Debatin, Jörg F.

Erhart, Peter

Ladd, Mark E.

McKinnon, Graeme C.

Romanowski, Benjamin J.

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Biopsy Needle Susceptibility Artifacts in Magnetic Resonance ImagingMark E. Ladd, Peter Erhart, Jörg F. Debatin, Benjamin J. Romanowski, Graeme C. McKinnonDepartement Medizinische Radiologie, MR-Zentrum, Universitätsspital Zürich, CH-8091 Zürich, SchweizIntroductionMagnetic resonance imaging is attracting attention äs apotential interventional monitoring modality. MRI-guidedbiopsies are one of the simpler interventional procedures.One possibility is to directly visualize the needle with MRI.However, its appearance is largely determined by thesusceptibility artifact.This paper presents the results of Computer simulationsof the image distortion resulting from the magneticsusceptibility difference between the needle and thesurrounding tissue. The simulations show not only anartifact size which is dependent on needle composition,orientation, and pulse sequence, but also a correspondingshift of the artifact center away from the actual needlecenter. This shift places limits on the accuracy of needle tipplacement.TheoryThe needle is modelled by assuming an infinite cylinderof permeability · and radius R in a uniform magnetic fieldB0. The cylinder has its axis parallel to the z-axis äs shown.The angle arepresents the orientation of the magnetic fieldwith respect to the needle, and the angle the orientation ofthe frequency encoding gradient'frequencyThe magnetic field extemal to the cylinder isysmor+ , [1]where e is the permeability of the material surrounding thecylinder.When objects with different magnetic susceptibilities areimaged, local magnetic field inhomogeneities are introducedwhich cause spatial and intensity distortions (1). Using Eq.[1], the distortion of voxels outside the cylinder can becalculated äs a function of a, ß, and the dimensionlessparameter[2]JrtquencyMethodeSpatial distortions were simulated for an image in the xz-plane and intensity profiles along the jt-axis were calculated.The distortion creates two peaks in the profile whichdelineate the apparent extent of the needle. From theseprofiles, two attributes were defined. The first was theartifact diameter, which was taken to be the distancebetween the two peaks. The second was the artifact shift,which was defined to be the difference between the cylindercenter and the midpoint of the two peaks.10/ Needle RadiusResultsBelow is a plot of artifact diameter, normalized to needlediameter, versus «(needle orientation) for various 77's. Thefrequency encoding was kept parallel to B0 (ß=ti). Asexpected, the artifact size increases with increasing 77. Also,since the susceptibility artifact is in the frequency encodingdirection, the artifact increases with a for a given . Thesecond plot shows the corresponding artifact shift,normalized to needle radius.202 Biomedizinische Technik · Band 41 · Ergänzungsband 1 · 1996φφs?b.2Τ)ωω*_£"δcgΟΐϊ30 60Needle Orientation (degrees)(ΟGC•σΦφcoοΟ 30 60 90Needle Orientation (degrees)DisctissionPassive visualization of veiy thin structures withoututilizing the susceptibility artifact is difficult in MRLHowever, structures which cause a large enough artifact tobe easily seen in an image distort the surrounding anatomyto such an extent that important Information for theInterventionist can be obscured. Further, these artifacts arcvery Orientation dependenLWith this Simulation tool, Orientation and pulse sequencedependent biopsy needle artifacts can be studied The aboveresults show that in addition to the problem of artifact sizenot being constant, the artifact is not always centered on thephysical needle. This could lead to false needle placement ifone relies only on passive visualization.AcknowledgmentsThis work was funded by General Electrical MedicalSystems, Milwaukee, Wisconsin, and the Swiss NationalScience Foundation, Grant Nr. 32-43284-95.References[1] K.M. L deke, et aL; Magn. Reson. Imaging 3, 329,(1985)Biomedizinische Technik · Band 41 · Erg nzungsband 1 · 1996 203

Biopsy Needle Susceptibility Artifacts in Magnetic Resonance Imaging

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