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.     

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.     

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.     

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.     

H Magnetic resonance imaging and P magnetic resonance spectroscopy in experimental filariasis

¹H Magnetic resonance imaging and ³¹P magnetic resonance spectroscopy (MRS) have been carried out in experimental rodent filariasis, i.e., Acanthocheilonema viteae infection in the rodent host, Mastomys coucha. The T₂-weighted image of the infected host shows fine hyperintense thread like structures of adult filariid nests in the cervical region. ³¹P MRS of normal and infected hosts, localized over the same region of interest, show seven major peaks corresponding to phosphomonoesters (including glucose-6-phosphate, fructose-6-phosphate, fructose-1-6-diphosphate, phosphorylcholine, and adenine monophosphate or AMP), inorganic phosphate, glycerophosphorylcholine, phosphoenolpyruvate, phosphocreatine and nucleoside di- and tri-phosphates. Concentrations of phosphomonoesters (PMEs) are higher in the normal rodent compared with the infected ones. In vivo ³¹P MRS provides a non-invasive assessment of tissue bioenergetics and phospholipid metabolism.     

High-field magnetic resonance imaging using solenoid radiofrequency coils

High-resolution magnetic resonance imaging using dedicated high-field radiofrequency micro-coils at 16.4 T (700 MHz) was investigated. Specific solenoid coils primarily using silver and copper as conductors with enamel and polyurethane coatings were built to establish which coil configuration produces the best image. Image quality was quantified using signal-to-noise ratio and signal variation over regions of interest. Benchmarking was conducted using 5-mm diameter coils, as this size is comparable to an established coil of the same size. Our 1.4-mm-diameter coils were compared directly to each other, from which we deduce performance as a function of conductor material and coating. A variety of materials and conductor coatings allowed us to choose an optimal design, which we used to image a kidney section at 10-micron resolution. We applied zero-fill extrapolation to achieve 5-micron resolution.     

Synthesis of water-soluble FeOOH nanospindles and their performance for magnetic resonance imaging

Water soluble FeOOH nanospindles with small size were synthesized by a simple hydrolysis method of inorganic salts and water bath treatment with different incubation time. The morphology, microstructure, magnetic resonance imaging (MRI) performance and cytotoxicity of the as-prepared FeOOH nanospindles were investigated, respectively. The results showed that the longitudinal length of FeOOH nanospindles was about 40-50 nm, and the incubation time had important effect for the morphology and production rate of FeOOH nanospindles. MRI test showed that the longitudinal and transverse relaxivities (r₁ and r₂ values) of FeOOH nanospindles were about 3.06 mM⁻¹ s⁻¹ and 5.06 mM⁻¹ s⁻¹, respectively. Furthermore, the experimental results of the Prussian Blue staining showed the clusters of FeOOH nanospindles in the cytoplasm of the labeled cells, and the cytotoxicity characterization indicated that FeOOH nanospindles have low cytotoxicity. Therefore, the as-prepared FeOOH nanospindles will have potential applications as T₁- and T₂-weighted MRI contrast agents.     

One-pot synthesis of amphiphilic superparamagnetic FePt nanoparticles and magnetic resonance imaging in vitro

Monodispersed amphiphilic FePt nanoparticles with the diameter of about 4 nm were synthesized by high temperature pyrolysis of iron(III) acetylacetonate and platinum(II) acetylacetonate. Their amphiphilicity is contributed to the tetraethylene glycol (TEG) and oleic acid (OA) on the surface, which is confirmed by FTIR and XPS spectra. They provide a superparamagnetic property with the saturation magnetization (Ms) of about 25 emu/g and the transverse relaxivity (r₂) of about 122.6 mM⁻¹ s⁻¹ in aqueous solutions. Furthermore, FePt nanoparticles show low cytotoxicity in living cells. They can be uptaken by HeLa cells effectively and result in the obvious decrease of T₂ relaxation time after internalization.     

Optimizing tissue contrast in magnetic resonance imaging

Magnetic resonance imaging demands that tissue contrast and signal-to-noise advantages be sought in each component of the imaging system. One component of magnetic resonance imaging in which contrast and signal-to-noise ratios are easily manipulated is in the choice of pulse sequences and interpulse delay times. This article provides a general method for determining the best choices of interpulse delay times in pulse sequences and applies that method to saturation recovery, inversion recovery, and spin-echo sequences. Saturation recovery and inversion recovery sequences with rephasing pulses, and tissues with unequal hydrogen densities are considered. Optimization of pulse sequences is carried out for the two distinct cases of (a) a fixed number of sequence repetitions and (b) a fixed total imaging time. Analytic expressions are derived or approximate expressions are provided for the interpulse delay times that optimize contrast-to-noise ratios in each pulse sequence. The acceptable range of interpulse delay times to obtain reasonable contrast using each pulse sequence is discussed.     

Developments in active noise control sound systems for magnetic resonance imaging

Magnetic resonance imaging (MRI) scanners can produce noise measuring over 130 dB SPL. This noise stimulates the auditory nervous system, limiting the dynamic range for stimulus driven activity in functional MRI (fMRI) experiments and can influence other brain functions. Even for structural scans it causes subject anxiety and discomfort in addition to the impediment to communications. Here we describe the realization and validation of a sound system for sound presentation inside an MRI scanner and the modifications to a standard active noise control technique for use in the noisy and compact environment of the scanner. This paper provides a review of the technology available for the presentation of audio stimuli in an MRI environment and the modifications required for the active control of scanner noise. Some of the content has been previously published [Chambers J, Akeroyd MA, Summerfield AQ, Palmer AR. Active control of the volume acquisition noise in functional magnetic resonance imaging: method and psychoacoustical evaluation. J Acoust Soc Am 2001;110(6):3041-54; Levitt H. Transformed up-down methods in psychoacoustics. J Acoust Soc Am 1971;49:467-77], but this paper goes further in describing the stages of development as the system performance was optimised. The performance of the system and both the objective and subjective reduction of the scanner noise are reported. Finally, we discuss recent improvements to the system that are currently being evaluated and describe the theory of opto-acoustical transducers that operate on the principle of light modulation. These are immune from, and do not create, electro-magnetic interference (EMI) and radio-frequency interference (RFI).     

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.     

Synthesis of Fe3O4 magnetic fluid used for magnetic resonance imaging and hyperthermia

Fe₃O₄ magnetic nanoparticles were prepared by co-precipitation from FeSO₄·7H₂O and FeCl₃·6H₂O aqueous solutions using NaOH as precipitating reagent. The nanoparticles have an average size of 12 nm and exhibit superparamagnetism at room temperature. The nanoparticles were used to prepare a water-based magnetic fluid using oleic acid and Tween 80 as surfactants. The stability and magnetic properties of the magnetic fluid were characterized by Gouy magnetic balance. The experimental results imply that the hydrophilic block of Tween 80 can make the Fe₃O₄ nanoparticles suspending in water stable even after dilution and autoclaving. The magnetic fluid demonstrates excellent stability and fast magneto-temperature response, which can be used both in magnetic resonance imaging and magnetic fluid hyperthermia.     

Design of an interventional magnetic resonance imaging coil for cerebral surgery

In clinical magnetic resonance imaging(MRI),the design of the radiofrequency(RF) coil is very important.For certain applications,the appropriate coil can produce an improved image quality.However,it is difficult to achieve a uniform B1 field and a high signal-to-noise ratio(SNR) simultaneously.In this article,we design an interventional transmitter-and-receiver RF coil for cerebral surgery.This coil adopts a disassembly structure that can be assembled and disassembled repeatedly on the cerebral surgery gantry to reduce the amount of interference from the MRI during surgery.The simulation results and the imaging experiments demonstrate that this coil can produce a uniform RF field,a high SNR,and a large imaging range to meet the requirements of the cerebral surgery.     

Calculation of electric fields induced near metal implants by magnetic resonance imaging switched-gradient magnetic fields

Electric (E) fields induced near metal implants by MRI switched-gradient magnetic fields are calculated by a new equivalent-circuit numerical technique. Induced E-field results are found for a metallic spinal-fusion implant consisting of two thin wires connected to the metallic case of a current generator as well as for its subsections: a bare U-shaped wire, an insulated U-shaped wire, a cut insulated wire, and a generator. The presence of the metallic implants perturbs the E field significantly. Near the ends of the bare U-shaped wire, the E field is 89.7 times larger than in the absence of the wire. The greatest E field concentration occurs near the ends of the cut insulated wire, where the E field is 196.7 times greater than in the absence of the wire. In all cases, the perturbation of the induced E field by the implanted wire is highly localized within a few diameters of the wire.     

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.     

Comparison of magnetic resonance imaging of inhaled SF6 with respiratory gas analysis

Magnetic resonance imaging of inhaled fluorinated inert gases ((19)F-MRI) such as sulfur hexafluoride (SF(6)) allows for analysis of ventilated air spaces. In this study, the possibility of using this technique to image lung function was assessed. For this, (19)F-MRI of inhaled SF(6) was compared with respiratory gas analysis, which is a global but reliable measure of alveolar gas fraction. Five anesthetized pigs underwent multiple-breath wash-in procedures with a gas mixture of 70% SF(6) and 30% oxygen. Two-dimensional (19)F-MRI and end-expiratory gas fraction analysis were performed after 4 to 24 inhaled breaths. Signal intensity of (19)F-MRI and end-expiratory SF(6) fraction were evaluated with respect to linear correlation and reproducibility. Time constants were estimated by both MRI and respiratory gas analysis data and compared for agreement. A good linear correlation between signal intensity and end-expiratory gas fraction was found (correlation coefficient 0.99+/-0.01). The data were reproducible (standard error of signal intensity 8% vs. that of gas fraction 5%) and the comparison of time constants yielded a sufficient agreement. According to the good linear correlation and the acceptable reproducibility, we suggest the (19)F-MRI to be a valuable tool for quantification of intrapulmonary SF(6) and hence lung function.