Post on 07-Feb-2021
transcript
Digital X-ray Detectors
Ho Kyung Kimhokyung@pusan.ac.kr
Pusan National University
Introduction to Medical Engineering
Outline
• Charge‐coupled device (CCD)
• Amorphous silicon (a‐Si) thin‐film‐transistor (TFT) pixel array
• Complementary metal‐oxide‐semiconductor (CMOS) pixel array
• Mammography
• Digital tomosynthesis
2
Digital radiography
• CCD‐based systems– Charge‐coupled device
• TFT‐based systems– Thin‐film transistor
• CMOS‐based systems– Complementary metal‐oxide‐semiconductor
3
CCD‐based systems
• 2‐D CCD detector arrays coupled with a scintillator such as Tl‐doped CsI or CsI(Tl)– CCD reads out the charge signals that are stored in light‐sensitive capacitors by a linear charge‐
transfer scheme– Inherent small pixel pitch & device size because of noise that is added in the charge‐transfer
mechanism w/ larger detector areas
4
5
• Remedies for the restricted FOV due to small‐area CCDs– Use of a lens to demagnify the scintillator image to be fitted to the CCD array by ~10:1
• e.g., from 44 cm 44 cm FOV to 4 cm 4 cm CCD w/ 3,000 3,000 pixels (pixel pitch = 0.013 mm)• > 99% loss of light at demagnification => higher quantum noise
– Slot‐scan geometry• multiple‐line linear CCD arrays long enough to cover the entire FOV in one dimension (horizontal)• coupled to a scintillator using fiber optic “light pipes” w/ demagnification of 2:1 or 3:1 (optional)• (342 68 pixels in a CCD array w/ a pixel pitch of 0.162 mm) 8 arrays = 44 cm (5.5 cm 8) 1.1 cm FOV• fan‐beam scanning using collimators (8–15 seconds)• breath hold to minimize motion artifacts
6
M. J. Yaffe & J. A. Rowlands | PMB | 1997
7
CCD‐based DR system CCD‐based time delay & integration system
TFT‐based systems
• Thin‐film transistors– Realized by depositing thin films of a semiconductor material, a dielectric, and metal conductors
onto glass– 2‐D TFT array in LCD & LED TV systems to control liquid crystal & light‐emitting diode in each pixel– 2‐D TFT array in flat‐panel detector to read out a charge that has accumulated at each pixel due to
x‐ray flux
8
• Amorphous materials– Availability in large area in a morphological form of short‐range order/long‐range disorder – Lower fabrication cost compared to crystalline counterparts – Better radiation hardness than crystalline counterparts– Worse electrical properties than crystalline counterparts– Charge trapping through dangling bonds
• Hydrogenated amorphous silicon, a‐Si:H
SiHydrogenated
Uncoupled
Void
9
Scintillators
• Particle‐in‐binder or powdered phosphors– Gd2O2S:Tb– Easy to manufacture– Very physically robust– Light scattering– Increasing thickness: reducing resolution &
increasing noise (Lubberts effect)– Depth‐dependent escape efficiency:
increasing noise (Swank noise)
• Structured phosphors– CsI:Tl– Light guiding– MG = ~ 200 𝜇m– DX = ~500–600 𝜇m
4 𝜇m
Phosphor
Binder Air gap
10
• TFT‐based flat‐panel detectors– Indirect detection method using scintillators e.g., CsI(Tl)
• Each pixel = TFT + amorphous silicon photodiode• X‐ray to light in CsI(Tl) & light to charge in a‐Si PD
– Direct detection method using photoconductors e.g., amorphous selenium• Each pixel = TFT + capacitor• X‐ray to electrons in a‐Se & induced charge in capacitor
11
12
• Lateral chest (120 kVp)
Courtesy J. Yorkston | Carestream Health
500 𝜇m CsI:Tl 500 𝜇m a‐Se
13
CMOS‐based systems
• CMOS active‐pixel sensor (APS)– lower cost, lower power consumption, tolerant to x‐ray irradiation, fast pixel readout– Pixel = PD + 3 more MOSFET transistors– Extra transistors to amplify the signal (conversion into voltage) prior to readout
• lower noise & faster readout– Pixel binning e.g., 4 4 pixels (e.g., 0.075‐mm pixel pitch to 0.3‐mm pixel pitch)
• much faster readout for real‐time applications– Indirect detection in conjunction with scintillators– 290 mm 230 mm FOV suitable for mammography
14
24.1 cm
17.1 cm
70 m
Pre‐ & post processing
Raw image Corrected gain/offset Interpolated bad pixels/lines Post-processed
Taken from J. A. Rowlands' Slides 15
Portable DR detectors
• Rechargeable battery power sources• Wireless Ethernet connectivity• Retrofit into existing table & wall‐mounted systems
16
Digital advantages
17
18
Fluoroscopy w/ a flat‐panel detector
M. Overdick | Philips | IWoRID 2002 19
Mammography
• For early detection of breast cancer– Direct detection of tumors– Detection of microcalcifications (tiny deposits of calcium), indicating the presence of breast cancer
• Features– Low‐energy x‐rays (about 30 kVp)– Beast compression to produce a reduced & more uniform (& somewhat more standardized across
patients) thickness organ for imaging– Automated intensity control system that optimizes detector exposure– Computer‐aided detection & diagnosis (CAD) algorithms
• Advances– Tomosynthesis– Stereo digital mammography
20
(relative to glandular)
21
22
The thickest part of the breast (at the chest wall) is positioned below the cathode, which helps equalize x‐ray intensities reaching the detector that are transmitted through the breast in the cathode‐anode direction
• Breast compression– to reduce scatter– to reduce radiation dose– to reduce exposure dynamic range
• uniform breast thickness– to spread out superimposed anatomy
• effective for spot compression
23
• Mammographic spectra
24
A AA A
A A
𝐾 17.5
𝐾 19.6
Less dense & thinner breast Denser & thicker breast
𝐾 20.2
𝐾 22.7
inappropriate
8 – 12 keV
• Digital tomosynthesis– 7.5 – 30 (15 – 40 images)
25
Wrap‐up
• Charge‐coupled device (CCD)
• Amorphous silicon (a‐Si) thin‐film‐transistor (TFT) pixel array
• Complementary metal‐oxide‐semiconductor (CMOS) pixel array
• Mammography
• Digital tomosynthesis
26