Modulation Transform Function Research Articles

Comprehensive MR imaging QA of 0.35 T MR-Linac using a multi-purpose large FOV phantom: A single-institution experience.

Magnetic resonance-guided radiotherapy (MRgRT) is desired for the treatment of diseases in the abdominothoracic region, which has a broad imaging area and continuous motion. To ensure accurate treatment delivery, an effective image quality assurance (QA) program, with a phantom that covers the field of view (FOV) similar to a human torso, is required. However, routine image QA for a large FOV is not readily available at many MRgRT centers. In this work, we present the clinical experience of the large FOV MRgRT Insight phantom for periodic daily and monthly comprehensive magnetic resonance imaging (MRI)-QA and its feasibility compared to the existing institutional routine MRI-QA procedures in 0.35 T MRgRT. Three phantoms; ViewRay cylindrical water phantom, Fluke 76-907 uniformity and linearity phantom, and Modus QA large FOV MRgRT Insight phantom, were imaged on the 0.35 T MR-Linac. The measurements were made in MRI mode with the true fast imaging with steady-state free precession (TRUFI) sequence. The ViewRay cylindrical water phantom was imaged in a single-position setup whereas the Fluke phantom and Insight phantom were imaged in three different orientations: axial, sagittal, and coronal. Additionally, the phased array coil QA was performed using the horizontal base plate of the Insight phantom by placing the desired coil around the base section which was compared to an in-house built Polyurethane foam phantom for reference. The Insight phantom captured image artifacts across the entire planar field of view, up to 400mm, in a single image acquisition, which is beyond the FOV of the conventional phantoms. The geometric distortion test showed a similar distortion of 0.45±0.01 and 0.41±0.01mm near the isocenter, that is, within 300mm lengths for Fluke and Insight phantoms, respectively, but showed higher geometric distortion of 0.8±0.4mm in the peripheral region between 300 and 400mm of the imaging slice for the Insight phantom. The Insight phantom with multiple image quality features and its accompanying software utilized the modulation transform function (MTF) to evaluate the image spatial resolution. The average MTF values were 0.35±0.01, 0.35±0.01, and 0.34±0.03 for axial, coronal, and sagittal images, respectively. The plane alignment and spatial accuracy of the ViewRay water phantom were measured manually. The phased array coil test for both the Insight phantom and the Polyurethane foam phantoms ensured the proper functionality of each coil element. The multifunctional large FOV Insight phantom helps in tracking MR imaging quality of the system to a larger extent compared to the routine daily and monthly QA phantoms currently used in our institute. Also, the Insight phantom is found to be more feasible for routine QA with easy setup.

Adaptive-weighted high order TV algorithm for sparse-view CT reconstruction.

With the development of low-dose computed tomography (CT), incomplete data reconstruction has been widely concerned. The total variation (TV) minimization algorithm can accurately reconstruct images from sparse or noisy data. However, the traditional TV algorithm ignores the direction of structures in images, leading to the loss of edge information and block artifacts when the object is not piecewise constant. Since the anisotropic information can facilitate preserving the edge and detail information in images, we aim to improve the TV algorithm in terms of reconstruction accuracy via this approach. In this paper, we propose an adaptive-weighted high order total variation (awHOTV) algorithm. We construct the second order TV-norm using the second order gradient, adapt the anisotropic edge property between neighboring image pixels, adjust the local image-intensity gradient to keep edge information, and design the corresponding Chambolle-Pock (CP) solving algorithm. Implementing the proposed algorithm, comprehensive studies are conducted in the ideal projection data experiment where the Structural similarity (SSIM), Root Mean Square Error (RMSE), Contrast to noise ratio (CNR), and modulation transform function (MTF) curves are utilized to evaluate the quality of reconstructed images in statism, structure, spatial resolution, and contrast, respectively. In the noisy data experiment, we further use the noise power spectrum (NPS) curve to evaluate the reconstructed images and compare it with other three algorithms. We use the 2D slice in the XCAT phantom, 2D slice in TCIA Challenge data and FORBILD phantom as simulation phantoms and use real bird data for real verification. The results show that, compared with the traditional TV and FBP algorithms, the awHOTV has better performance in terms of RMSE, SSIM, and Pearson correlation coefficient (PCC) under the projected data with different sparsity. In addition, the awHOTV algorithm is robust against the noise of different intensities. The proposed awHOTV method can reconstruct the images with high accuracy under sparse or noisy data. The awHOTV solves the strip artifacts caused by sparse data in the FBP method. Compared with the TV method, the awHOTV can effectively suppress block artifacts and has good detail protectionability.

Comparisons of Modulation Transform Function in 2 Monofocal Spherical Hydrophobic and Hydrophilic Intra Ocular Lenses

To compare Modulation Transform Function (MTF) between standard monofocal spherical hydrophobic acrylic Alcon SA60AT (Bausch and Lomb) and hydrophilic acrylic Rayner Superflex (620H) (Rayner) intra-ocular lenses (IOLs). This cross-sectional study was conducted on 68 patients who underwent cataract surgery. The hydrophobic Alcon SA60AT IOL was implanted in 39, and the hydrophilic Rayner Superflex (620H) was implanted in 29 eyes. The OPD Scan III (Nidek) was used to assess MTF in normal pupils under mesopic light conditions 1 and 3 months after the surgery. t-test showed no significant difference in mean MTF between the two IOLs in the 2 follow-ups (P=0.788). The results of repeated measure ANOVA for each type of IOL indicated that MTF increased significantly in the hydrophilic group versus the hydrophobic group in the 3rd month (P=0.033). Moreover, the results of repeated measure ANOVA showed that MTF was affected by the type of IOL and refractive error in the 3rd month (P=0.029, P=0.025). It seems that the material of IOL and post-surgical residual refractive error can affect the visual acuity of pseudophakic patients. Although the hydrophilic IOL provided a better MTF three months after the surgery, studies with longer follow-ups are required to confirm the results.

Assessment of a novel commercial large field of view phantom for comprehensive MR imaging quality assurance of a 0.35T MRgRT system

Consistent quality assurance (QA) programs are vital to MR‐guided radiotherapy (MRgRT), for ensuring treatment is delivered accurately and the onboard MRI system is providing the expected image quality. However, daily imaging QA with a dedicated phantom is not common at many MRgRT centers, especially with large phantoms that cover a field of view (FOV), similar to the human torso. This work presents the first clinical experience with a purpose‐built phantom for large FOV daily and periodic comprehensive quality assurance (QUASAR™ MRgRT Insight Phantom (beta)) from Modus Medical Devices Inc. (Modus QA) on an MRgRT system. A monthly American College of Radiology (ACR) QA phantom was also imaged for reference. Both phantoms were imaged on a 0.35T MR‐Linac, a 1.5T Philips wide bore MRI, and a 3.0T Siemens MRI, with T1‐weighted and T2‐weighted acquisitions. The Insight phantom was imaged in axial and sagittal orientations. Image quality tests including geometric accuracy, spatial resolution accuracy, slice thickness accuracy, slice position accuracy, and image intensity uniformity were performed on each phantom, following their respective instruction manuals. The geometric distortion test showed similar distortions of –1.7 mm and –1.9 mm across a 190 mm and a 283 mm lengths for the ACR and MRgRT Insight phantoms, respectively. The MRgRT Insight phantom utilized a modulation transform function (MTF) for spatial resolution evaluation, which showed decreased performance on the lower B0 strength MRIs, as expected, and could provide a good daily indicator of machine performance. Both the Insight and ACR phantoms showed a match with scan parameters for slice thickness analysis. During the imaging and analysis of this novel MRgRT Insight phantom the authors found setup to be straightforward allowing for easy acquisition each day, and useful image analysis parameters for tracking MRI performance.

Use of local noise power spectrum and wavelet analysis in quantitative image quality assurance for EPIDs.

To investigate the use of local noise power spectrum (NPS) to characterize image noise and wavelet analysis to isolate defective pixels and inter-subpanel flat-fielding artifacts for quantitative quality assurance (QA) of electronic portal imaging devices (EPIDs). A total of 93 image sets including custom-made bar-pattern images and open exposure images were collected from four iViewGT a-Si EPID systems over three years. Global quantitative metrics such as modulation transform function (MTF), NPS, and detective quantum efficiency (DQE) were computed for each image set. Local NPS was also calculated for individual subpanels by sampling region of interests within each subpanel of the EPID. The 1D NPS, obtained by radially averaging the 2D NPS, was fitted to a power-law function. The r-square value of the linear regression analysis was used as a singular metric to characterize the noise properties of individual subpanels of the EPID. The sensitivity of the local NPS was first compared with the global quantitative metrics using historical image sets. It was then compared with two commonly used commercial QA systems with images collected after applying two different EPID calibration methods (single-level gain and multilevel gain). To detect isolated defective pixels and inter-subpanel flat-fielding artifacts, Haar wavelet transform was applied on the images. Global quantitative metrics including MTF, NPS, and DQE showed little change over the period of data collection. On the contrary, a strong correlation between the local NPS (r-square values) and the variation of the EPID noise condition was observed. The local NPS analysis indicated image quality improvement with the r-square values increased from 0.80 ± 0.03 (before calibration) to 0.85 ± 0.03 (after single-level gain calibration) and to 0.96 ± 0.03 (after multilevel gain calibration), while the commercial QA systems failed to distinguish the image quality improvement between the two calibration methods. With wavelet analysis, defective pixels and inter-subpanel flat-fielding artifacts were clearly identified as spikes after thresholding the inversely transformed images. The proposed local NPS (r-square values) showed superior sensitivity to the noise level variations of individual subpanels compared with global quantitative metrics such as MTF, NPS, and DQE. Wavelet analysis was effective in detecting isolated defective pixels and inter-subpanel flat-fielding artifacts. The proposed methods are promising for the early detection of imaging artifacts of EPIDs.

SU‐C‐304‐05: Use of Local Noise Power Spectrum and Wavelets in Comprehensive EPID Quality Assurance

Purpose:As EPIDs are increasingly used for IMRT QA and real‐time treatment verification, comprehensive quality assurance (QA) of EPIDs becomes critical. Current QA with phantoms such as the Las Vegas and PIPSpro™ can fail in the early detection of EPID artifacts. Beyond image quality assessment, we propose a quantitative methodology using local noise power spectrum (NPS) to characterize image noise and wavelet transform to identify bad pixels and inter‐subpanel flat‐fielding artifacts.Methods:A total of 93 image sets including bar‐pattern images and open exposure images were collected from four iViewGT a‐Si EPID systems over three years. Quantitative metrics such as modulation transform function (MTF), NPS and detective quantum efficiency (DQE) were computed for each image set. Local 2D NPS was calculated for each subpanel. A 1D NPS was obtained by radial averaging the 2D NPS and fitted to a power‐law function. R‐square and slope of the linear regression analysis were used for panel performance assessment. Haar wavelet transformation was employed to identify pixel defects and non‐uniform gain correction across subpanels.Results:Overall image quality was assessed with DQE based on empirically derived area under curve (AUC) thresholds. Using linear regression analysis of 1D NPS, panels with acceptable flat fielding were indicated by r‐square between 0.8 and 1, and slopes of −0.4 to −0.7. However, for panels requiring flat fielding recalibration, r‐square values less than 0.8 and slopes from +0.2 to −0.4 were observed. The wavelet transform successfully identified pixel defects and inter‐subpanel flat fielding artifacts. Standard QA with the Las Vegas and PIPSpro phantoms failed to detect these artifacts.Conclusion:The proposed QA methodology is promising for the early detection of imaging and dosimetric artifacts of EPIDs. Local NPS can accurately characterize the noise level within each subpanel, while the wavelet transforms can detect bad pixels and inter‐subpanel flat fielding artifacts.

Optical photon transport in powdered‐phosphor scintillators. Part II. Calculation of single‐scattering transport parameters

Monte Carlo methods based on the Boltzmann transport equation (BTE) have previously been used to model light transport in powdered-phosphor scintillator screens. Physically motivated guesses or, alternatively, the complexities of Mie theory have been used by some authors to provide the necessary inputs of transport parameters. The purpose of Part II of this work is to: (i) validate predictions of modulation transform function (MTF) using the BTE and calculated values of transport parameters, against experimental data published for two Gd2O2S:Tb screens; (ii) investigate the impact of size-distribution and emission spectrum on Mie predictions of transport parameters; (iii) suggest simpler and novel geometrical optics-based models for these parameters and compare to the predictions of Mie theory. A computer code package called phsphr is made available that allows the MTF predictions for the screens modeled to be reproduced and novel screens to be simulated. The transport parameters of interest are the scattering efficiency (Q sct), absorption efficiency (Q abs), and the scatter anisotropy (g). Calculations of these parameters are made using the analytic method of Mie theory, for spherical grains of radii 0.1-5.0 μm. The sensitivity of the transport parameters to emission wavelength is investigated using an emission spectrum representative of that of Gd2O2S:Tb. The impact of a grain-size distribution in the screen on the parameters is investigated using a Gaussian size-distribution (σ = 1%, 5%, or 10% of mean radius). Two simple and novel alternative models to Mie theory are suggested: a geometrical optics and diffraction model (GODM) and an extension of this (GODM+). Comparisons to measured MTF are made for two commercial screens: Lanex Fast Back and Lanex Fast Front (Eastman Kodak Company, Inc.). The Mie theory predictions of transport parameters were shown to be highly sensitive to both grain size and emission wavelength. For a phosphor screen structure with a distribution in grain sizes and a spectrum of emission, only the average trend of Mie theory is likely to be important. This average behavior is well predicted by the more sophisticated of the geometrical optics models (GODM+) and in approximate agreement for the simplest (GODM). The root-mean-square differences obtained between predicted MTF and experimental measurements, using all three models (GODM, GODM+, Mie), were within 0.03 for both Lanex screens in all cases. This is excellent agreement in view of the uncertainties in screen composition and optical properties. If Mie theory is used for calculating transport parameters for light scattering and absorption in powdered-phosphor screens, care should be taken to average out the fine-structure in the parameter predictions. However, for visible emission wavelengths (λ < 1.0 μm) and grain radii (a > 0.5 μm), geometrical optics models for transport parameters are an alternative to Mie theory. These geometrical optics models are simpler and lead to no substantial loss in accuracy.

Analysis on axial squeeze distortion in resolution-priority integral imaging display systems

When the three-dimensional (3D) image is out of focus, a significant axial squeeze distortion is generally produced by position mismatching between the geometric convergences of the lenslet array and the convergence of each lenslet. In this paper, a rigorous explanation of a phenomenon that produces axial distortions is first presented. Then, the phenomenon is theoretically analyzed and the axial shifted position of the 3D image point is derived based on geometrical optics and the modulation transform function (MTF) theory. Simulated results show the dependence of 3D image axial shift error on the focal length of the lenslet array. Finally, a solution for eliminating the axial shift and the axial distortion is brought out and the suitable range of the focal length is discussed.

遥感相机动态调制传递函数与时间延迟积分CCD 行周期误差的关系

The error of line transfer frequency from Time Delay Integration(TDI)CCD has strong influence on dynamic Modulation Transform Function(MTF)and it is stronger when the integral grade is higher.Therefore,this paper develops an accurate mathematical model for dynamic MTF and line transfer period error rate was developed.Firstly,a typical line transfer timing of TDICCD was introduced and the window functions of one integral grade and multiple integral grade were derived.On these bases,the relationship between line transfer period error rate and dynamic MTF for this timing was established by Fourier transform of the window functions and a nonlinear parameter was derived in the function which is none in the traditional one.Then,simulations and experiments were performed,which indicate that whenΔT/T=0,the dynamic MTF value for this timing is 0.632 5times of static MTF with the number of phases b=4.It also indicates that there are obvious differences between dynamic MTF curves from proposed method and traditional method.Finally according to the requirement of the index,the maximum line transfer period error rates with different integral grades are given,which ensures that the decline of the dynamic MTF is less than 5%.

Improving image quality of x-ray in-line phase contrast imaging using an image restoration method

For practical application of x-ray in-line phase contrast imaging, a high-quality image is essential for object perceptibility and quantitative imaging. The existing approach to improve image quality is limited by high cost and physical limitations of the acquisition hardware. A useful image restoration algorithm based on fast wavelet transform is proposed. It takes advantage of degradation model and extends the modulation transform function (MTF) compensation algorithm from Fourier domain to wavelet domain. The modified algorithm is evaluated through comparison with the conventional MTF compensation algorithm. Its deblurring property is also characterized with the evaluation parameters of image quality. The results demonstrate that the modified algorithm is fast and robust, and it can effectively restore both the lost detail and edge information while ringing artifacts are reduced.

基于光学自由曲面的离轴三反光学系统

A freeform off-axis three-mirror system based on a freeform mirror was designed for the long focal length and wide-field off-axis three-mirror system in a space telescope.To obtain a system with a focal length of 4 500 mm,field of view of 11° and the ratio of the total length of system and the focal length in 1/3,the design and performance between traditional off-axis three-mirror system and freeform off-axis three-mirror system were compared.In the designed optical system,the freeform mirror was introduced to the secondary mirror to enhance system optimization and improve the balance capacity for optical aberration.After the comparison and optimization,the relative aperture of 1/9.5 was chosen.In the system,the average of wavefront error of all field of view is better than 0.030λ(λ=632.8 nm),and the average of the Modulation Transform Function(MTF) of all field of view is greater than 0.434(71.4 lp/mm) that is close to the diffraction limitation.Furthermore,the MTF has im-proved more than 5% compared to that of traditional TMA system and the departure of the secondary freeform mirror and theoretical spherical surface is only 1.1λ(λ=632.8 nm).The highly precision testing can be accomplished only by a standard sphere combined with the non-null testing method based on digital mask,which solves the problem of large-aperture convex freeform mirror testing and is helpful for the manufacturing of freeform secondary mirror.The off-axis three-mirror system based on freeform secondary mirror shows its advantages in smaller cubage,better realizability in engineering,better wavefront error and higher MTF.

Golay3望远镜系统的设计

The primary mirror of a two-reflective system for a telescopy was replaced by a Golay 3 sparse aperture to reduce its weight and volume while remaining high angular resolution.Firstly,the two-reflective system with a spherical primary mirror and a hyperbolic secondary mirror was designed according to the third-aberration theory.Then,the primary mirror was substituted by a Golay 3 sparse aperture,and the relationship between fill factor and sub-mirror radius was analyzed.The whole system was simulated by Zemax software to obtain the Modulation Transform Function(MTF) and the spot diagram of the system.Finally,two silicon correcting lenses were inserted behind the secondary mirror to optimize the radius of the correcting lens and imaging distances and to improve the field of the system.By Zemax simulation,it is shown that the cutoff frequency of the system has been decreased from 114.5lp/mm to 97.7lp/mm and to 77.8lp/mm as the fill factor reduces from 22.2% to 15%and to 10%,respectively.Moreover,inserting the correcting lens in front of the image plane can greatly enlarge the field of the system,which shows the RMS value of maximum spot radius without correcting lens is about 2.5times larger than that with a correcting lens in the field of 0.3°.

改进的离轴三反光学系统的设计

The key design of off-axial Three-mirror Anastigmat(TMA) systems was summarized,and an improved design of the off-axial TMA optical systems was proposed with a new optical structure.By taking two designed systems for examples,the optical elements of the TMA system were reasonably arranged,the secondary mirror was designed as a spherical surface,and the primary and the third mirrors were optimized with high-order aspheric coefficients.Obtained results show that the Modulation Transform Function(MTF) curve of the designed system approaches to the optical diffraction limit with an improved value of 0.6 at the spatial frequency of 50 pl/mm.Compared with the common off-axial TMA optical system,the Strehl ratio can be increased from 0.91 to 0.93.The surface tolerance is relaxed from λ/50 to λ/40,and the assembling tolerances of the primary mirror,second mirror and the third mirror can be relaxed by a factor of 4.The improved design makes the manufacture and assembly of the off-axial TMA optical system much easier,and helps the system to achieve excellent performance in the level of optical diffraction limit.These are beneficial to the popularization of off-axial TMA optical systems.