Comparison of two methods of noise power spectrum determinations of medical radiography systems

Noise in medical images is recognized as an important factor that determines the image quality. Image noise is characterized by noise power spectrum (NPS). We compared two methods of NPS determination namely the methods of Wagner and Dobbins on Lanex Regular TMG screen-film system and Hologic Lorad Selenia full field digital mammography system, with the aim of choosing the better method to use. The methods differ in terms of various parametric choices and algorithm implementations. These parameters include the low pass filtering, low frequency filtering, windowing, smoothing, aperture correction, overlapping of region of interest (ROI), length of fast Fourier transform, ROI size, method of ROI normalization, and slice selection of the NPS. Overall, the two methods agreed to the practical value of noise power spectrum between 10 -3-10-6mm2 over spatial frequency range 0-10mm-1

Comparison of denoising methods for digital mammographic image

We compared effects of denoising methods on digital mammographic images. The denoising methods studied were an adaptive Wiener filter and low–pass Gaussian filter. The denoising methods were applied as an image preprocessing techniques before enhancement. The performance of image denoising methods are based on Mean Squared Error (MSE) and Peak Signal To Ratio (PSNR) values

Detectability of fibrils by dilation technique in digital mammography

The detectability of fibrils in mammographic phantom images by morphological enhancement was analysed in the present study. Materials that mimic fibrils were imaged by a digital mammography machine at 28 and 29 kVP. The images were processed by a dilation technique to produce second set of images. Receiver operating characteristic analysis was performed to compare the detection performance from the two sets of images. As compared to original images, the 28 kVP's fibrils images from dilation technique become more prominence to be detected by observers. While at 29 kVP only a few observers can found the fibrils images from dilation technique. This study suggests morphological enhancement of mammography image did not increase the detection of low frequency signals of the images

Calculation of the modulation transfer function from the square wave response function data with an interactive curve fitting software

Two ways to calculate the modulation transfer function (MTF) of radiographic screen-film systems from the measured square wave response function (SWRF) data were investigated with an interactive curve fitting software. The measured SWRF data obtained by digitising a radiographic image of a bar pattern test object were fitted to a curve, and the fitted curve was used to calculate the MTF. Satisfactory MTF was obtained by using 12 terms in the calculation. A second version of the calculation included a correction for the normalization at 0.25 cycles/mm of the SWRF data. Measurements from a screen-film combination showed that the MTF of the first version was higher than the second by an average amount of 0.02 units for spatial range 0-3.5 cycles/mm, and on average the MTF of the first version was higher than the second by 10%. Both the SWRF data fitting and the MTF calculation were done within an interactive curve fitting software which made the calculation relatively easy to perform

Measurement of wiener spectrum of radiographic screen-film systems

A method to measure the Wiener spectrum of radiographic screen-film systems in a hospital setting is presented. Radiographs exposed to uniform X-rays are scanned by a microdensitometer, and optical density fluctuation data about the mean density are obtained by subtracting the mean density from the density values. The data are low-pass-filtered by averaging pairs of pixels, followed by low-frequency filtering to eliminate very low-frequency components. A slit trace is synthesised by averaging adjacent traces and the trace is segmented to segments of 256 data points per segment with overlap of 128 data points. Data in each segment are windowed and fast Fourier transformed. The Fourier coefficients are squared and normalized to obtain the power spectrum. On site measurement for Lanex Regular and Lanex Fine screens gives results that are comparable with published data

Some tests on the measurement of the modulation transfer function technique of screen-film system

A method to determine the modulation transfer function (MTF) of screen-film system was tested by measuring the MTF of a mammography and a conventional radiography systems. The method was further tested by measuring MTF obtained from x-rays exiting from two tissues equivalent phantoms. The MTF of the mammography system was higher than that of conventional radiography system confirming general correctness of the method. The MTFs obtained from the two equivalent phantoms agreed very well which indicate that the equivalent could be shown by the method. The study gives the experimental proof for the appropriateness of the method

Determination of noise equivalent quanta of medical screen-films

Noise equivalent quanta (NEQ) of Lanex Regular/T Mat G and Lanex Regular/T Mat L screen-film combinations were determined using modulation transfer function (MTF) and noise power spectrum (NPS) data. To accomplish this, average gamma of the radiograph was computed, the MTF and the NPS data were linearly interpolated and these values were used to compute the NEQ by means of a computer program. The computation shows that for spatial frequency 0–0.7 cycles/mm the NEQ of Lanex Regular/T Mat G is slightly lower than that of Lanex Regular/T Mat L, for spatial frequency 0.7–1.5 cycles/mm the NEQ of both are almost the same, and for frequencies greater than 1.5 cycles/mm the NEQ of the former is greater than that of the latter. Relatively, this indicates that low frequency signals show better on Lanex Rregula/T Mat L, but high frequency signals show better on Lanex Regular/T Mat G

Physical image quality evaluation of medical radiographs

The noise equivalent quanta (NEQ) has been recommended as a measure of physical quality of medical images. Digitised data of medical radiographs in a previous work have been analysed in terms of resolution via the modulation transfer function (MTF) and noise via the noise power spectrum (NPS), but not in terms of NEQ. As the NEQ is currently regarded as a basic device performance measure of the imaging system, it is very useful to be able to determine it. The average gamma of the radiograph was computed, both the MTF and the NPS data were interpolated and these values were used to compute the NEQ. The computation shows that for spatial frequency range 0–0.7 cycles/mm the NEQ of Lanex Regular/T Mat G screen-film combination is slightly lower than that of Lanex Regular/T Mat L, for 0.7–1.5 cycles/mm the NEQ of both are almost the same, and for frequencies greater than 1.5 cycles/mm the NEQ of the former is greater than that of the latter