Nat. Methods 3, 391–396 (2006). published online 20 April 2006; corrected after print 10 July 2006.

In the version of this article originally published, reference 12 was incorrect. The correct reference 12 is Querol, M., Chen, J.W., Weissleder, R. & Bogdanov, A. Jr. DTPA-bis-amide based MR sensor agents for peroxidase imaging. Org. Lett. 17, 1719–1722 (2005). This error has been corrected in the PDF version of the article.