Studio delle prestazioni di un sistema a fosfori per mammografia ...

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photons will reach the detector and the radiation dose to the tissue will be very high in order to obtain a signal to noise ratio similar to the one obtained with a more energetic beam. The choice of energy will therefore be a compromise between the requirements of low dose and high contrast [3]. At the energy values typical of radiography, the most important photons interactions are the photoelectric effect and scattering. In soft tissue the photoelectric cross section is larger than the scatter cross section for energies up to about 25 keV. The photoelectric cross section varies approximately as the fourth power of the atomic number and inversely as the third power of the photon energy and it shows discontinuities at the absorption edges. The photon scattering cross section varies more slowly with energy than does the photoelectric cross section and is approximately proportional to atomic number [4]. The voltage applied to a X-ray tube dedicated to mammography is usually between 20 and 30 kVp (Vmax), depending on the clinical diagnosis and patient morphology. The X-ray radiation is generated by Bremmstrahlung and by an X-ray emission characteristic of the anode material. The anode material used in mammography X-ray units is molybdenum, since, with respect to tungsten targets, it produces lower energy X-rays, which are more appropriate for imaging thinner body sections at high contrast. Usually the spectrum is attenuated by an added molybdenum filter (see Chapter 3.1 for the system under investigation) which reduces both the X-ray component above its K-edge of about 20 keV and the lower energy components. This is important since it reduces the soft component of the spectrum, which contributes only to the patient dose and not to image contrast. 1.2 The imaging plate IP The Imaging Plate is the detecting element of a CR system. It is a multilayer film composed of a support layer and a sensible layer made of photo-stimulable phosphors. Photo-stimulable phosphors, also known as storage phosphor, is one of the most successful detectors for digital radiography to date. The photo-stimulable phosphor (PSP) stores the absorbed X-ray energy in crystal structure traps. The trapped energy can be released in a later moment if stimulated by additional light 7
energy of the proper wavelength by the process of photo-stimulated luminescence (PSL here- after). The output signal shows a linear dependence to the absorbed dose. Furthermore the IP maintains this linearity over a wide range of exposures (four orders of exposure magnitude). The PSP is placed in a protective, light-tight cassette with the same geometrical characteristics of a film-based radiographic cassette, so that it can be exposed in a radiographic equipment. Using X-ray imaging techniques similar to screen film imaging, a latent image, in the form of trapped electrons is imprinted on the PSP receptor by absorption of the photons transmit- ted through the object. At this point, the unobservable latent image is processed by inserting the IP cassette into an image reader, where the PSP plate is extracted from the cassette and raster-scanned with a highly focused red laser light. A higher energy, low intensity blue photo- stimulated luminescence signal is emitted, the intensity of which is proportional to the number of x-ray photons that were absorbed in the local area of the receptor. The PSL signal is channeled to a photomultiplier tube, converted to a voltage, digitized with an analog to digital converter, and stored in a digital image matrix. After PSP detector is totally scanned, analysis of the raw digital data locates the pertinent areas of the useful image. Scaling of the data with well-defined computer algorithms creates a greyscale image that mimics the analog film image. Finally, the image is recorded on film, or viewed on a digital image monitor. The described steps of the PSP reading process are summarized in Fig. 1.3. PSP devices are based on the principles of photo-stimulated luminescence. When an X- ray photon deposits energy in the PSP material, the energy can be released by three different physical processes. Fluorescence is the prompt release of energy in the form of light. This process is the basis of conventional radiographic intensification screens. PSP imaging plates also emit fluorescence in sufficient quantity to expose conventional radiographic film [5] [6], however this is not the intended method of imaging. PSP materials store some of the deposited energy in defects in their crystal structure, thus they are sometimes called storage phosphors. This stored energy constitutes the latent image. Over time, the latent image fades spontaneously by the process of phosphorescence. If stimulated to light of the proper wavelength, the process of stimulated luminescence can release the trapped energy. The emitted light constitutes the signal for creating the digital image [7]. Many compounds possess the property of PSL. Few of these materials have characteristics 8
Page 1 and 2: UNIVERSITÀ DEGLI STUDI DI FIRENZE
Page 3 and 4: 3.6 Histogram analysis and IP sensi
Page 5 and 6: made, allowing the diffusion of the
Page 7 and 8: Figure 1.1: Block diagram of the im
Page 9: function and to replace the very ge
Page 13 and 14: Figure 1.4: Energy levels of the PS
Page 15 and 16: Figure 1.5: Single-side reading met
Page 17 and 18: digital levels. The system under in
Page 19 and 20: ¯ multiplying � Ü� Ý by a co
Page 21 and 22: frequency properties of our system.
Page 23 and 24: on the gain £ and on the function
Page 25 and 26: (a) (b) (c) -u N MTF pre MTF pre -u
Page 27 and 28: (a) (b) (c) -u N -u N -u N MTF p re
Page 29 and 30: -2u N Re { OTF } dig u1 u2 u3 uN 2u
Page 31 and 32: for every exposition. The output va
Page 33 and 34: The experimental data will be analy
Page 35 and 36: Console Value Measured Value (kVp)
Page 37 and 38: mAs Dose (�Gy) mAs Dose (�Gy) 2
Page 39 and 40: Al thickness Dose (mm) (�Gy) 0.0
Page 41 and 42: (a) (b) Figure 3.4: X-ray spectrum
Page 43 and 44: Figure 3.6: The graphical workstati
Page 45 and 46: theless a set of cassettes has been
Page 47 and 48: ing plate is very wide, a variable
Page 49 and 50: Chapter 4 Measurement of the physic
Page 51 and 52: Figure 4.1: Section along the short
Page 53 and 54: Figure 4.4: Pixel Value plotted as
Page 55 and 56: was filtered by a 30 mm block of PM
Page 57 and 58: 17 13 9 5 1 18 14 10 6 2 19 15 11 7
Page 59 and 60: y the pixel size (� �m) and the
Page 61 and 62:
Figure 4.13: Comparison between MTF
Page 63 and 64:
4.3 EMTF The phase dependence of Å
Page 65 and 66:
Figure 4.18: Differences between EM
Page 67 and 68:
Figure 4.20: 2 dimensional NPS The
Page 69 and 70:
The Noise Equivalent Quanta was com
Page 71 and 72:
Figure 4.26: NEQ scan vs NEQ subsca
Page 73 and 74:
Figure 4.29: DQEsubscan. In Fig. 4.
Page 75 and 76:
Conclusions The work performed in t
Page 77 and 78:
Bibliography [1] M.J. Yaffe and J.A
Page 79 and 80:
[21] “Characteristics of digital
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