3/06/2009IB Physics HL 21 X-Ray Medical Imaging Physics – IB Objectives I.2.1Define the terms attenuation coefficient and half-value thickness. I.2.2Derive

3/06/2009 IB Physics HL 21

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)


X-Ray Production

High voltage

Hot filamentcathode

Filament voltage

Anode(Tungsten)

X-rays

Vacuumchamber

...

Electrons

**Spinning** (Why?)


X-Ray Interaction with Matterand 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)



Orbital electronknocked out ofatomic orbitcreating ion

Incoming photonscatters offorbital electron

Photoelectric effect



Coherent scattering / Rayleigh scattering Atom not ionized nor excited

Incoming photonscatters offatom as a whole

Outgoing photonscatters offatom as a whole



Incoherent scattering / Compton scattering

Incoming photonscatters offsingle electron(as if electron werefree)

Electron scatteredout of atom

Outgoing photonafter scattering offelectron



Pair production Enough energy in initial beam to create e+e- pair

Incoming photonscatters off nucleus

Electron-positronpair created fromincoming photonand nuclear interaction

Nucleus interactswith incomingphoton e-

e+



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)


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

I dx

dI −=


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)


X-Ray Choice of Wavelength Choice of wavelength depends on what is being

imaged Bone Soft tissue

Also want to minimize absorbed energy


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?

In which of these materials does the X-ray intensity drop off more quickly?


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?


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


X-Ray Beam Techniques Tube voltage

Increasing tube voltage increases penetrating power of X-rays

Bremsstrahlung K, L

spectra


X-Ray Beam Techniques Beam current

Increasing beam current increases intensity of X-rays

Does not changepenetrating power


X-Ray Beam Techniques Target material

Changing target material changes characteristic K, L lines

Bremsstrahlungspectrum staysthe same (more orless)


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 gridbefore getting tofilm


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)


X-Ray – Coronary Arteries

From: http://www.ajronline.org/cgi/content-nw/full/179/4/911/FIG8


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)


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-raytube X-ray

sensitivefilm


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-raytube X-ray

film cassette


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


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-raytube X-ray

sensitive plate

Digitalscanningprocess


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-raytube

Computer-readableX-ray phosphor screen


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-raytube

X-ray sensitivephosphor screen


X-Ray Medical Imaging –Fundamental Ideas

What are they?


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


Idea of Multiple Scan Directions Imagine taking X-ray image of 2 x 2 square

Take image in horizontal direction

A B

C D

X-rays

8

10

Film

X-rayintensities

4 4

5 5



Take second image in vertical direction

A B

C D

X-rays

7 11Film

X-ray intensities

4 4

5 5

8

10



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

C D

7 11 X-ray intensities

3 5

4 6

8

10


Computed Tomography (CT) Scan Schematic

Use more then just 2 x 2 resolution Typical: 256 x 256


Computed Tomography (CT) Scanners


Computed Tomography Scanner

From http://en.wikipedia.org/wiki/Computed_Axial_Tomography


Computed Tomography Scanner - Internals



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)


Computed Tomography – Usage Brain scans

Bleeding Stroke Tumor

Other organs (soft tissue) Heart Kidneys Etc

Applications Tumors Trauma Structure



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


Computed Tomography –Fundamental Ideas

What are they?

Documents

3/06/2009IB Physics HL 21 X-Ray Medical Imaging Physics – IB Objectives I.2.1Define the terms attenuation coefficient and half-value thickness. I.2.2Derive