Effective Pixel Size Research Articles

3D simulation of macroscopic heat and mass transfer properties from the microstructure of wood fibre networks

A method to simulate the macroscopic permeability and thermal conductivity from the microstructure of wood fibre networks is presented. The simulations were done on high resolution microtomography data with an effective pixel size of 2.28 μm for material densities from 500 to 1360 kg m −3, and the stationary flow equations were solved with periodic boundary conditions. In addition, we present experimental data for permeability and thermal conductivity. Considering that no free parameters in the simulations were used for fitting with the experimental results, the simulated and experimentally determined permeability and thermal conductivity were in good agreement. Such finding opens the possibility to not only simulate the heat and mass transfer properties of real but also virtual fibre networks with good confidence.

Spatial resolution in Bragg‐magnified X‐ray images as determined by Fourier analysis

AbstractThe spatial resolution of an imaging system is most often experimentally determined by using model objects and idealized imaging conditions. This traditional approach does not properly take into account the influence of noise and the dependence of contrast on the object. The latter influence is essential especially in the case of magnified X‐ray images acquired by two fold asymmetric Bragg diffraction. For this phase sensitive technique the application of a criterion for spatial resolution based upon Fourier analysis is shown to provide reliable resolution values in a simple way, overcoming the limitations mentioned above. For the case of Bragg‐magnified imaging of an ant leg with 166 fold magnification (effective pixel size 0.094 µm), the physical resolution achieved in the image is found to be 0.4 µm. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

WE‐C‐L100J‐04: The Solid State X‐Ray Image Intensifier (SSXII): A Next‐Generation High‐Resolution Fluoroscopic Detector System

Purpose: Present the design for the new SSXII high‐resolution fluoroscope capable of overcoming lag, noise, and resolution limitations of current flat‐panel devices (FPDs). Method and Materials: The SSXII consists of an array of modules each featuring an electron multiplying (EM) CCD which views a structured phosphor through a fiber‐optic taper (FOT). The EMCCD operates like a standard frame‐transfer CCD; however, an additional row of multiplication elements enables on‐chip signal gains up to 2000X to overcome subsequent instrumentation‐noise degradation. The SSXII therefore has quantum‐limited performance at both fluoroscopic exposures with moderate gain, and radiographic exposures with low gain. The SSXII array design, through pixel binning and module selection, will enable rapid sequence and fluoroscopic imaging for either the full field‐of‐view (FOV) or high‐resolution regions‐of‐interest (ROIs). Results: A SSXII module was assembled with direct fiber‐optic coupling of the 350 micron thick CsI(Tl) phosphor and the EMCCD (Texas Instruments TC285SPD chip with 1004×1002 pixels). Operation at fluoroscopic and angiographic exposure levels was verified experimentally for gains of ∼80X and 1X, respectively, demonstrating sequences of a moving stent with no lag and bar‐pattern resolution up to 20 1p/mm with a 1:1 FOT. An array of four modules each with 6:1 FOTs will have an effective pixel size of 48 microns covering a FOV of 10×10 cm, sufficient for region‐of‐interest and neurovascular imaging. Larger arrays may be constructed to satisfy both cardiac imaging and general fluoroscopic applications. Module alignment, digital stitching, and distortion correction issues are being addressed. Conclusion: When assembled in an array of sufficient size, the new SSXII can be used in the same applications as current FPD's with advantages of high frame rates with no lag, high resolution due to the smaller pixels possible, and low‐effective‐noise, quantum‐limited fluoroscopic performance due to the on‐chip gain.(Support: UB Foundation, NIH grants R01‐EB002873, R01‐NS43924).

Hybrid photon detectors for the LHCb RICH

The LHCb Ring Imaging Cherenkov (RICH) counters use the pixel Hybrid Photon Detector (HPD) as a photo-sensitive device. Photo-electrons are produced in a semi-transparent multi-alkali photo-cathode (S20) and are accelerated by a voltage of 20 kV onto a pixelated silicon anode. The anode is bump-bonded to the LHCBPIX1 pixel readout chip which amplifies and digitises the anode signals at the LHC speed of 40 MHz. Using a demagnification of five, the effective pixel size at the HPD window is 2.5 × 2.5 mm 2 . Over the course of 18 months, 550 HPDs will undergo a quality-assurance programme to verify the specifications and to characterise the tubes. The tested parameters include the threshold and noise behaviour of the chip, the response to light emitting diode (LED) light, the demagnification of the electron optics, the leakage current and the depletion of the silicon sensor, the quality of the vacuum, the signal efficiency and the dark count rate. Results of tests of the first nine HPDs of the final design are presented and compared to the specifications.

Distance measurement sensor with PIN-photodiode and bridge circuit

The presented integrated optical distance measurement sensor works on the time-of-flight principle. The distance information is obtained from the correlation of received light and the transmitted signal. The PIN-bridge circuit concept ensures suppression of background light by equally charging and discharging the capacitor within one period, while integrating the wanted signal. The advantages of the included PIN-photodiode are high bandwidth f/sub 3 dB/>1.35 GHz together with high responsivity R=0.36 A/W at 660 nm. A single distance measurement is performed in 2 ms. With averaging, an accuracy of better than 1% is achieved for distances up to 3.7 m. Effective pixel size is 250/spl times/200 /spl mu/m/sup 2/ having a fill-factor of /spl sim/16%. The sensor was manufactured in a 0.6-/spl mu/m BiCMOS process.

Polarization effect on position accuracy of fluorophore localization

The technique of determining the position of individual fluorescent molecules with nanometer resolution, called FIONA, has become an important tool for several biophysical applications such as studying motility mechanisms of motor proteins. The position determination is usually done by fitting a 2-D Gaussian (x-y vs. photon number) to the emission intensity distribution of the fluorescent molecule. However, the intensity distribution of an emitting molecule depends not only on its position in space, but also on its three-dimensional orientation. Here, we present an extensive numeri-cal study of the achievable accuracy of position determination as a function of molecule orientation. We compare objectives with different numerical apertures and show that an effective pixel size of 100 nm or less per CCD pixel is required to obtain good positional accuracy. Nonetheless, orienta-tion effects can still cause position errors for large anisotropy, as high as 10 nm for high numerical aperture objectives. However, position accuracy is significantly better (< 2.5 nm) when using objectives with a numerical aper-ture of 1.2. Of course, probes with lower anisotropy decrease the positional uncertainty.

Development and characterization of a tissue equivalent plastic scintillator based dosimetry system

High precision techniques in radiation therapy, such as intensity modulated radiation therapy, offer the potential for improved target coverage and increased normal tissue sparing compared with conformal radiotherapy. The complex fluence maps used in many of these techniques, however, often lead to more challenging quality assurance with dose verification being labor-intensive and time consuming. A prototype dose verification system has been developed using a tissue equivalent plastic scintillator that provides easy-to-acquire, rapid, digital dose measurements in a plane perpendicular to the beam. The system consists of a water-filled Lucite phantom with a scintillator screen built into the top surface. The phantom contains a silver coated plastic mirror to reflect scintillation light towards a viewing window where it is captured using a charge coupled device camera and a personal computer. Optical photon spread is removed using a microlouvre optical collimator and by deconvolving a glare kernel from the raw images. A characterization of the system was performed that included measurements of linear output response, dose rate dependence, spatial linearity, effective pixel size, signal uniformity and both short- and long-term reproducibility. The average pixel intensity for static, regular shaped fields between 3 cm X 3 cm and 12 cm x 12 cm imaged with the system was found to be linear in the dose delivered with linear regression analysis yielding a correlation coefficient r2 > 0.99. Effective pixel size was determined to be 0.53 mm/pixel. The system was found to have a signal uniformity of 5.6% and a long-term reproducibility/stability of 1.7% over a 6 month period. The system's ability to verify a dynamic treatment field was evaluated using 60 degrees dynamic wedged fields and comparing the results to two-dimensional film dosimetry. Results indicate agreement with two-dimensional film dosimetry distributions within 8% inside the field edges. With further development, this system promises to provide a fast, directly digital, and tissue equivalent alternative to current dose verification systems.

X-ray beam-position monitoring in the sub-micrometre and sub-second regime

It is demonstrated that X-ray beam positions can be extracted from two-dimensional profiles with sub-pixel resolution. Beam-position measurements utilizing a self-designed low-cost two-dimensional detector have been performed at two synchrotron radiation beamlines of the Swiss Light Source. The effective detector pixel size was 4.8 microm and the resolution achieved for the beam position was about 5 nm. At a data rate of 25 frames per second, periodic variations of the beam position could be detected with a frequency resolution below 0.1 Hz. This allowed, for example, the influence of a turbo-pump in the X-ray optics hutch on the beam position to be quantified, and even minute variations related to the electron beam in the storage ring could be detected.

A photon counting CdTe gamma- and X-ray camera

A photon counting CdTe imaging camera suitable for gamma- and X-ray detection has been developed and tested. The current full active imaging area of the gamma/X-ray camera covers 44×44 mm 2. The camera is built of eight individual detector hybrids each consisting of a pixelated CdTe detector with dimensions of 22×11 mm 2 and solder bump-bonded to a photon counting custom-designed application specific integrated circuit (ASIC). The ASICs are realized in a mixed signal, 0.35 μm 4 metal 2 poly CMOS process. The effective pixel size (image pixel pitch) is 0.5 mm. To enable higher count rate imaging and to achieve better position resolution in X-ray CT scanning each pixel is divided both on the CdTe detector and on the ASIC into two sub-pixels with dimensions 0.25×0.5 mm 2. Every pixel circuit has two preamps each connected to one sub-pixel and feeding signal to a separate comparator. The digital pulses of the two distinct comparators are recorded by one common 8-bit counter. The amplifier offsets can be adjusted individually with 3-bit accuracy to compensate for process mismatch. A similar 3-bit gain tuning common to the two amplifiers in one pixel circuit is also implemented. A globally tuneable threshold voltage generated externally with high accuracy is used for energy discrimination. The camera can be operated both in the real time imaging mode with a maximum speed of 100 frames/s and in the accumulation mode with user adjustable counting time. Experimental data collected from a fully operational eight hybrid gamma/X-ray camera is presented and compared to simulated data. The camera exhibits excellent sensitivity and a dynamic range of 1:14,000,000. A sharp line spread function indicates the spatial resolution to be limited only by the pixel size (0.5 mm). A single pixel energy resolution of FWHM 4.7 keV at 122 keV (3.9%) was determined from measured 57Co spectra. The peak width of the spectrum combined from all pixels was somewhat larger due to calibration inaccuracy. The photopeak efficiency was estimated from 241Am spectra to be 60%. The developed gamma/X-ray camera is intended for back scattering imaging and as a building block in future nuclear medicine and CT scanning systems and promises highly improved performance in these applications in comparison to conventional technology.

Pixel hybrid photon detectors for the ring imaging Cherenkov detectors of LHCb

A Pixel Hybrid Photon Detector (pixel HPD) has been developed for the LHCb Ring Imaging Cherenkov (RICH) detectors. The pixel HPD is a vacuum tube with a multi-alkali photocathode, high-voltage cross-focused electron optics and an anode consisting of a silicon pixel detector bump-bonded to a CMOS readout chip; the readout chip is thus fully encapsulated in the device. The pixel HPD fulfils the stringent requirements for the RICH detectors of LHCb, combining single photon sensitivity, high signal-to-noise ratio and fast readout with an ∼8 cm diameter active area and an effective pixel size of 2.5 mm×2.5 mm at the photocathode. The performance and characteristics of two prototype pixel HPDs have been studied in laboratory measurements and in recent beam tests. The results of all measurements agree with expectations and fulfill the LHCb RICH requirements. In readiness for production of the ∼500 pixel HPDs for the RICH detectors, a test programme was designed and implemented to ensure component quality control at each stage of the manufacturing process.

Digital cameras: spatial image quality matters

Difficulty in understanding the relative performance and appreciating the numerous features of the ever-increasing range of digital cameras has led to the need for a simple objective test of camera quality. A method of measuring the modulation transfer function is described but, in practice, only a sector star pattern is needed to measure the spatial frequency bandwidth (SFB), which is related to resolution and the optimum print width (OPW) achievable with a particular camera, whether digital or film. The SFB of a camera divided by that of the eye in the plane of a print at arm's length and multiplied by the image format width gives the OPW. A value index (VI) is derived by dividing this number into the camera price. A pixel packing index (PPI) is the ratio of the true information content, obtained from the number of effective pixels present divided by the nominal information content, given by the stated number of pixels. It quantifies the proportion of pixels contributing to spatial image quality. Provisional results obtained using nine different cameras in the 1–4 MP range suggest that the SFB, and therefore the effective pixel size and spatial resolution, vary relatively little between cameras. A doubling of the OPW requires a fourfold increase in the number of pixels. The PPI over the full sensor area can be as little as 8%. The OPW is a useful parameter for optimising image quality settings and for comparing the quality of different cameras. The advantages are that users should be able to understand it, and it embodies both effective pixel size influencing spatial image quality and the number of pixels influencing acceptable print size. The VI, since it reflects the present highly volatile high-street prices, will only be of temporary value.

A tiled CCD detector with 2×2 array and tapered fibre optics for electron microscopy

Charge coupled devices (CCD)-based detectors have made a major impact on data collection in electron microscopy over the past few years. There have been a number of successful applications of CCDs in electron crystallography of two-dimensional protein crystal arrays but high-resolution imaging has been hampered by the relatively poor spatial resolution (and fewer independent pixels) compared to film. A partial solution to this problem, presented in this paper, are to design detectors with larger effective pixel sizes and with more pixels. A CCD detector with a much greater number of ‘independent’ pixels, achieved by tiling a 2×2 array of CCDs, each of which has 1242×1152 pixels is described here. The sensitive area of the detector, using fibre optics with a demagnification of 2.5 : 1, is 140×130 mm 2; the pixel size is 56 μm square and there is a total of ∼2500×2300 pixels.

Development of high spatial resolution X-ray CT system at BL47XU in SPring-8

High spatial resolution, micrometer range, X-ray CT system has been developed at SPring-8. The experiments were performed at the undulator beam line BL47XU. An “in-vacuum type” undulator is employed as an X-ray source, and the X-rays are monochromatized with a liquid nitrogen cooled Si(1 1 1) double crystal monochromator. High precision rotation stage with air bearing was used for sample rotation. The transmitted images were obtained with a two-dimensional image detector, which consists of a single crystal phosphor screen (Lu 2SiO 5 : Ce), an objective lens and a cooled CCD camera. In this system the smallest effective pixel size was set to 0.5 μm×0.5 μm. As a result of the experiments, three-dimensional images of a few micrometer-order texture has been successfully obtained with the developed CT system.

Silicon microstrip detectors for digital mammography — evaluation and spatial resolution study

Silicon microstrip detectors were used to build an experimental X-ray imaging setup. The detectors were used in an “edge-on” geometry, with the photons hitting the detector from the side. Efficiencies up to 90% at 20 keV photon energy could be achieved. The system was tested using a standard mammographic phantom. Images of modeled microcalcifications with various diameters down to 200 μm and images of modeled tumors were made. Spatial resolution of the system was studied on an X-ray test pattern with frequency of line-pairs between 1 and 10l p/mm. An appropriate scanning step combined with knowledge of the system's line spread function was used to deconvolve the measured image and increase the spatial resolution. In this way the effective pixel size was reduced as much as for a factor of ≈3.

A computerized analysis of masses on mammograms (in Japanese)

Two new schemes for analysis of mammographic images were developed, a diagnostic logic for classifying masses on mammograms and a multistage pendulum filter for detecting spicules around the mass. To construct a computer‐aided diagnosis scheme for detecting breast cancer requires a knowledge of the diagnostic process. We investigated the classification logic for the diagnosis of breast masses on mammograms in cooperation with a radiologist specialized in mammography. We tested our classification logic by use of 99 mammograms in which the diagnosis had been confirmed. The accuracy for classifying the mass as malignant or benign was very high (sensitivity of 84% and specificity of 96%). Existence of spicules around a mass is one of the important signs which characterize malignant tumors. We developed an automated method for detecting spicules. The detection was performed with a newly developed “multistage pendulum filter.” A “spicule value,” which indicates the probability of “spicule presence” around the mass, was calculated. This filter was evaluated on 71 digitized mammograms. Digitized mammograms were obtained with a laser film scanner with a 0.1 mm×0.1 mm pixel size and 1024 gray levels. The digitized data were then reduced to image with effective pixel size of 0.2 mm for detecting spicules. It detected most of the spicules seen by a radiologist. The sensitivity was 89% and the specificity was 80%, which demonstrates the effectiveness of our method.

Development of a reader for track etch detectors based on a commercially available slide scanner

NRPB has operated a routine neutron personal dosimetry service based on the electrochemical etch of PADC elements since 1986. Since its inception it has used an automated reader based on a video camera and real time analysis. A new and more powerful replacement system has been developed using a commercially available photographic slide scanner. This permits a complete image of the dosemeter to be grabbed in a single scan, generating a 2592x3888 pixel file which is saved for subsequent analysis. This gives an effective pixel size of 10x10 μm with an image of the entire dosemeter in one field of view. Custom written software subsequently analyses the image to assess the number of etched pits on the dosemeter and read the detector identification number (code). Batch scanning of up to 40 detectors is also possible using an autofeed attachment. The system can be used for electrochemically etched tracks for neutron detectors and chemically etched tracks for radon detectors.

Performance of a 2k CCD camera designed for electron crystallography at 400 kV

We discuss the performance of a charge-coupled device (CCD) camera that has been designed for use in electron crystallographic studies of proteins. There have been many previous publications describing the characteristics and performance of CCD-based cameras in electron microscopy; here we focus on characteristics relevant to protein studies at 400 kV. The low exposure that must be used in such studies produces a very poor signal-to-noise ratio, so any loss of signal-to-noise ratio in the recording process must be avoided. Images must contain a sufficient number of molecules to allow identification of the reciprocal lattice, thus requiring a large image format. Electron diffraction patterns may contain some spots with intensity around 10 −7 times that of the central beam, so the largest possible dynamic range is helpful. Some of the characteristics we discuss are most easily measured with crystals, but the conclusions also apply for other work such as single-particle analyses. The camera has been optimized for work at 400 kV with a P43 scintillator fiber-optically coupled to a CCD with 24 μm pixels. The scintillator in this camera is thicker than generally used at lower voltages, which provides an adequate signal level but slightly degrades the resolution. Operation at 400 kV leads to a point spread function that is broader than the CCD pixel size. Images are thus binned by a factor of two to double the effective pixel size, with the resulting loss of a factor of two in the size of areas that can be recorded in a single frame. A large CCD with a 2048×2048 pixel array is used to compensate for this loss and provide a sufficient signal for the crystallographic image processing used in this work. Images and electron diffraction patterns recorded on the CCD are compared with data recorded on photographic film. While the quality of the images recorded on the CCD at the low exposures required in protein studies is not quite as good as that on film, electron diffraction data recorded on the CCD are superior to that on film.

High pressure multiwire proportional and gas microstrip chambers for medical radiology

Application of MultiWire Proportional Chambers (MWPC) and MicroStrip Gas Chambers (MSGC) in medical radiography is discussed. These detectors are capable of detecting X-rays in counting mode with high efficiency, thus giving essential dose reduction compared to film/screen techniques. This was demonstrated on several Digital Radiographic Devices (DRD) with one-dimensional MWPC and scanning in the orthogonal direction. Effective pixel sizes of 1 mm and 0.5 mm for different devices with highly parallel readout systems has been achieved. The counting rate capability of DRDs is ∼ 500 kHz/pixel, which is enough to get high statistics of X-rays with a short exposure. Dose saving factors from 1 to 2 orders of magnitude for several common examinations were demonstrated.Further development of this approach can be made with MSGC which reproduce operation of MWPC in a much smaller scale. First tests of prototype chambers with 200 μm pitch of strips at high pressure have shown the possibility to reach a gain of ∼ 104 in a 6 bar Xe mixture. A proper choice of substrate material permits one to avoid charging problems at high fluxes. These features allow, one to build a counting device with 0.2 mm pixels for the detection of X-rays of 20–60 keV.

A new quasi-confocal image plate scanner with improved spatial resolution and ideal detection efficiency

Image plates (IP) can be used for electron detection in the EM like conventional negative material [N. Mori et al., Ultramicroscopy 25 (1988) 195]. The stored information can later be retrieved by a scanning process in form of a digitized image. Although the physics of the electron storage process of the IPs in theory predicts electron detection with high spatial resolution, high detection efficiency and high dynamic range, in practice the scanning process limits resolution and detection efficiency. At present the scanners yield only moderate spatial resolution — about 100 μm effective pixel size (i.e. about 200 μm line resolution [S. Isoda et al., Ultramicroscopy 41 (1992) 99]. Data on the detection efficiency have not been published so far. Here we report a scanner of novel design with an improved optical detection device which gives highly increased spatial resolution (40 μm effective pixel size) and an optimal detection efficiency with a DQE of about 0.9. These properties are comparable to values published on slow-scan CCD cameras. Using subsequent scans of the same IP (which could register an image, an electron diffraction pattern or a parallel EELS recording) a linear dynamic detection range of almost 5 orders of magnitude is possible. We discuss how these properties of the IP scanner system could be utilized in future applications.

Spatial Resolution of Solar Total Irradiance Variability: The YOHKOH White-Light Observations

The observations from the ACRIM instrument on board the Solar Maximum Mission, and other recent experiments, have allowed us to study the time variations of solar irradiance. These observations identified several mechanisms, ranging from sunspots to global oscillations produced by waves trapped in the solar interior (the p-modes) that cause variations in the total irradiance. In all cases the variations have small amplitudes in integrated light from the visible photosphere. The SXT instrument on board Yohkoh has provided a new data set that can resolve some of these variations spatially. Most of these data are in the form of whole-Sun images: rate about 5 per day; effective pixel size of 4.92 arc sec; passband 28Å centered at 4310Å; data interval beginning in September, 1991 and ending in October, 1992. These observations have several advantages over ground-based observations for the characterization of solar global variability, namely the lack of atmospheric effects and the long-term stability of the instrumentation. We present an assessment of these data in the context of the existing total irradiance data, and also discuss their application to determination of the figure of the Sun, particularly measurements of the variations of the solar radius.