Image-Intensifier Tube (II Tube) by T.R.B.

Conventional fluoroscopy and the Image intensifier tube are discussed. Each aspects of II tube are covered.

1. Image-Intensifier Tube Thumb Raj Baruwal Roll Number: 162

2. Contents : Introduction to Fluoroscopy Image intensifier tube Components Working principle physical characteristics of Image Intensifier tube Image quality and control Aspects of image in fluoroscopy References.

3. Fluoroscopy A “see –through” operation with motion . Used to visualize motion of internal fluid and structures. Operator controls activation of tube and position over patient. Early fluoroscopy gave dim image on fluorescent screen. Physician used to seared in dark room. Modern systems includes image intensifier with television screen display and choice of recording device.

4. Direct Fluoroscopy Sir Thomas Alva Edison invented Direct Fluoroscope in 1896,it has served as a valuable tool in medical imaging. In older fluoroscopic examination radiologist used to stand behind screen and view the picture . Radiologist were under high exposure; despite protective glass, lead shielding in stand ,apron and perhaps goggles. In the case of fluoroscopy , examinations without an image intensification or equivalent techniques are not justified and shall therefor be prohibited.

5. Fluoroscopy Fluoroscopy emits very faint image. So , the operator had to wear red googles for up to 30 minutes prior to the examination and the examination had to be performed in completely darkroom. The image brightness is very low , so the eye must be dark adapted . The retina of human eye consists of two types of receptors: rods and cones. Cone vision requires bright light or day light. Eventually, fluorescent screen was replaced by image intensifier tube.

6. Fluoroscopy Cont.. The layout of a fluoroscopic imaging system can vary The x-ray tube is usually hidden under the patient cough , and the image intensifier or other image receptors are set over the patient couch. With some fluoroscopes , the x-ray tube is over the patient couch, and the image receptor is under the patient couch . During fluoroscopy , the x-ray tube is operated at less than 5mA . Despite the lower mA , however , the patient dose is considerably higher during fluoroscopy than during radiographic examination because the x-ray beam expose the patient continuously for a longer time.

7. Fluoroscopy Cont.. The kilovolt peak ( kVp ) of operation depends entirely on the section of the body that is being examined . Fluoroscopic equipment allows the radiologist to select an image brightness level that is subsequently maintained automatically by varying the kVp , the mA , or sometimes both. This feature of the fluoroscope is called automatic brightness control (ABC) .

8. Fluoroscopy cont.. A radiologist may observe something that he or she would like to preserve for later study ; in this case, a permanent fixed image can be taken without interruption of the examination . One such method is known as a spot film .

9. Fluoroscopic X-ray Tube High KVP: 80-90 KVP Low mAs : 0.5-5mA The fluorescent x-ray tube is operated by foot horizontal to vertical position for upright examination. Some tables are composed of carbon fiber material which have great strength and reduce attenuation of the x-ray beam.

10. Remote Control fluoroscopy Wide range of imaging modalities with high workflow of the radiology unit. All imaging is performed in digital format due to the use of a flat panel detector working in fluoroscopy . The machine allows to perform routine investigations with minimal effective dose. Remote control is used to reduce radiation exposure of physicians and increase the service time of the equipment.

11. Mobile C-arm Fluoroscope A C-arm machine is an advanced medical imaging device based on x-ray technology . C-arm is called so due to its C-shaped arm, which is used to connect the x-ray source on one end and the detector on the other. C-arm fluoroscopes are widely used during orthopedic, urology , gastroenterology surgeries and emergency procedures.

12. Interventional Fluoroscope Interventional fluoroscopy uses ionizing radiation to guide small instruments such as catheters through blood vessel or other pathways in the body. Interventional fluoroscopy represents a tremendous advantage over invasive surgical procedures , because it requires only a very small incision, subsequently reduces the risk of infection and allows for shorter recovery time compared to surgical procedures.

13. Multipurpose Fluoroscopy System They can be used as a remote control system or as a system to perform simple interventional procedures.

14. Fluoroscopy X-ray generator X-ray tube Collimator Filters Patient table Grid Image intensifier Optical coupling Television system . Fluoroscope Imaging Chain

15. Image Intensifier(II) Tube Process of brightening the image during the fluoroscopy is intensification and the device used is image intensifier. It is a complex electronic device approximately 50cm long. Receives the image forming x-ray beam and converts it into visible light image of high intensity. Vacuum device which converts low intensity , full sized image into a high intensity miniaturized image. It increases the brightness of an image by 8000 times.

16. History Sir Thomas Alva Edison invented the fluoroscope in 1896, the year after Roentgen’s discovery of x-ray, using of Ba platino-cyanide as screen phosphor . The original fluoroscope was held by hand above the patient’s body in the path of the x-ray beam so, there were radiation hazards to the operator. The original fluoroscope was Cu activated ZnCdS screen . The image intensifier are developed in 1950s.

17. Some Important Dates 1937-fluoroscopic ii (brightness gain 100) was invented by Irving Langmuir. 1948-modified ii (brightness gain 1000) by Coltman . 1955-display of ii image using video camera and TV monitors. 1960-ii TV system replaced the complex mirror optic system of viewing and opened the way of routine angiography.

18. Components of Image Intensifier (ii) Tube Input Window Input Phosphor Photocathode Electrostatic accelerating lens (steering coils) Anode Output Phosphor Output window An evacuated glass or metal envelope.

19. Input Window It is the first vacuum window , typically 1mm thick made up of Aluminum . The vacuum window keeps the air out of the II tube and its curvature is designed to withstand the force of the air pressing against it. The vacuum window of a large FOV (35 cm) II supports over a ton of force from atmospheric air pressure. It receives x-ray photons emerging from the patient.

20. Input Phosphor After passing through the Al input window and substrate , x-rays strike the input phosphor , whose function is to absorb the x-rays and converts their energy into visible light . The input phosphor must be thick enough to absorb a large function of the incidents x-rays, but thin enough to not significantly degrade the spatial resolution of the image by the lateral dispersion of light through the phosphor.

21. Input Phosphor Dome shaped for electron optical reason. 25-57cm in diameter. It is composed of Cesium Iodide doped with Sodium ( CsI:Na ) The CsI crystal are approximately 400micr meter tall. Diameter of CsI crystals is 5 micro meter. Emits large number of light photons per x-ray quantum.

22. Input Phosphor The long , thin columns of CsI function as light pipes, channeling the visible light emitted within them towards the photocathode with less lateral spreading than an amorphous phosphor. The CsI crystals have a trace amount of sodium , causing it to emit blue light. For a 60 Ke V x-ray photon absorbed in the phosphor ,approximately 3000 light photons ( at about 420 nm wavelength ) are emitted. The k-edge of cesium (36KeV) and iodine (33KeV) are well positioned with respect to the fluoroscopic x-ray spectrum , which contribute to high x-ray absorption efficiency.

23. The Photo-cathode It is composed of an alloy of Antimony (Sb) ,Cesium(Cs) potassium(K) and sodium (Na). The shape of the photo cathode should conform intimately to the shape of input phosphor. Thin separating layer made up of glass or inactive material ensures the close contact between input phosphor and photo-cathode. The light photons emitted by input phosphor are absorbed by photocathode and release photoelectrons.

24. Photocathode With 10 % to 20% conversion efficiency , approximately 400 electrons are released from the photocathode for each 60 KeV x-ray photon absorbed in the phosphor.

25. Electrostatics Lenses The electron optics is used for focusing photoelectrons onto the output phosphor. Made up of series of negatively charged electrodes. Electrodes influences the velocity and direction of electron beam. A voltage of 25-35 KV is used to accelerate the electrons . As the photoelectrons travel to the output phosphor , they will cross at the focal point where the image is reversed . So, the output phosphor image is reversed from the input phosphor.

26. The Anode Cup A circular plate with a hole in the middle to allow the electrons through to the output phosphor. Located In the neck of image tube. Focus the electron beam to the output phosphor.

27. Output Phosphor It is smaller than that of input phosphor. Made up of silver activated ZnCdS . Which has a green (530nm) emission spectrum . Backed by a thin sheet of Al to prevent optical feedback. Its thickness lies 2.5 cm to 3.5 cm . Produces a fluorescent image when electrons strikes it which corresponds in detail to the original but it is much brighter.

28. Output window The last stage in the II tube the image signal passes through is the output window. The output window is part of the vacuum enclosure and must be transparent to the emission of light from the output phosphor . The output phosphor is coated directly onto the output window.

29. An evacuated glass or metal envelope Vacuumed glass or metal tube. 2.4 mm in thickness ,enclosed in a lead lined metal container. Must be light-tight , dry and dust leakage proof. Provide mechanical ,anti-magnetic and radiation protection to the tube.

30. Principles of Operation figure: Image intensifier tube

31. Principles of Operation

32. Principles of Operation X-ray emerging from the patient enter at the input window and strikes the input phosphor. The input phosphor scintillates and light photons are emitted. The brightness of light emitted is proportional to the intensity of incident x-ray photons.

33. Principles of Operation Adjacent to the input phosphor there lies a photocathode which produces electrons when light strikes it . Approximately 5 light photons are likely to produce 1 electron. A pattern of electron emission from the photocathode depicts the pattern of intensities in the transmitted x-ray beam. As soon electrons are ejected an electron image is formed.

34. Principles of Operation The electrons are accelerated through a potential difference of 25-35 kV in the body of image intensifier tube. The electrons are accelerated and focused by means of applied voltage and reach the output phosphor. The output phosphor fluorescence when electrons strike it. The light image so produced may be up to 8000-10000 times brighter than the input phosphor image.

35. Principles of Operation There are 2 reasons for the gain in brightness The acceleration of electrons : Electrons emitted from photo-cathode gain kinetic energy from the electric field inside image intensifier tube which is converted into light at output phosphor. Reduction in image size: Area of output phosphor is of input phosphor which increases the image brightness 100 times.  

36. Principle of Operation Behind the output screen, a digital CCD camera captures the image on the output window for further image processing purposes and display on a TV monitor. The CCD camera captures the image from image intensifier tube either through fiber optic or through lens system.

37. Physical Characteristics of Image Intensifier Brightness gain Flux gain Minification gain Conversion factor Magnification

38. Brightness gain Ability of the ii tube to increase the illumination level of the image. Ranges from 5000-30000 Brightness gain(BG)= flux-gain  

39. Flux-gain Ratio of number of light photons to the number of input x-rays quanta. Flux gain = Typical values are 50 to 100.  

40. Minification gain Minification Gain = Typical value vary from 40 to 90.  

41. Conversion factor Ratio of output phosphor illumination to the input exposure rate. Ranges from 50-300. Numerically equal to the 0.01 times the brightness gain i.e. 1 of brightness gain. Conversion factor(CF) =  

42. Magnification Magnification = Electrostatic lenses can change the magnification of the image by changing the focal point of photoelectrons. Magnification decrease minification gain. Selection of smaller diameter of input phosphor creates magnified image.  

43. Aspects of intensified image quality Un-sharpness Resolution Image Noise Distortion .

44. Un-sharpness It is difference in densities in an image. It is affected by scattered radiation from the patient, light scattered at input phosphor, output phosphor and light scattered within image intensifier tube itself. Can be minimized by using needle-shaped CsI micro-crystals in input phosphor. Also caused by divergence of electrons when moving from photo-cathode to output phosphor. Can be minimized by using more effective electrostatic focusing system.

45. Un-sharpness Internal scatter radiation in the form of x-rays, electrons and particularly light can reduce the contrast of image intensifier through a process called veiling glare . Some fraction of the light emitted by the output phosphor is reflected inside the glass window such stray light reflecting inside the output window contributes to veiling glare, which can reduce image contrast. This veiling glare is reduced by using a thick (about 14 mm) clear glass window , in which internally reflected light eventually strikes the side of the window ,which is coated with a black pigment to absorb the scattered light. Advanced image intensifiers have output phosphor designs that reduce veiling glare.

46. Vignetting Vignetting is a reduction in brightness at the periphery of the image. The portion of any image that results from the periphery of the input phosphor is inherently unfocused and suffers from vignetting . Since only the central region of the input phosphor is used in the magnification mode, spatial resolution is also improved. In the 25-cm mode CsI image-intensifier tube can image approximately 0.125mm objects (4Lp/mm) ; in the 10 cm mode , the resolution is approximately 0.08 (6Lp/mm) .

47. Spatial Resolution It is the ability to image two separate objects and visually distinguish one from the other. Spatial resolution refers to the ability to image small objects that has high subject contrast ,such as a bone-soft tissue interference ,a breast microcalcification ,or a calcified lung nodules. Spatial resolution improves with a reduction in screen blur, motion blur and geometric blur.

48. Spatial Resolution Spatial resolution for an image intensifier fluoroscopy system depends on the FOV (magnification mode) selected and the number of raster lines of the television system. Use of a small FOV and a large number of raster lines improves spatial resolution. The limiting resolution of the intensified image is between 5 and 7 lp /mm at the center of the image field and less than this at the periphery.

49. Contrast Resolution Contrast resolution is the ability to distinguish anatomical structures of similar subject contrast such as liver-spleen and gray-white matter . The less precise terms detail and recorded detail sometimes are used instead of spatial resolution and contrast resolution.

50. Noise A number of factors contribute to radiographic noise, including some that are under the control of radiologic technologist . Lower noise results in a better radiographic image because it improves contrast resolution. Screen-film radiographic noise has four components :film graininess , structure mottle , quantum mottle , and scatter radiation.

51. Noise Film graininess refers to the distribution in size and space of silver halide grains in the emulsion. Structure mottle is similar to film graininess but refers to the phosphor of the image intensifying tube. Film graininess and structure mottle are inherent in the screen-film image receptor. They are not under the control of the radiologic technologist , and they contribute very little to radiographic noise.

52. Noise Quantum mottle is somewhat under the control of the radiologic technologist and is a principal contributor to radiographic noise in many radiographic imaging procedures. Quantum mottle refers to the random nature by which x-rays interact with the image receptor. If an image is produced with just a few x-rays , the quantum mottle will be higher than if the image is formed from a large number of x-rays. The use of high mAs , low- kVp , and slower image receptors reduces quantum mottle.

53. Radiographic Quality Rules Fast image receptors have high noise, and low spatial resolution and low contrast resolution. High spatial resolution and high contrast resolution require low noise and slow image receptors. Low noise accompanies slow image receptors with high spatial resolution and high contrast resolution.

54. Noise It Is the grainy appearance in the image which is caused by insufficient x-ray radiation to produce uniform image. It is controlled by mA and time. Very small number of photons are used to activate fluorescent screen which creates noise and thus appears grainy. Structure mottle is caused due to the finite size of micro-crystal present in input phosphor, photocathode and output phosphor.

55. Distortion Fluoroscopic images, which are obtained using electron lenses, may suffer from all of the primary aberrations . Any deviation from the pattern created as electrons are emitted from photo-cathode results in distortion. More noticeable towards the edges of the image-field. To get rid from it we should keep the area of interest always in the center of the screen.

56. S-shaped distortion There is greater reduction in detail size at center than periphery which causes straight line at periphery to appear as curves ,convex towards the center of field. S-shaped or spiral or pocket handkerchief distortion occurs primarily as a result of Earth’s or other electronic device’s homogeneous magnetic fields. S-shaped distortion is variable and dependent upon the orientation of the ii tube. Correction may be accomplished using shielding or a coil creates a magnetic field in opposition to that created by Earth or other objects.

57. Pin-cushion distortion Pincushion distortion is the result of projecting the image with a curved input phosphor to the flat output phosphor. This is a lens distortion which causes straight line outside the image center to bend inwards. Distortion due to change in minification ratio over the full area of the image. Ratio becomes smaller towards periphery. Therefore , for the improved accuracy with distance measurements, it is best to position the desired anatomy in the central area of the FOV.

58. Pin-cushion distortion In pin-cushion distortion , image magnification increases with the distance from the optical axis. The visible effect is that lines that do not go through the center of the image are bowed inwards , towards the center of the image, like a pincushion. It is actually simply an exaggerated radius mapping for large radii in comparison with small radii.

59. Multifield Image Intensifier Most image intensifier are of the multifield type. Multifield image intensifiers provide considerably greater flexibility in all fluoroscopic examinations. Tri-field tubes come in various sizes , but perhaps the most popular is 25/17/12 cm. These numeric dimensions refer to the diameter of the input phosphor of the image intensifier tube.

60. Multifield Image Intensifier In the 25 cm mode , photoelectrons from the entire input phosphor are accelerated to the output phosphor. When a switch is made to the 17 cm mode , the voltage on the electrostatic focusing lenses increases; this causes the electron focal point to move farther from the output phosphor. Consequently , only electrons from the center 17 cm diameter of the input phosphor are incident on the output phosphor. Multifield image intensifier produces different magnification of the image.

61. Multifield Image Intensifier The principal result of this change in focal point is to reduce the field of view. The image appears magnified because it still fills the entire screen on the monitor. Use of the smaller dimension of a multifield image-intensifier tube always results in a magnified image , with a magnification factor in direct proportion to the ratio of the diameters. A 25/17/12 tube operated in the 12 cm mode produces an image that that is =2.1 times larger than the image produced in the 25 cm mode.  

62. Multifield Image Intensifier In the magnified field mode the minification gain is reduced , and fewer photoelectrons are incident on the output phosphor leading to the dimmer image. To maintain the same level of brightness , the x-ray tube mA is increased by the ABC, which increases the patient radiation dose. The increase in dose is approximately equal to the ratio of the area of the input phosphor used.

63. Multi-field image intensifier

64. Multi-field image intensifier

65. Multi-field image intensifier More flexible and standard components in digital fluoroscopy. Allows focal point to reduce FOV, magnify the image, help to gain brightness and quality. Most commonly used dual-field tube is 25 to 17cm i.e.(25/17) design and tri-field are 25/17/12 and 23/15/10.

66. Flat-panel Fluoroscopy Systems In fluoroscopy imaging systems, flat-panel detectors (FPDs ) are replacing image intensifiers as the modern solid-state image receptors; FPDs have potential to reduce patient radiation doses and to eliminate image degradation due to glare ,vignetting ,spatial distortions and defocusing effects.

67. References radiographic Imaging-6 th edition , John Ball & Tony Price.