# Tracking cursor movements to access voxel values

**URL:** https://discourse.slicer.org/t/tracking-cursor-movements-to-access-voxel-values/24568
**Category:** Development
**Tags:** python
**Created:** [July 29, 2022, 4:40pm UTC](https://discourse.slicer.org/t/tracking-cursor-movements-to-access-voxel-values/24568 "2022-07-29T16:40:48Z")
**Posts on this page:** 1
**Showing post:** 7

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### Author: ![siqueirl](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.slicer.org/siqueirl/32/12320_2.png) [@siqueirl](https://discourse.slicer.org/u/siqueirl)
#### Post date: [August 5, 2022, 1:53pm UTC](https://discourse.slicer.org/t/tracking-cursor-movements-to-access-voxel-values/24568/7 "2022-08-05T13:53:37Z")

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HI Kyle, thank you for helping us on this (I work with Leo).

I’m posting in case other people can benefit from our attempt - I believe we managed to get it working by simply moving the computationally-expensive calls outside the mouse movement loop. The console is now printing reasonable values for voxels at coordinates:

This is our current code:

```auto
import numpy as np

ras = [0, 0, 0]
volumeNode = slicer.util.getNode("MRHead")
# Apply that transform to get volume's RAS coordinates
transform_ras_to_volume_ras = vtk.vtkGeneralTransform()
slicer.vtkMRMLTransformNode.GetTransformBetweenNodes(None, volumeNode.GetParentTransformNode(), transform_ras_to_volume_ras)
# Get volume as slicer array
a = arrayFromVolume(volumeNode)

def onMouseMoved(observer,eventid):
    # Get point coordinate in RAS
    ras = [0, 0, 0]
    crosshairNode.GetCursorPositionRAS(ras)
    point_volume_ras = transform_ras_to_volume_ras.TransformPoint(ras[0:3])
    # Get voxel coordinates from physical coordinates
    volume_ras_to_ijk = vtk.vtkMatrix4x4()
    volumeNode.GetRASToIJKMatrix(volume_ras_to_ijk)
    point_ijk = [0, 0, 0, 1]
    volume_ras_to_ijk.MultiplyPoint(np.append(point_volume_ras, 1.0), point_ijk)
    point_ijk = [int(round(c)) for c in point_ijk[0:3] ]
    # Get markup's position
    x, y, z = point_ijk[0], point_ijk[1], point_ijk[2]
    value = a[z,y,x]
    print(value)

crosshairNode=slicer.util.getNode("Crosshair")
observerId = crosshairNode.AddObserver(slicer.vtkMRMLCrosshairNode.CursorPositionModifiedEvent, onMouseMoved)

```

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