Chapter 13 Medical Physics Flashcards
(66 cards)
1
Q
The piezoelectric effect is defined as:
A
The ability of particular materials to generate a potential difference (p.d.) by transferring mechanical energy to electrical energy
2
Q
A transducer is any device that
A
- converts energy from one form to another
3
Q
Piezoelectric Crystals
A
- At the heart of a piezoelectric transducer is a piezoelectric crystal
- Piezoelectric crystals are materials which produce a p.d. when they are deformed
- This deformation can be by compression or stretching
4
Q
If a p.d. is applied to a piezoelectric crystal, then it
A
deforms, and if the p.d. is reversed, then it expands
- If this is an alternating p.d. then the crystal will vibrate at the same frequency as the alternating voltage
- Crystals must be cut to a certain size in order to induce resonance
5
Q
One of the most common piezoelectric crystals is
A
quartz, which is made from a lattice of silicon dioxide atoms
- When the lattice is distorted, the structure becomes charged creating an electric field and, as a result, an electric current
- If an electric current is applied to the crystal, then this causes the shape of the lattice to alternate which produces a sound wave
- Due to the conventional direction of electric current, it will flow from the positive to the negative region of the crystal
6
Q
A molecule in a quartz crystal. When the compression and stretching alternates, an alternating e.m.f. is induced
A
7
Q
Applications of the Piezoelectric Transducer
- Microphone
A
- A piezoelectric microphone detects pressure variations in sound waves
- These can then be converted to an electrical signal for processing
8
Q
Applications of the Piezoelectric Transducer
- Ultrasound
A
- In a piezoelectric transducer, an alternating p.d. is applied to produce ultrasound waves and sent into the patient’s body
- The returning ultrasound waves induce a p.d. in the transducer for analysis by a healthcare professional
9
Q
- An ultrasound is defined as:
A
A high frequency sound above the range of human hearing
- This is above 20 kHz, although in medical applications the frequencies can be up to the MHz range
10
Q
An ultrasound transducer is made up of a
A
- piezoelectric crystal and electrodes which produce an alternating p.d.
- The crystal is heavily damped, usually with epoxy resin, to stop the crystal from vibrating too much
- This produces short pulses and increases the resolution of the ultrasound device
11
Q
The structure of an ultrasound transducer
A
12
Q
A piezoelectric crystal can act as both a receiver or transmitter of
A
ultrasound
- When it is receiving ultrasound, it converts the sound waves into an alternating p.d.
- When it is transmitting ultrasound, it converts an alternating p.d. into sound waves
13
Q
Generation:
An alternating p.d. is applied across a piezo-electric crystal, causing it to
A
- change shape
- The alternating p.d. causes the crystal to vibrate and produce ultrasound waves
- The crystal vibrates at the frequency of the alternating p.d., so, the crystal must be cut to a specific size in order to produce resonance
14
Q
Detection:
When the ultrasound wave returns
A
- the crystal vibrates which produces an alternating p.d. across the crystal
- This received signal can then be processed and used for medical diagnosis
15
Q
The frequency of the ultrasound is important because
A
- The higher the frequency of the ultrasound, the higher the resolution and the smaller structures that can be distinguished
16
Q
The ultrasound gives two main pieces of information about the boundary:
A
- Depth: the time between transmission and receipt of the pulse (the time delay)
- Nature: amount of transmitted intensity received (will vary depending on the type of tissue)
17
Q
- In an ultrasound scanner, the transducer sends out a beam of sound waves into the body
- The sound waves are
A
- reflected back to the transducer by boundaries between tissues in the path of the beam
- For example, the boundary between fluid and soft tissue or tissue and bone
- When these echoes hit the transducer, they generate electrical signals that are sent to the ultrasound scanner
- Using the speed of sound and the time of each echo’s return, the scanner calculates the distance from the transducer to the tissue boundary
- These distances can be used to generate two-dimensional images of tissues and organs
18
Q
- The acoustic impedance, Z, of a medium is defined as:
A
The product of the speed of the ultrasound in the medium and the density of the medium
- This quantity describes how much resistance an ultrasound beam encounters as it passes through a tissue
19
Q
- Acoustic impedance can be calculated using the equation:
A
Z = ρc
- Where:
- Z = acoustic impedance (kg m-2 s-1)
- ρ = the density of the material (kg m-3)
- c = the speed of sound in the material (m s-1)
20
Q
- Acoustic impedance can be calculated using the equation:
A
Z = ρc
- Where:
- Z = acoustic impedance (kg m-2 s-1)
- ρ = the density of the material (kg m-3)
- c = the speed of sound in the material (m s-1)
21
Q
Z = ρc
This equation tells us
A
- The higher the density of a tissue, the greater the acoustic impedance
- The faster the ultrasound travels through the material, the greater the acoustic impedance also
- This is because sound travels faster in denser materials
- Sound is fastest in solids and slowest in gases
- The closer the particles in the material, the faster the vibrations can move through the material
22
Q
At the boundary between media of different acoustic impedances, some of the wave energy is
A
- reflected and some is transmitted
23
Q
The greater the difference in acoustic impedance between the two media, the greater the
A
reflection and the smaller the transmission
- Two materials with the same acoustic impedance would give no reflection
- Two materials with a large difference in values would give much larger reflections
24
Q
- Air has an acoustic impedance of Zair = 400 kg m-2 s-1
- Skin has an acoustic impedance of Zskin = 1.7 × 106 kg m-2 s-1
- The large difference means ultrasound would be significantly
A
- reflected, hence a coupling gel is necessary
- The coupling gel used has a similar Z value to skin, meaning that very little ultrasound is reflected
25
* The intensity reflection coefficient α is defined as:
***The ratio of the intensity of the reflected wave relative to the incident (transmitted) wave***
26
intensity reflection coefficient α equation is
* Where:
* α = intensity reflection coefficient
* IR = intensity of the reflected wave (W m-2)
* I0 = intensity of the incident wave (W m-2)
* Z1 = acoustic impedance of one material (kg m-2 s-1)
* Z2 = acoustic impedance of a second material (kg m-2 s-1)
* This equation will be provided on the datasheet for your exam
* This ratio shows:
* If there is a large difference between the impedance of the two materials, then most of the energy will be reflected
* If the impedance is the same, then there will be **no reflection**
27
**Coupling Medium**
* When ultrasound is used in medical imaging, a coupler is needed between the transducer and the body
* The soft tissues of the body are much denser than air
* If air is present between the transducer and the body, then almost all the ultrasound energy will be reflected
* The coupling gel is placed between the transducer and the body, as skin and the coupling gel have a similar density, so little ultrasound is reflected
* This is an example of **impedance matching**
28
* Attenuation of ultrasound is defined as:
***The reduction of energy due to the absorption of ultrasound as it travels through a material***
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The attenuation coefficient of the ultrasound is expressed in
**decibels per centimetre** lost for every incremental increase in megahertz frequency
* Generally, 0.5 dB/cm is lost for every 1MHz
30
* The intensity I of the ultrasound decreases with distance *x*, according to the equation:
**I = I0 e−μx**
* Where:
* *I0* = the intensity of the incident beam (W m-2)
* *I* = the intensity of the reflected beam (W m-2)
* μ = the absorption coefficient (m-1)
* *x* = distance travelled through the material (m)
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The absorption coefficient μ, will vary from
* material to material
* Attenuation is not a major problem in ultrasound scanning as the scan relies on the reflection of the ultrasounds at boundaries of materials
32
X-rays are
* short wavelength, high-frequency part of the electromagnetic spectrum
* They have wavelengths in the range 10−8 to 10−13 m
* X-rays are produced when fast-moving electrons rapidly decelerate and transfer their kinetic energy into photons of EM radiation
33
**Producing X-rays**
* At the cathode (negative terminal), the electrons are released by **thermionic emission**
* The electrons are accelerated towards the anode (positive terminal) at high speed
* When the electrons bombard the metal target, they lose some of their kinetic energy by transferring it to photons
* The electrons in the outer shells of the atoms (in the metal target) move into the spaces in the lower energy levels
* As they move to lower energy levels, the electrons release energy in the form of **X-ray photons**
34
When an electron is accelerated, it gains energy equal to the
* **electronvolt**; this energy can be calculated using:
**Emax = eV**
* This is the **maximum energy** that an X-ray photon can have
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the maximum X-ray frequency fmax, or the minimum wavelength λmin, that can be produced is calculated using the equation:
* Where:
* e = charge of an electron (C)
* V = voltage across the anode (V)
* h = **Planck’s constant** (J s)
* c = **speed of light** (m s-1)
36
**Using X-rays in Medical Imaging**
* X-rays have been highly developed to provide detailed images of soft tissue and even blood vessels
* When treating patients, the aims are to:
* Reduce the exposure to radiation as much as possible
* Improve the **contrast** of the image
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**Reducing Exposure**
* X-rays are **ionising**, meaning they can cause damage to living tissue and can potentially lead to cancerous mutations
* Therefore, healthcare professionals must ensure patients receive the minimum dosage possible
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In order to reduce exposure, what are used
**aluminium filters** are used
* This is because many wavelengths of X-ray are emitted
* Longer wavelengths of X-ray are more penetrating, therefore, they are more likely to be absorbed by the body
* This means they do not contribute to the image and pose more of a health hazard
* The aluminium sheet **absorbs** these long wavelength X-rays making them safer
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* Contrast is defined as:
***The difference in degree of blackening between structures***
* Contrast allows a clear difference between tissues to be seen
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Image contrast can be improved by:
* Using the correct level of **X-ray hardness**: **hard X-rays** for bones, **soft X-rays** for tissue
* Using a **contrast media**
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* Sharpness is defined as:
***How well defined the edges of structures are***
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Image sharpness can be improved by:
* Using a narrower X-ray beam
* Reducing X-ray scattering by using a collimator or lead grid
* Smaller pixel size
43
Bones **absorb**
* X-ray radiation
* This is why they appear white on the X-ray photograph
* When the collimated beam of X-rays passes through the patient’s body, they are **absorbed** and **scattered**
44
* The attenuation of X-rays can be calculated using the equation:
**I = I0 e−μx**
* Where:
* *I0* = the intensity of the incident beam (W m-2)
* *I* = the intensity of the reflected beam (W m-2)
* μ = the linear absorption coefficient (m-1)
* *x* = distance travelled through the material (m)
* The attenuation coefficient also depends on the energy of the X-ray photons
45
The intensity of the X-ray **decays**
* **exponentially**
* The thickness of the material that will reduce the X-ray beam or a particular frequency to half its original value is known as the **half thickness**
46
**A good contrast is when:**
* There is a large difference between the intensities
* The ratio is much less than 1.0
47
**Computed Tomography Scanning**
* A simple X-ray image can provide useful, but limited, information about internal structures in a 2D image
* When a more comprehensive image is needed, a **computerised axial tomography** (CAT or CT) scan is used
48
The main features of the operation of a CT scan are as follows:
* An X-ray tube rotates around the stationary patient
* A CT scanner takes X-ray images of the **same slice**, at many different angles
* This process is **repeated**, then images of successive slices are combined together
* A computer pieces the images together to build a **3D image**
* This 3D image can be **rotated** and viewed from different angles
49
**Advantages of CAT Scans**
* Produces much more detailed images
* Can distinguish between tissues with similar attenuation coefficients
* Produces a 3D image of the body by combining the images at each direction
50
**Disadvantages of CAT Scans**
* The patient receives a much higher dose than a normal X-ray
* Possible side effects from the contrast media
51
* A **radioactive tracer** is defined as:
**A radioactive substance that can be absorbed by tissue in order to study the structure and function of organs in the body**
52
Radioactive **isotopes**, such as **technetium-99m** or **fluorine-18**, are suitable for
radioactive tracers
* They both bind to organic molecules, such as glucose or water, which are readily available in the body
* They both emit gamma (*γ*) radiation and decay into stable isotopes
* **Technetium-99m** has a short **half-life** of 6 hours (it is a short-lived form of Technetium-99)
* **Fluorine-18** has an even shorter half-life of 110 minutes, so the patient is exposed to radiation for a shorter time
53
* Positron Emission Tomography (PET) is:
**A type of nuclear medical procedure that images tissues and organs by measuring the** **metabolic activity** **of the cells of body tissues**
54
A common tracer used in PET scanning is a
glucose molecule with radioactive fluorine attached called **fluorodeoxyglucose**
* The fluorine nuclei undergoes ***β+*** **decay** – emitting a **positron** (*β+* particle)
55
The radioactive tracer is
* injected or swallowed into the patient and flows around the body
* Once the tissues and organs have absorbed the tracer, then they appear on the screen as a bright area for a diagnosis
* This allows doctors to determine the progress of a disease and how effective any treatments have been
* Tracers are used not only for the diagnosis of cancer but also for the heart and detecting areas of decreased blood flow and brain injuries, including Alzheimer's and dementia
56
**The Process of Annihilation**
* When a positron is emitted from a tracer in the body, it travels less than a millimetre before it collides with an electron
* The positron and the electron will **annihilate**, and their mass becomes pure energy in the form of two gamma rays which move apart in opposite directions
57
* Annihilation doesn’t just happen with electrons and positrons, annihilation is defined as:
**When a particle meets its equivalent antiparticle they are both destroyed and their mass is converted into energy**
* As with all collisions, the mass, energy and momentum are conserved
58
Once the tracer is introduced to the body it has a **short**
**half-life**, so, it begins emitting positrons (*β+*) immediately
* This allows for a short exposure time to the radiation
* A short half-life does mean the patient needs to be scanned quickly and not all hospitals have access to expensive PET scanners
59
In PET scanning:
* Positrons are emitted by the decay of the tracer
* They travel a small distance and annihilate when they interact with electrons in the tissue
* This **annihilation** produces a pair of gamma-ray photons which travel in opposite directions
60
**Detecting Gamma-Rays from PET Scanning**
* The patient lays stationary in a tube surrounded by a ring of detectors
* Images of slices of the body can be taken to show the position of the **radioactive tracers**
61
* The detector (for pet scanning) consists of two parts:
* **Crystal** **Scintillator** – when the gamma-ray (*γ*-ray) photon is incident on a crystal, an electron in the crystal is excited to a higher energy state
* As the excited electron travels through the crystal, it excites more electrons
* When the excited electrons move back down to their original state, the lost energy is transmitted as visible light photons
* **Photomultiplier** -The photons produced by the scintillator are very faint, so they need to be amplified and converted to an electrical signal by a photomultiplier tube
62
***Detecting gamma rays with a PET scanner***
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**Creating an Image from PET Scanning**
* The *γ* rays travel in straight lines in opposite directions when formed from a positron-electron **annihilation**
* This happens in order to **conserve momentum**
* They hit the detectors in a line – known as the **line of response**
* The tracers will emit lots of *γ* rays simultaneously, and the computers will use this information to create an image
* The more photons from a particular point, the more tracer that is present in the tissue being studied, and this will appear as a bright point on the image
* An image of the **tracer concentration** in the tissue can be created by **processing the arrival times** of the gamma-ray photons
64
**Calculating Energy of Gamma-Ray Photons**
* In the **annihilation** process, both mass-energy and momentum are conserved
* The gamma-ray photons produced have an energy and frequency that is determined solely by the mass-energy of the positron-electron pair
65
* The energy E of the photon is given by
**E = hf = mec2**
* Where:
* me = **mass of the electron** or positron (kg)
* h = **Planck's constant** (J s)
* f = frequency of the photon (Hz)
* c = the **speed of light** in a vacuum (m s**–**1)
66
* The momentum p of the photon is given by
* Where:
* me = **mass of the electron** or positron (kg)
* h = **Planck's constant** (J s)
* f = frequency of the photon (Hz)
* c = the **speed of light** in a vacuum (m s**–**1)
