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DEVIL PHYSICSTHE BADDEST CLASS ON CAMPUS

IB PHYSICS

TSOKOS OPTION I-2MEDICAL IMAGING

Reading Activity Answers

Reading Activity Option I-2 Answers.docx

IB Assessment Statements

Option I-2, Medical Imaging:

X-Rays

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,

xeII 0



X-Rays

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).



Ultrasound

I.2.7. Describe the principles of the generation and the detection of ultrasound using piezoelectric crystals.

I.2.8. Define acoustic impedance as the product of the density of a substance and the speed of sound in that substance.

I.2.9. Solve problems involving acoustic impedance.



Ultrasound

I.2.10. Outline the difference between A-scans and B-scans.

I.2.11. Identify factors that affect the choice of diagnostic frequency.



NMR and Lasers

I.2.12. Outline the basic principles of nuclear magnetic resonance (NMR) imaging.

I.2.13. Describe examples of the use of lasers in clinical diagnosis and therapy.

Objectives

State the properties of ionizing radiation

State the meanings of the terms quality of X-rays, half-value thickness (HVT), and linear attenuation coefficient

Perform calculations with X-ray intensity and HVT,

xeII 0

693.0HVT

Objectives

Describe the main mechanisms by which X-rays lose energy in a medium

State the meaning of fluoroscopy and moving film techniques

Describe the basics of CT and PET scans

Describe the principle of MRI

State the uses of ultrasound in imaging

State the main uses of radioactive sources in diagnostic medicine

Properties of Radiation

Two uses in medicine:

Diagnostic imaging (this lesson)

Radiation therapy (next lesson)


Types of Radiation:

Alpha (α)

Beta (β)

Gamma (γ)


Intensity – power as if it were radiated through a sphere

24 r

PI

Attenuation

Intensity drops exponentially when passed through a medium capable of absorbing it

The degree to which radiation can penetrate matter is the quality of the radiation

μ is a constant called the linear attenutationcoefficient

xeII 0

Attenuation

Attenuation depends not only on the material the radiation passes through, but also on the energy of the photons

Attenuation

Half-Value Thickness (HVT) – similar to radioactive decay law, the length that must be travelled through in order to reduce the intensity by a factor of 2

693.0HVT

Attenuation

Half-Value Thickness as a function of photon energy

Attenuation

X-rays absorbed via photoelectric and Compton effects

Photoelectric effect – X-ray photons absorbed by an electron which is then emitted by the atom or molecule

Compton effect – photon gives part of its energy to a free electron and scatters off it with a reduced energy and increased wavelength (elastic collision)

X-ray Imaging

First radiation to be used for imaging

Operate at voltage of around

15-30 kV for mammogram

50-150 kV for chest X-ray

X-ray Imaging

X-ray Imaging

Most energy lost through photoelectric effect

Photoelectric effect increases with atomic number of elements in tissue

Bone will absorb more X-rays than soft tissue

X-rays show a contrast between bone and soft tissue

Energy will pass through soft tissue and expose the film on the other side

Energy absorbed by bone tissue will cast a shadow

X-ray Imaging

When there is no substantial difference between Z-numbers in the material, patients are give a contrast medium, usually barium

Barium absorbs more X-rays to give a sharper image

X-ray Imaging

Image is sharper if:

Film is very close to patient

X-ray source is far from patient

Lead strips are moved back and forth between patient and film to absorb scattered X-rays

Low-energy X-rays removed by filtering

Intensifying screens used to enhance energy of photons passed through patient to reduce exposure time

X-ray Imaging

X-ray Imaging

X-rays on TV

Capability to project real-time X-ray images on a monitor

Advantages outweighed by increased exposure time/radiation dosage

Does have advantages for examining cadavers and inanimate objects (jet engines)

Computed Tomography (CT Scan)

Computed (axial) tomography or

Computer assisted tomography (CAT)

Still uses X-rays, but

Reduced exposure time

Greater sharpness

More accurate diagnoses


Thin X-ray beam directed perpendicular to the body axis

Beam creates an image slice that can be viewed from above

• Source then rotates to take a slice from a different angle


Many detectors are used to record the intensity of X-rays reaching them

Information is sent to a computer to reconstruct the image

Similar to digital camera processing

• Detector grids are also called pixels

Magnetic Resonance Imaging (MRI)

Based on a phenomenon called nuclear magnetic resonance

Superior to CT Scan

No radiation involved (don’t let ‘nuclear’ throw you)

But, much more expensive


Electrons, protons and most particles have a property called spin – See Eric

Particles with an electrical charge and spin behave like magnets – magnetic moment

In the presence of a magnetic field, the moment

Will align itself parallel (‘spin up’)

Or anti-parallel (‘spin down’) to the direction of the field


Hydrogen protons have specific energy levels

In the presence of a magnetic field, the energy level will change based on how the magnetic moment aligns with the field

Difference in energy levels is proportional to the external magnetic field strength


A radio frequency (RF) source (electromagnetic radiation) is introduced

If the frequency of the RF source corresponds to the difference in energy levels, the proton will jump to the higher state, then go back down and emit a photon of the same frequency


Detectors register the photon emissions and a computer can reconstruct an image based on the point of emission

Rate of photon emission important to identifying tissue type


Point of emission determined by using a second magnetic field to break up uniformity of original magnets used to align the spins

External magnetic field regulates photon emissions


Process dependent on hydrogen saturation

Newer techniques can measure rate at which protons return to ground state to better identify tissue type


Show and Tell

Dads MRI

Positron Emission Tomography (PET Scan)

Similar to a CT Scan

Involves annihilation of an electron and a positron (anti-particle of the electron) and detection of two photons that are then produced


Patients injected with radioactive substance that emits positrons during decay

Emitted positron collides with an electron in the patient’s tissue

Electron-positron collision annihilates in two photons each of energy 0.511 MeV

2 ee


Total momentum is conserved an the photons move in opposite directions with same velocity

Detectors can then located the point of emission

Can give a resolution of 1mm

Especially good for brain images

Ultrasound

Uses sound in the 1 to 10 MHz range – not audible

No radiation

No known adverse side effects

Can produce some images X-rays can’t (lungs)

Not as detailed as X-rays

Ultrasound

Sound emitted in short pulses and reflection off various surfaces is measured

Very similar to sonar and radar

Diffraction limits resolution size, d, to λ < d

Wavelength determined by speed of sound in tissue

In practice, with the frequencies used, pulse duration and not diffraction limits resolution

Ultrasound

Frequency determined by the type of organ tissue studied

Rule of thumb is f = 200(c/d) where c is speed of sound and d is depth (depth of 200 wavelengths

Ultrasound Transition into a body

an into different tissues means some of the waves will be reflected

Amount transmitted into second tissue depends on impedance of the two media

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Ultrasound

For the most energy to be transmitted, impedances should be as close as possible

Gel is used between transducer and body to improve impedance matching

Ultrasound

A-Scan

Ultrasound

Combined A-Scans

Diagnostic Uses of Radioactive Sources

Used to monitor organs and their functions

Measurement of body fluids

How food is digested

Vitamin absorption

Synthesis of amino acids

How ions penetrate cell walls

Radioactive iodine used to monitor thyroid functions

Diagnostic Uses of Radioactive Sources

Most commonly used is technetium-99

Horse example (27 minutes)

Abridged version

Radioisotopes Full Video.wmv

Radioisotopes Clip.wmv

Summary of Imaging Methods

Σary Review

State the properties of ionizing radiation

State the meanings of the terms quality of X-rays, half-value thickness (HVT), and linear attenuation coefficient

Perform calculations with X-ray intensity and HVT,

xeII 0

693.0HVT

Σary Review

Describe the main mechanisms by which X-rays lose energy in a medium

State the meaning of fluoroscopy and moving film techniques

Describe the basics of CT and PET scans

Describe the principle of MRI

State the uses of ultrasound in imaging

State the main uses of radioactive sources in diagnostic medicine



X-Rays

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,

xeII 0



X-Rays

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).



Ultrasound

I.2.7. Describe the principles of the generation and the detection of ultrasound using piezoelectric crystals.

I.2.8. Define acoustic impedance as the product of the density of a substance and the speed of sound in that substance.

I.2.9. Solve problems involving acoustic impedance.



Ultrasound

I.2.10. Outline the difference between A-scans and B-scans.

I.2.11. Identify factors that affect the choice of diagnostic frequency.



NMR and Lasers

I.2.12. Outline the basic principles of nuclear magnetic resonance (NMR) imaging.

I.2.13. Describe examples of the use of lasers in clinical diagnosis and therapy.

QUESTIONS?

#1-8

Homework

Stopped Here 4/10/14

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