MR imaging with ultrasmall superparamagnetic iron oxide particles in experimental soft-tissue infections in rats.
OBJECTIVE
To investigate the feasibility of macrophage magnetic resonance (MR) imaging in rats by using an experimental soft-tissue infection model.


METHODS
Thirteen rats with unilateral calf-muscle infection were imaged with a 4.7-T MR imager at an early chronic stage of infection (day 4 before contrast material injection, days 4-7 after injection). Eleven animals were imaged before and 3 and 24 hours after intravenous application of ultrasmall superparamagnetic iron oxide (USPIO), and eight animals were additionally imaged 48 hours and three animals 72 hours after USPIO application. Two infected rats served as controls. T1- and T2-weighted spin-echo and T2*-weighted gradient-echo sequences were applied. All animals were sacrificed, and histopathologic findings were correlated with findings on MR images. Electron microscopy was performed in two rats. For quantitative analysis, signal intensities on T2*-weighted images and T2 values on T2 maps were measured within regions of interest, and the temporal variation was analyzed by using the signed rank test.


RESULTS
Visualization of USPIO-loaded macrophages was most sensitive with a T2*-weighted sequence. USPIO distribution pattern and quantitative analysis of T2 and T2* effects 3 hours after USPIO application were significantly different (P <.05) from those at 24 and 48 hours, reflecting the dynamic transit of the particle accumulation from the intravascular to the intracellular compartment by means of macrophage phagocytosis. Local signal intensity alterations could be correlated with iron-loaded macrophages at histopathologic examination.


CONCLUSIONS
Activated macrophages in acute soft-tissue infection can be labeled with USPIOs and detected with MR imaging because of susceptibility effects.
Kaim
Achim H
AH
Department of Nuclear Medicine, University Hospital Zurich, Switzerland. akaim@uhbs.ch
Wischer
Thorsten
T
O'Reilly
Terence
T
Jundt
Gernot
G
Fröhlich
Johannes
J
von Schulthess
Gustav K
GK
Allegrini
Peter R
PR
D000038:Abscess / Q000175:diagnosis; D000818:Animals; D003287:Contrast Media; D003911:Dextrans; D005240:Feasibility Studies; D005260:Female; D052203:Ferrosoferric Oxide; D007501:Iron; D008262:Macrophage Activation; D008279:Magnetic Resonance Imaging; D058185:Magnetite Nanoparticles; D008854:Microscopy, Electron; D010087:Oxides; D010587:Phagocytosis; D051381:Rats; D017207:Rats, Sprague-Dawley; D018461:Soft Tissue Infections / Q000175:diagnosis*; D013203:Staphylococcal Infections / Q000175:diagnosis*
D003287:Contrast Media; D003911:Dextrans; D058185:Magnetite Nanoparticles; D010087:Oxides; C097921:ferumoxtran-10; D007501:Iron; D052203:Ferrosoferric Oxide