Medical Physics Flashcards
1
Q
- Describe how a CAT scan is produced, referring to the physics principles involved. (7 marks)
A
- A ring of detectors surround the patient
- Narrow, fan shaped X-ray beam
- An X-ray tube rotates around the patients many times per second
- This produces a thin cross-section
- X-rays are absorbed by tissue and bone of patient
- X-rays are absorbed by more high Z matter such as bone (or vice versa)
- The attenuation mechanism is mainly the photoelectric effect
- There is a possibility of using a contrast medium (such as barium)
- A CAT scan is better than a normal X-ray at differentiating other organs
- The patient is moved a small distance and the process is repeated
- A computer is used to process the image
- A 3-D image is produced
2
Q
- Explain, with reference to the cathode and anode of an X-ray tube (7 marks):
a. How X-rays are generated
b. The energy conversions that occur
A
The cathode is the negative heater and the anode is the positive target metal (e.g. tungsten).
Electrons are accelerated from the cathode to the anode.
The electrons gain kinetic energy from the cathode to the anode
The high-speed electrons hit the target metal and produce X-rays
Some of the kinetic energy of the electrons is transferred into X-ray photons
About 1% of the incident electron energy is converted into X-rays, the rest (99%) is transferred into heat in the target metal
The intensity against X-ray photon energy shows characteristic spectral lines and ‘broad background’ Bremsstrahlung radiation
The minimum wavelength λ of the X-rays can be determined using V×e=hc/λ
3
Q
- Explain how image intensifiers are used to improve the quality of the X-ray image (3 marks). QWC: In your answer, you should clearly explain the process involved which makes the image brighter.
A
- Absorption of X-rays (by silver halide molecules) by a photographic film
- Uses of fluorescent / scintillator / phosphor
- Photon releases electrons (that is accelerated onto a fluorescent screen)
- Number of electrons increased / multiplied
- QWC: Phosphor converts X-rays into increased number of visible light photons
4
Q
- Describe the use of a contrast medium, such as barium, in the imaging of internal body structures. Your answer should include: (8 marks)
a. How an image of an internal body structure is produced from an X-ray beam
b. An explanation of the use of contrast medium
c. Examples of the types of structure that can be imaged by this process
A
- X-ray photons are attenuated by high Z matter (e.g. bone)
- X-ray photons are attenuated by mainly the photoelectric effect (with energies around 1-100keV.
- The X-rays are detected by a film/ scintillation counter
- In the photoelectric effect the attenuation coefficient is proportional to Z3
- However X-rays do not show up soft tissues well
- Contrast medium has high Z so it absorb X-rays strongly
- It is usually taken orally/ as an enema/ can be injected
- They can image the digestive tract
5
Q
- Outline some of the main components of an MRI scanner (5 marks)
A
- There is a large superconducting magnet, which produces a strong external magnetic field.
- A set of gradient coils, which produce an additional magnetic field which varies across the length, width and depth of a patient’s body
- A radio frequency coil that transmits radio wave pulses into the patient
- A radio frequency coil that detects the signal emitted by the relaxing nuclei
- A computer that controls the gradient coils and RF pulses and which produces and displays images.
6
Q
- Outline the principles of magnetic resonance. Outline how the main components of an MRI scanner are used to obtain diagnostic information about the internal organs (7 marks)
A
- Protons have spin and behave like tiny magnets
- Protons precess about the magnetic field provided by the strong electromagnet
- Transmitting coils provide pulses of radio waves of frequency equal to the Larmor frequency
- The protons absorb the radio waves, resonate and flip into a higher energy state
- When protons flip back down to a lower energy state they emit photons of radio waves
- The relaxation time of the protons depends on the surrounding tissues
- The radio waves are picked up by the received coils
- The gradient coils alter the magnetic flux density through the body
- The gradient coils allow the position of nuclei in the body to be determined
- The Larmor frequency of the protons varies through the body
- The computer processes all the signals from the receiving coils and generates the image.
7
Q
- Name and describe the functions of the 4 main components in a gamma camera (8 marks).
A
- Collimator
- Only allows γ photons travelling along its axis to pass through – this ensures a sharp image. It is a honeycomb of lead tubes.
- Scintillation crystal
- Converts γ photons into many visible light photons
- Photomultiplier tubes
- Electrons are produced from the light photons
- Computer
- Converts the electrical signal from the photomultiplier tubes into an image
8
Q
- Describe the principles of the production of a short pulse of ultrasound using a piezoelectric transducer (5 marks)
A
- A voltage is applied to a piezoelectric crystal
- This makes the crystal change its shape
- The crystal material is quartz
- An alternating voltage makes the crystal vibrate
- The crystal resonates when the applied voltage has the same frequency as the natural frequency of the crystal.
- Resonant frequency = frequency of ultrasound (>20kHz)
- The crystal is damped to stop it vibrating when the voltage is switched off
- The application of alternating p.d. is short (which makes the pulse of ultrasound short)
- This is achieved by a backing material/ epoxy resin.
9
Q
- Explain why a coupling medium is required between the transducer and the skin of a patient when conducting ultrasound diagnosis (4 marks)
A
- There is a large difference in acoustic impedance between air and skin
- Therefore a significant fraction of ultrasound reflected
- A coupling medium is an impedance matching gel
- Reduces the reflection at the air-skin boundary
10
Q
- Describe how ultrasound scanning is used to obtain diagnostic information about internal structures of the body. In your description include the difference between an A-scan and a B-scan (4 marks)
A
- A pulse of ultrasound is sent into the body
- The wave is reflected at boundary of tissue
- The time delay is measured and used to determine the depth of the reflection
- The fraction of reflected tissue is used to identify the tissue
- An A-scan is in one direction only – it is only used for distance or depth finding
- B-scan uses a number of sensors or a sensor in different positions
11
Q
- Describe how ultrasound is used to determine the speed of blood in an artery (3 marks)
A
- Ultrasound is reflected off moving red blood cells
- The frequency of the ultrasound is changed
- The change of frequency is related the speed of the blood
12
Q
- Describe the use of medical tracers to diagnose the condition of organs. (2 marks)
A
- Tracer is injected into the body
- Tracer is absorbed by organ (or it shows a blockage)
- Gamma camera is used to detect radiation from the body
13
Q
- Describe the principles of positron emission tomography (PET) (5 marks)
A
- A positron emitting tracer is used
- The positron emitted quickly annihilates with an electron inside the patient
- Creating two gamma photons [must be photons here, not waves or rays]
- The photons travel in opposite directions
- The patient is surrounded by a ring of detectors
- The arrival time / delay time of the photons indicates location
- A 3-D image is created (by the computer connected to the detectors)
