Investigation of basic imaging properties in digital radiography. I. Modulation transfer function.

Purpose: To demonstrate a method of measuring the modulation transfer function (MTF) of digital imaging systems using the common star pattern test tool. The goal of this research is to develop a technique that brings precise and objective evaluation of spatial resolution to routine quality control in radiography and fluoroscopy. Method and Materials: Images of a 2‐degree star pattern were obtained on a variety of digital radiographic imaging systems, spanning a range of spatial resolution performance levels. One‐dimensional profiles from concentric circles around the center of the star were obtained from each star image. The circular profiles were unwrapped to produce a series of square waves, which were analyzed to produce the square wave response function (SWRF). Droege's method was used to compute the MTF from the SWRF. The MTF determined using the star pattern method was compared to published and quality control measurements of the MTF for several digital mammography and digital radiography systems. In implementing this method, important parameters that must be considered include the radial and angular sampling rates, the method for determining the zero‐frequency normalization and correction for beam nonuniformity. Results: The MTF computed from star pattern profile measurements closely matched the MTF obtained using other methods over the range of spatial frequencies between the Nyquist frequency and 1/3 of the Nyquist frequency. The star pattern MTF, which is a system measurement and includes the effects of detector and geometric unsharpness, was within 6–8% of reference values obtained from the scientific literature for two models digital mammography equipment. Conclusion: Images of a 2‐degree star pattern can produce estimates of the two‐dimensional system MTF. This method may offer a convenient means of assessing the spatial resolution of digital imaging systems for acceptance testing and quality control.

Until recently, advanced image quality metrics such as the frequency-dependent detective quantum efficiency, DQE (f), have been employed by a relatively small cadre of expert imaging scientists to compare the performance of digital radiography systems, often using differing methods. To address the potential for differences in measured DQE(f) arising from differences in test methodology, in 2003 the International Electro-technical Commission (IEC) published the first in a series of performance standards to define techniques for measuring the DQE(f) and its constitutent metrics, the modulation transfer function, MTF(f), and noise power spectrum, NPS(f). More recently, the increasing prevalence of digital radiographic systems in the clinical environment and availability of both commercial and public-domain resources have made DQE(f) evaluations both more accessible and increasingly relevant to the clinical medical physicist. This course will review the measurement and analysis methods for DQE evaluations of both general digital radiography and digital mammography systems, using the relevant IEC standard method as a guide. Learning Objectives: 1. Review the definition of the DQE(f) and its constitutent metrics the MTF(f) and NPS(f). 2. Gain an understanding of the IEC standard method for measuring detective quantum efficiency, for both general digital radiography and digital mammography imaging systems. 3. Learn which commercial and public-domain resources are available for measuring the DQE(f) of digital radiographic imaging systems.

Two main spatial frequency and quantitative calculations and measurements am, used to study the imaging system of three industrial digital radiography (IDR) modules for non-destructive testing (NDT), namely complementary metal oxide semiconductor (CMOS) flat-panel digital detector with 50 mu m pixel pitch, computed radiography (CR) with phosphor imaging plate with 25 mu m pixel pitch and laser-type film digitiser with 50 mu m pixel pitch. The modulation transfer function (MTF) and noise power spectrum (NPS) measurement and calculations were adapted on order to evaluate the image quality of IDR images. The MTF has been used to characterise the resolution properties of analogue and digital X-ray imaging systems and measure image deterioration due to optical factors, such as diffusion of image forming radiation. The NPS is used in determining noise in images produced and the choices of normalisation of the NPS vary slightly from researcher to researcher, but the underlying methodology is consistent. In order to easily compare these two measurements and calculations, the integral of up to the system Nyquist frequency was used as the final image quality quantitative evaluation. For the MTF measurement and calculation, four regions of interest (ROI) were, defined to portray the randomness in performing real testing. The four ROI are then compared with each other. For the NPS measurement and calculation, ten ROI were defined, scattered and averaged to have overall noise estimation. From the measurements and calculations which were done, the averaged MTF at 20% modulation for the laser-type film digitiser system is 6.55 cycles/mm, CR modulated 4.48 and CMOS modulated 2.83 cycles/mm. For NPS measurement and calculation, the result obtained in decreasing order is laser-type film digitiser, CMOS and CR. The laser-type film digitiser system modulates the best transfer function at 20% but does not have the lowest NPS. The lowest and stable NPS is the CR system but it has the lowest modulating capability at 20%. The study shows that in order to perform NDT by using the evaluated modules, the user must know the true capability, of the system and how, it is designed for specific application and discontinuity detection.

Quantitative evaluation of digital dental radiograph imaging systems

A digital x-ray imaging system was designed for small-animal studies. This system is a fiber-optics taper-coupled imaging system with two CCD arrays uniquely jointed. The x-ray source of the system has a small focal spot of 20 microm. This digital imaging system contains specially designed shelves to provide magnification levels, ranging from 1.5x to 5x. The system is characterized in terms of its properties of spatial resolution. An observer-based spatial resolution measurement was conducted with a line-pair target and a sector test pattern. The modulation transfer function of the system, with different magnifications, was studied by use of a 10-microm lead slit. The average resolutions at 50% and 5% modulations at 1x magnification were measured as 3.9 and 8.4 lp/mm, respectively, where lp indicates line pairs. With 5x magnification, the 50% and the 5% modulations provided 13.2- and 29.9-lp/mm, respectively, average spatial resolutions. The measurements showed consistency between the two individual CCD arrays; the difference in resolution between the two CCDs is less than 1%, even at high magnifications.

This study compared the diagnostic accuracy of digital radiographic images with conventional radiographs for the detection of experimentally induced root fractures. Horizontal fractures were induced by a mechanical force to the root of 15 single rooted teeth. Vertical fractures were induced in 15 single rooted teeth and 15 molars, respectively, by disk cutting. The teeth were mounted in dried mandibles and were radiographed with the parallel technique using a CCD-based Planmeca imaging system and F-speed films. Seven observers recorded their findings and then 2 pairs of them examined the same images together. Az values expressed the diagnostic accuracy of the imaging systems and the degree of agreement was estimated using the Cohen's kappa statistic. The areas under the ROC curves (Az) in single rooted teeth were 0.61 for conventional radiography and 0.64 for digital radiography. Digital imaging system did not perform significantly better than conventional one (p>0.01). Digital radiographs (Az: 0.72) provided higher detection accuracy in molars than conventional images (Az: 0.51) (p=0.0102). The digital images scored higher kappa values with narrower range than conventional. Observers in pairs did not perform significantly better than individually. The results were not affected by the observers but were affected by the cases (p<0.001). Digital images were equivalent to F-speed films for the detection of root fractures in single rooted teeth. The digital system performed significantly better than the conventional in detecting root fractures in molars.

엑스선 의료영상이 디지털 시스템으로 발전함에 따라 의료영상의 평가 방법 또한 디지털 시스템에 맞춰 새롭게 개선되었으며 그 중 MTF(Modulation Transfer Function) 측정법은 Fujita이론에 기초한 에지법으로 ISO에 규정되었다. 에지법의 기초인 Fujita이론은 MTF 측정을 위해 슬릿의 각도를 디지털 검출기의 픽셀 열에 대하여 <TEX>$1^{\circ}{\sim}2^{\circ}$</TEX> 기울여 영상을 획득한 후 LSF를 합성하는 것으로 샘플링 간격은 각도에 따라 크게는 1/54배, 작게는 1/28배 감소하게 된다. 본 연구에서 사용된 장비는 Simens사의 MAMMOMAT Inspiration으로 비정질 셀레늄(amorphous selenium) 기반의 <TEX>$0.085{\times}0.085mm^2$</TEX> 의 픽셀 사이즈를 갖는 검출기를 이용하여 presampling MTF를 측정하였다. 측정 방법은 Fujita의 슬릿법와 동일한 방법인 와이어법를 이용하였으며 측정된 영상에서 얻은 픽셀값을 이용하여 엑셀에서 이산 푸리에 변환을 시행하였다. 실험 결과 Fujita이론 대비 약 3배 이상의 샘플링 간격(sampling interval)의 경우 해상도 평가 지수인 10% MTF에서 약 85% 이하의 정확도를 보였으며 선예도 평가 지수인 50% MTF에서 약 93% 이하의 정확도를 나타내었다. 하지만 샘플링 간격이 Fujita 이론 대비 2배 정도 늘어난 경우는 50%와 10% MTF 모두 96% 이상의 정확도를 가진다는 것을 알 수 있었다. Assessments of medical image was improved in accordance with development of medical imaging systems. One of them is edge method to determining MTF(Modulation Transfer Function) based on the Fujita method. Fujita was reduced sampling interval used slightly angulated slit to the direction of pixel array and composited finely sampled LSF to determine presampling MTF. In this study, we investigate the effect of sampling interval on the MTF under a digital imaging system by changing wire angle. The wire method was equivalent to the slit method except signal appearance. A Simens's MAMMOMAT Inspiration with <TEX>$0.085{\times}0.085mm^2$</TEX> pixel size made by amorphous selenium was used and 96% accuracy on MTF in twice sampling interval compared with Fujita was obtained. However, three times of sampling interval showed 93% accuracy on 50% of MTF and 85% accuracy on 10% of MTF.

The presampling modulation transfer function (MTF) of a digital imaging system is commonly determined by measuring the system’s line spread function (LSF) using a narrow slit or differentiating the detector’s edge spread function (ESF) with an edge device. The slit method requires precise fabrication and alignment of a slit as well as a high radiation exposure. The edge method [3] is a complicated image processing procedure, requiring determination of the edge angle, reprojection, sub-binning, smoothing and differentiating the ESF, and spectral estimation. In this paper, a simple method is employed to evaluate the MTF using an edge device. The image processing procedures required by this method involve simply the determination of the over-sampling rate and the Fourier transform of the modified ESF. Differentiation and signal to noise ratio (SNR) improvement are jointly applied in the Fourier domain. The MTFs obtained by this simple method are compared to the theoretical MTF and the previously proposed more complicated edge method. The experimental results show that the proposed method provides a simple, accurate and convenient measurement of the presampling MTF for digital imaging systems.

Evaluating the use of exposure indicators in digital x-ray imaging system: Gauteng South Africa

This paper demonstrates a convenient experimental tech- nique for measuring the resolution of a digital imaging system. The spa- tial frequency response of the system is obtained by deriving the modu- lation transfer function (MTF) from the measured contrast transfer function (CTF). Basic signal-processing techniques are used to establish a relationship between the MTF and the CTF. Experimental methodology is discussed with respect to avoidance of shift variation in the CTF. Dis- crete Fourier transformation is applied for a power-spectrum analysis of the MTF. Using the CTF as a measure of performance, a series of ex- periments were carried out to characterize an imaging system based on an integrated CMOS camera. The experimental results showed that this approach can be applied in general, for the assessment of spatial reso- lution for digital imaging systems. © 1998 Society of Photo-Optical Instrumenta- tion Engineers. (S0091-3286(98)00609-6)

As a continuation of our digital radiographic sensor R&D, we have developed a digital gamma imaging system based upon the cadmium-telluride (CdTe) photoconductor for the applications of industrial gamma-imaging. The imaging system consists of a commercially-available CMOS pixel array of a 100 × 100 μm2 pixel size and a 5.4 × 151.0 mm2 active area, coupled with a 750-μm-thick CdTe photoconductor, and a collimated selenium (75Se) radioisotope of an about 2.3 × 1012 Bq activity and a physical size of 3.0 mm in diameter. In this study, we, for the first time, succeeded in obtaining useful gamma images of several test phantoms with the 75Se radioisotope from the imaging system and evaluated its imaging performance in terms of the line spread function (LSF), the modulation transfer function (MTF), the noise power spectrum (NPS), and the detective quantum efficiency (DQE). For comparison, we also tested its X-ray imaging performance with a microfocus X-ray tube of an about 5 μm focal spot size at an operation condition of 90 kVp and 100 μA. According to our test results, the designed imaging system seems to be promising for our ongoing application of nondestructive testings, provided that the effective source size of the 75Se radioisotope can be further limited.

The modulation transfer function (MTF) is well established as a metric to characterize the resolution performance of a digital radiographic system. Implemented by various laboratories, the edge technique is currently the most widespread approach to measure the MTF. However, there can be differences in the results attributed to differences in the analysis technique employed. The objective of this study was to determine whether comparable results can be obtained from different algorithms processing identical images representative of those of current digital radiographic systems. Five laboratories participated in a round-robin evaluation of six different algorithms including one prescribed in the International Electrotechnical Commission (IEC) 62220-1 standard. The algorithms were applied to two synthetic and 12 real edge images from different digital radiographic systems including CR, and direct- and indirect-conversion detector systems. The results were analysed in terms of variability as well as accuracy of the resulting presampled MTFs. The results indicated that differences between the individual MTFs and the mean MTF were largely below 0.02. In the case of the two simulated edge images, all algorithms yielded similar results within 0.01 of the expected true MTF. The findings indicated that all algorithms tested in this round-robin evaluation, including the IEC-prescribed algorithm, were suitable for accurate MTF determination from edge images, provided the images are not excessively noisy. The agreement of the MTF results was judged sufficient for the measurement of the MTF necessary for the determination of the DQE.

Prior studies on performance evaluation of digital radiographic systems have primarily focused on the assessment of the detector performance alone. However, the clinical performance of such systems is also substantially impacted by magnification, focal spot blur, the presence of scattered radiation, and the presence of an antiscatter grid. The purpose of this study is to evaluate an experimental methodology to assess the performance of a digital radiographic system, including those attributes, and to propose a new metric, effective detective quantum efficiency (eDQE), a candidate for defining the efficiency or speed of digital radiographic imaging systems. The study employed a geometric phantom simulating the attenuation and scatter properties of the adult human thorax and a representative indirect flat-panel-based clinical digital radiographic imaging system. The noise power spectrum (NPS) was derived from images of the phantom acquired at three exposure levels spanning the operating range of the clinical system. The modulation transfer function (MTF) was measured using an edge device positioned at the surface of the phantom, facing the x-ray source. Scatter measurements were made using a beam stop technique. The eDQE was then computed from these measurements, along with measures of phantom attenuation and x-ray flux. The MTF results showed notable impact from the focal spot blur, while the NPS depicted a large component of structured noise resulting from use of an antiscatter grid. The eDQE was found to be an order of magnitude lower than the conventional DQE. At 120 kVp, eDQE(0) was in the 8%-9% range, fivefold lower than DQE(0) at the same technique. The eDQE method yielded reproducible estimates of the system performance in a clinically relevant context by quantifying the inherent speed of the system, that is, the actual signal to noise ratio that would be measured under clinical operating conditions.

Several methods are available to measuring the presampled modulation transfer function (MTF) of a digital radiography system by using a slit camera, a slanted edge test device or a bar phantom. Many studies demonstrated the advantages of the edge method compared to the slit method. However, the method of processing to estimate the MTF is slightly different for each publication. The purpose of this study is to compare the effects of various processing steps with an edge image on estimating the presampled MTF. A digital image of a slanted edge test device was acquired and then examined using different image processing such as windowing the edge spread function (ESF), determining a region of interest (ROI) and locally smoothing the ESF data. The results of this study indicated that the impact of determining a ROI on the MTF is relatively modest and that the truncation of the ESF due to a finite window width causes an incorrect inflation error in the MTF estimate and loss of detail at high frequency. Moreover, the influence of ESF smoothing on the MTF is demonstrated, and more rigorous smoothing of the ESF produces a less noisy MTF and, consequently, a high data loss at high-frequency. In conclusion, an ESF processing technique can moderately impact the MTF estimates, and an adequate ROI and window width without a smoothing process is suggested.

Digital photography has many advantages over the conventional method. At present commercially available digital ophthalmological imaging systems utilise only a fraction of the advantages of this technique and some of them have fundamental conceptional problems. Images of a fundus camera, a slit lamp and a scanning laser ophthalmoscope were digitised using digital and video cameras and/or frame grabber cards. Digital images and movies were recorded via a personal computer. The image recording software was developed using an imaging library and open source software. The recorded images and video sequences were processed with three different commercially available programme packages. The images were archived in a around-the-clock accessible database. Our digital imaging system is capable to record normal and high resolution slit lamp images, colour fundus images in normal resolution, Fluorescein angiography images in normal and high resolution, digital simultaneous ICG and Fluorescein videoangiograms (without compression) and high resolution macro images. Different functions, e.g., contrast enhancement are integrated in the image capture software. Most of the parameters of the image acquisition are individually adaptable. The digital image archive supports multiple image and video formats. Using standard software and hardware components an imaging system can be constructed featuring functions usually not available in commercial ones. Since the system is adaptable, recording of images with a wide variety of parameters, e.g., for measurements for scientific experiments, is possible.