Overall Flashcards
What is the structure of a PMT in order of when the photons hit it?
Photocathode (which turns it into photoelectrons), then dynodes (amplifies signal) and finally to an anode
What does a focussing grid do in a PMT?
Ensures the electrons are electrons are focussed towards the next dynode
Is there lead shielding at the back of a gamma camera and why?
Yes to ensure the detected gamma rays only come from the patient
What is a collimator in basic terms for nuclear medicine?
A lead plate with thousands of small holes
How big are the holes of a collimator roughly?
A couple of millimetres in diameter (depends on type)
What is the purpose of a collimator in nuclear medicine?
Only gamma rays travelling in the direction of the holes can get through, which provides spatial localisation so an image can be formed
Why is the crystal sealed in an air tight frame?
Because it is hygroscopic, so air and light need to be kept out
What interactions happen in a scintillation crystal?
Photoelectric absorption or Compton scattering
Roughly how many light photons are produces from the scintillation of one gamma photon in a crystal?
Several thousand
What are scintillation crystals usually made of in gamma cameras?
Sodium iodide doped in thallium NaI(Tl)
Why is the inner surface of the encapsulation of a scintillation crystal covered in a diffuse white reflective coating?
The light will be reflected preferentially back out through the back of the camera so that more are collected
What do the photons meet after the scintillation crystal in a gamma camera?
A light guide
What are the three signals produced by the pulse arithmetic in a gamma camera?
Position (x and y) and energy
The light output in the crystal of a gamma camera is proportional to what?
The energy of the gamma ray
How is the position of a gamma ray worked out from a distribution of signal across PMTs (how does anger logic work here)?
The centroid of the distribution is found (colloquially known as centre of gravity)
Why does the position determined by anger logic have uncertainty and what is this known as?
Random fluctuations in the amount of light collected by each PMT and variations in the number of photoelectrons produced at the photocathode. Intrinsic spatial resolution
What is the value of the intrinsic spatial resolution of a gamma camera?
3 - 5 mm (3.5 mm according to lecturer)
What is the energy resolution of a gamma camera?
9% (according to lecturer)
How do you work out the x (and y) position of a gamma ray using anger pulse arithmetic?
Weighted sum of all the voltages (x1V1+x2V2+… where x1 is the position of PMT 1) divided by the sum of all the voltages (total energy)
How is the total energy of a gamma ray calculated using anger pulse arithmetic?
All the voltages summed from all PMTs
What are the advantages of a NaI(Tl) scintillation crystal?
Moderate density (good stopping power for gamma rays), high atomic number (most interactions at 140 keV are photoelectric absorption, good photopeak efficiency), light output proportional to energy absorbed, emits blue light which the crystal is transparent to and well matched to PMTs, can grow large crystals
Roughly how many scintillation photons are produced per keV in a NaI(Tl) crystal?
35 per keV
What is the wavelength and energy of the scintillation photons from a NaI(Tl) crystal?
415 nm (blue light) and 3 eV
How thick are standard crystals in gamma cameras?
3/8th of an inch (9.5 mm) - this is standard or 5/8th of an inch
What are the disadvantages of a NaI(Tl) scintillation crystal?
Poor energy resolution (compared to semiconductor detectors but better than some scintillators), crystal is hygroscopic, very fragile (mechanical stress or rapid temperature change)
When a gamma ray interacts with a scintillation crystal, what does it produce either by photoelectric absorption or compton scattering?
One secondary electron
What is the scintillation process in a gamma camera crystal?
The produced secondary electron from the gamma ray will move a short distance and result in ionisation or excitation of many atoms. Photons are emitted during de-excitation
What does the thallium doping do in a scintillation crystal?
It adds another bandgap in the energy levels that the electrons can use, so when the electron de-excites using this transition, it releases scintillation photons (rather than just heating)
What is the luminescence centre of the energy levels for a scintillation crystal?
Thallium doping
What does energy resolution mean when considering gamma cameras?
The ability to distinguish two different nearby energies and the variation in energy signal from gamma rays with the same energy
What does spatial resolution mean when considering gamma cameras?
The ability to distinguish fine detail in an image and the variation in position signal from gamma rays at the same position
How much energy is required from the gamma ray for each scintillation photon?
30 eV (so one 140 keV gamma ray results in 4700 scintillation photons)
What is the quantum efficiency value of the photocathode? (percentage of light photons that hit the PMT that produce a photoelectron)
20%
What does quantum efficiency mean?
The measure of the effectiveness of an imaging device to convert incident photons into electrons (eg with the PMT in gamma cameras)
Since Poisson statistics apply to the random fluctuations for the number of photoelectrons emitted in a PMT, what is the standard deviation on the value?
The square root of the mean
After the PMT, what does the current (the signal) do next in a gamma camera?
The signal is received by the pre-amplifier
What is the pre-amplifier in a gamma camera?
It converts the current produced at the anode of the PMT to a voltage pulse
Why is there a fundamental limit of energy resolution for a gamma camera signal?(which also affects position resolution)
Random variation in total PMT signal (and distribution of signals) from the small number of photoelectrons emitted from the photocathode (larger relative standard deviation as root n of a small number because quantum efficiency is low)
What happens if the crystal thickness is increased in gamma cameras?
Good stopping power (better for higher energies) but scintillation further from PMTs in general so broader light distribution (worse spatial resolution) but good uniformity (as less sensitive to PMT position, linear response)
What happens if the crystal in thinner in gamma cameras?
Worse stopping power but better spatial resolution (as narrow light distribution). Poor linearity so poor uniformity
What percentage of 140 keV gamma rays are stopped by a 9.5 mm (3/8th inch -standard) crystal in a gamma camera?
85%
What would happen with an increase in PMT size?
Less PMTs, large signals so lower noise (high SNR) but poor resolution
What would happen with an decrease in PMT size?
More PMTs, small signals so low SNR but better resolution
What is the optimum diameter of PMT?
50 to 75 mm
What is thresholding in gamma cameras? (relates to PMTs)
Only counts signals above a certain threshold and ignores lower ones because distant tubes give small signals, which has high noise and spoils resolution, so removing these gives a better SNR and resolution
Why is scatter rejection necessary?
Gamma rays can get scattered in the patient or crystal before being detected so, if they counted, they would reduce resolution as they are not in the correct place
What is scatter rejection in gamma cameras?
Setting an energy window around the photopeak to exclude scattered photons
Does the collimator reject scatter in gamma cameras?
NO
Does parallel hole collimators have any magnification?
No
What does a diverging hole collimator do to an image?
Minifies image
What is the use of a diverging hole collimator?
More necessary for older systems with smaller FOV crystals to image more of the patient
What does a converging hole collimator do to an image?
Magnifies images
What is the use of a converging hole collimator?
For smaller organs, like heart or brain, to view better
What does a pinhole collimator do to an image?
It inverts and highly magnifies the image
What is the use of a pinhole collimator?
Very small organs like the thyroid
Why are cast collimators better than foil collimators?
More stable and robust, so more likely to be uniform
What size hole diameter and septa do low energy collimators have?
Small holes (2 mm) and thin septa
What size hole diameter and septa do high energy collimators have?
Large holes (5 mm) and thick septa
Why do high energy collimators have large holes?
Since they have thick septa, they have reduced sensitivity, so this needs to be balanced with larger holes
What is septal penetration and do we want it?
Gamma rays going diagonally through the septa to the crystal and no because it spoils image resolution (star shaped point source)
How do we stop septal penetration?
Use a collimator with septal thickness that will stop the highest energy gamma rays emitted by the source
What radionuclides use a low energy collimator?
Technetium-99m, Iodine-123 (but not for absolute quantification because 1% 530 keV), Krypton-81m (190 keV, too high for some low energy collimators)
What radionuclides use a medium energy collimator?
In-111, Ga-67
What radionuclides use a high energy collimator?
I-131
What are the components of an analogue gamma camera?
Collimator, NaTl crystal, light guide, PMT (includes pre-amplifier to convert charge to voltage), pulse arithmetic, energy window, cathode ray tube (to display I think)
What is the equation for resolution in terms of D (hole diameter), L (hole length) and Z (distance from collimator to source) for a parallel hole collimator?
D + (D/L) * Z
What is the equation for sensitivity in terms of D (hole diameter) and L (hole length) for a parallel hole collimator?
D^2/L^2
What equation is used for system resolution squared?
The sum of intrinsic resolution squared and geometric resolution squared (due to collimator)
What is the magnification of a pinhole collimator?
L/Z (length of collimator divided by distance from collimator to source)
What is the resolution of a pinhole collimator?
It is the same equation as for parallel hole collimators
What is the sensitivity of a pinhole collimator?
D^2/Z^2 (diameter of hole squared divided by distance from collimator to source squared)
Does a pinhole collimator sensitivity depend on distance and why?
Yes it falls off rapidly with distance because there is only one hole so only the inverse square law applies)
When are LEHR collimators used?
Fine detail is required and enough time to acquire enough counts
What is the size of the septa and holes for LEHR collimators?
Small holes and thin septa
What is the size of the septa and holes for LEHS collimators?
Thin septa and large holes
When are LEHS collimator used?
Image time should be short (either for scans like cardiac or to stop discomfort) and don’t need to see fine detail
What are some potential collimator problems?
Poor construction (variation in hole size, septal thickness and hole angulation), damage in use (causes distortion), irregularities affect local sensitivity (apply sensitivity corrections for small deviations)
What are some potential crystal and light guide problems?
Non-uniform crystal stopping power (varied density or thickness), non-uniform light output from crystal (variation in Tl doping), non-uniform light transmission (crystal yellowing or poor optical coupling), light collection efficiency varies with position (gap and edges. Apply corrections)
What are some potential PMT problems?
Photocathode efficiency varies over tube face (better nearest the centre. Apply corrections), all tubes are slighting different (tuning needed), gain may change with time
What are some potential high voltage supply problems with gamma cameras?
Small change in high voltage produces large change in PMT gain (needs stabilised supply) and takes time to stabilise after turning on (keep high voltage switched on)
What are some potential electronic problems with gamma cameras?
Temperature variation (needs stabilisation), small signals from distant PMTs (more noise so set a threshold to exclude), failure of Anger arithmetic, signals overlap at high count rates (baseline restoration and pile-up rejection)
What contributes to dead time?
Scintillation (very small), amplifier and ADC (few micro seconds each)
What is the difference between paralysable and non-paralysable systems?
Paralysable = each new event restarts the dead time (any true event restarts it, even if not accepted)
Non-paralysable = only accepted (observed) events restart the dead time
Are most systems paralysable or non-paralysable?
Usually a mixture of both
What are the methods of image improvement?
Tuning, gain stabilisation, energy correction, linearity correction, sensitivity correction (changes counts up or down, sometimes called uniformity correction)
What effect does tuning correct for?
PMT differences
What effect does gain stabilisation correct for?
PMT drift
What effects does energy correction correct for?
Light output from crystal, crystal transparency, light collection with position, photocathode efficiency
What effects does linearity correction correct for?
Crystal transparency, light collection with position, photocathode efficiency
What effects does sensitivity correction correct for?
Collimator defects and crystal stopping power
What are the advantages of SPECT over planar imaging?
3D localisation, improved quantification (3D to see source, attenuation correction, scatter correction), improved contrast (difference between cold spots and normal in particular, eg lung perfusion and myocardial perfusion)
What are the requirements for SPECT imaging?
Camera must be able to rotate around patient, good collimators, good uniformity, adequate FOV, fixed pharmaceutical distribution (must be consistent across views), no patient motion
What are the two options for SPECT acquisition orbits?
Circular or non-circular (elliptical, pseudo-ellipse, auto-contour)
What are the SPECT acquisition parameters?
Collimator (high res preferred), rotation arc (360 or 180), acquisition time, matrix size (64 x 64 with zoom or 128 x 128), number of views (generally every 3 degrees)
Why is SPECT and CT combined?
Anatomical localisation, attenuation correction, SPECT aids interpretation of abnormalities seen on CT
What is the difference between correlative and hybrid imaging?
Correlative is using two modalities (e.g. SPECT-CT) on different occasions (separate machines) whereas hybrid imaging is two modalities in the same machine
What are the advantages of hybrid imaging?
Only one visit for the patient, patient positioning is the same for both images, automatic registration of images
What is the disadvantage of hybrid imaging?
Two pieces of equipment tied up at one time
What are the three uses of hybrid SPECT-CT? (levels of CT)
Attenuation correction, anatomical location, diagnosis
What is a solid state camera?
No vacuum devices (no PMTs). Could be considered semiconductor materials to replace PMTs to collect light, or no light at all (no scintillator)
What are three possible detector materials in gamma cameras?
Sodium iodide doped with thallium, caesium iodide doped with thallium and CZT (cadmium zinc telluride)
What is the light detection method for caesium iodide detector instead of PMTs?
Photodiode
What is the light detection method for CZT detector instead of PMTs?
Not needed as no scintillator
What are the advantages of solid state cameras?
Compact, no wasted edges (useful FOV to edge of detector), better energy resolution, high count rate capability, robust, stable, direct position imformation
What are the disadvantages of solid state cameras?
Still need collimators that limits the system resolution and cost
What are the two complex compartmental models considered?
Mamillary systems and catenary systems
What is the tracer and tracee? (tracer principle)
The tracee is the substance to be studied that is not readily available and the tracer is a small amount of another substance that can be easily observed. they should behave in the same way but the tracer being added should not disturb the system
What is the compartment in compartmental models?
The pool in which tracer and tracee distribute
What are the five requirements in order to use a compartmental model?
Tracer and tracee must behave in the same way (tracer principle), Addition of tracer must not disturb the system (tracer principle), Tracer and tracee must be well mixed (Concentration is the same throughout a compartment), Tracee must be conserved (Compartment volume is constant), Steady state must exist (Transport rates are constant)
What is the maximum and minimum count rate (and activity) for a sample in a well counter?
Max 20,000 cps (25 kBq) and min >1 cps (1Bq)
What makes a good radiopharmaceutical?
Follows biological pathway of interest and minimal other pathways. Goes to pathway of interest in a reasonable timeframe. Subsequent effective half-life in body. Chemically suitable (binds to radionuclide of choice, stable etc.). Shelf-life. Cost and availability. Ease of labelling
What makes a radionuclide suitable for use in diagnostic nuclear medicine?
Suitable gamma energy (leave body but stopped in gamma crystal). Minimal other emissions (lower dose). Half-life appropriate for biological uptake. High specific activity. Stable daughter products. Cost and availability
Is the CFOV or UFOV typically better in terms of image quality?
CFOV (central 75%)
What is approximately the maximum matrix size in square NM images?
256 x 256
To determine the required matrix size, how do you use the intrinsic resolution FWHM for this?
3 pixels per FWHM. So divided FWHM by 3 to get pixel size in mm and divide this by detector size to see which standard matrix size to use
If noise is an issue in a NM image, how can you change the matrix size to reduce the issue?
Reduce the matrix size
What matrix size is used for whole body images in NM?
256 x 1024
What two parameters could determine the length of a NM scan and why would you choose one over the other?
Time (if activity distribution is unknown) or counts (reproducible noise statistics)
What is one of the problems with whole body imaging if not corrected?
Areas at the start and end of the scan are only under the detector for a short period of time, so sensitivity is worse (as well as noise)
How is the problem (uneven sensitivity profile) in whole body imaging corrected for?
Electronic ramping so the patient is stationary at the start and end of the scan and acquisition window opens/closes one frame at a time. Or scan additional bed position
What is dynamic imaging?
Collection of short statics acquisition (frames)
Gated images are a special case of what?
Dynamic imaging
What is list mode?
Collect data as a list of events. Tracking location and time of detection and photon energy
What are methods for attenuation correction?
Conjugate counting, Chang’s method, transmission-based, and CT-based
What is the conjugate counting AC method?
The geometric mean of each head to get total signal at each point
Why is there filtration in CT in relation to AC?
Hardens beam to make it more monoenergetic (still a spectrum though) because linear attenuation coefficients (and therefore HU) depend on photon energy so less variation in HU (and AC) result if more monoenergetic
Why is the HU conversion to linear attenuation coefficient for a different energy photon (for AC) a bilinear graph?
The transition from where the photoelectric effect is dominant to the Compton effect being dominant because PE is less significant for higher energies (in NM or PET)
What is the IRR dose limit for the lens of the eye and the classification threshold?
Dose limit - 20 mSv. Classification threshold - 15 mSv
Does the exposure of carers and comforters have to be justified under IR(ME)R?
Yes
Where should you look for breastfeeding interruption times?
ARSAC and manufacturers guidance (can be different from each other)
How many stages are there for an EPR Environmental Impact Assessment?
3
What is the difference between the stages of an EPR Environmental Impact Assessment?
More generic data (basic calculations) to more detailed calculations, where it starts at stage 1 and moves up depending if the result (max critical group dose) is above a certain dose threshold
Under EPR permit contents, what value is calculated for sealed sources?
A/D (activity divided by danger activity)
Rank the importance of the following radiation protection measures: time, distance, shielding
Distance, time, shielding
What is the cap for in contamination monitors?
Could be two options: build up material or protection (need to check before using)
Are contamination monitors directional?
Yes but no completely
What source is used for extrinsic and intrinsic gamma camera uniformity QC?
Extrinsic - flood
Intrinsic - point
Is a curvature correction required for intrinsic uniformity test for gamma camera QC with a point source if not at a large distance (5 x FOV)?
Yes
How is the integral uniformity calculated?
100 times (Maximum - minimum pixel values in the whole of the FOV divided by max + min)
How is the differential uniformity calculated?
100 times maximum value of (Highest - lowest pixel values in any 5 pixel row or column divided by highest + lowest )
For count rate capability, what percentage reduction from input to output counts do we use as a result?
20% reduction of observed counts
Does ‘the set of operations (programming, coordinating, implementing) intended to maintain or to improve quality’ define QC or QA?
QC
What gas is typically used for the ionisation chamber in radionuclide calibrators and why?
Argon because it is an inert gas with well characterised ionisation relationship
Why is a particular voltage used in radionuclide calibrators (several hundred volts)?
if it was too low, it would be in the recombination region, so we want it in the saturation region, where changing the voltage no longer affects the results
Why do radionuclide calibrators have a peak in response at low photon energies (50 keV)?
Increase in the probability for photoelectric absorption. As the energy increase, it moves into the Compton range
What range of currents are measured in the ionisation chamber of radionuclide calibrators?
Micro amps to femto amps
What happens if there is a very high activity in a radionuclide calibrator?
Ion recombination due to there being so many ions created in the gas
What is the purpose of a sample holder for radionuclide calibrator measurements?
Centralises the source (consistency in position) and allows for easy manipulation and creates distance from the user
Why does the source height in a radionuclide calibrator affect the measurement?
If the source is closer to the opening, more emissions escape the calibrator due to the increasing solid angle
What detector type are sample counters?
Scintillators (usually NaI(Tl))
How do alpha beta counters work and why?
Mix the source with liquid scintillators then placed in a sensitive measurement device. this is needed because alpha and betas interact with the sample container itself so wouldn’t escape to be detected
What do we need for an ideal radiopharmaceutical for therapies?
Emissions (particle or gamma ray) for localisation, radiation protection, and imaging. Half-life (usually longer).
Same as diagnostic: Biological pathway, chemically suitable, shelf life, cost.
What are the typical particle emitters in MRT?
Alpha, beta, auger electrons (rare)
What is the units of linear energy transfer (LET)?
keV/micro metre
Do betas and alphas have high or low LET?
Alphas have high LET and betas have low LET
Which particle emitter can have a more uniform dose and why?
Betas as they have a longer range
What is the bystander effect?
Nearby cells can see the same effect even if not directly exposed
What is the half-life of I-131?
8 days
Can I-131 be used for theranostics?
Yes
What are the advantages of FBP?
Computationally very fast, reproducible, linear in response, long history of use
What are the disadvantages of FBP?
Struggles with non-standard scenarios and can’t compensate for noise effectively
For iterative reconstruction, does noise become a problem with higher or lower iterations?
Higher (beyond around 16 iterations)
What does order-subsets expectation maximisation (OSEM) do?
The projections are broken down into
subsets and then processed at that sub
level
What corrections can be added to the probability matrix that makes up the likelihood calculation in iterative reconstruction?
Attenuation, scatter, resolution, septal penetration
What are the main purposes of absolute quantification in SPECT?
To stage disease, to more accurately assess disease, and calculate dose to target and dose to organs at risk
What are the methods for scatter correction and which are we moving towards?
Compton window method (subtract fraction of a scatter energy window) and Monte Carlo. Moving towards Monte Carlo
What are activity recovery coefficients?
The ratio of the activity concentration measured compared to the known concentrations
What are Gibbs artefacts?
A ringing artefact as a result of reconstruction
If the iterations increase, is SUV_max higher or lower than with less iterations and why?
Higher because its noisier
What radiopharmaceuticals are used for VQ scans?
Ventilation - Krypton gas or alternative
Perfusion - Tc-99m MAA
Is ventilation or perfusion images acquired first in VQ scans and why?
Ventilation or else the perfusion scans would saturate the scan (sometimes ventilation is not done for pregnant patients to reduce dose)
Why is Tc-99m MAA used for VQ scans (perfusion)?
MAA particles are larger than than capillaries, so they are trapped in the alveolar capillary bed
What radiopharmaceutical is used for bone scans?
Tc-99m HDP or MDP or HMDP
What is the main radiopharmaceutical used for thyroid scans?
Pertechnetate (99mTcO4)
Are Tc-99m MAG3 renograms dynamic or static?
Dynamic
What imaging may be done in DMSA scans?
Static (ant, post, LPO, RPO) and sometimes SPECT
Why is the geometric mean used in DMSA scans? (square root of the product of posterior and anterior counts)
It accounts for the anatomical variation in renal tissue depth
What are In-111 octreotide scans for?
Neuroendocrine tumour localisation
What are some indications for F-18 FDG PET/CT?
Cancer staging and response assessment, RT planning, infection, fever of unknown origin, vasculitis, cardiac inflammation
In PET-CT, what is F-18 PSMA for and why?
Identify sites of active prostate cancer because PSMA is overexpressed in prostate cancer so uptake correlates with aggressiveness of disease
What is Lu-177 PRRT (Lutathera) for?
Neuroendocrine tumours (NETs) therapy
What is Lu-177 PSMA for?
Prostate cancer therapy (metastatic castration-resistant)
Where does Ra-223 therapy target?
Targets bone, specifically areas of
osteoblastic bone metastases
Why would we do dosimetry in nuclear medicine?
Provide the best option for patients (increased treatment efficacy) and optimisation/justification (risk quantification)
Which form of dose would you quote for a therapy?
Absorbed dose (Gy)
Which form of dose would you quote for a diagnostic procedure?
Effective dose (mSv)
What effects are NM therapies mostly concerned with and why?
Deterministic effects because considering doses to organs
What effects are diagnostic NM scans mostly concerned with and why?
Stochastic effects because concerned with risk to patient
For activity quantification, what do we do to try to recreate scatter and attenuation and what is the limitation?
Measure a known activity in a geometry similar to a patient but it is limited on how well it matches a patient
What are the intentions for the MIRD and ICRP methods?
Determine the stochastic risk from non-uniform exposure of internal emitters
What is the source and target organs in the MIRD method?
Source is where the emission happened. Target is where dose was deposited. A source organ can be its own target
What is the specific absorbed fraction (SAF)?
The energy deposited in target organ as a fraction of the total energy emitted by the source, normalised by weight of the target organ (units kg-1)
What are Dose Point Kernals (DPK)?
Experimentally or simulated dose distributions within a medium, where the initial source is a single point and the target is a uniform medium surrounding the point
What is DPK useful for?
Dose distributions within an organ. Good for therapy calculations and dose-volume-
histograms
What are the disadvantages of DPK?
Poor for cross organ dosimetry (as calculated within uniform medium and patients rarely uniform). Limited to particles (Photons have cross-organ irradiation). Minimal information for effective dose calculations
What is microdosimetry used for?
Particles with really short ranges (eg alpha and auger electrons)
Why is sterility so important in radiopharmacies?
Protect the product from the environment and operators, and protect operator from the product
How do you do a Mo-99 breakthrough test?
Put eluate in lead pot and measure in a calibrator and compare without pot. Mo-99 has 740 keV so this would still go through
Mo-99 breakthrough should be below what percentage?
0.1%
What labelling efficiency should radiopharmaceuticals have?
> 95%
At high count rates in PET, does trues, scatter or random prompt events dominate?
Randoms (varies with signal squared, whereas trues is linear and scatter is less than that)
What changes when you change radionuclide selection on a radionuclide calibrator?
Calibration factor changes
