Showing papers in "Topics in Magnetic Resonance Imaging in 2004"

SMASH, SENSE, PILS, GRAPPA: how to choose the optimal method.

Abstract: Fast imaging methods and the availability of required hardware for magnetic resonance tomography (MRT) have signifi- cantly reduced acquisition times from about an hour down to several minutes or seconds. With this development over the last 20 years, magnetic resonance imaging (MRI) has become one of the most im- portant instruments in clinical diagnosis. In recent years, the greatest progress in further increasing imaging speed has been the develop- ment of parallel MRI (pMRI). Within the last 3 years, parallel imag- ing methods have become commercially available, and therefore are now available for a broad clinical use. The basic feature of pMRI is a scan time reduction, applicable to nearly any available MRI method, while maintaining the contrast behavior without requiring higher gra- dient system performance. Because of its faster image acquisition, pMRI can in some cases even significantly improve image quality. In the last 10 years of pMRI development, several different pMRI re- construction methods have been set up which partially differ in their philosophy, in the mode of reconstruction as well in their advantages and drawbacks with regard to a successful image reconstruction. In this review, a brief overview is given on the advantages and disad- vantages of present pMRI methods in clinical applications, and ex- amples from different daily clinical applications are shown.

Perfusion imaging using arterial spin labeling.

Abstract: Arterial spin labeling is a magnetic resonance method for the measurement of cerebral blood flow. In its simplest form, the perfusion contrast in the images gathered by this technique comes from the subtraction of two successively acquired images: one with, and one without, proximal labeling of arterial water spins after a small delay time. Over the last decade, the method has moved from the experimental laboratory to the clinical environment. Furthermore, numerous improvements, ranging from new pulse sequence implementations to extensive theoretical studies, have broadened its reach and extended its potential applications. In this review, the multiple facets of this powerful yet difficult technique are discussed. Different implementations are compared, the theoretical background is summarized, and potential applications of various implementations in research as well as in the daily clinical routine are proposed. Finally, a summary of the new developments and emerging techniques in this field is provided.

Epidemiology of white matter lesions.

Abstract: This is an overview of the prevalence of, and risk factors for white matter lesions in older persons. The prevalence of white matter lesions is high and increases with age. Most risk factors that have been examined in association with white matter lesions are similar to those for cardiovascular disease, including elevated blood pressure, diabetes atherosclerosis, homocysteine levels, and markers of oxidative stress. The genetic contributions to white matter lesions are also reviewed.

Parallel imaging at high field strength: synergies and joint potential.

Abstract: MRI faces fundamental limitations in terms of sensitivity and speed. These limitations can be effectively tackled by the transition to higher field strengths and parallel imaging technology. Owing to largely independent physics, the two approaches can be readily combined. Considering the specific advantages and disadvantages of high field strength and parallel imaging, it is found that the combination is particularly synergistic. In the joint approach, the two concepts play different roles. Higher field strength acts as a source of higher baseline signal-to-noise ratio (SNR), while parallelization acts as a means of converting added SNR into a variety of alternative benefits. This interplay holds promise for a broad range of clinical applications, as recently illustrated by several imaging studies at 3 T. As a consequence, clinical MRI at 3 T and higher is expected to rely more on parallel acquisition than at lower field strength. The specific synergy with parallel imaging may even make 3 T the field strength of choice for a range of exams that conventionally work best at 1.5 T or less.

Perfusion MR imaging of brain tumors.

Abstract: Brain tumors rank second as the cause of cancer-related deaths in children and adults younger than 34 years old, and they are seen in adults of all ages. Primary malignant brain tumors are associated with the third highest cancer-related mortality rate and a disproportionate level of disability and morbidity. Considering this, accurate diagnosis and grading of brain tumors are critical to determining prognosis and therapy. Equally important is to evaluate for tumor status during therapy to assess for therapeutic response and treatment-related complications. Brain tumors can be characterized as a heterogeneous group of neoplasm with a correspondingly wide variation in malignant phenotype and a diverse array of imaging features. Magnetic resonance (MR) imaging with intravenous contrast agent is the test of choice to diagnose and monitor brain tumors before, during, and after therapy. Recent advances in imaging methods such as diffusion-weighted imaging, perfusion imaging, and spectroscopic imaging all have in common the ability to provide quantitative cellular, hemodynamic, and metabolic information that may enhance our understanding of brain tumor biology, help us to better assess treatment response, more accurately determine tumor activity during therapy, and differentiate recurrent tumor and treatment related complications. In this article, we will review the basics of brain tumor imaging and focus on the role of perfusion MR imaging in improving accurate diagnosis and monitoring brain tumors during therapy. Both strengths and shortcomings of perfusion MR imaging over standard anatomic MR imaging will be discussed as will important pitfalls of the technique.

MR spectroscopy of brain tumors.

Abstract: MR Spectroscopy provides a means to characterize the metabolite profiles of tumoral and non-tumoral lesions in the brain. This article aims to provide tools to increase our sensitivity and specificity of neurodiagnosis, particularly in combination with other advanced MRI techniques such as perfusion MR imaging.

Current concepts and advances in clinical parallel magnetic resonance imaging.

Abstract: Parallel imaging (PI) is one of the most promising recent advances in MRI technology and has, similar to the introduction of multidetector helical scanning in CT, revolutionized MR imaging. The speed of all conventional MRI methods has been limited by either gradient strength or their switching times. The basic idea in PI is to use some of the spatial information contained in the individual elements of a radiofrequency (RF) receiver coil array to increase imaging speed. These PI techniques are removing some of the previous limitations in speed of MRI scanners and set the basis for accelerated image formation. Initially, PI was motivated by the wish to accelerate image acquisition without reducing the spatial resolution of the image. However, depending on the application, it turned out that PI harbors several other advantages. Among those is the possibility for higher spatial resolution, shorter breath-holds or multiple averaging to diminish motion artifacts, reduced image blurring and geometric distortions, better temporal resolution, and means for navigator correction. This overview focuses on technical aspects, clinical applications, and ongoing research in different areas of the human body. The critical review demonstrates PI's great versatility as well as the current trends to use this unique technique in the majority of clinical scan protocols.

Diffusion tensor imaging in cerebral tumor diagnosis and therapy.

Abstract: Diffusion-weighted MR images and their analysis using the tensor model open up many new possibilities for tissue characterization, surgical planning, and treatment follow-up in patients with cerebral neoplasms. These possibilities are only just beginning to be fully explored. This article reviews the physical principles underlying diffusion tensor imaging (DTI); the various postprocessing methods available for DTI data in the context of tumor imaging; the commonly encountered patterns of tumor-related alteration to cerebral white matter, as depicted by directionally encoded color maps; and the current state of the art in DTI-based tumor diagnosis and treatment planning.

Cerebral perfusion imaging by bolus tracking

Abstract: Cerebral perfusion may be visualized by the dynamic imaging of an intravenously injected bolus (a few milliliters) of clinically approved gadolinium-containing contrast media. During its passage through the vasculature of the brain, the contrast agent induces magnetic field disturbances, which can be seen as signal loss on appropriately weighted dynamic MRI. This article deals with the quantitative analysis of such signal changes, first in terms of tracer concentration and then, via the mathematical approach of deconvolution, in terms of tissue microvascular physiology, culminating in quantitative estimates on a pixel-by-pixel basis of physiologic parameters, such as cerebral blood volume, mean transit time, and cerebral blood flow.

Pathological aging of the brain: an overview.

Abstract: The number of elderly people is increasing rapidly and, therefore, an increase in neurodegenerative and cerebrovascular disorders causing dementia is expected. Alzheimer disease (AD) is the most common cause of dementia. Vascular dementia, dementia with Lewy bodies, and frontotemporal dementia are the most frequent causes after AD, but a large proportion of patients have a combination of degenerative and vascular brain pathology. Characteristic magnetic resonance (MR) imaging findings can contribute to the identification of different diseases causing dementia. The MR imaging protocol should include axial T2-weighted images (T2-WI), axial fluid-attenuated inversion recovery (FLAIR) or proton density-weighted images, and axial gradient-echo T2*-weighted images, for the detection of cerebrovascular pathology. Structural neuroimaging in dementia is focused on detection of brain atrophy, especially in the medial temporal lobe, for which coronal high resolution T1-weighted images perpendicular to the long axis of the temporal lobe are extremely important. Single photon emission computed tomography and positron emission tomography may have added value in the diagnosis of dementia and may become more important in the future, due to the development of radioligands for in vivo detection of AD pathology. New functional MR techniques and serial volumetric imaging studies to identify subtle brain abnormalities may also provide surrogate markers for pathologic processes that occur in diseases causing dementia and, in conjunction with clinical evaluation, may enable a more rigorous and early diagnosis, approaching the accuracy of neuropathology.

Magnetic resonance imaging of perineural spread of head and neck malignancies.

Abstract: Perineural invasion is a common mechanism of spread of head and neck cancers. Imaging plays an important role in detection of this condition because a large number of patients with perineural spread (PNS) are clinically asymptomatic. Accurate detection of PNS requires an understanding of anatomy of commonly involved neural pathways. High level of suspicion on the part of the radiologist, awareness of common imaging signs of PNS and careful attention to imaging technique can aid in earlier detection of this condition.

Functional MRI in the Brain Tumor Patient

Abstract: Functional magnetic resonance imaging (fMRI) has been adopted almost universally by disciplines that endeavor to understand how the brain works. As basic scientists tune the technique, clinicians are increasingly able to apply brain mapping with fMRI to their clinical practice. We present here a guide to using fMRI in a clinical setting. We discuss the basic considerations of functional brain mapping in patients with brain tumors including: patient screening and training, paradigm design, data analysis and interpretation of the fMRI scans.

Perfusion MR imaging of extracranial tumor angiogenesis.

Abstract: Dynamic contrast-enhanced MRI (DCE-MRI) using small molecular weight gadolinium chelates enables noninvasive imaging characterization of tissue vascularity. Depending on the technique used, data reflecting tissue perfusion (blood flow, blood volume, mean transit time), microvessel permeability surface area product, and extracellular leakage space can be obtained. Insights into these physiological processes can be obtained from inspection of kinetic enhancement curves or by the application of complex compartmental modeling techniques. Combining morphologic and kinetic features can increase the accuracy of clinical diagnoses. Potential clinical applications include screening for malignant disease, lesion characterization, monitoring lesion response to treatment, and assessment of residual disease. Newer applications include prognostication, pharmacodynamic assessments of antivascular anticancer drugs, and predicting efficacy of treatment. For dynamic MRI to enter into widespread clinical practice, it will be necessary to develop standardized approaches to measurement and robust analysis approaches.

Superparamagnetic iron oxide nanoparticles: nodal metastases and beyond.

Abstract: Superparamagnetic iron oxide (SPIO) nanoparticles are unique MR contrast agents and are of great interest for their multiple potentials. SPIO nanoparticles have a higher diagnostic accuracy for detecting metastatic lymph nodes than conventional MR studies, particularly in head and neck. The impact of this unique MR contrast agent on treatment decision of patients with head and neck cancer needs to be investigated in comparison with contrast-enhanced CT. As MR technology advances, the accuracy of SPIO nanoparticles for detection of metastasis certainly improves; thus, 1 day we may be able to reliably detect metastases in stage N0 patients, so that treatment strategy is established for each individual patient. This article presents physiologic properties of SPIO, technical considerations and diagnostic accuracy for imaging with SPIO, and other potential applications of SPIO agents.

Parallel imaging in MR angiography.

Abstract: The recently developed techniques of parallel imaging with phased array coils are rapidly becoming accepted for magnetic resonance angiography (MRA) applications. This article reviews the various current parallel imaging techniques and their application to MRA. The increased scan efficiency provided by parallel imaging allows increased temporal or spatial resolution, and reduction of artifacts in contrast-enhanced MRA (CE-MRA). Increased temporal resolution in CE-MRA can be used to reduce the need for bolus timing and to provide hemodynamic information helpful for diagnosis. In addition, increased spatial resolution (or volume coverage) can be acquired in a breathhold (eg, in renal CE-MRA), or in otherwise limited clinically acceptable scan durations. The increased scan efficiency provided by parallel imaging has been successfully applied to CE-MRA as well as other MRA techniques such as inflow and phase contrast imaging. The large signal-to-noise ratio available in many MRA techniques lends these acquisitions to increased scan efficiency through parallel imaging.

MR imaging of brachial plexus.

Abstract: :The brachial plexus is a complex anatomic component originating from ventral rami of the lower cervical nerve roots from C5 to C8 and upper thoracic spinal nerve roots from T1, providing sensory and motor innervation to the upper extremities. As it is inaccessible to palpation, clinical eva

Quantitative MRI: hidden age-related changes in brain tissue.

Abstract: The advent of MRI has made a remarkable progress in the understanding of age-related brain changes providing a noninvasive tool to study in vivo the normally aging individuals at multiple time points. However, conventional MRI techniques are unable to detect and quantify age-related microstructural changes that have been documented at the post-mortem examination of brain tissues. More sophisticated, quantitative MR techniques such as magnetization transfer imaging, diffusion tensor imaging, and proton MR spectroscopy have been shown to be sensitive to microstructural and metabolic changes that occur in gray and white matter over the course of life span. This review highlights some of these innovative, quantitative MR techniques that are particularly relevant for the study of occult age-related brain tissue changes. Characterization of the in vivo patterns of molecular and cellular changes that occur in the normal aging brain is of crucial importance to understand the pathophysiology of normal cognitive decline and to interpret observed changes in neurodegenerative diseases.

Reduced data acquisition methods in cardiac imaging.

Abstract: In cardiac imaging, acquisition speed is of primary importance. While improved performance has mainly been achieved through improvements in gradient hardware in the past, further developments along this direction are limited due to physiological constraints such as the risk of peripheral nerve stimulation. With the introduction of parallel imaging, alternative means for increasing acquisition speed have become available. Using information from multiple receiver coils, images can be reconstructed from a sparsely sampled set of data. In practice, parallel imaging allows for 2- to 3-fold acceleration of the imaging process in typical cardiac applications. Further increases in acquisition speed are, however, difficult to achieve for current clinical field strengths and typical field of views. To address the limited gain in acquisition speed achievable with parallel imaging, a new set of methods has been proposed to take into account the similarity of image information at different time points during a dynamic series. Using these methods, 5- to 8-fold acceleration can be achieved in cardiac imaging. It is the purpose of this paper to review cardiac applications of reduced data acquisition methods with focus on parallel imaging and the recently developed k-t BLAST and k-t SENSE techniques.

Pediatric head and neck masses.

Abstract: Most neck masses in the pediatric head and neck region are benign. Congenital, developmental, and inflammatory lesions make up most of the masses in the pediatric head and neck. For example, neck masses due to inflammatory lymphadenitis are common in children because of the frequency of upper respiratory tract infections. Although many of the malignant tumors in children are found in the head and neck, they account for only a small portion of the neck masses. The choice of the imaging modality is based on a number of factors, several of which are unique to the pediatric population. Although the bulk of disease entities are adequately evaluated by CT, MRI can provide additional vital information in many cases. MRI provides better soft tissue characterization than CT, has multiplanar capabilities. In this article, we will attempt to provide an overview of conditions that present as neck masses.

Dynamic contrast-enhanced MR imaging.

Abstract: Dynamic contrast-enhanced magnetic resonance imaging is a useful clinical tool in evaluation of soft tissue neoplasm and lymph nodes in head and neck. It is thought to be a useful predictor of response to radiotherapy for head and neck carcinoma and used to monitor the treatment and distinguish post-therapeutic changes from recurrent mass with greater confidence. It can be used to distinguish between normal and malignant tissue and to differentiate a malignant lymphoma from other lymph nodal enlargements. The technique utilizes relative differences in microvasculature and microcirculation between malignant and non-malignant tissue to achieve greater contrast in signal imaging following bolus contrast administration. This article explains the underlying principles and imaging techniques for this new diagnostic tool. The clinical applications and technical challenges are discussed. The future challenges and some contradictions in results are also outlined.

Clinical perspectives in perfusion: neuroradiologic applications.

Abstract: Summary:Imaging of cerebral perfusion, particularly by the dynamic tracking of a bolus of gadolinium-based contrast agent, has emerged from the experimental laboratory and become a routine aspect of neuroradiologic practice. This article discusses the practical implementation of “perfusion” protocol

Artifact reduction using parallel imaging methods.

Abstract: Multiple receiver coils produce images with different but complementary views of a patient. This can be used to shorten scans times but there often remain image artifacts caused by patient motion or physiological processes such as flowing blood. This paper reviews how the extra information from the multiple coils can be used to reduce image artifacts. In one method, affected portions of data can be identified and discarded but enough information is still available to reconstruct an improved image. In other methods, the motion itself is determined and the corrupted data is then corrected, leading to an image with reduced artifacts. Results are presented from images corrupted by motion or by flowing blood. Multiple receiver coils produce images with different but complementary views of a patient. This can be used to shorten scans times but there often remain image artifacts caused by patient motion or physiological processes such as flowing blood. This paper reviews how the extra information from the multiple coils can be used to reduce image artifacts. In one method, affected portions of data can be identified and discarded but enough information is still available to reconstruct an improved image. In other methods, the motion itself is determined and the corrupted data is then corrected, leading to an image with reduced artifacts. Results are presented from images corrupted by motion or by flowing blood.

Magnetic resonance spectroscopy of head and neck neoplasms

Abstract: Magnetic resonance spectroscopy (MRS) is a validated noninvasive method for evaluation of possible malignant tumor and lymph nodes of the head and neck. From its roots as a budding research application, it has made the critical transition to a widespread clinical tool. MRS analyzes the tissue at a molecular level and searches for the presence of specific metabolites, which are markers for malignancy. Differentiation of benign from malignant neoplasm, detection of recurrence of malignant tumor and noninvasive treatment monitoring of treated or untreated tumor are some of the important utilities of MRS. One dimensional 1H-MRS is the most popular and promising technique for spectroscopic analysis while P-31 MRA and two-dimensional correlated spectroscopy (2D COSY) have also showed some promise. This article describes the application of magnetic resonance spectroscopy for evaluation of malignant tumors of the neck.

Parallel imaging techniques in functional MRI.

Abstract: Originally developed for increased scanning velocity in cardiac imaging, parallel imaging (PI) techniques have recently also been applied for the reduction of artifacts in single-shot techniques. In functional brain imaging (fMRI) techniques, PI has been used for several purposes. It has been applied to reduce the distortions caused by the length of the echo-planar imaging readout, diminution of the gradient-related acoustic noise, as a means to increase acquisition speed or to increase the achievable brain coverage per unit time. In this article, the different applications of PI techniques in fMRI are reviewed, together with the basic theoretical background and the recently developed hardware necessary to achieve rapid, high signal-to-noise ratio PI-fMRI.

Parallel imaging of the abdomen.

Abstract: Parallel imaging holds great potential for improving the quality of diagnostic abdominal MRI. The increased imaging speed afforded by parallel imaging can be translated into the obvious benefits of reduced scan time with set resolution and coverage, improved spatial resolution with set imaging time and coverage, increased anatomic coverage for a set imaging time and resolution, or some combination of the above. Additionally, the reduction in scan time can also allow some sequences that normally require multiple breath-holds to be performed with only one, or simply make breath-hold imaging possible for more patients. The decreased echo-train length allows for truer T2-weighting, less magnetic susceptibility artifact, and less blurring with echo-train imaging. Dynamic contrast-enhanced sequences can be acquired with improved temporal or spatial resolution. All of these potential advantages come with the trade-off of decreased signal-to-noise ratio, but for many patients, the benefits far outweigh the drawbacks and can vastly improve the diagnostic quality of abdominal MRI.

Beyond perfusion: cerebral vascular reactivity and assessment of microvascular permeability.

Abstract: Beyond perfusion, several other related methods are under development for a more complete characterization of different aspects of vascular function. This article will be divided into two parts focusing on two emerging imaging-based assays of vascular status: 1) the cerebrovascular reactivity to a vasodilatory stimulus and 2) the microvascular permeability to a blood-borne MR-visible tracer. Taken together, these approaches can be seen to extend our interrogation of vascular functional and structural integrity and to offer promising clinical and experimental applications.

Application of SENSE in clinical pediatric body MR imaging.

Abstract: Pediatric body magnetic resonance imaging can be challenging because of the intrinsic physiologic motions from high respiratory and heart rates. Unlike in adults, in preadolescent children, breath holding is often not an available option. In this paper, the authors describe their clinical experience using the recently available parallel imaging technique, sensitivity encoding (SENSE), to improve on the image quality of pediatric body MR examinations. The three major areas described include rapid imaging using SENSE to achieve time-resolved contrast-enhanced MRA, improvement in single-shot turbo spin echo imaging with SENSE, and decreasing respiratory motion artifact with combination of SENSE and multiple signal averages in various pulse sequences. Some practical suggestions of using SENSE in clinical pediatric body MR examinations are highlighted. The addition of SENSE has allowed for improved image quality and more efficient overall MR examinations for pediatric body imaging.

Qualitative MRI: evidence of usual aging in the brain.

Abstract: :The understanding of aging in our society with steadily increasing life expectancy is an important challenge in medical science since socioeconomic pressure increases in parallel. Magnetic resonance imaging is a most useful tool to explore age related changes in the central nervous system e

Improved perfusion and tracer kinetic imaging using parallel imaging.

Abstract: Dynamic contrast enhanced MRI is a valuable method to get functional information of the microvessel system. This article will describe the improvement in perfusion and tracer kinetic imaging by using parallel imaging techniques. The basic principles of kinetic analysis, the contrast mechanisms used and the scan techniques applied are addressed to give a better understanding of the influence of parallel imaging on dynamic contrast enhanced MRI. The effect of different parallel imaging techniques on scan efficiency and scan planning is part of the discussion. From the clinical applications shown, it can be concluded that parallel imaging is a very important contribution for improved dynamic contrast enhanced imaging.

Fast patient workup in acute stroke using parallel imaging.

Abstract: Treatment of ischemic stroke is a very frustrating topic for neurologists. Presently, the most promising therapy seems to be thrombolysis of the clot. However, this intervention is associated with complication risks, most significantly the risk of post-treatment hemorrhage. This risk of bleeding increases not only with the size of the ischemic brain tissue but also with the time-to-treatment interval. Studies suggest a time window of 3 hours for most effective treatment. Hence, there is demand for a rapid imaging workup, which thus far has been accomplished with computed tomography. Because of the risks associated with thrombolytic therapy, more detailed information is desirable. The distinction between patients with viable ischemically challenged neural tissue and those with complete infarcts is of great importance, and computed tomography is insufficient for this task. This is also true for outlining the etiology of stroke, which may impact treatment. For these tasks, magnetic resonance imaging has been proposed. However, comprehensive imaging protocols take time, which is limited in stroke treatment. Therefore, new imaging techniques are required that provide both in-depth information and short scanning times. Parallel imaging is uniquely suited for this purpose.