# Is vtkDICOMImageReader spacing reading wrong?

**URL:** <https://discourse.vtk.org/t/is-vtkdicomimagereader-spacing-reading-wrong/15879>\
**Category:** Development\
**Created:** [July 29, 2025, 4:42pm UTC](https://discourse.vtk.org/t/is-vtkdicomimagereader-spacing-reading-wrong/15879 "2025-07-29T16:42:13Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![adrian\_sager](https://discourse.vtk.org/user_avatar/discourse.vtk.org/adrian_sager/32/10164_2.png) [@adrian\_sager](https://discourse.vtk.org/u/adrian_sager)\
**Post date:** [July 29, 2025, 4:42pm UTC](https://discourse.vtk.org/t/is-vtkdicomimagereader-spacing-reading-wrong/15879/1 "2025-07-29T16:42:13Z")

</div>

I have a DICOM series sliced axially with:

```auto
(0020,0037) DS [1\0\0\0\1\0] # 12, 6 ImageOrientationPatient
(0028,0010) US 1024 # 2, 1 Rows
(0028,0011) US 180 # 2, 1 Columns
(0028,0030) DS [.25451123714447\.999946594238281] # 32, 2 PixelSpacing

```

The series loads properly in softwares like 3D Slicer or Horos.

However, when loading the image with `vtkDICOMImageReader`, the column and row spacings are swapped:

```c++
vtkSmartPointer<vtkDICOMImageReader> reader =
    vtkSmartPointer<vtkDICOMImageReader>::New();

reader->SetDirectoryName(path.c_str());
reader->Update();
imageData = reader->GetOutput();

```

`vtkDICOMImageReader` uses `DICOMAppHelper` where it gets the spacing like this:

```c++
void DICOMAppHelper::PixelSpacingCallback(DICOMParser* parser, doublebyte group, doublebyte element,
  DICOMParser::VRTypes, unsigned char* val, quadbyte)
{
  if (group == 0x0028 && element == 0x0030)
  {
    if (!val ||
      sscanf(reinterpret_cast<char*>(val), "%f\\%f", &this->PixelSpacing[0],
        &this->PixelSpacing[1]) != 2)
    {
      this->PixelSpacing[0] = this->PixelSpacing[1] = 0.0;
    }
  }
  else if (group == 0x0018 && element == 0x0050)
  {
    if (!val)
    {
      this->PixelSpacing[2] = 0.0;
    }
    else
    {
      this->PixelSpacing[2] =
        DICOMFile::ReturnAsFloat(val, parser->GetDICOMFile()->GetPlatformIsBigEndian());
    }
  }
}

```

But, in the same class `this->Dimensions[0]` corresponds to the columns and `this->Dimensions[1]` corresponds to the rows, which is backwards compared to `this->PixelSpacing[0]` (rows) and `this->PixelSpacing[1]` (columns).

Am I missing something? Is this a limitation of `vtkDICOMImageReader`?

---

<div class="post-metadata">

**Author:** ![dgobbi](https://discourse.vtk.org/user_avatar/discourse.vtk.org/dgobbi/32/18_2.png) [@dgobbi](https://discourse.vtk.org/u/dgobbi)\
**Post date:** [July 30, 2025, 2:50pm UTC](https://discourse.vtk.org/t/is-vtkdicomimagereader-spacing-reading-wrong/15879/2 "2025-07-30T14:50:09Z")

</div>

Yes, this is a bug. DICOM PixelSpacing is row/column and the DICOMAppHelper class should be switching it to VTK’s column/row ordering. I can push a fix to VTK’s gitlab for review later this week.

Note that vtkDICOMImageReader has many other limitations. It is unable to read compressed images, enhanced images, or images with embedded icons. It also parses sequences (SQ) and unknown (UN) values incorrectly, and doesn’t handle image orientation correctly. The only DICOM files it can read are very simple ones.

Some alternatives to the vtkDICOMImageReader are vtk-dicom (which I wrote, on github), ITK with an ITK-\>VTK bridge, or (if using Python) to read the file into a numpy array with pydicom and use a numpy-\>VTK bridge.