Nuclear Medicine & Functional Imaging Flashcards
(41 cards)
1
Q
What are the 2 major ways that nuclear medicine differs from other imaging techniques?
A
- it provides information about the physiology of particular organs or tissues rather than the anatomy of a region
- it uses an internal source of radiation that has been administered intravenously or orally, rather than an external source of radiation
2
Q
In general, how is a nuclear medicine scan performed?
A
- a radioactive substance attached to a metabolically active molecule (a radiopharmaceutical) is adminstered
- it is usually administered IV, but can also be given orally, inhaled or injected directly into an organ
- the radiation emitted from the patient is detected
- it exposes the patient to radiation, but often less than from a CT abdomen
- clinicians must hold a special licence (ARSAC) to prescribe radiopharmaceuticals
3
Q
What is a radiopharmaceutical?
A
radiopharmaceutical = carrier molecule + radioactive atom (radionuclide)
the carrier molecule is metabolically active
4
Q
What is a radionuclide?
A
radionuclides are unstable atoms
- isotopes are elements with differing neutron numbers
- atoms that have excess neutrons or deficient neutrons are “unstable” nuclei that become more “stable” by “decaying”
5
Q
What are the 3 different forms of “decay” that can occur to make an atom become more stable?
A
- “beta minus” decay
- “beta plus” decay
- gamma decay
6
Q
What is “beta minus” decay?
A
- this occurs when there are excess neutrons
- the excess neutrons are converted to a proton and an electron
- electrons are emitted as a “beta particle”
7
Q
What is meant by “beta plus” decay?
A
- this occurs when there are excess protons
- excess protons are converted to neutrons, which releases “positrons” by the process of positron emission
- positrons are electrons with a positive charge
- the positron meets an electron and is “annihilated”, which releases 2 gamma rays** that travel in **opposite directions from each other through body tissues
8
Q
What is meant by gamma decay?
What substance is this associated with?
A
- this is associated with nuclear medicine techniques that use technetium-99
- technetium-99 undergoes gamma decay, which involves nuclei transitioning to a more stable state and emitting a gamma ray
9
Q
Why is technetium-99m an ideal radionuclide for medical imaging?
A
- it has a half life of 6 hours
- this is the time required for the quantity to reduce by half its initial value
- this limits the amount of radiation exposure to the patient while allowing sufficient time to carry out procedures
- it is cheap and easy to produce
- it binds easily to pharmaceuticals
10
Q
What type of decay does technetium-99m undergo?
How is this utilised in medical imaging?
A
- it is a pure gamma emitter, which is less damaging to cells than B particles (harmful radiation)
- gamma rays have sufficient energy to pass through the patient to the detector, wheter they are converted to light
- the light is converted to an electrical signal
- the brightness depends on the number of gamma rays emitted
11
Q
What organs / tissues can technetium-99m be used to image?
A
- it can be attached to a wide range of pharmaceuticals that target different organs and tissues
- bone
- cerebrum
- myocardium
- lymph nodes
- renal system
- thyroid gland
- liver
- spleen
- bone marrow
12
Q
How is bone scanning (scintigraphy) performed?
Why might this be performed?
A
- administration of methylene diphosphonate labelled with technetium-99m followed by imaging 3-4 hours later
- this detects areas of bone that are metabolically abnormal
- the phosphate is taken up by osteoblast cells that produce bone
- increased uptake of Tc-99-MDP produces “hot spots” due to malignancy, trauma, infection, etc.
- you can distinguish between normal physiological uptake, high uptake (hotspots)** and **low uptake areas (coldspots)
13
Q
What is involved in sentinel node imaging?
A
- technetium-99m is attached to sulphur colloid particles and injected into a tumour
- the particles migrate through the lymphatic system to the nearest lymph node where they are phagocytosed and retained
- this allows identification of the sentinel lymph nodes which can aid the staging of cancer and planning of future surgery
14
Q
What is involved in gated cardiac blood pool imaging?
What is an alternative name for this and when is it performed?
A
- also referred to as multi-gated acquisition imaging (MUGA)
- the patient’s RBCs are labelled with technetium-99m, which enter the circulation
- images of the patient’s heart are taken in sync with the cardiac cycle
- this allows for assessment of ventricular function
15
Q
What is the purpose of V/Q scanning?
What is involved in this procedure?
A
- it is used to assess the blood flow and ventilation through the lungs, most commonly to look for PE
- the patient inspires diethylenetriamine pentaacetic acid (DTPA) labelled with xenon or technetium-99m
- they are given an IV injection of macroaggregated albumin labelled with technetium-99m
16
Q
How do PET scans work?
A
- it uses a radioactive tracer and generates 3D images - this is FDG (F-18 labelled deoxyglucose) radioisotope
- PET utilises beta plus decay, which produces positrons
- positrons react with electrons in the body and annihiliate each other
- this releases a small amount of energy and 2 gamma rays shoot off in opposite directions
- detectors in the PET scanner measure these gamma rays and use this information to create images of internal organs
17
Q
How are images created from gamma rays in the PET scan?
Why is the spatial resolution high?
A
- a ring containing multiple gamma cameras is used to detect gamma rays that interact with cameras at 180o to each other simultaneously
- this allows the source / location of the annihilation to be pinpointed
- as the positron travelled <2mm before annhiliation, the spatial resolution is high
18
Q
What are the applications of PET scanning?
What is the downside of this form of imaging?
A
- diagnosis and monitoring of cancer
- assessing response of malignancy to treatment
- detecting metastases
- functional imaging in neurodegenerative and psychiatric pathologies
- cardiac imaging
- it is expensive and the tracers have a short half life
19
Q
How is PET scanning used in cardiac imaging?
A
- damaged myocardial cells utilise glucose rather than fatty acids and therefore show up as “hotspots”
- PET scans can be used to assess myocardial viability and whether bypass or transplant surgery is appropriate
20
Q
What type of scan is shown here and how is it produced?
A
PET-CT
- the PET/CT scanner is a combination instrument and the patient remains in the same scanner for both PET and CT scans
- fusion of the images gives functional information + anatomical detail
21
Q
In a PET-CT scan, after CT is obtained, how is the PET scan then produced?
A
- patient is given an IV injection of FDG radioisotope (F-18 labelled deoxyglucose)
- FDG is a glucose analogue so its uptake in the body tissues is proportional to glucose metabolism
- FDG becomes concentrated in regions of high metabolic activity, producing “hotspots” e.g. malignancy, inflammation
- gamma rays are emitted in opposite directions and are detected by cameras
- the time difference between each ray being detected by each camera is measured and used to pinpoint where the annihilation reaction took place, allowing an image to be constructed
22
Q
What is FDG the best available tracer for?
A
- FDG is the best available tracer for detecting cancer and metastatic spread in the body
23
Q
A
24
Q
What does PET-CT improve accuracy of compared to using CT or PET alone?
A
- PET-CT is used in cancer staging
- it improves the accuracy of cancer staging than using PET or CT alone
25
What are the 2 major clinical applications of PET-CT?
* role in **_cancer staging_** by demonstrating **distant metastases**
* imaging and assessing **_primary malignancies_**, especially of the **bone**
* it is also good for the early assessment of **tumour response to chemotherapy** and **suspected recurrence of malignant disease**
26
Other than oncology, what are some other clinical applications of PET-CT?
* role in **myocardial assessment**
* developing role in **neuroimaging**, for both organic and psychiatric illness
* developing role in **infectious and inflammatory diseases**
27
What are the 5 relative contraindications with PET-CT?
* patients need to **lie still for up to 90 mins**
* relatively **contraindicated in pregnancy** due to high dose of radiation (20 mSv)
* interpretation can be difficult if the patient has **uncontrolled DM**
* **misregistration** is an issue if the **patient moves between scan components**
* need a letter to go through airport security within 7 days of injection
28
What are some other problems associated with PET-CT?
* patients must **fast for at least 4-6 hours** before the scan
* they must then **sit quietly** (no talking) for an hour whilst the radionuclide circulates
* patients are considered **radioactive sources** after administration of radionuclides:
1. must be kept in isolation in hospital
2. human waste is treated as radioactive
3. patients must avoid prolonged contact with children and pregnant women after leaving hospital
29
When might PET-MRI be used?
* this combines **_PET with MRI_** and is only available at specialist centres
* it has the **same principles as PET-CT** but with a **lower radiation dose**
* *it costs about 50% more than PET-CT and takes longer*
* it is excellent for **_brain imaging_** (tumours, focal cortical dysplasia, TLE) and has **excellent soft tissue detail**
* different tracers can be used (**11C-methionine, 68Ga-DOTA-TATE**)
30
What is SPECT?
How does it work?
**single photon emission computed tomography**
* a **radioactive tracer** is introduced into the patient
* the tracer is usually **_technetium-99m_**, but iodine, xenon, thallium and fluorine can be used
* tracers emit **_gamma rays_** (light that moves at a different wavelength to visible light)
* the gamma rays are detected by **gamma cameras** that **rotate around a patient** and they must lie still
* images are taken from different angles and 3D images are computer generated
31
What are the benefits of SPECT over PET?
* it is relatively cheap
* the tracers have a longer half life than those used for PET
32
What are the main applications of SPECT?
* it allows imaging of **_blood flow_** as the tracer **remains in the vasculature** and is **not taken up by the tissues**
* it is used in the diagnosis and monitoring of **_heart disease_** (myocardial ischaemia)
* it has roles in research and **functional brain imaging**
33
How is SPECT used for imaging myocardial perfusion?
* the **radionuclide** is taken up by **_mitochondria in cardiac cells_**
* the uptake of radionuclide is **_proportional to blood flow_**
* images are obtained **at rest** and **during activity** (e.g. stress test by exercise or giving adenosine)
* comparison of the images allows **ischaemic or infarcted regions** of the heart to be identified
34
How can SPECT be used in brain imaging?
* it can be used to evaluate **cerebral blood flow** using **_Tc-99m attached to exametazime_** which **passes through the BBB** and is **metabolised by neuronal cells**
* blood flow should be **_symmetrical_**
* **altered blood flow patterns** can indicate **_epilepsy, infarction_** or **_schizophrenia_**
* **_bilateral decrease_** in signal indicates **Alzheimer's disease**
35
36
What are the benefits of using PET and SPECT for diagnosing dementia?
* PET has better sensitivity and specificity for determining if patients had dementia
* SPECT has slightly better performance for identifying dementia with Lewy bodies
37
How has functional imaging begun to be used in neuropsychiatric conditions?
* **functional MRI, PET and SPECT** are used to study **brain activity** in several disorders in affected v. healthy control subjects:
1. Alzheimer's disease
2. schizophrenia
3. OCD
4. depression
* it has had a huge contribution to understanding the **structural and functional neuroanatomy** of these conditions and **efficacy of treatments**
38
What changes occur in the brain in OCD?
What is the neurodegenerative theory?
* several neurotransmitter pathways are implicated, including:
1. ***5HT***
2. ***dopamine***
3. ***GABA***
4. ***glutamate***
* the neurodegenerative theory suggests that possible **neuronal loss in inhibitory pathways** leads to **_functional hyperactivity in the cortico-limbic loop_**
39
What can be seen on imaging, CT & MRI and PET & SPECT scans in OCD?
**_CT & MRI:_**
* **reduced size** of the **caudate nucleus** (role in procedural learning)
**_fMRI, PET & SPECT:_**
* **increased activity** in the basal ganglia (particularly the **caudate**), anterior cingulate and orbitofrontal cortex
40
What is seen on scans in major depressive disorder (MDD)?
* structural and functional abnormalities in the limbic and prefrontal regions (emotional processing)
* volume reductions in the anterior cingulate cortex (ACC) and amygdala
* increased activity in the ACC in response to negative images
* abnormal and increased amygdala response to negative emotional stimuli
* reduced volume in the right dorsolateral prefrontal cortex (DLPFC) (cognitive ability)
* reduced hippocampal volume
41
What abnormal changes can be seen in schizophrenia?
**_MRI:_**
* abnormal grey matter density increases in the basal ganglia
* decreases in bilateral frontal, cingulate, temporal and insula cortices and thalamus
**_fMRI:_**
* abnormal activation of prefrontal, anterior cingulate and posterior cortical regions
