Synopsis

A resistive heater that is compatible with MR, CT, SPECT and PET imaging has been produced from a commercially available carbon-fibre sheet. Adequacy of temperature maintenance and insensitivity of MR and CT imaging to the presence and use of the heater is shown. Multimodal MR-CT-PET-SPECT imaging of the lower abdomen is demonstrated in vivo in the physiologically maintained and viable anaesthetised mouse.

INTRODUCTION

METHODS

Cradles that were size-matched to the RF coils used were CAD-drawn (Solidworks) and printed using ABS plastic (HP Designjet 3D). A 50 Ω resistive heater element formed from 0.75 mm thick carbon-fibre sheet (RS, 764-8700) cut into a U-shape with 2x7 mm wide legs, each 130 mm long and spaced by 1.5 mm, was glued to the underside of the cradles as shown in Fig. 1. 100 kHz AC at 1-2 W was applied using a power amplifier (Pioneer A-20) driven by a Pierce oscillator providing a cradle base temperature of ca. 37° C.

MRI was performed at 7 T using a 26 mm diameter, 100 mm long RF coil for system validation purposes, and with a 32 mm diameter, 45 mm long RF coil for multimodal imaging. MR-compatibility of the heater element was tested using global pulse-acquire spectroscopy, whole-body respiratory-gated bSSFP and FLASH imaging each with the heater turned on and off. For multi-modal imaging respiratory-gated FLASH MRI was performed repeatedly to effect DCE-MRI.

CT, PET and SPECT imaging were performed in a single gantry system (Vector4CT, MILabs) immediately before MRI, on animals that were held in the same cradles as for MRI. CT–compatibility was tested in vivo in the absence of any heater, and in the presence of the copper wire or carbon-fibre sheet heater elements. Hounsfield Units were measured to assess the impact of heater element composition on x-ray attenuation.

For validation of thermal stability and MR-compatibility, normal mice (n=3) were anaesthetised with isoflurane (1-4%) in oxygen-enriched air and placed on the imaging cradle where respiration rate was maintained at 40-60 breaths/minute. Rectal temperature was monitored for 50 minutes before animals were recovered. Homeothermic control was effected through manual switching of the current as the target temperature threshold was crossed.

Multimodal MR-CT-PET-SPECT imaging of kidneys was performed using anaesthesia and physiological maintenance as above, using [^99mTc]-mercaptoacetyltriglycine (SPECT), [¹⁸F]-fluorodeoxyglucose (PET), anatomical CT, and DCE-MRI. Images were aligned and overlaid using rigid-body registration. Further MR-CT-PET imaging has been performed successfully using this heating system in 55 mice undergoing study for other purposes.

RESULTS

Core temperature was maintained using carbon-fibre sheet heating at 36°C±0.9°C for 50 minutes and animals recovered from anaesthesia quickly.

NMR spectra, Fig. 2A, showed no lineshape distortions or frequency shift attributable to current flow through the heater. Similarly, no distortions attributable to the current were observed in respiratory-gated FLASH or bSSFP imaging (Figs. 2B and 2D, respectively). Residual artefacts result, primarily, from cardiac and gut motion.

Upon CT imaging the copper wire heater produced severe image streaking, Fig. 3A; organs were poorly-defined and poor estimations of CT intensities were made, Table 1. Conversely, no heater-related artefacts were observed when the carbon-fibre sheet heater was used, Fig. 3B, and absolute image quantification was equivalent to that measured in the absence of any heater, Table 1.

Multimodal PET/SPECT/CT/DCE-MRI imaging was performed and images could be easily co-registered, displayed, analysed and presented (Fig. 4).

DISCUSSION

CONCLUSION

Acknowledgements

References

1. Kersemans, V., Gilchrist, S., Allen, P. D., Beech, J. S., Kinchesh, P., Vojnovic, B., & Smart, S. C. (2015). A resistive heating system for homeothermic maintenance in small animals. Magnetic Resonance Imaging, 33(6), 847–851.

2. Gilchrist S, Gomes AL, Kinchesh P, Kersemans V, Allen PD, Smart SC (2016) An MRI-Compatible High Frequency AC Resistive Heating System for Homeothermic Maintenance in Small Animals. PLoS ONE 11(11): e0164920.

3. Bolus, D., Morgan, D. & Berland, L. (2017) Effective use of the Hounsfield unit in the age of variable energy CT. Abdom Radiol 42: 766.