Detector technology and image quality
3.4. Flat panel detectors
Flat panel detectors are integrated into the mammography x-ray unit and the detector dimensions are usually up to 24 x 30 cm to cover the whole field. Flat panel detectors consist of a base of a thin film transistor (TFT) read out array plus a convertor layer that convert the x-ray photons to a signal.
3.4.1. Indirect detector: Phosphor
Phosphors are commonly used in digital detectors, the x-ray photons are absorbed in the phosphor and light photons are then immediately emitted. The phosphor used in an indirect x-ray capture flat panel mammography system is normally thallium doped caesium iodide (CsI(Tl)) (fig. 3.1). When an x-ray photon is absorbed by the phosphor, generally over 1000 fluorescent light photons are released. There may also be a reflective top surface of the phosphor to reduce the loss of photons.
X-ray photon S tim u la te d light R e lle c tiv e la y e r B o n d in g lay er y / P h o to d io d e , T F T
s w itc h e s & d ata lines
G lass su b stra te
Fig. 3.1. Caesium iodide detector and TFT array; the stimulated light is emitted isotropically, diagram show effect of columnar structure on a few of the light photons
3.4.2. Direct x-ray capture detector: amorphous-selenium
Amorphous selenium (a-Se) is a photoconductor that produces electrons and holes following absorption of an x-ray photon and is used for mammography detectors (Zhao and Rowlands 1995). This is advantageous as the x-ray photons are directly converted to electrical charge without the need for any intermediate energy conversion stage such as light. An electric field is applied across the a-Se layer such that the electrons are attracted to the top surface and the holes are attracted to the read out array (fig. 3.1).
X-ray photons E lectric - . 1 Field + Electrode Dielectric layer Amorphous Selenium layer Electrodes, TFT switches & data lines
C le a rin g lig h t G la ss su b stra te
Fig. 3.2. Amorphous selenium detector with TFT read out
Amorphous selenium has a high detection fraction at mammographie energies (Zhao and Rowlands 1995). However, this type of detector is not used in general radiography due to the relatively low atomic number of selenium compared to other x-ray image detectors and consequently its sensitivity drops off rapidly with tube voltage (Lawinski et al 2005).
The fill factor (section 3.4.4) may be an indication of the proportion of signal that is lost. This is only true when the secondary quanta are light photons. For a-Se detectors, some of the initial holes produced will be trapped above the non-sensitive part of the TFT, however, this creates an electrostatic barrier and so the subsequent charge is repelled and moves to the read out electrodes. Zhao et al (2003) showed for a 85 pm pixel pitch detector that the effective fill factor was close to 100%.
There is an issue that some of the holes and electrons can become trapped in the bulk selenium, which can cause ghosting (Zhao et al 2005). Ideally the filled energy traps in the detector should be cleared before the next exposure or a ghost of the previous image may be seen. The traps are cleared by irradiating the detector with light from behind the TFT array after detector read out. An exception is in tomosynthesis, where there is not sufficient time to clear the detector between exposures and lag and ghosting effects can be measured (Mackenzie et al 2013).
3.4.3. Flat panel read out device
The electronics, consisting of a 2D array of elements sensitive to the secondary quanta plus data and control lines, are laid on a glass plate using sold state manufacturing techniques. These sensitive elements of the physical detector are often referred to as pixels (picture elements) although technically this should refer to the display. The detector elements are either electrodes or photodiodes, depending on the secondary quanta to be detected. The charge produced is stored in a capacitor until the image is read out. There are rows of control lines and when a voltage is applied to a row the TFT switches on that row
are activated and the stored charge is read out via columns of data lines to the external electronics. The signal is then amplified and digitised, before the next row of pixels is read out. The read out procedure is shown in fig. 3.3 where the control line of row 2 has a +10V applied and the data in that row is read out. The electronic components are manufactured using amorphous silicon (a-Si) technology and so these flat panels are often called a-Si/TFT arrays. -5V + 10V -5V Data lines / \
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1
1
1
_ / _ / .C ontrol line 3 I Pixels to be read Pixels being read X o n tro l line 2 I Empty pixelsControl line
External Electronics
Fig. 3.3. Read-out mechanism of a-Si/TFT array. Control line 2 has been activated and the pixels in that row are read out. Control line 1 had been previously activated and the associated pixels emptied.
3.4.4. Pixel fill factor and size ofpixels
A proportion of the area of each pixel is taken up by electronics (TFT switch, control and data lines) which reduces the space for detection of the secondary quanta. The proportion of sensitive area to the area of the detector is known as the fill factor. The size of the fill factor will depend on the pixel pitch and the design of the electronics, but figures of 75% for a 100 pm pixel pitch detector using caesium iodide (Csl) phosphor (Muller 1999) and 70% for a 85 pm pixel pitch detector using a-Se photoconductor (Polischuk et al 1999) have been reported. Generally the number of secondary quantum per absorbed x-ray photon is large and so the loss of some of the secondary quanta does not necessarily adversely affect the image quality. However, if smaller pixel pitches are required the fill factor will increase which may adversely affect the image quality.