Comparison of diffusion- and T2-weighted MRI for the early detection of cerebral ischemia and reperfusion in rats

The sensitivity of diffusion-weighted MRI was compared to that of T2-weighted MRI following temporary middle cerebral artery occlusion (MCA-O) for 33 min followed by 4 h of reperfusion in rats. Diffusion-weighted spin-echo images using strong gradients (b value of 1413 s/mm2) demonstrated a significant increase in signal intensity in ischemic regions as early as 14 min after onset of ischemia in comparison to the normal, contralateral hemisphere (p less than 0.05). This hyperintensity returned to baseline levels during reperfusion. T2-weighted images showed no evidence of brain injury during the temporary occlusion. In three rats subjected to permanent MCA-O, diffusion-weighted MRI demonstrated an increased signal intensity on the first image following occlusion and continued to increase during the 4-h observation period. T2-weighted images failed to demonstrate significant injury until approximately 2 h after MCA-O. Signal intensity ratios of ischemic to normal tissues were greater in the diffusion-weighted images than in the T2-weighted MR images at all time points (p less than 0.05). Close anatomical correlation was found between the early and sustained increase in diffusion-weighted MRI signal intensity and localization of infarcts seen on post-mortem histopathology.     

Early detection of regional cerebral ischemia in cats: comparison of diffusion- and T2-weighted MRI and spectroscopy

Diffusion-weighted MR images were compared with T2-weighted MR images and correlated with 1H spin-echo and 31P MR spectroscopy for 6-8 h following a unilateral middle cerebral and bilateral carotid artery occlusion in eight cats. Diffusion-weighted images using strong gradient strengths (b values of 1413 s/mm2) displayed a significant relative hyperintensity in ischemic regions as early as 45 min after onset of ischemia whereas T2-weighted spin-echo images failed to clearly demonstrate brain injury up to 2-3 h postocclusion. Signal intensity ratios (SIR) of ischemic to normal tissues were greater in the diffusion-weighted images at all times than in either TE 80 or TE 160 ms T2-weighted MR images. Diffusion- and T2-weighted SIR did not correlate for the first 1-2 h postocclusion. Good correlation was found between diffusion-weighted SIR and ischemic disturbances of energy metabolism as detected by 31P and 1H MR spectroscopy. Diffusion-weighted hyperintensity in ischemic tissues may be temperature-related, due to rapid accumulation of diffusion-restricted water in the intracellular space (cytotoxic edema) resulting from the breakdown of the transmembrane pump and/or to microscopic brain pulsations.     

Diffusion/perfusion MR imaging of acute cerebral ischemia

In vivo echo-planar MR imaging was used to measure apparent diffusion coefficients (ADC) of cerebral tissues in a comprehensive noninvasive evaluation of early ischemic brain damage induced by occlusion of the middle cerebral artery (MCA) in a cat model of acute regional stroke. Within 10 min after arterial occlusion, ADC was significantly lower in tissues within the vascular territory of the occluded MCA than in normally perfused tissues in the contralateral hemisphere. Sequential echo-planar imaging was then used in conjunction with bolus injections of the magnetic susceptibility contrast agent, dysprosium DTPA-BMA, to characterize the underlying cerebrovascular perfusion deficits. Normally perfused regions of brain were identified by a dose-dependent 35-70% loss of signal intensity within 6-8 s of contrast administration, whereas ischemic regions appeared relatively hyperintense. These data indicate that sequential diffusion/perfusion imaging may be useful in differentiating permanently damaged from reversibly ischemic brain tissue.     

MR imaging of cerebral ischemia: findings in the first 24 hours.

MR changes of cerebral ischemia have been shown to occur as early as 1-2 hr after vessel occlusion in experimental models of stroke. However, the MR findings in the early stages of ischemic stroke in the clinical population have not been well established. We studied 41 lesions in 39 patients in whom MR was performed within the first 24 hr after onset of ischemic symptoms. Twenty-five lesions were studied with gadopentetate dimeglumine. Vascular flow-related abnormalities, including absence of normal flow void and presence of arterial enhancement, were the earliest MR findings, detected within minutes of onset. Morphologic changes (brain swelling) on T1-weighted images without signal changes on T2-weighted images could be detected within the first few hours. Signal changes were not usually found before 8 hr on T2-weighted images or before 16 hr on T1-weighted images. In contrast to the absence of parenchymal enhancement typically found in cortical infarctions in the first 24 hr, a few lesions (including transient occlusions, partial occlusions, and isolated watershed infarctions) exhibited early, exaggerated parenchymal enhancement. We conclude that signal changes may not be reliable in detecting ischemic stroke within the first 8 hr after onset. Vascular abnormalities, when present, are the most reliable and earliest findings. Other MR findings of early ischemic stroke, including morphologic changes and early, exaggerated parenchymal enhancement, may also precede signal changes. Paramagnetic contrast administration often provides valuable information in the detection and evaluation of acute ischemia.     

Relationship between vascular enhancement, cerebral hemodynamics, and MR angiography in cases of acute stroke

BACKGROUND AND PURPOSE: The use of MR angiography and contrast-enhanced T1-weighted MR imaging in cases of acute cerebral ischemia may be helpful in the evaluation of middle cerebral artery (MCA) occlusion and leptomeningeal collaterals, respectively. The aim of our work was to investigate the relationship between MCA occlusion, T1-weighted vascular contrast enhancement, hemodynamic alterations, and tissue damage in cases of acute ischemic stroke. METHODS: We studied the MCA territory in 15 patients with acute ischemic stroke within 8 hr of symptom onset. The first MR imaging study (<8 hr after onset) comprised diffusion-weighted imaging, MR angiography, perfusion-weighted imaging, and contrast-enhanced T1-weighted MR imaging sequences. Follow-up MR imaging, performed 1 week later, consisted of MR angiography and T2-weighted fluid-attenuated inversion recovery MR imaging. RESULTS: Early MR angiography showed MCA stem occlusion in nine of 15 patients. Patients with MCA occlusion had significantly larger areas of abnormality on early diffusion-weighted images, significantly larger areas of altered hemodynamics, larger final lesion volumes, and poorer clinical outcome. Among the nine patients with MCA stem occlusion, vascular enhancement was marked in seven and absent in two who had complete MCA infarcts and poor clinical outcome. Among patients with MCA patency, vascular enhancement was marked in only one, mild in four, and absent in one. Patients with marked vascular enhancement had significantly larger regions of altered hemodynamics and significantly higher asymmetries in both regional cerebral blood volume and mean transit time because of increased values in the affected hemisphere. CONCLUSION: Among patients with stroke with MCA occlusion, marked vascular enhancement and increased blood volume indicate efficient leptomeningeal collaterals and compensatory hemodynamic mechanisms.     

Diffusion-weighted MR imaging of intracerebral masses: comparison with conventional MR imaging and histologic findings

BACKGROUND AND PURPOSE: The purposes of this study were to find the role of diffusion-weighted MR imaging in characterizing intracerebral masses and to find a correlation, if any, between the different parameters of diffusion-weighted imaging and histologic analysis of tumors. The usefulness of diffusion-weighted imaging and apparent diffusion coefficient (ADC) maps in tumor delineation was evaluated. Contrast with white matter and ADC values for tumor components with available histology were also evaluated. METHODS: Twenty patients with clinical and routine MR imaging/CT evidence of intracerebral neoplasm were examined with routine MR imaging and echo-planar diffusion-weighted imaging. The routine MR imaging included at least the axial T2-weighted fast spin-echo and axial T1-weighted spin-echo sequences before and after contrast enhancement. The diffusion-weighted imaging included an echo-planar spin-echo sequence with three b values (0, 300, and 1200 s/mm(2)), sensitizing gradient in the z direction, and calculated ADC maps. The visual comparison of routine MR images with diffusion-weighted images for tumor delineation was performed as was the statistical analysis of quantitative diffusion-weighted imaging parameters with histologic evaluation. RESULTS: For tumors, the diffusion-weighted images and ADC maps of gliomas were less useful than the T2-weighted spin-echo and contrast-enhanced T1-weighted spin-echo images in definition of tumor boundaries. Additionally, in six cases of gliomas, neither T2-weighted spin-echo nor diffusion-weighted images were able to show a boundary between tumor and edema, which was present on contrast-enhanced T1-weighted and/or perfusion echo-planar images. The ADC values of solid gliomas, metastases, and meningioma were in the same range. In two cases of lymphomas, there was a good contrast with white matter, with strongly reduced ADC values. For infection, the highest contrast on diffusion-weighted images and lowest ADC values were observed in association with inflammatory granuloma and abscess. CONCLUSION: Contrary to the findings of previous studies, we found no clear advantage of diffusion-weighted echo-planar imaging in the evaluation of tumor extension. The contrast between gliomas, metastases, meningioma, and white matter was generally lower on diffusion-weighted images and ADC maps compared with conventional MR imaging. Unlike gliomas, the two cases of lymphomas showed hyperintense signal on diffusion-weighted images whereas the case of cerebral abscess showed the highest contrast on diffusion-weighted images with very low ADC values. Further study is required to find out whether this may be useful in the differentiation of gliomas and metastasis from lymphoma and abscess.     

MR imaging of acute experimental ischemia in cats

The evolution of acute cerebral ischemia was documented by magnetic resonance (MR) imaging in 13 mongrel cats with occlusion of the middle cerebral artery through a transorbital approach. The animals were imaged under anesthesia at intervals from 30 min to 10 days after production of the lesion. An MR imager operating at 0.35 T was used with multislice, multi-spin-echo technique (TR = 500-2000 msec; TE = 28, 56 msec). The animals were sacrificed after imaging for pathologic correlation. Infarcts beyond 4 hr of age were visualized in all subjects. The earliest infarct was seen at 30 min (two cats) as an area of high signal intensity on T2-weighted images. In three other cats, however, 3-hr-old infarcts were not detectable. In one animal, a hemorrhage within a 1-week-old area of infarction was not characterized by MR imaging but was identified on CT scanning. The mass effect of the infarction appeared greatest at 2-4 days after infarction. The basal ganglia showed ischemic effects to best advantage. MR imaging offers previously unavailable sensitivity for the early noninvasive detection of cerebral ischemia in vivo.     

Demarcation of myocardial ischemia: magnetic susceptibility effect of contrast medium in MR imaging

Dysprosium diethylenetriamine-pentaacetic acid-bis (methylamide) (DTPA-BMA), a new nonionic contrast medium for magnetic resonance (MR) imaging, produces signal loss on T2-weighted images because of induced magnetic field gradients. The potential of this agent to delineate myocardial ischemia was investigated in 10 rats with acute (30 minutes) occlusion of the left coronary artery. T2-weighted MR images were acquired before and for 1 hour after intravenous administration of 1 mmol/kg of Dy-DTPA-BMA. Before administration of the contrast medium, signal intensity (SI) in the ischemic region was significantly greater than that in the normal myocardium; however, the borders of the ischemic region were not consistently distinct. The contrast medium caused marked decrease in SI of normal myocardium (18% +/- 3% of the control value), only slight decrease in the jeopardized region (76% +/- 6% of the control value), and no discernible effects on heart rate or blood pressure. Substantial contrast between normal and ischemic myocardium persisted for 1 hour. Moderate signal loss was observed in skeletal muscle. Dy-DTPA-BMA has the potential to demarcate the myocardial area in jeopardy as a region of high signal intensity because it erases signal preferentially in the normal myocardium.     

Early MR detection of experimentally induced cerebral ischemia using magnetic susceptibility contrast agents: comparison between gadopentetate dimeglumine and iron oxide particles.

PURPOSETo evaluate early patterns of MR changes in a rat model of cerebral ischemia using the first pass of two magnetic susceptibility contrast agents.METHODSOne hours after endovascular middle cerebral artery occlusion, all animals were examined in an experimental MR unit. After bolus application of gadopentetate dimeglumine and, 10 minutes later, of iron oxide particles, the MR changes of the first pass of these contrast agents were followed using a T2*-weighted fast low-angle shot sequence. Time-density curves of both contrast agents were analyzed and compared.RESULTSAfter bolus injection of either (paramagnetic) gadopentetate dimeglumine or superparamagnetic particles, nonischemic brain parenchyma decreased markedly in signal, whereas the ischemic brain area remained relatively hyperintense (and thus became clearly delineated). Only after application of gadopentetate dimeglumine did a mild reduction in signal occur in the ischemic hemisphere, although the main artery was occluded. An explantation for this phenomenon might be residual capillary perfusion (plasma flow), which is detectable only when the smaller (paramagnetic) contrast molecules are being used.CONCLUSIONSCerebral perfusion deficits can be detected 1 hour after vascular occlusion with T2*-weighted fast low-angle shot sequences and bolus injection of paramagnetic or superparamagnetic MR contrast agents. Gadopentetate dimeglumine may be used as a marker of microcirculatory plasma flow.     

Astrocytic swelling in the ipsilateral substantia nigra after occlusion of the middle cerebral artery in rats

BACKGROUND AND PURPOSE: Focal cerebral ischemia results in neuronal changes in remote areas that have fiber connections with the ischemic area. We reported previously that a high-signal-intensity lesion was observed in the substantia nigra after striatal infarction on T2-weighted MR images in both clinical and experimental cases. However, the origin of these changes in signal intensity remains unclear. The aim of this study was to investigate the nigral changes by examining the correlation between the apparent diffusion coefficient (ADC) and the tissue structure. METHODS: Sprague-Dawley rats were subjected to middle cerebral artery occlusion. Four days after the occlusion, when T2-weighted images revealed the presence of an area of high signal intensity in the ipsilateral substantia nigra, diffusion-weighted imaging was performed using a 4.7-T superconductive MR unit, and the ADCs were calculated and imaged. Histopathologic examination by both light and electron microscopy was performed on day 4 after surgery. RESULTS: Diffusion-weighted images showed an area of high signal intensity in the ipsilateral substantia nigra, and the ADC map revealed uniform reduction of the ADC in this area. Swelling of astrocytic end-feet was observed, especially in the pars reticulata. CONCLUSION: These findings suggest that MR changes in the ipsilateral substantia nigra after striatal injury consist mainly of swelling in the astrocytic end-feet.     

Diffusion MR Imaging of Postoperative Bilateral Acute Ischemic Optic Neuropathy

A 57-year-old woman experienced bilateral acute ischemic optic neuropathy after spine surgery. Routine MR imaging sequence, T2-weighted image, showed subtle high signal intensity on bilateral optic nerves. A contrast-enhanced T1 weighted image showed enhancement along the bilateral optic nerve sheath. Moreover, diffusion-weighted image (DWI) and an apparent diffusion coefficient map showed markedly restricted diffusion on bilateral optic nerves. Although MR findings of T2-weighted and contrast enhanced T1-weighted images may be nonspecific, the DWI finding of cytotoxic edema of bilateral optic nerves will be helpful for the diagnosis of acute ischemic optic neuropathy after spine surgery.     

Fluid-attenuated inversion recovery intraarterial signal: an early sign of hyperacute cerebral ischemia.

BACKGROUND AND PURPOSE: Early detection of arterial occlusion and perfusion abnormality is necessary for effective therapy of hyperacute cerebral ischemia. We attempted to assess the utility of the fast fluid-attenuated inversion recovery (fast-FLAIR) sequence in detecting occluded arteries as high signal (referred to as intraarterial signal) and to establish the role of fast-FLAIR in detecting ischemic penumbra of hyperacute stroke within 24 hours after ictus. METHODS: We studied 60 patients with hyperacute cerebral ischemia caused by occlusion of intracranial major arteries. We compared intraarterial signal on FLAIR images with time of flight (TOF) on MR angiograms, flow voids on T2-weighted images, hyperintense lesions on diffusion-weighted images, and results of follow-up CT or MR scans. RESULTS: In 58 (96.7%) patients, FLAIR detected intraarterial signals as early as 35 minutes after stroke onset. In 48 (80.0%) patients, intraarterial signal on FLAIR images coincided with lack of TOF on MR angiograms. In 41 (74.5%) of 55 patients, the intraarterial signals of fast T2-weighted imaging depicted occlusion better than did deficient flow void on T2-weighted images. In 25 (41.7%) of 60 patients, the area of intraarterial signal distribution was larger than the hyperintense lesion measured on diffusion-weighted images. Areas of final infarction had sizes between those of intraarterial signal distribution on FLAIR images and lesions measured on diffusion-weighted images. In 35 (87.5%) of 40 patients, areas of intraarterial signal distribution were equal to regions of abnormal perfusion. CONCLUSION: Intraarterial signal on FLAIR images is an early sign of occlusion of major arteries. FLAIR combined with diffusion-weighted imaging can be helpful to predict an area at risk for infarction (ischemic penumbra). FLAIR plays an important role for determining whether a patient should undergo perfusion study.     

MR imaging in acute stroke: diffusion-weighted and perfusion imaging parameters for predicting infarct size

PURPOSE: To investigate the predictive value of the ischemic lesion size, as depicted in the acute stroke phase on diffusion-weighted magnetic resonance (MR) images and time-to-peak (TTP) maps of tissue perfusion imaging, for infarct size, as derived from T2-weighted imaging in the postacute phase. MATERIALS AND METHODS: Fifty patients who underwent diffusion-weighted and perfusion imaging within 1-24 hours after stroke onset and a follow-up T2-weighted investigation after about 8 days were included. Lesion volumes were evaluated by using a semiautomatic thresholding technique. Volumetric results of acute diffusion-weighted and perfusion imaging were analyzed in comparison with follow-up T2-weighted images and in terms of the time difference between symptom onset and initial MR imaging. RESULTS: At diffusion-weighted imaging, the acute lesion defined by a signal intensity increase of more than 20%, compared with the contralateral side, showed the best correlation with the infarct size after 1 week. At perfusion imaging, the best predictor relative to the contralateral side was a delay of more than 6 seconds on TTP maps. Temporal analysis of volumetric results, which depended on the time difference between symptom onset and examination, revealed two patient subgroups. CONCLUSION: Diffusion-weighted imaging helped to predict the size of the lesion on T2-weighted images obtained after about 8 days in patients with a symptom onset of more than 4 hours (r = 0.96), while in patients with a symptom onset of less than 4 hours, perfusion imaging provided important additional information about brain tissue with impaired perfusion.     

MR detection of secondary changes remote from ischemia: preliminary observations after occlusion of the middle cerebral artery in rats.

PURPOSETo determine whether secondary MR changes occur in the thalamus or the substantia nigra after middle cerebral artery (MCA) occlusion in rats.METHODSSprague-Dawley rats were subjected to MCA occlusion. At varying intervals, proton density-, T1-, and T2-weighted images were obtained with a 4.7-T superconductive MR unit.RESULTST2-weighted images revealed an area of high signal intensity in the ipsilateral substantia nigra 4 days after occlusion. A lesion of low signal intensity appeared in the ipsilateral thalamus 7 days after surgery on proton density- and/or T2-weighted images.CONCLUSIONMR showed secondary changes in the thalamus and the substantia nigra after MCA occlusion in rats. MR imaging should provide more information on the neuropathology of the delayed neuronal degeneration after cerebral ischemia.     

Hyperacute ischemic stroke: middle cerebral artery susceptibility sign at echo-planar gradient-echo MR imaging

PURPOSE: To evaluate the accuracy of echo-planar T2*-weighted magnetic resonance (MR) sequences in detection of acute middle cerebral artery (MCA) or internal carotid artery (ICA) thrombotic occlusion. MATERIALS AND METHODS: Forty-two consecutive patients with stroke involving the MCA territory underwent MR imaging within 6 hours after clinical onset. MR examination included echo-planar T2*-weighted, diffusion-weighted (DW), and perfusion-weighted (PW) imaging and MR angiography. Presence or absence of the susceptibility sign on echo-planar T2*-weighted images, which is indicative of acute thrombotic occlusion involving MCA or ICA, was assessed in consensus by two observers blinded to clinical information and other MR imaging data. Differences in lesion volume on DW and PW images between patients with and those without the susceptibility sign were evaluated with the Mann-Whitney test. P <.05 was considered to indicate a significant difference. RESULTS: Thirty patients (71%) had a positive susceptibility sign that correlated with MCA or ICA occlusion at MR angiography in all cases (sensitivity, 83%; specificity, 100%). Mean lesion volume on PW images was higher in patients with a positive susceptibility sign (P =.01), but no significant differences were found in mean lesion volume on DW images. Cases in which the susceptibility sign was identified proximal to MCA divisional bifurcation (27 patients) showed a mean perfusion deficit of 83.9% of the total MCA territory (range, 50%-100%). CONCLUSION: Presence of the susceptibility sign proximal to MCA bifurcation provides fast and accurate detection of acute proximal MCA or ICA thrombotic occlusion.     

Rapid MR imaging of a vascular challenge to focal ischemia in cat brain

Deoxygenated blood was effectively used as a magnetic resonance (MR) susceptibility contrast agent to distinguish perfused and nonperfused (ischemic) regions in a focal ischemia model in cat brain at 2 T. Modulation of cerebral blood oxygenation levels in response to apnea was followed in real time with T2*-weighted (gradient-recalled) echo-planar MR imaging. Signal loss in the T2*-weighted images occurred only in perfused tissues as blood became globally deoxygenated. These data complemented information from diffusion-weighted and contrast agent bolus–-tracking images. In addition, observation of the signal recovery behavior on reventilation in both normal and ischemic brain offered potentially useful information about the state of the cerebral autoregulatory mechanism.     

CT and MR imaging findings in cerebral toxocaral disease

We report serial MR findings in a 54-year-old woman with eosinophilic meningoencephalitis due to Toxocara canis infection, a parasitic disease contracted through exposure with soil contaminated by the eggs of the roundworm. MR imaging revealed several enhancing subcortical and white matter lesions in both lobes. Antihelminthic chemotherapy yielded marked improvement of the neurologic deficits and cerebral lesions. The specific MR findings-low signal intensity on T1-weighted images, high signal intensity on T2-weighted images, and contrast enhancement-and the clinical and epidemiologic features of CNS involvement are herein reviewed.     

MR imaging of spontaneous spinal cord infarction

Magnetic resonance imaging was utilized to evaluate three women, one of whom was pregnant, with a clinical diagnosis of anterior spinal artery syndrome. In each case, development of spinal cord infarctions was spontaneous. Sagittal and axial MR images of the spine were obtained prior to and following administration of Gd-diethylenetriamine pentaacetic acid. Magnetic resonance showed no cord enlargement or signal abnormality on T1-weighted images prior to contrast medium administration. On T2-weighted images there was abnormal increase in signal intensity in the cord at levels corresponding to the neurologic deficits in all three patients. Variable contrast enhancement was demonstrated in these areas. Magnetic resonance imaging was positive, whereas CT-myelography, carried out in the two nonpregnant women, failed to demonstrate any abnormalities.     

Initial ischemic event: perfusion-weighted MR imaging and apparent diffusion coefficient for stroke evolution

PURPOSE: To prospectively determine if the degree of acute perfusion or diffusion abnormalities measured prior to treatment onset help predict the evolution of brain infarction on magnetic resonance (MR) images. MATERIALS AND METHODS: Local ethics committee approval and informed consent were obtained. On parametric maps obtained in 64 patients (mean age, 64 years +/- 13 [standard deviation]; 37 men and 27 women) with acute middle cerebral artery infarction, lesion volumetry was performed to determine time to peak, mean transit time, cerebral blood volume, and apparent diffusion coefficient obtained within 3 hours of symptom onset. The infarct lesions were assessed on T2-weighted MR images obtained at follow-up on day 8. Cerebrovascular changes were determined on MR angiograms. Inferential and correlation statistics were used. RESULTS: A perfusion delay of more than 6 seconds relative to the nonaffected hemisphere on time-to-peak maps helped to predict the lesion volume on T2-weighted images (r = 0.686, P < .001). In contrast, neither the volume nor the degree of the diffusion abnormality helped to predict the infarct volume (r < 0.46). This was because in one subgroup of patients there was an increase and in one subgroup there was a decrease in infarct volume on the T2-weighted images (P < .001). There was a greater prevalence (P < .02) of cerebral artery abnormalities in the patients with larger infarcts. Clinically, the neurologic impairment was more severe (P < .01) and the mean arterial pressure higher (P < .04) in these patients. CONCLUSION: The results suggest that in acute stroke the severity of the initial ischemic event as determined on time-to-peak maps indicates hemodynamic compromise in addition to internal carotid artery or middle cerebral artery occlusion, because of abnormalities in other cerebral arteries.     

Ultrafast magnetic resonance imaging: diffusion and perfusion

Echo-planar magnetic resonance imaging (MRI) can be used to measure apparent diffusion coefficients noninvasively in vivo, with scan times of 150 milliseconds or less, and to assess early ischemic effects in the feline experimental model, which has an occluded middle cerebral artery (MCA). The apparent diffusion coefficient in ischemic regions, which are identified later from vital staining, is significantly decreased from normal values within 1 hour after the MCA becomes occluded. A series of 10 echo-planar images that are progressively diffusion-weighted can be collected in 1 minute (effective TR of 6 seconds). Semilogarithmic plots of image intensity versus gradient strength factors (b values) were linear. Collecting sequential gradient echo-planar images during the passage of a bolus of contrast medium is also useful when assessing perfusion or vascular integrity before and during ischemic episodes. After intravenous injection of dysprosium-diethylene triamine penta-acetic acid-bis(methylamide), typical signal losses of 40% to 80% were observed and were dose-dependent. Areas of possible ischemia identified from diffusion-weighted images did not lose signal intensity with the use of contrast medium and were seen as regions of relative hyperintensity, clearly discernible from normally perfused tissues.