Most contrast-enhanced lymph node imaging to date has been performed with small (5 - 20 nm) ferrite particles known as ultra-small superparamagnetic iron oxides (US-SPIOs or USPIOs). After intravenous injection these nanoparticles are not immediately recognized by cells of the reticuloendothelial system (RES) and remain in circulation for several hours. This long blood half-life allows the ultra-small iron agents time to penetrate the vascular endothelium and leak into the interstitium. Thereafter, the particles are cleared by lymphatics, draining into lymph nodes and bone marrow.
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The distinction between small (SPIO) and ultrasmall (USPIO) iron oxide agents is somewhat arbitrary, but the latter usually denotes particles of size less than 50 nm. SPIO's are typically in the 50-200 nm range.
Positive-enhancement lymphotropic contrast agents are also in development using gadolinium with an appropriate chelator. See review by Choi and Moon below for further discussion.
Bashir MR, Bhatti L, Marin D, Nelson RC. Emerging applications for ferumoxytol as a contrast agent in MRI. J Magn Reson Imaging 2015; 41:884-898.
Choi SH, Moon WK. Contrast-enhanced MR imaging of lymph nodes in cancer patients. Korean J Radiol 2010; 11:383-394. (review) Feraheme™ package insert, from www.feraheme.com
Harisinghani MG, Barentsz J, Hahn PF, et al. Noninvasive detection of clinically occult lymph-node metastases in prostate cancer. N Engl J Med 2003; 348:2491-9. (early major paper, very remarkable sensitivity and specificity)
Heesakkers RAM, Jager GJ, Hövels AM, et al. Prostate cancer: detection of lymph node metastases outside the routine surgical area with ferumoxtran-10-enhanced MR imaging. Radiology; 2009; 251:408-414. (24% false positive rate in this study, leading to unnecessary surgical intervention).
Whatever happened to Feridex®? Aren't iron-containing contrast agents useful for liver MRI?