Extending 2-D BW slice image to its corresponding 3-D image?

Hi All,

Available Data:

1. m x n x 30 grayscale3D image, variable name im3d
2. m x n B/W image (which is, for example, is the BW image of slice number 15 in im3d, i. im3d(:,:,15) ). The 2D B/W image contains the ROI (Region of Interests) of the components that I want to look closer. Say this variable name is BW2D

Problem: How can one use the BW2D as a reference to create new 3D ROIs in the im3d ?

Rough Ideas:

1. Can I use the centroids of BW2D to get the ROI of corresponding im3d ? So this would be like using bwconncomp on the 3D image, but with the locations of the centroids specified, can it be done in MATLAB?
2. One could simply use BW2D to maskout im3d(:,:,15), and get all the grayscale values that contained within the ROI. Then with this information, simple thresholding can be done on im3d. But this would be not really desirable, as BW2D was generated using image registration, so no thresholding was involved, therefore I try to avoid thresholding.

So does no.1 can be done in MATLAB, or there are even better ideas?