Longitudinal functional magnetic resonance imaging in animal models

Abstract

Functional magnetic resonance imaging (fMRI) has had an essential role in furthering our understanding of brain physiology and function. fMRI techniques are nowadays widely applied in neuroscience research, as well as in translational and clinical studies. The use of animal models in fMRI studies has been fundamental in helping elucidate the mechanisms of cerebral blood-flow regulation, and in the exploration of basic neuroscience questions, such as the mechanisms of perception, behavior, and cognition. Because animals are inherently non-compliant, most fMRI performed to date have required the use of anesthesia, which interferes with brain function and compromises interpretability and applicability of results to our understanding of human brain function. An alternative approach that eliminates the need for anesthesia involves training the animal to tolerate physical restraint during the data acquisition. In the present chapter, we review these two different approaches to obtaining fMRI data from animal models, with a specific focus on the acquisition of longitudinal data from the same subjects.

Fig. 1

Typical surgical station for preparation of anesthetized animals. (A) Medical gases and vacuum scavenger; (B) Gas flowmeters and mixer; (C) Isoflurane vaporizer and compliance balloon; (D) Small animal ventilator; (E) Pulse oximeter/capnograph; (F) Temperature regulator, heated water bath and water mat; (G) Surgical microscope.

Fig. 2

(A) MRI-compatible animal bed, containing a stereotaxic head holder with ear pieces and a bite bar (B), to which the head of the animal is secured. Mechanical ventilation is provided by the gas lines. (C) Pulse oximetry sensor and rectal temperature probe. (D) Heated water mat (shown unfolded), which can be wrapped around the animal’s body to maintain temperature. (E) Multi-channel receive RF coils. (F) Multi-channel RF preamplifiers.

Fig. 3

Pooled data plot of ETCO₂ as measured with a capnograph versus PaCO₂ sampled from arterial blood (n= 34 rats, average of 6 points per animal) in normocapnia and hypercapnia. The dashed line is the line of identity, and the correlation coefficient between ETCO₂ and PaCO₂ is 0.77 (r² = 0.59). Even though the absolute value of ETCO₂ is influenced by the size of the animal relative to the total flow and volume of air in the ventilator, the length of the gas lines, and the flow of expired air into the capnograph, the significant correlation between ETCO₂ and PaCO₂ allows ETCO₂ values to be used as a relative index of changes in PaCO₂.

Fig. 4

Physiological and fMRI data acquisition traces recorded during a typical fMRI experiment of electrical stimulation of both forepaws of a rat anesthetized with α-chloralose. The experiment lasted 4 minutes. From top to bottom, the traces show the arterial blood pressure (red), respiratory pressure (green), EPI acquisition tics (blue), stimulation of the left (green) and the right (blue) forepaws, the MRI gradient temperature (magenta) and the heart rate derived from the ABP trace (blue).

Fig 5

Illustration of a restrained marmoset in the MRI-compatible bed. The body of the animal is loosely attached to a back cover via zip ties secured to a sleeveless jacket worn by the animal. The back cover is screwed to the side bars on the cradle, while the arms, legs and tail of the animal are free to move as she wishes. The head of the marmoset is secured to a two-piece, custom-built helmet made specifically for that individual alone. The chin piece on the bottom supports the chin of the animal, and the head piece on the top prevents head motion. Note that the helmet pieces are lined up with foam on the inside to provide a comfortable support to the entire head. The animal sits in the sphinx position looking out towards the back of the magnet. The bed is secured to the bed sliding mechanism on one end via the hanger.

Fig 6

Detail of the construction of the custom-fit helmet. Based on a 3D MRI of the entire head of the marmoset, a 3D model of the helmet is created consisting of two pieces: the chin piece (blue) to support the chin of the animal, and the head piece (red) that supports the head and prevents motion. Once the model is created it is sent to a 3D plastic printer that creates the helmet.

Fig. 7

Illustration of the three phases of training. In Phase 1 (top), the body of the animal is loosely attached to the MRI bed, and the animal is conditioned to staying in the bed for increasing periods of time. In phase 2 (middle), reinforcement of the training in phase continues while the animal gets used to MRI sounds. In phase 3 (bottom), the individualized helmet is introduced to restrain the head of the animal.

Fig. 8

Map of fMRI response evoked by electrical stimulation of the left arm of the marmoset. Eight contiguous coronal slices immediately posterior to the anterior commissure (AC) are shown in a representative session under awake (A) and propofol anesthesia (B). Robust BOLD responses can be detected in the thalamus (Tha.) and in the primary (SI) and secondary (SII) somatosensory cortex, although the responses are much more significant in the awake than in anesthetized animals.

Fig. 9

fMRI response obtained from the same animal in different MRI sessions in a time span of 10 months. (A) T-map demonstrating the main active areas of the brain, including SI, SII, and caudate putamen. (B)BOLD time-course in response to a 2 s electrical stimulus of both hands (2 mA, 0.3 ms, 64 Hz), obtained at five different times (0 weeks, 3 weeks, 6 weeks, 2 months and 10 months) post-acclimatization. Excellent reproducibility of the amplitude and temporal characteristics of the BOLD response is achieved.