Synopsis

Undiagnosed dehydration compromises health outcomes across many populations including the elderly, infants, soldiers and athletes. Here we introduce a miniature (~1000 cm³), portable (~4 kg), single-sided magnetic resonance (MR) sensor capable of identifying and quantifying fluid loss induced by dehydration. Multicomponent T2 relaxometry of skeletal muscle tissue enables the measurement of distinct fluid compartments. We first quantify fluid loss induced by dehydration with multicomponent T2 relaxometry using a standard MRI. We then extend these findings and demonstrate a portable, low-field MR sensor exhibits comparable performance via a similar measurement while offering significantly reduced sensor mass, measurement time, and system complexity.

Background and Purpose

Dehydration is associated with increased hospitalization and mortality, substandard postoperative outcomes, and decreased cognitive and physical performance [1–5]. Existing approaches towards identifying dehydration often require invasive sampling of bodily fluids or are easily confounded by external factors limiting widespread adoption [6].

We introduce a miniaturized (~1000 cm³), portable (~4 kg), single-sided magnetic resonance (MR) sensor. The primary component of this sensor (Figure 1) is an array of permanent magnets [7]. The sensor is capable of performing accurate multicomponent T2 relaxometry via CPMG measurements (Figure 2). We developed a mouse model of thermal dehydration and validated a shift in fluid from the lean tissue compartment via whole-body NMR (Figure 3). T2 relaxometry performed with both standard MRI (Figure 4) and our portable MR sensor (Figure 5) demonstrated that the MR signal produced by the slow component of muscle tissue can quantify fluid loss induced by dehydration.

Methods and Results

Portable, miniature, single sided MR sensor design and characterization

A portable, single sided MR sensor was designed and constructed with a permanent magnet array to generate a static magnetic field (B0) based on the Unilateral Linear Halbach array (Figure 1a-b) [7]. The sweet spot magnet design allows for high sensitivity measurements over a large uniform region using a field parallel to the surface of the sensor. The magnet produces a uniform region with a field strength of 0.28 Tesla located 2 to 7 mm from the surface of the sensor (Figure 1c). The sensitive region was characterized by scanning a small aqueous sample and spans a 12 x 5 x 2 mm region located directly above the RF coil (Figure 1d-e).

The portable MR sensor accurately measures T2 relaxation rate of a paramagnetic solution (aqueous CuSO₄) via CPMG as validated by benchtop NMR spectrometry (minispec mq7.5, Bruker, USA) (Figure 2a). A heterogeneous phantom consisting of two distinct solutions with unique T2 relaxation rates was measured with CPMG using the portable MR sensor (Figure 2b). Multicomponent T2 relaxometry successfully extracts the relaxation times and relative amplitudes of each component accurately and reproducibly via a biexponential model (Figure 2c-d). We utilize this ability to quantify distinct fluid compartments using multicomponent T2 relaxometry to measure changes in the distribution of fluid within tissue.

Mouse model of thermal dehydration with whole animal NMR validation

We utilized a mouse model of thermal dehydration via fluid restriction, elevated temperatures, high airflow, and low humidity (Figure 3a-b). This induced between 7 and 14% body weight loss over 5 to 10 hours (Figure 3c-e). Multicomponent T2 relaxometry performed via benchtop NMR relaxometry identified and quantified fluid loss in dehydrated mice via a decrease in the relative amplitude of the relaxation component assigned to lean muscle tissue (Figure 3f-h) [8]. This validated the mouse model and motivated further investigation into a dehydration measurement localized solely to muscle tissue [9].

T2 relaxometry with MRI localized to the muscle quantifies fluid depletion

T2-weighted MRI scans of the upper leg of mice were used to draw regions-of-interest on muscle tissue (Figure 4a-b). A composite T2 decay signal was formed from voxels corresponding to muscle tissue (Figure 4c). A decrease in the relative amplitude of the slow component in a biexponential fit predicts fluid loss induced by dehydration (Figure 4d). We demonstrate that a multicomponent T2 measurement localized to the muscle is capable of quantifying dehydration of the entire animal.

T2 relaxometry with portable MR sensor quantifies fluid depletion

T2 relaxometry was performed using our portable MR sensor on the upper leg of a mouse (Figure 5a). The signal primarily originated from the muscle tissue with some contribution from subcutaneous tissue and bone (Figure 5b). A decrease in the relative amplitude of the slow relaxation component in a triexponential fit of a CPMG measurement localized to the muscle quantified fluid loss induced by dehydration (Figure 5c-e).

Discussion and Conclusion

Our measurement of the relative volumes of tissue fluid compartments provides an indicator of fluid depletion. We first demonstrate that localizing multicomponent T2 relaxometry to muscle tissue, via a standard MRI, is capable of measuring dehydration as a shift in amplitude of the free fluid compartment. We then demonstrate that our portable MR sensor can make a comparable measurement.

Performing multicomponent T2 relaxometry localized to the skeletal muscle with a miniaturized MR sensor provides a novel, portable, non-invasive, physiologically relevant indicator of fluid loss induced by dehydration. Portable MR sensors have the potential for widespread adoption in reliable, real-time fluid volume status monitoring.

Acknowledgements

References

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