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X-Ray Medical Imaging Physics –
IB Objectives
I.2.1
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
1
IB Physics HL 2
X-Ray Production
Anode **Spinning**
(Tungsten)
(Why?)
Vacuum
chamber
...
High voltage
Hot filament
cathode
X-rays
Electrons
Filament voltage
3/06/2009
2
IB Physics HL 2
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
3
IB Physics HL 2
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
4
IB Physics HL 2
X-Ray Interaction with Matter
and Attenuation

Coherent scattering / Rayleigh scattering
 Atom not ionized nor excited
Outgoing photon
scatters off
atom as a whole
Incoming photon
scatters off
atom as a whole
3/06/2009
5
IB Physics HL 2
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
6
IB Physics HL 2
X-Ray Interaction with Matter
and Attenuation

Pair production
+  Enough energy in initial beam to create e e pair
Nucleus interacts
with incoming
photon
eElectron-positron
pair created from
incoming photon
and nuclear interaction
Incoming photon
scatters off nucleus
3/06/2009
e+
7
IB Physics HL 2
X-Ray Interaction with Matter
and Attenuation



3/06/2009
8
For carbon
(~people)
below 12 keV,
increasing
energy
decreases
interaction
Interaction
mainly from
photoelectric
effect
Bones (heavier
nuclei)
attenuate Xrays more than
soft tissue
(carbon)
IB Physics HL 2
X-Ray Attenuation Coefficient


Similar to radiation half-lives and decay coefficients
Decrease in intensity (W/m2) is proportional to initial
intensity:
dI
  I
dx
With solution: I = I0e-x



 is the linear attenuation coefficient (m-1)
 does depend on energy

3/06/2009
This gives the intensity at depth x meters
9
IB Physics HL 2
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
-x1/2
 I0/2 = I0e
-x1/2
 or 0.5 = e
or ln(0.5) = -x1/2
or  = ln(2) / x1/2 (just like radioactive decay)
3/06/2009
10
IB Physics HL 2
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
11
IB Physics HL 2
X-Ray Attenuation Sample Problem

The attenuation coefficient for an X-ray of a specific
wavelength through muscle is 0.045 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?

3/06/2009
In which of these materials does the X-ray
intensity drop off more quickly?
12
IB Physics HL 2
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
13
IB Physics HL 2
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
14
IB Physics HL 2
X-Ray Beam Techniques

Tube voltage
 Increasing tube voltage increases penetrating
power of X-rays
 Bremsstrahlung
 K, L
spectra
3/06/2009
15
IB Physics HL 2
X-Ray Beam Techniques

Beam current
 Increasing beam current increases intensity of Xrays
 Does not change
penetrating power
3/06/2009
16
IB Physics HL 2
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
17
IB Physics HL 2
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
18
IB Physics HL 2
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
19
IB Physics HL 2
X-Ray – Coronary Arteries
3/06/2009
From: http://www.ajronline.org/cgi/content-nw/full/179/4/911/FIG8
20
IB Physics HL 2
X-Ray Detection, Recording, and Display



Detection
 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
21
IB Physics HL 2
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
3/06/2009
X-ray
sensitive
film
22
IB Physics HL 2
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
3/06/2009
X-ray
film cassette
23
IB Physics HL 2
X-Ray Detection, Recording, and Display

Enhanced film cassette
 Intensifying screens contain X-ray sensitive
phosphors that create light when struck with Xrays
 Film displays X-rays detected by film and screen
3/06/2009
24
IB Physics HL 2
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
X-ray
tube
3/06/2009
Digital
scanning
process
X-ray
sensitive plate
25
IB Physics HL 2
X-Ray Detection, Recording, and Display

Computer Radiology
 Instead of film, X-rays detected by a computerreadable screen
 Computer reads screen, and stores image in
memory
 Computer display
X-ray
tube
Computer-readable
X-ray phosphor screen
3/06/2009
26
IB Physics HL 2
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
3/06/2009
X-ray sensitive
phosphor screen
27
IB Physics HL 2
X-Ray Medical Imaging –
Fundamental Ideas

What are they?
3/06/2009
28
IB Physics HL 2
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
29
IB Physics HL 2
Idea of Multiple Scan Directions

Imagine taking X-ray image of 2 x 2 square
 Take image in horizontal direction
A
C
4
5
B
D
X-rays
3/06/2009
8
4
5
X-ray
intensities
10
Film
30
IB Physics HL 2
Idea of Multiple Scan Directions

Imagine taking X-ray image of 2 x 2 square
 Take second image in vertical direction
X-rays
A
C
Film
7
3/06/2009
4
5
B
D
11
31
4
5
8
10
X-ray intensities
IB Physics HL 2
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
A
C

3/06/2009
3
4
B
D
5
6
8
10
X-ray intensities
11
7
This is the principle of Computed Tomography (CT)
32
IB Physics HL 2
Computed Tomography (CT) Scan
Schematic

Use more then just 2 x 2 resolution
 Typical: 256 x 256
3/06/2009
33
IB Physics HL 2
Computed Tomography (CT) Scanners
3/06/2009
34
IB Physics HL 2
Computed Tomography Scanner
3/06/2009 From http://en.wikipedia.org/wiki/Computed_Axial_Tomography
35
IB Physics HL 2
Computed Tomography Scanner Internals
3/06/2009 From http://en.wikipedia.org/wiki/Computed_Axial_Tomography
36
IB Physics HL 2
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
37
IB Physics HL 2
Computed Tomography – Usage



Brain scans

Bleeding

Stroke

Tumor
Other organs (soft tissue)

Heart

Kidneys

Etc
Applications

Tumors

Trauma

Structure
3/06/2009 From http://en.wikipedia.org/wiki/Computed_Axial_Tomography
38
IB Physics HL 2
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
39
IB Physics HL 2
Computed Tomography –
Fundamental Ideas

What are they?
3/06/2009
40
IB Physics HL 2