Evaluation of the pulmonary vasculature with three-dimensional magnetic resonance imaging techniques

Cardiac and respiratory motion during the long acquisition times required in 3d magnetic resonance imaging (MRI) can lead to excessive image degradation and consequently poor diagnosis and interpretation in the thoracic region. This paper addresses the issue of obtaining good image quality with 3d gradient echo (GE) imaging in the study of the pulmonary vascular system and its diseases. To study the pulmonary vascular system two approaches have been considered. First, a proton density/inflow weighted scan consisting of a syncopated 3d FLASH acquisition is used to provide a 3d pulmonary angiogram. Secondly, aT ₁ weighted scan using 3d IR-FLASH (inversion recovery FLASH) helps in determining the presence of pulmonary emboli. Multiple acquisitions and rectangular field of view are utilized to pseudogate to the respiratory period in order to reduce motion artifacts while keeping reasonable imaging times. Technical aspects on data collection during the approach to equilibrium and acquisition strategies in the presence of thoracic motion and its impact on vessel resolution are addressed. The method has proven successful for imaging volunteers and, more recently, in obtaining useful clinical information.     

Simultaneous image acquisition in magnetic resonance imaging

A simple multiplexing technique, implemented on a conventional NMR spectrometer is described. It shows that simultaneous acquisition of independent NMR signals with a unique detection chain is possible. Application is performed to proton imaging of objects in two non-interacting antennae. Address for correspondence: Laboratoire de Résonance Magnétique Nucleaire, Batiment 721, Université Claude Bernard Lyon I, 43, Boulevard du 11 Novembre 1918, 69622 Villeurbanne Cedex - France     

Clinical magnetic resonance imaging study of focal cerebral ischemia

Of the 47 patients with focal cerebral ischemia (FCI), 26 patients had completed stroke, 5 suffered from severe head injury, and 16 patients had anterior circulation aneurysms. Twenty-one stroke patients underwent bypass surgery and all traumatic patients underwent cerebral revascularization by omental graft. Aneurysm patients were surgically treated by neck clipping. In seven of them, aneurysm were occluded by nonferromagnetic (NFM) Ni-Ti alloy clips. In each case, comparative magnetic resonance imaging (MRI), computerized tomography (CT), and angiography (AG) findings were evaluated both before and after operation. MRI was much more sensitive than CT for detecting the ischemic tissue in all groups of patients. The difference in infarct area appreciated by MRI and CT in stroke patients turned out to be determined by the time between the onset of ictus and examination. MRI was particularly valuable for following up patients with FCI after cerebral revascularization by bypass surgery and omental graft. MRI was capable of replacing both postoperative AG and CT in patients operated on with NFM clips.     

Assessment of mulibrey nanism cardiopathy with functional magnetic resonance imaging

4. Conclusions It has been proposed that thickened pericardium is the main cause of the cardiac malfunction in Mulibrey nanism cardiopathy. In contrary, we showed that the pericardial thickness was normal in all our patients. Diastolic 1/3 filling was rapid, but in normal limits when taking into account the young age of our patient population. Although, LV time volume curves did not show remarkable pathologic changes, a restrictive physiology cannot be definitively excluded. LV end diastolic volumes and cardiac output were clearly reduced in our patients. It could be possible that in the course of time the reduced cardiac output affects clinical pattern of heart failure despite of normal systolic function. The reduced LA largest volume and atrial cyclic volume change may partly contribute to the low cardiac output observed in our patients.     

Application of magnetic resonance imaging in reptile medicine

Clinical examinations of reptiles are physically limited and therefore usually have to be complemented by other methods. This is especially true for Chelonians. A modern imaging technique like magnetic resonance imaging is well suited for this purpose. Its application and practical experiences with tortoises are presented.     

A brief review of hardware for catheter tracking in magnetic resonance imaging

Magnetic resonance imaging (MRI) has traditionally been used exclusively in a role for patient diagnosis. However, it is unlikely that this role is sufficient for its continued prominence in medical imaging. Instead, the more ambitious role in diagnosis and also therapy intervention will occur as demand for minimally invasive procedures increases. Fortunately, with recent improvement in technical specifications and creative pulse sequence design, MRI systems can now provide high quality near-real-time images that facilitate a variety of image-guided procedures, many based around delivery via catheters. While X-ray opacity is not available as a means for detecting the progression of the catheter in MRI systems today, a variety of novel hardware devices have been designed and used for MRI catheter tracking. This report provides a brief review of some fundamental methods for catheter tracking in MRI. Submitted at the ISMRM Hardware Workshop held 23–25 February 2001 in Cleveland, OH, USA.     

Turbo spin-echo sequences in magnetic resonance imaging of the brain: Physics and applications

Fast SE imaging provides considerable measure time reduction, high signal-to-noise ratios as well as similar contrast behavior compared to conventional SE sequences. Besides TR and TE_eff, echo train length (ETL), interecho time , and-space trajectory determine image contrast and image quality in fast SE sequences. True proton density contrast (CSF hypointense) and not too strong T2 contrast are essential requirements in routine brain MRI. A Turbo SE sequence with very short echo train length (ETL=3), short TE_eff and short interecho time (17 ms), and TR=2000 ms was selected for proton density contrast; a Turbo SE sequence with ETL=7, TE_eff=90 ms, =22 ms, and TR=3250 ms was selected for T₂-weighted images. Using both single-echo Turbo SE sequences yielded 50% measure time reduction compared to the conventional SE technique. Conventional SE and optimized Turbo SE sequences were compared in 150 patients resulting in very similar signal and contrast behavior. Furthermore, reduced flow artifacts in proton density—and especially in T₂-weighted Turbo SE images—and better contrast of high-intensity lesions in proton density-weighted Turbo SE images were found. Slightly reduced edge sharpness—mainly in T₂-weighted Turbo SE images—did not reduce diagnostic reliability. Differences between conventional and Turbo SE images concerning image contrast and quality are explained regarding special features of fast SE technique.Address for correspondence: Institut für Röntgendiagnostik, Klinikum der Universität Regensburg, Franz-Josef-Strauß-Allee 11, 93042 Regensburg, Germany. Additional reprints of this chapter may be obtained from the Reprints Department, Chapman & Hall, One Venn Plaza, New York, NY 10119.     

Interaction between grounding pads used for RF ablation therapy and magnetic resonance imaging

Objectives: To characterize artifacts and imaging problems in the presence of conductive grounding pads for RF ablation therapy as well as potential heating problems due to induction of eddy currents in the pads. Strategies for avoidance of those problems are developed. Materials and methods: Underlying principles of interactions between grounding pads and MR imaging are reported. Influential parameters, e.g., orientation in relation to the magnetic field, shape of the grounding pad, sequence type (spin-echo versus gradient echo) and magnetic field strength (0.2 T, 1.5 T, 3 T) were varied in systematic phantom studies. Heating effects due to induced eddy currents were estimated theoretically and measured by infrared imaging in an adapted set-up. Results: MR imaging artifacts are markedly dependent on the orientation and geometrical shape of the grounding pads. Visible signal extinction artifacts were more pronounced using spin-echo techniques than in gradient echo images and increased for higher field strengths. Suitable incisions in the grounding pad reduced eddy currents markedly and minimized image artifacts. Heating problems due to induced eddy currents by the RF transmitted for MR imaging were excluded by phantom measurements. Conclusions: Suitable positioning of the grounding pads and adaptation of their geometry provide clearly reduced artifacts in MR imaging.     

Magnetic resonance imaging of coronary arteries and coronary stenosis

Coronary artery imaging is an important investigation for the management of coronary artery disease. Alternative noninvasive imaging would be useful, but the small caliber and tortuosity of the coronary vessels and cardiac and respiratory motion create formidable imaging problems. We first studied 21 normal subjects and 5 with coronary artery disease established by X-ray contrast angiography, of whom 2 had undergone bypass grafting. Of these, 22 were imaged successfully. Identification of the artery was possible for the left main stem, left anterior descending, right coronary, and left circumflex arteries respectively in 95%, 91%, 95%, and 76%. The arterial diameter at the origin could be measured in 77%, 77%, 81%, and 63%. The mean ±SD arterial diameter in each case (4.8±0.8, 3.7±0.5, 3.9±0.9, and 2.9±0.6 mm) was not significantly different from reference values (allp=ns). The mean length of artery visualized was 10.4±5.2,46.7±22.8,53.7±27.9, and 26.3±17.5 mm. In 12 normal males, the total coronary area was 30.9±9.2 mm² and the ratio compared with body surface area was 16.4±4.4 mm² m^–2 (bothp=ns compared with reference values). In seven patients, with X-ray contrast coronary angiography, the proximal arterial diameter measured by magnetic resonance was 3.9±1.1 mm, and by X-ray contrast angiography 3.7±1.0 mm (p=ns). We then studied 17 patients with angina. Imaging of just the relevant artery was performed and analysis was blinded to the X-ray angiography results. Stenosis was identified on the magnetic resonance (MR) images by localized reduction in vessel signal intensity. Stenosis location by MR was assessed by measurement of its distance from a reference vessel, with correlation to the X-ray findings. X-ray coronary angiography showed 23 stenoses of which 15 (65%) were correctly located by blind assessment of the MR images. Of the eight remaining stenoses, a further 5 (63%) were correctly located on the MR images after retrospective comparison (overall sensitivity 87%). There were three lesions thought to represent stenosis by MR, which on review of the X-ray angiogram proved to be a minor stenosis <50% (two cases) or a tortuous vessel (one case). Greater signal loss was seen in the more severe stenoses. The stenosis length by MRI was greater than by X-ray (8.4 versus 5.1 mm,p<0.001). The overestimation of stenosis length may be due to turbulence.     

A time-efficient method for combinedt 1 andt 2 measurement in magnetic resonance imaging: Evaluation for multiparameter tissue characterization

A new magnetic resonance imaging high-resolution sequence is presented that allows for the collection of all data for determination ofT ₁ and as well as for multiexponentialT ₂ analysis within one measurement cycle.Noise preprocessing is performed in order to avoid systematic errors in relaxation parameter analysis and to increase the interexperimental reproducibility of the results. ForT ₂ analysis, an optimized Marquardt algorithm is used, in combination with image processing methods for both automatic detection of voxels with partial volume effects, and for speedup of the iterative nonlinear regression steps. Determination of longitudinal relaxation time is based on a sophisticated signal intensity ratio technique that computesT ₁ as the mean of up to eight individualT ₁ values, each weighted with its relativeT ₂ decay. Relative proton density is computed using results of the evaluations of both relaxation times. Validation of the method is accomplished by comparing phantom measurements with reference data acquired with spectroscopic sequences.In vivo examples of the computed parameter images taken from a study of experimental cerebral infarcts in rats are presented.The method allows one to acquire high-resolution parameter images within a measurement time that is tolerable even in clinical routine. Furthermore, the chosen evaluation concepts guarantee a short computation time. Therefore, an on-line computation of the parameter images and, in consequence, their direct use for diagnostic purposes appears feasible.     

The strengths of in vivo magnetic resonance imaging (MRI) to study environmental adaptational physiology in fish

Adaptational physiology studies how animals cope with their environment, even if this environment is subject to permanent fluctuations such as tidal or seasonal variations. Aquatic organisms are generally more prone to be exposed to osmotic, hypoxic and temperature challenges than terrestrial animals. Some of these challenges are more restraining in an aquatic environment. To date, very few studies have used in vivo magnetic resonance imaging (MRI) to uncover the physiological mechanisms that respond to or compensate for these challenges. This paper provides an overview of what has been accomplished thus far by using MRI to study the environmental physiology of fish. It introduces the reader to the use of small teleost fish such as carp (12 cm, 60 g) and eelpout (25 cm, 50 g) as models for such research and to provide new perceptions into the applicability of MRI tools based on new insights into the nature of MRI contrast. Representative MRI studies have made contributions to the identification of the lack of cell volume repair in stenohaline fish during osmotic stress. They have studied the underlying physiological mechanisms of brain anoxia tolerance in fish and have qualified the role of the cardio-circulatory system in setting thermal tolerance windows of fish.     

Potential use of the undersampling technique in the acquisition of nuclear magnetic resonance signals

This communication reviews the use of undersampling techniques to acquire NMR signals. Undersampling transforms bandpass free induction decay (FID) signals, centered at high frequencies, into lowpass signals or bandpass signals centered at much lower frequencies. Consequently, the analog electronic stages that perform the demodulation can be eliminated, gaining in stability and reducing the phase distortion while maintaining an equivalent or better signal to noise ratio when an adequate sampling rate is chosen. The technique has been tested on a BRUKER BIOSPEC BMT 47 40, and the results show that undersampling could be used to process NMR and MRI signals, extending the range of applications of the ‘digital radio’ techniques to NMR and MRI apparatus.     

High-resolution venography of the brain using magnetic resonance imaging

The purpose of this study was to evaluate a non-flow related magnetic resonance imaging method to visualize small veins independent of arteries in the human brain. A long TE, high-resolution 3D gradient echo MR acquisition was used to highlight venous information. The method is based on the paramagnetic property of deoxyhemoglobin and the resulting phase difference between veins and brain parenchyma at long echo times. The MR magnitude images were masked with a phase mask filter to enhance small structure visibility.. Venous information down to sub-pixel vessel diameters of several hundred microns is visible. Venous data are displayed in an angiographic manner using a minimum intensity projection algorithm. Both superficial veins and deep white matter veins are visible. The method has been successfully applied in volunteers. Preliminary results in patients with cerebral arteriovenous malformations indicate its potential in clinical applications. The proposed method is easy to implement and does not require administration of a contrast agent or application of specially designed rf pulses to highlight the veins. Rather it exploits the intrinsic magnetic properties (BOLD-effect) and the prolonged T ^_2* of venous blood. The method may be of diagnostic potential in the assessment of arteriovenous malformations or other vascular venous lesions. © 1998 Elsevier Science B.V. All rights reserved.     

Quantifying venous flow dynamics by flow-dephased and flow-rephased functional magnetic resonance imaging

By combining flow-dephased and flow-rephased diffusion weighting with blood oxygenation level dependent functional magnetic resonance imaging, it is possible to study flow dynamics in the venous network of the human brain. Thereby, ballistic flow, which conserves direction and velocity during echo time, is separated from diffusive flow with many changes in direction and velocity. By using this technique with very low diffusion/flow weighting, the mean velocity of ballistic flow was quantified in this study. The result of 10.9±3.2 cm/s strongly indicates that large venous vessels are the source of ballistic flow     

Magnetic resonance imaging of peripheral vascular disease and muscle atrophy in diabetes

Knowledge of the state of tissue hydration in patients suffering from peripheral vascular disease and neuropathy as a result of diabetes is important in their treatment. Further, because magnetic resonance imaging (MRI) is uniquely able to generate information about soft tissues and their water content, it is ideal for studying disorders of this kind. The feet and hands, often affected in diabetes, are ideal for studying fundamental aspects of the disease state and the response of patients to treatment. In this preliminary study, two related areas are reported: the measurement of diffusion coefficients in the finger and the visualization of the distribution of edema and muscle atrophy in the feet of people suffering from diabetes. Diffusion coefficients of water have been measured in the normal finger as a baseline study for a current patient study. It was found that the measured diffusion coefficient increased with subject age; this is not consistent with a direct-hydration model and it is conjectured that this could be linked to structural changes in proteins. Linked to this study, we have also imaged the feet of patients suffering from diabetes. Magnetization transfer has clearly demonstrated changes in muscle tissue with atrophy caused by motor neuropathy—in general, the amount of tissue water is increased as muscle volume decreases. Further, it is evident that these changes can be related to changes in cross-linking of protein and collagen molecules as muscle fibers become thinned, thus relating these studies to the diffusion coefficient measurements. The studies of the feet have also revealed artifacts in the images, consistent with the deposition of ferrous material in tissues. It is surmised that this is caused by hemosiderin deposits at ulcer sites associated with progress of the disease. MRI could be a useful tool for monitoring the distribution of ulcers below the skin surface and provide a means of determining the response of patients to treatment.     

Magnetic resonance functional imaging of the brain at 4 t

Blood Oxygenation Level Dependent (BOLD) contrast imaging of human brain function using echo-planar imaging at 4 T gives good freedom from motion artifact, high signal-to-noise ratio/unit time, and adequate spatial resolution. Studies were made of brain activation associated with perceptual and cognitive tasks of several minutes duration.Several cortical areas show task-dependent activity consistent across subjects, in images with a spatial resolution of 2.5 mm×2.5 mm×5 mm and a temporal resolution of up to 1 s. Multislice data were obtained at a rate of up to five slices per second. At 4 T, fractional changes of magnetic resonance (MR) image intensity up to 25% were observed.Novel cross-correlation methods, including the effect of the temporal point-spread function associated with the relatively slow hemodynamic response of the brain, allow activation maps of the brain to be generated with statistically meaningful thresholds.With appropriate data analysis, it is clear that oxygenation changes in large draining veins distant from active neural tissue do not dominate the changes observed, especially when brain tasks activating only a limited volume of gray matter are chosen. This is consistent with downstream dilution of blood oxygenation changes and direct optical observations of functional brain activity in animals.     

Comparison of sequences for depicting bone marrow alterations in osteomyelitis applied in a low field strength magnetic resonance imaging system

Objective/Patients: to investigate the efficacy of standard sequences of a low field system for the detection of osteomyelitis, we tested TlwI pre and post i.v. contrast, T2w and fat suppressed IR sequences. Design: on the basis of clinical and laboratory evidence, pathology reports, and three phase granulocyte scintigraphy, osteomyelitis was diagnosed in 18 of 21 patients with Charcot's joints. A consecutive low and high field magnetic resonance (MR) scan confirmed osteomyelitic bone marrow changes in the same osseous regions. These 18 diabetic patients were then studied on a 0.2 Tesla dedicated MR system (Esaote ArtoScan) using TlwI (SE: relaxation time (TR) 520/echo time (TE) 24: axial and coronal) before and after i.v. application of 0.1 mmol/1 Gd-DTPA/kg BW, T2w imaging (TSE: TR 3500/TE 80 or TR 2000/TE 120: axial), and fat suppressed inversion recovery (IR) imaging (short tau inversion recovery (STIR): TR 3000/TE 30/TI 80 or inversion recovery gradient echo (IRGE)/fat suppressed IRGE (GEFS): TR 1000/TE 16m 80: coronal). Results: the SE Tlw sequence showed a significantly higher contrast-to-noise ratio (CNR) before administration of i.v. contrast. The TSE T2w pulse sequence demonstrated bone marrow changes superiorily utilizing a TE of 120 ms (CNR=16.5±2.7 compared to 5.5±2.5 with TE=80 ms). The IRGE showed a higher CNR than the standard STIR (CNR=19.2±2.5 compared to 12.4±2.9). Conclusion: fat suppressed IRGE imaging and longer TE in T2w TSE sequences result in a significantly better, CNR in osteomyelitis. This way, using optimized sequences, low field systems are apt to depict bone marrow changes in the course of osteomyelitis.     

被动安检高分辨准光成像系统设计及仿真研究

被动安检设备没有电磁波辐射源,仅靠接收物体自身的辐射来识别目标。相比主动成像设备,无目标闪烁和对人体辐射等缺点,具有广阔的应用前景。准光成像系统是被动安检设备中的重要组成部分,直接关系到检测图像的分辨率、视场、有无光斑畸变等性能以及系统各部分之间的匹配。本工作采用被动焦面线阵扫描成像方式,设计了一个适用于被动安检设备的高分辨准光成像系统。通过仿真计算证实该系统成像带宽达4GHz,被成像人体目标各部位的空间分辨率为20~25mm,在2m×0.6m视场内基本无像差。因此,该准光系统应用于安检成像设备可以达到对违禁物品进行有效检测的目的,并且具有无辐射、高空间分辨率、大视场以及无像差等优点。     

Magnetic resonance imaging of regional cardiac function in the mouse

In this paper we introduce an improved harmonic phase (HARP) analysis for complementary spatial modulation of magnetization (CSPAMM) tagging of the mouse left ventricular wall, which enables the determination of regional displacement fields with the same resolution as the corresponding CINE anatomical images. CINE MRI was used to measure global function, such as the ejection fraction. The method was tested on two healthy mouse hearts and two mouse hearts with a myocardial infarction, which was induced by a ligation of the left anterior descending coronary artery. We show that the regional displacement fields can be determined. The mean circumferential strain for the left ventricular wall of one of the healthy mice was –0.09 ± 0.04 (mean ± standard deviation), while for one of the infarcted mouse hearts strains of –0.02 ± 0.02 and –0.10 ± 0.03 were found in the infarcted and remote regions, respectively.     

Characterization and detection of experimental rat gliomas using magnetic resonance imaging

Two different experimental rat brain tumours (F98 glioma and 9L glioma) were characterized using T1 and T2, apparent diffusion coefficient (ADC) and magnetization transfer ratio (MTR). Even though both tumours appeared homogenous at the early stage of growth, significant differences were measured for all parametric images between tumours and normal brain tissue. Irrespective of the sequence used, tumour lesion/normal parenchyma contrast for the non-infiltrative 9L was twice that of the infiltrative F98 glioma. The use of spin preparation via an inversion pulse in a fast spin echo sequence increases contrast by a factor of 20–30.