Purpose:To optimize contrast-to-noise and spatial resolution of a FLASH-based magnetization transfer (MT) protocol for visualization of substructures in human thalamus. Materials and Methods:Healthy adults were examined at 3 Tesla with a three-dimensional (3D) spoiled gradient-echo sequence. The signal-to-noise ratio (SNR) was increased by averaging eight bipolar echo acquisitions (mean echo time ϭ 12.3 ms; bandwidth ϭ 370 Hz/pixel). Three isotropic datasets with different weighting (proton density: flip angle/repetition time ϭ 7°/30 ms; T 1 : 20°/30 ms and MT: 10°/48 ms, Gaussian MT prepulse) yielded maps of T 1 , signal amplitude, MT ratio and MT saturation for comparison to MP-RAGE images. Measuring time was 23 min using partial k-space acquisition. First, the SNR of MT saturation maps in thalamus was optimized by means of the excitation flip angle. Then, noise and partial volume effects were traded off by means of the resolution. Finally, the contrast within the thalamus and to adjacent structures was compared between different maps. Results:The optimized MT saturation maps at 0.95 mm isotropic resolution provided the highest contrast. It was most prominent between structures of high axonal content (internal medullary lamina, ventral nuclei) and those containing predominantly neuronal somata (pulvinar, mediodorsal thalamus, geniculate bodies). Conclusion:Semiquantitative MT saturation maps provide an enhanced intra-thalamic contrast. The borders and nuclear groups of the thalamus are reliably delineated; individual assignment of singular nuclei seems feasible.

SUMMARY:Reliable identification of the subthalamic nucleus (STN) is a critical step in deep brain stimulation for Parkinson disease but difficult on T1-weighted stereotactic MR imaging. By simultaneous imaging of multiple gradient echoes, susceptibility contrast is added to conventional T1-weighted high-resolution MR image. Thus, the visibility of the STN is enhanced on a second colocalized dataset by exploiting the sensitivity of the T2*-relaxation to local iron deposits. The feasibility is underpinned by quantitative measurements on healthy adults.A n established neurosurgical therapy for Parkinson disease is to place electrodes for deep brain stimulation within the subthalamic nucleus (STN).1 Reliable identification of the anatomic borders of the STN is thus a critical step in stereotactic procedures.2,3 T2-weighted MR imaging showing the iron-rich structures as hypointensities has been suggested as additional information to the poor contrast of the STN on T1-weighted highresolution 3D MR imaging. 4,5 Exploiting the increased sensitivity of T2* to local iron deposits, a multigradient echo fast low-angle shot (FLASH) technique 6 is proposed to visualize the STN. This 3D MR technique enables simultaneous acquisition of T1-weighted images for stereotactic use and images with superimposed T2* contrast to localize the STN. TechniqueThe feasibility study was carried out on 16 healthy adults (8 men; age range, 23-31 years; mean age, 26 years) on a 3T MR system (Magnetom Trio; Siemens Medical Solutions, Erlangen, Germany) using an 8-channel phased-array head coil (MR Imaging Devices, Waukesha, Wis). Written informed consent as supervised by the local ethical committee was obtained.A multi-echo FLASH sequence (TR, 30 ms; flip angle, 20°) provided primarily T1-weighted 3D datasets of 0.95-mm isotropic resolution (FOV ϭ 243 mm; 176 sagittal partitions with 6/8 partial Fourier sampling in phase and section directions; 7:09 minutes). Eight gradient-echoes (TE ϭ 2.2/5.2/8.2/11.2/14.2/17.2/20.2/23.2 ms; bandwidth/pixel ϭ 370 Hz) provided additional T2*-weighted contrast increasing with TE. For comparison, multisection turbo spinecho images (TSE; 29 contiguous 2-mm axial sections; effective TE, 119 ms; TR, 3900 ms) were also obtained.The data were transferred to a stereotactic workstation (Sofamore Danek Stealth Station; Medtronic, Minneapolis, Minn) for coregistration of images with the Schaltenbrand-Wahren atlas.7 For quantitative comparison, T2* was determined in the STN and the reticulate formation by a region-of-interest analysis in 1 subject. ResultsThe T2*-signal intensity decay was markedly faster in the area of the STN than in the surrounding tissue. Figure 1 shows representative curve fits to the STN and the reference region of the reticulate formation. This corresponded with a contrast of approximately 25% at the longest TE. Thus, both STNs could be identified in all 16 of the subjects even when motion artifacts impaired the delineation (2 subjects). Figure 2 shows the midbrain region, where the solely T1-weighted...

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