Research on the Modulation Transfer Function Detection Method of a Bayer Filter Color Camera.

Bayer filter color cameras are more and more widely used in the field of aerospace remote sensing, but the Bayer filter causes great degradation in image quality; therefore, obtaining a means of achieving the high-precision measurement of the modulation transfer function (MTF) of Bayer filter color cameras is an urgent problem. In order to solve this problem, this paper develops a slanted-edge method via three steps: the detection of the slanted edge, the acquisition and processing of the edge spread function (ESF), and the acquisition and processing of the line spread function (LSF). A combination of the Canny operator and Hough transform is proposed for the detection of the slanted edge, which improves the fitting accuracy and anti-interference ability of the algorithm. Further, the Canny operator is improved by constructing an adaptive filter function and introducing the Otsu method, which can more effectively smooth the image and remove its false edges. A method of processing ESF data by combining cubic spline interpolation and Savitzky-Golay (SG) filtering is proposed, which reduces the effects of noise and the non-uniform sampling of ESF on MTF. A method of LSF processing using Gaussian function fitting is proposed to further reduce the effect of noise on MTF. The improved algorithm is verified by the MTF measurement test applied to a specific type of Bayer filter color space camera. The simulation and test results show that the improved slanted-edge method discussed in this paper has greater precision and a better anti-interference ability, and it can effectively solve the difficult problem associated with MTF detection in Bayer filter color space cameras.

Postural responses to sinusoidal modulations of viewpoint position in a virtual environment.

Visual self-motion information is known to contribute to postural control, but it is unclear precisely which aspects of visual motion information drive changes in posture. We report here results for standing humans which suggest that there is a speed of movement threshold that must be exceeded by a visual stimulus if a posture response is to be generated. We use signal-to-noise ratio (SNR) methods to measure the strength of steady-state visually evoked posture responses (SSVEPRs) to sinusoidal modulations of visual viewpoint position in a virtual environment (VE). Using threshold estimates found from data which show how posture responses depend on visual stimulus amplitude, we show that the sensitivity of the visuo-postural response system increases with the temporal frequency at which the position of one's viewpoint is modulated. We show further that there is a speed of movement threshold, on average 1.85 cm/s, which must be exceeded by a left-right modulation of viewpoint position if a posture response is to be generated. A comparison of visual stimulus visibility to posture response thresholds suggests that one tends to not make postural responses to visual stimuli that are unseen. Finally, we found small correlations between motion sickness in these experiments and both the time spent in the VE and the frequency of viewpoint movement.

Comparison of optical quality and distinct macular thickness in femtosecond laser-assisted versus phacoemulsification cataract surgery.

BACKGROUND: Optical quality and macular thickness changing optical quality is rarely reported after femtosecond laser-assisted cataract surgery (FLACS). In current research, we evaluated optical quality recovery and distinct macular thickness changes after FLACS and phacoemulsification cataract surgery (PCS). METHODS: A total of 100 cataract patients (100 eyes) were included (50 eyes for the FLACS group and 50 eyes for the PCS group). Modulation transfer function (MTF), point spread function (PSF) and dysfunctional lens index (DLI) were measured by a ray-tracing aberrometer (iTrace). Uncorrected distance visual acuity (UDVA) and corrected distance visual acuity (CDVA) were also assessed pre-operation,1 week and 1 month after surgery. The MTF values at spatial frequencies of 5, 10, 15, 20, 25 and 30 cycles/degree (c/d) were selected. We used optical coherence tomography (OCT) to assess the macular thickness of different regions pre-operatively and1month after the surgery. RESULTS: In PCS group, we found the statistically significant differences between pre-operation and post-operation in DLI (p < 0.0001), PSF (strehl ratio, SR) (p = 0.027) and MTF (p = 0.028), but not intraocular pressure (IOP) (p = 0.857). The differences between pre-operation and post-operation for DLI (p = 0.031), SR (p = 0.01) and IOP (p = 0.03), but not MTF (p = 0.128) were also found in FLACS group. The differences were statistically significant when the spatial frequencies were at 5, 10 and 25 (p = 0.013, 0.031 and 0.048) between pre-operation and post-operation in PCS group but not FLACS group at 1 month. In PCS group, we found the differences between pre-operation and post-operation in nasal inter macular ring thickness (NIMRT) (p = 0.03), foveal volume (FV) (p = 0.034) and average retinal thickness (ART) (p = 0.025) but not FLACS group at 1 month. CONCLUSION: FLACS is safe that did not cause significant increase of macular thickness in current study. However, it also cannot produce better optical quality. In contrast, PCS can produce macular thickness changes, but better optical quality recovery. The slightly retinal change may not affect optical quality.

Restoration and Calibration of Tilting Hyperspectral Super-Resolution Image.

Tilting sampling is a novel sampling mode for achieving a higher resolution of hyperspectral imagery. However, most studies on the tilting image have only focused on a single band, which loses the features of hyperspectral imagery. This study focuses on the restoration of tilting hyperspectral imagery and the practicality of its results. First, we reduced the huge data of tilting hyperspectral imagery by the p-value sparse matrix band selection method (pSMBS). Then, we restored the reduced imagery by optimal reciprocal cell combined modulation transfer function (MTF) method. Next, we built the relationship between the restored tilting image and the original normal image. We employed the least square method to solve the calibration equation for each band. Finally, the calibrated tilting image and original normal image were both classified by the unsupervised classification method (K-means) to confirm the practicality of calibrated tilting images in remote sensing applications. The results of classification demonstrate the optimal reciprocal cell combined MTF method can effectively restore the tilting image and the calibrated tiling image can be used in remote sensing applications. The restored and calibrated tilting image has a higher resolution and better spectral fidelity.

[Evaluation for Resolution Property of CT Image with Half-reconstruction Algorithm].

PURPOSE: The aim of this study was to investigate the resolution property in XY-plane for half-reconstruction computed tomography (CT) image by measuring 360° multi-directional modulation transfer functions (MTFs). MATERIALS AND METHODS: The 360° multi-directional MTFs were measured by use of a wire method to obtain line spread function with 15° interval in XY-plane. The MTFs of half-reconstruction CT image were measured with 100 mm off-center positions on the X- and Y-axis (X+100 mm, X-100 mm, Y+100 mm, and Y-100 mm) and compared with those of full-reconstruction CT image. We measured the MTFs of the half-reconstruction CT image at X+100 mm position with various X-ray tube positions of projection dataset. RESULTS: There were obvious differences for the MTFs of the half-reconstruction CT image between the tangential and radial directions at each measurement position. The dependences of the resolution property for the half-reconstruction CT image on positions and directions in XYplane were similar to those for the full-reconstruction CT image. The higher and the lower MTFs of the half-reconstruction CT image at X+100 mm position were measured with X-ray tube position of projection dataset at +X side and at -X side compared with those of the full-reconstruction CT image, respectively. CONCLUSION: We conclude that the half-reconstruction CT image had similar resolution property in XY-plane to the full-reconstruction CT image and showed dependency on the X-ray tube position of projection dataset for MTF in the tangential direction.

[Assessment Tool of Image Quality for Medical Liquid-crystal Displays Using Commercially Available Digital Cameras].

We developed a simplified tool for measuring image quality of medical liquid-crystal displays (LCDs) using a commercially available color digital camera. This tool implemented as a plug-in software for ImageJ (open-source image processing program) was designed to compute modulation transfer functions (MTFs) and Wiener spectra (WS) of monochrome and color LCDs from LCD photographed images captured by a camera. The intensities of the red (R), green (G), and blue (B) signals of the unprocessed image data depend on the spectral sensitivity of the image sensor used in the camera. In order to evaluate image quality based on LCD luminance, the plug-in software calibrates the RGB signals from the camera using measured luminance of the LCD and converts them into grayscale signals that correspond to the luminance of the LCD. The MTFs and WS are determined based on the line response from a one-pixel line image and the one-dimensional noise profiles acquired by scanning the uniform image using numerically synthesized slit, respectively. With this plug-in software for ImageJ, we are able to readily compute MTFs and WS of both monochrome and color LCDs from unprocessed image data of cameras. Our simplified tool is helpful to evaluate and understand the physical performance of LCDs for a large number of display users in hospitals and medical centers.

Image Quality and Clinical Usefulness of Ray-summation Image Reconstructed from CT Data, Compared with Digital Radiography.

Ray-summation (raysum) images reconstructed from computed tomography (CT) volume data resemble digital radiography (DR) images. Therefore, they have a potential to be used instead of DR images.The aim of this study was to compare the physical image quality evaluated by signal-difference-to-noise ratio (SDNR) and clinical usefulness between raysum and DR images. We employed an oval water phantom simulating adult abdomen for image quality measurement. Raysum images were reconstructed from CT volume data using an assumed x-ray quality of 70 keV. DR images were obtained using an indirect-type flat panel detector system. The normalized noise-power spectrum (NNPS) for various same dose indices (DR: entrance surface dose, CT: CT dose index volume) were measured from raysum and DR images. SDNRs were calculated from the results of NNPSs, modulation transfer function (MTF), and cartilage material contrast. Five experienced observers visually compared each pair of a clinical raysum image and a DR image for nine clinical cases (head, finger, pelvis, and foot). MTF of raysum was significantly lower than that of DR. SDNRs of DR were superior to those of raysum for each dose index, by an average factor of 1.24. For head and pelvis images, raysum images were comparable or a little superior compared with the DR images, because the radiation doses of raysum was much higher than those of DR. For finger and foot cases, the raysum images were inferior to DR images due to its lower resolution. Our results indicated a limited clinical usefulness of raysum compared with DR.

Evaluation of mean glandular dose and modulation transfer function for different tube potentials and target-filter combinations in computed radiography mammography.

BACKGROUND: Different target-filter combinations in computed radiography have different impacts on the dose and image quality in digital radiography. This study aims to evaluate the mean glandular dose (MGD) and modulation transfer function (MTF) of various target-filter combinations by investigating the signal intensities of X-ray beams. METHODS: General Electric (GE) Senographe DMR Plus mammography unit was used for MGD and MTF evaluation. The measured MGD was compared with the dose reference level (DRL), whereas the MTF was evaluated using ImageJ 1.46o software. A modified Mammography Accreditation Phantom RMI 156 was exposed using different target-filter combinations of molybdenum-molybdenum (Mo-Mo), molybdenum-rhodium (Mo-Rh) and rhodium-rhodium (Rh-Rh) at two different tube voltages, 26 kV and 32 kV with 50 mAs. RESULTS: In the MGD evaluations, all target-filters gave an MGD value of < 1.5 mGy. The one-way ANOVA test showed a highly significant interaction between the MGD and the kilovoltage and target-filter material used (26 kV: F (2,12) = 49,234, P = 0.001;32 kV: F (2,12) = 89,972, P = 0.001). A Tukey post-hoc test revealed that the MGD for 26 kV and 32 kV was highly affected by the target-filter combinations. The test of homogeneity of variances indicates that the MGD varies significantly for 26 kV and 32 kV images (0.045 and 0.030 (P < 0.05), respectively). However, the one-way ANOVA for the MTF shows that no significant difference exists between the target-filter combinations used with 26 kV and 32 kV images either in parallel or perpendicular to the chest wall side F (2,189) = 0.26, P > 0.05). CONCLUSION: Higher tube voltage and atomic number target-filter yield higher MGD values. However, the MTF is independent of the X-ray energy and the type of target-filter combinations used.

Modulation transfer function measurement of three-dimensional T1-weighted turbo spin echo sequence with low refocusing flip angles using single-plate method.

This study aimed to demonstrate the usefulness of modulation transfer function (MTF) measurements using the single-plate method to evaluate changes in resolution properties that are dependent on three parameters: echo train length (ETL), low refocusing flip angle (RFA), and start-up echo in three-dimensional T1-weighted turbo spin echoes (TSE) with a low RFA and to optimize these parameters. Although the MTFs were slightly degraded with an RFA of 120°, they were considerably degraded with an RFA of ≤ 90°. On the other hand, the MTF of low RFA was greatly improved by setting the start-up echo, allowing setting a long ETL. The single-plate method provided a clear and easy evaluation of the resolution properties of low RFA TSE. Furthermore, this method allows us to visualize changes in the signal intensity of each echo in k-space, depending on the sequence variation. These results suggest that the MTF measurement using the single-plate method is useful for evaluating the resolution properties of TSE sequences and optimizing the measured parameters.

Characterization of a Timepix detector for use in SEM acceleration voltage range.

Hybrid pixel direct electron detectors are gaining popularity in electron microscopy due to their excellent properties. Some commercial cameras based on this technology are relatively affordable which makes them attractive tools for experimentation especially in combination with an SEM setup. To support this, a detector characterization (Modulation Transfer Function, Detective Quantum Efficiency) of an Advacam Minipix and Advacam Advapix detector in the 15-30 keV range was made. In the current work we present images of Point Spread Function, plots of MTF/DQE curves and values of DQE(0) for these detectors. At low beam currents, the silicon detector layer behaviour should be dominant, which could make these findings transferable to any other available detector based on either Medipix2, Timepix or Timepix3 provided the same detector layer is used.

Exploration of the pulse pileup effects in a clinical CdTe-based photon-counting computed tomography.

BACKGROUND: High tube current generates a high flux of x-rays to photon counting detectors (PCDs) that can potentially result in the piling up of pulses formed by concurrent photons, which can cause count loss and energy resolution degradation. PURPOSE: To evaluate the performance of clinical photon-counting CT (PCCT) systems in high flux, potentially influenced by pulse pileup effects, in terms of task-generic image quality metrics. METHODS: A clinical phantom was scanned on a commercial PCCT scanner (NAEOTOM Alpha, Siemens) at 120 kV under fourteen different tube current levels (40-1000 mA) with a rotation time of 0.25 s and a pitch of 1. The dose levels corresponded to CTDIvol (32 cm phantom) of 0.79-19.8 mGy. CT sinograms were reconstructed using QIR-off mode (noniterative reconstruction algorithm), Br44 kernel, and a voxel size of 0.4102 × 0.4102 × 3 mm 3 $0.4102 \times 0.4102 \times 3{\mathrm{\ mm}}^3$ . imQuest, an open-source MATLAB-based software package was used to calculate noise power spectrum (NPS), task transfer function (TTF), contrast-to-noise ratio (CNR), and CT number according to AAPM Task Group 233 metrology. RESULTS: The 50% cut-off frequency of TTF (f50 ) remained mostly constant across all higher tube currents for all inserts, namely polyethylene, bone, air, and acrylic. Using the lowest two data points (40 and 80 mA), the expected relationship between noise magnitude and tube current was determined to be noise ∝ $ \propto \ $ mA-0.47 . The measured noise magnitude were up to 11.1% higher than the expected value at the highest tube current. The average frequency of NPS (fav ) decreased from 0.32 to 0.29 mm-1 as tube current increased from 40 to 1000 mA. No considerable effects were observed in CT number measurement of any insert; however, CT numbers for air and bone changed almost monotonically as tube current increased. Absolute CNR increased monotonically for all inserts; however, the difference between measured and expected CNRs were approximately -6% to 12% across all tube currents. CONCLUSIONS: Increasing tube currents did not affect the spatial resolution, but slightly affected the CT number and noise measurements of the clinical PCCT system. However, the effects were only considerable at clinically irrelevant tube currents used on a small 20-cm phantom. In general clinical practices, automatic exposure control techniques are used to decrease the variation of flux on the detector, which alleviates the chances of detector saturation due to high count rates. The observed effects could be due to pulse pileup, signal-dependent filtration of the system, or nonlinearities in the reconstruction algorithm. In conclusion, either the deadtime of the detector used in the photon-counting CT system is shorter such that count losses due to pulse pileup are negligible, or pulse pileup has inconsiderable effects on the image quality of clinical photon-counting CT systems in routine clinical practice due to possible corrections applied on the system.

Design of a Miniaturized Wide-Angle Fisheye Lens Based on Deep Learning and Optimization Techniques.

This paper presents the optimization design of a miniaturized five-element wide-angle fisheye lens using a deep learning algorithm. Zemax optical design software was used to simulate and optimize the wide-angle fisheye lens. A deep learning algorithm helped to find the best combination of different lens materials. We first used six lens elements as an initial configuration to design miniaturized wide-angle fisheye lenses using the optimization process. The optical system components were gradually decreased to five lens elements. Both OKP4HT and polymethyl methacrylate (PMMA) plastic aspheric lenses were selected to replace the second spherical glass lens in the original design. We propose two types of wide-angle fisheye lens designs with four spherical lenses and one aspheric lens. The results for these designs indicated a viewing angle of 174°, a total length of less than 15 mm, a spot size of less than 6 µm, lateral color within ±1 µm, field curvature within ±0.02 mm, and F-θ distortion of ±3.5%. In addition, the MTF value was larger than 0.4 at the spatial frequency of 100 cycles/mm.

[An Easy-to-use Method for CT Image Simulation with Parameter Optimization Using a Water Phantom].

PURPOSE: We developed an easy-to-use method to generate computed tomography (CT) images that simulate the images obtained when using an actual scanner. METHODS: The developed method generates images by simulating the data acquisition and image reconstruction processes of a scanner from a linear attenuation coefficient map of an object numerically generated. This approach is similar to general image simulation methods. However, we introduced adjustable parameters for the CT data acquisition process, for example, parameters related to X-ray attenuation in the anode of the X-ray tube and the bowtie filter. These parameters were optimized in advance by minimizing the difference between the simulated and measured images of a water phantom. To verify the validity of the developed method, a simulated image was generated for a torso phantom and then compared with the measured image of the phantom obtained using the scanner. RESULTS: The simulated and measured images of the torso phantom were in good agreement. The spatial resolution and noise characteristics of these two images were also comparable, further indicating the accuracy of the developed method. CONCLUSION: In the existing methods, various information/data related to an actual scanner, including difficult-to-acquire ones, were essential for image simulation. In the developed method, instead of determining the difficult-to-acquire information/data, we introduced adjustable parameters. Therefore, the developed method was easier to use than the existing methods.

Evaluating Optical Quality of a New Hydrophilic Enhanced Monofocal Intraocular Lens and Comparison to the Monofocal Counterpart: An Optical Bench Analysis.

INTRODUCTION: The aim of the study was to analyze the optical properties of a new hydrophilic enhanced monofocal intraocular lens (IOL) using optical bench analysis and compare it with its monofocal counterpart. METHODS: This laboratory study investigates the enhanced monofocal intraocular lens (L-333) and the monofocal counterpart (L-313) IOL by Teleon Surgical, Spankeren, Netherlands on the optical bench, using OptiSpheric IOL PRO2 (Trioptics, Germany) in order to assess the optical quality according to ISO 11979 with ISO-2 Cornea. IOLs (power 22.0 D) were evaluated regarding through frequency modulation transfer function (MTF), Strehl ratio (SR), and through focus MTF at 50 lp/mm using a 3.0-mm and a 4.5-mm aperture. Tilt and decentration were applied. In addition, wavefront measurements were obtained using WaveMaster® IOL 2 device (Trioptics, Germany) and analyzed. RESULTS: Centered: The MTF (mean) at 50 lp/mm (L-333/L-313) with 3.0 mm aperture was 0.606/0.724 and with 4.5 mm aperture 0.330/0.409. The SR (mean) with 3.0 mm aperture was 0.586/0.809 and with 4.5 mm aperture 0.330/0.348. Decentered by 1 mm: The MTF (mean) at 50 lp/mm (L-333/L-313) with 3.0 mm aperture was 0.485/0.705 and with 4.5 mm aperture 0.255/0.374. The SR (mean) with 3.0 mm aperture was 0.457/0.739 and with 4.5 mm aperture 0.185/0.268. Tilted by 5 degrees: The MTF (mean) at 50 lp/mm (L-333/L-313) with 3.0 mm aperture was 0.577/0.657 and with 4.5 mm aperture 0.345/0.336. The SR (mean) with 3.0 mm aperture was 0.583/0.702 and with 4.5 mm aperture 0.269/0.237. In through focus MTF and aperture of 3.0 mm, the L-333 showed a peak of 0.41 with some enlarged depth of power of about 2 D. For the aperture of 4.5 mm, the MTF values of L-313 and L-333 were slightly reduced; L-333 showed an MTF peak of 0.23 and some reduced depth of power of about 1.5 D. Wavefront measurements showed no major aberrations for the L-313, while a combination of moderate increase in Z 4-0 and Z 6-0 with opposite sign was revealed for the L-333. CONCLUSION: The enhanced monofocal Lentis Quantum (L-333) produces some enlarged depth of focus by combining spherical aberration of different order and opposite sign. The Lentis Quantum performs very well in comparison to the aspherical monofocal counterpart owing to its optical design. Results with large apertures were sufficient too, suggesting that  the lens is a good option in eyes with a wide pupil and thus in refractive surgeries of young patients.

Luminance white noise electroretinograms (wnERGs) in mice.

Purpose: To record and analyse electroretinograms (ERGs) to luminance stimuli with white noise temporal profiles in mice. White noise stimuli are expected to keep the retina in a physiologically more natural state than, e.g., flashes. The influence of mean luminance (ML) was studied. Methods: Electroretinograms to luminance temporal white noise (TWN) modulation (wnERGs) were measured. The white noise stimuli contained all frequencies up to 20 Hz with equal amplitudes and random phases. Responses were recorded at 7 MLs between -0.7 and 1.2 log cd/m2. Impulse response functions (IRFs) were calculated by cross correlating the averaged white noise electroretinogram (wnERG) responses with the stimulus. Amplitudes and latencies of the initial trough and subsequent peak in the IRFs were measured at each ML. Fourier transforms of the IRFs resulted in modulation transfer functions (MTFs). wnERGs were averaged across different animals. They were measured twice and the responses at identical instances in the 1st and 2nd recordings were plotted against each other. The correlation coefficient (r 2 repr) of the linear regression quantified the reproducibility. The results of the first and second measurement were further averaged. To study the underlying ERG mechanisms, the ERG potentials at the different MLs were plotted against those at the lowest and highest ML. The correlation coefficients (r 2 ML) were used to quantify their similarities. Results: The amplitudes of the initial (a-wave-like) trough of the IRFs increased with increasing ML. The following positive (b-wave-like) peak showed a minimum at -0.4 log cd/m2 above which there was a positive correlation between amplitude and ML. Their latencies decreased monotonously with increasing ML. In none of the IRFs, oscillatory potential (OP)-like components were observed. r 2 repr values were minimal at a ML of -0.1 log cd/m2, where the MTFs changed from low-pass to band-pass. r 2 ML values increased and decreased with increasing ML when correlated with responses obtained at the highest or the lowest ML, respectively. Conclusion: White noise electroretinograms can be reliably recorded in mice with luminance stimuli. IRFs resemble flash ERGs superficially, but they offer a novel procedure to study retinal physiology. New components can be described in the IRFs. The wnERGs are either rod- or cone-driven with little overlap.

The effects of x-ray beam hardening on detective quantum efficiency and radiation dose.

The goal of this preliminary study was to investigate the effects of x-ray beam hardening on the detective quantum efficiency (DQE) and the radiation dose of an inline x-ray imaging system. The ability to decrease the risk of harmful radiation to the patient without compromising the detection capability would more effectively balance the tradeoff between image quality and radiation dose, and therefore benefit the fields of diagnostic x-ray imaging, especially mammography. The DQE and the average glandular dose were both calculated under the same experimental conditions for a range of beam hardening levels, corresponding to no added beam hardening and two thicknesses each of Rhodium (Rh) and Molybdenum (Mo) filters. The dose calculation results demonstrate a reduction of 15% to 24% for the range of beam hardening levels. The comparison of all quantities comprising the DQE exhibit very close correlation between the results obtained without added beam hardening to the results corresponding to the range of beam hardening levels. For the specific experimental conditions utilized in this preliminary study, the results are an indication that the use of beam hardening holds the potential to reduce the radiation dose without decreasing the performance of the system. Future studies will seek to apply this method in a clinical environment and perform a comprehensive image quality evaluation, in an effort to further evaluate the potential of beam hardening to balance the tradeoff between dose and image quality.

[Comparison between Variance and Edge Methods for Measuring Image Resolution Properties in Digital Mammography].

We studied the image resolution properties in digital mammography using the variance and edge response function methods. The associated measurements were made using a polymethylmethacrylate phantom of 0-40 mm thickness while maintaining the incident dose to the detector constant, as well as with and without using the phantom but by varying the level of exposure to the detector. The results obtained using the two methods were compared on the object plane of the mammography unit with compensation for the magnification and rejection of the scattered radiation. In this unit, the imaging process of resolution enhancement may be performed according to the phantom thickness, shape of test device, exposure level, and so on. The modulation transfer function values of the variance method were slightly higher than those of the edge method, approximately 6% at 2 cycles/mm and 8% at 4 cycles/mm without resolution enhancement processing. We concluded that the variance method is more suitable than the edge method for the constancy test or quality control because of its lower coefficient of variation.

Detectability of the diagonal direction in the assessment of medical images using a high-brightness liquid crystal display monitor.

Liquid crystal display (LCD) monitors show a homogenous quadratic pattern in the number of pixels, size, and dimensions. However, their modulation transfer function (MTF) has a non-isotropic effect in the vertical, horizontal, and diagonal directions on the screen from the shape and pattern of arrangement of pixels. Moreover, the MTF of the human eye differs in spatial frequency response directivity among individuals. In this study, the high-brightness LCD monitor detectability was physically simulated and visually examined throughout the system, including the imaging system. Furthermore, the influence of anisotropy of the LCD monitor was evaluated. The MTF of the LCD monitor was measured by acquiring a bar pattern using a digital camera with sufficiently small pixels in the vertical, horizontal, and diagonal directions and by performing interpolation processing through waveform reproduction and frequency analysis. The detectability of the LCD monitor was verified throughout the system, including the imaging system. Radiographs of the rectangular wave chart (0.5-10 LP/mm) were obtained in the vertical, horizontal, and diagonal (45°) directions to assess the perceivable limit of the human eye (LP/mm). The spatial resolution of the LCD monitor in the diagonal direction was higher than that in the vertical or horizontal direction, which was in good agreement with the results of the profile analysis and visual evaluation.

Single-plate method for practical modulation transfer function measurement in magnetic resonance imaging.

A new "single-plate method" is presented for measuring the modulation transfer function (MTF) in magnetic resonance imaging (MRI). This method sets a slice plane perpendicular to a single-plate phantom to eliminate contamination effects from the direction perpendicular to the measurement direction in the image plane, which occur with a conventional ramp method. As no practical method for measuring the MTF has been established for MRI, we examined whether the MTF can be measured practically using the single-plate method for various fast imaging sequences. Furthermore, the MTFs of T1-weighted (T1W) fast spin echo (FSE) and conventional spin echo (CSE) images obtained using the single-plate method and ramp method were compared. The measured MTFs of T1W CSE images revealed rectangular shapes with a sharp decrease near the Nyquist cutoff frequency in both phase-encoding (PE) and frequency-encoding (readout, RO) directions. The measured MTFs of T1W FSE images obtained with centric-order acquisition showed symmetric step-function shapes reflecting k-space segmentation determined by the echo train length (ETL). The measured MTFs of T2-weighted (T2W) FSE images showed asymmetric step-function shapes reflecting differences in T2 decay of signals from samples. The MTFs obtained using the single-plate method significantly reduced the collapse caused by the contamination effect, which is observed in all the MTF measurements of the ramp method. The proposed "single-plate method" simplified the complicated MTF measurement procedure and eliminated the contamination effect. This method is expected to be useful for evaluating the resolution properties of MR fast imaging techniques with a complicated k-space trajectory.

Technical Note: Accuracy of MTF measurements with an edge phantom at megavoltage x-ray energies.

PURPOSE: Measurement of the modulation transfer function (MTF) is performed by evaluating the response of an imaging system to a predefined input. To obtain accurate results when using an edge phantom, the detector input signal must resemble an ideal step function. The MTF of megavoltage (MV) imagers used in radiotherapy has been measured with highly absorbing edge phantoms fabricated from thick metal blocks. This study investigates the influence of the edge phantom design on the accuracy of the resulting MTF. METHODS: The MTF of an electronic portal imaging device (EPID) was measured at 6 MV beam quality with four edge phantoms made of lead with 1.3, 3.3, 5.0, and 10.0 cm thickness. Monte Carlo simulations were carried out for these and a selection of tungsten phantoms to determine the photon fluence at the imaging plane and quantify the systematic error in the MTF introduced by the edge phantom design. RESULTS: The measured MTF depends on the design of the edge phantom. The detector input signal of a thin phantom is affected by secondary radiation from the phantom itself, causing an overestimation of the MTF. The amount of secondary radiation can be reduced by increasing the phantom thickness or introducing an air gap between the phantom and the detector. Both methods introduce geometric unsharpness, which can result in an underestimation of the true MTF. Edge phantoms made from 4.0 cm thick tungsten or 5.0 cm thick lead induce comparatively small systematic errors of below 3% or 5%, respectively. CONCLUSIONS: When MTF measurements are conducted at MV energies, even a highly absorbing edge phantom will introduce a systematic error of several percent. Direct comparison of MTFs obtained with different edge phantoms should therefore be treated with caution.