Overall Flashcards
1
Q
What is the structure of a PMT in order of when the photons hit it?
A
Photocathode (which turns it into photoelectrons), then dynodes (amplifies signal) and finally to an anode
2
Q
What does a focussing grid do in a PMT?
A
Ensures the electrons are electrons are focussed towards the next dynode
3
Q
Is there lead shielding at the back of a gamma camera and why?
A
Yes to ensure the detected gamma rays only come from the patient
4
Q
What is a collimator in basic terms for nuclear medicine?
A
A lead plate with thousands of small holes
5
Q
How big are the holes of a collimator roughly?
A
A couple of millimetres in diameter (depends on type)
6
Q
What is the purpose of a collimator in nuclear medicine?
A
Only gamma rays travelling in the direction of the holes can get through, which provides spatial localisation so an image can be formed
7
Q
Why is the crystal sealed in an air tight frame?
A
Because it is hygroscopic, so air and light need to be kept out
8
Q
What interactions happen in a scintillation crystal?
A
Photoelectric absorption or Compton scattering
9
Q
Roughly how many light photons are produces from the scintillation of one gamma photon in a crystal?
A
Several thousand
10
Q
What are scintillation crystals usually made of in gamma cameras?
A
Sodium iodide doped in thallium NaI(Tl)
11
Q
Why is the inner surface of the encapsulation of a scintillation crystal covered in a diffuse white reflective coating?
A
The light will be reflected preferentially back out through the back of the camera so that more are collected
12
Q
What do the photons meet after the scintillation crystal in a gamma camera?
A
A light guide
13
Q
What are the three signals produced by the pulse arithmetic in a gamma camera?
A
Position (x and y) and energy
14
Q
The light output in the crystal of a gamma camera is proportional to what?
A
The energy of the gamma ray
15
Q
How is the position of a gamma ray worked out from a distribution of signal across PMTs (how does anger logic work here)?
A
The centroid of the distribution is found (colloquially known as centre of gravity)
16
Q
Why does the position determined by anger logic have uncertainty and what is this known as?
A
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
17
Q
What is the value of the intrinsic spatial resolution of a gamma camera?
A
3 - 5 mm (3.5 mm according to lecturer)
18
Q
What is the energy resolution of a gamma camera?
A
9% (according to lecturer)
19
Q
How do you work out the x (and y) position of a gamma ray using anger pulse arithmetic?
A
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)
20
Q
How is the total energy of a gamma ray calculated using anger pulse arithmetic?
A
All the voltages summed from all PMTs
21
Q
What are the advantages of a NaI(Tl) scintillation crystal?
A
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
22
Q
Roughly how many scintillation photons are produced per keV in a NaI(Tl) crystal?
A
35 per keV
23
Q
What is the wavelength and energy of the scintillation photons from a NaI(Tl) crystal?
A
415 nm (blue light) and 3 eV
24
Q
How thick are standard crystals in gamma cameras?
A
3/8th of an inch (9.5 mm) - this is standard or 5/8th of an inch
25
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)
26
When a gamma ray interacts with a scintillation crystal, what does it produce either by photoelectric absorption or compton scattering?
One secondary electron (a free electron)
27
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 (valence to conduction band). Photons are emitted during de-excitation
28
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)
29
What is the luminescence centre of the energy levels for a scintillation crystal?
Thallium doping
30
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
31
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
32
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)
33
What is the quantum efficiency value of the photocathode? (percentage of light photons that hit the PMT that produce a photoelectron)
20%
34
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)
35
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
36
After the PMT, what does the current (the signal) do next in a gamma camera?
The signal is received by the pre-amplifier
37
What is the pre-amplifier in a gamma camera?
It converts the current produced at the anode of the PMT to a voltage pulse
38
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)
39
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)
40
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
41
What percentage of 140 keV gamma rays are stopped by a 9.5 mm (3/8th inch -standard) crystal in a gamma camera?
85%
42
What would happen with an increase in PMT size?
Less PMTs, large signals so lower noise (high SNR) but poor resolution
43
What would happen with an decrease in PMT size?
More PMTs, small signals so low SNR but better resolution
44
What is the optimum diameter of PMT?
50 to 75 mm
45
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
46
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
47
What is scatter rejection in gamma cameras?
Setting an energy window around the photopeak to exclude scattered photons
48
Does the collimator reject scatter in gamma cameras?
NO
49
Does parallel hole collimators have any magnification?
No
50
What does a diverging hole collimator do to an image?
Minifies image
51
What is the use of a diverging hole collimator?
More necessary for older systems with smaller FOV crystals to image more of the patient
52
What does a converging hole collimator do to an image?
Magnifies images
53
What is the use of a converging hole collimator?
For smaller organs, like heart or brain, to view better
54
What does a pinhole collimator do to an image?
It inverts and highly magnifies the image
55
What is the use of a pinhole collimator?
Very small organs like the thyroid
56
Why are cast collimators better than foil collimators?
More stable and robust, so more likely to be uniform
57
What size hole diameter and septa do low energy collimators have?
Small holes (2 mm) and thin septa
58
What size hole diameter and septa do high energy collimators have?
Large holes (5 mm) and thick septa
59
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
60
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)
61
How do we stop septal penetration?
Use a collimator with septal thickness that will stop the highest energy gamma rays emitted by the source
62
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)
63
What radionuclides use a medium energy collimator?
In-111, Ga-67
64
What radionuclides use a high energy collimator?
I-131
65
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)
66
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
67
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
68
What equation is used for system resolution squared?
The sum of intrinsic resolution squared and geometric resolution squared (due to collimator)
69
What is the magnification of a pinhole collimator?
L/Z (length of collimator divided by distance from collimator to source)
70
What is the resolution of a pinhole collimator?
It is the same equation as for parallel hole collimators
71
What is the sensitivity of a pinhole collimator?
D^2/Z^2 (diameter of hole squared divided by distance from collimator to source squared)
72
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)
73
When are LEHR collimators used?
Fine detail is required and enough time to acquire enough counts
74
What is the size of the septa and holes for LEHR collimators?
Small holes and thin septa
75
What is the size of the septa and holes for LEHS collimators?
Thin septa and large holes
76
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
77
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)
78
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)
79
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
80
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)
81
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)
82
What contributes to dead time?
Scintillation (very small), amplifier and ADC (few micro seconds each)
83
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
84
Are most systems paralysable or non-paralysable?
Usually a mixture of both
85
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)
86
What effect does tuning correct for?
PMT differences
87
What effect does gain stabilisation correct for?
PMT drift
88
What effects does energy correction correct for?
Light output from crystal, crystal transparency, light collection with position, photocathode efficiency
89
What effects does linearity correction correct for?
Crystal transparency, light collection with position, photocathode efficiency
90
What effects does sensitivity correction correct for?
Collimator defects and crystal stopping power
91
What are the advantages of SPECT over planar imaging?
3D localisation, improved quantification (separate source from background, attenuation correction, scatter correction), improved contrast (difference between cold spots and normal)
92
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
93
What are the two options for SPECT acquisition orbits?
Circular or non-circular (elliptical, pseudo-ellipse, auto-contour)
94
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)
95
Why is SPECT and CT combined?
Anatomical localisation, attenuation correction, SPECT aids interpretation of abnormalities seen on CT
96
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
97
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
98
What is the disadvantage of hybrid imaging?
Two pieces of equipment tied up at one time
99
What are the three uses of hybrid SPECT-CT? (levels of CT)
Attenuation correction, anatomical location, diagnosis
100
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)
101
What are three possible detector materials in gamma cameras?
Sodium iodide doped with thallium, caesium iodide doped with thallium and CZT (cadmium zinc telluride)
102
What is the light detection method for caesium iodide detector instead of PMTs?
Photodiode array
103
What is the light detection method for CZT detector instead of PMTs?
Not needed as no scintillator
104
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
105
What are the disadvantages of solid state cameras?
Still need collimators that limits the system resolution and cost
106
What are the two complex compartmental models considered?
Mamillary systems and catenary systems
107
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
108
What is the compartment in compartmental models?
The pool in which tracer and tracee distribute
109
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)
110
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)
111
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
112
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
113
Is the CFOV or UFOV typically better in terms of image quality?
CFOV (central 75%)
114
What is approximately the maximum matrix size in square NM images?
256 x 256
115
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
116
If noise is an issue in a NM image, how can you change the matrix size to reduce the issue?
Reduce the matrix size
117
What matrix size is used for whole body images in NM?
256 x 1024
118
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)
119
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)
120
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
121
What is dynamic imaging?
Collection of short statics acquisition (frames)
122
Gated images are a special case of what?
Dynamic imaging
123
What is list mode?
Collect data as a list of events. Tracking location and time of detection and photon energy
124
What are methods for attenuation correction?
Conjugate counting, Chang's method, transmission-based, and CT-based
125
What is the conjugate counting AC method?
The geometric mean of each head to get total signal at each point
126
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
127
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)
128
What is the IRR dose limit for the lens of the eye and the classification threshold?
Dose limit - 20 mSv. Classification threshold - 15 mSv
129
Does the exposure of carers and comforters have to be justified under IR(ME)R?
Yes
130
Where should you look for breastfeeding interruption times?
ARSAC and manufacturers guidance (can be different from each other)
131
How many stages are there for an EPR Environmental Impact Assessment?
3
132
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
133
Under EPR permit contents, what value is calculated for sealed sources?
A/D (activity divided by danger activity)
134
Rank the importance of the following radiation protection measures: time, distance, shielding
Distance, time, shielding
135
What is the cap for in contamination monitors?
Could be two options: build up material or protection (need to check before using)
136
Are contamination monitors directional?
Yes but no completely
137
What source is used for extrinsic and intrinsic gamma camera uniformity QC?
Extrinsic - flood
Intrinsic - point
138
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
139
How is the integral uniformity calculated?
100 times (Maximum - minimum pixel values in the whole of the FOV divided by max + min)
140
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 )
141
For count rate capability, what percentage reduction from input to output counts do we use as a result?
20% reduction of observed counts
142
Does 'the set of operations (programming, coordinating, implementing) intended to maintain or to improve quality' define QC or QA?
QC
143
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
144
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
145
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
146
What range of currents are measured in the ionisation chamber of radionuclide calibrators?
Micro amps to femto amps
147
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
148
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
149
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
150
What detector type are sample counters?
Scintillators (usually NaI(Tl))
151
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
152
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.
153
What are the typical particle emitters in MRT?
Alpha, beta, auger electrons (rare)
154
What is the units of linear energy transfer (LET)?
keV/micro metre
155
Do betas and alphas have high or low LET?
Alphas have high LET and betas have low LET
156
Which particle emitter can have a more uniform dose and why?
Betas as they have a longer range
157
What is the bystander effect?
Nearby cells can see the same effect even if not directly exposed
158
What is the half-life of I-131?
8 days
159
Can I-131 be used for theranostics?
Yes
160
What are the advantages of FBP?
Computationally very fast, reproducible, linear in response, long history of use
161
What are the disadvantages of FBP?
Struggles with non-standard scenarios and can't compensate for noise effectively
162
For iterative reconstruction, does noise become a problem with higher or lower iterations?
Higher (beyond around 16 iterations)
163
What does order-subsets expectation maximisation (OSEM) do?
The projections are broken down into
subsets and then processed at that sub
level
164
What corrections can be added to the probability matrix that makes up the likelihood calculation in iterative reconstruction?
Attenuation, scatter, resolution, septal penetration
165
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
166
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
167
What are activity recovery coefficients?
The ratio of the activity concentration measured compared to the known concentrations
168
What are Gibbs artefacts?
A ringing artefact as a result of reconstruction
169
If the iterations increase, is SUV_max higher or lower than with less iterations and why?
Higher because its noisier
170
What radiopharmaceuticals are used for VQ scans?
Ventilation - Krypton gas or alternative
Perfusion - Tc-99m MAA
171
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)
172
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
173
What radiopharmaceutical is used for bone scans?
Tc-99m HDP or MDP or HMDP
174
What is the main radiopharmaceutical used for thyroid scans?
Pertechnetate (99mTcO4)
175
Are Tc-99m MAG3 renograms dynamic or static?
Dynamic
176
What imaging may be done in DMSA scans?
Static (ant, post, LPO, RPO) and sometimes SPECT
177
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
178
What are In-111 octreotide scans for?
Neuroendocrine tumour localisation
179
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
180
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
181
What is Lu-177 PRRT (Lutathera) for?
Neuroendocrine tumours (NETs) therapy
182
What is Lu-177 PSMA for?
Prostate cancer therapy (metastatic castration-resistant)
183
Where does Ra-223 therapy target?
Targets bone, specifically areas of
osteoblastic bone metastases
184
Why would we do dosimetry in nuclear medicine?
Provide the best option for patients (increased treatment efficacy) and optimisation/justification (risk quantification), legal requirement for therapies
185
Which form of dose would you quote for a therapy?
Absorbed dose (Gy)
186
Which form of dose would you quote for a diagnostic procedure?
Effective dose (mSv)
187
What effects are NM therapies mostly concerned with and why?
Deterministic effects because considering doses to organs
188
What effects are diagnostic NM scans mostly concerned with and why?
Stochastic effects because concerned with risk to patient
189
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
190
What are the intentions for the MIRD (Medical Internal Radiation Dose committee) and ICRP methods?
Determine the stochastic risk from non-uniform exposure of internal emitters (MIRD is nuclear medicine specific, whereas ICRP is radiation protection in general, only subtle differences)
191
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
192
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)
193
What are Dose Point(/voxel) 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
194
What is DPK useful for?
Dose distributions within an organ. Good for therapy calculations and dose-volume-
histograms
195
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. It doesn't account for bremsstrahlung
196
What is microdosimetry used for?
Particles with really short ranges (eg alpha and auger electrons)
197
Why is sterility so important in radiopharmacies?
Protect the product from the environment and operators, and protect operator from the product
198
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
199
Mo-99 breakthrough should be below what percentage?
0.1%
200
What labelling efficiency should radiopharmaceuticals have?
>95%
201
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)
202
What changes when you change radionuclide selection on a radionuclide calibrator?
Calibration factor changes
203
What is the difference between the Chantler and the Brochner-Mortenson correction in GFR calculations?
Chantler correction is a multiplication by a constant factor whereas the Brochner-Mortensen correction is a quadratic correction
204
Why is either the Chantler and the Brochner-Mortenson correction needed in GFR calculations
The slope intercept method of calculating GFR does not account for the mixing phase
205
When does the Chantler correction for GFR calculations have increased error?
Increased error with increased GFR as the amount of clearance during the mixing phase increases with GFR
206
What are some of the reasons why the measured counts doesn't equate to the activity in a patient?
Attenuation, scatter, partial volume effects etc
207
What is the first step in the MIRD method?
Work out the dose to each organ
208
Where do Specific absorbed fraction (SAF) values as a function of energy come from?
Look up literature values available for standard phantoms with Monte Carlo simulations
209
What are S-values?
The combine the relevant yields, energies and SAF for a specific radionuclide
210
What is the units of S-values?
mGy/(MBq.s)
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With the MIRD scheme, what two values do we need to multiply to get the total absorbed dose to a target organ from that source organ? (need to do this for all source-target combinations for totals)
Dose per decay (S-value) and total decays (accumulated activity, A tilde)
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What are some issues with the MIRD scheme?
Assumes a homogenous distribution and mean dose across organs. S-values are specific to a phantom (representative of patient?)
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For the DPK method, what do we convolve with the experimental/simulated dose distributions within a medium to estimate dose distribution?
Patient activity distribution
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If there is a particle emitter, can we use the DPK scheme for dosimetry and why?
Probably yes, even though it doesn't account for bremsstrahlung or gamma emissions, but most of the dose will be from the particles anyway (maybe use MIRD for organs further away and this for dose within an organ)
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What are some methods in microdosimetry?
Modified MIRD schema, analytical models, Monte Carlo modelled dose distributions
216
How is foetal dosimetry performed typically? (eg VQ scan)
MIRD scheme to determine the dose to the uterus (typically quoted as absorbed dose). More relevant for early pregnancies.
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For day to day dosimetry, where can we look for a general dose to quote?
ARSAC NfG for tables of DRLs alongisde effective dose and dose to uterus
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What equation is used for the accumulated activity (A tilde) for the dose calculation using the MIRD scheme if there is instant uptake and no biological clearance (only physical half life)?
A_0 (activity at time = 0) divided by lambda (decay constant)
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For absorbed dose calculations using the MIRD scheme, what notation is used for absorbed dose from a source organ to a target organ?
D_Target<-source
220
For non-paralysable systems, what is the equation for the difference between true and observed count rate (R_true - R_obs)?
The product of the true count rate, observed count rate and dead time constant (R_true x R_obs x t)
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For paralysable systems, what is the equation for the observed count rate in terms of the true count rate and dead time constant?
The true count rate multiplied by e to the power of (minus the true count rate multiplied by the dead time constant)
222
What is the order of PMT gain?
1 million
223
Out of the linearity, energy and sensitivity corrections, which is last to be applied to an image?
Sensitivity
224
What is the result of having incorrect PMT gain (increased or decreased)?
Fewer counts in the energy window
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What is the process of creating an energy correction map?
Use a uniform source with known energy and measure the energy signal variation across the crystal and store as a map of correction factors
226
What does energy correction improve?
Energy resolution and uniformity (events not incorrectly rejected by energy window)
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The position signal should vary linearly with source position (good linearity), why may it not have this in reality?
Light lost between PMTs or at the edge of the crystal. Photocathode efficiency varies across the tube face
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Are linearity problems better or worse with a thin crystal?
Worse
229
What is the visual representation of poor linearity on an image?
Position signal is biased towards PMT centres so straight lines appear bent towards each PMT
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What are the negative effects of not having a linearity correction?
Affects resolution (slightly better between PMTs than at the centre) and causes small images non-linearities, which can cause large non-uniformity
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What is the process of creating a linearity correction map?
Use X and Y line sources and measure X and Y signal distortion across the crystal and store map of these correction factors
232
What part of Anger logic does the linearity correction apply to?
Position (x, y)
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What part of Anger logic does the energy correction apply to?
Energy (E)
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What is the process of creating a sensitivity correction map?
Image a uniform source and calculate correction factors proportional to the inverse of the counts at each point of the crystal and store a correction map
235
What does ADC stand for and what does it do?
Analogue-to-digital converters and it digitises the signal
236
Where is the ADC for digital gamma cameras and semi-digital cameras?
For each PMT in digital system or for X, Y and E for semi-digital system
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In a digital gamma camera, what comes after the ADC?
Signal processor (microprocessor) that calculates energy and position and applied corrections and energy windows
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What are the advantages of a digital gamma camera?
More reliable electronics, possibility of using non-Anger arithmetic, simple to applied corrections (better uniformity), easy to change algorithms for developments
239
What are the limitations of digital gamma cameras?
Still poor energy resolution and intrinsic spatial resolution, Still need collimators with poor geometric resolution
240
If there is two images with the same matrix size but one has a higher zoom, which one has smaller pixel sizes?
The one with higher zoom (as it makes FOV smaller but same matrix size means smaller pixel sizes)
241
Why are the heads sometimes put at 76 degrees in myocardial perfusion scans?
In L-mode (90 degrees), the heart can sometimes be outside the FOV and be clipped on one of its sides
242
What is the AC map for fused images and are different ones needed for each radionuclide?
A graph to convert CT numbers (x-axis) to attenuation coefficient at the photopeak energy (y-axis) and different AC maps needed for each radionuclide
243
What is the effective atomic number of NaI(Tl) and CZT (same number)?
50
244
For CZT solid state gamma cameras, what do the gamma photons create when they get absorbed by the CZT crystal?
Electron-hole pairs that creates an electric signal
245
For solid state cameras, do you get direct position information and what does this mean?
Yes so no need for pulse arithmetic and no need for linearity correction
246
In NM, the IRMER practitioner is typically who?
The ARSAC certificate holder
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