Physiological assessment of in vivo human knee articular cartilage using sodium MR imaging at 1.5 T

Osteoarthritis is a common joint disorder that is most prevalent in the knee joint. Knee osteoarthritis (OA) can be characterized by the gradual loss of articular cartilage (AC). Formation of lesion, fissures and cracks on the cartilage surface has been associated with degenerative AC and can be measured by morphological assessment. In addition, loss of proteoglycan from extracellular matrix of the AC can be measured at early stage of cartilage degradation by physiological assessment. In this case, a biochemical phenomenon of cartilage is used to assess the changes at early degeneration of AC. In this paper, a method to measure local sodium concentration in AC due to proteoglycan has been investigated. A clinical 1.5-T magnetic resonance imaging (MRI) with multinuclear spectroscopic facility is used to acquire sodium images and quantify local sodium content of AC. An optimised 3D gradient-echo sequence with low echo time has been used for MR scan. The estimated sodium concentration in AC region from four different data sets is found to be ~ 225 ± 19 mmol/l, which matches the values that has been reported for the normal AC. This study shows that sodium images acquired at clinical 1.5-T MRI system can generate an adequate quantitative data that enable the estimation of sodium concentration in AC. We conclude that this method is potentially suitable for non-invasive physiological (sodium content) measurement of articular cartilage.     

Multichannel transceiver dual-tuned RF coil for proton/sodium MR imaging of knee cartilage at 3 T

Sodium magnetic resonance (MR) imaging is a promising technique for detecting changes of proteoglycan (PG) content in cartilage associated with knee osteoarthritis. Despite its potential clinical benefit, sodium MR imaging in vivo is challenging because of intrinsically low sodium concentration and low MR signal sensitivity. Some of the challenges in sodium MR imaging may be eliminated by the use of a high-sensitivity radiofrequency (RF) coil, specifically, a dual-tuned (DT) proton/sodium RF coil which facilitates the co-registration of sodium and proton MR images and the evaluation of both physiochemical and structural properties of knee cartilage. Nevertheless, implementation of a DT proton/sodium RF coil is technically difficult because of the coupling effect between the coil elements (particularly at high field) and the required compact design with improved coil sensitivity. In this study, we applied a multitransceiver RF coil design to develop a DT proton/sodium coil for knee cartilage imaging at 3 T. With the new design, the size of the coil was minimized, and a high signal-to-noise ratio (SNR) was achieved. DT coil exhibited high levels of reflection S11 (～-21 dB) and transmission coefficient S12 (～-19 dB) for both the proton and sodium coils. High SNR (range 27-38) and contrast-to-noise ratio (CNR) (range 15-21) were achieved in sodium MR imaging of knee cartilage in vivo at 3-mm(3) isotropic resolution. This DT coil performance was comparable to that measured using a sodium-only birdcage coil (SNR of 28 and CNR of 20). Clinical evaluation of the DT coil on four normal subjects demonstrated a consistent acquisition of high-resolution proton images and measurement of relative sodium concentrations of knee cartilages without repositioning of the subjects during the same MR scanning session.     

In vivo 31P MRS of human brain at high/ultrahigh fields: a quantitative comparison of NMR detection sensitivity and spectral resolution between 4 T and 7 T

The primary goal of this study was to establish a rigorous approach for determining and comparing the NMR detection sensitivity of in vivo ³¹P MRS at different field strengths (B₀). This was done by calculating the signal-to-noise ratio (SNR) achieved within a unit sampling time at a given field strength. In vivo ³¹P spectra of human occipital lobe were acquired at 4 and 7 T under similar experimental conditions. They were used to measure the improvement of the human brain ³¹P MRS when the field strength increases from 4 to 7 T. The relaxation times and line widths of the phosphocreatine (PCr) resonance peak and the RF coil quality factors (Q) were also measured at these two field strengths. Their relative contributions to SNR at a given field strength were analyzed and discussed. The results show that in vivo ³¹P sensitivity was significantly improved at 7 T as compared with 4 T. Moreover, the line-width of the PCr resonance peak showed less than a linear increase with increased B₀, which leads to a significant improvement in ³¹P spectral resolution. These findings indicate the advantage of high-field strength to improve in vivo ³¹P MRS quality in both sensitivity and spectral resolution. This advantage should improve the reliability and applicability of in vivo ³¹P MRS in studying high-energy phosphate metabolism, phospholipid metabolism and cerebral biogenetics in the human at both normal and diseased states noninvasively. Finally, the approach used in this study for calculating in vivo ³¹P MRS sensitivity provides a general tool in estimating the relative NMR detection sensitivity for any nuclear spin at a given field strength.     

A practical multinuclear transceiver volume coil for in vivo MRI/MRS at 7 T

A practical multinuclear transceiver RF volume coil with improved efficiency for in vivo small animal ¹H/¹³C/²³Na MR applications at the ultrahigh magnetic field of 7 T is reported. In the proposed design, the coil's resonance frequencies for ¹H and ¹³C are realized by using a traditional double-tuned approach, while the resonant frequency for ²³Na, which is only some 4 MHz away from the ¹³C frequency, is tuned based upon ¹³C channel by easy-operating capacitive “frequency switches”. In contrast to the traditional triple-tuned volume coil, the volume coil with the proposed design possesses less number of resonances, which helps improve the coil efficiency and alleviate the design and operation difficulties. This coil design strategy is advantageous and well suitable for multinuclear MR imaging and spectroscopy studies, particularly in the case where Larmor frequencies of nuclei in question are not separate enough. The prototype multinuclear coil was demonstrated in the desired unshielded design for easy construction and experiment implementation at 7 T. The design method may provide a practical and robust solution to designing multinuclear RF volume coils for in vivo MR imaging and spectroscopy at ultrahigh fields. Finite difference time domain method simulations for evaluating the design and 7-T MR experiment results acquired using the prototype coil are presented.     

In vivo imaging of the human brain at 1.5 T with 0.6-mm isotropic resolution

We present high-resolution in vivo anatomical scans with 3D whole-brain coverage and an isotropic resolution of 0.6 mm, obtained at a clinical field of 1.5 T. The data are acquired in 10 independent scans over two sessions using a 3D magnetization-prepared, gradient echo sequence, modified to output phase images in addition to magnitude images. The independent scans are coregistered to correct for head motion, prior to performing complex averaging. The resolution of the final, averaged image, is found to be equal to the nominal one.     

Region and volume dependencies in spectral line width assessed by 1H 2D MR chemical shift imaging in the monkey brain at 7 T

High magnetic fields increase the sensitivity and spectral dispersion in magnetic resonance spectroscopy (MRS). In contrast, spectral peaks are broadened in vivo at higher field strength due to stronger susceptibility-induced effects. Strategies to minimize the spectral line width are therefore of critical importance. In the present study, ¹H 2D chemical shift imaging at short echo times was performed in the macaque monkey brain at 7 T. Large brain coverage was obtained at high spatial resolution with voxel sizes down to 50 μl being able to quantify up to nine metabolites in vivo with good reliability. Measured line widths of metabolites decreased from 14.2 to 7.6 Hz with voxel volumes of 3.14 ml to 50 μl (at increased spatial resolution). The line width distribution of the metabolites (7.6±1.6 Hz, ranging from 5.5 to 10 Hz) was considerably smaller compared to that of water (10.6±2.4 Hz) and was also smaller than reported in ¹H MRS at 7 T in the human brain. Our study showed that even in well-shimmed areas assumed to have minimal macroscopic susceptibility variations, spectral line widths are tissue-specific exhibiting considerable regional variation. Therefore, an overall improvement of a gross spectral line width — directly correlated with improved spectral quality — can only be achieved when voxel volumes are significantly reduced. Our line width optimization was sufficient to permit clear glutamate (Glu)–glutamine separation, yielding distinct Glu maps for brain areas including regions of greatly different Glu concentration (e.g., ventricles vs. surrounding tissue).     

In vivo structural analysis of articular cartilage using diffusion tensor magnetic resonance imaging

Purpose

The articular cartilage is a small tissue with a matrix structure of three layers between which the orientation of collagen fiber differs. A diffusion-weighted twice-refocused spin-echo echo-planar imaging (SE-EPI) sequence was optimized for the articular cartilage, and the structure of the three layers of human articular cartilage was imaged in vivo from diffusion tensor images.

Materials and Methods

The subjects imaged were five specimens of swine femur head after removal of the flesh around the knee joint, five specimens of swine articular cartilage with flesh present and the knee cartilage of five adult male volunteers. Based on diffusion-weighted images in six directions, the mean diffusivity (MD) and the fractional anisotropy (FA) values were calculated.

Results

Diffusion tensor images of the articular cartilage were obtained by sequence optimization. The MD and FA value of the specimens (each of five examples) under different conditions were estimated. Although the articular cartilage is a small tissue, the matrix structure of each layer in the articular cartilage was obtained by SE-EPI sequence with GRAPPA. The MD and FA values of swine articular cartilage are different between the synovial fluid and saline. In human articular cartilage, the load of the body weight on the knee had an effect on the FA value of the surface layer of the articular cartilage.

Conclusion

This method can be used to create images of the articular cartilage structure, not only in vitro but also in vivo. Therefore, it is suggested that this method should support the elucidation of the in vivo structure and function of the knee joint and might be applied to clinical practice.     

Resolution enhancement in in vivo NMR spectroscopy: detection of intermolecular zero-quantum coherences

Intermolecular zero-quantum coherences are insensitive to magnetic field inhomogeneities. For this reason we have applied the HOMOGENIZED sequence [Vathyam et al., Science 272 (1996) 92] to phantoms containing metabolites at low concentrations, phantoms with air inclusions, an intact grape, and the head of a rat in vivo at 750 MHz. In the 1H-spectra, the water signal is efficiently suppressed and line broadening due to susceptibility gradients is effectively removed along the indirectly detected dimension. We have obtained a 1H-spectrum of a 2.5 mM solution of gamma-aminobutyric acid in 12 min scan time. In the phantom with air inclusions a reduction of line widths from 0.48 ppm in the direct dimension to 0.07 ppm in the indirect dimension was observed, while in a deshimmed grape the reduction was from 1.4 to 0.07 ppm. In a spectrum of the grape we were able to resolve glucose resonances at 0.3 ppm from the water in 6 min scan time. J-coupling information was partly retained. In the in vivo spectra of the rat brain five major metabolites were observed.     

A practical approach to in vivo high-resolution diffusion tensor imaging of rhesus monkeys on a 3-T human scanner

The nonhuman primate brain study provides important supplemental means for human brain exploration since the two species share close anatomical and functional similarities. MR diffusion tensor imaging (DTI) in human brain has revealed exquisite details of brain structures especially in the brain white matter. However, most previous monkey brain DTI results lack the spatial resolution in comparison to the conventional tracing and postmortem imaging methods, especially when it is acquired in commonly available human MRI scanners of field strength of 3 T or lower. To meet the increasing demands for nonhuman primate DTI studies, we proposed an in vivo high-resolution monkey DTI acquisition protocol that is practically feasible and combined it with an improved postprocessing procedure for a 3-T human scanner. The acquisition protocol, susceptibility distortion correction method with phase reversal acquisition, and postprocessing steps were proved to be effective in our study of rhesus monkeys. Results from diffusion tensor estimations and fiber tractography at 1 x 1 x 1 mm(3) resolution were found to be comparable to previous ex vivo DTI studies with much longer acquisition times. Effects of image resolution were evaluated and it was confirmed that the partial volume effect due to the larger voxel size in low-resolution data biased the diffusion tensor estimation and produced erroneous fiber tractography. Our results suggest that in vivo high-resolution monkey brain DTI can be achieved within practical time, which allows accurate diffusion tensor estimation and fiber tractography in monkey brains, so that the complex anatomical structures within many small but important anatomic structures can be delineated.     

A microstrip transmission line volume coil for human head MR imaging at 4T

A high-frequency RF volume coil based on the use of microstrip transmission line (MTL) has been developed for in vivo 1H MR applications on the human head at 4T. This coil is characterized by major advantages: (i) completely distributed coil circuit, (ii) high-quality factor (Q), (iii) simple coil structure, and (iv) better sensitivity and less signal-intensity variation in the MR image of the human head compared with an RF shielded birdcage coil of similar coil size. The proposed MTL volume coil does not require additional RF shielding for preventing Q degradation from radiation losses due to the unique MTL structure; thus, it provides a maximal useable space inside the volume coil when compared with most volume coils available at high fields with the same overall coil size. The intrinsic B(1) distribution of the MTL volume coil effectively compensates for the dielectric resonance effect at 4T and improves the signal homogeneity in human head MR images in the transaxial planes. The results of this study demonstrate that the MTL volume coil design provides an efficient and simple solution to RF volume coil design for human MR studies at high fields.     

Central neurocytoma: proton MR spectroscopy and diffusion weighted MR imaging findings

Purpose

To present proton magnetic resonance spectroscopy and diffusion-weighted imaging (DWI) findings of central neurocytoma (CN).

Methods and Materials

Imaging findings of seven patients with the histopathological diagnosis of CN (five male and two female; age range, 21–28 years of age) were evaluated retrospectively. In addition to conventional magnetic resonance imaging features, we also assessed the metabolite ratios and tumor normalized apparent diffusion coefficient (NADC), which was calculated by dividing the tumor apparent diffusion coefficient (ADC) values by normal ADC. Approval from our institutional review board was obtained for this review.

Results

The tumor choline/creatine ratios were 5.17±2.38, while N-acetyl aspartate/choline and N-acetyl aspartate/creatine ratios were 0.33±0.15 and 1.84±1.38, respectively. On DWI, tumors had heterogeneous hyperintense appearances when compared with the contralateral parietal lobe white matter and tumor NADC values were 0.63±0.05.

Conclusion

Significantly increased choline/creatine and decreased N-acetyl aspartate/choline ratios with lower NADC values in CN resemble high-grade gliomas and complicate the diagnosis. Familarity its physiologic features would help to presurgical diagnosis of ventricular and exraventricular CNs.     

Cerebellar metabolite alterations detected in vivo by proton MR spectroscopy

The aim of our work was to evaluate the feasibility of in vivo single-voxel quantitative proton MR spectroscopy in order to identify possible alterations in the main metabolite concentrations due to some metabolic dysfunctions in the cerebellum of patients suffering from a particular form of migraine called “with aura.” Measurements of metabolite levels in the cerebellum disclosed reduced choline values (normalized both to N-acetyl-aspartate and creatine) in the patient group with respect to the age-matched control group. Our interest in this pathology is motivated by the fact that there are no available specific biochemical markers for migraine characterization, and the current diagnostic only takes advantage of the medical history and the clinical examination.     

磁共振成象新进展

磁共振成象(MRI)已经成为生命科学研究和医疗诊断的有力手段，因此荣获2003年诺贝尔生理学或医学奖．文章概述了磁共振成象的新近进展，包括医疗成象、脑功能成象、显微成象、活体磁共振波谱等方面．     

An intravascular loopless monopole antenna for vessel wall MR imaging at 3.0 T

The purpose of this study was to develop a novel intravascular loopless monopole antenna (ILMA) design specifically for imaging of small vessel walls. The ILMA consisted of an unshielded, low-friction guide wire and a tuning/matching box. The material of the guide wire was nitinol and it was coated with polyurethane. Because the guide wire was unshielded, it could be made thinner than the coaxial cable-based loopless intravascular antenna design. The material of the box was aluminum. In this study, the diameter of the guide wire was 0.5 mm and the length was 58.7 mm. The ILMA was used as a receiving antenna and body coil for transmission. To verify the feasibility of the ILMA, in vitro and in vivo experiments were performed on a 3.0-T magnetic resonance (MR) scanner. In vitro tests using the ILMA indicated that the proposed design could be used to image target vessel walls with a spatial resolution of 313 μm at the frequency coding direction and more than 100 mm of longitudinal coverage. In vivo tests demonstrated that the images showed the vessel walls clearly by using the ILMA and also indicated that the ILMA could be used for small vessels. The proposed antenna may therefore be utilized to promote MR-based diagnoses and therapeutic solutions for cardiovascular atherosclerotic diseases.     

T1 and proton density at 7 T in patients with multiple sclerosis: an initial study

Magnetic resonance imaging of cortical lesions due to multiple sclerosis (MS) has been hampered by the lesions' small size and low contrast to adjacent, normal-appearing tissue. Knowing cortical lesion T1 and proton density (PD) would be highly beneficial for the process of developing and optimizing dedicated magnetic resonance (MR) sequences through computer modeling of MR tissue responses. Eight patients and seven healthy control subjects were scanned at 7 T using a series of inversion recovery turbo field echo scans with varying inversion times. Regions of interest were drawn in white matter, gray matter, cortical lesions, white matter lesions and cerebrospinal fluid. White matter and gray matter T1s were significantly higher in MS patients than in controls. Cortical and white matter lesion T1 and PD are also presented for the first time. The advantages of ultrahigh field MR imaging will be important for future investigations in MS research and sequence optimization for the detection of cortical lesions.     

MRI methodological development of intervertebral disc degeneration: a rabbit in vivo study at 9.4 T

Intervertebral disc (IVD) degeneration is a complex process characterized by biochemical and structural changes in both the nucleus pulposus and the anulus fibrosus. In this study, we were able to obtain in vivo magnetic resonance (MR) images of the rabbit spine, with several MR imaging (MRI) contrasts (ρ, T₁ and T₂). We quantified several parameters (T₂, apparent diffusion coefficient, disc height and area) to differentiate between healthy and degenerative IVDs and to characterize the degeneration process. To our knowledge, there has not been any previous in vivo study of rabbit IVDs at high-field MRI (9.4 T).A custom radio frequency (RF) coil for 9.4 T was designed to match rabbit IVD morphology, to study the degeneration in vivo on a model of human lumbar disease. Our new probe, a custom half-birdcage-type coil, obtains the necessary exploration depth while meeting the requirements for signal homogeneity and sensitivity of the study. This design addresses some of the difficulties with constructing RF coils at high field strengths.     

NMR-microscopy with TrueFISP at 11.75T

The purpose of this paper is to demonstrate that a fully balanced gradient echo technique (TrueFISP) can be used for microscopic experiments at high static magnetic field strengths. TrueFISP experiments were successfully performed on homogeneous and inhomogeneous objects at 11.75T. High-resolution TrueFISP images were obtained from phantoms, plants, formalin-fixed samples, and from an isolated beating rat heart with an in-plane resolution of 78 micro m and a slice thickness of 500 micro m. The signal-to-noise ratio (SNR) gain of TrueFISP compared to conventional gradient echo or spin echo sequences will allow faster acquisition times or an improvement in spatial resolution for microscopic experiments.     

A practical guide to robust detection of GABA in human brain by J-difference spectroscopy at 3 T using a standard volume coil

gamma-Aminobutyric acid (GABA) is the major inhibitory neurotransmitter in human brain and has been implicated in several neuropsychiatric disorders. In vivo human brain GABA concentrations are near the detection limit for magnetic resonance spectroscopy ( approximately 1 mM), and because of overlap with more abundant compounds, spectral editing is generally necessary to detect GABA. In previous reports, GABA spectra edited by J-difference spectroscopy vary considerably in appearance. We have evaluated the factors that affect GABA spectra and the conditions necessary for robust acquisition of J-difference spectra from arbitrary brain regions. In particular, we demonstrate that variations in spectral quality can be explained in part by the incoherent addition of transients that results from shot to shot frequency and phase variations. An automated time-domain spectral alignment strategy that enables reproducible acquisition of high-quality GABA spectra at 3 T with a standard 30-cm T/R volume coil is presented. Representative GABA spectra from human frontal lobe, an area where susceptibility-induced frequency and phase variations are especially troublesome, that demonstrate the robustness of the acquisition and data handling strategy used in this study are presented.     

MR imaging of apparent 3He gas transport in narrow pipes and rodent airways

High sensitivity makes hyperpolarized ³He an attractive signal source for visualizing gas flow with magnetic resonance (MR) imaging. Its rapid Brownian motion, however, can blur observed flow lamina and alter measured diffusion rates when excited nuclei traverse shear-induced velocity gradients during data acquisition. Here, both effects are described analytically, and predicted values for measured transport during laminar flow through a straight, 3.2-mm diameter pipe are validated using two-dimensional (2D) constant-time images of different binary gas mixtures. Results show explicitly how measured transport in narrow conduits is characterized by apparent values that depend on underlying gas dynamics and imaging time. In ventilated rats, this is found to obscure acquired airflow images. Nevertheless, flow splitting at airway branches is still evident and use of 3D vector flow mapping is shown to reveal surprising detail that highlights the correlation between gas dynamics and lung structure.     

In vivo NMR imaging and T1 measurements of water protons in the human brain

Nuclear magnetic resonance (NMR) proton density images of the human brain have been made by the FONAR method. Spin-lattice relaxation times, T₁, of water hydrogen protons have been determined at random positions within frontal and temporal regions of the human brain. The primary purpose of this ongoing research is to accumulate a large data base of normal T₁ values for water protons in normal human brain tissue. Our experience to data includes 31 measurements on 18 volunteer subjects, and the mean value ± standard deviation is 215 ± 42 msec. In addition, two metastatic lesions of the brain were studied and found to have T₁ values longer than those for normal brain tissue.