Gain and offset calibration reduces variation in exposure-dependent SNR among systems with identical digital flat-panel detectors

Abstract

Purpose: The conditions under which vendor performance criteria for digital radiography systems are obtained do not adequately simulate the conditions of actual clinical imaging with respect to radiographic technique factors, scatter production, and scatter control. Therefore, the relationship between performance under ideal conditions and performance in clinical practice remains unclear. Using data from a large complement of systems in clinical use, the authors sought to develop a method to establish expected performance criteria for digital flat-panel radiography systems with respect to signal-to-noise ratio (SNR) versus detector exposure under clinical conditions for thoracic imaging.

Methods: The authors made radiographic exposures of a patient-equivalent chest phantom at 125 kVp and 180 cm source-to-image distance. The mAs value was modified to produce exposures above and below the mAs delivered by automatic exposure control. Exposures measured free-in-air were corrected to the imaging plane by the inverse square law, by the attenuation factor of the phantom, and by the Bucky factor of the grid for the phantom, geometry, and kilovolt peak. SNR was evaluated as the ratio of the mean to the standard deviation (SD) of a region of interest automatically selected in the center of each unprocessed image. Data were acquired from 18 systems, 14 of which were tested both before and after gain and offset calibration. SNR as a function of detector exposure was interpolated using a double logarithmic function to stratify the data into groups of 0.2, 0.5, 1.0, 2.0, and 5.0 mR exposure (1.8, 4.5, 9.0, 18, and 45 microGy air KERMA) to the detector.

Results: The mean SNR at each exposure interval after calibration exhibited linear dependence on the mean SNR before calibration (r2=0.9999). The dependence was greater than unity (m = 1.101 +/- 0.006), and the difference from unity was statistically significant (p <0.005). The SD of mean SNR after calibration also exhibited linear dependence on the SD of the mean SNR before calibration (r2 = 0.9997). This dependence was less than unity (m = 0.822 +/- 0.008), and the difference from unity was also statistically significant (p < 0.005). Systems were separated into two groups: systems with a precalibration SNR higher than the median SNR (N = 7), and those with a precalibration SNR lower than the median SNR (N= 7). Posthoc analysis was performed to correct for expanded false positive results. After calibration, the authors noted differences in mean SNR within both high and low groups, but these differences were not statistically significant at the 0.05 level. SNR data from four additional systems and one system from those previously tested after replacement of its detector were compared to the 95% confidence intervals (CI) calculated from the postcalibration SNR data. The comparison indicated that four of these five systems were consistent with the CI derived from the previously tested 14 systems after calibration. Two systems from the paired group that remained outside the CI were studied further. One system was remedied with a grid replacement. The nonconformant behavior of the other system was corrected by replacing the image receptor.

Conclusions: Exposure-dependent SNR measurements under conditions simulating thoracic imaging allowed us to develop criteria for digital flat-panel imaging systems from a single manufacturer. These measurements were useful in identifying systems with discrepant performance, including one with a defective grid, one with a defective detector, and one that had not been calibrated for gain and offset. The authors also found that the gain and offset calibration reduces variation in exposure-dependent SNR performance among the systems.

Figure 1

Test setup. Patient-equivalent phantom (LucAl Chest) imaged using an upright exposure station with measurement of free-in-air entrance exposure according to AAPM Report No. 31 (Ref. 3). Apparent nonuniformity is surface oxidation of the Al.

Figure 2

Box and whisker charts of exposure-dependent signal-to-noise ratio for 14 paired systems (a) before and (b) after calibration. Dark lines indicate the medians, boxes indicate quartiles, and whiskers show the range of data excluding minor (values that are between 1.5 and three times the interquartile range) and major outliers (values that are more than three times the interquartile range). Labeled points in (b) indicate two minor outliers from unit DC and one from Definium 3.

Figure 3

(a) Mean (squares) and (b) standard deviation (SD; triangles) of SNR of 14 paired systems after gain and offset calibration versus before calibration.

Figure 4

Exposure-dependent SNR data from 14 paired systems (a) before and (b) after calibration compared to 95% CIs (broken lines). Data from individual systems are depicted with the same symbols in both (a) and (b). Systems with data higher than the CI are shown in red, within the CIs are green, and below the CI are blue. Note that the number of systems outside the CIs decreased from 11 to 6 after calibration. The uncertainty of the individual measurements of SNR is ±5% at the 1.0 mR exposure level, which is approximated by the dimensions of the symbols. Symbols correspond to individual systems as defined in Table TABLE I..

Figure 5

Exposure-dependent SNR data for five additional systems compared to the CIs (broken lines) of 14 paired systems after calibration [from Fig. 4b]. Note that four of the systems conformed to the postcalibration CIs developed from the other 14 systems. The nonconformant system (I2) was replaced before it could be recalibrated. Symbols correspond to individual systems as defined in Table TABLE I..

Figure 6

Exposure-dependent SNR investigation of low-performing, nonconformant system. Unit Definium 3 is shown before (blue □) and after (blue +) calibration. In both cases, the most of the SNR measurements were below the CIs (broken lines) for 14 paired systems postcalibration [from Fig. 4b].With a substitute grid (green ▪), Definium 3 conforms to the CIs. Using definium 3’s grid (blue Δ), another unit (Definium 1) fails to conform to the CIs, although it conforms when using its own grid (green ▴).

Figure 7

Exposure-dependent SNR investigation of high-performing, nonconformant system. Unit DC was consistently higher than the calibrated CIs (broken lines) for the 14 paired systems [from Fig 4b]; before (□), after ( +  and ○), in high (_*), standard (◊), and low (X) vertical positions, and after grid replacement (Δ). The same system with a new detector (green •) conforms to the CIs.