A probe, with a distal end in proximity to a patient's internal organ tissue. A position sensor at the distal end generates position signals showing a location and an orientation of the distal end. An ultrasonic transducer transmits an ultrasound pulse along the orientation and receives ultrasound pulse reflections from the tissue. Location sensors are attached to the distal end, contact a surface of the tissue at respective locations in the distal end vicinity, and output respective location signals. A processor identifies ultrasound pulse reflections received from front and rear surfaces of the tissue. The processor processes the location signals to find a tissue front surface orientation angle. The processor then estimates a tissue thickness along an axis normal to the front surface based on a time elapsed between receipt of the reflections and an inclination of the direction of the orientation relative to the front surface orientation angle.
