Hi Bruce,

Thank you for your insightful comments.

Only one patient had the scan parameters of 1mm*0.86mm*0.86mm, and the rest all had 1mm*1mm*1mm including controls. So definitely, it won't be balanced.

I guess we have to let go the patient scanned differently.

Best

Keith

On Thu, Aug 1, 2013 at 2:44 PM, Bruce Fischl fischl@nmr.mgh.harvard.eduwrote:

Hi Keith

1x1 --> .86x.86 is a 25% reduction in SNR and the equivalent of scanning twice as long, so it's probably a relatively large effect. The very least you want to make sure you are balanced (that is, the worst thing you could do would be have all your patients with the .86mm scans and your controls with the 1mm). You could also do a small study by scanning some controls with both sequences and seeing if there is a spurious effect, which there might well be.

cheers Bruce

On Thu, 1 Aug 2013, Qi Wu wrote:

Hi Freesurfer experts,

...

I have a difficult decision to make and I don't know which direction is better. Your opinions are needed.

We are planning a paper on the treatment effects of a drug on the cortical plasticity in a group of patients ( the pre- vs. post- design). It has been a huge challenge to recruit patients for this subject. So far, after 3 years of active recruitment, we have only 13 complete set of pre- vs. post- data. But , we have another set of data which was collected at the earliest stage of the project with slightly different parameters. The main difference is the in-plane resolution; those 13 patients have a 1mm*1mm*1mm isovoxel resolution and that particular patients has a 1mm*0.86mm*0.86mm resolution.

To include the subject increases the power of the pre- vs. post analysis, as we are working in the range of relatively low S/N (only 13 pairs). We also know that we are open to attack if we include the subject with different scan parameters. Anyone knows how big the impact would be.

Is there anyway to estimate the impact? Is it easy to get away with the potential critique about this issue? Is it a must-not case?

Many thanks

Keith

Postdoc fellow Toronto Western Hospital Canada

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