X-Ray Fundamentals and basics of Diagnostic X-Ray Systems

Mar 17, 2019 | Publisher: Wim Spaak | Category: Technology |   | Views: 30 | Likes: 1

1 Spaak-Consultancy [wim.spaak@ziggo.nl] Consultancy & training on Medical X-ray Product Architecture, Design, Safety Risk Management X-Ray Fundamentals and basics of Diagnostic X-Ray Systems By: Wim Spaak wim.spaak@ziggo.nl Lecture: 2 Spaak-Consultancy [wim.spaak@ziggo.nl] 1. X-ray History 2. X-ray Physics 3. X-ray Dose 4. X-ray Generation 5. Geometry 6. Image Detection 7. From X-ray to Light Image 8. X-ray Control 9. Image Quality Elements 10. Image Processing & Storage 11. Overall System Lecture Chapters 3 Spaak-Consultancy [wim.spaak@ziggo.nl] Chapter 1 X-ray History 1. X-ray Discoverer 2. Rntgen's Experimental Equipment 4 Spaak-Consultancy [wim.spaak@ziggo.nl] X-rays discovered by Wilhelm Conrad Rntgen on 8 November 1895 University of Wrzburg 1.1 X-ray Discoverer In 1901 W.C. Rntgen (1845-1923) received the first Nobel Prize in physics for his discovery of X-rays 22 Dec 1895 X-ray picture of Anna Bertha Ludwig's hand (Mrs. Rntgen) Rntgen's office @ Wrzburg 5 Spaak-Consultancy [wim.spaak@ziggo.nl] 1.2 Rntgen's Experimental Equipment X-ray tube HT generator 6 Spaak-Consultancy [wim.spaak@ziggo.nl] Chapter 2 X-ray Physics 1. Electromagnetic Spectrum 2. Bremsstrahlung 3. Diagnostic X-ray Spectrum 4. X-ray Properties 5. X-ray Production 6. Efficiency of X-ray Production 7. X-ray Absorption 8. Hardening X-rays 9. Scatter Influence 7 Spaak-Consultancy [wim.spaak@ziggo.nl] 2.1 Electromagnetic Spectrum 8 Spaak-Consultancy [wim.spaak@ziggo.nl] 2.2 Bremsstrahlung Photon energy (keV) SpekCalc: spectrum @ 75 kV, 1.5mm Cu Photons per [cm2 mAs] @ 1m 9 Spaak-Consultancy [wim.spaak@ziggo.nl] Bremsstrahlung 2.3 Diagnostic X-ray Spectrum keV nm SpekCalc: spectrum @ 75 kV, 1.5mm Cu Photons per [cm2 mAs] @ 1m Photons per [cm2 mAs] @ 1m 10 Spaak-Consultancy [wim.spaak@ziggo.nl] SpecCalc: X-ray spectra calculation tool 2.3 Diagnostic X-ray Spectrum 11 Spaak-Consultancy [wim.spaak@ziggo.nl] 2.4 X-ray Properties 1. Can penetrate materials, depending on: Material nature (atomic number) Material density Thickness of the material X-ray hardness (wavelength) 2. Can luminescence certain materials (e.g. CsI) 3. Effects on photographic emulsions (blackening) 4. Can ionize gasses 5. Has effects on living tissues (biological effect) Reduces growth Destroys tissues Causes inflammations 6. X-ray beams travel in straight lines 12 Spaak-Consultancy [wim.spaak@ziggo.nl] 2.5 X-ray Production 75 kV 85 kV 105 kV Low kVp : relative long wavelength Low Penetration High Contrast High kVp : relative short wavelength High Penetration Low Contrast X-ray production: intensity depends on kVp and shifts in wavelength intensity is linear with mA linear with exposure time 13 Spaak-Consultancy [wim.spaak@ziggo.nl] 2.5 X-ray Production 75 kV 85 kV 105 kV Low kVp : relative long wavelength Low Penetration High Contrast High kVp : relative short wavelength High Penetration Low Contrast X-ray production: intensity depends on kVp and shifts in wavelength intensity linear with mA linear with exposure time So: mA and ms control the image brightness kV controls both image brightness and contrast 14 Spaak-Consultancy [wim.spaak@ziggo.nl] 2.6 Efficiency of X-ray Production Heat is an excitation rather than an ionization. Most kinetic energy of projectile e- is converted into heat (99%). Outer shell electrons immediately fall back to their normal energy level under emission of infrared radiation. The constant excitation and return of outer shell electrons are responsible for most of the heat generation. Production of heat in the anode increases directly with the increase of the tube current. Increasing kVp will also increase heat production. Efficiency of X-ray production At 60 kVp 0,5% At 100 kVp 1% 15 Spaak-Consultancy [wim.spaak@ziggo.nl] 2.7 X-ray Absorption Arthur Compton 16 Spaak-Consultancy [wim.spaak@ziggo.nl] 2.8 Hardening X-rays "Hardening" of the X-ray beam Removes low energy "soft" photons Increases the average beam energy Soft tissue penetration requires approximately 30-40 kilo electron volt (keV) photons Low energy photons cannot penetrate the patient Only contribute to patient dose 17 Spaak-Consultancy [wim.spaak@ziggo.nl] 2.9 Scatter Influence veiling glare and image contrast decay 18 Spaak-Consultancy [wim.spaak@ziggo.nl] Chapter 3 X-ray Dose 1. Unit-name Givers 2. X-Radiation Dose 3. Biological Effect of Radiation 4. Putting Dose into Perspective 19 Spaak-Consultancy [wim.spaak@ziggo.nl] 3.1 Unit-name Givers Wilhelm Conrad Rntgen a legacy unit of an ionization in air measurement, caused by X-ray radiation [R] Louis Harold Gray energy produced in unit volume of water by one rntgen of radiation [Gy] Rolf Maximilian Sievert biological effects of radiation [Sv] 20 Spaak-Consultancy [wim.spaak@ziggo.nl] unit Gy, Sv = J kg-1 3.2 X-Radiation Dose 1. Absorbed Dose [D] (Gy) Absorbed Dose is a measure of the energy deposited per unit mass by ionizing radiation. Absorbed Dose is used as an indicator of deterministic risk. The product of Absorbed Dose and the irradiated area is Dose Area Product (Gy.cm2) and is used as an indicator of stochastic risk. Entrance skin dose also takes into account the backscatter effect. 2. Kerma Dose [K] (Gy) { acronym for: Kinetic Energy Released per unit Mass } Kerma is expressed as the amount of energy released per mass of irradiated material. Kerma differs from Dose because Kerma is the energy released per mass, and Dose is the absorbed energy per mass. Kerma in in open air (Air Kerma) is of fundamental importance for the calibration of X-ray equipment. The measuring instrument used is known as "ionization chamber". 3. Equivalent Dose [H] (Sv) Used to assess how much biological damage is expected from the (absorbed) Dose (D). Equivalent Dose [H] is the product of the absorbed dose in tissue [D] and a dimensionless factor Q (quality factor), which is dependent on the radiation type [= 1 for X-rays]. 4. Effective Dose [E] (Sv) Used to assess the potential for long-term effects that may occur in the future and takes into account three factors: the absorbed dose of all organs of the body the relative harm level of the radiation the radiation sensitivities of each organ 1 Gy = 114,5 R 21 Spaak-Consultancy [wim.spaak@ziggo.nl] unit Gy, Sv = J kg-1 3.2 X-Radiation Dose 1. Absorbed Dose [D] (Gy) Absorbed Dose is a measure of the energy deposited per unit mass by ionizing radiation. Absorbed Dose is used as an indicator of deterministic risk. The product of Absorbed Dose and the irradiated area is Dose Area Product (Gy.cm2) and is used as an indicator of stochastic risk. Entrance skin dose also takes into account the backscatter effect. 2. Kerma Dose [K] (Gy) { acronym for: Kinetic Energy Released per unit Mass } Kerma is expressed as the amount of energy released per mass of irradiated material. Kerma differs from Dose because Kerma is the energy released per mass, and Dose is the absorbed energy per mass. Kerma in in open air (Air Kerma) is of fundamental importance for the calibration of X-ray equipment. The measuring instrument used is known as "ionization chamber". 3. Equivalent Dose [H] (Sv) Used to assess how much biological damage is expected from the (absorbed) Dose (D). Equivalent Dose [H] is the product of the absorbed dose in tissue [D] and a dimensionless factor Q (quality factor), which is dependent on the radiation type [= 1 for X-rays]. 4. Effective Dose [E] (Sv) Used to assess the potential for long-term effects that may occur in the future and takes into account three factors: the absorbed dose of all organs of the body the relative harm level of the radiation the radiation sensitivities of each organ 1 Gy = 114,5 R Gy does not describe the biological effects of the different radiations When we talk about fluoroscopy or exposure dose, e atually mean the Air Krma Dose, measured under calibration conditions, at the entrance plane of the Image Detector (see also 7.3, 7.4 and 11.3) 22 Spaak-Consultancy [wim.spaak@ziggo.nl] 3.2 X-Radiation Dose Effective dose [E] (Sv) Takes into account the specific organs and parts of the body that are exposed. All parts of the body and organs are not equally sensitive to the possible harmful effects of radiation, such as cancer induction and mutations. Effective dose is the sum of the weighted equivalent doses in all tissues and organs of the body. The factor by which the equivalent dose is weighted in tissue or organ is called "tissue weighting factor", which represents the relative contribution of that organ or tissue to the total damage resulting from uniform irradiation of the whole body. Oesophagus - 0.05 Thyroid - 0.05 Lungs - 0.12 Skin - 0.01 Breast - 0.05 Liver - 0.05 Stomach - 0.12 Colon - 0.12 Gonads - 0.20 tissue weighting factors example 23 Spaak-Consultancy [wim.spaak@ziggo.nl] 3.3 Biological Effect of Radiation Threshold dose (mGy) Direct effects that occur at radiation of the whole human body 50.000 Central nerve system syndrome. Mortality within several hours till one day Notice: The direct effects mentioned occur at radiation of the whole human body. With X-ray equipment, however, much less (perhaps only 10%) of the human body is exposed to X-rays. 24 Spaak-Consultancy [wim.spaak@ziggo.nl] 3.4 Putting Dose into Perspective The following list gives the radiation doses usually received every year by an average person in Ireland. The lifetime risk of a fatal cancer is also given at certain doses. Doses at and above 1 Sv, received during a short period of time, are given below, to illustrate the doses at which immediate harm to the body occurs. 1 Sv onset of early radiation effects 2 Sv threshold for early death 4 Sv 50% chance of survival 6 Sv early death. 1 Sv the average annual dose to a 'heavy' consumer of seafood from the Irish Sea 8 Sv the dose received on a return flight from Dublin to London 20 Sv the dose from a single chest X-ray 20 Sv 1 in 1.000.000 lifetime risk of fatal cancer 45 Sv the annual average dose from airline travel 240 Sv the annual average dose from radioactivity in food 300 Sv the annual average dose from gamma radiation from the ground 350 Sv the annual average dose from cosmic radiation 540 Sv the annual average dose from medical examinations 1.000 Sv 1 in 20.000 lifetime risk of fatal cancer 2.230 Sv the annual average dose from radon in the home and workplace 2.950 Sv the average total annual dose from all sources of ionizing radiation 10.000 Sv 1 in 2.000 lifetime risk of fatal cancer 25 Spaak-Consultancy [wim.spaak@ziggo.nl] Chapter 4 X-ray Generation 26 Spaak-Consultancy [wim.spaak@ziggo.nl] Chapter 4 X-ray Generation 1. X-ray Source 2. Focusing Cup 3. Effective Focal Spot 4. X-ray Tube Characteristics 5. Extra-focal Radiation 6. Heel Effect 7. X-ray Tube Housing 8. X-Generator 9. kV, mA, exposure time Control 10. X-ray Beam Collimation 27 Spaak-Consultancy [wim.spaak@ziggo.nl] 4.1 X-ray Source Often the anode is rotated to improve the short time X-ray power output. today's Philips MRC 200 0508 An X-ray tube produces X-rays by accelerating electrons to a high speed with a high-voltage field, which causing them to collide with the anode. The tube consists of a source of electrons the cathode which is usually a heated filament and a thermally robust anode, usually of tungsten, which is enclosed in an evacuated housing. 28 Spaak-Consultancy [wim.spaak@ziggo.nl] 4.2 Focusing Cup The focusing cup (made of Molybdenum) contains the filaments (mostly 2, a small one and a larger one). The filament emits electrons, all of which have a negative charge. Since negative repels negatively, the emitted electrons tend to diverge. Because this is counterproductive in X-ray tubes, the focusing cup has a negatively charged housing that "stimulates" the electrons to stay together. In essence, the force that causes the electrons to repel each other is overpowered by the repulsive force of the focusing cup and the electrons tend to converge rather than diverge. 29 Spaak-Consultancy [wim.spaak@ziggo.nl] 4.3 Effective Focal Spot Effective focal spot pinhole camera image Graphical representation of the intensity distribution of the left pinhole camera focal spot image. Shape caused by the cathode filament wiring 30 Spaak-Consultancy [wim.spaak@ziggo.nl] Philips MRC 200 GS-0310 kV range = 40 150 kV mA range = 0,2 mA 1 A Max power = 100 kW heat storage = 2,4 MHU ( HU = kV x mA x t ) 4.4 X-ray Tube Characteristics X-ray tube kV/mA characteristic MRC 200 power characteristic Basically an X-ray tube is a direct heated diode operating above saturation parameter T1 .. T3 = filament temperature 31 Spaak-Consultancy [wim.spaak@ziggo.nl] 4.4 X-ray Tube Characteristics Impact on an over powered X-ray tube Filament with hot spot 32 Spaak-Consultancy [wim.spaak@ziggo.nl] 4.4 X-ray Tube Characteristics Specific X-ray tube yield @ 1 meter 33 Spaak-Consultancy [wim.spaak@ziggo.nl] 4.5 Extra-focal Radiation Another phenomenon that influences the X-ray focus are the not insignificant amount of the electrons that hit the anode and are scattered backwards, leaving the anode again. When they do this, they will fall-back to the anode again and produce there the so-called "extra-focal radiation" (off-focus / a-focal) in anode areas, other than the actual focal spot. Decrease extra-focal radiation by: Place a collimator Lead diaphragm as close as possible to X-ray tube Use a metal housing Extra-focal radiation Attenuated direct focal radiation xtra-f l ra i n 34 Spaak-Consultancy [wim.spaak@ziggo.nl] 4.6 Heel Effect The Heel Effect is the X-ray focus intensity decrease that occurs when the angle between the anode surface of the X-ray tube's and the observation site decreases. This dependency on the angle of X-ray emission is due to attenuation of the X-radiation, arising from lower layers of the anode material (a few micrometers of tungsten) and the inherent filtering of the X-ray tube's output window and isolating oil. The Heel Effect shape is kV-dependent, since X-radiation attenuation depends on the X-ray spectrum. The figure shows the X-ray intensity distribution in different directions, with its typical hockey stick shape. (here for an X-ray Tube with a 20o anode angle). The energy density is considerable less on the anode side than on the cathode side. This fact is used in radiology: The thicker object part must therefore be placed on the cathode side. 35 Spaak-Consultancy [wim.spaak@ziggo.nl] 4.7 X-ray Tube Housing Made of cast steel and is usually lead-lined Provides for absorption of most off-focus radiation Filters low energy primary beam radiation (tube filter) Purposes: Controls leakage & off-focus radiation Isolates high voltages Helps to cool the tube tube filter 36 Spaak-Consultancy [wim.spaak@ziggo.nl] 4.8 X-Generator An X-generator powers and controls an X-ray Tube. 1. The main function of an X-generator is to supply the high voltage between the cathode and the anode of an X-ray tube to accelerate the electrons passing from cathode to anode. 2. In an X-ray Tube the electron emission is stimulated and controlled by the temperature of cathode filaments. The X-generator supplies and controls the current through the filament that heats it up. 3. The anode of a modern X-ray Tube is disk-shaped and rotates at high speed to prevent surface damage from the intense electron beam. The X-generator powers the electromotor of the anode and controls the rotational speed. 4. In some tubes the emission of electrons can be switched on and off by means of a grid switch. The X-generator controls the grid voltage or provides a switching signal to the grid switch electronics if they are installed in the tube's housing. 5. The X-generator monitors the thermal heating of the X-ray tube. 6. Between the system controller and the X-generator data and signal paths exists to exchange control and status information. 7. Often there is a direct connection between the Image Detection subsystem and the X-generator to provide feedback on the measured Average Grey Level (AGL), for dose control. 37 Spaak-Consultancy [wim.spaak@ziggo.nl] 4.8 X-Generator The HT Converter uses serial resonance and is a current transformer. As current-source it's an ideal HT X-ray transformer, because the X-ray tube is a current-consumer (the tube current hardly depends on it's kV-voltage). The serial resonance frequency is determined by the leakage inductance [L] of the HT transformer and a serial capacitor [C]. A higher resonance frequency results in a smaller HT transformer design and less kV-ripple. Thyristors T1 and T2 are "fired" with a variable time interval [Ttrigger] (which affects the average Iprim), to achieve and maintain the desired kV-voltage. When ID1 and IT2 overlap, they are merged into Iprim. kV t High Tension Converter principle IT1 ID1 ID2 IT1 IT2 ID1 38 Spaak-Consultancy [wim.spaak@ziggo.nl] 4.9 kV, mA, exposure time Control The X-Generator controls: kV in the range from 40 kV 150 kV mA in the range from 0,2 mA 1.000 mA exposure time in the range from 1 ms 1 s (for fluoroscopy) Not every combination of kV, mA and exposure time are possible, because the X-generator has some limitations: maximum power maximum cathode heat (limits the electron emission = mA) due to the saturation current the kV drop is linear with time When relative long high voltage cables are used between the X-generator and the X-ray tube, the cable capacity has influence on the high tension's falling edge. This is called the cable tail. For 20m HT-cable (2nF) @ 70kV, 50mA t-tail = 2,8 ms 39 Spaak-Consultancy [wim.spaak@ziggo.nl] 4.10 X-ray Beam Collimation X-ray beam collimation functions: beam limitation beam conditioning beam simulation Possible Collimator Functions: X-ray Shielding Rectangular collimation(shutters) Circular collimation(iris) Spectral Filtering(pre-filter) Spatial Filtering (wedges) Rotation(swivel) X-ray Simulation Light Source Image Distance Measurement Tool Dose Rate Measurement Tool Direct Shutter Control Aesthetic Cover Accessory Rails 40 Spaak-Consultancy [wim.spaak@ziggo.nl] 4.10 X-ray Beam Collimation X-ray focal spot Near focus shutters Spectral filters Back-up shutters Wedges Mirror X-ray simulation light Circular shutters (Iris) Main shutters Accessory rails Ionization chamber Collimator parts: 41 Spaak-Consultancy [wim.spaak@ziggo.nl] Chapter 5 Geometry 42 Spaak-Consultancy [wim.spaak@ziggo.nl] Chapter 5 Geometry 1. Geometry includes 2. Geometric Magnification 3. Object in the X-ray Beam 43 Spaak-Consultancy [wim.spaak@ziggo.nl] 5.1 Geometry includes Stand The mechanical part to which X-ray Tube and Image Receptor are attached, plus its local User Interface. Stands are designed with typical medical examinations in mind. Patient table and its local User Interface are also typical designed for special medical applications. Patient or other object in the beam is not part of the Geometry, but is included here for this presentation. 44 Spaak-Consultancy [wim.spaak@ziggo.nl] 5.2 Geometric Magnification Geometric magnification is the result of the diverging X-ray beams traveling in straight lines Is reduce by longer focus-to-receptor distance or shorter object-to-receptor distance Geometric magnification is utilized in some modalities e.g. mammography 45 Spaak-Consultancy [wim.spaak@ziggo.nl] 5.3 Object in the X-ray Beam X-ray object absorption also causes scatter (see 2.7: X-ray Absorption) Scatter causes veiling glare and reduced image contrast (see 2.9: Scatter influence) In a biplane system scatter from the opposite channel also ends up the own channel Appropriate collimation reduces scatter 46 Spaak-Consultancy [wim.spaak@ziggo.nl] Chapter 6 Image Detection 47 Spaak-Consultancy [wim.spaak@ziggo.nl] 1. Antiscatter Grid 2. Grid Characteristics 3. Aliasing and Moir Patterns 4. Conventional X-ray Image Detection 1) Image Intensifier TV Camera (II/TV) Image Detection 2) Flat Dynamic Image Detection 3) Screen type Film (used in the past), succeeded by a photostimulable phosphor (PSP) plate (not part of this presentation) Chapter 6 Image Detection 48 Spaak-Consultancy [wim.spaak@ziggo.nl] 6.1 Antiscatter Grid Antiscatter grids play an important role in improving image quality by transmitting a majority of primary radiation and selectively rejecting scattered radiation. Focused grids have lead strips oriented parallel to the center (along the central X-ray axis) and gradually run obliquely to the outer edge to adjust the divergence of the X-ray beam, from the focal spot to the detector, at a specific source to image (detector) distance (SID). 49 Spaak-Consultancy [wim.spaak@ziggo.nl] Grids are characterized by: grid ratio a good measure of the selectivity of primary to scatter transmission grid frequency a measure of the number of grid lines per distance unit focal distance is determined by the angle of the lead strip geometry focal range is an indicator of the flexibility of grid positioning distance from the focal spot, and is a function of the grid ratio and frequency bucky factor represents the increased dose for the patient when using a grid, compared to not using a grid 6.2 Grid Characteristics Scatter improvement shown with a pelvis phantom bucky factor = 8 75 kV - 24 mAs 75 kV - 3 mAs 50 Spaak-Consultancy [wim.spaak@ziggo.nl] Bucky factor: Bucky factor example for four different grid ratios, as a function of kVp, for a field of view of 30 cm and a water thickness of 30 cm. The bucky factor is the increase in entrance dose to a patient, which at the same kVp is necessary to compensate for the presence of the grid, and to achieve the same optical density. Higher grid ratios are more effective in reducing scatter, but require a higher bucky factor. 6.2 Grid Characteristics 51 Spaak-Consultancy [wim.spaak@ziggo.nl] 6.3 Aliasing and Moir Patterns Periodic structures, caused by interaction of the grid with the detector sampling. If the sampling frequency of the detector is low, compared to basic grid line frequencies, aliasing and Moir patterns can be a serious problem. Sufficient attenuation in the interaction of grid and detector. The Modulation Transfer Function (MTF) of the detector, in combination with the input modulation of the grid lines, should give a sufficiently low final modulation. The MTF of the detector must be measured or calculated very accurately at relatively high frequencies. 52 Spaak-Consultancy [wim.spaak@ziggo.nl] To avoid aliasing the grid frequency should be chosen close to the MTF cut- off frequency (modulation 0.2%) 6.3 Aliasing and Moir Patterns Aliasing caused by undersampling Image Detector pixel converted to the detector entrance plane MTF Image Detector [c/mm = lp/mm] 53 Spaak-Consultancy [wim.spaak@ziggo.nl] Conventional 1-dimensional filtering method 2-dimensional filtering method 6.3 Aliasing and Moir Patterns Information loss due to grid filtering 54 Spaak-Consultancy [wim.spaak@ziggo.nl] Fourier Transformation Fourier Transformation no filter 2D-filter 6.3 Aliasing and Moir Patterns Information loss due to grid filtering spatial domain frequency domain 55 Spaak-Consultancy [wim.spaak@ziggo.nl] 1. Image Intensifier 2. Image Intensifier Parameters 3. Image Intensifier - MTF 4. Image Intensifier Decay 5. Gx Dependencies 6. TV Camera 7. Inframing / Overframing 6.4.1 Image Intensifier TV Camera Image Detection 56 Spaak-Consultancy [wim.spaak@ziggo.nl] 6.4.1.1 Image Intensifier An X-ray image intensifier converts X-rays into visible light at higher intensity than only fluorescent screens. The image intensity is amplified in two ways: 1. Electric gain: electrons are accelerated 2. Minification gain: 57 Spaak-Consultancy [wim.spaak@ziggo.nl] II-format the diameter of the effective input window Gx II conversion factor, indicating the amount of output light, produced per entrance plane doserate. DQE The DQE combines the effects on modulation, spatial frequency and noise of an image receptor and can be used to compare different receptors in a more general way than the MTF alone. The parameter relates the Signal to Noise Ratio (SNR) of images represented by the imaging system (SNRout), to the SNR of the incident X-ray intensity pattern (SNRin) 6.4.1.2 Image Intensifier Parameters 58 Spaak-Consultancy [wim.spaak@ziggo.nl] 6.4.1.3 Image Intensifier - MTF Modulation Transfer Function (MTF) shows that the modulation decreases with increasing spatial frequency. A major advantage of the MTF concept is that for an image receptor with a number of image transduction stages, the total MTF can be obtained from the product of the individual component MTFs. Profile (in green) by an X-ray of a resolution test object depicted in scatter conditions. The no-scatter case is shown in blue for comparison. In the presence of scatter, we can expect that the reduction in contrast leads to reduced modulation at all spatial frequencies and a reduced ability to distinguish fine detail. 59 Spaak-Consultancy [wim.spaak@ziggo.nl] 6.4.1.4 Image Intensifier Decay The CsI scintillator causes decay. The time constant [t] of 150m CsI is 270sec. For an Image Intensifier, the output window adds an extra falling edge time constant of about one decade per decade. P20 60 Spaak-Consultancy [wim.spaak@ziggo.nl] The Image Intensifier's static transfer behavior is mainly caused by the CsI scintillator, which transfers X-rays into light. This transfer function depends on the X-ray spectrum, and thus on the object in the beam and the kV-value. 6.4.1.5 Gx Dependencies For an Image Intensifier, the Gx apparently decreases when the exposure time decreases. The Gx decrease increases when the X-ray intensity (the doserate in the pulse) increases. This effect is caused by the II output window phosphor decay and differences between Image Detection image integration time and the X-Generator X-ray control sample time. time 100% 75% 20msec 1msec kV Gx 75kV 61 Spaak-Consultancy [wim.spaak@ziggo.nl] 6.4.1.6 TV Camera The TV-camera is a professional medical version of a digital film- camera and falls outside the scope of this presentation Just some possible system interfaces: Image working point average E-value measurement within the region of interest (= the measuring field) Application factor indicates the signal space from average to maximum signal (= 'maximum signal' / 'image_working_point') 62 Spaak-Consultancy [wim.spaak@ziggo.nl] 6.4.1.7 Inframing / Overframing Overframing is used to adapt the circular Image Intensifier area to the rectangular image display. The degree of overframing depends on the medical application 63 Spaak-Consultancy [wim.spaak@ziggo.nl] 6.4.2 Flat Dynamic Image Detection 1. Flat X-ray Detector Technology 2. FDXD Processing 3. FDXD Processing result example 64 Spaak-Consultancy [wim.spaak@ziggo.nl] 6.4.2.1 Flat X-ray Detector Technology A/D X-rays Photo-diode Thin Film Transistor Caesium-Iodide Scintillator Glass substrate * * * Light 65 Spaak-Consultancy [wim.spaak@ziggo.nl] 1. Controls the detector read-out and other detector timing including the flash of the refresh light. 2. Corrects imperfections of the FDXD Detector, with: Darkcurrent (offset) correction per pixel temperature and acquisition-time dependent Gain correction per pixel Defect pixel / line correction by interpolation of neighbouring pixels Memory effect reduction/correction by refresh light flash Row-correlated noise correction Differential non-linearity correction A correction set is stored for each detector read-out mode. Darkcurrent correction data is collected when there is no X-ray. 6.4.2.2 FDXD Processing Refresh Light A-Si Photodiode Array CsI:Tl Scintillator 66 Spaak-Consultancy [wim.spaak@ziggo.nl] 6.4.2.3 FDXD Processing result example From "raw" detector read-out image to "processed" detector image "raw" detector image "processed" detector image FDXD processing 67 Spaak-Consultancy [wim.spaak@ziggo.nl] Chapter 7 From X-ray to Light Image 68 Spaak-Consultancy [wim.spaak@ziggo.nl] Chapter 7 From X-ray to Light Image 1. E-value in Photography 2. E-value in X-ray 3. X-Ray Doserate 4. X-Ray Dose 5. Image Working-point 6. Dose(rate) Calibration 7. E-curve Formulas 8. E-curve 9. P-curve 10. P-factor Usage 11. Patient Absorption Factor 12. Entrance Dose with real Patients 13. Entrance Dose Spectral Influence 14. Cable Tail Contribution 15. Effective Cable Tail Contribution 69 Spaak-Consultancy [wim.spaak@ziggo.nl] 7.1 E-value in Photography The E-value originates from the photography, where the film Density depends on the film Exposure, and the gamma of the film D E 1 2 0 3 1 2 3 where E = log10 ( I x t ) (light Intensity x exposure time) and D = log10 ( 1 / Transmission ) 70 Spaak-Consultancy [wim.spaak@ziggo.nl] 7.2 E-value in X-ray Is the integrated amount of light on the entrance plane of the Light Image Detector during an X-ray image Notice: Identical to film, the image (Density) in X-ray depends on the transfer function (gamma) of the Light Image Detector and the amount of light (E) on the entrance plane X-ray image Caesium Iodide Light Image Detector (I) where Light intensity 71 Spaak-Consultancy [wim.spaak@ziggo.nl] 7.3 X-Ray Doserate The doserate is the measured light intensity by the Light Image Detector, calibrated in Gy/s at the CsI detector entrance plane at a defined X-ray spectrum @ 75 kV and a homogeneous object of 1.5 mm Cu 1.5 mm Cu X-ray Light intensity measured Caesium Iodide Light Image Detector (I) light intensity kV = 75 mA continuous Image Detector entrance doserate 72 Spaak-Consultancy [wim.spaak@ziggo.nl] 7.4 X-Ray Dose The dose is the integrated measured light intensity by the Light Image Detector during the exposure time, calibrated in Gy at the CsI detector entrance plane at a defined X-ray spectrum @ 75 kV and a homogeneous object of 1.5 mm Cu 1.5 mm Cu X-ray Light measured Caesium Iodide Light Image Detector (E) light kV = 75 mA time Image Detector entrance dose 73 Spaak-Consultancy [wim.spaak@ziggo.nl] 7.4 X-Ray Dose The dose is the integrated measured light intensity by the Light Image Detector during the exposure time, calibrated in Gy at the CsI detector entrance plane at a defined x-ray spectrum @ 75 kV and a homogeneous object of 1.5 mm Cu 1.5 mm Cu X-ray Light measured Caesium Iodide Light Image Detector (E) light kV = 75 mA time Image Detector entrance dose Notice: In X-ray systems Exposure Dose measurement is in fact a Light Quantity measurement per exposure. Because of our Exposure Dose definition, E-value's are calibrated in (Air Kerma) Dose on the image detector's entrance plane @ 75 kV and 1.5 mm Cu object in the beam. 74 Spaak-Consultancy [wim.spaak@ziggo.nl] 7.5 Image Working-point Is the average image (density) inside the Light Image Detector's measuring field Notice: For an II/TV Image Detection System the Light Image Detector is either the photosensor or the TV-camera X-ray Light Caesium Iodide Light Image Detector (E) image working- point 75 Spaak-Consultancy [wim.spaak@ziggo.nl] 7.6 Dose(rate) Calibration In XRD systems the image working-point is stabilized by controlling the X-generator parameters (kV, mA, time) The Image Detection entrance dose(rate) is calibrated: @ 75kV / 1.5 mm Cu, by calibrating the measurement gain for the Light Image Detector's image working-point 1.5 mm Cu dose(rate) X-ray Light image Caesium Iodide Light Image Detector (E) working- point kV = 75 mA time 76 Spaak-Consultancy [wim.spaak@ziggo.nl] For X-rays, the E-value E :: kV p . mAs Notice: The P-factor depends on the kV-value and all objects in the X-ray beam Since P is also a function of kV, this formula can only be applied for small kV differences (where kV2 kV1) This formula shows that E is linear with mA and exposure time (s), but is influenced by the Power of the kV-value 7.7 E-curve Formulas An E-curve is a locus of identical E-values (all E-values have the same value) An E-curve is a natural logarithmic curve, which can be described as: mAs = e for E = constant or: ln (mAs) = a . kV b a . kV b X-ray Light (E) image Caesium Iodide Light Image Detector P is the derivative (slope) of the E-curve In formula: p = -b . a . kV b where: a and b are E-curve describing coefficients 77 Spaak-Consultancy [wim.spaak@ziggo.nl] 7.8 E-curve The X-ray parameter value relation, resulting in the same E-value for a given object in the X-ray beam Example of normalised E-curves: without anti-scatter grid no collimator filter Image Intensifier (CsI) Notice: for the same E-value @75 kV every 5 cm water increase requires 2 x mA increase 78 Spaak-Consultancy [wim.spaak@ziggo.nl] 7.9 P-curve P-curves for the previous shown E-curves: P-curves of previous shown example of normalised E-curves: without anti-scatter grid no collimator filter Image Intensifier (CsI) Notice: the 1.5mm Cu E-curve slope is much steeper then water E-curves 79 Spaak-Consultancy [wim.spaak@ziggo.nl] 7.10 P-factor Usage With the P-factor you can estimate the difference in E-value, due to a difference in kV-value Example: For 25 cm H2O the P-factor @ 75 kV is about 5 (shown in the previous graph) Then: a kV deviation (e.g. ripple) of 1% (= 750V !!) results in 5% E-value increase (1.015 1.05) An other example: @ 75 kV : 1.5 mm Cu absorption is equivalent to the absorption of 11 cm water (see 7.8 E-curve); however PCu = 7.3 and Pwater = 3.8 there 80 Spaak-Consultancy [wim.spaak@ziggo.nl] 7.11 Patient Absorption Factor Legal Patient Entrance Dose Limit = 87 mGy/min (10 R/min) = Air kerma dose 81 Spaak-Consultancy [wim.spaak@ziggo.nl] 7.12 Entrance Dose with Real Patients Since the image working-point is stabilized, the Image Detection entrance dose(rate) is determined by the transfer function of the Image Detector. Notice: The transfer function of an Image Detector has both static and dynamic behaviour parts, which depends on X-ray spectrum, intensity and time. Because of the X-ray spectrum, the actual Image Detection entrance dose(rate) is also influenced by the patient. X- ray Light Image working- point CsI Light Image Detector (E) Image Detection transfer function 82 Spaak-Consultancy [wim.spaak@ziggo.nl] Spectral Influence on Entrance Dose(rate) as a function of kV and object thickness, represented as a dose(rate) multiplication factor for identical image working-points. Calibrated dose(rate): @ 75 kV 1.5 mm Cu 7.13 Entrance Dose Spectral Influence Be aware that the image working-point is controlled and notice the Image Detector's spectral sensitivity impact (CsI) on the Image Detector entrance dose(rate) 83 Spaak-Consultancy [wim.spaak@ziggo.nl] 7.14 Cable Tail Contribution The green tail area covers (F x kV) mAs. However, only a part of this area contributes to the image working-point, depending on the spectral sensitive Image Detection transfer function, and thus depending on the objects in the X-ray beam. The green area below the orange line is the mAs component that contributes effectively to the image working-point. 84 Spaak-Consultancy [wim.spaak@ziggo.nl] 7.15 Effective Cable Tail Contribution Shown example for: 6 nF tot. capacity 20 cm H2O 60 kV 100 mA tpulse = 1.5 msec td = 0 @ 75% HT F x kV = 0,36 mAs Texp = time where td=0 = tpulse + 25% (FxkV/A) = 1.5 + 0.9 = 2.4 msec @ 100 mA resulting in: mAsgen = 0.24 mAs Real effective image contributing mAs here: calculated effective mAs-tail contributing area = 13.8% (FxkV) mAseff = 0.15 mAspulse + 0.05 mAstail = 0.20 mAs kV 0,00 0,05 0,10 0,15 0,20 0,25 effective X-Gen mAsPulse Tail Pulse Tail -16,7% In this example: 13.8% (FxkV) Real tail contribution = area under red line Tail contribution @ 75% kV 85 Spaak-Consultancy [wim.spaak@ziggo.nl] Chapter 8 X-ray Control 86 Spaak-Consultancy [wim.spaak@ziggo.nl] Chapter 8 X-ray Control 1. Measuring Field 2. X-ray Control Techniques 3. E-value Measurement 4. X-ray Control Loop 5. Exposure Control Curve 6. Continuous Fluoroscopy Control Curve 87 Spaak-Consultancy [wim.spaak@ziggo.nl] 8.1 Measuring Field The measuring field is that part of the image, that is used as sensor for X-ray control. In an II/TV subsystem the measuring field sensor is either a photosensor who "views" the II-output window, or a measuring field sensor inside the TV-camera. The photosensor measures average light intensity during an X-ray pulse The TV-camera measures after an acquired frame, and can be an average or top measurement. Measuring field shape: Generally the measuring field is a circular central measuring field, that covers a percentage of the whole image. Black/White exclusion is sometimes used, especially in surgical applications, where direct radiation and surgical tools (e.g. a drill who absorb all X-rays) are positioned inside the measuring field. 88 Spaak-Consultancy [wim.spaak@ziggo.nl] 8.2 X-ray Control Techniques Fluoroscopy is a method of image generation using X-ray, which is intended for: Real-time visualizing the patient's body for diagnostic and therapeutic purposes Providing a visual aid to the clinical user, while positioning the patient table and/or the stand towards and within the area of interest in the patient's body Navigating a catheter through the patient's vessels within the area of interest Assisting in performing proper beam limitation and beam attenuation, prior to an Exposure run Useful tool in interventional procedures such as angioplasty (PTCA) and coronary stenting, especially regarding Roadmap functionality Exposure is a method of image generation using X-ray, which is intended for: Visualizing the patient's body region of interest for diagnostic and therapeutic purposes Offering real-time image processing during the run (like image subtraction) Registering the resulting images on a digital image store, in order to document the examination for clinical purposes and to enable post-processing in a later stage Optionally performing an X-ray-coupled contrast medium injection, to visualize the patient's vessel pattern during the run Optionally combining X-ray-acquisition with geometrical movements (like in Bolus Chase or Digital Rotational Angiography) Possibility to make one 'SingleShot' exposure Registering and showing exposure- and patient-dose parameters Possibility to record physiological signals in parallel with the images 89 Spaak-Consultancy [wim.spaak@ziggo.nl] 8.2 X-ray Control Techniques X-ray control controls the output of the X-ray tube. Only 3 tube parameters can be controlled: kV, mA, exposure time (s) They are controlled manually or automatically, often with pre-programmed fixed values for one or more parameters, depending on the medical application (APR) Control techniques are (limited by the tube nomogram): 90 Spaak-Consultancy [wim.spaak@ziggo.nl] 8.2 X-ray Control Techniques : Mobile + Boost fluoro + RAD mode 91 Spaak-Consultancy [wim.spaak@ziggo.nl] 8.2 X-ray Control Techniques : Cardio 92 Spaak-Consultancy [wim.spaak@ziggo.nl] 8.2 X-ray Control Techniques : Vascular 93 Spaak-Consultancy [wim.spaak@ziggo.nl] 8.2 X-ray Control Techniques : URF 94 Spaak-Consultancy [wim.spaak@ziggo.nl] 8.2 X-ray Control Techniques : RAD 95 Spaak-Consultancy [wim.spaak@ziggo.nl] 8.3 E-value Measurement A photosensor measures the light intensity (I) of the II-output window, within the measuring field The cable tail also contributes to the E-value For phototiming the high tension (kV) must be switched-off, before the E-value reaches the target value of 100% High speed camera can also measure during an X-ray pulse 96 Spaak-Consultancy [wim.spaak@ziggo.nl] 8.4 X-ray Control Loop The X-ray control loop principle 97 Spaak-Consultancy [wim.spaak@ziggo.nl] 8.5 Exposure Control Curve E100% can be reached on every E-curve point, within the tube limitations area A second curve is needed to stabilize the X-ray control Philips MRC 200 GS-0310 Example: 80 nGy/fr 25 fr/s 30 cm H2O 100 cm SID 75 kWmax 40 mAmin 98 Spaak-Consultancy [wim.spaak@ziggo.nl] E100% is stabilized on the intersection of the E-curve and the medical application dependent control curve (the red curve) 8.5 Exposure Control Curve 75 kV Here an ideal Cardio control curve is shown, where the best iodine contrast is obtained @75 kV Philips MRC 200 GS-0310 Example: 80 nGy/fr 25 fr/s 30 cm H2O 100 cm SID 75 kWmax 40 mAmin 99 Spaak-Consultancy [wim.spaak@ziggo.nl] 8.6 Continuous Fluoroscopy Control Curve In "out of steam" situations the AGC of the TV-camera's can take care of the extra required image gain in this example the AGC gain @ 110 kV, 45 cm H2O need to be 4.6x higher than the non "out of steam" gain control curve out of steam Philips MRC 200 GS-0310 Example: 440 nGy/s 100 cm SID 660 Wmax 100 Spaak-Consultancy [wim.spaak@ziggo.nl] Chapter 9 Image Quality Elements 1. Resolution 2. Contrast 3. Dynamic Imaging 4. Noise 5. Artifacts Image Quality 101 Spaak-Consultancy [wim.spaak@ziggo.nl] 1. Resolution Definition 2. An X-ray Image has Un-sharp Edges 3. Measuring Resolution 4. Resolution & Contrast 5. How Geometry affects Resolution 6. How Image Detection affects Resolution 9.1 Resolution 102 Spaak-Consultancy [wim.spaak@ziggo.nl] 9.1.1 Resolution Definition The fineness of details that can be distinguished in an image Sharp: Having clear form and detail Also known as: Spatial Resolution in line pairs/millimeter (lp/mm), or c/mm) c/mm stands for cycles/mm and is identical to lp/mm Opposites: Unsharp: Indistinct or hazy in outline or appearance Blurry: Fuzzy, foggy or unfocussed Factors that affects resolution Geometry Object Image Detection Image processing Monitor 103 Spaak-Consultancy [wim.spaak@ziggo.nl] Intensity Position 9.1.2 An X-ray Image has Un-sharp Edges Lucite Lead 100% Object boundary Image boundary 100% 0% Intensity Position 104 Spaak-Consultancy [wim.spaak@ziggo.nl] 9.1.3 Measuring Resolution Resolution test patterns (also known as "phantoms") usually consist of strips of lead that have 100% object contrast Line pair / millimeter (lp/mm) test patterns 105 Spaak-Consultancy [wim.spaak@ziggo.nl] 9.1.4 Resolution & Contrast 100% Contrast 96% Contrast 100% Contrast 25% Contrast 106 Spaak-Consultancy [wim.spaak@ziggo.nl] Focal spot size OID (Object Input Distance) 9.1.5 How Geometry affects Resolution 107 Spaak-Consultancy [wim.spaak@ziggo.nl] 9.1.5 How Geometry affects Resolution OID (Object Input Distance) The Image Intensifier should be placed as close to the patient as possible (< 5 cm) for better image quality and a lower dose 108 Spaak-Consultancy [wim.spaak@ziggo.nl] 9.1.6 How Image Detection affects Resolution 400 pixels width 500 pixels height 100 pixels width 125 pixels height 40 pixels width 50 pixels height By the Pixel matrix 109 Spaak-Consultancy [wim.spaak@ziggo.nl] 9.2 Contrast 1. Contrast Definition 2. An X-ray Image suffers a Loss in Contrast 3. Measuring Contrast 4. kV is the Contrast Control 5. Scatter 6. Collimation reduces Scatter 7. Brightness & Contrast 8. Leeds Test Objects 110 Spaak-Consultancy [wim.spaak@ziggo.nl] 9.2.1 Contrast Definition The difference in brightness between the light and dark parts of an image Also known as: Latitude, dynamic range or gray scale Opposites: Flat: Lacking gray scale Factors that affects contrast Geometry Object Image Detection Image processing Monitor Black White Continuous Grayscale 20-step Grayscale 111 Spaak-Consultancy [wim.spaak@ziggo.nl] 9.2.2 An X-ray Image suffers a Loss in Contrast Intensity Position 100% 0% Intensity Position 95% 5% Object boundary Image boundary Lucite Lead 112 Spaak-Consultancy [wim.spaak@ziggo.nl] 9.2.3 Measuring Contrast Transmitted X-rays X-rays that are transmitted contribute to the image Absorbed X-rays Completely removed from the beam, X-rays that are absorbed cease to exist X-ray image The difference between absorbed and transmitted X-rays Lucite Lead 113 Spaak-Consultancy [wim.spaak@ziggo.nl] 9.2.4 kV is the Contrast Control 75 kV 85 kV 105 kV Low kVp : relative long wavelength Low Penetration High Contrast High kVp : relative short wavelength High Penetration Low Contrast Clinical images example, showing the influence of the kV-value on image contrast 114 Spaak-Consultancy [wim.spaak@ziggo.nl] 9.2.5 Scatter X-rays deflected into a random course, scattered radiation no longer contains useful information Because their direction is random, scatter does not display a usable image Scatter only contributes to image deterioration and loss of contrast Anti-scatter grid reduces scatter related contrast loss Patient Contrast filled blood vessels 115 Spaak-Consultancy [wim.spaak@ziggo.nl] 9.2.6 Collimation reduces Scatter Collimator wide open Larger image area Collimator used effectively Smaller image area And therefore also reduces loss of contrast 116 Spaak-Consultancy [wim.spaak@ziggo.nl] 9.2.6 Collimation reduces Scatter Collimator wide open Collimator used effectively 117 Spaak-Consultancy [wim.spaak@ziggo.nl] 9.2.7 Brightness & Contrast Decreasing brightness darkens the image (left), while increasing brightness lightens it (right) With decreasing contrast the colors in the image appear more muted and "flat" (left), while with increasing contrast it looks sharper and "crispy" (right) 118 Spaak-Consultancy [wim.spaak@ziggo.nl] 9.2.8 Leeds Test Objects These test objects are designed for quick quantitative assessments of image quality. 'TO 10' is intended for use with Fluoroscopy systems. 108 details (12 sizes x 9 contrasts) Size range 11mm to 0.25mm Contrast range 0.012 to 0.930 @ 70kV, 1.0mm Cu filtration 119 Spaak-Consultancy [wim.spaak@ziggo.nl] 9.3 Dynamic Imaging 1. Dynamic Imaging Definition 2. Photography and Motion 3. Dynamic Imaging Clinical Example 120 Spaak-Consultancy [wim.spaak@ziggo.nl] 9.3.1 Dynamic Imaging Definition The ability to image a moving object with high fidelity, such as the heart Dynamic imaging is measured in frames per second (fps) Also referred to as: Temporal Resolution Related terms: Motion blur, image lag and "stickiness" Factors that affects Dynamic Imaging Generator & tube / 15 or 30 fps Object (e.g. LCA vs. RCA) Detector Image processing 121 Spaak-Consultancy [wim.spaak@ziggo.nl] 9.3.2 Photography and Motion A short shutter speed (left) opens and closes the shutter so fast that a moving subject does not move very much during exposure. With slow speed (right) moving objects can move sufficiently to blur their image during the exposure of the image. 122 Spaak-Consultancy [wim.spaak@ziggo.nl] 9.3.3 Dynamic Imaging Clinical Example RCA motion blur example @ 30 fr/s: 8 ms / 3 ms X-ray pulse 8 millisecond pulse width 33 milliseconds between pulses 3 millisecond pulse width 33 milliseconds between pulses 123 Spaak-Consultancy [wim.spaak@ziggo.nl] 9.4 Noise 1. Noise Definition 2. All X-ray images have some Noise 3. Noise in an Image NOISE 124 Spaak-Consultancy [wim.spaak@ziggo.nl] 9.4.1 Noise Definition A disturbance, especially a random and persistent disturbance, that obscures or reduces the clarity of an image Also known as: Grain, quantum mottle, salt-and-pepper or "pixel-dust" Factors that affects noise Technique & geometry / higher mAs & large spot size Object / thin, shallow angles Image Detection / high DQE Image processing Mon

A PDF slide show lecture about the fundamentals of X-Rays and the basics of conventional diagnostic X-Ray Imaging Systems.

About Wim Spaak

Ritired now. Worked for more than 40 years in Holland at Philips Medical Systems as system designer and later on as department manager System Design Conventional X-ray Imaging Systems.

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