Patent Application: US-201615000137-A

Abstract:
an mpi - apparatus comprising a magnet system for generating a time - varying and position - dependent magnetic field and a detection system detecting signals from mpi contrast agents exposed to said magnetic field within a detection volume , said signals being suitable for reconstructing an image of the spatial and temporal distribution of said mpi contrast agents , is characterized in that the magnet system comprises an array with a plurality of permanent magnetic elements geometrically arranged in such a way that at least a part of the plurality of permanent magnetic elements are moved with sufficient speed in the vicinity of the detection volume to create the spatial and temporal magnetic field variations for inducing within the contrast agent the mpi signals recorded by the detection system . this avoids the high power requirements of current mpi scanners , while opening the way for higher spatial resolutions and variable scanning frequencies .

Description:
in the following , the invention is discussed in detail by way of examples : conventional ffp or ffl scanners generate the ffr by one set of electromagnets or permanent magnets ( selection field ) and then use another set of electromagnets ( drive field ) to move it in space . in some cases , additional slow sample movement is required to achieve full 3d encoding . the present proposal eliminates the drive field coils by moving the selection field magnet assembly instead . this will create a movement of the ffr relative to the imaged object . as the magnet assembly will typically have high moments of inertia , an oscillating motion would be impeded by the required large forces . therefore , the proposed design arranges the ffr creating magnets on the perimeter of two wheels mounted on a common axis with a certain gap size . b if neighboring magnet pairs are arranged to have opposite polarity , a sequence of ffps arranged in a circle is generated ( see fig4 ). movement of an ffp through a sample placed at any angular position between the rotating wheels will generate an electromagnetic signal whose time profile corresponds to the super - paramagnetic iron oxide nano - particles concentration profile along the ffp path ( first encoding direction ). by placing each magnet pair at different radial and lateral positions , each ffp will travel along a unique circular path during rotation . the magnet positions can be chosen in a way that the intersections of the ffp paths with a plane that includes the rotation axis ( imaging plane , ipl ) will form rectangular imaging grids on either side of the axis of rotation ( second and third encoding direction ). a cross section through the proposed scanner setup is shown in fig1 while fig2 shows an example of 100 ffp traversal positions through an ipl together with a suitable set of detection coils . the magnet layouts required for generating the ffp pattern in fig2 is shown in an axial view in fig3 and a view of the magnet layout in one quadrant of the wheel is shown in fig4 . for practically realizable sizes of wheels and magnets , the number of stations on the perimeter is limited . as already proposed for other mpi scanners ( see reference [ 10 ]), it is possible to add lateral shift coils to the scanner setup as shown in fig1 . in contrast to the focus field coils implemented in previous ffp scanners which need to provide large ffp shifts , these coils only need to provide small offset fields so they do not require high powers . by applying small current steps after each wheel rotation it is possible to reach intermediate ffp traversal paths . if neighboring magnets have identical polarity , and no gaps are left between , then a circular field free line ( ffl ) is obtained as shown for one quadrant . lateral and radial offsets of magnet pairs can be used to superimpose an undulating pattern on the ffl . the ffl intersects the ipl at two positions on either side of the rotation axis . during rotation , the undulating pattern will cause a smooth movement of the intersection points over an area determined by the magnet pattern . when a sample is positioned in the ffl path , the signal at any given point of time originates from the super - paramagnetic iron oxide nano - particles currently traversed by the ffl . for unraveling the super - paramagnetic iron oxide nano - particles distribution , the ffl must hit each spatial region multiple times during each cycle . additionally , an array of detection coils arranged along the rotation direction can be used to resolve the distribution via sensitivity encoding . to keep the rotating wheels balanced , the magnet pairs on opposite sides of the wheels preferably exhibit identical radial displacements . a straightforward setup uses a two - fold rotational symmetry around the axis of rotation or inversion symmetry about a point located between both wheels on the axis . in the first case , one rotation of the setup constitutes two field cycles , and identical field sequences are created at opposite sides of the axis . in the second case , one rotation constitutes one field cycle . as this provides more encoding positions , this is the preferred arrangement . the inversion symmetry of the magnet pairs implies that identical field geometries are present on opposite sides of the double wheel arrangement except for inversion of the axial field direction . if identical sets of detection coils are installed on opposite sides of the rotating wheel arrangement , then the passing permanent magnet induce identical voltages in opposite coil pairs . when these are interconnected with appropriate polarity , the signals cancel each other . by careful adjustment of the magnet positions , this cancellation can be optimized more easily than that in typical gradiometer coil setups . the design is a deviation from the development path of previous scanners , where the undesirable mechanical movement of the imaging object was replaced by adding more degrees of freedom to the magnetic shifting of the ffr . the novelty in the design presented here is that instead moving of the imaging object , the magnets defining the selection field are moved , and the arrangement of the magnets on rotating wheels allows to conveniently achieve ffr speeds comparable to those realizable by ac driven coils . the scanner design gives up the idea of sinusoidal field variations and ffp paths . instead , the ffps travel through the fov with constant speed along a circular path . hence , the resulting signals cannot be meaningfully be interpreted in terms of harmonics generation . receive filters are not required , as the non - harmonic excitation signal is perfectly filtered out by the compensation coil arrangement . in contrast to current ffp scanners , the full super - paramagnetic iron oxide nano - particles signal can be measured , allowing a straight - forward x - space reconstruction . the strongest available permanent magnets have flux densities of about b 0 = 1 . 3 t on their surface . for magnet poles that are large compared to the distance d between the gradient generating magnets , the maximum gradient strength can be estimated as g max = 2b 0 / d . neglecting space for the detection coils , the required pole gap is twice the field of view extent since the ffp that is located at the middle position between the magnets must be shifted over the entire field of view . for a field of view extent of 3 cm , one realistically obtains g max = 2 - 15 t / m . in a conventional ffp scanner , shifting the ffp of a 15 t / m gradient by drive field coils over a distance of ± 15 mm would require an unreasonable drive field amplitude of 225 mt . this highlights the advantage of the rotating wheel setup . for larger field of views and for smaller magnet segments , the achievable gradients are much smaller . this suggests that the new scanner layout may be particular useful for mpi microscopy . a wheel of 1 m diameter has a circumference of around 3 m . when using magnet slices of 3 cm thickness , one can place 100 such slices around the circumference , with 375 discrete field layouts per image ( see example figures ). rotation speeds up to 50 / s are realistic , implying an ffp speed of 150 m / s or 540 km / h . multiple acquisition stations can be installed around the wheels , allowing simultaneous studies on more than one subject . the magnet element sequence could include mostly homogeneous sections with a certain b field strength . by application of rf pulses synchronized to the rotation the acquisition of mr reference images will be possible . instead of two wheels with a gap , the magnets could be mounted on either side of a groove near the border of a single rotating wheel . longer sequences , and a straight field of view can be obtained if the magnet pairs are mounted on a “ conveyor belt ” that is guided along a rigid path . however , such a design is more difficult to implement . the two wheels could be driven by independent motors , thus realizing more complex field patters resulting from the interference of the installed magnetic fields . when alternating homogeneous fields are created , a variable frequency mps spectrometer can be obtained . by varying the field strength along the cycle , a spectrometer can be built that performs multi - frequency / multi - amplitude scanning during a single experiment . instead of two moving , magnet holding structures like a transport or conveyor belt or a wheel , only one moving structure may be used in connection with at least one electromagnet synchronized with the motion of the array of permanent magnets . the electromagnet would adapt the magnetic field accordingly to achieve a ffr for every different magnet position . the variation of permanent magnet position may be established in only one or two dimensions . a motorized sample holder may then shift the sample to bring further sample points into the detection volume . strong selection field gradients possible ( high resolution ) large field of view possible , in particular along wheel perimeter change of excitation frequency straightforward no expensive transmit amplifiers no expensive transmit filters no expensive , highly linear resonant circuit no cooling efforts straight - forward implementation of compensation coils low or no receive - filtering requirements the most similar scanner concept to the presented design is the twmpi scanner . there is also a similarity to the ultra - high gradient experiments presented by the twmpi group . the proposed new scanner design can be regarded as a generalized concept of the twmpi scanner with the following important new concepts and advantages : the coil array that produces the moving ffp is replaced by a set of permanent magnets arranged on rotating wheels . a variation in excitation frequency / scanning speed is easily realizable by changing the speed of rotation , as no resonant circuits are involved . the gradient orientation is perpendicular to the scanning direction . by placing magnet pairs at different radial and lateral offsets , a 3d scanner without shift coils is realized . the scanner can be realized both as an ffp and ffl scanner . by using inherent symmetry requirements of a rotating wheel design , a detection coil design can be realized that provides a strong suppression of the excitation signal without a direct gradiometer coil . the reported experiments on ultra - high gradient mpi used two magnet rings aligned on a single axis to generate a strong gradient field . a 1d line profile was realized by movement of a sample through the ffp located on the axis between the two rings . the setup will also exhibit a ffl between the rings , which is similar to the present proposal with the following major new concepts : the magnet rings are split into smaller segments which are placed at different offsets to provide a spatial encoding . instead of the ffp on the axis , the ffl ( or sequence of ffps ) between the actual rings are used for image generation . instead of a sample movement along the axis , the rings are rotated . the proposed new scanner design can be realized either as an ffp or ffl scanner . in an ffl setup , the following new concepts are present : no electromagnetic coils are required for the basic encoding and ffl movement . instead of a straight line , the ffl is realized as a circle with arbitrarily induced undulations . a variation in excitation frequency / scanning speed is easily realizable by changing the speed of rotation , as no resonant circuits are involved . by using inherent symmetry requirements of the rotating wheel design , a detection coil design can be realized that provides a strong suppression of the excitation signal without a direct gradiometer coil . the proposed new scanner design can be realized either as an ffp or ffl scanner . in an ffp setup , the following new concepts are present : no electromagnetic coils are required for the basic encoding and ffp movement . higher images resolutions are possible without sacrificing fov size . in contrast to early scanners with mechanical object movement , the rotating wheel concept allows fast 3d encoding . a new mpi scanner topology based on rotating permanent magnets is proposed which avoids the high power requirements of current mpi scanners , while opening the way for higher spatial resolutions and variable scanning frequencies . an apparatus for magnetic particle imaging at variable operating frequencies is proposed based on a set of moved , in particular rotating , permanent magnets for generating suitable time - varying magnetic fields and a set of detection and compensation coils for ideal suppression of excitation signals . 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