A histogram of the input image intensity should reveal two peaks, corresponding respectively to the signals from the background and the object. (Note: It is a core assumption of the current version of the 3DMA software that the input data set consists of 2 phases, a phase comprising the object of interest and a single other background phase.) Global thresholding consists of setting an intensity value (threshold) such that all voxels having intensity value below the threshold belong to one phase, the remainer belong to the other. Global thresholding is as good as the degree of intensity separation between the two peaks in the image. It is an unsophisicated segmentation choice.
The global thresholdling option in 3DMA allows the user to pick a single global threshold for a 3D image or separate thresholds for each 2D slice in the image. Some experimental options has also been provided to provide automatic choice of threshold by performing a binormal fit to the two-peak histogram and setting a threshold at the inerpeak minimum as determined by the normal fits.
The thresholding option outputs the segmented image slicewise, in a packed bit (0,1) format. All voxels having intensity below the threshold value are set to 0; the rest are set to 1.