6.5 Medical Physics Flashcards
What is the definition of Acoustic impedance?
The property of a material that determines reflected ultrasound intensity. Given by the product of speed of sound in the material and density.
Z = ρv
What is the purpose of a coupling gel?
To match the impedance of skin to ensure that most of the signal is transmitted.
If there is a large difference in acoustic impedance what happens to the ultrasound waves?
A large amount of reflection takes place.
Explain the piezoelectric effect for both emitting and receiving waves.
Emission: Piezoelectric crystal connected to an alternating e.m.f making it vibrate and hence emits ultrasound.
Reception: Ultrasound makes the piezoelectric crystal vibrate and this produces an alternating e.m.f
Explain a method using ultrasound to determine the speed of blood in an artery in the arm
Transducer placed at an angle to the artery / arm
Ultrasound (pulses) are reflected by (moving) blood (cells)
The frequency / wavelength (of ultrasound) is changed
Change in frequency is related to the speed (of blood) or change in wavelength is related to the speed
What is the reason for the acoustic insulator around the case of a Transducer?
to prevent external sounds from disturbing the signal.
For an ultrasound scanner, explain how the amplitude of each reflected pulse will vary.
The deeper the pulse goes the lower the amplitude as there is more energy lost.
In ultrasound scanning, explain what is meant by the impedance matching and how it may be achieved.
Coupling gel is used that has the same acousting impedance as skin
Reduced reflection at the skin
Explain how an A-scan could be used to measure the thickness of a patient’s eye lens.
Pulses of ultrasound sent into the eye
Reflections from front and back of lense and pulses displayed on oscilloscope
Thickness of lens determined from speed of ultrasound and time difference
What are the similarities and differences of an A scan and a B scan?
Similarities: Alternating voltage and piezo electric crystal are used to produce ultrasound pulses.
Uses time delay between reflected pulses to calculate thickness or distance from tissue boundaries
Differences: Amplitude scan measures thickness of tissues
Brightness scan creates a 2d image
What is the setup of a brightness scan.
it uses an array of transducers, each measuring amplitudes and plots each returning signal as a shaded pixel on a screen based on its amplitude. creates a 2d image.
What is meant by the doppler effect?
A change in the observed frequency because of the wave source having relative movement.
Explain what is meant by a photon and state one of its main properties.
A packet of EM energy.
Can travel in a vacuum with speed of 3x10^8
An X-ray scan of the heart and its blood vessels shows very poor contrast. Describe and explain a technique that can be used to reveal these blood vessels in an X-ray scan.
Contrast material iodine is injected into the vessels
The contrast material has large attenuation / absorption coefficient and has high Z (atoms)
(and hence reveal the outline of the blood vessels)
Discuss the advantages of using a gamma emitting tracer in the patient rather than a
beta-emitting tracer
Gamma radiation will pass through the patient (and hence can be detected) / beta particles will be absorbed by the patient (and hence cannot be detected)
Gamma radiation is not (very) ionising / gamma radiation does little damage to cells / beta particles are (very) ionising / beta particle damage cells
Describe how the components of a CAT scanner are used to produce a three-dimensional image of a patient.
X-ray tube rotates around (the patient) / X-ray beam passes through the patient at different angles
A thin X-ray beam is used
Image(s) of slice(s) / (cross) section(s) through the patient are taken
X-ray tube moves / spirals along (the patient)
The signals / information / pulses / data (from the detectors) are used by the computer (and its software) to produce a 3D image
Explain what is meant by an energy level of an atom
Discrete energy (of electrons in an atom) / quantised energy (of electrons in an atom) / permitted energy (states of electrons in an atom).
The attenuation coefficients for X-rays in bone and fat are 2.8 cm–1 and 0.90 cm –1
respectively. Two X-ray images are taken, one with bone and muscle and another with
muscle and fat. State and explain which image will give better contrast.
Bone and muscle have different (values for) μ hence better contrast.
or
Muscle and fat have similar (values for) μ hence poor contrast
The outline of the head of the foetus can be seen in Fig. 8.1. Describe and explain the principles of a B-scan.
Ultrasound reflected at boundary (between materials). B-scan takes place in different directions.
QWC: The intensity of the reflected ultrasound depends on the acoustic impedances of the materials (and this is greater when the difference between the acoustic impedances is greater).
The lighter regions in Fig. 8.2 show the active areas of the brain.
Fluorine-18 is a common tracer injected into a patient before a PET scan.
Explain the basic principles of PET scanning, including how the image is formed.
Any four from:
- The brain / body is surrounded by a ring of (gamma) detectors /gamma camera(s).
- The positrons (from the F-18 nuclei) annihilate electrons.
- The annihilation of a positron and an electron produces two (identical gamma) photons travelling in opposite directions.
- The delay time between these two photons / gamma rays is used to determine the location of the annihilation / F-18 / tracer.
- Computer connected to detectors / gamma camera and an image is formed by the computer (using the electrical signals from the detectors).
State two main properties of X-ray photons.
Any two from: Can travel in a vacuum Travel at the speed of light / c / 3 10 8 m s -1 in vacuum No charge / no (rest) mass Highly ionising
Name an element used as a contrast material in X-ray imaging. Explain why contrast materials are used in the diagnosis of stomach problems.
Barium / iodine
(Contrast medium absorbs X-rays because it) has large attenuation coefficient / has large absorption coefficient / has large Z values
Ideal for imaging the outline (of soft tissues)
Explain what is meant by activity.
Rate of decay / disintegration of nuclei
Name and describe the function of the main components of a gamma camera.
Collimator / lead tubes and
gamma (ray photons) travel along the axis of lead tubes
Scintillator / Sodium Iodide (crystal) and …….
gamma ray / gamma photon produces (many) photons of (visible) light
Photomultiplier (tubes) / photocathode and dynodes
and ……
(electrical) pulse / signal / electrons produced by
photon(s) of visible light
Computer and ……. signals / pulses /electrons (from photomultiplier tubes) are used to generate an image
QWC: Quality of image improved by narrower / thinner / longer collimators OR longer scanning time
State two main properties of ultrasound.
Longitudinal (wave)
Frequency (sound) above 20 kHz
Describe how the piezoelectric effect is used in an ultrasound transducer both to emit and
receive ultrasound.
Emission: (Piezoelectric film / crystal connected to an) alternating e.m.f / p.d / current making it vibrate / contract and expand / resonate (and hence emits ultrasound) (AW)
Reception: (Ultrasound makes the piezoelectric film / crystal) vibrate / contract and expand / resonate and this produces (alternating) e.m.f. / p.d / current (AW)
Explain why a gel is used between the ultrasound transducer and the patient’s skin during a
scan.
Without the gel, the ultrasound would be reflected (at the skin /air interface) or The gel allows (maximum) transmission of ultrasound (into the body)
Gel and skin has similar acoustic impedance / Z (values) or There is a large difference between the Z (values) of air and skin
Explain what is meant by the Compton effect.
(X-ray) photon interacts with an (orbital) electron
The (scattered) photon has a longer wavelength / lower frequency / lower energy AND The electron is ejected (from the atom at high speed)
Explain the purpose of using a contrast medium such as barium when taking X-ray images of
the body.
A contrast material has large attenuation coefficient / large atomic number / large Z (and hence easily absorbs X-rays)
Idea of revealing tissue
State one reason for using non-invasive techniques in medical diagnosis.
Less chance of infection
Describe the use of medical tracers to diagnose the condition of organs
Any two from:
- Tracer is injected into the body / placed inside the
body / circulates the body - Tracer is absorbed by organ / shows blockage
- Beta detector / gamma camera (is used to detect radiation from the body
Describe the principles of positron emission tomography (PET).
Any five from:
- A positron / beta-plus emitting tracer / source is used
- The positron annihilates with an electron (inside the patient)
- This produces two gamma photons
- The photons travels in opposite directions
- The patient is surrounded by a ring of gamma detectors
- The arrival times of the photons / delay time indicates location (of tumour inside the body)
- A 3-D image is created (by the computer connected to the detectors)
Describe the principles of ultrasound scanning.
- A (piezoelectric) crystal / transducer is used to send
pulse(s) of ultrasound (into the patient) - Wave / ultrasound / pulse / signal is reflected (at the boundary of tissue)
- The (intensity of the) reflected signal depends on the acoustic impedances (at the boundary)
- The (time of) delay is used to determine the depth / thickness
Explain the difference between an ultrasound A-scan and B-scan.
A-scan is one directional / B-scan involves different
directions or angles / B-scan consists of many A-scans / B-scan produces 2-D or 3-D image
State and describe one way in which X-ray photons interact with matter.
ANY ONE from X-rays interact with matter by:
the photoelectric effect where an (orbital) electron is ejected from atom / atom is ionised
Compton scattering where X-ray scattered by the interaction with (orbital) electron
Pair production where X-ray photon interacts
with the nucleus / atom and an electron and positron are produced
Explain how image intensifiers are used to improve the quality of the X-ray image.
Absorption of X-rays by (silver halide molecules) by a photographic film
Uses of fluorescent / scintillator/ phosphor
Photon releases electron (that is accelerated onto a fluorescent screen)
number of electrons increased /multiplied
Phosphor / Intensifier/ it converts X-ray
photon into increased number of ‘visible’
photons
Explain how contrast media are used to improve the quality of the X-ray image.
Different soft body tissue produce little difference in contrast/attenuation
(Contrast media with) high atomic number / Z used / iodine or barium (used to give greater contrast)
liquids injected or swallowed into soft tissue
areas / or examples of such
Describe briefly how X-rays are produced in an X-ray tube.
(Fast-moving) electrons hit a metal / an anode
The kinetic energy of the electrons is transferred into X-rays / photons / EM waves
Describe the Compton Effect in terms of an X-ray photon.
An X-ray photon interacts an electron (within the atom)
The electron is ejected and the energy / frequency of the (scattered) photon is reduced
Describe the operation of a computerised axial tomography (CAT) scanner. State one of the advantages of a CAT scan image over a conventional X-ray image.
X-ray beam passes through the patient at different angles / X-ray tube rotates around the patient
A thin fan-shaped beam is used (AW)
Images of ‘slices’ through the patient (in one plane are produced with the help of computer software)
X-ray tube / detectors are moved along (the patient for the next slice through the patient)
Advantage:
3D image / better contrast between different (soft) tissues
Describe in simple terms how X-ray photons are produced in a hospital X-ray machine.
Any two from:
- Electrons are accelerated through high voltage
- (High speed) electron(s) hit metal
- kinetic energy of electron(s) ‘produces’ X-ray (photons)
Explain why an X-ray photon has greater energy than a photon of visible light
E = hf and X-rays have higher frequency
X-ray photons interact with matter. One of the interaction mechanisms of the X-ray photons with atoms is known as the photoelectric effect. State another interaction mechanism.
Describe what happens to the X-ray photon interacting with a single atom using the mechanism you have stated
Compton (scattering)
Incoming photon collides with an electron, the electron is ejected and the photon is scattered / has lower energy
Or
Pair production
Incoming photon (disappears and) produces electron-positron pair
In the treatment of patients, explain what is meant by a non-invasive technique. State one of its advantages.
No entry into body / no cutting/incision of patient / no surgery
Lower risk of infection / less trauma
Explain what is meant by a medical tracer. Name a medical tracer commonly used to diagnose the function of organs.
Radioactive substance that is ingested / injected (into patient)
Technetium(-99m) / Iodine(-131) / fluorine(-18)
The main components of a gamma camera are the collimator, scintillator, photomultiplier tubes and the computer. Describe the function of each of these components.
Collimator – gamma (ray photons) travel along the axis of lead tubes or allows parallel gamma (ray photons travel to the scintillator)
Having thin / long / narrow (lead) tubes makes the image sharper / less blurred (QWC mark)
Scintillator – gamma ray photon produces many/thousands of photons of (visible) light
Photomultiplier - An electrical pulse is / electrons are produced from the light (photons)
Computer – Signals (from photomultiplier tubes) are used to produce an image
Describe the piezoelectric effect
The application of a p.d. across a material / crystal causes an expansion / contraction / vibration
Describe how ultrasound scanning is used to obtain diagnostic information about internal structures of a body. In your description include the differences between an A-scan and a B-scan.
Any two from:
- Pulses of ultrasound (sent into the body)
- Wave / ultrasound / pulse / signal is reflected (at boundary of tissue)
- Time of delay used to determine depth / thickness
- The fraction of reflected signal is used to identify the tissue
A-scan in one direction only / range or distance or depth finding
B-scan uses a number of sensors or a sensor in different positions / angles (to build up a 2D/3D image)
What is meant by acoustic impedance matching ? Explain why a gel is used to produce an effective ultrasound image.
(Acoustic) impedances of media are similar / identical
No / reduced reflection (at boundary) Or The gel allows maximum transmission of ultrasound (into the body)
Suggest why it is desirable to have ultrasound of short wavelength for a scan
Small wavelength means finer detail can be seen / greater resolution
Describe the use of image intensifiers and contrast media when X-rays are used to produce images of internal body structures.
- Intensifier used as X-ray would pass through film
- . Intensifie converts X-ray photon to many visible (light) photons (which are absorbed by film)
- *Lower exposure / fewer X-rays needed
- Iodine / barium (used as contrast material)
- *High Z number / large attenuation coefficient / large absorption coefficient (used to improve image contrast)
- Contrast media are ingested / injected into the body
- Sca shows outline / shape of soft tissue
Describe the advantages of a CAT scan compared to an X-ray image.
X-ray image is 2D / CT scan produces 3D image
Greater detail / definition / contrast with CT scan / ‘soft tissues can be seen’
Image can be rotated
Fluorine-18 is a positron emitter.
Name a medical imaging technique that uses fluorine-18 and state one benefit of the technique.
Used in PET (scans) Any one from: Used to diagnose function of organ / brain / body Detection of cancer / tumour Non-invasive / no surgery / no infection 3D (image)
Describe how the components of a computerised axial tomography (CAT) scanner can
produce high-quality images of the internal structures of a patient.
X-ray (tube) moves around the patient A thin (fan-shaped X-ray) beam is used (Images / scans of) cross-sections through the patient are taken Any one from: - A three-dimensional image is produced - (Soft) tissues can be identified
PET scanners are not available in all hospitals. This is because fluorine-18 requires expensive on-site particle accelerators and fluorine-18 has a very small ‘shelf-life’.
Suggest the impact this may have on the treatment and diagnosis of patients in the country.
Any sensible suggestion, e.g. ‘post-code’ lottery, some patients may not get the treatment because of where they live, longer waiting lists, etc.
Describe how a PET scanner is used to locate an area of increased activity within the patient.
The patient is surrounded by (gamma) detectors
or
Increased activity is where F-18 accumulates (AW)
The positrons (from the F-18) annihilate electrons (inside the patient)
Each annihilation produces two gamma photons travelling in opposite directions
The arrival times are used to locate position (of increased activity)
Describe the basic structure of an X-ray tube and explain how X-ray photons are produced.
You may draw a labelled diagram.
(Evacuated tube with) cathode / heater / filament and target / metal / anode
High voltage (supply) connected between cathode and anode
(Accelerated) electrons hit the target / metal / anode and their KE is transformed into X-ray (photons)
A beam of X-rays is directed at tissues in a patient.
The X-ray photons interact with the atoms of the tissues.
Simple scatter is one of the attenuation mechanisms.
Name and describe two other attenuation mechanisms.
Photoelectric (effect)
Photon (is absorbed and an) electron removed (from the atom)
Compton (scattering / effect)
Photon scattered / deflection with longer wavelength / low frequency / low energy and electron removed (from the atom)
Pair production
Photon (absorbed) and produce electron-positron (pair)
Explain how an ultrasound transducer can emit ultrasound.
You do not need to describe the design of the transducer.
Piezoelectric (effect) mentioned
An alternating p.d. applied to the film / crystal produces vibrations / resonance (and this in turn produces ultrasound)
Explain how the reflection of ultrasound at a boundary between two tissues depends on the physical properties of the tissues.
density (of tissue) or speed (of ultrasound in tissue) or acoustic impedance mentioned
Same Z, no reflection / different Z gives reflection
Ir/I0 = [Z1 – Z2]^2/[Z1 + Z2]^2 and intensity mentioned
What is the purpose of using technetium-99m for imaging organs?
emits gamma photons
detecting tumour
Describe how the fluorine-18 nuclei are located in a patient using a PET scanner.
The positrons / beta-plus particles annihilate electrons (within the patient)
Two gamma-photons are produced
these (photons / rays) travel in opposite directions
The difference in the arrival times at the detectors is used to locate the point of annihilation / nuclei
The attenuation coefficients of the cyst and the soft tissues in (b) were similar. This prevented imaging the cyst using a two-dimensional X-ray image.
Name a different X-ray technique that could be used to image the cyst. Explain the advantage of this technique.
CAT (CT) scan
Any one from
• A CAT scan will give 3D image
• A CAT scan gives better contrast
