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