We describe a simple, safe, and inexpensive image overlay system to assist surgical interventions inside a conventional CT scanner. The overlay system is mounted non-invasively on the gantry of the CT scanner and it consists of a seven degrees-of-freedom passive mounting arm, a flat LCD display, and a light brown acrylic plate as a half mirror. In a pre-operative calibration process, the display, half-mirror, and imaging plane of the scanner are spatially registered by imaging a triangular calibration object. Following the calibration, the patient is brought into the scanner, an image is acquired and sent to the overlay display via DICOM transfer. Looking at the patient through the half-mirror, the CT image appears to be floating inside the patient in correct size and position. This vision enables the physician to see both the surface and the inside of the patient at the same time, which can be utilized in guiding a surgical intervention. The complete system fits into a carry-on suitcase (except the mounting adapter), it is easy to calibrate, mounts non-invasively on the scanner, without utilizing vendor-specific features of he scanner.