Biomedical subjects

P M Robitaille

Publications and source records attributed to P M Robitaille.

64 records · Page 4Linked to original sources

PubMedDate not supplied

Macroscopic susceptibility in ultra high field MRI. II: acquisition of spin echo images from the human head.

The presence of magnetic susceptibility can lead to substantial geometric distortions when imaging the human head at 8 T. These are particularly significant in gradient echo images where susceptibility often results in a noticeable loss of MR signal in the temporal lobe, the frontal lobe, and the paranasal sinus regions. In this work, anterior coronal gradient and spin echo images were acquired from the frontal lobe and sinus regions. The spin echo was shown to significantly overcome the loss of signal observed in the corresponding gradient echo images, resulting in data of greatly increased quality. In conclusion, whereas susceptibility artifacts are significant in ultra high field MRI, they can be largely surmounted by using spin echo techniques, as had been previously demonstrated in studies at lower field strength.

Adipose Tissue↗

PubMedDate not supplied

RF penetration in ultra high field MRI: challenges in visualizing details within the center of the human brain.

PURPOSE: The purpose of this work is to discuss radio frequency (RF) penetration and its relevance to imaging the human head and to acquire images containing intricate structures located at the center of the brain with ultra high field MRI (UHFMRI). METHOD: A simple plane wave analysis of RF penetration was performed based on Maxwell equations as a function of frequency up to 900 MHz. Gradient-recalled images were acquired at 8 T (340 MHz) using an RF resonator operating in quadrature. Typical acquisition parameters were as follows: TR = 750 ms, TE = 17 ms, slice thickness = 2 mm, FOV = 20 x 20 cm, matrix = 1,024 x 1,024. The specific absorption rate was well below 1 W/kg. RESULTS: A simple analytical treatment, for a plane wave up to 900 MHz, reveals a lack of decreasing penetration depth with frequency beyond 200 MHz. Gradient-recalled echo images acquired from the human head displayed good contrast, homogeneity, and resolution. Importantly, excellent structural detail was observed on the resulting MR images, demonstrating that RF penetration is not a problem at 8 T. Images reveal excellent detail including the red nucleus, anterior commissure, fornix, mamillary body, pineal gland, and ependymal lining of the fourth ventricle. CONCLUSION: Structures located at the center of the human brain can be clearly visualized at 8 T with no detectable loss in signal intensity arising from RF penetration. The ability to examine these structures with UHFMRI will provide a powerful new modality for diagnostic radiology.

Absorption↗

PubMedDate not supplied

Human leptomeningeal and cortical vascular anatomy of the cerebral cortex at 8 Tesla.

PURPOSE: The purpose of this work was to describe the human leptomeningeal and cortical vascular anatomy as seen at high resolution on an 8 T UHFMRI system. METHOD: With a 1024 x 1024 matrix, axial gradient echo images of the cerebral cortex were acquired on a human volunteer at 8 T with TR 500 ms, TE 16 ms, flip angle 22.5 degrees, bandwidth 53 kHz, and slice thickness 2.84 mm. The same subject was evaluated at 1.5 T using similar parameters. The images were then reviewed in detail and compared with known cortical and leptomeningeal vascular anatomy. RESULTS: Two hundred forty micron in-plane resolution images of the human brain were acquired at 8 T without evident artifact from susceptibility distortions, RF penetration, or dielectric resonances. The CSF, gray matter, and white matter structures were well discerned. The microscopic leptomeningeal vascular anatomy was well visualized, and the course of small perforating cortical vessels could be followed from the cortical surface to the white matter junction. CONCLUSION: Initial 8 T images of the brain demonstrate detailed leptomeningeal and cortical vascular anatomy.

Arachnoid↗

PubMedDate not supplied

High resolution MRI of the deep brain vascular anatomy at 8 Tesla: susceptibility-based enhancement of the venous structures.

PURPOSE: The purpose of this work was to describe the deep vascular anatomy of the human brain using high resolution MR gradient echo imaging at 8 T. METHOD: Gradient echo images were acquired from the human head using a transverse electromagnetic resonator operating in quadrature and tuned to 340 MHz. Typical acquisition parameters were as follows: matrix = 1,024 x 1,024, flip angle = 45 degrees, TR = 750 ms, TE = 17 ms, FOV = 20 cm, slice thickness = 2 mm. This resulted in an in-plane resolution of approximately 200 microm. Images were analyzed, and vascular structures were identified on the basis of location and course. RESULTS: High resolution ultra high field magnetic resonance imaging (UHFMRI) enabled the visualization of many small vessels deep within the brain. These vessels were typically detected as signal voids, and the majority represented veins. The prevalence of the venous vasculature was attributed largely to the magnetic susceptibility of deoxyhemoglobin. It was possible to identify venous structures expected to measure below 100 microm in size. Perforating venous drainage within the deep gray structures was identified along with their parent vessels. The course of arterial perforators was more difficult to follow and not as readily identified as their venous counterparts. CONCLUSION: The application of high resolution gradient echo methods in UHFMRI provides a unique detailed view of particularly the deep venous vasculature of the human brain.

Arteries↗

PubMedDate not supplied

High resolution MRI of the deep gray nuclei at 8 Tesla.

PURPOSE: High resolution MR images obtained from a normal human volunteer at 8 T are utilized to describe the appearance of iron-containing deep gray nuclei at this field strength. METHOD: High resolution (1,024 x 1,024 matrix) near-axial gradient echo images of the deep gray nuclei were acquired on a human volunteer by using an 8 T scanner. The images were acquired using a transverse electromagnetic resonator operating in quadrature. The following parameters were utilized: TR = 750 ms, TE = 17 ms, flip angle = 45 degrees, receiver bandwidth = 50 kHz, slice thickness = 2 mm, FOV = 20 cm. The 8 T images were reviewed and correlated to the known anatomy of the deep nuclei by comparing them with images observed at lower field strength, published diagrams, and histologic sections. In addition, the appearance of the nuclei was related to the known imaging characteristics of brain iron at lower fields. RESULTS: The caudate, globus pallidus, putamen, thalami, substantia nigra, and red nuclei were clearly identified. The structures with the highest levels of iron, the globus pallidus, substantia nigra, and red nuclei, demonstrated significantly decreased signal, providing a map of iron distribution in the human brain. CONCLUSION: Preliminary imaging at 8 T demonstrates the ability to acquire ultra high resolution images of the deep nuclei, with signal characteristics believed to represent the distribution of brain iron. This may prove to be important in the early diagnosis of several neurodegenerative disorders.

Adult↗

PubMedDate not supplied

T1- and T2-weighted imaging at 8 Tesla.

In this work, both T1- and T2-weighted fast imaging methods at 8 T are presented. These include the modified driven equilibrium Fourier transform (MDEFT) and rapid acquisition with relaxation enhancement (RARE) methods, respectively. Axial MDEFT images were acquired with large nutation angles, both partially suppressing gray and white matter and permitting the visualization of vascular structures rich in unsaturated spins. Sagittal RARE images, acquired from the same volunteer, were highly T2-weighted, thus highlighting the CSF. At the same time, they provided good visualization of the corpus callosum, cerebellum, and gray and white matter structures. Importantly, both MDEFT and RARE images could be acquired without violating specific absorption rate guidelines.

Absorption↗

PubMedDate not supplied

Black body and transverse electromagnetic resonators operating at 340 MHz: volume RF coils for ultra high field MRI.

PURPOSE: The purpose of this work was to describe the newly formulated black body (BB) resonator with historical perspective and to outline the construction and assembly of the transverse electromagnetic (TEM) RF coil for use in ultra high field MRI (UHFMRI) studies at 340 MHz. METHOD: TEM and BB resonators were machined from acrylic and Teflon tubing, copper foil, and brass connectors. Tuning was accomplished through adjustable TEM elements. Variable Teflon-based capacitors were utilized to provide matching to the 50 omega line. The TEM resonator operated in quadrature, and the BB resonator operated in linear mode. The final resonators were fully adjustable from 63 to 430 MHz. Quality (Q) values were measured using a network analyzer over this frequency range for the unloaded and loaded coils. Coil performance was also evaluated using gradient and spin echo imaging at 8 T. RESULTS: Both resonators yielded excellent images from mineral oil phantoms, with good homogeneity throughout the imaging volume. The BB resonator was characterized with enhanced signal-to-noise ratio and greatly reduced RF power requirements relative to the TEM resonator. Images obtained from the human head at 8 T with the TEM resonator were also excellent. Tuning remains a tedious process. CONCLUSION: The TEM resonator provides an excellent RF coil for imaging studies up to 340 MHz. Its homogeneity reliability remains to be improved. In part as a result of its inability to sustain radiative loses, the BB resonator has extremely low RF power requirements. The BB resonator may have important uses in limiting RF power requirements and enhancing signal-to-noise ratio at other frequencies. Larger slightly modified versions may also prove useful in human imaging, depending on tolerances and final quality factors.

Acrylic Resins↗

PubMedDate not supplied

Ultra high resolution imaging of the human head at 8 tesla: 2K x 2K for Y2K.

PURPOSE: To acquire ultra high resolution MRI images of the human brain at 8 Tesla within a clinically acceptable time frame. METHOD: Gradient echo images were acquired from the human head of normal subjects using a transverse electromagnetic resonator operating in quadrature and tuned to 340 MHz. In each study, a group of six images was obtained containing a total of 208 MB of unprocessed information. Typical acquisition parameters were as follows: matrix = 2,000 x 2,000, field of view = 20 cm, slice thickness = 2 mm, number of excitations (NEX) = 1, flip angle = 45 degrees, TR = 750 ms, TE = 17 ms, receiver bandwidth = 69.4 kHz. This resulted in a total scan time of 23 minutes, an in-plane resolution of 100 microm, and a pixel volume of 0.02 mm3. RESULTS: The ultra high resolution images acquired in this study represent more than a 50-fold increase in in-plane resolution relative to conventional 256 x 256 images obtained with a 20 cm field of view and a 5 mm slice thickness. Nonetheless, the ultra high resolution images could be acquired both with adequate image quality and signal to noise. They revealed numerous small venous structures throughout the image plane and provided reasonable delineation between gray and white matter. DISCUSSION: The elevated signal-to-noise ratio observed in ultra high field magnetic resonance imaging can be utilized to acquire images with a level of resolution approaching the histological level under in vivo conditions. However, brain motion is likely to degrade the useful resolution. This situation may be remedied in part with cardiac gating. Nonetheless, these images represent a significant advance in our ability to examine small anatomical features with noninvasive imaging methods.

Brain↗

PubMedDate not supplied

High resolution MRI of the brainstem at 8 T.

A recently developed ultra high field MRI system operating at 8 T (UHFMRI) was applied for brainstem imaging. UHFMRI was performed in five healthy volunteers (three men, two women; age range 34--46 years). Sagittal and axial slices were obtained with the following settings: GE, TR 750--1,000 ms, TE 7-9 ms, FOV 20 cm, matrix 1,024 x 1,024 or 512 x 512 points, slice thickness 2 mm, resolution 195 or 390 microm/pixel. The brainstem structures were assigned based on anatomy and course. Images with good signal strength and homogeneity were acquired from the midbrain and the pons. Main intraaxial structures could be directly visualized. The periaqueductal gray matter and nuclei had higher signal intensity than the predominantly white matter structures such as the corticobulbar/corticopontine/corticospinal tracts, the sensory lemnisci, and the medial longitudinal fasciculus. Structures with high iron content such as the substantia nigra and the red nucleus were seen as prominent signal hypointensities. Numerous vessels traversing the brainstem including small perforators were also distinguished. It is concluded that UHFMRI enables the acquisition of high quality images of the upper brainstem with details approaching that of histologic specimen.

Adult↗

PubMedDate not supplied

Ultra high field MRI at 8 Tesla of subacute hemorrhagic stroke.

PURPOSE: Optimal treatment strategies and neurologic outcome after stroke depend on an accurate characterization of the lesion. There is a need for high resolution noninvasive imaging for assessment of the infarct size, perfusion, and vascular territory. MRI at the ultra high field (UHF) of 8 T offers unprecedented resolution, but its utility for stroke evaluation has not been determined yet. METHOD: A 55-year-old man with hypertension experienced sudden onset of speech arrest and right-sided hemiparesis that resolved in < 24 h with minimal neurologic deficit. MRI at 1.5 T showed initially a left posterior frontal lesion with subacute infarct (hyperintense on T2-weighted spin echo images) and right-sided frontal and periventricular lesions consistent with chronic infarct. There were many smaller white matter lesions. Delayed studies showed high signal changes involving the gray matter only on T1-weighted images. RESULTS: Gradient echo and rapid acquisition with relaxation enhancement (RARE) multislice images revealed a serpentine area of low signal in the left posterior frontal lobe gray matter suggestive of a hemorrhagic infarct, right-sided frontal lesion also showing iron deposits, multiple periventricular and cortical areas with abnormal high signal regions that were consistent with old infarcts, and numerous small vessels readily visible, more prominent on the right. CONCLUSION: MRI at 8 T displays lesions with a high resolution and striking anatomic details. Susceptibility to iron and sensitivity to detect blood products are increased at 8 T. The imaging characteristics at high field are different from those at low field, but both represent findings of iron products.

Humans↗