Photoacoustic imaging (PAI) is a promising medical imaging modality providing the spatial resolution of ultrasound imaging and the contrast of optical imaging. For linear-array PAI, a beamformer can be used as the reconstruction algorithm. Delay-and-sum (DAS) is the most prevalent beamforming algorithm in PAI. However, using DAS beamformer leads to low-resolution images as well as high sidelobes due to nondesired contribution of off-axis signals. Coherence factor (CF) is a weighting method in which each pixel of the reconstructed image is weighted, based on the spatial spectrum of the aperture, to mainly improve the contrast. We demonstrate that the numerator of the formula of CF contains a DAS algebra and propose the use of a delay-multiply-and-sum beamformer instead of the available DAS on the numerator. The proposed weighting technique, modified CF (MCF), has been evaluated numerically and experimentally compared to CF. It was shown that MCF leads to lower sidelobes and better detectable targets. The quantitative results of the experiment (using wire targets) show that MCF leads to for about 45% and 40% improvement, in comparison with CF, in the terms of signal-to-noise ratio and full-width-half-maximum, respectively.
Background: Chorioamnionitis from ascending bacterial infection through the endocervix is a potential risk factor for cerebral palsy. Tetrahydrobiopterin, an essential cofactor for nitric oxide synthase (NOS) and amino acid hydroxylases, when augmented in the fetal brain, prevents some of the cerebral palsy-like deficits in a rabbit hypoxia-ischemia model. Objectives: To study the effect of lipopolysaccharide (LPS)-induced intrauterine inflammation in preterm gestation on motor deficits in the newborn, and whether biosynthesis of tetrahydrobiopterin or inflammatory mediators is affected in the fetal brain. Methods: Pregnant rabbits at 28 days gestation (89% term) were administered either saline or LPS into both endocervical openings. One group underwent spontaneous delivery, and neurobehavioral tests were performed at postnatal day (P) 1 and P11, with some kits being sacrificed at P1 for histological analysis. Another group underwent Cesarean section 24 h after LPS administration. Gene sequences for rabbit biosynthetic enzymes of tetrahydrobiopterin pathways were determined and analyzed in addition to cytokines, using quantitative real-time polymerase chain reaction. Results: Exposure to 200 μg/kg/mL LPS caused a locomotion deficit and mild hypertonia at P1. By P11, most animals turned into normal-appearing kits. There was no difference in neuronal cell death in the caudate between hypertonic and nonhypertonic kits at P1 (n = 3–5 in each group). Fetal brain GTP cyclohydrolase I was increased, whereas sepiapterin reductase and 6-pyruvoyltetrahydropterin synthase were decreased, 24 h after LPS administration. Neuronal NOS was also increased. Regardless of the position in the uterus or the brain region, expression of TNF-α and TGF-β was decreased, whereas that of IL-1β, IL-6, and IL-8 was increased (n = 3–4 in each group). Conclusions: This is the first study using an ascending LPS-induced intrauterine inflammation model in rabbits, showing mostly transient hypertonia and mainly locomotor deficits in the kits. Not all proinflammatory cytokines are increased in the fetal brain following LPS administration. Changes in key tetrahydrobiopterin biosynthetic enzymes possibly indicate different effects of the inflammatory insult.
Visualization and detection of early-stage gynecological malignancies represents a challenge for imaging due to limiting factors including tissue accessibility, device ease of use, and accuracy of imaging modalities. In this work, we introduce a miniaturized phased-array ultrasound and photoacoustic endoscopic probe which is capable of providing structural, functional, and molecular data for the characterization of gynecologic disease. The proposed probe consists of a 64-element ultrasound phased-array transducer coupled to a fiber-optic light delivery system for co-registered ultrasound and photoacoustic imaging. The fabricated US and PA imaging endoscope’s diameter is 7.5 mm, allowing for potential passage through the cervical canal and thus an intimate contact with gynecological tissues such as the cervical canal and uterus. The developed endoscopic probe was tested and characterized in a set of tissue-mimicking phantoms. US and PA resolutions were measured experimentally using 200 μm diameter wires, resulting in apparent axial and lateral diameters of 289 μm and 299 μm for US, and 308 μm and 378 μm for PA, respectively. The probe’s abilities to operate in both discrete and integrated illumination/acquisition were tested in gelatin phantoms with embedded optical absorbers with the results demonstrating the ability to acquire volumetric dual-modal US and PA images.
The uterine cervix plays a central role in the maintenance of pregnancy and in the process of parturition. Cervical remodeling involves dramatic changes in extracellular matrix composition and, in particular, of collagen and water content during cervical ripening (a term that describes the anatomical, biochemical, and physiologic changes in preparation for labor). Untimely cervical ripening in early gestation predisposes to preterm labor and delivery, the leading cause of infant death worldwide. Inadequate ripening of the cervix is associated with failure of induction or prolonged labor. The current approach to evaluate the state of the cervix relies on digital examination and sonographic examination. Herein, we present a novel imaging method that combines ultrasound (US) and photoacoustic (PA) techniques to evaluate cervical remodeling by assessing the relative collagen and water content of this organ. The method was tested in vitro in extracted collagen phantoms and ex vivo in murine cervical tissues that were collected in mid-pregnancy and at term. We report, for the first time, that our imaging approach provides information about the molecular changes in the cervix at different gestational ages. There was a strong correlation between the results of PA imaging and the histological assessment of the uterine cervix over the course of gestation. These findings suggest that PA imaging is a powerful method to assess the biochemical composition of the cervix and open avenues to non-invasively investigate the composition of this organ, which is essential for reproductive success.
Significance: Photoacoustic imaging (PAI) has been greatly developed in a broad range of diagnostic applications. The efficiency of light to sound conversion in PAI is limited by the ubiquitous noise arising from the tissue background, leading to a low signal-to-noise ratio (SNR), and thus a poor quality of images. Frame averaging has been widely used to reduce the noise; however, it compromises the temporal resolution of PAI. Aim: We propose an approach for photoacoustic (PA) signal denoising based on a combination of low-pass filtering and sparse coding (LPFSC). Approach: LPFSC method is based on the fact that PA signal can be modeled as the sum of low frequency and sparse components, which allows for the reduction of noise levels using a hybrid alternating direction method of multipliers in an optimization process. Results: LPFSC method was evaluated using in-silico and experimental phantoms. The results show a 26% improvement in the peak SNR of PA signal compared to the averaging method for in-silico data. On average, LPFSC method offers a 63% improvement in the image contrast-tonoise ratio and a 33% improvement in the structural similarity index compared to the averaging method for objects located at three different depths, ranging from 10 to 20 mm, in a porcine tissue phantom. Conclusions: The proposed method is an effective tool for PA signal denoising, whereas it ultimately improves the quality of reconstructed images, especially at higher depths, without limiting the image acquisition speed.
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