FI_MRT_Final book biomedical instrumentation

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1. Fluoroscopic Imaging Systems Basic principles M R Talukder Department of Applied Physics and Electronic Engineering University of Rajshahi Book theke porte hbe eita

2. OBJECTIVES Differentiate fluoroscopic examinations from static diagnostic radiographic examinations Describe a typical basic fluoroscopic image chain Explain difference between fluoroscopic operation and a diagnostic x-ray tube Safety Principles

3. INTRODUCTION Since Thomas A. Edison invented the fluoroscope in 1896, it has served as a valuable tool in the practice of radiology It also a radiologic technique like X-ray A fluoroscope is used to visually examine the body or an organ An image intensifier is used to intensify improve image quality Direct visualization is obtained by which cardiac catheterization, thin needle biopsies of tumors, and localization of foreign bodies.

4. APPLICATIONS A radiologic technique in which a fluoroscope is used to visually examine the body or an organ . (A fluoroscope utilizes an X-ray tube and fluorescent screen, with the area to be viewed placed between the screen and the tube.) This immediate imaging, when coupled with an image intensifier, is invaluable in situations such as cardiac catheterization, thin needle biopsies of tumors, and localization of foreign bodies. The principal advantage of image-intensified fluoroscopy over earlier types of fluoroscopy is increased image brightness . Just as it is much more difficult to read a book in dim illumination than in bright illumination, it is much harder to interpret a dim fluoroscopic image than a bright one.

5. IMAGE INTENSIFIER The image-intensifier tube is a complex electronic device that receives the image-forming x-ray beam and converts it into a visible-light image of high intensity. The tube components are contained within a glass or metal envelope that provides structural support but more importantly maintains a vacuum. When installed, the tube is mounted inside a metal container to protect it from rough handling and breakage.

6. IMAGE INTENSIFIER

7. X-rays that exit the patient and are incident on the image-intensifier tube are transmitted through the glass envelope and interact with the input phosphor, which is cesium iodide ( CsI ). When an x-ray interacts with the input phosphor, its energy is converted into visible light; this is similar to the effect of radiographic intensifying screens. The CsI crystals are grown as tiny needles and are tightly packed in a layer of approximately 300 μm Each crystal is approximately 5 μm in diameter. This results in microlight pipes with little dispersion and improved spatial resolution. IMAGE INTENSIFIER

8. The image-intensifier tube is approximately 50 cm long. A potential difference of about 25,000 V is maintained across the tube between photocathode and anode so that electrons produced by photoemission will be accelerated to the anode IMAGE INTENSIFIER TUBE

9. Photocathode

10. The next active element of the image-intensifier tube is the photocathode , which is bonded directly to the input phosphor with a thin, transparent adhesive layer. The photocathode is a thin metal layer usually composed of cesium and antimony compounds that respond to stimulation of input phosphor light by the emission of electrons . The photocathode emits electrons when illuminated by the input phosphor. This process is known as photoemission . The term is similar to thermionic emission , which refers to electron emission that follows heat stimulation. Photoemission is electron emission that follows light stimulation. Photocathode

11. PHOTOCATHODE: MECHANISMS HV power supply

12. The anode is a circular plate with a hole in the middle through which electrons pass to the output phosphor, which is just the other side of the anode and is usually made of zinc cadmium sulfide. The output phosphor is the site where electrons interact and produce light . For the image pattern to be accurate, the electron path from the photocathode to the output phosphor must be precise. The engineering aspects of maintaining proper electron travel are called electron optics because the pattern of electrons emitted from the large cathode end of the image-intensifier tube must be reduced to the small output phosphor. The devices responsible for this control, called electrostatic focusing lenses, are located along the length of the image-intensifier tube. The electrons arrive at the output phosphor with high kinetic energy and contain the image of the input phosphor in minified form. ANODE

13. BRIGTNESS GAIN (B.G.) B.G. = Minification gain x Flux gain MOST INTENSIFIERS: 5,000 – 20,000 The increased illumination of the image is due to the multiplication of light photons at the output phosphor compared with x-rays at the input phosphor and the image minification from input phosphor to output phosphor. The ability of the image intensifier to increase the illumination level of the image is called its brightness gain. The brightness gain is simply the product of the minification gain and the flux gain . The interaction of these high-energy electrons with the output phosphor produces a considerable amount of light. Each photoelectron that arrives at the output phosphor produces 50 to 75 times as many light photons as were necessary to create it. This ratio of the number of light photons at the output phosphor to the number of x-rays at the input phosphor is the flux gain.

14. FLUX GAIN # OF PHOTONS AT THE OUTPUT PHOSPHOR # OF PHOTONS AT THE INPUT PHOSPHOR The interaction of these high-energy electrons with the output phosphor produces a considerable amount of light. Each photoelectron that arrives at the output phosphor produces 50 to 75 times as many light photons as were necessary to create it. This ratio of the number of light photons at the output phosphor to the number of x-rays at the input phosphor is the flux gain . The minification gain is the ratio of the square of the diameter of the input phosphor to the square of the diameter of the output phosphor. Output phosphor size is fairly standard at 2.5 or 5 cm. Input phosphor size varies from 10 to 35 cm and is used to identify image-intensifier tubes.

15. MINIFICATION GAIN SQUARE OF THE INPUT PHOSPHOR DIAMETER SQUARE OF THE OUTPUT PHOSPHOR DIAMETER Brightness gain is now defined as the ratio of the illumination intensity at the output phosphor, measured in candela per meter squared ( cd /m 2 ) to the radiation intensity incident on the input phosphor, measured in milliroentgens per second ( mR /s). This quantity is called the conversion factor and is approximately 0.01 times the brightness gain. The conversion factor is the proper quantity for expressing image intensification . Image intensifiers have conversion factors of 50 to 300. These correspond to brightness gains of 5000 to 30,000.

16. Mechanism : Differences between X ray and CT scan X-rays use a single beam of radiation to produce a two-dimensional image, while CT scans use multiple X-ray beams at different angles to create a three-dimensional image. The multiple angles of the CT scan allow for a more detailed image of the body’s internal structures, making it an ideal tool for detecting abnormalities in the lungs, heart, and other organs. Suitability : Differences between X ray and CT scan CT scans provide more detailed images than X-rays, as they create 3D images of the lungs and heart, compared to X-rays that only provide a 2D image. This means any abnormalities in the lungs or heart can be detected quickly. CT scans combine the benefits of both X-rays and MRI scans. Radiation : Differences between X ray and CT scan X-rays use ionizing radiation, which can cause cancer and other diseases such as leukemia or thyroid cancer if exposed for long periods or if receiving multiple exposures at once. However, X-rays use a low level of radiation that is generally safe for most people. X-rays are safer for pregnant women and children compared to CT scans. Patient-base : Differences between X ray and CT scan CT scans may not be suitable for everyone. For example, children may not be able to lie still enough for this test, and individuals with pacemakers may experience interference with the test results. Non-medical usage : Differences between X ray and CT scan X-rays are not only used for medical purposes but also for non-medical purposes, such as airport security screening or manufacturing quality control. Duration : Differences between X ray and CT scan X-rays are faster than CT scans as they do not require any contrast medium or radiation exposure like CT scans. However, X-rays do not provide enough detail for examining soft tissues such as muscles and ligaments Cost : Differences between X ray and CT scan Though there are wide ranges of x-rays and CT Scans, it wouldn’t be false to say that all the x-ray procedures cost much less than CT Scans.