MRI of macrophages in infectious knee synovitis.
OBJECTIVE
The objective of our study was to prospectively evaluate macrophage imaging using ultrasmall superparamagnetic iron oxide (USPIO)-enhanced MRI to depict bacterial knee infection in an experimental rabbit model.


METHODS
Unilateral knee infection was induced by intraarticular injection of Staphylococcus aureus in eight rabbits. The contralateral knees were used as internal controls. After a mean interval of 3 days, two MRI sessions (3-T MRI) were performed before and 24 hours after IV administration of USPIO. The protocol included T1-weighted spin-echo, T2-weighted spin-echo, and T2*-weighted gradient-echo images. A gadolinium-enhanced T1-weighted sequence was also performed in the first session to optimize detection of synovial hyperplasia. MR data were analyzed qualitatively and quantitatively and compared with histopathologic findings (H and E stain and Perls Prussian blue stain). Signal-to-noise ratio changes after USPIO administration were compared using Wilcoxon's signed rank test.


RESULTS
All inoculated knees presented infectious synovitis with intense infiltration of iron-loaded macrophages. In these infected knees, signal loss was determined visually and quantitatively on T1-weighted (p < 0.01), T2-weighted (p < 0.01), and T2*-weighted images (p < 0.01) 24 hours after USPIO administration, reflecting the presence of USPIO-loaded macrophages in the synovium. In contrast, no significant MR signal changes were observed in the control knees (p = 0.07-0.48), which presented a normal synovium without infiltration of iron-loaded macrophages.


CONCLUSIONS
Macrophage imaging using USPIO-enhanced MRI can depict infectious knee synovitis.
Bierry
Guillaume
G
Department of Radiology, University Hospital of Strasbourg, 67098 Strasbourg, France. guillaume.bierry@chru-strasbourg.fr
Jehl
François
F
Neuville
Agnès
A
Lefevre
Sophie
S
Robert
Philippe
P
Kremer
Stéphane
S
Dietemann
Jean-Louis
JL
D000818:Animals; D003287:Contrast Media; D003911:Dextrans; D004195:Disease Models, Animal; D052203:Ferrosoferric Oxide; D007091:Image Processing, Computer-Assisted; D007719:Knee Joint / Q000473:pathology; D008264:Macrophages / Q000473:pathology*; D008279:Magnetic Resonance Imaging / Q000379:methods*; D058185:Magnetite Nanoparticles; D008536:Meglumine; D009942:Organometallic Compounds; D011446:Prospective Studies; D011817:Rabbits; D018709:Statistics, Nonparametric; D013585:Synovitis / Q000382:microbiology / Q000473:pathology*
D003287:Contrast Media; D003911:Dextrans; D058185:Magnetite Nanoparticles; D009942:Organometallic Compounds; C097921:ferumoxtran-10; D008536:Meglumine; C072417:gadoterate meglumine; D052203:Ferrosoferric Oxide