A method for imaging the blood flow in a patient receiving radiomedical treatment, including forming a first and a second plurality of generally spherical biologically stable members, with each respective member of the first plurality includes a first non-radioactive isotope distributed substantially uniformly therein that, upon being subjected to an effective amount of neutron irradiation, emits a therapeutic intensity and amount of beta or gamma radiation and wherein each respective member of the second plurality includes a second non-radioactive isotope distributed substantially uniformly therethrough that, upon being subjected to an effective amount of neutron irradiation, emits a detectable intensity and amount of positron radiation. The first and second members are subjected to an effective amount of neutron radiation and the irradiated first and second pluralities of irradiated first and second members are introduced into a patient's circulatory system upstream of a desired treatment site. The second plurality of irradiated members is imaged via positron emission tomography. The second isotope, once activated by neutron irradiation, has a maximum half-life of about four days.