Method to remove surrounding body parts

How can i set a ROI which only selects the particular organ or body part.
for example in abdomen there are organs and other body part , how can i choose only one while doing segmentation using threshold.

In most imaging modalities, abdominal organs look quite similar to each other, so you cannot separate them by simple thresholding. You may try segmenting them using “Grow from seeds” effect or use other manual/semi-automatic methods. See some pointers for further help here: