Current technology in digital image production (CR/DR and other modalities) Jaroonroj Wongnil 25 Mar 2016

Transcription

1 Current technology in digital image production (CR/DR and other modalities) Jaroonroj Wongnil 25 Mar 2016

2 Current technology in digital image production (CR/DR and other modalities) 2/ Overview Digital Image Computed Radiography (CR) System overview and imaging plate Direct Radiography (DR) System overview and detector characteristics Tomography Digital tomosynthesis Dual energy imaging Stitching image

3 Current technology in digital image production (CR/DR and other modalities) 3/ What is a digital image? A digital image can be thought of as an array of pixels (or voxels in 3D imaging) that each take a discrete value The value assigned is dependent on the X-Ray intensity striking it Depending on its value, each pixel is assigned a shade of grey Pixel size may determine the limiting spatial resolution of the system

4 Current technology in digital image production (CR/DR and other modalities) 4/ Pixels and Image Matrices Pixel = Picture element

5 Current technology in digital image production (CR/DR and other modalities) 5/ Pixels and Image Matrices # of pixels = # of rows x # of columns Pixel Data p(i,j) Rows ( i ) Columns ( j )

6 Pixel size, Matrix size and FOV Pixel size = FOV/ # of rows (columns) Ex. 35 cm x 43 cm FOV, 200 µm pixels: 5 pixels/mm Maximal 2.5 lp/mm Matrix size = 1750 x 2150 Smaller pixel size Larger image matrix Larger pixel size Smaller image matrix Pixel Value Pixel Value Row # Column Number Row # Column Number Current technology in digital image production (CR/DR and other modalities) 6/

7 Current technology in digital image production (CR/DR and other modalities) 7/ Pixel size vs. resolution Small pixels = better resolution 200 micron pixels = 2.5 lp/mm 50 micron pixels = 10 lp/mm Resolution will also depend on the physics of the acquisition system technology Low MTF High MTF

8 Current technology in digital image production (CR/DR and other modalities) 8/ Binary System 1 Byte = 8 bits = = = = = 255 No. of gray levels = 2^(number of bits) 10 bits 0 to 1023 (2 10 or 1024 discrete pixel values) 14 bits 0 to 16,383 (2 14 or 16,384 discrete pixel values)

9 Current technology in digital image production (CR/DR and other modalities) 9/ Bit-depth Pixels that are capable of being represented by a larger number of discrete numerical values will be more sensitive to smaller changes in contrast Fundamental limit of low contrast sensitivity 16,383 Pixel Values 4096 Pixel Value

10 Current technology in digital image production (CR/DR and other modalities) 10/ 8-bit (2 8 or 256 shades of grey) 4-bit (2 4 or 16 shades of grey)

11 Current technology in digital image production (CR/DR and other modalities) 11/ Storage Requirements Amount of storage required depends directly upon the product of the matrix size (pixel size and FOV) and the bit-depth: DR chest: 3.76 M pixels, 14 bits deep ~ 6.5 MBytes/Image FFDM: 7.2M pixels, 10 bits deep ~ 9 MBytes/Image

12 Summary: Pixel and Bit-depth 1024 x x x x 16 8 bits/pixel 3 bits/pixel 2 bits/pixel 1 bit/pixel For a image of M x N matrix, k bytes/pixel, the memory taken to store the image is kxmxn bytes Current technology in digital image production (CR/DR and other modalities) 12/

13 Current technology in digital image production (CR/DR and other modalities) 13/ DR has wide dynamic range (latitude) Density (OD) Intensity (rel) Film/screen DR Exposure (mr) Low kv L=1.8 Under-Exposed S=750 High kv L=2.2 Over-Exposed S= mr Raw Plate Exposure 1000 mr

14 Current technology in digital image production (CR/DR and other modalities) 14/ Wide dynamic range Under and Over exposure Fewer photons More noise Obscures low-contrast details More photons = More signal strength (signal-to-noise ratio improves) Beautiful images! High patient dose! Wide dynamic range can lead to higher patient dose

15 Current technology in digital image production (CR/DR and other modalities) 15/ Detector energy sensitivity Martin Yaffe/Tony Seibert 1.0 Gd 2 O 2 S:Tb 120 mg/cm 2 (Lanex) BaFBr 100 mg/cm² (CR) A-Selenium 25 mg/cm CsI:Tl 45 mg/cm 2 (a-si/csi) Photon Energy (kev)

16 Current technology in digital image production (CR/DR and other modalities) 16/ Imaging in Radiology Radiography Digital Radiography Conventional/ Analog Computed Radiography (CR) Direct Radiography (DR) Indirect conversion Indirect conversion Direct conversion Storage Phosphor Scintillator-TFT Photoconductor Scintillator-CCD Image intensifier

17 Computed Radiography (CR) The most common technique for producing digital images Replace the analog screen-film to digital image Very similar equipment to conventional radiography Cassette Imaging Plate (IP), Photostimulable Phosphors/ Photostimulable storage phosphor (PSP) CR Image acquisition: 1. X-Ray Machine 2. Imaging plate, PSP plate 3. Imaging plate reader CR Reader/Digitizer 4. Console and Viewing Current technology in digital image production (CR/DR and other modalities) 17/

18 Current technology in digital image production (CR/DR and other modalities) 18/ CR Image Acquisition The National Institute for Occupational Safety and Health (NIOSH)

19 Current technology in digital image production (CR/DR and other modalities) 19/ Imaging Plate Phosphor Plate Cassette holder PSP currently in use for CR are crystals of alkaline earth and halides doped with Eu +2 BaFBr:Eu +2 BaFBr 0.85 I 0.15 :Eu +2 Ba 0.86 Sr 0.14 F 1.10 Br 0.84 I 0.06 :Eu +2 BaFBr 0.8 I 0.2 :Eu +2 BaFI:Eu +2 RuBr:Tl CsBr

20 Electron Trapping & Image Read Out Photon is absorbed by an electron Electron can move through conduction band They can then be trapped in Colour Centres which forms our latent image Electrons are actively stimulated to release their stored energy This is done by scanning the IP with an intense laser beam A red Laser is used as this matches the energy gap between Colour Centre and conduction band Light in the blue end of the visible spectrum is emitted Blue light photons are collected via a photomultiplier tube and digital image is produced Current technology in digital image production (CR/DR and other modalities) 20/ 20

21 Current technology in digital image production (CR/DR and other modalities) 21/ Readout process Sub scan direction (Plate translation) Scan direction (Laser beam deflection)

22 Available computed radiography technology Current technology in digital image production (CR/DR and other modalities) 22/

23 Current technology in digital image production (CR/DR and other modalities) 23/ CR Reader/Digitizer Various capabilities, sizes, throughput

24 Current technology in digital image production (CR/DR and other modalities) 24/ Direct Radiography (DR)...refers to the acquisition and capture of the X-Ray image without user intervention (automatic electronic processing and display) Indirect detector: a conversion of X-Rays into light by a scintillator, and light into electronic signal Direct detector: a conversion of X-Rays to electronic signal

25 DR: Acquisition Technology Indirect x-ray detection (IDR) X-Rays interact with an intensifying screen, and secondary photons interact with sensors (a-si TFT or CCD) The screen causes more blurring CsI is more commonly used to improve spatial resolution. Direct x-ray detection (DDR) X-Rays interact with photon sensors directly Photoconductor (a-se) on top of TFT array After exposure, e - s are generated and migrated through the Se layer (+) to the TFT layer for readout Unlike CR, no mechanical readout process is involved (self-reading) Current technology in digital image production (CR/DR and other modalities) 25/

26 Current technology in digital image production (CR/DR and other modalities) 26/ Flat Panel Detectors (IDR/DDR) For IDR, scintillator causes blurring CsI(Tl) needle phosphors to focus light down to the detector IDR w/ CsI+a-Si TFT No blurring in Se due to the E- field For Se detector, E-field can be designed to direct the e s to sensitive areas of TFT

27 Charged-Coupled Devices X-Rays on scintillator Convert visible light to electronic signal CCD chip = solid state / photo diode Use multiple CCDs to cover image area Also thicker than flat panels due to the optics between the phosphor and detector Current technology in digital image production (CR/DR and other modalities) 27/ 27

28 Current technology in digital image production (CR/DR and other modalities) 28/ CCD/CMOS CCD VS CMOS, Dallmeier Electronic GmbH & Co. KG CMOS Note: Complementary metal-oxide semiconductor (CMOS) detectors -- electronic noise

29 Current technology in digital image production (CR/DR and other modalities) 29/

30 Flat Panel Detectors Current technology in digital image production (CR/DR and other modalities) 30/

31 MTF OF DR Current technology in digital image production (CR/DR and other modalities) 31/

32 DQE Current technology in digital image production (CR/DR and other modalities) 32/

33 Current technology in digital image production (CR/DR and other modalities) 33/ Technical Setting: Set up good AEC setting with vendor Check appropriate Exposure Index Values - Can be a dose monitor Tech training for DR Technology - Hardware, Software - Guidelines for proper technical settings What we do Image Archiving/Viewing Method Storage/Archiving No problem, good we have good PACS Viewing We do need QC on monitors

34 Current technology in digital image production (CR/DR and other modalities) 34/ Improve the hardware More efficient light photon conversion Efficient light conversion compound More efficient detector More efficient signal read out What can we do more? Efficient scintillator shape - High efficiency columnar CsI Optimized columnar structured CsI Minimized optical blurring to give high MTF performance ALDAS TFT-PIN panel (Advanced Low Dose Amorphous Silicon) - Reduced noise by lowering the metal line impedance - Improved signal with high efficiency PIN photodiode

35 What can we do more? Improve the software More efficient dynamic range control Digital Radiography has wide dynamic range (poor contrast) Re-scaling is necessary Would be nice if we can re-scale but still see whatever we wish! More efficient tissue contrast enhancement More efficient noise reduction processing Dynamic Range Input Histogram Output Histogram Region of Interest Under-penetrated Area Region of Interest Under-penetrated Area Region of Interest Under-penetrated Area Courtesy from Samsung Electronics Current technology in digital image production (CR/DR and other modalities) 35/

36 Current technology in digital image production (CR/DR and other modalities) 36/ Software: Region based contrast enhancement High Contrast Low Noise High Contrast Contrast Enhanced

37 Current technology in digital image production (CR/DR and other modalities) 37/ CR vs DR Attribute Conventional/Analog CR DR Installation Cost $ $$ $$$ Radiation reduction not possible possible definite reduction possible Fragility Spatial resolution (high quality monitors) DQE Dynamic range + (S Shaped curve) +++ (Linear/Wide) +++ (Linear/Wide) Post processing not possible possible possible PACS & teleradiology not possible possible possible Image display time (up to 25 sec) +(<10 sec)

38 Advanced Acquisition & Processing Techniques Tomography Digital tomosynthesis Acquisition from several projection angles Reconstruction of tomographic slices Dual energy imaging Tissue selective imaging Differential attenuation with energy Stitching image Current technology in digital image production (CR/DR and other modalities) 38/ 38

39 Tomography Conventional radiography superimposes structures In tomography, only structures in a selected plane of the patient, parallel to the film, are imaged sharply everything else above and below appears blurred to the point they become unrecognisable Blurring is produced by simultaneous movement of at least two of; The tube, The film and/or the patient Actively exploit motion unsharpness that we normally wish to minimise! Patient dose Linear tomography less effective for linear structures lying in the plane of movement Current technology in digital image production (CR/DR and other modalities) 39/

40 Digital Tomosynthesis Acquisition from several projection angles Wider angle of projections = better 3D resolution More projections = slower to acquire Data processing is manufacturer specific Current technology in digital image production (CR/DR and other modalities) 40/

41 Tomosynthesis Ben Johnson, Barts and the London NHS Trust, UKMPG 2011 Like tomography, the basic idea is that in plane structures are in focus, out of plane structures blurred Unlike tomography, reconstruct a number of different projections to get a series of images through the volume (like CT) Current technology in digital image production (CR/DR and other modalities) 41/ 41

42 Dual energy imaging A subtraction technique where images are taken at high and low kv in rapid succession Used in general radiography (e.g. chests), CT, fluoro Subtracting low kv image from high kv minimises visibility of bone and improves soft-tissue contrast Remove ribs in chest radiography! Conversely, subtract high kv from low kv image displays bony anatomy in greater detail Current technology in digital image production (CR/DR and other modalities) 42/

43 Dual energy technology CR and DR based solutions available for general radiography CR; Use two image plates with a Cu plate in between Acquire two images at the same time (no artefacts due to motion) Cu plate filters the beam that leaves the first plate ( low energy ) to give high energy image Little energy separation using this technique results in relatively low SNR Current technology in digital image production (CR/DR and other modalities) 43/

44 Dual energy technology DR; Low energy image (60 kvp)+read-out, High-energy image (120kVp)+read-out Large energy separation = higher SNR Relatively long read-out times mean motion artefacts are common Current technology in digital image production (CR/DR and other modalities) 44/

45 Dual energy imaging Current technology in digital image production (CR/DR and other modalities) 45/

46 Dual Energy CT Technological challenges Constant data free from motion and contrast changes Need largest practical energy separation and detector optimisation Commercial solutions Multiple rotations at different kvp Dual x-ray source Rapid switching kvp Energy sensitive detectors (not double exposure) Current technology in digital image production (CR/DR and other modalities) 46/

47 Stitching image Detector Detector A single exposure covers the entire anatomical region Source-tilting method, where the X-ray source tilts and aims to the detector Courtesy from Carestream Source-translation method, where the X-ray source translates during exam Current technology in digital image production (CR/DR and other modalities) 47/ 47

48 Conclusions CR is the most flexible and cost-effective technology for digital acquisition Direct digital radiographic devices have advantages in efficiency and throughput The different natures of various DR technologies, radiographic techniques may need to modified for different systems New opportunities Image acquisition and image processing tools Imaging technology innovation for diagnosis and intervention All technologies are becoming faster, better, cheaper Image Quality and Radiation dose Patient Current technology in digital image production (CR/DR and other modalities) 48/

49 Acknowledgement Organizing Committee Annual meeting of the 51 st The Royal College of Radiologists of Thailand and The Radiologists Society of Thailand (RCRT & RST) 2016 Radiology Department Bumrungrad International Hospital Chamaree Chuapetcharasopon, M.D., Royal College of Radiologists of Thailand Napapong Pongnapang, Ph.D., Department of Radiological Technology Faculty of Medical Technology Mahidol University, Bangkok, Thailand 49

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