computed radiography, digital radiography

computed radiography, digital radiography

1. Computed radiography and digital radiography and digital subtraction angiography Moderator – Dr Vijayalakshmi MDRD Presentor – Dr Swetha

2. Computed radiography Computed radiography is also called as photostimulatable phosphor imaging It is the bridge between classical radiography and digital radiography CR is used almost like conventional imaging plate in a casette which must be processe d in CR reader

3. CR system Image plate X ray or gamma source Image reader Photo multiplier tube Computer console or work station

4. Principle In computed radiography the imaging paltes are inserted in a radiographic casette holder and exposed to x rays The energy of incoming radiation is stored in a special phosphor layer and are excited into a higher energy state forming a latent image

5. Then a specialised machine called scanner is then used to read out the latent image from the plate by stimulating it with a finely focused laser beam The light is collected by a light guide and trasmitted to a highly sensitive photomultiplier device and converted to a digital signal using analog to digital converter

6. Steps of CR processing

7. Imaging plate It looks like a intensyfying screen which is seen in conventional radiography Made of photo stimulatable phosphor Instead of emitting light when exposed to x rays ,this photo stimulatable phosphor has the property to store energy and produce digital image on exposure to laser light

8. Image scanner Plate is scanned by a laser causing emission of blue light from the plate which is then detected by a photomultiplier After exposed and scanning the phosphor plate is erased by exposure to bright Light from the digitaliser And then the casette is further ready for another exposure

9. Photomultiplier tube A photo multiplier tube useful for light detection of very weak signals is a photo emissive device in which the absorbtion of a photon results in the emmision of an electron These detectors work by amplifying the electrons generated by a photocathode exposed to a photon flux

10. Advantages Post processing manipulation and storage of images is easy Repeat examinations are reduced consistent high quality images

11. Ability to deliver images quickly To acquire images Increased savings Reduction in exposure and processing time Digital magnification can be done Global data exchange

12. Disadvantages Initial cost Additional cost that is service and maintenance Plate sensitive to fogging

13. Digital radiography Digital radiography is a form of X-Ray imaging in which digital x-ray sensors are used instead of traditional photographic film Digital Radiography uses two types of flat panel detectors – direct and indirect

14. Flat panel detectors

15. Direct digital radiography X-rays are directly registered by a flat panel detector There is no need for intermediate cassette in both direct and indirect radiography

16. Direct digital radiography It is made from a layer of semiconductor materials on the top of TFT array The electrons released in the detector layer from the x ray interaction are used to form the image directly

17. Indirect digital radiography X-ray strikes schintillator producing light Photo diode array converts light to electrical signals These electrical signals pass through TFT array and produces image Photo detector is made up of amorphous silicon

18. The photo conductor is made up of amorphous selenium Selenium has higher atomic number than sillicon Due to electric field selenium direct detectors are made much thicker

19. Advantages Receptor provides direct digital output Potentially lower patient dose than CR High latitude as per CR No processor or reader is required Less than 15 seconds Much less work for technologist

20. Digital subtraction angiography It is a digital imaging technique for better visualisation of contrast will blood vessels In this technique pre contrast imagesare digitally subtracted from post contrast image to provide background separation

21. Process involved in DSA 1. Preprocessing – digitalisation analogue to digital conversion 2 . Processing – subtraction techniques 3 . Post processing – manipulation to enhance visibility 4 . Information extraction

22. Processing Mask mode subtraction Time interval difference subtraction Dual energy subtraction Hybrid subtraction

23. Mass mode subtraction Most widely used process of subtraction In this temporal subtraction is done It involves initial acquisition of a frame of region of interest Then a second images taken and stored as a mask image This mask images subtracted from subsequently acquired images and show only contrast will structures

24. If any moment occurs after acquisition of mass image mis registration occurs in the subtracted images This can be overcome to some extent by pixel shifting

25. Time interval difference subtraction Another mode of temporal subtraction where a consecutive previous frame is subtracted from current frame This technique is very useful in cardiac imaging where there is rapid motion

26. Dual energy subtraction Another technique in which the region of interest is exposed to higher and lower kV at very short intervals Then the higher kV image is subtracted from lower kV image to produce the image

27. This technique has advantage of elimination of motion artefacts Does not require the acquisition of images before arrival of contrast material as in Mask subtraction Provides selective cancellation

28. Problems of dual energy subtraction More complex x ray machine is needed to rapidly switch kV and mA Due to difference in beam hardening in soft tissue and bone improper subtraction results

29. Hybrid subtraction It is a combination of dual energy and temporal subtraction It has advantage that it eliminate effect of patient motion

30. Techniques used in DSA Road map technique Fluoroscopy fade technique Dynamic 3D road mapping technique

31. Road map technique This technique is very useful in placement of catheters and guidelines in complex and small Vessels Static fluoroscopic images subtracted from densely opacified vessel Motion after road map acquisition may affect subsequent intervention

32. Fluoroscopy fade technique In this technique reference DSA images overlayed on a real time fluoroscopic image

33. Dynamic 3D road mapping This new development allows projection of 3D reconstructed vessel on live 2D floroscopic image

34. Peripheral DSA Can be performed with single contrast injection using stepping technique Two types – stepping table technique . Stepping gantry method

35. Stepping table technique In this , table moves into Three stations with X ray tubes with detector remaining fixed

36. Stepping gantry method In this the tube detector or image intensifier moves keeping the table fixed

37. In both method precontrast images are accquired at different Stations and stored as mask images Subsequently matching post contrast images are taken at the same position And subtracted from the corresponding mask image

38. Uses Significantly reduce the use of contrast Reduce the examination time

39. Increased chance of movement between pre and post contrast images

40. Post processing Mask pixel shift Remask Image summation Stacking Landmarking

41. Mask pixel shift It is a software modification system Used when smaller patient motion is required This technique maybe manual or automatic

42. Remask It is a similar feature where another mask images selected which is temporary closer to the contrast image This is useful one patient motion occurs prayer to contrast image but after initiation of the acquisition

43. Image summation It is a property in which two or more frames of DSA acquisition are summed into the single image

44. Stacking Similar technique used in carbon dioxide angiography In this fragmented boluses of the gases added to produce a complete picture of the opacified structure

45. Land marking It is a feature in which lesser intensed your original images added to the subtracted image This provides anatomical landmarks in the subtracted images useful in subsequent intervention

46. Thank you