DICOM PS3.17 2020b - Explanatory Information

RRRR.2 Example Encoding of OBJ & MTL (Current)
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## RRRR.2 Example Encoding of OBJ & MTL

This Section contains example excerpts for encoding OBJ files and associated preview icons (optional), materials library file (MTL) (optional), and texture map images (optional).

### RRRR.2.1 Example A

A patient, Kevin Franz-Lopez, with Medical Record Number 547892459, will shortly be undergoing a complex partial nephrectomy to remove lesions on their left kidney. A 3D manufacturing model (encoded in OBJ) was created to manufacture a surgical planning aid representing the patient's unique anatomy.

A model was constructed from a CT dataset (CT1). The model was created on July 16, 2017 at 1:04:34 PM. The model was expressed as a single OBJ file (kidneymodel.obj) which makes use of two texture maps encoded using PNG (ntissue.png and fluid.png) and one texture map encoded using JPEG (distissue.jpg). The relationship between the OBJ and texture maps is captured in the materials list file (matlist.mtl). This set of files corresponds to the Encapsulated MTL and DICOM Images elements in Figure RRRR.1-1.

A preview icon was created showing the rendered 3D object for inclusion with the OBJ file when encapsulated.

Table RRRR.2-1 shows the Encapsulated OBJ.

Table RRRR.2-1. Encapsulated OBJ Example A

Attribute Name | Tag | Example Value | Comments
--- | --- | --- | ---
Content Date | (0008,0023) | 20170716 |
Acquisition DateTime | (0008,002A) | 20170716 13:00:34 |
Content Time | (0008,0033) | 13:00:34 |
... |  |  |
Modality | (0008,0060) | M3D |
...
Series Description | (0008,103E) | Nephrectomy Planning Models |
Referenced Instance Sequence | (0008,114A) |  |
%item |  |  |
>Referenced SOP Class UID | (0008,1150) | 1.2.840.10008.5.1.4.1.1.104.5 | Encapsulated MTL file SOP class
>Referenced SOP Instance UID | (0008,1155) | 2.999.89235.5951.35894.751 | UID of the encapsulated MTL file (see below) supporting this OBJ model
>Relative URI Reference Within Encapsulated Document | (0068,7005) | "matlist.mtl" | Relative URI that preserved the MTL file's original filename as referenced from within the OBJ file.
%enditem |  |  |
...
Patient Name | (0010,0010) | Franz-Lopez^Kevin |
Patient ID | (0010,0020) | 547892459 |
...
Image Laterality | (0020,0062) | L |
Burned In Annotation | (0028,0301) | YES | In this example, the creator of the model inscribed the patient's medical record number on a side of the model, to avoid the possibility of a wrong patient error.
Recognizable Visual Features | (0028,0302) | NO |
MIME Type of Encapsulated Document | (0040,0012) | model/obj |
%item |  |  |
>Referenced SOP Class UID | (0008,1150) | 1.2.840.10008.5.1.4.1.1.2.1 | Referenced object is an Enhanced CT Image Storage
>Referenced SOP Instance UID | (0008,1155) | 2.999.89235.5951.35894.153 | The multi-frame CT image from study CT1
%enditem |  |  |
...
Measurement Units Code Sequence | (0040,08EA) |  |
%item |  |  |
|  | (mm, UCUM, "mm") |
%enditem |  |  |
...
Document Title | (0042,0010) | Kidney Model |
Encapsulated Document | (0042,0011) | Byte stream representing the OBJ file. |
Source Instance Sequence | (0042,0013) | A sequence referencing CT1 source images |
Model Modification | (0068,7001) | NO |
Model Mirroring | (0068,7002) | NO |
Model Usage Code Sequence | (0068,7003) |  |
%item |  |  |
|  | (129013, DCM, "Planning Intent") |
%enditem |  |  |
Icon Image Sequence | (0088,0200) | Sequence containing an image | A pre-rendered view of the model

Since the above OBJ file contains a reference to a materials library (MTL) file, the MTL's contents must likewise be encapsulated in DICOM, as shown in Table RRRR.2-2.

Table RRRR.2-2. Encapsulated MTL Example A

Attribute Name | Tag | Example Value | Comments
--- | --- | --- | ---
SOP Instance UID | (0008,0018) | 2.999.89235.5951.35894.751 | UID referenced in the Referenced Instance Sequence of the Encapsulated OBJ object in the table above
...
Modality | (0008,0060) | M3D |
...
Series Description | (0008,103E) | Nephrectomy Planning Models |
...
Content Date | (0008,0023) | 20170716 |
Content Time | (0008,0033) | 13:00:34 |
...
Referenced Image Sequence | (0008,1140) |  |
%item |  |  |
>>Referenced SOP Class UID | (0008,1150) | 1.2.840.10008.5.1.4.1.1.7.4 | Multi-frame True Color Secondary Capture SOP class
>>Referenced SOP Instance UID | (0008,1155) | 2.999.89235.5951.35894.841 | UID reference to texture image used for normal kidney tissue (Multi-frame True Color Secondary Capture Instance)
>>Referenced SOP Class UID | (0008,1150) | 1.2.840.10008.5.1.4.1.1.7.4 | Multi-frame True Color Secondary Capture SOP class
>>Relative URI Reference Within Encapsulated Document | (0068,7005) | "ntissue.png" | Relative URI that preserved the first texture map's original filename as referenced from within the MTL file.
>>Referenced SOP Instance UID | (0008,1155) | 2.999.89235.5951.35894.842 | UID reference to texture image used for diseased kidney tissue (Multi- frame True Color Secondary Capture Instance)
>>Relative URI Reference Within Encapsulated Document | (0068,7005) | "distissue.png" | Relative URI that preserved the second texture map's original filename as referenced from within the MTL file.
>>Referenced SOP Class UID | (0008,1150) | 1.2.840.10008.5.1.4.1.1.7.4 | Multi-frame True Color Secondary Capture SOP class
>>Referenced SOP Instance UID | (0008,1155) | 2.999.89235.5951.35894.843 | UID reference to texture image used for fluid (Multi- frame True Color Secondary Capture Instance)
>>Relative URI Reference Within Encapsulated Document | (0068,7005) | "fluid.jpg" | Relative URI that preserved the third texture map's original filename as referenced from within the MTL file.
%enditem |  |  |
...
Patient Name | (0010,0010) | Franz-Lopez^Kevin |
Patient ID | (0010,0020) | 547892459 |
...
MIME Type of Encapsulated Document | (0040,0012) | model/mtl |
Document Title | (0042,0010) | Kidney Model Materials |
Encapsulated Document | (0042,0011) | Byte stream representing the MTL file. |
...

The example MTL file contains references to three texture images (see Referenced Image Sequence above) and these likewise need to be encoded in DICOM (if they are not natively DICOM). The Multi-frame True Color Secondary Capture Instance is used to represent such texture images in DICOM, regardless of the original format in which the texture map image was stored.

In our example, the pixel data is read from the PNG files "ntissue.png" and "distissue.png", and the JPEG file "fluid.jpg". Corresponding DICOM Multi-Frame True Color Secondary capture images are created for each of these texture maps, as is shown in Figure RRRR.2-1. The original filenames are preserved in the URI Within Encapsulated Document values within the Encapsulated MTL's Referenced Image Sequence.

Figure RRRR.2-1. Example of Converting Texture Map Images into DICOM Images and back again

An abbreviated version of the first of these three object's DICOM headers is shown in Table RRRR.2-3, focusing on how these relate to use with an MTL Instance.

Table RRRR.2-3. Multi-frame True Color Secondary Capture Texture Map Example A

Attribute Name | Tag | Example Value | Comments
--- | --- | --- | ---
...
Modality | (0008,0060) | TEXTUREMAP | Indicates that the image is a texture map, and not some other image taken of the patient
...

It is important to note that when de-encapsulating MTL file, the texture map images must be restored to both their original file name and file format (as indicated by the corresponding URI Within Encapsulated Document attribute values contained in the Encapsulated MTL instance that references the texture map images. This is done so that the file name references inside the MTL, which will be read by downstream OBJ-capable software, will still be valid. Thus, in our example, our first texture map DICOM image must be converted back into a PNG (as indicated by the file extension in Relative URI Reference Within Encapsulated Document value) and saved to the file system as "ntissue.png" in the same location as the OBJ and MTL files.

The two other texture map images would be encoded in a manner like the one above.

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DICOM PS3.17 2020b - Explanatory Information