Hello @zivy .
Iāve incorporated your inputs and my code looks like this. Right now Iāve hard coded many values as Iām trying to get the code to work for 1 PET Scan. The source of the values are mentioned as a comment.
np_vol = pet_resampled[0]
sitk_vol = sitk.GetImageFromArray(pet_resampled[1])
type(sitk_vol)
#Adding components of spatial objects
sitk_vol.SetDirection([1,0,0,0,1,0,0,0,1]) #Identity Matrix
sitk_vol.SetSpacing([5,5,5]) # Since pixel spacing is 5 x 5 and the slice thickness is 5mm
sitk_vol.SetOrigin([337.88238150935,510.94804353575,768]) # Values from Image Patient Position (IPP) from DICOM
#Function to convert the centre index to origin, from index to millimeter. Created this as was unable to get the TransformContinuousIndexToPhysicalPoint to work
def idx_mm(orig,idx,spacing):
mm_x= idx[0]*spacing[0] + orig[0]
mm_y= idx[1]*spacing[1] + orig[1]
mm_z= idx[2]*spacing[2] + orig[2]
cent = [mm_x,mm_y,mm_z]
return cent
center_of_volume_idx = np.array([np_vol.shape[0]/2,np_vol.shape[1]/2,np_vol.shape[2]/2])
centre_of_vol_mm = idx_mm([337.88238150935,510.94804353575,768],center_of_volume_idx,[5,5,5]) # Output = [47.11761849064999, 170.94804353575, 428.0]
axis = [0,1,0]
degrees = 20
radians = np.pi * degrees / 180
versor_tx = sitk.VersorRigid3DTransform()
versor_tx.SetCenter(centre_of_vol_mm)
versor_tx.SetRotation(axis, radians)
#Defining the grid
grid = sitk.GridImageSource()
grid.SetOutputPixelType(sitk.sitkUInt16)
grid.SetSize([180 X 512 X 512]) #same as np_vol.shape
grid.SetSigma([0.5, 0.5,0.5]) # Used value from code example. What does this mean?
grid.SetGridSpacing([5.0, 5.0,5.0]) #same as Spacing of sitk_vol
grid.SetOrigin([337.88238150935,510.94804353575,768]) #Value of IPP
grid.SetSpacing([5.0, 5.0,5.0]) # Used value from code example. Unclear about difference from GridSpacing
#Performing rotation
rotated_sitk = sitk.Resample(grid, sitk_vol, versor_tx ,sitk.sitkCosineWindowedSinc, 0,sitk.sitkUInt16)
rotated_ndarr= sitk.GetArrayFromImage(rotated_sitk)
plt.imshow(rotated_ndarr[:,:,101]) # Plotting a random slice from rotated object shows Blank
plt.imshow(np_vol[:,:,101]) # Whereas Plotting a random slice from original nd array of the volume showed values of the PET scan
I have the following questions:

Are the identified sources of information for the hardcoded values in line with the correct interpretation?
Eg: while defining the sitk volumne as a spatial object,
Origin = IPP
Spacing = Pixel_spacing_X, Pixel_spacing_Y, Slice_Thickness
Direction Cosine matrix = 100(for x), 010 (for y) and 001 (for z)

Could you explain what the parameters used while defining the Grid mean? The documentation is not very clear about this.

While performing the rotation which interpolator should one use?

Finally on plotting the rotated object, I get a blank output  This is documented as one of the common errors, and is attributed to setting the wrong grid  But the details of the parameters expected by the Grid and what they mean are fuzzy to me.
Thanks for humoring the plethora of questions. Coming from a Python background, Iām finding it difficult to understand the Doxygen documentation, and am relying on the Jupyter examples which contain very few examples of 3D rotation.