Philipp Bernhardt

X-ray spectrum optimization of full-field digital mammography : Simulation and phantom study

In contrast to conventional analog screen-film mammography new flat detectors have a high dynamic range and a linear characteristic curve. Hence, the radiographic technique can be optimized independently of the receptor exposure. It can be exclusively focused on the improvement of the image quality and the reduction of the patient dose. In this paper we measure the image quality by a physical quantity, the signal difference-to-noise ratio (SDNR), and the patient risk by the average glandular dose (AGD). Using these quantities, we compare the following different setups through simulations and phantom studies regarding the detection of microcalcifications and tumors for different breast thicknesses and breast compositions: Monochromatic radiation, three different anode/filter combinations: Molybdenum/molybdenum (Mo/Mo), molybdenum/rhodium (Mo/Rh), and tungsten/rhodium (W/Rh), different filter thicknesses, use of anti-scatter grids, and different tube voltages. For a digital mammography system based on an amorphous selenium detector it turned out that, first, the W/Rh combination is the best choice for all detection tasks studied. Second, monochromatic radiation can further reduce the AGD by a factor of up to 2.3, maintaining the image quality in comparison with a real polychromatic spectrum of an x-ray tube. And, third, the use of an anti-scatter grid is only advantageous for breast thicknesses larger than approximately 5 cm.

New technologies to reduce pediatric radiation doses

X-ray dose reduction in pediatrics is particularly important because babies and children are very sensitive to radiation exposure. We present new developments to further decrease pediatric patient dose. With the help of an advanced exposure control, a constant image quality can be maintained for all patient sizes, leading to dose savings for babies and children of up to 30%. Because objects of interest are quite small and the speed of motion is high in pediatric patients, short pulse widths down to 4 ms are important to reduce motion blurring artifacts. Further, a new noise-reduction algorithm is presented that detects and processes signal and noise in different frequency bands, generating smooth images without contrast loss. Finally, we introduce a super-resolution technique: two or more medical images, which are shifted against each other in a subpixel region, are combined to resolve structures smaller than the size of a single pixel. Advanced exposure control, short exposure times, noise reduction and super-resolution provide improved image quality, which can also be invested to save radiation exposure. All in all, the tools presented here offer a large potential to minimize the deterministic and stochastic risks of radiation exposure.

Spatial Frequency-Dependent Signal-to-Noise Ratio as a Generalized Measure of Image Quality

A generalized, objective image quality measure can be defined for X-ray based medical projection imaging: the spatial frequency-dependent signal-to-noise ratio SNR = SNR(u,v). This function includes the three main image quality parameters, i.e. spatial resolution, object contrast, and noise. The quantity is intimately related to the DQE concept, however its focus is not to characterize the detector, but rather the detectability of a certain object embedded into a defined background. So also effects from focus size and radiation scatter can be quantified by this method. The SNR(u,v) is independent of basic linear post-processing steps such as appropriate windowing or spatial filtering. The consideration of the human visual system is beyond the scope of this concept. By means of this quantity, different X-ray systems and setups can be compared with each other and with theoretical calculations. Moreover, X-ray systems (i.e. detector, beam quality, geometry, anti-scatter grid, basic linear post-processing steps etc.) can be optimized to deliver the best object detectability for a given patient dose. In this paper SNR(u,v) is defined using analytical formulas. Furthermore, we demonstrate how it can be applied with a test phantom to a typical flat panel detector system by a combination of analytical calculations and Monte Carlo simulations. Finally the way this function can be used to optimize an X-ray imaging device is demonstrated.

Absorbers for medical x-ray detectors with optimum spatial resolution: a simulation study

The requirements for medical X-ray detectors tend towards higher spatial resolution, especially for mammography. Therefore, we have investigated common absorber materials with respect to the possible intrinsic limitations of their spatial resolution. Primary interaction of an incident X-ray quantum is followed by a series of processes: Rayleigh scattering, Compton effect, or the generation of fluorescence photons and subsequent electrons. Lateral diffusion of carriers relative to their drift towards the electrodes also broadens the point-spread function. One consequence is that the spatial resolution of the detector, expressed in terms of the modulation transfer function (MTF), is reduced. Monte Carlo simulations have been carried out for spectra with tube voltages of 28-120 kV using the program ROSI (Roentgen Simulation) based on the well-established EGS4 algorithm. The lateral distribution of deposited energy has been calculated in typical materials such as Se, CdTe, HgI2, and PbI2 and used to determine the line spread function. The complex absorption process is found to determine the spatial resolution of the detector considerably. The spectrum at energies closely above the K-edge of the absorber material tends to result in a reduced MTF. At energies above 50 keV, electron energy loss increasingly reduces spatial resolution in the high frequency range. The influence of fluorescence is strongest in the 5-20 lp/mm range. If a very high spatial resolution is required, a well-adapted semiconductor should be applied.