Laser Surgery Medical Imaging: X-Rays and Lasershadron.physics.fsu.edu/.../Lasers_Mar_31.pdf ·...

BiomedicalPhysics II

MedicalImaging andRadiationX-Rays

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Lasers in MedicalImaging

Biomedical Physics II - PHZ 4703

Medical Imaging: X-Rays and Lasers

03/31/2020

My Office Hours:Wednesday 11:00 AM - Noon

212 Keen Building



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Outline

1 Medical Imaging and RadiationX-RaysCT Scanning

2 LasersLaser SurgeryLasers in Medical Imaging



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Observations about MedicalImaging and Radiation

• They manage to work right through your skin.

• Imaging involves radiation of various sorts.

• Some imaging radiation is itself hazardous.

• Some radiation involves radioactivity.

• Some radiation involves accelerators.



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Producing X-Rays

Electrons are acceleratedto high energies, and usedto bombard a metal target.This results in

• white radiation,

• and characteristicradiation.



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Producing X-Rays

https://www.youtube.com/watch?v=IsaTx5-KLT8

https://www.youtube.com/watch?v=IsaTx5-KLT8



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Using X-Rays for Imaging

X-ray photons interact with tissue and bone throughfour major processes.

QuestionHow does electromagnetic radiation actually interactwith matter?



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Using X-Rays for Imaging

X-ray photons interact with tissue and bone throughfour major processes:

1 Elastic scattering• Atom acts as antenna (Blue Sky):

Absorbing and reemitting without keeping energy⇒ No effect on atom: not important in therapy

• X-ray tube filters away lowest energy photons andbouncing X-ray photons

2 Compton scattering

3 Electron-positron pair production

4 Photoelectric effect⇒ Makes X-ray imaging possible.



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Conventional X-Ray ImagingAbsorption of X-rays: Photoelectric effect

• More absorption for dense and high-Z materials (Pb shielding).

• Denser tissues / bones absorp more X-rays.

• Softer tissues can be imaged by injecting contrasting agent.



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X-Ray Imaging

• Any atom that blocks X-rays casts a shadow.• Many-electron atoms produce shadows.• Few-electron atoms cast essentially no shadows.

• All atoms Rayleigh scatter X-rays, causing haze.

• X-ray imaging observes shadows of large atoms.

• Haze can be filtered by collimating structures.



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X-Ray ImagingX-ray detection• Photographic plate• Photostimulable phosphors• Geiger counter• Scintillators• Semiconductor detectors

Common applications of X-ray imaging• Skeletal systems• Dental• Chest X-ray• Abdominal X-ray• Cardiovascular (with contrasting agent)

https://www.youtube.com/watch?v=gsV7SJDDCY4

https://www.youtube.com/watch?v=gsV7SJDDCY4



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X-Ray CrystallographyX-ray crystallography: Technique used for determining atomic andmolecular structure of a crystal, in which crystalline structure causesthe incident X-ray beam to diffract into many specific directions.



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CT Scanning

• Separate X-ray imagesproduce CT database.

• Computer can recreateoriginal 3-D from database.

• Computer typically plotscross sections.



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CT Scans: Brain



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CT ImprovementDiagnostic Applications

• Head / brain

• Chest / pulmonary angiogram (CTPA)

• Cardiac (temporal and spatial resolution)

• Abdominal and Pelvic



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CT Scanning



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Adverse Effects

Diagnostic X-rays (primarily from CT scans due to the large doseused) increase the risk of developmental problems and cancer inthose exposed. X-rays are classified as a carcinogen by both theWorld Health Organization’s International Agency for Research onCancer and the U.S. government. It is estimated that 0.4 % of currentcancers in the United States are due to computed tomography (CTscans) performed in the past and that this may increase to as highas 1.5–2 % with 2007 rates of CT usage. (Wikipedia)



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Adverse Effects

Diagnostic X-rays (primarily from CT scans due to the large doseused) increase the risk of developmental problems and cancer inthose exposed. X-rays are classified as a carcinogen by both theWorld Health Organization’s International Agency for Research onCancer and the U.S. government. It is estimated that 0.4 % of currentcancers in the United States are due to computed tomography (CTscans) performed in the past and that this may increase to as highas 1.5–2 % with 2007 rates of CT usage. (Wikipedia)

Deformity of hand due to an X-ray burn.These burns are accidents. X-rays were notshielded when they were first discovered andused, and people received radiation burns.



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Summary

X-rays are part of the electromagnetic spectrum, with wavelengthsshorter than visible light. Different applications use different parts ofthe X-ray spectrum.



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Question

When an X-ray photon is absorbed in a tumor, it maycause the tumor cell to die because the photon

1 causes the cell to crystallize into a brittle solidthat cannot tolerate any deformation.

2 carries enough energy to damage molecules andcause chemical injury to the cell.

3 magnetizes the cell and causes it to stick topassing killer cells of the immune system.

4 polarizes the cell and causes its vertical electricfield to become horizontal.



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Question

Imaging X-rays go easily through living tissue but not soeasily through bone. This is primarily because the calciumatoms in bone have

1 many more neutrons in their nuclei than the atoms intissue have.

2 many more electrons than the atoms in tissue have.

3 an odd number of electrons while those in tissuehave even numbers of electrons.

4 an even number of electrons while those in tissuehave odd numbers of electrons.



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Outline

1 Medical Imaging and RadiationX-RaysCT Scanning

2 LasersLaser SurgeryLasers in Medical Imaging



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Spontaneous Emission

• Excited atoms normally emit light spontaneously.

• Photons are uncorrelated and independentÜ Incoherent light.



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Stimulated Emission

Excited atoms can be stimulated into duplicating passing light.

• Light at the resonance frequency interacts with the excitedatoms by stimulating them to make a transition back into thelower energy state.

• Photons are correlated and identicalÜ Coherent light.



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Laser Amplification

• Stimulated emission can amplify light.

• Laser medium contains excited atom-like systems.

• Photons must have appropriate wavelength, polarization, andorientation to be duplicated.

• Duplication is perfectÜ Photons are clones.



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Laser Oscillation

• Laser medium in a resonator produces oscillator.

• A spontaneous photon is duplicated repeatedly.

• Duplicated photons leak from semitransparent mirror.

• Photons from oscillator are identical.



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Laser States: Equilibrium

Under equilibrium conditions:More atoms are in a lower energy state than are ina higher one.

• Light beam (at resonant frequency) passesthrough collection of atomsÜ Photons taken out, light beam attenuated.



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Inverted Population

Cycle helps the laser develop a populationinversion between its upper and lower laserstates.



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Optical Pumping

What provides the energy needed to shift atoms or atom-likesystems in the laser medium from their ground states to theirexcited states in order to develop the population inversion?



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Flashlamp-Pumped LaserAmplifier



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And what about ...



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And what about ...

Unfortunately, forget it:From the point of viewof power, such a laser isessentially impossible.



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Properties of Laser Light

• Coherent – identical photons

• Controllable wavelength/frequency – colors

• Controllable spatial structure – narrow beams

• Controllable temporal structure – short pulses

• Energy storage and retrieval – intense pulses

• Giant interference effects



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Properties of Laser Light

• Coherent – identical photons

• Controllable wavelength/frequency – colors

• Controllable spatial structure – narrow beams

• Controllable temporal structure – short pulses

• Energy storage and retrieval – intense pulses

• Giant interference effects

Ü Apart from these issues, laser light is just light.

https://phet.colorado.edu/en/simulation/legacy/lasers

https://phet.colorado.edu/en/simulation/legacy/lasers



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Laser Surgery

Lasers are very useful as a surgical tool:

• Focused on a small area, can burn off and vaporize selectedtissue without much damage to neighboring areas.Ü Reduced bleeding, pain, infections.

• However, precise control of light intensity, duration, andpositioning of focal point required.



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Laser Eye Surgery

Lasers are very useful as a surgical tool:

• Focused on a small area, can burn off and vaporize selectedtissue without much damage to neighboring areas.Ü Reduced bleeding, pain, infections.

• Laser light can be focused through the iris onto the boundaryof the detached or torn region of the retina.

Tissue is burnt, subsequentscarring “welds” the retina tothe underlying tissue.

Diabetic RetinopathyDisorders in blood circulationÜ Possible leaks in retinal

blood vessels:Laser light can seal leak.



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LASIK Technique

Laser-Assisted in Situ KeratomileusisLaser surgical procedure that reshapes the cornea with the aimof correcting focusing problems associated with myopia, hyperopia,and astigmatism.

The computer that controls the laser isprogrammed for amount and locationof the corneal tissue to be removed.

• A flap is cut in the front part ofthe cornea.

• Mid-part of the cornea isreshaped.



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OCTOptical Coherence TomographyOCT is based on interferometry, where light reflected or scattered offthe subject under investigation interfere with light from a referencearm. The light from both arms originate from the same light source,consequently the two beams will interfere if path length difference ofthe two arms in the interferometer is within the coherence length ofthe optical signal.



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OCTOptical Coherence Tomography

• Light that has been multiply scattered from the tissue has lostits phase correlation with the illuminating laser beam (and withreference) Ü No interference pattern with reference beam.

• Low-intensity interference pattern contains information aboutposition and reflectivity of source cell.



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OCTOptical Coherence Tomography

• Typical resolution of an OCT instrument is about 10 µm.

• Can form images of cells up to 2 or 3 mm inside the tissue.

Laser Surgery Medical Imaging: X-Rays and Lasershadron.physics.fsu.edu/.../Lasers_Mar_31.pdf ·...

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Transcript of Laser Surgery Medical Imaging: X-Rays and Lasershadron.physics.fsu.edu/.../Lasers_Mar_31.pdf ·...