For example:
- on a waveform (1D dataset), a time or frequency interval
- on an image (2D dataset), the boundaries of an object
- in a volume (3D dataset), the contours or surfaces outlining an object
- in a time-volume (4D dataset), the outline of an object at or during a particular time interval
The concept of an ROI is commonly used in medical imaging. For example, the boundaries of a tumor may be defined on an image or in a volume, for the purpose of measuring its size. The endocardial border may be defined on an image, perhaps during different phases of the cardiac cycle, say end-systole and end-diastole, for the purpose of assessing cardiac function.
There are three fundamentally different means of encoding an ROI:
- burned in to the dataset, with a value that may or may not be outside the normal range of normally occuring values
- as separate purely graphic information, such as with vector or bitmap (rasterized) drawing elements, perhaps with some accompanying plain (unstructured) text annotation
- as separate structured semantic information (such as coded value types) with a set of spatial and/or temporal coordinates
Medical imaging standards such as DICOM provide general and application-specific mechanisms to support various use-cases.
For DICOM images (two or more dimensions):
- burned in graphics and text may occur within the normal pixel value range (e.g., as the maximum white value) (deprecated)
- bitmap (rasterized) graphics and text may be present in unused high bits of the pixel data or in a separate attribute (deprecated)
- vector graphics may be encoded in separate image attributes as (deprecated)
- unstructured vector graphics and text as well as bitmap (rasterized) overlay graphics may be encoded in a separate object as a that references the image object to which it is to be applied
- structured data may be encoded in a separate object as a in the form of a tree of name-value pairs of coded or text concepts possibly associated with derived quantitative information can reference spatial and/or temporal coordinates that in turn reference the image objects to which they apply
- reference locations may be encoded as in the form of spatial coordinates with an associated coded purpose, either as pixel coordinates by reference to specific images, or as coordinates in a named 3D cartesian space
- pixels (possibly non-contiguous) may be classified into encoded in a '''segmentation''' object as either binary or probabilistic values in a raster (which is not required to have the same spatial sampling or extent as the images from which the segmentation was derived)
For DICOM radiotherapy:
- contours of objects may be defined as , either as pixel coordinates by reference to specific images, or as coordinates in a named 3D cartesian space
For DICOM time-based waveforms:
- burned in values may occur with the waveform (deprecated)
- annotations may be encoded in a separate attribute can select multiple time points or a range of time points, either by sample number or specified time
- structured data may be encoded in a separate object as a in the form of a tree of name-value pairs of coded or text concepts possibly associated with derived quantitative information can reference temporal coordinates that in turn reference the waveform objects to which they apply
HL7 CDA ( Clinical Document Architecture ) also has a subset of mechanisms similar to (and intended to be compatible with) DICOM for referencing image-related spatial coordinates as observations; it allows for a circle, ellipse, polyline or point to be defined as integer pixel-relative coordinates referencing an external multi-media image object, which may be of a a consumer rather than medical image format (e.g., a GIF , PNG or JPEG ).
As far as non-medical standards are concerned, in addition to the purely graphic markup languages (such as PostScript or PDF ) and vector graphic (such as SVG ) and 3D (such as VRML ) drawing file formats that are widely available, and which carry no specific ROI semantics, some standards such as JPEG 2000 specifically provide mechanisms to label and/or compress to a different degree of fidelity, what they refer to as regions of interest.
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