**2. Detectors for digital mammography**

#### **2.1 Flat panel Systems – DR**

Flat panel systems (DR) have an active matrix of electronic detectors where each element absorbs the radiation transmitted through mammary tissue, producing an electrical signal proportional to the intensity of the X-rays.

#### **2.1.1 Indirect capture of an image**

In the indirect capture of an image, a flat screen scintillator, a photodiode circuitry layer, and a TFT array are used (Fig. 1). Caesium iodide crystals (CsI(Tl)) are the scintillators usually employed. CsI(Tl) crystals are structured in an array of thin needles that guide the light photon reducing the light diffusion within the scintillator layer. The light is captured by the elements of the photodiode matrix (amorphous silicon), which converts the light into electrical current. These amorphous silicon sensors (a-Si) are connected to a matrix of thin-film transistors (TFT) which store the information of each pixel up to the moment of its reading by the scan circuit in the detector (Vedantham, 2000; Suryanarayanan, 2004; Peixoto, 2009).

#### **2.1.2 Direct capture of an image**

In the direct capture mode, an amorphous selenium plate (a-Se) photoconductor is used to convert the incident X-ray photons into electron-hole pairs (Yaffe, 1997, Peixoto, 2009). Each charge of the electron-hole pair created is attracted by the corresponding electrode under the action of the strong electric field applied between the electrodes. The created charge is accumulated and stored by a TFT matrix (Fig. 2).

#### **2.2 Computed radiography system – CR**

Computed radiography (CR) is a process comparatively similar to the conventional screenfilm system. The film is replaced by a plate (IP) made up of photostimulable phosphorus

guarantee a constant high quality of the mammography examination (ICRU, 2009; Ng, 2005). Conventional film/screen mammography is being gradually substituted by digital technology in most countries. Consequently, there is an important activity related with developing quality control protocols adapted to this new digital technologies (CEC,2006;

Data retrieved from programmes in the Netherlands (Beckers, 2003), Sweden (Leitz, 2001), Norway (Pedersen, 2000) and the UK (NHSBSP, 2003) show that the levels of DG in screenfilm mammography range between 0.8 and 2.5 mGy for a 5.3 cm compressed breast thickness. Thus, several national and international protocols have established an accepted DG limit of 2.5 mGy for a 5.3 cm standard breast thickness. Data from a European survey (Report EUR 14821, 2001) on radiation doses developed in 56 mammography institutions showed DG values ranging from 1.0 to 3.0 mGy for 6.0 cm thick breasts. This value was

This chapter is devoted to describe the relevant parameters and procedures for the quality control of digital mammography systems making the necessary distinctions among the two

Flat panel systems (DR) have an active matrix of electronic detectors where each element absorbs the radiation transmitted through mammary tissue, producing an electrical signal

In the indirect capture of an image, a flat screen scintillator, a photodiode circuitry layer, and a TFT array are used (Fig. 1). Caesium iodide crystals (CsI(Tl)) are the scintillators usually employed. CsI(Tl) crystals are structured in an array of thin needles that guide the light photon reducing the light diffusion within the scintillator layer. The light is captured by the elements of the photodiode matrix (amorphous silicon), which converts the light into electrical current. These amorphous silicon sensors (a-Si) are connected to a matrix of thin-film transistors (TFT) which store the information of each pixel up to the moment of its reading by the scan circuit in

In the direct capture mode, an amorphous selenium plate (a-Se) photoconductor is used to convert the incident X-ray photons into electron-hole pairs (Yaffe, 1997, Peixoto, 2009). Each charge of the electron-hole pair created is attracted by the corresponding electrode under the action of the strong electric field applied between the electrodes. The created charge is

Computed radiography (CR) is a process comparatively similar to the conventional screenfilm system. The film is replaced by a plate (IP) made up of photostimulable phosphorus

SEFM, 2008;NHSBSP, 2009; IAEA, 2011).

established from measurements using an acrylic simulator.

**2. Detectors for digital mammography** 

proportional to the intensity of the X-rays.

**2.1.1 Indirect capture of an image** 

**2.1.2 Direct capture of an image** 

accumulated and stored by a TFT matrix (Fig. 2).

**2.2 Computed radiography system – CR** 

**2.1 Flat panel Systems – DR** 

technologies (computed radiography (CR) and flat panel detectors (DR)).

the detector (Vedantham, 2000; Suryanarayanan, 2004; Peixoto, 2009).

Fig. 1. Indirect method of image acquisition with CsI(Tl)/a-Si. The CsI scintillators hold needle structures and work as channels which guide the light perpendicularly to the surface of the photodiodes (Peixoto, 2009).

Fig. 2. Method of direct acquisition of an image with a-Se (Peixoto, 2009).

(PSP) which is introduced into a cassette of similar characteristics than the one used with the film.

Inside the cassette, the photostimulable phosphorus plate is used to absorb and store the energy of the X-ray transmitted through the breast, thus producing a 'latent image'. The energy stored in the phosphorous plate is associated to the electrons raised to excited levels of energy in which they hold trapped ("F-centre"). This is the non-observable electronic latent image, where the number of electrons trapped is proportional to the number of incident X-ray photons (Marcelino V.A. Dantas., 2010). As follows, the cassette is inserted into the reading unit (Fig. 3). Inside this unit, the plate is scanned with a low energy intense laser light (~ 2 eV) which is highly focused. The electrons trapped in the phosphorus photostimulable matrix (PSP) are stimulated by the laser energy, and a significant fraction

Image Quality Requirements for Digital Mammography in Breast Cancer Screening 119

The residual latent image information is erased through an intense light which removes the electrons not released by the laser stimulation, and the IP returns to the cassette "reset" and

The diagram of the whole process involving the image acquisition with a CR system is

Fig. 5. Acquisition process, processing and visualisation of mammography images CR

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

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

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

ready to be reused.

shown in Fig. 5 (left).

(Alvarenga, 2008).

size).

**3.1 Characteristics of the breast** 

proportion of adipose tissue increases with age.

returns to the lowest energy level with a simultaneous emission of a higher energy photostimulated luminescence (PSL) (~ 3 eV). The intensity of the PSL, proportional to the number of electrons emitted, is captured by a light guide system near the IP (Fig. 4). A photomultiplier tube (PMT) at the output of the light guide amplifies and converts the PSL into a corresponding output voltage (Rowlands, 2002; Dantas, 2010).

Fig. 3. Image digitiser for CR Systems (Alvarenga, 2008).

Fig. 4. Image acquisition for the CR system: (a) single reading; (b) double reading (Peixoto, 2009).

returns to the lowest energy level with a simultaneous emission of a higher energy photostimulated luminescence (PSL) (~ 3 eV). The intensity of the PSL, proportional to the number of electrons emitted, is captured by a light guide system near the IP (Fig. 4). A photomultiplier tube (PMT) at the output of the light guide amplifies and converts the PSL

Fig. 4. Image acquisition for the CR system: (a) single reading; (b) double reading (Peixoto,

into a corresponding output voltage (Rowlands, 2002; Dantas, 2010).

Fig. 3. Image digitiser for CR Systems (Alvarenga, 2008).

2009).

The residual latent image information is erased through an intense light which removes the electrons not released by the laser stimulation, and the IP returns to the cassette "reset" and ready to be reused.

The diagram of the whole process involving the image acquisition with a CR system is shown in Fig. 5 (left).

Fig. 5. Acquisition process, processing and visualisation of mammography images CR (Alvarenga, 2008).
