= Image region of interest
An <image region of interest> in <ImageJ> is an explicit image selection whose geometry and image or slice association are used by subsequent operations. This is the program-level realization of a <region of interest>; it is not necessarily an object identified automatically by <image segmentation>. The guide describes selecting, measuring, saving and restoring selections with the ROI Manager.<ImageJ User Guide by Tiago Ferreira and Wayne Rasband>[Sections19 and 30.14.5, selection tools and ROI Manager, Add/Measure/Open/Save]{ref}
For an original two-frame example, let
$$
I_1=\begin{pmatrix}10&20\\30&40\end{pmatrix},\qquad I_2=\begin{pmatrix}40&30\\20&10\end{pmatrix}.
$$
A fixed selection of the left column has mean 20 in the first frame and 30 in the second. A rule selecting values at least 25 instead selects the lower row in $I_1$ and upper row in $I_2$; its mean is 35 in both frames. Thus a fixed geometric selection and a value-derived selection can produce different measurements on exactly the same frames.
A saved selection is not a copy of all underlying pixel data. Reusing it requires the corresponding images and a consistent coordinate interpretation. Record whether the same geometry is applied across an <image stack> or each frame receives a newly generated selection. The ROI Manager can retain slice associations, while its multi-measure operation can apply selections across slices; these are choices to specify, not evidence that an object has remained stationary.<ImageJ User Guide by Tiago Ferreira and Wayne Rasband>[Section30.14.5, slice association, Multi Measure and Remove Slice Info]{ref}
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