BASICS OF FLUOROSCOPY

Transcription

1 Medical Physics Residents Training Program BASICS OF FLUOROSCOPY Dr. Khalid Alyousef, PhD Department of Medical Imaging King Abdulaziz Medical City- Riyadh

2 Edison examining the hand of Clarence Dally with a fluoroscope 1896

3 Fluoroscopy for tuberculosis

4 The purpose of fluoroscopy is to provide a real-time x-ray imaging system. The two principle components are the image intensifier and the CC-TV image system. Image intensifiers have increased in size from the older 15cm systems to the modern 40cm systems. It s now possible to view an entire abdomen of the patient on the image intensifier. The development of High Definition TV will also help to improve the resolution of fluoroscopy systems.

5 Since the procedures requiring fluoroscopy may take some time, the exposure to the patient must be low. The x-ray exposure rate from fluoroscopy is several orders of magnitude less than that of general x-ray. Since the duration of the exposure is much longer and the penetration (kv) must still be maintained, it is the tube current that must be reduced. X-ray tube currents are usually much less for fluoroscopy, usually times less. The exposure rate for fluoroscopy is therefore also much less.

6 Relatively few x-rays are used to produce a fluoroscopic image. The resolution will therefore be less than that for radiographic imaging. Relative to general x-ray, the speed of the fluoroscopy systems are much much higher. The speed of the system will also have an effect on the rate at which the image can be refreshed. Frame rates of 30 frames per second are usually the limit.

7 REAL TIME IMAGING Dynamic studies require an imaging device that can follow events at a frame rate that is high enough so that a diagnosis can be made. A barium swallow study for example requires a fast enough frame rate to follow the peristaltic motion of the esophagus. Dynamic imaging in fluoroscopy provides adequate temporal resolution (frames per second) to visualize this function. Even higher frames rates are used for cardiac imaging, but this also requires a higher exposure rate.

8 POSITIONING Fluoroscopy is most commonly used for positioning. Following the progress of barium through the GI system or catheters as they pass through vessels are the procedures most commonly supported by fluoroscopy. Whilst the basic design of fluoroscopic systems is similar, systems can be designed for particular types of procedures. Small facilities will require a general purpose unit for a wide range of procedures whilst large facilities may have systems that are specifically designed for specialist tasks (e.g. cardiac, vascular, neuro, etc.)

9 FLUOROSCOPY EQUIPMENT

10 } More sensitive detector system than film. Low Current: ma Long exposure Controlled by foot switch } Tube capable of prolonged current

11 Image Intensification Tube Components Input screen and photocathode Electrostatic lenses Magnification tubes Anode Output phosphor

12 IMAGE INTENSIFIER This device is the major component of the fluoroscopy system. Its function is to convert the small number of x-rays incident upon it into visible light images. The primary technique is to amplify the small signal into a signal that can be viewed. The very early systems used a primitive x-ray fluorescent screen onto which x-rays were directed through the patient and viewed from behind. These very old systems produced huge radiation doses to the patient and the radiologist.

13 The modern image intensifier has 4 principle components:- 1. Vacuum bottle to keep all air out 2. Input layer to convert the incident x-rays into electrons 3. Electronic lenses to focus the electrons 4. Output phosphor to convert the energy of the electrons into visible light.

14 Focusing Electrodes Output Phosphur Incoming X-rays Electron Beam Anode Vacuum Tube

15 IMAGE INTENSIFIER Vacuum Bottle Whilst the task of the vacuum bottle seems simple, the design of the bottle is complex. The input phosphor may be up to 40cm in diameter and is able to withstand up to a ton of pressure from the outside atmosphere. Whilst maintaining the vacuum the bottle must also allow x- rays to penetrate. The loss of vacuum is usually the cause of degradation in the performance of the Image Intensifier. The Image Intensifier is a high cost item.

16 Input Layer The input layer is actually 4 layers. The first is a thin aluminum window built into the vacuum bottle. This window has a curved design to withstand the high pressures of the vacuum. This layer is usually about 1mm thick. The second is a substrate layer that supports the input phosphor but is also able to allow the penetration of as many x-rays as possible. This is also the first component of the electronic focusing system and is made from a 0.5mm layer of aluminum.

17 Input Layer The third layer is the input phosphor. It s function is to convert x-rays into visible light. This is similar in function to the screen in a film/screen radiographic system. It is also subject to the same compromising situation with regard to it s thickness (i.e. resolution vs sensitivity). Most Input Phosphors are made from CsI. CsI is constructed of thin needle-shaped light pipes, ideal for directing the light with minimal spreading. The K-edges of Cesium (36keV) and Iodine (33keV) allow a high x-ray absorption efficiency.

18 X-Ray Support CsI Needles Light Photocathode Electrons

19 IMAGE INTENSIFIER Input Layer The fourth layer is the photocathode. The primary function of this layer is convert light photons from the phosphor to photoelectrons. This layer is usually constructed by a thin alloy layer of antimony and alkali metals. The efficiency of the photocathode is approximately 10-20%. For a single 60keV x-ray photon absorbed in the phosphor, approximately 400 electrons are released from the photocathode.

20 Focusing Electrodes Aluminium Cover Output Phosphur Input Phosphur Anode Light Pipe Photocathode Vacuum Tube

21 IMAGE INTENSIFIER Electronic Lens System The purpose of the lens system is to focus the electrons produced by the photocathode and accelerate them towards the output phosphor. Electrons are released from the photocathode with little energy so a relatively large potential is applied to accelerate the electrons to the output phosphor. As each electron strikes each electrode in the lens system more electrons are produced, increasing the flux of electrons that finally strike the output phosphor. In order to maintain an equal path length for all electrons, the surface of the photocathode must be curved.

22 Output Phosphor The purpose of the output phosphor is to convert the electrons striking it into visible light. The output phosphor is usually made of zinc cadmium sulfide and emits green light at a wavelength of 530nm The output phosphor is thin (4-8 mm) to maintain the highest possible spatial resolution. Each electron striking the output phosphor will produce approximately 1000 light photons. The image produced by the output phosphor is highly minified. The input phosphor is 30-40cm in diameter while the output phosphor is just 2.5cm diameter.

23 Output Window The final stage of the image intensifier is the output window. The output window is part of the vacuum bottle and must allow as much light as possible to pass from the output phosphor. The glass window must transmit as much light as possible with as little reflected light as possible. Reflected light can cause a loss of image contrast and is called veiling glare. Output windows are now designed to transmit light and trap the small fraction of reflected light before it can travel back to the output phosphor.

24 IMAGE INTENSIFIER CHARACTERISTICS 1. Conversion Factor The conversion factor is a measurement of the light output relative to the input radiation exposure. Light Output is measured in units of Candela/m 2 (Cd/m 2 ) Input Exposure Rate is measured in units of mr/sec Conversion Factor = Light Input/Exposure Rate (Cd-s/mR-m 2 ) Typical conversion factors are in the range of The conversion factor will degrade with time and is another indication for the replacement of the II.

25 2. Brightness Gain This is a measure of the increase in light output from the output phosphor. The Brightness Gain is the product of the Flux Gain and the Minification Gain. Typically times. Brightness Gain = Flux Gain x Minification Gain The Flux Gain is the increase in the number of light photons emitted by the output phosphor compared to the input phosphor. Usually a factor of Minification Gain is the geometric change in the image size from the input to output phoshors. Minification Gain = (d input /d output ) 2

26 3. Contrast Ratio This is the ability of the II do identify contrast of a large object. Contrast is primarily determined by the amount of veiling glare, modern systems have reduced this and therefore enhanced the contrast of fluoroscopy systems. Contrast Ratio is determined by measuring the light output at the center of the FOV, with and without a lead disk to filter to block x-rays incident on the II. In an ideal system the light behind the lead disk should be zero and the contrast ratio should be infinite. In modern systems the actual values are in the range of

27 A Contrast Ratio = A/B B

28 4. Field of View (Magnification Modes) The physical size of the input phosphor of most modern II s is in the range of 23-40cm. The larger the II diameter, the more expensive the system. The larger systems are very useful for imaging the larger sections of the body, i.e. abdomen. It is however also useful for be able to magnify sections of the image to smaller fields of view. This is made possible by the electric field of the electric lens system. Modifying the voltage on this system effectively reduces the field of view.

29 5. Detected Quantum Efficiency The Detected Quantum Efficiency (DQE) is a parameter that relates the statistical quality of the output signal to that of the input x-ray signal. A perfect system would have a DQE of 100%. Noise will always reduce DQE. The DQE will also be highly dependant on the x-ray spectrum of the incident radiation. Typical DQE values are in the range of 60-80%. The advantage of a high DQE is that low noise images can be produced at the same exposure rate.

30 6. Limiting Spatial Resolution The resolution of a modern II ranges from 4-5lp/mm for a 23cm FOV up to 7lp/mm for higher magnification systems. The limiting spatial resolution is easily measured with a line/pair phantom. Whilst this measurement may be subjective, it is still a useful guide in the clinical setting.

31 Field Size Input Field size typically in the range 23-57cm. Size can be reduced further electronically to 14 17cm. Large field sizes for imaging the abdomen. Resolution decreases with increasing field size 23cm (5lp/mm) to 57cm (3lp/mm). Output Phosphor typically 2.5cm diameter. Input Phosphor µm thick Cesium Iodide (CsI) Like screens, resolution decreases as phosphur thickness increases.

32 Output Phosphor 4-8µm thick to preserve the spatial resolution of the image Zinc Cadmium Sulphide Usually emits light in the green section of the visible spectrum (530nm) Each electron striking the output phosphor produces about 1000 light photons.

33 Image Intensifier (II) Light emitted by the input phosphor is absorbed by the photocathode to produce photoelectrons. Photoelectrons are absorbed in the output phosphor and converted to light Image brightness increased by minimizing the image and producing many light photons for each x-ray absorbed. Brightness Gain = Minimization Gain x Flux Gain Minimization Gain = Flux gain = Therefore, Brightness Gain =

34 Image Quality Resolution in the range 3-5lp/mm Resolution will decrease at the edges Since fluoroscopy is performed at low doses, images are noisy. Artifacts Lag due to continued luminescence (minor problem) Veiling Glare Light scatter in the output image adds to the total background of the image. Pincushion distortion of the image at the edges of the II Vignetting low brightness at the edges of the II

35 Optical System TV Camera Photospot or Cine Camera

36 Radiation Exposure Whilst the radiation dose rate from fluoroscopy is much lower than from conventional x-ray, the exposure times from fluoroscopy is much longer. The radiation dose rate to the skin can be quite high and is limited to 100mGy/minute. Hence, most fluoroscopy systems will have an alarm to warn the user of excessive radiation doses.

37 Digital Fluoroscopy An analogue-to-digital converter can be used to digitize the TV output of the system. The digital images can be stored on a computer system and the images can be further processed Multiple images can be stored as a series at frame rates of up to 30 frames/second on a digital system. Digital Subtraction Angiography (DSA) involves the subtraction of a background image from an angiographic image.

38 Digital Fluoroscopy Image matrix of 1024x1024 pixels

39 Digital Subtraction Angiography A Post-Contrast Image is subtracted from a Pre-Contrast Mask Image. The result is an image of the contrast only and gives the best visualisation of vessels. Post processing is possible on the resulting images. Image matrix sizes usually 512 x 512 and 1024 x 1024

40 Image Intensifier Artifacts 1. Veiling Glare This is predominantly the result of light scatter from the output window of the Image Intensifier. This scattered light will have an affect on the entire area of the field of view and adds to the entire background of the image. Modern II s have a design that reduces this effect and there is little that can be done by the technologist to reduce this. Measuring the Contrast Ratio will indicate the particular amount of Veiling Glare resulting from the output window design.

41 2. Pincushion Distortion This is a form of spatial distortion that warps the appearance of the image. This is due to mapping the image from a curved input phosphor to the flat output phosphor. This distortion results in a slightly higher magnification at the edges of the field of view. Viewing the affect of pincushion distortion can best be done using a grid or screen with a regular spacing of lines.

42 Input Image Output Image

43 3. Vignetting The brightness of the entire field of view will vary from the center to the edge. The brightest section of the image will be at the center. This is brought about in a similar way to that of the pincushion artifact. Magnification will not only create the pincushion effect but also reduce the intensity of the image at the edges. Even though the image is minified by the electron focusing system, the loss of brightness at the edges will still be apparent.

44 4. S Distortion The S-Distortion is a spatial warping of the image along an s- shaped axis through the image. This type of distortion is subtle and the effect of pincushion distortion is usually more pronounced. S-distortion is the result of stray magnetic fields that affect the electron focusing lens system. When electrons are accelerated they will be influenced by magnetic fields. If the image intensifier is rotated the s-distortion will follow the direction will change position and align with the magnetic field.

45 OPTICAL COUPLING Distribution Mechanism. The image coming from the Output Phosphor requires a pair of lenses to focus the image onto the TV system. In simple systems this is carried out by a single fully enclosed connection. In more complex systems, the image from the Output Phosphor may be directed to more than one device. An Optical Distributor is used to direct the image through a mirror system to multiple devices. This mirror system is called a beam splitter. No mechanical devices are used in the beam splitter, it is simply a system of mirrors.

46 OPTICAL COUPLING Aperture Effect on Dose Rate and Noise. The light output of the Output Phosphor varies greatly as the exposure rate of the II changes. The image brightness that the TV system can view is limited. An aperture is placed between the optical coupling mechanism and the TV system to adjust the light input to the TV system. Adjusting the aperture will require an adjustment to the input exposure to the II in order to obtain good image quality. Hence this will also affect the radiation dose and image noise.

47 Video Camera Lens Accessory Port Aperture Mirror Output Window Output Phosphor

48 Optical System TV Camera Photospot or Cine Camera

49 VIDEO CAMERAS The output image of the optical distributor is connected to a CC-TV system on most modern fluoroscopy systems. The major disadvantage of using CC-TV for viewing is that the image resolution of the CC-TV system will be less than that of the Output Phosphor. Disadvantage: Since CC-TV resolution is low, this image is only used for positioning, permanent images are recorded using a different system attached to the accessory port.

50 Advantages of video camera: The CC-TV monitor is large and can be viewed from a large distance. The CC-TV monitor will remain stationary, even if the II is moved relative to the patient. A number of monitors can be set-up to view the same image from the CC-TV camera in multiple locations. The CC-TV image can easily be recorded on video tape for later viewing The distance between the II and the CC-TV monitor can be large, thereby reducing crowding in the fluoroscopy suite.

51 Components of video camera: The components of the CC-TV system are the TV camera, TV monitor and the camera control unit. The camera converts the 2-dimensional image into a single signal of varying voltage. The voltage is proportional to the brightness of the output image in the same relative location of the filed of view of the camera. This video signal is transmitted along a cable to the TV monitor and the one-dimensional signal is converted back to a 2- dimensional image by the monitor.

52 VIDEO CAMERAS Image Production: TV camera design has aimed to develop a system that reduces image lag. Image Lag becomes a problem when large bright objects move quickly across the field if view, leaving a comet tail artifact. Modern cameras have largely overcome this problem.

53 VIDEO CAMERAS New Developments Modern CCD (charge couple device) cameras use an array to record an image instead of an electron beam. CCD camera produce images similar to a computer matrix with a 512x512 or 1024x1024 pixel image. CCD cameras are very rugged and this technology is used in many of the modern home video cameras. CCD cameras produce virtually no lag. The introduction of HDTV will also benefit fluoroscopy. Higher resolution TV systems will be able to record the high resolution output phosphor images.

54 AUTOMATIC BRIGHTNESS CONTROL The exposure rate of a fluoroscopic system is controlled by the Automatic Exposure Control to ensure the brightness of the image remains constant. The exposure rate will vary greatly depending on the body part being imaged. The system monitors the brightness of the output phosphor and controls the kv and ma of the x-ray generator to maintain constant brightness. A light sensitive device (usually a photodiode) is used to measure the light output of the output phosphor. The signal from the light sensitive device feeds back to the x- ray generator.

55 Whilst the Automatic Brightness Control has the ability to change both the kv and ma, most systems have a predetermined sequence for modifying kv and ma. An increase in kv will result in a decrease in image contrast so if extra x-rays are required, ma is usually altered first before kv. It is also important to maintain low patient doses. Lower patient doses are achieved by limiting the increase in ma before increasing kv. It is always a trade-off between image contrast and patient dose.

56 SYSTEM RESOLUTION The overall resolution of a complex system is the product of the resolution of the components that make-up the system. The component with the poorest resolution will have the largest detrimental effect on overall system resolution. Of the three major components of the fluoroscopic imaging system (Image Intensifier, Optics and TV camera), the TV camera has the poorest resolution. Optics have the least effect on resolution, ahead of the Image Intensifier. Plotting the Modulation Transfer Function of each component will indicate the contribution of each component to spatial resolution.

57 IMAGE ACCESSORIES FOR FLUOROSCOPY Photospot Camera This camera is attached to the accessory port of the optical distributor. The camera records the image on radiographic film, usually in a small 100x100mm square format. The image is created from a single radiographic pulse of x-rays and the film ejected after exposure. The resolution of the photospot film will actually be higher than that of the TV image. This is generally an old technology replaced now by digital devices.

58 IMAGE ACCESSORIES FOR FLUOROSCOPY Digital Photospot This is a high resolution TV camera system which digitizes the image and stores it in a computer system. The image is created by a single radiographic exposure from the x-ray generator The resulting image is displayed on a computer screen. The resolution of this system is that of the TV System and is usually 1023 lines vertical. Hardcopy imaging can be printed on a laser printer from the computer image. Digital photospot cameras are in common use on modern fluoroscopy systems.

59 IMAGE ACCESSORIES FOR FLUOROSCOPY Cine Radiography A cine camera is attached to the accessory port of the optical distributor. The camera can record a very rapid sequence of images, usually on 35mm film. Cine radiography is primarily used in cardiology where frames rates of up to 120 frames/second are required. Images are recorded from sequences of short radiographic pulses from specially designed x-ray generators. X-ray tube loading is usually quite high for these systems.

60 RADIATION EXPOSURE Skin entrance exposure is the primary concern in fluoroscopy. Organ doses are reduced by the attenuation of overlying soft tissue. Skin doses of 5-50mGy/min are typical skin doses for fluoroscopy. Maximum allowable doses vary from country to country from mGy/min. Skin erythema is a possible outcome from high dose fluoroscopy. Patient doses can be reduced by reducing fluoroscopy time and increasing the field of view.