devil physics the baddest class on campus ib physics...
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DEVIL PHYSICSTHE BADDEST CLASS ON CAMPUS
IB PHYSICS
TSOKOS OPTION I-2MEDICAL IMAGING
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
IB Assessment Statements
Option I-2, Medical Imaging:
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).
IB Assessment Statements
Option I-2, Medical Imaging:
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.
IB Assessment Statements
Option I-2, Medical Imaging:
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.
IB Assessment Statements
Option I-2, Medical Imaging:
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)
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 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
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
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-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
IB Assessment Statements
Option I-2, Medical Imaging:
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).
IB Assessment Statements
Option I-2, Medical Imaging:
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.
IB Assessment Statements
Option I-2, Medical Imaging:
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.
IB Assessment Statements
Option I-2, Medical Imaging:
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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