The next generation of picture archiving and communication systems (PACS) will make use of more sophisticated modality interfaces which address not just the issues of acquiring images but the consistent entry of patient information and the improvement of departmental workflow. This article examines the components, features, and technical specifications of tomorrow’s PACS.
is a Johns Hopkins University trained physicist who has been in
the medical imaging industry since 1980. He is the author of
numerous technical papers in the fields of high-energy physics,
heavy ion physics, and PACS. Dr. DeJarnette is currently
president of DeJarnette Research Systems, Inc., in Towson,
he earliest PACS systems were installed in the mid-to-late 1980s.
These prototype systems were limited by computer technology, cost,
rudimentary workflow concepts, and limited modality interface
technology. In most instances they were little more than
large-scale video acquisition and storage systems.
AT&T/Philips was the primary vendor for the first generation
systems. Only a few dozen were ever installed, and few if any are
in active use today. The U.S. military was heavily involved in the
evaluation of the AT&T/Philips product offering (Commview) at
Georgetown University Hospital in Washington DC.
The U.S. military learned from the Commview experience. In 1991,
a request for proposals (RFP) was issued to the medical imaging
industry to build a second generation PACS for deployment in Army
and Airforce medical facilities worldwide. This RFP came to be
known as the MDIS (Medical Diagnostic Information System). In
September of 1992, the contract was awarded to a joint venture
between Loral Aerospace and Siemens Gammasonics. MDIS was to be
developed with a strong reliance on communication standards,
particularly the ACR-NEMA V2, and later, digital imaging and
communications in medicine (DICOM).
A large number of the resultant second generation PACS systems
from various manufacturers have been installed. These systems were
characterized by the utilization of ACR-NEMA V2 and early DICOM
implementations for the acquisition of images from modalities;
limited integration of hospital information systems (HIS) and
radiology information systems (RIS); limited image viewing
paradigms; limited network printing capabilities; and largely
proprietary communications in all aspects of the system except for
image acquisition. Companies such as Loral/Siemens (now GEMS),
E-Systems (EMED division, now Access Radiology), Kodak (Vortech),
Agfa, Cemax-Icon (now Kodak), Olicon (now ALI), 3M (now Kodak) and
Siemens Medical Systems (Erlangen, Germany) offered the second
generation systems; most are still in use today.
The industry is now at the beginning of the next generation--the
third generation--of PACS solutions. Third generation solutions are
characterized by a heavy reliance on interface and communication
standards throughout the system, like the DICOM standard and the
Health Level 7 (HL-7) standard.
DICOM is the direct descendant of the ACR-NEMA V2 standard. This
standard was developed by industry under the auspices of a joint
venture between the American College of
Radiology (ACR) and the National Equipment Manufacturers
Association (NEMA). DICOM is in fact the ACR-NEMA V3 standard. The
name was changed in 1993 to DICOM in order to disassociate it from
any one standards body and to gain widespread international
HL-7 is purely medical informatics meant to standardize the
transfer of information between the HIS products of different
manufacturers. In order to optimize the workflow efficiency of an
institution making use of PACS, the PACS must be integrated with
the hospital information system and the radiology information
Some near-third generation solutions, outgrowths of second
generation systems, are currently in the marketplace. These systems
are limited by their second-generation design concepts and
architectures, which reduce performance. The U.S. Military and the
Veterans Administration, two early adopters of PACS technology, are
again helping to drive PACS concepts and design. The U.S. Military
has recently awarded a new PACS contract vehicle, known as
DIN-PACS, to replace the earlier MDIS contract. This contract is
for the deployment of third generation PACS throughout the
military; it calls for heavy usage of communication and application
standards such as DICOM and HL-7.
What have we learned?
There have been numerous lessons learned from the deployment of
first and second generation systems. These lessons account for all
components in a PACS.
--All first and nearly all second generation PACS systems have been
characterized as "central storage" systems, as opposed to
"distributed storage" systems. Central storage systems have the
advantages of reduced hardware costs and easier maintenance.
However, such systems have the disadvantages of poor scalability
and single point failure. Performance of central storage systems is
also relatively poorer, as the architecture requires that all
acquired images move to a central storage device before being
distributed to display stations. This deficiency can be made up for
by utilizing higher bandwidths and higher cost communications
The cost advantage of a central storage system is being eroded.
As the cost of storage components comes down, there is less reason
to use a central storage system. This was a significant advantage
in the past, but no longer. Such systems, designed for a single
radiology department, are less well suited to the distributed
environment radiology will require in the future. Moving images to
a central site for storage increases the likelihood of catastrophic
disk failure and increases communication costs.
The MDIS systems deployed in the early-to-mid 1990s are perfect
examples of central storage architecture. These systems made use of
image acquisition gateways to acquire images from modalities. These
images had to be forwarded on to the central storage facility
(WSU--working storage unit), where they would be made known to the
system database and would be stored on a redundant array of
independent disks (RAID), backed up by an optical jukebox. This
method provides higher performance with less chance of catastrophic
disk failure. The database co-resided with the image storage on a
single computer. This central storage facility would serve as a
central server for the PACS workstations, all connected in a
point-to-point fashion over proprietary high-speed fiber optic
The image retrieval performance of this type of system was far
and away the best in its day and remains one of the highest
performing systems available. However, this architecture did have a
number of weaknesses. The co-location of the image database and
image storage made the central archive a single point of failure.
The tight integration of the database and the storage system made
upgrading to new storage technology difficult, expensive, and of
high impact from a clinical perspective. Upgrading these early
systems required that the entire PACS facility be brought down for
periods of up to a few weeks. It is really this issue that,
arguably, most distinguishes central storage architectures from
distributed storages. Distributed storage architectures completely
de-couple the image storage components from the image database
(management) components; image database components are potentially
redundant and self-replicating. The independent storage components
are suitably distributable to minimize image movement within the
--Distributed storage systems can minimize communications bandwidth
requirements, reduce the likelihood of catastrophic failure, have a
minimal impact on clinical operations and, when properly designed,
can make upgrade to new storage technology less expensive.
As anyone who has lived through a PACS deployment will tell you,
you can be certain that your archive will grow over time. Storage
technology is a rapidly advancing science. The storage system you
purchase today will be functional but technically obsolete within a
few years. Therefore, your PACS system must allow for easy
migration to future storage technologies, or you will find yourself
re-purchasing your PACS sooner than you thought. This is the single
greatest advantage of physically separating your image management
(database) and image storage components.
Even today, when a vendor attempts to sell you a DICOM archive,
this single device is designed to include short-term storage,
long-term storage, and the database to manage these images. In a
true distributed storage architecture, these components are
de-coupled, creating individual components known as "short-term
store," "image manager (the database)," and "long-term archive."
The long-term archive is the storage component whose upgrade must
be the most carefully planned. It is this upgrade (or technology
migration of this component) which was not considered early on by
the designers of PACS.
The difficulty in migrating to new storage technology,
especially long-term storage technology, arises out of interfacing
any new storage technology to the existing PACS. The electronic and
software interfaces of today's technology and tomorrow's technology
may be very different. In a tightly coupled PACS archive component,
integration of a new long term storage device (at this "hardware
level") generally will require a significant amount of software
rewriting, re-integration, and perhaps even hardware modifications.
These tasks can make such an upgrade a very expensive proposition
for the vendor, which means that the customer will most certainly
incur some significant costs.
In a distributed storage architecture, the long-term archive
stands as a separate entity. Its interface, is a "system level"
interface, not a "hardware level" interface. These system level
interfaces are known as "hierarchical storage management" (HSM)
systems (they have also been referred to as application storage
management [ASM] systems). HSMs are designed to make the long-term
archive look like a network disk, from a software engineering
perspective. Typically, these systems are designed to work with a
hardware configuration consisting of a small, high speed disk
subsystem, a database, and the long-term storage hardware. The HSM
manages the migration of stored objects (files) between the high
speed disk subsystem and long-term storage. The HSM also presents a
simple "file handle" interface (a type of file naming system) to
the rest of the world. A distributed storage architecture PACS
makes use of this file handle interface. A DICOM long-term archive
is built by layering DICOM storage and DICOM query/move services on
top of the HSM.
The following section serves as a simple DICOM dictionary,
fundamental to any thorough discussion of PACS and their
SOP: service operator pair
--A SOP is a pair of software entities: one provides the ability to
make requests for a particular service, the other provides the
service. In DICOM, one entity is known as an SCU (service class
user) and the other as an SCP (service class provider).
Storage class SCU
--This is an entity which makes a request of another entity
(storage class SCP) that an image be stored. This component is the
image sender; the storage class SCP is the receiver.
Query class SCU
--This is an entity which makes a query request of a query class
SCP. The query in question is for a list of images with certain
attributes. Typically, a query class SCU would be an imaging
workstation and the query class SCP would be an image database that
would know where the images are stored.
Move class SCU
--This is an entity which makes a request of a move class SCP to
move a set of images to another location. The move class SCP is
generally also a storage class SCU entity, such that it receives a
request to move a set of images to another location and does so by
acting as a storage class SCU. The images are moved to a storage
class SCP, which could be the same entity which made the initial
query. If a storage class SCU makes a request to move images to
some entity other than itself, it is referred to as a "third party
--This entity acts as a query class SCP and a move class SCP. Such
an entity has the ability to respond to queries for lists of images
with specific attributes and then to move those images to another
location, based on a request from a move class SCU.
The distinction between a PACS archive and a long-term archive
may seem a trivial and semantic distinction, though this is not
actually the case. It is true that one could build a PACS archive
(which includes the long-term storage, short-term storage, and the
image management database) which makes use of HSM technology. The
result would be the two databases--a PACS image management database
and the HSM database--co-residing. From an engineering perspective,
this is problematic in terms of performance, machine loading, and
other technicalities; it is also more costly. Inevitably, the
vendor engineers choose to have one database which serves both the
image management and storage migration (HSM) functions. It is in
making this choice that we (vendors and customers) are backed into
a corner, making migration to future storage technology expensive
Simple migration to future storage hardware requires that the
current archive (storage system) not have a great deal of
application knowledge resident. Combining the two databases into
one mixes an application knowledge database (image management
database) with what could be an application knowledge-free database
(HSM database). There would now be no clean high level interface to
new storage technology. This interface is embedded in the single
database, which increases the engineering work to be done if one
migrates to new storage technology.
The real difference between what has been termed a PACS archive
and a long-term archive is the level of application knowledge which
is resident on these devices. A PACS archive has knowledge of the
storage application it is performing, whereas a long-term archive
as defined in this article has no application-specific knowledge. A
long-term archive stores neutral objects. These objects may be
images, but the archival system is not aware of this; it is the
image manager (database) that has this knowledge. This division of
application knowledge makes migration to future long-term storage
technology easier from an engineering standpoint and less obtrusive
from the clinical standpoint. This type of long-term archive is
known as a neutral object application habitat (NOAH).
It is generally recognized that a PACS without an RIS makes
little sense. One of the major benefits of a PACS, workflow
improvement, cannot be recognized without the integration of the
PACS with the radiology information system. Enterprise-wide
benefits are attained with the integration of the PACS/RIS and an
In first generation PACS, such integration was rare; integration
was more common with second generation systems but was still highly
limited. Today, it is not uncommon to see an RIS and a PACS
integrated in a unidirectional fashion, where information flows
from the RIS to the PACS but not back. Bi-directional PACS/RIS
integration is far less common.
Generally, an RIS will pass ADT (admission, discharge, transfer)
and order information to the PACS. It is also desirable that
reports be passed from the RIS to the PACS and that study status
and preliminary report information be sent back to the RIS. Today,
second generation PACS do this by making use of multiple databases.
On a basic level there would be a PACS image database and a RIS
database sharing (duplicating) information. In the majority of
second generation systems, however, three databases are required to
achieve this integration: an RIS database, a PACS image database,
and a middleware engine database, such as a PACS broker--an entity
which acts as an interface between the HIS/RIS systems and a PACS.
It enables the transfer of patient information, radiology
department orders, and ADT information to the PACS. All of these
databases maintain a common subset of data.
Multiple databases in a PACS/RIS system are not cause for
concern. However, the two or three databases required for PACS/RIS
integration in second generation systems is highly undesirable, as
the maintenance involved in the sharing of a common subset of data
is problematic. Such databases can become unsynchronized, and they
invariably do. This increases the maintenance burden and can lead
to clinical confusion. Therefore, it is ideal to have the PACS
image management database and the RIS database be one in the same.
It is important not to view PACS and RIS as independent systems or
products; PACS is a value-added addition to a RIS and a RIS is a
value-added addition to a PACS.
Some late second generation PACS/
teleradiology systems have made use of what has come to be known as
"Web technology." These early Web systems have proven useful in
many cases. What is perceived as Web technology has come to be
associated with the Internet, Web browsers, and low cost
applications. However, this is not a useful definition. In
actuality, it is a software technology which allows for the
widespread distribution of information and standardizes the
interaction between information systems. Web technology does not
imply usage of the Internet, and does not, in and of itself, imply
lower cost. Web technology can be used over the Internet, over
intranets, or over other communication infrastructures which
The Internet, however, has proven an excellent infrastructure
for teleradiology, where cost is a driving concern. There has been
some usage of pure Web technology in this application. Low cost,
general purpose Web browsers have been used to view radiological
images. These browsers, however, are suboptimal for this task. To
improve the viewing capabilities of these browsers, "add ons"
(applets) have been developed to remove some of the limitations.
For example, the ability to window/level cross sectional images
requires an add on. In general, teleradiology viewing applications
based on Web technology are poorer in terms of functionality and
performance than dedicated teleradiology viewing products.
It is generally thought that the usage of Web technology has
brought the cost of at home teleradiology systems down. In the case
of equal functionality and performance, the development cost of a
radiological image viewing enabled Web browser is the same as the
development cost of a dedicated non-Web based teleradiology viewer.
The widespread availability and usage of the Internet, TCP/IP and
other standards, as well as an increasing market is what drives
costs lower in this application, not the usage of Web based
The remote HIS/RIS data entry and review application and the
external distribution of radiology reports are true web
applications, which are gaining acceptance. The amount of
information moved in these applications is far less than is moved
in a teleradiology application. The information moved, patient
data, reports, and scheduling information is better suited to the
development of specialized application applets.
Both the internet and pure Web technology will find increasing
usage in the next generation of PACS.
First generation modality interfaces were achieved primarily
through video acquisition techniques, whereas second generation
interfaces made use of ACR-NEMA V2, proprietary digital, and early
DICOM storage interfaces. Such interfaces allowed for the
exportation of image information into the PACS. The "double entry"
problem, that of entering one set of patient information into the
RIS and an incorrect variant of that same information into the
modality, was not significantly addressed by first or second
generation systems. Some second generation modality interface
gateways ("protocol converters") made use of DICOM modality
worklist interfaces to rationalize the RIS and modality patient
information. However, such interfaces were the exceptions to the
Modality image acquisition, as a technical problem in PACS, has
largely been solved by the widespread implementation of the DICOM
standard and the availability of image "protocol converters," used
to interface pre-DICOM modalities. The modality interface problem
facing third generation PACS is that of truly solving the "double
entry" problem and improving workflow. Although most, if not all,
modalities manufactured today support the DICOM standard, they
support only the DICOM storage SOP portion of the standard. A
smaller number of modalities support DICOM worklist management SOP
or DICOM detached study management SOP, which are required to solve
the "double entry" problem; even fewer modalities manufactured
today support the DICOM modality performed procedure step SOP,
which is used to track the status of image acquisition
The next generation modality interfaces ("protocol converters")
will have to address these informatics problems. Such devices will
have to provide for the acquisition of images, the rationalization
of the patient information with the RIS patient information, and
the reporting of modality workflow status. These interfaces will
provide a complete DICOM interface to the modality. In the case of
some modalities, these interfaces will be supplied as a part of the
The best examples of such modality interfaces can be found in
the world of computed radiography. These interfaces present a
modality worklist to the operator, associate records from the
worklist with the digitized images, provide image quality control,
transmit the digitized images to multiple destinations, print
images to both local and network printers, and report modality
The most visible PACS component is the display station. In first
and early second generation systems, many vendors and customers
alike thought that the most important component in the PACS was the
display station, though we now realize that there is not really any
single most important component.
First generation PACS displays grew out of experience with
multimodality display technologies. These systems were largely
suitable for viewing cross sectional modalities. With the advent of
second generation PACS, more thought was given to the viewing of
radiographic images. The optimal display and viewing paradigms
differ dramatically for cross sectional images and radiographic
Early in the deployment of PACS, the debate raged as to how many
monitors were needed to make a useful display station. That debate
has largely been settled, with most vendors and customers feeling
that for diagnostic purposes, a dual head, high brightness, 2 K *
2.5 K display station is suitable. Four-head systems, common in
first and second generation PACS, are no longer thought to be
necessary (and certainly not cost effective) by the majority of
PACS customers. Some researchers argue that even lower cost display
stations are all that is needed for diagnostic purposes, though
this is yet to be proven.
Today, PACS display stations are broken into three types, based
on functionality: 1) primary diagnostic displays; 2) clinical
review displays; and 3) low cost teleradiology displays. Viewing
paradigms differ among these three types of displays. All first and
most second generation display stations were generally designed
with viewing paradigms not optimized for primary diagnostic
reading. Such systems served the roles of clinical review and
teleradiology display much better than that of primary viewing.
These systems are generally characterized by large areas of the
display monitor being taken up by distracting logos, icons, menus
and the like. In fact, researchers have done eye motion studies on
such display stations and determined that a radiologist spent up to
25% of their reading time not looking at the images, but rather
searching for menus, controls, and interacting with these controls.
Results of the above mentioned research pointed the way to a new
generation of display stations.
The new display stations are characterized by a "dark and stark"
appearance with few visible controls and distracting non-image
information on the display monitor. Previously, viewing controls
consisted of drop down menus and icons, each one needed to indicate
that a specific operation was to be performed. In such systems it
was not uncommon to have to work through three or more menus or
selections to perform some simple function. The new generation of
display stations has put the most common controls on a three-button
mouse, making the mouse buttons context sensitive to the state of
the display. This new control paradigm, termed "heads-up imaging,"
allows the viewer to spend more time concentrating on the images
and less time looking for the controls.
The next generation of picture archiving and communication
systems will differ dramatically from what we have seen in their
first and second generation. The new systems will make use of more
sophisticated modality interfaces, which address not just the
issues of acquiring images, but the consistent entry of patient
information and the improvement of departmental workflow. In the
future, PACS will be much better integrated with RIS and HIS.
Additionally, the new systems will be the first with product
offerings that include a full-featured RIS at a reasonable
In any sophisticated technical product or technology, it is not
until the arrival of 3rd or 4th generation products that the early
promise of that product or technology is realized. From here on,
PACS will bring significantly improved radiology department
workflow through improved display station viewing paradigms and the
tighter integration of HIS/RIS and PACS. The new generation will be
characterized by higher performance at a lower cost, lower cost
maintenance, and lower cost technology upgrade and migration.
Finally, PACS will see a widespread adoption, both internally in
radiology and in the wider medical enterprise, by hospitals and the
external medical community.
1. Stewart BK, Langer SG, Taira RK:
DICOM image integration into an electronic medical record using
thin viewing clients. Medical Imaging 1998. PACS Design and
Evaluation. Proceedings of SPIE 3339: 322-328, 1998.
2. Krupinski, EA, Lund PJ:
Comparison of film vs monitor viewing of CR films using eye
position recording, pp 269-279. Proceedings from SCAR, 1996.
3. Horii, SC, Grevera G, Feingold E, et al:
Prototype controls for a plain radiography workstation. Medical
Imaging 1998: PACS Design and Evaluation. Proceedings of SPIE 3339:
4. Stockham CD, DeJarnette WT, Levy VH, et al:
Implementation and experiences with a minimal GUI diagnostic
display station, Proceedings of SPIE, Medical Imaging, 1999.