An evaluation of reconstructed ACL impingement on PCL using MRI

Using magnetic resonance imaging (MRI), we evaluated 42 reconstructed anterior cruciate ligaments (ACLs) for impingement on corresponding posterior cruciate ligaments (PCLs) in the knee-extended position. Thirty-one single-bundle ACL reconstructions and 11 double-bundle ACL reconstructions were performed. MR examinations were performed at 3 and 12 months after arthroscopic ACL reconstructions. Using oblique axial MRIs of reconstructed ACLs, we evaluated the shape of the PCL and divided them into two groups: an impingement-positive group and an impingement-negative group. Using sagittal images, we measured the PCL index (Liu's method) and examined the correlation between the degree of impingement and the index. At 3 months after surgery, 14 of 31 single-bundle ACL reconstructions and 5 of 11 double-bundle ACL reconstructions were regarded as positive impingement on PCLs. At 12 months after surgery, 17 of 31 single-bundle ACL reconstructions and 5 of 11 double-bundle ACL reconstructions were regarded as positive impingement on PCLs. At 3 months in single-bundle reconstructions and at 3 and 12 months in all reconstructions, the PCL index of the impingement-positive group was significantly lower than that of the negative group. This study indicated that reconstructed ACLs in the impingement-positive group pressed the PCLs more posteriorly than did the impingement-negative group.     

Optimization of spoiler gradients in flash MRI

The FLASH technique for fast magnetic resonance (MR) imaging often employs strong magnetic field gradients, called spoiler gradients, to dephase the transverse magnetization after it has been measured. Otherwise, image artifacts can develop. The effectiveness of spoiler gradients at suppressing these artifacts was evaluated experimentally on two-dimensional MR images of a uniform phantom and patients. It was informative to compare the magnetization immediately before the RF excitation in each phase encoding step. Only spoiler gradients in the slice selection direction were effective. Spoiler gradients that decreased steadily from a large amplitude in the first phase encoding step to zero in the last minimized the transverse magnetization and suppressed the image artifact, without changing the image contrast.     

3.0-T MRI evaluation of patients with chronic liver diseases: initial observations

PURPOSE: To describe the use of 3.0-T magnetic resonance imaging (MRI) for the evaluation of chronic liver diseases. MATERIALS AND METHODS: Two groups of patients who had chronic liver diseases and underwent 3.0-T MRI for evaluation of the liver were included in the study. The first group of patients included 66 consecutive patients (33 male, 33 female; mean age+/-standard deviation, 56+/-11). The second group of patients included 30 consecutive patients (18 males, 12 females; mean age+/-standard deviation, 53+/-10) in whom Variable-Rate Selective Excitation (VERSE) pulses and improved adjustments procedure were used during the acquisitions. Imaging findings of chronic liver diseases, predetermined artifacts and image quality of all individual sequences in the first group and predetermined artifacts and image quality of T2-weighted sequences in the second group were reviewed retrospectively and independently by two reviewers. chi-Square tests were used to compare the findings between two groups of patients and individual sequences. Kappa statistics were used to determine the extent of agreement between the reviewers. RESULTS: Fifteen dysplastic nodules in 6 of 66 (9%) patients and 12 hepatocellular carcinomas in 11 of 66 (17%) patients were detected. Excluding motion artifacts, three-dimensional (3D) T1-weighted gradient-echo (GE) sequence was the least affected sequence by the artifacts. Image quality of T1-weighted 3D-GE sequences was excellent in 43 of 66 (65%) patients. In-phase and out-of-phase T1-weighted spoiled GE (SGE) images were fair in 62 of 66 (94%) and 61 of 66 (92%) patients, respectively. The image quality of short tau inversion recovery (STIR) and half-Fourier rapid acquisition with relaxation enhancement (RARE) sequences were fair in 31 of 66 (47%) and 53 of 66 (80%) patients. STIR and half-Fourier RARE sequences in the second group demonstrated significantly better image quality (P=.03 and P<.0001). CONCLUSION: 3.0-T MRI allows the acquisition of very high quality postgadolinium 3D-GE sequence, which permitted the detection and characterization of lesions in the setting of chronic liver diseases. The use of VERSE pulses and improved adjustments procedure improved the image quality of T2-weighted sequences. In-phase/out-of-phase SGE sequences are at present of fair quality.     

The suppression of selected acoustic frequencies in MRI

A study is made of certain dominant frequencies in the acoustic noise spectrum of the magnetic resonance imaging system. Motivated by both spring and string ideas, we investigate whether the contributions to the sound from certain frequencies can be canceled by the appropriate gradient pulse sequence design. From both simulations and experiments, vibrations resulting from an impulsive force associated with a ramping up of a gradient pulse are shown to be cancelled immediately upon the application of another impulsive force coming from the subsequent appropriately timed ramping down of that pulse. A general approach to suppression of multiple-frequency contributions involving a series of gradient pulses with variable timings is given for the cancellations between pairs of impulsive forces. Various examples are confirmed through string simulations, MRI experiments, and linear response theory. This also provides a foundation to explain some results in previous papers on this subject. The method suggests that a variety of pulse profiles and timing combinations can be used to attenuate important contributions to the acoustic spectrum.     

Exploiting the wavelet structure in compressed sensing MRI

Sparsity has been widely utilized in magnetic resonance imaging (MRI) to reduce k-space sampling. According to structured sparsity theories, fewer measurements are required for tree sparse data than the data only with standard sparsity. Intuitively, more accurate image reconstruction can be achieved with the same number of measurements by exploiting the wavelet tree structure in MRI. A novel algorithm is proposed in this article to reconstruct MR images from undersampled k-space data. In contrast to conventional compressed sensing MRI (CS-MRI) that only relies on the sparsity of MR images in wavelet or gradient domain, we exploit the wavelet tree structure to improve CS-MRI. This tree-based CS-MRI problem is decomposed into three simpler subproblems then each of the subproblems can be efficiently solved by an iterative scheme. Simulations and in vivo experiments demonstrate the significant improvement of the proposed method compared to conventional CS-MRI algorithms, and the feasibleness on MR data compared to existing tree-based imaging algorithms.     

匹配型保偏光纤自动对轴技术

The auto alignment technique is studied for the PM fiber (PMF) of matching refractive index(MRI). The polarization observation by lens effect tracing technique(POL) is introduced. Based on the POL,an auto alignment system is designed and its performance is then tested. Different light intensity distribution characteristic is found on the different observation surfaces in a series of experients. The widest range of adjustment is achieved when the observation surface corresponds to a light intensity distribution with five characteristic points. This means it is easier to focus.The five point method is presented after analysis of the curves of the five characteristic points with respect to the azimuth angle of the optical fiber.By comparison with the POL,this method is easier to be realized and has higher measurement accuracy,so it is suitable for auto alignment system.     

Claustrophobia in MRI: the role of cognitions

Purpose

This study aimed to investigate the role of cognitive and behavioural factors in the experience of claustrophobia in the context of magnetic resonance imaging (MRI) scanners.

Materials and Methods

One hundred and thirty outpatients attending an MRI unit completed questionnaires before and after their scans. Specific measures of experience in the scanner included subjective anxiety, panic symptoms, strategies used to stay calm and negative cognitions (such as ‘I will suffocate’ and ‘I am going to faint in here’). Other general measures used included anxiety, depression, health anxiety and fears of restriction and suffocation.

Results

The amount of anxiety experienced during the scan was related to the perceived amount of time spent having physical symptoms of panic. Cognitions reported concerned the following: suffocation, harm caused by the machine and lack of perceived control. The number of strategies patients used to cope in the machine was also a related factor. Neither position in the scanner, nor head coil use nor previous experience of being in the scanner was related to levels of anxiety.

Conclusion

The cognitions identified here may be used to construct a measure to identify those unable to enter the scanner or those most likely to become claustrophobic whilst undergoing the procedure and to further inform future brief, effective interventions.     

Solving the problem of concomitant gradients in ultra-low-field MRI

In ultra-low-field magnetic resonance imaging (ULF MRI), spin precession is detected typically in magnetic fields of the order of 10-100 μT. As in conventional high-field MRI, the spatial origin of the signals can be encoded by superposing gradient fields on a homogeneous main field. However, because the main field is weak, gradient field amplitudes become comparable to it. In this case, the concomitant gradients forced by Maxwell's equations cause the assumption of linearly varying field gradients to fail. Thus, image reconstruction with Fourier transformation would produce severe image artifacts. We propose a direct linear inversion (DLI) method to reconstruct images without limiting assumptions about the gradient fields. We compare the quality of the images obtained using the proposed reconstruction method and the Fourier reconstruction. With simulations, we show how the reconstruction errors of the methods depend on the strengths of the concomitant gradients. The proposed approach produces nearly distortion-free images even when the main field reaches zero.     

Quantitative evaluation of optimal imaging parameters for single-cell detection in MRI using simulation

Super-paramagnetic iron oxide (SPIO) nanoparticles are actively investigated to enhance disease detection through molecular imaging using magnetic resonance imaging (MRI). Detection of the cells labeled by SPIO depends on the MRI protocols and pulse sequence parameters that can be optimized. To evaluate the sensitivity and specificity of the image acquisition methods and to obtain optimal imaging parameters for single-cell detection, we further developed an MRI simulator. The simulator models an object (tissue) at a microscopic level to evaluate effects of spatial distribution and concentration of nanoparticles on the resulting image. In this study, the simulator was used to evaluate and compare imaging of the labeled cells by the gradient-echo (GE), true-FISP [fast imaging employing steady-state acquisition (FIESTA)] and echo-planar imaging (EPI) pulse sequences. Effects of the imaging and object parameters, such as field strength, imaging protocol and pulse sequence parameters, imaging resolution, cell iron load, position of SPIO within the voxel and cell division within the voxel, were investigated in the work. The results suggest that true-FISP has the highest sensitivity for single-cell detection by MRI.     

Correlation of MRI findings to histology of acetaminophen toxicity in the mouse

Acetaminophen (APAP) toxicity is responsible for approximately half of all cases of acute liver failure in the United States. The mouse model of APAP toxicity is widely used to examine mechanisms of APAP toxicity. Noninvasive approaches would allow for serial measurements in a single animal to study the effects of experimental interventions on the development and resolution of hepatocellular necrosis. The following study examined the time course of hepatic necrosis using small animal magnetic resonance imaging (MRI) following the administration of 200 mg/kg ip APAP given to B6C3F1 male mice. Mice treated with saline served as controls (CON). Other mice received treatment with the clinical antidote N-acetylcysteine (APAP+NAC). Mouse liver pathology was characterized using T1- and T2-weighted sequences at 2, 4, 8 and 24 h following APAP administration. Standard assays for APAP toxicity [serum alanine aminotransaminase (ALT) levels and hematoxylin and eosin (H&E) staining of liver sections] were examined relative to MRI findings. Overall, T2 sequences had a greater sensitivity for necrosis and hemorrhage than T1 (FLASH) images. Liver injury severity scoring of MR images demonstrated increased scores in the APAP mice at 4, 8 and 24 h compared to the CON mice. APAP+NAC mice had MRI scores similar to the CON mice. Semiquantitative analysis of hepatic hemorrhage strongly correlated with serum ALT. Small animal MRI can be used to monitor the evolution of APAP toxicity over time and to evaluate the response to therapy.     

Enhancing the discrimination accuracy between metastases, gliomas and meningiomas on brain MRI by volumetric textural features and ensemble pattern recognition methods

Three-dimensional (3D) texture analysis of volumetric brain magnetic resonance (MR) images has been identified as an important indicator for discriminating among different brain pathologies. The purpose of this study was to evaluate the efficiency of 3D textural features using a pattern recognition system in the task of discriminating benign, malignant and metastatic brain tissues on T1 postcontrast MR imaging (MRI) series. The dataset consisted of 67 brain MRI series obtained from patients with verified and untreated intracranial tumors. The pattern recognition system was designed as an ensemble classification scheme employing a support vector machine classifier, specially modified in order to integrate the least squares features transformation logic in its kernel function. The latter, in conjunction with using 3D textural features, enabled boosting up the performance of the system in discriminating metastatic, malignant and benign brain tumors with 77.14%, 89.19% and 93.33% accuracy, respectively. The method was evaluated using an external cross-validation process; thus, results might be considered indicative of the generalization performance of the system to "unseen" cases. The proposed system might be used as an assisting tool for brain tumor characterization on volumetric MRI series.     

The value of MRI in evaluating perirectal and pelvic disease

MRI of the perirectal region is facilitated by the superb soft tissue contrast, multiplanar imaging capability, lack of respiratory motion artifact and absence of clip artifact which can hamper visualization by CT scan. MRI provides distinct advantages over CT scanning without the need for ionizing radiation or the injection of intravenous contrast material. This study reviews the findings in 18 consecutive patients with a variety of perirectal pathologies including rectal carcinoma (3), gynecologic neoplasm (8), sacral lesions (2), pelvic arteriovenous malformations (2), inflammatory bowel disease (2), and a pelvic kidney (1). In the perirectal region, MR was useful to show normal tissue planes, benign processes which can mimic neoplasm, intrapelvic extension of malignancy and adenopathy.     

Assessment of MRI issues for the Argus II retinal prosthesis

Objective

The objective was to evaluate magnetic resonance imaging (MRI) issues (magnetic field interactions, heating, artifacts and functional alterations) at 1.5 T and 3 T for the Argus II Retinal Prosthesis (Second Sight Medical Products, Sylmar, CA, USA).

Materials and Methods

Standardized protocols were used to assess magnetic field interactions (translational attraction and torque; 3 T, worst case), MRI-related heating (1.5 and 3 T), artifacts (3 T; worst case) and functional changes (1.5 and 3 T) associated with MRI.

Results

The magnetic field interactions were acceptable. MRI-related heating, which was studied at a relatively high, MR system-reported whole body averaged specific absorption rates, will not pose a hazard to the patient under the conditions used for testing. While artifacts were “moderate” in relation to the dimensions of the Argus II Retinal Prosthesis, optimization of MRI parameters can reduce the size of the artifacts. Exposures to MRI conditions at 1.5 and 3 T did not damage or alter the functional aspects of the Argus II Retinal Prosthesis.

Conclusions

In consideration of the test results, a patient with the Argus II Retinal Prosthesis may undergo MRI at 1.5 T or 3 T when specific guidelines and MRI conditions are followed, including those advised by the manufacturer.     

Computer aided measurement of melanoma depth of invasion in microscopic images

This paper presents a novel computer aided technique for measurement of melanoma depth of invasion. Melanoma is the deadliest form of skin cancer with worldwide increasing incidences. For a conclusive diagnosis of melanoma, skin biopsies should be examined under a microscope. Visual inspection of microscopic samples is often subjective, time-consuming, cumbersome and prone to human errors. This fact demonstrates the necessity of developing an automated method which assists pathologists in evaluating histopathological samples more accurately in the busy clinical environment. To the best of our knowledge, this is the first time that a computer-assisted diagnosis algorithm has been applied in measurement of melanoma invasion depth. The proposed method uses a clustering algorithm for granular layer extraction and a pre-trained SVM classifier for detection of malignant melanocytes. The experimental results with average error of 3.9 μm demonstrate that the proposed method is reliable and effective.     

Primary lymphoma of the cervix: MRI findings with gadolinium

MRI evaluation of primary cervical lymphoma has not been reported. We report such a case of primary cervical lymphoma, a lesion well seen and well delineated from normal tissue by MRI. Although primary lymphoma of the cervix is a rare entity, the disease does exist and can be well demonstrated by MRI. We evaluated the MR appearance of this lesion with both nonenhanced and gadolinium-enhanced imaging.     

Reproducibility and biases in high field brain diffusion MRI: An evaluation of acquisition and analysis variables

Diffusion tensor imaging (DTI) of in-vivo human brain provides insights into white matter anatomical connectivity, but little is known about measurement difference biases and reliability of data obtained with last generation high field scanners (> 3 T) as function of MRI acquisition and analyses variables. Here we assess the impact of acquisition (voxel size: 1.8 × 1.8 × 1.8, 2 × 2 × 2 and 2.5 × 2.5 × 2.5 mm³, b-value: 700, 1000 and 1300 s/mm²) and analysis variables (within-session averaging and co-registration methods) on biases and test-retest reproducibility of some common tensor derived quantities like fractional anisotropy (FA), mean diffusivity (MD), axial and radial diffusivity in a group of healthy subjects at 4 T in three regions: arcuate fasciculus, corpus callosum and cingulum. Averaging effects are also evaluated on a full-brain voxel based approach. The main results are: i) group FA and MD reproducibility errors across scan sessions are on average double of those found in within-session repetitions (≈ 1.3 %), regardless of acquisition protocol and region; ii) within-session averaging of two DTI acquisitions does not improve reproducibility of any of the quantities across sessions at the group level, regardless of acquisition protocol; iii) increasing voxel size biased MD, axial and radial diffusivities to higher values and FA to lower values; iv) increasing b-value biased all quantities to lower values, axial diffusivity showing the strongest effects; v) the two co-registration methods evaluated gave similar bias and reproducibility results. Altogether these results show that reproducibility of FA and MD is comparable to that found at lower fields, not significantly dependent on pre-processing and acquisition protocol manipulations, but that the specific choice of acquisition parameters can significantly bias the group measures of FA, MD, axial and radial diffusivities.     

MRI assessment of drug-induced fluid accumulation in humans: validation of the technology

PURPOSE: The purpose of this study was to evaluate the feasibility of using proton and sodium magnetic resonance imaging (MRI) to detect fluid accumulation produced by fludrocortisone and nifedipine - two drugs known to cause salt/water retention by different mechanisms. MATERIALS AND METHODS: Twelve young healthy male subjects were randomly assigned to one of two groups and treated with either fludrocortisone or nifedipine for 14 or 25 days, respectively. The change in sodium MRI, as well as in proton T(2) value and T(1)-weighted signal intensity in the calf following postural change [referred to here as 'postural delta signal'(PDS)], was evaluated before, during and after drug administration. The changes in MRI PDS were compared to conventional physiological parameters, including body weight, calf volume and pitting edema. RESULTS: When compared to the baseline pretreatment values, the subjects treated with fludrocortisone showed a 5.5% increase in sodium MRI PDS (P=.01), a 2-ms increase in proton T(2) PDS of the gastrocnemius muscle (P=.06) and a body weight gain of 2.3% (P=.001) within 1 week. In the nifedipine-treated subjects, the sodium MRI PDS increased by 6% versus baseline (P=.03), while the proton T(2) PDS of the gastrocnemius muscle increased by 3.7 ms (P=.01), associated with a 0.5% weight gain (P=.55), within 3 weeks. No significant changes were noted in the T(1)-weighed images following postural change. Measurements of calf circumference, volume and pitting edema did not show consistent changes associated with the drug administration. CONCLUSION: The postural change in sodium MRI and proton T(2) signals provides a sensitive method for detecting the fluid accumulation produced by fludrocortisone and nifedipine. The MRI results are consistent with treatment-induced increases in extracellular fluid volume and correlate well with the observed weight gain. These findings support the potential utility of MRI for the evaluation of medication-induced fluid retention.     

Statistical methods for detecting activated regions in functional MRI of the brain

Two statistical tests for detecting activated pixels in functional MRI (fMRI) data are presented. The first test (t-test) is the optimal solution to the problem of detecting a known activation signal in Gaussian white noise. The results of this test are shown to be equivalent to the cross-correlation method that is widely used for activation detection in fMRI. The second test (F test) is the optimal solution when the measured data are modeled to consist of an unknown activation signal that lies in a known lower dimensional subspace of the measurement space with added Gaussian white noise. A model for the signal subspace based on a truncated trigonometric Fourier series is proposed for periodic activation–baseline imaging paradigms. The advantage of the second method is that it does not assume any information about the shape or delay of the activation signal, except that it is periodic with the same period as the activation–baseline pattern. The two models are applied to experimental echo-planar fMRI data sets and the results are compared.