LinuxWhat is the ACR file format?
An .acr file is an ACR-NEMA medical image - a binary raster image in the pre-DICOM format jointly defined by the American College of Radiology (ACR) and the National Electrical Manufacturers Association (NEMA). The ACR-NEMA Standard 300 provided an early standardized way to store and exchange digital medical images from modalities such as X-ray, CT, MRI, and ultrasound.
Unlike the modern .dcm DICOM format, ACR-NEMA files have no file preamble or magic bytes - the binary data begins immediately at offset zero as a series of Tag-Length-Value (TLV) data elements encoding patient demographics, study metadata, and raw uncompressed pixel data. Byte order could be either little-endian or big-endian depending on the vendor implementation, which contributed to significant interoperability problems. The format originally defined only a point-to-point RS-422 hardware interface (50-pin connector) with no network capabilities.
Images are typically 8-12 bits per pixel grayscale; CT slices were commonly 512 × 512 at 12 bpp, providing 4,096 gray levels. ACR-NEMA version 1 files sometimes carry the .an1 extension; version 2 files .an2.
DICOM 3.0 superseded ACR-NEMA entirely by adding a proper 128-byte file preamble with DICM magic bytes, network protocols, and a far richer metadata model. Legacy .acr files are still encountered in archives of early digital radiology departments and can be opened by tools such as MicroDicom, Weasis, and 3D Slicer.
Security & safety
RISK: LOWACR-NEMA files contain sensitive patient medical data (protected health information / PHI). The primary concern is privacy, not malware: these files should be handled per HIPAA (US) or equivalent regulations. The binary format itself has no executable code and poses no malware risk.
Format details
in a nutshell- Adobe Camera Raw settings (ACR) - Adobe Camera Raw may create .acr sidecar files storing RAW image adjustment settings - unrelated to medical imaging.
- ACRobot configuration file - Some applications (ACRobot) use .acr for script or configuration files.
Programs that open ACR files
Windows3 apps
MicroDicom
Free File > Open and select the .acr file. MicroDicom supports ACR-NEMA alongside DICOM. Free for non-commercial use.
Weasis
Open-source Open via File > Open files and select the ACR file. Cross-platform DICOM/ACR-NEMA viewer. 
XnView MP
Free XnView MP can open some ACR-NEMA files as it supports DICOM. Drag & drop or File > Open. 
macOS2 apps
Horos
Open-source Free macOS medical imaging viewer supporting DICOM/ACR-NEMA. Open via File > Open. Technical details
deep spec| Standard | ACR-NEMA 300 (versions 1 and 2); superseded by DICOM 3.0 |
| Encoding | Binary Tag-Length-Value (TLV) data elements |
| Byte order | Little-endian or big-endian (implementation-dependent per vendor) |
| File preamble | None - no magic bytes; TLV data elements begin at offset 0 |
| Compression | None (uncompressed pixel data) |
| Color depth | 8-12 bpp grayscale; typically 12 bpp (4,096 gray levels) |
| Typical image dimensions | 512 x 512 (CT/MRI); up to 2,048 x 2,048 (digital radiography) |
| Typical file size | 256 KB - 30 MB |
| Supported modalities | X-ray, CT, MRI, ultrasound |
| Original hardware interface | Point-to-point RS-422 with 50-pin connector (hardware spec, not a network protocol) |
| Version file extensions | .an1 (version 1), .an2 (version 2), .acr (general) |
| Developer | American College of Radiology (ACR) & National Electrical Manufacturers Association (NEMA) |
| Superseded by | DICOM 3.0 - added 128-byte file preamble, DICM magic bytes, and network protocol support |
| Released | 1985 (ACR-NEMA Standard 300 version 1); version 2 in 1988 |
| Latest version | ACR-NEMA version 2 (1988); replaced in 1993 by DICOM 3.0 |
| Open standard | Yes · royalty-free |
| Specification | dicom.nema.org |
ACR conversions
Community Q&A
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