Functional Imaging Flashcards
What are the types of functional imaging?
- PET
- fMRI
- SPECT
What is the application of functional imaging in RT?
- staging and decision making
- treatment plan optimisation
- measuring RT response
What are the problems with CT based anatomical imaging for staging?
- limited spatial resolution and sensitivity
- cant resolve metastatic disease or nodal involvement
What are the problems with CT based anatomical imaging for planning?
- CTV delineation and assumuptions of uniform radiosensitivity (assume each cell will respond the same)
- OAR contouring (functionality assumed to be uniform)
- plan optimisation
What are the problems with CT based anatomical imaging for treatment reponse?
- where was the dose actually delivered (hope matches plan but know not 100% accurate)
- response of disease and normal tissues both during and post treatment
What is functional imaging in context of oncology?
- mapping in 3D the distribution of a tumour, tissue or functional feature and provide information about the clinical repsonse of tumours of healthy tisues to ionising radiation
What MRI is being increasinlgy used for functional imaging?
- dynamic contrast-enhanced MRI and spectroscopy techniques
What are the commonents of SPECT?
- injection of y emitting radiotopes (Tc99m)
- gamma camera (functional imaging)
- CT (anatomical imaging)
What is a problem PET/CT?
- when decays emitts positrons compared to gamma emits for SPECT
- the positron moves mm before it interacks so there is a cloud of uncertainity from where they orginate
What are the two types of MRI imaging?
- T1: gross anatomy
- T2: biological pathology
What are the types of functinoal MRI?
- magnetic resonance spectroscopy imgaing
- dynamic contrast enhanced MRI
- diffusion-weighted MRI
- fast pulse sequence
- hyperpolarisation
How does nuclear medicine work?
- inject the patent with gamma emitting radiotracer
- detect where the radiotracer goes in the body
What does nuclear medicine measure?
- the distribution of a radionuclide in the body
- distribution of radionuclide or radiopharmaceutical should correlate with a biological process
- gamma camera
How does PET work?
- 18F decays emitting positrons
- positrons annihilate with electrons at the end of their tracks (1-2mm) creating two annihilation photons
- conservation of energy and momentum (photons energy 511 keV, 180 degrees to each other)
- detection of two photons in coincidence allows one to reconstuct a tomographic image
What is the coincidence measurement in PET?
- detection of two simultaneous events within a temporal time window (10 nanoseconds)
- assume annihilation took place along the line connecting the two events
- as annihilation doesn’t occur in the centre of body, gamma rays will have different time of detection
What is true coincidences in PET?
- 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
- true signcal that one wants to detect
What is the scattered radiation coincidence?
- 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
What is 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
What is the standard uptake value?
- calculate the SUV that are an index of tracer uptake that can be compared between subjects
What is the equation for SUV?
= TU/Q
- where TU (MBq/ml) is the tumour uptake from the image
- Q (MBq/g) is the injected dose per unit mass of the patient
What is the application of PET/CT for NSC lung cancer?
- staging changed when used PET/CT fused compared to CT alone
- shows nodal volumes a lot better
What are the lung scan using Krypton 81m gas?
- ventilation
- get pateitn to breath in gas and see how the gas goes into lung
What is the application of PET/CT for hypoxia and glucose metabolism?
- hypoxic cells have increased radio and chemo resitance
- FDG evaluates glucose metabolism
- FMISO (18F fluoromisonidazole) better for correlation for tumour hypoxia
What is dynamic contrast enhanced MRI?
- sequential imaging following the injections of a suitable para-magnetic contrast agent (Gadalidium based)
- do fast imaging to show the uptake then wash-out the contrast agent
- fast gradient echo sequences (decreases quality but interested in gunctionality)
What are T2 DCE-MRI good for?
- T2 images can be used to measure tissue perfusion and blood volume
What are T1 DCE-MRI good for?
- T1 used for extravascular extra-celluar space
What is diffusion weighted MRI?
- image contrast that correlates with difference in tissue-water mobility
- can infer the apparent diffusion coefficient from the measured signal
- 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
What is DW-MRI good for?
- measure treatment response
- treatment can cause change in Apparent Diffusion Coefficient (ADC) due to swelling, tumour lysis and necrosis
What are the commonly used MR nuclei?
- hydrogen
- phosphorus-31
- fluorine-19
- carbon-13
How do the MR nuclei work?
- they are present in tissue and concentrations depend on metabolism present in tissue
- there are pre-defined levels and thus measuring the concentration can see the strength of each element which shows abnormal tissue
How does magnetic resonance spectroscopic imaging work?
- the precession frequency for each of the nuclei has a small shift that depends on the local chemical environment
- MR spectrum contains peaks at different frequencies
- obtain an MR spectrum for eahc voxel
- the spectra will depend on the metabolic processes in the tissues
- changes in metabolism can be messaured (e.g. in tumour)
What is hyperpolarisation MRI?
- Nuclei are driven to a very high degree of polarisation increasing the MR
signal temporarily - He-3 and Xe-129 gas are used
- patient breathes gas then fast imaging to see where gas is going
What is the application of functional MRI for brain in RT?
- used to assist in GTV and CTV delineation
- OAR avoidance
- measurement of response to therapy through diffusion maps (if diffusion changes fro pre to post, then indicate reponse)
What is the application of functional MRI for lung in RT?
- fast image sequence give high spatial and temporal resoluation
- tumour mobility for dynamic imaging
- effects of motion on dosimetry using probability density function (dump dose where lung is not functioning)
- hyperpolarised He-3 determine lung function
- measurement of response to RT
What is the application of functional MRI for prostate in RT?
- MRI superior
- extent of diease and contouring
- multiple combinations of MRI sequences, T2, DCE, DW and MRS
- blood oxygen level dependent sequences for measuring hypoxia
What are the PET nuclides?
- 15O, 2 mins HL, cerebral blood flow
- 11C, 20 mins HL, tumour protein synthesis
- 13N, 10 mins HL, myocardial blood flow
- 18F, 110 mins HL, glucose metabolism
