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.     

Damages to the extracellular matrix in articular cartilage due to cryopreservation by microscopic magnetic resonance imaging and biochemistry

To investigate the damages to the extracellular matrix in articular cartilage due to cryopreservation, the depth-dependent concentration profiles of glycosaminoglycans (GAGs) in 34 cartilage specimens from canine humeral heads were imaged at 13-μm pixel resolution using the in vitro version of the dGEMRIC protocol in microscopic MRI (μMRI). In addition, a biochemical assay was used to determine the GAG loss from the tissue to the solution where the tissue was immersed. For specimens that had been frozen at −20°C or −80°C without any cryoprotectant, a significant loss of GAG (as high as 56.5%) was found in cartilage, dependent upon the structural zones of the tissue and the conditions of cryopreservation. The cryoprotective abilities of dimethyl sulfoxide (DMSO) as a function of its concentration in saline and storage temperature were also investigated. A 30% DMSO concentration was sufficient in preventing the reduction of GAG in the tissue at the −20°C storage temperature, but a 50% concentration of DMSO was necessary for the −80°C cryopreservation. These imaging results were verified by the biochemical analysis.     

Measurement of the extracellular volume of human melanoma xenografts by contrast enhanced magnetic resonance imaging

The magnitude of the extracellular volume fraction (ECV) of tumors is of importance for the transport of macromolecular therapeutic agents from the vessel wall to the tumor cells. The aim of this study was to develop a method for measurement of tumor ECV by contrast enhanced MRI. Tumors of two human amelanotic melanoma xenograft lines (A-07 and R-18) grown intradermally in Balb/c nu/nu mice were used as model system, and muscle tissue was used as control. The renal arteries of the mice were ligated prior to i.v. administration of Gd-DTPA, and an MRI protocol for calculating Gd-DTPA concentration in tissue was followed. ECV was calculated from the Gd-DTPA concentrations in the tissue and in a plasma sample. In muscle tissue, the concentration reached a constant level after 1 min and the ECV was calculated to be 0.12 (+/- 0.01), consistent with values reported in the literature. Individual tumors showed large differences in the uptake of Gd-DTPA. The Gd-DTPA concentration in the tissue at 40 min after the Gd-DTPA administration was used to calculate tumor ECV. The ECV was found to differ significantly among regions of individual tumors and among individual tumors. The ECV ranged from 0.075 to 0.33 for A-07 tumors and from 0.016 to 0.097 for R-18 tumors. The intra- and intertumor heterogeneity in ECV was confirmed by histologic findings, showing that contrast enhanced MRI is suitable for non-invasive studies of the ECV in experimental tumors without necrosis.     

Optimizing tissue contrast in magnetic resonance imaging

Magnetic resonance imaging demands that tissue contrast and signal-to-noise advantages be sought in each component of the imaging system. One component of magnetic resonance imaging in which contrast and signal-to-noise ratios are easily manipulated is in the choice of pulse sequences and interpulse delay times. This article provides a general method for determining the best choices of interpulse delay times in pulse sequences and applies that method to saturation recovery, inversion recovery, and spin-echo sequences. Saturation recovery and inversion recovery sequences with rephasing pulses, and tissues with unequal hydrogen densities are considered. Optimization of pulse sequences is carried out for the two distinct cases of (a) a fixed number of sequence repetitions and (b) a fixed total imaging time. Analytic expressions are derived or approximate expressions are provided for the interpulse delay times that optimize contrast-to-noise ratios in each pulse sequence. The acceptable range of interpulse delay times to obtain reasonable contrast using each pulse sequence is discussed.     

Phase detection and contrast loss in magnetic resonance imaging

Several recent articles have assessed the relative efficiency of nuclear magnetic resonance (NMR) pulse sequences. One consideration that has received little attention is the effect on image contrast of displaying images without information on the sign of the reconstructed signals. The radiofrequency receivers currently used on most NMR imaging systems are quadrature detectors that preserve both the magnitude and sign of the NMR signal. Usually, however, sign or phase information is not used in the final image presentation. We point out that in imaging sequences that may have negative signals, such as inversion recovery, this loss of sign information produces a reduction in contrast between some tissues in an NMR image. We discuss the tissue parameters and interpulse delay times that result in contrast loss in inversion recovery and indicate the extent of contrast loss. We point out that for some tissues with unequal hydrogen spin densities, the region of contrast loss coincides with the region where maximum contrast would occur if sign information were preserved.     

Bilateral filtering of magnetic resonance phase images

High-pass filtering is required for the removal of background field inhomogeneities in magnetic resonance phase images. This high-pass filtering smooths across boundaries between areas with large differences in phase. The most prominent boundary is the surface of the brain where areas with large phase values inside the brain are located close to areas outside the brain where the phase is, on average, zero. Cortical areas, which are of great interest in brain MRI, are therefore often degraded by high-pass filtering. Here, we propose the use of the bilateral filter for the high-pass filtering step. The bilateral filter is essentially a Gaussian filter that stops smoothing at boundaries. We show that the bilateral filter improves image quality at the brain's surface, without sacrificing contrast within the brain.     

High resolution proton magnetic resonance of selenophene

The high resolution proton magnetic resonance of selenophene has been studied at 100 Mc/s. The spectrum was analysed as an A₂X₂ system and the following coupling constants were obtained: J ₂₃ = 5·35, J ₂₄ = 1·05, J ₃₄ = 3·56, J ₂₅ = 2·47 c.p.s. Coupling was also observed between ⁷⁷Se and the α and β protons and coupling constants of 48 c.p.s. (α) and 9·5 c.p.s. (β) were obtained. The τ values were 2·30 (α) and 2·88 (β). The results are compared with values available for furan, pyrrole and thiophene and the conclusion is drawn that selenophene is probably a planar molecule.     

光磁共振测量地磁场水平分量方法评述与改进

比较了在光磁共振实验中不加射频场、固定射频场、改变射频场3类测量地磁场水平分量的实验方法.提出利用反向测量、线性拟合等方法进行改进,一定程度上克服了原来实验方法的缺点.     

Transport of contrast agents in contrast-enhanced magnetic resonance angiography

Contrast-enhanced magnetic resonance angiography (CEMRA) often appears to display vessels with good resolution, but it does not adequately visualize the throat of a tight stenosis. Image quality in CEMRA is also governed by the timing of contrast injection and data acquisition. We developed a numerical technique to predict the image appearance of a target vessel by taking into account the passage of contrast agent. Experiments were conducted on a phantom with both centric and linear ordering. An effort to prolong the duration in which the contrast passes by employing a double injection technique gives a wider window of opportunity to acquire quality images.