For SPECT-based internal dose estimation, a portable detector is used to sample activity. The portable detector may selectively use far-field or near-field imaging for a SPECT scan. A camera and/or gyroscope assist in determining emission location and/or aligning activities from the different times. The time-activity curve or another dose is estimated using activities from different times where the activity for at least one time is from the portable detector, which may allow for more frequent sampling of activity and more accurate dose estimation.