Reduction of signal fluctuation in functional MRI using navigator echoes

Functional magnetic resonance imaging is sensitive to signal fluctuations due to physiological motion and system instability. In this paper, motion-related signal fluctuations are studied, and a method that uses navigator echoes to monitor and compensate for signal fluctuations in a gradient-echo sequence is described. The technique acquires a “navigator” signal before the application of the phase-encoding and readout gradients and corrects the phase of the subsequently acquired imaging data. This technique was implemented on a 4 Tesla whole body system and validated on normal volunteers. With this technique, temporal fluctuations in image intensity were substantially reduced and improved functional activation maps were obtained.     

Development of a compact MRI system for trabecular bone volume fraction measurements.

A compact MRI system for measuring trabecular bone volume fraction (TBVF) of the calcaneus was developed with the use of a 0.21 T permanent magnet and portable MRI console. The entire system weighed < 600 kg and was installed in a 2 m x 2 m space. Two cross-sectional spin-echo images of a heel acquired with external reference phantoms (total measurement time = 5 min) were used to quantify the TBVF of the calcaneus. The linearity and reproducibility of the measurements were evaluated by means of proton density-adjusted phantoms. Comparative measurements with quantitative ultrasound (QUS) in groups of healthy female volunteers showed a relatively high positive correlation (R(2) = 0.4539, 0.2693) between TBVF and the speed of sound (SOS). These results demonstrate the potential of this new system for measuring bone density. Magn Reson Med 52:440-444, 2004. Copyright 2004 Wiley-Liss, Inc.     

Cardiac MRI in mice at 9.4 Tesla with a transmit-receive surface coil and a cardiac-tailored intensity-correction algorithm

PURPOSE: To evaluate the use of a transmit-receive surface (TRS) coil and a cardiac-tailored intensity-correction algorithm for cardiac MRI in mice at 9.4 Tesla (9.4T). MATERIALS AND METHODS: Fast low-angle shot (FLASH) cines, with and without delays alternating with nutations for tailored excitation (DANTE) tagging, were acquired in 13 mice. An intensity-correction algorithm was developed to compensate for the sensitivity profile of the surface coil, and was tailored to account for the unique distribution of noise and flow artifacts in cardiac MR images. RESULTS: Image quality was extremely high and allowed fine structures such as trabeculations, valve cusps, and coronary arteries to be clearly visualized. The tag lines created with the surface coil were also sharp and clearly visible. Application of the intensity-correction algorithm improved signal intensity, tissue contrast, and image quality even further. Importantly, the cardiac-tailored properties of the correction algorithm prevented noise and flow artifacts from being significantly amplified. CONCLUSION: The feasibility and value of cardiac MRI in mice with a TRS coil has been demonstrated. In addition, a cardiac-tailored intensity-correction algorithm has been developed and shown to improve image quality even further. The use of these techniques could produce significant potential benefits over a broad range of scanners, coil configurations, and field strengths.     

Skeletal age assessment in children using an open compact MRI system

MRI may be a noninvasive and alternative tool for skeletal age assessment in children, although few studies have reported on this topic. In this article, skeletal age was assessed over a wide range of ages using an open, compact MRI optimized for the imaging of a child's hand and wrist, and its validity was evaluated. MR images and their three‐dimensional segmentation visualized detailed skeletal features of each bone in the hand and wrist. Skeletal age was then independently scored from the MR images by two raters, according to the Tanner–Whitehouse Japan system. The skeletal age assessed by MR rating demonstrated a strong positive correlation with chronological age. The intrarater and inter‐rater reproducibilities were significantly high. These results demonstrate the validity and reliability of skeletal age assessment using MRI. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

Development of a compact MRI system for measuring the trabecular bone microstructure of the finger.

A compact MRI system for measuring the trabecular bone (TB) microstructure of the finger using a high-field-strength (1.0T) permanent magnet was developed. The entire system was installed in a 0.6 mx1.2 m space. One male and 36 female subjects participated in the imaging experiments. The TB of the distal phalanx of the middle finger was imaged at a voxel resolution of (160 microm)3 using a three-dimensional (3D) driven equilibrium spin-echo (SE) imaging sequence (imaging time=approximately 14 min). The image data sets obtained yielded two distinct peaks for the bone and marrow in image intensity histograms when no motion was present. The structural parameters obtained through 3D image analysis show that this compact system is potentially useful for evaluating bone quality.     

自旋回波及其改良序列在肺实质磁共振成像中的应用

目的　选择肺实质磁共振功能成像合适的序列。方法　选择健康志愿者 8人 ,健康家犬 8条 ,对自旋回波及其改良序列进行调试、组合。结果　选择的序列是单激发快速自旋回波序列和单激发反转快速自旋回波序列。结论　适合肺实质磁共振功能成像的序列是中心相位编码方式的单激发快速自旋回波序列和单激发反转快速自旋回波序列     

MRI一站式综合扫描对活体肝移植的应用价值

 目的 研究MRI一站式综合扫描对活体肝移植的应用价值.方法 选择活体肝移植的患者及供体各20名.进行MRI扫描,包括3.0T磁共振采用常规血管四期增强扫描.磁共振平扫采用flash序列及TSE序列扫描.血管增强后选择与平扫相同的T,vibe压脂序列进行扫描.依据个体需要,在注射后40-120 min之间进行延迟扫描.以手术所见为参考标准,比较MRI平扫与一站式MRI综合扫描(MRI平扫+MRCP+血管增强+肝实质增强+延迟)对病灶检出的敏感度,评估其诊断价值.结果 MRI平扫对供体/受体血管变异、胆道变异和癌变的检出敏感性分别为40.0%、72.22%、75.0%.而一站式MRI扫描对病灶检出的敏感性都为100.0%,两种方法对后两种病变的敏感性比较差异有统计学意义(P<0.05).结论 在活体肝移植中一站式MRI综合扫描检查具有重要价值.     

Novel approaches to low-cost MRI

This paper presents a combination of speculative approaches, some related to earlier work and some apparently novel, which show great promise in providing a new class of MRI machines that would be considerably less expensive. This class would have advantages and disadvantages as compared to existing MRI, over and above that of low cost. The disadvantages include the apparent inability to perform classic spectroscopy, and limited flexibility in the area of selective excitation. The advantages include a fundamental immunity to inhoinogenelty and susceptibility problems, the ability to create a wide class of machines that are designed for specific anatomy-related applications, the ability to design open machines for physician access, and improved capability for high speed imaging. Generic to all of the methods presented are a pulsed polarizing field and an oscillatory readout bias field. The pulsed field initially polarizes the magnetic moments. Sinat it is not on during the readout operation it has negligible homogeneity requirements since changes in the field amplitude will merely shade the image intensity. During readout a relatively low bias field is used. To enable the use of a relatively inhomogeneous bias field, an osciliatory field is used that has a zero average value. This prevents any long-term buildup of phase errors due to a frequency error associated with inhomogeneity. Thus the average bias frequency will be determined solely by the frequency rather than the amplitude of the bias field. Three methods are described, all including the above features. The first two involve imaging in the laboratory frame, while the third involves imaging in the rotating frame. The second approach requires no RF excitation and the third approach uses RF bias and gradient signals. Some approaches to slice selection are described.     

3-T MRI safety assessments of magnetic dental attachments and castable magnetic alloys

Objectives:

To assess the safety of different magnetic dental attachments during 3-T MRI according to the American Society for Testing and Materials F2182-09 and F2052-06e1 standard testing methods and to develop a method to determine MRI compatibility by measuring magnetically induced torque.

Methods:

The temperature elevations, magnetically induced forces and torques of a ferromagnetic stainless steel keeper, a coping comprising a keeper and a cast magnetic alloy coping were measured on MRI systems.

Results:

The coping comprising a keeper demonstrated the maximum temperature increase (1.42 °C) for the whole-body-averaged specific absorption rate and was calculated as 2.1 W kg⁻¹ with the saline phantom. All deflection angles exceeded 45°. The cast magnetic alloy coping had the greatest deflection force (0.33 N) during 3-T MRI and torque (1.015 mN m) during 0.3-T MRI.

Conclusions:

The tested devices showed minimal radiofrequency (RF)-induced heating in a 3-T MR environment, but the cast magnetic alloy coping showed a magnetically induced deflection force and torque approximately eight times that of the keepers. For safety, magnetic dental attachments should be inspected before and after MRI and large prostheses containing cast magnetic alloy should be removed. Although magnetic dental attachments may pose no great risk of RF-induced heating or magnetically induced torque during 3-T MRI, their magnetically induced deflection forces tended to exceed acceptable limits. Therefore, the inspection of such devices before and after MRI is important for patient safety.     

恶性脑膜瘤的CT、MRI影像分析

目的探讨恶性脑膜瘤的CT、MRI的影像学特征,以提高诊断正确率,为临床手术提供可靠依据。方法回顾性分析近几年来我院经手术病理证实的恶性脑膜瘤(35例)的CT、MRI资料。结果恶性脑膜瘤:(1)T2WI信号及密度不均29例(82.1%);(2)T1WI信号不均27例(77.1%);(3)边缘不规则21例(60%);(4)粗短的硬脑膜尾征19例(54.2%);(5)颅骨受侵8例(22.8%);(6)瘤内钙化14例(40%);(7)多发2例(占5.7%)。结论恶性脑膜瘤的影像学表现具有一定的特征性,定性诊断需要综合分析,注意与良性脑膜瘤的鉴别。     

圆柱型Halbach永磁魔环研究进展

受局部饱和和局部退磁效应的影响以及磁材料性能的限制,永磁体的气隙磁场一般难以达到4T。Halbach永磁魔环结构磁材料利用效率高,且能够产生比剩磁更高的静磁场,因而近年来在磁共振成像领域得到了广泛关注。本文针对Halbach永磁魔环的构成形式,介绍了永磁魔环、阵列永磁魔环、永磁双魔环和单边永磁魔环的研究现状,分析了这些魔环结构由于永磁磁块形状、尺寸,永磁材料受温度变化影响,和加工工艺等因素引起的磁场均匀性较差等主要问题,并对圆柱型Halbach永磁魔环在NMR、MRI和便携式分析仪器等方面的应用进行了展望。     

钯银合金对磁共振成像影响的初步研究

目的: 探讨口腔修复用钯银合金对磁共振成像的影响.材料和方法:在临床随机选取中切牙至第二磨牙的钯银合金桩核4个,金属全冠2个,烤瓷全冠1个.放置于受检者右侧腹部,采用临床常用序列(SSFSE;FRFSE;GRE; DWI)扫描.然后,选择2位口腔中有多个钯银合金固定修复体的志愿者,采用颅脑磁共振常用序列(T1Flair;FSE;GRE;DWI)和参数分别在1.5T和3.0T的磁场中扫描.结果:钯银合金在1.5T磁场中行腹部和颅脑磁共振检查时无明显伪影形成,对磁共振成像无影响.在3.0T磁场中在牙齿周围有轻微伪影出现,对脑部、上下颌骨均无影响.结论:①钯银合金固定修复体在磁共振检查时没有热效应和投射效应,对患者和仪器是安全的.装有钯银合金固定修复体的患者可以做磁共振检查.②钯银合金固定修复体在做MRI检查时在低场强的磁场中没有显著的伪影.在高场强的磁场中,有轻微伪影,但只局限于牙齿,不影响周边组织结构及器官的成像.③建议给装有钯银合金固定修复体的患者做磁共振检查时尽量使用低磁场,并且最好不要用GRE序列.     

Coronary MRI with a Respiratory Feedback Monitor: The 2D Imaging Case

The inability to return the heart to the same position for all breath-holds during 2D coronary MR imaging can result in imaging different locations than desired. This can lead to problems such as (i) missing a whole vessel, or a part of it, (ii) misaligning segments of vessels imaged in different breath-holds, and (iii) degrading image quality when a single slice is acquired in multiple breath-holds. To reduce inconsistencies in the breath-hold level, we designed a respiratory feedback monitor (RFM) that uses a bellows to monitor the circumference of the subject's chest. When the circumference of the subject's chest is within preset limits, an audio signal alerts subjects to hold their breath at that position. Use of the RFM significantly reduces the problems caused by inconsistent breath-holds and the number of breath-holds for an examination in 2D coronary MR imaging.     

核磁弥散成像及增强扫描在诊断非缺血性颅脑病变中的意义

目的探讨核磁弥散成像及增强扫描在诊断非缺血性颅脑病变中的意义。方法收集经临床证实的非缺血性颅脑病变56例,均作常规MRI T2WI、T1WI、增强扫描及DWI,分析其DWI信号及增强表现。结果32例颅脑肿瘤、11例感染性病变、9例血肿、4例多发性硬化病例,病变在DWI上呈高或稍高信号。结论常规MRI T2WI、T1WI扫描图像,弥散成像及增强扫描在非缺血性颅脑病变的定性诊断中具有重要意义。     

MRI in central nervous system infections: A simplified patterned approach

Recognition and characterization of central nervous system infections poses a formidable challenge to the neuro-radiologist. Imaging plays a vital role, the lesions typically being relatively inaccessible to tisue sampling. The results of an accurate diagnosis are endlessly rewarding, given the availability of excellent pharmacological regimen. The availability of numerous magnetic resonance (MR) sequences which provide functional and molecular information is a powerful tool in the hands of the radiologist. However, the plethora of sequences and the possibilities on each sequence is also intimidating, and often confusing as well as time consuming. While a large number of reviews have already described in detail the possible imaging findings in each infection, we intend to classify infections based on their imaging characteristics. In this review we describe an algorithm for first classifying the imaging findings into patterns based on basic MR sequences (T1, T2 and enhancement pattern with Gadolinium), and then sub-classify them based on more advanced molecular and functional sequences (Diffusion, Perfusion, Susceptibility imaging, MR Spectroscopy). This patterned approach is intended as a guide to radiologists in-training and in-practice for quickly narrowing their list of differentials when faced with a clinical challenge. The entire content of the article has also been summarised in the form of flow-charts for the purpose of quick reference.     

Coronary artery imaging at 0.5 t using segmented 3d echo planar imaging

The application of segmented 3D gradient echo EPI at 0.5 T for coronary artery imaging is described. Experiments were performed using fat suppression, ECG triggering, and a patient-controlled breath-holding scheme. This approach provides a sufficient signal-to-noise ratio for thin contiguous slices in conjunction with a phased array cardiac receive coil. Wide 3D volumes, covering the proximal branches of the coronary tree, were measured with a high spatial resolution. Such data sets can be used for subsequent vessel segmentation. Furthermore, data out of narrow 3D volumes were obtained containing fewer slices angulated in the direction of a selected coronary artery. This provides a good visualization of the selected vessel over several centimeters without the need for segmentation.     

Three-dimensional chemical shift-selective MRI of a ruptured intracranial dermoid cyst

We report a man with a ruptured intracranial dermoid cyst, suffering from headache, nausea, vomiting and a generalised seizure. MRI was performed before and 2 weeks after surgical resection. On T1-weighted images the tumour gave high signal, as did fatty material in the frontal and parietal brain sulci. Identification of this hyperintense material as lipids was possible by chemical-shift-selective 3 D gradient-echo imaging, which provided excellent contrast between the subarachnoid lipids and the adjacent normal brain, with a good spatial resolution. Possible complications of subarachnoid and intraventricular lipid particles after dermoid cyst rupture are discussed and the diagnostic value of 3 D chemical-shift-selective additional to conventional T1-weighted spin-echo images in identification of even small amounts of fat is emphasised.