to increase coherence, then filtered. Rotated phase angles are determined by the local strain and spatial position. This form of correlation coefficient filtering enhances true correlation coefficient peaks in large strain applications if decorrelation due to deformation does not completely destroy the coherence among neighboring correlation coefficients. The assumed strain used in the filter can also deviate from the true strain and still be effective. Further improvement in displacement estimation can be expected by combining correlation coefficient filtering with a new Viterbi-based displacement estimator.

Cardiac activation mapping using ultrasound current source density imaging (UCSDI).

We describe the first mapping of biological current in a live heart using ultrasound current source density imaging (UCSDI). Ablation procedures that treat severe heart arrhythmias require detailed maps of the cardiac activation wave. The conventional procedure is time-consuming and limited by its poor spatial resolution (5-10 mm). UCSDI can potentially improve on existing mapping procedures. It is based on a pressure-induced change in resistivity known as the acousto-electric (AE) effect, which is spatially confined to the ultrasound focus. Data from 2 experiments are presented. A 540 kHz ultrasonic transducer (f/# = 1, focal length = 90 mm, pulse repetition frequency = 1600 Hz) was scanned over an isolated rabbit heart perfused with an excitation-contraction decoupler to reduce motion significantly while retaining electric function. Tungsten electrodes inserted in the left ventricle recorded simultaneously the AE signal and the low-frequency electrocardiogram (ECG). UCSDI displayed spatial and temporal patterns consistent with the spreading activation wave. The propagation velocity estimated from UCSDI was 0.25 +/- 0.05 mm/ms, comparable to the values obtained with the ECG signals. The maximum AE signal-to-noise ratio after filtering was 18 dB, with an equivalent detection threshold of 0.1 mA/ cm(2). This study demonstrates that UCSDI is a potentially powerful technique for mapping current flow and biopotentials in the heart.

Analysis of correlation coefficient filtering in elasticity imaging.

Correlation-based speckle tracking methods are commonly used in elasticity imaging to estimate displacements. In the presence of local strain, a larger window size results in larger displacement error. To reduce tracking error, we proposed a short correlation window followed by a correlation coefficient filter. Although simulation and experimental results demonstrated the efficacy of the method, it was not clear why correlation coefficient filtering reduces tracking error since tracking error increases if normalization before filtering is not applied. In this paper, we analyzed tracking errors by estimating phase variances of the cross-correlation function and the correlation coefficient at the true time lag based on statistical properties of these functions' real and imaginary parts. The role of normalization is clarified by identifying the effect of the cross-correlation function's amplitude fluctuation on the function's imaginary part. Furthermore, we present analytic forms for predicting axial displacement error as a function of strain, system parameters (signal-to-noise ratio, center frequency, and signal and noise bandwidths), and tracking parameters (window and filter sizes) for cases with and without normalization before filtering. Simulation results correspond to theory well for both noise-free cases and general cases with an empirical correction term included for strains up to 4%.

Noninvasive ultrasound elasticity imaging (UEI) of Crohn's disease: animal model.

Inflammation occurs in episodic flares in Crohn's disease, which are part of the waxing and waning course of the disease. Healing between flares allows the intestine to reconstitute its epithelium, but this healing results in the deposition of fibrotic scar tissue as part of the healing process. Repeated cycles of flares and healing often lead to clinically significant fibrosis and stenosis of the intestine. Patients are treated empirically with steroids, with their many side effects, in the hope that they will respond. Many patients would be better treated with surgery if we could identify which patients truly have intestinal fibrosis. Ultrasound elasticity imaging (UEI) offers the potential to radically improve the diagnosis and management of local tissue elastic property, particularly intestinal fibrosis. This method allows complete characterization of local intestine tissue with high spatial resolution. The feasibility of UEI on Crohn's disease is demonstrated by directly applying this technique to an animal model of inflammatory bowel disease (IBD). Five female Lewis rats (150-180g) were prepared with phosphate buffered solution (PBS) as a control group and six were prepared with repeated intrarectal administration of trinitrobenzenesulfonic acid (TNBS) as a disease group. Preliminary strain measurements differentiate the diseased colons from the normal colons (p < 0.0002) and compared well with direct mechanical measurements and histology (p < 0.0005). UEI provides a simple and accurate assessment of local severity of fibrosis. The preliminary results on an animal model also suggest the feasibility of translating this imaging technique directly to human subjects for both diagnosis and monitoring.