DICOM PS3.3 2020d - Information Object Definitions

C.8.17.14 Ophthalmic Optical Coherence Tomography En Face Image Module (Current)
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#### C.8.17.14 Ophthalmic Optical Coherence Tomography En Face Image Module

Table C.8.17.14-1 specifies the Attributes that describe the Ophthalmic Optical Coherence Tomography En Face Image Module.

Table

Table C.8.17.14-1. Ophthalmic Optical Coherence Tomography En Face Image Module Attributes

Attribute Name | Tag | Type | Attribute Description
--- | --- | --- | ---
Image Type | (0008,0008) | 1 | Image identification characteristics. See Section C.8.17.14.1.5 for specialization.
Instance Number | (0020,0013) | 1 | A number that identifies this SOP Instance.
Bits Allocated | (0028,0100) | 1 | Number of bits allocated for each pixel sample. See Section C.8.17.14.1.6 for specialization.
Bits Stored | (0028,0101) | 1 | Number of bits stored for each pixel sample. See Section C.8.17.14.1.6 for specialization.
High Bit | (0028,0102) | 1 | Most significant bit for pixel sample data. Shall be one less than the value in Bits Stored (0028,0101). See Section C.8.17.14.1.6 for specialization.
Samples per Pixel | (0028,0002) | 1 | Number of samples (planes) in this image. Enumerated Values: 1
Photometric Interpretation | (0028,0004) | 1 | Specifies the intended interpretation of the pixel data. See Section C.8.17.14.1.4 for specialization.
Pixel Representation | (0028,0103) | 1 | Data representation of pixel samples. Enumerated Values: 0
Pixel Spacing | (0028,0030) | 1 | Nominal physical distance at the focal plane (in the retina) between the center of each pixel, specified by a numeric pair - adjacent row spacing (delimiter) adjacent column spacing in mm. See Section 10.7.1.3for further explanation of the value order. ### Note Since a patient's retina is curved and the image representation is planar, there can be an error in using Pixel Spacing (0028,0030) for measurements in the periphery of the image. En face imaging does not support wide field measurements.
Content Time | (0008,0033) | 1 | The time the image pixel data creation started.
Content Date | (0008,0023) | 1 | The date the image pixel data creation started.
Source Image Sequence | (0008,2112) | 1 | A Sequence that identifies the Images that were used to derive this Image. One or more Items shall be included in this Sequence. See Section C.12.4.1.2 and Section C.8.17.14.1.1 for further explanation.
>Include Table 10-3 “Image SOP Instance Reference Macro Attributes” |
>Purpose of Reference Code Sequence | (0040,A170) | 1 | Describes the purpose for which the reference is made, that is what role the source image or frame(s) played in the derivation of this image. Only a single Item shall be included in this Sequence.
>>Include Table 8.8-1 “Code Sequence Macro Attributes” | DCID 7202 “Source Image Purposes of Reference”. If the derived en face image is based upon OPT structure information, the Concept Code shall be (128250, DCM, "Structural image for image processing"). If the derived en face image is based upon OPT flow information, the Concept Code shall be (128251, DCM, "Flow image for image processing").
Derivation Algorithm Sequence | (0022,1612) | 1 | Software algorithm that performed the derivation. Only a single Item shall be included in this Sequence.
>Include Table 10-19 “Algorithm Identification Macro Attributes” | DCID 4270 “OCT-A Processing Algorithm Families” shall be used for Algorithm Family Code Sequence (0066,002F)
Ophthalmic Image Type Code Sequence | (0022,1615) | 1 | En face image type used to identify this SOP Instance Only a single Item shall be included in this Sequence. See Section C.8.17.14.1.3 for further explanation.
>Include Table 8.8-1 “Code Sequence Macro Attributes” | DCID 4271 “En Face Image Types”
Ophthalmic Image Type Description | (0022,1616) | 3 | Description of the en face image type.
Window Center | (0028,1050) | 1 | Window Center for display. See Section C.11.2.1.2 for further explanation. ### Note When Bits Allocated (0028,0100) is 8, the value is typically always set to 127 or 128.
Window Width | (0028,1051) | 1 | Window Width for display. See Section C.11.2.1.2 for further explanation. ### Note When Bits Allocated (0028,0100) is 8, the value is typically always set to 256.
Ophthalmic FOV | (0022,1517) | 3 | The horizontal field of view used to capture the ophthalmic image, in degrees. The field of view is the maximum image size displayed on the image plane, expressed as the angle subtended at the exit pupil of the eye by the maximum dimension 2r (where r equals the radius).
Referenced Surface Mesh Identification Sequence | (0022,1620) | 1 | Reference to the surface mesh(s) used in the creation of this SOP Instance. One or more Items shall be included in this Sequence. See Section C.8.17.14.1.2 for further explanation.
>Referenced SOP Instance UID | (0008,1155) | 1 | Referenced SOP Instance that contains the surface segmentation used in the creation of this SOP Instance.
>Referenced Surface Number | (0066,002C) | 1 | Reference to a Surface Number (0066,0003) present in Surface Sequence (0066,0002).
>Segmented Property Type Code Sequence | (0062,000F) | 1 | Sequence defining the specific property the surface represents. Only a single Item is permitted in this Sequence ### Note "Property" is used in the sense of meaning "what the surface represents", whether it be a physical or biological object, be real or conceptual, having spatial, temporal or functional extent or not. I.e., it is what the segment "is" (as opposed to some feature, Attribute, quality, or characteristic of it, like color or shape or size).
>>Include Table 8.8-1 “Code Sequence Macro Attributes” | BCID 4273 “Retinal Segmentation Surfaces”.
>Surface Mesh Z-Pixel Offset | (0022,1658) | 1 | Offset in number of pixels along the z axis by which the mesh data has been shifted when generating this SOP Instance. The mesh data is the Attribute Point Coordinates Data (0066,0016) of the surface mesh referenced by Attribute Referenced SOP Instance UID (0008,1155). ### Note If no offset is used the value is set to 0.
Ophthalmic Axial Length | (0022,1019) | 3 | The axial length measurement, in mm.
Ophthalmic Axial Length Method | (0022,1515) | 3 | The method used to obtain the Ophthalmic Axial Length. Enumerated values: MEASURED Measured axial length. ESTIMATED An estimated value based upon performing the examination (i.e., based upon surrogate markers of axial length). POPULATION A length that represents a population norm (i.e., not based upon a measured axial length or surrogate markers of axial length).
Lossy Image Compression | (0028,2110) | 1 | Specifies whether an Image has undergone lossy compression (at a point in its lifetime). Enumerated Values: 00 Image has NOT been subjected to lossy compression. 01 Image has been subjected to lossy compression. Once this value has been set to 01 it shall not be reset. See Section C.7.6.1.1.5.
Lossy Image Compression Ratio | (0028,2112) | 1C | Describes the approximate lossy compression ratio(s) that have been applied to this image. See Section C.7.6.1.1.5.2. Required if Lossy Image Compression (0028,2110) is "01".
Lossy Image Compression Method | (0028,2114) | 1C | A label for the lossy compression method(s) that have been applied to this image. See Section C.7.6.1.1.5.1. Required if Lossy Image Compression (0028,2110) is "01".
Presentation LUT Shape | (2050,0020) | 1C | Specifies an identity transformation for the Presentation LUT, such that the output of all grayscale transformations defined in the IOD containing this Module are defined to be P-Values. Enumerated Values: IDENTITY output is in P-Values Required if Photometric Interpretation (0028,0004) is MONOCHROME2
Calibration Image | (0050,0004) | 3 | Indicates whether a reference object (phantom) of known size is present in the image and was used for calibration. Enumerated Values: YES NO
Burned In Annotation | (0028,0301) | 1 | Indicates whether or not image contains sufficient burned in annotation to identify the patient and date the image was acquired. Enumerated Values: YES NO
Recognizable Visual Features | (0028,0302) | 1 | Indicates whether or not the image contains sufficiently recognizable visual features to allow the image or a reconstruction from a set of images to identify the patient. Enumerated Values: YES NO

##### C.8.17.14.1 Ophthalmic Optical Coherence Tomography En Face Image Module Attribute Descriptions

###### C.8.17.14.1.1 Source Image Sequence

An OCT en face image is derived from images obtained using OCT technology. The Source Image Sequence (0008,2112) shall convey the SOP Instances used to derive this en face SOP Instance.

If Attribute Purpose of Reference Code Sequence (0040,A170) is set to (128250, DCM, "Structural image for image processing"), the Source Image Sequence will reference an Ophthalmic Tomography SOP Instance.

If Attribute Purpose of Reference Code Sequence (0040,A170) is set to (128251, DCM, "Flow image for image processing"), the Source Image Sequence will reference an Ophthalmic Optical Coherence Tomography B-scan Volume Analysis SOP Instance.

A typical example of the image processing stages performed to generate en face images is shown in Figure C.8.17.14-1.

Figure C.8.17.14-1. Example of the Image Process Performed to Generate En Face Images

Figure Legend:

1. OCT proprietary B-scan data (possibly a DICOM Raw Data Instance)

2. Volumetric structural ophthalmic tomography image (Ophthalmic Tomography Image Instance)

3. OCT angiographic flow volume information (Ophthalmic Optical Coherence Tomography B-scan Volume Analysis Instance)

4. OCT surface mesh (Surface Segmentation Instance)

5. Structural en face image (Ophthalmic Optical Coherence Tomography En Face Image Instance)

6. En Face angiographic flow image (Ophthalmic Optical Coherence Tomography En Face Image Instance)

Stage 1: OCT technology is used to acquire a volumetric data set from a retinal region of interest. This volumetric data set (A) consists of multiple B-scans in a raster pattern, and multiple frames are acquired at each B-scan location. The B-scans are acquired in the manufacturer's proprietary format for analysis and storage. If this information is stored in DICOM, it can use the Raw Data Storage SOP Class.

Stage 2: The OCT proprietary B-scan data (A) (or DICOM Raw Data SOP Instance) is then analyzed to derive the volumetric structural ophthalmic tomography image (B). From (B) one or more OCT surface meshes (D) are generated to delineate the anatomical boundaries. The difference in signal between the frames of each individual B-scan is analyzed to produce the OCT angiographic flow volume information (C).

Stage 3: Clinicians typically make their assessment based upon two types of OCT en face images. The structural OCT en face image (E) is derived by using pixel information in (B) and two surface meshes (C). The vascular OCT en face image (F) may be derived using the volumetric structural ophthalmic tomography image (B) , the OCT surface mesh (D) and the OCT angiographic flow volume information (C).

En face images are typically derived by the acquisition modality that generated the Ophthalmic Tomography Image, Surface Segmentation and Ophthalmic Optical Coherence Tomography B-scan Volume Analysis SOP Instances or by image workstations that received the respective Ophthalmic Image, Surface Segmentation and Ophthalmic Optical Tomography B-scan Volume Analysis SOP Instances via DICOM Storage.

### Note

Image workstations receiving ophthalmic tomography images may choose to evaluate the structural ophthalmic tomography image and generate a different set of segmented surfaces than defined by an acquisition device. The surface segmentation information can be stored in a separate Surface Segmentation SOP Instance.

The Ophthalmic Tomography Image, Surface Segmentation, Ophthalmic Optical Coherence Tomography B-scan Volume Analysis and the Ophthalmic Optical Coherence Tomography En Face Image SOP Instances all reside in different DICOM Series. They share the same spatial Frame of Reference which is identified in Attribute Frame of Reference UID (0020,0052) (i.e., the value of Frame of Reference UID (0020,0052) is the same in each SOP Instance). Figure C.8.17.14-2 illustrates the relationships between the OCT angiography based SOP Instances.

Figure C.8.17.14-2. Relationships Between OCT-A Based SOP Instances

###### C.8.17.14.1.2 Referenced Surface Mesh Identification Sequence

Referenced Surface Mesh Identification Sequence (0022,1620) identifies one or more segmentation surfaces used to generate the derived en face image. The segmented surfaces are described in the SOP Instance identified by Referenced SOP Instance UID (0008,1155) (e.g., Surface Segmentation Storage SOP Instance).

###### C.8.17.14.1.3 Ophthalmic Image Type Code Sequence

Implementations may generate many different types of derived en face images. Figure C.8.17.14-3 illustrates various derived enface image types. The Ophthalmic Image Type Code Sequence (0022,1615) is used to identify the type of derived en face image.

Figure C.8.17.14-3. Examples of En Face Images Types

###### C.8.17.14.1.4 Photometric Interpretation

Specifies the intended interpretation of the pixel data.

Enumerated Values:

MONOCHROME2

PALETTE COLOR

###### C.8.17.14.1.5 Image Type

The Image Type (0008,0008) Attribute (General Image Module, Section C.7.6.1) identifies important image characteristics in a multiple valued Data Element. For the Ophthalmic Optical Tomography En Face Image IOD, Image Type is specified as a Type 1 Attribute and further specialized as follows:

- Value 1 shall identify the Pixel Data Characteristics in accordance with Section C.7.6.1.1.2.

  Enumerated Values for Value 1:

  DERIVED

- Value 2 shall identify the Patient Examination Characteristics in accordance with Section C.7.6.1.1.2.

  Enumerated Values for Value 2:

  PRIMARY

- Value 3 shall identify any Image IOD specific specialization in accordance with Section C.7.6.1.1.2 (optional).

  Defined Terms for Value 3:

  MONTAGE

    identifies a Montage Image

- Other Values that are implementation specific in accordance with Section C.7.6.1.1.2 (optional)

### Note

A Montage Image is constructed out of several individual images, which also can be exchanged separately. The images used to create the montage image will be included in the Source Image Sequence (0008,2112) if those images are also exchanged. A Montage Image is identified as Image Type DERIVED\PRIMARY\MONTAGE.

###### C.8.17.14.1.6 Bits Allocated, Bits Stored, and High Bit

These Attributes shall be determined based upon the Photometric Interpretation (0028,0004):

Photometric Interpretation (0028,0004) | Bits Allocated (0028,0100) | Bits Stored (0028,0101) | High Bit (0028,0102)
--- | --- | --- | ---
MONOCHROME2 PALETTE COLOR | 8 | 8 | 7
16 | 12 | 11
16 | 16 | 15

###### C.8.17.14.1.7 Relationship Between Ophthalmic Tomography Image and Ophthalmic Optical Coherence Tomography B-scan Volume Analysis IODs

When generating an angiographic en face Image SOP Instance implementations need to understand the relationship between the Ophthalmic Tomography Image SOP Instance(s) and the Ophthalmic Optical Coherence Tomography B-scan Volume Analysis SOP Instance.

The Ophthalmic Optical Coherence Tomography B-scan Volume Analysis SOP Instance, which is a multi-frame SOP Instance, references one or more Ophthalmic Tomography Image SOP Instances using the Derivation Image Macro. The Derivation Image Macro defines Attributes at the Frame Level (i.e. each frame in the Ophthalmic Optical Coherence Tomography B-scan Volume Analysis SOP Instance, references an Ophthalmic Tomography Image SOP Instance and the Ophthalmic Tomography Image SOP Instance frame number that was used to generate the specific Ophthalmic Optical Coherence Tomography B-scan Volume Analysis frame).

Examples are shown in Section UUU.2 “Relationship Between Ophthalmic Tomography Image and Ophthalmic Optical Coherence Tomography B-scan Volume Analysis IODs” in PS3.17.

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DICOM PS3.3 2020d - Information Object Definitions