Introduction

Cellular-interstitial water exchange, often characterized by the kinetic parameter intracellular water residence time (τ_i), has been shown to be a promising biomarker for cellular energy turnover.¹ Recently the Active Contrast Encoding (ACE) MRI method was introduced to measure τ_i by using more than one flip angle during dynamic scan with contrast injection.³ In this study, we generated whole tumor τ_i parameter maps and compared them with 18F-FDG PET standardized uptake values to investigate the feasibility of using τ_i as an imaging marker for cellular metabolic activity.

Methods

Six to eight-week-old C57BL6 mice (n=7) with GL261 mouse glioma xenograft models were used in this study. MRI scans were performed on a Bruker 7T micro-MRI system, with a ¹H four-channel phased array receive-only MRI CryoProbe with a volume transmit coil. The 3D-UTE-GRASP method² was used to acquired dynamic contrast-enhance data (TR/TE = 4 / 0.028 ms) with image matrix = 128 x 128 x 128, and field of view = 20 x 20 x 20 mm³. In order to achieve active contrast encoding for τ_i measurement, this sequence was continuously run to acquire 154,080 spokes with two flip angles (51,360 spokes for each flip angle segment 8^o - 25^o - 8^o) for 10 minutes and 13 seconds. Pre-contrast T₁ mapping was obtained using the 3D-UTE-GRASP sequence with 38,328 spokes (12,776 spokes for each flip angle segment 8^o - 2^o - 12^o), for a total acquisition time of 153 s at the same resolution as the DCE scan. The joint compressed sensing and parallel imaging reconstruction was implemented based on the 3D-UTE-GRASP algorithm² with temporal frame resolution T = 5 s/frame. Arterial input function (AIF) was obtained following the Principal Component Analysis (PCA) method used in our previous study³ with the independently measured T₁ map. The T₁ weighted images were also used to manually segment whole tumors. Pharmacokinetic model analysis was carried out for the whole tumor with the same Two Compartment Exchange Model (TCM)⁴ and Three Site Two Exchange (3S2X)⁵ Model for τ_i estimation with the two-flip angle approach³. Five parameters, interstitial space volume fraction (v_e), vascular space volume fraction (v_p), blood flow (F_p), permeability surface area product (PS), and intracellular water life time (τ_i) were estimated from the model fit. Transfer constant (Ktrans) was calculated from PS and F_p.

Dynamic 18F-FDG PET/CT scans were performed on an Inveon small-animal PET/CT scanner (Siemens Medical Solutions), immediately following the DCE-MRI scan. The mice were administered an 18F-FDG bolus (6.45-11.1 MBq/0.1 mL) via tail vein injection 1 minute into the 60 minute scan. Whole tumors were manually segmented from co-registered PET/CT images. Dynamic tracer uptake was measured following decay correction and CT-based attenuation correction. The SUV of the whole tumor ROI was calculated as: [decay corrected activity (Bq/mL)] / [animal weight (g)]/[injected dose (Bq)].

All animals were treated in strict accordance with the National Institutes of Health Guide for the Care and Use of Laboratory Animals.

Results

Figure 1 depicts the tumor heterogeneity captured by the whole tumor kinetic parameter maps modeled from the DCE images. Figure 2 illustrates the 18F-FDG activity in the whole mouse based on the summed frames of the dynamic 18F-FDG PET/CT scan data, with a red arrow indicating location of the tumor. Figure 3 (a) depicts an example 3D-UTE-GRASP MRI image overlaid with the τ_i parameter map of the tumor ROI. Figure 1 (b) depicts the 18F-FDG activity in the same subject and region from the follow up 18F-FDG PET/CT scan. Figure 4 shows the comparison of whole tumor mean standardized uptake value (SUV_mean) with each kinetic parameter map median values in highly enhancing pixels of whole tumor ROI with positive correlations for: (a) F_p, R² =0.6346, (b) PS, R² =0.6346, (c) K^trans, R² = 0.6346, (d), negative correlations for: v_e, R² =0.6346, (e) v_p, R2 =0.6346, (f) τ_i, R² =0.6263 and (g) demonstrates little correlation between whole tumor volume with SUV_mean, R² = 0.00893.

Discussion

As the τ_i parameter represents the intracellular water residence time, higher τ_i values would indicate slower interstitial-cellular water exchange. Water exchange has been implicated as a biomarker of cellular metabolic activity.¹ The standardized uptake value from 18F-FDG PET is widely used as a semi-quantitative method of measuring glucose uptake and phosphorylation in tissue, an indicator of cellular metabolic activity. The negative correlation between τ_i and SUV reinforces τ_i as a sensitive parameter for cellular metabolic activity, suggesting that the water exchange is facilitated by trans-membrane active water transport. The weak correlation between tumor volume and SUV further supports the relationship between τ_i, SUV and tumor metabolism as FDG uptake is more likely driven by energy turnover than the bias of larger tumors acting as an FDG sink.

Conclusion

In this proof-of-concept study, we found that τ_i measured from ACE-MRI has a strong correlation with the SUV from 18F-FDG PET, suggesting the feasibility of using τ_i as an imaging marker for cellular metabolic activity without using a radioactive contrast agent. Future studies are warranted to further characterize τ_i as biomarker for cellular metabolic activity with a larger cohort and its potential for grading tumor aggressiveness and response to treatment.

Acknowledgements

NIH R01CA160620, NIH R01CA219964, P41EB017183, NIH/NCI 5P30CA016087