DEVIL PHYSICS
THE BADDEST CLASS ON CAMPUS
IB PHYSICS
TSOKOS OPTION I-2
MEDICAL IMAGING
Reading Activity Answers
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,
I  I 0e
 x
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 Xrays, half-value thickness (HVT), and linear
attenuation coefficient
 Perform calculations with X-ray intensity and
HVT,
I  I 0e
 x
HVT 
0.693

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
P
I
2
4r
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
I  I 0e
 x
 μ is a constant called the linear attenutation
coefficient
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
HVT 
0.693

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
e  e  2


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
Z  v
It
4Z1Z 2

2
I 0 Z1  Z 2 
I r Z1  Z 2 

2
I 0 Z1  Z 2 
2
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 Xrays, half-value thickness (HVT), and linear
attenuation coefficient
 Perform calculations with X-ray intensity and
HVT,
I  I 0e
 x
HVT 
0.693

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,
I  I 0e
 x
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?
Homework
#1-8
Stopped Here 4/10/14