1. FRCR: Physics Lectures Diagnostic Radiology
Digital detectors, tomography and dual energy imaging
Dr Tim Wood
Clinical Scientist

2. Overview What is a digital image? Computed Radiography (CR)
The advantages of digital
The disadvantages
Computed Radiography (CR)
The theory of CR
The advantages and disadvantages
Digital Radiography (DR)
The theory of DR
DR vs CR
Tomography
Dual energy imaging

3. The story so far…
We know how X-rays are made in the X-ray tube and how they interact with the patient
We know how we control the quality and intensity of the X-ray beam, and hence patient dose, with kVp, mAs, filtration and distance
We discussed the main descriptors of image quality
Contrast
Spatial Resolution
Noise
Discussed ways to improve contrast by minimising scatter and using contrast agents
Remember, there is always a balance between patient dose and image quality – fit for the clinical task!
Film is a dying medium for X-ray imaging…

4. Digital imaging

5. 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

6. What is a digital image?

7. Why bother with digital?
Film has been used since the beginning, so why are we changing to digital techniques?
Increased latitude and dynamic range
Images can be accessed simultaneously at multiple workstations
Viewing stations can be set up in any location
Uses digital archives rather than film libraries
Images quicker to retrieve and less likely to be lost
Post processing
Softcopy reporting – lower cost if do not print
No need for dangerous processing chemicals

8. Disadvantages of digital
Initial cost
Problems with interconnectivity
Lack of information and set up of automatic exposure control (AEC)
Lack of link between exposure and brightness
Potential for dose creep (see following slides)
Generally poorer limiting spatial resolution when compared with film

9. Dynamic Range - Film
With conventional film, too low a dose will results in a ‘thin’ film
Too high a dose results in a very dark film
Fixed and limited dynamic range – must match exposure parameters to the film being used
Gives a measure of control over patient dose!

10. Dynamic Range - Digital
With digital, too low a dose will still produce a recognisable image (just a bit noisy!)
Similarly, too high a dose will produce a recognisable image (but with very little noise!)
Consequences:
Less retakes = GOOD
Dose creep = BAD – must pay special attention to digital imaging to ensure doses are optimised

11. Detector Dose Indicators (DDI)
The restricted latitude of film gives a clear indication of dose
Too dark a film = overexposure
Too light a film = underexposure
Film has ‘in-built’ quality control of exposure
Digital images will be presented with the greyscale optimised no matter what dose is given
Will always see a recognisable image, but the noise will vary
Can result in dose creep
Increased dose (lower noise) is not punished by the detection medium, so tendency to go for slowly increasing image quality – NOT ACCEPTABLE!

12. DDI
The DDI has been introduced for digital imaging as an indication of the level exposure on a broad region of the detector
Analogous to the OD of film
The definition of DDI is manufacturer specific!
Some manufacturers have high DDI = underexposure
Some the other way round
Some are a function of the log of dose
Some are linear…
Manufacturers will provide an indication of acceptable range of DDI, but local departments must validate these – DRLs and OPTIMISATION
Operators should monitor DDI of patient exposures to ensure doses remain acceptable

13. Digital Imaging Techniques
Computed Tomography (CT)
Radionuclide imaging
Film scanner
Computed radiography (CR)
Direct digital radiography (DR)
Flat panel fluoroscopy
Electronic portal imaging device (EPID)

14. Computed Radiography

15. Computed Radiography (CR)
The most common technique for producing digital images
Was the first digital technique available commercially
Exploits storage phosphors which emit light that is proportional to the intensity of the X-rays that hit it, when they are stimulated by a laser beam
Primary reason for being the most common technique is that it is the cheapest (at least in the short term)
Old X-ray sets used for film-screen radiography can be used, provided exposure factors and AECs are adjusted for the new type of detector

16. CR Components

17. CR Stage 1: Image Capture
Image receptor is a laser stimulable phosphor, known as an image plate (IP)
Capture image by irradiating an IP in the same way as conventional film
Does not need a new X-ray system when replacing film-screen (just make sure automatic exposure controls are re-calibrated)
Typically ~40% of X ray photons are absorbed
IPs retain majority of absorbed X-ray energy as a pattern of electrons in meta-stable energy states
The spatial distribution of stored electrons is equivalent to the pattern of absorbed x rays – latent image

18. Electron Trapping 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

19. CR Stage 2: Image Read Out
Electrons are actively stimulated to release their stored energy
This is done by scanning the IP with an intense laser beam

20. CR Stage 2: Image Read Out
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
Hence, optical separation of input and output light photons
Means a colour filter can be used to prevent laser photons contaminating the output signal
Blue light photons are collected via a photomultiplier tube and digital image is produced

21. Read Out
Stimulation of IPs with laser causes trapped electrons to transfer to conduction band
These then relax to the ground state, emitting blue light photons

22. How Does CR work? Conduction band Electron traps X Rays are absorbed
Valence band

23. Photo stimulated luminescence
Conduction band
Red laser light
Blue light
Valence band

25. The read/erasure cycle
The image plate is removed from the cassette inside the CR reader
Scanning or laser achieved with a rotating mirror
The light guide (with optical filter) directs the emitted blue light to a photomultiplier tube, which measures the intensity of the light (proportional to the number of X-rays absorbed)
Whilst repeatedly scanning the plate, it is moved through the laser beam
Once scanned, the residual signal is removed by exposing the plate to a very bright light source (erasure cycle)
Takes about s to read and erase an image plate

26. CR Reader cycle
Analogue output

27. Image quality Pixel size limits spatial resolution in CR
For small plates (detail required) ~5.5 lp/mm
Large plates (detail not essential) ~3.5 lp/mm
(Film-screen ~8-12 lp/mm)
Other limits to resolution in CR;
Scattering of laser light in the phosphor layer results in detected light from a larger area than expected
Divergence of light emitted before detection
Increases with thickness of phosphor
Some phosphors have needle-like structure to guide light (like an optical fibre), but quite brittle so not for general use

28. CR image quality

29. Image quality
Image processing (e.g. edge enhancement) may improve visibility of fine detail
Contrast is determined by the image processing and LUT that is applied (and the window/level the user decides upon)

30. Digital Radiography

31. Digital Radiography
Directly acquire the data in digital format (no separate read-out phase like with CR)
Improves throughput of X-ray systems – could be important in chest clinic, mammo, etc
Most expensive method, as it requires complete dedicated X-ray system
Main technologies:
Phosphor coupled to a read out device – Indirect conversion
a-Se/TFT array – Direct conversion flat panels

32. Detective Quantum Efficiency
DQE reflects the efficiency of photon detection and the noise added
Every photon detected and no noise added, DQE = 100%
DQE for DR ~65%
DQE for CR and film-screen ~30%
In principle, DR could be used with lower patient doses as it is more efficient at using what is available!

33. Indirect conversion
Indirect conversion involves converting the X-rays into visible light (in a phosphor), and detecting the resulting light photons (akin to film-screen radiography!)
Either amorphous silicon (a-Si) photodiode TFT array, or CCD for readout
Sharpness limited by both pixel pitch of readout array, and spread of light in phosphor
Usually CsI(Tl) needle phosphors to focus light down to the detector (like mini-fibre optics to minimise spread)
Needle phosphors can be thicker (more efficient)

35. Indirect flat panels Phosphor => X-ray to light photons
Light photons detected in photodiode array => light photons to electrical charge
Read out by the amorphous silicon TFT array (discussed after direct conversion)
Can be manufactured as a single panel up 45 x 45 cm2, but in practice tend to be made up of four smaller detectors ‘stitched’ together
Tiled detectors
Requires image processing and interpolation to cover the join between panels

37. CCD detectors
CCD light detectors (like in a camera) can only be manufactured in relatively small sizes
Usually need multiple CCDs to cover image area (‘tiled detector’), or slot scanning technique
Also thicker than flat panels due to the optics between the phosphor and detector

38. Direct conversion flat panels
Amorphous Selenium (a-Se) is a photoconductor
Converts X-rays directly to electrons
Deposited directly onto amorphous silicon TFT array
No phosphor, hence no light spread
Resolution governed by effective pixel pitch

39. Flat Panel Physics X ray photons reach the panel
These are converted to an electrical charge
Either in the phosphor/photodiode arrangement, or photoconductor layer
Charge read-out by the TFT array
An image is instantaneously produced on the computer screen
Grey levels depend on the charge from each sensor and the LUT/window/level settings

40. The TFT array Amorphous Silicon thin-film transistor array
Transistors amplify electrical signals
Electrical charge is stored in the TFT array until release by applying a high potential
Each row of detectors is connected to the same activating potential (gate-line control), and each column to a charge measuring device (read-out electronics)
The activating potential is applied row-by-row, so the timing of the detected signal determines the position of the pixel from which it originates
Each pixel ~100 μm

41. The TFT array

42. Modern flat panels

43. Advantages of DR over CR
Image displayed immediately to operator in room
Faster
Greater throughput of patients as no intermediate read-out phase
Slightly better resolution (CR limited by laser spot size and scatter)
Harder wearing imaging device (as long as you don’t drop it!)
Or at least that’s the theory…

44. Disadvantages of DR over CR
Much more expensive
Need to refurbish X-ray room
Less flexible?
Originally, DR with fixed detectors, in carefully controlled ambient conditions, wired directly into the system
Now have mobile DR units with wireless detectors
BUT need to make sure you can get the images off to PACS – secure wireless network? Fixed connection points?

45. And now for something a bit different…
Tomography and Dual Energy Imaging

46. Tomography
Conventional radiography superimposes structures on the film/detector
Results in;
Inability to determine the depth of structures
Limited ability to resolve the shape of structures
Reduction in contrast
Can resolve depth by using orthogonal projections
In other situations conventional or mechanical tomography may be useful (or going full 3D with CT, MRI, etc)

47. Tomography
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!

48. Linear tomography
The tube and film/detector are linked by an extensible rod, hinged about a pivot
During exposure, the film/detector moves in a straight line (e.g. right-to-left) along rails, whilst the tube moves the other way
Whilst moving, the tube is rotated so that the central ray always points toward the pivot point
Structures that are in the same plane as pivot point (focal plane) will not move when projected onto the film, producing a sharp image
The projections of structures outside the focal plane will move around on the film, and hence are blurred in the final image

49. Linear tomography
Structures in focal plane maintain same position on the film = sharp image

50. Linear tomography
Structures above and below the focal plane will move from one end of the film to the other = blurred image

51. Linear tomography
The further the object is from the focal plane, the greater the blurring
Structures lying within a plane of thickness t will be sufficiently sharp to recognise
Everything outside this will be too blurred and low contrast
Cut-height (the focal plane position) is adjusted by lowering or raising the pivot
t is controlled by the tomographic angle and height of pivot (higher = thinner thickness of cut)
Narrower angles reduces the degree of blurring, and hence increase t (and vice-versa)
0° = conventional projection radiography = everything in focus!

52. Linear tomography Typical angle = 40°, equivalent to t~3 mm
However, thin slices and the spread of off-focus anatomy over the whole film reduces contrast
Use low kV (consistent with penetrating the patient)
Most useful for high contrast structures e.g. bony structures in the ear
Patient dose is higher compared with projection radiography
Linear tomography less effective for linear structures lying in the plane of movement
Use alternative, more complex movements (e.g. circular, elliptical, etc)
Tomography equipment becoming less common with the availability of CT

53. Tomography

54. Panoramic Dental Radiography

55. 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
Low kV = high contrast between bone and soft tissue (photoelectric effect)
High kV = image contrast determined by tissue density rather than atomic number (Compton scatter)
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

56. Dual energy imaging
Images (shown here) and good video of how it works can be found at:

57. Dual energy imaging

58. 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

59. Dual energy technology

60. Dual energy technology
DR;
Low energy image (60 kVp), read-out, high-energy image, read-out
Large energy separation = higher SNR
Relatively long read-out times mean motion artefacts are common (involuntary and voluntary)

61. Dual energy technology

62. Dual energy spectra

63. Dual energy imaging

64. Motion artefacts

65. Lesion conspicuity

66. Dual Energy CT Technological challenges Commercial solutions
Constant data free from motion and contrast changes
Need largest practical energy separation and detector optimisation
Commercial solutions
Multiple rotations at different kVp
Single spiral with alternating kVp
Dual x-ray source
Rapid switching kVp
Energy sensitive detectors (not double exposure)

67. Dual Energy CT Applications
Virtual non-contrast imaging
Bone segmentation and removal
Tissue typing?
Mono-energetic imaging – PET/SPECT apps?
Specialist breast CT?
Not particularly proven yet – a technology looking for an application?

68. Dual Energy CT – Manufacturers
Siemens
Dual source GE
Rapid kVp switching
Toshiba
Single spiral with alternating kVp
Philips
Acquire two lots of data at high and low kVp, register and use dual energy software
BUT, they are developing dual-energy detectors (dual layer and photon-counting)