Noise-equivalent Number Of Quanta Research Articles

Characterization of the acquisition modes implemented in Pixirad-1/Pixie-III X-ray Detector: Effects of charge sharing correction on spectral resolution and image quality

X-ray Photon Counting Detectors (XPCDs) with thick semiconductor sensors and small pixel sizes suffer from a charge-sharing effect which can induce multiple counts from a single interacting photon. This issue degrades not only the energy resolution, but it also worsens the spatial resolution. Multiple counts can be removed by acting on the discriminator threshold or they can be corrected by means of specific acquisition modalities implemented in the readout electronics. In this paper we present the results of the characterization of the Pixirad-1/Pixie-III device, a XPCD carrying a 650μm thick CdTe sensor with a small pixel (62×62μm2). The Pixie-III readout system includes programmable energy thresholds and it implements three acquisition modes: a pure photon counting mode and two modes specially designed to correct charge-sharing effects. The measured energy resolution of the three acquisition modes for different energies are here reported. Moreover, we characterize the imaging performance for different combinations of acquisition modes and thresholds by measuring the presampling Modulation Transfer Function (MTF), the Normalized Noise Power Spectrum (NNPS) and by assessing the Noise Equivalent number of Quanta (NEQ).

Linear analysis of single-shot dual-energy computed tomography with a multilayer detector

The sandwich-like multilayer detector can have a unique characteristic when it employs two detector layers having different spatial-resolution properties to each other. Reconstruction of dual-energy (DE) images using a weighted log-subtraction technique then results in the unsharp masking-like effect on the resultant images. We have developed a simple model describing the signal and noise in the sandwich detector-based DE computed tomography (CT) using a linear-systems theory. Applying the sandwich detector to a laboratory micro-CT system, we measured the Fourier metrics, such as modulation-transfer function, noise-power spectrum, and noise-equivalent number of quanta (NEQ), using thin wire and water phantoms. The agreements between the measured Fourier metrics and the theoretical models were excellent. The DE-CT for a postmortem mouse showed an enhanced details of bone structures, and the theoretical model explained the reason as showing a shift of the peak position of NEQ to high-frequency regions.

OA158] Noise equivalent number of quanta (NEQ): A tool for choosing the optimal reconstruction algorithm in computed tomography (CT)

Purpose Evaluate optimal reconstruction algorithm and voxel size for a breast CT system using NEQ. Methods CT scans of a wire phantom and a homogeneous cylindrical sample have been performed using a tomographic acquisition system for breast-CT with synchrotron radiation. By setting a monochromatic beam of 38 KeV , the images have been acquired using a CdTe photon-counting detector with a pixel size of 60 μ m. The optimal CT reconstruction algorithm has been investigated comparing the Filtered Back Projection (FBP), the Simultaneous Algebraic Reconstruction Technique (SART) and the Simultaneous Iterative Reconstruction Technique (SIRT). CT images have been reconstructed using two voxel sizes ( V 60 = 60 3 μ m3 and V 120 = 120 3 μ m3). The spatial resolution and the noise have been evaluated on CT reconstructions respectively trough the modulation transfer function (MTF) and the normalized noise power spectrum (NNPS). MTF has been evaluated with the thin wire method and the NNPS has been measured on the CT reconstructions of the homogeneous sample. The NEQ is defined as the ratio between the squared MTF and the NNPS. It is a measure of the signal to noise ratio in frequency domain and has been used to compare the performances of the considered reconstruction algorithms. Results For both used voxel sizes, compared to SART and FBP algorithms, SIRT algorithm shows highest values of NEQ for all accessible frequencies. Apart from the reconstruction algorithm, compared to V 60 , V 120 lead to better NEQ values for frequencies below 3.3 mm - 1 thus, for this system, details with diameter lower than 300 μ m are better resolved by the smallest voxel size available. Conclusions Summarizing information about spatial resolution and noise, NEQ is an extremely comprehensive metric which contains all the key issues for the global image quality assessment. For a given CT system, the comparison between NEQ curves measured on CT reconstructions provides a general method for choosing the best reconstruction algorithm and the voxel size to be used for achieving optimal performances in terms of spatial resolution and noise.

Planar cone-beam computed tomography with a flat-panel detector

For a dedicated x-ray inspection of printed-circuit boards (PCBs), a bench-top planar cone-beam computed tomography (pCT) system with a flat-panel detector has been built in the laboratory. The system adopts the tomosynthesis technique that can produce cross-sectional images parallel to the axis of rotation for a limited angular range. For the optimal operation of the system and further improvement in the next design, we have evaluated imaging performances, such as modulation-transfer function, noise-power spectrum, and noise-equivalent number of quanta. The performances are comparatively evaluated with the coventional cone-beam CT (CBCT) acquisition for various scanning angular ranges, applied tube voltages, and geometrical magnification factors. The pCT scan shows a poorer noise performance than the conventional CBCT scan because of less number of projection views used for reconstruction. However, the pCT shows a better spatial-resolution performance than the CBCT. Because the image noise can be compensated by an elevated exposure level during scanning, the pCT can be a useful modality for the PCB inspection that requires higher spatial-resolution performance.

Dose sensitivity of three phantoms used for quality assurance in digital mammography

Technical quality assurance (QA) is one of the key issues in breast cancer screening protocols. For this QA task, three different methods are commonly used to assess image quality. The European protocol suggests a contrast-detail phantom (e.g. the CDMAM phantom), while in North America the American College of Radiology (ACR) accreditation phantom is proposed. Alternatively, phantoms based on image quality parameters from applied system theory such as the noise-equivalent number of quanta (NEQ) are applied (e.g. the PAS 1054 phantom). The aim of this paper was to correlate the changes in the output of the three evaluation methods (CDMAM, ACR and NEQ) with changes in dose. We varied the time–current product within a range of clinically used values (40–140 mAs, corresponding to 3.5–12.4 mGy entrance dose and detector dose of 32–110 μGy). For the ACR phantom, the examined parameter was the number of detected objects. With the CDMAM phantom we chose the diameters 0.10, 0.13, 0.20, 0.31 and 0.5 mm and recorded the threshold thicknesses. With respect to the third method, we evaluated the NEQ at typical spatial frequencies to calculate the relative changes in NEQ. Plotting NEQ versus dose increment shows a linear relationship and can be described by a linear function (with R > 0.99). Every manually selectable current– time product increment can be detected. With the ACR phantom, the number of detected objects increases only in the lower dose range and reaches saturation at about 9 mGy entrance dose (80 μGy detector dose). The CDMAM can detect a 50% increase in dose over the examined dose range with all five diameters, although the increases of threshold thickness are not monotonous. We conclude that an NEQ-based method has the potential to replace the established detail phantom methods to detect dose changes in the course of QA.

Ideal‐observer detectability in photon‐counting differential phase‐contrast imaging using a linear‐systems approach

To provide a cascaded-systems framework based on the noise-power spectrum (NPS), modulation transfer function (MTF), and noise-equivalent number of quanta (NEQ) for quantitative evaluation of differential phase-contrast imaging (Talbot interferometry) in relation to conventional absorption contrast under equal-dose, equal-geometry, and, to some extent, equal-photon-economy constraints. The focus is a geometry for photon-counting mammography. Phase-contrast imaging is a promising technology that may emerge as an alternative or adjunct to conventional absorption contrast. In particular, phase contrast may increase the signal-difference-to-noise ratio compared to absorption contrast because the difference in phase shift between soft-tissue structures is often substantially larger than the absorption difference. We have developed a comprehensive cascaded-systems framework to investigate Talbot interferometry, which is a technique for differential phase-contrast imaging. Analytical expressions for the MTF and NPS were derived to calculate the NEQ and a task-specific ideal-observer detectability index under assumptions of linearity and shift invariance. Talbot interferometry was compared to absorption contrast at equal dose, and using either a plane wave or a spherical wave in a conceivable mammography geometry. The impact of source size and spectrum bandwidth was included in the framework, and the trade-off with photon economy was investigated in some detail. Wave-propagation simulations were used to verify the analytical expressions and to generate example images. Talbot interferometry inherently detects the differential of the phase, which led to a maximum in NEQ at high spatial frequencies, whereas the absorption-contrast NEQ decreased monotonically with frequency. Further, phase contrast detects differences in density rather than atomic number, and the optimal imaging energy was found to be a factor of 1.7 higher than for absorption contrast. Talbot interferometry with a plane wave increased detectability for 0.1-mm tumor and glandular structures by a factor of 3-4 at equal dose, whereas absorption contrast was the preferred method for structures larger than ∼0.5 mm. Microcalcifications are small, but differ from soft tissue in atomic number more than density, which is favored by absorption contrast, and Talbot interferometry was barely beneficial at all within the resolution limit of the system. Further, Talbot interferometry favored detection of "sharp" as opposed to "smooth" structures, and discrimination tasks by about 50% compared to detection tasks. The technique was relatively insensitive to spectrum bandwidth, whereas the projected source size was more important. If equal photon economy was added as a restriction, phase-contrast efficiency was reduced so that the benefit for detection tasks almost vanished compared to absorption contrast, but discrimination tasks were still improved close to a factor of 2 at the resolution limit. Cascaded-systems analysis enables comprehensive and intuitive evaluation of phase-contrast efficiency in relation to absorption contrast under requirements of equal dose, equal geometry, and equal photon economy. The benefit of Talbot interferometry was highly dependent on task, in particular detection versus discrimination tasks, and target size, shape, and material. Requiring equal photon economy weakened the benefit of Talbot interferometry in mammography.

Relationship between radiation dose reduction and image quality change in photostimulable phosphor luminescence X-ray imaging systems

The aim of the study was to clarify the change in image quality upon X-ray dose reduction and to re-analyse the possibility of X-ray dose reduction in photostimulable phosphor luminescence (PSPL) X-ray imaging systems. In addition, the study attempted to verify the usefulness of multiobjective frequency processing (MFP) and flexible noise control (FNC) for X-ray dose reduction. Three PSPL X-ray imaging systems were used in this study. Modulation transfer function (MTF), noise equivalent number of quanta (NEQ) and detective quantum efficiency (DQE) were evaluated to compare the basic physical performance of each system. Subjective visual evaluation of diagnostic ability for normal anatomical structures was performed. The NEQ, DQE and diagnostic ability were evaluated at base X-ray dose, and 1/3, 1/10 and 1/20 of the base X-ray dose. The MTF of the systems did not differ significantly. The NEQ and DQE did not necessarily depend on the pixel size of the system. The images from all three systems had a higher diagnostic utility compared with conventional film images at the base and 1/3 X-ray doses. The subjective image quality was better at the base X-ray dose than at 1/3 of the base dose in all systems. The MFP and FNC-processed images had a higher diagnostic utility than the images without MFP and FNC. The use of PSPL imaging systems may allow a reduction in the X-ray dose to one-third of that required for conventional film. It is suggested that MFP and FNC are useful for radiation dose reduction.

Influence of the Crossover Effect on X-Ray Tube Voltage Dependence of Quantum Mottle

AbstractThe influence of the crossover effect on the tube voltage dependence of the quantum mottle for the front and back emulsion has been investigated. A new method is proposed for separating the Wiener spectra of the screen mottle for the front and back emulsions with the crossover effect into those of the quantum mottle and the structure mottle and a method for obtaining the detective quantum efficiency (DQE) and the noise-equivalent number of quanta (NEQ).The numbers of incident X-ray photons with the crossover effect were smaller than those without the effect for all tube voltages studied. However, at tube voltages below about 75 or 80 kV, the Wiener spectral values of the quantum mottle with the crossover effect at 0 mm 1 were smaller than those without the effect. The Wiener spectral value increased with the tube voltage and, at tube voltages above about 75 or 80 kV, exceeded that without the effect. Because the Wiener spectrum of the quantum mottle is proportional to the reciprocal of the NEQ, th...

Assessment of Signal Detectability for Conventional and Reversal-Processed Medical X-Ray Films Obtained without Intensifying Screens

The detectability of the conventional and reversal-processed images obtained without intensifying screens was assessed by using the maximum information content (Imax) generated from receiver operating characteristic (ROC) curve data and the noise-equivalent number of quanta (NEQ). It was demonstrated, from the Imax values versus background density, that reversal-processed images have a very high detectability at lower density levels, but exhibit a rapid decrease in detectability at higher density levels. On the other hand, the detectability of conventional images was not as strongly affected by the density. Therefore, the detectability of both conventional and reversal-processed images varies with the density, and conventional images are superior to reversal-processed images at higher densities. This was confirmed by measureing the NEQ.

NEQ (u) の測定誤差の検討

A noise-equivalent number of quanta (u) (NEQ(u)) which included a gamma, a modulation transfer function (MTF) and a Wiener spectrum has provided useful means for evaluating the total image quality. However, the image quality evaluated by the NEQ(u) may be influenced by measuring errors of the gamma, the MTF and the Wiener spectrum, because the NEQ(u) is calculated from these three factors. Then, we measured fluctuations of measuring errors of these three factors employing two screen film systems of different speed and investigated effects for the NEQ(u). As the result, the NEQ(u) was influenced greatly by fluctuations of measured Wiener spectrum. But, fluctuations of NEQ(u) of these systems were quite little compared with NEQ(u) value of other systems. We concluded that the image quality of both systems can be distinguished clearly by using of NEQ(u) although the difference of the speed between two systems is quite little.

DQEおよびNEQと信号検出理論の検出能との対応と走査開口について

In this paper, the general formulas concerning detective quantum efficiency (DQE) and noise-equivalent number of quanta (NEQ) were theoretically derived. DQE and NEQ become maximum in value when the scanning aperture of the micro-densitometer is the same size and form of the subject. Mathematically and strictly, DQE(u) and NEQ(u) were formally defined, in the integration of spatial frequency area by applying the inequality of Schwarz. Each integration of the product of DQE(u) or NEQ(u) due to the subjective power spectrum (or of the scanning aperture) is DQE_<max> and NEQ_<max>, respectively. Where, NEQ_<max> is qA・DQE_<max>. The detectability d' on the signal detection theory corresponds to √<NEQ_<max>>. Therefore, consequently, in the surrounding of the next important formula, [numerical formula] derived from the signal detection theory it became not only easy to understand but also close to be connected with many sorts of the results regarding the fields of the stochastic matrix theory, the frequency analysis theory, the photophysics and the statistical psychophysics. NEQ has a strong correlation to NEQ_<max> and the ability of image quality evaluation, but each image quality is necessarily compared with both NEQs, measured by circular apertures of larger and smaller diameters than approximate 0.3 mm under the conditions used in this paper.

Effective exposure level and diagnostic performance in endodontic radiography

Image quality is limited by the information capacity of the image-forming system and can be computed from three parameters: contrast, resolution, and noise. These parameters can be combined to yield a single measure which determines the maximum amount of information obtainable from any x-ray system and is called the noise-equivalent number of quanta (NEQ) per unit area. The effects of image quality, expressed as noise-equivalent number of quanta (NEQ) per unit area, on the radiographic performance by dentists reading the position of an endodontic file in a root canal were studied. Three different speed films were used in conjunction with a fixed screen. NEQ values ranged between 0.64 × 10 4 and 14.3 × 10 4 per square millimeter. Components of variance associated with the position of the tooth apex and the tip of an endodontic file in a root canal were compared for the effect of different NEQs and observers. Results show that the standard deviation in locating a file tip and tooth apex may be a linear function of log NEQ. The estimated standard deviation associated with changes in NEQ was found to be relatively small compared to that observed among dentists. These findings indicate that a significant reduction in exposure would have a relatively small effect on the precision of endodontic distance measurements.

Image Quality of Reversal-Developed X-Ray Film with Reference to Density

The image quality of medical X-ray film treated by reversal development is studied. Evaluation of the image quality is performed by determining the characteristic curve, the contrast transfer function (CTF), the noise-equivalent number of quanta (NEQ), and the detective quantum efficiency (DQE), all of which depend on the density. The CTF of reversal-developed film is better than that of negative film in the low-exposure domain. There is no notable difference between the DQE of reversal-developed film and that of negative film. The S/N ratio of the output (NEQ) of reversal-developed film is almost the same or larger than that of negative film over the complete exposure range.

The Photographic Process as a Photon Counting Device

AbstractIn the application of photography to problems involving low light levels and limiting detection criteria, it can be instructive to consider the photographic process simply as a photon counter. For in spite of the conclusions from low measured DQE values that the photographic process is inefficient in this respect, in this way the principles involved in systems optimisation can be understood without the difficulties which may arise when considering the photographic process as a density meter. The physical significance of the noise-equivalent number of quanta is discussed, and numerical examples are given for the S/N ratio gains which might be made using DQE rather than gamma-to-signa ratio criteria. It is indicated why the DQE for grain counts may be higher than that for density measurementS.