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X-ray Imaging Overview.pdf

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					        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!!!




                             1895




                                                               1
   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 kVp2
 • 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
    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=Noe -(μ /ρ)ρx
• The thickness of material
  – N=Noe –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
     Cs = (A-B)/A
      A = Noe –μ x
      B = Noe –μ (x+z)
     Cs = 1 - e –μ z




                                              3
              Noise interferes with our ability to
                                 detect contrast

    σ = √N
  SNR = N/σ = √N


Exposure        Photons     Noise
  (mR)        /100μ pixel    (%)

  1.0            1333        2.7

  0.1            133         8.6

  0.01            13        27.4




           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
 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
          Hurter & Driffield (H&D) curve
               (aka “Characteristic Function”)




                Contrast (γ)

          Speed (Sensitivity)


                      Latitude


                                          OD = log10(I/Io)




        Scatter is a problem in ordinary
                            radiography


                                C0 = (A-B)/A
                                 C = C0 / (1+S/P)




                     Scatter control methods




Scatter reduction grid                  Air gap technique




                                                             6
                                    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
     GREEN ?                 BLUE ?              VIOLET?




                                                                  388 nm ?

                                                                  413 nm ?




                  LAVENDER?                      INDIGO?




         Photostimulable Phosphor Reader
         Rotating polygon mirror                  Analog-to-Digital Converter
                                   Photomultiplier tube
                                                                             ?
                         Light guide              Amplifier
 Laser

                    fast scan



              Latent Image           slow scan


         Imaging plate




   Both Indirect and Direct Digital Detectors
depend on Thin Film Transistor (TFT) arrays




                                                  • Indirect
                                                  • Direct




                                                                                 8
Mammography is a specialized form
                   of radiography




   Mammography is performed with
      specialized x-ray generators




Mammography utilizes special target
          and filter combinations




                                      9
        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 screen-
                   film systems




                                           10
   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
  What’s wrong with digital imaging
                          anyhow?


                         • Limited
                           spatial
                           resolution




Fluoroscopy provides real-time, full-
       motion radiographic imaging




                  • Humans detect
                    moving objects better
                    than stationary objects
                    against a noisy
                    background!




                                              12
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
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
          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
“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
        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
     Digital tomosynthesis for mammography,
                               chest, and body




                                Courtesy JA Seibert, UC-Davis




          Tomosynthesis Noise vs Dose Setting

Default                                              Modified




0.8 mAs/exposure                     5.0 mAs/exposure
20 mAs total                              75 mAs total




                        Artifacts—Surgical clip
          Scout
                              Scout Tomo




                                                                18
                   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
                    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
Desktop CR system for portal
                radiography




   Film vs CR portal imaging




          CR Portal imaging




                               21
  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

				
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