Dispositivi e sistemi per la radiografia digitale

Radiografia digitale
Radiografia digitale
Sistema ideale: Acquisizione istantanea di una immagine
in forma digitale con il minimo numero di raggi X
Figure 1. Chart illustrates a digital radiography system. After image exposure, the
imaging data are digitally processed and
stored in a digital archive. A centralized
image management system is used for further distribution of the images to viewing
stations, information systems, and electronic patient records.
Computed Radiology
(Storage Phosphorous Radiology)
a systematic overview of
ctors. CCD chargepanel detector, TFT
Figure 3. D rawing illustrates a CR system based on
storage-phosphor image plates. Image generation is
separated into two steps. First, the image plate ( IP) is
exposed to x-ray energy, part of which is stored within
the detective layer of the plate. Second, the image plate
is scanned with a laser beam, so that the stored energy
Computed Radiology
(Storage Phosphorous Radiology)
Radiografia analogica vs
digitale
Radiografia digitale
le 1
ameters for Digital X-Ray Imaging Systems. (Data From Rowlands and Yorkston)
Specifiche
collecting pixels depending on the depth of absorption.
r an a-Se photoconductor of thickness 200–1000 m and
3) The dark current should be negligibly small. T
means the contacts to the photoconductor should
Necessità di larga area
• Soluzioni
– accoppiamento ottico
“fosfori+lenti+Fotomoltiplicatore/CCD”
– matrici attive in selenio/silicio amorfo
Necessità di larga area
• Soluzioni
– accoppiamento ottico
“fosfori+lenti+Fotomoltiplicatore/CCD”
– matrici attive in selenio/silicio amorfo
Conversione diretta
Conversione indiretta
Matrice di sensori attivi a conversione diretta
Fotoconduttori
Requisiti per fotoconduttori
Alto coefficiente di assorbimento: d<<L
Alta sensibilità
No ricombinazione
No trappole: mtE>> L
Bassa corrente dark: no iniezione dagli elettrodi, no generazione termica
No degradazione
Facilità di deposizione
Profondità di penetrazione
Table 2 summarizes the absorption dep
didate X-ray photoconductor materials a
20 keV (mammographic
X-ray) and 60
n
» 3 requires th
The minimization of n
dosage
such that the most of the radiation is ab
thickness or
. This means that
and, hence, the particular imaging app
-n
cations of the K andnL»edges
3- 4of the X-r
material. The K edge of a-Se is 12.7 keV
ularly useful for mammographic applicati
keV. For mammorgraphy,
diology with mean photon energy of 60 k
For comparison, the corresponding thick
detector are about 60 and 540 m, resp
as is increased, there is an increased p
freed charges will be trapped as they drift
tances to reach the electrodes, i.e., the s
come schubweg-limited, as discussed in S
d»E
dµZ
Fig. 8. Attenuation depth ( m) versus photon energy (keV)
for various materials. Attenuation coeffients calculated by
the authors using elemental mass attenuation coefficients and
V. X-RAY SENSITIVITY
The total collectable charge
gene
Energia di ionizzazione
W± = 2.8× Eg + E phonon
E phonon = 0.5eV
Fig. 9. EHP creation energy versus energy bandgap
materials.
for various
Fig. 10.
v
energies as measu
obtained from ra
Sm, 48.8 fluoresc
from Am-241), 5
m. 74.1-keV fluor
m [14].
Sensibilità
Q
S=
AX
Materiali per fotoconduttori
Matrice di sensori in selenio amorfo
Sensore in selenio amorfo
Polarizzazione negativa
tic diagram representing the equivalent circuit of the photoconductive pixel detector.
Source
Drain
n a-Si:H
i a-Si:H
Gate insulator
Substrato
Gate
Polarizzazione positiva
Polarizzazione positiva
Matrice di sensori in silicio amorfo
Matrice di sensori in silicio amorfo
Matrici di sensori in a-Si:H: Scintillatori
• Sono scintillatori inorganici: NaI, CsI, Bi4Ge3O12 (noto come BGO),
PbWO4, BaF2...
• Il meccanismo di scintillazione negli scintillatori inorganici è
caratteristico della struttura a bande elettroniche che si trovano nei
cristalli.
Spesso si hanno 2 costanti di tempo:
1.ricombinazione rapida dai centri di
attivazione (ns-μs)
2.ricombinazione ritardata (trappole)
(~100 ms)
Matrice di sensori in silicio amorfo
Scintillatori
X-ray
Reflector
• Segnale dipende da:
– coefficiente di conversione
– efficienza quantica
– self-absorption
S
SCATTERING
SELF ABSORPTION
Matrice di sensori in silicio amorfo
CsI:Tl
•
•
•
•
•
Spessori 550µm
Buon assorbimento dei raggi X (65000 ph/MeV)
Struttura colonnare: guide d’onde
1,0
MTF=50% @ 1 lp/mm
Emissione luce nel verde
0,8
0,6
a-Si:H
CsI:Tl
0,4
0,2
0,0
400 500 600 700 800
wavelength / nm
Matrice di sensori in silicio amorfo
ITO/Metal
n a-Si:H
i a-Si:H
p a-Si:H
Metal/ITO
Substrato
Matrice di sensori in silicio amorfo
Matrice di sensori in silicio amorfo
Row
Driver
Switch
PD
Vbias
Source
Drain
n a-Si:H
i a-Si:H
Gate insulator
Substrato
Gate
Matrice di sensori in silicio amorfo
Tecnologia planare
• Limited Fill-factor limitato (<75%)
TOP VIEW
CROSS SECTION
Tecnologia Multi-livello
• Fill-factor >90%
TOP VIEW
CROSS SECTION
66
ble 2 Characteristics of
me flat-panel detector sysms currently available
Differenti prodotti
Canon CXDI-11
Scintillator 200-µm thick (terbium-doped gadolinium dioxide sulphide)
Photodiode (semiconductor -type photoelectric converter made from hydrogenated amorphous
silicon a-Si:H)
TFT (made from hydrogenated amorphous silicon)
Each pixel consists of an a-SI TFT and a metal insulator semiconductorphotoelectric converter .
Indirect conversion of X-rays
2688´2688 pixels
43´43 cm
160-µm pixel size
4096 grey-scale image (14-bit resolution)
Trixell (Philips, Siemens, Thomson)
Structured scintillator 550-µm thickness (thallium doped cesium-iodide, CsI:Tl)
Photodiode (amorphous silicon)
Switching diode
Indirect conversion of X-rays
3000´3000 pixels
43´43 cm
143-µm pixel size
4096 grey-scale image (14-bit resolution)
General Electric Medical Systems (Milwaukee, W is.)
Scintillator (cesium-iodide)
Photodiode (amorphous silicon)+TFT
Indirect conversion of X-rays
2048´2048 pixels
41´41 cm
200-µm pixel size
DirectRay (Hologic, Kodak, Rochester , N.Y.)
Amorphous selenium 500-µm (photoelectric layer)
Capacitor+amorphous silicon TFT
Direct conversion of X-rays
2560´3072 pixels
35´43 cm
139-µm pixel size
2567
Prestazioni FPD
Table 3 Phantom studies with flat-panel detectors (FPD). ROC receiver operating characteristics; AUC area under curve of the ROC experiment; SPR storage phosphor radiography system; SFS screen-film systems
Reference
Type of study
Detector type
Results
[16]
Simulated bone erosions in a hand phantom
(holes of different sizes drilled in polymethyl
acrylate panels which were superimposed on
a hand phantom, i.e. low-contrast detection
task). ROC study with four observers, 7200
observations
CDRAD contrast detail phantom (four
alternative forced choice experiments).T est
signals: holes of different diameters and
depths up to 2.0 mm (i.e. high and low
contrast with different diameters)
CDRAD 2.0 phantom. Comparison of FPD
(at 400, 600 and 800 equivalent speed), SPR
(AC-3 with ST-V plates, at approximately
equivalent 200 speed) and a Kodak 160 speed
SFS. Digital images evaluated at a Sectra
Workstation with different monitors. Four
observers, each reading three images for each
setting. Comparison of image quality figure.
Determination of skin entrance doses
Comparison of images obtained with FPD
and a 400-speed SFS (Insight VHC, Kodak).
Anthropomorphic chest phantom with
simulated lung structure and superimposed
nodular, micronodular, linear and reticular
patterns. 480 observation fields for each
modality. Four observers. ROC analysis
Detection of foreign bodies (glass with and
without lead, bone fragments, aluminium,
iron, copper, gravel, graphite) of dif ferent
sizes superimposed to fresh porcine meat.
Comparison of FPD (no spatial frequency
processing) at different simulated speed
classes to a 400-speed SFS (Lanex
Regular/T-MAT Plus DG film, Kodak). 400
observation fields per modality . Four
observers. One ROC curve for all foreign
bodies, no separate evaluation
Detection of cortical bone defects and
fractures. 232 tubular deer -bones with
mechanically induced fractures on 1 10 of
them and cortical bone defects on 1 12 of
them. Comparative images obtained with
identical exposure with the FPD and
400-speed SFS (Lanex Regular/T-MAT Plus
DG film, Kodak) and reduced exposure for
the FPD. Four observers
Prototype of the
Trixell/Siemens
detector
Compared with a FSS (Lanex regular screen and
T-MAT Plus DG film; Kodak), better diagnostic
performance of the FPD with same dose ( p<0.05).
No significant difference for FPD images obtained
with reduced dose (30% and 50% dose reduction)
GE Revolution
XQ/I
Compared to both Insight regular and Insight HC
the FPD shows better detection of test signals. T est
signals with low contrast are more frequently
detected with the FPD even when the dose is reduced
by 20%
FPD has equal image quality at less than half the
dose when compared with SPR, and at approximately
at one-fifth the dose when compared with SFS
[17]
[18]
[19]
[20]
[21]
Trixell/Philips
Reduced size
(15´15 cm)
prototype of
Trixell/Siemens
detector
For dose equivalent images, FPD performs better than
SFR for linear structures and micronodular opacities,
whereas no significant dif ference is detected for
nodules and reticular patterns. No significant
difference between full-dose SFS images and
half-dose FPD images was found
Reduced size
(15´15 cm)
prototype of
Trixell/Siemens
detector
At a simulated speed of 400, the FPD system
performs significantly better than the SFS. At a
simulated speed of 800 and 1200, no significant
difference between FPD and the 400-speed SFS was
detected. At a simulated speed of 1600, the FPD
system was significantly inferior to the 400-speed
SFS
Reduced size
(15´15 cm)
prototype of
Trixell/Siemens
detector
No significant difference could be found for the
detection of cortical defects and fractures, even with
dose reduced images obtained with the FPD detector
(at one-half, two-thirds and one-fifth of the dose).
Very high AUC; thus small dif ferences may not have
been detected
Prestazioni FPD
Sistemi statici
Large size (43cm x 43cm / 17" x 17") for high projection flexibility
even with large patients
Resolution up to 3.5 lp/mm, 143 μm pixel size
Sistemi dinamici
• Amplificatore di brillanza
– Pesante
– Sensibile ai campi magnetici
– Schermo curvo
Sistemi dinamici
• Flat detector
– Leggero
– Assenza di distorsione
– Miglior contrasto
– Miglior uso della dose