Optical imaging [1
] has received great attention in biomedicine because of its rich contrast and nonionizing radiation. However, due to strong light scattering, pure optical imaging modalities suffer from either shallow penetration depth (e.g., confocal microscopy [2
], two-photon microscopy [3
], and optical coherence tomography [4
]) or poor spatial resolution (e.g., diffuse optical tomography (DOT) [5
]). The maximum penetration depth of optical microscopy using ballistic or quasi-ballistic photons is typically limited to one optical transport mean free path (~1 mm). Using diffusive photons, DOT with model-based reconstruction is able to provide both optical scattering and absorption parameters, so the penetration depth is extended to a few centimeters. However, this technique struggles with poor spatial resolution, typically 1/5 of the imaging depth. This fundamental issue of light diffusion has hindered pure optical imaging techniques from achieving widespread clinical application.
Photoacoustic (PA) imaging [1
] has overcome the drawback of pure optical imaging by taking advantage of rich optical contrast and ultrasonic spatial resolution for deep imaging. It is capable of high-resolution structural, functional, and molecular imaging free from speckle artifacts. More importantly, because of its ultrasonic detection mechanism, the penetration depth and spatial resolution are scalable even beyond the optical transport mean free path in optically scattering media. Centimeter-scale imaging depths have been achieved. Oraevsky et al.
demonstrated PA imaging in tissue mimicking phantoms and biological tissues at penetration depths exceeding 5 cm [7
]. By enhancing the optical contrast with indocyanine green, Ku et al.
photoacoustically imaged objects embedded at depths of greater than 5 cm in biological tissues [8
Recently, PA imaging has been proposed as a noninvasive method of identifying sentinel lymph nodes and guiding fine needle aspiration or core needle biopsies. Sentinel lymph node biopsy (SLNB) is the emerging standard for axillary lymph node staging in clinically node-negative breast cancer patients [9
]. Axillary staging is critical in planning appropriate treatment and estimating patient prognosis. The current SLNB technique requires injection of blue dyes and/or radioactive tracers, followed by surgical removal of sentinel nodes for pathological examination. Compared with the current surgical SLNB, the photoacoustically guided minimally invasive approach has the potential to significantly reduce the impact on patients. Song et al.
imaged in vivo
deeply positioned (>3 cm) rat sentinel lymph nodes (SLNs) stained with either methylene blue or gold nanocages [12
]. We have previously reported in vivo
PA and ultrasound (US) mapping of SLNs in rats using a clinical US array [15
]. An US probe combined with a fiber-based light delivery system enabled hand-held scanning analogous to ultrasonography [16
]. Further, this hand-held probe enabled photoacoustic image-guided needle insertion [17
]. Combined US and PA imaging systems provide US imaging for locating lymph nodes and PA imaging for identifying which nodes are sentinel based on the accumulation of blue dye. US imaging alone cannot identify which lymph nodes are sentinel nodes.
In this paper, we demonstrate deeply penetrating PA imaging using a hand-held PA/US probe with a modified clinical US array system. We successfully imaged a tube filled with methylene blue (~30 mM) at a depth of 5.2 cm in chicken breast tissues. This imaging depth was achieved using a light fluence on the tissue surface of only 3 mJ/cm2
, 1/7 of the ANSI safety limit [19
]. Further, we report noninvasive in vivo
imaging of deeply positioned (~2 cm) methylene-blue-dyed SLNs and metal needles in rats.