The Many Types of Magnetic Resonance Examination

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Magnetic resonance, unlike computed tomography (CT) or other imaging modalities, is a collection of techniques that enable many aspects of the structure, biochemistry, and function of the brain to be identified completely noninva-sively. The scanner is still the standard 'workhorse' for the acquisition of anatomic images. Even for this standard procedure, there are a number of choices that have to be made by the operator in order to acquire the most informative information. It is, therefore, important when considering a specific pathology that the technique is 'optimized' in order to identify the pathologic substrate that is the subject of the investigation. There are many choices of imaging sequence, orientation, slice thickness, and imaging time, all of which can contribute to optimization of the imaging. This often needs to be defined for each pathologic entity that is to be investigated, and it cannot be assumed that simple or 'routine' imaging is adequate for the acquisition of images that will allow the identification of all important abnormalities.

Magnetic resonance techniques can also do more than noninvasively acquire pictures of slices through the brain. Three-dimensional data sets can be acquired, which allows reformatting of the two-dimensional image in any plane and with any slice thickness. This is akin to the ability to reformat (or remontage) the electroencephalogram (EEG) data after it is acquired if it is acquired digitally. This is important for dealing with partial volume effects and for volume measurement techniques. Techniques are also available that can help to quantify tissue characteristics such as the T2 or T1 relaxation times. This requires different sequences and provides different information from that obtained with standard anatomic imaging techniques. MR can also identify biochemical aspects of a selected region of the brain by MR spectroscopy (MRS) and can provide low-resolution images of these metabolites using chemical shift imaging (CSI). These techniques can be used to identify specific metabolites and their relative concentrations in specific areas of the brain (Chapter 13).

Functional MRI (fMRI) takes MR technology into the field of imaging brain function as well as structure. The noninvasive nature of MR techniques and the high spatial resolution of MR hold great promise for the investigation of many aspects of brain function, both in normal states and in pathologic conditions. fMRI is now available on most clinical scanners in conjunction with the rapid imaging technique of echo-planar imaging. Although there is much yet to learn about this technique and its implications, it extends the importance of MR into many new areas. This provides a practical means of achieving the dream of being able to link brain structure and function by noninvasive investigations in the same subject during the same integrated examination.

These are the techniques that give rise to the clinical information that is the subject of this book. Before considering these techniques and their clinical applications specifically, it is important to discuss some general principles that apply to all these techniques. As we have already emphasized, the aim of this is to introduce concepts that we believe should be familiar to all who spend time using imaging techniques in the care of patients with epilepsy. It is not intended to provide a detailed explanation of basic principles or to be a substitute for a text in this area.

Terminology

The importance of nuclear magnetic resonance (NMR) in the field of neurology, and in particular in the field of epilepsy, can be compared to the impact on medicine that has followed from such major discoveries and developments as the X-ray and the EEG. The basic principle upon which all NMR experiments have been based was described by Block and Purcell in the mid 1940s. That is, if certain nuclei are placed in a magnetic field, they are able to 'absorb' energy in a specific radio frequency range, and signal can be recorded as a result of this energy exchange as these nuclei return to their original state. This phenomenon is the basic principle of NMR. In recent years, the word 'nuclear' has been dropped because of its association with the concept of radiation. MR techniques do not rely on ionizing radiation and are not associated with exposure to any radioactivity at all.

Some Basic Principles of Magnetic Resonance Physics Magnetic Properties of Atomic Nuclei

The technique of MRI is based on the intrinsic properties of atomic nuclei of charge, spin, and magnetism. The magnetic property is especially enhanced in certain naturally occurring nuclei such as !H (protons), 31P (phosphorus), and 23Na (sodium) and these are the nuclear species that are commonly investigated with NMR. The nuclei can be considered as tiny bar magnets and, when positioned in an external magnetic field, will align themselves along the direction of the field in the same way that a compass needle aligns itself with magnetic north. But the nucleus differs from this simple model in that it possesses another intrinsic property, nuclear spin (or spin angular momentum), and can be considered as spinning continuously around the nuclear axis. The combination of nuclear magnetism and spin confers properties on the nuclei that have an analogy with a spinning top that has tipped from the vertical direction. In the same way that the top rotates around both its own axis and the vertical direction of the gravitational field, the nucleus rotates around both the direction of an external magnetic field and its own axis (Fig. 2.1). This frequency of the precession around the magnetic field is characteristic of the particular nucleus and the direct linear relationship between frequency and the strength of the magnetic field is fundamental to NMR. For example, the improvement of signal-to-noise ratio (SNR) in MR images acquired in a 3 T scanner is improved with respect to images acquired in a standard 1.5 T scanner by a direct result of the doubling of the fundamental precession frequency that this relationship predicts.

In fact, two discrete possibilities exist for the direction of the precession: one for the nucleus aligned along the direction of the external magnetic field (represented in Fig. 2.1) and another one in the opposite direction (anti-aligned).

FIG. 2.1. A nucleus in the presence of an external magnetic field, B0. As a consequence of the intrinsic properties of spin and magnetism, it will also rotate around not only its own axis but also the axis of the external magnetic field. This is known as precession.

As expected, the former state of alignment is preferred and the majority of nuclei in a large 'ensemble' or collection of nuclei will assume this state (low energy). However, a certain proportion of the nuclei will be anti-aligned (high energy) and it is this population difference that underlies the phenomenon of NMR. In the presence of a pulse of electromagnetic waves whose energy corresponds to the difference between these two states, the energy from these waves will be absorbed by the system - a process of resonance (hence nuclear magnetic 'resonance') - and the populations of spins in the two states will be equalized. The frequency region of the electromagnetic spectrum (in the order of tens of megahertz) that is suitable for this purpose is known as radiofrequency (RF).

This perturbed state is temporary and, once the application of the electromagnetic waves is terminated, the nuclei will return to the initial state. The energy previously absorbed by the system will hence be returned in the form of a signal that can be detected by a suitably placed receiver coil. By imposing conditions on the system using magnetic field gradients (see later), this signal will reflect not only the nuclear species from which it originated but also its spatial location. A spatially encoded map of these signals - an MR image - can thereby be formed. The nucleus of the hydrogen atom ('H) - the proton - is especially suitable for this technique and since water (H20) forms more than 80% of the human body, imaging with 'proton MRI' has revolutionized the clinical imaging of soft tissue.

For a more detailed discussion of the principles of physics that give rise to the MR signal, and how this is then manipulated to create an image, refer to one of the texts listed at the end of this chapter.

The Energy Used in Magnetic Resonance and Its Biological Effects

The energy which is used in the process by which the MR image is acquired is not known to cause any harmful biologic effects. As compared to both X-rays (as in CT) and radioisotopes (as in positron-emission tomography, PET, and single-photon-emission CT, SPECT), the energy used in MR imaging is nine orders of magnitude (1,000,000,000 times) less than the energy of X-rays and radioisotopes. It is known to be largely the high energy of the radiation (in X-rays) that causes biologic damage to cells (particularly their DNA), and therefore, even from first principles, the extremely low energy of the radiofrequency electromagnetic radiation employed in MR studies (when compared to X-rays) is far less likely to cause significant biologic damage.

X-Rays Compared to Magnetic Resonance

It is worth keeping in mind that the basic principle of X-rays and NMR signals are fundamentally different.

X-ray images stem from the interaction between the X-rays themselves and the electron cloud of the atoms. This is a 'diffraction technique' and the resolution that can be obtained is therefore related to the wavelength of the radiation (X-rays are high-frequency electromagnetic radiation with high energy). If MR were to use this principle, then the wavelength is so long (low-frequency and low-energy electromagnetic radiation) that the smallest object that could be seen using radiofrequency diffraction (at these energy levels) would be of the order of meters in size. In contrast to this, the NMR is a 'resonance technique' and stems from intrinsic properties of the nucleus described in the previous section.

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