Patent ID: 6858848

Claim:
An instrument for obtaining 2-dimensional information about the distribution of mass of an object, the instrument including; a source of positrons, shielded so that the positrons annihilate and produce co-linear gamma-rays pairs in the immediate vicinity of the source, said source being located with respect to an object to be examined that at least some of the gamma-rays impinge on the object; a gamma-ray shield surrounding a gamma-ray detector having an aperture for facing at the object to be examined, said gamma-ray detector being located on the same side of the object as the source of positrons, and said gamma-ray detector being capable of detecting the arrival position of gamma-rays of said gamma-ray pairs travelling directly from the source and the arrival position of gamma-rays of said gamma-ray pairs after scattering from the object and passing through said aperture; and means for determining 2-dimensional information about the object from the direction of flight of the directly detected gamma-ray and the arrival position of the scattered gamma-ray of each gamma-ray pair.