8.2.6 MPEG2 MP@HL Image Compression
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### 8.2.6 MPEG2 MP@HL Image Compression

MPEG2 Main Profile at High Level (MP@HL) corresponds to what is commonly known as HDTV ('High Definition Television'). DICOM provides a mechanism for supporting the use of MPEG2 MP@HL Image Compression through the Encapsulated Format (see PS3.3). Annex A defines a Transfer Syntax that references the MPEG2 MP@HL Standard.

### Note

MPEG2 compression is inherently lossy. The context where the usage of lossy compression of medical images is clinically acceptable is beyond the scope of the DICOM Standard. The policies associated with the selection of appropriate compression parameters (e.g., compression ratio) for MPEG2 MP@HL are also beyond the scope of this standard.

The use of the DICOM Encapsulated Format to support MPEG2 MP@HL compressed pixel data requires that the Data Elements that are related to the Pixel Data encoding (e.g., Photometric Interpretation, Samples per Pixel, Planar Configuration, Bits Allocated, Bits Stored, High Bit, Pixel Representation, Rows, Columns, etc.) shall contain values that are consistent with the characteristics of the compressed data stream, with some specific exceptions noted here. The Pixel Data characteristics included in the MPEG2 MP@HL bit stream shall be used to decode the compressed data stream.

### Note

These requirements are specified in terms of consistency with what is encapsulated, rather than in terms of the uncompressed pixel data from which the compressed data stream may have been derived.

When decompressing, should the characteristics explicitly specified in the compressed data stream be inconsistent with those specified in the DICOM Data Elements, those explicitly specified in the compressed data stream should be used to control the decompression. The DICOM data elements, if inconsistent, can be regarded as suggestions as to the form in which an uncompressed Data Set might be encoded.

The requirements are:

- Planar Configuration (0028,0006) shall be 0

  ### Note

  The value of Planar Configuration (0028,0006) is irrelevant since the manner of encoding components is specified in the MPEG2 standard, hence it is set to 0.

- Samples per Pixel (0028,0002) shall be 3

- Photometric Interpretation (0028,0004) shall be YBR_PARTIAL_420 or MONOCHROME2

- Bits Allocated (0028,0100) shall be 8

- Bits Stored (0028,0101) shall be 8

- High Bit (0028,0102) shall be 7

- Pixel Representation (0028,0103) shall be 0

- Rows (0028,0010) shall be either 720 or 1080

- Columns (0028,0011) shall be 1280 if Rows is 720, or shall be 1920 if Rows is 1080.

- The value of MPEG2 aspect_ratio_information shall be 0011 in the encapsulated MPEG2 data stream corresponding to a 'Display Aspect Ratio' (DAR) of 16:9.

- The DICOM attribute Pixel Aspect Ratio (0028,0034) shall be absent. This corresponds to a 'Sampling Aspect Ratio' (SAR) of 1:1.

- Cine Rate (0018,0040) and Frame Time (0018,1063) or Frame Time Vector (0018,1065) shall be consistent with the limitations of MP@HL, as specified in Table 8-2.

Table 8-2. MPEG2 MP@HL Image Transfer Syntax Frame Rate Attributes

Video Type | Spatial resolution layer | Frame Rate (see Note 2) | Frame Time (see Note 3)
--- | --- | --- | ---
30 Hz HD | Single level, Enhancement | 30 | 33.33 ms
25 Hz HD | Single level, Enhancement | 25 | 40.0 ms
60 Hz HD | Single level, Enhancement | 60 | 16.17 ms
50 Hz HD | Single level, Enhancement | 50 | 20.00 ms

### Note

1. The requirements on rows and columns are to maximize interoperability between software environments and commonly available hardware MPEG2 encoder/decoder implementations. Should the source picture have a lower value, it should be re-formatted accordingly by scaling and/or pixel padding prior to MPEG2 encoding.

2. The frame rate of the acquiring camera for '30 Hz HD' MPEG2 may be either 30 or 30/1.001 (approximately 29.97) frames/sec. Similarly, the frame rate in the case of 60 Hz may be either 60 or 60/1.001 (approximately 59.94) frames/sec This may lead to small inconsistencies between the video timebase and real time.

3. The Frame Time (0018,1063) may be calculated from the frame rate of the acquiring camera. A frame time of 33.367 ms corresponds to 29.97 frames per second.

4. The value of chroma_format for this profile and level is defined by MPEG as 4:2:0.

5. Examples of screen resolutions supported by MPEG2 MP@HL are shown in Table 8-y. Frame rates of 50 Hz and 60 Hz (progressive) at the maximum resolution of 1080 by 1920 are not supported by MP@HL. Interlace at the maximum resolution is supported at a field rate of 50 Hz or 60 Hz, which corresponds to a frame rate of 25 Hz or 30 Hz respectively as described in Table 8-y.

6. An MPEG2 MP@HL decoder is able to decode bit streams conforming to lower levels. These include the 1080 by 1440 bit streams of MP@H-14, and the Main Level bit streams used in the existing MPEG2 MP@ML transfer syntax in the Visible Light IOD.

7. MP@H-14 is not supported by this transfer syntax.

8. The restriction of DAR to 16:9 is required to ensure interoperability because of limitations in commonly available hardware chip set implementations for MPEG2 MP@HL.

Table 8-3. Examples of MPEG2 MP@HL Screen Resolution

Rows | Columns | Frame rate | Video Type | Progressive or Interlace
--- | --- | --- | --- | ---
1080 | 1920 | 25 | 25 Hz HD | P
1080 | 1920 | 29.97, 30 | 30 Hz HD | P
1080 | 1920 | 25 | 25 Hz HD | I
1080 | 1920 | 29.97, 30 | 30 Hz HD | I
720 | 1280 | 25 | 25 Hz HD | P
720 | 1280 | 29.97, 30, | 30 Hz HD | P
720 | 1280 | 50 | 50 Hz HD | P
720 | 1280 | 59.94, 60 | 60 Hz HD | P

One fragment shall contain the whole MPEG2 bit stream.

### Note

1. If a video stream exceeds the maximum length of one fragment (approximately 4 GB), it may be sent as multiple SOP Instances, but each SOP Instance will contain an independent and playable bit stream, and not depend on the encoded bit stream in other (previous) instances. The manner in which such separate instances are related is not specified in the standard, but mechanisms such as grouping into the same Series, and references to earlier instances using Referenced Image Sequence may be used.

2. This constraint limits the length of the compressed bit stream to no longer than 232-2 bytes.

The Basic Offset Table in the Pixel Data (7FE0,0010) shall be empty (present but zero length).

### Note

The Basic Offset Table is not used because MPEG2 contains its own mechanism for describing navigation of frames. To enable decoding of only a part of the sequence, MPEG2 manages a header in any group of pictures (GOP) containing a time_code - a 25-bit integer containing the following: drop_frame_flag, time_code_hours, time_code_minutes, marker_bit, time_code_seconds and time_code_pictures.

The container format for the video bitstream shall be MPEG-2 Transport Stream, a.k.a. MPEG-TS (see [ISO/IEC 13818-1]) or MPEG-4, a.k.a. MP4 container (see [ISO/IEC 14496-12] and [ISO/IEC 14496-14]).

Any audio components present within the MPEG2 MP@HL bit stream shall comply with the restrictions as for MPEG2 MP@ML as stated in Section 8.2.5.

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