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IB PHYSICS

TSOKOS OPTION I-2MEDICAL IMAGING

Reading Activity Answers

IB Assessment Statements

Option I-2, Medical Imaging:X-RaysI.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

IB Assessment Statements

Option I-2, Medical Imaging:X-RaysI.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).

IB Assessment Statements

Option I-2, Medical Imaging:UltrasoundI.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.

IB Assessment Statements

Option I-2, Medical Imaging:UltrasoundI.2.10. Outline the difference

between A-scans and B-scans.I.2.11. Identify factors that affect

the choice of diagnostic frequency.

IB Assessment Statements

Option I-2, Medical Imaging:NMR and LasersI.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)

Properties of Radiation

Types of Radiation: Alpha (α) Beta (β) Gamma (γ)

Properties of Radiation

Intensity – power as if it were radiated through a sphere

24 rPI

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 attenutation coefficient

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

693.0

HVT

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

Computed Tomography (CT Scan) 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

Computed Tomography (CT Scan) 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

Magnetic Resonance Imaging (MRI) 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

Magnetic Resonance Imaging (MRI) 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

Magnetic Resonance Imaging (MRI) 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

Magnetic Resonance Imaging (MRI) 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

Magnetic Resonance Imaging (MRI) 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

Magnetic Resonance Imaging (MRI) Process dependent on hydrogen

saturation Newer techniques can measure rate at

which protons return to ground state to better identify tissue type

Magnetic Resonance Imaging (MRI) Show and Tell

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

Positron Emission Tomography (PET Scan)

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

Positron Emission Tomography (PET Scan) 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

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

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

IB Assessment Statements

Option I-2, Medical Imaging:X-RaysI.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

IB Assessment Statements

Option I-2, Medical Imaging:X-RaysI.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).

IB Assessment Statements

Option I-2, Medical Imaging:UltrasoundI.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.

IB Assessment Statements

Option I-2, Medical Imaging:UltrasoundI.2.10. Outline the difference

between A-scans and B-scans.I.2.11. Identify factors that affect

the choice of diagnostic frequency.

IB Assessment Statements

Option I-2, Medical Imaging:NMR and LasersI.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?

#1-8Homework

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