Comparison of sequences for depicting bone marrow alterations in osteomyelitis applied in a low field strength magnetic resonance imaging system

Objective/Patients: to investigate the efficacy of standard sequences of a low field system for the detection of osteomyelitis, we tested TlwI pre and post i.v. contrast, T2w and fat suppressed IR sequences. Design: on the basis of clinical and laboratory evidence, pathology reports, and three phase granulocyte scintigraphy, osteomyelitis was diagnosed in 18 of 21 patients with Charcot's joints. A consecutive low and high field magnetic resonance (MR) scan confirmed osteomyelitic bone marrow changes in the same osseous regions. These 18 diabetic patients were then studied on a 0.2 Tesla dedicated MR system (Esaote ArtoScan) using TlwI (SE: relaxation time (TR) 520/echo time (TE) 24: axial and coronal) before and after i.v. application of 0.1 mmol/1 Gd-DTPA/kg BW, T2w imaging (TSE: TR 3500/TE 80 or TR 2000/TE 120: axial), and fat suppressed inversion recovery (IR) imaging (short tau inversion recovery (STIR): TR 3000/TE 30/TI 80 or inversion recovery gradient echo (IRGE)/fat suppressed IRGE (GEFS): TR 1000/TE 16m 80: coronal). Results: the SE Tlw sequence showed a significantly higher contrast-to-noise ratio (CNR) before administration of i.v. contrast. The TSE T2w pulse sequence demonstrated bone marrow changes superiorily utilizing a TE of 120 ms (CNR=16.5±2.7 compared to 5.5±2.5 with TE=80 ms). The IRGE showed a higher CNR than the standard STIR (CNR=19.2±2.5 compared to 12.4±2.9). Conclusion: fat suppressed IRGE imaging and longer TE in T2w TSE sequences result in a significantly better, CNR in osteomyelitis. This way, using optimized sequences, low field systems are apt to depict bone marrow changes in the course of osteomyelitis.     

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.     

A novel selective-excitation RF pulse for magnetic resonance imaging

OBJECT: A selective-excitation radiofrequency (RF) pulse that uses hard pulses composed of a sequence of composite pulses with positive and negative phases (P/N pulse) is proposed herein. Because the amplitude of the RF signal is unchanged during the excitation, RF amplification can be accomplished using a nonlinear RF power amplifier (i.e., class C or D type). MATERIALS AND METHODS: In this article, Fourier series have been first used to analyze the equivalence between the proposed P/N pulse and the conventional soft pulse on selective excitation. Subsequently, computer simulations based on density-matrix theory are used to compare the excitation profiles of both the soft and the P/N pulses. RESULTS: The excitation profiles of the P/N pulses have been measured experimentally through a home-built 0.3-T magnetic resonance imaging (MRI) system. In addition, several slices of images have been obtained as proofs by using the multislice two-dimensional spin echo sequence through replacement of the conventional soft pulse by the proposed P/N pulse. CONCLUSION: Because the perfect selectivity of the proposed P/N pulse, it can be used for imaging studies to improve the efficiency of amplification at the lowest cost.     

Magnetic field gradient system for nuclear magnetic resonance microimaging

In this study we present an orthogonal magnetic field gradient system for nuclear magnetic resonance (NMR) microimaging applications. The construction details are given for a prototype assembly for proton microscopy inside a 50-mm vertical bore magnet, which is designed to fit into a commercial 300-MHz NMR probe. This system has been used to acquire images of the human spinal cordin vitro. Its performance has been evaluated and compared to that predicted by computer simulation.     

Automatic slice alignment method for cardiac magnetic resonance imaging

Objectives

Automatic slice alignment is important for easier operation and shorter examination times in cardiac magnetic resonance imaging (MRI) examinations. We propose a new automatic slice alignment method for six cardiac planes (short-axis, vertical long-axis, horizontal long-axis, 4-chamber, 2-chamber, and 3-chamber views).

Materials and methods

ECG-gated 2D steady-state free precession axial multislice images were acquired using a 1.5-T MRI scanner during a single breath-hold. The scanning time was set to <20 s in 23 volumes from 23 healthy volunteers. In this method, the positions of the mitral valve, cardiac apex, left ventricular outflow tract, tricuspid valve, anterior wall of the heart, and right ventricular corner are detected to determine the positions of six reference planes by combining knowledge-based recognition and image processing techniques. In order to evaluate the results of automatic slice alignment for the short-axis, 4-chamber, 2-chamber, and 3-chamber views, the angular and positional errors between the results obtained by our proposed method and by manual annotation were measured.

Results

The average angular errors for the short-axis, 4-chamber, 2-chamber, and 3-chamber views were 3.05°, 4.52°, 7.28°, and 5.79°, respectively. The average positional errors for the short-axis (base), short-axis (apex), 4-chamber, 2-chamber, and 3-chamber views were 6.61°, 3.80°, 1.55°, 1.52°, and 1.48°, respectively.

Conclusion

The experimental results showed that our proposed method can detect the cardiac planes quickly and accurately. Our method is therefore beneficial to both patients and operators.     

Comparison of calibration systems for magnetic field measurementequipment

The current ANSI/IEEE standard specifies two possible methods for magnetic meter calibration: a single square loop or a round Helmholtz coil. However it: (a) does not specify the size of the coil; and (b) does not provide a comparison of the field uniformities between the two suggested systems. In this paper, the z-directed magnetic field equations for four calibration system designs, the single round loop, the round Helmholtz coil, the single square loop, and the square Helmholtz coil are derived and presented     

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.     

Current technical development of magnetic resonance imaging

The impact of MRI continues to grow due to progress in all phases of the development cycle. Since its initial use for human imaging approximately 20 years ago, magnetic resonance imaging (MRI) has developed into a widely used clinical imaging modality. Now, at the start of the 21st century, the number of MRI systems worldwide is in excess of 10,800. With an average of over ten patients examined per day per machine, the number of clinical studies per day is well over 100,000. Along with X-ray imaging, ultrasound, computed X-ray tomography, and nuclear medicine, MRI is well recognized as a commonly used medical imaging modality. In spite of this significant growth over the last two decades, technical and application development continues. The purpose of this article is to identify the current development of MRI and to attempt to indicate future trends. In some sense this is an update of a similar technical assessment of MRI made four years ago     

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.     

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     

7 Tesla compatible in-bore display for functional magnetic resonance imaging

Background and methods

A liquid crystal display was modified for use inside a 7 T MR magnet. SNR measurements were performed using different imaging sequences with the monitor absent, present, or activated. fMRI with a volunteer was conducted using a visual stimulus.

Results and discussion

SNR was reduced by 3.7 %/7.9 % in echo planar/fast-spin echo images when the monitor was on which can be explained by the limited shielding of the coated front window (40 dB). In the fMRI experiments, activated regions in the visual cortex were clearly visible. The monitor provided excellent resolution at minor SNR reduction in EPI images, and is thus suitable for fMRI at ultra-high field.     

Measurement techniques for magnetic resonance imaging of fast relaxing nuclei

In this review article, techniques for sodium (²³Na) magnetic resonance imaging (MRI) are presented. These techniques can also be used to image other nuclei with short relaxation times (e.g., ³⁹K, ³⁵Cl, ¹⁷O). Twisted projection imaging, density-adapted 3D projection reconstruction, and 3D cones are preferred because of uniform k-space sampling and ultra-short echo times. Sampling density weighted apodization can be applied if intrinsic filtering is desired. This approach leads to an increased signal-to-noise ratio compared to postfiltered acquisition in cases of short readout durations relative to T ₂ ^* relaxation time. Different MR approaches for anisotropic resolution are presented, which are important for imaging of thin structures such as myocardium, cartilage, and skin. The third part of this review article describes different methods to put more weighting either on the intracellular or the extracellular sodium signal by means of contrast agents, relaxation-weighted imaging, or multiple-quantum filtering.     

Micro- to macroscale magnetic resonance imaging of glioma

Magnetic Resonance Materials in Physics, Biology and Medicine -     

Low-molecular-weight paramagnetic 19F contrast agents for fluorine magnetic resonance imaging

Objective

¹⁹F MRI requires biocompatible and non-toxic soluble contrast agents with high fluorine content and with suitable ¹⁹F relaxation times. Probes based on a DOTP chelate with 12 magnetically equivalent fluorine atoms (DOTP-tfe) and a lanthanide(III) ion shortening the relaxation times were prepared and tested.

Methods

Complexes of DOTP-tfe with trivalent paramagnetic Ce, Dy, Ho, Tm, and Yb ions were synthetized and characterized. ¹⁹F relaxation times were determined and compared to those of the La complex and of the empty ligand. In vitro and in vivo ¹⁹F MRI was performed at 4.7 T.

Results

¹⁹F relaxation times strongly depended on the chelated lanthanide(III) ion. T₁ ranged from 6.5 to 287 ms, T₂ from 3.9 to 124.4 ms, and T₂* from 1.1 to 3.1 ms. All complexes in combination with optimized sequences provided sufficient signal in vitro under conditions mimicking experiments in vivo (concentrations 1.25 mM, 15-min scanning time). As a proof of concept, two contrast agents were injected into the rat muscle; ¹⁹F MRI in vivo confirmed the in vivo applicability of the probe.

Conclusion

DOTP-based ¹⁹F probes showed suitable properties for in vitro and in vivo visualization and biological applications. The lanthanide(III) ions enabled us to shorten the relaxation times and to trim the probes according to the actual needs. Similar to the clinically approved Gd³⁺ chelates, this customized probe design ensures consistent biochemical properties and similar safety profiles.

     

A powerful graphical pulse sequence programming tool for magnetic resonance imaging

A powerful graphical pulse sequence programming tool has been designed for creating magnetic resonance imaging (MRI) applications. It allows rapid development of pulse sequences in graphical mode (allowing for the visualization of sequences), and consists of three modules which include a graphical sequence editor, a parameter management module and a sequence compiler. Its key features are ease to use, flexibility and hardware independence. When graphic elements are combined with a certain text expressions, the graphical pulse sequence programming is as flexible as text-based programming tool. In addition, a hardware-independent design is implemented by using the strategy of two step compilations. To demonstrate the flexibility and the capability of this graphical sequence programming tool, a multi-slice fast spin echo experiment is performed on our home-made 0.3 T permanent magnet MRI system     

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.     

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.