Blood oxygen level-dependent magnetic resonance imaging of the kidneys: influence of spatial resolution on the apparent R2* transverse relaxation rate of renal tissue.

OBJECTIVES

The aim of this study was to quantify the influence of image resolution on the apparent transverse relaxivity (R2*) of the magnetic resonance (MR) signal in human renal tissue in vivo and in phantom measurements.

MATERIALS AND METHODS

This prospective study included 17 healthy volunteers (age, 32 ± 8 years, 6 women). Parametrical R2* maps were computed via monoexponential fitting of multiecho 2-dimensional fast-field echo data measured at 1.5 T (repetition time [TR], 150 milliseconds; flip angle [FA], 40°; minimum echo time [TE], 4.6 milliseconds; ΔTE, 5 milliseconds; 16 echoes) and at 3 T (TR, 140 milliseconds; FA, 70°; minimum TE, 2 milliseconds; ΔTE, 5 milliseconds; 16 echoes) with varying nominal volumes of the encoded voxels (from 5.76 to 36.0 mm). For each voxel size, mean R2* values were computed in regions of interest drawn in the left and right renal parenchyma. For data acquired using minimum voxel size, the mean R2* values were computed over the cortex and medulla separately. The squared 2-norm of the residuals was computed to evaluate the goodness of the pixel-wise exponential fits. Six multiecho MR images of a water phantom were acquired using a 2-dimensional fast-field echo sequence (FA, 50°; TR, 108 milliseconds; TE, 4 milliseconds; ΔTE, 20 milliseconds) at 3 T after shim adjustment and in the presence of a uniform background gradient of 40 μT/m. The nominal voxel size was varied in a range between 2 and 12.5 mm.

RESULTS

Mean R2* values of 13.04 ± 0.71 s (right renal cortex) and 16.47 ± 1.92 s (right renal medulla) were computed at 1.5 T. At 3 T, the R2* of the right medulla was 28.27 ± 1.52 s and the cortical R2* was 19.22 ± 2.32 s. Comparable relaxivity values were found over the left kidney at both field strengths. Increasing R2* values were observed for increasing voxel volume in both the water phantom and renal tissue data. At a constant slice thickness of 4 mm, the decrease in the in-plane resolution from 1.2 × 1.2 mm to 3.0 × 3.0 mm led to a maximum increase of the renal R2* of 15% at 1.5 T and of 12% at 3 T. Increasing the slice thickness from 3 to 8 mm at a constant in-plane resolution of 1.5 × 1.5 mm resulted in a maximum increase of the renal R2* of 30% at 1.5 T and of 26% at 3 T. On the other hand, increasing the voxel size improved the goodness of the fit implied by the smaller residuals.

CONCLUSIONS

The phantom experiments and in vivo acquisitions of healthy renal tissue documented a significant dependence of the apparent R2* relaxation rate on the spatial resolution of the MR imaging data. In clinical practice, the voxel volume for the quantification of renal R2* should be optimized in a compromise between minimizing the effects of macroscopic field inhomogeneity and maintaining a sufficiently high signal-to-noise ratio and goodness of fit. When comparing quantitative R2* among different publications, the influence of the spatial resolution should be taken into account.