The Michener Institute for Applied Health Sciences recently played host to a one-day epilepsy symposium: Evaluation and Prevention. The event featured a number of speakers, who covered a wide variety of topics relevant to epilepsy, including: surgery in epilepsy, classifications of seizures, and epilepsy in childhood and adulthood.

The seminar primarily focused on technological advancements in epilepsy treatment, particularly in the field of imaging. With the advent of Computer Axial Tomography [CAT] scanning and Magnetic Resonance Imaging [MRI], the detection of seizures and their effects has improved considerably. With these advancements, we are able to obtain clearer images of the brain, allowing specialists to pinpoint the exact source of the seizure, thereby making treatment more swift and effective.

This article will discuss two of these imaging techniques – CAT and MRI, briefly dwelling on how they work and their application in the detection of epilepsy.

Computer Axial Tomography [CAT]

The role of CAT and MRI is simple – to create an image of a part of the body that is not usually visible to the naked eye. It is interesting to note that both use similar technology as that applied in x-rays, although with a slight twist. The first of the two systems to evolve was the CAT scan, which is credited as being the brainchild of Godfrey Houndsfield in 1972. The premise behind CAT is simple: to take many x-ray exposures at a variety of angles. All of these one-dimensional images are then put together using sophisticated computer software to create a 'picture' of two-dimensional slices of the exposed area. Modern CAT computer systems are now capable of combining these two-dimensional slices to produce a remarkably clear three-dimensional reconstruction of the area in question

A CAT image, therefore, is more than a photograph or an x-ray. In those two cases, all of the structures that appear are stacked on top of each other. This prevents us from obtaining potentially useful information about, in the case of epilepsy, the source of a seizure. Conversely, the three-dimensional nature of the CAT image provides us with information about the specific structures and materials in the path of the light beams, thereby allowing for a more accurate diagnosis of the situation.

Magnetic Resonance Imaging [MRI]

To put it simply, MRI has revolutionized medical imaging as we know it, and has become one of the most sought after procedures the world over. Although MRI may appear, at first glance, to have evolved from CAT, it works in an entirely different way. MRI relies on a medical biophysical application known as Nuclear Magnetic Resonance [NMR] (the terms are often used interchangeably): the science behind which can be rather complex. For the purposes of this article, we shall present an overly-simplified model of how NMR [and thus MRI] works.

Atoms are comprised of three key particles: protons, electrons, and neutrons. The human body, as an inhomogeneous sample, is comprised of many, many atoms, and more importantly for us, many protons. Each of these protons has what physicists refer to as spin – a type of motion that is almost identical to that of a spinning top. This type of motion is known as precession, and we measure the movement by determining the frequency of the precession – that is, how many times the top spins in a given time interval.

While spinning, the protons oscillate back and forth. This motion causes protons to release electromagnetic waves, creating a magnetic field around each proton. Consequently, we can say that the overall magnetic field of a region of the body is simply the sum of all of the small magnetic fields in that particular area. Furthermore, we can measure the frequencies of the different regions by inducing a current in a small coil – a pickup coil – that is placed close to this overall magnetic field.

It was the suggestion of Dr. Paul Lauterbur and Dr. Peter Mansfield that we use these different frequencies and different magnitudes of the signals to distinguish different parts of this inhomogeneous sample by applying what is known as a monotonic gradient to the magnetic field. The gradient is made up of more spinning, oscillating protons, all precessing within a small range of frequencies. When this gradient is applied, it will activate the same frequencies in a slice of the sample by exciting the nuclear spins of the corresponding protons. Each area on this slice will have protons oscillating at different frequencies. Since MRI picks up signals from the full 360° possible, computers similar to those used in CAT are then used to reconstruct these different areas, and produce a remarkably clear image.

To obtain the image, the computer calculates two values known as relaxation times. Different tissues have different values for these relaxation times, and consequently, different levels of contrast. By applying different gradients in different directions, a variety of these relaxation times can be calculated, allowing the computer to obtain enough information to produce an image.

The final 'image' is not simply presented on a piece of paper. Rather, the computer generates a three-dimensional representation, allowing the user to rotate about a particular area and examine it from any angle. It now becomes possible to take a virtual tour of one's knee, for instance, the same way one could a car or building on the Internet!

Imaging in Epilepsy

According to Stephanie Holowka, lead technologist of Magnetoencephalography (MEG) and 3D imaging at the Hospital for Sick Children and a speaker at the Epilepsy Symposium, MRI has been a revolutionary step in the improving the detection of epilepsy. This is true for a number of reasons. First of all, MRI demonstrates comparison of tissues on a molecular level, which allows us to better see subtle alterations in the brain tissue and related structures. Often times, even what appear to be small changes can have a crucial impact on an individual. Furthermore, MRI is definitive in seizure evaluation, particularly in determining its source, in addition to being risk-free and harmless to the patient.

However, MRI is not without its share of drawbacks, which centre particularly in the design of the apparatus. MRI machines are designed to take an image of the entire body. It is not possible to, like an x-ray, simply obtain an image of a certain region. As a result, for a successful, useable image to be produced, the patient is required to keep still for upwards of 30 minutes – a task that can prove to be rather difficult if dealing with a person experiencing frequent seizures or a child. Furthermore, the person is required to be placed in a particularly narrow chamber, which may not be particularly good news for sufferers of claustrophobia.

Ms. Holowka was also quick to point out that CAT is not often the best option when dealing with epilepsy. Though it produces accurate images, the type of image that is produced differs greatly from an MRI. Where MRI is analogous to a movie of the inside of one's body, a CAT image is more closely modeled to a three-dimensional photograph. This key difference prevents CAT from detecting the subtle alterations discussed earlier. As a result, CAT is most effective in diagnosing larger-scale malformations, whether in the brain or elsewhere.

To conclude, imaging has been a vital step towards improving our understanding of the nature of epilepsy and seizures. We are now better able to diagnose potential cases of this condition, and develop treatment that is quicker and more effective. Continued improvements in technology enable us to improve the quality of images that we are able to obtain, in addition to making the MRI apparatus more economical and cost-effective, thereby allowing more people to have access to this important technology.

For those interested in exploring the science behind these imaging techniques in greater detail, several sources have been provided below for your reading pleasure, all of which are conveniently available online.

Computer Axial Tomography [CAT]:

Key, Anthony. 2005. S2.6.2: Computer Axial Tomography [CAT, or CT Scan]. X-Ray Production, Characteristics, and Use. University of Toronto. Available for download at: http://www.physics.utoronto.ca/%7Ekey/PHY138/Suppl.Notes/SNII%20-%20X-rays.pdf

Michael, Greg. 2001. X-ray Computed Tomography. Physics Education. 36, 442-451. Available for download at: http://www.physics.utoronto.ca/~key/PHY138/xray_computed_tomography.pdf

Oldham, Mark. 2001. Radiation Physics and Applications in Therapeutic Medicine. Physics Education. 36: 460-467. Available for download at: http://www.physics.utoronto.ca/~key/PHY138/Radiation_Physics_Oldham.pdf

An interactive site that describes information regarding CT scans can be found at: http://www.colorado.edu/physics/2000/index.pl. [Click on Einstein's Legacy and then on CAT Scan].

Magnetic Resonance Imaging [MRI]:

Keevil, Stephen F. 2001. Magnetic resonance imaging in medicine. Physics Education. 36:476-485. Available for download at: http://www.physics.utoronto.ca/~key/PHY138/Suppl.Notes/Magnetic_Resonance_Imaging_in_Medicine.pdf

Key, Anthony. 2005. Magnetic Resonance Imaging. Website: PHY138Y Nuclear and Radiation Section. University of Toronto. Available for download at: http://www.physics.utoronto.ca/~key/PHY138/Suppl.Notes/SNVI - Magnetic Resonance Imaging.pdf

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Last Modified: 06/30/2006 10:54:09 AM