Biomedical subjects

Petra Schmalbrock

Publications and source records attributed to Petra Schmalbrock.

At least 19 recordsLinked to original sources

PubMed2007

In-depth study of the electromagnetics of ultrahigh-field MRI.

In this work, numerical and experimental studies of the transverse electromagnetic (TEM) resonator modes at ultrahigh-field (UHF) MRI are performed using an in-house finite difference time domain package at 340 MHz and using an 8 T whole-body MRI system. The simulations utilized anatomically detailed human head mesh and a spherical head-sized phantom, while the experiments included an electromagnetically equivalent (to simulations) phantom and in vivo human head studies. An in-depth look at the homogeneity of the transmit-and-receive fields and local and global polarization of the electromagnetic waves inside the cavity of the head coil, and also the current distribution obtained on the resonator elements, is provided for several coil modes when the coil is empty and loaded. Based on the numerical and experimental results, which are in excellent agreement, an electromagnetic characterization of loading radio-frequency (RF) head coils during a UHF MRI experiment is provided. The possibility of using the aforementioned modes for specific types of imaging application is briefly reviewed.

Electromagnetic Fields↗

PubMed2006

Blipped multi gradient-echo slice excitation profile imaging (bmGESEPI) for fast T2 * measurements with macroscopic B0 inhomogeneity compensation.

With the rapid development of human MRI at field strengths > or = 7 T, knowledge of T(2) (*) relaxation times at such field strengths is needed to optimize acquisition parameters and understand relaxation mechanisms in many applications. However, standard T(2) (*) measurements (e.g., using conventional multiecho gradient-echo (GE) sequences) are affected by macroscopic static magnetic field (B(0)) inhomogeneities, which are particularly severe at high field strength. The multi-GE slice excitation profile imaging (mGESEPI) method was developed for T(2) (*) measurements in the presence of macroscopic B(0) inhomogeneity, but it requires excessive acquisition times at field strengths > or = 7 T. In this paper a more efficient technique, named blipped mGESEPI (bmGESEPI), is proposed. To demonstrate its advantages, T(2) (*) maps were acquired using a conventional multiecho GE method, the mGESEPI method, and the bmGESEPI method in postmortem and in vivo human brains at 8 T.

Algorithms↗

PubMed2006

Growth of benign and malignant schwannoma xenografts in severe combined immunodeficiency mice.

OBJECTIVES: Models for the development of new treatment options in vestibular schwannoma (VS) treatment are lacking. The purpose of this study is to establish a quantifiable human VS xenograft model in mice. STUDY DESIGN AND METHODS: Both rat malignant schwannoma cells (KE-F11 and RT4) and human malignant schwannoma (HMS-97) cells were implanted near the sciatic nerve in the thigh of severe combined immunodeficiency (SCID) mice. Additionally, human benign VS specimens were implanted in another set of SCID mice. Three-dimensional tumor volumes were calculated from magnetic resonance images over the next 6 months. RESULTS: Mice implanted with malignant schwannoma cells developed visible tumors within 2 weeks. Imaging using a 4.7-tesla magnetic resonance imaging and immunohistopathologic examination identified solid tumors in all KE-F11 and HMS-97 xenografts, whereas RT4 xenografts consistently developed cystic schwannomas. VS xenografts demonstrated variability in their growth rates similar to human VS. The majority of VS xenografts did not grow but persisted throughout the study, whereas two of 15 xenografts grew significantly. Histopathologic examination and immunohistochemistry confirmed that VS xenografts retained their original microscopic and immunohistochemical characteristics after prolonged implantation. CONCLUSIONS: This study describes the first animal model for cystic schwannomas. Also, we demonstrate the use of high-field magnetic resonance imaging to quantify VS xenograft growth over time. The VS xenografts represent a model complimentary to Nf2 transgenic and knockout mice for translational VS research.

Animals↗

PubMed2006

An fMRI study of semantic priming: modulation of brain activity by varying semantic distances.

OBJECTIVE: Our purpose was to study the effect of semantic priming at varying semantic distances on brain activation during a lexical decision experiment, using functional magnetic resonance imaging (fMRI). BACKGROUND: Neuroimaging studies have demonstrated decreased brain activation for primed versus unprimed stimuli in language areas due to semantic priming, suggesting facilitated semantic retrieval. However, the effect of varying semantic distances on brain activation has not been studied. Therefore we examined direct and indirect priming effects on cerebral activation to provide information regarding spread of activation in the semantic network. METHODS: Participants were presented with closely, distantly, and unrelated word pairs during fMRI, and asked to make a lexical decision on the second word. RESULTS: Behavioral measurements demonstrated significant priming effects for all semantic distances. Imaging results showed modulation of brain activation due to different semantic relationships in the left inferior frontal gyrus, bilateral middle frontal gyrus and anterior temporal lobe, and consisted of decreased magnitude of activation when primed stimuli were processed compared with unprimed stimuli, with the greatest effect observed for closely related words. CONCLUSIONS: This study demonstrates graduated effects of semantic priming on fMRI in semantic but not attentional brain regions, contributing to explain how semantic knowledge is organized and retrieved. These findings support the network model for organization of the semantic lexicon.

Adult↗

PubMed2006

Clinical magnetic resonance imaging of brain tumors at ultrahigh field: a state-of-the-art review.

With the advancement of the magnetic resonance (MR) technology, the whole-body ultrahigh field MR system operated from 7 to 9.4 T becomes feasible for the routine patient imaging in clinical settings. The associated potentials and challenges from the perspectives of technology, physics, and biology as well as clinical application of the ultrahigh field MR systems are different from those systems operated at 3 T, 1.5 T, or lower field strength. In this article, we will present our initial experiences of brain tumor imaging using the 7 and 8 T whole-body MR systems at the Ohio State University Medical Center and provide a brief overview pertinent to the ultrahigh field clinical MR systems.

Brain Neoplasms↗

PubMed2005

Effects of static and radiofrequency magnetic field inhomogeneity in ultra-high field magnetic resonance imaging.

To characterize the severe static (B(0)) and radiofrequency (B(1)) magnetic field inhomogeneity in ultra-high field (> or =7 T) magnetic resonance imaging, gradient echo (GE) and spin echo (SE) images of in vivo and postmortem human brains were acquired. The B(0) and B(1) inhomogeneity were experimentally mapped and/or numerically simulated, and correlated with the image artifacts. Whereas B(0) inhomogeneity affects predominantly GE images near air/tissue interfaces, B(1) inhomogeneity affects SE images more severely and shows non-intuitive patterns. Mapping of the B(0) and B(1) inhomogeneity is important in characterizing image artifacts. This will help develop better B(0) and B(1) inhomogeneity correction methods.

Adult↗

PubMed2005

High-resolution 8 Tesla imaging of the formalin-fixed normal human hippocampus.

The purpose of this study was to evaluate the capacity of high-resolution magnetic resonance imaging (MRI) to visualize the normal anatomic features of the human hippocampus in vitro, using high field imaging equipment, parameters, and acquisition times appropriate for imaging human subjects in vivo. This research compared high field, high-resolution MRI of formalin-fixed normal human hippocampus specimens to histologic sectioning of the same hippocampus samples. Four specimens were evaluated using an 8 Tesla (T), 80 cm bore whole-body MRI scanner equipped with a 12.7 cm single strut transverse electromagnetic resonator (TEM) coil. Hahn spin echo images were acquired with a repetition time (TR) of 800 msec, echo times (TE) of 20, 50, 90, and 134 msec, and an acquisition time (TA) of 3.25 min. The image quality was superb with demonstration of most of the features of the hippocampus. High field, high-resolution MRI can be used to depict multiple layers of the formalin-fixed human hippocampus in vitro using an 8 T whole-body scanner, a TEM coil, and short acquisition times compatible with human imaging in vivo.

Aged↗

PubMed2005

Electromagnetic perspective on the operation of RF coils at 1.5-11.7 Tesla.

In this work experimental and numerical studies of the MR signal were performed at frequencies ranging from 64 MHz to 485 MHz, utilizing three different MRI coils: a single-strut transverse electromagnetic (TEM)-based coil, a TEM resonator, and a high-pass birdcage coil. The experimental analyses were conducted using 1.5 and 8 Tesla whole-body systems and volume RF head coils. The simulation data were obtained utilizing an in-house-developed finite difference time domain (FDTD) model. Pertinent data from the numerical and experimental setups were compared, and a remarkable agreement between the two methods was found that clearly demonstrates the effectiveness of the FDTD method when it is applied rigorously. The numerical and experimental studies demonstrate the complexity of the electromagnetic (EM) fields and their role in the MR signal. These studies also reveal unique similarities and differences between the transmit and receive field distributions at various field strengths. Finally, for ultra high-field operations, it was demonstrated mathematically, numerically, and experimentally that highly asymmetric inhomogeneous images can be acquired even for linear excitation, symmetrical load geometries, and symmetrical load positioning within the coil.

Electromagnetic Fields↗

PubMed2005

Use of ultra-high-resolution data for temporal bone dissection simulation.

OBJECTIVES: For the past 5 years, our group has been developing a virtual temporal bone dissection environment for training otologic surgeons. Throughout the course of our development, a recurring challenge is the acquisition of high-resolution, multimodal, and multi-scale data sets that are used for the visual as well as haptic (sense of touch) display. This study presents several new techniques in temporal bone imaging and their use as data for surgical simulation. METHODS: At our institution (OSU), we are fortunate to have a high-field (8 Tesla) magnetic resonance imaging (MRI) research magnet that provides an order of magnitude higher resolution compared to clinical 1.5T MRI scanners. Magnetic resonance imaging has traditionally been superb at delineating soft tissue structure, and certainly, the 8T unit does indeed do this at a resolution of 100-200 microm(3). To delineate the bony structure of the mastoid and middle ear, computed tomography (CT) has traditionally been used because of the high signal-to-noise ratio delineating bone signal from air and soft tissue. We have partnered with researchers at other institutions (CCF) to make use of a "microCT" that provides a resolution of 214 x 214 x 390 micrometers of bony structure. RESULTS: This report provides a description of the 2 methodologies and presentation of the striking image data capable of being generated. See images presented. CONCLUSIONS: Using these 2 new and innovative imaging modalities, we provide an order of magnitude greater resolution to the visual and haptic display in our temporal bone dissection simulation environment.

Cadaver↗

PubMed2004

In vivo measurement and imaging of tumor oxygenation using coembedded paramagnetic particulates.

Tumor tissue oxygenation is an important parameter that is positively correlated to the chemo- or radiation treatment outcome of certain tumors. Hence, methods to accurately and noninvasively determine the concentration of oxygen (pO2) in tumors will be valuable. In this study, electron paramagnetic resonance (EPR) spectroscopy, utilizing microcrystalline particulates of lithium phthalocyanine (LiPc), was used to perform repeated measurements of pO2 as a function of tumor growth. We permanently embedded the particulates in the tumor by coimplanting them with RIF-1 tumor cells during inoculation in mice. This procedure enabled repeated measurements of oxygen concentration in the tumor to be obtained for >2 weeks during its growth phase. The particulates were stable and nontoxic to the tumor cells. Both an in vitro clonogenic assay and an in vivo tumor growth rate examination in C3H mice showed no apparent effect on cell proliferation or tumor growth rate. The measurements indicated that the pO2 of the tumor decreased exponentially with tumor growth and reached hypoxic levels ( approximately 4 mmHg) within 4 days after inoculation of the tumor cells. Spatial EPR imaging revealed a nonuniform distribution of the embedded particulates, which were localized mainly in the middle of the tumor volume. Oxygen mapping of the tumor, obtained by spectroscopic EPR imaging, showed significant variation of pO2 within the tumor. In summary, EPR spectroscopy and imaging with an embedded oximetry probe enabled accurate and repeated measurements of pO2 to be obtained in growing tumors under nonperturbing conditions.

Analysis of Variance↗

PubMed2004

Functional magnetic resonance imaging studies of pain: an investigation of signal decay during and across sessions.

BACKGROUND: Several investigations into brain activation caused by pain have suggested that the multiple painful stimulations used in typical block designs may cause attenuation over time of the signal within activated areas. The effect this may have on pain investigations using multiple tasks has not been investigated. The signal decay across a task of four repeating pain stimulations and between two serial pain tasks separated by a 4-min interval was examined to determine whether signal attenuation may significantly confound pain investigations. METHODS: The characteristics of the brain activation of six subjects were determined using whole brain blood oxygenation level-dependent functional magnetic resonance imaging on a 1.5-T scanner. Tasks included both tingling and pain induced by transcutaneous electrical stimulation of the median nerve. The average group maps were analyzed by general linear modeling with corrected cluster P values of less than 0.05. The time courses of individual voxels were further investigated by analysis of variance with P values of less than 0.05. RESULTS: Significant differences between pain and tingling were found in the ipsilateral cerebellum, contralateral thalamus, secondary somatosensory cortex, primary somatosensory cortex, and anterior cingulate cortex. Highly significant signal decay was found to exist across each single pain task, but the signal was found to be restored after a 4-min rest period. CONCLUSIONS: This work shows that serial pain tasks can be used for functional magnetic resonance imaging studies using electrical nerve stimulation as a stimulus, as long as sufficient time is allowed between the two tasks.

Adult↗

PubMed2004

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.

Brain Neoplasms↗

PubMed2003

Experimental treatment of Epstein-Barr virus-associated primary central nervous system lymphoma.

Primary central nervous system lymphoma (PCNSL) that arises in immune-deficient patients is an aggressive B-cell neoplasm that is universally associated with the EBV. Patients with EBV(+) PCNSL face a particularly poor prognosis with median survival times of 2-12 months despite aggressive management with radiation therapy. We have developed a preclinical model of EBV(+) PCNSL to explore strategies that specifically target EBV-infected B lymphoblasts in vivo. Stereotactic implantation of EBV-transformed human lymphoblastoid B-cell lines into the caudate nucleus of the nude rat resulted in lethal CNS tumor burden manifested by the onset of focal neurological symptoms within 21 days. Histological evaluation at autopsy revealed a multifocal, perivascular human EBV(+) lymphoblastic B-cell infiltrate that displayed a latency type III EBV gene expression profile similar to PCNSL that develops in some immune-deficient patients. Radiation (1600 cGy) of lymphoblastoid B-cell lines resulted in up-regulation of the EBV thymidine kinase (EBV-TK) transcript and sensitization of these cells to drug-induced apoptosis using nucleoside analogs. Enhanced expression of EBV-TK mRNA in EBV(+) PCNSL tumors by radiation therapy occurred in a dose-dependent fashion. In vivo trials using the nude rat PCNSL model demonstrated significantly improved mean survival time (MST) with single fraction whole-brain radiotherapy (WBRT) and antiviral therapy consisting of zidovudine (AZT) and ganciclovir (GCV; MST 41.3 +/- 3.3 days; P = 0.05), compared with either antiviral therapy (MST 32.1 +/- 1.1 days) or WBRT alone (MST 22 +/- 0.8 days). We found constitutive and abundant EBV-TK mRNA expression in a stereotactic core biopsy specimen from a solid organ transplant patient with EBV(+) PCNSL. Withdrawal of immunosuppression did not result in disease regression. This patient achieved a complete response after therapy with high-dose AZT and GCV in the absence of WBRT, and remains in remission on oral maintenance AZT/GCV therapy 3 years after diagnosis. These results suggest that antiviral therapies can be effectively explored in vivo using a preclinical animal model of human EBV(+) PCNSL with subsequent translation to patients with EBV(+) PCNSL.

Animals↗

PubMed2003

Enhanced gray and white matter contrast of phase susceptibility-weighted images in ultra-high-field magnetic resonance imaging.

PURPOSE: To evaluate if magnetic susceptibility sensitive phase postprocessed images can be used to enhance the inherent brain/gray white matter contrast in gradient echo (GE) images at 8-Tesla (T) magnetic resonance (MR). MATERIALS AND METHODS: Phase and magnitude images of high-resolution GE MR 8-T images were created. Comparisons were made between the magnitude, the product of the magnitude and phase, and pure phase images. RESULTS: The pure phase images significantly improved the contrast between the gray and white matter structures. In general, the higher the iron content or subvoxel field inhomogeneities, the higher was the contrast, and the greater were the resultant phase shifts. The phase images best demonstrated anatomy that was not apparent on the standard magnitude images. CONCLUSION: Phase imaging can significantly improve the demonstration of the internal anatomical brain structures over standard magnitude GE imaging techniques at high field.

Brain↗

PubMed2003

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.

Aged↗

PubMed2002

Three-dimensional numerical simulations of susceptibility-induced magnetic field inhomogeneities in the human head.

Three-dimensional numerical simulations of the static magnetic field in the human head were carried out to assess the field inhomogeneity due to magnetic susceptibility differences at tissue interfaces. We used a finite difference method and magnetic permeability distributions obtained by segmentation of computed tomography images. Computations were carried out for four models, consisting of the head and the neck; the head, neck, and shoulders; the head, neck, and thorax; and the head tilted backwards, including the neck and the shoulders. Considerable magnetic field inhomogeneities were observed in the inferior frontal lobes and inferior temporal lobes, particularly near the sphenoid sinus and the temporal bones. Air/tissue interfaces at the shoulders were found to induce substantial magnetic field inhomogeneities in the occipital lobes and the cerebellum, whereas air/tissue interfaces in the lungs appeared to have less influence on the magnetic field in the brain. Tilting the head backwards could significantly reduce the field inhomogeneities superior to the planum sphenoidale as well as in the occipital lobes and the cerebellum.

Artifacts↗

PubMed2002

Virtual temporal bone dissection: an interactive surgical simulator.

OBJECTIVE: Our goal was to integrate current and emerging technology in virtual systems to provide a temporal bone dissection simulator that allows the user interactivity and realism similar to the cadaver laboratory. STUDY DESIGN: Iterative design and validation of a virtual environment for simulating temporal bone dissection. SETTING: University otolaryngology training program with interdisciplinary interaction in a high-performance computer facility. RESULTS: The system provides visual, force feedback (haptic), and aural interfaces. Unlike previous "fly through" virtual systems, this environment provides a richer emulation of surgical experience. CONCLUSION: The system provides a high level of functional utility and, through initial evaluations, demonstrates promise in adding to traditional training methods. SIGNIFICANCE: The system provides an environment to learn temporal bone surgery in a way similar to the experience with cadaver material where the subject is able to interact with the data without constraints (nondeterministic). Eventually, it may provide the "front end" to a large repository of various temporal bone pathologies that can be accessed through the Internet.

Algorithms↗

PubMed2002

Functional magnetic resonance imaging as a tool for investigating amblyopia in the human visual cortex: a pilot study.

PURPOSE: This study investigates interocular differences in the level and extent of cortical activation in amblyopic and normal subjects. SUBJECTS AND METHODS: Blood oxygenation level dependent functional magnetic resonance imaging (FMRI) was performed at 1.5 T. A total of 5 subjects with amblyopia and 6 control subjectswere recruited. Visual stimuli included a homogeneous field flickering at 8 Hz and vertical sinusoidal gratings with spatial frequencies of 0.5, 1, and 2 cycles per degree of visual angle (counterphased at 8 Hz). Baseline images were taken in darkness. Stimuli were presented monocularly and binocularly. Data analysis was constrained to the occipital cortex, including striate and extrastriate areas. An absolute percent difference measure defined interocular differences in terms of total area, average level of activation, and a pooled activation parameter. RESULTS: Subjects with amblyopia exhibited a significantly larger (P <.05) interocular activation difference compared to controls. The level of activation driven by monocular stimulation of the amblyopic eye was reduced by 8.25% +/- 1.3% relative to the dominant eye. Controls showed an interocular difference in level of activation of 4.82% +/- 0.74%. The total area of activation driven by the amblyopic eye was reduced by 34.86% +/- 6.25% relative to the dominant eye. Controls showed an interocular difference in total area of activation of 20.80% +/- 3.67%. Subjects with amblyopia also manifested significantly greater (P <.05) cortical area and level of activation differences between the binocular and monocular states; the dominant eye response differed less from the binocular response than did the amblyopic eye response. CONCLUSION: FMRI is sensitive to amblyopia-related deficits in the human occipital cortex and, therefore, has potential as a tool for basic amblyopia research.

Adolescent↗