Improvement In Lateral Resolution Research Articles

Quantitative comparison of frequency-domain and delay-and-sum optoacoustic image reconstruction including the effect of coherence factor weighting

Image reconstruction in optoacoustic imaging is often based on a delay-and-sum (DAS) or a frequency domain (FD) algorithm. In this study, we performed a comprehensive comparison of these two algorithms together with coherence factor (CF) weighting using phantom and in-vivo mouse data obtained with optoacoustic microscopy. For this purpose we developed an FD based definition of the CF. Our results reveal the equivalence of DAS and FD, with and without CF weighting, in terms of spatial resolution and contrast-to-noise ratio (CNR) but highlight the clear advantage of FD in terms of computational cost, making it preferable for 3D reconstruction or real-time applications. An important additional result of this research is that, contradictory to previous studies, CF weighting does not lead to any improvement in lateral resolution.

Two-Dimensional Spatial Coherence for Ultrasonic DMAS Beamforming in Multi-Angle Plane-Wave Imaging

Ultrasonic multi-angle plane-wave (PW) coherent compounding relies on delay-and-sum (DAS) beamforming of two-dimensional (2D) echo matrix in both the dimensions PW transmit angle and receiving channel to construct each image pixel. Due to the characteristics of DAS beamforming, PW coherent compounding may suffer from high image clutter when the number of transmit angles is kept low for ultrafast image acquisition. Delay-multiply-and-sum (DMAS) beamforming exploits the spatial coherence of the receiving aperture to suppress clutter interference. Previous attempts to introduce DMAS beamforming into multi-angle PW imaging has been reported but only in either dimension of the 2D echo matrix. In this study, a novel DMAS operation is proposed to extract the 2D spatial coherence of echo matrix for further improvement of image quality. The proposed 2D-DMAS method relies on a flexibly tunable p value to manipulate the signal coherence in the beamforming output. For p = 2.0 as an example, simulation results indicate that 2D-DMAS outperforms other one-dimensional DMAS methods by at least 9.3 dB in terms of ghost-artifact suppression. Experimental results also show that 2D-DMAS provides the highest improvement in lateral resolution by 32% and in image contrast by 15.6 dB relative to conventional 2D-DAS beamforming. Nonetheless, since 2D-DMAS emphasizes signal coherence more than its one-dimensional DMAS counterparts, it suffers from the most elevated speckle variation and the granular pattern in the tissue background.

Optical coherence refraction tomography.

Optical coherence tomography (OCT) is a cross-sectional, micron-scale imaging modality with widespread clinical application. Typical OCT systems sacrifice lateral resolution to achieve long depths of focus for bulk tissue imaging, and hence tend to have better axial than lateral resolution. Such anisotropic resolution can obscure fine ultrastructural features. Furthermore, conventional OCT suffers from refraction-induced image distortions. Here, we introduce optical coherence refraction tomography (OCRT), which extends the superior axial resolution to the lateral dimension, synthesising undistorted cross-sectional image reconstructions from multiple conventional images acquired with angular diversity. In correcting refraction-induced distortions to register the OCT images, OCRT also achieves spatially resolved refractive index imaging. We demonstrate >3-fold improvement in lateral resolution as well as speckle reduction in imaging tissue ultrastructure, consistent with histology. With further optimisation in optical designs to incorporate angular diversity into clinical instruments, OCRT could be widely applied as an enhancement over conventional OCT.

A Confocal Reflection Super-Resolution Technique to Image Golgi-Cox Stained Neurons.

Metal-based Golgi-Cox (GC) staining is an established method used to visualise neurons with great morphological detail. Although GC stained samples are imaged routinely under transmitted light microscopy, this method is unable to yield information on the three-dimensional structure of dendrites and neurons and thus help reveal the connective properties of the central nervous system. Although a few studies have attempted simultaneous visualisation of GC staining and antigen-specific fluorescent labelling under a confocal reflection technique, the resolution of both confocal reflection and fluorescence modalities used to acquire GC reflection and fluorescently stained antibody signals are still limited by the diffraction limit of light at about 220 nm. Here, we report a confocal reflection super-resolution technique (CRSR) to break this diffraction barrier, which is achieved by minimising the pinhole size from 1 airy unit (AU) to 0.1 AU. This is achieved by minimising or closing the confocal pinhole size and is possible in this reflection modality, unlike fluorescence, because it is not a photon limited technique. Utilising the lowest wavelength of light available in the system (405 nm), the CRSR technique results in ∼30% lateral and axial resolution improvement. We also show that the CRSR technique can be used in conjunction to visualise both GC and immunofluorescence targets to create precise and improved three-dimensional visualisation and analysis. In addition, using these superresolution confocal reflection data sets from GC in CRSR mode significantly reduced the data overestimation, improving the accuracy of statistical analysis of dendritic spine density and average spine dimensions. Combining the 0.1 AU setting with deconvolution routines, the signal-to-noise ratio and resolution could further be improved an additional ∼20-25%, yielding CRSR images with resolutions up to 2-fold over the diffraction limit both laterally and axially. The improved precision of both visualisation and quantification of subdiffraction limited dendritic spines using the CRSR technique may prove to be critical in investigations that concern changes in detailed neuron morphology under central nervous system disease conditions such as multiple sclerosis and Alzheimer's disease. LAY DESCRIPTION: For over a century, Golgi-Cox (GC) has been a leading staining technique in the field of neuroscience, used to visualise neurons with great morphological detail. GC stained brain or spinal cord samples are conventionally visualised under transmitted light techniques. This limits the view of Golgi-staining to a two-dimensional image. A recent report showed that Golgi staining can be visualised in three-dimensions using the reflection modality of the confocal microscope. This visualisation also allows for the simultaneous acquisition of immunofluorescence signals. However, the reported resolution of Golgi staining confocal reflection is limited by the diffraction limit of light, which is around 220 nm. Here, we report a superresolution confocal reflection technique (CRSR) that achieves superresolution by minimising the pinhole size used in confocal microscopy. The CRSR technique results in ∼30% lateral and axial resolution improvement. Adding a deconvolution step in the final processing could improve the SNR and resolution even further up to 2-fold improvement in resolution over the diffraction limit both laterally and axially. We hope that this improved visualisation will help in investigations that concern changes in detailed neuron morphology under central nervous system disease conditions such as multiple sclerosis and Alzheimer's disease.

Lateral Resolution Improvement in Ultrasound Imaging System using Compressed Sensing: Initial Results.

Compressed-Sensing (CS) has been applied to ultrasound imaging to reduce data or to reduce the data acquisition time. There appears to be no report that uses CS framework to reduce the number of active receive elements in Conventional Focused Beamforming (CFB). Thus, in our previous work, a novel undersampling scheme based on Gaussian distribution was investigated and reported for reducing the number of active receive elements and data in CFB. In this paper, we exploit the Gaussian sampling based CS framework to improve the lateral resolution (LR) of the ultrasound system without increasing the system's complexity and cost. A notable difference from our previous work being the use of waveatom as the sparsifying basis, instead of 2D-Fourier basis, and analysis of the proposed framework for different receive aperture sizes. Simulation data for this study were generated using Field II, and experimental data were acquired from an in-vitro cyst phantom using Verasonics V-64 ultrasound scanner. The results indicate that the proposed framework of choosing a limited number of receive elements from a receive aperture length that is three or four times the corresponding active aperture size obtained from the same number of consecutive receive elements yields nearly twice an improvement in LR and about 27% increase in contrast to that of CFB reference image. Thus, the findings suggest a possibility to improve the LR of the current ultrasound system without increasing the system complexity, which will be significant for affordable point-of-care ultrasound systems.

Compressed sensing reconstruction of synthetic transmit aperture dataset for volumetric diverging wave imaging

A high volume rate and high performance ultrasound imaging method based on a matrix array is proposed by using compressed sensing (CS) to reconstruct the complete dataset of synthetic transmit aperture (STA) from three-dimensional (3D) diverging wave transmissions (i.e. 3D CS-STA). Hereto, a series of apodized 3D diverging waves are transmitted from a fixed virtual source, with the ith row of a Hadamard matrix taken as the apodization coefficients in the ith transmit event. Then CS is used to reconstruct the complete dataset, based on the linear relationship between the backscattered echoes and the complete dataset of 3D STA. Finally, standard STA beamforming is applied on the reconstructed complete dataset to obtain the volumetric image. Four layouts of element numbering for apodizations and transmit numbers of 16, 32 and 64 are investigated through computer simulations and phantom experiments. Furthermore, the proposed 3D CS-STA setups are compared with 3D single-line-transmit (SLT) and 3D diverging wave compounding (DWC). The results show that, (i) 3D CS-STA has competitive lateral resolutions to 3D STA, and their contrast ratios (CRs) and contrast-to-noise ratios (CNRs) approach to those of 3D STA as the number of transmit events increases in noise-free condition. (ii) the tested 3D CS-STA setups show good robustness in complete dataset reconstruction in the presence of different levels of noise. (iii) 3D CS-STA outperforms 3D SLT and 3D DWC. More specifically, the 3D CS-STA setup with 64 transmit events and the Random layout achieves ~31% improvement in lateral resolution, ~14% improvement in ratio of the estimated-to-true cystic areas, a higher volume rate, and competitive CR/CNR when compared with 3D DWC. The results demonstrate that 3D CS-STA has great potential of providing high quality volumetric image with a higher volume rate.

Improvement in lateral resolution of through-transmission scanning acoustic tomography using capacitive micromachined ultrasound transducer

We present the first through-transmission images in scanning acoustic tomography (SAT) using a capacitive micromachined ultrasound transducer (CMUT) as a receiving probe. To improve the lateral resolution, we developed broadband and highly sensitive CMUT cells. An ultra-narrow gap structure was implemented in the cells, and it consisted of an evacuated gap of 30 nm and insulating layers of 100 nm between the electrodes. The equivalent distance in vacuum between the electrodes was 55 nm, which is much smaller than that of the typical CMUT cell between 100 and 300 nm. Approximately 10,000 CMUT cells were placed in a circle with a diameter of 2 mm to create the unfocused receiving probe. The center frequency was found to be 30 MHz, and the −3-dB bandwidth was from 14 to 47 MHz broad (fractional bandwidth: 109%). The CMUT receiving probe was installed into a SAT system in through-transmission mode. The signal-to-noise ratio of the SAT system with the CMUT probe was 70% higher than that with a 25-MHz piezoelectric one. In a 2.3-mm thick ball grid array (BGA) package, the through-transmission images taken with the CMUT probe were clearer than those with a piezoelectric one. In comparing the received pulses, we found the CMUT probe was more sensitive than the piezoelectric one by 10 dB in the frequency range of 20 to 30 MHz. The high sensitivity at high frequencies caused the CMUT probe to have higher lateral resolution than the piezoelectric one.

Fingerprint Imaging System Based on Capacitive Micromachined Ultrasonic Transducer by Using Impediography Method Including Direct Touch and Waveguide Methods.

Fingerprint imaging is a powerful tool in biometric identification systems. This paper proposes a fingerprint imaging method that involves applying an impedance signal to a fingerprint via ultrasonic impediography using a capacitive micromachined ultrasonic transducer (CMUT). We fabricated CMUT sparse array which has a center frequency of 6.25 MHz and collapse voltage of 50 V by local oxidation of silicon process. We develop a prototype by using waveguide and ultrasonic impediography method based on the CMUT. By using a waveguide made from a hard material, the prototype can ensure device protection and image lateral resolution improvement. The proposed technique successfully images the acoustic input impedance differences between the ridges and valleys of a fingerprint. The images created using direct touch and waveguide models are compared and analyzed. In addition, we present a phenomenon in the waveguide system in terms of energy loss by using numerical simulation. Therefore, this study facilitates ultrasonic fingerprint impediography measurement based on the basic direct touch and quartz glass waveguide system.

Improving Spatial Resolution Using Incoherent Subtraction of Receive Beams Having Different Apodizations.

In ultrasonic imaging, reduction of lateral sidelobes can result in an improved image with less distortion and fewer artifacts. In general, apodization is used to lower sidelobes in exchange for increasing the width of the main lobe, and thus decreasing lateral resolution. Null subtraction imaging (NSI) is a nonlinear image processing technique that uses different receive apodizations on copies of the same RF data to maintain low sidelobe levels while simultaneously improving lateral resolution. The images created with three different apodization functions are combined to form an image with low sidelobe levels and apparent improvements in lateral resolution compared to conventional rectangular apodization. To evaluate the performance of this technique for different imaging tasks, experiments were performed on an ATS539 phantom containing wire targets to assess lateral resolution and cylindrical anechoic and hyperechoic targets to assess contrast. NSI images were compared against rectangular apodized images and minimum variance beamformed images. In experiments, the apparent lateral resolution was observed to improve by a factor of more than 35× when compared to rectangular apodization. Image quality was assessed by the estimation of lateral resolution (-6-dB receive beamwidth), main-lobe-to-sidelobe ratio, and contrast-to-noise ratio (CNR). Imaging with NSI using a focal number of 2 (f/2), the -6-dB beamwidth on receive as measured from a small wire target in the ATS phantom was 0.03λ compared to 2.79λ for rectangular apodization. Sidelobes were observed to decrease by 32.9 dB with NSI compared to rectangular apodization. However, the ability to observe the contrast of anechoic and hyperechoic targets reduced when utilizing the NSI scheme, i.e., the CNR decreased from -3.05 to -1.01 for anechoic targets and 1.65 to 0.45 for the hyperechoic targets.

Sparse Convolutional Beamforming for Ultrasound Imaging.

The standard technique used by commercial medical ultrasound systems to form B-mode images is delay and sum (DAS) beamforming. However, DAS often results in limited image resolution and contrast that are governed by the center frequency and the aperture size of the ultrasound transducer. A large number of elements lead to improved resolution but at the same time increase the data size and the system cost due to the receive electronics required for each element. Therefore, reducing the number of receiving channels while producing high-quality images is of great importance. In this paper, we introduce a nonlinear beamformer called COnvolutional Beamforming Algorithm (COBA), which achieves significant improvement of lateral resolution and contrast. In addition, it can be implemented efficiently using the fast Fourier transform. Based on the COBA concept, we next present two sparse beamformers with closed-form expressions for the sensor locations, which result in the same beam pattern as DAS and COBA while using far fewer array elements. Optimization of the number of elements shows that they require a minimal number of elements that are on the order of the square root of the number used by DAS. The performance of the proposed methods is tested and validated using simulated data, phantom scans, and in vivo cardiac data. The results demonstrate that COBA outperforms DAS in terms of resolution and contrast and that the suggested beamformers offer a sizable element reduction while generating images with an equivalent or improved quality in comparison with DAS.

Spatially-incoherent annular illumination microscopy for bright-field optical sectioning

Numerous advanced microscopic imaging techniques have been proposed for optical sectioning, but they generally employ a complex and costly optical system. Here we report a microscopy termed spatially-incoherent annular illumination microscopy (SAIM). It allows for simple, effective, non-fluorescence, and bright-field optical sectioning. The proposed technique is implemented by installing an annular array of light emitting diodes (LEDs) on a standard bright-field microscope for illumination. The LED array produces distinctive illumination, that is, each LED provides coherent, large-angle oblique illumination while all LEDs generate spatially-incoherent annular illumination. Such a distinctive illumination can improve both lateral resolution and axial resolution. The improvement of lateral resolution is due to the coherent and large-angle oblique illumination. The spatially-incoherent annular illumination can improve the axial resolution. It is because, for defocused structures, each LED results in a blurred image with a different lateral shift and all LEDs result in an incoherent stagger superposition of the defocused images. The superposition looks much more blurred, which improves the contrast of the in-focus image remarkably. We experimentally demonstrate that SAIM is able to provide bright-field optical sections with 600-nm axial resolution and 150-nm lateral half-pitch resolution by using a 525-nm wavelength LED array and an objective with 100X, numerical aperture (NA) 1.25.

Multisensor Imaging-From Sample Preparation to Integrated Multimodal Interpretation of LA-ICPMS and MALDI MS Imaging Data.

Laterally resolved chemical analysis (chemical imaging) has increasingly attracted attention in the Life Sciences during the past years. While some developments have provided improvements in lateral resolution and speed of analysis, there is a trend toward the combination of two or more analysis techniques, so-called multisensor imaging, for providing deeper information into the biochemical processes within one sample. In this work, a human malignant pleural mesothelioma sample from a patient treated with cisplatin as a cytostatic agent has been analyzed using laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) and matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS). While LA-ICPMS was able to provide quantitative information on the platinum distribution along with the distribution of other elemental analytes in the tissue sample, MALDI MS could reveal full information on lipid distributions, as both modes of polarity, negative and positive, were used for measurements. Tandem MS experiments verified the occurrence of distinct lipid classes. All imaging analyses were performed using a lateral resolution of 40 μm, providing information with excellent depth of details. By analyzing the very same tissue section, it was possible to perfectly correlate the obtained analyte distribution information in an evaluation approach comprising LA-ICPMS and MALDI MS data. Correlations between platinum, phosphorus, and lipid distributions were found by the use of advanced statistics. The present proof-of-principle study demonstrates the benefit of data combination for outcomes beyond one method imaging modality and highlights the value of advanced chemical imaging in the Life Sciences.

Investigation on improvement of lateral resolution of continuous wave STED microscopy by standing wave illumination.

In this paper, we report the enhancement of resolution of continuous wave (CW) stimulated emission depletion (STED) microscopy by a novel method of structured illumination of an excitation beam. Illumination by multiple excitation beams through the specific pupil apertures with high in-plane wave vectors leads to interference of diffracted light flux near the focal plane, resulting in the contraction of the point spread function (PSF) of the excitation. Light spot reduction by the suggested standing wave (SW) illumination method contributes to make up much lower depletion efficiency of the CW STED microscopy than that of the pulsed STED method. First, theoretical analysis showed that the full width at half maximum (FWHM) of the effective PSF on the detection plane is expected to be smaller than 25% of that of conventional CW STED. Second, through the simulation, it was elucidated that both the donut-shaped PSF of the depletion beam and the confocal optics suppress undesired contribution of sidelobes of the PSF by the SW illumination to the effective PSF of the STED system. Finally, through the imaging experiment on 40-nm fluorescent beads with the developed SW-CW STED microscopy system, we obtained the result which follows the overall tendency from the simulation in the aspects of resolution improvement and reduction of sidelobes. Based on the obtained result, we expect that the proposed method can become one of the strategies to enhance the resolution of the CW STED microscopy.

Silver Filler Pre-embedding to Enhance Resolution and Contrast in Multidimensional SEM: A Nanoscale Imaging Study on Liver Tissue.

Contemporarily, serial block-face scanning electron microscopy (SBF-SEM) has emerged as an immensely powerful nanoscopic imaging technique, capable of generating large-volume three-dimensional information on a variety of biological specimens in a semiautomated manner. Despite the plethora of insights and advantages provided by SBF-SEM, a major challenge inherent to the technique is that of electron charging, which ultimately reduces attainable resolution and detracts from overall image quality. In this chapter, we describe a pre-embedding approach that involves infiltration of tissue with a highly conductive silver filler suspension following primary fixation. Such an approach is demonstrated to improve overall sample conductivity, resulting in the minimization of charging under high-vacuum conditions and an improvement in lateral resolution and image contrast. The strength of this sample preparation approach for SBF-SEM is illustrated on liver tissue.

Application of Synthetic Aperture Focusing Technique for inspection of plate-like structures using EMAT generated Lamb waves

The main challenge for guided wave inspection is exact defect characterization and sizing. EMAT generated Lamb waves usually have low signal-to-noise ratio which reduces the defect detection, characterization and sizing capabilities. That's why in most cases the method is used only as a screening tool. The Synthetic Aperture Focusing Technique is a process that increases the signal-to-noise ratio by numerically focusing the acoustic fields. In this paper the application of SAFT is tested over EMAT generated Lamb waves. The improvement of lateral resolution and signal-to-noise ratio is evaluated. Results are presented as a comparison between standard B-scan and SAFT processed data.

High-Affinity Functional Fluorescent Ligands for Human \u03b2-Adrenoceptors

Visualization of the G-protein coupled receptor (GPCR) is of great importance for studying its function in a native cell. We have synthesized a series of red-emitting fluorescent probes targeting β-adrenergic receptor (βAR) that are compatible with confocal and Stimulated Emission Depletion (STED) microscopy as well as with Time-Resolved Fluorescence Resonance Energy Transfer (TR-FRET) binding assay in living cells. The probe based on the agonist BI-167107 and fluorescent dye KK114 demonstrates nanomolar binding affinity and up to nine-fold β2AR selectivity over β1AR. Carazolol-derived probes are fluorogenic and allow no-wash imaging experiments. STED microscopy of β2ARs stained at the native expression level on pancreatic CAPAN cells provides two-fold improvement in lateral optical resolution over confocal mode and reveals the formation of receptor microdomains. These probes retain their functional (agonist or antagonist) properties, allowing simultaneous modulation of cyclic adenosine monophosphate (cAMP) levels and receptor internalization as well as imaging receptor localization.

Improvement of Lateral Resolution of 3D-Relief Interferometric Measurement by Deconvolution

The way of the lateral resolution improvement for 3D nanorelief measurement is proposed. Algorithm for digital processing of measurement data based on deconvolution operation using Wiener filtration is presented. Results of theoretical researches, numerical and optical experiments are given.

Operational deployment of compressive sensing systems for seismic data acquisition

Compressive sensing (CS) provides a new basis for sampling that can increase sampling efficiency for seismic data acquisition by an order of magnitude. A major challenge for this new technology is to show that theoretical increases in sampling efficiency can be translated to real efficiency gains in the field. Along with efficiency gains, data quality must be preserved in order to gain acceptance of a new acquisition technology. CS designs require solution of large optimization problems that are consistent with compressive sampling theory. We refer to our optimization framework for CS-based acquisition design and processing as compressive seismic imaging (CSI). We illustrate our CSI framework on example projects for ocean-bottom node, narrow-azimuth marine streamer, and land vibroseis acquisition. The ocean-bottom-node project was conducted in the UK North Sea during the difficult winter season. A CSI dual-source design was used to significantly reduce shooting time for this project. The project was completed on time, under budget, and with data quality that exceeded the quality of an overlapping uniformly sampled survey. The narrow-azimuth marine CSI survey project was acquired in offshore Australia for field development purposes. Nonuniform CSI sampling was used to increase sampling efficiency for both sources and cables, resulting in significant improvements in data quality and lateral resolution. The land vibroseis project was conducted on the North Slope of Alaska. In this case, the goal was to acquire a development survey of sufficient size within a short time window. Nonuniform CSI sampling was used to support the use of 10 or more vibrators shooting simultaneously, along with improving sampling efficiency for both sources and receivers. Compared to conventional designs, the CSI survey achieved an order of magnitude improvement in field acquisition efficiency and step-function improvements in data quality. These examples show that theoretical improvements in sampling efficiency from CS can make real and significant impacts on seismic data acquisition and processing.

Label-free super-resolution with coherent nonlinear structured-illumination microscopy

Structured-illumination microscopy enables up to a two-fold lateral resolution improvement by spatially modulating the intensity profile of the illumination beam. We propose a novel way to generalize the concept of structured illumination to nonlinear widefield modalities by spatially modulating, instead of field intensities, the phase of the incident field while interferometrically measuring the complex-valued scattered field. We numerically demonstrate that for second-order and third-order processes an almost four- and six-fold increase in lateral resolution is achievable, respectively. This procedure overcomes the conventional Abbe diffraction limit and provides new possibilities for label-free super-resolution microscopy.

Structured illumination for wide-field Raman imaging of cell membranes

Although the diffraction limit still restricts their lateral resolution, conventional wide-field Raman imaging techniques offer fast imaging speeds compared with scanning schemes. To extend the lateral resolution of wide-field Raman microscopy using filters, standing-wave illumination technique is used, and an improvement of lateral resolution by a factor of more than two is achieved. Specifically, functionalized surface enhanced Raman scattering nanoparticles are employed to strengthen the desired scattering signals to label cell membranes. This wide-field Raman imaging technique affords various significant opportunities in the biological applications.