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Figure 5.8. Pattern of USPIO-induced signal loss. Axial MR images (T2*W [TE=5.6 ms]) through the common carotid artery. a, b Diffuse USPIO effect. c, d Focal USPIO signal effect. a, c Pre-USPIO images. b, d Thirty-six hours after infusion (*lumen, arrow heads: USPIO signal effect). Images provided by courtesy of Gillard. Reprinted from Trivedi et al. (2006), with permission of Lippincott Williams & Wilkins.

focal signal loss and Perl's positive cell count (p = -0.63, P < 0.001) (Trivedi et al., 2006). These analysis might have been hampered by the varying delay between infusion and surgery of 6.9 ± 4.8 days with a range of 2-18 days so that with a fixed delay these correlations might be higher.

The temporal effect on signal loss in MR images was investigated by Trivedi et al. (2004). Largest signal loss was found 24 and 36 hours post-USPIO infusion, while the magnitude of signal loss was lower after 48 hours and there was no significant signal loss 72 hours post-contrast. This confirmed earlier findings by Kooi et al. (2003) who found significant signal loss after 24 hours, while no significant decrease occurred 72 hours post-infusion. In histology, USPIO uptake could still be visualized, even though the time between infusion and surgery was on average 5-7 days (Kooi et al., 2003; Trivedi et al., 2006).

The amount of USPIOs in plaque at these time points however seems to be too low to visualize with MRI.

All studies so far have been performed on symptomatic patients. An initial study determined signal loss in the carotid vessel wall on the symptomatic as well as the contralateral asymptomatic side (Tang et al., 2006). It was shown that in 19 of 20 patients bilateral signal loss was observed (Tang et al., 2006), highlighting the possible systemic nature of vulnerable plaque (Buffon et al., 2002; Madjid et al., 2004; Mauriello et al., 2005). Future studies are warranted to investigate differences in USPIO uptake between symptomatic and asymptomatic patients, to determine the effect of drugs, such as statin therapy on USPIO-enhanced images and to study the relation between USPIO uptake and recurrent strokes.

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