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Diffusion Magnetic Resonance Imaging: Its Principle and Applications 102 THE ANATOMICAL RECORD (NEW ANAT.) 257:102–109, 1999 FEATURE ARTICLE Diffusion Magnetic Resonance Imaging: Its Principle and Applications SUSUMU MORI* AND PETER B. BARKER Diffusion magnetic resonance imaging (MRI) is one of the most rapidly evolving techniques in the MRI field. This method exploits the random diffusional motion of water molecules, which has intriguing properties depending on the physiological and anatomical environment of the organisms studied. We explain the principles of this emerging technique and subsequently introduce some of its present applications to neuroimaging, namely detection of ischemic stroke and reconstruction of axonal bundles and myelin fibers. Anat Rec (New Anat) 257:102–109, 1999. ௠ 1999 Wiley-Liss, Inc. KEY WORDS: brain imaging; magnetic resonance imaging; diffusion MRI; diffusion tensor imaging; stroke; fiber reconstruction It is truly amazing to realize that more in its acute phase.14 Around the same ous types of MRI techniques have been than two decades after the invention time, scientists had also noticed that designed to use the difference in such of magnetic resonance imaging there is a peculiar property of water MRI properties of water in different (MRI),8 this technology is still evolv- diffusion in highly ordered organs such tissues to differentiate regions of inter- ing with considerable speed. The tech- as brains.13,11,5,6,19 In these organs, wa- est. nique of diffusion-weighted imaging ter does not diffuse equally in all direc- In order to explain the concept of (DWI) is one of the most recent prod- tions, a property called anisotropic the conventional MRI, we use the anal- ucts of this evolution. Briefly speak- diffusion. For example, brain water ogy of a gyroscope (Fig. 1). In an MRI ing, this approach is based on the diffuses preferentially along axonal fi- experiment, we first excite water pro- measurement of Brownian motion of ber directions. We now believe that it tons in a sample (or human in our molecules. It has been long, but not is possible to use this diffusion prop- case) with the imposition of a strong widely, known that nuclear magnetic erty as a probe to study the structure magnetic field. This is similar to start- resonance is capable of quantifying of spatial order in living organs non- ing the rotation of millions of gyro- diffusional movement of molecules.17 invasively. In this tutorial, We will In the 1980s, a method that combines explain the physical principles of this this diffusion measurement with MRI emerging technology and introduce its was introduced, which is now widely present applications. called diffusion imaging.18,10,9 This technique can characterize water diffu- sion properties at each picture ele- CONVENTIONAL MRI ment (pixel) of an image. The first Before explaining diffusion MRI, we important application of diffusion MRI will briefly go over principles of con- emerged at center stage of the MRI ventional MRI—proton density and community in early 1990s when it was T2-weighted imaging—because they discovered that DWI can detect stroke share some important analogies with diffusion MRI. In MRI, we usually observe water protons, because they are by far the dominant chemical spe- Drs. Mori and Barker are faculty mem- bers in the Department of Radiology, cies observable by magnetic reso- The Johns Hopkins University School nance. MRI is an extraordinarily versa- Figure 1. Analogy of MRI signal to a gyro- of Medicine, Baltimore, Maryland. tile technique because of its capability scope. After excitation of protons in MRI, the *Correspondence to: Susumu Mori, signal behaves like a gyroscope that pre- Ph.D., Johns Hopkins University School of producing various types of contrast of Medicine, Department of Radiology, 217 in the images, or so-called, weighting. cesses at a fixed rate. If the position of the Traylor Bldg., 720 Rutland Ave., Baltimore, gyroscope is projected to a horizontal plane, MD 21205. Fax: (410) 614-1948; E-mail: Water protons have characteristic MRI such precession can be presented as a rotat- [email protected] properties depending on their physi- ing vector. The position of the vector is called cal and chemical environments. Vari- phase. FEATURE ARTICLE THE ANATOMICAL RECORD (NEW ANAT.) 103 several mechanisms through which DIFFUSION WEIGHTING the signal eventually diminishes, or Weighting of MRI by diffusion can relaxes. The T relaxation can be ex- 2 also be explained using the analogy to plained by a loss of coherence or syn- the gyroscope. Just as the rate of the chrony between the gyroscope rota- precession of the gyroscope is propor- tions. Right after the excitation, all the tional to the strength of gravity, in gyroscopes have the same phase (Fig. MRI the rate of the precession is pro- 2). However, as time goes by, the phases portional to the strength of the mag- of gyroscopes become randomized be- net. For example, in a typical MRI cause each gyroscope precesses at magnet of 1.5 tesla (T), the rate of the slightly different speed due to various precession is about 64 MHz. Because reasons such as local non-homegene- Figure 2. Mechanism of T2 relaxation. Phase the strength of magnetic field is kept of each proton is gradually randomized after ity of the magnetic field. Because what as homogeneous as possible, this pre- excitation due to slightly different precession we observe is the vector sum with cession rate is also very homogeneous rates. As a result, the vector sum (indicated different phases, this randomization by thick arrows) decreases over the time, across the magnet. This homogeneity which means signal loss in MRI. of the phase leads to the loss of signal can be disturbed linearly by using a in MRI, which is called T2 relaxation. so-called pulsed field gradient. The Depending on the location of water strength (slope) of the gradient, its scopes simultaneously. The gyroscopes protons related, for example, to patho- direction, and the time period can be will start to precess, and it is this logical conditions, the time required controlled. As an example, Figure 5 precession-equivalent of water pro- for T2 relaxation varies, resulting in shows a diagram of an x gradient. As a tons that produces signals (electric different degrees of signal loss. This in result of this gradient application, pro- currents in a receiver) in MRI. If the turn can be used for the diagnosis of tons start to precess at a different rate movement of the gyroscope is pro- certain diseases. The exact mecha- along the x-axis. With an analogy to jected to a horizontal plane, the preces- nism that confers longer or shorter T 2 the T2 relaxation process (Fig. 2), such sion can be presented as a rotating relaxation is not completely under- differences in the precession rate lead vector as shown in Figure 1. The posi- stood. One thing we are sure of is that to dispersion of the phase and signal tion of this vector is called the phase. when water is in an environment loss (Fig. 6). However, if another gradi- In a standard proton density image, where it can freely tumble (e.g., less ent pulse is subsequently applied with the visual contrast is determined by viscosity or less macromolecules with the same direction and time period the concentration of water, or the num- which to interact), the relaxation tends but of opposite magnitude, such dis- ber of gyroscopes in our analogy. to take longer. One typical example is persion can be refocused or re-phased, Namely, the more water in a given the formation of edema, which leads therefore, the first gradient is called region, the brighter the region will to significant slowing of the relaxation the dephasing gradient and the second appear. The more frequently used, but and a prolonged T2-weighted signal. one the rephasing gradient. more difficult to understand, proto- The T2 weighting can be obtained by From Figure 6, it can be understood cols generate so-called relaxation- inserting a weighting period in be- that this refocusing can not be perfect weighted images, such as T2-weighted tween the excitation and data acquisi- if the protons moved in between a pair images. After the excitation, there are tion (Fig. 3) and this time period of the gradient applications. Thus, by (strictly speaking from the time point applying a pair of gradient pulses after of excitation to the beginning of the the excitation and before the data ac- Water protons have acquisition) is called echo time (TE). quisition, we can sensitize the image Depending on the TE, the amount of (make the resultant image sensitive) characteristic MRI T weighting varies. We can obtain a 2 properties depending on heavily weighted image by increasing their physical and TE, while the use of the shortest pos- sible TE produces minimally T2- chemical environments. weighted images. Examples of the Various types of MRI lightly weighted (short TE) and heavily T2-weighted (long TE) images are techniques have been shown in Figure 4. Regions in the designed to use the brain that have slow T2 relaxation show up bright, such as cerebrospinal difference in such MRI fluid (CSF). White matter has faster T 2 Figure 3. Mechanism of T2 weighing. By insert- properties of water in relaxation and consequently looks ing a waiting period in between excitation darker. The minimally T2-weighted im- and data acquisition, we can obtain relax- different tissues to age in Figure 4a is referred as ‘‘proton ation weighting. For T2 weighting, a scheme density,’’ which means the image is not called spin-echo is inserted (in this case, the differentiate regions of spin-echo time is identical to the T -weighting weighted by anything but water con- 2 interest.
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