MR imaging visualization of the cerebral microvasculature: a comparison of live and postmortem studies at 8 T.

SUMMARY: In this study, we compared 8-T gradient-echo MR images of the microvasculature in the live human brain with images of the unembalmed and embalmed postmortem brain. Small vessels were well visualized in the live brain and even better seen in the unembalmed postmortem brain, but they could not be visualized in the embalmed postmortem brain. These findings are important for direct comparisons of the microvasculature on 8-T MR images and on histologic sections.

Susceptibility-based imaging of glioblastoma microvascularity at 8 T: correlation of MR imaging and postmortem pathology.

BACKGROUND AND PURPOSE: Imaging methods are currently being optimized in an attempt to assess and monitor angiogenesis in vivo. The purpose of this investigation was to determine whether areas of apparently increased tumor vascularity, as identified on 8-T gradient-echo (GE) imaging of a known glioblastoma multiforme (GBM), corresponds to foci of increased microvascularity on histopathologic analysis. METHODS: We performed postmortem in situ, high-resolution GE 8-T MR imaging of the brain in a 53-year-old woman with GBM. Ten histopathologic specimens in the region of the tumor bed were studied by using hematoxylin-eosin and reticulin stains. MR and histopathologic results were assessed and compared for microvascular size and density. RESULTS: 8-T GE images showed small, penetrating vessels in the gray matter and white matter. The images, however, were partly inhomogeneous as a result of local magnetic field inhomogeneities adjacent to the skull base and aerated paranasal sinus structures. 8-T MR images demonstrated serpiginous areas of signal intensity loss, which were thought to represent areas of increased microvascularity. Areas of lower microvascularity in the tumor bed corresponded to areas of lower vascularity on histopathologic sections with smaller vessel diameters. There was concurrence between vascular size predicted by histopathologic analysis and 8-T MR imaging in nine of nine biopsy samples. Vascular density agreed in seven of nine biopsy samples. CONCLUSION: Our pilot data suggest that microvascularity in GBM can be identified by use of high-resolution, GE, 8-T MR imaging.

Visualization of microvascularity in glioblastoma multiforme with 8-T high-spatial-resolution MR imaging.

We used 8-T high-spatial-resolution gradient-echo MR imaging to directly visualize microvascularity in pathologically proved glioblastoma multiforme. Images were compared with 1.5-T high-spatial-resolution fast spin-echo T2-weighted images and digital subtraction angiograms. Preliminary data indicate that 8-T high-spatial-resolution MR imaging may enable the identification of areas of abnormal microvascularity in glioblastoma multiforme that are not visible with other routine clinical techniques.

Epoxy-resin injection of the cerebral arterial microvasculature: An evaluation of the limits of spatial resolution in 8 Tesla MRI.

The purpose of this study was to quantify the spatial resolution of microscopic arteries on magnetic resonance images acquired at 8 Tesla (T). Techniques similar to those used for standard MRI of the human brain in vivo at 8 T were utilized to generate high-resolution gradient echo (GE) images of a whole postmortem human brain whose common carotid arterial system had been injected with an epoxy-resin. Single slice images, along with summed images of up to 5 contiguous slices, were then compared to color digital photographs detailing the distribution of the arterial system on the surface of the same injected brain. There was excellent MR visualization of the microscopic cerebral arteries down to a spatial resolution of 200 microm. Through the use of an 8 T whole-body MRI scanner and standard GE imaging sequences, microscopic arterial structures can be clearly resolved down to a dimension of 200 microm.

8.0-Tesla human MR system: temperature changes associated with radiofrequency-induced heating of a head phantom.

PURPOSE: To investigate if the heat induced in biological tissues by typical radio frequency (RF) energy associated with an 8.0-Tesla magnetic resonance (MR) system causes excessive temperature changes. MATERIALS AND METHODS: Fluoroptic thermometry was used to measure temperatures in multiple positions in a head phantom made of ground turkey breast. A series of experiments were conducted with measurements obtained at RF power levels ranging from a specific absorption rate (SAR) of up to 4.0 W/kg for 10 minutes. RESULTS: The highest temperature increases were up to 0.7 degrees C. An inhomogeneous heating pattern was observed. In general, the deep regions within the phantom registered higher temperature increases compared to the peripheral sites. CONCLUSION: The expectation of an inhomogeneous RF distribution in ultra high field systems (> 4 T) was confirmed. At a frequency of 340 MHz and in-tissue RF wave length of about 10 cm, the RF inhomogeneity was measured to create higher temperatures in deeper regions of a human head phantom compared to peripheral tissues. Our results agree with the computational electromagnetic calculations for such frequencies. Importantly, these experiments indicated that there were no regions of heating that exceeded the current FDA guidelines.

Randomized comparison of cognitive function in humans at 0 and 8 Tesla.

PURPOSE: To discover whether there was a measurable alteration in cognitive performance in humans when exposed to a static magnetic field of 8 Tesla (T). MATERIALS AND METHODS: Twenty-five normal human subjects were evaluated at both 0.05 and 8 T in a randomized order. Six standardized neuropsychological tests were administered and auditory reaction times were assessed. The cognitive assessment included measures of learning and retention, verbal fluency (spontaneous word generation), auditory attention, and auditory working memory. Alternate test forms were utilized to reduce practice effects. The sequential order of testing, 0.05 T first vs. 8 T first exposure, was randomized. The data was analyzed using univariate comparisons for correlated means to assess potential differences under the two conditions. RESULTS: There were no clinically significant differences in any of the measures. On a measure of recognition memory the subjects performed significantly better in the 0.05T condition, but the difference was extremely small, not clinically meaningful, and likely due to statistical artifact. CONCLUSION: This study shows that exposure of the brain to high magnetic fields of up to 8 T does not appear to alter human cognitive performance.

Limits of 8-Tesla magnetic resonance imaging spatial resolution of the deoxygenated cerebral microvasculature.

PURPOSE: To quantify the minimum magnetic resonance imaging (MRI) spatial resolution of the visible deoxygenated microscopic vessels of the human brain at 8 T. MATERIALS AND METHODS: This study compared 8-T gradient echo (GE) images of a human cadaver brain having an in-plane resolution of 195 x 195 microm to corresponding digital photographs of 205 cryomicrotome sections of the same cadaver brain, along with summed images of 25 contiguous cryomicrotome sections. One-millimeter-thick GE images of a 1-cm-thick unfixed whole coronal brain section were acquired using techniques similar to those commonly utilized for 8-T human imaging in vivo. RESULTS: There was excellent MR visualization of the deoxygenated microscopic vessels within the brain down to a resolution of approximately 100 microm. CONCLUSION: By taking advantage of magnetic susceptibility-based blood oxygenation level-dependent (BOLD) contrast, deoxygenated microscopic blood vessels smaller than the pixel dimensions used for imaging can be visualized using a whole-body 8-T MRI system.