A method, including registering a computerized tomography (CT) scan of a patient with a tracking system configured to track a location and an orientation of a sensor in the patient, and displaying the CT scan on a screen. The method further includes tracking coordinates of a probe having the sensor inside the patient using the tracking system. Based on the tracked coordinates, an icon representative of the location of the sensor is overlaid on the CT scan on the screen. The method also includes overlaying on the CT scan a ray, issuing from the icon in a direction corresponding to the orientation of the sensor, and traversing the screen to a termination point chosen responsively to a gray level distribution of the CT scan.