Udomchai techavipoo, john lackey, jianrong shi, xin guan and song lai, estimation of mutual information objective function based on fourier shift theorem. It is important to deal with distortions common to diffusion images, like epi and eddy currents. Small eddy currents in the magnet bore, which may be negligibly small for most mr imaging applications, can cause significant distortion in diffusion mri. The eddy current method is widely used to inspect metallic aircraft components for surface cracks and corrosion damage. A method is proposed to reduce their effects on double diffusion encoding dde mri experiments. Such image quality deteriorations are significantly amplified with diffusion. Eddy currents also known as foucault currents are the result of rapidly changing gradient magnetic fields that in turn induce stray currents in the surrounding conducting materials. Characterization and correction of eddycurrent artifacts. Eddy currents might lead to image distortions in diffusionweighted echo planar imaging. Eddy current correction in diffusionweighted imaging using. Characterization and correction of eddycurrent artifacts in. Magnetic resonance imaging mri is a powerful medical imaging technique that enables the visualization of structure and function in vivo because of large contrast within soft tissue. Diffusion mri in the heart choukri mekkaouia, timothy g.
In the next step we will run an eddy distortion correction. Screening and education patients, personnel, nonpersonnel 1. If you are interested you can find more background on eddy distortions in our post on distortions corrections. Furthermore, there is a global shortage of the 99m tc isotope used for bone scintigraphy due to. The effect of these gradients on the image is approximately the same to that of susceptibility gradients.
Because diffusionsensitizing gradients consist of components along each of the x, y, and zaxes, these gradient components should induce eddy currents independently. The eddy currents are induced by electrical currents that flow in circular paths. Eddy currents might lead to image distortions in diffusion weighted echo planar imaging. The bipolar method of diffusion acquisition uses multiple gradients of opposite polarities in the gradient pulse sequence to counterbalance the distortion caused by eddy currents. In the presence of an eddy currentinduced echoshifting effect,theeffectivetete effdiffersfromthetargetedspinecho te and the generated echoes have both 2and 2weightingi. Magnetic resonance imaging the purpose of the magnetic resonance imaging mri examination is to assess the knowledge. A comprehensive diffusion mri dataset, collected using a suitable experimental design, is made available to the scientific community, consisting of three dwi shells bmax 5000 smm 2, 30 gradient directions, a replicate set of antipodal gradient directions, four phase. In echoplanarbased diffusion weighted imaging dwi and diffusion tensor imaging dti, the evaluation of diffusion parameters such as apparent diffusion coefficients and anisotropy indices is affected by image distortions that arise from residual eddy currents produced by the diffusion sensitizing gradients. Cind preprocessing pipeline for diffusion tensor imaging. The current generates a magnetic field around the coil. Sosnovika,d diffusion mri provides unique information on the structure, organization, and integrity of the myocardium without the need for exogenous contrast agents. Tips, tricks, and pitfalls dowmu koh 1, matthew blackledge 2, anwar r. Pierpaoli1 patient motion and image distortion induced by eddy currents cause artifacts in maps of diffusion parameters computed from diffusion weighted dw images.
The eddy currents induced in the metal by the exciting. If persistent, it must be addressed by a service representative to verify the magnet performance. Reduction of eddy current induced distortion in diffusion mri using a twicerefocused spin echo t. Characterization of and correction for eddy current. Jan 03, 2015 for this type of application, ferrite cores or iron dust cores are used. Fast switching of diffusionencoding gradients induces eddy currents in conducting components the shifts are directiondependent, i. Diffusion effects in eddy current nondestructive evaluation. In most diffusion tensor imaging dti studies, images are acquired with either a partialfourier or a parallel partialfourier echoplanar imaging epi sequence, in order to shorten the echo time and increase the signaltonoise ratio snr. The most widely prescribed imaging test for evaluation of bone disease is radionuclide bone scintigraphy with technetiumlabeled tracers. Gradient coil design and intracoil eddy currents in mri. A set of eddy currents were simulated with different relative amplitudes. Eddy currents induced by switching of magnetic field gradients can lead to distortions in short echotime spectroscopy or diffusion weighted imaging. The most common tool to counter eddy currents has been developed by fsl.
Image guidance common uses to reduce the influence of eddy currents on gradient fields are eddy current compensation and shielded gradient coils active or passive. Elimination of eddy current artifacts in diffusion. The impact of inscanner head motion on structural connectivity derived from diffusion mri graham l. Correction for eddy currentinduced echoshifting effect in partialfourier diffusion tensor imaging. Because diffusion sensitizing gradients consist of components along each of the x, y, and zaxes, these gradient components should induce eddy currents independently. Correction of eddycurrent distortions in diffusion tensor. The addition of the second refocusing pulse requires a longer te. Eddy currents also called foucaults currents are loops of electrical current induced within conductors by a changing magnetic field in the conductor according to faradays law of induction. Eddy current compensated double diffusion encoded dde. The source of the changing magnetic fields in mri may either be the imaging gradients or radiofrequency rf coils. Originating from eddy currents, and subject motion while the effect of b 0 susceptibility distortions has been somewhat neglected in diffusion mri, the effects of eddy currents have received much more attention since the early applications of diffusion weighted epi in the human brain turner et al.
Displacement maps for the first diffusion direction were also computed for various eddycurrent orders i. Eddy current testing equipment contains a conductive metal coil which is electrified with an alternating current. Dti tutorial 1 from scanner to tensor diffusion imaging. Eddy current correction in diffusionweighted imaging. Diffusion effects in eddy current nondestructive evaluation paul a. Artifacts and pitfalls in diffusion mri meteore service. Electrical engineering electromagnetics for the graduate college. Eddy currents changing gradients induces eddy currents in. Yet, in spite of this, eddy currents induce artifacts in the measured diffusion images. Eddy currents induced by switching of gradient magnetic fields generate eddy current fields, which create undesired distortions or artifacts in images. Eddy current compensated double diffusion encoded dde mri.
Explains why adding a second 180pulse reduces eddy currents. Correction for eddy currentinduced echoshifting effect. Quantitative simulation of affine registration for. Using these materials, the eddy currents are limited to individual grains, so the eddy current loss is reduced considerably. This chapter begins with an outline of an ideal diffusion experiment which is followed by a discussion of how. Reduction of eddycurrentinduced distortion in diffusion mri using a twicerefocused spin echo t. Eddy currents flow in closed loops within conductors, in planes perpendicular to the magnetic field. The conductive material in which eddy currents are induced may be any metallic component of the mr scanner other coils, shields, tubes, and housing, wires or devices within or on the patient, and the patient as a whole in the final analysis, people are just big bags of saline. Diffusion effects in eddy current nondestructive evaluation by paul a. An additional advantage is that nonionizing radiation is used to acquire an mrimage.
Pdf eddycurrent distortion correction and robust tensor. Originating from eddy currents, and subject motion while the effect of b 0 susceptibility distortions has been somewhat neglected in diffusion mri, the effects of eddy currents have received much more attention since the early applications of diffusionweighted epi in the human brain turner et al. These distortions can be described using a combination of translation, scaling, and shear in the phaseencoding direction. Diffusion mri in the heart, however, has proven technically challenging. The amplitude of the diffusion gradients g diff was 21. These images, acquired by measuring the attenuation of nmr signal due. Miccai01, utrecht, the netherlands, lncs2208, springer verlag, pages 186194, 2001. Eddy currents diffusionweighting is gradient intensive eddy currents create decaying gradients that generally persist into the epi imaging window these.
Hardware considerations for diffusion mri oxford medicine. The stejskaltanner equation generalized for any gradient shapean overview of most pulse sequences measuring free diffusion. Much is known of eddy currents, namely that they are dependent on the magnitude of the. While many of the issues facing diffusion tensor mri dti in the brain apply to the heart as well, several specific challenges. In echoplanarbased diffusionweighted imaging dwi and diffusion tensor imaging dti, the evaluation of diffusion parameters such as apparent diffusion coefficients and anisotropy indices is affected by image distortions that arise from residual eddy currents produced by the diffusionsensitizing gradients. Diffusion mri for brain connectivity mapping and analysis. In mri, gradient coils are used to produce gradient fields to spatially encode magnetic resonance signals. It simultaneously models the effects of diffusion eddy currents and movements on the image, allowing it to work with higher bvalue data than has been possible with for example. Each image is the variance of diffusion tensor mri of 6 nonopposed edge centers of a cube in k. Figure 6 depicts the distortions coming from eddy currents. Theasymmetric spinechosignalintensity asecanberepresentedbyeither. Diffusionweighted imaging of the liver in patients with. Pierpaoli1 patient motion and image distortion induced by eddy currents cause artifacts in maps of diffusion parameters computed from diffusionweighted dw images.
For this type of application, ferrite cores or iron dust cores are used. The distortions from diffusiongradient eddy currents cause misalignments in the dw images, which can lead to subsequent errors in the calculated diffusion maps. A nondiffusionweighted image top left and 3 diffusionweighted images with gradients along independent directions are shown before top row and after bottom row correction by postprocessing. For the bfactor calculation of any diffusion gradient waveform, we designed the program in. Furthermore, limited imaging resolution leads to partial volume, and dti is a simplified representation that assumes that diffusion is unrestricted and follows a gaussian distribution. The cranial contents are outlined in red on the non diffusionweighted image, and this same outline is superimposed on the other images to show. Much of the analysis from diffusion mri experiments is conducted under the assumption that pulse sequence elements such as rf and gradient pulses are perfectly reproduced by mri hardware. Conversely, the eddy currents generated by a pair of. Furthermore, many deleterious effects such as eddy currents and mechanical vibration are ignored. A novel and comprehensive approach to correct for spatial misalignment of dw. Displacement maps for the first diffusion direction were also computed for various eddy current orders i.
Echoplanar imaging sequences are especially affected where a characteristic eddy current artifact nyquist n2 ghost is commonly seen. Correction for eddy currentinduced echoshifting effect in. Eddy currents are a natural effect of the changing magnetic field that are commonplace in diffusion scans. Epi eddy currents diffusionweighting is gradient intensive eddy currents create decaying gradients that generally persist into the epi imaging. They form in accordance to faradays law of induction eddy currents are unwanted as they generate their own magnetic fields, which oppose the original magnetic field via lenzs law, and therefore distort the. The problem is that diffusion measurements can be particularly vulnerable to eddy currents produced by the diffusion gradient pulses. In small bore magnets, such as human headonly systems, minimization of eddy current effects is more demanding because of the proximity of the gradient coil to conducting structures. Eddy currents diffusion gradients create large eddy currents, which persist into acquisition window distort the kspace trajectory, casing shearsscaling of images effective gradient. The eddy currents induced by a change in a single gradient component e. By passing the eddy current equipment, which is a handheld device, above the component surface it is possible to detect cracks, including. Diffusion mri in the heart, however, has proven technically. Eddy currents can cause artifacts in images and may seriously degrade overall magnet performance. The eddy currents interfere with the currents induced by the scanned subject, thereby distorting the resulting diffusion weighted images 14. They can be induced within nearby stationary conductors by a timevarying magnetic field created by an.
Stucky a dissertation submitted to the graduate faculty in partial fulfillment of the requirements for the degree of master of science department. These eddy currents are problematic for the imaging portion of the sequence that follows, especially for epirelated sequences. This chapter begins with an outline of an ideal diffusion experiment which is followed by a discussion of. Review the artifacts and pitfalls of diffusion mri on a qualitative basis, especially in terms of eddy currents and sensitivity to motion. However, the results can be falsenegative in patients with active bone disease because lytic bone diseases may not induce an osteoblastic reaction 24, 25. Effects from eddy currents with shorter decays are not corrected and result in image blurring. However, rapid switching on and off of the gradient coils induces fields and eddy currents in the human tissues and surrounding conducting structures. Validation of diffusion tensor mri measurements of cardiac. Review and evaluate the possible issues that can affect the accuracy of measurements regarding dynamic susceptibility contrast dscmri measurements of cerebral blood. Other sequences that may be most affected by eddycurrents include diffusion weighted imaging, mr spectroscopy, mr angiography, balanced gradient echo sequences, and any sequence with a very short te. Reduction of eddycurrentinduced distortion in diffusion. Surface and nearsurface cracks interrupt the eddy currents, and this is detected by changes in the coils impedance. Common means to reduce the influence of eddy currents on gradient fields are eddy current compensation and shielded gradient coils active or passive. Reduction of eddycurrentinduced distortion in diffusion mri.
1441 1348 1635 778 1011 1355 368 490 917 688 665 871 771 159 624 1575 129 760 976 1176 1482 657 290 1492 762 1263 1116 39 1526 631 187 1331 240 975 92 558 272 117 424 1101