Principle of X-Ray Systems

Transcription

1 Principle of X-Ray Systems Hossein Ebrahimi Nasab PHYSICS OF X-RAYS Nature of X-rays Energy unit Interaction with matter INTERACTION WITH THE MATTER In vacuum: photon move along a straight line In materials, photon can be - transmitted (no interaction) - absorbed (they disappear, transferring their energy to the material), - scattered: they are deviated, with or without loss of energy X Ray Imaging INTERACTION WITH THE MATTER Absorption increases with the density of the material it is what we want to assess, and also what causes biological damage Transmission is what we can see photographically It is what we use to assess the amount of radiation absorbed Scatter is undesirable It interferes with the measurement of the transmitted radiation High density tissues (bones) result in low transmission Scattered radiation reduces the quality of the image History November 8, 1895 Wilhelm Conrad ROENTGEN, a German physicist, discovers accidentally unknown rays, able to pass through his hand and image the bones and his ring He called them. X-rays He was awarded the first Nobel Prize for physics in 1901 One year later, Elihu Thomson demonstrates use of X-rays for diagnosis of fractures and location of foreign objects and Eddy C. Jerman starts the first school for Radiographers PARTICLES ENERGY (Definition) When an electron is submitted to an electrical field, it is accelerated and gets a speed (an energy) proportional to the applied voltage. This energy is expressed in ev : Electron Volt When this electron reaches the positive electrode, it has a kinetic energy of 1000eV or 1 KeV KeV is a unit of energy used for all kinds of particles PRODUCING AN IMAGE The X-ray image displays the part of the X-ray beam not absorbed by the tissues. The less the absorption, the darker the film. The radiograph displays these differences in tissue density 1

2 Technology The heated filament produces a cloud of free electrons These electrons are attracted and accelerated towards the anode by the electrical field (kv) The flow of electrons is an electrical current (ma) The electrical power (100 kv x 100 ma = 10 kw) is converted into heat (99%) and X-ray power (1%) X-ray Spectrum: Variation with ma At a given energy, the rate of photon production is proportional to the ma 1 ma10 ma50 ma100 ma500 ma1000 ma the shape of the spectrum remains the same when ma is varied The Main Component HV Generator Power supply Exposure Exposure parameters control Image receptor Static (Radiography) Film Dynamic (Fluoroscopy) Image Intensifier + video camera X-ray tube Production of X-rays Collimator Beam size control Static & dynamic Digital detector+ Workstation X-ray Spectrum: Variation with Anode Material it changes the emission efficiency (continuous spectrum) FILTRATION Beam hardening (low energy filtration) Any material in the beam path absorbs more of the lower energy radiation (harmful for the patient) than the higher energy Regulation require a minimum of 2.5 mm Aluminum equivalent filtration THE X-RAY TUBE it determines the presence of characteristic radiations and their energy insert Tube casing (housing) X-ray Spectrum: Variation with kv kv changes the emission efficiency it changes the shape of the spectrum : maximum energy of photons in the spectrum (in kev) has the same value as tube HV (in kv) it has a significant effect on the relative amplitude of characteristic radiations Take Away 99% of the electrical power is converted into heat The X-ray beam is multi-energetic (spectrum) Continuous spectrum Specific peaks kv control mainly the maximum energy ma control the intensity of the beam Anode material defines the specific peaks Anode/filter combination enhances specific energies Tube casing (housing) The x-ray tube insert is mounted inside a protective housing. The housing is made of aluminum and lined with lead. The housing: Controls scatter and leakage radiation Isolates high voltage Cools the tube 2

3 X-RAY TUBE COMPONENT: Frame X-RAY TUBE COMPONENT: Anode The outer part of the insert (sometimes called the enclosure or frame) can be made of glass, steel, or copper. The tube is inserted inside a casing and immersed in oil for electrical insulation and cooling The casing also shields x-rays emitted in all directions The enclosure: Provides a structural base for the cathode and anode Provides high-voltage insulation Allows a vacuum to be maintained X-rays are produced by energy conversion when electrons strike the anode. The anode consists of 3 major components: A target, which receives a stream of electrons from the cathode A rotor of an induction motor, which holds the target and drives its rotation at a rate of approximately 10,000 rpm. This rotation dissipates the heat generated during the production of x-ray A bearing assembly, which supports the rotor and target assembly, and provides smooth rotation of the target Insert Compeonent Tube insert consist of the: Frame Cathod Anode X-RAY TUBE COMPONENT: Cathode(1) The cathode provides the electron stream for x-ray production. The cathode consists of: Filaments: which are coils of tungsten wire that, when heated, provide the electron stream for x-ray production Focusing cup: which surrounds the filament and concentrates, or focuses, the electron beam Rotore X-RAY TUBE COMPONENT: Cathode(2) Dual Focus Cup Some focusing cups have two filaments. X-ray Generation Filament and Cathode Anode Electrons e - beam X-Rays beam The large filament: used for higher power levels needed for dense anatomy. (High ma, high kv, or both) The small filament: used for lower dosage levels, which provides better resolution. The process by which the x-ray tube produces x-rays is very inefficient. Only one percent of the energy produced is converted to x-rays; the rest is dissipated as heat. 3

4 Focal Spot Characteristics Actual Focal Spot: The electron beam from the tube filament lands on the angled target surface and creates the focal spot. This area (rectangular area) called the actual focal spot. انتخاب ظرفيت حرارتي مناسب ظرفيت حرارتي 300KHU توان دفع حرارتي 40KHU/min ظرفيت حرارتي بدنه تيوب 1250KHU در صورت عدم دفع حرارت توسط تيوب در صورتي كه تابش ها تركيبي از نماهاي فوق باشد با متوسط 3000HU براي هر تابش مي توان 100 تابش داشت The actual focal spot is the area on the anode target where electrons strike, and where x-rays and heat are produced با توان دفع حرارتي 40KHU/min مي توان 4 عكس مهره كمري در دقيقه داشت Focal Spot Characteristics Effective Focal Spot: The area from which the x-ray beam appears to come. Viewed from the perspective of the central ray. Anode Heat Capacity Energy= Kv*mA*time Take Away An X-ray tube is characterized by: Focal spot size (single or dual) The smaller the focus, the better the spatial resolution Anode rotation speed Maximum heat storage capacity (khu or kj) The x-ray tube's effective focal spot is commonly called the focal spot. Focal Spot Characteristics Focal Spot Size and Shape: Length of the tube filament Diameter of the filament wire coil Width and length of the focusing cup slot Depth of the focusing cup Focal Spot Size and Target Angle: as the target angle increases, the effective focal spot size increases. The slope of the anode target allows a larger area to be heated while keeping the apparent area from which x-rays are produced as small as possible. مقدار انرژي حرارتي ايجاد شده در يك تابش اندام KV mas HU دست زانو قفسه سينه مهره كمري رخ شكم (چاق) مهره كمري جانبي The Generator The main functions of the x-ray generator are: Control kv applied to the x-ray tube, thereby controlling x-ray beam penetration Control the amount of ma in the tube, thereby controlling the quantity of x-ray beams Control exposure time and switching 4

5 Types Of Generators(1) Single phase generator Three phase generator High frequency generator Battery powered generator Capacitor discharge Types Of Generators(4) High frequency generator: Generator Control Console The control console variables include: kv ma Exposure time Types Of Generators(2) Single phase generator: Types Of Generators(5) Battery powered generator: Generator Parameters Importance of High KV & ma KV Penetration Good For Thick Patient ma Resolution High Contrast Image Power of Generator = Kilo Watt (KW) = KV x ma / 1000 mas = ma x Time (ma x sec) Types Of Generators(3) Three phase generator: Types Of Generators(6) Capacitor discharge: Tube Exposure Radiation Power Voltage Current Time Amount Unit kw kv ma sec mas Clinical Meaning Shorter Time No Penetration Contrast avoids motion blur Accumulated Dose Amount Correlation W = VxA mas=maxsec 5

6 Generator Classes Middle Power Generators Around 50kW For General Purpose Fluoroscopy is possible High Power Generators Around 80~100kW For long time exposure For high ma Fluoroscopy Low Power Generators Example : Less than 20kW Limited with Radiography General Radiology The collimator: Collimator(1) Controls the dimensions of the x-ray beam Aligns the x-ray beam, the patient, and the bucky prior to exposure Limits patient and technician exposure to radiation Reduces scatter radiation Cassette Tray The cassette tray is a mechanical component of the x-ray system. Purpose : Holds the cassette Centers the cassette System Overview Collimator(2) Two pairs of shutters control the beam dimensions. Patient Centering A light and mirror are built into the collimator to pre-position the x-ray beam. The simulation light allows the technologist to pre-center the beam on anatomical landmarks. Notice the cross hair on the patient's shoulder. Bucky A bucky provides the housing for three assemblies: Cassette tray Grid Automatic exposure control A bucky can be in a horizontal (table) or vertical (chest stand) orientation. 6

7 Automatic Exposure Controller (A.E.C) Automatic Exposure Control measures the dose of radiation that strikes the X-ray film behind the patient, and turns the X-ray system off when the predetermined dose for that screen-film combination has been reached. This assures that only the smallest required dose is administered. The resulting images all show a uniform blackening, and the danger is reduced that the X-ray examination might have to be repeated owing to an error in the image. In this way, automatic exposure timing also indirectly reduces the dose. Grid Design A radiographic grid is composed of strips of lead, separated by aluminum, a material that is relatively transparent to x- rays. The graphic shows how grid removes scatter radiation. Because the scatter radiation, represented by dotted lines, moves in various directions, it is largely absorbed by the lead strips. The transmitted x-rays pass untouched through the grid and reach the film. A E C In 1913, Gustav Bucky introduced the radiographic grid, a device placed between the patient and the cassette. The purpose of the grid is to: Reduce the amount of scatter radiation reaching the film Increase contrast in the x-ray image Scatter radiation is a noise factor. It causes a fogging which impairs radiographic quality, lessening image contrast. Scatter provides no useful information. Grid Grid Pattern(1) X-ray grids are available with either focused or parallel lead strips. Focused Grid: The lead strips of this grid are angled. A line drawn through each lead strip intersects at a point a specified distance from the grid, as shown here. Two important characteristics of the focused grid are: The distance from the grid to the point of intersection is called the focal distance of the Grid The density of the x-ray image is uniform from edge to edge, provided the grid is properly centered and leveled relative to the THE RADIOGRAPHIC FILM CASSETTE: The X-Ray Film GRID TYPES GRID SPECIFACTIONS Grid Pattern(2) Parallel Grid When the lead strips are not angled, they are all perpendicular to the face of the grid. This type of grid is termed parallel. Parallel grids have limited use because: The uniform density of an x-ray exposure is limited to a width of about 10 cm The advantage of a parallel grid is: significant only for an extremely long focal distance Latent Image Formation When the radiation interacts with the silver halide crystals in the film emulsion, the image on the film is produced. The image, which is not visible before processing, is called the latent image. An example of another type of latent image is fingerprints. If you touch an item, you leave your fingerprints even though you cannot see them on that item. When that item is treated, your fingerprints become visible. 7

8 Film Speed Film speed refers to the amount of radiation required to produce a radiograph of standard density (darkness). Film speed is determined by the following factors: The size of the silver halide crystals The thickness of the emulsion The presence of special radiosensitive dyes (Cont d) X-Ray Film Processing Processing is a series of steps that changes the latent image on the exposed film into a radiograph by producing a visible image on the film. Proper processing is just as important as exposure technique in producing diagnostic-quality radiographs. Radiographs that are nondiagnostic because of poor processing techniques must be retaken, exposing the patient to unnecessary radiation. Rinsing Rinsing of the films is necessary to remove the developer from the film so that the development process stops. Usually, agitating the film rack for 20 seconds is sufficient. This must be done under safelight conditions. Film Speed (Cont d) The film speed determines how much exposure time is required to produce the image on the film. A fast film requires less radiation; the film responds more quickly because the silver halide crystals in the emulsion are larger. The larger the crystals, the faster the film speed. This is the same principle as film speed on photographic film. F-speed film, the newest and fastest film on the market today, reduces radiation exposure to the patient by 20% to 60% compared with E-speed and D-speed film. The Five Steps in Processing Development Rinsing Fixation Washing Drying Fixing The acidic fixing solution removes the unexposed silver halide crystals from the film emulsion. The fixer also hardens the film emulsion during this process. For permanent fixation, the film is kept in the fixer for a minimum of 10 minutes. However, films may be removed from the fixing solution after 3 minutes for viewing. Films that are not properly fixed will fade and turn brown in a short time. Leaving films in the fixer for a long time (e.g., over a weekend) can remove the image from the film. Intensifying Screen An intensifying screen intensifies or increases the effect of the radiation and thus decreases the amount of exposure time needed. The intensifying screen is coated with a material called phosphor that gives off light when struck by x-radiation. The film inside the cassette is sandwiched between the intensifying screens and is affected by both the light produced by the phosphor and the x-radiation. Developing Developing is the first step in processing films. A chemical solution called the developer is used. The purpose of the developer is to chemically reduce the exposed silver halide crystals to black metallic silver. The developer solution also softens the film emulsion during this process. Washing After fixation, a water bath is used to wash the film. The washing step requires about 20 minutes to thoroughly remove all excess chemicals from the emulsion. 8

9 The final step in film processing is the drying of the films. Films may be air-dried at room temperature in a dust-free area or placed in a heated drying cabinet. Films must be completely dried before they can be handled for mounting and viewing. Drying Components of the Automatic Processor The processor housing covers all of the component parts. The film feed slot is for the unwrapped films to be inserted into the automatic processor. The roller film transporter is a system of rollers that rapidly moves the film through the compartments. The developer and fixer compartments holds the solutions.the film is transported directly from the developer into the fixer without a rinsing step. The water compartment holds circulating water. The drying chamber holds heated air and dries the wet film. INFLUENCE OF ENERGY (kv) Image Quality(2) The Automatic Processor Automatic film processing is a fast and simple method used to process x-ray films. Other than opening the film packet, all steps of film processing are handled by the automatic processor. Automatic film processing requires only 4 to 6 minutes for the development, fixing, washing, and drying of a film, whereas manual processing techniques require approximately 1 hour. (Cont d) Image Quality(3) INFLUENCE OF ENERGY (kv) The choice of kv depends on the patient anatomy and the object to be imaged Thick or dense part of the body: High kv, ex. Abdomen, pelvis: 80 to 100 kv Small or flat parts of the body: Low kv, ex. Breast, hand: 30 to 50 kv Then the necessary mas is elected to produce the clinically useful image The Automatic Processor (Cont d) The automatic processor maintains the correct temperature of the solutions and adjusts the processing time. Proper maintenance of the automatic processor reduces the chance of errors during film processing. Image Quality(1) The higher the energy, INFLUENCE OF ENERGY (kv) the higher the transmission ratio, the better the penetration Image Quality(4) INFLUENCE OF FOCAL SPOT SIZE: PENUMBRA Sharp projection from a fine focal spot High energy may be necessary in dense or thick areas (abdomen, pelvis) Very low energies are rarely of use and are harmful to the patient Large focal spot is responsible for blurr, leading to inability to distinguish between small, close objects LOSS OF SPATIAL RESOLUTION 9

10 Image Quality(5) The blurr effect, or Penumbra, can be decreased by: Increasing the Source to Object Distance Placing the image receptor as close as possible to the patient Take Away High energy (kv) = high penetration, low dose Low energy = high contrast Very low energy X-rays are undesirable There is an optimum energy to image each object The focal spot should be as small as possible. but thermal constraints Scattered radiation can be reduced by a grid, but it results in dose increase Mobile Radiology 10