Accurate MTF measurement in digital radiography using noise response

Abstract

Purpose:The authors describe a new technique to determine the system presampled modulation transfer function ͑MTF͒ in digital radiography using only the detector noise response. Methods: A cascaded-linear systems analysis was used to develop an exact relationship between the two-dimensional noise power spectrum ͑NPS͒ and the presampled MTF for a generalized detector system. This relationship was then utilized to determine the two-dimensional presampled MTF. For simplicity, aliasing of the correlated noise component of the NPS was assumed to be negligible. Accuracy of this method was investigated using simulated images from a simple detector model in which the "true" MTF was known exactly. Measurements were also performed on three detector technologies ͑an x-ray image intensifier, an indirect flat panel detector, and a solid state x-ray image intensifier͒, and the results were compared using the standard edge-response method. Flat-field and edge images were acquired and analyzed according to guidelines set forth by the International Electrotechnical Commission, using the RQA 5 spectrum. Results: The presampled MTF determined using the noise-response method for the simulated detector system was in close agreement with the true MTF with an averaged percent difference of 0.3% and a maximum difference of 1.1% observed at the Nyquist frequency ͑f N ͒. The edgeresponse method of the simulated detector system also showed very good agreement at lower spatial frequencies ͑less than 0.5 f N ͒ with an averaged percent difference of 1.6% but showed significant discrepancies at higher spatial frequencies ͑greater than 0.5 f N ͒ with an averaged percent difference of 17%. Discrepancies were in part a result of noise in the edge image and phasing errors. For all three detector systems, the MTFs obtained using the two methods were found to be in good agreement at spatial frequencies less than 0.5 f N with an averaged percent difference of 3.4%. Above 0.5 f N , differences increased to an average of 20%. Deviations of the experimental results largely followed the trend seen in the simulation results, suggesting that differences between the two methods could be explained as resulting from the inherent inaccuracies of the edgeresponse measurement technique used in this study. Aliasing of the correlated noise component was shown to have a minimal effect on the measured MTF for the three detectors studied. Systems with significant aliasing of the correlated noise component ͑e.g., a-Se based detectors͒ would likely require a more sophisticated fitting scheme to provide accurate results. Conclusions: Results indicate that the noise-response method, a simple technique, can be used to accurately measure the MTF of digital x-ray detectors, while alleviating the problems and inaccuracies associated with use of precision test objects, such as a slit or an edge.

Spatial resolution in CBCT machines for dental/maxillofacial applications—what do we know today?

“…15 Several methods have been described for the accurate calculation of sensor resolution via pre-sampled MTF. 16 The MTF of the detector can be obtained by square-shaped stripe patterns ( Figure 1) on the basis of noise response 17 or the Fourier transform of a slanted edge or slit image 18 via its line spread function (LSF). By using the Fourier transform of the LSF, the MTF can be computed.…”

Estimation of the two‐dimensional presampled modulation transfer function of digital radiography devices using one‐dimensional test objects

“…[11][12][13] Recent advances have yielded promising results for extraction of the 2D MTF from NPS data, but the precision and accuracy of these results come at the cost of intensive cascaded systems analysis and fitting of measured data to an assumed functional form of the MTF. 9 By comparison with existing 2D MTF test objects, namely, 2D arrays of pinholes, 5,11 our techniques that estimate the 2D MTF from multiple measurements with 1D devices have certain benefits. First, 1D test devices have been used extensively in laboratory and clinical settings, and their use is well understood and widely accepted.…”

“…In a recent effort to measure the 2D MTF without the use of physical test devices, Kuhls-Gilcrist et al derived an exact relationship between the 2D NPS and presampled MTF. 9 Although accurate, the technique requires in-depth cascaded systems analysis to produce a functional form of the system MTF for fitting purposes. Still, several authors have reported on efforts to develop 2D resolution test objects.…”

Advances in Technological Design to Optimize Exposure and Improve Image Quality

“…screen-film) by sinusoidal test patterns using Coltman's method [18] or by using sharp-edge test phantoms on digital systems [68]. Alternatively, the analysis of the noise response can be used for this purpose [47].…”