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Rauschrobuste Verbesserung schwacher Strukturen in digitalen Radiographien durch nichtlineare Multiskalen-Filterung
Abstract
Heutige digitale Radiographiesysteme benutzen zur Verbesserung der Bilddarstellung meist Algorithmen, die auf der unscharfen Maske („unsharp masking“) basieren, wobei die zu filternde Radiographic in zwei bis drei Frequenzbänder zerlegt wird. Dies ermöglicht sowohl eine Verbesserung des Schärfeeindrucks der Radiographic (Verstärkung des hochfrequenten Bandes) als auch eine Harmonisierung des Bildes (durch relative Abschwachung des tieffrequenten Bandes). Allerdings erlaubt die Methode keine Verstärkung schwacher Strukturen mittlerer Größe. Wir stellen ein Verfahren vor, welches die Radiographic in eine Vielzahl von Frequenzbändern zerlegt und so Objekte größenabhängig voneinander trennt. In jedem Frequenzband können so schwach kontrastierende Strukturen identifiziert und verstärkt werden. Die Zerlegung erfolgt durch hierarchisch wiederholte Anwendung des in der unscharfen Maske verwendet en Verfahrens. Besonderer Wert wird auf Rauschrobustheit des Verfahrens gelegt. Seine Leistungsfähigkeit wird durch eine vergleichende klinische Studie belegt.
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Today's digital radiography systems mostly use unsharp masking-like image enhancement techniques based on splitting input images into two or three frequency channels. This method allows to enhance very small structures (edge enhancement) as well as enhancement of global contrast (harmonization) . However, structures of medium size are not accessible by such enhancement. We develop and test a nonlinear enhancement algorithm based on hierarchically repeated unsharp masking, resulting in a multiscale architecture allowing consistent access to structures of all sizes. The algorithm is noise-resistant in the sense that it prevents unacceptable noise amplification. Clinical tests performed in the radiology departments of two major German hospitals so far strongly indicate the superior performance and high acceptance of the new processing.
During 1990 The UK Department of Health installed two Photostimulable Phosphor Computed Radiography (PPCR) systems in the General Infirmary at Leeds with a view to evaluating the clinical and physical performance of the technology prior to its introduction into the NHS. An issue that came to light from the outset of the projects was the radiologists reservations about the influence of the standard PPCR computerized image processing on image quality and diagnostic performance. An investigation was set up by FAXIL to develop an algorithm to produce single format high quality PPCR images that would be easy to implement and allay the concerns of radiologists.
We describe a technique for image encoding in which local operators of many scales but identical shape serve as the basis functions. The representation differs from established techniques in that the code elements are localized in spatial frequency as well as in space. Pixel-to-pixel correlations are first removed by subtracting a lowpass filtered copy of the image from the image itself. The result is a net data compression since the difference, or error, image has low variance and entropy, and the low-pass filtered image may represented at reduced sample density. Further data compression is achieved by quantizing the difference image. These steps are then repeated to compress the low-pass image. Iteration of the process at appropriately expanded scales generates a pyramid data structure. The encoding process is equivalent to sampling the image with Laplacian operators of many scales. Thus, the code tends to enhance salient image features. A further advantage of the present code is that it is well suited for many image analysis tasks as well as for image compression. Fast algorithms are described for coding and decoding.
This contribution discusses a selection of today's techniques and future concepts for digital x-ray imaging in medicine. Advantages of digital imaging over conventional analog methods include the possibility to archive and transmit images in digital information systems as well as to digitally process pictures before display, for example, to enhance low contrast details. After reviewing two digital x- ray radiography systems for the capture of still x-ray images, we examine the real time acquisition of dynamic x- ray images (x-ray fluoroscopy). Here, particular attention is paid to the implications of introducing charge-coupled device cameras. We then present a new unified radiography/fluoroscopy solid-state detector concept. As digital image quality is predominantly determined by the relation of signal and noise, aspects of signal transfer, noise, and noise-related quality measures like detective quantum efficiency feature prominently in our discussions. Finally, we describe a digital image processing algorithm for the reduction of noise in images acquired with low x-ray dose.
A digital chest radiography system has been developed, with a detector based on the photoelectric properties of amorphous selenium. The selenium layer is deposited on a cylindrical aluminium drum, large enough to cover the full field of view for chest imaging. The electrostatic charge image which is formed on the selenium surface after x-ray exposure is read out by electrometer probes using fast drum rotation. For a physical evaluation of the attainable image quality, the characteristic curve, the modulation transfer function, and the noise spectra were measured. From these measurements, the signal-to-noise properties of the detector in terms of detective quantum efficiency (DQE) and noise equivalent quanta (NEQ) were derived. The results show that the selenium-based detector has a wide dynamic range and a significantly better DQE than screen-film and storage phosphor systems for spatial frequencies below the Nyquist limit (2.7 lp/mm). As a consequence, the detectability of small, low-contrast details is considerably improved.
Digital radiography enhancement by nonlinear multiscale processing. Eingereicht bei Medical Physics
- M Stahl
- T Aach
- Tm Buzug
- S Dippel
- U Neitzel
Noise resistant weak structure enhancement for digital radiography, angenommen bei SPIE
- M Stahl
- T Aach
- S Dippel
- Tm Buzug
- R Wiemker
- U Neitzel