Nuclear Medicine Flashcards
What is nuclear medicine?
- Imaging modality focusing on the use of radiopharmaceuticals
- Diagnosis and Treatment
- Based on physiologic function of organs or tissues
function vs anatomy
The diagnostic information obtained from imaging the distribution of radiopharmaceuticals is functional
PET function vs. anatomy
- Limited anatomic Information
- Less spatial resolution
- High sensitivity and specificity for molecular processes
MRI function vs. anatomy
- Excellent soft tissue contrast
- High spatial resolution
- Poor sensitivity for molecular processes
what are radiopharmaceuticals
- Referred to as radiotracer/tracer
Introduced into the body: - Injection (intravenous, intradermal, intrathecal)
- Inhalation
how do radiopharmaceuticals work?
- Selected based on ability to localize in organs/tissues
- Undergo radioactive decay to emit gamma rays
- Emissions allow for the detection of the tracer’s presence
NM images
- Scintillation detectors detect gamma emissions
- Emissions are transformed into images
- Lowest amount of radiotracer used to reduce exposure without compromising image quality
nuclear medicine professionals
- Radiologist
- Nuclear Medicine Technologist
- Physicist
- Radiochemist
radiopharmacy
- Naturally occurring radionuclides have very long half-lives
- Limited availability
- High absorbed patient dose
- Radionuclides produced in particle accelerators (cyclotrons) and nuclear generators
radionuclide definition
radioactive material used to tag the pharmaceutical
pharmaceutical definition
biologically active compound chosen because of its preferential localization in or function of an organ
radionuclides
- 11C
- 13N
- 15O
- These elements are the predominant constituents of natural compounds found in the body
- Can replace their stable isotopes in substrates, metabolites, drugs and other biologically active compounds
common radionuclides
- Commonly used radionuclide, 18F (fluorine), can replace hydroxyl group in many molecules
- Achieved without disrupting any bodily biochemical processes and mechanisms
- Emits positrons
isotope production
- Radioisotopes and radiotracers need to be produced on the same day as the scan
- Radiochemist produces radioisotope with cyclotron
- Step 2 involves attaching the radioisotope to a biomolecule
radiopharmaceutical properties
- Easy to produce
- Readily available
- Low cost
- Easily administered
- Concentrate in specific organs or tissues
- Have an activity level that is high enough that only small amounts are needed
- Has the requisite half-life
- Emit gamma radiation suitable for detection by gamma cameras (or positrons suitable for PET scanning)
- Needs to be sterile (regulated as drugs) to be injectable
Isotope half life?
- Less stable radionuclide = short half life
- Exponential decay
what are the most common isotope half life?
11C: 20 min
18F: 110 min
What are the two modes of imaging in nuclear medicine?
- Positron emission tomography - PET scanner
- Single photon emission imaging - gamma camera
what is positron emission tomography
anti electron radioactivity emitted from an unstable nucleus inside the subject section imaging
steps of PET
- isotope production
- chemistry, purification and formulation
- imaging and analysis
how is signal produced in PET?
radionuclide decays into a positron and an isotope
annihilation radiation
- when a positron combines with an electron
- two 511 keV photons and goes straight out from collision
what is coincidence detection?
detection of pairs of 511 keV photons by 3D ring of PET detectors
18F
fluorodeoxyglucose (FDG)
- hydroxyl group (OH) has been replaced with radioactive fluorine (18F) to form FDG
- malignant tumors tend to preferentially metabolize glucose
- FDG can be used to identify sites of malignancy
FDG
- measures glucose metabolism
- clinically about 95% of PET scans
- gold standard for oncological imaging
- brain mapping
radio[pharmaceutial physiological processes
- receptor/ligand based
- enzyme/substrate based
both get “stuck” at the cell
pet sensitivity
- fraction of the annihilation pairs that are detected
- also affected by the number and type of detector
true coincidences
- both annihilation photons escape without interaction with tissue and strike detectors
random coincidences
- two annihilation photons from separate emissions strike detectors at the same time
- falsely get attributed to a different original event
scatter coincidences
- one or both annihilation photons get scattered in tissue
PET Scanner Blocks
- basic compartment of a PET scanner
- composed of a scintillation crystal coupled with 4 PMTs
- organized in a row to form detector modules - aligned side by side to form a ring
what is LoR?
Line of response - line between the 2 annihilations photons
detector rings
- to increase z-axis coverage, several rings are combined
- 800-1000 detectors per ring
signal detection and processing
- scintillation crystals detect gamma rays
- converts gamma rays into low energy visible photons
- photosensor (PMT) used to convert light to an electrical signal
scintillation crystal of PET
LSO:Ce
- high mass density and atomic number
- effective at stopping photons and has a high efficiency of light output
collimators PET
- located at the face of the detector, where photons first enter
- filters gamma rays not perpendicular to the detector and removes scatter
- made of material with high Z, like lead
how is collimator sensitivity determined?
- determined by the fraction of photons transmitted through the collimator and strike the detector
- depending on the magnification level and sensitivity required, different collimators are used for different exams
PET detector module measures what 3 aspects of the signal?
- the time is takes the gamma rays to hit the detector
- position
- energy
signal detection and processing
- coincidence unit recognizes the true events generated by the same annihilation event occurring along a specific LoR
image reconstruction
- filtered back projection
- iterative reconstruction
oncology imaging
- Majority of PET scans are done to diagnose, stage, or restage cancer
- 18F-FDG is the radiopharmaceutical of choice
- PET plays an important role in differentiating benign/malignant processes
- Effectively monitor therapeutic interventions by non-invasively assessing the metabolic response of tissues
PET scan colour expression
- shades of grey (hot spot darker)
- colour tables (hot spot red)
- linear colour tables (hot spot light)
SPECT
- single photon emission computer tomography
- single photon (gamma ray) is emitted as opposed to positrons
Which radionuclide is most used in SPECT?
99m Tc (technetium)
what is technetium?
- produced in nuclear generators
- parent - 99 Mo (half life of 66.7 hours)
- daughter - 99Tc (half life of 6 hours)
- isomeric transition yields a gamma photon (140 keV)
Technetium as a radiopharmaceutical - tagging
- technetium can be labeled onto a variety of pharmaceuticals
- tagged to macro aggregated albumin: lung scan
- tagged to methyl-diphosphate: bone scans for fractures
99mTc MAA
- after IV injection, this substance follows the pathway of blood flow to the lungs, where it is trapped in capillaries
- blood clots prevent distribution beyond the clot
- images show a cold spot
SPECT use
- most common for cardiac perfusion, brain, liver, ,tutor and bone studies
- detectors rotate 360 around the patient
- image data can be reconstructed in multiple planes and 3D
- any gamma emitting radionuclides
Gamma Camera
- AKA scintillation or Anger camera
- used to image game radiation emitting radioisotopes (scintigraphy)
- original was on detector that rotated around patient
- today systems may have 2 or 3 detector heads - dual head is most common
Static/spot view
- snapshot of radiopharmaceutical distribution in the body
- obtained in various orientations around a structure
- low radiopharmaceutical activity levels to minimize patient dose
- images must be acquired for a pre-set amount of time or a minimum umber of counts or radioactive emissions
whole body scanning
- sweep images allow the patient and detector to move with respect to each other and produce whole body images
- dual-head camera designed to acquire simultaneous anterior and posterior acquisitions
- used primarily for metastatic tumors, bones or infection imaging
dynamic imaging
- acquisition of multiple sequential images at defined intervals or over time
- sequential images are collected and aligned together to show function over time
- a dynamic or “flow” study of a structure is generally used to evaluate blood perfusion in the tissue
what are the hybrid imaging systems
SPECT/CT
PET/CT
hybrid imaging systems
- provide increased diagnostic accuracy
- misregulation possible due to difference in patient positioning
- hardware approach developed where both SPECT and PET scanners were combined with CT scanners
- Simultaneously acquire PET/SPECT functional images and CT anatomic images
- Both modalities are co-registered or exactly matched in size and position
Hybrid imaging
- position of suspected tumors can be recognized more easily
- suspicious metabolically active areas can be identified anatomically
- metabolic and anatomic evaluation after therapy can now be accomplished in one imaging session
Radioimmunotherapy
- Antibodies specifically designed to localize on the surface of different types of cancer cells can be tagged with a radioisotope and imaged
- If the antibody successfully localizes on the tumor site, the radioisotope may be replaced with a beta-emitting therapeutic radioisotope
- Current studies are looking to treat non-Hodgkin lymphoma with beta-emitting therapeutic radioisotopes
Radiation safety
- differ from safety measures used in general radiography
- radionuclides continuously emit radiation
- TLD badges and rings to monitor exposure to the body and hands
- Designated prep area due to high activity called hot labs
- Isolated ventilation, protective lead and lead glass shielding for vials and syringes, and gloves
Patient prep and post care
- Minimal, with most studies requiring no special prep
- All metal objects inside or outside clothing must be removed because they may attenuate photons
- The waiting time between dose administration and imaging varies
- After completion, patient may generally resume normal activity
What are the different collimators used in nuclear medicine and how do they affect the image?
parallel - normal
diverging - minimize image
converging - magnify image
pin hole - magnify image and increase spatial resolution
