Volume Loader

Authors: Holoscan Team (NVIDIA)
Supported platforms: x86_64, aarch64
Language: C++
Last modified: March 9, 2026
Latest version: 1.0.0
Minimum Holoscan SDK version: 0.6.0
Tested Holoscan SDK versions: 0.6.0, 1.0.3, 2.0.0, 2.1.0
Contribution metric: Level 1 - Highly Reliable

The volume_loader operator reads 3D volumes from the specified input file.

Supported Formats

The operator supports these medical volume file formats:

• MHD (MetaImage)
• Detached-header format only (.mhd + .raw)
• NIFTI
• NRRD (Nearly Raw Raster Data)
• Attached-header format (.nrrd)
• Detached-header format (.nhdr + .raw)

You must convert your data to one of these formats to load it with VolumeLoaderOp. Some third party open source tools for volume file format conversion include:

• Command Line Tools
• the Insight Toolkit (ITK) (PyPI, Image IO Examples)
• SimpleITK (PyPI)
• Utah NRRD Utilities (unu)
• GUI Applications
• 3D Slicer
• ImageJ

API

holoscan::ops::VolumeLoaderOp

Operator class to read a volume.

Parameters

• file_name: Volume data file name
• type: std::string
• allocator: Allocator used to allocate the volume data
• type: std::shared_ptr<Allocator>

Outputs

• volume: Output volume data
• type: nvidia::gxf::Tensor
• spacing: Physical size of each volume element
• type: std::array<float, 3>
• permute_axis: Volume axis permutation of data space to world space, e.g. if x and y of a volume is swapped this is {1, 0, 2}
• type: std::array<uint32_t, 3>
• flip_axes: Volume axis flipping from data space to world space, e.g. if x is flipped this is {true, false, false}
• type: std::array<bool, 3>
• extent: Physical size of the the volume in world space
• type: std::array<float, 3>

API Reference

Python

VolumeLoaderOp

The volume_loader operator reads 3D volumes from the specified input file.

The operator supports these file formats: * MHD https://itk.org/Wiki/ITK/MetaIO/Documentation * NIFTI https://nifti.nimh.nih.gov/ * NRRD https://teem.sourceforge.net/nrrd/format.html

Constructor Parameters

Parameter	Type	Required	Description
fragment	Fragment	Required	The fragment that the operator belongs to.
allocator	holoscan.resources.Allocator	Required	Allocator used to allocate the volume data
file_name	str	Optional	Volume data file name
name	str	Optional	The name of the operator.

Methods

• initialize: Initialize the operator.
• setup: Define the operator specification.