W5: Positron-emission tomography (PET)

What is the rationale for measuring brain function in mental health?

Brain disorders may not show structural deficits; functional changes may precede anatomical changes; functional measurements help understand pathological mechanisms.

What is a tracer in PET imaging?

A tracer is a small drop with biochemical properties that move through body compartments at certain rates.

What are the three strict requirements for tracer measurements in PET?

Must be small mass that doesn't affect the system; must move between compartments in a defined way; presence of instrumentation to measure concentration over time.

What is typically contained in a PET tracer?

A substrate (biological element of interest) and a label (which emits a signal for measurement).

What is the main advantage of PET over CT and MRI for tracer measurements?

PET requires very small amounts of tracer and allows accurate quantification of radioactivity concentration.

Why are large molecules like iodine and gadolinium limited for use as PET tracers?

They are too big, change biochemical properties of substrates, do not cross the blood-brain barrier, and may interfere with the system.

What is the role of a cyclotron in PET?

A particle accelerator that produces radioisotopes for use as tracers in PET imaging.

What happens during the decay of a positron?

The positron travels a distance and annihilates with an electron, creating two gamma rays that travel in opposite directions.

What is the purpose of correction methods in PET gamma detection?

To account for attenuation, scattering, and random events that can affect signal accuracy.

What are the two main methods for reconstructing images from PET data?

Filtered Back Projection (FBP) and Iterative Reconstruction (IR).

What does the Kety-Schmidt method measure?

Cerebral blood flow (CBF) based on gas concentration in the brain.

What is the main source of energy for the brain?

Oxidation of glucose.

How does the glucose metabolism process start in the brain?

Glucose is transported across the blood-brain barrier and phosphorylated in mitochondria.

What does deoxyglucose do in glucose tracing?

It accumulates in the mitochondria after phosphorylation and stops glucose entry, functioning as a poison.

What was the significant development in the 1970s regarding glucose metabolism measurement?

The deoxyglucose method proposed by Louis Sokoloff, estimating the metabolic rate of glucose using PET.

What is quantification in experimental science?

The process of transforming experimental measures into a set of parameters describing the system quantitatively.

What are the two classes of PET studies?

Research PET studies and Diagnostic (clinical) PET studies.

What is static PET imaging?

A single acquisition of the target volume at a specific time after tracer injection.

What is Standardized Uptake Value (SUV) in PET?

A normalization measure reflecting tracer uptake relative to the subject’s body weight.

What is a main limitation of PET imaging?

Time constraints, particularly in evaluating dynamic processes with static imaging.

What is the purpose of dynamic PET imaging?

To collect multiple images over time to fully characterize tracer kinetics.

What does the total signal in PET imaging consist of?

Contributions from free tracer, radiotracer bound to blood components, and radiolabelled metabolites.

What are the four groups of non-invasive methods in PET quantification?

Image-derived input functions, alternative modeling strategies, population-based input functions, and venous input functions.

What is the first phase of PET imaging data preprocessing?

Motion correction, which addresses motion problems within each imaging frame.

What are voxel-wise and region-wise analyses in PET?

Voxel-wise analysis examines individual voxels, while region-wise analysis averages activities of groups of voxels.

What are some techniques used for biomarker detection in preclinical research?

Western blotting, ELISA, RT-qPCR, and immunohistochemistry.

What is Western blotting used for?

To separate and identify proteins and detect changes in specific proteins of interest.

What occurs during the RT-qPCR process?

RNA is reverse-transcribed to cDNA, which is then amplified and quantified based on fluorescence increase.

What is a sandwich ELISA?

A type of ELISA that uses a specific antibody pre-coated on a plate to capture target antigens.

What does immunohistochemistry allow researchers to visualize?

The localization of specific antigens or proteins in tissue sections.

What are imaging biomarkers used to indicate?

Normal biological processes, diseases, or responses to therapeutic interventions.

What are the steps involved in producing PET images?

Producing radioactivity with a cyclotron, absorbing tracers, positron annihilation, and gamma ray detection.

How do coloured PET images represent brain activity?

The colour indicates the intensity of the energy signal, where red signifies the highest activity.

What is translational research in the context of PET?

It expresses the benefits that modern technologies can bring to the discovery and development of new therapies.

What role do preclinical models play in medical research?

They are crucial for integrating nuclear medicine/PET techniques with clinical studies.

How is glucose metabolism visualized using FDG PET?

By tracking the flow and uptake of fluorodeoxyglucose which highlights glucose utilization in tissues.

What is the difference between bio-specimen and bio-signal biomarkers?

Bio-specimen biomarkers include molecular and genetic markers, while bio-signal biomarkers include imaging.

Why is it important to validate PET biomarkers?

To ensure they accurately characterize biological conditions and processes.

What is compartmental analysis in PET?

A method that uses first-order differential equations to describe tracer kinetics within tissues.

What is the ultimate purpose of quantification in PET?

To isolate specific contributions of tracer signals from other effects for accurate interpretation.