METHODS FOR VIEW ANGLE TILT IMAGING OF INTERVENTIONAL DEVICES PROVIDING ADJUSTMENT OF DEVICE CONTRAST

Abstract: Imaging of patients, intervention devices (48) by MRI systems can be achieved using a combination of view angle tilting, and magnetization manipulation. In one embodiment the volume elements (20, 21) appear to be disconnected (27) in the MRI image. The gap (27) provides contrast between the device (48), and the patient. This method allows the contrast of device (48) to be increased or decreased.

Description: CLAIMS What is claimed is : 1. A method for view-angle tilt magnetic resonance imaging of an obj ect having a localized magnetic susceptibil ity variation, the method comprising the steps of : a ) applying to the obj ect an excitation RF pulse simultaneous with a f irst field gradient to select a slice and determine an excitation profile ; b) applying to the obj ect a refocusing RF pulse s imultaneous with a second f ield gradient to ref ocus the s lice and determine a refocus ing profile , wherein the second f ield gradient has an amplitude different from the first field gradient , and wherein the ref ocus ing RF pulse has a dif ferent bandwidth compared to the excitation RF pul se such that an exc itation prof i le and refocusing prof ile are substantially overlapping in a region outside the magnetic susceptibility variation; c) applying to the object a tilted readout field gradient; whereby the excitation profile and refocusing profile are partially not overlapping in the region of the magnetic susceptibility variation, thereby providing magnetic susceptibility contrast in the region of the magnetic susceptibility variation. 2. The method of claim 1 wherein the excitation profile and refocusing profile have the same thickness and location outside the magnetic susceptibility variation. 3. The method of claim 1 wherein additional refocusing RF pulses, second field gradients, and tilted readout gradients are applied to comprise a rapid acquisition and relaxation enhancement sequence . 4. The method of claim 1 further comprising the step of varying an amplitude difference between the first field gradient and second field gradient to provide a des ired amount o f magne t i c susceptibility contrast . A method for view-angle tilt magnetic resonance imaging of an obj ect having a localized magnetic susceptibility variation, the method comprising the steps of : a ) applying to the obj ect a excitation RF pulse simultaneous with a first field gradient to select a slice and determine an excitation profile ; b) applying to the obj ect a refocusing RF pulse simultaneous with a second f ield gradient to refocus the slice and determine a refocusing profile , wherein the second field gradient has a direction opposite from the f irst field gradient , and wherein the refocusing RF pulse bandwidth is selected and the second f ield gradient magnitude is selected such that the excitation profile and the ref ocus ing pro f i le are subs tantial ly overlapping in a region outside the magnetic susceptibility variation; c) applying to the object a tilted readout field gradient; whereby the excitation profile and refocusing profile are partially not overlapping in the region of the magnetic susceptibility variation, thereby providing magnetic susceptibility contrast in the region of the magnetic susceptibility variation. 15 6. The method of claim 5 wherein the excitation profile and refocusing profile have the same thickness and location outside the magnetic susceptibility variation. 7. The method of claim 5 wherein additional refocusing RF pulses, second field gradients, and tilted readout gradients are applied to comprise a rapid acquisition and relaxation enhancement sequence . 8. The method of claim 5 further comprising the step of varying an amplitude difference between the first field gradient and second field gradient to provide a desired amount of magnetic susceptibility contrast. A method for view-angle tilt magnetic resonance imaging of an object having a localized magnetic susceptibility variation, the method comprising the steps of: a) applying to the object an excitation pulse simultaneous with a first field gradient, and a refocusing pulse simultaneous with a second field gradient, and a tilted readout gradient such that a spin echo signal is produced from a volume of interest within the region of the magnetic susceptibility variation; b) capturing the spin echo signal during a detection time which is offset with respect to the echo time; whereby magnetic susceptibility contrast is provided in the region of the magnetic susceptibility variation. 16 10. The method of claim 9 further comprising the step of varying the offset to provide a desired amount of magnetic susceptibility contrast. 11. The method of claim 9 wherein additional refocusing RF pulses, second field gradients, and tilted readout gradients are applied to comprise a rapid acquisition and relaxation enhancement sequence. 12. A method for view-angle tilt magnetic resonance imaging of an object having a localized magnetic susceptibility variation, the method comprising the steps of: a) applying to the object an excitation RF pulse simultaneous with a first field gradient; b) applying to the object a refocusing RF pulse simultaneous with a second field gradient; c) applying to the object a tilted readout gradient tilted at an angle selected to provide incomplete removal of the image distortion produced by the magnetic susceptibility variation; whereby magnetic susceptibility contrast is provided in the region of the magnetic susceptibility variation. 13. The method of claim 12 further comprising the step of adjusting the tilt angle to provide a desired amount of magnetic susceptibility contrast. 14. The method of claim 12 wherein additional refocusing RF pulses, second field gradients, and tilted readout gradients are applied to comprise a rapid acquisition and relaxation enhancement sequence . 17 5. A method for view angle tilt magnetic resonance imaging of an object having a localized magnetic susceptibility variation, the method comprising the steps of: a) applying to the object an excitation RF pulse simultaneous with a first field gradient, wherein the excitation RF pulse has a nonlinear phase; b) applying to the object an inverted field gradient such that a gradient echo is produced; c) applying to the object a tilted readout gradient tilted at an angle selected to provide complete removal of the image distortion produced by the magnetic susceptibility variation. 16. The method of claim 15 further comprising the step of varying an amplitude of the nonlinear component to provide a desired amount of magnetic susceptibility contrast. 17. The method of claim 15 wherein the excitation RF pulse has a quadratic phase . 18. The method of claim 17 further comprising the step of varying the quadratic component of the excitation pulse such that magnetic susceptibility contrast is adjusted. 19. A method for view angle tilt magnetic resonance imaging of an object having a localized magnetic susceptibility variation, the method comprising the steps of: a) applying to the object an RF half pulse simultaneous with a first field gradient; b) simultaneously turning off the RF half pulse and first field gradient; c) after step (b) , waiting an interval time 18 d) after step (c) , capturing a signal from spins resonant with the RF half pulse; e) adjusting the interval time to provide a desired amount of magnetic susceptibility contrast; f) applying to the object a tilted readout gradient tilted at an angle selected to provide complete removal of the image distortion produced by the magnetic susceptibility variation. 19 Methods for View Angle Tilt Imaging of Interventional Devices Providing Adjustment of Device Contrast RELATED APPLIATIONS The present application claims priority from copending United States patent application serial number 08/946,544, filed on October 7, 1997. FIELD OF THE INVENTION This invention relates generally to magnetic resonance imaging techniques. More particularly, it relates to methods of imaging devices used in a patient during interventional magnetic resonance imaging procedures which allow independent adjustment of device contrast and tissue contrast. BACKGROUND OF THE INVENTION Magnetic resonance imaging is commonly used to image internal organs of patient or interior of an object. MRI is performed by placing the object to be imaged in a highly uniform and strong (e.g. 1.5 Tesla) magnetic field. The magnetic field causes the atomic nuclei (which possess a magnetic moment) to become aligned with the magnetic field. The nuclei (spins) precess about the magnetic field direction at a rate which is proportional to the magnetic field. For hydrogen nuclei (commonly used as the diagnostic species in MRI) in a 1.5 Tesla magnetic field, the precession frequency is about 64 MHz. In performing imaging, a magnetic field gradient is applied to the object simultaneously with an RF excitation pulse. The magnetic field gradient causes nuclei in different locations to precess at different frequencies (since the precession frequency is dependent upon magnetic field strength) . The RF pulse has a well-defined frequency spectrum and polarization, and so causes nuclei having only certain precession frequencies (i.e. in certain locations) to become excited. The process is designed such that a rotating magnetic moment (localized within the volume of interest) is produced. The rotating magnetic moment provides a signal which is easily detectable with a nearby antenna. The entire process of applying gradients and RF pulses is repeated using a sequence of readout gradient encodings to collect a data set that can be reconstructed into an image . MRI is helpful in guiding interventionl devices such as needles into a patient. MRI enables the accurate positioning of the device. Such interventional devices often have a magnetic susceptibility which is different from the magnetic susceptibility of surrounding human tissue. Although usually very small (e.g. about 100 parts per million) , this difference in magnetic susceptibility causes problems when performing MRI, which requires a highly uniform field. The interventional device causes the magnetic field to become distorted in magnitude (directional changes are negligible for MRI purposes). Therefore, when imaging is performed, the resultant image is distorted in the vicinity of the interventional device. Distortions make it difficult to properly locate an interventional device in a desired location. One solution to the problem of interventional device imaging has been to make the interventional devices out of a material which has a magnetic susceptibility that closely matches that of human tissue (e.g. carbon fiber) . An interventional device with such a magnetic susceptibility will not distort the magnetic field. However, interventional devices made of such materials typically do not work as well in performing their intended function (e.g. acquiring biopsy samples) as devices made of conventional materials. Another proposed solution to interventional device distortions concentrated on changing the pulse sequence to achieve the best contrast between the device and patient. See ewin, J. et al . "Needle Localization in MR-Guided Biopsy and Aspiration: Effects of Field Strength, Sequence Design, and Magnetic Field Orientation" in Ameri can Journal of R o en t gen o l ogy , vol. 166, pgs . 1337-1345, 1996, and "Interactive MR-Guided Breast Lesion Localization-A Phantom Study" by B. Daniel in Proc . , In terna tional Soci ety for Magnetic Resonance in Medicine, New York, 1996, pg. 1733. A problem with these techniques is that neither the tissue- tissue contrast nor the device-tissue contrast is optimized in the same pulse sequence. Total inhomoqeneitv correction including chemical shifts and susceptibility by view angle tilting, by Z.H. Cho, D.J. Kim, and Y.K. Kim, in Med. Phys . 15 (1 ) Jan/Feb 1988 , discloses a successful technique for removing the distortions caused by variations in magnetic susceptibility. The method includes the step of applying an additional magnetic field gradient during readout of MR signals to change the angle of MR data readout. This angular tilt corrects for displacements and distortions caused by local variations in magnetic field strength. Reduction of image distortion in the presence of metal , by A.J. McGowan, A.L. Mackay, Q.S. Xiang, D.G. Connell, D.L. Janzen, and P.L. Munk, in the 1997 Proceedings of the International Society of magnetic Resonance in Medicine annual meeting, Vancouver, Canada, Abstract #1973 discloses that the method of view angle tilting can be used to reduce or eliminate distortions caused by metal objects (e.g. a hip joint replacement) in human tissue. A problem with removing magnetic susceptibility distortions by means of view-angle tilting is that the distortions are often removed completely, rendering the device nearly invisible. This is undesirable because it is often useful to see the device so that it can be guided into its proper location (e.g. when taking a biopsy tissue sample) . Imaging parameters such as gradient echo imaging or spin echo imaging may be adjusted to increase the device-tissue contrast so that the device may be seen, but this often precludes the use of parameters which are optimized for tissue-tissue contrast. Therefore, it would be an advance in the art of magnetic resonance imaging to provide a method of using view angle tilting which allows independent control of device-tissue contrast and tissue-tissue contrast. OBJECTS AND ADVANTAGES OF THE INVENTION Accordingly, it is a primary object of the present invention to provide a method for magnetic resonance imaging of interventional devices which 1) allows adjustment of device-tissue contrast independent of adjustment of tissue contrast. SUMMARY OF THE INVENTION This object and advantage is attained by six embodiments of the present invention in which view angle tilt magnetic resonance imaging is used to image an object (e.g. a patient) having a localized magnetic susceptibility variation (e.g. an interventional device) . In a first embodiment of the present invention, a method is provided in which an excitation RF pulse and a first field gradient is applied to the object to select a slice. Then, a refocusing RF pulse and a second field gradient are applied to refocus the slice. The first field gradient and second field gradient have different amplitudes. Also, the excitation pulse and refocusing pulse have different bandwidths such that the excitation profile and refocusing profile are overlapping in a region outside the magnetic susceptibility variation. Finally, a tilted readout field gradient is applied to the object. The excitation profile and refocusing profile are overlapping outside the magnetic susceptibility variation, and only partially overlapping or not overlapping in the region of the magnetic susceptibility variation. Preferably, the excitation profile and refocusing profile have the same thickness and location (i.e. completely overlap one another) in regions outside the magnetic susceptibility variation. The difference in gradient amplitudes determines the magnetic susceptibility contrast. In a second embodiment of the present invention, an excitation pulse and first field gradient are applied to the object, thereby selecting a slice. Then, a refocusing pulse and second field gradient are applied to the object, thereby refocusing the slice. Finally, a tilted readout field gradient is applied to the object. The second field gradient has a direction opposite the first field gradient. The excitation profile and refocusing profile are overlapping outside the magnetic susceptibility variation, and only partially overlapping or not overlapping in the region near the magnetic susceptibility variation. The first and second gradients may have the same or different amplitudes. Preferably, the excitation profile and refocusing profile have the same thickness and location (i.e. completely overlap one another) in regions outside the magnetic susceptibility variation. The difference in gradient amplitudes determines the magnetic susceptibility contrast. In a third embodiment of the present invention, RF pulses and field gradients are applied to the object such that a spin echo is produced from a localized volume of interest. The spin echo MR signal is detected during a detection time which is offset from the spin echo time. This results in a dephasing of the nuclear spins within the region of the magnetic susceptibility variation, and a corresponding loss of MR signal from this region. The amount of offset determines the magnetic susceptibility contrast. In a fourth embodiment of the present invention, the method of view angle tilting is used wherein the tilt angle of the readout gradient is selected to provide incomplete reduction of the image distortion produced by the magnetic susceptibility variation. The tilt angle determines the magnetic susceptibility contrast. In a fifth embodiment of the present invention, a single excitation pulse is applied simultaneously with a first field gradient. Preferably, the excitation pulse has a nonlinear phase profile. Most preferably, the excitation pulse has a quadratic phase profile. The amplitude of the nonlinear component of the excitation pulse determines the magnetic susceptibility contrast. In a sixth embodiment of the present invention, a RF half pulse is applied simultaneously with a first field gradient. Then, the first field gradient and RF half pulse are abruptly turned of and the resulting signal is detected after waiting an interval time (i.e. an echo time) after the termination of the RF half pulse and field gradient. The length of the interval time determines the magnetic susceptibility contrast. DESCRIPTION OF THE FIGURES Fig. 1 (Prior Art) shows a pulse sequence which uses a tilted view angle. Figs. 2A-2B (Prior Art) illustrate the effect of view angle tilting on the distortion causes by three needles . Fig. 3 illustrates method A of the present invention. Fig 4A, which illustrates method A, is an MRI image of three needles using a conventional 90° RF pulse. Fig 4B, which illustrates method A, is an MRI image of three needles using a 90° RF pulse having a decreased bandwidth. Fig 5A, which shows method B, is an MRI image of three needles using a conventional 90° RF pulse. Fig 5B, which shows method B, is an MRI image of a needle with the slice select gradient inverted. Fig 5C, which shows method B, is an MRI image of a needle within an animal model with a conventional 90° RF pulse. Fig 5D, which shows method B, is an MRI image of a needle within an animal model with the slice select gradient inverted. Fig. 6 illustrates method C of the present invention. Fig 7A, which illustrates method C, is an MRI image of three needles using a 180° refocusing RF pulse offset toward the 90° RF pulse by 0 μs . Fig 7B, which illustrates method C, is an MRI image of three needles using a 180° refocusing RF pulse offset toward the 90° RF pulse by 500 μs . Fig 7C, which illustrates method C, is an MRI image of three needles using a 180° refocusing RF pulse offset toward the 90° RF pulse by 1000 μs . Fig 7D, which illustrates method C, is an MRI image of three needles using a 180° refocusing RF pulse offset toward the 90° RF pulse by 1500 μs . Fig 7E is an MRI image of an animal model with view angle tilting only. Fig 7F is an MRI image of an animal model with view angle tilting and method C where the 180° refocusing RF pulse is offset toward the 90° RF pulse by 1000 ms . Figs 8A-8B (Prior Art) illustrate methods of view angle tilting. Fig 9A, which illustrates method E, is a chart showing the transmit magnitude of a linear phase sine RF pulse Fig 9B, which illustrates method E, is a chart showing the transmit phase of a linear phase sine RF pulse Fig 9C, which illustrates method E, is a chart showing the transmit magnitude of a quadratic phase RF pulse Fig 9D, which illustrates method E, is a chart showing the transmit phase of a quadratic phase RF pulse Fig 10A, which illustrates method E, is a chart showing a linear phase. Fig 10B, which illustrates method E, is a chart showing a quadratic phase. Fig IOC, which illustrates method E, is a chart showing a linear + quadratic phase. Fig 11, which illustrates method F, is a chart showing interleaved half pulses . DETAILED DESCRIPTION In all the embodiments of the present invention, the method of view angle tilting is used to reduce or eliminate distortions caused by localized magnetic susceptibility variations . For more information concerning view angle tilting reference can be made to Total Inhomogeneity Correction Including Chemical Shifts and Susceptibility by View Angle Tilting, by Z.H. Cho, D.J. Kim, and Y.K. Kim, in Med. Phys . 15 (1 ) Jan/Feb 1988 . Fig 1 shows a rapid acquisition and relaxation enhancement (RARE) pulse sequence with view angle tilting applied. Figs. 2A and 2B show the results of view angle tilting applied to the imaging of three needles 48A, 48B, 48C having different diameters and materials . The three example needles are from left to right: 48A - 18 gauge Inconel (65% Ni, 22% Cr, 9% Mo, and other trace elements) needle shaft with no cutting edge from Manan Medical Products, 48B - 14 gauge surface-treated titanium biopsy needle from Daum Gmbh, 48C - 22 gauge Inconel Technicut aspiration needle from Manan Medical Products . As all three needles 48A, 48B, 48C are manufactured from metal alloys, they distort the local magnetic field. Fig 2A shows the three needles 48 imaged without view angle tilting, while Fig 2B shows the three needles 48A, 48B, 48C imaged with view angle tilting. In Fig 2A, notice the large artifacts caused by the local magnetic fields generated by needles 48A, 48B, 48C. Artifacts make it very difficult to determine the actual diameter and position of needles. In Fig 2B, artifacts are reduced due to the use of view angle tilting. Signal loss due to off-resonant spins only occurs in the actual locations of needles 48A, 48B, 48C, not in the surrounding area. The artifact reduction allows accurate determination of the position of needles 48A, 48B, 48C with respect to the tissues of patient. Although view angle tilting is a very useful technique for removing distortion caused by interventional devices (such as needles 48), it reduces visibility of devices. Depending on the interventional device, its orientation within patient, and the imaging sequence used, artifact produced by the device can be too faint, rendering the device invisible. The present invention solves this problem by providing several methods of adjusting the contrast between device and patient. These methods allow a health care practitioner to increase or decrease artifact of device 48 as necessary. The first steps of producing a net magnetization of patient and applying a radiofrequency (RF) excitation pulse in the presence of a first slice select field gradient to select a slice are common to all methods . In method A shown in Fig. 3, the slice 26 is shown. A refocusing RF pulse is applied to patient and device 48 in the presence of a second field gradient to refocus the slice 26. The second field gradient has a different amplitude than the first field gradient. This allows adjustment of contrast of device 48. The refocusing RF pulse can have a length which allows for selection of the same on-resonant slice, such that off-resonant spins near device 48 (i.e. within region 24) are not refocused and thus do not appear in the resulting image. Fig 4A shows an image of needles 48A, 48B, 48C acquired with a conventional 90° RF pulse. Fig 4B shows an image of needles 48A, 48B, 48C having greater device contrast, which is achieved by decreasing the bandwidth or increasing the length of the 90° RF pulse. In method B, the refocusing RF pulse is applied to patient 46 and device 48 in the presence of a second field gradient which has an orientation opposite the first field gradient. In this method, fewer off-resonant spins near device 48 are refocused. Spins that are refocused are not distorted. Inversion of the second field gradient reduces the amount of spins near the device that experience both RF pulses because off-resonance spins are spatially shifted in the opposite direction from the on-resonant spins . See Fig 5B for an image showing greater contrast of needles 48A, 48B, 48C due to inversion of the amplitude of the second slice select gradient. When compared with Fig 5A, which shows an image of needles 48A, 48B, 48C acquired with a conventional 90° refocusing RF pulse, it is obvious that method B provides greater contrast of devices 48. This method is also illustrated in Fig 5D, as compared to conventional imaging with view angle tilting in Fig 5C . Figs. 5C and 5D show artifacts of needle 48D in a animal model. In Fig 5C, no view angle tilting is applied and artifact is very large, making the actual position of needle 48D unclear. Fig 5D was taken with view angle tilting. However, this image is not optimal because artifact of needle 48D is not as visible as desired. In method C, a refocusing pulse is applied to produce a spin echo signal. As shown in Fig. 6, The detection time is not coincident with the time of the spin-echo. The detection time 45 during which the spin echo signal is detected is offset 46 compared to the spin echo time. This offset 46 results in a phase cancellation of spins within the vicinity of device 48. The phase cancellation can be adjusted by varying the offset 46. Due to the large local magnetic field variations around 10 needles 48A, 48B, 48C, even small offsets result in an increase in device contrast. Fig 7A - 6D show images of needles 48A, 48B, 48C taken with offsets of 0 μs , 500 μs, 1000 μs, and 1500 μs, respectively. Notice the increase in device contrast is proportional to the increase in offset. This method is again illustrated in an animal model in Fig 7F. The device contrast of 48D is greater than that shown in Fig 7E where only view angle tilting was used. Fig. 8A shows a diagram which illustrates a conventional imaging method according to the prior art . Volume elements 20, 21 are located in a region of perturbed magnetic field (e.g. close to a magnetic susceptibility variation) . Although volume elements 20, 21 are mechanically connected in physical space, they appear to be disconnected in an MRI image 24 due to the magnetic field perturbation. The MRI image is produced line by line in a readout direction 22 determined by a readout field gradient. Fig. 8B illustrates the method of view angle tilting according to the prior art. The readout gradient is tilted by an angle θ 23, thereby tilting the readout direction 22. The tilt 23 of the readout direction makes the volume elements appear connected (i.e. as they actually are), thereby eliminating the distortion which made the elements 20, 21 appear disconnected. Method D of the present invention is illustrated in Fig. 8C. In the method of Fig. 8C, the readout gradient is tilted by an amount which does not fully correct for distortion caused by device 48. As a result, some device distortion is allowed to remain in the image. The tilt angle 25 is different from the angle 23 which would result in complete correction of the distortion. The volume elements 20, 21 appear to be disconnected 27 in the MRI image 24. The gap 27 provides contrast between the device 48 and a patient. This method allows the contrast of device 48 to be increased or decreased. 11 In method E, the RF excitation pulse has a non-linear phase such that the first slice has a nonlinear phase profile. Only a single RF pulse is used, and an inverted gradient is applied after the excitation pulse to produce a gradient echo. Figs. 9A and 9B show the amplitude and phase of a linear RF sine pulse. Figs. 9C and 9D show the amplitude and phase of a nonlinear phase RF pulse according to method E of the present invention. Fig 10A (Prior Art) shows a schematic of a conventional linear phase RF pulse which is used in the presence of a linear field gradient, which results in the slice having a linear phase profile. A slice with a linear phase profile produces a zero sum signal in the vicinity of the device 48. Fig 10B shows the quadratic phase profile of a quadratic RF pulse used in a homogenous field. The quadratic phase results in phase cancellation across the slice and sums up to 50% of the signal, causing a 50% signal loss. However, when quadratic phase RF pulses are used in a non-homogeneous field, such as a linear magnetic field around device 48, the spins have the sum of the linear and quadratic phase and sum up to 50% of the signal, as shown in Fig IOC. As this 50% signal is around the area of device 48, device contrast is maintained and can be manipulated by altering the magnitude of the quadratic component of the RF excitation pulse. In method F, shown in Fig. 11, a RF half pulse 60 is used to increase sensitivity to off-resonant shifts caused by the local magnetic fields of device 48. The use of such half pulses for measuring on-resonance (i.e. resonant with RF pulse) spins is disclosed in US Patent 5,150,053 to Pauly. Abruptly turning off the RF pulse 60 and gradient 62 at time 64 allows a signal to be detected from the on-resonance spins within a slice. Interval time 66 (typically less than 200 microseconds) is required to shut off the RF transmitter and turn on a receiver. In a typical application, many RF pulses can be used sequentially. Each half pulse has the same RF waveform, but with the gradient waveform inverted 68 in every other pulse. The use of half pulses reduces the echo time, 12 which reduces intervoxel dephasing of spins and thus reduces signal loss around device 48. A decrease in signal loss allows adjustment of device contrast by simply increasing or decreasing the interval time 66. The last method is the same as methods A-D, but has with additional RF refocusing pulses interspersed with NMR detection. The present invention comprises the combination of view angle tilting with any one of methods A - F. Thus it provides a number of ways in which to remove device distortion while adjusting device contrast, all of which do not significantly affect tissue imaging. The results are MRI images which exhibit accurate interventional device positioning within a patient. It will be clear to one skilled in the art that the above embodiment may be altered in many ways without departing from the scope of the invention. For example, all types of interventional devices fabricated from a broad range of materials can be imaged using the described method. Accordingly, the scope of the invention should be determined by the following claims and their legal equivalents. 13

Claims: CLAIMS What is claimed is: 1 1. A method for view-angle tilt magnetic resonance imaging of 2 an object having a localized magnetic susceptibility 3 variation, the method comprising the steps of: 45a) applying to the object an excitation RF pulse 6 simultaneous with a first field gradient to select 7 a slice and determine an excitation profile; 8 b) applying to the object a refocusing RF pulse 9 simultaneous with a second field gradient to 10 refocus the slice and determine a refocusing 11 profile, wherein the second field gradient has an 12 amplitude different from the first field gradient, 13 and wherein the refocusing RF pulse has a 14 different bandwidth compared to the excitation RF 15 pulse such that an excitation profile and 16 refocusing profile are substantially overlapping 17 in a region outside the magnetic susceptibility 18 variation; 19 c) applying to the object a tilted readout field 20 gradient; 21 22 whereby the excitation profile and refocusing profile 23 are partially not overlapping in the region of the 24 magnetic susceptibility variation, thereby 25 providing magnetic susceptibility contrast in the 26 region of the magnetic susceptibility variation. 27 1 2. The method of claim 1 wherein the excitation 2 profile and refocusing profile have the same 3 thickness and location outside the magnetic 4 susceptibility variation. 5 1 3. The method of claim 1 wherein additional 2 refocusing RF pulses, second field gradients, and 14 BNSDOCIDKWO 9917656A1 I > 3 tilted readout gradients are applied to comprise a 4 rapid acquisition and relaxation enhancement 5 sequence . 6 1 4 . The method of claim 1 further comprising the step 2 of varying an amplitude difference between the 3 first field gradient and second field gradient to 4 provide a des ired amount o f magnet i c 5 susceptibility contrast . 6 1 5. A method for view-angle tilt magnetic resonance imaging of 2 an obj ect having a localized magnetic susceptibility 3 variation, the method comprising the steps of : 4 5 a ) applying to the obj ect a excitation RF pulse 6 simultaneous with a first field gradient to select 7 a slice and determine an excitation profile; 8 b) applying to the obj ect a refocusing RF pulse 9 simultaneous with a second field gradient to 10 refocus the slice and determine a refocusing 11 profile , wherein the second field gradient has a 12 direction opposite from the first field gradient , 13 and wherein the refocusing RF pulse bandwi dth is 14 selected and the second field gradient magnitude 15 is selected such that the excitation prof ile and 16 the ref ocus ing prof ile are subs tantial ly 17 overlapping in a region outside the magnetic 18 susceptibility variation; 19 c) applying to the object a tilted readout field 20 gradient; 21 22 whereby the excitation profile and refocusing profile 23 are partially not overlapping in the region of the24magnetic susceptibility variation, thereby 25 providing magnetic susceptibility contrast in the 26region of the magnetic susceptibility variation. 27 15 Ru[sigma]nnnn [Lambda][Pi] qqi7fWfiAi I =, 1 6 . The method of claim 5 wherein the exc itation 2 prof ile and ref ocusing prof ile have the same 3 thickness and location outs ide the magnetic 4 susceptibility variation . 5 1 7 . The method o f c laim 5 wherein addi t ional 2 refocusing RF pulses , second f ield gradients , and 3 tilted readout gradients are applied to comprise a 4 rapid acquisition and relaxation enhancement 5 sequence . 6 1 8. The method of claim 5 further comprising the step 2 of varying an amplitude difference between the 3 first field gradient and second field gradient to 4 provide a desired amount of magnetic 5 susceptibility contrast. 6 1 9. A method for view-angle tilt magnetic resonance imaging of 2 an object having a localized magnetic susceptibility 3 variation, the method comprising the steps of: 4 5 a) applying to the object an excitation pulse 6 simultaneous with a first field gradient, and a 7 refocusing pulse simultaneous with a second field 8 gradient, and a tilted readout gradient such that 9 a spin echo signal is produced from a volume of 10 interest within the region of the magnetic 11 susceptibility variation; 12 b) capturing the spin echo signal during a detection 13 time which is offset with respect to the echo 14 time ; 15 16 whereby magnetic susceptibility contrast is provided 17 in the region of the magnetic susceptibil ity 18 variation . 19 16 BNSDOCID: <WO 9917656A1 I > 1 10. The method of claim 9 further comprising the step 2 of varying the offset to provide a desired amount 3 of magnetic susceptibility contrast. 4 1 11. The method of claim 9 wherein additional 2 refocusing RF pulses, second field gradients, and 3 tilted readout gradients are applied to comprise a 4 rapid acquisition and relaxation enhancement 5 sequence. 6 1 12. A method for view-angle tilt magnetic resonance imaging of 2 an object having a localized magnetic susceptibility 3 variation, the method comprising the steps of: 4 5 a) applying to the object an excitation RF pulse 6 simultaneous with a first field gradient; 7 b) applying to the object a refocusing RF pulse 8 simultaneous with a second field gradient; 9 c) applying to the object a tilted readout gradient 10 tilted at an angle selected tc provide incomplete 11 removal of the image distortion produced by the 12 magnetic susceptibility variation; 13 14 whereby magnetic susceptibility contrast is provided 15 in the region of the magnetic susceptibility 16 variation. 17 1 13. The method of claim 12 further comprising the step 2 of adjusting the tilt angle to provide a desired 3 amount of magnetic susceptibility contrast. 4 1 14. The method of claim 12 wherein additional 2 refocusing RF pulses, second field gradients, and 3 tilted readout gradients are applied to comprise a 4 rapid acquisition and relaxation enhancement 5 sequence . 6 17 RMQnn i[pi] 7R[sigma]fi[Delta]i i 1 15. A method for view angle tilt magnetic resonance imaging of 2 an object having a localized magnetic susceptibility 3 variation, the method comprising the steps of: 4 5 a) applying to the object an excitation RF pulse 6 simultaneous with a first field gradient, wherein the 7 excitation RP pulse has a nonlinear phase; 8 b) applying to the object an inverted field gradient such 9 that a gradient echo is produced; 10 c) applying to the object a tilted readout gradient tilted 11 at an angle selected to provide complete removal of 12 the image distortion produced by the magnetic 13 susceptibility variation. 14 1 16. The method of claim 15 further comprising the step of 2 varying an amplitude of the nonlinear component to 3 provide a desired amount of magnetic susceptibility 4 contrast. 5 1 17. The method of claim 15 wherein the excitation RF pulse 2 has a quadratic phase. 3 1 18. The method of claim 17 further comprising the step 2 of varying the quadratic component of the 3 excitation pulse such that magnetic susceptibility 4 contrast is adjusted. 5 1 19. A method for view angle tilt magnetic resonance imaging of 2 an object having a localized magnetic susceptibility 3 variation, the method comprising the steps of: 4 5 a) applying to the object an RF half pulse simultaneous 6 with a first field gradient; 7 b) simultaneously turning off the RF half pulse and first 8 field gradient; 9 c) after step (b) , waiting an interval time 18 BNSDOCID:<WO 9917656A1 I 10 d) after step ( c ) , capturing a signal from spins resonant 11 with the RF half pulse; 12 e ) adjusting the interval time to provide a desired amount 13 of magnetic susceptibility contrast ; 14 f ) applying to the object a tilted readout gradient tilted 15 at an angle selected to provide complete removal of 16 the image distortion produced by the magnet ic 17 susceptibility variation . U

Inventors: BUTTS ROSEMARY K, PAULY JOHN M, BUTTS, ROSEMARY, K., PAULY, JOHN, M.
Applicants: UNIV LELAND STANFORD JUNIOR, THE BOARD OF TRUSTEES OF THE LELAND STANFORD JUNIOR UNIVERSITY
Application year: 1998
Main classification: A61B5
IPC codes: A61B5, G01R33