Angular Scan Range Research Articles

Integrated VCSEL-Microlens Scanner With Large Scan Range

We present an integrated beam steering system with a microlens scanner and a vertical cavity surface-emitting laser (VCSEL) source. The scanner and the VCSEL are assembled with an alignment sphere process and with an accuracy of 121 ±7 μm defined by precision microspheres. The microlens scanner employs an electrostatic comb-drive actuator with prebent springs to extend the maximum displacement to 83 μm; 60% larger than that with standard folded springs. This translates to an angular scan range of ±9.6° with the double sided comb-drive scanner. The mechanical resonance with lens is measured to be 236 Hz.

Two-Element Phased-Array Acousto-Optic Deflector at High Diffraction Efficiency: Scanning Range Broadening

An acousto-optic deflector with a two-element phased-array piezoelectric transducer is investigated at high diffraction efficiency (close to 100%). Significant broadening of the frequency band (and, thus, the angular scanning range) relative to the well-known low efficiency regime is theoretically predicted and experimentally demonstrated. Experiments with an off-axis deflector based on paratellurite (TeO2), a 1.06 µm light radiation and 6 mm interaction length gave frequency band from 17 to 48 MHz at an efficiency of no less than 94% of the maximum value.

Analysis and accurate reconstruction of incomplete data in X-ray differential phase-contrast computed tomography

X-ray differential phase-contrast computed tomography (DPC-CT) is a powerful physical and biochemical analysis tool. In practical applications, there are often challenges for DPC-CT due to insufficient data caused by few-view, bad or missing detector channels, or limited scanning angular range. They occur quite frequently because of experimental constraints from imaging hardware, scanning geometry, and the exposure dose delivered to living specimens. In this work, we analyze the influence of incomplete data on DPC-CT image reconstruction. Then, a reconstruction method is developed and investigated for incomplete data DPC-CT. It is based on an algebraic iteration reconstruction technique, which minimizes the image total variation and permits accurate tomographic imaging with less data. This work comprises a numerical study of the method and its experimental verification using a dataset measured at the W2 beamline of the storage ring DORIS III equipped with a Talbot-Lau interferometer. The numerical and experimental results demonstrate that the presented method can handle incomplete data. It will be of interest for a wide range of DPC-CT applications in medicine, biology, and nondestructive testing.

Extension of the angular scanning range of the acousto-optic deflector with a two-element phased-array piezoelectric transducer

The acousto-optic deflector with a two-element phased-array piezoelectric transducer is studied at an almost limiting diffraction efficiency. Significant broadening of the working acoustic frequency band (and angular scanning range) relative to the well-studied low-efficiency regime is theoretically predicted and experimentally demonstrated. For the off-axial deflector based on paratellurite (TeO2), a radiation wavelength of 1.06 μm, and a two-element transducer with a total length of 6 mm, the working frequency band is 17–48 MHz (with the lower frequency determined by the electromechanical properties of the transducer) at an efficiency of no less than 94% of the maximum value.

Region-of-interest reconstruction for a cone-beam dental CT with a circular trajectory

Dental CT is the most appropriate and accurate device for preoperative evaluation of dental implantation. It can demonstrate the quantity of bone in three dimensions (3D), the location of important adjacent anatomic structures and the quality of available bone with minimal geometric distortion. Nevertheless, with the rapid increase of dental CT examinations, we are facing the problem of dose reduction without loss of image quality. In this work, backprojection-filtration (BPF) and Feldkamp–Davis–Kress (FDK) algorithm was applied to reconstruct the 3D full image and region-of-interest (ROI) image from complete and truncated circular cone-beam data respectively by computer-simulation. In addition, the BPF algorithm was evaluated based on the 3D ROI-image reconstruction from real data, which was acquired from our developed circular cone-beam prototype dental CT system. The results demonstrated that the ROI-image quality reconstructed from truncated data using the BPF algorithm was comparable to that reconstructed from complete data. The FDK algorithm, however, created artifacts while reconstructing ROI-image. Thus it can be seen, for circular cone-beam dental CT, reducing scanning angular range of the BPF algorithm used for ROI-image reconstruction are helpful for reducing the radiation dose and scanning time. Finally, an analytical method was developed for estimation of the ROI projection area on the detector before CT scanning, which would help doctors to roughly estimate the total radiation dose before the CT examination.

Noise spatial nonuniformity and the impact of statistical image reconstruction in CT myocardial perfusion imaging

To achieve high temporal resolution in CT myocardial perfusion imaging (MPI), images are often reconstructed using filtered backprojection (FBP) algorithms from data acquired within a short-scan angular range. However, the variation in the central angle from one time frame to the next in gated short scans has been shown to create detrimental partial scan artifacts when performing quantitative MPI measurements. This study has two main purposes. (1) To demonstrate the existence of a distinct detrimental effect in short-scan FBP, i.e., the introduction of a nonuniform spatial image noise distribution; this nonuniformity can lead to unexpectedly high image noise and streaking artifacts, which may affect CT MPI quantification. (2) To demonstrate that statistical image reconstruction (SIR) algorithms can be a potential solution to address the nonuniform spatial noise distribution problem and can also lead to radiation dose reduction in the context of CT MPI. Projection datasets from a numerically simulated perfusion phantom and an in vivo animal myocardial perfusion CT scan were used in this study. In the numerical phantom, multiple realizations of Poisson noise were added to projection data at each time frame to investigate the spatial distribution of noise. Images from all datasets were reconstructed using both FBP and SIR reconstruction algorithms. To quantify the spatial distribution of noise, the mean and standard deviation were measured in several regions of interest (ROIs) and analyzed across time frames. In the in vivo study, two low-dose scans at tube currents of 25 and 50 mA were reconstructed using FBP and SIR. Quantitative perfusion metrics, namely, the normalized upslope (NUS), myocardial blood volume (MBV), and first moment transit time (FMT), were measured for two ROIs and compared to reference values obtained from a high-dose scan performed at 500 mA. Images reconstructed using FBP showed a highly nonuniform spatial distribution of noise. This spatial nonuniformity led to large fluctuations in the temporal direction. In the numerical phantom study, the level of noise was shown to vary by as much as 87% within a given image, and as much as 110% between different time frames for a ROI far from isocenter. The spatially nonuniform noise pattern was shown to correlate with the source trajectory and the object structure. In contrast, images reconstructed using SIR showed a highly uniform spatial distribution of noise, leading to smaller unexpected noise fluctuations in the temporal direction when a short scan angular range was used. In the numerical phantom study, the noise varied by less than 37% within a given image, and by less than 20% between different time frames. Also, the noise standard deviation in SIR images was on average half of that of FBP images. In the in vivo studies, the deviation observed between quantitative perfusion metrics measured from low-dose scans and high-dose scans was mitigated when SIR was used instead of FBP to reconstruct images. (1) Images reconstructed using FBP suffered from nonuniform spatial noise levels. This nonuniformity is another manifestation of the detrimental effects caused by short-scan reconstruction in CT MPI. (2) Images reconstructed using SIR had a much lower and more uniform noise level and thus can be used as a potential solution to address the FBP nonuniformity. (3) Given the improvement in the accuracy of the perfusion metrics when using SIR, it may be desirable to use a statistical reconstruction framework to perform low-dose dynamic CT MPI.

Time-Resolved Interventional Cardiac C-arm Cone-Beam CT: An Application of the PICCS Algorithm

Time-resolved cardiac imaging is particularly interesting in the interventional setting since it would provide both image guidance for accurate procedural planning and cardiac functional evaluations directly in the operating room. Imaging the heart in vivo using a slowly rotating C-arm system is extremely challenging due to the limitations of the data acquisition system and the high temporal resolution required to avoid motion artifacts. In this paper, a data acquisition scheme and an image reconstruction method are proposed to achieve time-resolved cardiac cone-beam computed tomography imaging with isotropic spatial resolution and high temporal resolution using a slowly rotating C-arm system. The data are acquired within 14 s using a single gantry rotation with a short scan angular range. The enabling image reconstruction method is the prior image constrained compressed sensing (PICCS) algorithm. The prior image is reconstructed from data acquired over all cardiac phases. Each cardiac phase is then reconstructed from the retrospectively gated cardiac data using the PICCS algorithm. To validate the method, several studies were performed. Both numerical simulations using a hybrid motion phantom with static background anatomy as well as physical phantom studies have been used to demonstrate that the proposed method enables accurate reconstruction of image objects with a high isotropic spatial resolution. A canine animal model scanned in vivo was used to further validate the method.

Smart Two-Dimensional Laser-Based Display

In this paper, is proposed a smart design for a two-dimensional (2-D) optical display using 2-D laser scanning and an electronically programmable three-dimensional (3-D) beamforming lens element. Specifically, an electronically controlled variable focal length lens (ECVFL) adjusts its focus power to produce the smallest focused beam spot on the display screen within the designed variable screen distance range. Since the screen distance is known, the angular scan range for the scan mirrors can be computed to generate the desired number of scanned spots in the 2-D display. Thus a smart display with the smallest pixel spot size is formed leading to the highest spatial resolution display for any given distance. Furthermore, if the scan angles are increased, a larger size display of higher pixel count can be formed without sacrificing display pixel size at this larger display size. A proof-of-concept smart display optics system is designed and demonstrated for a red 633-nm laser wavelength and an optimal display variable screen distance of 20-150 cm, giving optimized display pixel sizes of 51.92 and 538.1 μm, respectively.

Surface plasmon resonance instrument as a refractometer for liquids and ultrathin films

A surface plasmon resonance (SPR) setup in Kretschmann configuration is being utilized as a refractometer for both liquids as well as ultrathin films. The SPR signal detection technology used is based on a goniometer approach providing a wide angular scan range which facilitates highly accurate liquid and gas phase measurements. Attention was paid to improve sample handling and preparation. In order to avoid cross-contamination between measurements an easily removable and exchangeable molded PDMS flow cell was used during the measurements. By careful choice of components for liquid handling the dead volume of the system could be reduced down to some microliters. The angular change and thus the refractive index for sucrose, ethylene glycol (EG) and ethanol solutions with different concentrations, the thickness and refractive index of deposited Langmuir-Blodgett (LB) films, and the interaction kinetics between a biotin containing self-assembled monolayer (SAM) and streptavidin were determined. The measured refractive indices of sucrose, EG and ethanol solutions corresponded well with literature values. LB films were characterized by measuring the complete SPR curve in an angular scan range from 40 to 78°. A two-color SPR approach combined with two-media measurements was successfully employed for simultaneous and unambiguous determination of both refractive index and thickness of stearic acid monolayers. The thickness obtained for the stearic acid monolayer was 2.66 nm, and the refractive indices at 635 and 670 nm were 1.5800 and 1.4138, respectively. The developed sensor-plate holder enabled functionalization of the SPR gold chip outside the instrument, therefore making the sample handling more flexible. The affinity constant obtained for the streptavidin–biotin interaction was 1.01 × 10 −8 M. The total angle SPR method used in this study clearly shows its potential to be used as a refractometer for both liquids and ultrathin films, as well as for traditional liquid phase biomolecular kinetic studies.

Task-based assessment of breast tomosynthesis: Effect of acquisition parameters and quantum noisea)

Tomosynthesis is a promising modality for breast imaging. The appearance of the tomosynthesis reconstructed image is greatly affected by the choice of acquisition and reconstruction parameters. The purpose of this study was to investigate the limitations of tomosynthesis breast imaging due to scan parameters and quantum noise. Tomosynthesis image quality was assessed based on performance of a mathematical observer model in a signal-known exactly (SKE) detection task. SKE detectability (d') was estimated using a prewhitening observer model. Structured breast background was simulated using filtered noise. Detectability was estimated for designer nodules ranging from 0.05 to 0.8 cm in diameter. Tomosynthesis slices were reconstructed using iterative maximum-likelihood expectation-maximization. The tomosynthesis scan angle was varied between 15 degrees and 60 degrees, the number of views between 11 and 41 and the total number of x-ray quanta was infinity, 6 X 10(5), and 6 x 10(4). Detectability in tomosynthesis was compared to that in a single projection. For constant angular sampling distance, increasing the angular scan range increased detectability for all signal sizes. Large-scale signals were little affected by quantum noise or angular sampling. For small-scale signals, quantum noise and insufficient angular sampling degraded detectability. At high quantum noise levels, angular step size of 3 degrees or below was sufficient to avoid image degradation. At lower quantum noise levels, increased angular sampling always resulted in increased detectability. The ratio of detectability in the tomosynthesis slice to that in a single projection exhibited a peak that shifted to larger signal sizes when the angular range increased. For a given angular range, the peak shifted toward smaller signals when the number of views was increased. The ratio was greater than unity for all conditions evaluated. The effect of acquisition parameters on lesion detectability depends on signal size. Tomosynthesis scan angle had an effect on detectability for all signals sizes, while quantum noise and angular sampling only affected the detectability small-scale signals.

The quantitative potential for breast tomosynthesis imaging

Due to its limited angular scan range, breast tomosynthesis has lower resolution in the depth direction, which may limit its accuracy in quantifying tissue density. This study assesses the quantitative potential of breast tomosynthesis using relatively simple reconstruction and image processing algorithms. This quantitation could allow improved characterization of lesions as well as image processing to present tomosynthesis images with the familiar appearance of mammography by preserving more low-frequency information. All studies were based on a Siemens prototype MAMMOMAT Novation TOMO breast tomo system with a 45 degrees total angular span. This investigation was performed using both simulations and empirical measurements. Monte Carlo simulations were conducted using the breast tomosynthesis geometry and tissue-equivalent, uniform, voxelized phantoms with cuboid lesions of varying density embedded within. Empirical studies were then performed using tissue-equivalent plastic phantoms which were imaged on the actual prototype system. The material surrounding the lesions was set to either fat-equivalent or glandular-equivalent plastic. From the simulation experiments, the effects of scatter, lesion depth, and background material density were studied. The empirical experiments studied the effects of lesion depth, background material density, x-ray tube energy, and exposure level. Additionally, the proposed analysis methods were independently evaluated using a commercially available QA breast phantom (CIRS Model 11A). All image reconstruction was performed with a filtered backprojection algorithm. Reconstructed voxel values within each slice were corrected to reduce background nonuniformities. The resulting lesion voxel values varied linearly with known glandular fraction (correlation coefficient R2 > 0.90) under all simulated and empirical conditions, including for the independent tests with the QA phantom. Analysis of variance performed on the fit line parameters revealed statistically significant differences between the two different background materials and between 28 kVp and the remaining energies (26, 30, and 32 kVp) for the dense experimental phantom. How ever, no significant differences arose between different energies for the fatty phantom, nor for any of the many other combinations of parameters. These strong linear relationships suggest that breast tomosynthesis image voxel values, after being corrected by our outlined methods, are highly positively correlated with true tissue density. This consistent linearity implies that breast tomosynthesis imaging indeed has potential to be quantitative.

Optical micro-endoscopes for 3D in-vivo imaging

Optical coherence tomography (OCT) is a medical optical imaging technique that can achieve high-resolution 3D images (1 15 m)1 by detecting infrared light scattered by body tissue. It has an imaging depth range of about 3mm in highly-scattered tissue and has already been widely used for external imaging, e.g., for eye diseases and skin cancer diagnosis. It would be useful to apply OCT to internal imaging, since about 85% of all cancers originate from the surface layers of internal organs. As these layers are within its imaging depth range, OCT can detect cancer at an early stage as well as precisely determine tumor margins. However, it is very challenging to apply this technique to internal organs because of stringent size and imaging time constraints. One promising approach is to use microelectromechanical systems (MEMS) scanning micromirrors. Several research groups have demonstrated actuation mechanisms to generate rotational scanning for micromirrors for endoscopic OCT. Among them, electrostatic micromirrors can be very fast and consume low power, but the angular scan range is limited and the required driving voltage is too high ( 100V) to be safe for patients. Electromagnetic micromirrors have also been used for OCT probes, but the assembly process is complicated and the potential electromagnetic interference caused by the use of permanent magnets is another concern. Another problem these mirrors share is that they all have a small fill factor (the ratio of the mirror area to the device area). For a given resolution, the required mirror area is fixed. Thus, a small fill factor leads to a large device, which in turn increases the imaging probe size. Electrothermal mirrors, on the other hand, can achieve a large scan range at low drive voltage, but most of them have large initial tilt angles and significant shift of rotation centers, causing serious optical alignment and coupling problems. Figure 1. Scanning electron micrograph of an electrothermal MEMS mirror. Chip size: 2mm 2mm.

Image reconstruction for fan-beam differential phase contrast computed tomography

Recently, x-ray differential phase contrast computed tomography (DPC-CT) has been experimentally implemented using a conventional tube combined with gratings. Images were reconstructed using a parallel-beam reconstruction formula. However, parallel-beam reconstruction formulae are not applicable when the parallel-beam approximation fails. In this paper, we present a new image reconstruction formula for fan-beam DPC-CT. There are several novel features of the new image reconstruction formula: (i) when the scanning angular range of data acquisition is larger than π + γm (γm is the full fan angle), the entire field of view can be exactly reconstructed; (ii) when the scanning angular range is smaller than π + γm, a local region of interest (ROI) can be exactly reconstructed; (iii) it enables an exact reconstruction for a local ROI when the projection data are truncated at some view angles; (iv) it enlarges the imaging field of view when the detector is asymmetrically placed. In this last case, the data are truncated from every view angle. Numerical simulations have been conducted to validate the new reconstruction formula.

Theoretical Study of Light Refraction in Three-Dimensional Photonic Crystals

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> A rigorous theory for solving general 3-D photonic crystal refraction problem is developed. The power flow direction and transmission intensity of each mode refracted at a periodic surface are rigorously analyzed and determined from matrix equations. We further illustrate how to apply our method to design a low-index-contrast 3-D superprism structure that exhibits efficient transmission over a wide angular scanning range at wavelengths near 1550 nm. </para>

MO‐D‐330A‐04: Exploitation of Data Redundancy in a Reduced Fan‐Beam Scan

Purpose: We aim to devise new weighting schemes to improve the signalto‐noise ratio (SNR) in images reconstructed from data acquired in reduced fan‐beam scans by fully utilizing the redundant information. Method and Materials: Recently, we have developed a backprojection‐filtration (BPF) algorithm, which can reconstruct ROI‐images from the transversely truncated data in a reduced‐scan with a scanning angular range less than that in a short‐scan. However, the measured data in a practical reduced scan contain some redundant information. In this work, we devised two weighting schemes to appropriately incorporate the redundant data. Applying the weighting schemes to the BPF algorithm, we derived two new algorithms, derivative of weighted BPF (WD‐BPF) and weighted‐derivative BPF (DW‐BPF) algorithms. Both of these two algorithms can be used to improve the SNR in reconstructed images. Results: The ROI‐images are reconstructed from the truncated projection data in a short‐scan and a reduced‐scan, respectively. For the reduced‐scan, some shading artifacts appear in the images reconstructed by use of the WD‐BPF algorithm. This is caused by the numerical error in the derivative of the discontinuous weighting function. In contrast, artifact‐free images can be obtained by use of the DW‐BPF algorithm. For the short‐scan, both algorithms can obtain artifact‐free images, because the weighting function is smooth. The results in the noise study show that image noise obtained by use of WD‐BPF and DW‐BPF algorithms are similar within the ROI. Conclusion: We propose two weighting schemes for handling the data redundancy in reduced‐scan fan‐beam CT. Our results demonstrate that the proposed two algorithms can utilize the redundant data to improve the signal‐noise ratio. Moreover, the noise properties of these two algorithms are similar to each other.

Multiplexed holographic memory by use of fiber bundle referencing

We propose a volume holographic storage technique, in which a fiber bundle is used to guide the reference beam. Multiplexing is implemented by changing the incident direction of the laser beam upon the fiber bundle in the reference arm. In the technique, we make the system more compact by using a wedge prism to change the direction of the laser beam. This method has a large accessible angular scanning range and a large geometric storage bandwidth. Multiple images are stored in a LiNbO3:Fe crystal with an angular separation of 0.05° between successive holograms using a wedge prism with a deviation angle of 8°. This method is useful as a new image storage technique because of its compactness and simplicity.

Superwide-angle coverage code-multiplexed optical scanner.

A superwide-angle coverage code-multiplexed optical scanner is presented that has the potential to provide 4 pi-sr coverage. As a proof-of-concept experiment, an angular scan range of 288 degrees for six randomly distributed beams is demonstrated. The proposed scanner achieves its superwide coverage by exploiting a combination of phase-encoded transmission and reflection holography within an in-line hologram recording-retrieval geometry. The basic scanner unit consists of one phase-only digital mode spatial light modulator for code entry (i.e., beam scan control) and a holographic material from which we obtained what we believe is the first-of-a-kind extremely wide coverage, low component count, high speed (e.g., microsecond domain), and large aperture (e.g., > 1-cm diameter) scanner.

High-speed two-dimensional laser scanner based on Bragg gratings stored in photothermorefractive glass

A high-speed free-space wavelength-multiplexed optical scanner with high-speed wavelength selection coupled with narrowband volume Bragg gratings stored in photothermorefractive (PTR) glass is reported. The proposed scanner with no moving parts has a modular design with a wide angular scan range, accurate beam pointing, low scanner insertion loss, and two-dimensional beam scan capabilities. We present a complete analysis and design procedure for storing multiple tilted Bragg-grating structures in a single PTR glass volume (for normal incidence) in an optimal fashion. Because the scanner design is modular, many PTR glass volumes (each having multiple tilted Bragg-grating structures) can be stacked together, providing an efficient throughput with operations in both the visible and the infrared (IR) regions. A proof-of-concept experimental study is conducted with four Bragg gratings in independent PTR glass plates, and both visible and IR region scanner operations are demonstrated.

The relation of scan range and reflection shape in single-crystal 1D profile measurement

Changes in shape of 1D profiles of small-single-crystal Bragg reflections have been examined in terms of the shapes of the components which, convoluted together, generate the profile. In most practical cases, operational features require truncation of the angular scan range of measurement and the conventional linear formula for scan range, ω = a + b tan θ, is then not strictly valid. A more appropriate relationship involves a combination of root mean square (RMS) and linear (LIN) forms, ω = [(p′)2 + (q′ tan θ)2]1/2 + (p′′ + q′′ tan θ) where p′ is associated with the leading and trailing edges of the distribution of the combined θ-invariant components and p′′ with its plateau width while q′ is associated with the leading and trailing edges of the distribution of the wavelength component and q′′ with the separation of its outer peaks if there are more than one. For operational purposes, this relationship can be substituted with adequate precision by ω = [c2 + (d tan θ)2]1/2, but the parameters c and d do not then have a simple relationship to the θ-invariant and θ-variant components. Use of a conventional linear formula when a RMS one is the relevant one can mean that, in the lower θ range, the estimate of integrated intensity will be too high and, in the higher θ range, it will be too low, so that, with increasing θ, a positive then a negative systematic error is introduced and not merely a negative error as the conventional interpretation of truncation holds. The conclusions of the present analysis are tested against experimental data where the conventional treatment for truncation failed [Eisenstein & Hirshfeld (1983). Acta Cryst. B39, 61-75]. For the estimation of integrated intensity, and hence of structure factors, which are consistent over the operational range of θ, a RMS formula for the scan range is advisable.

Reducing the fan-beam scanning angular range

A fan-beam X-ray CT scanner is developed which needs to scan the object in an angular range equal to (180 degrees +fan angle) instead of the 360 degrees angular range currently required by straightforward reconstruction techniques. By using the techniques of limited-angle image reconstruction, but otherwise without using additional equipment and measurements, the quality of the image reconstructed from such a fan-beam scan is improved to be essentially identical to that from a 360 degrees fan-beam scan. The proposed method can reduce the dosage to objects and scanning time by roughly 50%.