The volume of archived digital images is increasing and will continue to rise more steeply than film storage volumes did in the past. Many filmless facilities have been caught off guard by this increase, which has been stimulated by many factors. The most significant is investment in new modalities that are digital and compliant with the Digital Imaging and Communications in Medicine (DICOM) standard. This investment brings more images into the filmless network. Computed radiography and digital radiography technologies are becoming more affordable, so more plain-film studies are now digital. A huge volume driver is the increase in images per study from multislice technology. New multislice CT scanners, for example, can generate as many as 800 to 1000 images per examination.
Storage requirements also are affected by disaster-recovery initiatives and state retention mandates. Health Insurance Portability and Accountability Act (HIPAA) guidelines for disaster recovery and data storage require the existence of a backup of imaging data that can be quickly recovered in case of disaster. This can be accomplished in a number of ways, but they all mean that a second copy of the data must be stored, which is most easily accomplished electronically.
THE TRUE COST OF STORAGE
Burgeoning imaging data volume presents both cost and technology challenges. The total cost of long-term ownership is an extremely important consideration, but is not always easy to calculate. It is important to look at both initial costs and recurring costs. As a facility considers implementing a picture archiving and communications system (PACS), it is difficult to predict future needs. Therefore, there must be a balance between scalability and cost containment. If cost were the only consideration, without regard for future scale, future costs associated with outgrowing the technology could negate the original return on investment. There are several factors that contribute to the total cost of ownership, and that the enterprise should consider as it evaluates any archiving solution. As the amount of information increases, and dependence on these data increases, it becomes more important for the data to be retrieved quickly and easily. Some archiving options have a lower cost but have slow access times. The time of radiologists and technologists is valuable. As part of any cost analysis, data-access times for archived information should be evaluated.
Although most types of storage media have been on the market for a long time, their storage formats and capacities are continuously being improved. This means that the drives that read the media also change. A 5-year-old tape cartridge may no longer be readable using current drive technology. The data must then be migrated to the newer technology, and that can introduce additional costs. Ever-increasing storage capacity also means increased demands on the storage setting. More physical space is required, and some solutions require more space than others. Some storage technologies also have specific requirements for power, cooling, service access, security, and so forth. These data-center environmental issues must be addressed, and associated costs must be considered. As a facility moves toward becoming a filmless enterprise, its dependence upon reliable, robust systems increases. The redundancy and reliability built into the system must strike a balance between budget constraints and a comfortable degree of risk.
The trend favoring online storage is facilitated by a continuous decrease in hard-disk cost per MB. In 1998, some analysts correctly predicted that the price per MB for hard disk storage would drop dramatically by 2002, based on a 10-year trend leading from $11.54 in 1988 to to 4.3 cents in 1998. Today, the overall average price per MB continues to decline. For that reason, online hard-disk archiving has become a viable storage solution for the primary archiving of images.
EVALUATING AN ARCHIVE SOLUTION
The two basic approaches to archiving are single tier and multitier; each has benefits.
The traditional approach to archiving has been a multitier approach in which all images are stored at the deepest archiving level (usually a jukebox with tape or optical disks). This was an inexpensive storage method, but at the expense of immediate access. To improve performance, only the images used most frequently were stored on high-cost, fast-access media, usually a redundant array of inexpensive disks (RAID) or direct attached storage (DAS). Those less frequently accessed were shifted to lower-cost, slower-retrieval media. Images from the deep archive were moved back to the intermediate and online levels (usually RAID) if they were needed more quickly.
As the cost of hard-disk storage declines, there is an increasing trend toward a single-tier archiving approach. All the images are stored on a single medium that can be accessed very quickly (this is usually RAID, or network storage). A redundant backup copy of the images is then stored on another, less expensive medium, such as tape or modular optical disk (MOD). In this approach, online storage is increased incrementally as volume grows.
Network storage is simply a system architecture whereby a server, which is dedicated to nothing more than file storage and sharing, sits on the network. Data stored on the network are made available directly to clients on the network. Two methods of archiving fall into this category: network attached storage (NAS) and storage area network (SAN). These network storage devices do not provide any of the activities that a server in a server-centered system typically provides, such as image management, compression, or routing. Network storage allows more hard-disk storage space to be added to a server-centered network without shutting the host server down for maintenance and upgrades. Unlike DAS, network storage devices do not need to be connected directly to the server itself, but can exist anywhere on a limited-area network.
Whether a single-tier or multitier approach offers the lower long-term cost of ownership really depends on a number of factors.
COST OF OWNERSHIP
For fast access to all images with high scalability, the single-tier approach is recommended. Since all images are online, all studies, both current and historical, are immediately available without the need for prefetching. The end user is then empowered to select and view the prior studies that are relevant. The major benefit of the single-tier approach, especially when using NAS or SAN, is that the storage capacity can be increased as needed, without interruption to systems and work flow. The initial investment consists only of the infrastructure and storage needed for 1 to 2 years. As time passes and storage requirements grow, more storage can be added easily. Rather than making a premium investment in total hard-drive capacity today, the organization can take advantage of the ever-decreasing cost of drives, adding more as prices decline in the future. For smaller enterprises, NAS or SAN can be cost prohibitive, and a multitier approach might be more suitable.
The cost benefit of a multitier approach sometimes outweighs the decrease in scalability and performance. In environments where little growth is expected, these latter factors are less significant. Both scalability and performance can be achieved, but there are issues to consider. First, to achieve good performance using a multitier approach, prefetching is a must. Algorithms must be developed to determine when (and which) prior studies need to be migrated from the deep archive to online storage. This introduces complexity and limitations to the number of prior studies that may be available to the end user. To achieve scalability in pure jukebox environments, additional jukeboxes can be added. Because the cost of housing and robotics can be high, it is usually most cost effective to plan for a jukebox large enough to accommodate 5 years’ imaging volume. Reliability is a critical factor in any archiving solution. If the system is down, clinicians cannot get to images, and radiologists cannot do their jobs. Both single-tier and multitier systems can be configured for reliability. Single-tier systems are inherently more reliable than multitier systems by virtue of the fact that data are stored redundantly on hard drives. In a multitier approach, buyers should consider only solutions that offer highly reliable removable media. There are many vendors with jukebox solutions, some more reliable than others. Jukeboxes have more moving parts and robotics. The more moving parts, the more a system is prone to failure. Although some vendors’ solutions are self calibrating, many offer jukeboxes requiring calibration. Organizations should be sure to ask the PACS vendor what jukebox technology is used and what reliability record it has shown.
Backup is a critical component of any disaster-recovery plan. Redundancy, both in system architecture and data, is a major consideration, especially with the advent of HIPAA guidelines.
With respect to system architecture, critical components within a server must be redundant. Beyond that, a failover server can be employed in case the primary server fails. Failover is a mechanism by which an operation automatically switches to a standby database, server, or network when the primary device fails. Ideally, these two servers should be in physically different locations in case of a disaster. Fully automated failover is not a prerequisite for an effective system in medical imaging, but there must be a mechanism in place to avoid downtime. Data redundancy can be achieved using a copy of data on removable media. The media should then be stored in a physically different location. Automated forms of backup can be considered, such as a redundant jukebox or NAS. Backup copies of the database should also be maintained.
It is necessary to strike a balance between cost and the level of risk mitigation that the organization is willing to undertake. As the organization makes decisions on its archiving solution, consider that it will need access to imaging data 7 years from now (or longer). Bearing the costs of technology compliance over time can be overwhelming. When thinking about any storage medium, one should ask what the expected future advances will be, whether those will be backward compatible, and how easily the data can be migrated from the old medium to newer media. If the organization chooses not to migrate from old drive technology, however, it will not gain performance. Servicing old removable media drives can also be a costly challenge. With a single-tier approach, media/drive compatibility is a less important issue. With hard-disk drive technology, the drive and disk are inseparable (the drive can always read the disk).
Whenever possible, simplicity and minimization of system administration are needed. A system is more cost effective when monitoring and administration are easy and convenient, with both local and remote access. Notification and alerts are also critical because they can head off potential problems. These goals can be met with both single-tier and multitier approaches, but the cost of ownership is less with a higher level of administrative simplicity. The physical space of an archive is important to consider. Multitier solutions rely heavily on physical components that require considerably more floor space than hard-drive based single-tier solutions. NAS packs 2.25 times more storage than MOD. Since the cost space is real, this expense should also be factored into the long-term cost-of-ownership analysis. For single-tier system, the initial cost is the server and NAS or SAN infrastructure. For smaller and mid-sized facilities, SAN is still cost-prohibitive because of its infrastructure costs. The recurring costs in a single-tier approach are service and additional storage increments. With the multitier approach,the initial cost is often higher than for NAS. These costs include a server, some form of short-term storage (usually RAID), and the long-term archive (usually a jukebox). The recurring costs would also be service and the cost of additional media or more jukeboxes.
For most imaging enterprises, a single-tier archiving approach is the best solution. With the cost of hard drives declining, NAS is a very feasible solution today. It is highly reliable, it offers immediate access to all examinations, and it easily scales as imaging volume grows. Best of all, media-obsolescence challenges need not be a concern.
For backup storage, a MOD or tape jukebox can be implemented. This requires a smaller investment as it will only be used for a redundant copy of the data. There is no need to keep the data online and available. If further system redundancy is desired, multiple servers should be considered.
If the enterprise already has a SAN in place for other purposes, take advantage of it. Otherwise, SAN is neither affordable nor is it necessary for an effective PACS.
The multitier approach still has its merits for smaller enterprises, but with a detailed long-term cost-of-ownership analysis, NAS will probably still come out on top as the solution of choice for many imaging facilities.
Barbara Dumery is senior product manager, eMed Technologies, Lexington, Mass.