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C.5 Waveform Information Model​

The part of the composite information object that carries the waveform data is the Waveform Information Entity (IE). The Waveform​ IE includes the technical parameters of waveform acquisition and the waveform samples.​

The information model, or internal organizational structure, of the Waveform IE is shown in Figure C.5-1. A waveform information​ object includes data from a continuous time period during which signals were acquired. The object may contain several multiplex​ groups, each defined by digitization with the same clock whose frequency is defined for the group. Within each multiplex group there​ will be one or more channels, each with a full technical definition. Finally, each channel has its set of digital waveform samples.​

Patient

1 is the subject of 1-n

Study

Figure C.5-1. DICOM Waveform Information Model​

C.6 Harmonization With HL7​

This Waveform IE definition is harmonized with the HL7 waveform semantic constructs, including the channel definition Attributes​ and the use of multiplex groups for synchronously acquired channels. The use of a common object model allows straightforward​ transcoding and interoperation between systems that use DICOM for waveform interchange and those that use HL7, and may be​ viewed as an example of common semantics implemented in the differing syntaxes of two messaging systems.​

This section describes the congruence between the DICOM Waveform IE and the HL7 version 2.3 waveform message format (see​ HL7 version 2.3 Chapter 7, sections 7.14 - 7.20).​

C.6.1 HL7 Waveform Observation​

Waveforms in HL7 messages are sent in a set of OBX (Observation) Segments. Four subtypes of OBX segments are defined:​

•​The CHN subtype defines one channel in a CD (Channel Definition) Data Type​

•​The TIM subtype defines the start time of the waveform data in a TS (Time String) Data Type​

•​The WAV subtype carries the waveform data in an NA (Numeric Array) or MA (Multiplexed Array) Data Type (ASCII encoded​ samples, character delimited)​

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•​The ANO subtype carries an annotation in a CE (Coded Entry) Data Type with a reference to a specific time within the waveform​ to which the annotation applies​

Other segments of the HL7 message definition specify patient and study identification, whose harmonization with DICOM constructs​ is not defined in this Annex.​

C.6.2 Channel Definition​

TheWaveformModuleChannelDefinitionsequenceAttribute(003A,0200)isdefinedinharmonizationwiththeHL7ChannelDefinition​ (CD) Data Type, in accordance with the following Table. Each Item in the Channel Definition sequence Attribute corresponds to an​ OBX Segment of subtype CHN.​

Table C.6-1. Correspondence Between DICOM and HL7 Channel Definition​

In the DICOM information object definition, the sampling frequency is defined for the multiplex group, while in HL7 it is defined for​ each channel, but is required to be identical for all multiplexed channels.​

Note that in the HL7 syntax, Waveform Source is a string, rather than a coded entry as used in DICOM. This should be considered​ in any transcoding between the two formats.​

C.6.3 Timing​

In HL7, the exact start time for waveform data is sent in an OBX Segment of subtype TIM. The corresponding DICOM Attributes,​ which must be combined to form the equivalent time string, are:​

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C.6.4 Waveform Data​

The DICOM binary encoding of data samples in the Waveform Data (5400,1010) corresponds to the ASCII representation of data​ samples in the HL7 OBX Segment of subtype WAV. The same channel-interleaved multiplexing used in the HL7 MA (Multiplexed​ Array) Data Type is used in the DICOM Waveform Data Attribute.​

Because of its binary representation, DICOM uses several data elements to specify the precise encoding, as listed in the following​ Table. There are no corresponding HL7 data elements, since HL7 uses explicit character-delimited ASCII encoding of data samples.​

C.6.5 Annotation​

InHL7,WaveformAnnotationissentinanOBXSegmentofsubtypeANO,usingtheCE(CodedEntry)DataTypeCE.Thiscorresponds​ precisely to the DICOM Annotation using Coded Entry Sequences. However, HL7 annotation ROI is to a single point only (time refer-​ ence), while DICOM allows reference to ranges of samples delimited by time or by explicit sample position.​

C.7 Harmonization With SCP-ECG​

TheSCP-ECGstandardisdesignedforrecordingroutinerestingelectrocardiograms.SuchECGsarereviewedpriortocardiacimaging​ procedures, and a typical use case would be for SCP-ECG waveforms to be translated to DICOM for inclusion with the full cardiac​ imaging patient record.​

SCP-ECG provides for either simultaneous or non-simultaneous recording of the channels, but does not provide a multiplexed data​ format (each channel is separately encoded). When translating to DICOM, each subset of simultaneously recorded channels may be​ encoded in a Waveform Sequence Item (multiplex group), and the delay to the recording of each multiplex group shall be encoded​ in the Multiplex Group Time Offset (0018,1068).​

The electrode configuration of SCP-ECG Section 1 may be translated to the DICOM Acquisition Context (0040,0555) sequence items​ using TID 3401 “ECG Acquisition Context” and Context Groups 3263 and 3264.​

The lead identification of SCP-ECG Section 3, a term coded as an unsigned integer, may be translated to the DICOM Waveform​ Channel Source (003A,0208) coded sequence using CID 3001 “ECG Leads”.​

Pacemaker spike records of SCP-ECG Section 7 may be translated to items in the Waveform Annotations Sequence (0040,B020)​ with a code term from CID 3335 “ECG Annotations”. The annotation sequence item may record the spike amplitude in its Numeric​ Value and Measurement Units Attributes.​

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D SR Encoding Example (Informative)​

This Annex was formerly located in Annex K “SR Encoding Example (Informative)” in PS3.3 in the 2003 and earlier revisions of the​ Standard.​

The following is a simple and non-comprehensive illustration of the encoding of the Informative SR Content Tree Example in PS3.3.​

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