A Carbon-Fiber Sheet Resistor for MR-, CT-, SPECT-, and PET-Compatible Temperature Maintenance in Small Animals

Abstract

A magnetic resonance (MR)-, computed tomography (CT)-, single-photon emission computed tomography (SPECT)-, and positron emission tomography (PET)-compatible carbon-fiber sheet resistor for temperature maintenance in small animals where space limitations prevent the use of circulating fluids was developed. A 250 Ω carbon-fiber sheet resistor was mounted to the underside of an imaging cradle. Alternating current, operating at 99 kHz, and with a power of 1-2 W, was applied to the resistor providing a cradle base temperature of ∼37°C. Temperature control was implemented with a proportional-integral-derivative controller, and temperature maintenance was demonstrated in 4 mice positioned in both MR and PET/SPECT/CT scanners. MR and CT compatibility were also shown, and multimodal MR-CT-PET-SPECT imaging of the mouse abdomen was performed in vivo. Core temperature was maintained at 35.5°C ± 0.2°C. No line-shape, frequency, or image distortions attributable to the current flow through the heater were observed on MR. Upon CT imaging, no heater-related artifacts were observed when carbon-fiber was used. Multimodal imaging was performed and images could be easily coregistered, displayed, analyzed, and presented. Carbon fiber sheet resistors powered with high-frequency alternating current allow homeothermic maintenance that is compatible with multimodal imaging. The heater is small, and it is easy to produce and integrate into multimodal imaging cradles.

Keywords: MR-CT-PET-SPECT compatibility; carbon fiber; heater; multimodal imaging.

Figure 1.

Carbon fiber sheet–resistive heater embedded in a 3D-printed, flat-base multimodal imaging cradle. Proportional–integral–derivative (PID) controller (A); bottom view of the cradle (B); top view of the cradle (C); and thermal image of the cradle surface (D). The cradle contains a heater and (*) a mouthpiece assembly consisting of a base block, a vertical and horizontal adjustable mouth bar, and an anesthetic gas delivery tube, (¦) is a pressure balloon for respiration monitoring, (#) is an optical fiber for temperature monitoring, and (†) is a cover sheet to contain anesthetic gas. The current wires and metal couplings to the carbon-fiber are located beyond the imaging field of view (FOV) so do not present image distortions.

Figure 2.

Homeothermic maintenance of core temperature in CBA mice using a carbon-fiber sheet resistor powered with high-frequency AC under PID control. The target rectal temperature was set to 36.0°C. Core temperature of 4 mice placed into the SPECT/(PET)SPECT/CT scanner (A). Core temperature of 4 mice placed into the MRI scanner (B). The arrow indicates when respiratory-gated balanced steady-state free-precession (bssfp) imaging was initiated to replicate the additional heat load of high-duty-cycle magnetic resonance imaging (MRI) scanning.

Figure 3.

Impact of current flow through the carbon-fiber sheet heater element on MRI of a water gel phantom. PRESS pulse-acquire spectroscopy with the heater turned off and on (A). The image shows the placement of the 2-mm-cubic PRESS voxel that was located 3, 7, and 5 mm off-isocentre in the z, y, and x axes, respectively, such that it was positioned as close to one of the heater legs as possible; absolute value display of the spectrum showing the water resonance with the heater turned off (top trace) and the difference spectrum from acquisitions made with and without current flow through the heater (bottom trace) (B). The intensity of the difference spectrum is scaled 10× higher than the top trace and the absolute value display was used for clarity of display of the residual subtraction error. T1-weighted 2D FLASH MRI on a water gel phantom with the heater turned off (left column) and on (right column) (C).

Figure 4.

Impact of current flow through the carbon-fiber sheet heater element on in vivo MRI of the mouse. T1-weighted in vivo whole-body respiration-gated 2D FLASH MRI with the heater turned off (top) and on (middle) (A). The bottom image displays the difference image between both images. Cardiorespiratory-gated whole-body 3D FLASH MRI with the heater turned off (left column) and on (right column) (B). The average intensity over a square, placed close to the heater surface, was plotted for 20 repetitions with the heater turned on (black dots) or off (orange dots) (C).

Figure 5.

Impact of the heater material on computed tomography (CT) image quality. CT imaging using the 150-µm-diameter copper wire heater (A) or the carbon-fiber sheet heater (B). Streak artifacts owing to the presence of the heater are absent when using the carbon-fiber heater, and the image intensities remain intact.

Figure 6.

Multimodal imaging of a mouse using the carbon-fiber sheet–resistive heater embedded in a 3D-printed, flat-base, multimodal imaging cradle. The skeleton, kidneys, and major vessels to the kidneys (*) are marked up. The skeleton (white) was imaged by CT, while ¹¹¹In-citrate (red), ¹⁸F-fluorodeoxyglucose (purple), and gadodiamide (green) were used for SPECT, (PET)SPECT, and DCE-MRI of the kidneys, respectively. Each panel shows an additional layer of the coregistered, segmented image: CT + MRI (A), CT + MRI + SPECT (B), CT + MRI + (PET)SPECT (C), CT + MRI + (PET)SPECT + SPECT (D).