Magnetic resonance imaging (MRI), formerly referred to as magnetic resonance tomography (MRT) or nuclear magnetic resonance (NMR), is a method used to visualize the inside of living organisms as well as to detect the composition of geological structures. It is primarily used to demonstrate pathological or other physiological alterations of living tissues and is a commonly used form of medical imaging. MRI has also found many novel applications outside of the medical and biological fields such as rock permeability to hydrocarbons and certain non-destructive testing methods such as produce and timber quality characterization. ^* The devices used in medicine are expensive, costing approximately $1 million USD per tesla for each unit (common field strength ranges from 0.3 to 3 teslas), with several hundred thousand dollars per year of upkeep costs.

Background

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Nomenclature

Magnetic resonance imaging was developed from knowledge gained in the study of nuclear magnetic resonance. The original name for the medical technology is nuclear magnetic resonance imaging (NMRI), but the word nuclear is almost universally dropped. This is done to avoid the negative connotations of the word nuclear, and to prevent patients from associating the examination with radiation exposure, which is not one of the safety concerns for MRI. Scientists still use NMR when discussing non-medical devices operating on the same principles.

More on [ Magnetic resonance imaging ]

pubmed: 0730-725X

Improvement of spectral density-based activation detection of event-related fMRI data.
Ngan SC, Hu X, Tan LH, Khong PL Related Articles Improvement of spectral density-based activation detection of event-related fMRI data. Magn Reson Imaging. 2009 Sep;27(7):879-94 Authors: Ngan SC, Hu X, Tan LH, Khong PL For event-related data obtained from an experimental paradigm with a periodic design, spectral density at the fundamental frequency of the paradigm has been used as a template-free activation detection measure. In this article, we build and expand upon this detection measure to create an improved, integrated measure. Such an integrated measure linearly combines information contained in the spectral densities at the fundamental frequency as well as the harmonics of the paradigm and in a spatial correlation function characterizing the degree of co-activation among neighboring voxels. Several figures of merit are described and used to find appropriate values for the coefficients in the linear combination. Using receiver-operating characteristic analysis on simulated functional magnetic resonance imaging (fMRI) data sets, we quantify and validate the improved performance of the integrated measure over the spectral density measure based on the fundamental frequency as well as over some other popular template-free data analysis methods. We then demonstrate the application of the new method on an experimental fMRI data set. Finally, several extensions to this work are suggested. PMID: 19535208 [PubMed - indexed for MEDLINE]
On MRI turbulence quantification.
Dyverfeldt P, GÃ¥rdhagen R, Sigfridsson A, Karlsson M, Ebbers T Related Articles On MRI turbulence quantification. Magn Reson Imaging. 2009 Sep;27(7):913-22 Authors: Dyverfeldt P, Gårdhagen R, Sigfridsson A, Karlsson M, Ebbers T Turbulent flow, characterized by velocity fluctuations, accompanies many forms of cardiovascular disease and may contribute to their progression and hemodynamic consequences. Several studies have investigated the effects of turbulence on the magnetic resonance imaging (MRI) signal. Quantitative MRI turbulence measurements have recently been shown to have great potential for application both in human cardiovascular flow and in engineering flow. In this article, potential pitfalls and sources of error in MRI turbulence measurements are theoretically and numerically investigated. Data acquisition strategies suitable for turbulence quantification are outlined. The results show that the sensitivity of MRI turbulence measurements to intravoxel mean velocity variations is negligible, but that noise may degrade the estimates if the turbulence encoding parameter is set improperly. Different approaches for utilizing a given amount of scan time were shown to influence the dynamic range and the uncertainty in the turbulence estimates due to noise. The findings reported in this work may be valuable for both in vitro and in vivo studies employing MRI methods for turbulence quantification. PMID: 19525079 [PubMed - indexed for MEDLINE]
Relaxo-volumetric multispectral quantitative magnetic resonance imaging of the brain over the human lifespan: global and regional aging patterns.
Saito N, Sakai O, Ozonoff A, Jara H Related Articles Relaxo-volumetric multispectral quantitative magnetic resonance imaging of the brain over the human lifespan: global and regional aging patterns. Magn Reson Imaging. 2009 Sep;27(7):895-906 Authors: Saito N, Sakai O, Ozonoff A, Jara H The objective of this study was to determine the T1, T2 and secular-T2 relaxo-volumetric brain aging patterns using multispectral quantitative magnetic resonance imaging, both globally and regionally, and covering an age range approaching the full human lifespan. Fifty-one subjects (28 males, 23 females; age range: 0.5-87 years) were studied consisting of 18 healthy volunteers and 33 patients. Patients were selected after carefully reviewing their radiology reports to have either normal-by-MRI findings (25 patient subjects) or small focal pathology less than 6 mm in size (eight patient subjects). All subjects were MR imaged at 1.5 T with the mixed turbo spin echo pulse sequence. The soft tissues inside the cranial vault, termed intracranial matter (ICM), were segmented using a dual-clustering segmentation algorithm. ICM segments were further divided into six subsegments: bilateral anterior cerebral, posterior cerebral and cerebellar subsegments. T1, T2 and secular-T2 relaxation time histograms of all segments were generated and modeled with Gaussian functions. For each segment, the volumes of white matter, gray matter and cerebrospinal fluid were calculated from the T1 histograms. The age-related tendencies of three quantitative MRI parameters (T1, T2 and secular-T2) and the fractional tissue volumes showed four distinct periods of life, specifically a maturation period (0-2 years), a development period (2-20 years), an adulthood period (20-60 years) and a senescence period (60 years and older). For all ages, the anterior cerebral subsegment exhibited consistently longer gray matter T1s and shorter white matter T1s than the posterior cerebral and cerebellar subsegments. Volumetric age-related changes of the cerebellar subsegment were more gradual than in the cerebral subsegments. This study shows that relaxometric and volumetric age-related changes are synchronized and define the same four periods of brain evolution both globally and regionally. PMID: 19520539 [PubMed - indexed for MEDLINE]
Fast low-angle positive contrast steady-state free precession imaging of USPIO-labeled macrophages: theory and in vitro experiment.
Mascheri N, Dharmakumar R, Zhang Z, Paunesku T, Woloschak G, Li D Related Articles Fast low-angle positive contrast steady-state free precession imaging of USPIO-labeled macrophages: theory and in vitro experiment. Magn Reson Imaging. 2009 Sep;27(7):961-9 Authors: Mascheri N, Dharmakumar R, Zhang Z, Paunesku T, Woloschak G, Li D The feasibility of imaging macrophages labeled with ultrasmall superparamagnetic iron-oxide nanoparticles (USPIO) with fast low-angle positive contrast steady-state free precession (FLAPS) was investigated through theory and in vitro experiment. Human macrophage cells were labeled with USPIO and imaged at 1.5 T. The metric "visibility," which combines magnitude and spatial extent of positive contrast, was used to evaluate the images. Negative contrast steady-state free precession (SSFP) and gradient-echo (GRE) imaging were also evaluated. Positive contrast was observed for relatively high concentrations of labeled cells for flip angles less than alpha=25 degrees . Theoretical and experimental results indicate that positive visibility (VIS(POS)) was maximized at alpha=10 degrees and 15 degrees. Low flip angle SSFP also provided negative contrast comparable to standard SSFP and GRE imaging. Results suggest that USPIO-labeled macrophages are capable of producing the conditions necessary for positive contrast with FLAPS at clinical field strength (1.5 T) and resolution (0.8x0.8x3 mm(3)). PMID: 19520536 [PubMed - indexed for MEDLINE]
A fully automated algorithm under modified FCM framework for improved brain MR image segmentation.
Sikka K, Sinha N, Singh PK, Mishra AK Related Articles A fully automated algorithm under modified FCM framework for improved brain MR image segmentation. Magn Reson Imaging. 2009 Sep;27(7):994-1004 Authors: Sikka K, Sinha N, Singh PK, Mishra AK Automated brain magnetic resonance image (MRI) segmentation is a complex problem especially if accompanied by quality depreciating factors such as intensity inhomogeneity and noise. This article presents a new algorithm for automated segmentation of both normal and diseased brain MRI. An entropy driven homomorphic filtering technique has been employed in this work to remove the bias field. The initial cluster centers are estimated using a proposed algorithm called histogram-based local peak merger using adaptive window. Subsequently, a modified fuzzy c-mean (MFCM) technique using the neighborhood pixel considerations is applied. Finally, a new technique called neighborhood-based membership ambiguity correction (NMAC) has been used for smoothing the boundaries between different tissue classes as well as to remove small pixel level noise, which appear as misclassified pixels even after the MFCM approach. NMAC leads to much sharper boundaries between tissues and, hence, has been found to be highly effective in prominently estimating the tissue and tumor areas in a brain MR scan. The algorithm has been validated against MFCM and FMRIB software library using MRI scans from BrainWeb. Superior results to those achieved with MFCM technique have been observed along with the collateral advantages of fully automatic segmentation, faster computation and faster convergence of the objective function. PMID: 19395212 [PubMed - indexed for MEDLINE]
Image correction during large and rapid B(0) variations in an open MRI system with permanent magnets using navigator echoes and phase compensation.
Li J, Wang Y, Jiang Y, Xie H, Li G Related Articles Image correction during large and rapid B(0) variations in an open MRI system with permanent magnets using navigator echoes and phase compensation. Magn Reson Imaging. 2009 Sep;27(7):988-93 Authors: Li J, Wang Y, Jiang Y, Xie H, Li G An open permanent magnet system with vertical B(0) field and without self-shielding can be quite susceptible to perturbations from external magnetic sources. B(0) variation in such a system located close to a subway station was measured to be greater than 0.7 microT by both MRI and a fluxgate magnetometer. This B(0) variation caused image artifacts. A navigator echo approach that monitored and compensated the view-to-view variation in magnetic resonance signal phase was developed to correct for image artifacts. Human brain imaging experiments using a multislice gradient-echo sequence demonstrated that the ghosting and blurring artifacts associated with B(0) variations were effectively removed using the navigator method. PMID: 19369023 [PubMed - indexed for MEDLINE]

Magnetic Resonance in Medicine

Parallel excitation in the human brain at 9.4 T counteracting k-space errors with RF pulse design
Xiaoping Wu, J. Thomas Vaughan, Kâmil U[gbreve]urbil, Pierre-François Van de Moortele Wed, 16 Dec 2009 10:09:00 -0000
Multidimensional spatially selective radiofrequency (RF) pulses have been proposed as a method to mitigate transmit B1 inhomogeneity in MR experiments. These RF pulses, however, have been considered impractical for many years because they typically require very long RF pulse durations. The recent development of parallel excitation techniques makes it possible to design multidimensional RF pulses that are short enough for use in actual experiments. However, hardware and experimental imperfections can still severely alter the excitation patterns obtained with these accelerated pulses. In this note, we report at 9.4 T on a human eight-channel transmit system, substantial improvements in two-dimensional excitation pattern accuracy obtained when measuring k-space trajectories prior to parallel transmit RF pulse design (acceleration ×4). Excitation patterns based on numerical simulations closely reproducing the experimental conditions were in good agreement with the experimental results. Magn Reson Med, 2010. © 2009 Wiley-Liss, Inc.
3D compressed sensing for highly accelerated hyperpolarized 13C MRSI with in vivo applications to transgenic mouse models of cancer
Simon Hu, Michael Lustig, Asha Balakrishnan, Peder E. Z. Larson, Robert Bok, John Kurhanewicz, Sarah J. Nelson, Andrei Goga, John M. Pauly, Daniel B. Vigneron Wed, 16 Dec 2009 09:57:00 -0000
High polarization of nuclear spins in liquid state through hyperpolarized technology utilizing dynamic nuclear polarization has enabled the direct monitoring of 13C metabolites in vivo at a high signal-to-noise ratio. Acquisition time limitations due to T1 decay of the hyperpolarized signal require accelerated imaging methods, such as compressed sensing, for optimal speed and spatial coverage. In this paper, the design and testing of a new echo-planar 13C three-dimensional magnetic resonance spectroscopic imaging (MRSI) compressed sensing sequence is presented. The sequence provides up to a factor of 7.53 in acceleration with minimal reconstruction artifacts. The key to the design is employing x and y gradient blips during a fly-back readout to pseudorandomly undersample kf-kx-ky space. The design was validated in simulations and phantom experiments where the limits of undersampling and the effects of noise on the compressed sensing nonlinear reconstruction were tested. Finally, this new pulse sequence was applied in vivo in preclinical studies involving transgenic prostate cancer and transgenic liver cancer murine models to obtain much higher spatial and temporal resolution than possible with conventional echo-planar spectroscopic imaging methods. Magn Reson Med, 2010. © 2009 Wiley-Liss, Inc.
Heilum-3 MR q-space imaging with radial acquisition and iterative highly constrained back-projection
Rafael L. O'Halloran, James H. Holmes, Yu-Chien Wu, Andrew Alexander, Sean B. Fain Tue, 01 Dec 2009 09:47:00 -0000
An undersampled diffusion-weighted stack-of-stars acquisition is combined with iterative highly constrained back-projection to perform hyperpolarized helium-3 MR q-space imaging with combined regional correction of radiofrequency- and T1-related signal loss in a single breath-held scan. The technique is tested in computer simulations and phantom experiments and demonstrated in a healthy human volunteer with whole-lung coverage in a 13-sec breath-hold. Measures of lung microstructure at three different lung volumes are evaluated using inhaled gas volumes of 500 mL, 1000 mL, and 1500 mL to demonstrate feasibility. Phantom results demonstrate that the proposed technique is in agreement with theoretical values, as well as with a fully sampled two-dimensional Cartesian acquisition. Results from the volunteer study demonstrate that the root mean squared diffusion distance increased significantly from the 500-mL volume to the 1000-mL volume. This technique represents the first demonstration of a spatially resolved hyperpolarized helium-3 q-space imaging technique and shows promise for microstructural evaluation of lung disease in three dimensions. Magn Reson Med, 2010. © 2009 Wiley-Liss, Inc.

Magnetic Resonance Materials in Physics, Biology and Medicine

Eddy current effects on a clinical 7T-68 cm bore scanner
Thu, 10 Dec 2009 15:09:43 -0000
Abstract Introduction  Eddy currents induced by switching of magnetic field gradients can lead to distortions in short echo-time spectroscopy or diffusion weighted imaging. In small bore magnets, such as human head-only systems, minimization of eddy current effects is more demanding because of the proximity of the gradient coil to conducting structures. Methods  In the present study, the eddy current behavior achievable on a recently installed 7 tesla—68 cm bore head-only magnet was characterized. Results  Residual effects after compensation were shown to be on the same order of magnitude as those measured on two whole body systems (3 and 4.7 T), while using two to three fold increased gradient slewrates. Content Type Journal ArticleCategory Research ArticleDOI 10.1007/s10334-009-0192-0Authors Nils Kickler, Ecole Polytechnique Fédérale de Lausanne Laboratory for Functional and Metabolic Imaging Lausanne SwitzerlandWietske van der Zwaag, Ecole Polytechnique Fédérale de Lausanne Laboratory for Functional and Metabolic Imaging Lausanne SwitzerlandRalf Mekle, Ecole Polytechnique Fédérale de Lausanne Laboratory for Functional and Metabolic Imaging Lausanne SwitzerlandTobias Kober, Ecole Polytechnique Fédérale de Lausanne Laboratory for Functional and Metabolic Imaging Lausanne SwitzerlandJose P. Marques, Ecole Polytechnique Fédérale de Lausanne Laboratory for Functional and Metabolic Imaging Lausanne SwitzerlandGunnar Krueger, Advanced Clinical Imaging Technology, Siemens Medical Solutions-CIBM Lausanne SwitzerlandRolf Gruetter, Ecole Polytechnique Fédérale de Lausanne Laboratory for Functional and Metabolic Imaging Lausanne Switzerland Journal Magnetic Resonance Materials in Physics, Biology and MedicineOnline ISSN 1352-8661Print ISSN 0968-5243
Absolute quantification of perfusion using dynamic susceptibility contrast MRI: pitfalls and possibilities
Thu, 03 Dec 2009 18:21:52 -0000
Abstract  Absolute quantification of cerebral blood flow, cerebral blood volume and mean transit time is desirable in the determination of tissue viability thresholds and tissue at risk in acute ischaemic stroke, as well as in cases where a global reduction in cerebral blood flow is expected, for example, in patients with dementia or depressive disorders. Absolute values are also useful when comparing sequential examinations of tissue perfusion parameters, for example, in the monitoring and follow-up of various kinds of therapy. Regardless of the method employed, a number of assumptions and approximations must be made to obtain absolute measures of perfusion. Furthermore, the different stages of data acquisition and processing are associated with various degrees of uncertainty. In this review, the problems of particular relevance to absolute quantification of cerebral perfusion parameters using dynamic susceptibility contrast magnetic resonance imaging are discussed, and possible solutions are outlined. Content Type Journal ArticleCategory Review ArticleDOI 10.1007/s10334-009-0190-2Authors Linda Knutsson, Lund University, Lund University Hospital Department of Medical Radiation Physics 221 85 Lund SwedenFreddy StÃ¥hlberg, Lund University, Lund University Hospital Department of Medical Radiation Physics 221 85 Lund SwedenRonnie Wirestam, Lund University, Lund University Hospital Department of Medical Radiation Physics 221 85 Lund Sweden Journal Magnetic Resonance Materials in Physics, Biology and MedicineOnline ISSN 1352-8661Print ISSN 0968-5243
3D T1-mapping for the characterization of deep vein thrombosis
Sat, 28 Nov 2009 07:04:38 -0000
Abstract Purpose  The aim of this work was to investigate fast T 1-mapping for the characterization of deep vein thrombosis (DVT). Methods  The accuracy and reproducibility of the T 1-mapping sequence was tested in phantoms and in 8 healthy volunteers on a 1.5 T clinical scanner using a 32-channel array coil. Furthermore, the feasibility of the technique was tested in 5 patients diagnosed with DVT by measuring the volume and T 1 values of the thrombus at 5 time points over a period of 6 months. Results  The results of the phantom and volunteer study showed a high accuracy and reproducibility for the quantification of T 1. The resolution of the T 1-maps was high enough to identify small anatomical structures. T 1 values derived for normal blood and various other tissues were comparable to those reported in the literature. In all patients, the T 1 times of thrombi showed decreased values (T 1 = 843 Â± 91 ms) in the acute phase and recovered back to normal values of blood (T 1 = 1,317 Â± 36 ms) after 6 months. Conclusions  Measurement of all relevant T 1 values of acute thrombi and normal blood achieved accurate and reproducible results in vivo. Fast T 1 quantification of the thrombus can provide information about tissue characteristics such as thrombus resolution. Such a quantitative MRI technique may be valuable in studying the factors that influence natural resolution and in evaluating treatment effects that enhance this process. Content Type Journal ArticleCategory Research ArticleDOI 10.1007/s10334-009-0189-8Authors Ulrike Blume, King’s College London, St Thomas Hospital Division of Imaging Sciences, The Rayne Institute 4th Floor Lambeth Wing London SE1 7EH UKJames Orbell, St Thomas Hospital Academic Department of Surgery, Cardiovascular Division London UKMatthew Waltham, St Thomas Hospital Academic Department of Surgery, Cardiovascular Division London UKAlberto Smith, St Thomas Hospital Academic Department of Surgery, Cardiovascular Division London UKReza Razavi, King’s College London, St Thomas Hospital Division of Imaging Sciences, The Rayne Institute 4th Floor Lambeth Wing London SE1 7EH UKTobias Schaeffter, King’s College London, St Thomas Hospital Division of Imaging Sciences, The Rayne Institute 4th Floor Lambeth Wing London SE1 7EH UK Journal Magnetic Resonance Materials in Physics, Biology and MedicineOnline ISSN 1352-8661Print ISSN 0968-5243 Journal Volume Volume 22 Journal Issue Volume 22, Number 6 / December, 2009

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Magnetic resonance imaging: Referred to as magnetic resonance tomography (MRT) or, in chemistry nuclear magnetic resonance (NMR), is a non-invasive method used to render images of the inside of an object. The scanners used in medicine cost approximately US$ 1 million per tesla (T) and have a typical field strength of 0.3 to 3 T, with several hundred thousand dollars paid per year just for maintenance.