Ionization Chamber with Remote Preamplifier for Automatic Exposure Systems Used in X-Ray Diagnosis

Abstract

efficiency and exposure dose decrease in the X ray detec tor plane. The sensitivity of automatic exposure units of digital X ray imaging systems should be substantially increased [1]. The problem of sensitivity increase is made quite difficult by increased requirements for X ray trans parency of media located between patient and X ray input detector plane. According to GOST R IEC 61223 3 1 2001, the attenuation factor of automatic exposure dose control system detector is 1.11 [2]. Distance between patient plane and input X ray detector plane should be reduced to decrease X ray image geometric unsharpness and radiation load on patient. This can be achieved by reducing the ionization chamber width. Within the last decade, the ionization chamber width in X ray appara tuses was decreased from 12 10 to 6 mm against the back ground of virtually invariable sensitivity per area of meas ured (dominant) zone of the radiation detector. The sen sitivity losses were 30 40% and were compensated by low noise high sensitivity electronics. The sensitivity of the electronics increased by an order of magnitude during recent time. In automatic exposure units of digital X ray imaging systems, the ionization chamber is installed before the X ray radiation detector (X ray film cassettes or digital panel) and is connected to an automatic exposure system or to a preliminary amplifier using a signal cable. Possible schemes of ionization chamber connection to an auto matic exposure system are shown in Fig. 1. Connection version (Fig. 1a) was used in X ray apparatuses of the past century but is not used any longer. This is due to requirements for patient radiation load decrease and recent progress in enhancement of sensitiv ity of X ray apparatuses. Sensitivity of X ray detector (ionization chamber) should be increased adequately. A preamplifier installed near the ionization chamber pro vides an economical solution to this problem. Two con nection versions (Figs. 1b and 1c) are presently used. In the connection version shown in Fig. 1b, the preamplifi er is installed directly on the ionization chamber housing. In the connection version shown in Fig. 1c, the preampli fier is installed close to the ionization chamber. Advantages and disadvantages of these versions were dis cussed at length in [1]. Biomedical Engineering, Vol. 42, No. 5, 2008, pp. 236 238. Translated from Meditsinskaya Tekhnika, Vol. 42, No. 5, 2008, pp. 13 15. Original article submitted April 11, 2008.