What types of features/properties a pixel in MRI image may have?

I have to classify voxels based on their features/properties. I know that, in gray scale images - like MRI image - have position, intensity as a property for each pixel. What are the other features or properties a pixel may have in MRI image?

This paper by Folkesson et al. mentions voxels in MRI image also have `three jet` property, which consist of all first, second and third order derivative with respective to (x,y,z), eigenvalue and eigen vector.

I did not get what `three jet` means here, and how to get it. Will someone please help me understand what exactly it means and how to get it. Here is the paragraph in paper:

In summary, our candidate features are the intensity, the po- sition, the three-jet, eigenvalues, and eigenvectors of both the Hessian and the structure tensor and the third-order tensor in the gradient direction. All features except the position are cal- culated at three different scales (0.65, 1.1, and 2.5 mm), and the scales are in mm instead of number of voxels for handling scans with different resolutions.

[Walter Roberson]

https://en.wikipedia.org/wiki/Jet_(mathematics)