In Vivo Comparison of Three Quantitative MRI Techniques to Measure Brain Iron

Introduction Correlation of quantitative MRI with brain iron may provide a useful tool in the diagnosis and prognosis of neuropathology such as Multiple Sclerosis, Alzheimer’s and Parkinson’s Disease. Several different MR measures of brain iron have been proposed ([1] and references therein). The aim of this work was to compare three of the most promising, namely: transverse relaxation rate, magnetic susceptibility and magnetic field correlation (MFC) mapping. Methods Imaging: A gradient-echo spin-echo hybrid (GESE) [2] for R2 measurement, a 3DGRASS susceptibility weighted sequence [3] and a symmetric and asymmetric spin-echo for MFC[4] were implemented on a Signa HDx 3T MRI (General Electric, Milwaukee, WI) scanner. To perform fair comparisons between techniques, published protocols were adjusted to achieve comparable brain coverage (32, 2 mm thick slices, and FoV 25.6x25.6cm), under the constraint of approximately equal, and patient tolerable scan times (6-8 minutes). All images were acquired axially on three healthy volunteers (aged 4550). Sequence-specific parameters were: GESE: TR/TE=2750/34ms, echo spacing=0.86ms, matrix=128x128, #gradient echoes=32; SWI: TR/TE=35ms/20ms, matrix=384x384; MFC: TR/TE=3000/48ms, matrix=128x128, 180 time-shift=0, -3.5, -7, -10.5, -14ms, #repeats=7, #shots=4. Processing: GESE: magnitude images for each gradient echo were reconstructed offline (Viewit, NCSA, Champaign, IL) and fitted for R2 as described in ref. [2], SWI: phase and susceptibility were calculated as described in [5] and [6]. MFC: All volumes were co-registered and repeat volumes averaged prior to fitting the expected signal equation as described in [4]. Regions of interest (ROI) were drawn within the red nucleus (RN), substantia nigra (SN), globus pallidus (GP), Putamen (Put), Thalamus (Thal) and frontal white matter (fWM) and plotted against reference iron levels [7]. Results Figure 1 shows representative R2, susceptibility and MFC maps (top row). R2 measures in the ROIs (Fig. 1, bottom left) are consistent with previous studies [8], however MFC values are approximately double those previously measured [4]. Magnetic susceptibility is approximately half measured elsewhere [6] but has the greatest coefficient of determination in this group of 3 subjects. R is the least for R2 and although precision of R2 appears to be improved (smaller standard deviation (sd)), in the Put, RN and SN, mean values ±1sd do not lie on the line of regression.