Ms. Kirsi M. Nykanen Profile

Kirsi M. Nykanen

Image Quality Specialist at GE Healthcare

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Proceedings Article | 5 June 2003 Paper

Physical characteristics of a clinical prototype for full-field digital mammography with an a-Se flat-panel detector

Barbara Lazzari, Giacomo Belli, Cesare Gori, Kirsi Nykanen

Proceedings Volume 5030, (2003) https://doi.org/10.1117/12.479989

KEYWORDS: Modulation transfer functions, Sensors, Digital mammography, Image quality, Prototyping, Spatial frequencies, Polymethylmethacrylate, Quantum efficiency, Mammography, Signal attenuation

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The purpose of this study was to evaluate physical characteristics of a clinical prototype for full-field digital mammography (FFDM) with an amorphous selenium (a-Se) flat-panel detector (FPD) and to compare these results with the currently available systems for FFDM. The effective FPD area consists of 2816 x 2016 pixel matrix having a pixel pitch of 85μm. This yields to theoretical maximum spatial frequency of ~5.9lp/mm. The basic performance of Instrumentarium clinical prototype direct mammography system has been assessed on the basis of measured response curve, the modulation transfer function (MTF), the noise power spectrum (NPS), the noise equivalent quanta (NEQ) and the detective quantum efficiency (DQE) in the clinical setting. The system shows a linear response curve over a dynamic range from 0.4 mR to 57 mR. The presampling MTF was found to be approximately 0.91, 0.72 and 0.50 at 2, 4 and 5.9 (Nyquist frequency) lp/mm. The NEQ is linearly related to the exposure starting from about 8 mR above which value the system can be considered noise quantum limited. The DQE, evaluated in clinical conditions (28kVp Mo-Mo spectrum hardened by 4cm of PMMA) is at close to zero spatial frequency approximately 69% at 46.4 mR and 61% at 8.3 mR. Below 8 mR the DQE(0) falls to 54.4%, 46% and 32.5% at 5.2, 3.0 and 1.4 mR respectively due to structured and electronic noise. The results of quantitative analysis demonstrate a high MTF as we expected due to direct conversion technology and a high DQE over the exposure range from 8 mR to 50 mR. The NEQ shows that the system can be considered noise quantum limited above 8 mR suggesting the exposure level that should be set in the clinical practice to ensure an adequate image quality.

Proceedings Article | 5 June 2003 Paper

Advantages of gridless full-field digital mammography

Kirsi Nykanen, Samuli Siltanen

Proceedings Volume 5030, (2003) https://doi.org/10.1117/12.479944

KEYWORDS: Mammography, Breast, Point spread functions, Sensors, X-rays, Image quality, Scattering, Digital mammography, Tissues, Signal detection

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The purpose of the study was to find out whether the image quality in full-field digital mammography can be improved while lowering the patient dose by removing the anti-scatter grid. Moreover, a fast approximate computational algorithm was developed for determining the scattered field in a real mammogram. The method is non-iterative, robust against noise, and works without modification for any scatter-to-primary ratio. Furthermore, it is computationally effective since it is based on fast Fourier transform (FFT). It was found out that the wide dynamic range of digital detectors leads to decrease in patient dose from 10.9% up to 46.6% at breast thickness of 2cm and from 0.8% up to 40.8% at breast thickness of 4cm depending on the efficiency of the removed grid. At constant patient dose the increase in contrast-to-noise ratio is 5.8% - 36.9% and 0.4%-30.0% accordingly at those two breast thickness. The convolution-based X-ray scatter model was considered. The developed scatter removal method was demonstrated with simulated mammograms and applied to clinical full-field digital mammograms acquired with a high-end digital flat panel detector based on amorphous selenium. Errors in reconstructed scattered fields were 0.3% in case of an ideal simulated mammogram and 7.4% in case of a real simulated mammogram (3cm breast). Applications where the scattered field needs to be determined include 3-D mammography and dual-energy breast imaging. In screening mammography gray-scale optimization eliminates the effect of scattering.

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