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PACS Training - DICOM Basics 101

What You Need To Know as a Prospective PACS Analyst

DICOM SOP Classes.  This is an important term a successful PACS administrator must understand. In this module, we’ll define what a DICOM SOP class is.  We will briefly review some of the most common SOP classes. We will also discuss specific DICOM SOPs such as PR, SR, and KO… and explain how they appear in PACS. 

By understanding SOP classes, you will be able to troubleshoot more advanced issues within your PACS and imaging environment.

DICOM Basics

DICOM is short for Digital Imaging and Communications in Medicine. It is a standard for storing and transmitting medical images. It was developed in the early 1990s and has since become the accepted standard for medical imaging. DICOM offers several key features and benefits that make it an essential tool for medical professionals.

  1. First, DICOM files can be viewed as high-resolution images and can be viewed on a variety of devices. This allows doctors to view images in different ways, depending on their needs.
  2. Second, DICOM images can be compressed, which reduces the amount of space required to store them.
  3. Finally, DICOM images can be transmitted electronically, making it possible to share them with colleagues around the world.

These features make DICOM an essential tool for doctors and other medical professionals.

SOP Classes in DICOM

A Service-Object Pair (SOP) Class is defined as follows.  The SOP class is the pairing of an Information Object Definition (IOD) and a DICOM Service Element (DIMSE). The DIMSE is a set of commands such as “Store” or “Move” for objects such as medical images. 

The IOD and DIMSE make up the SOP class.  In simple layman’s terms, the IOD is the image.  The DIMSE is the action. 

IOD + DIMSE = SOP class
Image + Action = SOP class

Confusing right?  Let’s dig deeper.

What are SOP classes, and what do they mean for DICOM files/images?

SOP Classes are service objects that encapsulate the DICOM image files. Each SOP Class is identified by a unique UID (Universal Unique Identifier). The UID is a string of numbers that serve as a pointer to the corresponding DICOM file.

When a DICOM image file is acquired at the modality, the SOP Class UID is generated and assigned to that file. You will likely receive an error message if you attempt to view or open a DICOM image without the corresponding SOP Class UID. In order to view or open a DICOM image, you must have the correct SOP Class UID associated with that image. For example, on a CT modality.  When images are captured, the software in the modality assigns a UID of 1.2.840.10008. This is the SOP class UID for “CT Image Storage”. In most cases, the correct SOP Class UID is already configured by default on new modalities.

There are many different types of SOP Classes. Each serving a different purpose. For example, there are commonly used SOP classes for image objects from modalities such as MR (Magnetic Resonance), CT (Computed Tomography), and X-Ray. There are also proprietary SOP classes for specific vendors’ equipment, such as GE (General Electric) and Philips. In addition, there are classes for specific DICOM applications, such as PACS (Picture Archiving and Communication System).

Common SOP ClassesSOP Class UIDModality
Computed Radiography Image Storage1.2.840.10008.
Digital X-Ray Image Storage – For Presentation1.2.840.10008.
Digital Mammography X-Ray Image Storage – For Presentation1.2.840.10008.
CT Image Storage1.2.840.10008.
Enhanced CT Image Storage1.2.840.10008.
Ultrasound Multi-frame Image Storage1.2.840.10008.
MR Image Storage1.2.840.10008.
Enhanced MR Image Storage1.2.840.10008.
X-Ray Angiographic Image Storage1.2.840.10008.
Nuclear Medicine Image Storage1.2.840.10008.
Positron Emission Tomography Image Storage1.2.840.10008.
Enhanced PET Image Storage1.2.840.10008.

The wide variety of SOP Classes means that there is a corresponding class for almost any DICOM file or image. When attempting to view or open a DICOM image file, always check to ensure that your PACS or VNA supports the SOP Class UID associated with that file. Otherwise – you may not be able to view or open the file.

What?  Okay let’s sum this up.

IOD + DIMSE = SOP class
Image + Action = SOP class
For example
MR image + Storage = MR Image Storage SOP class UID 1.2.840.10008.

Got it?

How is this useful?  When installing a new PACS you must ensure the PACS can support this MR SOP class for storage (along with other SOP classes from your other modalities).  Storage for the above-mentioned MR image may be standard for most PACS but not all PACS support the newer enhanced MR image SOP. Be sure to check!

How do DICOM SOP classes differ from one another, and why is this important to know?

There are many different types of DICOM SOP classes, each with its own unique purpose. For example, the “Image Storage” SOP class stores image pixel data, while the “Basic Text Structured Report” SOP stores textual information. DICOM files containing images and other medical data are stored in individual SOP classes. These classes differ from one another in terms of file format, compression, resolution, and other factors

For prospective PACS administrators, recognizing which SOP class is used for each medical file is essential for understanding how data is processed and stored by the respective systems.  More seasoned imaging professionals not only know how to recognize these SOP classes but also how to configure them…

As such, knowing which SOP class to use is important during installation of modalities and implementation of PACS. One must know which SOP class to assign to images that are captured… to ensure proper acquisition and storage into the respective PACS where it can also be viewed.  For example, if you need to store high-resolution images, you will want to use a different SOP class than if you only need to store low-resolution images. Using the wrong SOP class can result in lost data or compromised image quality.

DICOM SOP Classes without Pixel Data

DICOM SOP classes without image pixel data include PR, SR, KO and others. Each of these has its unique set of characteristics and is used to represent different data types.

What are DICOM PR, SR, KO files?

DICOM SOPs for PR, SR, and KO provide additional information about an image. PR refers to presentation states, which provide additional information about an image, such as measurements or annotation. SR is short for structured reports, which provide detailed information about the content of an image. Finally, KO files are key objects that flag certain images in a study. 

Let’s make something clear here.  PR, SR, KOs are actually the modality attribute (0080,0060) DICOM tag .  Not the SOP Class name. Though as a technologist or a PACS administrator, we may reference these files as simply a PR, SR or KO.  That’s because it is simpler to describe these files by their modality tag.  When viewing a study list in most DICOM or PACS viewers they usually appear as the following:

Patient NamePatient IDPatient DOBAccessionStudy DescriptionModalityStudy Date
Patient, Test1122334409/09/915022369Chest CTCT, PR, SR10/12/22
Patient, Test1122334409/09/915034876EchocardiogramUS, SR10/10/22
Patient, Fake1100110001/02/855044391MRI BrainMR, KO10/11/22
Patient, Fake1100110001/02/855044330Breast MammographyMG, SR10/01/22

What does a technologist see when they look at this?

From a brief glance a technologist can quickly gather that there are two patients on the study list.  One patient had a CT and ultrasound procedure.  The other patient had an MRI and a Mammogram.  They may gather this from study description and the modality fields.

What does an imaging informatics professional see?

Perhaps a PACS administrator could gleam more information from this.  The seasoned professional may infer the following from experience. 

  • For the CT, they’d know there are presentation states (PR) and structured reports (SR) in the study by looking at the modality field.
    • They may infer that the presentation states are the commonly used SOP class GSPS (Grayscale Softcopy Presentation State Storage SOP Class)
    • They may infer that the SOP class for the structured report is a RDSR (X-Ray Radiation Dose SR Storage SOP class)
  • For the Ultrasound, they can infer the SR is contains measurement values.  This is commonly stored as the Comprehensive SR Storage SOP class.
  • For the MR, the KO would mean there is a key object SOP present. That would be the Key Object Selection Document Storage SOP class.
  • Lastly for the Mammo, the SR could possibly be a CAD (Mammography CAD SR Storage SOP Class)

The SOP Class names for these PR, SR and KO files could fall under the following categories.  Here are just some possible SOP Classes

Presentation State SOP Class NamesSOP Class UIDModality
Grayscale Softcopy Presentation State Storage1.2.840.10008.
Color Softcopy Presentation State Storage1.2.840.10008.
Key Object SOP Class Names
Key Object Selection Document Storage1.2.840.10008.
Structured Report SOP Class Names
Mammography CAD SR Storage1.2.840.10008.
Basic Text SR Storage1.2.840.10008.
Enhanced SR Storage1.2.840.10008.
Comprehensive SR Storage1.2.840.10008.
X-Ray Radiation Dose SR Storage1.2.840.10008.
Radiopharmaceutical Radiation Dose SR Storage1.2.840.10008.
Enhanced X-Ray Radiation Dose SR Storage1.2.840.10008.5.​1.​4.​1.​1.​88.​76SR

Does it make more sense now that imaging professionals refer to these non-image DICOM files by the modality tag rather than the SOP Class?

How do DICOM files like PR, SR, and KO appear in PACS?

DICOM files like PR, SR, and KO can appear in PACS (Picture Archiving and Communication System) in a number of ways. For example, they may be readable as a DICOM header or included as part of a DICOM object sequence.

In addition, some PACS implementations may also provide a way to view DICOM files from a separate application window. Regardless of how they are accessed, these non-image DICOM files provide important information about the image data that they represent.

For instance, a PR defines the characteristics of a particular image, while an SR contains detailed information about the study in which the image was acquired.  For example a comprehensive SR in an Ultrasound study will detail measurement information.  While the PR can contain annotations on the Ultrasound images.

Why is it important to understand these non-pixel DICOM files? 

In some PACS, some non-image objects are not included in the image count.  For example, an x-ray can be stored to PACS with three DICOM objects.  Two x-ray images (PA and Lateral view) and a radiation dose structured report (SR).  However, in the PACS viewer the study may appear to have 2 images in the image-count column.  This can be confusing to the PACS administrator. 

Let’s take the example of comparing a legacy PACS to a VNA viewer. Prior to launching the study, an end user can see details of the study on the study list.

Patient NamePatient IDPatient DOBAccessionStudy DescriptionModalityStudy DateImage Count
Patient, Test1122334409/09/915022354Chest X-RayDX, SR10/12/222
Sample PACS Study List

In other DICOM viewers such as a VNA viewer, the non-image objects for the exact same study may show up in the image count.  Showing 3 images. 

Patient NamePatient IDPatient DOBAccessionStudy DescriptionModalityStudy DateCount
Patient, Test1122334409/09/915022354Chest X-RayDX, SR10/12/223
Sample VNA Study List

This will be confusing to end users who only expect to see two diagnostic images.  The PACS administrator must be able to explain that the SR may show up as a DICOM image in the count but since there is no pixel data, it may not display as a viewable image.

Not all PACS or VNAs behave this way. It depends on the vendor. The PACS administrator must understand how their particular PACS displays DICOM objects in order to set expectations for the end users.


DICOM SOP classes are important parts of the DICOM standard. They play a significant role in the communication and storage of medical images. By understanding the different types of DICOM files and SOP classes, a PACS administrator or radiologic technologist can better support their medical imaging workflows.

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