Acute Infarction
Early (within the first 6 hours after stroke) CT signs of brain ischemia are subtle and difficult to detect . On conventional MR images, early (within the first 6 hours after stroke) morphologic signs (produced by tissue swelling) are detected in 50% of acute infarctions; however, signal abnormalities are not detected . With diffusion-weighted imaging of acute infarction (within the first 6 hours after stroke), 94% sensitivity and 100% specificity have been reported .
What's Going On?The interruption of cerebral blood flow results in rapid (within minutes) breakdown of energy metabolism and ion exchange pumps. This leads to a massive shift of water from the extracellular into the intracellular compartment (cytotoxic edema) and produces a typical high-intensity area on diffusion-weighted images.
Note! Diffusion-weighted imaging is the best technique for confirming a diagnosis of stroke in time to treat it with appropriate therapy. The value of diffusion-weighted and perfusion MR imaging in patient treatment decisions has not yet been clearly defined.
Early (within the first 6 hours after stroke) CT signs of brain ischemia are subtle and difficult to detect . On conventional MR images, early (within the first 6 hours after stroke) morphologic signs (produced by tissue swelling) are detected in 50% of acute infarctions; however, signal abnormalities are not detected . With diffusion-weighted imaging of acute infarction (within the first 6 hours after stroke), 94% sensitivity and 100% specificity have been reported .
What's Going On?The interruption of cerebral blood flow results in rapid (within minutes) breakdown of energy metabolism and ion exchange pumps. This leads to a massive shift of water from the extracellular into the intracellular compartment (cytotoxic edema) and produces a typical high-intensity area on diffusion-weighted images.
Note! Diffusion-weighted imaging is the best technique for confirming a diagnosis of stroke in time to treat it with appropriate therapy. The value of diffusion-weighted and perfusion MR imaging in patient treatment decisions has not yet been clearly defined.
Old Infarction
Five to 10 days after acute infarction, the reduced values (by an average of about 50%) of the apparent diffusion coeficient (ADC) will increase above normal values. On diffusion-weighted images, this is characterized by a "flip-flop" from hyperintense lesion to iso/hypointense lesion. On ADC maps, the lesion becomes iso/hyperintense (and therefore more conspicuous). This behavior is useful in the following situations:
In patients with multiple ischemic lesions, T2-weighted imaging is frequently unable to help differentiate between acute and old lesions
A new extension of a previous ischemic lesion is frequently difficult to identify on T2-weighted images
Five to 10 days after acute infarction, the reduced values (by an average of about 50%) of the apparent diffusion coeficient (ADC) will increase above normal values. On diffusion-weighted images, this is characterized by a "flip-flop" from hyperintense lesion to iso/hypointense lesion. On ADC maps, the lesion becomes iso/hyperintense (and therefore more conspicuous). This behavior is useful in the following situations:
In patients with multiple ischemic lesions, T2-weighted imaging is frequently unable to help differentiate between acute and old lesions
A new extension of a previous ischemic lesion is frequently difficult to identify on T2-weighted images
Tumors
Diffusion-weighted imaging has been used occasionally in the evaluation of intraaxial tumors. Brunberg et al (1) reported that apparent diffusion coefficient (ADC) and index of diffusion anisotropy determinations allow distinction between normal white matter areas and necrosis or cyst formation, edema, and solid enhancing tumor. Tien et al (2) found that areas of enhancing tumor were markedly hyperintense on diffusion-weighted images and that it was possible to distinguish areas of predominantly nonenhancing tumor from areas of predominantly peritumoral edema when the abnormality was located in the white matter aligned in the direction of the diffusion-weighted gradient.In our experience (3), diffusion-weighted imaging has little (if any) value in the differentiation between tumor and edema and, most important, in the detection of tumor invasion of white matter and edema when the differentiation cannot be done on conventional MR images. The behavior of inflammatory granulomas and abscesses on diffusion-weighted images may be used in differential diagnosis with necrotic tumors.In a case of multiple mucormucosis granulomas in a patient with acquired immunodeficiency syndrome, the lesions were hyperintense on diffusion-weighted images, with ADC values in the range of 0.39. In a case of streptococcus abscess, the hyperintensity on diffusion-weighted images was even higher, with a very low ADC value of 0.29 (see next topic). This behavior allowed differential diagnosis with necrotic glioblastoma, which was suggested by clinical and conventional MR examinations.
Diffusion-weighted imaging has been used occasionally in the evaluation of intraaxial tumors. Brunberg et al (1) reported that apparent diffusion coefficient (ADC) and index of diffusion anisotropy determinations allow distinction between normal white matter areas and necrosis or cyst formation, edema, and solid enhancing tumor. Tien et al (2) found that areas of enhancing tumor were markedly hyperintense on diffusion-weighted images and that it was possible to distinguish areas of predominantly nonenhancing tumor from areas of predominantly peritumoral edema when the abnormality was located in the white matter aligned in the direction of the diffusion-weighted gradient.In our experience (3), diffusion-weighted imaging has little (if any) value in the differentiation between tumor and edema and, most important, in the detection of tumor invasion of white matter and edema when the differentiation cannot be done on conventional MR images. The behavior of inflammatory granulomas and abscesses on diffusion-weighted images may be used in differential diagnosis with necrotic tumors.In a case of multiple mucormucosis granulomas in a patient with acquired immunodeficiency syndrome, the lesions were hyperintense on diffusion-weighted images, with ADC values in the range of 0.39. In a case of streptococcus abscess, the hyperintensity on diffusion-weighted images was even higher, with a very low ADC value of 0.29 (see next topic). This behavior allowed differential diagnosis with necrotic glioblastoma, which was suggested by clinical and conventional MR examinations.
Inflammation
The differential diagnosis between intracerebral necrotic tumors and cerebral abscesses is frequently impossible on conventional MR images. In our experience, the abscesses are hyperintense on diffusion-weighted images (1) with a very low ADC value of 0.3-0.4. Therefore, the diffusion-weighted imaging and ADC findings may be useful in the differential diagnosis of "ring-enhancing" cerebral masses. The presence of a central area of hyperintensity on diffusion-weighted images and very low ADC values strongly suggest the presence of pus and abscess. These characteristics are also present in acute infarction. However, ring enhancement in acute ischemic stroke is unusual, and ADC values are higher after 8 hours. The ring-enhancing mass with central hypointensity on diffusion-weighted images and increased ADC values suggest cerebral glioma or metastasis (2). We may only hypothesize about the low ADC values of pus (related to fibrinogen accumulation?), but this reduced diffusion may explain the reduced activity of antibiotics in the presence of suppuration.
The differential diagnosis between intracerebral necrotic tumors and cerebral abscesses is frequently impossible on conventional MR images. In our experience, the abscesses are hyperintense on diffusion-weighted images (1) with a very low ADC value of 0.3-0.4. Therefore, the diffusion-weighted imaging and ADC findings may be useful in the differential diagnosis of "ring-enhancing" cerebral masses. The presence of a central area of hyperintensity on diffusion-weighted images and very low ADC values strongly suggest the presence of pus and abscess. These characteristics are also present in acute infarction. However, ring enhancement in acute ischemic stroke is unusual, and ADC values are higher after 8 hours. The ring-enhancing mass with central hypointensity on diffusion-weighted images and increased ADC values suggest cerebral glioma or metastasis (2). We may only hypothesize about the low ADC values of pus (related to fibrinogen accumulation?), but this reduced diffusion may explain the reduced activity of antibiotics in the presence of suppuration.
Hemorrhage
Diffusion-weighted and T2-weighted echo-planar images are also exquisitely sensitive to hemorrhage and may be used to detect and distinguish between acute hemorrhagic and nonhemorrhagic stroke. Decreased ADC was demonstrated in both acute hemorrhagic and in nonhemorrhagic stroke. However, in contrast to patients with nonhemorrhagic stroke, whose lesions were iso- or hyperintense on T2-weighted echo-planar images, lesions in patients with hemorrhagic stroke were of mixed hypo- and hyperintensity on T2-weighted echo-planar images (1).
Diffusion-weighted and T2-weighted echo-planar images are also exquisitely sensitive to hemorrhage and may be used to detect and distinguish between acute hemorrhagic and nonhemorrhagic stroke. Decreased ADC was demonstrated in both acute hemorrhagic and in nonhemorrhagic stroke. However, in contrast to patients with nonhemorrhagic stroke, whose lesions were iso- or hyperintense on T2-weighted echo-planar images, lesions in patients with hemorrhagic stroke were of mixed hypo- and hyperintensity on T2-weighted echo-planar images (1).
Myelination
Diffusion-weighted imaging is exquisitely sensitive to the direction of the axons in white matter. The measured diffusion coefficients appear to be larger when measured along the fibers (in the range of 1.0 × 10-3 mm2/sec) than across the fibers (in the range of 0.6 × 10-3 mm2/sec). Diffusion-weighted imaging will probably be a valuable technique in identification of myelination disorders (1), although the exact value of this technique has not yet been established (2-4).
References
Diffusion-weighted imaging is exquisitely sensitive to the direction of the axons in white matter. The measured diffusion coefficients appear to be larger when measured along the fibers (in the range of 1.0 × 10-3 mm2/sec) than across the fibers (in the range of 0.6 × 10-3 mm2/sec). Diffusion-weighted imaging will probably be a valuable technique in identification of myelination disorders (1), although the exact value of this technique has not yet been established (2-4).
References
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