# RuntimeError with number of expected elements for DisplacementFieldTransform
**URL:** https://discourse.itk.org/t/runtimeerror-with-number-of-expected-elements-for-displacementfieldtransform/6076
**Category:** Beginner Questions
**Tags:** transforms, metadata, simpleitk, python
**Created:** [July 18, 2023, 5:30pm UTC](https://discourse.itk.org/t/runtimeerror-with-number-of-expected-elements-for-displacementfieldtransform/6076 "2023-07-18T17:30:44Z")
**Posts on this page:** 2
**Page:** 1
### Author:  [@rtlim](https://discourse.itk.org/u/rtlim)
#### Post date: [July 18, 2023, 5:30pm UTC](https://discourse.itk.org/t/runtimeerror-with-number-of-expected-elements-for-displacementfieldtransform/6076/1 "2023-07-18T17:30:44Z")
I’m trying to create a DisplacementFieldTransform using a MetaImage that I’ve saved, but I’m getting a runtime error and I haven’t found anything online related to it. This is what the error says:
```auto
RuntimeError: Exception thrown in SimpleITK new_DisplacementFieldTransform: D:\a\1\sitk\Code\Common\include\sitkImageConvert.hxx:45:
sitk::ERROR: Expected number of elements in vector image to be the same as the dimension!
```
The MetaImage that I’m using is a .mhd file that represents a deformation vector field. My goal is to apply the deformation field to an image to see how the image changes. A simplified version of my Python code is pasted below.
```auto
ref_img = sitk.ReadImage('\path\to\ref_img.mhd')
dvf_img = sitk.ReadImage('\path\to\dvf_img.mhd', sitk.sitkVectorFloat64)
displacementTx = sitk.DisplacementFieldTransform(dvf_img) # the code is breaking on this line
displacementTx.SetInterpolator(sitk.sitkNearestNeighbor)
resampler = sitk.ResampleImageFilter()
resampler.SetTransform(displacementTx)
resampler.SetReferenceImage(ref_img)
deformed = resampler.Execute(ref_img)
```
I’ve looked at the size of the array that I get from the .mhd file and compared it to the original Image, and they have the same number of elements. The number of components per pixel is also 1 for the image. I’m very new to SITK so maybe it’s a simple solution that I haven’t thought of, but I haven’t been able to find any sort of documentation on this error. Any help would be greatly appreciated!
---
### Author:  [@zivy](https://discourse.itk.org/u/zivy)
#### Post date: [July 18, 2023, 5:54pm UTC](https://discourse.itk.org/t/runtimeerror-with-number-of-expected-elements-for-displacementfieldtransform/6076/2 "2023-07-18T17:54:48Z")
Hello @rtlim,
The error is not with respect to the image size, it is referring to the image dimensionality. An nD displacement field is expected to have values that are nD vectors. Code below recreates the issue (change the `number_of_elements_in_vector` to match `dim` and the error is resolved):
```auto
import SimpleITK as sitk
dim = 2
number_of_elements_in_vector = 3
dvf_img = sitk.Image([2]*dim, sitk.sitkVectorFloat64, number_of_elements_in_vector)
print(f"Number of elements in vector: {dvf_img.GetNumberOfComponentsPerPixel()}")
print(f"Vector field dimension: {dvf_img.GetDimension()}")
displacementTx = sitk.DisplacementFieldTransform(dvf_img)
```
You will need to print the information for the `dvf_img` to see what exactly is causing the mismatch in your case.