Responsemean whan to use and when to avoid

[m.bart]

Dearest MRtrixers!

I am asking you, whether or not did you ever thought about using responsemean after dwi2response. The description here is, of course, general. I am aware it is highly individual, based on the data specifications. So I want to ask about your specific experience - when did you used it/ when do you recommend using it?

My data are from MR-negative epilepsy patients (+ HC). In the end, I will compare them on an individual level as “singleton-vs-group”. Since there is an assumption that this patients have some sturctural irregularities somewhere in the brain (we dont know where and if it is visible afte some analysis on DWI data and if it implies the AFDs), I am thinkig about 2 options:

• no responsemean in this study

• or (more creative one) - making average response for all HC (I won’t include epi patients) - but poor patient will be in a statistical analysis (one patients vs HC group) the only one with different response used.

Have please anybody any ideas/experiences?

Thanks for all,

Michaela.

[maxpietsch]

For analysis, you’d most likely want a single common set of response functions. If you were using multiple subject- or group-specific responses, you’d need to take the differences between these responses into account when interpreting differences in the ODFs which is not trivial nor recommended.

We typically recommend the group-average as the most representative set of response functions unless you have good reasons to believe that the group-average response would not be representative of a tissue type. If in doubt, I’d take the average across the control subjects so that the interpretation of the responses (and derived ODFs) are tied to normal tissue properties. But note that even when there are subtle differences in the responses between groups, using the group average response is not necessarily a bad strategy as the mismatch between signal and average response could still manifest in group differences in the estimated ODFs (the constraints in CSD allowing). There are a few threads discussing this in more detail if you are interested:

Response function for group analysis

Dear Mrtrix experts, I would like to create an atlas from a group of N controls. All the acquisitions have the same parameters (number of directions, b-0 value, etc…). Do I need to estimate the response function for each subject with the dwi2response command then estimate the CSD with the N different response functions? Or the best way is to have the same average response function for everyone (and does it exist a command to average the response function)? Thanks in advance! Best regards Fé…

Registering FODs

It depends. If it is purely for rough visualisation / artistic purposes, I’d say it is fine to use averaged ODFs from subject-specific response functions. When response functions are estimated from the data, one reason for using a single response function is to fix the basis with which the signal is deconvolved across all images so that we can focus on analysing the coefficients (ODFs), without the need to also take the subject-specific ODFs into account. Your question really boils down to wh…

[m.bart]

Dear Max,
thank you for your response. I am in doubt, mainly because I will somehow compare (as I mentioned) 1-patient-vs-all-HC in the end. So, technically, If I will make group response from all patients and all HC, my group response will be affected by epileptic patients that are not included in a statistics (I hope It’s clear) … but calcutation of separate group response and separate analysis on 30+ different subsets (each time one different patient and all HC) sounds like pretty time and space consuming idea So I will most probably stick to the group response calculated from HC.

Thanks for your time!