Positron attenuation is estimated. Positrons attenuate differently than x-rays, so measuring positron attenuation may assist in diagnosis or material study. To measure positron attenuation, a positron beam is formed using a magnetic field. The annihilations along the beam within an object are measured using positron emission tomography. The rate of annihilation and integration of the rate of annihilation along the positron beam may be used to determine positron attenuation.