Abstract:
A method and apparatus generate a pre-polarizing magnetic field having a rise-time of less than about 10 microseconds and/or a fall-time of less than about 10 microseconds for immersing a tissue sample in the pre-polarized magnetic field to polarize an animal tissue sample whereby magnetic gradient and/or radio-frequency pulses may be applied in order to read out the location and/or state of the spins. A method and apparatus deliver such magnetic fields through planar coils. A method and apparatus enable guidance and propulsion of magnetic fluids.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 12/488,105 filed Jun. 19, 2009, which relies for priority on U.S. Provisional Application No. 61/074,397 filed Jun. 20, 2008, the contents of which are incorporated herein by reference. 
    
    
     FIELD OF THE DISCLOSED EMBODIMENTS 
     The present invention concerns an apparatus and a method for improving magnetic resonance imaging and spectroscopy of living tissues or inanimate objects. 
     BACKGROUND 
     Conventionally, Magnetic Resonance Imaging (MRI) has been used to visualize characteristics of soft tissue of the human body and to diagnose diseased tissue. 
     Pre-polarized Magnetic Resonance Imaging (PMRI) has been promoted as a method of constructing low-cost imaging systems. The basic principle of the PMRI method is to immerse or otherwise subject an object of interest in a transient magnetic field. The purpose of said immersion is to polarize and/or align spins in the object along the direction of the transient magnetic field. For example, living animal tissue is largely composed of water molecules containing hydrogen nuclei or protons. When such tissue is immersed in a magnetic field, some of the protons align with the direction of the field. This alignment may also occur for certain inanimate objects, for example, petroleum deposits in rock samples. A transient magnetic field takes the place of (or can augment) the static magnetic field (that is typically employed in conventional MRI systems). 
     In PRMI, following the application of the transient magnetic field, a set of magnetic gradient and/or radio-frequency pulses are typically applied, in order to read out the location and/or state of the spins. For example, a radio frequency transmitter may be used to provide an electromagnetic field whereby photons of this field having resonance frequency, flip the spin of the aligned protons. After the transient magnetic field is turned off or reduced in magnitude, the protons decay to the original spin-down state and the difference in energy between the two states is released as a photon. These photons produce a signal which can be detected by a scanner. 
     It is conventionally known that application of a high transient magnetic field during the polarization portion of the pulse sequence results in an improved signal (see for example, A Macovski, S Conolly: “Novel Approaches to Low-Cost MRI”, in Magnetic Resonance in Medicine 30:221-230, the subject matter of which is incorporated herein by reference in its entirety) because more spins are aligned; as a result, the application of this field subsequently results in output of a more significant signal as they return to their equilibrium state. 
     Reducing the fall-time of the transient magnetic field may be also advantageous, because the magnetization (due to the aligned polarized spins) does not have a chance to decay much before the readout sequence is completed. Speed of the overall pulse sequence is important in order to reduce overall scan time. 
     Furthermore, reducing the overall scan time may be desirable for economic reasons (for example, in order to study more patients in a fixed period of time) and/or physiological considerations (for example, to reduce the effect of breathing or cardiac motion). 
     The principle of PMRI has been applied to anatomical studies, as well as to explosives detection (see, for example, M Espy, M Flynn, J Gomez, C Hanson, R Kraus, P Magnelind, K Maskaly, A Matlashov, S Newman, T Owens, M Peters, H Sandin, I Savukov, L Schultz, A Urbaitis, P Volegoc, V Zotev: in “Ultra-Low Field MRI for the Detection of Liquid Explosives Using SQUIDs”, published in IEEE/CSC &amp; ESAS European Superconductivity New Forum 8:1-12 (2009), the subject matter of which is incorporated herein by reference in its entirety). 
     Additionally, a variation of the pre-polarizing principle is denoted as field-cycling MRI, in which the magnitude of the transient magnetic field is varied, in order to provide information about magnetic decay properties of the object of interest (see, for example, K M Gilbert, W B Handler, T J Scholl, J W Odegaard, B A Chronik, in “Design of field-cycled magnetic resonance systems for small animal imaging”, published in Physics of Medicine and Biology 51:2825-2841 (2006) the subject matter of which is incorporated herein by reference in its entirety). 
     Furthermore, PMRI systems have been proposed as methods of examining neuronal activity in vivo (see, for example, R S Wijesinghe and B J Roth, in “Detection of Peripheral Nerve and Skeletal Muscle Action Currents Using Magnetic Resonance Imaging”, published in the Annals of Biomedical Engineering 37(11):2402-2406 (2009), the subject matter of which is incorporated herein by reference in its entirety). 
     SUMMARY 
     In accordance with disclosed embodiments of this disclosure, an apparatus and method are provided for creating strong pulsed currents to drive magnetic fields with short rise- and fall-times (see IN Weinberg, inventor: “Apparatus and Method for Decreasing Bio-Effects of Magnetic Gradient Fields”, PCT/US2009/047960, the subject matter of which is incorporated herein by reference in its entirety). This disclosure relates primarily to the generation of magnetic gradients by the apparatus and method, in which the configuration of circuits and switches is used to drive gradient coil configurations (such as opposed Helmholtz pairs). 
     In the description of the embodiments, the term “coil” is used to imply a set of conductors arranged to generate a magnetic field, whether the conductors are in a solenoidal configuration or in some other configuration. 
     Circuits and/or switches as previously disclosed by I N Weinberg (PCT/US2009/047960) can also be used to drive magnetic fields with solenoids, Golay coils, or non-opposed Helmholtz coil pairs, or other coils. Examples of such alternative coil configurations include planar configurations of magnetic field generation coils, which can reduce the footprint of an MRI scanner (see, for example, B Aksei, L Marinelli, B D Collick, C Von Morze, P A Bottomley, C J Hardy, in “Local Planar Gradients With Order-of-Magnitude Strength and Speed Advantage”, published in Magnetic Resonance in Medicine 58(1):134-143 (2007), the subject matter of which is incorporated herein by reference in its entirety) and implement cost-effective imaging of small body parts (e.g., breast, foot, knee) or samples. The cost-effectiveness of such alternative coil configurations (especially when used with the previously described apparatus and method of IN Weinberg&#39;s application PCT/US2009/047960) is realized as a result, in part, of the ability to suffuse a relatively small volume with magnetic energy, thereby reducing power and/or cooling requirements. 
     The present disclosure embodiments involve application of fast switches and methods, as previously disclosed by IN Weinberg, to drive solenoids and/or coils for use in fast-cycling and/or pre-polarized magnetic resonance imaging and/or spectroscopy systems. Present disclosure embodiments also include the application of fast switches and methods to drive planar gradient coils. In both of these embodiments, magnetic fields of very high magnitudes may be applied with minimal or no bio-effects, due to the very short rise- and/or fall-times possible with the disclosed methods. Such high gradients may be used to propel, repel, or contain magnetizable or magnetic particles in the body (heretofore denoted as “magnetic particles”). 
     It is known that magnetic particles can be bound, either permanently or temporarily, to other agents with biochemical specificity or with therapeutic effect. The short durations of the pulsed magnetic fields may be used to monitor the progress of the magnetic particles in the body. 
     The currently disclosed embodiments include the use of fast switches and methods, as previously disclosed by IN Weinberg, to eliminate or reduce the bio-effects during the application of the pre-polarizing magnetic pulse, and/or during subsequent (e.g., readout) magnetic pulses. A potential advantage of fast readout pulses, when used in combination with pre-polarizing pulses, may be that the spins are read out during the readout sequence while the magnetization from the pre-polarizing pulse is still in effect. 
     The currently disclosed embodiment may reduce acoustic noise as compared to conventional MRI systems because of the more rapid pulse sequences resulting in sounds at higher frequencies than are produced by conventional MRI systems. Since the human ear is less sensitive to high frequencies than to low frequencies (see, for example, Acoustics-Normal equal-loudness-level contours. Internal Organization for Standardization (ISO) 226:203, the subject matter of which is incorporated herein by reference in its entirety), the net effect is a perceived reduction in acoustic noise. 
     It is an object of the currently disclosed embodiments to provide a method and apparatus for improving magnetic resonance imaging. 
     It is another object of the currently disclosed embodiments to provide a method and apparatus for improving magnetic resonance imaging by generating a pre-polarizing magnetic field having at least one of a rise-time of less than 10 microseconds and a fall time of less than 10 microseconds; and immersing a tissue sample in the pre-polarized magnetic field to polarize the tissue sample. 
     It is further object of the currently disclosed embodiments that the pre-polarizing magnetic pulse has a rise-time of less than 10 microseconds and a fall time of less than 10 microseconds. 
     A further object of the currently disclosed embodiments is that at least one magnetic pulse following a pre-polarizing magnetic pulse has a rise-time of less than 10 microseconds and a fall time of less than 10 microseconds. 
     Another object of the currently disclosed embodiments generates a first magnetic pulse having at least one of a rise-time of less than 10 microseconds and a fall time of less than 10 microseconds to create a first magnetic field, immerses a tissue sample in the first magnetic field to polarize the tissue sample, and generates a second magnetic field to read out characteristics of the tissue sample. 
     A further object of the currently disclosed embodiments is that the first magnetic pulse may fall to a level that is non-zero, in order to act as an evolution field during the application of subsequent magnetic pulses, in which at least one of the subsequent magnetic pulses has a rise-time of less than 10 microseconds and/or a fall time of less than 10 microseconds. 
     A further object of the currently disclosed embodiments is that the first magnetic pulse may remain in order to act as an evolution field during the application of subsequent magnetic pulses, in which at least one of the subsequent magnetic pulses has a rise-time of less than 10 microseconds and/or a fall time of less than 10 microseconds. 
     Another object of the currently disclosed embodiments is that the first magnetic pulse has a magnitude that is stronger than that of subsequent magnetic pulses. 
     The short rise- and fall-times of subsequent magnetic pulses may be advantageous in that the duration of the pulse sequence may be reduced, thereby reducing the need to cool the coil or coils responsible for generating the magnetic pulses and permitting the characterization or description of short-duration processes or of objects in rapid motion or vibration. 
     A further object of the currently disclosed embodiments is that the first magnetic pulse has a rise-time of less than 10 microseconds and a fall time of less than 10 microseconds. 
     A further object of the currently disclosed embodiments is that at least one magnetic pulse following a first magnetic pulse has a rise-time of less than 10 microseconds and a fall time of less than 10 microseconds. 
     Another object of the currently disclosed embodiments is that the rise-time of less than 10 microseconds and fall time of less than 10 microseconds are produced using at least one multi-stage high-voltage switch. 
     Another object of the currently disclosed embodiments is that at least one multi-stage high-voltage switch is used to drive at least one planar coil to generate at least one magnetic pulse. 
     Another object of the currently disclosed embodiments is the application of magnetic storage materials such as ceramics or ferrites in order to sustain the first magnetic pulse or subsequent magnetic pulses in order to minimize the amount of current through the coil or coils responsible for generating the first or subsequent magnetic pulses. 
     Another object of the currently disclosed embodiments is the use of at least one multi-stage high-voltage switch in generating at least one magnetic pulse in order to reduce perceived acoustic noise from coils used to create the magnetic pulse. 
     Another object of the currently disclosed embodiments is the generation of at least one magnetic pulse following a pre-polarizing magnetic pulse uses at least one multi-stage high-voltage switch to drive at least one planar coil. 
     Another object of the currently disclosed embodiments is the generation of at least one magnetic pulse following a pre-polarizing magnetic pulse that uses at least one multi-stage high-voltage switch to reduce perceived acoustic noise for given magnetic field strengths. 
     Another object of the currently disclosed embodiments includes the generation of a first magnetic pulse uses at least one multi-stage high-voltage switch enabling shorter magnetic field pulse durations to reduce perceived noise level of MRI by raising the bulk of sound frequencies to a level above the optimal human hearing range. 
     Another object of the currently disclosed embodiments involves increasing the level of the bulk of generated sound frequencies to above approximately 10 kHz. 
     Other aspects of the presently disclosed embodiments will be apparent from the discussion that follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be described in connection with one or more drawings, in which: 
         FIG. 1  is a depiction of an apparatus of the present invention; 
         FIG. 2  is an example of a pulse sequence of the invention; 
         FIG. 3  is a flow chart of operation of the invention; and 
         FIG. 4  is an illustration of acoustic noise reduction resulting from the invention. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention will now be described in connection with one or more contemplated embodiments. The embodiments discussed are not intended to be limiting of the scope of the present invention. To the contrary, the embodiments described herein are intended to be exemplary of the broad scope of the present invention. In addition, those skilled in the art will appreciate certain variations and equivalents of the embodiments described herein. The present invention is intended to encompass those equivalents and variations as well. 
     In  FIG. 1 , an apparatus for pre-polarized MRI is shown, and has elements corresponding to the IN Weinberg application No. PCT/US2009/047960. With this overview in mind, reference is now made to  FIG. 1 .  FIG. 1  provides a schematic diagram of a first contemplated embodiment of a MRI device  10  according to the present invention. 
     The MRI device  10  includes a power source  12 . The power source  12  may be any type of generator suitable for generating power to be provided to the one or more of the components connected thereto. The generator may provide an alternating current (AC) or a direct current (DC) or both, as should be appreciated by those skilled in the art. The precise output of the power source  12  is not critical to the operation of the present invention. Moreover, the power output of power source  12 , once generated, may be converted to different types (e.g., AC or DC) as required by individual components of the system. 
     In  FIG. 1 , the power source  12  is illustrated as providing power to each of the various components of the MRI device  10  of the present invention. It is noted, however, that the depicted arrangement is meant to be illustrative only. As should be appreciated by those skilled in the art, the individual components of the MRI device  10  may receive power from a centralized source, such as the power source  12 . Alternatively, the various components may receive power from alternative power sources. Accordingly, the depiction of a single power source  12  is not intended to be limiting of the invention. 
     In addition, as detailed below, the MRI device  10  of the present invention is illustrated and discussed with reference to single communication lines (or links) extending between the various components. The illustration of single communication lines is meant to simplify the discussion and illustration of the various embodiments of the invention. As should be appreciated by those skilled in the art, there may be multiple communication lines between the various components of the MRI device  10  as required for their operation. Moreover, the communication lines are not intended to be limited to wired and optical links. To the contrary, some or all of the communication lines may be wireless, as required or desired for operation of the MRI device  10 . The communication lines may also provide power, and may include transformers, rectifiers, switches, and other components in order to boost voltage, provide isolation from voltage spikes, synchronize timing, or perform other electrical functions. 
     In one contemplated embodiment of the present invention, the power source  12  may include a plurality of power sources  12 , each of which generates power with different characteristics, as required by the device(s) and/or components associated therewith. 
     As depicted in  FIG. 1 , power from the power source  12  travels in two directions. Power from the power source  12  is conducted first along a communications line  14  to a capacitor  16 . Intermediary components along the power line  14 , such as a high-voltage transformer and rectifier circuit, are optionally implied in the diagram as described above. Power from the power source  12  is carried second along a communication line  18  to a processor  20 . 
     The capacitor  16  may be of any size or type as would be appreciated by those skilled in the art. As is its nature, the capacitor  16  stores a charge inputted from the power source  12 . That charge is eventually discharged (fully or partially), as discussed in greater detail below. It is understood that the term capacitor includes not only the class of discrete components, but also the class of pulse-forming lines, in which the capacitor is combined with a transmission line. 
     While  FIG. 1  illustrates a single capacitor  16 , a plurality of capacitors  16  may be employed without departing from the scope of the present invention. In one contemplated embodiment, the MRI device  10  relies upon a plurality of capacitors  16  for its operation. As should be appreciated by those skilled in the art, plural sets of capacitors  16  may be employed to generate one or more magnetic fields and/or gradients. The generated magnetic field may be a polarizing pulse, or may be in the form of a gradient as part of a read-out pulse sequence, or may be a sustained magnetic field that continues during the read-out sequence. 
     In the second flow path, power from the power source  12  is provided to the processor  20  through communication lines  18 . The processor  20  may be of any type suitable for executing instructions, generating data, receiving data, storing data, analyzing data, processing data and the like. In one contemplated embodiment, the processor  12  may be a personal computer. In other embodiments, the processor  12  may be a mainframe computer, a portable computer, a Personal Data Assistant (PDA) or any other similar device. The exact design and functionality of the processor  12  is not critical to operation of the present invention. Accordingly, the processor  12  may be of any type suitable for the operation of the MRI device  10 . 
     The capacitor  16  is connected, via a communication line  22 , to a switch, where the term switch refers to one or more high-power solid-state switch modules as previously disclosed in I N Weinberg&#39;s application No. PCT/US2009/047960. Accordingly, when the capacitor  16  discharges the stored charge, the stored charge passes through the communication line  22  to the switch  24 . 
     The switch  24  is connected, via a communication line  26 , to a coil  28 . Accordingly, when the capacitor  16  is discharged, energy from the capacitor  16  is passed to the coil  28 , which generates a magnetic field  30 . 
     The coil  28  need not be a single coil. To the contrary, it is contemplated that the coil  28  may include a plurality of coils  28 , each of which is capable of generating all or part of the magnetic field  30 . Moreover, as should be appreciated by those skilled in the art, where plural coils  28  are employed, the coils  28  need not be of the same type or size. To the contrary, it is contemplated that, where plural coils  28  are employed, they may be differ from one another to produce magnetic field gradients of differing magnitudes, periods, etc. The coil  28  may be a solenoid or other coil that can produce a relatively uniform magnetic field (i.e., not a gradient) for use in pre-polarizing an object or for read-out (in the case of an integrated read-out/gradient coil type (see, for example, Z H Cho and J H Yi, in “A Novel Type of Surface Gradient Coil”, published in Journal of Magnetic Resonance 94:471-485 (1991), the subject matter of which is incorporated herein by reference in its entirety)). The coil may be planar, in order to reduce overall size of the system. 
     As also shown in  FIG. 1 , the MRI device  10  includes an RF transmitter  32 . As discussed briefly above, the Radio Frequency (RF) transmitter  32  generates radio waves  34 . While one RF transmitter  32  is illustrated, it is contemplated that a plurality of RF transmitters  32  may be employed without departing from the scope of the present invention. Moreover, where plural RF transmitters  32  are employed, they may be of different sizes, types, etc. 
     As illustrated, the magnetic field  30  and the RF waves  34  are directed at a tissue sample  36 . While the tissue sample  36  may be a portion of an organism, it may also be a complete organism. The sample may alternatively be an inanimate object, such as a rock containing petroleum samples. 
     During or after interaction of the magnetic field  30  and the RF waves  34  with the tissue  36 , the tissue  36  generates a responsive signal  38  that is detected by the detector  40 . As should be appreciated by those skilled in the art, the signal  38  may encompass a multitude of different signals from the tissue  36 . The detector  40  detects the signals  38  and passes the signals  38  to the processor  20  via the communication line  42 . The processor  20  receives and processes the signals  38  to generate an image representative of the composition of the tissue  36 . The detector may be a coil as is used in many MRI devices, or may be a magnetometer or Super-conducting QUantum Interference Device (SQUID), as has been advocated for polarized MRI and field-cycling MRI by Espy et al. Alternatively detector  40  need not be a separate component, because MRI device  10  can be configured so that RF transmitter  32  may function (fully or in part) as detector  40 . 
     As should be appreciated by those skilled in the art, the processor  20  may not be the device that processes the signals  38  to generate the image of the tissue  36 . To the contrary, the detector  40  may be combined with a suitable imaging device. In still another embodiment, the imager may be a component separate from the processor  20  and the detector  40 . Still further embodiments are contemplated to fall within the scope of the present invention. 
     With continued reference to  FIG. 1 , the MRI device  10  includes communication line  42 . Communication line  42  is illustrated as a central bus that connects the processor  20  to the capacitor via communication line  44 , to the switch, via communication line  46 , to the coil, via communication line  48 , and to the RF transmitter, via communication line  50 . A central bus, however, is not required to practice the invention. To the contrary, multiple connections may be established between the components of the MRI device  10  without departing from the scope of the invention, as discussed above. 
     It is noted that the communication lines  14 ,  18 ,  22 ,  26 ,  42 ,  44 ,  46 ,  48 ,  50  all may conduct data and/or power. The communication lines, therefore, are meant to illustrate multi-modal connections between the various components of the MRI device  10 . As noted above, each of the communication lines  14 ,  18 ,  22 ,  26 ,  42 ,  44 ,  46 ,  48 ,  50  may be replaced with one or more separate connections, as required or desired. The communication lines  14 ,  18 ,  22 ,  26 ,  42 ,  44 ,  46 ,  48 ,  50  may be unidirectional or bidirectional as required or desired. 
     With respect to the communication lines  42 ,  44 ,  46 ,  38 ,  50 , it is contemplated that the processor  20  will provide operating instructions to one or more of the components to which it is connected. The processor  20 , therefore, is contemplated to incorporate control functionality over one or more of the components, as should be appreciated by those skilled in the art. It is also contemplated that controls may be fed from one component to another, as required or desired for operation of the MRI device  10 . 
     In  FIG. 2 , an example of the pulse sequence for pre-polarized MRI in accordance with the invention is shown by, for example, P Morgan, S Conolly, G Scott, A Macovski, in “A Readout Magnet for Prepolarized MRI”, published in Magnetic Resonance in Medicine 36:527-536 (1996), the subject matter of which is incorporated herein by reference in its entirety. In this figure, ordinate axis  60  represents the pre-polarizing field generated by the circuits, switches, and coils of the invention and ordinate, axis  64 , generally represents the magnitudes of a set of pulsed magnetic field gradients and/or radiofrequency pulses which may be generated by the circuits, switches, and coils of the invention. The time evolution of these magnitudes is represented by abscissa axes  62  and  66  respectively. The pre-polarizing pulse  68  is shown with rise-time  70  and fall-time  72 , which can be very short according to the invention, so as not to induce bio-effects. After fall-time  72 , a reduced magnetic field  74  remains. This field  74  may be generated by the circuits, switches, and coils of the invention, or may be produced by other means, such as a permanent magnet. The pre-polarizing field  68  or the remaining field  74  may occasionally change in order to implement field-cycling. After the fall-time  72 , a set of gradient pulses (which may be produced by the circuits and switches of the invention) and/or radiofrequency pulses are applied in order to read-out the spins previously polarized by pulse  68 . A representation of the signal acquired by a digitizer is included as  76 , again following the publication by P Morgan et al. The signal might be acquired from a tuned coil or from a magnetometer, as in Espy et al. 
     In  FIG. 3 , a flow-chart  78  is provided to illustrate the method of operation of the invention for imaging particles such as protons or electrons. The operation begins at  80 , after which a polarizing magnetic field is generated at  82  with a magnitude of at least 1 milliTesla by the circuits and/or switches of the invention, and (if used on living beings) with the change occurring in a time frame small enough to fail to solicit a response from neurological tissue. The polarizing magnetic field is sustained for at least one microsecond  84 . The polarizing magnetic field is then reduced at  86 , again (if used on living beings) with the change occurring in a time frame small enough to fail to solicit a response from neurological tissue. An imaging pulse sequence follows the polarizing magnetic field, said imaging pulse sequence typically involving the application of radiofrequency and magnetic gradient pulses to the tissue. 
     The magnetic gradients for either the pre-polarizing pulse or for subsequent magnetic gradient pulses that may be generated by the circuits and/or switches of the invention may be repeated according to a decision performed at  88 , until such time as the pulse sequence is terminated at  90 . Although  FIG. 3  describes the pulse sequence as ending at  90 , it is understood that the pulse sequence flow-chart of  FIG. 3  may be repeated many times with similar or different values of magnetic fields, gradients, and RF pulses in the course of acquiring an image or measurement of a tissue or sample. 
     In  FIG. 4 , an illustration is provided to show the reduction in acoustic noise expected with the invention. Graph  92  shows the acoustically-weighted frequency spectrum (in dbA) of a conventional MRI pulse sequence, with peak frequency at 767 Hertz. This calculation modeled sound pressure levels for echo-planar sequence at 4 T with standard pulse sequence (slew-rate 200 T/m/s, and maximum gradient strength 40 milliTesla per meter). Graph  94  shows the acoustically-weighted frequency spectrum of an accelerated MRI pulse sequence, taking advantage of the shorter rise- and fall-times available with the circuits and/or switches of the invention, with peak frequency at 12.6 kHz. This second calculation modeled a 14 μs rise/fall time and a gradient maximum of 1,780 milliTeslas per meter, which is a much higher gradient strength than the conventional MRI but results in similar perceived loudness. 
     The higher gradient fields produced by the disclosed embodiments without soliciting a response from neurological tissue may be used to propel magnetic fluids, where the term magnetic fluids is understood to include collections of magnetic particles. It is understood that the size of the particles, whether micro- or nano-, is included in the term magnetic particles, and that conductive particles impelled with magnetic forces are also included in the class of magnetic particles. By alternating the propulsive pulse sequences with pulse sequences designed to image the magnetic fluids, the invention may be used to guide delivery of the magnetic fluids within tissues.