Novel Air Stimulation MR-Device for Intraoral Quantitative Sensory Cold Testing

Abstract

The advent of neuroimaging in dental research provides exciting opportunities for relating excitation of trigeminal neurons to human somatosensory perceptions. Cold air sensitivity is one of the most frequent causes of dental discomfort or pain. Up to date, devices capable of delivering controlled cold air in an MR-environment are unavailable for quantitative sensory testing. This study therefore aimed at constructing and evaluating a novel MR-compatible, computer-controlled cold air stimulation apparatus (CASA) that produces graded air puffs. CASA consisted of a multi-injector air jet delivery system (AJS), a cold exchanger, a cooling agent, and a stimulus application construction. Its feasibility was tested by performing an fMRI stimulation experiment on a single subject experiencing dentine cold sensitivity. The novel device delivered repetitive, stable air stimuli ranging from room temperature (24.5°C ± 2°C) to -35°C, at flow rates between 5 and 17 liters per minute (l/min). These cold air puffs evoked perceptions similar to natural stimuli. Single-subject fMRI-analysis yielded brain activations typically associated with acute pain processing including thalamus, insular and cingulate cortices, somatosensory, cerebellar, and frontal brain regions. Thus, the novel CASA allowed for controlled, repetitive quantitative sensory testing by using air stimuli at graded temperatures (room temperature down to -35°C) while simultaneously recording brain responses. No MR-compatible stimulation device currently exists that is capable of providing non-contact natural-like stimuli at a wide temperature range to tissues in spatially restricted areas such as the mouth. The physical characteristics of this novel device thus holds promise for advancing the field of trigeminal and spinal somatosensory research, namely with respect to comparing therapeutic interventions for dentine hypersensitivity.

Keywords: QST; cold air stimulation; dentine hypersensitivity; fMRI.

Figure 1

Schematic display of the CASA components (graphical objects not shown in proportional scale). Outside scanner room: (1) Air source (2) Computer-controlled air jet delivery system with three outlets: cooling system, stimulation air, and pneumatic control of air switch. Inside scanner room: (3) Cold exchanger [note: the two gas circuits for cooling (3.1) and stimulation (3.2) are completely separated] (4) Air switch composed of mobile slider (in red) and spring (5) Only one target tooth is exposed to air while the others are shielded by impression material (transparent green). The various colors of the connecting tubes reflect different air temperatures (T°) in the tubes: room T° (yellow); approx. −196°C (light blue); target T° for tooth stimulation (dark blue).

Figure 2

(A) Psychophysical testing of a sensitive tooth during MR scan in a single subject. Left x-axis: Stimulation temperature in °C (blue dots), Right y-axis: Subject ratings on modified BORG scale (red dots), x-axis: time (minutes). Pain threshold was defined as BORG score >2.5. (B) The BORG scale was presented on a screen and consisted of non-linearly distributed numerical (from 0 to 12), verbal (from none to maximum) and color-coded (green, non-painful; yellow, painful; red, clearly painful) descriptors of the sensory perception.

Figure 3

Brain activity evoked by noxious cold air stimulation (T-Contrast “Noxious cold air stimulation >no stimulation,” p < 0.05, FWE-corrected) evoked neural activity in wide regions of the brain. (A) Sagittal view; (B) coronal view; (C) axial view. 1, Cerebellum; 2, Insula; 3, Thalamus; 4, cingulate cortex; 5, Frontal cortex; 6, somatosensory cortex. X,Y,Z-coordinates are shown in MNI space.