Introduction

The availability of advanced imaging techniques has revolutionized the evaluation of many clinical neurological disorders. Similarly, the availability of advanced imaging techniques has enhanced the utility of animal models related to the study of these disorders. Currently, the most available and useful imaging techniques in animal models of neurological disorders are those related to magnetic resonance imaging (MRI) applications, computerized tomography (CT), and positron emission tomography (PET). This chapter will introduce the basic concepts related to these various imaging modalities and then discuss their application to neurological disorders with a focus on acute ischemic brain injury.

Magnetic resonance imaging

A wide variety of MRI techniques are currently available for use in animals and patients. The range of MRI modalities and their main uses are provided in Table 9.1. The initial MRI modalities employed were T1- and T2-weighted imaging that evaluated the density of water proton spins. Water protons in relatively unrestricted fluid spaces have higher T1 and T2 values, while these protons in more restricted environments such as brain edema, hemorrhages, or tumors have lower values. These two MRI modalities are associated with an increase in interstitial water content of the brain, as seen with the development of vasogenic edema. Conventional T1 and T2 MRI have been used widely in clinical imaging for two decades and have also been used extensively in animal models of brain ischemia, tumors, traumatic injury, and multiple sclerosis (MS).

Human beings owe a great deal to animals. From the earliest periods of history of mankind, animals have been used by humans for food, clothing, tool making, and for several other purposes. Primitive artists painted animal figures onto stone surfaces; animal figures became parts of religions and tribal identities. Over time, some animals were domesticated, serving as regular sources of meat and milk; additionally, animals were used in farmwork and for transport. Dogs were used to defend property and were trained for rescue missions. Cats were used as pets as early as the ancient Egyptian Kingdom. Interesting additional missions have been given to animals such as searching for illicit drugs, explosives, and mushrooms. Some areas where we are still strictly dependent on animals include the drug industry (e.g., insulin isolated from swine pancreas), but there are also areas subject to intense debate (e.g., fur farming, fox hunting, and the cosmetics industry).

We are very much dependent on animals in medical research and in clinical surgery training. Neurological diseases comprise a major health problem all over the world and their importance continues to grow as the population ages and as neurology moves from being largely a diagnostic field to one with more therapeutic approaches. Neurological diseases already absorb approximately one-fourth of health budgets in industrialized countries. It is urgent to develop novel effective therapies for neurological diseases: the aging of the population will increase the number of neurological patients whereas the labor force available in the health sector appears to be decreasing.

Stroke is a leading cause of death in Western countries, with a mortality rate higher than most forms of cancer and now the commonest cause of long-term adult disability. Stroke diagnosis and management were revolutionized by the widespread introduction of computed tomographic (CT) scanning in the 1970s. CT scanning sensitively excludes cerebral hemorrhage, but early ischemic changes can be subtle. In the first few hours after stroke onset, when acute therapies such as thrombolysis are being considered, CT is often normal, although acute ischemic changes have become better recognized in recent years. Conventional magnetic resonance imaging (MRI) became widely available in most countries a decade after the advent of CT scanning, but has had a limited role in stroke diagnosis and management. Although MRI provides far better imaging of posterior fossa structures and facilitated non-invasive angiography (MRA), its sensitivity in acute stroke is not much better than CT. Other functional imaging techniques such as single photon emission computed tomography (SPECT) and positron emission tomography (PET) have been valuable research tools, but have not been of routine clinical use in the management of stroke.

Since the 1990s, the increasingly widespread availability of echoplanar MRI technology facilitated the introduction of diffusion-weighted imaging (DWI), perfusion imaging (PWI) and magnetic resonance spectroscopy (MRS). Diffusion-weighted imaging allows the hyperacute evaluation of the ischemic core within minutes of stroke onset and the distinction between acute and chronic ischemic lesions.