Magnetic resonance spectroscopy in vivo

Nuclear magnetic resonance (NMR) has become an important non-invasive investigative technique in medicine and biology. The most recent development has been the ability to perform magnetic resonance spectroscopy (MRS) in selected regions within the human body. Such volumes can be selected by techniques which fall into the following broad catagories: surface coil methods, surface coils with depth selection, volume selection and chemical shift mapping. The latter two methods use magnetic field gradients, present on magnetic resonance imaging systems, to select the volume. MRS can be used to measure phosphorus and proton metabolites and hence study tissue biochemistry in-vivo.     

Birdcage volume coils and magnetic resonance imaging: a simple experiment for students

Background

This article explains some simple experiments that can be used in undergraduate or graduate physics or biomedical engineering laboratory classes to learn how birdcage volume radiofrequency (RF) coils and magnetic resonance imaging (MRI) work. For a clear picture, and to do any quantitative MRI analysis, acquiring images with a high signal-to-noise ratio (SNR) is required. With a given MRI system at a given field strength, the only means to change the SNR using hardware is to change the RF coil used to collect the image. RF coils can be designed in many different ways including birdcage volume RF coil designs. The choice of RF coil to give the best SNR for any MRI study is based on the sample being imaged.

Results

The data collected in the simple experiments show that the SNR varies as inverse diameter for the birdcage volume RF coils used in these experiments. The experiments were easily performed by a high school student, an undergraduate student, and a graduate student, in less than 3 h, the time typically allotted for a university laboratory course.

Conclusions

The article describes experiments that students in undergraduate or graduate laboratories can perform to observe how birdcage volume RF coils influence MRI measurements. It is designed for students interested in pursuing careers in the imaging field.

     

Proton/Deuterium Magnetic Resonance Imaging of Rodents at 9.4T Using Birdcage Coils

The purpose of the present study was to fabricate a volume coil for proton/deuterium magnetic resonance imaging (MRI) in rodents at 9.4 T. Two birdcage radiofrequency (RF) coils have been designed for proton/deuterium MRI: the rungs of two concentric birdcages were azimuthally interleaved with each other for better decoupling, and the two coils were tuned to 400.3 and 61.4 MHz for ¹H/²H resonance at 9.4 T. Compared to a commercially available coil, the proposed ¹H/²H RF coil provides reasonable transmission efficiency and imaging signal-to-noise ratio (SNR); the relationships among imaging parameters such as SNR, voxel size, and deuterium oxide concentrations have been quantitatively studied, and the linear correlation results together with the spectroscopic data in vivo indicate its feasibility in deuterium metabolic imaging (DMI) in vivo. Our study indicates that using the birdcage design for MRI signal excitation combined with surface coil array for signal reception can facilitate DMI investigations more effectively towards future pre-clinical and clinical applications. As a noninvasive method by measuring nonhydrogen nuclear deuterium signals to reflect metabolite information, DMI will feature prominently in future precision medicine through the whole process of diagnosis, treatment, and prognosis. © 2021 Bioelectromagnetics Society.     

Uniform magnetic fields and double-wrapped coil systems: improved techniques for the design of bioelectromagnetic experiments.

A common mistake in biomagnetic experimentation is the assumption that Helmholtz coils provide uniform magnetic fields; this is true only for a limited volume at their center. Substantial improvements on this design have been made during the past 140 years with systems of three, four, and five coils. Numerical comparisons of the field uniformity generated by these designs are made here, along with a table of construction details and recommendations for their use in experiments in which large volumes of uniform intensity magnetic exposures are needed. Double-wrapping, or systems of bifilar windings, can also help control for the non-magnetic effects of the electric coils used in many experiments. In this design, each coil is wrapped in parallel with two separate, adjacent strands of copper wire, rather than the single strand used normally. If currents are flowing in antiparallel directions, the magnetic fields generated by each strand will cancel and yield virtually no external magnetic field, whereas parallel currents will yield an external field. Both cases will produce similar non-magnetic effects of ohmic heating, and simple measures can reduce the small vibration and electric field differences. Control experiments can then be designed such that the only major difference between treated and untreated groups is the presence or absence of the magnetic field. Double-wrapped coils also facilitate the use of truly double-blind protocol, as the same apparatus can be used either for experimental or control groups.     

Cranial MRI of small rodents using a clinical MR scanner

Increasing numbers of small animal models are in use in the field of neuroscience research. Magnetic resonance imaging (MRI) provides an excellent method for non-invasive imaging of the brain. Using three-dimensional (3D) MR sequences allows lesion volumetry, e.g. for the quantification of tumor size. Specialized small-bore animal MRI scanners are available for high-resolution MRI of small rodents' brain, but major drawbacks of this dedicated equipment are its high costs and thus its limited availability. Therefore, more and more research groups use clinical MR scanners for imaging small animal models. But to achieve a reasonable spatial resolution at an acceptable signal-to-noise ratio with these scanners, some requirements concerning sequence parameters have to be matched. Thus, the aim of this paper was to present in detail a method how to perform MRI of small rodents brain using a standard clinical 1.5 T scanner and clinically available radio frequency coils to keep material costs low and to circumvent the development of custom-made coils.     

经颅磁刺激参数与结构要件的影响分析

为了进一步探索经颅磁刺激工作机理并改进或研制出新的经颅磁刺激激励源.本文从经颅磁刺激的原理推导出了磁场、感应电流及激励源原理电路电流的表达式,利用大脑-线圈和大脑-线圈-铁芯两种经颅磁刺激模型分析影响因素与头模型各组织的磁场和感应电流分布.对比分析表明电流的性质,线圈半径,线圈激励特性与铁芯对感应电流分布与电磁场分布有着本质的影响.对经颅磁刺激参数及结构要件的研究与分析可用于指导刺激线圈参数及激励源电路参数的设置,以及探索新的激励源制作.     

Development of a dynamic imaging method for gravitropism in pea sprouts using clinical magnetic resonance imaging system

Although magnetic resonance imaging (MRI) is a useful technique, only a few studies have investigated the dynamic behavior of small subjects using MRI owing to constraints such as experimental space and signal amount. In this study, to acquire high-resolution continuous three-dimensional gravitropism data of pea (Pisum sativum) sprouts, we developed a small-bore MRI signal receiver coil that can be used in a clinical MRI and adjusted the imaging sequence. It was expected that such an arrangement would improve signal sensitivity and improve the signal-to-noise ratio (SNR) of the acquired image. All MRI experiments were performed using a 3.0-T clinical MRI scanner. An SNR comparison using an agarose gel phantom to confirm the improved performance of the small-bore receiver coil and an imaging experiment of pea sprouts exhibiting gravitropism were performed. The SNRs of the images acquired with a standard 32-channel head coil and the new small-bore receiver coil were 5.23±0.90 and 57.75±12.53, respectively. The SNR of the images recorded using the new coil was approximately 11-fold higher than that of the standard coil. In addition, when the accuracy of MR imaging that captures the movement of pea sprout was verified, the difference in position information from the optical image was found to be small and could be used for measurements. These results of this study enable the application of a clinical MRI system for dynamic plant MRI. We believe that this study is a significant first step in the development of plant MRI technique.     

Low stray ELF magnetic field exposure system for in vitro study

An exposure facility for wide application to cell exposure to an ELF (extremely low frequency) magnetic field was developed. It is suitable for conducting experiments under a high-intensity, variable-frequency magnetic field, on the biological effects of the ELF magnetic field in an in vitro study. The exposure system consists of Merritt's 4-square coil as a basic component to generate the required magnetic field intensity of 10 mT at 50 Hz with spatial field uniformity less than +/-3% in a 400 mm cube. Concentric compensation coils are adopted to eliminate the effects of stray fields on sham (control) samples in the vicinity of the exposure system. The uniformity of the magnetic field in the exposure coil, the increase in the power supply capacity due to the existence of compensation coils, and the stray field estimation were investigated carefully. After fabricating the system, performance tests were carried out and all the characteristics were found to be satisfactory. In addition, the ideal configuration for a concentric coil system was proposed.     

50-Hz magnetic field exposure system for small animals

The design, construction, and results of evaluation of an animal-exposure system for the study of biological effects of extremely low frequency (ELF) magnetic fields are described. The system uses a square coil arrangement based on a modification of the Helmholtz coil. Due to the cubic configuration of this exposure system, horizontal and vertical magnetic fields as high as 0.3 mT can be generated. Circularly polarized magnetic fields can also be generated by changing the current and phase difference between two sets of coils. Tests were made for uniformity of the magnetic field, stray fields, sham-exposure ratio of stray field, changes of temperature and humidity, light intensity and distribution inside the animal-housing space, and noise due to air-conditioning equipment. Variation of the magnetic field was less than 2% inside the animal housing. The stray-field level inside the sham-exposure system is less than 2% of experimental exposure levels. The system can be used for simultaneous exposure of 48 rats (2 to a cage) or 96 mice (4 to a cage). © 1993 Wiley-Liss. Inc.     

In vivo biomicroscopy of the skin with high-resolution magnetic resonance imaging and high frequency ultrasound.

Noninvasive imaging and characterization of the skin is of great interest in dermatology. In order to get relevant diagnostic information, high-resolution imaging techniques have to be applied. Ultrasonic imaging is a potential method for this purpose where the special requirements concerning the spatial resolution make it essential to apply high frequency ultrasound (HFUS). Alternatively, magnetic resonance imaging (MRI), being a very promising imaging modality, also shows the perspective of becoming a valuable diagnostic tool in dermatology. However, to account for the small dimensions of the structures under observation, very specialized system designs have to be developed. In this paper, a HFUS imaging system working in the 50 MHz and 100 MHz range is applied for high-resolution skin imaging. Furthermore, a commercial MRI-system was equipped with specially designed low noise rf (radio frequency) coils with minimized volume, and customized imaging sequences were applied to optimize the signal-to-noise ratio. With HFUS and high-resolution magnetic resonance (HR-MR) imaging complementary imaging techniques for in vivo biomicroscopy of the skin are available.     

Noninvasive tumor volume measurement using magnetic resonance (MR) imaging and an open sided saddle coil

A magnetic resonance (MR) imaging scanner operated at 0.5 T with a specially constructed receiving coil was used to measure volumes of primary spontaneous tumors in rats and guinea pigs. The coil was used to improve the signal to noise ratio (S/N) of the MR images of tumors in these small animals. The tumor volume was determined by the summation of the volume of contiguous slices or ellipsoid approximation. The accuracy of the volume measurement was better when the numerical integration was used in calculating the slice volume. The open sided saddle (OSS) coil used as the receiving coil gave better S/N than that of the standard head coil.     

Surface coil magnetic resonance imaging

Detection of MR signals with surface coils provides increased signal-to-noise ratio for superficial structures relative to detection by circumferential coils, permitting improved spatial resolution. Different geometries of surface coils can be used for different regions. Coils that are flat or curved to fit body contours are good for general imaging, with a range of coil sizes useful for structures of different size or depth. Solenoidal coils are useful for imaging protruding structures such as breasts, while smaller versions of conventional circumferential coils that can be slipped over limbs are useful for imaging extremities.     

Modular Coils with Low Hydrogen Content Especially for MRI of Dry Solids

IntroductionRecent advances have enabled fast magnetic resonance imaging (MRI) of solid materials. This development has opened up new applications for MRI, but, at the same time, uncovered new challenges. Previously, MRI-invisible materials like the housing of MRI detection coils are now readily depicted and either cause artifacts or lead to a decreased image resolution. In this contribution, we present versatile, multi-nuclear single and dual-tune MRI coils that stand out by (1) a low hydrogen content for high-resolution MRI of dry solids without artifacts; (2) a modular approach with exchangeable inductors of variable volumes to optimally enclose the given object; (3) low cost and low manufacturing effort that is associated with the modular approach; (4) accurate sample placement in the coil outside of the bore, and (5) a wide, single- or dual-tune frequency range that covers several nuclei and enables multinuclear MRI without moving the sample.

Materials and Methods

The inductors of the coils were constructed from self-supporting copper sheets to avoid all plastic materials within or around the resonator. The components that were mounted at a distance from the inductor, including the circuit board, coaxial cable and holder were manufactured from polytetrafluoroethylene.

Results and Conclusion

Residual hydrogen signal was sufficiently well suppressed to allow ¹H-MRI of dry solids with a minimum field of view that was smaller than the sensitive volume of the coil. The SNR was found to be comparable but somewhat lower with respect to commercial, proton-rich quadrature coils, and higher with respect to a linearly-polarized commercial coil. The potential of the setup presented was exemplified by ¹H / ²³Na high-resolution zero echo time (ZTE) MRI of a model solution and a dried human molar at 9.4 T. A full 3D image dataset of the tooth was obtained, rich in contrast and similar to the resolution of standard cone-beam computed tomography.     

Magnetic resonance imaging using a clinical whole body system: an introduction to a useful technique in small animal experiments.

A clinical whole body magnetic resonance imaging (MRI) system with high resolution coils was used to obtain non-invasive images of the living rat. The results demonstrate the feasibility of the set-up and the advantages of this new imaging technique: detailed information, no extra costs, longitudinal studies without killing animals and simple anaesthesia. It is concluded that in small animal experimentation, this use of high resolution coils in whole body magnetic resonance systems may be particularly helpful in establishing effects of experimental procedures. Whenever non-invasive visualization is required, especially in longitudinal animal studies, e.g. biomaterial research or tumour investigation, this use of MRI will offer challenging possibilities.     

The Travelling-Wave Primate System: A New Solution for Magnetic Resonance Imaging of Macaque Monkeys at 7 Tesla Ultra-High Field

Introduction

Neuroimaging of macaques at ultra-high field (UHF) is usually conducted by combining a volume coil for transmit (Tx) and a phased array coil for receive (Rx) tightly enclosing the monkey’s head. Good results have been achieved using vertical or horizontal magnets with implanted or near-surface coils. An alternative and less costly approach, the travelling-wave (TW) excitation concept, may offer more flexible experimental setups on human whole-body UHF magnetic resonance imaging (MRI) systems, which are now more widely available. Goal of the study was developing and validating the TW concept for in vivo primate MRI.

Methods

The TW Primate System (TWPS) uses the radio frequency shield of the gradient system of a human whole-body 7 T MRI system as a waveguide to propagate a circularly polarized B1 field represented by the TE11 mode. This mode is excited by a specifically designed 2-port patch antenna. For receive, a customized neuroimaging monkey head receive-only coil was designed. Field simulation was used for development and evaluation. Signal-to-noise ratio (SNR) was compared with data acquired with a conventional monkey volume head coil consisting of a homogeneous transmit coil and a 12-element receive coil.

Results

The TWPS offered good image homogeneity in the volume-of-interest Turbo spin echo images exhibited a high contrast, allowing a clear depiction of the cerebral anatomy. As a prerequisite for functional MRI, whole brain ultrafast echo planar images were successfully acquired.

Conclusion

The TWPS presents a promising new approach to fMRI of macaques for research groups with access to a horizontal UHF MRI system.     

Cancer promotion in a mouse-skin model by a 60-Hz magnetic field: I. Experimental design and exposure system.

The rationale for selection of an animal model, the experimental design, and the design and evaluation of an exposure system used in studies of 60-Hz magnetic fields are described. The studies were conceived to assay development of cancer and immune responsiveness in mice exposed to magnetic fields. The exposure system utilized a quadrupole-coil configuration to minimize stray magnetic fields. Four square-wound coil provided a uniform field within a volume occupied by 16 animal cages. The magnetic field had a mean flux density of 2 mT that varied less than +/- 10% within the volume occupied by animals' cages. The flux density decreased to less than 0.1 microT at a distance of 2 m from the coils. In each exposure system 32 animals could be housed in plastic cages.     

Safety concerns related to magnetic field exposure

The recent development of superconducting magnets has resulted in a huge increase in human exposure to very large static magnetic fields of up to several teslas (T). Considering the rapid advances in applications and the great increases in the strength of magnetic fields used, especially in magnetic resonance imaging, safety concerns about magnetic field exposure have become a key issue. This paper points out some of these safety concerns and gives an overview of the findings about this theme, focusing mainly on mechanisms of magnetic field interaction with living organisms and the consequent effects.     

The Wellcome Foundation lecture, 1981. Nuclear magnetic resonance imaging in medicine: physical principles

In recent years nuclear magnetic resonance (n.m.r.) has become a means of providing excellent images of the interior of the human body which are proving useful in medical practice. The development of n.m.r. imaging, much of which was pioneered in Britain, is outlined. Proton image resolution of human anatomy is comparable with X-ray computed tomography images, but without the hazard of ionizing radiation. There is improved soft tissue discrimination and pathological contrast through the basic imaging parameters of the proton density and the relaxation times T1 and T2, whose differences from one tissue to another are exploited by use of appropriate radiofrequency pulse sequences. Images may be obtained directly of transverse, coronal and sagittal sections of the head and body. Single slices or multiple slices may be imaged and imaging may be done in three dimensions. The lecture describes the more important imaging techniques and gives illustrative examples of images obtained. The efficient use of time in n.m.r. imaging is discussed, particularly mentioning the multiecho-multislice procedure and the development of real-time n.m.r. imaging. Magnetic field strengths in current use for proton n.m.r. imaging range from 0.02 to 2 T. At the lower end of the range resistive magnets are used, while for higher fields superconducting magnets are needed. A considerable improvement in image quality is obtained by use of special receiver coils.     

Functional and morphological cardiac magnetic resonance imaging of mice using a cryogenic quadrature radiofrequency coil

Cardiac morphology and function assessment by magnetic resonance imaging is of increasing interest for a variety of mouse models in pre-clinical cardiac research, such as myocardial infarction models or myocardial injury/remodeling in genetically or pharmacologically induced hypertension. Signal-to-noise ratio (SNR) constraints, however, limit image quality and blood myocardium delineation, which crucially depend on high spatial resolution. Significant gains in SNR with a cryogenically cooled RF probe have been shown for mouse brain MRI, yet the potential of applying cryogenic RF coils for cardiac MR (CMR) in mice is, as of yet, untapped. This study examines the feasibility and potential benefits of CMR in mice employing a 400 MHz cryogenic RF surface coil, compared with a conventional mouse heart coil array operating at room temperature. The cryogenic RF coil affords SNR gains of 3.0 to 5.0 versus the conventional approach and hence enables an enhanced spatial resolution. This markedly improved image quality - by better deliniation of myocardial borders and enhanced depiction of papillary muscles and trabeculae - and facilitated a more accurate cardiac chamber quantification, due to reduced intraobserver variability. In summary the use of a cryogenically cooled RF probe represents a valuable means of enhancing the capabilities of CMR of mice.     

Conditioning domestic chickens to a magnetic anomaly

Young domestic chicks of two strains, ISA brown layers and White Leghorn X Australorps, were trained to associate a magnetic anomaly with food. This was done by feeding them in their housing boxes from a dish placed above a small coil that produced a magnetic anomaly roughly six times as strong as the local geomagnetic field. Unrewarded tests began on day 9 after hatching. In a square arena, two corresponding coils were placed underneath two opposite corners. One coil, the control coil, was double-wrapped producing no net magnetic field, while the other in the opposite corner produced a local magnetic anomaly similar to that experienced during feeding. The chicks favoured the corner with the anomaly from day 10 after hatching onward. Both strains of chickens showed this preference, indicating that they could sense the local changes in the magnetic field.