Diffusion-Weighted MR Imaging of the Brain ⅐ 333 DW MR Imaging Characteristics of Various Disease Entities

State of the Art Pamela W. Schaefer, MD Diffusion-weighted MR P. Ellen Grant, MD 1 R. Gilberto Gonzalez,MD, Imaging of the Brain PhD Diffusion-weighted magnetic resonance (MR) imaging provides image contrast that Index terms: is different from that provided by conventional MR techniques. It is particularly Brain, diffusion sensitive for detection of acute ischemic stroke and differentiation of acute stroke Brain, infarction, 10.781 from other processes that manifest with sudden neurologic deficits. Diffusion- Brain, infection, 10.20 Brain, injuries, 10.41, 10.42 weighted MR imaging also provides adjunctive information for other cerebral dis- Brain, ischemia, 10.781 eases including neoplasms, intracranial infections, traumatic brain injury, and de- Brain, MR, 10.12141, 10.12144 myelinating processes. Because stroke is common and in the differential diagnosis of Brain neoplasms, 10.31, 10.32 most acute neurologic events, diffusion-weighted MR imaging should be considered Sclerosis, multiple, 10.871 an essential sequence, and its use in most brain MR studies is recommended. State of the Art Radiology 2000; 217:331–345 Abbreviations: ADC ϭ apparent diffusion coefficient CSF ϭ cerebrospinal fluid Diffusion-weighted (DW) magnetic resonance (MR) imaging provides potentially unique DW ϭ diffusion weighted information on the viability of brain tissue. It provides image contrast that is dependent on the molecular motion of water, which may be substantially altered by disease. The 1 From the Neuroradiology Division, method was introduced into clinical practice in the middle 1990s, but because of its Massachusetts General Hospital, GRB demanding MR engineering requirements—primarily high-performance magnetic field 285, Fruit St, Boston, MA 02114-2696. gradients—it has only recently undergone widespread dissemination. The primary appli- Received April 30, 1999; revision re- cation of DW MR imaging has been in brain imaging, mainly because of its exquisite quested July 14; revision received No- vember 8; accepted November 15. sensitivity to ischemic stroke—a common condition that appears in the differential diag- Address correspondence to R.G.G. nosis in virtually all patients who present with a neurologic complaint. (e-mail: [email protected]). Because DW MR imaging uses fast (echo-planar) imaging technology, it is highly © RSNA, 2000 resistant to patient motion, and imaging time ranges from a few seconds to 2 minutes. As a consequence, DW MR imaging has assumed an essential role in the detection of acute brain infarction and in the differentiation of acute infarction from other disease processes. DW MR imaging is also assuming an increasingly important role in the evaluation of many other intracranial disease processes. BASIC CONCEPTS OF DW MR IMAGING In this section, the basic concepts involved in DW MR imaging will be briefly reviewed. For more detailed descriptions of the physics of DW imaging, a number of excellent reviews (1–5) are available. Stejskal and Tanner (6) provided an early description of a DW sequence in 1965. They used a spin-echo T2-weighted pulse sequence with two extra gradient pulses that were equal in magnitude and opposite in direction. This sequence enabled the measurement of net water movement in one direction at a time. To measure the rate of movement along one direction, for example the x direction, these two extra gradients are equal in magnitude but opposite in direction for all points at the same x location. However, the strength of these two balanced gradients increases along the x direction. Therefore, if a voxel of tissue contains water that has no net movement in the x direction, the two balanced gradients cancel each other out. The resultant signal intensity of that voxel is equal to its signal intensity on an image obtained with the same sequence without the DW gradients. However, if water molecules have a net movement in the x direction (eg, due to diffusion), they are subjected to the first gradient pulse at one x location and the second pulse at a different x location. The two gradients are no longer equal in magnitude and no longer cancel. The difference in gradient pulse magnitude is propor- tional to the net displacement in the x direction that occurs between the two gradient pulses, and faster-moving water protons undergo a larger net dephasing. The resultant signal intensity of a voxel of tissue containing moving protons is equal to its signal 331 intensity on a T2-weighted image decreased by an amount related to the rate of diffusion. The signal intensity (SI) of a voxel of tissue is calculated as follows: SI ϭ SI0 ϫ exp ͑Ϫb ϫ D͒, (1) where SI0 is the signal intensity on the T2-weighted (or b ϭ 0 sec/mm2) image, the diffusion sensitivity factor b is equal to ␥2G2␦2(⌬Ϫ␦/3), and D is the diffusion coefficient. ␥ is the gyromagnetic ratio; G is the magnitude of, ␦ the width of, and ⌬ the time between the two balanced DW gradient pulses. Figure 1. Anisotropic nature of diffusion in the brain. Transverse DW MR images (b ϭ 1,000 2 According to Fick’s law, true diffusion sec/mm ; effective gradient, 14 mT/m; repetition time, 7,500 msec; minimum echo time; matrix, 128 ϫ 128; field of view, 200 ϫ 200 mm; section thickness, 6 mm with 1-mm gap) with the is the net movement of molecules due to diffusion gradients applied along the x (Gx, left), y (Gy, middle), and z (Gz, right) axes demon- a concentration gradient. With MR imag- strate anisotropy. The signal intensity decreases when the white matter tracts run in the same ing, molecular motion due to concentra- direction as the DW gradient because water protons move preferentially in this direction. Note tion gradients cannot be differentiated that the corpus callosum (arrow on left image) is hypointense when the gradient is applied in the from molecular motion due to pressure x (right-to-left) direction, the frontal and posterior white matter (arrowheads) are hypointense gradients, thermal gradients, or ionic in- when the gradient is applied in the y (anterior-to-posterior) direction, and the corticospinal tracts (arrow on right image) are hypointense when the gradient is applied in the z (superior-to-inferior) teractions. Also, with MR imaging we do direction. not correct for the volume fraction available or the increases in distance traveled due to tortuous pathways. Therefore, when measuring molecular motion with noted in the developing brain before T1- ing the magnitude of the ADC are used in DW imaging, only the apparent diffusion and T2-weighted imaging or histologic clinical practice. coefficient (ADC) can be calculated. The evidence of myelination becomes evi- DW gradient pulses are applied in one signal intensity of a DW image is best dent (13,14). It is likely that in addition direction at a time. The resultant image expressed as to axonal direction and myelination, has information about both the direction other physiologic processes, such as ax- and the magnitude of the ADC (Fig 1). To SI ϭ SI0 ϫ exp ͑Ϫb ϫ ADC͒. (2) olemmelic flow, extracellular bulk flow, create an image that is related only to With the development of high-perfor- capillary blood flow, and intracellular the magnitude of the ADC, at least three mance gradients, DW imaging can be streaming, may contribute to white mat- of these images must be combined. The performed with an echo-planar spin- ter anisotropy. The anisotropic nature of simplest method is to multiply the three echo T2-weighted sequence. With the diffusion in the brain can be appreciated images created with the DW gradient original spin-echo T2-weighted sequence, by comparing images obtained with DW pulses applied in three orthogonal direc- even minor bulk patient motion was gradients applied in three orthogonal di- tions. The cube root of this product is the enough to obscure the much smaller mo- rections (Fig 1). In each of the images, the DW image (Fig 2). It is important to un- lecular motion of diffusion. The substitu- signal intensity is equal to the signal in- derstand that the DW image has T2- tion of an echo-planar spin-echo T2- tensity on echo-planar T2-weighted im- weighted contrast as well as contrast due weighted sequence markedly decreased ages decreased by an amount related to to differences in ADC. To remove the T2- imaging time and motion artifacts and the rate of diffusion in the direction of weighted contrast, the DW image can be increased sensitivity to signal changes the applied gradients. Images obtained divided by the echo-planar spin-echo T2- due to molecular motion. As a result, with gradient pulses applied in one direc- weighted (or b ϭ 0 sec/mm2) image to the DW sequence became clinically fea- tion at a time are combined to create DW give an “exponential image” (Fig 3). Al- sible to perform. Other methods of per- images or ADC maps. The ADC is actually ternatively, an ADC map, which is an forming DW MR imaging without echo- a tensor quantity or a matrix: image whose signal intensity is equal to planar gradients have also been devel- the magnitude of the ADC, can be cre- ADC ADC ADC oped. These include DW sequences based xx xy xz ated (Fig 4). ADC ϭ ADCyx ADCyy ADCyz . (3) on a single-shot gradient and spin-echo Instead of obtaining images with b ϭ 0 ͫ ADCzx ADCzy ADCzz ͬ or single-shot fast spin-echo techniques sec/mm2 and with b ϭ 1,000 sec/mm2 (7,8). “Line-scan” DW and spiral DW The diagonal elements of this matrix and solving for ADC using Equation (2), sequences have also been developed (9– can be combined to give information one usually determines the ADC graphi- 12).

Load more