4 T Actively detuneable double-tuned 1H/31P head volume coil and four-channel 31P phased array for human brain spectroscopy

Typically 31P in vivo magnetic resonance spectroscopic studies are limited by SNR considerations. Although phased arrays can improve the SNR; to date 31P phased arrays for high-field systems have not been combined with 31P volume transmit coils. Additionally, to provide anatomical reference for the 31P studies, without removal of the coil or patient from the magnet, double-tuning (31P/1H) of the volume coil is required. In this work we describe a series of methods for active detuning and decoupling enabling use of phased arrays with double-tuned volume coils. To demonstrate these principles we have built and characterized an actively detuneable 31P/1H TEM volume transmit/four-channel 31P phased array for 4 T magnetic resonance spectroscopic imaging (MRSI) of the human brain. The coil can be used either in volume-transmit/array-receive mode or in TEM transmit/receive mode with the array detuned. Threefold SNR improvement was obtained at the periphery of the brain using the phased array as compared to the volume coil.     

Radio-frequency probe for 1H decoupled 31P MRS of the head and neck region.

For optimal performance of 31P MRS at 1.5 Tesla, the use of a double resonant probe is essential to enable the application of 1H decoupling and Nuclear Overhauser Enhancement. This note describes the design, evaluation and safety validation of a versatile and compact probe optimized for 1H decoupled 31P MRS studies of tumors close to the surface of the body, in particular the head and neck region.     

Pilocarpine treatment of xerostomia in head and neck patients

We studied the effect of pilocarpine hydrochloride, a parasympathicomimetic agent, on major salivary gland output and subjective responses in 40 patients with salivary hypofunction. Pilocarpine increased salivary output or gave significant symptomatic relief in 21 of the 40 patients. The women fared better than the men. Side effects were uncommon, were generally mild, and caused no treatment interruption. These results indicate that pilocarpine is effective in relieving the signs and symptoms of postradiation xerostomia.     

An optimized solenoidal head radiofrequency coil for low-field magnetic resonance imaging

Applications of low-field magnetic resonance imaging (MRI) systems (<0.3 T) are limited due to the signal-to-noise ratio (SNR) being lower than that provided by systems based on superconductive magnets (≥1.5 T). Therefore, the design of radiofrequency (RF) coils for low-field MRI requires careful consideration as significant gains in SNR can be achieved with the proper design of the RF coil. This article describes an analytical method for the optimization of solenoidal coils. Coil and sample losses are analyzed to provide maximum SNR and optimum B₁ field homogeneity. The calculations are performed for solenoidal coils optimized for the human head at 0.2 T, but the method could also be applied to any solenoidal coil for imaging other anatomical regions at low field. Several coils were constructed to compare experimental and theoretical results. A head magnetic resonance image obtained at 0.2 T with the optimum design is presented.     

Effects of coil dimensions and field polarization on RF heating inside a head phantom

Deterioration of radiofrequency (RF) inhomogeneity with increasing static magnetic field in magnetic resonance imaging (MRI) is one of the fundamental challenges preventing their clinical rendition and posing safety hazards. Variation in RF coil designs could help redistribute RF energy absorption over the imaged object. This work is intended to determine experimentally the difference in RF heating produced within a human head phantom by in situ measurement of RF inhomogeneity as a function of coil design utilized at 8 T. The heating patterns of 1/4 wavelength (long) and 1/8 wavelength 11-cm (short) transverse electromagnetic (TEM) coils loaded with a homogeneous human head phantom at 340 MHz were evaluated. In addition, different transmit/receive (T/R) configurations were used in search for the possibility of "hot-spot" formation. Fluoroptic thermometry was used to measure temperatures in multiple positions in a head phantom made of ground turkey breast for RF powers corresponding to a specific absorption rate (SAR) of 4.0 W/kg for 10 min. Numerical simulations were performed to study the general RF power deposition patterns in phantoms at 340 MHz including the effects of field polarization. The temperature increases varied from 0 to 0.8 degrees C for the long RF coil, while the short RF coil produced a maximum temperature change of 0.5 degrees C. Similar to ultra high-field electromagnetic simulations, these measurements revealed low peripheral and high deep-tissue heating at 8 T. The findings indicated that the largest temperature changes for both cases were less than 1 degrees C. While these results showed an increase in localized heating due to RF pulses at 8 T, they highlight that RF inhomogeneity could be redistributed using different RF coil designs through which the hot spots could be made cooler.     

MRI assisted treatment planning for radiation therapy of the head and neck

Improved visualization of head and neck tumors has been demonstrated with the use of magnetic resonance imaging (MRI). Using standard plastic radiation therapy immobilization casts and an MR positive surface marker system developed in this institution, we have utilized MRI as an adjunct to the simulation of complex radiation treatments for tumors of the head and neck. This technique includes an indirect display of field margins and/or isodose curves over selected MR images. The lack of induced artifact from the immobilization cast, improved delineation of tumor extension from normal anatomy and the ability to image in arbitrary planes without changing patient positioning favor the use of MR over CT for radiation therapy planning in the head and neck, while ensuring reproducibility of the treatment plan at subsequent therapy sessions.     

MR elastography of the head and neck: Driver design and initial results

Purpose

The purpose was to describe the design and fabrication of a driver suitable for magnetic resonance elastography (MRE) of the head and neck and to assess its performance in evaluating human parotid gland, lymph nodes and thyroid at 3.0 T.

Materials and methods

A head and neck driver was fabricated using a commercial transducer, headrest mould and piston extension. Driver performance was tested using a motion-sensitized spin-echo MRE pulse sequence. Six healthy volunteers and three patients (two metastatic nodes and one papillary carcinoma) were evaluated using MRE. Viscoelastic maps were computed to obtain storage modulus (G’) and loss modulus (G”) of the normal parotid and thyroid, metastatic node and thyroid cancer. Reproducibility was assessed by coefficient of variation.

Results

All subjects completed MRE examination without discomfort. Initial G’ and G” values were as follows: normal parotid gland, 1.12 kPa and 0.48 kPa; thyroid, 0.58 kPa and 0.42 kPa; metastatic node, 0.66 kPa and 0.58 kPa; and thyroid cancer, 0.17 kPa and 0.28 kPa. Based on parotid data, the coefficient of variation for G’ and G” was 4.7% and 9.8%.

Conclusion

A new MRE driver for head and neck was successfully implemented, and our initial results suggested the device was suitable for the mechanical assessment of tissues in the head and neck.     

Short echo time proton spectroscopy of human brain using a gradient head coil.

Short echo time, single voxel localized proton spectroscopy was accomplished using a stimulated echo (STEAM) sequence running on a Siemens 1.5-T system with a head coil incorporating the Z and Y gradients. Spectra from the temporal lobe, the cerebellum and mid brain were acquired from a group of normal volunteers using the following parameters: voxel size = 8 ml, TE = 22 msec, 512 signal averages and TR = 1.7 sec. STEAM spectra acquired with the small diameter gradients showed significantly fewer artifacts at short TE, allowing the observation of glutamate/glutamine, GABA, taurine, and inositol in addition to the prominent resonance of choline, creatine/phosphocreatine and N-acetylaspartate (NAA). The levels of chlorine, creatine and NAA were found to be significantly different in the three regions of the brain examined.     

A microstrip transmission line volume coil for human head MR imaging at 4T

A high-frequency RF volume coil based on the use of microstrip transmission line (MTL) has been developed for in vivo 1H MR applications on the human head at 4T. This coil is characterized by major advantages: (i) completely distributed coil circuit, (ii) high-quality factor (Q), (iii) simple coil structure, and (iv) better sensitivity and less signal-intensity variation in the MR image of the human head compared with an RF shielded birdcage coil of similar coil size. The proposed MTL volume coil does not require additional RF shielding for preventing Q degradation from radiation losses due to the unique MTL structure; thus, it provides a maximal useable space inside the volume coil when compared with most volume coils available at high fields with the same overall coil size. The intrinsic B(1) distribution of the MTL volume coil effectively compensates for the dielectric resonance effect at 4T and improves the signal homogeneity in human head MR images in the transaxial planes. The results of this study demonstrate that the MTL volume coil design provides an efficient and simple solution to RF volume coil design for human MR studies at high fields.     

A purpose-built neck coil for black-blood DANTE-prepared carotid artery imaging at 7 T

Atherosclerotic plaques in the bifurcation of the carotid arteries can pose a significant health risk due to possible plaque rupture and subsequent stroke. The assessment of plaques, and evaluation of the risk they pose, can be performed with Black-Blood (BB) vessel wall magnetic resonance imaging. However, resolution at standard clinical field strengths (up to 3 T) is limited, hampering reliable assessment and diagnosis. The aim of this study was to investigate the benefits of 7 T MRI using a BB application that has been successful at clinical field strengths. Therefore, for BB imaging, each sequence was preceded with ‘Delay Alternating with Nutation for Tailored Excitation’ (DANTE) preparation pulses for blood signal suppression. A coil comprising a 4-channel Tx array was designed and built to provide the required excitation coverage for the DANTE train; and a 4-channel Rx array was constructed to target the carotid bifurcation. Human and phantom results showed satisfactory blood suppression and comparable SNR and CNR to 3 T, therefore demonstrating the feasibility of the application at 7 T. However, the imposed SAR restrictions led to long scan times and subsequent motion artifacts. Thus, more accurate local SAR supervision schemes are required which could lead to a further improvement of BB DANTE vessel wall imaging at 7 T.     

Correlation study between DKI and conventional DWI in brain and head and neck tumors

PurposeTo investigate the relationship between the Diffusion Kurtosis Imaging (DKI) parameters and conventional metrics provided by Diffusion-weighted imaging (DWI) in patients affected by Brain or Head and Neck (HN) cancer.MethodsTen patients affected by brain tumor and nine patients with HN tumor underwent a pre-treatment MR examination at 3 T. The largest tumor section was manually contoured by two expert neuroradiologists. The apparent diffusion coefficient (D_app) and apparent diffusional kurtosis (K_app) parameters were determined at the voxel level by using the DKI model, and compared to the apparent diffusion coefficient (ADC) and the tissue diffusion coefficient (D_mono) obtained from mono-exponential fitting methods. The Akaike Information Criteria (AIC) was calculated to assess the quality of the fitting methods. Cross-correlations between all the variables were assessed using the Spearman rank test.ResultsIncreased K_app values were found in each lesion. All parameters were strongly related, in particular an inverse relationship emerged between median values of K_app and D_app/D_mono/ADC in both patient groups, while D_app showed positive correlations with D_mono and ADC. From the analysis at the voxel level, significant inverse associations were found between K_app and D_mono within the lesions, while a weak or moderate association emerged between K_app and ADC or D_app.ConclusionsA significant association between the apparent diffusional kurtosis K_app and the tissue diffusion coefficient D_mono emerged for both brain and HN tumors at 3 T, suggesting that both variables may consistently reflect deeper insight into the microstructural characteristics of tumors.     

Diagnosis of head and neck squamous cell carcinoma using Raman spectroscopy: tongue tissues

Raman spectroscopy is a powerful optical technique capable of providing the structural information at the molecular level. Thus, the technique can be used to detect biochemical changes associated with carcinogenesis and identify the biomolecules involved in cancer. We studied the Raman spectral characteristics of normal, carcinoma in situ, and invasive squamous cell carcinoma (SCC) tissues of tongue, and identified the spectral features that can discriminate these three tissue types. We found that the intensities of Raman bands assignable to tryptophan increase while those attributable to protein keratin decrease when tissue changes from normal to invasive SCC. The variation observed in the intensity of many discriminating peaks including those of tryptophan and keratin as tissue changes from normal to carcinoma in situ and then to invasive SCC suggests that Raman spectroscopy can be used to monitor progression of the disease. We have also analyzed the data with multivariate statistical methods such as principal component analysis and discriminant function analysis. These chemometric methods clearly separate the whole data into three distinct groups consistent with results of pathology. We were able to detect with 91% success rate the normal and carcinoma in situ tissues and with 89% accuracy the invasive SCC tissues of the tongue. Copyright © 2011 John Wiley & Sons, Ltd.     

An integrated head immobilization system and high-performance RF coil for fMRI of visual paradigms at 1.5 T.

A flexible quadrature radiofrequency coil that maximizes the signal-to-noise ratio over the field of view of the human brain has been integrated into a head immobilization and visor system for fMRI at 1.5 T. Head motion is reduced by the visor that incorporates a head clamp and a simple visual sighting system that provides feedback on head position. This system is demonstrated in serial images by correction of deliberate head motions. The sensitivity at the cortical surface of fMRI using blood oxygenation level dependent contrast is increased significantly above that of the commercial rigid volume RF coil under the same acquisition conditions. This improved performance is demonstrated using visual activation and eye movement paradigms.     

Short echo time proton spectroscopy of the brain in healthy volunteers using an insert gradient head coil

An insert gradient head coil with built-in X, Y, and Z gradients was used for localized proton spectroscopy in the brain of healthy volunteers, using short echo time stimulated echo acquisition mode (STEAM) sequences. Volume of interest size was 3.4 ml, repetition time was 6.0 s, and echo times were 10 and 20 ms, respectively. Good quality proton spectra with practically no eddy current artefacts were acquired allowing observation of strongly coupled compounds, and compounds with short T₂ relaxation times. The gradient head coil thus permits further studies of compounds such as glutamine/glutamate and myo-inositols. These compounds were more prominent within grey matter than within white matter. Rough estimations of metabolite concentrations using water as an internal standard were in good agreement with previous reports.     

EMuS超导俘获线圈样机模型线圈的研制及测试

为了验证覆铝卢瑟福超导线缆的绕制工艺,在实验型μ子源(EMuS)超导俘获线圈样机正式生产前,须完成截面相同的300 mm短样模型线圈的研制和测试。本文介绍了EMuS超导俘获线圈样机模型线圈绕制生产线搭建、线圈绕制、线圈装配和VPI等工艺流程,并对VPI后的模型线圈进行了切割取样和测试,结果表明：模型线圈切片的线缆排列整齐、紧密,无气孔、干绝缘等缺陷;绝缘的电学性能和机械强度满足设计要求,模型线圈的绕制工艺满足EMuS超导俘获线圈样机制造的工程需求。     

用于近场集成光学头微透镜器件的灰度掩模技术

讨论了制作适用于近场集成光学头中的凸形、凹形微透镜和折衍射复合微透镜的灰度掩模技术。定性地给出了与几种典型的凸形、凹形微透镜和折衍射复合微透镜对应的灰度掩模版的设计实例 ,以及将它应用于光刻操作的情况 ,为采用灰度掩模技术制作适用于近场集成光学头中的微透镜器件奠定了基础。灰度掩模技术在微透镜器件的制作方面具有重要的应用前景 ,有助于简化制备工艺 ,降低制作成本 ,优化微透镜阵列的结构参数。