KEYWORDS: Breast, Signal to noise ratio, Image quality, Modulation transfer functions, Digital mammography, X-rays, Spatial frequencies, Breast cancer, Mammography, Polymethylmethacrylate
Breast density has a close relationship with breast cancer risk. The exposure parameters must be appropriately chosen for
each breast. However, the optimal exposure conditions for digital mammography are uncertain in clinical. The exposure
parameters in digital mammography must be optimized with maximization of image quality and minimization of
radiation dose. We evaluated image quality under different exposure conditions to investigate the most advantageous
tube voltage. For different compressed breast phantom thicknesses and compositions, we measured the Wiener spectrum
(WS), noise-equivalent number of quanta (NEQ), and detective quantum efficiency (DQE). In this study, the
signal-to-noise ratios were derived from a perceived statistical decision theory model with the internal noise of eye-brain
system (SNRi), contrived and studied by Loo et al.1 and Ishida et al.2 These were calculated under a fixed average
glandular dose. The WS values were obtained with a fixed image contrast. For 4-cm-thick and 50% glandular breast
phantoms, the NEQ showed that high voltages gave a superior noise property of images, especially for thick breasts, but
the improvement in the NEQ by tube voltage was not so remarkable. On the other hand, the SNRi value with a Mo filter
was larger than that with a Rh filter. The SNRi increased when the tube voltage decreased. The result differed from those
of WS and NEQ. In this study, the SNRi depended on the contrast of signal. Accuracy should be high with an intense,
low-contrast object.
KEYWORDS: Signal to noise ratio, X-rays, Modulation transfer functions, Digital mammography, Interference (communication), Mammography, Image quality, Breast, Spatial frequencies, Visualization
The use of digital mammography systems has become widespread recently. However, the optimal exposure parameters
are uncertain in clinical practice. We need to optimize the exposure parameter in digital mammography while
maximizing image quality and minimizing patient dose. The purpose of this study was to evaluate the most beneficial
exposure variable-tube voltage for each compressed breast
thickness-with these indices: noise power spectrum, noise
equivalent quanta, detective quantum efficiency, and signal-to-noise ratios (SNR). In this study, the SNRs were derived
from the perceived statistical decision theory model with the internal noise of eye-brain system (SNRi), contrived and
studied by Loo LN1), Ishida M et al. 2) These image quality indices were obtained under a fixed average glandular dose
(AGD) and a fixed image contrast. Our results indicated that when the image contrast and AGD was constant, for
phantom thinner than 5 cm, an increase of the tube voltage did not improve the noise property of images very much. The
results also showed that image property with the target/filter Mo/Rh was better than that with Mo/Mo for phantom
thicker than 4 cm. In general, it is said that high tube voltage delivers improved noise property. Our result indicates that
this common theory is not realized with the x-ray energy level for mammography.
KEYWORDS: Signal to noise ratio, Curium, Modulation transfer functions, X-rays, Photons, Mammography, Spatial frequencies, Digital mammography, Computing systems, Sensors
Recently, with developments in medicine, digital systems such as computed radiography (CR) and flat-panel
detector (FPD) systems are being employed for mammography instead of analog systems such as the screen-film system.
Phase-contrast mammography (PCM) is a commercially available digital system that uses images with a magnification
of 1.75x. To study the effect of the air gap in PCM, we measured the scatter fraction ratio (SFR) and calculated the
signal-to-noise ratio (SNR) in PCM, and compared it to that in conventional mammography (CM). Then, to extend the
SNR to the spatial frequency domain, we calculated the noise equivalent quanta (NEQ) and detective quantum efficiency
(DQE) used by the modulation transfer function (MTF), noise power spectrum of the pixel value (NPSΔPV), gradient of
the digital characteristic curve, and number of X-ray photons. The obtained results indicated that the SFR of the PCM
was as low as that of the CM with a grid. When the exposure dose was constant, the SNR of the PCM was the highest in
all systems. Moreover, the NEQ and DQE for the PCM were higher than those for the CM (G-) in the spatial frequency
domain over 2.5 cycles/mm. These results showed that the number of scattered X-rays was reduced sufficiently by the air
gap in the PCM and the NEQ and DQE for PCM were influenced by the presampled MTF in the high-spatial-frequency
domain.
Mammography techniques have recently advanced from those using analog systems (the screen-film system) to those using digital systems; for example, computed radiography (CR) and flat-panel detectors (FPDs) are nowadays used in mammography. Further, phase contrast mammography (PCM)-a digital technique by which images with a magnification of 1.75× can be obtained-is now available in the market. We studied the effect of the air gap in PCM and evaluated the effectiveness of an antiscatter x-ray grid in conventional mammography (CM) by measuring the scatter fraction ratio (SFR) and relative signal-to-noise ratio (rSNR) and comparing them between PCM and the digital CM. The results indicated that the SFRs for the CM images obtained with a grid were the lowest and that these ratios were almost the same as those for the PCM images. In contrast, the rSNRs for the PCM images were the highest, which means that the scattering of x-rays was sufficiently reduced by the air gap without the loss of primary x-rays.
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