Inge Manuela Kalis^{1}, Axel Joachim Krafft^{2}, and Michael Bock^{2}

BOLD MRI can be applied as an indirect measure of the oxygenation level changes in the kidneys while performing an experiment with a functional renal challenge. These changes are detected by the relaxation rate R2* in the renal cortex and medulla. For R2* analysis different methods are proposed, such as the conventional manual ROI method and a compartmental method. Here, these methods and two further compartmental methods are compared to each other by analyzing full time-resolved renal BOLD MR experiments in healthy volunteers.

Time-resolved renal BOLD MRI was performed in 6
healthy volunteers who had no food or water at least 6 h before the exam. Up to
7 baseline R2* measurements were applied before the volunteers drank up to 1000 ml
tap water in the magnet. Then another 25-40 BOLD data sets were collected over
about 50 min. The experiments were done on a 3T whole body system (Siemens
PRISMA, Erlangen, Germany) applying a 2D multi-echo GRE sequence using the
following imaging parameters: TR = 35 ms, TE1 = 2.42 ms, ΔTE = 2.66 ms, 12 echoes, matrix: 192x192, voxel size: 2.2x2.2x5.0 mm^{3},
a = 25°, GRAPPA with 24
reference lines, bandwidth = 810 Hz/Px, TA = 7.6 s. One coronal slice per
kidney was acquired, and the KALIBRI method was
implemented for renal motion correction^{10}.

Renal medullar and cortical R2* values are assessed using the manual ROI method (mROI) and 3 compartmental methods. With mROI small regions of interest (ROIs) were drawn manually into medullar and cortical regions, and the mean R2* is calculated for those two tissues separately. The same ROIs are placed into all data of a volunteer, and R2* is assessed for each measurement time point.

In the compartmental methods, it is assumed that the
R2* distribution of the whole kidney shows a bimodal separation into medullar
and cortical regions. The sum of 2 probability distribution functions is fitted
into the histogram evaluating the mean R2* values of cortex and medulla: Ebrahimi
et al.^{8} empirically assumed the sum of a Gaussian and a Gamma distribution for
the cortical and medullary distribution, respectively. Here, the cortical part
is also described by a Gaussian distribution, while the medullary part is either
another Gaussian (GG), a Poisson (GP), or a Gamma distribution function (GΓ). The fitting procedure was
repeated for each time point in the dynamic data set. High R2* values from the
calyx system and from renal blood vessels were excluded by setting R2*
thresholds. For method comparison Bland-Altman plots for bias and variation analysis
were created and Pearson correlation coefficients were calculated.

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Cortical and medullar
R2* values of the left and the right kidney,
assessed with the mROI and the 3 compartmental methods for each measured time
point. Here, only the results from volunteer 2 are shown. The vertical blue bars
define the drinking period.

R2* maps of the left
kidney from volunteer 1 and 2 at one measurement time point after water load.
The histograms show the according R2* distributions including the fitted
functions of the 3 compartmental methods with their mean values and standard
deviations, defining the medullar and cortical R2* values.

Diagrams summarizing
the correlations between each compartmental method with the mROI method. For
each kidney of all volunteers, the correlation coefficients (bars) with
according *p*-values, written above the
bars, are represented in the upper row, and the biases between the R2* results
(shown as squares) and the corresponding lines of agreements (as error bars)
are shown in the lower row. The green colored *p*-values sign the correlations as significant (*p*<0.05).