MMed Physics, Task 2
Question 1:
Name the different layers of input phosphor of an image intensifier tube and briefly give the purpose of each layer.
a) Input Phosphor and Photocathode
i) Cesium Iodide (CsI).
(1) The input fluorescent screen in image intensifiers is CsI. CsI is deposited on a thin aluminium substrate by a process called “vapour deposition”.
(2) Image quality is dramatically better with CsI (then with the older Zinc-cadmium sulphide screens). Minimal lateral light diffusion, and minimal light scattering by the “needle-shaped deposition perpendicular to the substrate”. Three physical characteristics of CsI make it superior:
(a) The vertical orientation of the crystals
(b) A greater packing density, and
(c) A more favourable effective atomic number
(3) CsI can be vacuum deposited, it requires no inert binder, so more active material can be packed into a given space. The packing density is three times greater than zinc-cadmium sulphide. Phosphor thickness has been reduced from 0.3mm to 0.1mm with CsI. The principle advantage of a thinner phosphor layer combined with needle shaped crystals is improved resolution. The resolution with CsI is 4lp/mm.
(4) The K-edges closer to the x-ray beam average energy.
(a) The mean energy of an x-ray beam is approximately 1/3 of its peak energy, depending on the kVp and filtration. Most fluoroscopy in adults are done between 80 to 120 kVp, which has a mean energy between 30 to 40 keV.
(b) The k-edges of Cesium (36keV) and Iodide (33.2keV) are almost ideal. CsI absorbs almost 2/3 0f the incident beam, even though CsI is only one third as thick.
ii) Photocathode. Photoemissive metal. When light from the fluorescent screen strikes the photocathode, photoelectrons are emitted in numbers proportional to the brightness of the screen. Light from CsI is applied directly to the photocathode.
Question 2:
Name the different layers of output phosphor of an image intensifier tube and briefly give the purpose of each layer
i) Output Phosphor.
(1) Silver-activated zinc-cadmium sulphide
(2) A thin layer of aluminium
ii) Silver-activated zinc-cadmium sulphide. Diameter of the output image is reduced to 0.5 to 1 inch. Because the electrons are greatly accelerated, they emit more light photons from the output screen then were originally present at the input screen. The number of light photons is increased 50 fold.
A thin layer of aluminium is plated onto the fluorescent screen to prevent light from moving retrograde through the tube and activating the photocathode. The aluminium layer is very thin, and will not prevent high energy electrons from passing through it to the output phosphor
The glass tube image intensifier is 2 to 4 mm thick and is enclosed in a lead lined container to prevent the operator from stray radiation.
The output phosphor image is viewed directly through a series of lenses and mirrors or indirectly through Close Circuit Television
Question 3:
Which factors degrade contrast in an image intensifier tube?
a) Contrast. Two factors diminish contrast in an image intensifier
i) The input screen does not absorb all the photons in the x-ray beam. Some are transmitted through the intensifier tube, and few are eventually absorbed by the output screen. These transmitted photons contribute to the illumination of the output phosphor but not to the image formation. They produce background of fog that reduces image contrast.
ii) Retrograde light flow from the output screen. Most retrograde light flow is blocked by a thin layer of aluminium on the back of the screen. Although the aluminium layer is extremely thin it would absorb electrons that convey the fluoroscopic image. Some light photons penetrate the aluminium and pass back through the image tube to activate the photocathode. These electrons produce “fog” and further reduce image contrast. The contrast tends to deteriorate as an image intensifier ages,
b) Lag. Lag is defined as the persistence of luminescence after the x-ray stimulation has been terminated. Older image tubes had long lad times, 40ms. CsI tubes lag times are 1ms.
c) Distortion. The center of the image intensifier screen has better resolution, a brighter image, and less geometric distortion.
i) “Pincushion effect”. Peripheral electrons do not strike the output phosphor where they ideally should. They tend to flare out from their ideal course. The result is unequal magnification, which produces peripheral distortion. The further the electrons from the center the more difficult to control. (this makes it difficult to evaluate straight lines esp when tying to reduce a fracture).
ii) Vignetting Unequal magnification also causes unequal illumination. The center is brighter than the periphery. A fall-off in brightness at the periphery of an image is called vignetting.
iii) Unequal focus. Resolution is better in the center of the screen.
Question 4:
What is DAP? Give a typical value
DAP is a descriptor of radiation dose. Dose Area Product. DAP is a product of the irradiated surface area multiplied by the radiation dose at the surface.
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Christensen’s Physics of Diagnostic Radiology, 4th Edition, Thomas, et al. Image Intensifiers, 166-172
Lesson 12, Fluoroscopic Imaging, MMed Physics
X rays were discovered because of their ability to cause fluorescence, and the first image of a human part was observed fluoroscopically.
2) The fluoroscope
a) Visual Physiology
Rods, night vision, peripheral or scotopic vision for dim light. Fovea has no rods - Entirely from peripheral vision. Less acute then photopic vision of cones. Most sensitive to blue green light.
Rod vision is improved by motion.
Cones, bright light, central vision or photopic vision for daylight. Cones are concentrated in the fovea.
Dark adaptation is not destroyed by red light so radiologists used to use red goggles before fluoroscopy to protect the dark adaptation.
3) Image Intensifier Design
The components of an x-ray image intensifier. The tube itself is an evacuated glass envelope, a vacuum tube, which contains four basic elements:
Input phosphor and photocathode
Electrostatic focusing lens
Accelerating anode
Output phosphor
a) Input Phosphor and Photocathode
i) Cesium Iodide (CsI). The input fluorescent screen in image intensifiers is CsI. CsI is deposited on a thin aluminium substrate by a process called “vapour deposition”. Minimal lateral light diffusion, and minimal light scattering by the “needle-shaped deposition perpendicular to the substrate”. Image quality is dramatically better with CsI (then with the older Zinc-cadmium sulphide screens).
(1) Three physical characteristics of CsI make it superior:
(a) The vertical orientation of the crystals
(b) A greater packing density, and
(c) A more favourable effective atomic number
(2) CsI can be vacuum deposited, it requires no inert binder, so more active material can be packed into a given space. The packing density is three times greater than zinc-cadmium sulphide. Phosphor thickness has been reduced from 0.3mm to 0.1mm with CsI. The principle advantage of a thinner phosphor layer combined with needle shaped crystals is improved resolution. The resolution with CsI is 4lp/mm.
(3) The K-edges closer to the x-ray beam average energy. The mean energy of an x-ray beam is approximately 1/3 of its peak energy, depending on the kVp and filtration. Most fluoroscopy in adults are done between 80 to 120 kVp, which has a mean energy between 30 to 40 keV. The k-edges of Cesium (36keV) and Iodide (33.2keV) are almost ideal. CsI absorbs almost 2/3 0f the incident beam, even though CsI is only one third as thick.
ii) Photocathode. Photoemissive metal. When light from the fluorescent screen strikes the photocathode, photoelectrons are emitted in numbers proportional to the brightness of the screen. Light from CsI is applied directly to the photocathode.
b) Electrostatic Focusing Lens. The lens is made up of a series of positively charged electrodes that are usually plated onto the inside surface of the glass envelope. Electron focusing inverts and reverses the image. This is called point inversion. Figure 12-3. The input phosphor is curved to ensure that all electrons travel the same distance.
i) Accelerating Anode. The anode is located in the neck of the image tube. Its function is to accelerate electrons emitted from the photocathode towards the output screen. The anode has a positive potential of 25 to 35 kV relative to the photocathode and accelerates electrons at a tremendous velocity.
ii) Output Phosphor. Silver-activated zinc-cadmium sulphide. Diameter of the output image is reduced to 0.5 to 1 inch. Because the electrons are greatly accelerated, they emit more light photons from the output screen then were originally present at the input screen. The number of light photons is increased 50 fold.
A thin layer of aluminium is plated onto the fluorescent screen to prevent light from moving retrograde through the tube and activating the photocathode. The aluminium layer is very thin, and will not prevent high energy electrons from passing through it to the output phosphor
The glass tube image intensifier is 2 to 4 mm thick and is enclosed in a lead lined container to prevent the operator from stray radiation.
The output phosphor image is viewed directly through a series of lenses and mirrors or indirectly through Close Circuit Television
4) Brightness Gain
The brightness gain tends to deteriorate as an image intensifier ages. Brightness gain is the ratio of two illuminations:
Brightness gain = intensifier luminescence / Patterson B-2 luminesence
Conversion factor used to enhance reproducibility:
Conversion factor = (cd/m2) / (mR/s)
a) Minification Gain. The brightness gain from Minification is produced by a reduction in image size. The quality of the gain depends on the relative areas of the input and output screens.
Minification = ( d1 / do ) 2
d1 is the diameter of the input screen
do is the diameter of the output screen
b) Flux Gain. Flux gain increases the brightness of the fluoroscopic image by a factor of 50. For each photon at the input screen, 50 photons are emitted by the output screen.
Total Brightness gain = Flux gain x Magnification gain
Total brightness gain is the product of the voltage or the flux gain and magnification gain. 15 cm diameter input image intensifier tube ; output diameter 2,54 cm has a flux gain of 42. The brightness of the tube:
Magnification gain = (input diameter)2 / (Output diameter) 2 = 15 2/2.542 = 34,9
Total Brightness gain = Flux gain x Magnification gain = 42 x 34,9 = 1466
5) Imaging Characteristics
a) Contrast. Two factors diminish contrast in an image intensifier
i) The input screen does not absorb all the photons in the x-ray beam. Some are transmitted through the intensifier tube, and few are eventually absorbed by the output screen. These transmitted photons contribute to the illumination of the output phosphor but not to the image formation. They produce background of fog that reduces image contrast.
ii) Retrograde light flow from the output screen. Most retrograde light flow is blocked by a thin layer of aluminium on the back of the screen. Although the aluminium layer is extremely thin it would absorb electrons that convey the fluoroscopic image. Some light photons penetrate the aluminium and pass back through the image tube to activate the photocathode. These electrons produce “fog” and further reduce image contrast. The contrast tends to deteriorate as an image intensifier ages,
b) Lag. Lag is defined as the persistence of luminescence after the x-ray stimulation has been terminated. Older image tubes had long lad times, 40ms. CsI tubes lag times are 1ms.
c) Distortion. The center of the image intensifier screen has better resolution, a brighter image, and less geometric distortion.
i) “Pincushion effect”. Peripheral electrons do not strike the output phosphor where they ideally should. They tend to flare out from their ideal course. The result is unequal magnification, which produces peripheral distortion. The further the electrons from the center the more difficult to control. (this makes it difficult to evaluate straight lines esp when tying to reduce a fracture).
ii) Vignetting Unequal magnification also causes unequal illumination. The center is brighter than the periphery. A fall-off in brightness at the periphery of an image is called vignetting.
iii) Unequal focus. Resolution is better in the center of the screen.
6) Multiple Field Image Intensifiers.
a) Large Field View Image Intensifier Tubes
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