Ultrasound

Ultrasound uses high frequency sound waves for imaging. One advantage of ultrasound is that it is relatively cheap. Also, ultrasound also uses no ionizing radiation, making it safe for long-term exposure. The main challenge in intraoperative ultrasound is the ultrasound transducer. The shape and size of the transducer is highly specific to the application. Otherwise ultrasound is well-suited to the operating room.
Ultrasound is compatible with any surgery tool, unlike MRI. Like any other operating room tool, size is an important constraint for ultrasound. Although the ultrasound transducer is usually very small, the entire set of ultrasound equipment can be quite bulky.

Sources:

Unsgaard, G.; "Brain Operations Guided by Real-time Two-dimensional Ultrasound: New Possibilities as a Result of Improved Image Quality," Neurosugery Online, 2002