shows representative post-ethanol 13C metabolic maps of Pyr, Lac, and Ala from the 3D volume covering kidney and liver superimposed onto corresponding 1H 3D SPGR slices. The central 12 of the 24 reconstructed slices are shown here. The 13C images were averaged over all of the 16 time-points acquired during the 80-s scan. The time series of 13C-Lac maps from the kidney and liver slices shown in illustrate the temporal dynamics and spatial distribution of this metabolite. The fast acquisition achieved by spiral MRSI enabled a temporal resolution of 5 s for dynamic imaging. These figures present an example of time-resolved volumetric imaging, which allows the simultaneous characterization of metabolic profiles from different tissues.
Figure 1 Representative post-ethanol 13C metabolic maps of pyruvate, lactate and alanine superimposed on corresponding 1H MRI images from the 3D volume covering kidneys and liver. The 13C images are averaged over all 16 time-points acquired. The central 12 of (more ...)
Figure 2 Time series of lactate maps from kidney and liver slices illustrate the metabolite dynamics and distribution. The labeled time indicates time from beginning of the pyruvate injection. The threshold for the display of 13C images was 15% of the maximum (more ...)
The baseline (pre-ethanol) SNR values were calculated for the time-averaged Pyr, Lac and Ala in liver and kidney. For liver, the SNR of Pyr, Lac, and Ala were 63.8±10.7 (mean±standard deviation, n=5), 37.5±6.6, and 29.3±3.2 respectively. For kidney, the SNR of Pyr, Lac, and Ala were 304.7±79.1, 70.4±8.6, and 30.8±3.4 respectively. The kidney SNR was measured in the right kidney, which lies relatively close to the aorta and vena cava. The pyruvate level in vasculature is much higher than that in the kidney, and at the 5 mm spatial resolution used here, there can be some pyruvate signal contribution from the vasculature into the kidney ROI. This may be the cause for the high inter-subject variability seen in the pyruvate SNR in kidney. This signal contamination from vasculature is much lower for lactate and alanine, and hence could also lead to underestimation of apparent kpl and kpa. Imaging with higher spatial resolution would help alleviate this problem.
shows the time-averaged lactate spectra, normalized to pyruvate, pre- and post-ethanol in liver and in kidney. The spectra from the liver show higher lactate signal post-ethanol compared to pre-ethanol, as expected from the NADH accumulation due to ethanol metabolism. The alanine signal was unchanged. The kidney lactate signal pre- and post-ethanol was also similar. The spectra shown here are from the lactate reconstruction where lactate is corrected for the chemical shift phase accrual and reconstructed ‘in-focus’ while the aliased resonances are blurred
Representative time-averaged 13C lactate spectra for liver (a) and kidney (b). The liver spectra show higher lactate signal post-ethanol compared to pre-ethanol while the kidney spectra showed similar lactate signal pre- and post-ethanol.
plots the time-courses of the mean Pyr, Lac, and Ala signals from ROIs in the liver and the kidney, measured pre- and post-ethanol, from one animal. The corresponding fits with the apparent rate constants are also shown. The liver plots clearly show increased lactate signal post-ethanol compared to pre-ethanol, while there was little change in the kidney lactate signal. Alanine signal also was similar before and after ethanol in both organs. The estimated apparent kpl
rate constants for all 5 rats are summarized in and in . The BALs for the 5 animals ranged from 81.1 to 101.6 mg/dL. The average apparent liver kpl
increased from 0.049 ± 0.007 s−1
(mean ± standard deviation) pre-ethanol to 0.064 ± 0.012 s−1
post-ethanol. This increase in kpl
was found to be statistically significant (P<0.044), and is consistent with the hypothesis that NADH levels are rate limiting for liver Pyr-to-Lac conversion at the given Pyr dose [21
]. As a control, no significant changes were observed for the labeled lactate production in kidney or vasculature ROIs or for alanine production in any of the tissues. The inter-subject variability leads to high standard deviations in , but the intra-subject comparison between pre- and post-ethanol in clearly shows the change in rate constants.
Figure 4 Representative time course data, curve fits, and calculated apparent rate constants, from the liver acquired pre-ethanol (a) and 45 min post-ethanol (b), and from the kidney pre-ethanol (c) and post-ethanol (d). Higher lactate was measured in the liver (more ...)
Figure 5 Estimated apparent kpl and kpa rate constants in liver, kidney, and vasculature in the respective slices. Conversion of labeled pyruvate to lactate increased in the liver after ethanol administration as seen by the increase in kpl values. There was no (more ...)
Table 1 Average apparent kpl and kpa rate constants with corresponding standard deviations in liver, kidney and vasculature ROIs from 5 animals. Conversion of labeled pyruvate to lactate increased in the liver after ethanol administration as seen by the increase (more ...)
Compared to the slice-selective MRS measurements in [21
], the organ specific apparent kpl
values obtained here at baseline were considerably higher in the liver but similar in the kidney. For example, the average apparent kpl
at baseline in liver was 0.049 ± 0.007 s−1
for the 3D MRSI and 0.014 ± 0.003 s−1
for the slice-selective MRS in ref. 21
. The corresponding values in kidney were 0.018 ± 0.004 s−1
for the 3D MRSI and 0.012 ± 0.002 s−1
for the slice-selective MRS. The greater difference in liver kpl
is presumably because the fraction of Pyr signal contribution from the vasculature to the total Pyr signal in the slice is higher for the liver slice than for the kidney slice. This effect is also seen in the Pyr images in . The Pyr signal in the liver is much lower compared to that in the blood vessel in the same slice, and is below the threshold used for display of the 13
C maps. However, the Pyr signal in the kidney relative to the vessel in the kidney slice is easily visible. Due to the large contribution of the vascular Pyr signal, the whole-slice ROI is a better comparison to the slice-selective estimates. For the whole-slice time-courses generated from the 3D MRSI for liver and kidney slices, the apparent kpl
were 0.015 ± 0.003 s−1
for liver and 0.016 ± 0.006 s−1
for kidney, and the apparent kpa
were 0.010 ± 0.002 s−1
for liver and 0.011 ± 0.003 s−1
for kidney, all of which are similar to the slice-selective MRS estimates from [18
]. These values demonstrate how the organ-specific rate constants are underestimated by slice-selective dynamic data due to the large vascular Pyr component.
The three-site exchange model used for fitting does not account for the RF excitation scheme used for data acquisition. The slice-selective measurements in [21
] used a single RF excitation with a 5° flip angle for each time-point, while the 3D imaging here used 36 excitations of 5.6° each, resulting in a much higher effective flip angle per time-point. Also, the 3D sequence had an acquisition duration of 4.5 s and a TR of 5 s with inflow occurring throughout, as compared to the short 409.6 ms readout and TR of 3 s in [21
]. These differences in the RF sampling schemes and the inflow effects could lead to differences in the absolute values of the apparent rate constant and T1
decay estimates. However, the impact of the RF sampling is the same for both pre- and post-ethanol measurements, and should have only a small effect on the change in apparent kpl
from pre- to post-ethanol infusion.
A key difference from the fitting in [21
] was that instead of modeling the bolus input function by a trapezoid, this study used the measured Pyr time-curve directly as the bolus input into the model. As the measured Pyr time-curve includes the T1
decay information, it obviates the need to separately fit for Pyr T1,
and so apparent T1pyr
was fixed to 1 s. Good fits were obtained with apparent T1lac
= 12 s and T1ala
= 15 s for liver and kidney, and apparent T1lac
= 20 s for vasculature. These values were chosen empirically and kept the same for all animals.
The increase in apparent liver kpl
after infusion of ethanol (post-ethanol/pre-ethanol) observed here was 1.33 ± 0.11 SEM compared to the approximately 2-fold (2.0 ± 0.5 SEM) increase reported in [21
]. This smaller effect may be due to the use of 80-mM Pyr instead of 100-mM Pyr as in that study, whereby the LDH enzyme activity may not be completely saturated. Though the total dose was comparable in both studies (240 μmol vs.
250 μmol), the lower concentration Pyr solution may be just at the boundary between the linear and saturated regimes of LDH enzyme activity, especially given the time-varying Pyr concentration during the ~15 s bolus injection.
The analysis in [21
] used separate control measurements with saline injection instead of ethanol injection to confirm that the increased lactate production in the liver was an effect of ethanol. Based on those results, additional saline control measurements were not repeated for this study. The alanine kinetics as well as the lactate kinetics in kidney and in vasculature serve as self-control within each animal.