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Radiation physics basics Clémence Driol IQ/system engineer GE Healthcare Gerhard Brunst Clinical Leader X-ray Europe GE Healthcare Agenda X-ray Basics X-ray generation, interaction and detection Image Quality and Dose Radiation dose definitions, ALARA 2 Radiation Physics Basics 5/29/2015 X-ray Basics Introduction Nature of X-rays • X-Rays are electro-magnetic waves, like light and radio frequency waves; • Like all other waves, X-rays are also Photons; this aspect is predominant • Energy of X-Rays is usually measured in eV and mostly its multiple: keV • For medical imaging, energy of photons ranges from ~10 keV to less than 150 keV. (visible light ~2eV) Fig: electro-magnetic spectrum • X-rays are ionizing radiations 4 Radiation Physics Basics 5/29/2015 X-ray dose and image quality Inseparable • Images are created by the interaction of xrays with materials • During this interaction x-rays leave some energy in the patient • dose at the Image Detector is about 100 to 1000 less than dose entering the patient • X-ray production, interactions with matter, and detection are random in nature, so there is statistical variation which we see as image “noise”. • Object size and radiographic attenuation affect our ability to visualize them against the noisy background. 5 Radiation Physics Basics 5/29/2015 X-ray Basics X-ray generation X-ray Tube • Electrons are accelerated in a high electrical field (kV) • When they hit the anode they have an energy of keVp, • most electrons (99%) just cause heat in the anode • 1% will create a photon of high energy, that‘s an x-ray photon of x-ray energy keV < keV p (the electron energy) 7 Radiation Physics Basics 5/29/2015 X-ray beam generation: atomic view Bremsstrahlung 1.0 X-ray X ray are generated by sudden deceleration of energetic electrons on the anode: Bremsstrahlung. Normalized Counts 0.8 Characteristic Radiation: Spectral lines characteristic of the anode material but not important for radiography and interventional procedures 0.6 0.4 Continuous spectrum e*kVp (energy of the electron) 0.2 mA = # photons 0 0 20 40 kV Contrast 60 80 Energy (keV) 100 120 kV Penetration Fig: X-ray Spectrum Distributed spectrum = multi-energetic photons 8 Radiation Physics Basics 5/29/2015 X-ray beam generation: in X-ray tube Vacuum Power applied to the tube FILAMENT CATHODE 1% X-ray beam 99 % HEAT • Rotating anode to distribute the heat • Size of the anode to accumulate the heat for some time (Anode heat capacity) • Cooling system to maintain continuous fluoro (heat dissipation) 9 Radiation Physics Basics 5/29/2015 Which X-ray Spectrum to Use? soft x-rays (low energy, low keV) are absorbed in a short distance into the body - dose to patient but no use for imaging medium x-rays are partially absorbed, differently in different tissue material - they provide information about the body hard x-rays penetrate all material, - don‘t provide image information X-ray absorption vs photon energy 10 Radiation Physics Basics 5/29/2015 Which X-ray Spectrum to Use? soft x-ray photons can be blocked by filtration (Cu of various thicknesses) avoid very x-ray photons by using moderate kV X-ray absorption vs photon energy 11 Radiation Physics Basics 5/29/2015 Shaping the X-ray beam: Filtration Spectrum after 25 cm Water Spectrum at patient entrance 4.00E+02 7.00E+04 3.50E+02 3.00E+02 5.00E+04 Photon Nr Photon Nr 6.00E+04 4.00E+04 3.00E+04 2.00E+04 2.50E+02 2.00E+02 1.50E+02 1.00E+02 1.00E+04 5.00E+01 0.00E+00 0.00E+00 15 25 35 45 55 65 75 85 15 25 35 Energy (keV) regular filtration 0.2 mm Cu additional filtration 45 55 65 75 85 Energy (keV) regular filtration 0.2 mmCu additional filtration Softer radiation does not contribute significantly to image Spectral filtration (Cu) eliminates this radiation before it reaches the patient BUT – Potentially reduces image contrast because harder spectrum BUT - Requires higher X-Ray Tube power to be effective 12 Radiation Physics Basics 5/29/2015 Tube Take Away • 99% of the electrical power is converted into heat (tube must be cooled, specs for heat dissipation) • The X-ray beam is multi-energetic (continuous spectrum) • kV controls mainly the maximum x-ray energy • mA controls the intensity of the beam • filter removes low energy x-rays, shifts peak to higher energy 13 Radiation Physics Basics 5/29/2015 X-ray Basics X-ray absorption in the body Interaction of X-rays with matter • In vacuum, photons move along a straight line, no attenuation • In materials, photons can be - Transmitted, no interaction - Absorbed, they disappear, transferring their energy to the material - Scattered, they are deviated, and keep the same energy (elastic scatter), or transfer a fraction of their energy to the material (inelastic scatter) • Interactions can be multiple • Images are created by the interaction of X-rays with materials • During this interaction, X-rays leave some energy in the material: energy at the image receptor is 100 to 1000 times less than energy entering the object (patient) 15 Radiation Physics Basics 5/29/2015 Interaction of X-rays with matter (patient) Absorption: Scatter: Incident photon Incident photon E0 In the photoelectric effect, an X-ray is absorbed by an atomic electron, which is ejected from the atom causing ionization. Photoelectric effect is the most contributor for the image formation Scattered Photon Eω In Compton scattering, an incident X-ray interacts with an outer-shell electron. The electron is ejected from the atom, causing ionization. A scattered x-ray photon emerges at an angle ω relative to the incident photon’s trajectory (scatter) or it gets absorbed in further interactions. Compton effect contributes to the image but also cause fog on the image (reduced contrast) and dose to the operator. 16 Radiation Physics Basics 5/29/2015 Interaction of X-rays with matter X-ray imaging principle Low density tissue (lung with lots of air): low absorption, high transmission medium density tissue (soft tissue): medium absorption, medium transmission Dense tissue (bone): high absorption, low transmission Contrast is the difference in signal due to either different attenuating thickness of different attenuating material. 17 Radiation Physics Basics 5/29/2015 Absorption On their path more and more of the x-ray photons are absorbed Dose within the patient similar to an exponential curve but stretched due to beam hardening 18 Radiation Physics Basics 5/29/2015 Attenuation of X-rays with AEC 1 10.0 2 20.0 AEC brings dose at patient exit to the required level for good image quality 10.0 40.0 By doubling the object thickness for same exit dose entrance dose is doubled 19 Radiation Physics Basics 5/29/2015 Attenuation of X-rays 3 10.0 3 80.0 Adding another doubling layer cause another doubling of dose when exit dose is maintained 10.0 160.0 4x the same thickness required 8x the entrance dose than 1x the thickness 20 Radiation Physics Basics 5/29/2015 Interaction of X-rays with matter Effect of scattered radiation in image object to detect • Contrast reduction due to added offset in the image • How to minimize scattered radiation? Minimize generation of scatter • Collimate to the smallest viewing area that is required (less area that contributes to scatter) • primary radiation scattered radiation image receptor Minimize patient thickness by avoiding steep angles when possible Reduce scatter effect • • Add air-gap Use anti-scatter grid antiscatter grid Anti-scatter grid reduce contrast loss but slightly increase the dose. Remove-it for pediatric patients which cause little scatter. 21 Radiation Physics Basics 5/29/2015 Quantum noise “rain” of photons Quantum noise is due to the quantified nature of X-rays (made of a finite quantity of photons) 10 impacts 100 impacts 500 impacts 1000 impacts To get a clear image, we need enough x-ray photons: • sufficient dose per image in general higher dose results in better image quality 22 Radiation Physics Basics 5/29/2015 X-ray Basics Image capture CsI Detector Technology 1 photon X (~0.15-0.3Å) X ray absorption 3000 light photons (~5000Å) Light emission Scintillator: CsI-Needles X-ray Photon Light absorption ~1450 e- electron – hole generation Conversion to digital signal Visible Light Pixel Size Amorphous Silicon Detector 25 Radiation Physics Basics 5/29/2015 Image Quality DQE = Detective Quantum Efficiency System must be optimized for spatial resolution, contrast, and noise at the same time Contrast Noise Spatial resolution 26 Radiation Physics Basics 5/29/2015 Detector Efficiency DQE DQE = Detective Quantum Efficiency Detector DQE = Information Out Information In Patient DQE measures how much of the latent X-ray image that reaches the detector is accurately captured to form a viewable image. sensitivity: transfer of signal and of contrast into the image XRT DQE combines the measures of spatial resolution, sensitivity and noise. 27 Radiation Physics Basics 5/29/2015 Image Generation Take away • Images are created by the interaction of X-rays with materials in the body • During this interaction X-rays leave some energy in the body • Scatter radiation reduce relative contrast and need to be reduced. • X-ray production, interactions with matter, and detection are random in nature, so there is statistical variation which we see as image “quantum noise”. • DQE is the best scientifically accepted standard for assessing X-ray detector dose efficiency. 28 Radiation Physics Basics 5/29/2015 Image Quality and Dose ALARA concept, Radiation risks X-ray dose and image quality: Relations For each examination • dose must be sufficient to provide the required image quality for the diagnostic purpose • but dose should not be higher than necessary ALARA As low as reasonably achievable 30 / GE Healthcare Radiation dose Physics Definitions PATIENT DOSE ABSORBED DOSE: Energy per unit mass in this location (J/kg=Gy) Gray Nature of Radiation DOSE EQUIVALENT: Dose of x-rays that would have same Biological Effect (Sv) Sievert Localization vs. anatomy measured ORGAN DOSE: Dose Equivalent integrated over the organ (Sv) xxx weighting by risk PATIENT ENTRANCE DOSE (measurable) • Free in Air Skin Exposure (or Air KERMA) (Gy, R) • generally without backscatter + Integration EFFECTIVE DOSE: Sum of Organ Doses, weighted by risk specific to the organ (Sv) Biological effect 31 / GE Healthcare effective dose cannot be measured, but is estimated in model calculations Displayed Dose parameters Cumulated Air Kerma Focal Spot 67cm on DiscoveryTM Reference Isocenter Point 15cm • Live Monitor Air Kerma is cumulated over the course of exam and displayed • • Reference Monitor Air Kerma is a local dose property By standard, it is determined for a fixed distance to the x-ray tube for isocentric C-arm (Interventional Reference Point or Patient Entrance Reference Point) The cumulated air kerma is related to the patient skin dose to some extent: • The reference point is only an “average” representation of the patient skin location • The accumulation is performed over all gantry angulations and table positions 32 GE Healthcare confidential & proprietory Displayed Dose parameters Cumulated Dose Area Product (DAP) • Focal Spot Area2 Area1 AirKerma1 DAP is the product of Air Kerma and exposed area and is cumulated over the course of exam and displayed • it is independent of distances to the focal spot • DAP should be more representative of long-term risk (stochastic risk increases with dose and with exposed area, both are captured in DAP) • The accumulation is performed over all gantry angulations and table positions • Several guidance levels based on DAP AirKerma2 DAP = AirKerma1 * Area1 = AirKerma2 * Area2 Live Monitor Reference Monitor 33 GE Healthcare confidential & proprietory Dose vs Dose-Area Product (DAP) ESAK (Entrance Surface Air Kerma) vs AP (Dose Area Product): Quantity Dose (ESAK) Pro • Estimates deterministic risk of skin burn, >> particularly important for interventional procedures • Required reporting by JCAHO1 in some regions. DAP • Coarse estimate of lifetime cancer risk (but does not take into account the different sensitivity of the various exposed organs) Con Subject to inaccuracies: • Source-skin distance • Peak skin dose when using multiple gantry positions and FOVs less useful to estimate skin burn risk' (a small area exposed to a high dose is reported the same as a large area with a smaller dose) • Easy to measure Both Dose (ESAK) and DAP measures have their use. Dose (ESAK) is best to assess deterministic skin injury risk. DAP is a coarse estimate of (stochastic) lifetime cancer risk Joint Commission on Accreditation of Healthcare Organizations 34 GE Healthcare confidential & proprietory Whose responsibility? Clinical imaging optimization: Benefit Cost = Clinical Objective ~ = Risk Clinical Image Quality Imaging Dose Efficiency = Dose Rewrite the Equation: Clinical Image Quality Imaging Dose Efficiency Manufacturer’s Responsibilities: Clinician’s Responsibilities: To decide his/her thresholds of acceptable image quality, by procedure type. • Maximize Imaging Dose Efficiency • Provide wide range of Dose/IQ selections to operator • Provide Dose Feedbacks to clinician for awareness and decision making. • Automate the process as much as possible. x Dose Clinician’s Responsibilities to • Manage dose controls during procedures. • Monitor dose during procedure Clinicians AND Manufacturer have key roles in Patient Dose Optimization Maximization of Imaging Dose Efficiency is primarily the responsibility of the 35 manufacturer GE Healthcare confidential & proprietory Auto Exposure Control AutoEx Automatic Dose Setting: Principles of Operation Detector Average Signal is read from detector Patient Auto Ex XRT AutoEx™ Calculates Effective Patient Thickness (EPT), based on: • Average Detector Signal • kVp, mA, Pulse Width • Spectral Filter • FOV, SID Generator kVp mA Pulse width Spectral Filter Focal spot Using new EPT, AutoEx Determines Technique & Spectral Filter for next exposure from trajectory table, and sends to generator. High level of automation, optimization and flexibility. Adapts to a wide range of imaging situations. 36 Radiation Physics Basics 5/29/2015 Dose Take Away • X-ray is Electro-magnetic Radiation, with a spectrum far beyond UV, much more energetic, making it able to penetrate matter • An X-ray image reflects the difference in transmission through the different tissues • X-ray is an ionizing radiation – This property is used to detect and measure it – It is also responsible for biological damage • The absorbed dose is expressed in Gray • Effective Dose (biological effect) is measured in Sievert (SI units) 37 Radiation Physics Basics 5/29/2015 Radiation physics basics Clémence Driol IQ/system engineer GE Healthcare Gerhard Brunst Clinical Leader X-ray Europe GE Healthcare