19 Jul

CT Data – Finally talking to a radiologist

So I finally sat down with one of our radiologists to pick his brain about what is going on with this data and why do I get the data that I get.  I’d mention him here because he was tremendously helpful, but I don’t want anything I say to make more work for anyone.

What did I learn?

1) Variable slice thickness is most likely a myth.  

The reason I said this is because he told me that the data is likely acquired using 0.625 or 1.25 millimeter (mm) slice thicknesses (really thin cheese).  That data is then usually outputted to whatever slice thickness the radiologist needs to read.  That can usually go anywhere from 5mm to 10mm.  Changing this slice thickness allows radiologists, who read a whole lot of scans per day, only have to read 30-60 slices vs. the 400 or so that the raw data contain.  

Now, back to the variable slice thickness.  From what I can tell, this happens because there isn’t a lot of action (disease/injury) going on in some parts of the brain, so they don’t need to review those places.  Also, there is a bit of liability where “you can’t miss what you can’t see”; you may have seen something on the thinner sliced scans.  So overall, I get it – you don’t want to spend your time reading 1000 slices at a time and that’s totally reasonable.

That said, we discussed the fact that the raw data should be on their PAC system or a parallel server of storage.  This data can usually be retrieved by asking one of the techs for it.  Also, you could theoretically ask for all the data in 5mm slice thicknesses; we’re considering standardizing this in our protocol of our recently-funded upcoming Phase III clinical trial: MISTIE III.  (Side note: why do we reference phases of clinical trials by Roman numerals?).

2) Gantry tilt.  So this is really to protect the eyes during conventional CT scanning.  I mean who wants to come out of a stroke to only hear “You’re fine…but you may have some cataracts”.  There has been some research on trying to correct it using some projections (http://www.ncbi.nlm.nih.gov/pubmed/17282566, as well as a few others but this has some good pictures).

3) Acquisition.  So my analogy with the cheese is still valid for conventional scanning, but helical or spiral CT scanning is more common.  Think spiral ham.  The scan is taken while the X-ray tube spins around the table the patient is on.  There are reasons to use this method over conventional scanning, but I’m not going to mention them here.  

4) Registration – so the discussion of registration was somewhat light.  We discussed the use of SPM and FSL in image registration, but the discussion was slighted to more MRI which don’t have some of the same problems.  We did discuss registering MRI to CT, but this is commonly done using BrainLab, an intra-operative program that helps image guidance, and is done by using external landmarks on the head.  While this is a method that may prove worthwhile, not all CT have the landmarks – usually only the operative scans do.

So overall there’s a lot out there left to do, but it’s good to hear that there is some rhyme and reason to getting data in this way.  Next steps – make friends with some techs to find out how the best way to get a multi-center trial to tell it’s coordinators that they need to request these.