Nuclear Medicine Flashcards
(118 cards)
1
Q
What is nuclear medicine?
A
The administration of internal quantities of radioactive material for diagnosis and therapy.
2
Q
What is the difference between nuclear medicine diagnosis and therapy?
A
In nuclear medicine diagnosis the emitted radiation from the administered radiopharmaceutical is observed from outside the body, whereas, in therapies it the radiopharmaceutical is delivered to a target within the body for energy depostion.
3
Q
What is the typical energy range of a diagnostic radiopharmaceutical?
A
kBq - MBq
4
Q
What is the typical energy range of a therapeutic radiopharmaceutical?
A
MBq - GBq
5
Q
State the 7 types of nuclear medicine
A
- Planar/static
- Dynamic/gated
- Whole body
- SPECT (single photon emission tomography)
- PET (positron emission tomography)
- Non-imaging
- Therapy
6
Q
Are nuclear medicine scans typically quantitative or qualitative?
A
Qualitative
7
Q
Are nuclear medicine scans functional or anatomical?
A
Functional
8
Q
State 3 common forms of multimodality imaging in nuclear medicine
A
SPECT/CT
PET/CT
PET/MR
9
Q
What is the purpose of multimodality imaging?
A
To combine both anatomical and functional information to improve image quality.
10
Q
Describe an ideal diagnostic radionuclide
A
One with the ability to produce a good image quality in a timely manner (< 20 minutes) with minimal radiation dose.
11
Q
Describe an ideal therapeutic radionuclide
A
One that targets radiation dose to a biological pathways and supplies minimal radiation protection issues for others.
12
Q
What considerations have to be made when choosing a radionuclide?
A
- Half life (long enough to transport and use but not so long that it causes excess dose after useful period)
- Emissions (suitable energy for detection without excessive dose)
- Cost
- Ease of manufacture
- Toxicity
- Chemistry (can it bind to pharamaceuticals and remain stable)
13
Q
Which radionuclide is most commonly used in diagnostic nuclear medicine? Why?
A
Technitium-99m (the metastable version of Tc):
- Mostly emits 140.5 keV gamma rays
- Can be produced ‘on site’ from a Mo-99 generator
- 6 hour half-life so almost completely decayed within 24hrs
- Can be easily transported locally
14
Q
State 4 commonly used therapeutic radionuclides
A
I-131
Y-90
Lu-177
Ra-223
15
Q
Why do therapeutic radionuclides typically emit ‘particles’ (e.g. alpha or beta emissions)?
A
- They have a short range in tissue
- The deposit energy locally
16
Q
Are therapeutic half-lives long or short?
A
Long (several days)
17
Q
What is a gamma camera (scintillation camera)?
A
A detector that measures the location and energy of incoming gamma rays within a 40-50cm FOV. It processes one event at a time (single photon emission computed tomography) with a very high sensitivity.
18
Q
What is the sensitivity of a gamma camera?
A
Hundreds of counts per MBq
19
Q
Gamma cameras have _______ heads that can rotate around an __________ relative to one another, giving different ________ of __________.
A
Multiple
Isocentre
Angles
Acquisition
20
Q
Gamma cameras have a ________ couch with a ___ attenuation that can move during __________.
A
Moveable
Low
Acquisition
21
Q
What component of a gamma camera detects radiation?
A
The ‘high Z’ scintillation crystals
22
Q
What are the scintillation crystals used in gamma cameras typically made of?
A
NaI doped with TI
23
Q
How do scintillation crystals detect radiation?
A
The energy from gamma rays is deposited in the crystal, which releases a secondary electron. This electron interacts with other electrons to cause ionisation in other atoms. Some of the electrons move to doping sites, drop into valence spaces, and release visible light photons which are detected after passing through a photomultiplier tube (PMT).
24
Q
How does image quality change as scintillation crystals get thicker?
A
The gamma camera is more sensitive but has less intrinsic resolution.
25
What is a photomultiplier tube (PMT)?
A device that uses the principle of electron cascades to convert light detection from a scintillation crystal into a measurable signal.
26
How can each individual photon be precisely located despite the large size of the PMTs in a gamma camera?
The gamma camera uses Anger logic based on the understanding that each gamma ray produces thousands of light photons within the crystal, so each event is detected by multiple PMTs. Each PMT sees a different amount of light depending on the distance from the event, allowing the interaction point to be estimated with mm accuracy.
27
What is a collimator?
A device that filters out gamma rays that are not perpendicular to the scintillator crystal in a gamma camera in order to provide directional information and improve the point spread function, thus improving image quality.
28
What are the 4 typical types of collimator?
29
What are the pros and cons of using a collimator?
+ Improves imaging performance
+ Improves PSF (giving the image directional information)
- Drastic reduction in sensitivity
- Small angle of acceptance
30
What are the walls of a collimator typically made of?
Lead
31
Why do gamma cameras require several different collimators?
To account for different gamma energies and resolution requirements. Typically, there are low-, medium-, and high-energy collimators and high resolution and all-purpose collimators.
32
Give the equation for collimator resolution
R_c = collimator resolution
d = collimator hole diameter
L = collimator length
D = distance from collimator face to source
33
How does collimator hole diameter impact image resolution?
Increased diameter = worse resolution
34
How does collimator length impact image resolution?
Increased length = better resolution
35
How does the distance from collimator face to the source impact image resolution?
Increased distance = worse resolution
36
Why should a patient be placed as close to the gamma camera head as possible?
Because resolution decreases with distance from the collimator.
37
Give the equation for the overall spatial resolution of a gamma camera
R_s = spatial resolution
R_i = intrinsic resolution
R_e = extrinsic (collimator) resolution
38
What determines the intrinsic resolution of a gamma camera?
Light spread (Anger logic)
39
What determines the extrinsic resolution of a gamma camera?
Collimator design
40
Is the sensitivity of a gamma camera dependent on the distance from the source? Why?
No. For parallel-hole collimators, gamma camera sensitivity remains roughly constant with distance because the decreased photon acceptance per hole is balanced by more holes being able to detect the source.
41
True or false: gamma camera sensitivity is only dependent on collimator geometry.
True
42
What are the two types of solid state gamma cameras?
1) CsI crystal and silicon photodiode: the detector directly captures visible light, meaning that no PMT or Anger logic is required.
2) CZT semiconductor: the detector directly converts the gamma ray energy into charge without a light conversion phase.
43
State the pros and cons of solid state gamma cameras
+ Increased energy resolution (increased signal per event)
+ Higher count rates
+ More compact
+ Intrinsic resolution is only limited by element size
- Cost
- Fixed collimation
44
How is the digital image (pixel) size set for a gamma camera?
As gamma cameras use a continuous crystal, the data has to be digitally binned into user-defined matrix sizes. This allows users to 'zoom' into images (at the cost of signal-to-noise ratio).
45
State 3 ways that scan length can be determined for a gamma camera
1) Time (typically minutes)
2) Counts (typically millions of counts)
3) Heartbeats (when linked to ECG)
46
When is it useful to measure scan length in terms of time?
When uptake is unknown
47
When is it useful to measure scan length in terms of counts?
When uptake is expected and good statistics are required
48
When is it useful to measure scan length in terms of heartbeats?
When it is necessary to 'bin' counts into parts of the heart cycle to create dynamic images
49
How can scatter impact gamma camera image quality?
It can reduce image contrast by increasing noise (if it still manages to pass through the collimator after scattering)
50
How can scatter events be removed from a gamma camera?
Gamma emissions are monoenergetic when produced, hence, events (like scatter events) with a lower energy than expected can be removed.
51
How can the allowed energy window of a gamma camera be determined?
By measuring the FWHM of the photopeak and excluding any events outside this window.
52
True or false: only one energy window can be set per scan for a gamma camera.
FALSE: multiple energy windows can be utilised, which is especially useful if imaging a radionuclide with multiple photopeak or when imaging multiple radionuclides simultaneously.
53
Why is it important to optimise an energy window?
Too narrow: reduced sensitivity
Too wide: inclusion of scattered photons
54
Why is it important to measure the amount of radionuclide administered to a patient?
- Image quality depends on the number of detected emissions, which depends of the quantity of radioactivity
- Radiation dose depends on the number of emissions
- Quantification of dose requires knowledge of administered radioactivity
55
How can the quantity of a radionuclide be measured?
Using a radionuclide calibrator
56
How does a radionuclide calibrator work?
The source emits radiation, causing ionisations in the calibration chamber. This applies a potential difference to the chamber, resulting in a measureable current which is amplified and corrected for the properties of the specific radionuclide. Finally, the radioactivity of the source is displayed in units of radioactivity.
57
State 3 causes of non-uniformity in nuclear medicine
- Crystal construction
- PMT gain
- Non-linearity in anger logic
58
What are the 3 types of correction that gamma cameras use?
1) Energy correction
2) Linearity correction
3) Uniformity correction
59
What is gamma camera energy correction?
Energy correction ensures that the camera interprets the detected gamma photon’s energy correctly, even if the detector's response varies across the field.
60
Why do gamma cameras need energy correction?
PMT output varies across the detector, particularly for events that happen in between the PMTs; this variation needs to be corrected for to produce an accurate image.
61
What is gamma camera linearity correction?
Linearity correction is a calibration process used to correct for spatial distortions in the gamma camera image, ensuring that the positions of detected events match their true physical locations.
62
Why do gamma cameras need linearity correction?
To ensure that the positions of detected events match their true positions.
63
What is gamma camera uniformity correction?
Uniformity correction is a calibration process that ensures the gamma camera produces a uniform image when imaging a uniform source, correcting for variations in detector response across the field of view.
64
Why do gamma cameras need uniformity correction?
To identify any non-uniform responses in a gamma camera and correct them to prevent artefacts in patient imaging.
65
State 2 common ways to carry out uniformity correction
1) Using a flat/flood source of either Tc-99m or Co-57
2) Using a point source with no collimator, at a distance 5.5x the FOV, and with an applied curvature correction
66
What are the 2 types of uniformity?
- Integral uniformity
- Differential uniformity
67
What FOVs are used to test uniformity?
1) Central FOV (central portion of image)
2) Useful FOV (entire image including edges)
68
Why is it important to test the centre of rotation of a gamma camera?
Image reconstruction assumes that all views are projected equally back to the isocentre, however, gamma camera heads are heavy so there may be mechanical strain that offsets the centre of rotation.
69
How can the centre of rotation of a gamma camera be tested?
By imaging a point source and measuring the deviation
70
State 6 QA tests that are carried out on a gamma camera other than uniformity, linearity, COR, and energy
1) Spatial resolution (intrinsic and extrinsic)
2) Count rate response
3) Sensitivity
4) Energy resolution
5) Multiple window registration
6) SPECT-CT registration
71
What does SPECT stand for?
Single
Photon
Emission
Computed
Tomography
72
What is SPECT?
A nuclear medicine imaging technique in which a gamma camera detects, processes and outputs a single photon at a time whilst the CT collects dose information from a spectrum. The information collected from several projections is then reconstructed as a 3D image using the resulting volumetric data.
73
What are the pros and cons of SPECT imaging?
+ 3D localisation of a source
+ Improved quantification
+ Background removal
- Requires physical capabilities of the gamma camera
- Requires very good uniformity
- Requires stable pharmaceutical distribution throughout scan
- Requires a good centre of rotation
74
What are the two rotation types for SPECT imaging?
- Circular (fixed diameter)
- Contoured (camera moves close to patient at all angles)
75
What are the pros and cons of circular rotation for SPECT?
+ Easy to set up
- Constant but poor resolution
76
What are the pros and cons of contoured rotation for SPECT?
+ Better resolution
- More complex
- Needs a way to identify where the patient is (typically a light beam and touch sensitive pads)
77
What are the two configurations for SPECT acquisition?
H-mode and L-mode
78
What are the two types of image reconstruction techniques used in SPECT?
- Iterative reconstruction
- Filtered back-projection (FBP)
79
What is an attenuation correction map?
A 3D dataset used in nuclear medicine imaging (especially SPECT and PET) to correct for the loss of signal caused by photon attenuation as gamma rays pass through the body.
80
What are the pros and cons of SPECT/CT?
+ Localisation of radionuclide
- Increased patient dose
- Extra scan time
81
What are planar nuclear medicine scans?
Planar (2D) imaging, analogous to general X-ray imaging, in which the detector and patient remain still.
82
When are planar nuclear medicine scans useful?
When specific anatomy is of interest (as the camera has a limited FOV) and when there is little concern about background uptake or overlying structures.
83
Planar nuclear medicine scans are not useful for _______ radiopharmaceutical distributions.
Dynamic
84
Give an example of a planar nuclear medicine scan
Thyroid scans to visualise thyroid function
85
What is wholebody nuclear medicine imaging?
Planar imaging of the entirety of a patient, in which the detector is stationary but the couch moves between the two detector heads.
86
When are wholebody nuclear medicine scans useful?
To see radionuclide distribution throughout the entirety of a patient (i.e. if looking for an unknown cause of symptoms).
87
Give an example of a wholebody nuclear medicine scan
A bone scan to look for abnormal bone growth (e.g. metastases)
88
What is a dynamic nuclear medicine scan?
A scan that is taken to view a rapidly changing biodistribution, acquired by taking a series of short planar scans in quick succession.
89
Give an example of a dynamic nuclear medicine scan
- Renogram to see uptake and clearance in the kidneys
- Gastric emptying
90
When is SPECT imaging useful?
To see the distribution of a radiopharmaceutical which is complex within an organ (i.e. variations would be missed in planar studies). This can be accompanied by a CT scan to acquire volumetric distribution information.
91
Give an example of a SPECT scan in nuclear medicine
Blood flow within the brain
92
What is non-imaging in nuclear medicine?
Diagnostic procedures that use radiopharmaceuticals but do not produce images. Instead, they involve measuring radioactivity in a sample, organ, or region to assess physiological function or uptake quantitatively.
93
Give an example of a non-imaging procedure in nuclear medicine
Glomerular filtration rate (a measurement of how well the kidneys are functioning)
94
What is therapeutic nuclear medicine?
Nuclear medicine procedures that use a higher-dose radionuclude (normally a beta or alpha emitter) to treat certain diseases.
95
Give an example of a therapeutic nuclear medicine procedure
Treatment of neuroendocrine tumours by administration of Lu-177, emitting beta particles which destroy cancerous tissue
96
How are neuroendocrine tumours diagnosed?
Using a PET scan
97
What is positron emission tomography?
A subtype of nuclear medicine that uses radionuclides that emit positrons, which annihilate with electrons to give back to back 511 keV gamma rays. This requires a different scanner design to a gamma camera.
98
What radionuclide is typically used for PET scanning? Why?
FDG: it is a positron emitter and has a short half-life of 110 minutes
99
Describe the process of a PET scan
1) A positron-emitting radionuclide/radiopharmaceutical is administered to a patient
2) Positrons travel a few mm post-decay
3) The positrons collide with an electron and form positronium, which decays into two back-to-back 511 keV gamma rays
4) The gamma rays are detected by opposing crystals within the detector ring
5) An energy window is used to confirm both gamma rays are 511 keV
6) If detection happens within a small time window (few ns) then they are assumed to have come from the same decay event
100
Describe the structure of a PET scanner detector ring
- Made of BGO, LYSO, or LSO crystals as they have a good stopping power
- Crystals are etched to provide light paths that show the PMTs where each event occurred
- The crystals are assembled in a ring
- There is no collimator, making the scanner more sensitive
101
What is time of flight?
A PET reconstruction algorithm that considers the subtle difference in arrival times of two gamma rays to determine approximately where an annihilation event happened. This improves image contrast.
102
What are the 3 possible lines of response following a PET annihilation event?
103
What is a standardised uptake value
A semi-quantitative measure used in PET imaging to estimate how much radiotracer is taken up by a specific tissue relative to the injected dose and the patient's body characteristics. It depends of how much concentration was injected, when it was injected, and patient weight.
104
Give the equation for standardised uptake value (SUV)
SUV = standardised uptake value
Conc = the activity concentration in the tissue/voxel of interest (kBq/ml)
A_DC = decay-corrected injected dose of radiotracer (kBq or MBq)
W = patient's body weight (kg)
105
What are the 3 types of SUV?
SUV_mean: the mean value of voxels within an ROI
SUV_max: the largest single voxel within an ROI
SUV_peak: the average of N voxels surrounding the largest SUV voxel
106
Give the equation for total dose administered
Total dose = dose rate x time
107
Dose rate is approximately equal to ____ and ______ of emissions.
Rate
Energy
108
Why is the total dose from internal radiation hard to calculate?
Biological pathways have different uptake and clearance rates and different organs receive different doses.
109
__________ dose is absorbed dose weighted for radiation type.
Equivalent
110
Give the equation for effective dose
111
Define S-factor
A quantity used to calculate the mean absorbed dose in a target organ (t) from a given activity of a radioisotope distributed within a source organ (s). This factor is normalised to activity.
112
Define specific absorbed fraction (SAF)
The fraction of emitted energy which is deposited in a target organ.
113
What is cumulated activity?
The total number of emissions from an organ during the lifetime of a radioactive material, found by calculating the acrea under a time-activity curve.
114
Give the equation for cumulated activity
115
What two factors impact internal radioactivity?
1) Radioactive decay
2) Biological pathways (uptake, clearance, equilibrium)
116
Radioactive material remains in the body until it is ________ or _______.
Excreted
Decays
117
Define target organ dose
The sum of dose from source organs.
118
How is target organ dose calculated?
By calculating the dose to all target organs, correcting for weighting factors, and summing for effective dose.
