Hello everyone,
I’ve been playing with the MRtrix toolkit, particularly commands like dwi2response
, dwi2fod
, for Constrained Spherical Deconvolution (CSD) and have been finding it to be incredibly powerful. Thank you to everyone involved for your hard work!
I do have some questions that might seem naive, but they’re puzzling me. My current work involves using only 6 diffusion MRI measurements (6 directions), which meet the minimum requirement for SH-order 2. However, I noticed that the dwi2response
command can calculate response function SH coefficients up to order 10 (its default order), and dwi2fod csd
can generate SH-order 8 Fiber Orientation Distribution Functions (fODFs), which effectively have 45 channels. Following this spherical deconvolution, I used the shconv
command on the response function and fiber ODF to perform a spherical convolution, resulting in an SH-order 8 image of the “reconstructed” signal. This is where I’m confused – my original data is only SH-order 2, yet the reconstructed signal appears to contain much more detailed data. Also, when I tried to use the amp2sh
command to transform my 6 measurements into SH-order 8 data, it was unsuccessful, stating that transformation is only possible to SH-order 2, the maximum l_max value. (I also tried using Dipy
for this projection, but it yielded poor results with anomalously high values at higher orders, which seems illogical.)
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How should I understand what’s happening in this process and interpret the results from higher-order reconstructions when starting with minimal data? Is it valid to consider the original SH-2 data and the reconstructed SH-8 data equivalent?
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Are there methods to effectively & directly generate an SH-8 data projection from just 6 measurements? And how do we assess the accuracy and reliability of higher-order spherical harmonic projections/reconstructions?
Thank you in advance for your insights!
Best regards,
Rizhong