  Objective. Magnetic Particle Imaging (MPI) is capable of high temporal
resolution measurements of the spatial distribution of magnetic nanoparticles
and therefore well suited for perfusion imaging, which is an important tool in
medical diagnosis. Perfusion imaging in MPI usually requires a fresh bolus of
tracer material to capture the key signal dynamics. Here, we propose a method
to decouple the imaging sequence from the injection of additional tracer
material, without further increasing the administered iron dose in the body
with each image. Approach. A bolus of physiological saline solution without any
particles (negative contrast) diminishes the steady-state concentration of a
long-circulating tracer during passage. This depression in the measured
concentration contributes to the required contrast dynamics. The presence of a
long-circulating tracer is therefore a prerequisite to obtain the negative
contrast. As a quantitative tracer based imaging method, the signal is linear
in the tracer concentration for any location that contains nanoparticles and
zero in the surrounding tissue which does not provide any intrinsic signal.
After tracer injection, the concentration over time (positive contrast) can be
utilized to calculate dynamic diagnostic parameters like perfusion parameters
in vessels and organs. Every acquired perfusion image thus requires a new bolus
of tracer with a sufficiently large iron dose to be visible above the
background. Main results. Perfusion parameters are calculated based on the time
response of the proposed negative bolus and compared to a positive bolus.
Results from phantom experiments show that normalized signals from positive and
negative boli are concurrent and deviations of calculated perfusion maps are
low. Significance. Our method opens up the possibility to increase the total
monitoring time, while minimizing the iron dose per acquired image.
