Medical Image AnalysisMedical Image AnalysisInteraction of Electromagnetic Radiation with Matter in Medical Imaging

Figures come from the textbook: Medical Image Analysis, by Atam P. Dhawan, IEEE Press, 2003.

X-rays◦Roentgen received the first Nobel Prize

for his discovery of X-rays in 1901.X-ray Computed Tomography

◦Godfrey Hounsfield and Allen Cormack in 1972

◦Share the Nobel Prize for Medicine in 1979

PET◦The clinical use of Positron Emission

Tomography was demonstrated by G. Brownell in 1953

Figures come from the textbook: Medical Image Analysis, by Atam P. Dhawan, IEEE Press, 2003.

Electromagnetic (EM) Electromagnetic (EM) RadiationRadiationWave

Figures come from the textbook: Medical Image Analysis, by Atam P. Dhawan, IEEE Press, 2003.

Figure comes from the Wikipedia, www.wikipedia.org.

c

Electromagnetic (EM) Electromagnetic (EM) RadiationRadiationQuantum or photon

Figures come from the textbook: Medical Image Analysis, by Atam P. Dhawan, IEEE Press, 2003.

Figure comes from the Wikipedia, www.wikipedia.org.

Electromagnetic (EM) Electromagnetic (EM) RadiationRadiationPenetrationScattering

◦A partial loss of energy and a change of the direction

Photoelectric absorption

Figures come from the textbook: Medical Image Analysis, by Atam P. Dhawan, IEEE Press, 2003.

keV /24.1/ hchE

EM Radiation for Image EM Radiation for Image FormationFormationX-rays

◦The X-ray photons usually travel in a straight line and are attenuated, depending on the density and atomic properties of the matter in the medium

◦Bone, soft tissue, fluidGamma-ray emission

◦Radioactive tracer in the object◦Metabolic or functional

Figures come from the textbook: Medical Image Analysis, by Atam P. Dhawan, IEEE Press, 2003.

Radiation Interaction with Radiation Interaction with MatterMatter

Figures come from the textbook: Medical Image Analysis, by Atam P. Dhawan, IEEE Press, 2003.

Photon Energy (keV)0

0500

100

1.0

(cm2/g)

Rayleigh Scattering

Photoelectric Absorption Scattering

Compton Scattering

Total Mass Attenuation Coefficient

Figure 3.1. The mass attenuation coefficients of water under the 511 keV energy range.

Radiation Interaction with Radiation Interaction with MatterMatterCoherent or Rayleigh scattering

◦Elastic collision of the photon with the matter that causes a slight change in the direction of the photon travel with no loss of energy

◦Low-energy photons in the range of a few kiloelectron volts

Figures come from the textbook: Medical Image Analysis, by Atam P. Dhawan, IEEE Press, 2003.

Figures come from the textbook: Medical Image Analysis, by Atam P. Dhawan, IEEE Press, 2003.

Figure comes from the Wikipedia, www.wikipedia.org.

Figures come from the textbook: Medical Image Analysis, by Atam P. Dhawan, IEEE Press, 2003.

Figure comes from the Wikipedia, www.wikipedia.org.

Radiation Interaction with Radiation Interaction with MatterMatterPhotoelectric absorption

◦A photon loses its energy by interacting with a tightly bound electron in the body matter, which is subsequently ejected from the atom due to the increased kinetic energy

◦Emission of a fluorescent radiation◦Low-energy photons are absorbed by M

and L shells of the atomic structure, while the high-energy photons are absorbed in the inner K-shell

Figures come from the textbook: Medical Image Analysis, by Atam P. Dhawan, IEEE Press, 2003.

Figures come from the textbook: Medical Image Analysis, by Atam P. Dhawan, IEEE Press, 2003.

Figure comes from the Wikipedia, www.wikipedia.org.

Radiation Interaction with Radiation Interaction with MatterMatterCompton scattering

◦Photon energies are comparable to the electron rest energy of 511 keV

Pair production◦Above 1.022 MeV

Figures come from the textbook: Medical Image Analysis, by Atam P. Dhawan, IEEE Press, 2003.

Figures come from the textbook: Medical Image Analysis, by Atam P. Dhawan, IEEE Press, 2003.

Figure comes from the Wikipedia, www.wikipedia.org.

Radiation Interaction with Radiation Interaction with MatterMatterCompton scattering

◦An inelastic collision of a photon with an outer-shell electron with a negligible binding energy

◦After the collision, the photon with reduced energy is deflected while the electron with an increased energy is ejected from the atom

◦The deflections in scattering events cause uncertainties in photon localization as it becomes difficult to keep the desired radiation transmission path

Figures come from the textbook: Medical Image Analysis, by Atam P. Dhawan, IEEE Press, 2003.

Figures come from the textbook: Medical Image Analysis, by Atam P. Dhawan, IEEE Press, 2003.

Figure comes from the Wikipedia, www.wikipedia.org.

Radiation Interaction with Radiation Interaction with MatterMatterPair production

◦A high-energy photon of the order of 1 Mev interacts near the nucleus of an atom in a manner similar to the positron emission in a radioactive decay

◦Not used in diagnostic radiology

Figures come from the textbook: Medical Image Analysis, by Atam P. Dhawan, IEEE Press, 2003.

Figures come from the textbook: Medical Image Analysis, by Atam P. Dhawan, IEEE Press, 2003.

Figure comes from the Wikipedia, www.wikipedia.org.

Radiation Interaction with Radiation Interaction with MatterMatterDiagnostic radiological imaging

with X-rays at 20keV or higher energy levels◦The sum of Photoelectric Absorption

and Compton Scattering

Figures come from the textbook: Medical Image Analysis, by Atam P. Dhawan, IEEE Press, 2003.

CompPhotoRayl 12 EZRayl 33 EZPhoto

1

E

A

ZN

m

avComp

Linear Attenuation Linear Attenuation CoefficientCoefficient

Assume the medium is homogeneous and the radiation beam is obtained from a monochromatic energy source

Figures come from the textbook: Medical Image Analysis, by Atam P. Dhawan, IEEE Press, 2003.

tino eNN

)(),,(),(),,,(

EdeEyxIyxIdzEzyx

ind

tind eyxIyxI 0),(),(

Figures come from the textbook: Medical Image Analysis, by Atam P. Dhawan, IEEE Press, 2003.

Photon Energy (keV)0

0500

100

5.0

(cm2/g)

Fat

Compact Bone

50

Figure 3.2. The mass attenuation coefficients of compact bone and fat.

Radiation DetectionRadiation DetectionSpectrometric detectors

◦Ionization and scintillation

Figures come from the textbook: Medical Image Analysis, by Atam P. Dhawan, IEEE Press, 2003.

Radiation DetectionRadiation DetectionIonized chambers and

proportional counters

Figures come from the textbook: Medical Image Analysis, by Atam P. Dhawan, IEEE Press, 2003.

CNqV /

ioneEN /

Figures come from the textbook: Medical Image Analysis, by Atam P. Dhawan, IEEE Press, 2003.

+ Anode

- Cathode

Quantum

Collimator

+

-ions

Figure 3.3: A schematic diagram of an ionization chamber based detector.

Radiation DetectionRadiation DetectionSemiconductor detectors

◦The particle energy is transformed into electric pulses at the junction region of the semiconductor material

◦Apply positive voltage to the n region and negative voltage to the p region

◦The depletion layer serves as an ionization chamber

◦A quantum interacting with the surface of the detector will create electron-hole pairs in the depletion layer

Figures come from the textbook: Medical Image Analysis, by Atam P. Dhawan, IEEE Press, 2003.

Figures come from the textbook: Medical Image Analysis, by Atam P. Dhawan, IEEE Press, 2003.

Figure comes from the Wikipedia, www.wikipedia.org.

Radiation DetectionRadiation DetectionAdvantages of semiconductor

detectors◦Fabricated with smaller e~3.6eV for

siliconE value becomes independent of

the mass and charge of the particleProvide small charge collection

time (<10ns)Cause very small recombination

losses due to fast charge collectionFigures come from the textbook: Medical Image Analysis, by Atam P. Dhawan, IEEE Press, 2003.

Radiation DetectionRadiation DetectionScintillation detectors

◦A scintillation phosphor and a photomultiplier tube

◦The charged particles interact with the scintillation material to excite molecules

◦The excited molecules emit optical photons during the relaxation process to return to the ground state

◦Photomultiplier tubes are used to amplify the optical photon intensity and produce voltages proportional to the energy of the particle creating scintillation

Figures come from the textbook: Medical Image Analysis, by Atam P. Dhawan, IEEE Press, 2003.

+ Anode- Photocathode

Quantum

Collimator dynodes

Figure 3.4: A schematic diagram of photomultiplier tube.

Figures come from the textbook: Medical Image Analysis, by Atam P. Dhawan, IEEE Press, 2003.

Figure comes from the Wikipedia, www.wikipedia.org.

Radiation DetectionRadiation DetectionThe number of electrons emitted

by the photocathode can be given as

Figures come from the textbook: Medical Image Analysis, by Atam P. Dhawan, IEEE Press, 2003.

phEwEkgN /00