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A low-cost setup for orientation-dependent post-mortem MRI under temperature control
Niklas Wallstein1, Roland Müller1, André Pampel1, and Harald E. Möller1,2
1NMR Methods & Development Group, Max Planck Institute for Human Cognitive and Brain Sciences, Leipzig, Germany, 2Felix Bloch Institute for Solid State Physics, Leipzig University, Leipzig, Germany

Synopsis

Keywords: New Devices, New Devices, Orientation-dependent MRI, Post-mortem MRI, Temperature control, Temperature-dependent MRI, Tiltable Coil

Motivation: Systematic variation of external parameters can provide insight into whether theoretical models appropriately describe MRI contrast.

Goal(s): Our goal was to develop a cost-effective setup for comprehensive temperature- and orientation-dependent relaxation and magnetization-transfer experiments in post-mortem tissue on a clinical scanner.

Approach: A remotely tiltable Helmholtz coil was integrated into a thermally insulated box, where the temperature can be adjusted by a heated airflow.

Results: Robust coil performance, accurate adjustment of the sample orientation relative to B0 (±1°), and stable temperature conditions (±0.5 °C) were achieved. Theoretically expected temperature dependencies of T1 and diffusivity in agarose were experimentally reproduced.

Impact: Well-defined variations of the temperature (between ambient temperature and 45 °C) and sample orientation (between 0 and 90° relative to B0) in MRI experiments with small post-mortem tissue specimens were achieved on a clinical scanner with a cost-effective setup.

Introduction

Post-mortem MRI is often used as a surrogate for examinations in vivo because it allows subsequent validation or correlation with the results of destructive methods (e.g., histology). However, quantitative transfer of results is limited by different experimental conditions. Namely, acquisitions at body temperature require precise adjustment of the sample temperature, which is usually not available on clinical scanners. Studies on small tissue sections or whole organs, such as the brain, also allow more flexible imaging conditions. This includes acquisitions in arbitrary sample orientations to investigate the anisotropy of contrast parameters, whereas changes in the orientation of the human head in vivo are very limited in a head coil. Understanding the orientation dependence of relaxation times,1–4 magnetization-transfer (MT) parameters,5 or magnetic susceptibility6,7 has gained increasing interest as it can be used to test tissue models.8 The goal of the current work was to develop a versatile setup that allows precise setting and control of sample temperature in coils for post-mortem examinations on a clinical scanner. A mechanism for rotation of the sample relative to the direction of the static field was also incorporated for added flexibility.

Methods

Temperature control unit: A cuboid box made of glued wood panel (beech, 18mm thick), which fits on the patient table, was used to define a volume with adjustable temperature. The size (235 mm × 295 mm × 380 mm, xyz) was sufficient to accommodate coils for small brain specimens (e.g., marmoset).9 Elevated temperature was achieved by a stream of heated air from a heat exchanger located outside the magnet room. To avoid dust accumulation, the airflow was drawn from the magnet room via a waveguide by an adjustable (via an external phase-angle controller) multi-purpose vacuum cleaner (WD 3P, Kärcher, Winnenden, Germany) and directed through a heater and into the box via another waveguide and approx. 12 m of flexible tubing (35 mm inner diameter) thermally insulated with polyurethane foam. The heater consisted of a simple heating plate and an aluminum heat sink encased in sheet metal as a heat exchanger. To avoid hysteresis effects, the heating plate’s bimetal regulator was set to maximum, and a second phase-angle controller was used for power adjustment. Temperature monitoring was performed at four distinct positions using a fiberoptic system (Optocon; Weidmann, Dresden, Germany).

RF coil: For the current experiments, a small TxRx Helmholtz coil10 (16 mm radius and loop spacing) that achieves a high Q ≈ 470 with minimal radiation damping11 was installed in the box. Additional mechanical components were 3D-printed allowing a precise sample rotation (worm drive with 2° rotation per crank revolution) without removing the setup from the isocenter (Figure 1). The accuracy of the orientation-angle adjustment was better than ±1°.

Sample: A 5-mm NMR tube was filled with a 16-mm long piece of 3% low-melting agarose embedded in Fomblin.

MR experiments: 1D data were acquired on a MAGNETOM Skyrafit (Siemens Healthineers, Erlangen, Germany) at 22, 28, 31, 36 and 44 °C unless otherwise noted, including the following techniques: (i) A flip-angle array with nominal excitation angles αnom between 0° and 270° (increments of 45°) at room temperature to obtain local B1+ scaling factors, σ, from fits of the normalized signal amplitude to sinσαnom . (ii) Inversion-recovery experiments (rectangular 40µs inversion and 20µs readout pulse, TR = 15 s) with 23 logarithmically spaced inversion times between 770 μs and 10 s. (iii) Diffusion-weighted acquisitions (Stejskal-Tanner, TR = 10 s, TE = 60 ms, b = 750 s/mm2, 60 independent diffusion directions, 7 acquisitions with b = 0).

Results and Discussion

The 1D transmit field varied by only ±4% along the sample length (Figure 2) and was almost invariant of the orientation angle of the coil (±1.5%; Figure 3).
A detailed evaluation revealed a high temperature stability (±0.5 °C) inside the box and the NMR tube, once a steady state was reached (after approx. 15 min; Figure 4 ). Proof-of-concept measurements of the sample’s longitudinal relaxation time and the mean diffusivity (MD) yielded the expected dependences, T1 ~ exp(-ε/T) and MD ~ exp(-ε/T), over the investigated temperature range (T1: R2=0.99973; MD: R2=0.99998 Figure 5).

Conclusion

A low-cost, precise and robust setup consisting of a temperature-control unit and a rotatable Helmholtz coil was developed to support temperature- and orientation-dependent studies of contrast parameters in small samples on a clinical scanner. This could be beneficial for comprehensive investigations of orientation-dependent relaxation1–4, MT5, or magnetic susceptibility6,7 in cerebral white matter and the role of temperature in studies of relaxation12-13 or inhomogeneous MT14,15.

Acknowledgements

We thank Henrik Grunert for his support of the construction.

References

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  2. Schyboll F, Jaekel U, Weber B, Neeb H. The impact of fibre orientation on T1-relaxation and apparent tissue water content in white matter. Magn. Reson. Mater. Phy. 2018; 31(4): 501–510.
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  10. Müller R, Wallstein N, Pampel A, Möller HE. A 3T Helmholtz coil with either reduced or maximized radiation damping effects. Proceedings of the 31st Annual Meeting of ISMRM, Toronto, ON, Canada, 2023. p. 5278.
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Figures

FIGURE 1. Simplified illustration of the temperature control unit combined with a rotatable Helmholtz coil. Airflow is created by a controllable vacuum cleaner and directed through a regulable heater (hotplate and heat exchanger with external phase control) outside the scanner room. A wooden box provides sufficient thermal isolation inside the magnet.


FIGURE 2. B1+ profiles along the 5mm NMR tube (nominal resolution 0.153 mm) measured for different orientations of the coil and sample. While a small overall shift of the mean B1+ is visible, the general shape of the profile is independent of the orientation.


FIGURE 3. Average of the voxel-wise B1+ scaling factor σ as a function of the rotation angle with respect to the B0 field. The error bars indicate standard deviations. A subtle variation of the transmit efficiency (≤1.5%) is apparent but negligible in realistic experiments.


FIGURE 4. Demonstration of performance of the temperature control unit in a sample temperature range between 22 °C and 44 °C. The exemplary 30-min intervals indicated by gray shading yield excellent temperature stability (±0.2 °C within the NMR tube).


FIGURE 5. Temperature dependence of (A) the longitudinal relaxation time T1 and (B) the mean diffusivity MD of 3% low-melting agarose (logarithmic plots). The symbols indicate mean values of the voxel-wise quantity with standard deviations indicated by the error bars. Red lines show the results of fits to Arrhenius laws.


Proc. Intl. Soc. Mag. Reson. Med. 32 (2024)
1594
DOI: https://doi.org/10.58530/2024/1594