Table C.8.17.14-1 specifies the Attributes of the Ophthalmic Optical Coherence Tomography En Face Image Module, which describe the Ophthalmic Optical Coherence Tomography En Face Image Module.
Table C.8.17.14-1. Ophthalmic Optical Coherence Tomography En Face Image Module Attributes
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Image identification characteristics. See Section C.8.17.14.1.5 for specialization. |
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Number of bits allocated for each pixel sample. See Section C.8.17.14.1.6 for specialization. |
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Number of bits stored for each pixel sample. See Section C.8.17.14.1.6 for specialization. |
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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. |
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Specifies the intended interpretation of the pixel data. See Section C.8.17.14.1.4 for specialization. |
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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. |
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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. |
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>Include Table 10-3 “Image SOP Instance Reference Macro Attributes” |
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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. |
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DCID 7202 “Source Image Purpose 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"). |
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>Include Table 10-19 “Algorithm Identification Macro Attributes” |
DCID 4270 “OCT-A Processing Algorithm Family” shall be used for Algorithm Family Code Sequence (0066,002F) |
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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. |
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See Section C.11.2.1.2 for further explanation. |
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Window Width for display. See Section C.11.2.1.2 for further explanation. |
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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). |
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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. |
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Referenced SOP Instance that contains the surface segmentation used in the creation of this SOP Instance. |
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Reference to a Surface Number (0066,0003) present in Surface Sequence (0066,0002). |
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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). |
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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). |
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The method used to obtain the Ophthalmic Axial Length. Enumerated values: |
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Specifies whether an Image has undergone lossy compression (at a point in its lifetime). Enumerated Values: Once this value has been set to 01 it shall not be reset. See Section C.7.6.1.1.5. |
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Describes the approximate lossy compression ratio(s) that have been applied to this image. |
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A label for the lossy compression method(s) that have been applied to this image. |
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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. Required if Photometric Interpretation (0028,0004) is MONOCHROME2. |
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Indicates whether a reference object (phantom) of known size is present in the image and was used for calibration. |
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Indicates whether or not image contains sufficient burned in annotation to identify the patient and date the image was acquired. |
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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. |
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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.
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OCT proprietary B-scan data (possibly a DICOM Raw Data Instance)
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Volumetric structural ophthalmic tomography image (Ophthalmic Tomography Image Instance)
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OCT angiographic flow volume information (Ophthalmic Optical Coherence Tomography B-scan Volume Analysis Instance)
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Structural en face image (Ophthalmic Optical Coherence Tomography En Face Image Instance)
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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 (D). The OCT angiographic flow en face image (F) may be derived using the OCT angiographic flow volume information (C) and the OCT surface mesh (D).
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.
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).
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.
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:
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Value 1 shall identify the Pixel Data Characteristics in accordance with Section C.7.6.1.1.2.
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Value 2 shall identify the Patient Examination Characteristics in accordance with Section C.7.6.1.1.2.
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Value 3 shall identify any Image IOD specific specialization in accordance with Section C.7.6.1.1.2 (optional).
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Other Values that are implementation specific in accordance with Section C.7.6.1.1.2 (optional)
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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.
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.