Talk:Positron emission tomography
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Are the scan times quoted for PET/CT machines correct? The machine I have experience of - it's a couple of years old - requires significantly longer - the CT part takes approximately 10 to 14 minutes for a H&N CT, the slow time mainly being a result of the fact that the CT is very much a low-grade CT scanner. I am not sure what other machines do, and obviously times are very much dependent on the protocol being used, area being imaged, etc., but (in my experience) the CT part takes significantly longer than 30 seconds. zebdee
Answer: With todays multi-detector helical CT scans, the scan time is much faster. We can do an "eyes to thighs" CT scan in less than a minute. NucsDoc
- And one should add in the context of combined PET/CT that this is always the case, since these are always the newer scanners and have the latest CT features, which are always VERY fast. The 1-minute full CT scan is typical there. It's the PET part that takes 10 or 15 minutes a section and will be much harder to speed up with future tech (though of course it will happen). SBHarris 09:33, 4 November 2006 (UTC)
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[edit] PET results
What are the percentages of false positives for PET scans?
Answer: PET is not specific, it merely tells you which areas of the body have faster metabolism than others. This means it picks up inflammation, infection and injuries as well as tumors. For this reason, PET is only suitable for diagnosing tumors in conjunction with other tests. Its other main role is staging of tumors i.e. assessing the extent of a tumor that has already been found by other means.
For example, a CT scan of the chest finds a single nodule in one lung. The question is then, "is this cancer?". PET scanning in this situation can tell you whether this nodule is likely, or unlikely to be tumor.
There was a recent review of PET imaging in lung cancer[1]. In the case of the solitary pulmonary nodule scenario, the specificity of a "positive" PET scan was 82%-94% depending on the precise PET technique used.
For staging of lung cancer - the most important role for PET is determining whether a lung tumor has spread to the center of the chest (the mediastinum). However, even in this case there is a significant false positive rate (PPV : 74-93%; false positive rate: 44.6%). There is also a significant false-negative rate in this situation (up to 11.7%), so PET is usually used in conjuction with other tests, including exploratory surgery (mediastinoscopy). ChumpusRex 15:04, 5 March 2006 (UTC)
[edit] New image
Hello, I come from french Wikipedia, I upload a new image for oncology part. Enjoy !
[edit] Data units for PET Images
PET Images are measured in different units like Bacqurel per mili litre, SUV (Standard Uptake Value), Percentage and Counts/sec What does each of these units mean?
Answer: Becquerel per mililitre is a measure of the concentration of radioactity within a region. One Bequerel is one count/sec.
The problem with this unit is that the value depends on how much radioactive tracer was given to the patient, the size of the patient, how much time has passed since the injection (due to radioactive decay), etc.
The SUV is a method for partially compensating for this. It corrects for the total amount of radioactivity given, and the approximate volume of the patient (volume is very difficult to measure, so it is estimated from weight).
There are a variety of other methods used for calculating SUV, some correct for blood glucose concentration, some use lean body weight instead of total body weight, some estimate body surface area.
Percentage is a cruder method of correction. The voxel with the highest activity is found, and it's activity is called 100%. The activity in other regions is then presented as a fraction of that.
There still some debate as to whether the SUV is a useful measurement, from the point of diagnosis. Some abnormal areas (e.g. tumors), will have a higher uptake (and hence SUV) than normal tissues, and studies have suggested that the higher the activity of an abnormality, the more likely it is to be signficant. Some studies have looked at 'threshold' values of SUV, below which an abnormality is unlikely to be significant (or above which an abnormality is likely to be significant). E.g. if PET can be used to assess a nodule in the lung. If the SUV of the nodule is higher than 2.5, some studies have suggested that it's likely to be cancer.
However, given the different interpretations of SUV, different scan techniques, different times between injection and scanning at different hospitals, etc. some people argue that the SUV is relatively useless - as it's whole point is to allow comparisons between scans done at different times and at different sites.
ChumpusRex 21:43, 10 June 2006 (UTC)
[edit] Antimatter?
I was just curious why there is a link to antimatter in the "See also" section. I couldn't see how the two were related, so I think there needs to be some mention of antimatter in the article if it relates, or the link should be deleted. cøøkiə Ξ (talk) 04:13, 22 September 2006 (UTC)
- Positrons are antimatter. This is clear if the wiki link to positron is followed. However, I see your point that it would be sensible to mention this in the article. ChumpusRex 12:14, 22 September 2006 (UTC)
[edit] Kinetic Analysis of PET-CT scan
right now i am working on a project.the topic is 'Development of a parameter which will distinguish between the TB n cancer' and this has to be done by kinetic analysis of PET-CT scan.
[edit] PET Reconstruction
I haven't seen anything in this page related to the reconstruction process, which is a very important component of PET, because of the low data quality of the process. Do you think that's worth including here? User:Sjayanthi November 3rd, 2006
- Absolutely, if you know something about it. If it's arcane, you can put it in its own section toward the end of the article, as something like "Image reconstruction techniques important to PET" and reference it higher in the section which first mentions how the scanner opperates in more general terms. Actually, there are many ways of doing this. The main article on PET scanning should probably not be too much longer, but long sections on ANY subtopic in PET scanning can be summarized in their own short sections, with a main article referenced there. Then, you can put in a lot more detail in that main article. This is how Wikipedia expands. The main rules here are: don't delete, improve! Next is BE BOLD in adding stuff. The worst that will happen if you know what you're talking about, (provided you don't hit a malicious delete-man), is that your section will be edited down, and the extra material offloaded to a subarticle. Don't worry too much about writing quality-- we need good information worst. Writing can be improved by secondary writers who have a lesser understanding of material. So go for it. SBHarris 09:14, 4 November 2006 (UTC)
[edit] Annihilation in body
I'm not much of an expert in these fields, being a year 9 science student, however in the article under the sub-heading 'How the scanner operates', it reads "the positron travels a few millimetres before encountering and annihilating with an electron." Could somebody clarify for everybody (especially me) which material (the body or any other material) the electron is a part of? Because I had always thought that an annihilation of a sub-atom in the body would result in the atom concerned becoming a positively charged ion, and causing disruption in the atomic structure of the organs being built from these atoms. Thanks. 210.11.82.107 09:28, 19 November 2006 (UTC)
- Electrons wiped out by such positrons come from any old atom out there, and they do indeed become cations (positive ions) as a result. However, we're not talking about large numbers compared normal amounts of cations in the body (look at all those Na+,K+ and Mg++ floating around in all your fluids!). So it's no problem. If you calculate the number of moles of ions created from a normal PET scan in an hour, it's tiny-- something on the order of 10^12, which is a picomole compared with millimolar concentrations of your normal cations. Or 100 nanocoulombs with assocated currents of 50 picoamps, compared with microamps produced in your heart and brain.
Far, far worse damage comes from the gamma, which is ionizing also, but the damage is not caused by the ion per se (it's called that only because this is how these things were DETECTED originally), but rather from the high energy interactions which provide enough energy to break DNA. A simple ion contacting DNA won't do that. DNA is ionized (lots of negatives) and associates with positively ionized histones to be stored. Lots of ions in water are not bad, and in biology are normal. By the way, the atom the positron come from becomes a negative ion also, but this is soon fixed, as it picks up an H+ from solution and becomes a sugar hydroxyl. Again, no sweat, and generally no damage (due to the very small amounts of stuff made). SBHarris 08:47, 16 November 2006 (UTC)
Great, thanks a million. A+, here I come! 210.11.82.107 09:28, 19 November 2006 (UTC)
There is another interesting potential decay pathway of the positron - instead of annihilating with a free electron, the positron may strip off and then bind with (by coulomb attraction) a weakly bound electron, with the consequent formation of positronium (Ps), which is also sometimes referred to as "a light hydrogen atom". Ps can exist in either of 2 quasi-stable states - a triplet spin 1 state (3S1) which eventually decays by annihilation with a spin opposed electron from a neighboring atom in a process known as "pick-off annihilation", or in a singlet spin 0 state (1S0) which eventually decays by self combination. Note that with the decay of the spin 0 state, the process occurs in approximately 100 ps with two gamma photons being created and emitted at 180 degrees, while the spin 1 state decay occurs much more slowly in approximately 100 ns, and creates 3 gamma photons. (see http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=pubmed&cmd=Retrieve&dopt=AbstractPlus&list_uids=14761021&query_hl=3&itool=pubmed_docsum and http://www.ph.unimelb.edu.au/photo/people/leighton/thefinalcheck.pdf) Duedilly 06:12, 11 December 2006 (UTC)
[edit] Annihilation after some time?
i just want to know about the line that says "the positron travels a few millimetres before encountering and annihilating with an electron." but i thought that as soon as a positron encounters an electron, it annihilates...and electrons are present anywhere and everywhere.so how is that positrons are able to travel upto distances as large as milimeters inside the body...please somebody explain this...urgent ,specialy for me...i have a related project even...
- The distance any GIVEN particle travels in ANY material is related to its energy (all other things being equal), due to the fact the short wavelength particles, which of course are high energy particles, just "look" smaller, so have a smaller cross section. High energy electrons go a couple of mm in skin for radiotherapy, and Mev electrons up to several cm. A positron's no different. SBHarris 16:08, 28 March 2007 (UTC)
