Structure Mottle Research Articles

Proportionality between Wiener spectra of quantum mottleand the squares of modulation transfer functions

Rossmann proposed that the Wiener spectra of the quantum mottle of radiographs made using screen-film systems were proportional to the squares of the modulation transfer functions (MTFs) of the screen-film systems. On the other hand, Lubberts theoretically pointed out that the shape of the Wiener spectrum of the quantum mottle depended on the sum of the squares of the MTFs for different depths in the screen phosphor layer, rather than the square of the sum of the MTFs for the different depths, i.e. the square of the MTF of the screen-film systems. The purpose of this study is to experimentally investigate the proportionality between the Wiener spectra of the quantum mottle and the squares of the MTFs of screen-film systems using two screen-film systems having different screen thicknesses. For this purpose, we determined correction factors for the square of the MTF of the screen-film system in the Wiener spectrum of the quantum mottle at each spatial frequency when the Wiener spectral values of the screen mottle were separated into those of the quantum mottle and structure mottle. The correction factor is the ratio of the normalized Wiener spectrum of the quantum mottle to the square of the MTF of the screen-film system. As a result, for a thin screen, the correction factors were unity for all spatial frequencies; on the contrary, for a thick screen, the factor increased with spatial frequency. By calculating the theoretical correction factors using the models for the MTF and Wiener spectrum of the quantum mottle of Nishikawa and Yaffe based on Lubberts' theory, we verified that our experimental results agreed with Lubberts' theory. Furthermore, by obtaining the screen thickness dependence of the theoretical correction factors for the two screens, we showed that, for screens thinner than 0.02 mm, Rossmann's theory can be applied to the relationship between the Wiener spectrum of the quantum mottle and the MTF of the screen-film system, whereas for screens thicker than 0.02 mm, Lubberts' theory should be applied.

Extended random‐dot models for radiographic image noise analysis [In Japanese

The object of this thesis was to study the basic characteristics of radiographic image noise using a microscopic theoretical model. Radiographic images obtained by screen‐film systems contain various noise sources: quantum mottle, structure mottle and film granularity. These noise sources affect the signal detectability in the radiographic image. A two‐dimensional microstructure model for radiographic image noise is proposed, i.e., an extended random‐dot model which obeys triple Poisson process. Rigorous expressions of mean transmittance and autocorrelation function were obtained. By using numerical calculations, it is shown that this model can explain basic characteristics of radiographic image noise. Next, a random‐dot model containing a signal is used to evaluate the theoretical detectability of computer‐generated random‐dot images, i.e., idealized models of nuclear‐medicine images. Theoretical detectability of a random‐dot model containing a signal are calculated and compared with the results of receiver operating characteristics analysis (ROC). The results of ROC analysis were in agreement with the calculated theoretical detectability.

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...

X-Ray Tube Voltage Dependence of Wiener Spectra of Screen Structure Mottle Recorded on Front and Back Emulsions

A low-speed screen/film system was used to investigate structure mottle, i.e., the density fluctuation of an x-ray film due to inhomogeneous screen structure. The tube voltage dependence of the Wiener spectra of the structure mottle was separately determined for front and back film emulsions. The Wiener spectral values of the structure mottle of the front emulsion were greater than those of the back emulsion at lower tube voltages. The spectral values of the structure mottle of the front emulsion decreased with tube voltage, while those of the back emulsion increased. We explain these phenomena by the behavior of the Wiener spectra of the relative spatial fluctuation of fluorescence intensity due to following reasons: (1) spatial thickness fluctuation of the screen produces a relative spatial fluctuation of x-ray energy absorbed in the screen, and (2) as the distance between the emulsion of the film and the average position of the x-ray absorption in the screen lengthens, the number of random scattering and absorption of light photons increase.

Density Dependence of Rossmann’s Three Factors in Wiener Spectra of Radiographic Mottle Made Using Rare-Earth Screen-Film Systems

AbstractThe density dependence of Rossmann's three factors in Wiener spectra of radiographic mottle has been experimentally determined for typical rare-earth screen-film systems. For this purpose we have given a new equation for the density dependence of the Wiener spectrum of the structure mottle. Using this equation we proposed a new method for separating the screen mottle into the structure mottle and the quantum mottle. The consistency of our method was verified.At frequencies near 0 mm–1, which are important for visual granularity, the Wiener spectral value of the quantum mottle decreases as the density increases. On the contrary, the Wiener spectral value of the film granularity rapidly increases and exceeds that of the quantum mottle. The structure mottle also exceeds the quantum mottle and passes through a maximum as the density increases. The resultant radiographic mottle also passes through a maximum.We have found that the structure mottle is not dominant but cannot be neglected at any density. ...

Development of a high-resolution digital tomosynthesis system and its clinical application.

Conventional tomography has been criticized for its low contrast caused by scattered radiation and tomographic blur. The authors developed a digital tomosynthesis system as a substitute for conventional tomography. With the image processor in the system, two types of postprocessing can be performed to improve the quality of synthesized tomograms. One-dimensional spatial frequency filtering effectively reduces linear tomographic blur and enhances image information when an appropriate filter is used. Correction of structure mottle helps enhance the image clarity. The system allows reconstruction of tomograms at any desired depth and performance of tomosynthetic angiography and dual-energy subtraction. The system solves some of the problems associated with plain tomography and dual-energy subtraction and provides diagnostic clinical images.

各種画像評価因子が DQE(u) におよぼす効果

Detective quantum efficiency (DQE (u)) has provided a useful means for evaluating imaging systems. In this paper, we have simulated the effects of various image factors (modulation transfer function (MTF (u)), gamma and absorption efficiency) on DQE (u). Regarding the simulations, the ideal value of the evaluated imaging factors were substituted Rossmann's equation. This study revealed that DQE(u) at high frequencies is reduced by the lower MTF (u) and DQE (u) at low frequencies is reduced by the lower absorption efficiency and the structure mottle. Gamma also changes all over DQE (u). Although these results are similar to those of NEQ (u), there is a meaningful difference between DQE (u) and NEQ (u). Since DQE (u) represents detective quantum efficiency whereas NEQ (u) relates to image quality itself. Therefore, even if large DQE (u) exists, it dose not necessarily give a evaluation of good image quality evaluation.

Basic imaging properties of a large image intensifier-TV digital chest radiographic system.

The basic imaging properties of a large (57 cm) image intensifier (I.I.)-TV digital imaging system were examined to determine the effects of various physical parameters on the quality of the digital chest images obtained, and also to explore the clinical usefulness of the system. The characteristic curve of the digital system, which relates the output pixel value to the input relative x-ray intensity, was measured with an aluminum stepwedge. MTFs were determined using slit images, and the veiling-glare fraction was measured with a lead-disk technique. Noise Wiener spectra were obtained from uniformly exposed images. The current limitations of the large II-TV digital chest system are its low spatial resolution, and the presence of large amounts of veiling glare and structure mottle. Advantages of this system over other digital chest imaging systems include the high speed of image data acquisition and the capability of "real-time" dynamic imaging of the chest at a radiation dose comparable to that in conventional radiography of the chest.

Investigation of basic imaging properties in digital radiography 10. Structure mottle of II-TV digital imaging systems.

Single-frame images obtained with image intensifier (II)-TV digital systems contain a large amount of structure mottle. In the present study, we examined several II-TV digital systems by use of Wiener spectral analysis and noted considerable variation of the structure mottle over the wide spatial frequency range. We found that the structure mottle in these systems may originate in the input phosphor, the output phosphor, and/or the electronic components, and that the Wiener spectra of structure mottle seem to depend on the specific combination of these components. The results of observer performance studies indicated that structure mottle can significantly decrease the detection of low-contrast objects in a single-frame image when the exposure incident on the II is greater than approximately 0.1 mR. In addition, we showed that the structure mottle can be removed by subtraction of a uniformly exposed mask. This simple procedure will improve the quality of radiologic images obtained with II-TV digital systems. Note, however, that the structure mottle is largely eliminated by subtraction in digital subtraction angiography (DSA) images.

Investigation of basic imaging properties in digital radiography. 7. Noise Wiener spectra of II-TV digital imaging systems.

We used Wiener spectral analysis in order to investigate the different noise sources and the effects of various parameters such as pixel size, image intensifier (II) field size, and exposure level on the noise in an II-TV digital system. The digital Wiener spectra in terms of relative x-ray intensity were determined directly from the digital noise data in terms of pixel values, by use of the characteristic curve of the imaging system. From averaged, subtracted, and/or combination images, the amount of structure mottle relative to the amount of quantum mottle was estimated. We found that a substantial amount of structure mottle was included in our II-TV digital subtraction angiography system, whereas the electronic noise of the TV system was quite small relative to the quantum and structure mottle. The effects of time jitter on the noise in single-frame images (consisting of multiple video frames) and in subtracted and averaged images were also investigated.