Performance of a new star-bar phantom designed for MTF calculations in x-ray imaging systems.

PURPOSE: A new phantom, designed and manufactured for modulation transfer function (MTF) calculations is presented in this work. The phantom has a star-bar pattern and is manufactured in stainless steel. Modulation transfer function determinations are carried out with the new phantom and with an edge phantom to compare their performance and to compare them with previous theoretical predictions. METHODS: The phantoms are imaged in an x-ray imaging system using different beam qualities and different entrance air KERMA. Methods, previously developed for synthetic images and simulations, are adapted to real measurements, solving practical implementation issues. RESULTS: In the case of the star-bar, in order to obtain optimal MTF determinations it is necessary to accurately determine the center of the pattern. Also, to avoid underestimates in MTF calculations, the length in pixels of each of the scanning circumferences must be an integer multiple of the number of cycles in the pattern. Both methods, star-bar and edge, give similar mean values of the MTF in all cases analyzed. Also, the dependence with frequency of the experimental MTF standard deviation (SD) agrees with the theoretical expressions presented in previous works. In this regard, the precision is better for the star-bar method than for the edge and differences in precision between both methods are higher for the lowest beam quality. CONCLUSIONS: The star-bar phantom can be used for MTF determinations with the advantage of having an improved precision. However, precision is reduced when the radiation quality increases. This fact suggests that, for the highest beam qualities, materials with an attenuation coefficient greater than that of steel should be used to manufacture the phantom.

Efficient detrending of uniform images for accurate determination of the noise power spectrum at low frequencies.

The noise power spectrum (NPS) of a digital x-ray imaging device is usually estimated from the average of periodograms of regions of interest (ROIs) in images obtained with uniform radiation fields. In order to mitigate low frequency trends, present in the images and not arising from stochastic processes, detrending methods are applied to the images before being Fourier transformed. The most common of these methods subtracts a second-order polynomial fit from the image. In this work, it is shown that the characteristics of low frequency trends can deviate from the quadratic dependence on spatial coordinates. This results in large residual trends that give rise to important correlations in the detrended images and produce an inaccurate rise of the NPS calculations at low frequencies. A new detrending method of uniform images is presented. The method operates in the subbands of a wavelet transform, removing the low frequency contents of the uniform image. To do this, the approximation subband of the highest level of the wavelet transform is cancelled. The effect on the NPS calculations for three digital detectors is shown and the importance of the parameters of the wavelet transform is discussed. The main result states that the performance of the new method improves those of two polynomial detrending methods commonly used and is close to the performance of the subtraction of uniform exposure images method. Finally, guidelines for the implementation of the procedure, like the number of levels in the wavelet decomposition, are provided. As the number of levels in the wavelet transform increases, the removal of trends is restricted to lower frequencies. The selection of the number of levels should be guided by the shape of the autocorrelation function of the detrended image, which has to resemble the shape expected from the propagation of noise through the imaging chain.

Technical Note: An oversampling procedure to calculate the MTF of an imaging system from a bar-pattern image.

PURPOSE: Line-pair resolution phantoms are used to determine the spatial resolution of medical imaging systems. In some cases, these phantoms are used to determine the maximum number of line-pairs per mm that the system can resolve. In other cases, a numerical determination of the modulation transfer function (MTF) is carried out by means of the analysis of the variance of ROIs on the image. In this note, a new procedure is implemented to calculate the presampled MTF of an imaging system. METHODS: Images of a commercial line-pair phantom are acquired in a flat panel detector. After applying an edge detector and a Radon transform to the image, the direction of the bars in the phantom is calculated. Then, an area of the image that excludes the ends of the bars is determined. Every pixel in this area is used to obtain an oversampled profile on which the MTF is calculated. Every group of line-pairs conforms a periodic wave on this profile. Each of these waves is the output of the system for a square wave input with the same frequency. After extracting a wave, the MTF value for its frequency is calculated as the ratio of its first odd harmonic to the first odd harmonic of the square wave input. The amplitude of this square wave is obtained from two uniform areas on the phantom image. RESULTS: The results obtained are compared to those obtained following the standard edge method, as recommended by the IEC, and show a very good agreement between both methods, in both main directions of the detector and all the dose ranges analyzed. CONCLUSIONS: The presented method is shown accurate and can be used to extend the conventional use of line-pair phantoms in conventional radiology.