Unwrapping magnetic resonance phase maps with Chebyshev polynomials

A phase-unwrapping algorithm, based on the method of moments, is introduced in this work. The proposed algorithm expands the phase map in terms of a two-dimensional Chebyshev series. The expansion coefficients are calculated by exploiting the orthogonality of Chebyshev polynomials of the first kind. The performance of the proposed phase-unwrapping algorithm is tested on a synthetic phase map and experimental phase maps of a uniform phantom, a human brain and a mouse torso, all acquired from 3-T magnetic resonance (MR) scanners. To impose additional burdens on the algorithm, we introduced magnetic field inhomogeneities to the phantom and human brain data by an external gradient coil. The proposed phase-unwrapping algorithm is found to perform well on the phantom data set in a low signal-to-noise ratio (SNR) environment and on the synthetic data set. The proposed algorithm is also found to perform well in in vivo data sets of the human brain and mouse torso. Results obtained from the in vivo MR data sets show that the proposed algorithm produced unwrapped phase maps that are nearly identical to those produced by a widely used phase-unwrapping algorithm, PRELUDE 2D in the fMRI Software Library.     

Multicontext wavelet-based thresholding segmentation of brain tissues in magnetic resonance images

A novel segmentation method based on wavelet transform is presented for gray matter, white matter and cerebrospinal fluid in thin-sliced single-channel brain magnetic resonance (MR) scans. On the basis of the local image model, multicontext wavelet-based thresholding segmentation (MCWT) is proposed to classify 2D MR data into tissues automatically. In MCWT, the wavelet multiscale transform of local image gray histogram is done, and the gray threshold is gradually revealed from large-scale to small-scale coefficients. Image segmentation is independently performed in each local image to calculate the degree of membership of a pixel to each tissue class. Finally, a strategy is adopted to integrate the intersected outcomes from different local images. The result of the experiment indicates that MCWT outperforms other traditional segmentation methods in classifying brain MR images.     

Artifacts in magnetic resonance imaging after surgical resection of brain tumors

Background

Since the advent of magnetic resonance imaging, metal artifacts have posed an important diagnostic problem in different fields of medicine. However, this has not been systematically studied in patients undergoing surgery for brain tumors.

Objective

This study was planned to assess whether metal artifacts can occur in patients undergoing brain surgery without metallic implants.

Methods

Of 40 individuals who could be included because of having a pre- and postoperative MRI and a postoperative computed tomography (CT) scan or a conventional skull X-ray for the detection of metallic artifacts, 26 patients agreed to participate in this study and gave informed consent.

Results

Twenty-six subjects, 12 males and 14 females, with an age range of 12 to 54 years, were included in the study. Four patients were found to have gross metal particles in their postoperative brain CTs and were excluded. Of the remaining 22 subjects, 7 patients (31.8%) had metallic artifacts.

Conclusion

Our study showed that simple bone drilling or chiseling during surgical manipulation of skull bones may result in separation of very tiny metal particles which can remain in the surgical site and cause artifacts in postoperative MRIs. This finding appeared to be independent of factors such as age, sex, tumor/incision site, tumor size, pathologic tumor type, total radiation dose, operation–MRI time interval and sequence of MRI.     

Detection of toxoplasmic lesions in mouse brain by USPIO-enhanced magnetic resonance imaging

The objective of this study was to examine the feasibility of detecting toxoplasmic brain lesions in a mouse model of cerebral toxoplasmosis by ultrasmall superparamagnetic particles of iron oxide (USPIO)-enhanced magnetic resonance imaging (MRI). Toxoplasmosis encephalitis was induced in Kunming mice by intracerebral injection of Toxoplasma gondii tachyzoites. T₂- and T₂*-weighted MRI was performed 1, 3, 4, 5 and 6 days after infection before USPIO injection; immediately after USPIO injection; and 24 h later. A comparison of USPIO enhancement and Gd-DTPA enhancement was made in three toxoplasmic mice 4 days after infection. Hematoxylin and eosin staining and Prussian blue staining were performed to detect inflammatory reactions and presence of iron in and around the toxoplasmic brain lesions. Nonenhanced T₂-/T₂*-weighted imaging detected few abnormalities in the brain up to 5 days. Most mice developed prominent hydrocephalus at 6 days. Gd-DTPA-enhanced imaging showed prominent enhancement of the cerebral ventricles but revealed only few space-occupying lesions in the parenchyma. USPIO-enhanced T₂*-weighted imaging showed improved detection of toxoplasmic brain lesions that were invisible to nonenhanced T₂-/T₂*-weighted imaging and gadolinium-enhanced imaging. Most of the enhancing lesions showed nodular enhancement immediately after USPIO injection, some of which changed appearance 24 h later, having a ring enhancement at the outer rim. It can be concluded that USPIO enhancement of the toxoplasmic lesions may reflect blood–brain barrier impairment and/or inflammatory reactions associated with these lesions. USPIO-enhanced imaging may be used in combination with gadolinium-enhanced imaging to provide better characterization of toxoplasmic brain lesions and, potentially, improve the differential diagnosis of toxoplasmosis encephalitis.     

A simple and accurate relative alternative magnetic susceptibility measurement technique

Investigation of relative AC magnetic susceptibility interests for many magnetic transition studies such as superconductor transition. A technique based on mutual or self inductive measure provides a fast and relatively easy (no contact) way to determinate the temperature of any transition affecting the magnetic susceptibility. The half Wheatstone inductive/resistive bridge is used instead of the usual RLC quarter bridge in order to balance the bore inductance of the coil. A comparison between quarter and half bridge measurements illustrates the accuracy of our device.     

Magnetic nanostructures as amplifiers of transverse fields in magnetic resonance

We introduce the concept of amplifying the transverse magnetic fields produced and/or detected with inductive coils in magnetic resonance settings by using the reversible transverse susceptibility properties of magnetic nanostructures. First, we describe the theoretical formalism of magnetic flux amplification through the coil in the presence of a large perpendicular DC magnetic field (typical of magnetic resonance systems) achieved through the singularity in the reversible transverse susceptibility in anisotropic single domain magnetic nanoparticles. We experimentally demonstrate the concept of transverse magnetic flux amplification in an inductive coil system using oriented nanoparticles with uni-axial magnetic anisotropy. We also propose a composite ferromagnetic/anti-ferromagnetic core/shell nanostructure system with uni-directional magnetic anisotropy that, in principle, provides maximal transverse magnetic flux amplification.     

Three-dimensional magnetic resonance microscopy of materials

Several aspects of magnetic resonance microscopy are examined employing three-dimensional (3D) back-projection reconstruction techniques in combination with either simple Bloch-decay methods or MREV-8 multiple-pulse line narrowing techniques in the presence of static field gradients. Applications to the areas of ceramic processing, catalyst porosity measurements and the characterization of polymeric materials are presented. The focus of the discussion centers on issues of sensitivity and resolution using this approach compared with other methods. Advantages and limitations of 3D microscopy over more commonly employed slice selection protocols are discussed, as well as potential remedies to some of the inherent limitations of the technique.     

Assessment of mitral regurgitation by magnetic resonance imaging

To evaluate the potential of magnetic resonance imaging (MRI) in detection and quantification of mitral regurgitation, 26 pts. with echocardiographically or angiographically documented mitral regurgitation were examined using a 0.5 Tesla superconducting magnet. In each patient a multislice-multiphase study in a sagittal-coronal double angulated projection (four-chamber view equivalent) was performed to assess left and right ventricular volumes, ejection fraction and regurgitant fraction. Additionally a blood flow sensitive cine-study (fast field echo: FFE) was done to visualize direction and area of regurgitant jet. MRI data were compared with quantitative and qualitative assessment of mitral regurgitation by angiography, 2D echocardiography, Doppler sonography and color flow mapping. Using the FFE mode MRI was able to detect the regurgitant jet as a typical signal loss within the left atrium in all patients. The ratio of regurgitant jet area/left atrium area as determined by MRI showed a correlation with a comparable ratio from color Doppler sonography of R = 0.87 (p < 0.001). There was also good agreement in semiquantitative grading of mitral regurgitation between MRI and angiography (R = 0.77, p < 0.001). The determination of left and right ventricular stroke volume allowed the calculation of the regurgitant fraction, which showed a correlation with invasively determined regurgitation fraction of R = 0.84 (p < 0.001). These data provide additional information that MRI may be useful as a noninvasive technique to detect and quantify mitral regurgitation.     

Entropy changes accompanying the magnetic phase transitions in low Si-doped Ce2Fe17−xSix Alloy

A summary of the results of ac susceptibility and isothermal magnetization measurements on polycrystalline samples of Ce₂Fe_17−xSi_x with nominal composition of x=0.0, 0.1, 0.2 and 1.0 is presented. These data reveal that the substitution of small amounts of Si for Fe produce a significant increase in temperature at which ferromagnetism appears, to the extent that, at x=1, characteristics of the anti ferromagnetic to paramagnetic transition (at temperature T_N) have disappeared completely. The nature of the various magnetic phase transitions — identified through the use of Arrott plots — and the accompanying magnetic entropy change, ΔSm, are both affected significantly by small amounts of Si substitution. In particular, while the peak entropy change is modest (occurring at x=0.1), the temperature interval over which a substantial entropy change occures is significant, approaching 150 K, an important criterion for improving the overall effectiveness of such materials for magnetic refrigeration.     

Automated segmentation of lateral ventricles from human and primate magnetic resonance images using cognition network technology

Automatic segmentation of different types of tissue from magnetic resonance images is of great importance for clinical and research applications, particularly large-scale and longitudinal studies of brain pathology. We developed a fully automated algorithm for the segmentation of lateral ventricles from cranial magnetic resonance images. This problem is of interest in the study of schizophrenia, dementia and other neuropsychiatric disorders. Our algorithm achieves comparable results to expert human raters. The theoretical approach, which is based on an emerging object-oriented technology that has been adapted and evaluated to process 3D data for the first time, may, in the future, be transferred to other important problems of magnetic resonance image analysis like gray/white matter segmentation.     

Three dimensional magnetic resonance imaging by magnetic resonance force microscopy with a sharp magnetic needle

An electropolished magnetic needle made of Nd(2)Fe(14)B permanent magnet was used for obtaining better spatial resolution than that achieved in our previous work. We observed the magnetic field gradient |G(Z)|=80.0G/microm and the field strength B=1250G at Z approximately 8.8 microm from the top of the needle. The use of this needle for three dimensional magnetic resonance force microscopy at room temperature allowed us to achieve the voxel resolution to be 0.6 microm x 0.6 microm x 0.7 microm in the reconstructed image of DPPH phantom. The acquisition time spent for the whole data collection over 64 x 64 x 16 points, including an iterative signal average by six times per point, was about 10 days.     

Wavelet-based characterization of vertebral trabecular bone structure from magnetic resonance images at 3 T compared with micro-computed tomographic measurements

Trabecular bone structure and bone density contribute to the strength of bone and are important in the study of osteoporosis. Wavelets are a powerful tool in characterizing and quantifying texture in an image. The purpose of this study was to validate wavelets as a tool in computing trabecular bone thickness directly from gray-level images. To this end, eight cylindrical cores of vertebral trabecular bone were imaged using 3-T magnetic resonance imaging (MRI) and micro-computed tomography (microCT). Thickness measurements of the trabecular bone from the wavelet-based analysis were compared with standard 2D structural parameters analogous to bone histomorphometry (MR images) and direct 3D distance transformation methods (microCT images). Additionally, bone volume fraction was determined using each method. The average difference in trabecular thickness between the wavelet and standard methods was less than the size of 1 pixel size for both MRI and microCT analysis. A correlation (R) of .94 for microCT measurements and that of .52 for MRI were found for the bone volume fraction. Based on these results, we conclude that wavelet-based methods deliver results comparable with those from established MR histomorphometric measurements. Because the wavelet transform is more robust with respect to image noise and operates directly on gray-level images, it could be a powerful tool for computing structural bone parameters from MR images acquired using high resolution and thus limited signal scenarios.     

Stray field magnetic resonance tomography using ferromagnetic spheres

The methodology for obtaining two- and three-dimensional magnetic resonance images by using azimuthally symmetric dipolar magnetic fields from ferromagnetic spheres is described. We utilize the symmetric property of a geometric sphere in the presence of a large externally applied magnetic field to demonstrate that a complete two- or three-dimensional structured rendering of a sample can be obtained without the motion of the sample relative to the sphere. Sequential positioning of the integrated sample-sphere system in an external magnetic field at various angular orientations provides all the required imaging slices for successful computerized tomographic image reconstruction. The elimination of the requirement to scan the sample relative to the ferromagnetic tip in this imaging protocol is a potentially valuable simplification compared to previous scanning probe magnetic resonance imaging proposals.     

Semiautomated detection of cerebral microbleeds in magnetic resonance images

Cerebral microbleeds (CMBs) are increasingly being recognized as an important biomarker for neurovascular diseases. So far, all attempts to count and quantify them have relied on manual methods that are time-consuming and can be inconsistent. A technique is presented that semiautomatically identifies CMBs in susceptibility weighted images (SWI). This will both reduce the processing time and increase the consistency over manual methods. This technique relies on a statistical thresholding algorithm to identify hypointensities within the image. A support vector machine (SVM) supervised learning classifier is then used to separate true CMB from other marked hypointensities. The classifier relies on identifying features such as shape and signal intensity to identify true CMBs. The results from the automated section are then subject to manual review to remove false-positives. This technique is able to achieve a sensitivity of 81.7% compared with the gold standard of manual review and consensus by multiple reviewers. In subjects with many CMBs, this presents a faster alternative to current manual techniques at the cost of some lost sensitivity.     

Research and progress in magnetic resonance imaging of triple-negative breast cancer

Triple-negative breast cancer (TNBC), which characterized by distinct biological and clinical pathological features, has a worse prognosis because the lack of effective therapeutic targets. Breast MR is the most accurate imaging modality for diagnosis of breast cancer currently. MR imaging recognition could assist in diagnosis, pretreatment planning and prognosis evaluation of TNBC. MR findings of a larger solitary lesion, mass with smooth mass margin, high signal intensity on T₂-weighted images and rim enhancement are typical MRI features associated with TNBC. Further work is necessary about the clinical application of dynamic contrast-enhanced MR imaging (DCE-MRI), DWI and MRS.     

Phase contrast and time-of-flight magnetic resonance angiography of the intracerebral arteries at 1.5, 3 and 7 T

Purpose

Time-of-flight (ToF) and phase contrast (PC) magnetic resonance angiographies (MRAs) are noninvasive applications to depict the cerebral arteries. Both approaches can image the cerebral vasculature without the administration of intravenous contrast. Therefore, it is used in routine clinical evaluation of cerebrovascular diseases, e.g., aneurysm and arteriovenous malformations. However, subtle microvascular disease usually cannot be resolved with standard, clinical-field-strength MRA. The purpose of this study was to compare the ability of ToF and PC MRA to visualize the cerebral arteries at increasing field strengths.

Materials and Methods

The Institutional Review Board-approved study included eight healthy volunteers (age: 36±10 years; three female, five male). All subjects provided written informed consent. ToF and PC MRAs were obtained at 1.5, 3 and 7 T. Signal intensities of the large, primary vessels of the Circle of Willis were measured, and signal-to-noise ratios were calculated. Visualization of smaller first- and second-order branch arteries of the Circle of Willis was also evaluated.

Results

The results show that both ToF and PC MRAs allow the depiction of the large primary vessels of the Circle of Willis at all field strengths. Ultrahigh field (7 T) provides only small increases in the signal-to-noise ratio in these primary vessels due to the smaller voxel size acquired. However, ultrahigh-field MRA provides better visualization of the first- and second-order branch arteries with both ToF and PC approaches. Therefore, ultrahigh-field MRA may become an important tool in future neuroradiology research and clinical care.     

Single-point imaging with a variable phase encoding interval

A modified single-point imaging (SPI) technique using a variable phase encoding interval is proposed. This method is based on the minimization of the phase encoding interval for further signal-to-noise ratio (SNR) optimization. This is particularly beneficial when the maximum gradient amplitude limits an optimal phase encoding interval, and the resulting SNR suffers from T(2)-related signal attenuation. Theoretical calculation of the SNR and simulation of the point spread function (PSF) for the different experimental parameters are presented. Experiments using a rubber sample (T(2)* approximately 73 micros) and a tooth (bi-exponential relaxation: T(2,1)*=111 micros and T(2,1)*=872 micros) showed a significant increase in SNR (>3 and >2, respectively) when compared with images acquired with conventional SPI.     

Basic concepts for nuclear magnetic resonance imaging

The basic concepts necessary to understand the physical basis of NMR imaging are presented in this didactic article. It is intended as a starting point for the radiologist or medical physicist who is addressing the topic of NMR for the first time. The basis of the NMR phenomena is described with introduction of the concepts of magnetic moment, magnetic fields, magnetic resonance, net magnetic moment of a sample, NMR excitation and NMR emission. The equipment necessary to observe these NMR properties of matter is summarized as well as the procedures for basic pulsed NMR experiments. The physical concepts for spatial localization of NMR emissions are introduced with physical analogies to stringed musical instruments. Several alternative imaging modalities are compared with greatest emphasis on the inversion recovery technique which yields images weighted by tissue T₁ values. The six subsystems of an NMR imaging device (primary magnet, computer, radio equipment, magnetic gradient, data storage and display subsystems) are described in an overview fashion. The paper is followed by a series of study questions to test the reader's comprehension of basic NMR imaging concepts.