Limits of detection of SPIO at 3.0 T using T₂* relaxometry

Abstract

T 2 * relaxometry for quantitative MR imaging is strongly hampered by large-scale field inhomogeneities, which lead to signal losses and an overestimation of the relaxation rate R 2 *. This is of particular importance for the sensitive detection of iron oxide contrast agent distributions. To derive an accurate measurement of T 2 *, a main field inhomogeneity correction is applied: the main field inhomogeneity is derived from multislice T 2 * relaxometry data and used as an initial value for an iterative optimization, by which the relaxation signal is corrected for each voxel. These corrected T 2 * maps show reduced influence of the local field variation and contain information about the local SPIO concentration. The method was tested on phantoms and the limit of detection of SPIO labeled cells using T 2 * relaxometry was estimated in volunteers to be 120 Quantitative imaging by means of T 2 * relaxometry is becoming an increasingly requested tool in many areas of MRI. It is applied for BOLD contrast studies (1,2) as well as for the detection of changes in cancellous bone structure (3). The evaluation of perfusion by means of bolus transit methods (4) and the assessment of iron content in brain and liver can also profit from accurate T 2 * mapping methods (5,6). The sensitive detection of iron oxide particlebased contrast agents has become a growing application, especially within the scope of molecular imaging MR. The distribution of these iron oxides is usually determined by acquiring T 2 and T 2 *-weighted images. In Ref. (7) it was already suggested that EPI sequences could be used to study localized magnetic inhomogeneities, since they are introduced by SPIO particles. Hence, a multigradient readout approach can be utilized to come one step closer toward quantification of iron oxide particle distributions by measuring the T 2 * relaxation time.SPIO particles are phagocytosed by macrophages in the liver or lymph nodes. They are also used to label cells ex vivo and track them throughout the body after injection (8,9). For cellularly compartmentalized SPIO particles T 2 is less influenced than T 2 *, which is explained by the static dephasing theory (10). To gain high sensitivity for the detection of compartmentalized SPIOs, the reversible relaxation rate T 2 * should be measured (11). Therefore, this study focuses on the measurement of T 2 *. This includes the challenge that the T 2 * relaxation rate is not only influenced by SPIO contrast agents, but also by macroscopic susceptibilities that arise from air/tissue interfaces, like the lung/liver or the sinus/brain interface. These disturbances cause enhanced signal decay and scale linearly with the field strength, which intensifies their influence at 3.0 T. Therefore, susceptibility artifacts lead to overestimated relaxation rates or obscure small concentrations of SPIO-labeled cells.Several methods have been proposed to correct for the macroscopic magnetic susceptibility influence. The most obvious method involves increasing the spatial resolution (12). Compen...

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