MRI’s main clinical use is making internal anatomical images. Almost all biological MR images are images of water. More specifically, we are imaging the protons in water. Water is convenient to use because it is the most plentiful substance in the body, and because protons give the highest NMR signal of any atomic species.

Back to the top

What kinds of magnetic fields are used in MRI?

There are three. You should know about two types already: the static magnetic field and pulsed (RF) magnetic fields. The third type, a gradient magnetic field, is used to cause a linear variation in magnetic field strength (and therefore Larmor frequency). The gradient can be oriented in any direction that we want it to be. Gradients, which are switched on and off during the course of the imaging experiment, have many uses, including:

· Creating signals

· Selective excitation

· Spatial encoding

· Diffusion imaging

· Disrupting unwanted MR signal patterns

Back to the top

What kinds of MR signals are used for MRI?

MR imaging uses echo signals. An echo is an MR signal that is “returned” after the FID has decayed. There are several types of MR echoes, which are distinguished by what causes the return of signal. If we use a pulsed magnetic field at the Larmor frequency, it’s called a spin echo. This pulse is often called a “refocusing” pulse because it brings back into phase any magnetic moments that have dephased because of magnetic field inhomogeneities. Another common type of echo is a gradient echo. A gradient echo is caused by first turning a gradient magnetic field on, during the FID. This disrupts the phases of the magnetic moments. Then the gradient is reversed, which restores the phases. A gradient echo results from the restoration of the phases.

Back to the top

What properties of the MR signal are you concerned with in MRI?

Its magnitude, frequency, phase, and origin in the body.

Back to the top

How can you tell the signal from one part of the body apart from the signal from another part of the body?

First, we selectively excite the protons in a narrow band (“slice”) of tissue. To do this, we apply a gradient, which causes a linear variation in Larmor frequency. Then we give an RF pulse that is designed to only excite the protons in a narrow frequency band. That band corresponds to the slice.

To tell things apart within the slice, we also use gradients. We apply a gradient while the signal is being read to cause a linear variation in Larmor frequency along one direction within the slice. This gradient is called the “read-out” or frequency-encoding” gradient. We spatially encode signals along the other direction by applying a gradient in between the FID and the echo. This causes a linear variation in phase (so this gradient is called the “phase-encoding gradient”). Phase encoding requires that we acquire many echoes, with a different gradient strength each time. The data acquired for each phase-encoding step are written into a raw data matrix called k-space.

In conventional imaging, one “line” of k-space is recorded per echo. In echo-planar imaging (EPI), all of k-space is recorded in a single echo.

Finally, we process the k-space data. We use a two-dimensional Fourier transform for this purpose. The transformed data get displayed as a grayscale image.

Back to the top

What are some common MRI terms?

TR (Repetition Time): In conventional imaging, TR is the time between phase encodings. In EPI, the effective TR is the time between images. Typical values range from 100 to 4000 ms.

TE (Echo Time): The time between the initial pulse and the peak of the echo. Typical values range from 5 to 150 ms.

Slice Thickness: The thickness (in mm or cm) of tissue that is selectively excited for each slice. Typical values range from 0.5 – 2 cm.

FOV (Field of View): The spatial dimensions (in mm or cm) of the slice. Typical values range from 10 ´10 to 40´40 cm.

Matrix Size: The size of the k-space matrix. For example, there might be 128 points read out along the frequency encoding direction and 128 phase-encoding steps. Typical values range from 64´64 to 512´512 and are almost always a power of two.

Voxel (Volume Element): The smallest unit of spatial resolution in an image.

In-plane Resolution: The area of each voxel within the slice plane; for 25.6´25.6 cm FOV and a 128´128 matrix, the in-plane resolution is 4 mm².

Voxel Size: The volume of each voxel; equal to the in-plane resolution times the slice thickness.

Number of Excitations: the number of times the entire imaging sequence is run, for purposes of signal averaging.

T₁: T₁ is the longitudinal relaxation time constant, which describes how long it takes for the system to return to equilibrium after it has been disrupted by an RF pulse. In muscle, the T₁ of water protons is about 1 s.

T₂: T₂ is the transverse relaxation time constant, which describes how long it takes for the signal to decay. At 30 – 40 ms, the T₂ of muscle water is much shorter than its T₁.

Back to the top

How can you tell things apart in images?

There are several ways of generating contrast in images. First, things can be distinguished on the basis of how much stuff is there (proton density). To generate this type of contrast, TR must be long (relative to T₁) and TE must be short (relative to T₂). Second, things can be distinguished using relaxation times. To develop contrast on the basis of T₂, a spin-echo image with a long TR and long TE are used. To generate contrast on the basis of T₁, a short TR and short TE are used.

There are many other ways of generating contrast, including making images that are sensitive to diffusion or to the exchange of magnetization between proteins and water.

Back to the top

In addition to imaging anatomy, what else is MRI good for?

There exists a whole family of MRI methods called “physiologic MRI.” Some methods, like diffusion MRI, can be used to make quantitatively accurate images of the physiological process. Others, like functional MRI of the brain and of muscle, are simply sensitive to physiologic processes that are associated with the activation of the tissue.

Back to the top

What is functional MRI (fMRI) of the brain?

When a region of the brain is activated, it receives increased blood flow – so much, in fact, that the venous and capillary concentrations of deoxyhemoglobin actually decreases. This is significant in MRI because deoxyhemoglobin is paramagnetic, and therefore causes minute variations in magnetic field strength. These inhomogeneities cause the signal to decay faster. So when the flow goes up and the deoxyhemoglobin concentration goes down, you get more signal from more stuff. The result is a slight (~2%) increase in MR signal from active regions of the brain. Gradient echoes are particularly sensitive to magnetic field inhomogeneity and so are often used in fMRI.

Back to the top

What is muscle functional MRI (mfMRI)?

Exercising muscles appear brighter in the image grayscale. In part this is due to an increase in the T₂ of intracellular water (longer T₂ = more signal = brighter in the image). The T₂ increases appear to be caused by changes in the chemical behavior of water due to cellular energy metabolism. Also, there are important extracellular effects of exercise, such as volume increases and blood flow and oxygenation changes, that influence the signal too.

mfMRI therefore seems to indicate muscle involvement during exercise, with an excellent inherent spatial sensitivity. Right now, our incomplete understanding of what causes the mfMRI response precludes equating it with any single physiological variable (such as neural activation, metabolism, or blood flow). Another limitation of mfMRI is caused by the fact that after exercise begins, it takes about 2 – 3 minutes for the signal to reach a plateau. Even more inconvenient is the fact that it takes 30 – 45 minutes for the signal intensity to recover after exercise. This means that it is not sensitive to the timing of muscle activations and deactivations during an activity.

Back to the top

Could you tell me a little more about diffusion MRI?

In diffusion MRI, gradients are applied on each side of a refocusing RF pulse. The first gradient disrupts the magnetic phases of the protons, and the second one restores the phases of all stationary protons. The restoration of signal is incomplete for protons that have moved (diffused) during the elapsed time, however. Diffusion MRI can, of course, be used to measure diffusion coefficients. Because the cellular diffusion of water corresponds to cell geometry in muscle, diffusion MRI can also be used to make inferences about muscle architecture. It is a much lengthier measurement than other ways of evaluating muscle architecture, however.

Back to the top

What are some the main biomedical MRI journals?

If your institution has electronic subscriptions to these journals, you can follow the links to go to the journal home page and downloads current articles:

Journal of Magnetic Resonance Imaging

Magnetic Resonance Imaging

Magnetic Resonance in Medicine

Magnetic Resonance Materials in Physics, Biology, and Medicine

NMR in Biomedicine

You will also find MRI papers in applied physiology journals.

Back to the top

Where can I go for more information?

Websites:

The International Society for Magnetic Resonance in Medicine maintains an extensive list of MRI tutorial and educational websites.

The spectroscopyNOW.com website has information about MRI and many other spectroscopic methods.

Books:

Questions and Answers in MRI by Allan Elster and Jonathan Burdette.

Magnetic Resonance Imaging by David Stark and William Bradley.

Back to the top

Back to the About the Technologies We Employ page.

Back to the NINS Home.