Determination of the two-dimensional detective quantum efficiency of a computed radiography system

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

PURPOSE

The modulation transfer function (MTF) of medical imaging devices is commonly reported in the form of orthogonal one-dimensional (1D) measurements made near the vertical and horizontal axes with a slit or edge test device. A more complete description is found by measuring the two-dimensional (2D) MTF. Some 2D test devices have been proposed, but there are some issues associated with their use: (1) they are not generally available; (2) they may require many images; (3) the results may have diminished accuracy; and (4) their implementation may be particularly cumbersome. This current work proposes the application of commonly available 1D test devices for practical and accurate estimation of the 2D presampled MTF of digital imaging systems.

METHODS

Theory was developed and applied to ensure adequate fine sampling of the system line spread function for 1D test devices at orientations other than approximately vertical and horizontal. Methods were also derived and tested for slit nonuniformity correction at arbitrary angle. Techniques were validated with experimental measurements at ten angles using an edge test object and three angles using a slit test device on an indirect-detection flat-panel system [GE Revolution XQ∕i (GE Healthcare, Waukesha, WI)]. The 2D MTF was estimated through a simple surface fit with interpolation based on Delaunay triangulation of the 1D edge-based MTF measurements. Validation by synthesis was also performed with simulated images from a hypothetical direct-detection flat-panel device.

RESULTS

The 2D MTF derived from physical measurements yielded an average relative precision error of 0.26% for frequencies below the cutoff (2.5 mm(-1)) and approximate circular symmetry at frequencies below 4 mm(-1). While slit analysis generally agreed with the results of edge analysis, the two showed subtle differences at frequencies above 4 mm(-1). Slit measurement near 45° revealed radial asymmetry in the MTF resulting from the square pixel aperture (0.2 mm × 0.2 mm), a characteristic which was not necessarily appreciated with the orthogonal 1D MTF measurements. In simulation experiments, both slit- and edge-based measurements resolved the radial asymmetries in the 2D MTF. The average absolute relative accuracy error in the 2D MTF between the DC and cutoff (2.5 mm(-1)) frequencies was 0.13% with average relative precision error of 0.11%. Other simulation results were similar to those derived from physical data.

CONCLUSIONS

Overall, the general availability, acceptance, accuracy, and ease of implementation of 1D test devices for MTF assessment make this a valuable technique for 2D MTF estimation.

A method for the determination of the two-dimensional MTF of digital radiography systems using only the noise response

We present a new method that enables the determination of the two-dimensional MTF of digital radiography systems using the noise response measured from flat-field images. Unlike commonly-used methods that measure the one-dimensional MTF, this new method does not require precision-made test-objects (slits/edges) or precise tool alignment. Although standard methods are dependent upon data processing that can result in inaccuracies and inconsistencies, this method based on the intrinsic noise response of the imager is highly accurate and less susceptible to such problems. A cascaded-linear-systems analysis was used to derive an exact relationship between the noise power spectrum (NPS) and the presampled MTF of a generalized detector system. The NPS was then used to determine the two-dimensional MTF for three systems: a simulated detector in which the "true" MTF was known exactly, a commercial indirect flat-panel detector (FPD), and a new solid-state x-ray image intensifier (SSXII). For the simulated detector, excellent agreement was observed between the "true" MTF and that determined using the noise response method, with an averaged deviation of 0.3%. The FPD MTF was shown to increase on the diagonals and was measured at 2.5 cycles/mm to be 0.086±0.007, 0.12±0.01, and 0.087±0.007 at 0, 45, and 90°, respectively. No statistically significant variation was observed for the SSXII as a function of angle. Measuring the two-dimensional MTF should lead to more accurate characterization of the detector resolution response, incorporating any potential non-isotropy which may result from the physical characteristics of the sensor, including the active-area shape of the pixel array.

Method of simulating dose reduction for digital radiographic systems

The optimisation of image quality vs. radiation dose is an important task in medical imaging. To obtain maximum validity of the optimisation, it must be based on clinical images. Images at different dose levels can then either be obtained by collecting patient images at the different dose levels sought to investigate-including additional exposures and permission from an ethical committee-or by manipulating images to simulate different dose levels. The aim of the present work was to develop a method of simulating dose reduction for digital radiographic systems. The method uses information about the detective quantum efficiency and noise power spectrum at the original and simulated dose levels to create an image containing filtered noise. When added to the original image this results in an image with noise which, in terms of frequency content, agrees with the noise present in an image collected at the simulated dose level. To increase the validity, the method takes local dose variations in the original image into account. The method was tested on a computed radiography system and was shown to produce images with noise behaviour similar to that of images actually collected at the simulated dose levels. The method can, therefore, be used to modify an image collected at one dose level so that it simulates an image of the same object collected at any lower dose level.