**3. Parameters with the greatest impact on dose and image quality**

The objective of mammography is to provide the early detection of cancer, and therefore the image quality is a fundamental aspect. The image with a suitable diagnostic-quality has to be acquired with a radiation dose as low as possible. Nowadays, it is widely accepted that mean glandular dose (DG) is the best indicator to estimate the risk associated with breast irradiation in mammography (see definition in SectionV). Both factors, image quality and DG, are depending on breast characteristics (glandularity and thickness), exposure factors (beam quality, exposure time and compression force), detector features and mammography system performance (automatic exposure control) and characteristics (geometry, focal spot size).

#### **3.1 Characteristics of the breast**

Breast composition varies among women due to different proportion of glandular, fibrous and adipose tissue. The composition also changes with the age of the woman such that the proportion of adipose tissue increases with age.

Image Quality Requirements for Digital Mammography in Breast Cancer Screening 121

For both digital and screen-film detection techniques, the energy spectrum of the X-ray beam (including their filtering) is of a great concern when evaluating the performance of the mammographic systems, demanding careful and accurate quality controls in a clinical

Fig. 6. X-ray spectra for 30 kVp operating potential for Mo/Mo (a), Mo/Rh (b), Rh/Rh (c),

The attenuation coefficient of the glandular tissue is similar to that of the tumour tissue which makes difficult the visualization of smaller tumours; low energy X-ray beam is needed to demonstrate the subtle density differences between non-calcified normal and abnormal tissues. X-ray tubes of mammography systems are equipped with special

and W/Rh (d) source/filter assemblies (NCRP 149, 2005).

**3.4 X-ray spectrum** 

environment.

Differences among the x-ray attenuation properties of the different breast tissues can be observed at the breast images. Glandular and fibrous tissues are visualised in mammography as radio-opaque whereas the adipose tissues are observed as radio-lucent (dark). Therefore, given the same compressed breast thickness a dense breast (having a higher proportion of glandular tissues) absorbs a higher amount of radiation than an adipose breast.

The lesions of interest for diagnosis are microcalcifications, masses, asymmetries and distortions of the breast architecture. Microcalcifications are small (100 µm) and "easily" detected regardless of breast density. The masses tend to have low contrast, making it difficult to detect. Therefore, mammography must have - besides optimal resolution - good contrast, which makes visible those anatomic structures and pathological signs which have very similar densities.

#### **3.2 The compression system**

A proper compression of the breast is fundamental to provide a good quality image. Breast compression brings the structures close to the detector enhancing sharpness, prevents breast movement, reduces the breast thickness penetrated by X-rays and reduces the scattered radiation. All these factors improve the contrast and even reduce the absorbed dose (Karellas, 2004). In addition, the exposure factors in modern mammography systems are automatically selected in base of the compressed breast thickness. Quality control programmes have to include procedures to verify the compression force and the accuracy of the breast thickness determined by the system.

#### **3.3 Automatic exposure control**

In mammography, the automatic exposure control (AEC) (also known as "photo timer") cuts off the exposure when arrives to the AEC radiation sensor, which lies below the antiscatter grid and the image receptor, the necessary dose resulting in optimum optical density or pixel value. The sensor can be placed at several positions (3 in most of the units) between the thorax and the nipple in depending of the breast size.

Flat panel detectors of digital mammography units operate as AEC sensors. In this case, the information can be derived from the whole area of the detector or from predefined regions.

The AEC of most modern mammography units uses the information associated with a preexposure to determine the attenuation of the breast. This information along with the breast thickness automatically detected (compressor) determines all the exposure factors (anode/filter, kV, mAs).

The AEC is committed to provide images with an appropriate optical density, independently from the beam quality and the characteristics of the breast. Thus, there is a guarantee that the information will be registered in the linear region of the characteristic curve of the film.

In digital systems, the main role of the AEC is to assure that signal noise ratio (SNR) and contrast noise ratio (CNR) are adequate throughout the image and that the dose values comply with recommendations.

#### **3.4 X-ray spectrum**

120 Imaging of the Breast – Technical Aspects and Clinical Implication

Differences among the x-ray attenuation properties of the different breast tissues can be observed at the breast images. Glandular and fibrous tissues are visualised in mammography as radio-opaque whereas the adipose tissues are observed as radio-lucent (dark). Therefore, given the same compressed breast thickness a dense breast (having a higher proportion of

The lesions of interest for diagnosis are microcalcifications, masses, asymmetries and distortions of the breast architecture. Microcalcifications are small (100 µm) and "easily" detected regardless of breast density. The masses tend to have low contrast, making it difficult to detect. Therefore, mammography must have - besides optimal resolution - good contrast, which makes visible those anatomic structures and pathological signs which have

A proper compression of the breast is fundamental to provide a good quality image. Breast compression brings the structures close to the detector enhancing sharpness, prevents breast movement, reduces the breast thickness penetrated by X-rays and reduces the scattered radiation. All these factors improve the contrast and even reduce the absorbed dose (Karellas, 2004). In addition, the exposure factors in modern mammography systems are automatically selected in base of the compressed breast thickness. Quality control programmes have to include procedures to verify the compression force and the accuracy of

In mammography, the automatic exposure control (AEC) (also known as "photo timer") cuts off the exposure when arrives to the AEC radiation sensor, which lies below the antiscatter grid and the image receptor, the necessary dose resulting in optimum optical density or pixel value. The sensor can be placed at several positions (3 in most of the units) between

Flat panel detectors of digital mammography units operate as AEC sensors. In this case, the information can be derived from the whole area of the detector or from predefined

The AEC of most modern mammography units uses the information associated with a preexposure to determine the attenuation of the breast. This information along with the breast thickness automatically detected (compressor) determines all the exposure factors

The AEC is committed to provide images with an appropriate optical density, independently from the beam quality and the characteristics of the breast. Thus, there is a guarantee that the information will be registered in the linear region of the characteristic

In digital systems, the main role of the AEC is to assure that signal noise ratio (SNR) and contrast noise ratio (CNR) are adequate throughout the image and that the dose values

glandular tissues) absorbs a higher amount of radiation than an adipose breast.

very similar densities.

**3.2 The compression system** 

**3.3 Automatic exposure control** 

regions.

(anode/filter, kV, mAs).

comply with recommendations.

curve of the film.

the breast thickness determined by the system.

the thorax and the nipple in depending of the breast size.

For both digital and screen-film detection techniques, the energy spectrum of the X-ray beam (including their filtering) is of a great concern when evaluating the performance of the mammographic systems, demanding careful and accurate quality controls in a clinical environment.

Fig. 6. X-ray spectra for 30 kVp operating potential for Mo/Mo (a), Mo/Rh (b), Rh/Rh (c), and W/Rh (d) source/filter assemblies (NCRP 149, 2005).

The attenuation coefficient of the glandular tissue is similar to that of the tumour tissue which makes difficult the visualization of smaller tumours; low energy X-ray beam is needed to demonstrate the subtle density differences between non-calcified normal and abnormal tissues. X-ray tubes of mammography systems are equipped with special

Image Quality Requirements for Digital Mammography in Breast Cancer Screening 123

Fig. 7. Modulation transfer function of mammography systems (ICRU, 2009).

(i.e., the sd is related to the square root of the Kerma) (Chevalier, 2010).

The main component of noise in radiographic images is the quantum noise, which is associated with the statistical fluctuations in the photons' fluence on the detector and with the random variations in the absorption. The simplest way to define the noise is through a measurement of the standard deviation (sd) of the number of photons absorbed (N) in a region of the detector. This figure complies with Poisson statistics, and therefore sd = N0.5

The structural noise in the digital detectors emerges mainly from the lack of homogeneity in the sensitivity of the elementary detectors (i.e., from the fixed spatial variation of the image detecting structure), which means that it is also proportional to the dose. Moreover, this noise causes the appearance of a structured background in the image, which is usually removed through the flat field techniques. These techniques include the creation of a corrective mask from a direct and uniform image of the X-ray beams (Chevalier, 2010).

In digital systems, what has to be added to these two types of noise is the electronic one, which emerges from the electronic readout outside the pixels and in the amplification of the signal and which does not depend on the dose. The electronic noise owes essentially to the dark noise in the detectors and decreases if the temperature of the surroundings is lowered or reduced. Therefore, the digital equipment has to operate in temperatures at intervals

The initial operation which usually occurs is a "flat-field", a correction of the uniformity of gain. The non-uniformity of the sensitivity of the detector is corrected through a gain map and is also used to correct all the images acquired. Moreover, if an element of a single (pixel) detector is defective its signal can be replaced with a reasonable combination of adjacent detector signals. This is acceptable if the defective detectors are isolated and only few of them are faulty. Detectors of the CR type presents a lack of uniformity due to the heal effect

**4.2 Noise** 

between 20-30 ºC.

that is very depending on the X-ray unit.

**4.3 Uniformity** 

anode/filter combinations, such as Mo/Mo or Mo/Rh, operating in the 25-35 kVp range. The spectra of several anode/filter combinations (Fig. 6) show the important proportion of of X-rays characteristics (17.5 and 19.7 keV) from the molybdenum target and the strong suppression of the spectrum at energies >20 keV because of the k-shell absorption edge of the molybdenum filter (Fig. 6a) or else higher than 23 keV because of the Rh filter (Fig. 6b). The characteristics of the new digital detectors make it possible to use other anode/filter combinations such as Rh/Rh (Fig. 6c), W/Rh (Fig. 6d), W/Ag and W/Al which have some advantages for imaging dense or thick breasts. In addition, the breast doses associated with these combinations are lower than those delivered with Mo/Mo or Mo/Rh.

The adequate selection of the spectrum (beam) may reduce the dose values above 20% (Young, 2006; Dance, 2000; Riabi, 2010). The threshold value for breast thickness where the spectrum is changed depends on the AEC calibration which is performed by technical services who install the equipment (which should be done together with those medical staff who use the equipment). The correct selection of the X-ray beam will strongly influence the dose and image quality.
