PET Flashcards
what is Nuclear Medicine
medical speciality which applies unsealed radioactive substances administered in the form of radiopharmaceutical, for the diagnosis and treatment of diseases
non invasive imaging aimed at capturing functional and metabolic images of the target body tissue
how is it administered
a small dose of a radioisotope is administered to the patient in the form of a radiopharmaceutical or tracer, which is designed to enter the cells of the target organ
radioactive decay
produces the electromagnet radiation we use for imaging
decay is spontaneous process aimed at achieving stability in the atom
decay can result in the emission of energy in the form of electromagnetic radiation or the emission of particles
half life
the length of time that it takes for an element to decay to half its activity
what is the half life for F18 and Ga68
F18 = 110 minutes
Ga68 = 68 minutes
SPECT alone pitfalls
- low resolution leading to decreased reporting confidence
- acquisition times long —> motion becomes a problem
- lack of anatomical landmarks
how are x-rays and y-rays produced & their difference
- they are both electromagnetic radiation, however, one originates from the nucleus and other from the orbital electrons
- X-ray = when an electron transitions from an excited state back to ground state, energy is released in the form of an electromagnetic radiation
- Y-ray = when a nucleon transitions from excited state back to ground state, energy is released in the form of an electromagnetic radiation
what is PET
- positron emission tomography
- provides metabolic and functional imaging
- employs the use of short lived positron emitting isotopes
- uses annihilation coincidence detection
positron decay
- the proton in the nucleus is transformed into a neutron and a positive charged electron (positron)
- the positively charged electron and a neutrino are ejected from the nucleus
SPECT imaging
camera rotates around the patient recording multiple images that are then reconstructed into a 3D data set
SPECT CT
combines the low resolution of SPECT with high resolution of CT, low dose CT is applied usually around 20mAs and 80-100kV
why is SPECT CT important to RT
- CT images for SPECT are typically LDCT and their purpose is for attenuation correction, image co-registration for the purpose of pathology localisation
- because they are co-registered to the pathological or metabolic region of interest the CT can be imported into RT planning systems
what is ALARA
as low as reasonably achievable
ALARA - for patients
- we keep their activity levels administered both from radiopharmaceutical administrations and those from CT as low as practical
- this means giving considerations to prior imaging and how to best achieve an accurate and interpretable image for the radiologist
ALARA - for staffs and carers
- we ensure that the patients meet regulation guidelines prior to release to public
- we ensure that we educate
how do we achieve ALARA
- distance (maintaining around 100cm is a good “safe” distance to communicate from when required
- time (reduce time in close contact)
- shielding
positrons
is the antiparticle of an ordinary electron
positron decay
a proton in a nucleus is transformed into a neutron and a positively charged electron or a positron
the positively charged electron and a neutrino are ejected from the nucleus
motivation for PET/CT
the need to be able to identify areas of increased radiopharmaceutical (metabolic information) with patient-specific anatomy under identical “conditions”
pitfalls of PET/CT
- differences in breathing patterns
- CT - Breath hold
- PET - tidal breathing
- Mis-registration
- high density contrast agents (eg barium) and metallic objects (eg pacemaker, hip replacement)
- overestimation of FDG activity if CT data is used for attenuation correction
- 511keV vs CT energies
standardised uptake value (SUV)
most commonly used quantitative analytical parameter in clinical practice
allows comparison of 18F-FDG uptake to be made between the target tissues and normal tissues
ROI compared to liver - 1.5 to 2.5 SUV
basic patient prep for 18F-FDG
- 6 hour fast
- nothing to eat or drink other than plain water with no additives
- refrain from strenuous exercise 24-48 hours prior to scan
- diabetics must discuss current therapy and special requirements with the department to ensure insulin levels are correct
rigid registration
LDCT from PET session fused to the simulation/planning CT
spatial transformation is then applied to the respective PET
deformable registration
allows for registration beyond translation and rotation
ultimately, the deformable algorithms ‘warp’ the PET data set to match the reference image (small vectors used maximise the similarities between data sets)
pitfalls FDG imaging
treatment adaptation - modification to improve clinical outcome
- FDG - poor at early response to assessment
morphologica imaging also slow to demonstrate response
high and low uptake of 18F-FDG for the brain
high - cortical and cerebellar grey matter, nuclei and thalamus
low - white matter and ventricular system
pros for PET in RT
provides info for GTV delineation
able to diagnose residual malignant mass or recurrent tumour
Very safe
allergic reactions are rare
cons for PET in RT
ionising radiation
false results if chemical balances within the body are not normal (diabetic)
may not detect small cancers (<7mm)
requires high capital cost and ongoing cost
Standard Biodistribution of
18F-FDG (all patients)
Liver > Spleen
Mediastinum (blood vessels)
Kidneys, Ureters and Bladder
Muscles and Bone Marrow
Standard Biodistribution of
18F-FDG (variable in patients)
heart
stomach and intestines
uterus and ovaries
thymus
secretory mammary gland
normal oesophagus
extraocular muscles
Ga68 half life and clinical application
68 minutes
neuroendocrine tumours
F18 half life and clinical application
110 minutes
glioma
congenital hyperinsulinism
parkinsons disease
neuroendocrine tumours
How is Ga68 used in prostate cancer?
Ga68 measures the PSMA for prostate cancer which give a better indication of tumour progression and location compared to PSA test
