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Neuroimaging 1 > PET > Flashcards

PET Flashcards

1
Q

What is nuclear medicine?

A

Using radioactive tracers (radiopharmaceuticals) to obtain diagnostic information or perform targeted radiotherapy

Imaging a tracer NOT organs or tissue inside the body

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2
Q

What is the basic idea of PET?

A

Take a pharmaceutical which traces physiology we’re interested in or it localises in an organ we’re interested in and then we take a radioactive nuclide which emits radiation and we essentially bond them together (biochemical bonding)

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3
Q

What does PET stand for?

A

Positron Emission Tomography

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4
Q

What is a positron?

A

Positively charged electron

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5
Q

What happens during PET imaging?

A

If you have something that emits positrons, it meets a negatively charged electron.

When they meet they anihilate and produce two photons in 180 degrees opposite to each other

If we can detect those and we know they happen about the same time, then we know we have detected that PET isotope and that is essentially all we do in PET imaging

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6
Q

What is a PET scanner?

A

Essentially a ring of detectors around the patient

Patients have a PET emitter inside and that will anihilate and produce the two gamma photons that go 180 degrees together and they will be detected at opposite sides of the scanner

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7
Q

What is a true event?

A

Every time we get an anihilation, the two photons hit the scanner at the same time, creating a peak in the data and because they happen at the same time we can be sure they cam from the same place and its called a true event

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8
Q

What happens when we have a true event?

A

We get a line of conincidence so essentially we know that PET event/anihilation happened somewhere along that line

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9
Q

How do we create an image from these anihilations?

A

There are millions of these pairs of electrons that are flying out from the patient the whole time and we add up all the lines of conincidence to create the image

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10
Q

What happens if a photon from another anihilaiton get detected?

A

In this case we have anihilation, 2 photons going in opposite directions but then we have a second one that happens.

In this case one of the photons from the first anihilation gets stopped, scattered and absorbed, as does one from the second

So we are just left with two photons that are still flying towards the detector

Again we get two peaks from these different photons but how do we tell this is not a true coincidence that we want to measure?

-We just use the timing - because it was a little late we can tell that its not come from the same pair, we call this a random event and ignore it

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11
Q

How do we create an image in 3D

A

Through an iterative reconstruction

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12
Q

What do you have inside the ring of detectors?

A

Lots of different banks of crystals and electronics all together in a big ring around the patient

Put the patient on the bed, drive the bed into the centre of the field of view, the image is acquired continually as the protons come out of the patients head, dont have to do much just let detection happen

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13
Q

What are the two types of PET image?

A

Static
Dynamic

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14
Q

What are dynamic PET images?

A

Involves the continuous imaging over a pre-defined period immediately after injection, acquiring kinetic information.

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15
Q

What are static PET images?

A

Traditional clinical use of PET technology employs a static approach to the image acquisition, where imaging is acquired at a single time frame, after the patient is injected with the radiotracer

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16
Q

WHy might we want a dynamic scan?

A

Interested in how that tracer moves in different parts of the brain or body

17
Q

What are some recent advancements in PET imaging?

A

Traditional pet detectors were quite thin, a recent advancement is a bigger FOV (multi-ring) which essentially adds more banks of detectors
- scan quicker, much more sensitive can be done in minutes with very high signal

Time of flight is the lines of responses where the interactions occur- electronics are getting so fast that we now can isolate where this happens along the line of response even more so
-means we get even better signal to noise ratio and better resolution as well

18
Q

What is a limitation of PET?

A

Poor spatial resolution
- Research into the design of the detector block - crystals allow you to detect the gamma photons and allow you to turn it into light or UV and some electronics at the back that generate the image, currently trying to increase spatial resolution as it is limited

But we do get to image function with this type of scan

19
Q

How does PET compare to MRI?

A

The sensitivity is much much higher so you only need really small amounts of tracer to get a high signal which is a main advantage

20
Q

What is attenuation?

A

Loss of signal due to absorption or scatter of photons in tissue

21
Q

How is attenuation a limitation of PET?

A

Photons dont just pass through the patient without having any kind of interaction, they can get attenuated or blocked by your organs and they might not get detected

Things like bone which are denser are going to be more likely to scatter photons and if it scatters them out of the scanner then we lose the signal whereas lung for example won’t scatter them as nmuch

This means we bias the image depending on where the anihilation reaction happens e.g. in the centre of the body vs near the skin
-we need to be aware of this and try to correct for this otherwise our final image will be biased and the numbers we get out of it will be wrong

22
Q

How can we compensate for attenuation?

Attenuation correction

A

PET-CT

Can take a CT image at the same time

CT image is a map of tissue densities and photon attenuation

Compensates for the effect of signal loss due to scatter and absorption

Essential for quantitation

23
Q

Why is PET MRI harder to corrrect for attenuation than PET CT?

A

PET MRI isnt related to tissue density or photon attenuation

Have to use more convoluted methods that don’t work as well

24
Q

What are the parameters you can change in a PET scan?

A

Required axial field of view (e.g. brain, half a body)

Time per bed position- scan the body in sections because the field of view is anly about 30cm- so we have to move the patient through slowly

Bed overlap- the sensitivity in the FOV is maximum in the middle for geometric reaons so we have to smooth it out over time to maintain sensitivity across the whole of the length of that image

25
Q

What is the signal to noise ratio like for PET?

A

Image signal to noise ratio improves over time, the longer the patient is there the better the image is because the photons are hitting the detectors the whole time so we’re getting a better and better picture, the longer you scan the better the image

But how long can your patient sit still is the trade off in nuclear medicine

26
Q

What do we inject into patients ?

A

Four common ones:

15O 2min half life

13N 10 min half life

11C 20 min half life

18F 110 min half life

27
Q

What is a big problem for PET?

A

Half life

28
Q

What is half life?

A

The amount of time it takes to half the signal you get from injecting it into the patient

29
Q

What is a cyclotron?

A

Can only do scans for long periods of time if we have a cyclotron

A cyclotron is a particle accelerator that produces the radioactive component of the tracer used in PET/CT imaging

30
Q

How do we account for half life as a limitation of PET?

A

Most PET scans use 18F because it has a half life of nearly 2hrs so you can drive it part way across the count and still have quite a lot of useful stuff you can still inject into the patient

31
Q

What are the two commercially avaiable tracers?

A

FDG
FEC

32
Q

What is FDG?

A

Glucose analogue- follows the same metabolic pathway as glucose until inside the cell, at which point metabolism stops and FDG is trapped

Activity in the cell builds up in proportion to the rate of glucose metabolism

33
Q

What can FDG be useful for clinically?

A

Help with the differential diagnosis of AD, epilepsy localisation, tumour characterisation etc

Can be used to map patterns of glucose metabolism for normal/disease brain functions

34
Q

What are Amyloid tracers?

A

e.g. F18- florbetaben

Binds to beta amyloid plaques in the brain

Gets accumulated but then also does wash out so we have to be careful to image at a very specific timepoint

35
Q

What are the clinical benefits of amyloid tracers?

A

Can help with differential diagnosis of AD

In research can used used e.g. to assess the impact of anti-AB drugs

Neuroimaging 1 (18 decks)

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