Diego S. Dumani

The applications of ultrasound imaging are often limited due to low contrast, which arises from the comparable acoustic impedance of normal tissues and disease sites. To improve the low contrast, we propose a contrast agent called gas-generating laser-activatable nanorods for contrast enhancement (GLANCE), which enhances ultrasound imaging contrast in two ways. First, GLANCE absorbs near-infrared lasers and generates nitrogen gas bubbles through the photocatalytic function of gold nanorods and photolysis of azide compounds. These gas bubbles decrease the acoustic impedance and highlight the injection site from the surrounding tissues. Second, GLANCE exhibits photoacoustic properties owing to the gold nanorods that emit photoacoustic signals upon laser irradiation. Additionally, GLANCE offers several benefits for biomedical applications such as nanometer-scale size, adjustable optical absorption, and biocompatibility. These distinctive features of GLANCE would overcome the limitations of conventional ultrasound imaging and facilitate the accurate diagnosis of various diseases.

Photoacoustic (PA) imaging (PAI) is an emerging imaging modality that takes advantage of the optical absorption properties of tissue to generate high-resolution in vivo images of anatomy and exogenous chromophores.¹ PAI works through the PA effect: pulsed light delivered to a sample is absorbed, thereby causing thermoelastic expansion and generating acoustic pressure waves.^2,3 These acoustic waves are detected by an ultrasound transducer and reconstructed to form an image. In the near-infrared (NIR) window, oxyhemoglobin and deoxyhemoglobin have relatively high optical absorption coefficients compared to other endogenous chromophores, enabling PAI to detect and highlight blood-rich tissues in vivo with a spatial resolution comparable to ultrasound imaging.^{4 – 8} The high contrast and sensitivity of these blood-rich tissues are responsible for PAI’s suitability as a functional and anatomical imaging modality. Additionally, PA contrast agents are designed with optical absorption properties that can be exploited with multispectral (i.e., multiwavelength) PAI.^{9 – 14}

Photothermal therapy (PTT) is a type of cancer treatment capable of damaging tumors using laser irradiation. This procedure can be a promising approach to complement current cancer therapies, due in part to its minimal invasiveness. One of the challenges of photothermal therapy is the potential collateral damage to the surrounding healthy tissue, as well as excessive temperature increase in the target tumor region that can cause tissue carbonization and evaporation. With the aim of increasing the performance of photothermal therapy in damaging targeted tumor while keeping healthy nearby tissue unaffected, this research proposes the use of a feedback control system that considers the cumulative thermal damage to both types of tissue. Two separate control algorithms (fuzzy logic and PI) were designed and tested in silico using simulations made in MATLAB® and Python. Results showed that both controllers successfully accomplished the proposed goals. Therefore, the feasibility of using these automated systems to improve the efficacy and safety of PTT was demonstrated.

Lymph node mapping is important in cancer immunotherapy because the morphology of lymph nodes is one of the crucial evaluation criteria of immune responses. We developed new theragnostic glycol-chitosan-coated gold nanoparticles (GC-AuNPs), which highlighted lymph nodes in ultrasound-guided photoacoustic (US/PA) imaging. Moreover, the ovalbumin epitope was conjugated GC-AuNPs (OVA-GC-AuNPs) for delivering tumor antigen to lymph node resident macrophage. In vitro studies proved the vigorous endocytosis activity of J774A.1 macrophage and consequent strong photoacoustic signals from them. The macrophages also presented a tumor antigen when OVA-GC-AuNPs were used for cellular uptake. After the lingual injection of GC-AuNPs into healthy mice, cervical lymph nodes were visible in a US/PA imaging system with high contrast. Three-dimensional analysis of lymph nodes revealed that the accumulation of GC-AuNPs in the lymph node increased as the post-injection time passed. Histological analysis showed GC-AuNPs or OVA-GC-AuNPs located in subcapsular and medullar sinuses where macrophages are abundant. Our new theragnostic GC-AuNPs present a superior performance in US/PA imaging of lymph nodes without targeting moieties or complex surface modification. Simultaneously, GC-AuNPs were able to deliver tumor antigens to cause macrophages to present the OVA epitope at targeted lymph nodes, which would be valuable for cancer immunotherapy.

A minimally invasive method to detect lymph node metastases using glycol-chitosan-coated gold nanoparticles and ultrasound-guided photoacoustic imaging.

Photoacoustics has emerged as a high-contrast imaging modality that provides optical absorption maps inside of tissues, therefore complementing morphological information of conventional ultrasound. The laser-generated photoacoustic waves are usually envelope-detected, thus disregarding the specific waveforms generated by each photoabsorber. Here we propose a sparsity-promoting image reconstruction method that allows the estimation of each photoabsorber's source-time function. Preliminary studies showed the ability to reconstruct the optical absorption map of an in silico vessel phantom. By using a sparsity-promoting imaging method, absorption maps and source-time functions can still be recovered even in situations where the number of transducers is decreased by a factor of six. Moreover, the recovery is able to attain higher resolution than conventional beamforming methods. Because our method recovers the source-time function of the absorbers, it could potentially also be used to distinguish different types of photoabsorbers, or the degree of aggregation of exogenous agents, under the assumption that these would generate different source-time functions at the moment of laser irradiation.

Photoacoustic imaging using exogenous contrast agents has emerged as a hybrid technique that enables the deep imaging of optical properties of tissues with high spatial resolution. The power of this imaging technique can be greatly enhanced by the use of contrast agents that absorb at near-infrared wavelengths and whose optical properties can be modulated in response to the local environment. We have designed contrast agents consisting of gold nanoparticles coated with anisotropic silica nanoshells. The tunable aggregation of these janus particles in cell culture media resulted in a dramatic amplification of photoacoustic signals in the near-infrared region. We also demonstrated imaging using these contrast agents in mammalian cells, including macrophages and breast cancer cells as well as in vivo. The ability to modulate janus particle aggregation in response to a range of stimuli in combination with the high resolution and deep penetration of multiwavelength photoacoustic imaging are attractive for a broad range of applications in diagnostic imaging and theranostics.

We have developed a preclinical 3D imaging instrument integrating photoacoustic tomography and fluorescence (PAFT) addressing known deficiencies in sensitivity and spatial resolution of the individual imaging components. PAFT is designed for simultaneous acquisition of photoacoustic and fluorescence orthogonal projections at each rotational position of a biological object, enabling direct registration of the two imaging modalities. Orthogonal photoacoustic projections are utilized to reconstruct large (21 cm³ ) volumes showing vascularized anatomical structures and regions of induced optical contrast with spatial resolution exceeding 100 µm. The major advantage of orthogonal fluorescence projections is significant reduction of background noise associated with transmitted or backscattered photons. The fluorescence imaging component of PAFT is used to boost detection sensitivity by providing low-resolution spatial constraint for the fluorescent biomarkers. PAFT performance characteristics were assessed by imaging optical and fluorescent contrast agents in tissue mimicking phantoms and in vivo. The proposed PAFT technology will enable functional and molecular volumetric imaging using fluorescent biomarkers, nanoparticles, and other photosensitive constructs mapped with high fidelity over robust anatomical structures, such as skin, central and peripheral vasculature, and internal organs.