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Digital Imaging and Communications in Medicine (DICOM)

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214<br />

Chapter 9 <strong>DICOM</strong> Associations<br />

<strong>in</strong>g <strong>DICOM</strong>-illegal <strong>in</strong> this behavior; but if your software does it, it <strong>in</strong>vites serious<br />

problems <strong>and</strong> creates noticeable overhead.<br />

The number of concurrent <strong>DICOM</strong> associations accepted by any AE is often<br />

also controlled by <strong>DICOM</strong>/PACS vendors. That is, the AE is programmed not<br />

to process more than a limited number of transactions at any given time. This<br />

has noth<strong>in</strong>g to do with <strong>DICOM</strong> <strong>and</strong> is done ma<strong>in</strong>ly for software licens<strong>in</strong>g purposes;<br />

the more concurrent associations supported, the more vendors charge.<br />

If you work <strong>in</strong> a complex environment with many <strong>in</strong>teract<strong>in</strong>g devices <strong>and</strong> you<br />

are faced with this licens<strong>in</strong>g limit, I would recommend that you to get at least<br />

ten concurrent associations supported; if you can afford more, buy more.<br />

Concurrent users<br />

Many vendors also set a limit for concurrent users who can work on their<br />

system. This is particularly common for server architectures <strong>in</strong> which the<br />

same server (for image render<strong>in</strong>g or voice recognition) can be used simultaneously<br />

by multiple users.<br />

The ma<strong>in</strong> issue here is how well your vendor controls the user concurrency.<br />

For example, ten users might be currently logged <strong>in</strong>, but only four are work<strong>in</strong>g<br />

<strong>and</strong> the other six simply forgot to log off, or went to lunch, or switched<br />

to someth<strong>in</strong>g else. In well-designed software, only four currently active<br />

users should be counted as concurrent; that is, us<strong>in</strong>g the system at a given<br />

time. In real life unfortunately, most vendors would count all ten users as<br />

concurrent, even though no resources are allocated to six of them.<br />

Always ask your prospective vendors to def<strong>in</strong>e their <strong>in</strong>terpretation of “concurrent”,<br />

whether for users or for associations; sometimes this might differ<br />

from what it should really be.<br />

One of the best examples of both <strong>DICOM</strong>-compliant <strong>and</strong> proprietary tweaks<br />

would be the C-Store protocol (reviewed <strong>in</strong> 7.3). While other SOPs deal mostly<br />

with connectivity <strong>and</strong> searches, C-Store transmits the image data. Naturally,<br />

large volumes of digital images call for highly efficient transmission techniques,<br />

<strong>and</strong> C-Store has become the most frequent target for enhancements.<br />

With<strong>in</strong> the realm of <strong>DICOM</strong>, the major improvement to C-Store was the<br />

use of data compression. It was achieved with image compression Transfer Syntaxes,<br />

as already discussed <strong>in</strong> 6.2. Image compression does not change the overall<br />

structure of the <strong>DICOM</strong> data object conta<strong>in</strong><strong>in</strong>g the image. Instead, it simply<br />

compresses the contents of the “Pixel Data” element (7FE0,0010), which holds<br />

image pixels. Because image pixels account for the bulk of the data transmission,<br />

<strong>and</strong> can usually be compressed even losslessly up to 3–4 times, C-Store<br />

with compressed images can significantly improve data transmission on a slow<br />

network. Therefore, various image compression techniques, both public <strong>and</strong><br />

proprietary, cont<strong>in</strong>uously contribute to improv<strong>in</strong>g <strong>DICOM</strong> network <strong>and</strong> storage<br />

efficiency (Fig. 70).

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