Quantitative Ultrasound Biomarkers to Assess Nonalcoholic Fatty Liver Disease.

PURPOSE: To assess diagnostic performance of quantitative ultrasound (QUS) biomarkers in assessing hepatic steatosis. METHODS: We prospectively recruited 125 participants (mean age 54 years) who underwent liver QUS, magnetic resonance imaging (MRI), and laboratory tests within 30 days in this IRB approved study. Based on MRI-proton density fat fraction (MRI-PDFF) and MRE, we divided 125 participants into normal liver, nonalcoholic fatty liver (NAFL) and liver fibrosis (≥F1) groups. We examined diagnostic performance of ultrasound attenuation coefficient (AC), normalized local variance (NLV), superb microvascular imaging-based vascularity index (SMI-VI), and shear wave velocity (SWV) for determining hepatic steatosis and fibrosis using area under receiver operating characteristic curve (AUC). We also analyzed correlations of QUS biomarkers to MRI using Spearman correlation coefficient. RESULTS: We observed significant differences in AC, NLV, and SMI-VI among the three groups (22 participants with normal liver, 78 with NAFL, and 25 with liver fibrosis). AUC of AC, NLV, and SMI-VI for determining ≥ mild steatotic livers (MRI-PDFF ≥5%) was 0.95, 0.90, and 0.92, respectively. AUC of SWV for determining ≥ F1 liver fibrosis was 0.93. The correlation of MRI-PDFF was positive to AC (r = 0.91) and negative to NLV (r = -0.74), SMI-VI (r = -0.8) in NAFL group. There was a significant difference in regression slope of AC to MRI-PDFF in livers with and without ≥F1 (0.84 vs 0.91, P = .02). CONCLUSIONS: QUS biomarkers have high sensitivity and specificity to determine and grade hepatic steatosis and detect liver fibrosis. The effect of liver fibrosis on the performance of QUS biomarkers in quantifying liver fat content warrants further investigation.

Radiological Society of North America/Quantitative Imaging Biomarker Alliance Shear Wave Speed Bias Quantification in Elastic and Viscoelastic Phantoms.

OBJECTIVES: To quantify the bias of shear wave speed (SWS) measurements between different commercial ultrasonic shear elasticity systems and a magnetic resonance elastography (MRE) system in elastic and viscoelastic phantoms. METHODS: Two elastic phantoms, representing healthy through fibrotic liver, were measured with 5 different ultrasound platforms, and 3 viscoelastic phantoms, representing healthy through fibrotic liver tissue, were measured with 12 different ultrasound platforms. Measurements were performed with different systems at different sites, at 3 focal depths, and with different appraisers. The SWS bias across the systems was quantified as a function of the system, site, focal depth, and appraiser. A single MRE research system was also used to characterize these phantoms using discrete frequencies from 60 to 500 Hz. RESULTS: The SWS from different systems had mean difference 95% confidence intervals of ±0.145 m/s (±9.6%) across both elastic phantoms and ± 0.340 m/s (±15.3%) across the viscoelastic phantoms. The focal depth and appraiser were less significant sources of SWS variability than the system and site. Magnetic resonance elastography best matched the ultrasonic SWS in the viscoelastic phantoms using a 140 Hz source but had a - 0.27 ± 0.027-m/s (-12.2% ± 1.2%) bias when using the clinically implemented 60-Hz vibration source. CONCLUSIONS: Shear wave speed reconstruction across different manufacturer systems is more consistent in elastic than viscoelastic phantoms, with a mean difference bias of < ±10% in all cases. Magnetic resonance elastographic measurements in the elastic and viscoelastic phantoms best match the ultrasound systems with a 140-Hz excitation but have a significant negative bias operating at 60 Hz. This study establishes a foundation for meaningful comparison of SWS measurements made with different platforms.

Three-dimensional US for Quantification of Volumetric Blood Flow: Multisite Multisystem Results from within the Quantitative Imaging Biomarkers Alliance.

Background Quantitative blood flow (QBF) measurements that use pulsed-wave US rely on difficult-to-meet conditions. Imaging biomarkers need to be quantitative and user and machine independent. Surrogate markers (eg, resistive index) fail to quantify actual volumetric flow. Standardization is possible, but relies on collaboration between users, manufacturers, and the U.S. Food and Drug Administration. Purpose To evaluate a Quantitative Imaging Biomarkers Alliance-supported, user- and machine-independent US method for quantitatively measuring QBF. Materials and Methods In this prospective study (March 2017 to March 2019), three different clinical US scanners were used to benchmark QBF in a calibrated flow phantom at three different laboratories each. Testing conditions involved changes in flow rate (1-12 mL/sec), imaging depth (2.5-7 cm), color flow gain (0%-100%), and flow past a stenosis. Each condition was performed under constant and pulsatile flow at 60 beats per minute, thus yielding eight distinct testing conditions. QBF was computed from three-dimensional color flow velocity, power, and scan geometry by using Gauss theorem. Statistical analysis was performed between systems and between laboratories. Systems and laboratories were anonymized when reporting results. Results For systems 1, 2, and 3, flow rate for constant and pulsatile flow was measured, respectively, with biases of 3.5% and 24.9%, 3.0% and 2.1%, and -22.1% and -10.9%. Coefficients of variation were 6.9% and 7.7%, 3.3% and 8.2%, and 9.6% and 17.3%, respectively. For changes in imaging depth, biases were 3.7% and 27.2%, -2.0% and -0.9%, and -22.8% and -5.9%, respectively. Respective coefficients of variation were 10.0% and 9.2%, 4.6% and 6.9%, and 10.1% and 11.6%. For changes in color flow gain, biases after filling the lumen with color pixels were 6.3% and 18.5%, 8.5% and 9.0%, and 16.6% and 6.2%, respectively. Respective coefficients of variation were 10.8% and 4.3%, 7.3% and 6.7%, and 6.7% and 5.3%. Poststenotic flow biases were 1.8% and 31.2%, 5.7% and -3.1%, and -18.3% and -18.2%, respectively. Conclusion Interlaboratory bias and variation of US-derived quantitative blood flow indicated its potential to become a clinical biomarker for the blood supply to end organs. © RSNA, 2020 Online supplemental material is available for this article. See also the editorial by Forsberg in this issue.

Propagation Imaging in the Demonstration of Common Shear Wave Artifacts.

In this article, we describe our experience with shear wave propagation imaging (SWPI) as an adjunct to 2-dimensional (2D) shear wave elastography (SWE) in a cohort of patients being evaluated for diffuse liver disease. Two-dimensional SWE has been extensively studied in previous publications; however, 2D SWE using propagation images has not been widely described in the literature to date. We observed that when certain artifacts occurred on the color elastograms, highly characteristic changes to shear wave propagation contours were seen, which can help clarify the cause of the artifacts. To our knowledge, the use of SWPI to explain the etiology of artifacts has never been published before. The artifacts described in this article include the capsule reverberation artifact, penetration limitation or dropout artifact, artifact due to blood vessels, shadowing artifact, tissue motion artifact, and near-field distortion/precompression artifact. Hence, the purpose of this article is to show examples of common artifacts seen on 2D SWE as depicted on corresponding SWPI to demonstrate that both types of image displays are complementary to each other.

Comparison of Superb Microvascular Imaging With Color Flow and Power Doppler Imaging of Small Hepatocellular Carcinomas.

OBJECTIVES: The purpose of this study was to compare Superb Microvascular Imaging (SMI; Toshiba America Medical Systems, Tustin, CA) with conventional color flow Doppler (CFD) and power Doppler (PD) imaging in the liver to distinguish between malignant and benign liver neoplasms. METHODS: After Institutional Review Board approval (number 449984-ED), patients undergoing routine pre-radiofrequency ablation planning ultrasound examinations for suspected hepatocellular carcinomas (HCCs) of less than 2 cm in diameter between January 1, 2015, and July 1, 2016, were prospectively identified. Four readers reviewed the ultrasound images independently for the presence or absence of flow centrally and along the periphery of the lesion. RESULTS: Higher peripheral vessel grades were found on SMI than CFD (P < .001) and PD (P < .001) imaging: in particular, more grade 2 (39% versus 16% and 11%, respectively) and grade 3 (8% versus 0% and 0%). Overall, more central and peripheral vessels were found on SMI than CFD and PD imaging for both HCC lesions (P < .001) and benign lesions (P < .001). Vascular grades were significantly higher in HCC lesions than benign lesions in nearly all cases, although the corresponding area under the curve values were relatively low, at 0.54 to 0.59 for the central vessel grades and 0.63 to 0.64 for the peripheral vessel grades. CONCLUSIONS: More central and peripheral vessels were found around liver lesions on SMI than on CFD and PD imaging. Although there was significantly more vascularity on the periphery of malignant liver lesions than benign lesions, the overall diagnostic performance based on this criterion alone was relatively low, with an area under the curve of 0.64.

Ultrasound Attenuation Imaging vs MRI-PDFF, Echogenicity and Liver Function for Assessing Degree of Steatosis in NAFLD and Non-NAFLD Patients.

ABSTRACT: Nonalcoholic fatty liver disease (NAFLD) is a primary cause of parenchymal liver disease globally. There are currently several methods available to test the degree of steatosis in NAFLD patients, but all have drawbacks that limit their use.The objective of this study is to determine if a new technique, ultrasound (US) attenuation imaging (ATI), correlates with magnetic resonance proton density fat fraction imaging and hepatic echogenicity as seen on gray scale US imaging.Fifty-four patients were recruited at the University of Washington Medical Center from individuals who had already been scheduled for hepatic US or magnetic resonance imaging (MRI). All participants then underwent both hepatic MRI proton density fat fraction and US. Ultrasound images were then evaluated using ATI with 2 observers who individually determined relative grayscale echogenicity.Analysis showed positive correlation between ATI- and MRI-determined fat percentage in the case group (Spearman correlation: 0.50; P = 0.015). Furthermore, participants with NAFLD tended to have a higher ATI than controls (median: 0.70 vs 0.54 dB/cm/MHz; P < 0.001).This study demonstrates that US ATI combined with grayscale imaging is an effective way of assessing the degree of steatosis in patients with moderate to severe NAFLD.

The effect of water intake on ultrasound tissue characteristics and hemodynamics of adult livers.

AIM OF THE STUDY: To assess the effect of water intake on ultrasound tissue characteristics and hemodynamics of adult livers. MATERIAL AND METHODS: In February 2020, we prospectively performed ultrasound shear wave elastography and attenuation imaging (ATI) of the liver parenchyma, and spectral Doppler sonography of the portal vein and hepatic artery in 19 adult healthy volunteers (10 men and 9 women, mean age 27 years, mean body mass index 24.65 kg/m2). We measured liver shear wave velocity (SWV, m/s), shear wave dispersion (SWD, m/s/kHz), attenuation coefficient (dB/cm/MHz), main portal vein velocity (PVV, cm/s), hepatic artery peak systolic velocity (PSV, cm/s), and end diastolic velocity (EDV, cm/s) immediately before and at different time points (15, 30, 45, and 60 minutes) after water intake (1.0 l water and 1.5 l water for body weight < 150 lbs. and ≥ 150 lbs., respectively). RESULTS: The differences in SWV, PVV, hepatic artery PSV and EDV before and after water intake were significant (p < 0.01) whereas the differences in SWD and ATI were not (p > 0.05) based on repeated measures ANOVA tests. The values of SWV, PVV, PSV, and EDV reached a peak at 30-45 minutes and returned to baseline 60 minutes after water intake. We observed positive correlations of SWV with PVV, PSV, and EDV in linear regression analyses (r 2 > 0.73). CONCLUSIONS: Water intake affects the liver stiffness and hemodynamics. No water intake at least one hour prior to liver ultrasound elastography and Doppler sonography is recommended.

Sentinel lymph nodes in the rat: noninvasive photoacoustic and US imaging with a clinical US system.

PURPOSE: To evaluate in vivo sentinel lymph node (SLN) mapping by using photoacoustic and ultrasonographic (US) imaging with a modified clinical US imaging system. MATERIALS AND METHODS: Animal protocols were approved by the Animal Studies Committee. Methylene blue dye accumulation in axillary lymph nodes of seven healthy Sprague-Dawley rats was imaged by using a photoacoustic imaging system adapted from a clinical US imaging system. To investigate clinical translation, the imaging depth was extended up to 2.5 cm by adding chicken or turkey breast on top of the rat skin surface. Three-dimensional photoacoustic images were acquired by mechanically scanning the US transducer and light delivery fiber bundle along the elevational direction. RESULTS: Photoacoustic images of rat SLNs clearly help visualization of methylene blue accumulation, whereas coregistered photoacoustic/US images depict lymph node positions relative to surrounding anatomy. Twenty minutes following methylene blue injection, photoacoustic signals from SLN regions increased nearly 33-fold from baseline signals in preinjection images, and mean contrast between SLNs and background tissue was 76.0 +/- 23.7 (standard deviation). Methylene blue accumulation in SLNs was confirmed photoacoustically by using the optical absorption spectrum of the dye. Three-dimensional photoacoustic images demonstrate dynamic accumulation of methylene blue in SLNs after traveling through lymph vessels. CONCLUSION: In vivo photoacoustic and US mapping of SLNs was successfully demonstrated with a modified clinical US scanner. These results raise confidence that photoacoustic and US imaging can be used clinically for accurate, noninvasive imaging of SLNs for axillary lymph node staging in breast cancer patients.

Comparison of superb microvascular imaging to conventional color Doppler ultrasonography in depicting renal cortical microvasculature.

PURPOSE: The aim of the study was to compare Superb Microvascular Imaging (SMI) to conventional color Doppler ultrasonography (CDUS) and power Doppler ultrasonography (PDUS) in depicting kidney cortical microvasculature. METHODS: Using 3.5 MHz curve-linear array ultrasound transducer and manufacturer recommended color Doppler settings (Canon Medical Systems) for KIDNEY sonography, we performed kidney CDUS, PDUS, and SMI in 20 healthy adults (10 man and 10 women, mean age 27 years). Color Doppler signals in all color Doppler images were quantified by counting color pixel intensity, calculating area ratio of color to total region of interest (AR), and measuring distance of cortical end vessel to the kidney capsule. We used one-way analysis of variance (ANOVA) and post-hoc to test the difference in color pixel intensity, AR, and distance of cortical end vessel to the kidney capsule among SMI, CDUS, and PDUS and in all paired groups. RESULTS: The differences in color pixel intensity, AR, and distance of cortical end vessel to the kidney capsule were significant among CDUS, PDUS, and SMI, as well as in all paired groups (P < 0.001). Color pixel intensity and AR in SMI were significantly higher than CDUS and PDUS (p < 0.001). The distance of cortical end vessel to the kidney capsule in SMI was significantly less than in CDUS and PDUS (p < 0.001). Inter- and intra-observer reliability of quantifying color Doppler images was good (Intraclass correlation coefficient: 0.79-0.92). CONCLUSION: Our results suggest that SMI seems more sensitive than CDUS and PDUS in depicting kidney cortical microvasculature.

Bubble-based acoustic radiation force using chirp insonation to reduce standing wave effects.

Bubble-based acoustic radiation force can measure local viscoelastic properties of tissue. High intensity acoustic waves applied to laser-generated bubbles induce displacements inversely proportional to local Young's modulus. In certain instances, long pulse durations are desirable but are susceptible to standing wave artifacts, which corrupt displacement measurements. Chirp pulse acoustic radiation force was investigated as a method to reduce standing wave artifacts. Chirp pulses with linear frequency sweep magnitudes of 100, 200 and 300 kHz centered around 1.5 MHz were applied to glass beads within gelatin phantoms and laser-generated bubbles within porcine lenses. The ultrasound transducer was translated axially to vary standing wave conditions, while comparing displacements using chirp pulses and 1.5 MHz tone burst pulses of the same duration and peak rarefactional pressure. Results demonstrated significant reduction in standing wave effects using chirp pulses, with displacement proportional to acoustic intensity and bubble size.

Bubble-based acoustic radiation force elasticity imaging.

Acoustic radiation force is applied to bubbles generated by laser-induced optical breakdown (LIOB) to study viscoelastic properties of the surrounding medium. In this investigation, femtosecond laser pulses are focused in the volume of gelatin phantoms of different concentrations to form bubbles. A two-element confocal ultrasonic transducer generates acoustic radiation force on individual bubbles while monitoring their displacement within a viscoelastic medium. Tone burst pushes of varying duration have been applied by the outer element at 1.5 MHz. The inner element receives pulse-echo recordings at 7.44 MHz before, during, and after the excitation bursts, and crosscorrelation processing is performed offline to monitor bubble position. Maximum bubble displacements are inversely related to the Young's moduli for different gel phantoms, with a maximum bubble displacement of over 200 microm in a gel phantom with a Young's modulus of 1.7 kPa. Bubble displacements scale with the applied acoustic radiation force and displacements can be normalized to correct for differences in bubble size. Exponential time constants for bubble displacement curves are independent of bubble radius and follow a decreasing trend with the Young's modulus of the surrounding medium. These results demonstrate the potential for bubble-based acoustic radiation force methods to measure tissue viscoelastic properties.

Dual-Modality Photoacoustic and Ultrasound Imaging System for Noninvasive Sentinel Lymph Node Detection in Patients with Breast Cancer.

The detection of regional lymph node metastases is important in cancer staging as it guides the prognosis of the patient and the strategy for treatment. Sentinel lymph node biopsy (SLNB) is an accurate, less invasive alternative to axillary lymph node dissection. The sentinel lymph node hypothesis states that the pathological status of the axilla can be accurately predicted by determining the status of the first lymph nodes that drain from the primary tumor. Physicians use radio-labeled sulfur colloid and/or methylene blue dye to identify the SLN, which is most likely to contain metastatic cancer cells. However, the surgical procedure causes morbidity and associated expenses. To overcome these limitations, we developed a dual-modality photoacoustic and ultrasonic imaging system to noninvasively detect SLNs based on the accumulation of methylene blue dye. Ultimately, we aim to guide percutaneous needle biopsies and provide a minimally invasive method for axillary staging of breast cancer.

Performance characterization of an integrated ultrasound, photoacoustic, and thermoacoustic imaging system.

We developed a novel trimodality system for human breast imaging by integrating photoacoustic (PA) and thermoacoustic (TA) imaging techniques into a modified commercial ultrasound scanner. Because light was delivered with an optical assembly placed within the microwave antenna, no mechanical switching between the microwave and laser sources was needed. Laser and microwave excitation pulses were interleaved to enable PA and TA data acquisition in parallel at a rate of 10 frames per second. A tube (7 mm inner diameter) filled with oxygenated bovine blood or 30 mM methylene blue dye was successfully detected in PA images in chicken breast tissue at depths of 6.6 and 8.4 cm, respectively, for the first time. The SNRs at these depths reached ∼24 and ∼15 dB, respectively, by averaging 200 signal acquisitions. Similarly, a tube (13 mm inner diameter) filled with saline solution (0.9%) at a depth of 4.4 cm in porcine fat tissue was successfully detected in TA images. The PA axial, lateral, and elevational resolutions were 640 µm, 720 µm, and 3.5 mm, respectively, suitable for breast cancer imaging. A PA noise-equivalent sensitivity to methylene blue solution of 260 nM was achieved in chicken tissue at a depth of 3.4 cm.

Multimodal sentinel lymph node mapping with single-photon emission computed tomography (SPECT)/computed tomography (CT) and photoacoustic tomography.

The identification of cancer cells in the lymph nodes surrounding a tumor is important in establishing a prognosis. Optical detection techniques such as fluorescence and photoacoustic tomography (PAT) have been reported in preclinical studies for noninvasive sentinel lymph node (SLN) mapping. A method for validation of these techniques is needed for clinical trials. We report the use of a multimodal optical-radionuclear contrast agent as a validation tool for PAT in a preclinical model. Methylene blue (MB) was radiolabeled with (125)I for multimodal SLN mapping and used in conjunction with MB to assess the feasibility of multimodal SLN mapping in a rat model by PAT and single-photon emission computed tomography (SPECT). MB provided sufficient contrast for identifying SLNs noninvasively with a PAT system adapted from a clinical ultrasound imaging system. The signal location was corroborated by SPECT using (125)I labeled MB. The translation of PAT into the clinic can be facilitated by a direct comparison with established imaging methods using a clinically relevant dual SPECT and photoacoustic imaging agent. The new high-resolution PAT is a promising technology for the sensitive and accurate SLN detection in cancer patients.

Multi-scale molecular photoacoustic tomography of gene expression.

Photoacoustic tomography (PAT) is a molecular imaging technology. Unlike conventional reporter gene imaging, which is usually based on fluorescence, photoacoustic reporter gene imaging relies only on optical absorption. This work demonstrates several key merits of PAT using lacZ, one of the most widely used reporter genes in biology. We show that the expression of lacZ can be imaged by PAT as deep as 5.0 cm in biological tissue, with resolutions of ∼1.0 mm and ∼0.4 mm in the lateral and axial directions, respectively. We further demonstrate non-invasive, simultaneous imaging of a lacZ-expressing tumor and its surrounding microvasculature in vivo by dual-wavelength acoustic-resolution photoacoustic microscopy (AR-PAM), with a lateral resolution of 45 µm and an axial resolution of 15 µm. Finally, using optical-resolution photoacoustic microscopy (OR-PAM), we show intra-cellular localization of lacZ expression, with a lateral resolution of a fraction of a micron. These results suggest that PAT is a complementary tool to conventional optical fluorescence imaging of reporter genes for linking biological studies from the microscopic to the macroscopic scales.

In vivo three-dimensional photoacoustic imaging based on a clinical matrix array ultrasound probe.

We present an integrated photoacoustic and ultrasonic three-dimensional (3-D) volumetric imaging system based on a two-dimensional (2-D) matrix array ultrasound probe. A wavelength-tunable dye laser pumped by a Q-switched Nd:YAG laser serves as the light source and a modified commercial ultrasound imaging system (iU22, Philips Healthcare) with a 2-D array transducer (X7-2, Philips Healthcare) detects both the pulse-echo ultrasound and photoacoustic signals. A multichannel data acquisition system acquires the RF channel data. The imaging system enables rendering of co-registered 3-D ultrasound and photoacoustic images without mechanical scanning. The resolution along the azimuth, elevation, and axial direction are measured to be 0.69, 0.90 and 0.84 mm for photoacoustic imaging. In vivo 3-D photoacoustic mapping of the sentinel lymph node was demonstrated in a rat model using methylene blue dye. These results highlight the clinical potential of 3-D PA imaging for identification of sentinel lymph nodes for cancer staging in humans.

Performance benchmarks of an array-based hand-held photoacoustic probe adapted from a clinical ultrasound system for non-invasive sentinel lymph node imaging.

Clinical translation of photoacoustic (PA) imaging can be facilitated by integration with commercial ultrasound (US) scanners to enable dual-modality imaging. An array-based US scanner was modified for hand-held PA imaging. The performance was benchmarked in terms of signal-to-noise ratio (SNR), axial spatial resolution and sensitivity. PA images of a tube, filled with methylene blue (MB; approx. 30 mM) and placed at various depths in chicken tissue, were acquired. A 5 cm penetration depth was achieved with an 18.6 dB SNR using a laser fluence of 3 mJ cm(-2), only one-seventh of the safety limit (20 mJ cm(-2)). An axial resolution of approximately 400 µm was maintained at all imaging depths. The PA sensitivity to MB placed 2.3 cm deep in chicken tissue was less than 100 µM. Further, after intradermal injection of MB (approx. 30 mM), a rat sentinel lymph node was clearly identified in vivo, beneath a 3.8 cm thick layer of chicken breast. The accumulated concentration of MB in the node was estimated to be approximately 7 mM. The noise-equivalent sensitivities (approx. 2 cm depth) were 17 and 85 µM, ex vivo and in vivo, respectively. These results support the use of this PA system for non-invasive mapping and image-guided needle biopsy of sentinel nodes in breast cancer patients.

Quantitative photoacoustic imaging: correcting for heterogeneous light fluence distributions using diffuse optical tomography.

The specificity of molecular and functional photoacoustic (PA) images depends on the accuracy of the photoacoustic absorption spectroscopy. The PA signal is proportional to the product of the optical absorption coefficient and local light fluence; quantitative PA measurements of the optical absorption coefficient therefore require an accurate estimation of optical fluence. Light-modeling aided by diffuse optical tomography (DOT) can be used to map the required fluence and to reduce errors in traditional PA spectroscopic analysis. As a proof-of-concept, we designed a tissue-mimicking phantom to demonstrate how fluence-related artifacts in PA images can lead to misrepresentations of tissue properties. To correct for these inaccuracies, the internal fluence in the tissue phantom was estimated by using DOT to reconstruct spatial distributions of the absorption and reduced scattering coefficients of multiple targets within the phantom. The derived fluence map, which only consisted of low spatial frequency components, was used to correct PA images of the phantom. Once calibrated to a known absorber, this method reduced errors in estimated absorption coefficients from 33% to 6%. These results experimentally demonstrate that combining DOT with PA imaging can significantly reduce fluence-related errors in PA images, while producing quantitatively accurate, high-resolution images of the optical absorption coefficient.

Deeply penetrating in vivo photoacoustic imaging using a clinical ultrasound array system.

Using a hand-held photoacoustic probe integrated with a clinical ultrasound array system, we successfully imaged objects deeply positioned in biological tissues. The optical contrasts were enhanced by methylene blue with a concentration of ~30 mM. The penetration depth reached ~5.2 cm in chicken breast tissue by using 650-nm wavelength, which is ~4.7 times the 1/e optical penetration depth. This imaging depth was achieved using a laser fluence on the tissue surface of only 3 mJ/cm(2), which is 1/7 of the American National Standards Institute (ANSI) safety limit (20 mJ/cm(2)). The noise equivalent sensitivity at this depth was ~11 mM. Further, after intradermal injection of methylene blue in a rat, a sentinel lymph node was easily detected in vivo, beneath a 2-cm thick layer of chicken breast. Also, blood located 3.5 cm deep in the rat was clearly imaged with intrinsic contrast. We have photoacoustically guided insertion of a needle into a rat sentinel lymph node with accumulated methylene blue. These results highlight the clinical potential of photoacoustic image-guided identification and needle biopsy of sentinel lymph nodes for axillary staging in breast cancer patients.

Handheld array-based photoacoustic probe for guiding needle biopsy of sentinel lymph nodes.

By modifying a clinical ultrasound array system, we develop a novel handheld photoacoustic probe for image-guided needle biopsy. The integration of optical fiber bundles for pulsed laser light delivery enables photoacoustic image-guided insertion of a needle into rat axillary lymph nodes with accumulated indocyanine green (ICG). Strong photoacoustic contrast of the needle is achieved. After subcutaneous injection of the dye in the left forepaw, sentinel lymph nodes are easily detected, in vivo and in real time, beneath 2-cm-thick chicken breast overlaying the axillary region. ICG uptake in axillary lymph nodes is confirmed with fluorescence imaging both in vivo and ex vivo. These results demonstrate the clinical potential of this handheld photoacoustic system for facile identification and needle biopsy of sentinel lymph nodes for cancer staging and metastasis detection in humans.