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Diffusion-weighted MR Imaging of the Brain¹

¹ From the Neuroradiology Division, Massachusetts General Hospital, GRB 285, Fruit St, Boston, MA 02114-2696. Received April 30, 1999; revision requested July 14; revision received November 8; accepted November 15. Address correspondence to R.G.G. (e-mail: rggonzalez@partners.org).

View larger version (64K): [in a new window]   Figure 1. Anisotropic nature of diffusion in the brain. Transverse DW MR images (b = 1,000 sec/mm2; effective gradient, 14 mT/m; repetition time, 7,500 msec; minimum echo time; matrix, 128 x 128; field of view, 200 x 200 mm; section thickness, 6 mm with 1-mm gap) with the diffusion gradients applied along the x (Gx, left), y (Gy, middle), and z (Gz, right) axes demonstrate anisotropy. The signal intensity decreases when the white matter tracts run in the same direction as the DW gradient because water protons move preferentially in this direction. Note that the corpus callosum (arrow on left image) is hypointense when the gradient is applied in the x (right-to-left) direction, the frontal and posterior white matter (arrowheads) are hypointense 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) direction.

View larger version (60K): [in a new window]   Figure 2. Calculation of signal intensity on an isotropic DW image (b = 1,000 sec/mm2; effective gradient, 14 mT/m; repetition time, 7,500 msec; minimum echo time; matrix, 128 x 128; field of view, 400 x 200 mm; section thickness, 6 mm with 1-mm gap). The signal intensities of the three transverse images (Gx, Gy, and Gz), each with a diffusion gradient applied in one of three orthogonal directions, are multiplied together. Here the DW gradients were applied along the x, y, and z axes. The signal intensity of the isotropic DW image (bottom) is essentially the cube root of the signal intensities of these three images multiplied together. Note that both T2-weighted contrast and the rate of diffusion contribute to the signal intensity of the isotropic DW image.

View larger version (65K): [in a new window]   Figure 3. Removal of T2-weighted contrast. To remove the T2-weighted contrast in the isotropic transverse DW image (b = 1,000 sec/mm2; effective gradient, 14 mT/m; repetition time, 7,500 msec; minimum echo time; matrix, 128 x 128; field of view, 200 x 200 mm, section thickness, 6 mm with 1-mm gap), the transverse DW image (DWI) is divided by the transverse echo-planar spin-echo T2-weighted (EP SET2) image. The resultant image is called the exponential image because its signal intensity is exponentially related to the ADC.

View larger version (61K): [in a new window]   Figure 4. Creation of an ADC map. One method of creating an ADC map is to mathematically manipulate the exponential (Exp.) image. The appearances on the transverse DW image (DWI), exponential image, and ADC map, as well as the corresponding mathematic expressions for their signal intensities, are shown. Image parameters are b = 1,000 sec/mm2; effective gradient, 14 mT/m; repetition time, 7,500 msec; minimum echo time; matrix, 128 x 128; field of view, 200 x 200 mm; section thickness, 6 mm with 1-mm gap. SI = signal intensity, SIo = signal intensity on T2-weighted image.

View larger version (86K): [in a new window]   Figure 5. Time course of an ischemic infarction. Images demonstrate the evolution of an ischemic infarction involving the left cerebellar hemisphere and left middle cerebellar peduncle. Both transverse DW images (DWI; b = 1,000 sec/mm2; effective gradient, 14 mT/m; repetition time msec/echo time msec, 6,000/108; matrix, 256 x 128; field of view, 400 x 200 mm; section thickness, 6 mm with 1-mm gap) and transverse ADC maps are displayed. The patient underwent MR imaging 6 hours after the onset of acute neurologic symptoms. At 6 hours, the lesion (arrows) is hyperintense on the DW images and hypointense on the corresponding ADC map. The lesion becomes progressively more hyperintense on DW images, reaching its maximum hyperintensity at the 58-hour time point, when it also reaches its maximum hypointensity on ADC maps. At 7 days, there is ongoing resolution of the lesion on both DW images and ADC maps. By 134 days, there is subtle hypointensity on the DW image and hyperintensity on the ADC images.

View larger version (103K): [in a new window]   Figure 6. Diffusion-perfusion mismatch after left middle cerebral artery stroke. The patient was imaged 3.8 hours after a witnessed sudden onset of a right hemiparesis. Transverse DW images (DWI; b = 1,000 sec/mm2; effective gradient, 14 mT/m; 6,000/108; matrix, 256 x 128; field of view, 400 x 200 mm; section thickness, 6 mm with 1-mm gap) demonstrate hyperintensity in the subcortical region, including in the lenticular nucleus and corona radiata (arrowheads, right-hand image in top row). Transverse cerebral blood volume (CBV) images (spin-echo echo-planar technique; 0.2 mmol/kg gadopentetate dimeglumine [Magnevist; Berlex Laboratories, Wayne, NJ]; 51 images per section; 1,500/75; matrix, 256 x 128; field of view, 400 x 200 mm; section thickness, 6 mm with 1-mm gap) demonstrate decreased dynamic cerebral blood volume in the region of hyperintensity on the DW images. However, there are areas of abnormal cerebral blood volume (arrows) that appear relatively normal on the DW study. Follow-up study performed 10 hours after the onset of symptoms demonstrates an increase in the size of the DW imaging abnormality (arrowheads, right-hand image in fifth row) as it extends into the region of brain that was previously normal on DW images but abnormal on cerebral blood volume images.

View larger version (131K): [in a new window]   Figure 7. Reversible ischemic lesion. Top: The patient was imaged approximately 2 hours after the onset of a witnessed acute neurologic deficit. Top left: Transverse DW image (DWI; b = 1,000 sec/mm2; effective gradient, 14 mT/m; 6,000/108; matrix, 256 x 128; field of view, 400 x 200 mm; section thickness, 6 mm with 1-mm gap) shows an area of hyperintensity (arrow) in the left posterior frontal and anterior parietal lobes. Top middle: A region of hypointensity (arrow) corresponding to this area is seen on the transverse ADC image (arrow). Top right: No definite abnormality is seen on the transverse fast spin-echo T2-weighted MR image (4,000/104; echo train length, eight; matrix, 256 x 192; field of view, 200 x 200 mm; section thickness, 5 mm with 1-mm gap; one signal acquired). The patient was treated with intravenous recombinant tissue plasminogen activator, with resolution of the neurologic symptoms. Bottom: Follow-up images obtained 3 days later demonstrate near interval resolution of the abnormalities on the 2-hour DW image and ADC map. No definite lesion was identified on the follow-up T2-weighted image. Of note, the decrease in ADC was approximately 20% of the normal value. Lesions that become confirmed infarctions typically demonstrate a 50% reduction in ADC.

View larger version (111K): [in a new window]   Figure 8. Differentiation of acute white matter infarction from nonspecific small-vessel ischemic changes. This patient had onset of symptoms 2 days prior to imaging. Top: Transverse DW images (DWI; b = 1,000 sec/mm2; effective gradient, 14 mT/m; 6,000/108; matrix, 256 x 128; field of view, 400 x 200 mm; section thickness, 6 mm with 1-mm gap) in the top row clearly demonstrate the acute infarction (arrowheads) in the putamen and corona radiata. Bottom: Fluid-attenuated inversion recovery (FLAIR) images (10,000/141; inversion time, 2,200 msec; echo train length, eight; matrix, 256 x 192; field of view, 240 x 240 mm; section thickness, 5 mm with 1-mm gap; one signal acquired) demonstrate multiple white matter lesions in which acute (arrowhead) and chronic lesions (arrows) cannot be differentiated.

View larger version (81K): [in a new window]   Figure 9. Hyperperfusion syndrome after carotid endarterectomy. The patient developed neurologic symptoms referable to the left hemisphere several days after undergoing a left carotid endarterectomy. The CT scan was abnormal, and the question of infarction was raised. Left: Transverse fast spin-echo T2-weighted MR image (4,000/104; echo train length, eight; matrix, 256 x 192; field of view, 200 x 200 mm, section thickness, 5 mm with 1-mm gap; one signal acquired) demonstrates numerous areas of abnormal high signal intensity (arrow) in the left hemisphere. Infarctions remained in the differential diagnosis. Middle: Transverse DW MR image (b = 1,000 sec/mm2; effective gradient, 14 mT/m; 6,000/108; matrix, 256 x 128; field of view, 400 x 200 mm; section thickness, 6 mm with 1-mm gap) reveals predominant isointensity in the left hemisphere with small areas of slight hypointensity and slight hyperintensity (arrow). ADC images (not shown) demonstrated no areas of restricted diffusion. Right: Transverse three-dimensional time-of-flight MR angiogram (49/6.9; 20° flip angle; matrix, 256 x 192; field of view, 200 x 200 mm; section thickness, 1 mm) demonstrates excellent flow-related enhancement (arrow) in the left hemisphere. A diagnosis of hyperperfusion syndrome with vasogenic edema was established on the basis of DW imaging findings. The patient was treated conservatively and recovered fully.

View larger version (78K): [in a new window]   Figure 10. Postoperative residual epidermoid tumor. The patient underwent resection of a large left middle cranial fossa epidermoid tumor that extended into the posterior fossa. Transverse T1-weighted (left) (650/16; matrix, 256 x 192; field of view, 200 x 200 mm; section thickness, 5 mm with 1-mm gap; one signal acquired) and fast spin echo T2-weighted (middle) (4,000/104; echo train length, eight; matrix, 256 x 192; field of view, 200 x 200 mm; section thickness, 5 mm with 1-mm gap, one signal acquired) MR images do not allow clear differentiation of residual mass from the resection cavity. Right: Transverse DW MR image (b = 1,000 sec/mm2; effective gradient, 14 mT/m; 6,000/108; matrix, 256 x 128; field of view, 400 x 200 mm; section thickness, 6 mm with 1-mm gap) clearly demonstrates a hyperintense mass (black arrow) adjacent to the left pons and a smaller amount of mass (white arrow) in the left middle cranial fossa, consistent with residual epidermoid tumor. CSF (arrowhead) in the resection cavity is markedly hypointense.

View larger version (72K): [in a new window]   Figure 11. Pathologically proved cerebral abscess. Left: A complex signal intensity pattern is visible in the right occipital and temporal lobes on the fast spin-echo T2-weighted MR image (4,000/104; echo train length, eight; matrix, 256 x 192; field of view, 200 x 200 mm; section thickness, 5 mm with 1-mm gap; one signal acquired). Middle: Ring-enhancing lesion (arrows) in the right occipital lobe is demonstrated on the gadolinium-enhanced T1-weighted MR image (650/16; matrix, 256 x 192; field of view, 200 x 200 mm; section thickness, 5 mm with 1-mm gap; one signal acquired). Right: DW MR image (b = 1,000 sec/mm2; effective gradient, 14 mT/m; 6,000/108; matrix, 256 x 128; field of view, 400 x 200 mm; section thickness, 6 mm with 1-mm gap) demonstrates the characteristic restricted diffusion of pyogenic abscess (arrows). Note the hyperintensity (arrowhead) in the left occipital horn due to a loculated collection of pus in this location.

View larger version (82K): [in a new window]   Figure 12. Herpes encephalitis proved with results of polymerase chain reaction test. DW MR images (DWI; b = 1,000 sec/mm2; effective gradient, 14 mT/m; 6,000/108; matrix, 256 x 128; field of view, 400 x 200 mm; section thickness, 6 mm with 1-mm gap). demonstrate restricted diffusion bilaterally in the temporal lobes (short arrows), inferior frontal lobes (long arrows), and insulae (arrowheads), which is a typical distribution for herpes encephalitis.

View larger version (128K): [in a new window]   Figure 13. Pathologically proved Creutzfeldt-Jakob disease. Top: Transverse T2-weighted MR images (4,000/104; echo train length, eight; matrix, 256 x 192; field of view, 200 x 200 mm; section thickness, 5 mm with 1-mm gap; one signal acquired) demonstrate hyperintensity of the basal ganglia. Bottom: Transverse DW MR images (DWI; b = 1,000 sec/mm2; effective gradient, 14 mT/m; 6,000/108; matrix, 256 x 128; field of view, 400 x 200 mm; section thickness, 6 mm with 1-mm gap) show marked hyperintensity involving the basal ganglia bilaterally (arrowheads) and portions of the bilateral cortical ribbon (arrows).

View larger version (115K): [in a new window]   Figure 14. Severe head trauma resulting in diffuse axonal injury. Top: Transverse T2-weighted MR images (4,000/104; echo train length, eight; matrix, 256 x 192; field of view, 200 x 200 mm; section thickness, 5 mm with 1-mm gap; one signal acquired) demonstrate multiple white matter hyperintensities (arrows). Bottom: Transverse DW MR images (DWI; b = 1,000 sec/mm2; effective gradient, 14 mT/m; 6,000/108; matrix, 256 x 128; field of view, 400 x 200 mm; section thickness, 6 mm with 1-mm gap) demonstrate the lesions (arrows) with increased conspicuity. The hyperintensity is consistent with restricted diffusion. Note abnormalities (arrowheads) that extend to the cortex posteriorly.

View larger version (98K): [in a new window]   Figure 15. Hematoma in a patient with a right hemisphere glioblastoma who had undergone prior resection and who had developed a hematoma in the right frontal lobe. The patient was hospitalized for progression of symptoms and development of fever. A ring-enhancing lesion at the site of the prior hematoma was seen on a gadolinium-enhanced T1-weighted MR image (not shown) in the right frontal lobe. Left: DW MR image (b = 1,000 sec/mm2; effective gradient, 14 mT/m; 6,000/108; matrix, 256 x 128; field of view, 400 x 200 mm; section thickness, 6 mm with 1-mm gap) demonstrates a hyperintense lesion (arrow) in the right frontal lobe. Right: On the ADC image, the lesion is hypointense (arrow), which is consistent with restricted diffusion. The lesion was drained, and old hemorrhage was demonstrated. There was no evidence of infection.