Determination of the Detective Efficiency of X-Ray Diagnostic Systems by Visual Assessment of the Maximal Resolution

Abstract

Determination of the detective efficiency of X ray diagnostic systems is a rather difficult task. It has been widely discussed in the literature [1 3, 7]. An internation al standard unifies the method for determination of the detective efficiency of X ray detectors, in particular, matrix detectors used in medical X ray diagnosis [8]. The standard can be used only under laboratory conditions and requires the use of special expensive equipment, which makes it virtually impossible to apply this standard during acceptance testing, as well as in maintenance test ing of X ray apparatuses at the site of their use. A rela tively simple method for determination of detective effi ciency of X ray diagnostic systems from visually evaluat ed minimal radiation contrast was suggested in our previ ous work [2]. This method is based on the use of a con trast sensitivity test object and an X ray dosimeter provid ing measurement within the required sensitivity range. In [2], the detective efficiency was determined only at quasi zero frequency. The goal of this work was to extend the range of applicability of this method to virtually all spatial frequencies used in X ray diagnosis without eliminating such advantages of the method as simplicity and possibil ity of use for acceptance, periodical, and maintenance tests. The improved method is based on the use of a reso lution test object and an X ray dosimeter installed at the input of the X ray receiver to measure the exposure dose.The signal/noise ratio is the main characteristic determining the efficiency of detection. It can be described by the following equation:where (1 − exp(−µd)) = K is the radiation contrast pro vided by a lead test object with given thickness d and extinction coefficient µ for given effective energy of X ray radiation. Anode voltage should be set to 70 kV, and a 20 mm Al filter should be placed at the X ray collimator output. This corresponds to half attenuation Al layer thickness 7 mm and effective energy of 50 keV. For this energy, the mass coefficient µ/ρ of radiation extinction in lead is 7.94 cm 2 /g [7]. Lead density ρ Pb is 11.3 g/cm 3 , so that the linear extinction coefficient µ is 89.7 cm -1 . Taking into account that the stripes of resolution test objects used for testing of medical X ray equipment are usually 50, 70, or 100 µm thick, the theoretically calculated value of radi ation contrast for effective energy of 50 keV is 0.36 (36%), 0.47 (47%), or 0.59 (59%), respectively.The frequency-contrast characteristic (FCC) of an X ray diagnostic system is determined by its focus size, geometric characteristics of the test procedure, distance between focus and resolution test object, and distance between resolution test object and input plane of the receiver, as well as the parameters of the X ray detection and imaging system. In Eq. (1), FCC characterizes the rate of decrease in radiation contrast with increasing fre quency of lead stripes in the test object. For an ideal sys tem, FCC = 1 for all spatial frequencies. N (quantum/ mm 2 ) is the is the nu...

A study of digital X-ray detection systems with optical image transfer

“…where g 1 is conversion coefficient of image formation, i.e., the number of photons generated upon X ray beam absorption by screen; g 2 is efficiency of optical coupling; g 3 is detection quantum efficiency of video camera with CCD matrix, g 3 = 0.45 [2]. Detection quantum efficiency is a function of spatial frequency and exposure level.…”

“…The KRTs 1 digital X ray system development was described in more detail in [2]. The KRTs 1 digital X ray system development was described in more detail in [2].…”