Complex sentence: The measurement is usually made by external ultrasound, occasionally by internal, invasive ultrasound catheters, see IVUS, measurements of the thickness of the wall of blood vessels can also be using other imaging modalities.
Split into simpler sentences: The measurement is usually made by external ultrasound and occasionally by internal, invasive ultrasound catheters; see IVUS. Measurements of the wall-thickness of blood vessels can also be done using other imaging modalities.