A microscope slide probe for high resolution imaging at 11.7 tesla

We present details of a novel RF coil for use in NMR microscopy. This coil is an inductively coupled surface coil which is built into a standard microscope slide. The coil is highly sensitive and is also designed so that the sample to be imaged can be viewed under an optical microscope. This facility allows comparison of optical and NMR micrographs, as well as accurate sample positioning. Using the slide coil, images of onion epidermal cells with an in-plane resolution of 4.5 μm have been produced.     

In vivo high resolution coronal MRI of the wrist at 8.0 tesla

The wrists of asymptomatic volunteers were imaged at 8 and 1.5 T. The signal-to-noise ratio, spatial resolution, and soft tissue contrast of the 8 T images were much greater. The increased signal-to-noise ratio available at 8 T allows acquisition of high quality, high resolution images and visualization of intricate anatomy at scan times acceptable for clinical imaging.     

MR imaging of enucleated human eyes at 1.4 tesla

Proton magnetic resonance images obtained for nine human eyes with various pathology were correlated with histological findings. One eye with retinal gliosis, three eyes with malignant melanoma, one eye with extraocular squamous cell carcinoma, one pair of eyes with incipient senile cataracts, and one pair of eyes with diabetic cataracts were examined at a field strength of 1.4 T using spin-echo and inversion-recovery signal acquisition protocols. Eyes were examined unfixed and within 24 h of enucleation. Most images were characterized by a homogeneous vitreous and a lens made conspicuous by its low-signal intensity. The anterior chamber, ciliary body, and optic nerve could be seen, but the retina and choroidal layers could not be distinguished. The dynamic range of vitreal signal was quite wide and allowed all lesions in this series to be well-contrasted against the vitreous. In addition, lenticular edema accompanying cataract formation gave a strong signal. Signal differences were apparent between paired cataractous lenses with a 1.6% difference in water content. Magnetic resonance imaging is a modality that promises good contrast for ocular imaging and sensitive detection of incipient cataractous change.     

Fast interleaved echo-planar imaging with navigator: High resolution anatomic and functional images at 4 tesla

Echo-planar Imaging (EPI) Is sensitive to magnetic field inhomogeneities, which lead to signal loss and geometric distortions of the image. Magnetic field inhomogeneities induced by susceptibility differences, as encountered in the human body, increase with the magnetic field strength, thus, complicating implementation of high resolution EPI techniques on high magnetic field systems. These problems were overcome by using a fast multishot high resolution EPI method that uses variable flip angles, center-out k-space sampling, and navigator echoes. This approach maximizes signal-to-noise ratio, reduces flow artifacts, and permits correction of intersegment amplitude and phase variations, providing high spatial and temporal resolution. This scheme can be implemented with a single magnetization preparation for contrast that precedes the segments. The utility of this ultrafast segmented EPI technique with navigator is demonstrated for anatomic and functional imaging studies on the human brain at 4 T.     

Magnetic resonance imaging of tibial classic adamantinoma at 2 tesla

Adamantinoma is a rare malignant neoplasm arising most often in the tibia and is locally aggressive. Conservative surgical treatment frequently is followed by recurrence and magnetic resonance (MR) imaging plays an important role in preoperative planning. Magnetic resonance features of this tumor have not been described in detail. We report three cases of classic tibial adamantinoma examined at 2 Tesla. High-resolution images with findings that may influence management are discussed.     

Diffusion imaging of the prostate at 3.0 tesla

OBJECTIVES: We sought to assess the efficacy of diffusion imaging in the differential diagnosis of prostatic carcinoma using a 3.0 T scanner and parallel imaging technology. MATERIALS AND METHODS: Diffusion-weighted images were acquired using a single shot echo-planar imaging sequence with b = 0 and 500 seconds/mm. Apparent diffusion coefficient (ADCy) values were calculated in tumor and healthy-appearing peripheral zone for 62 patients. Diffusion tensor images were also acquired in 25 patients and mean diffusivity and fractional anisotropy determined. RESULTS: Significant differences were noted between prostatic carcinoma (1.33 +/- 0.32 x 10(-3) mm2/s) and peripheral zone (1.86 +/- 0.47 x 10(-3) mm2/s) for ADCy. Significant differences between the 2 tissue types were also noted for mean diffusivity and fractional anisotropy. Utilizing a cut-off of 1.45 x 10(-3) mm/s for mean diffusivity, a sensitivity of 84% and a specificity of 80% were obtained. CONCLUSIONS: Diffusion imaging of the prostate was implemented at high magnetic field strength. Reduced ADC and increased fractional anisotropy values were noted in prostatic carcinoma.     

Role of very high order and degree B(0) shimming for spectroscopic imaging of the human brain at 7 tesla

With the advent of ultrahigh field systems (7T), significant improvements in spectroscopic imaging (SI) studies of the human brain have been anticipated. These gains are dependent upon the achievable B₀ homogeneity, both globally (σB, over the entire regions of interest or slice) and locally (σB, influencing the linewidth of individual SI voxels within the regions of interest). Typically the B₀ homogeneity is adjusted using shim coils with spatial distributions modeled on spherical harmonics which can be characterized by a degree (radial dependence) and order (azimuthal symmetry). However, the role of very high order and degree shimming (e.g., 3rd and 4th degree) in MRSI studies has been controversial. Measurements of σB and σB were determined from B₀ field maps of 64 × 64 resolution. In a 10 mm thick slice taken through the region of the subcortical nuclei, we find that in comparison to 1st–2nd degree shims, use of 1st–3rd and 1st–4th degree shims reduces σB by 29% and 55%, respectively. Using a SI voxel size of ～1cc with an estimate of σB from 3 × 3 × 3 B₀ map pixels in this subcortical region, the number of pixels with σB of less than 5 Hz increased from 24 to 59% with 1st–3rd and 1st–4th over 1st–2nd degree shims, respectively. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.     

Magnetic resonance imaging of the pancreas at 3.0 tesla: qualitative and quantitative comparison with 1.5 tesla

OBJECTIVES: We sought to perform a preliminary comparison of signal-to-noise ratio (SNR) and image quality for magnetic resonance imaging (MRI) of the pancreas at 1.5 and 3 T. MATERIALS AND METHODS: Two imaging cohorts were studied using a T2-weighted, single-shot fast spin-echo pulse sequence and a T1-weighted, fat-suppressed 3D gradient-echo pulse sequence. In the first cohort, 4 subjects were imaged using identical imaging parameters before and after contrast administration at 1.5 and 3.0 T. The SNR was quantified for the pancreas as well as for the liver, spleen, and muscle. In a second cohort of 12 subjects in whom the receiver bandwidth was adjusted for field strength, SNR measurements and qualitative rankings of image quality were performed. RESULTS: In the study cohort using identical imaging parameters at both magnetic field strengths, the mean (SD) ratios of SNR at 3.0 to 1.5 T of the single-shot fast spin-echo images for the pancreas, liver, spleen, and muscle were 1.63 (0.39), 1.82 (0.39), 1.45 (0.18), 2.01 (0.16), respectively. For the precontrast fat-suppressed 3D gradient-echo sequence, the corresponding ratios were 1.28 (0.29), 1.26 (0.30), 1.16 (0.27), and 1.76 (0.45), respectively; for the arterial phase, the corresponding ratios were 2.02 (0.28), 1.60 (0.42), 1.47 (0.26), and 1.94 (0.32), respectively; and for the delayed postcontrast phase, the corresponding ratios were 1.63 (0.51), 2.01 (0.25), 1.66 (0.06), and 2.31 (0.47), respectively. The SNR benefit of 3.0 T was significantly greater on contrast-enhanced as compared with noncontrast T1-weighted 3D gradient-echo images. In the second study cohort, SNR was superior at 3.0 T, although the use of a reduced readout bandwidth at 1.5 T substantially diminished the advantage of the higher field system. With qualitative comparison of images obtained at the 2 magnetic field strengths, the fat-suppressed 3D gradient-echo images obtained at 3.0 T were preferred, whereas the single shot fast spin-echo images obtained at 1.5 T were preferred because of better signal homogeneity. CONCLUSIONS: Our results in a small cohort of volunteers and patients demonstrate a marked improvement in SNR at 3.0 T compared with 1.5 T (by a factor of 2 in some cases) when identical imaging parameters were used. The SNR advantage at 3.0 T is diminished but persists when the receiver bandwidth is adjusted for magnetic field strength. The results suggest that 3.0 T may offer promise for improved body MRI, although further technical development to optimize SNR and improve signal homogeneity will be needed before its full potential can be achieved.     

正常人小腿肌肉3 T MR扩散张量成像的参数优化

目的 探讨在3 T MR上应用体部相控阵线圈加脊柱表面线圈作为接收线圈进行小腿肌肉DTI的可行性,并优化序列参数,探索理想的层厚和b值.方法 采用完全随机设计方法,先随机选取5名健康志愿者,将脑部DTI序列应用于小腿进行肌肉DTI,根据DTI原始图像及后处理图像存在问题进行初步参数优化并应用于下一步研究;再随机选取5名健康志愿者进行不同层厚的小腿肌肉DTI,对其后处理图像进行质量评分,评分高者为理想层厚并应用于下一步研究;另外随机选取5名健康志愿者进行不同b值的小腿肌肉DTI,对其后处理图像进行质量评分,评分高者为理想b值.图像质量评分采用随机设计的多组秩和检验.结果 将脑部DTI序列应用于小腿进行肌肉DTI,其原始图像及后处理图像存在3个方面问题:原始图像信噪比(SNR)极低,各肌肉显示不清;出现局部肌肉信号丢失,尤以胫骨前肌为著;化学位移和shost伪影比较明显.4.5和6 mm层厚的肌肉显示评分分别(7.0±0.O)、(8.6 4-0.9)和(9.0±0.0)分,信号丢失评分分别(5.0±0.0)、(12.8±2.6)和(13.8±2.2)分,总评分分别(22.0±0.0)、(30.1±3.8)和(31.0±4.1)分,差异均有统计学意义(F值分别为21.000、30.544和12.390;P值均<0.05).4 mm层厚的肌肉显示、信号丢失及总评分均低于5 mm和6 mm层厚(q值分别为4.896、6.120、6.327、7.138、3.863和4.043;P值均<0.05).b值为400 s/mm2<的肌肉显示、信号丢失及总评分分别为(9.0±0.0)、(14.0±2.2)和(33.0±2.2)分,分别高于b值为800 s/mm2[分别为(7.0±0.0)、(6.2±2.2)、(21.8±3.4)分]和1000 s/mm2[分别为(7.0±0.O)、(5.0±0.0)、(20.6±2.2)分]的评分(q值分别为3.873、3.873、6.650、7.672、7.101和5.917;P值均<0.05);b值为600 s/mm2的肌肉显示、信号丢失及总评分分别为(8.2±1.1)、(13.0±2.3)和(30.8±3.8)分,分别高于b值为800和1000 s/nun2的评分(q值分别为3.873、3.873、5.797、6.820、5.326和5.917;P值均<0.05),与b值为400 s/mm2的评分差异无统计学意义(q值分别为2.582、0.852、1.775;P值均>0.05).结论 初步研究结果表明应用3 T MR进行正常人小腿肌肉DTI是可行的.通过优化DTI序列,应用体部相控阵线圈加脊柱表口自I线圈作为接收线圈能获得可以接受的SNR.进行小腿肌肉成像时,以层厚为5 mm、b值为400 s/mm2的图像质量为佳.     

Readout-segmented EPI for rapid high resolution diffusion imaging at 3 T

Readout mosaic segmentation has been suggested as an alternative approach to EPI for high resolution diffusion-weighted imaging (DWI). In the readout-segmented EPI (RS-EPI) scheme, segments of k-space are acquired along the readout direction. This reduces geometric distortions due to the decrease in readout time. In this work, further distortion reduction is achieved by combining RS-EPI with parallel imaging (PI). The performance of the PI-accelerated RS-EPI scheme is assessed in volunteers and patients at 3T with respect to both standard EPI and PI-accelerated EPI. Peripherally cardiac gated and non-gated RS-EPI images are acquired to assess whether motion due to brain pulsation significantly degrades the image quality. Due to the low off-resonance of PI-driven RS-EPI, we also investigate if the eddy currents induced by the diffusion gradients are low enough to use the Stejskal-Tanner diffusion preparation instead of the twice-refocused eddy-current compensated diffusion preparation to reduce TE. It is shown that non-gated phase corrected DWI performs equally as well as gated acquisitions. PI-driven DW RS-EPI images with substantially less distortion compared with single-shot EPI are shown in patients-allowing the delineation of structures in the lower parts of the brain. A twice-refocused diffusion preparation was found necessary to avoid blurring in the DWI data. This paper shows that the RS-EPI scheme may be an important alternative sampling strategy to EPI to achieve high resolution T2-weighted and diffusion-weighted images.     

Ultra high field MRI at 8 Tesla of subacute hemorrhagic stroke

PURPOSE: Optimal treatment strategies and neurologic outcome after stroke depend on an accurate characterization of the lesion. There is a need for high resolution noninvasive imaging for assessment of the infarct size, perfusion, and vascular territory. MRI at the ultra high field (UHF) of 8 T offers unprecedented resolution, but its utility for stroke evaluation has not been determined yet. METHOD: A 55-year-old man with hypertension experienced sudden onset of speech arrest and right-sided hemiparesis that resolved in < 24 h with minimal neurologic deficit. MRI at 1.5 T showed initially a left posterior frontal lesion with subacute infarct (hyperintense on T2-weighted spin echo images) and right-sided frontal and periventricular lesions consistent with chronic infarct. There were many smaller white matter lesions. Delayed studies showed high signal changes involving the gray matter only on T1-weighted images. RESULTS: Gradient echo and rapid acquisition with relaxation enhancement (RARE) multislice images revealed a serpentine area of low signal in the left posterior frontal lobe gray matter suggestive of a hemorrhagic infarct, right-sided frontal lesion also showing iron deposits, multiple periventricular and cortical areas with abnormal high signal regions that were consistent with old infarcts, and numerous small vessels readily visible, more prominent on the right. CONCLUSION: MRI at 8 T displays lesions with a high resolution and striking anatomic details. Susceptibility to iron and sensitivity to detect blood products are increased at 8 T. The imaging characteristics at high field are different from those at low field, but both represent findings of iron products.     

Multicoil array for high resolution imaging of the breast

An RF multicoil array for high resolution imaging of the breast is presented. The RF multicoil is incorporated into a dedicated breast imaging apparatus that holds either breast in gentle compression during imaging. Simulations and imaging experiments demonstrate the high sensitivity and RF homogeneity of the multicoil array. Direct comparisons are made between the multicoil and a surface coil. In vivo images are presented to demonstrate the homogeneity of the RF field of the multicoil over the entire breast. An in vivo image of both high temporal and spatial resolution is shown, which demonstrates the sensitivity of the RF multicoil array that may be achieved in a practical imaging experiment.     

Advances in magnetic resonance imaging of the pelvis at 0.15 tesla

The recent development of improved commercial radiofrequency coils and multiecho, multislice software for low field strength magnetic resonance systems has markedly increased the clinical utility of magnetic resonance imaging (MRI) of the pelvis at low field strengths. An evaluation of 70 patients with a variety of pelvic lesions and 14 normal volunteers who were studied using 0.15 T resistive magnet scanner revealed that anatomic structures and a variety of mass lesions could be clearly depicted in transaxial, sagittal and coronal planes using this updated system. Accurate characterization of lesions was possible in many instances using T2 weighted multiecho scans with echo time (TE) ranging from 30 ms to 120 ms (45 ms-180 ms using a reduced bandwidth technique). T1 weighted multislice scans demonstrated anatomic structures to best advantage and calculation of T1 and T2 relaxation times frequently facilitated more accurate differential diagnosis, particularly in the case of ovarian lesions.