【DSE Physics E4】Medical Physics 醫學物理學｜Vision & Hearing｜Ionizing & Non-Ionizing imaging

Medical Physics 醫學物理學

Vision and Hearing
Non-ionizing Medical Imaging
Ionizing Medical Imaging

Chapter 1 Vision and Hearing

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Our eyes

Similar to a convex lens and a screen
⇒ A real image is formed on the screen so that we can see
Only visible light (400 nm ≤ λ ≤ 750 nm) can be detected

Basic structure of our eyes

refracted by cornea
pass through pupil, refracted by lens
fall on retina
signal sent to brain via optic nerve

Refraction system

Cornea does most of eye′s focusing;
Lens is only for fine tuning
Difference in refractive indices between cornea and air is the greatest.
To catch a sharp image, ciliary muscle may contract or relax to change the shape of lens

Light sensitive cells

rods	cones
sensitive to different light intensities	for perception of colours
mainly for vision in darkness	mainly for vision in bright conditions

The cones are less numerous than the rods and distributed unevenly, most concentrated at the yellow spot.

Rods and cones

Rods and cones convert light into electrical signals (nerve impulses)
⇒ signals are sent to brain
via optic nerves
The brain interprets signals and produces vision

Two special spots on retina

Blind spot: the point where optic nerves leave retina
⇒ no light sensitive cells
⇒ cannot detect light falling on there
Yellow spot: the point where cones are most concentrated
⇒ very sensitive to light

Power of a lens

A thicker lens usually has a shorter focal length f
A thicker lens is more powerful in bending light

Power of a lens

Power of a lens P:reciprocal of focal length f (in metres)

$$P=\frac1f$$

Unit: dioptre (D), 1 D = 1 m⁻¹

The more powerful the lens, the shorter its focal length
⇒ larger degree to which it bends light rays
By ‘real is positive’ convention,
power of convex lens is positive

Compound optical system (more than one lens)

If lenses are thin and close together,
power of combination = sum of individual power

Lens formula and the eye

$$P=\frac1f=\frac1u+\frac1\nu $$

Treat cornea and lens as a single lens of power P
Treat v as the size of eyeball

Accommodation

Our eye changes the shape of lens in order to focus on an object (produces a sharp image)
⇒ accommodation
When viewing near objects, ciliary muscle contracts
⇒ lens becomes thicker
When viewing distant objects, ciliary muscle relaxes
⇒ lens becomes thinner

limits of lens

far point	near point
farthest position at which an object can be focused	closest position at which an object can be focused
infinity (for a normal person)	25 cm away (for a normal person)

Resolving power

Even if we can focus on an object, we may not see all the details of it
e.g. two headlights on a car may appear as one light spot
if the car is very far away
our eyes cannot resolve two objects when their angular separation is too small

Formula of Resolving power

For our eye to resolve two objects, their minimum angular separation has to be

$$\theta_{\min}\approx\frac{1.22\lambda}{D}\quad\text{(in radians)}$$

where
λ: wavelength of light
D: diameter of pupil

Spectral response

Spectral response with receptor absorption curves
Sensitivities of cones and rods vary considerably with λ
In human eyes, for light of the same intensity, green light appears brighter than others
Percept all visible colours as three coloured lights with different intensities
Rods are most sensitive to light at 510 nm (blue–green) and least sensitive in red zone

In a dimly lit environment,

only rods but no cones are used
we cannot perceive any colours but only shades of grey

Defects of vision and corrections

In general, defects of vision are caused by eyes that are too powerful or too weak

Short sight

A short-sighted person can only see a near object clearly
For a distant object, its image is formed in front of retina and therefore appears blurry

Possible causes:

eye too powerful, or eyeball too long
To correct short sight, reduce power of the eye with a concave lens
(∵ P < 0)

Long sight

A person with long sight can only see a distant object clearly
For a near object, its image is formed behind retina and therefore appears blurry

Possible causes:

eye too weak (not powerful enough), or eyeball too short
To correct long sight,
increase power of the
eye with a convex lens
(∵ P > 0)

Old sight

When a person gets older, lenses in the eyes lose elasticity

⇒ Lenses may not be able to change their shapes well enough to accommodate to object

For a person with both old sight and short sight

to view a near object, ↑ Peye
⇒ convex lenses (for old sight)
to view a distant object, ↓ Peye
⇒ concave lenses (for short sight)

∴ glasses with bifocal lenses

Our ears

Convert incoming sound waves into electrical signals
Can detect frequencies between 20 and 20 000 Hz

Basic structure of our ears

Hearing mechanism

When eardrum is struck by some sound waves,

it vibrates at same frequency as the waves, causing ear bones to move
vibration is transmitted to inner ear via oval window

Pressure amplification

Pressure is amplified in two ways:

Ear bones amplify force by
≈ 1.3 times
Area of eardrum ≈ 17 times area of oval window

Conversion to electrical signals

Vibrations from middle ear are transmitted to cochlea

Conversion to electrical signals

Vibrations from middle ear are transmitted to cochlea

Look at cochlea

coiled tube consisting of three channels filled with liquids
Upper and lower channels are connected at apex
Vibrations enter cochlea via oval window, travel through upper to lower channel, and finally leave via round window

When cochlea is uncoiled

non-uniform basilar membrane
between upper and lower channels
When a vibration is transmitted into cochlea, some parts of membrane vibrate more than others ⇒ resonance

Mode of resonance

Basilar membrane near base vibrates more at high f
Basilar membrane near apex vibrates more at low f

Mode of resonance

Basilar membrane near base vibrates more at high f
Basilar membrane near apex vibrates more at low f

Sensitivity of our ears

for sound waves from 60 to 1000 Hz, two sounds with Δf ≈ 2-3 Hz can be distinguished

Cells on basilar membrane are connected to auditory nerves
When cells are stimulated, they send signals to the brain
⇒ produces sense of hearing

Sound intensity level: Intensity

How much energy passes through a unit area per unit time
unit: W m⁻²

A sound becomes more intense if

same amount of energy passes through a smaller area
energy is transferred more rapidly
more energy is transferred through same size of area

Decibel

Our ears respond to a wide range of sound intensities,
from 10⁻¹² to 10² W m⁻²
To compare two sounds of different intensities IA and IB,
use a logarithmic scale:

$$\log_{10}I_A-\log_{10}I_B$$

Intensity of softest sound we can just barely hear is
I₀ ≈ 10⁻¹² W m⁻²

$$L=10\cdot\log_{10}\left(\frac{I_A}{I_B}\right)\quad\mathrm{(in~dB)}$$

Typical sound intensity levels

the softest sound we can barely hear	0 dB
whisper	20 dB
conversation at 0.5 m	65 dB
busy street	85 dB	causes hearing damage after long exposure
loud rock music	120 dB	painful to ears
jet engine at 50 m	140 dB
space shuttle engine	200 dB	immediate permanent hearing damage

Perception of sound: Hearing range

The intensity level of softest sound we can barely hear is about 0 dB (= 10⁻¹² W m⁻²)
Such intensity level may vary from frequency to frequency

Loudness and phon

Sounds that give same sense of
loudness vary with frequency ⇒ Curves of equal loudness
curves are higher up at both ends
⇒ ear is less sensitive to sound of low and high frequencies

Use phon scale to measure loudness
(which depends on listener)
On phon scale, a pure note of 1000 Hz is used for reference

Noise and hearing

Damage to middle ear, or cochlea and nerves may arise hearing loss ⇒ hearing range may shrink
Noise is one of causes of such damage

Depending on exposure time to noise and its intensity, hearing loss can be temporary or permanent
e.g. exposure to noise of 85 dB for 8 hours or more
can cause hearing loss
⇒ louder sounds, more immediate damage

For age-related loss

the curve shifts upwards

For noise-related loss

the curve shifts upwards
sometimes ‘crest’ is formed ⇒ hearing was damaged more seriously for a certain range of frequencies

Chapter 2 Non-ionizing Medical Imaging

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What is medical imaging?

Medical imaging makes use of various kinds of waves to create visual representation of our body

Some waves are ionizing while some are non-ionizing.

Image quality

Limited by

wavelength of the waves used
how fine the detector is
technique used to create visual representation from signals

Invasive and non-invasive

Invasive means having a cut or a hole* (or a risk of having them) on a patient’s body
But even non-invasive methods may harm the human body
e.g. exposed to ionizing radiation

Ultrasound scans

Ultrasound waves are used to produce medical images

Ultrasound scans

Ultrasound waves are used to produce medical images

Ultrasound is a kind of sound waves
Above upper limit of human audible frequency range
i.e. > 20 kHz

Echolocation

Key principle of ultrasound imaging

Emit ultrasound pulses

⇒ analyse echoes

⇒ locate the target

Piezoelectric effect

A special kind of crystal is compressed/stretched

⇒ voltage is developed across its two ends

Detecting ultrasound

Crystal changes shape when hit by ultrasound

⇒ varying voltage is developed

⇒ electrical signal is produced

Producing ultrasound

Apply varying voltage

⇒ crystal shape is changed

⇒ ultrasound is emitted

Transducer

A device for producing and detecting ultrasound
Apply ac voltage ⇒ produce ultrasound
Measure developed voltage by CRO ⇒ detect ultrasound

Travelling of ultrasound in media

Soft tissue & organ:
high percentage of water
Bone: mainly solid

Speed

soft tissue ≈ organ < bone

Acoustic impedance

measures how easy ultrasound waves travel through a medium

Acoustic impedance Z

$$\mathbb{Z}=\rho\mathbb{C}$$

ρ: density of medium
c: ultrasound speed in medium

Unit: kg m⁻² s⁻¹ or Rayl

In general, higher density ⇒ higher wave speed

⇒ Z typically increases with wave speed

Intensity reflection coefficient

shows how much waves are reflected
Intensity reflection coefficient α

$$\alpha=\frac{I_r}{I_0}=\frac{(Z_2-Z_1)^2}{(Z_2+Z_1)^2}$$

Note:

Larger Z difference ⇒ larger α
α < 1

Coupling gel

has acoustic impedance similar to skin
avoid reflection from air–skin boundary

Attenuation

Intensity gradually decreases when it travels in a medium
Reason:
- Energy being absorbed by the medium
- Ultrasound waves being scattered by the medium

Attenuation

Intensity gradually decreases when it travels in a medium
Reason:
- Energy being absorbed by the medium
- Ultrasound waves being scattered by the medium

Higher ultrasound frequency

⇒ higher attenuation

⇒ shorter it can travel

Pulse-echo technique

Thickness of tissue 1 is given by

$$2s=ct\quad\Rightarrow\quad s=\frac{ct}2$$

Ultrasound scans

Information from echoes

Echo strength
stronger echoes ⇒ larger acoustic impedance difference
Receiving time
later time ⇒ from deeper boundary

A-scan

A stands for amplitude
Only produce 1-D graph

A-scan measures amplitudes of echoes
⇒ plots a graph of amplitude against time

B-scan

B stands for brightness
Can produce 2-D image
B-scan uses bright dots to represent amplitude
Transducer sends out fan beam to construct 2-D image

Resolution vs penetration

Axial resolution

resolution along direction of beam
shorter pulse duration
⇒ better axial resolution

Lateral resolution

resolution normal to direction of beam
higher frequency
⇒ better lateral resolution

Choosing suitable frequency

Higher frequency
⇒ higher resolution & higher attenuation
For structures near surface,
use high frequency (7–12 MHz)
For deeper structures,
use low frequency (3–5 MHz)

Ultrasound Advantages and limitations

Advantages

Non-ionizing & relatively small health effect
Can distinguish soft tissues
Can detect movements inside body
Can produce 3-D images
Inexpensive & readily available

Limitations and precautions

Cannot scan structures covered by bones & gases
Energy is absorbed by tissues & converted to heat
(has to monitor the power transmitted carefully)
Gas bubbles may form & damage surrounding tissues if burst
(has to monitor pressure in the region carefully )

Endoscopy

Based on total internal reflection of light

Total internal reflection

Occurs when

n₁ > n₂ (i.e. towards optically less dense medium) &
θ₁ > c

Optical fibre

Basic structure

A glass fibre core surrounded by a cladding of slightly lower refractive index

Working principle

n_core > n_cladding & θ₁ > c
⇒ total internal reflection occurs
Light ray is guided along the fibre

Endoscope

Contains two bundles of optical fibres

Incoherent fibre bundle
Coherent fibre bundle

Besides sending light, endoscope can also

pass down tools, e.g. forceps for cutting tissues
pass down air/water for inflating/flushing organs

Incoherent fibre bundle

sends light to the organ for illumination

Coherent fibre bundle

sends reflected light back to transmit images

Image formation

Coherent fibre bundle

relative positions of fibres are the same between two ends
reproduces image at eyepiece

Incoherent fibre bundle

cannot reproduce image
used for illumination (∵ cheaper)

Resolution

Light transmitted in each fibre mixes up
Each fibre forms a basic unit

For fixed diameter,

more & finer fibres

⇒ higher resolution

& the fibres can bend more

Uses

Examine hollow organs, e.g. stomach & colon
Directly insert into organs through natural openings,
e.g. throat & rectum
Used in keyhole surgery
Only a few small cuts are made in body
Can pass down surgical instruments

Endoscope Advantage

Allow doctor to inspect the inner surfaces of organs
along tubes of the body
No ionizing radiation is used
Used in keyhole surgery to speed up recovery
(∵ only small cuts are made)
Small tissue samples can be taken out from the body

Limitations and precautions

Requires anaesthesia
May require fasting to empty the organs
Field of view is relatively narrow
Can only view the inner surface of an organ with cavity
Invasive, though minimal
May cause internal bleeding/allergic reaction

Chapter 3 Ionizing Medical Imaging

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Ionizing radiation

Radiation with very high energy that can ionize an atom by knocking electrons out

⇒ can damage/kill living tissues

For EM waves,

the higher the frequency, the more energy they carry

Ionizing radiation can penetrate deep into a material
because they have

a small size (in form of particles) or
a short wavelength (in form of waves)

X-rays

High-frequency EM waves
Can blacken a photographic film ⇒ method to detect
Characterized by

Frequency range — 10¹⁶ to 10¹⁹ Hz
Energy range — 100 eV to 100 keV

Production of X-rays

Electron gun shoots high-speed electrons to a heavy metal target
Electrons decelerate rapidly during the hit
Some KE taken away from electrons is released as X-ray

Attenuation

When a beam of X-rays travels in a medium, its intensity gradually decreases, similar to that of ultrasound.

Causes and factors

X-ray beam will lose energy as it ionizes the medium during its travel
X-rays are scattered when they travel in a medium

scattered = bounced off (actually absorbed and re-emitted) in random direction

X-rays are attenuated more by a medium when

medium has a higher density
medium is made of element with a higher atomic number
X-rays with a lower f are more easily scattered

Linear attenuation coefficient

For an X-ray beam of initial intensity I₀ ,

after it travels in a uniform medium for a distance of x, its transmitted intensity will drop to

$$I=I_0\cdot\mathrm{e}^{-\mu x}$$

where
μ: linear attenuation coefficient (unit: cm⁻¹)

The larger the coefficient, the faster the X-ray beam is attenuated to a lower intensity

Half-value thickness

Consider a graph of I against x,

I decreases by half when it travels a
half-value thickness (x½ or HVT)

⇒ blocks of x½, 2x½ & 3x½ thick can
reduce I to 1/2, 1/4 and 1/8

⇒ transmitted I becomes negligible if
the block is 10x_½ thick

Consider relation between x½ and μ

When intensity is halved, i.e. I/I0 = ½

X-ray radiographic imaging

X-rays are attenuated to different degrees when passing through different materials

Working principle

Analogy:

We cannot see an object behind an umbrella, as our sight is blocked
if intense light comes from behind, a projection of object can be seen

⇒ X-ray image is a projection of structures inside our body

Making of an X-ray image

When an image is taken, X-ray beam passes through our body
Tissues of our body have various compositions & thicknesses:

⇒ X-rays are attenuated to different degrees

⇒ X-rays of different intensities emerge from body

⇒ Detectors (e.g. films) visualize emergent X-rays

Appearance on image & attenuation

The more X-rays the detector is exposed to, the darker the image that is produced
OR, the more an X-ray beam is attenuated by a human body, the lighter the image that is produced

Matching tissues to X-ray image

e.g. a chest X-ray image

⇒ provides a high contrast between bones and neighbouring soft tissues

The formation of a radiographic image is the result of the different degrees of attenuation of X-rays by different body tissues.

Artificial contrast medium

In an X-ray image, we can clearly distinguish bones from neighbouring soft tissues, as their contrast is sharp

For certain organs, still possible to make them temporarily opaque by using an artificial contrast medium

⇒ compound with a large μ

⇒ can be taken orally or injected into body

e.g. to contrast between stomach and nearby tissue,

⇒ a patient take in an artificial contrast medium orally (known as a barium meal)

⇒ the medium coats inner surface of stomach

⇒ stomach becomes opaque to X-rays

⇒ a clear image is produced

Advantages

Relatively inexpensive as compared with other imaging methods, e.g. CT scan, magnetic resonance imaging (MRI)
Simple and fast
Good resolution of bony structures

Disadvantages and limitations

Ionizing, has radiation hazard
Poor contrast of soft tissue structures
Images of body structures overlap each other

Disadvantages and limitations

Ionizing, has radiation hazard
Poor contrast of soft tissue structures
Images of body structures overlap each other
Use of artificial contrast medium can be unpleasant;
Some may be allergic to iodine compounds (a common kind of artificial contrast medium)

Computed tomographic scan

X-ray radiographic imaging cannot image a cross section of a body
Use computed tomography (CT) to do so

CT images

CT scanner

Bulkier than a X-ray imaging machine
Basically a big gantry (like a huge doughnut) and a table

When a patient receives a CT scan, he lies on a table.

X-ray tube inside gantry rotates around patient
Array of X-ray detectors opposite to X-ray tube

How CT images are formed

X-ray tube emits a fan of X-ray beams towards patient at centre
When X-ray beams pass through a slice of patient’s body, they are attenuated
Detectors on opposite side pick up beams and determine how much X-ray beams are attenuated
X-ray tube rotates around, repeating process

As beams pass through tissues of different composition
and thickness,

beams are attenuated by different degrees
a series of emergent intensities at different angles obtained

With help of computers, transform intensities into an array of numbers (μavg of a small part of every body slice)
Numbers are displayed as pixels of different
shades of grey on a screen
⇒ An image of cross section of body is reconstructed

Resolution of image depends on # pixels

⇒ Common resolution: 1024 × 1024 or above

Smaller detectors can increase amount of X-ray data obtained, and hence resolution

CT image and attenuation coefficient

Show both soft tissues and bones
By convention, body parts that highly attenuate X-rays will appear white

A single CT image can only show the plane of body
With powerful computers, images from multiple cross sections can be combined to form a cross sectional image in perpendicular plane

A CT image is a map of attenuation coefficients of body tissues. These coefficients can be displayed in a grey or colour scale by a computer.

Back projection

Consider a certain body part,

when X-rays are directed at it, they are attenuated by different degrees
using shades of grey from light to dark to represent attenuation from high to low, a pattern is obtained

Repeat the process from different angles
⇒ a series of patterns obtained
When patterns are overlapped, bones are reconstructed (light coloured spots) on image ⇒ back projection

In CT, back projection is a method of reconstructing the image of a body by measuring the data of the attenuated X-rays from multiple projections.

Advantages

Can image cross section & locate abnormality inside body
Good contrast between various body tissues
⇒ can take images of bones, soft-tissues and blood at the same time

Disadvantages

A patient receiving a CT scan absorbs more X-rays than X-ray imaging
More expensive than X-ray imaging

Comparison

	CT image	X-ray image
image	• reconstructed from multiple projections (as X-ray tube rotates around patient) • more detailed structural information • show soft tissues with good contrast	• single projection of a human body (with overlapping structures) • less detailed structural information • cannot show soft tissues with good contrast
exposure to radiation	• take a longer time ⇒ patient is exposed to more X-rays	• take a shorter time ⇒ patient is exposed to less X-rays
cost	• much more expensive ⇒ equipment is more complicated	• much cheaper

Radionuclide imaging

Waves emitted from the body?

For both ultrasound scan and X-ray imaging, waves are directed at interior of a patient’s body from exterior
Radionuclide imaging (RNI) is in reverse

For radionuclide imaging,

radionuclides are mixed with suitable chemicals and taken into a patient’s body
radionuclides will accumulate at specific organs
⇒ produce images of that organ by detecting radiation emitted

Suitable radionuclides

Kinds of radiation

Which radiation should be used for medical diagnosis?

α sources cannot be used
∵ cannot pass through a human body
∵ highly ionizing and readily damage body tissues
Choose a γ source
∵ has highest penetrating power and
least ionizing power

Decay process

T_1/2 of a suitable radionuclide should not be too long
∵ no serious health effects
T_1/2 cannot be too short
∵ medical diagnosis takes time to be carried out
Radionuclide should decay into a stable product

Other factors

Also consider

chemical properties
pharmaceutical properties
cost

Technetium-99m

One of most common radionuclides in imaging because

It decays by emitting γ rays only

$$_{43}^{99\mathrm{m}}\mathrm{Tc}\longrightarrow_{43}^{99}\mathrm{Tc}+\gamma $$

It decays into a stable nuclide
(technetium-99)
γ rays emitted have suitable
energy and can be easily detected

One of most common radionuclides in imaging because

Its half-life is 6 hours, not too long
or too short
Non-toxic
Can be easily attached to various
chemicals, forming suitable tracers for
imaging different organs
Relatively inexpensive

Effective half-life

When a sample of radionuclides is taken into a patient’s body, its activity drops for two reasons

First reason: radioactive sample
decays exponentially

⇒ with physical half-life T_½p

When a sample of radionuclides is taken into a patient’s body, its activity drops for two reasons

Second reason: sample is gradually removed from body due to biological processes (e.g. urination)

⇒ Usually a logarithmic process, or decay exponentially

⇒ with biological half-life T_½b

∴ Actual time for activity inside the body to be halved
should be shorter than either T½p or T_½b

⇒ effective half-life T_½e

$$\begin{aligned}\frac{1}{T_{1/2\text{e}}}&=\frac{1}{T_{1/2\text{p}}}+\frac{1}{T_{1/2\text{b}}}\end{aligned}$$

Activity A inside a human body (after a radioactive sample is uptaken) will drop exponentially with time t:

$$A=A_0•\mathrm{e}^{-kt}$$

where
A₀ : initial activity
k : effective decay constant, which is

$$k=\frac{\ln2}{T_{1/2\text{e}}}=\ln2\cdot\left(\frac1{T_{1/2\text{p}}}+\frac1{T_{1/2\text{b}}}\right)$$

Imaging process

Radioactive tracer

To produce an image,

put a radioactive tracer into a patient’s body (by injection, ingestion or inhalation)
tracer accumulate in target organ
detect radiation emitted and produce images

A tracer usually consists of

a radionuclide tagged with a chemical compound
single radionuclide,
e.g. iodine-123 and xenon-133 (gas)

⇒ Tracer can accumulate in a specific organ

Gamma camera

A tool to detect gamma rays more effectively than films
Four main components:

Main components:

Collimator (a lead plate which has many holes) restricts gamma rays to enter the camera at a right angle
Scintillator converts incoming gamma rays into visible light flashes
Photomultiplier tubes (PMT) convert visible light flashes into electrical pulses (counts)
Positioning logic circuit collects electrical pulses and determines the positions in human body that emit γ rays
⇒ Dots are drawn on final image to represent these positions

To form an image of the organ,
γ rays are collected over a period of time (~30 to 120 s)

Radionuclide images

Image and resolution

By gamma camera, each gamma ray is converted into a dot on screen
An image is accumulation of dots over a certain period of time
More dots, better image resolution

Radionuclide images

Image and resolution

By gamma camera, each gamma ray is converted into a dot on screen
An image is accumulation of dots over a certain period of time
More dots, better image resolution

Injected tracer will accumulate in
target organs
The darker the region on the image, the greater the amount of tracer that has accumulated

Regions as seen on image

Hot spot (dark region) indicates an accumulation of tracer that is more intense than normal
Cold spot (light region) indicates the lack of a tracer
Both can be due to abnormalities in a body

e.g. Black dots pointed to by arrows are hot spots

due to abnormal growth of tissues which causes an intense uptake of tracer
black dots may be tumours

e.g. A radioactive tracer is injected into bloodstream of patient.

Two cold spots are seen in the lung

⇒ due to lack of tracers in these two areas of lungs

⇒ may be caused by blood clots

Dynamic imaging

A series of images can be taken over a period of time to illustrate uptake of tracer into a particular organ
e.g. kidney scan of a patient

Tracer has accumulated in right kidney but not left kidney

Recording change in terms of image and counts per second

From curve: how the activity changes with time
From images: right kidney may have kidney stones which affect its normal function

Advantages

Can evaluate function of organs, also monitor function of an organ following a medical treatment
Can detect disease early (∵ pathological changes often occur before structural damages)
Can detect disease efficiently
e.g. bone tumours can be detected quickly even if their locations are not yet known

Disadvantages

Poor images resolution
Post a health risk (∵ A radioactive tracer is introduced into body)
Costly compared with ultrasound and X-ray imaging
Usually non-specific diagnosis
e.g. a cold spot on the image of a thyroid may be due to a tumour (solid mass of tissues) or a cyst (sac filled with fluids or gas)

RNI images and X-ray images Comparison

RNI images and X-ray images apply different electromagnetic waves (γ rays and X-rays)

	RNI image	X-ray image
principle	• produced from radiation emitted from a specific organ • a tracer has to be introduced into patient’s body • non-invasive	• produced by projecting X-rays through a body part (and detecting how much X-rays are transmitted through) • nothing has to be put into patient’s body • non-invasive
strength	• can study functions of organs rather than structures • can detect diseases before structural damage occurs	• can show structural detail • cannot show functions of organs
time	more time is needed (0.5 to 2 minutes)	less time is needed (1 s) (CT scan = 10 s to 60 s)
resolution	good	poor

Safety precautions for ionizing radiation

Effective dose

To measure hazard, we cannot only measure how much ionizing radiation our bodies absorb

⇒ measure overall biological effect

For overall biological effect on entire human body, consider

Absorbed dose received
Type of radiation to be exposed
Tissues or organs exposed to radiation

Effective dose (measured in sievert, Sv)

measures overall biological effects on a person resulting from exposure to ionizing radiation

effective dose = absorbed dose x radiation weighting factor x tissue weighting factor

Biological effects

example of effects	occurring time	characteristic of effects
skin damage	acute	deterministic
damage to reproductive system
damage to blood forming system
damage to digestive system
damage to central nervous system
cataract	latent
damage to immune system
cancer		stochastic
genetic effects		stochastic

Acute or latent

When a person receives an effective dose of < 1 Sv,

he may suffer from radiation sickness within a short time (acute effects)
some effects may occur on him 6 months after dose is received (latent effects)

Deterministic or stochastic

For deterministic effects

effect will not happen unless received dose exceeds
a certain value
severity increases with received dose
e.g. If lens of eye receives an effective dose of 5 Sv,
a cataract will result

For stochastic effects

occur by chance
the higher the radiation dose received, the larger the chance of having these effects
e.g. Risk of developing cancer for radiation workers is
4% per Sv, but severity of effects is independent of dose

Biological effects due to imaging

In general, a patient receives a greater radiation dose if

a larger part of his body is exposed to radiation
he is exposed to radiation for a longer time
Effective dose absorbed during RNI may vary with the tracers being used

Safety precautions

Three main principles (JOD) that govern precautions

Justification of a practice

Justify every radiological procedure with
‘the benefit against the risk’
Always be a net benefit

Optimization of protection

Radiation doses should be kept As Low As Reasonably Achievable (ALARA)
e.g. radiographer should keep patient dose to a minimum as long as exposure is sufficient to produce a good diagnostic image

Dose limitation

e.g. annual dose limit for occupational radiation workers is 20 mSv

Minimizing dose uptake

Principle to minimize dose received by staff and patients

Time
Distance — keep as far away as
possible from sources
⇒ well designed procedures
and radiation areas
Shielding — around radiation sources
⇒ apply dense materials such as lead and concrete

Comparison of imaging methods

	RNI image	endoscopy	X-ray radiographic imaging	X-ray CT scan	RNI
invasive or non-invasive	non-invasive	invasive	non-invasive	non-invasive	non-invasive
radiation used	ultrasound	visible light	X-rays	X-rays	γ rays
radiation type	non-ionizing	non-ionizing	ionizing	ionizing	ionizing
radiation production	piezoelectric effect	light bulbs	X-ray tube	X-ray tube	radionuclides
principle	reflection of ultrasound waves	reflection of visible light	transmission and attenuation of X-rays from a single direction	transmission and attenuation of X-rays from multiple directions	emission of γ rays
major strength	no ionizing radiation and good resolution for soft tissues	no ionizing radiation	good resolution of bony structures	good contrast between various body tissues	functional study
major disadvantage and limitation	cannot scan structures covered by bones or air	can only view the inner surfaces of hollow organs	poor resolution for soft tissues; structures may overlap	relatively large dose of radiation	poor resolution and diagnosis not specific
effective dose received	nil	nil	small	medium	medium
time for each image	immediate	immediate	immediate	medium	long
allow real-time imaging	yes	yes	yes (using fluoroscopy method)	no	no
uses	diagnostic & surgical	diagnostic & surgical	diagnostic & surgical	diagnostic	diagnostic
cost	low	medium	low	medium	high

Physics 選修課題 E1

Physics 選修課題 E2

Physics 選修課題 E3

Physics 選修課題 E4

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