Overview. Professor Roentgen was a Physicist!!! The Physics of Radiation Oncology X-ray Imaging

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1 The Physics of Radiation Oncology X-ray Imaging Charles E. Willis, Ph.D. DABR Associate Professor Department of Imaging Physics The University of Texas M.D. Anderson Cancer Center Houston, Texas Overview Review fundamentals of x-ray imaging Ordinary radiographic examinations Mammography Fluoroscopy Special x-ray imaging examinations Professor Roentgen was a Physicist!!!

2 X-rays are produced when electrons slam into a target! Two kinds of x-rays generated Braking radiation (Bremsstrahlung) Characteristic Radiation A spectrum of energies is produced 90 kvp The quantity of x-rays produced depends on technical factors (a.k.a. technique ) Accelerating potential (kilovolts peak, kvp) Exposure α kvp 2 Beam/tube current (milliampere-seconds, mas) X-rays go out in all directions Some are blocked by the tube housing Some are blocked by the collimator blades Some are allowed to travel toward the patient and detector A light indicates where the radiation field is projected 2

3 At diagnostic energies, x-rays interact with matter mainly by two processes Photo-electric effect Compton scattering X-rays are attenuated differently according to Their energy The composition of material Density (ρ) Atomic number (Z) N=N o e -(μ /ρ)ρx The thickness of material N=N o e μ x HVL= 0.693/μ X-ray imaging is a quest for contrast! We can visualize anatomic features because they attenuate x-rays to different extents C s = (A-B)/A A = N o e μ x B = N o e μ (x+z) C s = 1 -e μ z 3

4 Noise interferes with our ability to detect contrast σ = N SNR = N/σ = N Exposure (mr) Photons /100μ pixel Noise (%) Ordinary radiographic examinations are cleverly designed To position the patient in an x-ray beam of sufficient quality and quantity to project the anatomy of interest onto a flat detector of radiation. Additional projections (views) are obtained in order to visualize clinical features in three dimensions without overlying anatomic structures Basic geometry of projection radiography 4

5 Technique affects patient exposure in ordinary radiography What about the radiation detector? The vast majority of ordinary radiographic exams are captured on film. This is NOT the case at M. D. Anderson! Although you could make radiographic images by direct exposure of film, (cardboard cassettes for extremities), this requires a lot of radiation. Even Roentgen used a fluorescent intensifier to convert the x-rays into light that the photographic emulsion would be more sensitive to. Screen-film cassette Fluorescence from intensification screens exposes dual-emulsion film (AgBr and AgI) Latent image on film is chemically developed (reduction) Image is density from metallic silver (Ag) 5

6 Hurter & Driffield (H&D) curve (aka Characteristic Function ) Contrast (γ) Speed (Sensitivity) Latitude OD = log 10 (I/I o ) Scatter is a problem in ordinary radiography C 0 = (A-B)/A C = C 0 / (1+S/P) Scatter control methods Scatter reduction grid Air gap technique 6

7 Reduce scatter: leave less radiation in the patient Scatter increases patient dose and degrades image quality The amount of scatter depends on the volume of tissue irradiated at the same time Collimating beam into a fan and scanning anatomy reduces scatter and patient dose Differs from scatter reduction grid which imposes dose penalty (Bucky Factor) Filmless radiography at M. D. Anderson Fuji Computed Radiography (CR) GE Digital Radiography (DR) CR is based on the physical process of photo-stimulated luminescence (PSL) X-rays contribute energy to the electrons by the photoelectric effect Electrons can give up energy (violet light) by emitting light immediately (fluorescence) by emitting light slowly (phosphorescence) Some electrons can retain (store) their energy crystal defects can trap excited electrons electrons can escape the traps when exposed to the proper wavelength (red) light (photo-stimulated luminescence) electrons can also escape by thermal mechanisms 7

8 GREEN? BLUE? VIOLET? 388 nm? 413 nm? LAVENDER? INDIGO? Photostimulable Phosphor Reader Rotating polygon mirror Analog-to-Digital Converter Photomultiplier tube? Laser Light guide Amplifier fast scan Latent Image slow scan Imaging plate Both Indirect and Direct Digital Detectors depend on Thin Film Transistor (TFT) arrays Indirect Direct 8

9 Mammography is a specialized form of radiography Mammography is performed with specialized x-ray generators Mammography utilizes special target and filter combinations 9

10 Compression is essential in mammography Lower scatter Reduced radiation dose Shorter exposure time Geometric magnification: sometimes crucial for diagnostic mammography Small focal spot (0.1mm) Best resolution on anode side (nipple) Best penetration on cathode side (chest wall): heel effect Mammography uses special screenfilm systems 10

11 Mammography operates at the limits of radiographic technology Quality Assurance is extremely important! Mammography Quality Standards Act (MQSA) Film Viewing Conditions are also critical! Stereotactic breast biopsy Digital detectors are common in small FOV mammography: new for full FOV Intensifying screen coupled to CCD via fiberoptic taper Slot-scan device 11

12 What s wrong with digital imaging anyhow? Limited spatial resolution Fluoroscopy provides real-time, fullmotion radiographic imaging Humans detect moving objects better than stationary objects against a noisy background! 12

13 Components of a fluoroscopy system What s different in these two pictures? The input window of the image intensifier (II) converts x-rays into electrons The output window of the II converts electrons into light 13

14 The optical distributor allows imaging on video and film Digital detectors can substitute for the II The tradeoff for real-time, full-motion imaging is lousy resolution That s why radiologists record their findings with still images! 14

15 In general, fluoroscopy systems come in two flavors GI Suites Peripheral Angiography Suites Cardiac Catheterization Suites Biplane Angiographic Systems Portable Fluoroscopy C arms Radiation management issues with fluoroscopy Patient dose Beam filtration Low frame rate pulsed fluoro Low dose Automatic Brightness Control (ABC) Last-image(frame)-hold Largest FOV Avoid Lead-foot syndrome Limit associated images (photospot, cine, overheads) 2 X 7.5/30 = ½ Exposure rate 7.5/30 = ¼ Exposure rate 15

16 Mag-mode provides more detail but requires more x-ray exposure Radiation management issues with fluoroscopy Personnel Dose Scatter Lead aprons Portable lead glass shields Reduce fluoro time Lead eyeglasses, thyroid shield, ceiling mounted shields Don t remove the lead curtains! Time, Distance, Shielding High dose rate fluoro ( specially activated fluoroscopy ) can be especially hazardous 16

17 Special x-ray imaging exams Geometric Tomography (conventional tomography, IVP) Digital Tomosynthesis Temporal Subtraction (Digital Subtraction Angiography) Dual-Energy Subtraction Portal Imaging Conventional tomography Digital Tomosynthesis Projections are shifted and summed to emphasize features in the focal plane 17

18 Digital tomosynthesis for mammography, chest, and body Courtesy JA Seibert, UC-Davis Tomosynthesis Noise vs Dose Setting Default Modified 0.8 mas/exposure 20 mas total 5.0 mas/exposure 75 mas total Scout Artifacts Surgical clip Scout Tomo 18

19 Artifacts Processing cutoff Usually due to failure to perform scout before tomo sequence Temporal Subtraction Dual-energy Subtraction two commercial methods Single exposure, dual detector Single detector, dual exposure 19

20 Dual-energy subtraction Low energy High energy Bone-only Soft tissue Portal imaging Documenting beam position and adjusting coverage of the irradiated field in treatment Used in both photon and electron beam therapy Lead, steel, or Cu filter typical: Why? 20

21 Desktop CR system for portal radiography Film vs CR portal imaging CR Portal imaging 21

22 How is Diagnostic Imaging Physics distinct from Therapeutic Radiological Physics? Therapy Physics involves imaging for documenting beam position and adjusting coverage of the irradiated field in treatment The purpose of imaging is to control the application of radiation rather than to assess the condition of the patient anatomy. The quality of imaging needs only to be sufficient for proper registration with anatomic landmarks The quantity of radiation and patient population differs between Diagnostic and Therapy Physics The quantity of radiation involved in a therapeutic procedure is usually orders of magnitude higher than a diagnostic examination. The number of patients subjected to diagnostic procedures is orders of magnitude higher than patients undergoing therapy. Patients undergoing therapeutic procedures are already ill: patients undergoing diagnostic procedures are not necessarily ill. Example: consequences of a single misadministration are different! Thanks for your time and attention! 22