Nuclear Medicine & Functional Imaging Flashcards
What are the 2 major ways that nuclear medicine differs from other imaging techniques?
- 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
In general, how is a nuclear medicine scan performed?
- 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
What is a radiopharmaceutical?
radiopharmaceutical = carrier molecule + radioactive atom (radionuclide)
the carrier molecule is metabolically active
What is a radionuclide?
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”
What are the 3 different forms of “decay” that can occur to make an atom become more stable?
- “beta minus” decay
- “beta plus” decay
- gamma decay
What is “beta minus” decay?
- 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”
What is meant by “beta plus” decay?
- 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
What is meant by gamma decay?
What substance is this associated with?
- 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
Why is technetium-99m an ideal radionuclide for medical imaging?
- 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
What type of decay does technetium-99m undergo?
How is this utilised in medical imaging?
- 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
What organs / tissues can technetium-99m be used to image?
- 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
How is bone scanning (scintigraphy) performed?
Why might this be performed?
- 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)
What is involved in sentinel node imaging?
- 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
What is involved in gated cardiac blood pool imaging?
What is an alternative name for this and when is it performed?
- 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
What is the purpose of V/Q scanning?
What is involved in this procedure?
- 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
How do PET scans work?
- 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
How are images created from gamma rays in the PET scan?
Why is the spatial resolution high?
- 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
What type of scan is shown here and how is it produced?
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
What is FDG the best available tracer for?
- FDG is the best available tracer for detecting cancer and metastatic spread in the body
What does PET-CT improve accuracy of compared to using CT or PET alone?
- PET-CT is used in cancer staging
- it improves the accuracy of cancer staging than using PET or CT alone
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
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
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
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
