A desktop magnetic resonance imaging system

Modern magnetic resonance imaging (MRI) systems consist of several complex, high cost subsystems. The cost and complexity of these systems often makes them impractical for use as routine laboratory instruments, limiting their use to hospitals and dedicated laboratories. However, advances in the consumer electronics industry have led to the widespread availability of inexpensive radio-frequency integrated circuits with exceptional abilities. We have developed a small, low-cost MR system derived from these new components. When combined with inexpensive desktop magnets, this type of MR scanner has the promise of becoming standard laboratory equipment for both research and education. This paper describes the development of a prototype desktop MR scanner utilizing a 0.21 T permanent magnet with an imaging region of approximately 2 cm diameter. The system uses commercially available components where possible and is programmed in LabVIEW software. Results from 3D data sets of resolution phantoms and fixed, newborn mice demonstrate the capability of this system to obtain useful images from a system constructed for approximately $13 500.     

Dynamic magnetic resonance imaging of tumor perfusion

This work presents approaches and biomedical challenges of dynamic contrast enhanced magnetic resonance imaging (DCE-MRI). DCE-MRI using small molecular weight gadolinium chelates enables noninvasive imaging characterization of tissue vascularity. Depending on the technique used, data reflecting tissue perfusion (blood flow, blood volume, mean transit time), microvessel permeability surface area product, and extracellular leakage space can be obtained. Insights into these physiological processes can be obtained from inspection of kinetic enhancement curves or by the application of complex compartmental modeling techniques. Potential clinical applications include screening for malignant disease, lesion characterization, monitoring lesion response to treatment, and assessment of residual disease. Newer applications include prognostication, pharmacodynamic assessments of antivascular anticancer drugs, and predicting efficacy of treatment. For dynamic MRI to enter into widespread clinical practice, it will be necessary to develop standardized approaches to measurement and robust analysis approaches. These include the need for commercial equipment manufacturers to provide robust methods for rapidly measuring time-varying change in T1 relaxation rates, incorporation of arterial input function into kinetic modeling processes, robust analysis software that allows input from a variety of MRI devices, and validated statistical tools for the evaluation of heterogeneity.     

Distributed large-scale simulation of magnetic resonance imaging

The concept and the implementation of a parallelized and spin-based simulator for magnetic resonance (MR) imaging is presented. The dynamics of magnetization are modeled using the Bloch equation covering arbitrary radiofrequency (RF) pulses, gradients, main-field inhomogeneity, and relaxation. A temporal decomposition of a given sequence is introduced, leaning to basic sequence elements called atoms. A concept of spatial sampling of the object by spins is proposed, in the course of which Shannon's sampling theorem must be respected. In biomedical MR imaging, spins can be modeled as noninteracting entities, permitting an efficient parallelization of the simulation. The simulator ParSpin was implemented on a heterogeneous, interconnected cluster of workstations based on existing message passing libraries. The communication overhead has been kept moderately small. The aggregate computing performance of many processors enables the research into very complex problems (e.g., three-dimensional or steady-state MR experiments requiring up to 10⁶ spins). Additionally, ParSpin allows a comprehensive visualization for educational purposes.     

Current status and future technical advances of ultrasonic imaging

Discusses current ultrasound technology in routine clinical practice, state-of-the-art technology in advanced practice, emerging technologies, and challenges in further developing the technologies. It is concluded that ultrasound imaging is an essential component in modern medical process. It is complementary to other imaging modalities; often, it is the best or even the only applicable method     

Adapted waveform encoding for magnetic resonance imaging

Magnetic resonance imaging is a very flexible modality; the images produced depend critically on the values of many parameters. We have presented adapted waveform encoding as a tool for the intelligent utilization of this flexibility in several specific imaging tasks. We have seen that waveform encoding can be implemented with existing scanners, and have analyzed performance with regard to several figures of merit. We noted several performance advantages which may be attributed to the edge localization properties of the wavelet bases that can be employed in this encoding technique. Finally, a particular instance of the technique was presented in which the encoding basis was chosen to reflect statistical regularities in a particular class of standard diagnostic studies. This basis captures most of the variability of the class in the first basis elements. This can be exploited for reduction of imaging times and for progressive imaging     

A desktop magnetic resonance imaging system.

Modern magnetic resonance imaging (MRI) systems consist of several complex, high cost subsystems. The cost and complexity of these systems often makes them impractical for use as routine laboratory instruments, limiting their use to hospitals and dedicated laboratories. However, advances in the consumer electronics industry have led to the widespread availability of inexpensive radio-frequency integrated circuits with exceptional abilities. We have developed a small, low-cost MR system derived from these new components. When combined with inexpensive desktop magnets, this type of MR scanner has the promise of becoming standard laboratory equipment for both research and education. This paper describes the development of a prototype desktop MR scanner utilizing a 0.21 T permanent magnet with an imaging region of approximately 2 cm diameter. The system uses commercially available components where possible and is programmed in LabVIEW software. Results from 3D data sets of resolution phantoms and fixed, newborn mice demonstrate the capability of this system to obtain useful images from a system constructed for approximately $13,500.     

Simultaneous image acquisition in magnetic resonance imaging

A simple multiplexing technique, implemented on a conventional NMR spectrometer is described. It shows that simultaneous acquisition of independent NMR signals with a unique detection chain is possible. Application is performed to proton imaging of objects in two non-interacting antennae. Address for correspondence: Laboratoire de Résonance Magnétique Nucleaire, Batiment 721, Université Claude Bernard Lyon I, 43, Boulevard du 11 Novembre 1918, 69622 Villeurbanne Cedex - France     

Micro- to macroscale magnetic resonance imaging of glioma

Magnetic Resonance Materials in Physics, Biology and Medicine -     

Application of magnetic resonance imaging in reptile medicine

Clinical examinations of reptiles are physically limited and therefore usually have to be complemented by other methods. This is especially true for Chelonians. A modern imaging technique like magnetic resonance imaging is well suited for this purpose. Its application and practical experiences with tortoises are presented.     

Non-breath-hold lung magnetic resonance imaging with real-time navigation

Magnetic resonance imaging (MRI) with navigating techniques based on consecutive breath-holds demand a level of respiratory control that is often beyond the capability of patients with lung disease. The objectives of this investigation were to develop and evaluate a navigating technique for lung MRI that does not rely on patient cooperation. Navigating techniques were implemented at 0.5 T using conventional imaging techniques of short echo-time and imaging during normal breathing in the diastolic phase of the cardiac cycle. A column of spins, orthogonal to the diaphragm, was excited both immediately before and after the imaging segment. These signals were processed in real time to provide the position of the lung-diaphragm interface. An imaging segment was considered correctly acquired only when the interface position was within the acceptance window both before and after the acquisition of the segment. A distribution of lung-diaphragm interface positions obtained during normal respiration was employed to define the acceptance window. In the case of multislice techniques, the position of the lung-diaphragm interface immediately before the imaging segment was also employed to decide which phase-encoding step to acquire next, therefore reducting the apparent frequency of the respiratory motion. A distribution of interface position, updated in real time, served as a reference for the allocation of phase-encoding steps according to diaphragm position. The lung images obtained represent a significant advance in image quality, improving further the ability of MR to detect and monitor pulmonary disease. Motion artifacts were reduced, and images reliably demonstrated smaller vessels, which are not normally visible without navigation.     

ViP MRI: virtual phantom magnetic resonance imaging

Object

The ability to generate reference signals is of great benefit for quantitation of the magnetic resonance (MR) signal. The aim of the present study was to implement a dedicated experimental set-up to generate MR images of virtual phantoms.

Materials and methods

Virtual phantoms of a given shape and signal intensity were designed and the k-space representation was generated. A waveform generator converted the k-space lines into a radiofrequency (RF) signal that was transmitted to the MR scanner bore by a dedicated RF coil. The k-space lines of the virtual phantom were played line-by-line in synchronization with the magnetic resonance imaging data acquisition.

Results

Virtual phantoms of complex patterns were reproduced well in MR images without the presence of artifacts. Time-series measurements showed a coefficient of variation below 1 % for the signal intensity of the virtual phantoms. An excellent linearity (coefficient of determination r ² = 0.997 as assessed by linear regression) was observed in the signal intensity of virtual phantoms.

Conclusion

Virtual phantoms represent an attractive alternative to physical phantoms for providing a reference signal. MR images of virtual phantoms were here generated using a stand-alone, independent unit that can be employed with MR scanners from different vendors.     

Nuclear magnetic resonance imaging of electrical trees in PE

Test samples of LDPE (low-density polyethylene) were electrically aged until PD (partial discharge) and electrical treeing occurred. Investigations on the spatial distribution of microscopic domain sizes in the lamellar morphology of the samples were carried out applying a novel method of proton NMR imaging which probes magnetization transfer by spin diffusion. Spatial changes of LDPE morphology are detected due to the appearance of PD     

Functional magnetic resonance imaging of real and sham acupuncture

Functional magnetic resonance imaging (fMRI) is an ideal method to noninvasively explore the whole brain for cortical activation related to acupuncture. The results from this study demonstrate that fMRI can identify specific cortical regions associated with acupuncture stimulation. The choice of acupuncture stimulation using visual or auditory points simplifies the interpretation of the results because the anatomic correlates are well defined. The magnitude and extent of the activations were different for the auditory and the visual stimulation; this may be due to the relative potency of the acupuncture points used, the overall imbalance associated with these systems, and the interpretation of the acupuncture point by the acupuncturist. Nevertheless, these data reflect significant blood-flow changes within the appropriate regions of the brain in response to stimulation of acupuncture points in the lower leg. With the appropriate experimental design, a clearer understanding of the neural substrate associated with acupuncture is likely to provide considerable insights into acupuncture treatment. This type of scientific approach to investigating and validating alternative medicine techniques is critical for the widespread acceptance of these methods.     

Clinical magnetic resonance imaging study of focal cerebral ischemia

Of the 47 patients with focal cerebral ischemia (FCI), 26 patients had completed stroke, 5 suffered from severe head injury, and 16 patients had anterior circulation aneurysms. Twenty-one stroke patients underwent bypass surgery and all traumatic patients underwent cerebral revascularization by omental graft. Aneurysm patients were surgically treated by neck clipping. In seven of them, aneurysm were occluded by nonferromagnetic (NFM) Ni-Ti alloy clips. In each case, comparative magnetic resonance imaging (MRI), computerized tomography (CT), and angiography (AG) findings were evaluated both before and after operation. MRI was much more sensitive than CT for detecting the ischemic tissue in all groups of patients. The difference in infarct area appreciated by MRI and CT in stroke patients turned out to be determined by the time between the onset of ictus and examination. MRI was particularly valuable for following up patients with FCI after cerebral revascularization by bypass surgery and omental graft. MRI was capable of replacing both postoperative AG and CT in patients operated on with NFM clips.     

Automatic prostate segmentation of magnetic resonance imaging using Res-Net

Magnetic Resonance Materials in Physics, Biology and Medicine - Segmenting the prostate from magnetic resonance images plays an important role in prostate cancer diagnosis and in evaluating the...     

High-resolution venography of the brain using magnetic resonance imaging

The purpose of this study was to evaluate a non-flow related magnetic resonance imaging method to visualize small veins independent of arteries in the human brain. A long TE, high-resolution 3D gradient echo MR acquisition was used to highlight venous information. The method is based on the paramagnetic property of deoxyhemoglobin and the resulting phase difference between veins and brain parenchyma at long echo times. The MR magnitude images were masked with a phase mask filter to enhance small structure visibility.. Venous information down to sub-pixel vessel diameters of several hundred microns is visible. Venous data are displayed in an angiographic manner using a minimum intensity projection algorithm. Both superficial veins and deep white matter veins are visible. The method has been successfully applied in volunteers. Preliminary results in patients with cerebral arteriovenous malformations indicate its potential in clinical applications. The proposed method is easy to implement and does not require administration of a contrast agent or application of specially designed rf pulses to highlight the veins. Rather it exploits the intrinsic magnetic properties (BOLD-effect) and the prolonged T ^_2* of venous blood. The method may be of diagnostic potential in the assessment of arteriovenous malformations or other vascular venous lesions. © 1998 Elsevier Science B.V. All rights reserved.     

Artefacts in magnetic resonance imaging caused by dental material

A common problem in computer tomography (CT) based imaging of the oral cavity is artefacts caused by dental restorations. The aim of this study was to investigate whether magnetic resonance imaging (MRI) of the oral cavity would be less affected than CT by artefacts caused by typical dental restorative alloys. In order to assess the extent of artefact generation, corresponding MRI scans of the same anatomic region with and without dental metal restorations were matched using a stereotactic frame. MRI imaging of the oral and maxillofacial region could be performed without reduction of the image quality by metallic dental restorations made from titanium, gold or amalgam. Dental restorations made from titanium, gold or amalgam did not reduce the image quality of the MRI sequence used in imaging of the oral and maxillofacial region for dental implant planning. In this respect MRI is superior to CT in implant planning.     

Towards active guidewire visualization in interventional magnetic resonance imaging

Improving the visibility of interventional devices is of paramount importance if MRI-guided fluoroscopy is to become a reality. Passive visualization is problematic in that the susceptibility-induced artifacts are material- and orientation-dependent. Here a concept is presented for making interventional devices visible. It involves fitting a device with a straight-wire antenna. As the sensitivity of such an antenna is highest for signal sources in the immediate neighborhood, using the antenna for reception gives an outline image. In this manner a guidewire or other interventional device could be made MRI-visible. The image appearance of a straight-wire antenna depends on the orientation of the device with respect to the main magnetic field and imaging plane. This phenomena is discussed theoretically and documented with MR images.     

Semi-quantitative approach to estimating GFR by magnetic resonance imaging

Objective: The purpose of this study was to compare a semi-quantitative approach to estimating glomerular filtration rate (GFR) by magnetic resonance imaging with radionuclide calculation of GFR, and to investigate whether spin echo or gradient echo is more suitable for estimating GFR.Methods and patients: Fourteen kidneys of seven patients (GFR ranging from 26 to 57 ml/min) were studied. Spin echo and gradient echo sequences interleaving each other at every excitation were used. After intravenous injection of gadopentetate dimeglumine, serial scans were performed. The signal intensities measured in the regions of interest were converted to time-transverse relaxation rate changes for both spin echo (ΔR2) and gradient echo (ΔR2^*). The areas under the time-ΔR2 and time-ΔR2^* curves were calculated as a semi-quantitative index of GFR for both spin echo and gradient echo images, and the results were compared by GFR measured by radionuclide imaging.Results: The semi-quantitative index of the GFR calculated from spin echo images showed a significant correlation with the GFR measured by radionuclide imaging (r=0.85,P<0.001). On the other hand, the semi-quantitative index of the GFR calculated from gradient echo images showed no such correlation (r=0.46,P=0.10).Conclusion: Spin echo sequences may be more suitable than gradient echo sequences for the evaluation of GFR.     

Real-time direct volume rendering in functional magnetic resonance imaging

Direct volume rendering is a visualization method that allows display of all information hidden in three-dimensional data sets of, for example, computed tomography or magnetic resonance imaging (MRI). In contrast to commonly used surface rendering methods, these algorithms need no preprocessing but suffer from a high computational complexity. A real-time rendering system, VIRIM (Vitec: Visualization Technology GmbH, Mannheim, Germany), cuts down rendering times of minutes on normal workstations to an interactive rate of 1 second or less. The immediate visual feedback allows interactive steering of the visualization process to achieve insight into the internal three-dimensional structure of objects. Additional information is obtained by using an interactive gray-value segmentation tool that both allows segmentation of the data set according to bone, tissue, and liquor and display of multifunctional data sets (e.g., functional MRI [fMRI] data sets). Thus, real-time direct volume rendering allows segmentation and volume data processing of functional and anatomical MR data sets simultaneously. As this method can be integrated in the clinical routine, it is of great importance for real-time motion artifact detection and the interpretation of fMRI data acquired during cognitive experiments with normal subjects and psychiatric patients. Because of the free programmability of VIRIM, more complex matching procedures are currently being investigated for future implementation.