Digital Imaging and Communications
in Medicine (DICOM)

What is DICOM?

Digital Imaging and Communications in Medicine (DICOM) is an international, nonproprietary standard that specifies the protocols that are used to facilitate the exchange of medical images and related data in healthcare systems.

DICOM also defines the file formats for data management and assists in managing workflows in image capturing processes like X-rays. DICOM has a built-in communication protocol and it supports TCP/IP. In DICOM jargon, the capture of a medical image is called an acquisition and imaging equipment units are called acquisition devices.

What are the main DICOM functions?

The DICOM standard is concerned with five main functions in medical imaging: to transmit and persist images and related data between endpoints; to query and retrieve files; to perform specific actions like printing or archiving; to support digital imaging workflows; and to provide high quality images for diagnostics.

In digital imaging systems, these functions have the goal of enabling interoperability between hardware and software from different equipment manufacturers and software vendors.

What is the DICOM data model?

Medical imaging workflow

The DICOM data model supports a hierarchical medical imaging workflow that consists of several processes. The first process represents an initial patient visit and registration. The second process consists of one or more studies such as procedures or medical examinations. In the third process, each study is connected to one or more series, or modality, for example an X-ray. The fourth and final process in the medical imaging workflow represents the conformance of an acquired image and related data with the DICOM standard. 

What is a modality?

Sometimes, an imaging device is referred to as a modality, for example an X-ray machine, and the operators of imaging equipment are called modality operators but this is not technically correct.

A modality is one type of property, or attribute, of a DICOM data object. A DICOM data object is a real-world information entity (IE) and can be a patient, study, medical device, physician’s visit, series, patient schedule, document, diagnostic interpretation, or even a print queue.

A modality attribute describes the type of image that is required from a patient. Examples of modalities include radiography, mammography, angiography, fluoroscopy, computer tomography, and nuclear medicine. Modalities are stored with their abbreviations like US for ultrasound and ES for endoscopy.

Modalities include properties for objects that are not directly related to procedures or to types of images, for example a hard copy of a physician’s report has a modality with the abbreviation HC. In this case, the modality attribute does not describe the type of image stored but serves to identify the DICOM file type.

Besides the modality attribute, other attributes of an IE include elements like the patient’s name, the name of the consulting physician, the examination date, the duration of procedure, or the equipment used.

At its lowest level, an attribute consists of a tag, which is a code that identifies the attribute, a value representation (VR) that describes the data type and format, and a value. For example, a consulting physician will have the tag (0008,009C) and a VR of PN, which indicates that this attribute is a person’s name.

What is a DICOM service-object pair (SOP) class?

In DICOM, SOPs are used to uniquely identify instances of an IE and to specify how they are acted upon. SOPs are made up of an information object definition (IOD) and  DICOM service elements (DIMSE).

IODs describe the attributes of objects and include information about an image and related details, for example a physician’s case notes, radiation dosages, and the acquisition device used.

DIMSEs are commands, for example GET, FIND, and STORE, which application entities (AEs) use to act on objects. An AE is an endpoint or service in a medical imaging system such as an acquisition device that creates an image or software that requests an image.

DICOM does not use a versioning system and existing SOP classes are never updated. DICOM evolves through the addition of new SOP classes.

What are DICOM modules?

DICOM modules specify the attributes of data elements in a DICOM model.

In the DICOM workflow, DICOM modules are grouped into categories: patient, study, series, and image. These categories are broken down further into subcategories, for example computed tomography (CT) images, magnetic resonance (MR) images, and ultrasound (US) images fall in the image category. Patient, device, and specimen are modules in the CT images subcategory, for example the patient module specifies the attributes of a patient in a study.

What is a DICOM data set?

A DICOM data set is an object and an instance of an IE. A data set includes a number of elements that are relevant to the IE and one image attribute that contains pixel data.

Although a DICOM object can only have one attribute that contains pixel data, it may contain multiple frames to store multidimensional images and cine loops. Pixel data is compressed using common compression standards like JPEG, JPEG 2000, and lossless JPEG.

DICOM data sets are files that usually have a .dcm file extension. DICOM files can be exported to other formats like JPEG.

What are DICOM services?

The DICOM standard provides several services that specify how data is exchanged, how and where data is stored and archived, and how data is retrieved in a picture archiving and communication system (PACS).

The DICOM store service specifies how images and other objects such as documents are sent to a PACS.

The DICOM print service standardizes the way images are printed from different devices and specifies the print output presentation, for example an x-ray film.

DICOM-structured reporting allows the exchange of text reports in common formats like Extensible Markup Language (XML). 

The DICOM modality worklist service allows a modality to retrieve scheduling information for a procedure and it also acts as a task list. For example, an image acquisition device gets the information it needs to perform the acquisition from a provider like a radiology information system (RIS). This information is populated in the image metadata and can contain the type of, and reason for, the procedure, the physician’s details, and the type of acquisition equipment used.

The modality performed procedure step (MPPS) service, a relatively new service in the DICOM standard, works in conjunction with the worklist. The MPPS is a network transaction that sends image data and other relevant information about a completed procedure to a PACS or to an RIS. This data includes information about what was actually done during the image acquisition phase, not just what was scheduled to be done in the worklist.

DICOM viewers

Viewing DICOM files on iOS, Windows, or Linux is possible using several applications such as GIMP. Images may also be viewed online with most web browsers using open-source viewers or commercial viewers.

What does
the DICOM
standard cover?

The DICOM standard covers digital imaging procedures and protocols and data formatting in five areas: image management, image interpretation, network print management, procedure management, and offline storage management.

DICOM image management involves the exchange of files that include information related to images, for example what procedure was used, image interpretation, and printing instructions.

DICOM image interpretation is concerned with connections between images and user observations made in free text, audio, or links to coordinates where related material may be found.

DICOM network image management makes use of explicit semantics, which means that devices that send and receive images understand the structure of the exchanged information. This enables the exchange of images based on attributes and not only the file name as is the case with traditional FTP applications.

DICOM print management allows workstations and image acquisition devices to share printers on a DICOM network and specifies the SOP classes that need to be implemented to conform to the DICOM specifications for printing.

DICOM offline storage management allows users to exchange files using removable storage media.

The DICOM standard also defines the transport layer protocol. 

Where is DICOM
used?

DICOM is widely used for most types of medical imaging, for example magnetic resonance imaging (MRI), computed tomography (CT) scans, mammograms, and ultrasounds.

DICOM is used in most medical fields. Separate working groups focus on fields such as cardiology, dentistry, radiotherapy, dermatology, dentistry, ophthalmology, security, clinical trials, dermatology, pathology, conformance, web technology, and veterinary science.

Why is DICOM important?

Compliance

Legal compliance and quality control are enabled by the MPPS service, which tracks and records radiation information, among other things. DICOM was originally designed to assist medical practitioners who wanted to be able to correctly plan dosages for radiation therapy.

DICOM has enumerated values for some data to ensure compliance with the standard, for example right and left are mandatory options for the laterality attribute.  

Workflows

DICOM services allow optimized procedure scheduling, exposure durations, and accurate reporting about workflows.  

Diagnostics

Traditional image standards like JPEG and TIFF do not provide patient and procedure information so they are not useful for diagnostic purposes. DICOM data sets include relevant information about the patient, the examination, the equipment used, the medical practitioners and equipment operators involved, and the scope of the examination.

Data accuracy

Data accuracy is enabled by a single point of data entry, which is at the acquisition equipment endpoint.

The DICOM protocol enables acquisition operators to make decisions based on accurate information provided by MPPS messages, for example to cancel or to modify a procedure.

MPPS messages also transmit exceptions and error messages and automatically warn other systems and users about errors. 

Administration

The MPPS collects information about consumables like film, devices, and supplies and therefore enables automated billing and inventory management.

The DICOM storage commitment service ensures that images are stored separately from the acquisition equipment, which can be freed up to acquire new images in different procedures. The archiving system allows medical personnel to view files by checking them out and locking them for update.

Digital storage saves physical space and materials.

What are the disadvantages of using DICOM?

One disadvantage is the number of optional fields that it allows. There are more than 2000 attributes that can be added to a DICOM file. This increases the possibility of errors if data is inconsistently entered or is incomplete or inaccurate.

The DICOM grayscale standard display function specifies the luminance or density level acquisition equipment must use for the best quality images. However, when displaying images acquired on a device that was provided by one vendor on a device that was created by a different vendor, there may be differing amplitude ranges that could result in poor image quality. These parameters may need to be adjusted manually.

DICOM may be vulnerable to cyberattacks. In 2019, a Cylera researcher found a bug in DICOM that could allow hackers to embed malicious code in seemingly HIPAA-compliant images. This flaw would enable malware to be hidden in medical files and spread to other parts of the system. The solutions to mitigate the risk of attack are to use antivirus discovery tools and to implement network monitoring software.

Conclusion

 
The DICOM standard includes specifications for the management of different kinds of data, not only images. A typical DICOM file includes information about the series and study it belongs to, the patient, and, where applicable, information about the related image.

DICOM enables cross-vendor interoperability between imaging devices and digital imaging software, for example it allows an image created using a scanner from one vendor to be archived using software developed by a different vendor and evaluated on a viewing device created by a different vendor.

DICOM services mitigate user error in the capture and transfer of image data and ensure standard procedures are followed in the processing of data. DICOM services shorten the turnaround time between a procedure and the evaluation of the image data, which is vital in emergency trauma cases.