Hi
In mark up there is only rectangular box ROI, I need circle ROI? to measure the signal in MRI
You can use the Paint or Scissors segment editor effect which have circle based drawing methods to create your segmentation using the Segment Editor module. You can then use the segmentation in the Segment Statistics module to get stats based on the intensity values it covers.
This process of segmenting to get stats is how you previously did things as in: