  The recent demonstration that nanoparticles associated with various
biological molecules and pharmacological agents can be administered
systemically to humans, without toxicity from the particles, has opened a new
era in the targeting of such particles to specific tissues in the body for the
imaging and therapy of disease. The majority of particles used for this purpose
contain iron and are detected in the body by magnetic resonance imaging. We
believe a superconducting quantum interference device (SQUID) could provide
quantitative and spatial information relevant to localization of
superparamagnetic nanoparticles directed to a specific cell target in vivo. We
envision a scanning system consisting of a DC induction field, a transport
device, and an array of planar first order gradiometer coils coupled to DC
SQUID amplifiers. We performed a set of computer simulations using
experimentally determined values for concentrations of paramagnetic particles
achievable in specific tissues of the mouse in vivo and concentrations of
particles linked to monoclonal antibodies specific to antigens of two human
cancer cell lines in vitro. An instrument to target distance of 10 centimeters
was selected so that for an average adult scanning both the anterior and
posterior surfaces could provide coverage of most of the body. The simulations
demonstrate the feasibility of SQUID magnetometry for monitoring achievable
concentrations of superparamagnetic particles in vivo and raise the possibility
of using this approach to detect and localize collections of abnormal cells
targeted by such particles.
