Thursday, 5 July 2012

What are the heart, CT and PET scans like?

Heart Scan (Electrocardiogram & Echocardiogram)


The heart scan is pretty straight-forward really, at least from a patient perspective, as you just have a few sensors connected to your chest that read your heart rate etc (electrocardiogram), and at the same time an ultrasound scanner is used to look at your heart in action (echocardiogram) - The relevant data can be recorded for later review, e.g. by a doctor.

All you really do during the heart scan is lay in a certain position on a bed, and when requested change position or control your breathing in a specific way, e.g. take a deep breath.  It is probably all over in 5 to 15 minutes.

You don't really feel anything during the heart scan, other than: the gel they put on the ultrasound scanner feels a bit cold when it first touches your skin; the pressure from the ultrasound scanner being pushed against your chest can be a bit uncomfy; and if you have a hairy chest then you are in for a bit of a waxing when the sensors come off!

CT Scan


A CT scan can vary a bit depending on the area of the body being examined, but in a nutshell you lay on a bed that slides back and forth through a quite thin donut shaped scanner (you're unlikely to feel claustrophobic), and is probably all over in 5 to 15 minutes.  It works by essentially taking lots of x-rays from different angles, so that they can all be combined on a computer into a 3D model; the end result being a much clearer/more detailed view than that possible with a simple/single x-ray.

I have only ever had the Chest, Abdomen and Pelvis type of CT scan, with the exception of the first time when I also had the Head type of CT scan, so can really only speak to that in terms of the full experience, but for me it has always involved a few things that I've not mentioned yet:

  • I needed to drink a bit less than a litre of contrast fluid, so that a clearer view could be seen on the CT scan.  The contrast fluid looks a bit like a white washing detergent, and has a similar consistency; Having never tasted washing detergent, I can't say for sure, but the contrast fluid probably doesn't taste much better, even though it does seem to have had a bit of an orange like flavouring added to it.  I did once get a different contrast fluid that was more like water in consistency and I think colour too, which did taste better, but was still not exactly something that you'd order at the bar.  There are a couple of things to keep in mind with the contrast fluid, one is that you get about thirty minutes to drink it, so you better be thirsty, and two is that you really need to make use of those thirty minutes; I once downed-it quite quickly, and lets just say as laxatives go it is quite an effective one.

  • During the CT scan, I needed a contrast dye to be inserted intravenously, so that a clearer view could be seen, hence I needed a cannula in my arm.  I was on one occasion told you may feel the contrast dye when it goes in, as it is at quite high pressure, but having had several CT scans I've only ever felt it once; although, I must admit, when I did feel it, it was more uncomfortable than expected, but it was over quickly so was nothing really.  The one thing I have always noticed though is that when the contrast dye goes in, you get a kind of warm sensation go down your body, a funny taste in your mouth, and it feels a bit like you just wet yourself!

  • During the CT scan, you are occasionally asked to take a deep breath and hold it for a short period; It's only a few seconds, but it can sometimes feel longer.  It's similar to being asked to stay still during the CT scan, as it's not really a problem staying still for a few minutes, until you suddenly start to imagine that your nose is really itching and desperately needs scratching!

Once the CT scan is complete, you are asked to wait for around fifteen minutes before the cannula is removed from your arm, just in case you have some kind of allergic reaction that requires treatment.  I have never had a reaction myself, nor have I ever seen any other patients have one whilst I have been waiting, so I assume they don't happen very often.  Anyway, once the cannula is removed, you are free to go.

PET Scan


A PET scan is similar to a CT scan in that you lay down on a bed that slides back and forth through a fairly thin donut shaped scanner (you're unlikely to feel claustrophobic), but it does take longer, i.e. 15 to 45 minutes, and it works in a very different way.  It relies on a radioactive tracer being added into your blood stream, and as that tracer decays the scanner can detect the resulting radiation and pin-point its origin; This is helpful as the cancer cells tend to absorb more of the radioactive tracer than normal cells, hence the cancer cells show-up as hot spots (areas of higher activity) on the PET scan.

In order for a PET scanner to work effectively, you are not allowed to eat or drink anything except water (of which you may have to drink plenty to ensure that you are well hydrated) for roughly four to six hours before being scanned, as this ensures that the sugar levels in your blood are not too high; your sugar levels will be tested before proceeding with the PET scan.  Assuming your sugar levels are fine, then you are given the radioactive tracer into a cannula in your arm, after which you need to sit still and remain quiet for around an hour, as this enables the cancer cells to absorb the radioactive tracer - If you moved around and talked, normal cells would absorb more of the radioactive tracer than they would have done otherwise, which would lead to the cancer cells not showing-up as well (if it all).

Once the radioactive tracer has had time to be absorbed by the cancer cells, you will shortly be taken through to the PET scanner.  I have only had a couple of PET scans myself, but I was always walked through to the toilets first, as given I was well hydrated it was best to go then, i.e. before spending around thirty minutes in the scanner where you need to lay still.  The thing I remember the most about the whole PET scan experience is that the person that walked me to the toilets asked me to walk at least a metre behind him, presumably because I was essentially radioactive due to the tracer added to my blood stream - I was a bit unsure whether he was joking at first, as I was thinking how radioactive can I really be, but I soon found out he was quite serious when I got too close to him as he opened a door for me.  That brings me to an interesting point actually, as after a PET scan you remain slightly radioactive for several hours, so they recommend that you stay away from children and anyone that is pregnant, due to their higher susceptibility to the radiation - This can pose a bit of a problem though, as if you use public transport to get home, and a woman sits down next to you, it can be a bit awkward to ask if she is pregnant, especially if you are hoping to avoid getting slapped!

If you are interested in more detail of the science behind it all, then in the vast majority of cases the tracer is a radioactive sugar, specifically fluorodeoxyglucose (FDG).  As the radioactive tracer decays, which is quite quickly as it has a short half-life (time for half of it to decay), it emits positrons (the antiparticle counterpart of an electron) and when those positrons collide with an electron, they annihilate each other, and that results in gamma rays firing in opposite directions, which the scanner can detect to pin-point their origin - The annihilation will take place very close to the position of the decay.

Some of the PET scanners are actually combined PET and CT scanners, and this is useful as it enables both scans to take place simultaneously, and in doing so allow for the PET scan to be overlayed on the CT scan, so that it is easier to determine where exactly the hot spots are in the patient's body; If the scans are done separately, then the patient may be in a slightly different position, which makes the scans harder to overlay.

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