10/25/12
Fluoroscopy & Angiography: Image Intensifiers, Flat Panels, Dose
Robert G. Gould, Sc.D. Professor and Vice Chair Department of Radiology and Biomedical Imaging University of California San Francisco
Fluoroscopy vs Radiography • Used for dynamic imaging • X-ray tube operated at lower currents – Factor of 50 to 100+ less • X-ray quanta/cm2 – Radiography: 107/cm2 – Fluoroscopy: < 104 /cm2 • Statistical difference in the quality of the image • Higher noise: lower signal
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Fluoroscopy: Image Intensifier
Digital Video Video Processor • Electro-optical camera amplification of low light level signal Image • Digitize video signal Intensifier Video – Image processing Monitor – Flexible frame rates – Electronic subtraction Collimator X-ray tube
X-Ray Image Intensifier
• Convert X-ray energy into visible light image Housing • Concentrates energy from Input screen! the input phosphor onto smaller output phosphor Photocathode! Output! screen – “minification” gain • Adds kinetic energy – 15-50 X more brightness Vacuum! • Increases brightness per enclosure unit area by several thousand Electron lens
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X-Ray Image Intensifier: Gain
• Minification gain, GM 2 2 Gm =(Dinput) / (Doutput) ~ 16 – 200
• Electronic (flux) gain, GE , produced by electron acceleration
GE ~ 50
• Total gain, GT
GT = GM x GE ~ 5000
X-Ray-to-Light Conversion • Fluorescent layer of CsI – Transform x-ray I K-edge energy into light
energy Cs Intensity Relative – Good x-ray stopping K-edge power (Zeff~54, ρ = 4.5 gm/cm3) – Hydroscopic
• Structured phosphor AttenuationCoefficient Mass 20 30 40 50 60 70 X-Ray Energy (KeV)
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Photoconduc ve Layer (Photocathode)
• Thin layer of Housing antimony trisulfide Input screen! sputtered directly onto CsI Photocathode! Output! screen • Conversion efficiency of 10 - 20% Vacuum! – A 60 KeV photon enclosure absorbed in phosphor yields ~400 electrons Electron lens
Image Intensifier
• Electron optics – Electrons emitted by photo- emissive layer • Charged particles • Effected by magnetic or electrical fields • Kept coherent by electrical potentials across tube Housing Input screen! • Output screen, ZnCdS Photocathode! Output! – Convert KE of electrons to light screen – Much smaller than input screen ~ 3 cm diameter Vacuum! enclosure – Efficient
• Charged electrons easily stopped Electron lens
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Image Intensifier Mag Modes
• Change in electro- Full! FOV optical focusing Field – Object appears larger • Improve resolution • Less minification gain – More dose to achieve Mag! FOV Mode same brightness
Image Distor on
• Geometric – results from the projection of the x-ray image onto the curved surface of the image intensifier – worse for large fields-of-view – partially corrected by electronic lens system • S - distortion – results from stray magnetic fields (e.g. the field of the earth) – use mu-metal shield around the II
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Automa c Exposure Control (AEC) • Feedback mechanism that maintains a constant brightness level from the center portion of the output screen – Adjusts the X-ray technique factors (mA and KV)
Fluctuating patient entrance dose Constant input exposure Constant light level
X-ray tube + KV Patient Image intensifier + mA Feedback loop
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Flat Panel Angiographic Systems
• Pixilated detector – Digitization within the detector • Dynamic range >> analog systems • AEC not the same as in II based systems • Complex
Flat Panel Angiographic Systems
• Scintillator/amorphous silicon detector – Thin film transistor (TFT) active array – Each pixel individually addressable • Readout of the TFT array Amplifiers between X-ray pulses
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Angiographic Systems
Flat Panel Detector Image Intensifier
• Pixellated detector • Image intensifier – Pixel dimensions systems: digitize determined at detector video signal – Array of light sensitive • Pixel dimensions detectors covered by determined by ADC light emitting phosphor process (indirect detection) • Charge conversion • Light generated by X- within video system rays is converted into charge within detector
Flat Panel Mag Modes
• FP pixelated to finest resolution but readout binning occurs for large FOVs – Maximum presentation is 10242 x10242 during fluoro
2048 pixels
2048 2 2 40 cm pixels 1024 x 1024 display
40 cm
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Flat Panel Mag Modes
• Resolution improves until pixel matrix Improvement equals display matrix 2048 1024 – 16-20 cm FOV No inherent • Further FOV change 1024 reductions, no inherent change in resolution 2048 • No change in gain!
Automa c Exposure Control Considera ons (Vendor Spin) • Purposes – Provide ‘optimal’ image quality for all patient thickness and viewing angles – Maintain appropriate patient dose • Stay within regulations at all times – Eliminate all manual adjustment of technique factors • Fast and easy regulation – Provide long x-ray tube life • Improve cost effectiveness
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Automa c Exposure Control Considera ons • NOT a dose saving system • Design factors are to maintain a given image quality (contrast-to-noise factor) while staying within equipment capabilities and patient dose regulations – X-ray tube limits – User preference • Can have low dose settings – Limit patient dose to fraction of regulatory limits
Automa c Exposure Control Variables
• Tube potential – Sensitivity of the detector is varies with beam energy • Beam filtration – Use both Cu and Al • mAs per pulse – Both mA and pulse width vary
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Variable X-ray Beam Filtra on
Not all conditions possible due to x-ray tube limits Constant: Patient dose rate Patient thickness Pulse width 0.9 mm Cu
Increasing filter thickness 0.4 mm Cu 0.1 mm Cu Increasing mA IncreasingmA
Increasing kVp
Pa ent Dose Rate
4.4 cGy/min
15 fps normal 7.5 fps normal
15 fps low dose 2.2 cGy/min 7.5 fps low dose
Patientdose rate Variable slope
Patient thickness
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Pa ent Dose Rate: Pulsed Fluoro
< 2x reduction ~ 2x reduction No difference!
4.4 cGy/min
15 fps normal
7.5 fps normal Patientdose rate
Patient thickness
Pulse Rate
• Effect on patient dose depends on other factors – Patient dose does not always scale with rate – mAs per pulse, filtration, and kVp can all change
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Flat Panels: Dose Rate vs Field Size
• Increases with 1/(field size) – 15 x15 cm2 has 4x dose over 30 x 30 cm2 • No change in gain • Empirically determined • No change from II based systems
Measures of Dose: Kerma
• Kinetic energy released per unit mass • Air kerma: measure of the dose to air • Unit is the Gray (Gy) • At diagnostic x-ray energies, 114 R of exposure will produce 1 Gy in air • 5 R/min = 44 mGy/min
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Dose Area Product (DAP) aka Kerma Area Product (KAP) • Kerma (mGy) x Area (cm2) – Units: mGy-cm2 • Independent of distance from the source • Each x-ray source equipped with DAP meter
Interven onal Reference Point
• Air kerma at a point located on the central ray, 15 cm from the
isocenter towards the x-ray Central Ray tube
– C-arm geometry angio systems Isocenter 15 cm – ~ 30 cm from the image detector Reference point – Cumulative kerma correlates with peak skin dose – Does not account for panning of the beam X-ray tube – Does not include scatter
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Fluoroscopy Equipment Manufactured A er June-06
• Must display at the fluoroscopist's working position the air kerma rate (AKR) and cumulative air kerma for each x-ray tube at the reference point(s) – Display dose rate during fluoroscopy (mGy/min) – Display cumulative dose within 5 sec after exposure ends • AKR normally < 44 mGy/min
Dose Indicators: Philips
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Recommended Dose Thresholds
Dose Metric Threshold
Peak skin dose 3000 mGy
Reference dose 5000 mGy
Kerma-area product 300,000-500,000 mGy-cm2
Fluoroscopy time 40-60 min
• If any threshold exceeded, patient notified and told to look for skin symptoms
Typical Skin Dose: Selected interven onal Procedures
Procedure ~ Dose, mGy Reference TIPS 2168 Miller et al., 2003, JVIR Nephrostomy 258 Miller et al., 2003, JVIR Neuroembolization 1977 Miller et al., 2003, JVIR Spine embolization 3739 Miller et al., 2003, JVIR IVC filter placement 193 Miller et al., 2003, JVIR Biliary drainage 781 Miller et al., 2003, JVIR Hepatic embolization 1959 Dauer et al., 2009, JVIR PTCA & CA 2000 Balter, et al., 2008, Med Phys Coronary intervention 1407 Suzuki et al., 2006, Circ
After CRCPD PUB #E-10-7 (2010)
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Methods to Reduce Pa ent Dose • Minimize x-ray beam on time – Use last image hold – Use virtual (electronic) collimation • Position the x-ray source and the detector optimally – Inverse square law • Collimate the beam • Understand and use pulsed fluoroscopy • When possible use store fluoro loop instead of an angiographic (cine) run – 8-10x reduction • Vary the entrance port site • Use the least amount of magnification possible – Avoid steeply angled projections if possible
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