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X-Ray Medical Imaging Physics – IB Objectives

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1 X-Ray Medical Imaging Physics – IB Objectives
Define the terms attenuation coefficient and half-value thickness. I.2.2 Derive the relation between attenuation coefficient and half-value thickness I.2.3 Solve problems using the equation I = I0e-x I.2.4 Describe X-ray detection, recording, and display techniques I.2.5 Explain standard X-ray imaging techniques used in medicine I.2.6 Outline the principles of computed tomography (CT) 3/06/2009 IB Physics HL 2

2 . . . X-Ray Production Anode (Tungsten) **Spinning** Vacuum chamber
(Why?) . . . High voltage X-rays Hot filament cathode Electrons Filament voltage 3/06/2009 IB Physics HL 2

3 X-Ray Interaction with Matter and Attenuation
X-rays interact with matter in four ways Photoelectric effect (photon in – electron out) Coherent scattering off atom as a whole (photon in – photon out) Compton scattering off electron (photon in – electron + photon out) Pair production (photon in – electron + positron out) (E > 1 MeV) 3/06/2009 IB Physics HL 2

4 X-Ray Interaction with Matter and Attenuation
Photoelectric effect Orbital electron knocked out of atomic orbit creating ion Incoming photon scatters off orbital electron 3/06/2009 IB Physics HL 2

5 X-Ray Interaction with Matter and Attenuation
Coherent scattering / Rayleigh scattering Atom not ionized nor excited Incoming photon scatters off atom as a whole Outgoing photon scatters off atom as a whole 3/06/2009 IB Physics HL 2

6 X-Ray Interaction with Matter and Attenuation
Incoherent scattering / Compton scattering Electron scattered out of atom Incoming photon scatters off single electron (as if electron were free) Outgoing photon after scattering off electron 3/06/2009 IB Physics HL 2

7 X-Ray Interaction with Matter and Attenuation
Pair production Enough energy in initial beam to create e+e- pair Nucleus interacts with incoming photon e- Electron-positron pair created from incoming photon and nuclear interaction Incoming photon scatters off nucleus e+ 3/06/2009 IB Physics HL 2

8 X-Ray Interaction with Matter and Attenuation
For carbon (~people) below 12 keV, increasing energy decreases interaction Interaction mainly from photoelectric effect Bones (heavier nuclei) attenuate X-rays more than soft tissue (carbon) 3/06/2009 IB Physics HL 2

9 X-Ray Attenuation Coefficient
Similar to radiation half-lives and decay coefficients Decrease in intensity (W/m2) is proportional to initial intensity: With solution: I = I0e-x  is the linear attenuation coefficient (m-1)  does depend on energy This gives the intensity at depth x meters 3/06/2009 IB Physics HL 2

10 X-Ray Half-Value Thickness
Similar to the radioactive decay half-life, we can define a half-value thickness at which the beam drops to one-half its initial intensity I0/2 = I0e-x1/2 or 0.5 = e-x1/2 or ln(0.5) = -x1/2 or  = ln(2) / x1/2 (just like radioactive decay) 3/06/2009 IB Physics HL 2

11 X-Ray Choice of Wavelength
Choice of wavelength depends on what is being imaged Bone Soft tissue Also want to minimize absorbed energy 3/06/2009 IB Physics HL 2

12 X-Ray Attenuation Sample Problem
The attenuation coefficient for an X-ray of a specific wavelength through muscle is cm-1 What is the half-value thickness? The half-value thickness of bone, for the same X-ray, is 150 times smaller What is its attenuation coefficient? In which of these materials does the X-ray intensity drop off more quickly? 3/06/2009 IB Physics HL 2

13 X-Ray Attenuation Sample Problem (Cont’d)
If the initial X-ray intensity is 2.00 W/m2, what is its intensity after traveling through 13.0 cm of muscle? How much is absorbed by the muscle? What is the intensity of the X-ray after traveling through 3.47 cm of bone? 3/06/2009 IB Physics HL 2

14 X-Ray Beam Techniques Improve penetrating quality of beam by absorbing out low-energy X-rays With large attenuation coefficients, X-rays get absorbed easily by soft tissue Use ~1 mm to 1 cm of Al 3/06/2009 IB Physics HL 2

15 X-Ray Beam Techniques Tube voltage
Increasing tube voltage increases penetrating power of X-rays Bremsstrahlung K, L spectra 3/06/2009 IB Physics HL 2

16 X-Ray Beam Techniques Beam current
Increasing beam current increases intensity of X-rays Does not change penetrating power 3/06/2009 IB Physics HL 2

17 X-Ray Beam Techniques Target material
Changing target material changes characteristic K, L lines Bremsstrahlung spectrum stays the same (more or less) 3/06/2009 IB Physics HL 2

18 X-Ray Imaging Techniques
Putting a lead grid in front of imaging material will improve the sharpness of the image Scattered X-rays are absorbed by grid before getting to film 3/06/2009 IB Physics HL 2

19 X-Ray Imaging Techniques
Direct image Bone (white) Higher energy X-ray Soft tissue (gray) Lower energy X-ray Gaps – air (black) Contrast medium Opaque material outlines soft tissue Barium, bismuth (intestines) Iodine (blood) 3/06/2009 IB Physics HL 2

20 X-Ray – Coronary Arteries
3/06/2009 From: IB Physics HL 2

21 X-Ray Detection, Recording, and Display
Film, image-enhanced film, digital computer-read screens and detectors Recording Film, digital film, computer memory Display Film, computer display, television (real-time) display (~fluoroscopy) 3/06/2009 IB Physics HL 2

22 X-Ray Detection, Recording, and Display
Film Person placed between X-ray tube and film Film is detection, recording, and display mechanism all in one X-ray tube X-ray sensitive film 3/06/2009 IB Physics HL 2

23 X-Ray Detection, Recording, and Display
Enhanced film (basically all modern X-rays) Person placed between X-ray tube and film Film is placed in cassette with X-ray sensitive phosphors Provides better image Film as recording and display device X-ray tube X-ray film cassette 3/06/2009 IB Physics HL 2

24 X-Ray Detection, Recording, and Display
Enhanced film cassette Intensifying screens contain X-ray sensitive phosphors that create light when struck with X-rays Film displays X-rays detected by film and screen 3/06/2009 IB Physics HL 2

25 X-Ray Detection, Recording, and Display
Digital Radiology Instead of normal film, X-rays detected by a plate sensitive to X-rays Plate is “read” by laser Stored in computer memory Computer display Digital scanning process X-ray tube X-ray sensitive plate 3/06/2009 IB Physics HL 2

26 X-Ray Detection, Recording, and Display
Computer Radiology Instead of film, X-rays detected by a computer-readable screen Computer reads screen, and stores image in memory Computer display X-ray tube Computer-readable X-ray phosphor screen 3/06/2009 IB Physics HL 2

27 X-Ray Detection, Recording, and Display
Real-Time Displays Observe operation of heart, intestines, throat, etc. Instead of film, X-rays detected by phosphors on screen Television camera observes phosphor screen Display real-time image on television screen X-ray tube X-ray sensitive phosphor screen 3/06/2009 IB Physics HL 2

28 X-Ray Medical Imaging – Fundamental Ideas
What are they? 3/06/2009 IB Physics HL 2

29 Drawbacks of Normal X-Ray Scans
X-rays show only one view of body Shadow of everything between X-ray tube and film Difficult to interpret soft-tissue images -> Idea: take X-ray scans in multiple directions 3/06/2009 IB Physics HL 2

30 Idea of Multiple Scan Directions
Imagine taking X-ray image of 2 x 2 square Take image in horizontal direction A B 8 10 4 5 X-ray intensities C D X-rays Film 3/06/2009 IB Physics HL 2

31 Idea of Multiple Scan Directions
Imagine taking X-ray image of 2 x 2 square Take second image in vertical direction X-rays A B 8 10 4 5 C D Film 7 11 X-ray intensities 3/06/2009 IB Physics HL 2

32 Idea of Multiple Scan Directions
Imagine taking X-ray image of 2 x 2 square Use both intensities to determine relative X-ray absorption Show relative absorption with different shading This is the principle of Computed Tomography (CT) A B 8 10 3 5 4 6 C D 7 11 X-ray intensities 3/06/2009 IB Physics HL 2

33 Computed Tomography (CT) Scan Schematic
Use more then just 2 x 2 resolution Typical: 256 x 256 3/06/2009 IB Physics HL 2

34 Computed Tomography (CT) Scanners
3/06/2009 IB Physics HL 2

35 Computed Tomography Scanner
3/06/2009 IB Physics HL 2 From

36 Computed Tomography Scanner - Internals
3/06/2009 IB Physics HL 2 From

37 Computed Tomography – 2D to 3D
X-ray imaging system can move along the body CT scans in cross-section Can build up 3D model of body Instead of pixels (picture elements): voxels (volume elements) 3/06/2009 IB Physics HL 2

38 Computed Tomography – Usage
Brain scans Bleeding Stroke Tumor Other organs (soft tissue) Heart Kidneys Etc Applications Tumors Trauma Structure 3/06/2009 IB Physics HL 2 From

39 Computed Tomography – Risk Balancing
CAT scans and X-rays use ionizing radiation Ionizing radiation is damaging to tissue Normal X-rays give some multiples of background radiation dosage CAT scans give significantly more than normal X-rays Balance help to patient from scan vs risk of damage (cancer) from X-rays 3/06/2009 IB Physics HL 2

40 Computed Tomography – Fundamental Ideas
What are they? 3/06/2009 IB Physics HL 2

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