DICOM and Waveform Data

T. Becker, R. Simon, Kiel, Germany


Contents:
 Introduction
 Digital Storage
 DICOM Extension
 Time Synchronisation
 Outlook

Introduction
 
DICOM was originally created as a protocol for image data exchange. In the meantime, DICOM is the accepted standard to store and exchange angiographic image data in most US and European cathlabs. Additionally DICOM can already be used for ultrasound images, MR, CT and other imaging modalities.

The standard, however, is expanding towards the inclusion of other, non-image digital data related to a patient and an examination (from Digital Imaging COmmunication in Medicine to Digital Imaging and COmmunication in Medicine). One important step into this direction is to define a protocol for the storage and exchange of ECG and hemodynamic curve data. The requirements are completely different from storing image data. The waveforms are not only to be stored as images, like a printout on a sheet of paper, but all raw data shall also be available in digital format. This means, that later measurements will be possible without loosing quality.

Digital Storage
 
Most hemodynamic systems already handle and store the acquired data in digital format, which has many advantages. Data can be reproduced and transferred without any loss of quality. Digital archiving and data transfer over networks can give physicians access to data, that have been acquired at any place at any time in the institution.

Current hemodynamic systems, however, use proprietary protocols for data storage: even small differences can make the exchange between systems impossible, so that typically data can be reviewed only on the acquisition system itself. The practical and logistic implication of this restriction are obvious. In contrast, a physician should be able to review hemodynamic data together with other related data (images, waveforms and reports) on a remote workstation via network connections. For this purpose, all data have to be stored in one common and standardised format. The DICOM waveform extension is created exactly for this purpose.

DICOM Extension
 
Supplement no. 30 on waveforms has been developed to integrate waveform storage into DICOM. This includes ECG, electrophysiological and hemodynamic curve data, such as pressures flow signals, independent from sampling frequency, amplitude and system sensitivity. Furthermore, audio signals such as voice comments can be entered.

In analogy to the storage of image data, it is possible (and required) to add patient and study information, that allow the correct relation to a patient, an examination and a status of the patient. Thus, image data and waveform data of one examination can be reviewed on one workstation at the same time.

Time Synchronisation
 
When image and waveform data are available on the same system, it should be possible to review both types of data in a synchronised mode side by side. For example, a physician wants to know the pressure related to a particular angiographic image within the cardiac cycle. For this purpose an exact time synchronisation is necessary.

One way is to timestamp all acquired data with the accurate acquisition time. Thus, a review system would be able to display an image of an angiographic run and a pressure value of a curve, that have been acquired at the same time. Currently, however, time stamps can only be created using the internal clocks of the acquisition systems and at present, these internal clocks are not synchronised. Contemporary imaging systems do not care much about the real time in the real world. Hopefully, future systems will be able to accept an external time synchronisation from a master clock in the catheterization laboratory. This master clock itself could be triggered for example from atomic clocks using existing protocols like the Global Positioning System. But at the moment we are not able to use this technique.

Figure 1: Both hemodynamic and imaging system receive a time signal from a master clock in cathlab. Using this time, the acquired data can be time stamped and displayed in a synchronised mode.
 
For the time being, an alternative mechanism has been embedded in the waveform supplement. All state-of-the-art imaging system for angicardiography can create a signal for each picture that is acquired. Actually this signal coincides with X-Ray tube activation ("X-ray on") and thus marks the start of the acquisition of a particular angiographic frame in an image run. This signal may be recorded by the hemodynamic system as an additional channel. All acquired hemodynamic data will then have a fixed temporal relationship to the image data and by digitally processing of the X-ray time marks, a viewing system can display both image and waveform data side by side in a synchronised mode.

Figure 2: The master clock triggers only the hemodynamic system. The imaging system generates a reference signal (X-Ray On), that can be acquired by the hemodynamic system. Using this signal, a time synchronised display is possible.

Figure 3 displays an example of left ventricular angiocardiography with synchronous acquisition of one ECG lead, left ventricular pressures, and the "X-ray" on signal .

The bars (in the right part above the ECG, in the left part within the ECG signal at bottom) indicate the instant of the acquisition of the angiographic frame.


 
Figure 3: Hardcopy of a DICOM viewer displaying an angiographic image and waveform data side by side in a synchronised mode.

Outlook
 
Supplement 30 on waveforms is currently in the state of a final draft to be submitted to the DICOM Standards Committee for final ballot, after which it will be an official extension of the standard applicable for everyone in within the DICOM environment.