DICOM PS3.3 2020a - Information Object Definitions |
Page 1025 |
C.8.17.10.1.1 Reference Coordinates
A frame can be described in terms of its "position" on another image. In the case of ophthalmic tomography (OPT) images with longit- udinal orientation, the OPT image corresponds to a vertical slice along a trajectory on the referenced image. Each column of the OPT frame is nominally perpendicular to the image. Therefore, each column of the OPT frame can also correspond to a particular pixel on anassociatedimage.ItisnormalinOPTanalysistoindicatethelocationoftheOPTslicebyshowingthecorrespondingpixellocations on the image. For Transverse frames, the position is indicated by two corners of a rectangle.
Note
The simple geometric relationship Modules are not practical to use for describing the relationship between OPT and retinal photography images. The retinal surface is highly curved, and the optical path is influenced by the lens and other eye structures. It is impractical to measure all of the geometric and optical properties of the eye to the degree needed to establish the relationship by geometric computation. It is easy to capture a retinal image and the slice path on that image as part of the OPT acquisition.
The OPT columns for a retinal slice are not strictly perpendicular to the retina and there are minor systematic distortions as a result. This DICOM Functional Group does not attempt to capture the data needed to measure or correct for these effects, except for the presence of the Maximum Along-scan Distortion Attribute (0022,0038).
The relationship between the columns in the image frame and the reference image are illustrated in Figure C.8.17.10-1.
Frames can be captured using a linear scan with uniform column spacing. This permits the column position to be described by giving theendpointsofthelinesegmentthatwasscanned.FigureC.8.17.10-1showssuchascanlabeledas"OCTFrame1".Whendescribing this frame using the Linear Column Locations Functional Group Macro, the Sequence Attributes would contain:
a.The Attributes of the Image SOP Instance Reference Macro would point to a frame of the reference image.
b.Reference Coordinates (0022,0032) would contain values that give the row and column coordinates for the image pixel on the referenced image that corresponds to the first column of the OPT Frame 1 and the row and column coordinates that corresponds to the last column of the OPT Frame 1.
Framescanalsobetakenalongcurvedpaths,orwithnon-uniformspacing.Theseframescannotbedescribedbyjustthetwoendpoints. These frames are described by specifying the referenced image pixel coordinates for each column in the frame. The frame labeled "OCT Frame 2" illustrates this kind of relationship. The Non-linear Column Locations Functional Group Macro description for this frame would contain:
a.The Attributes of the Image SOP Instance Reference Macro would point to a frame of the reference image.
b.Reference Coordinates (0022,0032) would contain 2N integer values. Their contents would be:
1-2 (e,f)
…other values 2L-1, 2L (g,h)
…other values 2N-1, 2N (x,y)
A three dimensional OPT raster scan image would be described by a series of Linear Column Location Sequences, one for each frame.
Note
One expected use of column location is the automatic generation of annotation graphics display. The slice location can be indicatedanddynamicallycontrolledonthereferenceimagewhileexaminingtheobservedslices.Thisreplacestherelatively inflexible practice of creating a modified retinal image with burned in annotation to indicate the slice locations.
Include Table C.8.17.8-2 “Ophthalmic Acquisition Parameters Macro Attributes”
BCID 4208 “Mydriatic Agent”.
DCID 82 “Units of Measurement”.
>Include Table 8.8-1 “Code Sequence Macro Attributes” 
BCID 4210 “Ophthalmic Tomography Acquisition Device”.
