Radiology Physics Lectures: Digital Radiography. Digital Radiography. D. J. Hall, Ph.D. x20893

Transcription

1 Digital Radiography D. J. Hall, Ph.D. x20893

2 Background Common Digital Modalities Digital Chest Radiograph x 4096 x 12 bit CT x 512 x 12 bit SPECT x 128 x 8 bit MRI x 256 x 8 bit US x 512 x 8 or 24 bits Viewing Station 2k x 2k x 12 bits

3 Computed Radiography Photostimulable phosphor Energy trapped on plate Readout at later time BaFBr or BaFI Flexible plate stored in cassette Exposed to x-rays like film Processed in special reader

4 Computed Radiography Photostimulable phosphor - readout

5 Computed Radiography Reader

6 Computed Radiography - emission wavelengths

7 Computed Radiography - How it works!

8 Computed Radiography - Dynamic Range

9 Charge Coupled Devices Make images from visible light Made of Silicon Visible light liberates electrons Electrons accumulate in individual pixel cells Accumulated charge readout pixel by pixel Requires coupling between light source and CCD Used for fluoroscopy and cine-angiography Large FOV imaging loses light Proportional to areas of CCD and light source

10 Charge Coupled Devices - readout

11 Charge Coupled Device - with Image Intensifier

12 Charge Coupled Device - with fiberoptic coupling

13 Charge Coupled Device - with lens coupling

14 Flat Panel Detectors Photodetector sensitive to visible light Coupled to x-ray intensifying screen Gd 2 O 2 S or CsI CsI grown in columnar crystals to improve efficiency X-rays absorbed in screen give off visible light Visible light absorbed in photodetector Fill factor determines efficiency Detector size determines best spatial resolution 125 μm -> 4 cycles/mm 100 μm -> 5 cycles/mm

15 Flat Panel Detectors - general configuration

16 Flat Panel Detectors - fill factor

17 Flat Panel Detectors - readout process

18 Direct detection flat panel systems Photoconductor layer on TFT array Direct detection of x-rays - x-ray to electron Not x-ray to light to electron Typically Selenium Electrons follow E-field lines Relatively thick detectors

19 Direct detection flat panel systems

20 Digital mammography Full-field digital mammography Mosaic of CCD detectors TFT flat panel detectors Slot-scan detector 1D detector array

21 Digital mammography - slot scan detector

22 Radiology Physics Lectures: Digital Radiography Digital Mammography - FFDM Digital Imaging Detector Large dynamic range Reasonable spatial resolution (300 μm) Digital image -> input to CAD system Expensive ~ $300k

23 Digital Mammography - FFDM Digital Detector Film-Screen

24 Digital mammography - slot scan detector readout

25 Patient Dose Considerations - Film vs. Digital Need comparable DQE Detected Quantum Efficiency CR ~ equivalent to 200 speed film system Flat Panel Detectors ~ 2x - 3x faster than CR

26 Hard Copy vs. Soft Copy Resolution varies with modality: SPECT & PET x 128 pixels MRI x 256 pixels US x 512 pixels CT x 512 pixels Digital angio x 1024 pixels Digital chest x 4300 pixels Digital mammo x 4000 pixels Monitor must reproduce data fidelity

27 Image Processing Correct for flaw in image Dead pixels Non-linearities in transfer function Dark noise Non-uniformities across field Store flat-field image for corrections

28 Image Processing - correction raw corrected

29 Image Processing - global adjustment wide narrow inverted

30 Image Processing - convolution Mathematical process Permits image enhancement Computationally efficient Various operations (kernels): Soft tissue - smoothing Bone - edge enhancement

31 Image Processing - convolution

32 Image Processing - convolution original 11x11 smooth edge enhance edge - smooth

33 Image Processing - histogram equalization original equalized

34 Contrast versus Spatial Resolution

35 Geometric Tomography

36 Geometric Tomography

37 Geometric Tomography - slice sensitivity profile

38 Digital Tomosynthesis

39 Temporal Subtraction

40 Dual Energy Subtraction One-pulse sandwiched detector Two-pulse single detector

41 Dual Energy Subtraction Low High Bone Soft tissue

42 Review Question

43 Review Question

44 Review Question

45 Review Question

46 Review Question

47 Review Question