Presentation on theme: "RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY"— Presentation transcript:
1 RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY Part No...., Module No....Lesson NoModule titleIAEA Training Material on Radiation Protection in Diagnostic and Interventional RadiologyRADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGYL 6: X Ray productionPart …: (Add part number and title)Module…: (Add module number and title)Lesson …: (Add session number and title)Learning objectives: Upon completion of this lesson, the students will be able to:…. (Add a list of what the students are expected to learn or be able to do upon completion of the session)Activity: (Add the method used for presenting or conducting the lesson – lecture, demonstration, exercise, laboratory exercise, case study, simulation, etc.)Duration: (Add presentation time or duration of the session – hrs)Materials and equipment needed: (List materials and equipment needed to conduct the session, if appropriate)References: (List the references for the session)IAEA Post Graduate Educational Course in Radiation Protection and Safe Use of Radiation Sources
2 Basic elements of the X Ray source assembly Generator : power circuit supplying the required potential to the X Ray tubeX Ray tube and collimator: device producing the X Ray beam6: X Ray production
4 X Ray tube componentsCathode: heated filament which is the source of the electron beam directed towards the anodetungsten filamentAnode (stationary or rotating): impacted by electrons, emits X RaysMetal tube housing surrounding glass (or metal) X Ray tube (electrons are traveling in vacuum)Shielding material (protection against scattered radiation)6: X Ray production
5 X Ray tube components housing cathode 1: long tungsten filament 2 : short tungsten filament3 : real size cathode1: mark of focal spot6: X Ray production
6 Cathode structure (I)Cathode includes filament(s) and associated circuitrytungsten material : preferred because of its high melting point (3370°C)slow filament evaporationno arcingminimum deposit of W on glass envelopeTo reduce evaporation the emission temperature of the cathode is reached just before the exposurein stand-by, temperature is kept at ± 1500°C so that 2700°C emission temperature can be reached within a second6: X Ray production
7 Cathode structure (I) Modern tubes have two filaments a long one : higher current/lower resolutiona short one : lower current/higher resolutionCoulomb interaction makes the electron beam divergent on the travel to the anodelack of electrons producing X Rayslarger area of target usedfocal spot increased lower image resolutionFocalisation of electrons is crucial !6: X Ray production
8 X Ray tube characteristics Anode mechanical constraintsMaterial : tungsten, rhenium, molybdenum, graphiteFocal spot : surface of anode impacted by electronsAnode angleDisk and annular track diameter (rotation frequency from 3,000 to 10,000 revolutions/minute)Thickness mass and material (volume) heat capacityAnode thermal constraintsInstantaneous power load (heat unit)Heat loading time curveCooling time curve6: X Ray production
9 This conflict is solved by slanting the target face Anode angle (I)The Line-Focus principleAnode target plate has a shape that is more rectangular or ellipsoidal than circularthe shape depends on :filament size and shapefocusing cup’s and potentialdistance between cathode and anodeImage resolution requires a small focal spotHeat dissipation requires a large spotThis conflict is solved by slanting the target face6: X Ray production
10 Anode characteristic 1 : anode track 2 : anode track 6: X Ray production
11 Induction motor Works on the principle similar to the transformer. Electromagnetic induction.Current flowing in the stator develops a magnetic field.Stator windings are sequentially energized so that the induced magnetic field rotates on the axis of the stator.This causes the rotor to rotate.
12 Line focus principleThe area of the x-ray tube anode from which the x-ray photons are emitted.This is called the actual focal spot
13 THE BETTER THE RESOLUTION Anode angle (II)Angle‘ AngleActual focalspot sizeActual focalspot sizeIncident electronbeam widthIncident electronbeam widthIncreasedapparentfocal spot sizeApparent focal spot sizeFilmFilmTHE SMALLER THE ANGLETHE BETTER THE RESOLUTION6: X Ray production
14 Line focus principleWas incorporated into x-ray tube targets to allow a large area for heating while maintaining a small focal spot.The effective focal spot is the area projected onto the patient and film.
15 Line focus principleFocal spot sizes always make reference to the effective focal spot.The lower the target angle, the smaller the effective focal spot size.
16 Line focus principleThe advantage of the line-focus principle is that it provides the detail of a small focal spot while allowing a large amount of heat dissipation.
17 Line focus principleThe unfortunate bi-product of the line-focus principle is the “anode heel effect”
18 Anode heel effectConstruction phenomenon that causes the x-ray photons exiting the tube on the cathode side to have a greater energy value than those exiting the tube on the anode side.
19 Anode heel effectMore energy absorption occurs at the anode heel resulting in less energy value from the incident photons at the anode heel.This is used to advantage when imaging anatomical parts that are unequal in thickness and densities throughout their respective lengths.
20 Using the anode heel effect The following anatomical parts may be imaged using the anode heel effect:Thoracic vertebraeHumerusFemurTibia & fibulaForearm
21 Anode heel effect (I)Anode angle (from 7° to 20°) induces a variation of the X Ray output in the plane comprising the anode-cathode axisAbsorption by anode of X photons with low emission angleThe magnitude of influence of the heel effect on the image depends on factors such as :anode anglesize of filmfocus to film distanceAnode aging increases heel effect6: X Ray production
22 Anode heel effect (II) The heel effect is not always a negative factor It can be used to compensate for different attenuation through parts of the bodyFor example:thoracic spine (thicker part of the patient towards the cathode side of the tube)mammography6: X Ray production
23 Focal spot size and imaging geometry Focal spot finite size image unsharpenedImproving sharpness small focal spot sizeFor mammography focal spot size 0.4 mm nominalSmall focal spot size reduced tube output (longer exposure time)Large focal spot allows high output (shorter exposure time)Balance depends on organ movement (fast moving organs may require larger focus)6: X Ray production
24 It supplies the X-ray tube with : X-ray generator (I)It supplies the X-ray tube with : Current to heat the cathode filament Potential to accelerate electrons Automatic control of exposure (power application time) Energy supply 1000 X-ray beam energy (of which 99.9% is dissipated as thermal energy)6: X Ray production
25 X-ray generator (II)Generator characteristics have a strong influence on the contrast and sharpness of the radiographic imageThe motion unsharpness can be greatly reduced by a generator allowing an exposure time as short as achievableSince the dose at the image plane can be expressed as:D = k0 . Un . I . TU: peak voltage (kV)I: mean current (mA)T: exposure time (ms)n: ranging from about 1.5 to 36: X Ray production
26 Tube potential wave form (I) Conventional generatorssingle 1-pulse (dental and some mobile systems)single 2-pulse (double rectification)three 6-pulsethree 12-pulseConstant potential generators (CP)HF generators (use of DC choppers to convert 50Hz mains into voltages with frequencies in the kHz range) “Inverter technology”6: X Ray production
27 Tube potential wave form (II) Single phase single pulsekV ripple (%)100%Single phase 2-pulse13%Three phase 6-pulse4%Three phase 12-pulseLine voltage0.01 s0.02 s6: X Ray production
28 The choice of the number of pulses (I) Single pulse : low power (<2 kW)2-pulse : low and medium power6-pulse : uses 3-phase mains, medium and high power (manual or automatic compensation for voltage drop)12-pulse : uses two shifted 3-phase system, high power up to 150 kW6: X Ray production
29 The choice of the number of pulses (II) CP : eliminates any changes of voltage or tube currenthigh voltage regulators can control the voltage AND switch on and off the exposurevoltage can be switched on at any moment (temporal resolution)kV ripple <2% thus providing low patient exposureHF : combines the advantages of constant potential and conventional generatorreproducibility and consistency of tube voltagehigh frame rate possible6: X Ray production
30 Automatic exposure control Optimal choice of technical parameters in order to avoid repeated exposures (kV, mA)Radiation detector behind (or in front of) the film cassette (with due correction)Exposure is terminated when the required dose has been integratedCompensation for kVp at a given thicknessCompensation for thickness at a given kVp6: X Ray production
31 Automatic exposure control X Ray tubeCollimatorBeamSofttissueBoneAirPatientTableGridCassetteAEC detectors6: X Ray production
32 X-ray equipment operation mode and application (II) Radiography and TomographySingle and 3 generators (inverter technology)output : 30 kW at 0.3 focus spot sizeoutput : kW at 1.0 focus spot sizeselection of kV and mAs , AECRadiography and FluoroscopyUnder couch equipment, three generator (inverter technology) - continuous output of Woutput : 50 kW at 1.0 focus size for spot filmoutput : 30 kW at 0.6 for fluoroscopy (high resolution)priority given to contrastautomatic settings of kV6: X Ray production
33 X-Ray equipment operation mode and application (III) Radiography and FluoroscopyOver couch equipment, three phase generator (inverter technology) - continuous output of at least 500 Woutput : focus size for spot filmoutput : for fluoroscopy (high resolution)priority given to contrastautomatic settings of kVCardiac angiographyThree phase generator - continuous output 1kWoutput : focus sizeoutput : focus sizeframe rate : up to 120 fr/s6: X Ray production
35 Protective housingX-ray tube is always mounted inside a lead-lined protective housing that is designed to:Prevent excessive radiation exposure.Prevent electric shock to the patient and operator (technologist).
36 Protective housingIncorporates specially designed high-voltage receptacles.Provides mechanical support for the x-ray tube and protects it from damage.Some tube housings contain oil in which the tube is bathed.Some tube housings contain a cooling fan to air-cool the tube.When properly designed, they reduce the level of leakage radiation to less than 100 mR/hr at 1 meter when operated at maximum conditions.