Digital image acquisition and processing in medical x-ray imaging

This contribution discusses a selection of today's techniques and futurc 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 intormation 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 süll 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 descibe a digital image processing algorithm for the reduction of noise in images acquired with low x-ray dose. @ 19ss SptE and tS&T. [s1 01 7-e909(99)00401 -8] 1 lntroduction and Overview In this paper, we discuss selected current topics of digital image acquisition and processing in medicine, focusing on x-ray projection imaging. A key feature of digital imaging is the inherent separation of image acquisition and display. Whereas analog screen/film combinations (Fig. 1) use film as a medium for both image recording and viewing, digitally acquired images can be processed in order to correct accidental overor underexposure, or to enhance diagnostically relevant information before display. Also, digital images can be stored and fransmitted via picture archiving and communication systems (PACS),' and be presented on different output devices, like film printers or cathode ray tube (CRT) monitors (softcopy view in g). *Partly presented m m inyited paper at th€ Intemational Symposium on Elecuonic Photography (ISEP: a PHOTOKINA event), Cologne, Gemmy, Sept. 21-22, 1996. iT. Aacb is now witb rhe Medical University of Luebeck, hstitute for Sipal Processhg md hmess Control, Rataburger Allee 160, D-23538 Luebeck, Gemmy. Paper 97010 reeived Apr. 1, 1997; revised mmuscript rrceived May i, 1998; accepred for publication June 1, 1998. 1017-9909/99/510.00 O 1999 SPIE and IS&T. The separation of image acquisition and display in a digital system is illustrated by comparing analog and digital acquisition of silgle high resolution projection images (xray radiograpäy). The principle of the imaging serup is sketched in Fig. 2. X radiation passes through the patient before exposing a detector. Widely used for image detection are analog screen/film combinations as shown in Fig. 1, which consist of a fiIm sheet sandwiched between thin phosphor intensifying screens. The phosphor screens convert the incoming x radiation into visible light blackening the film, which, after developing, is examined by viewing on a lightbox. Well-established digital alternatives include storage phosphor systems (SPS),'* also known as computed radiography (CR) systems, and a selenium-detector based digital chest radiography system [(DCS), "Thoravision"].''o In CR systems, the image receptor is a photostimulable phosphor plate, which absorbs and stores a significant portion of the incoming x-ray energy by trapping electrons and holes in elevated energy states. The stored energy pattern can be read out by scanning the plate with a laser beam. The emitted luminescence is detected by a photomultiplier and subsequently digitized. Common plate sizes are 35X35 cm2 sampled by a 1760X 1760 matrix, 24x30 cm2 sampled by a 7576X1976 matrix, and for high resolutions 18 x24 cmz sampled by a 1770x2370 matrix. The resulting Nyquist frequencies are between 2.5 and 5 lp/mm. An example CR image is given in Fig. 3. The detector of a DCS consists of an amorphous selenium layer evaporated onto a cylindrical aluminum drum. Exposure of the drum to x radiation generates an electrostatic charge image, which is read out by electrometer sensors. Maximum size of the sampled image matrix is 2166 x2448 pixels, with a Nyquist frequency of 2.7 lp/mm. In analog as well as in digital systems, the acquired radiographs are degraded by nonideal system properties. These include limitations of contrast and resolution, and are described for instance by the modulation transfer function (MTF). Other undesired effects are spatially varying detector sensitivity and unwanted offsets. Additional degradations can be introduced by accidental overor underexpoJournal of Electronic Imaging / January 1999 / Vol. B(1) / 7 Aach, Schiebel, and Spekowius