Picture archiving and communication system

An image as stored on a picture archiving and communication system (PACS)

The same image following contrast adjustment, sharpening and measurement tags added by the system

In medical imaging, picture archiving and communication systems (PACS) are computers, commonly servers, dedicated to the storage, retrieval, distribution and presentation of images. The medical images are stored in an independent format. The most common format for image storage is DICOM (Digital Imaging and Communications in Medicine). Electronic images and reports are transmitted digitally via PACS; this eliminates the need to manually file, retrieve or transport film jackets. A PACS consists of four major components: the imaging modalities such as CT and MRI, a secured network for the transmission of patient information, workstations for interpreting and reviewing images, and long and short term archives for the storage and retrieval of images and reports. Combined with available and emerging Web technology, PACS has the ability to deliver timely and efficient access to images, interpretations and related data. PACS breaks down the physical and time barriers associated with traditional film-based image retrieval, distribution and display.

Types of images

Most PACSs handle images from various medical imaging instruments, including ultrasound (US), magnetic resonance (MR), positron emission tomography (PET), computed tomography (CT), endoscopy (ENDO), mammograms (MG), digital radiography (DR), computed radiography (CR) etc. (see DICOM Application areas).

Uses

PACS has two main uses:

• Hard copy replacement: PACS replaces hard-copy based means of managing medical images, such as film archives. With the decreasing price of digital storage, PACSs provide a growing cost and space advantage over film archives in addition to the instant access to prior images at the same institution. Digital copies are referred to as Soft-copy.
• Remote access: It expands on the possibilities of conventional systems by providing capabilities of off-site viewing and reporting (distance education, telediagnosis). It enables practitioners in different physical locations to access the same information simultaneously for teleradiology.

PACS is offered by virtually all the major medical imaging equipment manufacturers, medical IT companies and many independent software companies. Basic PACS software can be found free on the internet.

One difficult area in PACS is interpreting the DICOM image format. DICOM does not fully specify the metadata tags stored with images to annotate and describe them, so vendors of medical imaging equipment have latitude to create DICOM-compliant files that differ in the meaning and representation of this metadata. A feature common to most PACS is to read the metadata from all the images into a central database, allowing the PACS user to retrieve all images with a common feature no matter the originating instrument. The differences between vendors' DICOM implementations make this a difficult task.

A PACS can store volume data from exams and reconstruct 3D images

Some medical modality vendors have defined private DICOM tags to introduce added features. Tags like this are permitted according to DICOM protocol and will not impact on the images in most cases, but will not operate when the image is viewed on a different platform.

Architecture

Typically a PACS consists of a central server that stores a database containing the images connected to one or more clients via a LAN or a WAN which provide or utilize the images.

More and more PACS include web-based interfaces to utilize the Internet as their means of communication, usually via VPN (Virtual Private Network) or SSL (Secure Sockets Layer). The client side software is often using ActiveX, JavaScript and/or Java. These are considered only suitable for very small practices that do not view studies containing more than one or two images, as web-based viewers are sandboxed by the parent web browser and cannot allocate enough memory to view even modestly sized image data sets. More robust PACS clients are full applications which can utilize the full resources of the computer they are executing on.

Definitions vary, but most claim that for a system to be truly web based, each individual image should have its own URL.

Client workstations can use local peripherals for scanning image films into the system, printing image films from the system and interactive display of digital images. PACS workstations offer means of manipulating the images (crop, rotate, zoom, window, level and others).

Modern radiology equipment and modalities feed patient images directly to the PACS in digital form. For backwards compatibility, most hospital imaging departments and radiology practices employ a film digitizer.

PACS image backup is a critical, but sometimes overlooked, part of the PACS Architecture (see below). HIPAA requires that backup copies of patient images be made in case of image loss from the PACS. There are several methods of backing up the images, but they typically involve automatically sending copies of the images to a separate computer for storage, preferably off-site.

Querying

The communication with the PACS server is done through dicom objects that are similar to dicom images, but with different tags. A query typically looks as follows:

• The client establishes the network connection to the PACS server.
• The client prepares a query object which is an empty dicom dataset object.
• The client fills in the query object with the keys that should be matched. E.g. to query for a patient ID, the patient ID tag is filled with the patient's ID.
• The client creates empty tags (tags with zero length string values) for all the tags it wishes to receive from the server. E.g. if the client wishes to receive an id that it can use to receive images (see image retrieval) it should create the tag SOPInstanceID (0008,0018) in the query object.
• The query object is sent to the server.
• The server sends back to the client a list of response dicom objects.
• The client extracts the tags that are of interest from the response dicom objects.

Image retrieval

Images are retrieved from a PACS server through a C-MOVE request, as defined by the DICOM network protocol. This request specifies where an image instance should be sent through an identifier known as the destination AETITLE. The server must be configured about the mapping of the AETITLE to a TCP/IP address and port, and as a consequence the server must know in advance all the AETITLEs that it will ever be requested to send images to.

Image backup

Digital medical images are typically stored on a Picture Archiving and Communication System (PACS) for retrieval. Computer images are fragile and can be lost very quickly. It is important (and required in the USA by the Security Rule's Administrative Safeguards section of HIPAA) that facilities have a backup copy of the images. Some companies, such as Medstrat, provide off-site back-up services that cover this requirement.

While each facility is different, the goal in image backup is to make it automatic and as easy to administer as possible. The hope is that the copies won't ever be needed. But, as with other disaster planning, they need to be available if needed.

Ideally, copies of images should be streamed off-site as they are created. (If using the internet, the Security Rule's Technical Safeguards section of HIPAA requires that the images be encrypted during transmission.) Depending on bandwidth and image volume, this may not be practical. Other options include removable media (hard drives, DVDs or other media that can hold many patients' images) and/or separate computers. These copies need to be protected.

Hardware RAID (Redundant Array of Inexpensive Discs) is a very powerful and inexpensive storage solution which provides much faster disk access while completely protecting against the failure of one (or even two) discs in the physical RAID array. In the event of a physical disc failure, the RAID array notifies an administrator via SNMP while continuing to serve data, albeit at reduced throughput. Typically, the administrator can replace the failed drive (i.e "hot-swap" it) without bringing down the array and without interruption to users. The array then rebuilds the array incorporating the new disc and performance returns to normal. Data written on RAID is subject to the same virus or software problems as the original image, but it is protected from physical hard drive failure.

A SAN (Storage Area Network) is a more expensive implementation of RAID hosted on a dedicated device which can accommodate large numbers of physical drives (dozens+) in a single array. This array presents as a single unit of space which can be sub-divided as desired. A resulting logical drive is connected to a server via iSCSI or Fiber Channel and shows up as a single drive on that server. SANs are fast, flexible, expandable and very robust.

Data stored on a RAID array of any sort is typically backed up to tape (which leaves a window of vulnerability between backups and restoring the files from tape can be a slow, complex process) or copied, in real time, to a slower, inexpensive disc in another machine at another location.

To guard against user error and provide users with the ability to revert to earlier versions of a file, Windows Server 2003 and higher offers the Volume Shadow Copy Service , a service that transparently maintains earlier file versions (subject to space limitations) and allows the user to choose earlier versions.

In the event that it is necessary to reconstruct a PACS from the backup images, the backup system should be able to be turned into a "super modality" that simply blasts all of its images back to the PACS. This will allow the PACS to continue receiving current images while also rebuilding its historical images at the same time.

When migrating images from one PACS to another, it is sometimes very difficult to get the old PACS to blast the images to the new one. This is another application where backup can be used to "restore" the images to the new PACS.

Backup infrastructure is often expensive, semi-autonomous and frequently results in extended downtime. However, it should still be flexible enough to be used for immediate partial or full restores, as well as performing the migration of images to a new PACS.

Integration

A chest image displayed via a PACS

A full PACS should provide a single point of access for images and their associated data. That is, it should support all digital modalities. Integration with digital mammography has thus far taken a different course than other modalities, however. When an institution implements full field digital mammography (FFDM), it can choose between two options for reading, manipulating, and reporting on the mammograms. The first option is to integrate the digital mammography device with the institution’s general PACS. The second option is to purchase a mammography-only specialty workstation or mini-PACS from the FFDM vendor and thereby isolate FFDM on its own technological island. Most institutions have chosen the latter option to date

Some experts consider the failure to integrate a mistake. The specialty workstations/mini-PACS have limited capabilities compared to a full-featured PACS, critics note. They negatively affect the efficiency of the radiology department in other ways, and also add unnecessary expenses and space requirements because the institution is using redundant technology and data sources .

One of the leading critics of the non-integrated approach is radiologist Michael Trambert, who has presented at the Radiological Society of North America conference and published articles on the benefits of integrating digital mammography with an institution’s existing PACS. Trambert bases his advocacy of integration on his experience and research at a clinic in Santa Barbara, Calif. At the clinic, he had access to specialty workstations for digital mammography, as well as a regular PACS workstation that integrated digital mammography with the institution’s general PACS. The specialty workstations were from Hologic, the institution’s FFDM vendor. The integration had been accomplished by DR Systems, the institution’s general PACS vendor and a PACS industry’s pioneer in digital mammography/PACS integration.

In his articles, Trambert has noted the following advantages of integrating digital mammography with a general PACS:

• Ability to easily compare mammograms to other modalities such as breast MRIs or ultrasounds on a single piece of equipment
• Avoiding the expense of a separate specialty workstation/mini-PACS and associated storage device, while leveraging the institution’s investment in its existing PACS
• Saving space in the radiology department instead of adding redundant equipment
• Enabling referring physicians and radiologists to access all radiologic studies via a single user interface, rather than learning a new interface for a specialty workstation/mini-PACS ,

In August of 2004, DR Systems was the first to announce that it had received FDA clearance for diagnostic reading of digital mammography images on a PACS . Since that time, other PACS vendors including Ramsoft, VEPRO, CareStream Health, GE Healthcare, Cedara, FUJIFILM, Philips Healthcare, Sectra, Emageon, and Siemens Medical Solutions have also obtained FDA clearance for FFDM.

A full PACS should also interface with existing hospital information systems: Hospital information system (HIS) and Radiology Information System (RIS). There are several data flowing into PACS as inputs for next procedures and back to HIS as results corresponding inputs:

In: Patient Identification and Orders for examination. These data are sent from HIS to RIS via integration interface, in most of hospital, via HL7 protocol. Patient ID and Orders will be sent to Modality (CT,MR,etc) via Dicom protocol (Worklist). Images will be created after images scanning and then forwarded to PACS Server. Diagnosis Report is created based on the images retrieved for presenting from PACS Server by physician/radiologist and then saved to RIS System.
Out: Diagnosis Report and Images created accordingly. Diagnosis Report is sent back to HIS via HL7 usually and Images are sent back to HIS via DICOM usually if there is a DICOM Viewer integrated with HIS in hospitals (In most of cases, Clinical Physician gets reminder of Diagnosis Report coming and then queries images from PACS Server).

Interfacing between multiple systems provides a more consistent and more reliable dataset:

• Less risk of entering an incorrect patient ID for a study – modalities that support DICOM worklists can retrieve identifying patient information (patient name, patient number, accession number) for upcoming cases and present that to the technologist, preventing data entry errors during acquisition. Once the acquisition is complete, the PACS can compare the embedded image data with a list of scheduled studies from RIS, and can flag a warning if the image data does not match a scheduled study.
• Data saved in the PACS can be tagged with unique patient identifiers (such as a social security number or NHS number) obtained from HIS. Providing a robust method of merging datasets from multiple hospitals, even where the different centers use different ID systems internally.

An interface can also improve workflow patterns:

• When a study has been reported by a radiologist the PACS can mark it as read. This avoids needless double-reading. The report can be attached to the images and be viewable via a single interface.
• Improved use of online storage and nearline storage in the image archive. The PACS can obtain lists of appointments and admissions in advance, allowing images to be pre-fetched from nearline storage (for example, tape libraries or DVD jukeboxes) onto online disk storage (RAID array).

Recognition of the importance of integration has led a number of suppliers to develop fully integrated RIS/PACS. These may offer a number of advanced features:

• Dictation of reports can be integrated into a single system. The recording is automatically sent to a transcript writer's workstation for typing, but it can also be made available for access by physicians, avoiding typing delays for urgent results, or retained in case of typing error.
• Provides a single tool for quality control and audit purposes. Rejected images can be tagged, allowing later analysis (as may be required under radiation protection legislation). Workloads and turn-around time can be reported automatically for management purposes.

DICOM Viewers

There are several DICOM Viewers available both free and proprietary. Some of the DICOM Viewers include: Myrian by Intrasense, Medstrat, eFilm, K-Pacs, DICOM Works, OsiriX, SureVistaVision , UniPACS, Syngo Imaging, VRRender, ImageJ and MicroDicom. Various viewers can connect directly to a PACS server or retrieve images from local storage. Of note, OsiriX is an open-source DICOM viewer.

History

The principles of PACS were first discussed at meetings of radiologists in 1982. Various people are credited with the coinage of the term PACS. Cardiovascular radiologist Dr Andre Duerinckx reported in 1983 that he had first used the term in 1981. Dr Samuel Dwyer, though, credits Dr Judith M. Prewitt for introducing the term.

Dr Harold Glass, a medical physicist working in London in the early 1990s secured UK Government funding and managed the project over many years which transformed Hammersmith Hospital in London as the first filmless hospital in the United Kingdom. Dr Glass died a few months after the project came live but is credited with being one of the pioneers of PACS.

Regulatory concerns

In 2008, USFDA published an guidance for industry under which the system has been regulated against 21 CFR 892.2050.

See also

• X-ray
• DICOM
• Digital Mammography and PACS
• Medical device
• Medical imaging
• Medical software
• Computed axial tomography
• Telemedicine
• Electronic health record (EHR)

External links

• [2] A comprehensive & very user friendly PACS system
• Teleradiology, PACS and DICOM Software List of free PACS and DICOM software available on the web
• DICOM info and a list of free tools
• History of PACS
• Medstrat A widely used PACS system
• PACS PACS News And Products.
• USC IPILab Research Article on Backup
• Veterinary PACS
• www.webpax.com
• www.ictforhealth.com PACS Training Courses, Educational products, Seminars & CBT