Quantitative single-voxel spectroscopy: the reciprocity principle for receive-only head coils

PURPOSE: To correct MR spectra for local changes in the coil sensitivity for a widely used coil setup, consisting of a transmitting body coil and a receive-only head coil. MATERIALS AND METHODS: The method relies on the reciprocity principle for the body coil and a correction factor for signal amplitudes between body coil and head coil. The correction is based either on the local flip angle dependence of the stimulated echo acquisition mode signal (TFC) or on the automatic RF calibration (RFC). Water phantoms of different volumes were used to simulate variable coil loads, and B1 field inhomogeneities were assessed by varying the voxel position. Furthermore, the correction was tested by longitudinal measurements in one volunteer. RESULTS: The correction in vitro yields a reduction of the variation coefficient of the water signal by about 77% (TFC) and 66% (RFC) for different coil loads, as well as 55% (TFC) for variable voxel positions. Slightly lower reductions were assessed for the variation coefficients of the metabolite signals in vivo. CONCLUSION: This approach adequately compensates for local changes in coil sensitivity, when acquiring MR spectra with a receive-only head coil.     

Magnetic resonance imaging in neonatal encephalopathy

Magnetic resonance imaging may provide invaluable information in the term born neonate with encephalopathy. However, both hardware and sequences may need adaptation from normal adult protocols. Sedation is often required to obtain good quality imaging, but anaesthesia is not necessary in this population. The perinatal history may predict the pattern of brain lesions, which, in turn, may be used to predict the neurodevelopmental outcome. Image interpretation is not easy and requires a full clinical history in addition to experience of both normal and abnormal neonatal brain appearances. Lesions evolve rapidly, and perinatally acquired leasions are at the most obvious 1-2 weeks from delivery. Early imaging in the first few days from presentation should always include diffusion-weighted sequences to identify early ischaemic change. Advanced techniques such as venography, angiography and perfusion-weighted imaging may be useful in certain situations, and serial imaging may help differentiate perinatal-acquired lesions from other pathologies.     

B(1) destructive interferences and spatial phase patterns at 7 T with a head transceiver array coil.

RF behavior in the human head becomes complex at ultrahigh magnetic fields. A bright center and a weak periphery are observed in images obtained with volume coils, while surface coils provide strong signal in the periphery. Intensity patterns reported with volume coils are often loosely referred to as "dielectric resonances," while modeling studies ascribe them to superposition of traveling waves greatly dampened in lossy brain tissues, raising questions regarding the usage of this term. Here we address this question experimentally, taking full advantage of a transceiver coil array that was used in volume transmit mode, multiple receiver mode, or single transmit surface coil mode. We demonstrate with an appropriately conductive sphere phantom that destructive interferences are responsible for a weak B(1) in the periphery, without a significant standing wave pattern. The relative spatial phase of receive and transmit B(1) proved remarkably similar for the different coil elements, although with opposite rotational direction. Additional simulation data closely matched our phantom results. In the human brain the phase patterns were more complex but still exhibited similarities between coil elements. Our results suggest that measuring spatial B(1) phase could help, within an MR session, to perform RF shimming in order to obtain more homogeneous B(1) in user-defined areas of the brain.     

Occupational safety: effects of workplace radiofrequencies on hearing function

BACKGROUND: The effects of radio frequency (RF) and microwave radiation on humans have been the subject of continuous investigation. Clinical investigations related to occupational RF/microwave exposure have been reported by investigators (1). Since one of the major groups occupationally exposed to RF and microwave radiation includes those working in radio broadcasting and TV transmitter stations, this study investigates whether RF affects auditory systems of people exposed to RF. METHODS: The study is carried out with people working in radio broadcasting stations and living in employee residential houses close to the broadcasting stations. All subjects in the control group were similar in age, work regime, socioeconomic status, and lack of experience in working with RF sources. Brainstem Evoked Response Audiometer (BERA) and Pure Tone Audiometry (PTA) were used to measure the effects of RF under investigation on hearing functions of the subjects. In BERA measurements, I-III, III-V and I-V interpeak latencies were evaluated. In pure tone audiometric measurements, 250 Hz, 500 Hz, 1000 Hz, 2000 Hz, 4000 Hz and 8000 Hz frequencies of hearing threshold were measured in subjects of experimental and control groups. Interpeak latencies and bone conduction hearing thresholds of subjects in the experimental group were compared with those of the control group. RESULTS: BERA results showed that I-III, I-V and III-V interpeak latencies of people occupationally exposed to RF were not higher than subjects in control groups (p>0.05). Results of BERA indicated no statistically significant differences between exposure and control subjects. In audiometric evaluation, hearing threshold of people occupationally exposed to RF were found higher than the control group subjects for frequencies of 4000 Hz and 8000 Hz in terms of bone and air conduction of right and left ear (p < 0.01). CONCLUSIONS: The results of traditional audiometer indicated that RF promotes sensorineural hearing loss and affects cochlea parts related to 4000 Hz and 8000 Hz. These findings may have immediate implications and considerations for workplace safety in order to provide an occupationally safe environment to employees working in such settings.     

A wavelet-based approximation of surface coil sensitivity profiles for correction of image intensity inhomogeneity and parallel imaging reconstruction

We evaluate a wavelet-based algorithm to estimate the coil sensitivity modulation from surface coils. This information is used to improve the image homogeneity of magnetic resonance imaging when a surface coil is used for reception, and to increase image encoding speed by reconstructing images from under-sampled (aliased) acquisitions using parallel magnetic resonance imaging (MRI) methods for higher spatiotemporal image resolutions. The proposed algorithm estimates the spatial sensitivity profile of surface coils from the original anatomical images directly without using the body coil for additional reference scans or using coil position markers for electromagnetic model-based calculations. No prior knowledge about the anatomy is required for the application of the algorithm. The estimation of the coil sensitivity profile based on the wavelet transform of the original image data was found to provide a robust method for removing the slowly varying spatial sensitivity pattern of the surface coil image and recovering full FOV images from two-fold acceleration in 8-channel parallel MRI. The results, using bi-orthogonal Daubechies 97 wavelets and other members in this family, are evaluated for T1-weighted and T2-weighted brain imaging.     

Thermal—electrical finite element modelling for radio frequency cardiac ablation: Effects of changes in myocardial properties

Finite element (FE) analysis has been utilised as a numerical tool to determine the temperature distribution in studies of radio frequency (RF) cardiac ablation. However, non of the previous FE analyses clarified such computational aspects as software requirements, computation time or convergence test. In addition, myocardial properties included in the previous models vary greatly. A process of FE modelling of a system that included blood, myocardium, and an ablation catheter with a thermistor embedded at the tip is described. The bio-heat equation is solved to determine the temperature distribution in myocardium using a commercial soft-ware application (ABAQUS). A Cauchy convergence test (∈=0.1°C) was performed and it is concluded that the optimal number of elements for the proposed system is 24610. The effects of changes in myocardial properties (±50% electric conductivity, +100%/−50% thermal conductivity, and +100%/−50% specific heat capacity) in both power-controlled (PCRFA) and temperature-controlled RF ablation (TCRFA) were studied. Changes in myocardial properties affect the results of the FE analyses of PCRFA more than those of TCRFA, and the maximum changes in lesion volumes were −58.6% (−50% electric conductivity), −60.7% (+100% thermal conductivity), and +43.2% (−50% specific heat).     

Moment and direction of the spoiler gradient for effective artifact suppression in RF-spoiled gradient echo imaging.

Radiofrequency (RF) -spoiled gradient echo sequences were developed with the aim to produce images with T(1) weighted contrast within short acquisition time. Over the past two decades, this type of sequence has proven to be a robust technique and represents a reliable workhorse in clinical MRI. This study presents an analysis of ghost artifacts, which appear occasionally in RF-spoiled gradient echo images. It is demonstrated that the artifacts result from intrinsically emerging signal oscillations, which can be damped down by the application of spoiler gradients.     

日立AIRIS-ⅡMRI射频发射线圈不良的调整方法对图像质量影响的探讨

通过对MRI射频发射线圈不良的调整方法对图像质量的影响的探讨,以反射功率、阻抗、相位为参数对发射线圈进行处理和调整,研究测量图像的信噪比,总结出图像质量最佳的调整方法和有效途径。     

MRI thermometry: Fast mapping of RF‐induced heating along conductive wires

Conductive implants are in most cases a strict contraindication for MRI examinations, as RF pulses applied during the MRI measurement can lead to severe heating of the surrounding tissue. Understanding and mapping of these heating effects is therefore crucial for determining the circumstances under which patient examinations are safe. The use of fluoroptic probes is the standard procedure for monitoring these heating effects. However, the observed temperature increase is highly dependent on the positioning of such a probe, as it can only determine the temperature locally. Temperature mapping with MRI after RF heating can be used, but cooling effects during imaging lead to a significant underestimation of the heating effect. In this work, an MRI thermometry method was combined with an MRI heating sequence, allowing for temperature mapping during RF heating. This technique may provide new opportunities for implant safety investigations. Magn Reson Med, 2008. © 2008 Wiley‐Liss, Inc.     

Significant precision improvement for temperature mapping.

MR temperature measurements are important for applications such as the evaluation of thermal therapies and radiofrequency (RF) coil heating effects. In this work the spherical mean value (SMV) method has been applied to significantly improve the precision of MR temperature mapping in a homogeneous gel phantom. Temperature-increase maps of the phantom were obtained with three-dimensional (3D) MR phase difference mapping after heating with the RF coil. The temperature-increase distribution in most regions in the phantom is a harmonic function with the mean value property. Based on this property, the precision of temperature-increase maps was improved up to sixfold with the SMV method. Comparison of this method with conventional smoothing, further precision improvement, and the in vivo application of the SMV method are discussed.