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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The direction cosines of the first row and the first column with respect to the patient. See Section C.7.6.2.1.1 for further explanation. |
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Specifies the location of this image in terms of location on a referenced localizer image. |
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>Include Table 10-3 “Image SOP Instance Reference Macro Attributes” |
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Image coordinates for the points on the referenced image that correspond to the Top Left Hand Corner (TLHC) and the Bottom Right Hand Corner (BRHC) of the En Face Image. Exactly two pairs of values where the first value of each pair is the row (vertical) offset and the second value of each pair is the column (horizontal) offset. Specified with sub-pixel resolution such that the origin at the Top Left Hand Corner (TLHC) of the TLHC pixel is 0.0\0.0, the Bottom Right Hand Corner (BRHC) of the TLHC pixel is 1.0\1.0, and the BRHC of the BRHC pixel is Rows\Columns (see Figure C.10.5-1, except that row and column order is reversed). The values must be within the range 0\0 to Rows\Columns of the referenced image. See Section C.8.17.10.1.1. Alignment of an En Face Image is equivalent to that of a transverse OPT Image. |
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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 4274 “En Face 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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Description of the volume or boundary surfaces used to select the en face image data from the source image(s). One or two Items shall be included in this Sequence. See Section C.8.17.14.1.2 for further explanation. |
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Part of the en face volume described by this Item. Enumerated Values: If value is ENTIRE, this Item shall be the only Item in the Ophthalmic En Face Volume Descriptor Sequence (0022,1627). Otherwise, two Items shall be included in the Ophthalmic En Face Volume Descriptor Sequence (0022,1627), one with value ANTERIOR and the other with value POSTERIOR. |
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Reference to the segmentations used in the selection of the en face data. One or more Items shall be included in this Sequence. Required if segmentation is used to select the en face data volume or surface. See Section C.8.17.14.1.2 for further explanation. |
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>>Include Table 10-3 “Image SOP Instance Reference Macro Attributes” |
Reference to a segmentation SOP Instance and one or more segments thereof in Referenced Segment Number (0062,000B). Referenced Segment Number (0062,000B) shall be present, even if the referenced segmentation SOP Instance contains only a single segment. |
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Sequence defining the specific property the segmentation represents. The Items in this Sequence shall be copied from the Segmented Property Type Code Sequence (0062,000F) of the referenced segmentation. The number of Items in this Sequence shall equal the number of values in Referenced Segment Number (0062,000B). |
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>>>Include Table 8.8-1 “Code Sequence Macro Attributes” |
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Sequence defining the modifier of the property type of this segment. |
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>>>>Include Table 8.8-1 “Code Sequence Macro Attributes” |
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Offset in pixels from the referenced segmentation surface, in the direction from the top towards the bottom of the source image frames. If no referenced segmentation surface is specified in this Item of Ophthalmic En Face Volume Descriptor Sequence (0022,1627), the offset is from the top of the source image frames. |
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A description of processing performed to construct the surface, such as interpolation between referenced segmented surfaces. |
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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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In this Section, the term "surface segmentation" (uncapitalized) is a generic reference to any type of segmentation that describes a surface. It includes both the Surface Segmentation IOD or SOP Class (capitalized) and the Height Map Segmentation IOD or SOP Class.
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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OCT surface segmentation (e.g., Height Map Segmentation or Surface Segmentation 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 segmentations (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 segmentationes (D). The OCT angiographic flow en face image (F) may be derived using the OCT angiographic flow volume information (C) and the OCT surface segmentation (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.
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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 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.
The Items of the Ophthalmic En Face Volume Descriptor Sequence (0022,1627) identify the parameters used to select the data volume (slab) from the source image(s) for the derived en face image. The en face image data may be selected by a volumetric segmentation specified in a single Item of the Ophthalmic En Face Volume Descriptor Sequence (0022,1627), or by specifying an anterior and a posterior surface respectively in two Items.
Anterior and posterior surfaces may each be specified by
If a referenced segmentation does not extend to the full pixel matrix of the en face image, the en face pixel values outside the extent of the segment are implementation specific. If the surfaces intersect, or reverse (anterior surface behind the posterior), the en face pixel values at the points of intersection or reversal are implementation specific.
Surface Offset (0066,0005) specifies an offset from the referenced segmentation surface to the surface of the en face image slab. If no segmentation is referenced in the same Item of the Ophthalmic En Face Volume Descriptor Sequence (0022,1627), the offset is from the top of the source image frames (i.e., the boundary surface is flat relative to the source image volume). The offset is a fractional number of pixels relative to the top of the frames of the source image, i.e., a positive number indicates offset toward the bottom of the frame.
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The two en face data boundary surfaces may be specified relative to the same referenced segmentation surface, potentially with different offsets. The reference is duplicated in the two Items of the Ophthalmic En Face Volume Descriptor Sequence (0022,1627).
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An ENTIRE en face data volume may be specified by two referenced surface segments in a single Item of the Ophthalmic En Face Volume Descriptor Sequence (0022,1627). The two referenced segments may be specified in two Items of the Referenced Segmentation Sequence (0008,114C), or by a single Item that identifies two surfaces in Referenced Segment Number (0062,000B).
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An en face data boundary surface may be specified by a combination (e.g., interpolation) of multiple referenced segments. The multiple referenced segments may be specified in multiple Items of the Referenced Segmentation Sequence (0008,114C), or by a single Item that identifies multiple surfaces in Referenced Segment Number (0062,000B). The nature of the combination may be described in Surface Processing Description (0066,000B).
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An application that wishes to specify an offset that has been determined in real world units, e.g., 0.015 mm above the segmented surface, must calculate the offset in pixels by using the pixel measures (row spacing) of the segmentation derivation image to convert from real world distances to fractional number of pixels, and use that value in the Surface Offset (0066,0005) Attribute.
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This Module allows the creating application to record its processing for purposes of provenance and traceability. It does not necessarily provide sufficient information for a receiving application to reproduce an identical en face image.
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.