In a conventional system, the x-ray source is a high-voltage vacuum-tube device that operates by accelerating electrons towards an x-ray target (anode) within the source. Electrons are stopped in the target where their kinetic energy is converted to x-ray radiation and heat. Because only about 1% of the electron energy is converted to x-rays, the source must operate at high power to produce sufficient x-ray radiation. Almost all of the power is converted to heat, meaning that the dissipation of heat from the x-ray target is a major design challenge. Because a stationary target would melt under the intense electron beam, the source is rotated at high speed inside a vacuum to distribute the heat in a circular track on the x-ray target, using thermal radiation for cooling. This inefficient cooling limits a conventional system to approximately 20 seconds of operation in the high-power diagnostic (cine) mode. The user must then wait several minutes for the target to cool sufficiently before imaging can resume.

Conventional rotating-anode x-ray source		NovaRay scanning x-ray source

NovaRay’s innovative approach to this problem has resulted in a system that provides direct liquid cooling of the target. The NovaRay x-ray source is a high-voltage vacuum-tube device that accelerates electrons towards an x-ray target within the source. As opposed to conventional systems, however, NovaRay’s source uses a moving electron beam and a stationary target, similar to the cathode-ray tube used in a television, except that it produces x-ray photons rather than light. The electron beam is steered from outside the vacuum by electromagnets. Where the electron beam strikes the x-ray target, it produces x-rays in all directions. The collimator, which is a plate with an array of holes, allows only those x-rays that are headed towards the detector to emerge. The remaining x-rays are absorbed within the collimator. As in a conventional system, our electron beam produces a large amount of heat in the x-ray target. The scanning electron beam distributes heat over the large area of the target, which is efficiently cooled by conduction to the liquid coolant flowing across the back side of the target. As the liquid coolant can carry away more heat than the electron beam deposits, NovaRay’s source can operate continuously at full power.