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

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Figure 3.7: The IP cassette. both manufactured by FUJI for computed radiography. This in order to increase the amount of X-ray absorption and to reduce the photon noise. In general, increasing the thickness of the phosphor layer reduces sharpness. However the FUJI IP compensates this effect by minimizing the phosphor grain size, which enhances sharpness [10]. A comparison between the single side reading and dual side reading IP layer structure is shown in Fig. 3.8. Figure 3.8: The IP layer structure for the FUJI High Resolution single and dual side reading plates. For all the measurements done, described in Chapter 4, the same IP has been used. Never- 41
theless a set of cassettes has been exposed to the same dose and the results have been compared in order to be sure of the uniformity of the response over a larger set of IPs. Prior of every exposition, the IP is inserted in the cassette reader and submitted to a complete cancellation cycle, so to minimize possible ghosts effects. Great care has been put in reading the IP with the same delay (approximately 30 seconds) after each exposure session so to reduce the decay effect of the stored information. 3.5 Image readout process The IP readout process starts when the cassette is inserted in the FCR5000 MA reader. The IP is internally extracted from the cassette box. A laser beam is focused on the sensitive part of the IP and scan the detector surface in a direction parallel to the longer side (scan direction). When the first row has been read, the plate moves a step corresponding to a pixel size in a direction parallel to the shorter side (sub-scan direction) so to start a new row acquisition. The laser source is a semiconductor device with a spectral emission centered on 660 nm and a power of 60 mW. The light beam is separated in two components by a beam splitter just outside the laser output window. The less intense components is directed onto a photo-detector that provides the reference signal necessary to deal with possible intensity fluctuations during the scanning operation. This is a key feature since the IP response is a function of the laser intensity arriving on the photostimulable phosphor layer. The main beam is directed towards an optical system (polygonal mirror, collimating lens, deflection mirror) that has the main goal to steer the bean on the desired portion of the IP and to maintain an uniform speed and focus on the plate. The laser spot size is about 50 �m, corresponding to the pixel size dimensions. The scan speed has an upper limit set by the phosphor decay time constant, equal to ��s. This poses a minimum limit to the time required for reading an IP with 50 �m pixel size and a 18cm¢24cm size (3540 ¢4740 pixels), that is about 12 sec. The total read-out time is in any case dominated by the feed-load-eject mechanical procedure. When the laser reaches the end of the scan line, it is repositioned to the beginning of a new line by the polygonal mirror. In the meantime the plate moves in an orthogonal direction with a 50 �m step. This process ends when the entire detector surface has been exposed to the laser light. The light emitted is proportional to the PSL 42
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 and 10: function and to replace the very ge
Page 11 and 12: energy of the proper wavelength by
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: Figure 3.6: The graphical workstati
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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