Lecture 2 + 3: Instrumentation Flashcards
Sketch a gamma camera. (7 marks)
- Don’t forget to include the bed and PC etc.
- Collimator
- Scintillation crystal (NaI(Tl))
- PMTs
- Electronics and Anger positioning network
- PC/display
- lead shielding
Describe how collimators are used in NM and the difference between LEAP and LEHR. (5 marks)
- Used to localise the radionuclide via the line of sight principle as radiation from sources used in nuclear medicine is emitted in all directions
- They consist of lead sheets permeated with thousands of tiny holes.
- LEHR collimators have long thin holes which increase the resolution but decrease the sensitivity. The septal thickness is generally varied with the energy of the gamma rays.
- LEAP are shorter and wider holes to increase the sensitivity.
For a parallel hole collimator what variables in terms of design affect the resolution and sensitivity and how? (12 marks)
Easiest to draw it and then think about the effects:
- Increasing the length of holes, L, will increase res but decrease sensitivity.
- Increasing the hole diameter, D, will increase the sensitivity but decrease the res.
- Increasing the distance from the patient to the collimator will not affect the sensitivity but will decrease the resolution.
- Increasing the septa thickness, T, will decrease the sensitivity but not affect the resolution provided the hole size remains the same.
What is a pinhole collimator and when is it used? (3 marks)
- Pinhole collimator is angled so that the holes point onto a focussed area.
- This reduces the FOV but increases the resolution, results in magnification of the ROI.
- It is used for small field applications such as thyroid imaging.
What are the pro’s and con’s of using a NaI(Tl) crystal as a detector in MI? (7 marks)
Cons: Fragile and hygroscopic ( absorbs water) which can degrade the crystal.
Pros:
- A high number of photons are produced per keV which gives good energy resolution.
- High attenuation coefficient around 140 keV which makes it ideal for use with Tc-99m.
- High density which makes it an efficient absorber.
- The amount of light produced is proportional to the energy of the photon incident upon it, so can be used to determine energy of the photon.
- It is transparent to the light it produces, so it efficient.
- It has a short excited state which increases the count rate which it can detect and reduces pulse pile up effects.
For a scintillation crystal in a gamma camera, why is there a trade-off with the thickness of the crystal? (2 marks)
- Increasing the thickness increases the efficiency as more photons will be detected and more light will be produced
- But this reduces the resolution of the system as a thicker crystal results in more spread of the signal in the crystal as well as scatter, reducing knowledge of the event of the localisation of the event.
How does a PMT in a gamma camera work? (6 marks)
Draw a sketch:
- 200-300 V PD between the dynodes
- an entrance window and a photo-cathode allow the photons to enter the PMT and be converted to electrons.
- dynodes are used to accelerate the electrons and multiply the number produced for each dynode by a factor of ~ 10
- there are around 7 dynodes typically which results is in a gain in signal by 10^7
- the electrons are then received by an anode and converted into a signal
- there are negative focussing electrodes to ensure that the electrons are directed toward the dynodes
What is meant by the term quantum efficiency? (2 marks)
- QE = number of photo-electrons detected / number of photons incident
- occurs in photocathode in PMT
- typically 20-30%
What is the energy resolution of a detector? (3 marks)
- The FWHM of the signal photo-peak
- Caused by statistical variations (scattering of photons decreases the energy)
- Improves at higher energies due to less lateral scatter (1/E^0.5)
What signal processing is carried out in a gamma camera? (6 marks)
- Correction circuits are used for spatial distortion and energy drift
- Summation of the signal gives the energy of the gamma ray.
- Signal thresholding is carried out to remove background and improve localisation and reduce dead-time
- Energy window is set typically around photo-peak +/- 10% to ensure only RN of interest is detected and to reject scatter
- Anger positioning network is used to determine the location of the scintillation event by using relative contribution from PMTs
What is the protocol for bone scintigraphy? (6 marks)
- 600 MBq of Tc-99m MDP
- Use a LEHR collimator to optimise resolution as clinically the location of the bone mets is more crucial than quantification of the signal
- MDP goes to region of interest which is areas or bone growth/crystallisation
- Image patient 2-3 hrs post IV injection
- Use geometric mean to analyse results as the signal varies less with distance compared to the aritmetic mean
Breifly, what QA is carried out on gamma cameras and how? (4 marks) Why do we do it? (2 marks)
- Energy resolution -> measure spectrum of source such as Tc-99m and measure FWHM of signal.
- Sensitivity -> measure set amount of activity at a set distance and compare the number of counts received.
- Uniformity -> remove collimators and image point source at a large distance from detector. Should result in a uniform image. Measure uniformity by looking at SD in regions of interest.
- Spatial Resolution -> image a line source and measure the FWHM of a line profile take across it. FWHM = spatial resolution.
QA used to track changes in performance parameters over-time and ensure that system performance is not degrading over-time. Required to maintain optimal, or at least clinically acceptable, performance.
