# ACPC transform question

Hello,

I have been trying to figure out how to transform a scan so that it aligns to the ACPC plane. I’ve tried using this module (https://www.slicer.org/wiki/Documentation/4.8/Modules/ACPCTransform) and placing fiducial points on different slices to transform the scan, but have been unsuccessful. Can anyone explain how I can do this? Also, is there a way to overlay a grid on the scan or in some way calculate just horizontal distance with the ruler?

Thank you!

Hi Alexis,

You can try this code on the Python console:

``````import numpy as np
import SampleData

def getSampleVolume():
sampleDataLogic = SampleData.SampleDataLogic()
return volumeNode

def getMatrixToACPC(ac, pc, ih):
# Anteroposterior axis
pcAc = ac - pc
yAxis = pcAc / np.linalg.norm(pcAc)
# Lateral axis
acIhDir = ih - ac
xAxis = np.cross(yAxis, acIhDir)
xAxis /= np.linalg.norm(xAxis)
# Rostrocaudal axis
zAxis = np.cross(xAxis, yAxis)
# Rotation matrix
rotation = np.vstack([xAxis, yAxis, zAxis])
# AC in rotated space
translation = -np.dot(rotation, ac)
# Build homogeneous matrix
matrix = np.eye(4)
matrix[:3, :3] = rotation
matrix[:3, 3] = translation
return matrix

def getTransformNodeFromNumpyMatrix(matrix, name=None):
# Create VTK matrix object
vtkMatrix = vtk.vtkMatrix4x4()
for row in range(4):
for col in range(4):
vtkMatrix.SetElement(row, col, matrix[row, col])
# Create MRML transform node
'vtkMRMLLinearTransformNode')
if name is not None:
transformNode.SetName(name)
transformNode.SetAndObserveMatrixTransformToParent(vtkMatrix)
return transformNode

# I defined these on MRHead
ac = np.array([-0.0641399910762672, 17.61291529545006, 5.009494772024041])
pc = np.array([-0.5843405105866637, -10.4779127581112, 3.044020167647495])
ih = np.array([-1.1045410300970602, 1.746799450383008, 45.04402016764749])

volumeNode = getSampleVolume()
matrix = getMatrixToACPC(ac, pc, ih)
transformNode = getTransformNodeFromNumpyMatrix(matrix, name='World to ACPC')

# Create markups node with AC, PC and an interhemispheric point
markupsNode.SetName('AC-PC-IH')

# Apply transform to volume node and markups node
volumeNode.SetAndObserveTransformNodeID(transformNode.GetID())
markupsNode.SetAndObserveTransformNodeID(transformNode.GetID())

# Fit image to slices
applicationLogic = slicer.app.applicationLogic()
applicationLogic.FitSliceToAll()

# Center views on AC
markupsLogic = slicer.modules.markups.logic()
acIndex = 0
markupsLogic.JumpSlicesToNthPointInMarkup(markupsNode.GetID(), acIndex)
``````

By the way, looking forward to showing examples like these in a Jupyter Notebook https://discourse.slicer.org/t/jupyter-notebooks-are-now-usable-in-3d-slicer/3438

These may help (they are accurate for Slicer 3.6, but the behavior is not that much different on Slicer 4.9):

If you still have problems, then please describe what you did exactly, what you expected to happen, and what happened instead.