Functional And Molecular Imaging Flashcards
What is functional imaging
Mapping in 3D the distribution of tumour, tissue or functional feature
Provide information about the clinical response of tumours or healthy tissues to ionising radiation
Usually means NUCLEAR MEDICINE: PET/SPECT
Types of functional imaging
PET/CT
SPECT/CT
MRI
Problems with CT based anatomical imaging
CT has a limited spatial resolution and sensitivity - cannot visual true extent of tumour
Often can’t resolve Mets or nodal involvement
Assumes uniform radiosensitivity
SPECT CT
Injection of gamma emitting radioisotopes
Uses a gamma camera
TC99 @stable state
PET/CT
Different radioactive decay than SPECT -> positron decay
MRI sequence types
T1 images - gross anatomy
T2 images - biological pathology
MRSI - Magnetic resonance spectroscopy imaging
DCE-MRI - Dynamic contrast enhanced MRI
DWI- Diffusion weighted mRI
Fast pulse sequences
Hyperpolarisation
What does nuclear medicine involve
Measure the distribution of a
radionuclide in the body
● Distribution of radionuclide or
radiopharmaceutical should
correlate with a biological
process
● Gamma camera
Half life
Need a half life that is not too long - otherwise patient will be radioactive for too long
Not too short- otherwise it can’t be tracked in imaging
Technetium - 99 is 6hr half life - the most appropriate
PET advantages
Compared to other nuclear med techniques:
Spatial resolution and quantification is better
PET problems
Coincidence measurement: scattered radiation and random coincidence
True coincidence
Coincidences simultaneously detected on both
detectors resulting from the same annihilation of a
positron and corresponding to the 511 keV energy
photons not having undergone any scatter
Scattered radiation
Photons from the same
annihilation
– Due to scattering the
assumption that the
annihilation took place on a
line joining the two detection
points is incorrect
– Energy of scattered photon
will be lower than 511 keV
Random coincidences
Photons emitted by different
annihilations but detected in
the same time window
– Background noise
– Reduced by lowering
coincidence time window
– Axial filters or septa
How to reduce random coincidences
Reducing time window
Using filters and collimator (however it may impact true coincidences)
SUV
Standard uptake value: index of tracer uptake that can be compared between subjects
Tu is the tumour uptake from the image
Q is the injected dose per unit mass of the patient
Cyclotron
Accelerate protons -> throw at targets -> creates radioistopes
Tracer needs to be produced on site or close to PET centre - 6 hour travel time would require an initial activity of 2.8GBq
Clinical applications of PET/CT
NSCLC
Nodal disease
Ventilation and perfusion: highlight blood flow and lung function
Hypoxia and glucose metabolism
– Hypoxic cells in tumours have an increased radioresistance (3x) and
chemo-resistance
F18 and FMISO can evaluate glucose metabolism and correlation for tumour hypoxia
However hypoxia is not the sole cause of increase in glucose metabolism observed in cancer cells *specific to tumour type
Breast cancer - bone + supraclav lns mets
Dynamic Contrast Enhanced MRI (DCE-MRI)
dynamic contrast enhanced MRI
Sequential imaging following injection of a suitable para-magnetic contrast agent (Gd based)
Image the uptake and wash out of contrast agents
Can measure tissue perfusion and blood volume (T2) as well as extra vascular extra cellular space (T1)
Diffusion Weighted MRI (DW-MRI)
Movement of water molecules in tissue is restricted as motion is limited by interactions with cell membranes and macro molecules
Tumours with high cellularity the motion of water is more restricted
Can be used to measure treatment response - can cause change in ADC due to cell swelling and necrosis
Magnetic Resonance Spectroscopic Imaging (MRSI)
magnetic resonance spectroscopy imaging
Measures concentrations of hydrogen, phosphorus, fluorine and carbon - the image shows the distribution of these metabolites across the image
Measures changes in metabolism which indicate presence of ontological abnormalities
Dynamic imaging
Fast sequences
Used for lung - to see motion of tumour
T2-weighted scans have long TR and are slower than
gradient echo T1-weighted sequences
● Single shot techniques can be used
○ Echo planar imaging - sub-second images
Hyperpolarisation
can be used for lung function measurement - patient inhales hyperpolarised gas
Sensitivity of mRI can be increased using Hyperpolarisation
Nuclei are driven to a very high degree of polarisation, increasing the MR signal temporarily
This is not used clinically
Clinical sites- Functional MRI
Brain
Lung
Prostae
Role of contrast in fMRI
DCE-MRI images
Better contrast between soft tissue
Edge detection for tumours
challenges of fMRI
Accessibility
Contraindications to contrast
Extensive safety protocols
Brain - fMRI
used frequently to assist in GTV and CTV delineation in planning
Measurement of response to therapy - diffusion maps
Can do correlation between diffusion coefficient pre-post treatment
Lung - fMRI
fast imaging sequences can give high spatial and temporal resolution (10 images per sec)
Can check tumour mobility through dynamic imaging
Hyperpolarised to determine lung function - avoidance of functioning lung during planning
Measurement of response to radiation
Effects of motion on dosimetry using probability density function (PDF)
Prostate - fMRI
MRI is superior
Extent of disease and contouring
Mix sequences: T2, DCE, DW and MRS for cancer detection and mapping
BOLD: blood oxygen level dependent sequences for measuring hypoxia
PET radionuclides half life
15O - 2 min - FOR BRAIN
11C - 20 MIN - FOR BRAIN
13N - 10MIN - For protein
18F - 110min - glucose metabolism
SPECT CT problems
- photo electric effect stops the signal from reaching the detector
- scattered photons do not pass through collimator due to angle: leads to signal loss
- compton scattered photon reaches detector but degrades quality of image
Why use in RT
Decision making and staging
Treatment plan optimisation
Measuring treatment response
