w10 Flashcards
Anatomical Imaging methods vs Molecular Imaging methods
Anatomical Imaging: x ray, CT, MRI
Molecular Imaging: PET, SPECT, functional MRI w/ BOLD
describe the system input and output data in PET imaging
input: arterial plasma time activity curve. acquired from arterial blood input function.
output: PET measurement tissue time activity curve. acquired from image reconstruction over time.
what are isotopes
Same element, different mass number (different neutrons)
what is the fundamental reason for radioactive decay
If a nucleus is proton or neutron deficient, then it undergoes a transformation to a more stable state and releases radioactive decay in the process
how is radioactive decay characterised
Half-life of decay
Particle, γ (gamma) and X-ray emissions
N0 = the initial amount of active substance / Exponential decay function
what is the unit of unit of radioactive activity and what is its meaning
Becquerel (Bq)
One Bq is defined as one transformation (or decay or disintegration) per second
For a fixed mass of radioactive material, how does the radioactive activity change with time
exponential decay function
what is a cyclotron and why is it relevant to medical imaging
Cyclotrons accelerate charged particles using a high-frequency, alternating voltage (potential difference)
used to make radioactive tracers for PET
What PET tracers are used for
Glucose metabolism Bone metabolism Prostate cancer Neuroendocrine Tumors Alzheimer's Disease Brain epilepsy Myocardial perfusion Blood flow/perfusion Oxygen consumption
18F-FDG 18F-Fluoride 68Ga-PSMA 68Ga-Dotatate 11C-PIB 11C-Flumazenil 13N-Ammonia 15O-Water 15O-O2
how are tracers produced for SPECT
Parent tracer decays to daughter tracer using a generator (check table for SPECT tracers)
what is the main imaging device used in nuclear medicine
The gamma (Anger) camera
What is a gamma ray
A gamma ray is a packet of electromagnetic energy (photon) emitted by the nucleus of some radionuclides following radioactive decay
how are gamma rays converted to electrical pulses
photomultiplier tube
used for light detection of very weak signals, is a photoemissive device in which the absorption of a photon results in the emission of an electron
what is Scatter and Collimation in SPECT
Scatter: increases the uncertainty of the origin of emitted photons
Collimation: blurs already poorer spatial resolution ??? directs light to a straight line / makes light parallel
What are the specific energy windows to detect the following gamma rays
123 I
201 Tl
99m Tc
159 keV
70 keV
140 keV
this means you can simultaneously detect multiple tracers using multiple energy window
what are the different types of Collimator
hexagonal holes
square holes
triangular holes
What are the advantages and disadvantages of SPECT
Low resolution and high noise
Low cost and wide-spread
Simultaneous measurements
A large number of available tracers for diverse purposes
describe the process of annihilation in PET
PET tracer will emit positrons due to decay in the body
Annihilation is the process that occurs when a subatomic particle collides with its respective antiparticle, in PET: positron+ with electron-
what is the energy of a positron / electron
511 keV
what is Coincidence Detection in PET
The simultaneous detection of 2 opposite photons within a specified time window is referred to as a coincidence
A line connecting the 2 coincidence detectors is called a Line of Reponses (LOR)
what are the two ways of calculating tissue time activity curves (Parameter Estimation)
ROI-derived - tissue time activity curve (TTAC) less noisy
Voxel-wise - doesn’t require prior knowledge on how to define ROI
what are compartmental models (what is the connection???)
describes a system as a series of compartments.
are widely employed to solve a broad spectrum of
physiologic and clinical problems related to the distribution of materials in living systems
what are the basic characteristics of compartments
Conservation of mass
Finite number of variables can describe the system
mass balance principle
what are some variables that can be used to describe compartments
compartment mass qi
transfer rate from the jth compartment kji
transfer rate to the jth compartment kij.
Influx rate from outside the system, kio
Excretion rate into outside the system, koi
describe properties of transfer rates (kij)
kij is constant, does not depend upon any Qi
kij is described by a saturated relationship: Michaelis-Menten equation
controlled by the arrival compartment: Langmuir relationship
describe the two different types of transfer rates
Nonlinear model: transport rate is nonlinear for most physiological systems e.g. facilitated diffusion and receptor binding systems.
Linear model: Advantage: mathematical properties of the linear model. If the dynamic system is in steady state and is time-invariant over the study duration, the administration of a small amount of tracer will not disturb the steady state. Therefore, the linear fractional transfer rate can be assumed.
in what situation is it suitable to abstract (combine) compartments
As the transport of FDG across the cell membrane is considered fast compared to the transport across the capillary membrane and the phosphorylation reaction, the interstitial and cellular compartments for FDG can be combined into a single compartment.
what are the assumptions for the Kinetic Model of FDG
All the rates are constant (rate constant).
The tissue compartment is homogeneous.
Arterial plasma concentrations (plasma time-activity curve from blood sampling) are equal to their capillary plasma concentration;
FDG and glucose are distributed in a single compartment
The tissue extraction fraction is small compared to whole system => linear model.
what are the three compartments in the Kinetic Model for FDG
FDG in plasma;
FDG in tissue;
FDG-6-PO4 in tissue
what are the variables in the Kinetic Model for FDG
Cp(t), Ce(t), Cm(t), Ct(t) = Ce(t) + Cm(t)
K1 : rate constant of FDG forward transcapillary membrane transport
k2 : rate constant of FDG reverse transcapillary membrane transport
k3 : rate constant of FDG phosphorylation
k4 : rate constant of FDG-6-PO4 dephosphorylation. If rate of constant, K1, is 0.1/min, it means 10% of FDG in plasma is transported into tissue per min
what is the functional parameter related to glucose metabolism
CMR Glc: cerebral metabolic rate of glucose consumption (check equations)
what is the format of a tissue time activity curve (TTAC)
formatted as list of frames
For one frame (tmin, tduration, y) there are three variables: mid-time, sampling duration, and activity
mid times and sampling durations make up the sample schedule
what is the proccess for simulating a TTAC
Load the plasma time activity curve
Specify K1, k2, k3, k4 for white and grey matter
Define sample schedule for TTAC
Derive Ct(t) respectively for each sample interval using an equation (Check notes)
The intrinsic characteristics of functional imaging lead to high level of noise, i.e. low signal to noise ratio (SNR).
Computer simulations should take noise into account. what are two methods of estimating the noise distribution
Projection data: Poisson distribution
Reconstruction data: Gaussian distribution
describe the kinetic model for Glucose Metabolism
Tracer: 18F-FDG
Functional parameter is CMR Glc or CMRglu
three compartments: FDG in plasma; FDG in tissue; FDG-6-PO4 in tissue
k4 is assumed to be ignored i.e. the dephosphorylation
rate of FDG-6-PO4 is 0
describe the kinetic model for blood flow
Tracer: 15O-H2O
Two-compartment and two-parameter model (plasma and tissue)
Functional parameter: K1
what is a receptor
a protein on the cell membrane (or within the cytoplasm or cell nucleus) that binds to a specific molecule (a ligand), such as a neurotransmitter, hormone, or
other substance, and initiates the cellular response to the ligand.
describe the compartment model for receptor binding
Cp is the concentration of ligand in plasma.
Cf is the free concentration of ligand.
Cns is the concentration of nonspecifically bound ligands.
Cs is the tissue concentration of specifically bound ligands
OR Cf+ns is the concentration of free and non-specifically bound ligand
Functional parameter: BP = k3/k4
the compartment models for receptor binding can be four or three compartments, why would you use the three compartment model
The three-tissue model is a priori only nonuniquely identifiable i.e. there are multiple solutions for each parameter.
To ensure unique identifiability, assume the exchange rates between the free tissue and nonspecific binding pools are sufficiently rapid.
