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Vocabulary

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

A

AIC, Akaike Information Criterion

Increasing the complexity of the model (adding more parameters) improves the fit to the data, although the improvement may not be statistically significant. As the reliability of individual parameter estimates generally degrades with increasing number of model compartments, the model producing the smallest AIC with given data set should be selected.

Angular compression

Technique used to reduce the amount of data in a sinogram. This technique has been shown to have little effect near the center of the field of view. However, it could cause loss of precision. When angular compression is applied, data in the sinogram are summed in the angular direction. You can apply angular compression when the sinogram is acquired, and/or when it is reconstructed.

Antagonist

A chemical compound whose physiological effect is the opposite of the effect of the original molecule.

Argument

Anything that follows a command. These are typically options and parameters to pass to a program, as well as filenames. In the program user help, the obligatory arguments are shown inside '< >' characters and optional arguments inside '[ ]' characters. You must never write these characters to the command line.

B

Batch file

Batch file (script) is a collection of commands in a file. It's a way to run many commands in a batch mode. It is recommended that you write a batch file (or script in Solaris) to analyse all PET studies; then it is easier to analyze all studies again, if needed. And when script is saved with your data and results, it is also a good documentation on what you actually have done to your data.

Becquerel

The SI unit of radioactivity. 1 Bq is equal to 1 disintegration/second. Calibration units in TPC.

Binding

Binding is the class of processes that consist of two molecular species combining, usually non-covalently, to form a single complex. Examples are hormones and neurotransmitters binding to their receptors on the cell surface and fatty acids binding to albumin. PET can quantitate the rates of physiological or biochemical processes in vivo. There are three types of dynamic biological processes: translocation, transformation, and binding. Binding is actually a special case of transformation, but as a widespread biological phenomenon it deserves a separate category.

Binding potential

Binding potential (BP) is the ratio of B_max (receptor density) to K_D (radioligand equilibrium dissociation constant), as defined by Mintun et al. Ann Neurol. 1984; 15: 217-227.
BP = B_max / K_D = receptor density x affinity.
In vitro at tracer doses, BP equals the ratio of specifically bound ligand to its free concentration.

B_max

B_max is the total density (concentration) of receptors in a sample of tissue. To refer to the concentration of available (free) receptors, terms Bavail or B'max can be used.

C

Catenary model

Catenary model is a one-dimensional chain of compartments, each connected only to the immediate right and immediate left, i.e. compartments are in series. Name comes from the Latin word for chain. Mixed mammillary/catenary models are commonly used in nuclear medicine.

Compartment

Compartment is a chemical species in a physical place (in the sense used by modellers); so neither glucose or interstitial space is a compartment, but glucose in interstitial space is one. Inside a compartment the tracer is considered to be distributed uniformly.

Compartmental model

Compartmental model assumes that injected isotope exists in the body in a fixed number of physical or chemical states (compartments) with specified interconnections among them; the arrows indicate the possible pathways the tracer can follow. Physiologically the flux of material represents transport from one location to another or a chemical transformation or both. Compartmental models can be described in terms of a set of linear, first-order, constant-coefficient, ordinary differential equations (ODEs). By convention, in the nuclear medicine literature, the first compartment is the blood or plasma pool.

CT

Computerized tomography (CT) is a computer-assisted medical imaging technique that generates visual cross-sectional images by exposing a subject to an x-ray beam that rotates around the subject and then recording the attenuation of those beams that pass through the body.

Curie

A unit of radioactivity equal to 3.7 x 10¹⁰ nuclear decays per second. Calibration units in TPC.

D

DFT

Data Format (in) Turku, time-activity curve format used in TPC.

Distribution volume

(Total) distribution volume (DV or volume of distribution, V_T) is defined as the ratio of the tracer concentration in tissue to that in plasma at equilibrium. Therefore, a V_T equal to 15 means that the tracer is being concentrated in tissue by 15:1.

Dynamic scan

Scan acquired over multiple (time) frames.

E

Effect size

Effect size is a way of quantifying the effectiveness of a particular intervention, relative to some comparison. It is the standardized mean difference between the two groups: Effect size = (Mean of experimental group - Mean of control group) / Standard deviation. In studies where there is a large control group, its SD should be used in calculation of effect size. If there is not a true control group, or the control group is small, it is better to use a pooled estimate of SD: SDpooled = sqrt(((NE-1)*SDE2 + (NC-1)*SDC2 ) / (NE+NC-2)).

Emission

The release of radioactivity from a radioactive source.

Event

A generic term referring to the PET scanner's detection of a gamma ray. There are four types of events: prompt, delayed (or random), true, and multiple.

F

f₁

Free fraction of radioligand or drug in plasma; see f_P.

f₂

Free tissue fraction; see f_ND.

Fitting

Fitting or parameter estimation: Finding a set of parameters that with measured input function will produce simulated output which is as close as possible to a given set of actual PET measurements. The criterion for goodness of fit is the weighted least squares function. An optimization algorithm is used for iteratively moving from one set of parameters to a better set until progress is stalled or until a fixed maximum number of iterations has passed. If the criterion function has multiple local minima, the iterative search may end up at any one of these. If no constraints are imposed on the parameters, the minimum could correspond to a physically unrealizable set of parameters, e.g. negative rate constants or vascular volume fraction greater than 1.

f_ND

Tissue free fraction (f_ND or f₂) is the fraction of radioligand or drug that is freely dissolved in tissue water. It is expressed relative to the non-displaceable tissue concentration (C_ND):
C_FT=f_ND·C_ND
In a two-tissue compartment model f_ND affects both k₂ and k₃.

f_P

Free fraction of radioligand or drug in plasma (f_P or f₁) is the fraction of of the ligand that is not bound to plasma proteins at equilibrium. Thus, the concentration of radioligand freely diffusible in plasma water and available for transport into tissue can be calculated as
C_FP=f_P·C_P

Frame

Division of the data matrix that indicates where the image is located in time.

ftp

File transfer protocol used to copy files over network. For example Filezilla can be used to copy files from/to the ftp server.

Functional imaging

Techniques for obtaining maps that represent physiological and metabolic processes performed by the organs of the body.

G

Gate

Division of the data matrix that indicates with which group of synchronized frames the image is associated. Often used in heart studies, to minimize the effect of the heart-beat motion.

Gjedde-Patlak plot

Gjedde-Patlak plot (or Patlak plot) is a multiple-time graphical analysis for irreversible tracers, e.g. [¹⁸F]FDG. It produces net influx rate (K_i). Either arterial plasma or reference region curve can be used as model input.

Graphical analysis

Graphical plots are compartment model independent methods, which deliver less detailed information, but are often less prone to errors. A regression line is fitted to the data after certain integral transformations. The most frequently used graphical methods are the Gjedde-Patlak plot for irreversible tracers and the Logan plot for reversible tracers.

Grey matter

Portions of the nervous system with a grey colour, which mainly contains the bodies of nerve cells. See also white matter.

H

Half-life

The time during which the radioactivity decreases by one-half.

Head curve

The count rate values sampled at a specific interval (usually 1 s) for the scan duration.

I

Input function

Input function (delivery function) describes the concentration of the authentic (original) compound in arterial plasma as a function of time.

ICC

Intraclass correlation coefficient (ICC) evaluates the within-subject variablity relative to the between-subject variability. ICC ranges between -1 (no reliability) and 1 (maximum reliability).

Intranet

Internal network, also used in TPC.

J

K

K_D

K_D is the (radioligand) equilibrium dissociation constant. Affinity of ligand binding is the inverse of K_D.

L

Linearity

In compartmental models, from the linearity of the tracer follows that kinetic measurements are the convolution of the tracer input function and the response function of the system. If the input function is also known, the response function of the system can in theory be deduced by deconvolving the input function from the measured kinetics of the system.

Logan plot

Logan plot is a multiple-time graphical analysis for reversible tracers. It produces volume of distribution (DV, V_T) in case arterial plasma is measured and used as model input, or distribution volume ratio (DVR) in case a reference region curve is used as model input.

M

Mammillary model

Mammillary model (from the Latin word for breast) has a central compartment surrounded by others connected only to the central compartment (like a sow nursing piglets), i.e. compartments are in parallel. Mixed mammillary/catenary models are commonly used in nuclear medicine.

Mask image

Image, where each voxel has it's own label.

Model

A mathematical model is a simultaneous system of differential equations and associated algebraic equations that defines the state variables and rate laws for a particular physical, chemical or biological system. A model is an analogue that behaves like the real system.

Modelling

Modelling gives us tools for investigating, understanding, and predicting various phenomena in science. In PET the mathematical models describe the dynamic behavior of the tracer in terms of mathematical representations. Known information about the tracer and process (biochemical reaction sequence, membranes that must be crossed, etc.) is incorporated by restricting the representation to a limited set of functions.

Molecular imaging

Molecular imaging is the visualization, characterization, and measurement of biological processes at the molecular and cellular levels in humans and other living systems. To elaborate; Molecular imaging typically includes 2- or 3-dimensional imaging as well as quantification over time. The techniques used include radiotracer imaging/nuclear medicine, MR imaging, MR spectroscopy, optical imaging, ultrasound, and others. (definition by MICoE and SNM).

MRI

Magnetic resonance imaging (MRI) is a computer-assisted medical imaging technique for creating cross-sectional images by exposing a subject to radio waves in the presence of a powerful magnetic field and measuring signals emitted by certain atoms or molecules (like water) in responce to this treatment.

MSC

Model selection criterion (MSC) is a modification of Akaike information criterion (AIC), which is independent on the magnitude of the data. Larger MSC means better fit.

MTGA

Multiple-time graphical analysis, MTGA, is a method where PET time-radioactivity curves of region-of-interest and "model input" (reference region or arterial plasma) are normalized and plotted in a graph. The slope of the linear part of the plot represents the net uptake rate of the tracer (so called Gjedde-Patlak plot for irreversible tracers) or volume of distribution (so called Logan plot for reversible tracers).

N

Non-displaceable uptake

Non-displaceable (ND) radioligand uptake is the sum of non-specific (NS) and free (F) radioligand concentrations in tissue. This uptake can not be abolished by adding large amounts of non-labelled ("cold") ligand that binds to the same receptor.

O

Option

In addition to the arguments that you must pass to a program, there may be optional arguments or switches, usually starting with dash, '-'.

P

Parameter

Parameters of the model are the transfer rates between compartments, and other constants in compartmental model, e.g. vascular fraction.

Parametric image

An image, where pixels represent values of some parameter (e.g. blood flow). When traditionally modelling is done using regional time activity information, a parametric image is created so that the modelling is done separately in each pixel.

PVE

Partial volume effect (PVE): when the volume of the structure of interest is small in comparison to the resolution of the scanner, the radioactivity in the structure is artifactually reduced from its true value, while it is artifactually increased in its surrounding region.

Patlak plot

Patlak plot (or Gjedde-Patlak plot) is a multiple-time graphical analysis for irreversible tracers, e.g. [¹⁸F]FDG. It produces net influx rate (K_i). Either arterial plasma or reference region curve can be used as model input.

Permission

Determines who has permission (access) to read, write, and execute a file. You can see each file's permission with the attrib (DOS/Windows) or ls -l (Unix/Linux) command. You can change the permission with the attrib (MSDOS) or chmod (Unix/Linux) command.

PET

Positron emission tomography Pixel from pixel element. The dots that comprise an image.

Plane

The plane is one aspect of an image's matrix. The plane indicates where the image is located relative to the patient's position in the scanner gantry. When a patient is placed in the gantry head first, plane numbers increase in an inferior direction.

Q

R

Rate constant

Rate constant denotes the fraction of the total tracer that will leave the compartment per unit time (fractional clearance). It has the unit of inverse time. Rate constants can still have values over 1.0; if the concentration declines to zero, the flow of material will also be zero, since the amount of substance transferred per unit time is equal to the product of rate constant and the amount of tracer in the originating compartment. Rate constants are related to classical enzyme kinetics.

RC

Repeatability coefficient (RC) is calculated as RC = 2 * SD(scan1 - scan2) in PET test-retest study.

Receptor

A protein or complex of proteins on the surface of a cell that binds to a specific chemical.

Reference tissue

Reference tissue is a region without specific binding of the ligand.

Resolution

The smallest detectable distance between two points.

ROI

Region-of-interest.

RRT

Relative residence time, RRT, is used to measure the clearance of PET tracer from region-of-interest relative to reference region. In practise, it is the negative of the intercept in the Logan plot (multiple-time graphical analysis for reversible binding) with reference tissue input. See Shoghi-Jadid et al. Am J Geriatr Psychiatry 2002; 10(1): 24-35.

S

Script

Script (batch file) is a collection of commands in a file. It's a way to run many commands in a batch mode. It is recommended that you write a script (or batch file in Windows) to analyse all PET studies; then it is easier to analyze all studies again, if needed. And when script is saved with your data and results, it is also a good documentation on what you actually have done to your data.

Simulation

Simulation as a modelling term means the calculation of the perfect (noise-free) data from solving the equations for a particular model with a given blood input function and a given set of parameters.

Specific binding

Specific binding is defined as the radioligand binding to the target receptor, not including the binding to other macromolecular components or radioligand that is not bound at all (free) in the tissue sample.

SPECT

Single photon emission computerized tomography (SPECT, SPET) is a computer assisted medical imaging technique for generating cross-sectional images of a subject, combining the use of radiotracers with the computer technology used in computed tomography.

SRRM

Simplified reference region model, same as simplified reference tissue model (SRTM).

SRTM

Simplified reference tissue model is commonly applied to estimate BPND without blood sampling.

Standard output (stdout)

This is where most output from commands is sent. By default, the standard output is the screen. To save output sent to the screen, you must redirect it with > and | commands.

Supine

Lying on the back with face upward.

T

TAC

Time (radio)activity (concentration) curve. A group of ROIs across time frames. A time activity curve defines the same ROI (may be as small as one pixel) across different times, or frames.

TPC

Turku PET Centre

Tomography

The visual presentation of cross-sectional slices through an object.

Tracer

Tracer is a positron emitting isotope labeled molecule, that is either structurally related to the natural substrate or involved in the dynamic process. Tracer is introduced to system in a trace amount i.e. with a high specific activity; process being measured is not perturbed by it. In general, the amount of tracer is at least a couple of orders of magnitude smaller than the natural substance, tracee. Dynamic process is evaluated in a steady state: rate of process is not changing with time, and amount of natural substance is constant during the evaluation period. Steady state of the tracer is not required. When these requirements are satisfied, the processes can be described with first-order rate constants.

Transformation

Transformation is the class of processes that results in conversion of one molecular species to another (biochemical reactions). These may be catalyzed by enzymes, or may be spontaneous reactions. The other classes of dynamic biological processes are translocation and binding.

Translocation

Translocation is the class of processes that results in movement of a chemical species from one location to another. Examples are diffusion and traversing a membrane. The other classes of dynamic biological processes are transformation and binding.

TRV

Test-retest variability (TRV) is calculated as the absolute value of the difference between test and retest values, divided by the mean of both measurements. The percentage mean TRV ± SD is usually reported.

U

Uncertainty

Uncertainty is a parameter, associated with the result of a measurement, that characterises the dispersion of the values that could reasonably be attributed to the measurand (International Vocabulary of Basic and General Terms in Metrology, VIM, ISO, Geneva, 1993). Results without uncertainty should not be taken seriously.

V

VOI

Volume-of-interest

Volume of distribution

See distribution volume.

W

White matter

Portions of the nervous system with a whitish colour which consists mainly of the axons of nerve cells that are wrapped by the whitish protein myelin. See also grey matter.

Wildcard

Wildcards can take on various values. The most common wildcards symbols are '*' and '?'. For example, in Windows command-prompt window you could use wildcard '*' to list all ECAT 7 images with command dir *.v.