Medical physics Flashcards
Intensity
Power per cross sectional area
Examples of non-invasive techniques
MRI - Magnetic Resonance Imaging
CAT - Computerised Axial Tomography
PET - Positron emission tomography
X-rays
Endoscope
Ultrasound
Gamma Cameras
Why are non-invasive techniques important?
They do not require surgery. Surgery is not good because:
- damage is caused to the body + it must recover.
- surgeries may have complications + result in death
- expensive and take longer to diagnose problem
Describe the production of an ultrasonic pulse by a piezoelectric crystal
- The application of a p.d. across a material/ crystal
- causes an expansion / contraction / vibration
- that is equal to the frequency of the alternating current [for detection of ultrasound: reverse this process]
How is ultrasound used to image the body?
- 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
What’s the difference between A-scans and B-scans?
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)
Advantages of Ultrasound
Non-invasive technique
Does not involve the use of ionising radiation; patients can have many ultrasonic scans without risk to their health.
Scans are quick to perform
Disadvantages of Ultrasound
Difficult to image through bone (e.g. chest cavity is difficult to image)
Difficult to image boundaries between two soft tissues (as they often have very similar acoustic impedances)
Why is a gel 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)
Calculating acoustic impedance
Z = ρc
acoustic impedance = density of material x speed of ultrasound in material
Calculating the fraction of reflected ultrasound
I0 is original intensity
Ir is the reflected intensity
z1 and Z2 are the acoustic impedances of the two materials (order not important due to squaring)
What is ultrasound?
(Longitudinal) sound waves with frequency >20kHz
[The ultrasound used in medicine is around 2MHz with wavelengths as small as 1mm]
Describing how Doppler ultrasound can be used to measure the speed of blood flowing in a blood vessel.
- Ultrasonic pulse is reflected by moving blood cells
- The frequency of the reflected pulse is altered — increased if the blood is moving towards the emitter/detector (The Doppler effect)
- Speed of blood can be calculated from change in frequency.
What is ionising radiation?
Ionising radiation means radiation capable of removing electrons from neutral atoms.
This process can cause mutations in and can kill living cells by chemically altering the DNA molecules within their nuclei.
What are X-rays?
Any two from:
(X-rays) are EM waves
Travel at speed of light / 3 x 108 m/s (in a vacuum)
Travel in a vacuum / empty space
Transverse waves
Can cause ionisation
Have wavelength of about 10-10 m
(X-rays are high energy) photons
How are medical x-rays produced?
- Electrons emitted from a heated cathode
- In a vacuum
- Electrons accelerated towards a tungsten [high m.p.] anode ‘target’
- Rapid deceleration of fast-moving electrons when the impact the anode results in braking radiation in the x-ray region of the EM spectrum
- KE of electrons converted into EM energy and heat
- X-ray photos emerge in all directions
- Casing with a ‘Window’ is used to allow only a collimated beam to emerge from the device [collimated = rays emerge parallel to one another]
What is characteristic radiation?
- During the production of X-rays, on a graph of intensity against wavelength, these are sharp peaks in the broad spectra.
- X-rays excite the electrons into higher energy levels. The relaxation of these electrons produce very specific wavelengths of radiation.
- Does not affect diagnostic uses for X-rays
In what ways can X-rays interact with matter?
The photoelectric effect where an (orbital) electron is ejected from atom / atom is ionised
Compton scattering where X-ray scattered / deflected by the interaction with (orbital) electron. Photon has lower energy + electron ejected.
Pair production where X-ray photon interacts with the nucleus / atom and an electron and positron are produced. (Photon disappears)
How to calculate the intensity of X-rays after passing through a material?
I = I0e-µx
I0is the inital intensity
x is the thickness of the material (m)
µ is the attenuation (absorption) coefficient (m-1)
How does an image intensifier work?
- X-rays incident on the first phosphor screen. X-ray photons produce a greater number of visible light photons.
- The photons strike a photocathode to release electrons.
- Electrons accelerated + focused by +vely charged anode so they strike a second phosphor screen, which gives out visible light.
- The final image can be viewed on a monitor / sent to a computer.
How are X-rays captured on film?
- Use an intensifier screen.
- These are sheets of material that contain a phosphor / scintillator.
- Film is sandwiched between two screens.
- Each X-ray photon can produce 100s of light photons (darkening the silver halide based film)
- Metal back reflects X-rays that have passed through back onto the film for maximum efficiency.
What role do contrast media play in producing x-ray images?
Different soft body tissue produce little difference in contrast/attenuation
(Contrast media with) high atomic number used / barium (used to give greater contrast)
Liquids injected or swallowed into soft tissue areas
Describe the use of image intensifiers and contrast media when X-rays are used to produce images of internal body structures
- Different soft body tissue produce little difference in contrast/attenuation
- Intensifier used as X-ray would pass through film
- The intensifier is a phosphor: 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
- Scan shows outline / shape of soft tissue
Advantages and disadvantages of X-ray imaging
Scans are quick to perform
BUT
X-rays are ionising radiation. There is a risk of cell damage and cancer associated with ionising radiation; patient’s exposure must be monitored and limited.
Poor soft tissue contrast
Explain how the production of a CAT (Computerised Axial Tomography) scan differs from a simple X-ray image
Simple X-ray is one directional / produces single image
CT image(s) taken at different angles: X-ray tube is rotated
Computer processes data Image constructed from many slices
The images are combined to build up a 3D Image
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
Explain what is meant by a medical tracer. Name a medical tracer commonly used to diagnose the function of organs
Radioactive substance that is injected (into patient) Technetium(-99m) / Iodine(- 131) / fluorine (-18)
[Usually gamma emitter used, unless involved in a PET scan requiring a beta + emitter. Both ionising.]
Describe the use of medical tracers to diagnose the condition of organs.
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 function of each of the components in the gamma camera.
Collimator: lead tubes allow only parallel gamma ray photons to travel to the scintillator.
Having thin / long / narrow (lead) tubes makes the image sharper
Scintillator: gamma ray photon produces very many photons of (visible) light
Photomultiplier: An electrical pulse is produced from the light (photons)
Computer: Signals from the photomultiplier tubes are combined to produce an image.
Describe the principles of positron emission tomography (PET).
- 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
The principals of MRI
- Some nuclei behave as small magnets / nuclei line up in the magnetic field
- Need for a strong magnetic field
- The frequency of precession is known as Larmor frequency
- Application of RF pulses at the Larmor frequency produces resonance flip energy states
- RF pulse tumed off nuclei relax / flip back (and emit RF signal)
- Relaxation time is the time taken for this to occur
- RF detected (by coil receiver) and processed
- Use of non-uniform mag field gradient field means different Larmor frequencies throughout the body.
- To locate position of nuclei in body
- Difference in the relaxation times for hydrogen in different tissues / materials
Advantages / disadvantages of MRI (comparing with X-rays)
Advantages:
- not ionising radiation (as with X-rays)
- better soft tissue contrast
Disadvantages:
- heating effect of metal objects
- effect on cardiac pacemakers
- takes a long time to perform MRI scan
How do photomultiplier tubes work?
Incident photon from scintillator eject a single electron from photocathode.
Electron is accelerated towards a positive plate (dynode) at +100V
Impact releases a few electrons
Electrons accelerated towards a +200V dynode, ejecting more electrons.
Process continues until an avalanche of electrons is produced, which is enough to trigger a single electrical pulse to the computer.
