PET Flashcards
What is nuclear medicine?
- applies unsealed radioactive substances, administered in the form of radiopharmaceuticals
Used for the diagnosis and treatment of disease
Non-invasive imaging aimed at capturing functional and metabolic images of the target body tissue.
What is the process of Nuclear Medicine
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.
The radioisotope decays to emit energy in the form of
gamma radiation that can be detected by the gamma
camera or PET/CT Scanner.
Scanned data represents the distribution and uptake of the tracer in the body
What is radioactive decay?
◦ 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.
Unit of radioactive decay
The unit of radioactivity is the Becquerel (Bq)
One Bq. corresponds to 1 decay per second
What is the half-life
Half-life is the length of time that it takes for an element to decay to half its activity.
When a nucleon is transitioning from excited state back to ground state, energy is released in the form of…
electromagnetic radiation - Υ ray (gamma ray)
When an electron transitions from the excited state back to the ground state, energy is released in the form of…
electromagnetic radiation - X rays
What is SPECT Imaging
Camera rotates around the patient recording multiple images that are then reconstructed into a 3-D data set
Cross sectional imaging
Disadvantages of sole SPECT imaging
◦ Low resolution leads to decreased reporting confidence
◦ Acquisition times long, motion a problem
◦ Lack of anatomical landmarks
What is the benefit of SPECT CT
Combines the low resolution of SPECT with High Resolution of CT
Provides specificity to the image interpretation
Allows for image attenuation correction, localisation and registration
How is SPECT CT beneficial to RT
CT images for SPECT are typically LDCT’s and their purpose is for attenuation
correction, image co-registration for the purpose of pathology localisation
Once the CT is imported and registered to the RT planning data the nuclear
medicine metabolic data can then be accurately used to assist with
treatment planning
How is SPECT CT beneficial 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
How to maintain radiation safety
Time - reduce time spent surrounding the patient
Distance - maintain distance between the patient and yourself (distance greater than 1.0m shown to be effective when communicating)
attempt to schedule around other procedures especially ultrasound which does require close contact
PET overview
◦ provides metabolic and functional imaging
◦ Uses short lived positron emitting isotopes
◦ uses annihilation coincidence detection
How are short-lived positron emitting radiopharmaceuticals created?
Cyclotron - stores isotropes used to make radiopharmaceuticals
A cyclotron is a type of particle accelerator which repeatedly propels a beam of charged particles (protons) in a circular path
What is positron decay
Positron decay is a way for an atom with too many protons to achieve a more relaxed state
P + -> n + e + + neutrino + energy
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
What is a positron?
A positron is the antiparticle of an ordinary electron
What happens to a positron once ejected from a nucleus?
After ejection from nucleus it loses kinetic energy in collisions with atoms in surrounding matter and comes to rest within millimetres of the site of origin (10 -9sec)
The positron then combines with an ordinary electron in an annihilation reaction
In which its mass and that of the ordinary electron are converted into energy
This energy appears in the form of two 0.511MeV annihilation photons
PET/CT relation to annihilation photons
PET uses annihilation coincidence detection to create the image
This is based on two 511kev photons are emitted in opposite directions following the annihilation of a positron and an ordinary electron
Why is attenuation correction important in PET/CT
Photons from deep in the tissue are absorbed –> therefore need to correct for depth
Where can the attenuation coefficient be applied in PET/CT
Where the tissue structure is relatively uniform’ e.g. the head, a standard attenuation value can be used uniformly across the structure
What was the motivation for PET/CT
The motivation for the development of PET/CT was the need to be able to identify areas of increased
radiopharmaceutical (metabolic information) with patient-specific anatomy under identical “conditions”.
How can malignant tumours be visualised on a PET/CT
Increased glycolysis is a distinct marker of malignant tumours compared to normal tissue
Typical biodistribution of FDG throughout the body
The Brain
High Uptake - Cortical and cerebellar grey matter, nuclei, and thalamus
Low/No Uptake - White matter and ventricular system
Body
Liver > Spleen
Mediastinum (blood vessels)
Kidneys, Ureters and Bladder
Muscles and Bone Marrow
What is the standardised uptake value
SUV is a relatively simple, reproducible and well-established
index for quantifying glucose utilisation by measuring the
activity of tracer in the lesion or organ corrected for the
patient’s weight and dose of FDG injected.
provide an objective assessment of therapy response on serial PET scans
Patient preparation for FDG imaging
- 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 discus current therapy and special requirements with the department to
insure insulin levels are correct
Why is lorazepam sometimes utilised for PET scan
- Reduces muscle activation within
the neck and relieves tension. Also used for patients
How can brown fat affect PET imaging
- Mitochondria-packed brown fat cells burn energy and rapidly produce heat
- 18F-FDG uptake in brown fat is a potential confounder in the interpretation of PET images, because the presence of brown fat may obscure or lead to erroneous diagnosis of lesions
Beta blockers can be given an hour to scan
What is brown fat
Brown fat (brown adipose tissue) is a type of body fat that regulates your body temperature in cold conditions
How to ensure excellent cardiac suppression on PET
- 48 hours prior to imaging we ask the patient to follow a high fat, low carb diet -> ensurs heart is using fatty acids and glucose as an energy source when FDG is administered
- Heparin may also be used 15 minutes prior to injection to stop non-specific myocardial uptake by increasing free fatty acid availability
Steps used with tumours located around/within the heart
How does crystal size in the PET detector correlate to spatial resolution and detectability
Reducing crystal size –> improve spatial resolution and detectability
What are the benefits of Biograph Vision Quadra PET CT scanner
Decreases the time of scan which will allow us to decrease the dose administered to the patient significantly to achieve reductions in ionising radiation
FOV -> 106 cm
What is a PET/MRI
combination of the metabolic information from a PET scan with the superior soft tissue and morphologic imaging offered by MRI
What is rigid-coregistration?
LDCT from PET session is fuse to the simulation/planning CT
Spatial transformation is then applied to the respective PET (translation or rotation)
What is deformable co-registration
- Allows for registration beyond translation and rotation
- deformable algorithms “warp” the PET data set to match the reference image (QA is needed when applying to check PET and LDCT perfectly aligned)
Disadvantages of Hybrid Imaging
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 correctionq
Where do meningiomas arise from
Meningiomas arise from the dura of the brain / spinal cord
Effects of Meningioma and epidemiology
Are the most common intracranial tumour – accounting for
almost 30% of all primary intracranial tumours (higher incidence in female 3:2)
Progressive enlargement of the tumour and compression of
the neural tissue lead to clinical manifestations such as seizure disorders, focal neurological deficits and neuropsychological decline
Precision Medicine
tumours are heterogenous -> radiopharmaceuticals are chosen by facotring in the biology, physiology and molecular profiles
Disadvantages of FDG usage
Tumour must have upregulation of the Glut1 receptor to be
detected
- poor at early response to assessment
What gene does meningiomas overexpress
Meningomas demonstrate overexpression of SSR2
Because of this, Ga68 DOTATATE which acts as a SSTR2
analogue has been found to be of value
What is Ga 68 DOTATATE
- Ga 68 Dotatate is routinely used for diagnosis and staging of well differentiated neuroendocrine tumours..
- It provides functional imaging by acting as a somatostatin analogue
- DOTATATE binds to somatostatin receptors (SSTR) and in particular, binds to SSTR sub-type
Benefits of GA68 DOTATATE use for Meningiomas
Ga68 DOTATATE PET/CT has been shown to assist with
◦ target volume delineation
◦ Stereotactic surgical planning
◦ Diagnosis of small meningiomas
◦ Monitoring tumour growth rate
◦ Evaluation in recurrence in the presence of scar formation (MRI non-
specific due to contrast enhancement of scar tissue)
Ga68 DOTATATE PET/CT is far superior to CT or MRI when there is low
lesion contrast (infiltrative lesions) or the imaging is prone to artefact
such as from base of skull
This study demonstrated that Ga-DOTATATE PET has
potential to improve planning contours with more precise
target delineation and as a consequence reduce treatment
toxicity for patients (OAR)
What radiopharmaceuticals are used in prostate cancer
GA68 PSMA
F18 PSMA
What is Ga68 PSMA
GA68 PSMA is the most commonly used
agent for prostate cancer imaging
We use this to image tumours with
increased prostate-specific-membrane-
antigen (PSMA)
What is a PSMA
PSMA is a transmembrane protein primarily
present in all prostatic tissues
Which malignancies have increased expression of PSMA
◦ Renal Cell Carcinoma
◦ Colorectal
◦ Prostate malignancy
◦ high PSMA expression on
the angiogenic neovasculature of solid
tumors (GBM)
◦ Adenoid Cystic Carcinoma
When should GA68 PSMA be considered
Primary staging in high-risk disease before surgical procedures or planning external beam radiation
◦ Gleason score >7, PSA > 20ng/mL, clinical stage T2c-3a)
In this setting when the likelihood of lymph node or bony
disease is increased PSMA PET has shown to be superior to
CT, MRI or Bone Scans alone.
What analogs should be utilised when PET imaging a glioblastoma
PET using Amino Analogs have been proposed as a way to
provide prognosis and assess for tumour progression
Epidemiology of Glioblastoma
GBM account for 70% of gliomas
1000 Australians affected annually
Morbidity is devastating both physically and cognitively
Benefits of PET Amino Acid Analog Imaging
Amino acid (AA) PET is promising in brain tumours
◦ The AA tracers show promise in distinguishing disease recurrence from pseudoprogression (an increase in the size of the primary tumor or the appearance of a new lesion followed by tumor regression)
AA transport is increased in malignant transformations due to cell
proliferation
◦ High tumour uptake and low background
◦ AA uptake may be present in non-enhancing sites of tumour
◦ AA are beneficial in demonstrating the site and extent of tumour
◦ May guide stereotactic biopsy or radiotherapy
F18 and Ga68 half life and energy
511 keV
F18 - 110 min
Ga68 - 68 min
Motion in PET effect on RT
Blurring in PET images can be used to RT advantage to define the GTV
Application of F-FDOPA
Large neutral amino acids analogue that detects:
1. Glioblastoma
2. Parkinson’s disease
3. Congenital hyper insulinism
Use of FMISO and FDOPA in RT
Use of radiosensitiers such as stereotactic boost to hypoxic regions
