Technical Note Simultaneous Imaging and R2* Mapping Using a Radial Multi-Gradient-Echo (rMGE) Sequence Stefanie Winkelmann, MS, 1 * Tobias Schaeffter, PhD, 2,3 Steffen Weiss, MS, 2 Holger Eggers, MS, 2 and Olaf Doessel, PhD 1 Purpose: To demonstrate a rapid MR technique that com- bines imaging and R2* mapping based on a single radial multi-gradient-echo (rMGE) data set. The technique pro- vides a fast method for online monitoring of the adminis- tration of (super-)paramagnetic contrast agents as well as image-guided drug delivery. Materials and Methods: Data are acquired using an rMGE sequence, resulting in interleaved undersampled radial k- spaces representing different echo times (TEs). These data sets are reconstructed separately, yielding a series of im- ages with different TEs used for pixelwise R2* mapping. A fast numerical algorithm implemented on a real-time re- construction platform provides online estimation of the re- laxation rate R2*. Simultaneously the images are summed for the computation of a high-resolution image. Results: Convenient high-resolution R2* maps of phan- toms and the liver of a healthy volunteer were obtained. In addition to stable intrinsic baseline maps, the proposed technique provides particularly accurate results for the high relaxation rates observed during the presence of (super-)para- magnetic contrast agents. Assuming that the change in R2* is proportional to the concentration of the agent, the technique offers a rough estimate for dynamic dosage. Conclusion: The simultaneous online display of morpho- logical and parametric information permits convenient, quantitative surveillance of contrast-agent administration. Key Words: real-time imaging; R2* mapping; radial multi- gradient-echo; contrast-agent quantification; image-guided delivery J. Magn. Reson. Imaging 2006;24:939 –944. © 2006 Wiley-Liss, Inc. THE NEED FOR REAL-TIME R2* mapping is of increas- ing interest in noninvasive and dynamic studies of (su- per-)paramagnetic contrast agent delivery for both diag- nostic and therapeutic purposes. Paramagnetic or superparamagnetic contrast agents (e.g., iron-oxide par- ticles) generate tissue contrast by exploiting the local sus- ceptibility effect, which results in a signal void due to the low T2* or equivalently high relaxation rate (R2* = 1/T2*). Hence, iron-based contrast agents can be localized by means of R2* parametric maps. Furthermore, the local concentration of agent can be quantified because it is known to be proportional to the local increase R2* of the relaxation rate, considering the static dephasing regime in which diffusion effects are negligible (1). In diagnostic tumor imaging this linear relationship between concen- tration and R2* can be exploited for the quantitative imaging of tumor vascularity (2). In a similar application for tumor therapy, Nijsen et al (3) used the concentration- related R2* effect of paramagnetic radioactive holmium- microspheres to monitor an intra-arterial radionuclide during liver-tumor treatment. Another important applica- tion for the use of ultrafast R2* mapping is to monitor the delivery of labeled stem cells in the treatment of myocar- dial infarction, a topic that is currently of great interest (4 – 6). In all of the above-mentioned applications, a method that combines real-time imaging and R2* map- ping would facilitate direct therapy control and optimiza- tion, particularly for drug dosage determination. In the current paradigm, mapping is performed after imaging, and nonquantitative T2*-weighted images are used to dy- namically monitor the delivery of the agent (2–5). In some studies T2* quantification was performed once before and once after the delivery. The T2* parametric maps were either obtained from independent experiments at differ- ent TEs (7) or, more efficiently, by multi-gradient-echo (MGE) sequences that acquire images at different TEs simultaneously (2,5,8,9). However, the acquisition of an appropriate number of high-resolution single-echo im- ages in real time is still a major problem (10,11). Real-time mapping would be advantageous for better dynamic mon- itoring of procedures. In this work we investigate the use of radial k-space sampling in combination with an MGE experiment for the fast simultaneous computation of high-resolution images and R2* maps. The k-space sampling pattern consists of interleaved undersampled radial k-space data sets for each TE. These are reconstructed sepa- rately, yielding images representing different TEs (sin- gle-echo images), which can be simultaneously used for imaging and R2* mapping. A numeric mapping algo- 1 Institute of Biomedical Engineering, University of Karlsruhe, Karlsruhe, Germany. 2 Philips Research Laboratories, Hamburg, Germany. 3 Division of Imaging Sciences, King’s College London, London, United Kingdom. *Address reprint requests to: S.W., Roentgenstr. 24-26, 22335 Ham- burg, Germany. E-mail: Stefanie.Winkelmann@philips.com Received August 25, 2005; Accepted June 30, 2006. DOI 10.1002/jmri.20712 Published online 6 September 2006 in Wiley InterScience (www. interscience.wiley.com). JOURNAL OF MAGNETIC RESONANCE IMAGING 24:939 –944 (2006) © 2006 Wiley-Liss, Inc. 939