Hidemasa Uematsu

Magnetic resonance microimaging of intraaxonal water diffusion in live excised lamprey spinal cord

Anisotropy of water diffusion in axon tracts, as determined by diffusion-weighted MRI, has been assumed to reflect the restriction of water diffusion across axon membranes. Reduction in this anisotropy has been interpreted as degeneration of axons. These interpretations are based primarily on a priori reasoning that has had little empirical validation. We used the experimental advantages of the sea lamprey spinal cord, which contains several very large axons, to determine whether intraaxonal diffusion is isotropic and whether anisotropy is attributable to restriction of water mobility by axon surface membranes. Through the application of magnetic resonance microimaging, we were able to measure the purely intraaxonal diffusion characteristics of the giant reticulospinal axons (20–40 μm in diameter). The intraaxonal apparent diffusion coefficients of water parallel (longitudinal ADC, l-ADC) and perpendicular (transverse ADC, t-ADC) to the long axis were 0.98 ± 0.06 (10−3 mm2/sec) and 0.97 ± 0.11 (10−3 mm2/sec), respectively. In white matter regions that included multiple axons, l-ADCs were almost identical regardless of axon density in the sampled axon tract. By comparison, t-ADCs were reduced and varied inversely with the number of axons (and thus axolemmas) in a fixed cross-sectional area. Thus, diffusion was found to be isotropic when measured entirely within a single axon and anisotropic when measured in regions that included multiple axons. These findings support the hypothesis that the cell membrane is the primary source of diffusion anisotropy in fiber tracts of the central nervous system.

MR imaging at high magnetic fields

Recently, more investigators have been applying higher magnetic field strengths (3-4 Tesla) in research and clinical settings. Higher magnetic field strength is expected to afford higher spatial resolution and/or a decrease in the length of total scan time due to its higher signal intensity. Besides MR signal intensity, however, there are several factors which are magnetic field dependent, thus the same set of imaging parameters at lower magnetic field strengths would provide differences in signal or contrast to noise ratios at 3 T or higher. Therefore, an outcome of the combined effect of all these factors should be considered to estimate the change in usefulness at different magnetic fields. The objective of this article is to illustrate the practical scientific applications, focusing on MR imaging, of higher magnetic field strength. First, we will discuss previous literature and our experiments to demonstrate several changes that lead to a number of practical applications in MR imaging, e.g. in relaxation times, effects of contrast agent, design of RF coils, maintaining a safety profile and in switching magnetic field strength. Second, we discuss what will be required to gain the maximum benefit of high magnetic field when the current magnetic field (< or = 1.5 T) is switched to 3 or 4 T. In addition, we discuss MR microscopy, which is one of the anticipated applications of high magnetic field strength to understand the quantitative estimation of the gain benefit and other considerations to help establish a practically available imaging protocol.

Apparent Diffusion Coefficient in Pancreatic Cancer: Characterization and Histopathological Correlations

To clarify the components primarily responsible for diffusion abnormalities in pancreatic cancerous tissue.

18F-FDG PET of Common Enhancing Malignant Brain Tumors

OBJECTIVE The purpose of our study was to determine whether (18)F-FDG PET can be used to differentiate among common enhancing brain tumors such as lymphoma, high-grade glioma, and metastatic brain tumor. MATERIALS AND METHODS We evaluated 34 patients with an enhancing brain tumor on MRI, including seven lymphomas, nine high-grade gliomas, and 18 metastatic tumors. All patients also underwent FDG PET. For PET image analysis, regions of interest were placed over the tumor (T), contralateral cortex (C), and white matter (WM). Average and maximum pixel values were determined at each site. On the basis of these measurements, average and maximum standard uptake values (SUV(avg) and SUV(max)) were calculated, along with activity ratios (T/C(avg), T/WM(avg), T/WM(max), and T/C(max)), and comparisons among lesions were then made. RESULTS All parameters were significantly higher for lymphoma than for other tumors (p < 0.01). High-grade gliomas showed significantly higher SUV(avg) and SUV(max) than metastatic tumors (p < 0.05). Other parameters did not differ between lesion types. SUV(max) was the most accurate parameter for distinguishing lymphomas. Using an SUV(max) of 15.0 as a cutoff for diagnosing CNS lymphoma, only one high-grade glioma was found as a false-positive (SUV(max), 18.8). CONCLUSION FDG PET may be useful for differentiating common enhancing malignant brain tumors, particularly lymphoma versus high-grade glioma and metastatic tumor. FDG PET can provide useful information for distinguishing between lymphoma and other malignant enhancing brain tumors and is recommended when differential diagnoses are difficult to narrow using MRI alone.

Double-echo perfusion-weighted MR imaging: basic concepts and application in brain tumors for the assessment of tumor blood volume and vascular permeability

Perfusion-weighted magnetic resonance (MR) imaging using contrast agents plays a key role in characterizing tumors of the brain. We have shown that double-echo perfusion-weighted MR imaging (DEPWI) is potentially useful in assessing brain tumors. Quantitative indices, such as tumor blood volume, are obtained using DEPWI, which allows correction of underestimation of tumor blood volume due to leakage of contrast agents from tumor vessels, in addition to simultaneous acquisition of tumor vessel permeability. This article describes basic concepts of DEPWI and demonstrates clinical applications in brain tumors.

High field body MR imaging:Preliminary experiences

Whole-body magnetic resonance (MR) scanners at high field strengths (> or =3 T) have been introduced in expectation of a larger signal-to-noise ratio (SNR), which would decrease the length of scan time or improve the spatial resolution. In this paper, the advantages and disadvantages of the high field MR imaging are discussed. Although the building of the radio frequency (RF) coil, safety and the specific absorption rate (SAR) are issues, the application of high field MR imaging is promising. The optimization of all parameters including injection rate of Gd-DTPA is necessary for high field MR imaging to obtain maximal results; however, we hope that high field MR imaging can be used in routine clinical applications in the future.

Yamanouchi Magnetic Compression Anastomosis for Bilioenteric Anastomotic Stricture after Living-donor Liver Transplantation

Yamanouchi magnetic compression anastomosis (YMCA) is a novel interventional method that creates an anastomosis between the bile duct and small intestine. The method uses two magnets to compress the stricture transmurally, causing gradual ischemic necrosis of the stricture. This ischemic necrosis creates an anastomosis between the two magnets. The present report describes two cases in which YMCA was successfully applied to treat bilioenteric anastomotic stricture after living-donor liver transplantation. These two patients exhibited good long-term clinical courses.

Visualization of the cervical spinal cord with FDG and high-resolution PET.

PURPOSE Our aim was to evaluate the visibility of the cervical spinal cord with [18F]2-fluoro-2-deoxyglucose (FDG) and a high-resolution PET scanner and to quantify the glucose utilization by the cervical cord. METHOD Twenty-one normal subjects and three cervical myelopathy patients were studied. The visibility of the cervical spinal cord in sagittal and coronal sections was evaluated. The metabolic rate of glucose (MRGlu) and standardized uptake value (SUV) of FDG in the cord were calculated. RESULTS The entire cervical spinal cord was clearly visualized in 57% of the subjects: the upper cord in 81%, the middle cord in 73%, and the lower cord in 57%. The MRGlu of the normal cord was 1.93 +/- 0.37 mg/100 g/min. SUV was constant across all the vertebral levels and negatively correlated with subject age. In the myelopathy patients, the SUV of the entire cervical cord was lower than in the age-matched normal subjects. CONCLUSION These preliminary results indicate that the cervical spinal cord can be visualized as a normal structure in routine head and neck PET imaging and that FDG-PET may provide quantitative information about spinal cord disorders.

Tissue characterization of glioma by proton magnetic resonance spectroscopy and perfusion-weighted magnetic resonance imaging : glioma grading and histological correlation

BACKGROUND AND PURPOSE Our intent was to clarify the usefulness of proton magnetic resonance spectroscopy (MRS) and perfusion-weighted magnetic resonance imaging (PW-MRI) in the grading of glioma. METHODS Twenty-three consecutive patients with gliomas were investigated by both proton MRS and PW-MRI. For quantitative analysis, the metabolite data of the gliomas were estimated using the LCModel software. Receiver operating characteristic (ROC) curve analyses were also performed to assess which metabolite parameter was optimal for discrimination of glioma grade. From the PW-MRI data, the value of blood volume was measured on the parametric map corresponding to the location of MRS analysis. We then compared tumor blood volume with the amount of choline (Cho). RESULTS The mean Cho/creatine (Cre) ratio was useful to discriminate between Grades II and III, and the mean lactate (Lac)/Cre ratio was found to be significantly different between Grades III and IV. ROC curve analysis showed that measurements involving Cho were superior indices for grading glioma compared with blood volume information. Furthermore, the correlation between tumor blood volume and the amount of choline was statistically significant. CONCLUSION MRS may provide valuable information for glioma grading.

Quantification of pulmonary perfusion with MR imaging: recent advances

Recent advances in magnetic resonance pulmonary perfusion imaging are reviewed, focusing on magnetic resonance perfusion imaging using gadolinium contrasts agents or spin labeling of blood using naturally flowing spins as the source of intravascular signal. These recent developments in magnetic resonance imaging have made it possible to analyze data quantitatively which holds significant potential for clinical imaging of lung perfusion and opens windows to functional MR imaging of the lung. We believe that fast magnetic resonance functional imaging will play an important role in the assessment of pulmonary function and the pulmonary disease process.