General Questions Regarding Distribution of B Values for Grid scheme Converted to q Space

8 views
Skip to first unread message

William McCuddy

unread,
Mar 30, 2024, 12:18:29 PMMar 30
to DSI Studio
Hello Frank, 

The Philips research team put together a DTI sequence based on your 256-grid scheme converted to q space. A summary of the b value sampling in the sequence they provided is below. I understand that there is no need to oversample at the low b-values, but was curious about the outliers b=2560, b=1920, and b=1280 with so few samples. Is this problematic?

Picture1.png

A summary of the primary (abs) vector orientations:
Left: 0, Right: 108, Up: 53, Down: 30, Front: 35, Back: 30

Assuming my python script is correct, I suppose the lack of left pointing vectors is due to the bipolar acquisition scheme needed to correct for eddy current distortions at the sequence level  because post-processing correction requires a higher number of samples per b-value. 

So really, the actual directions would be:
Left: 54, Right: 54, Up: 53, Down: 30, Front: 35, Back: 30
Is this correct?

I might be oversimplifying things too much, but with these assumptions it seems that the acquisition should be left to right and not anterior to to posterior; however I know that is not the case.  

Am I missing something?

python script:

import numpy as np

# List of vectors [excluding B0]
vectors = np.array([
[0.8, -0.6, 0],
[0.8, 0, 0],
#...other vectors
[0.8, 0, -0.6],
])

# Initialize counters for each direction
left = right = up = down = front = back = 0

# Loop through the list of vectors
for vec in vectors:
    x, y, z = vec
    # Check the primary direction of the vector
    primary_direction = np.argmax(np.abs(vec))
    if primary_direction == 0:  # x-axis
        if x > 0:
            right += 1
        elif x < 0:
            left += 1
    elif primary_direction == 1:  # y-axis
        if y > 0:
            up += 1
        elif y < 0:
            down += 1
    elif primary_direction == 2:  # z-axis
        if z > 0:
            front += 1
        elif z < 0:
            back += 1

# Display the results
print(f"Left: {left}, Right: {right}, Up: {up}, Down: {down}, Front: {front}, Back: {back}")

Frank Yeh

unread,
Mar 30, 2024, 12:59:19 PMMar 30
to wtmc...@gmail.com, DSI Studio

The Philips research team put together a DTI sequence based on your 256-grid scheme converted to q space. A summary of the b value sampling in the sequence they provided is below. I understand that there is no need to oversample at the low b-values, but was curious about the outliers b=2560, b=1920, and b=1280 with so few samples. Is this problematic?

The grid points distribution is very different from shell-based. The low samplings at certain b values are entirely okay.
 

So really, the actual directions would be:
Left: 54, Right: 54, Up: 53, Down: 30, Front: 35, Back: 30
Is this correct?

It is hard to tell based on these numbers. I would plot the q-space sampling points in a 3D view to see if there is any potential issue. 


I might be oversimplifying things too much, but with these assumptions it seems that the acquisition should be left to right and not anterior to to posterior; however I know that is not the case.  

It does not matter, because the q-space signals are assumed to be symmetric w.r.t. the origin.
 

William McCuddy

unread,
Apr 2, 2024, 1:24:31 PMApr 2
to DSI Studio
Thanks for your response. 3D plots below. Many more L-R directions (green lines) and no -x values. 
Screenshot 2024-04-02 100706.pngScreenshot 2024-04-02 100825.png

The reason I'm looking into this is because I will likely need to shorten the scan time by removing some directions and/or B values and/or n number of samples for particular B values and I want to keep results as similar to the 258 dir acquisition as possible.

I noticed provide a 101 dir table here but it does not contain the b-values. So I'm trying to work out the best way to assign specific B-values to each of the 101 directions. I assume it is not random?

Travis
dti_vectors_input.txt

Frank Yeh

unread,
Apr 2, 2024, 1:41:58 PMApr 2
to wtmc...@gmail.com, DSI Studio
This is half sphere acquisition. Here needs to plot its corresponding full sphere:

For each sample 'q', the plot needs to duplicate another point at -q
This is because the DWI signal at q is assumed to be the same as -q.



--
You received this message because you are subscribed to the Google Groups "DSI Studio" group.
To unsubscribe from this group and stop receiving emails from it, send an email to dsi-studio+...@googlegroups.com.
To view this discussion on the web visit https://groups.google.com/d/msgid/dsi-studio/44fa70be-ae2b-4af5-a0db-901cf769479dn%40googlegroups.com.
Reply all
Reply to author
Forward
0 new messages