Integration of the RF coil and the bed is a key challenge, due to the wide variety of coil designs from a number of different manufacturers. While coil integration may not be possible across all coil platforms, the bed should at least address the need for using different coils for different applications, including both volume and surface coils (see Note 4). One approach is the use of mounting hardware that allows the independent placement of both the coil and the animal bed (Fig. 3). An alternative approach is use of an animal bed platform that can accommodate interchangeable RF coils (Fig. 4).

While RF coil integration can provide an optimal method for addressing the unique demands of MRI as it relates to animal position and ease of animal preparation, it can also lead to necessary compromise in sensitivity as the coil may need to be larger than ideal, due to having to accommodate hardware for animal positioning. Another disadvantage of this approach is that it may restrict compatibility with imaging systems across other modalities, unless a modular approach is taken, where the bed itself can be separated from the coil. A core facility that operates in multiple disease areas and in a variety of models may prefer manageable compromises on sensitivity to enable maximum flexibility for use with different coils and across modalities. On the other hand, a smaller facility focused on more specific research topics may prefer customized animal management for each RF coil in use.

Today, the vast majority of imaging applications rely on anesthesia (5–8). The breathable halogenated ether isoflurane (6, 7) is the most commonly used anesthetic due to rapid effect, rapid recovery time, excellent tolerance by mammalian species, and low cost. Sevoflurane (7) is another halogenated ether used for this purpose. Both have little or no known toxic side effects as they are not metabolized. They are also relatively insoluble in the blood, which promotes rapid recovery when the gas delivery is turned off.

Isoflurane can be delivered in either oxygen or air, with dosage level accurately controlled in real time by a vaporizer that mixes vapor-phase isoflurane with the breathable gas of choice. This provides advantages over injectable methods of anesthetic, as the dosage level can be easily adjusted based on the specifics of the species, model, and treatment being used. The dose can also be adjusted over time after an initially high induction dose of up to 4–5% concentration, allowing fine control of respiratory rate and health of a research animal over several hours if necessary. Modern imaging labs are therefore normally fitted with centrally supplied compressed air and/or oxygen, multiple isoflurane vaporizers and gas piping that integrates the anesthetic with the imaging system(s). It is then a simple matter to open the appropriate valve to deliver the anesthetic to the induction chamber, animal bed or imaging bore.

The predominant alternative to breathable anesthesia is injectable anesthetic and most commonly the dissociative anesthetic ketamine is used in combination with the sedative/relaxant, xylazine. Use of ketamine/xylazine anesthesia can be preferred in cases where cardiac physiology is critical, as isoflurane is known to affect cardiac function (9). Ketamine/xyalzine also results in more relaxed respiration, compared with isoflurane, which can cause erratic respiration. In MRI, this can lead to undesirable motion artifacts, and therefore isoflurane anesthesia may not be preferred in thoracic imaging applications. The potential physiological side effects of short- or long-duration anesthesia and use in oxygen versus air should be well understood within the context of the end points being quantified (8). Animal dehydration during anesthesia is a primary concern and can be addressed by the use of eye lubrication and post-anesthetic intraperitoneal saline injection for rehydration and recovery.

A basic anesthesia system comprises just a few simple components, including these:

However, design and implementation of a more elaborate system, while more costly, can result in large increases in animal anesthesia reliability, efficiency, and flexibility, and also in enhanced occupational health and safety of laboratory workers. Thought should be given to capacity for induction of multiple animals simultaneously, capacity for lab bench animal procedures under anesthesia (e.g., catheterization), and delivery of anesthetic via ceiling drops at the magnet bore opening and other animal work areas. New products are now available that provide management of multiple anesthetic lines via a small consolidated unit with easy adjustment and accurate control of flow rates, and switching between multiple lines that control supply for induction, procedure, and imaging (Fig. 5). The following components are features of an optimally designed MRI anesthesia system:

While cheap and commonly available materials can provide functional anesthesia delivery temporarily, it pays to use tested materials that will resist wear and rupture, preventing compromised anesthesia delivery and potential exposure hazards. While isoflurane is considered a non-toxic chemical, studies have demonstrated potential safety concerns and minimizing environmental exposure of technicians during usage should be an area of focus for an MRI lab. For example, induction boxes that utilize active suction to minimize exposure during chamber opening are now available (see Note 6). Monitoring actual exposure through sensors that are worn on external clothing can be a reliable way to demonstrate acceptable exposure and track improved exposure in problem areas (see Note 7).

While in vivo MRI is predominantly achieved using anesthetized animals, the use of MRI in conscious animals is increasing in parallel with recent advancements in MRI-compatible motion restriction devices (see also Chapter 17). Imaging in conscious animals is a particularly important consideration in functional MRI applications where brain activation in response to stimulus is key to the experiment. As functional MRI protocols are extended increasingly to small animal models, imaging in awake animals will also increase (10–12). Devices for this purpose are usually based on traditional stereotactic technology (Fig. 6) such as bite bars, nose cones, and ear bars (3, 13).

Of course, these methods have more general applications in motion restriction for MRI, even in anesthetized animals. Respiratory motion is a large concern in MRI, where even small motion during the scan can translate to significant phase artifacts that increase data variability or may even render a slice or entire image as being non-quantifiable. Respiratory motion commonly extends throughout the anesthetized animal’s body and can even create severe motion artifacts in brain imaging. In small animals, even the use of tape and immobilization splints can localize full body respiratory motion to the thorax, immobilizing the head or lower abdomen. Fortunately, the incorporation of simple mechanisms for motion restriction such as bite bars and nose cones are now commonly incorporated into MRI animal bed and/or RF coil design negating the need for an enormous inventory of surgical tapes in today’s imaging lab! These features not only help to reduce animal respiratory motion, but also inherently increase the reproducibility of the animal position. For example, stereotactic devices ensure the animal’s head is fixed in space both during a procedure and between procedures, providing inherent registration of images and minimizing animal preparation and setup time.

Maintenance of animal body temperature is an important consideration during imaging procedures, especially during the use of anesthesia which generally causes a decrease in core body temperature. There are therefore a variety of MR-compatible techniques that are used for warming of the animal while in the RF coil and magnet. These include the following: