JP3571365B2 - Magnetic resonance tracker - Google Patents

Description

こゝで“＋”は選ばれた方向に印加される磁界勾配パルスを表わし、“−”は同じ軸に沿って、反対の極性で略同一の勾配パルスが印加されることを表わす。
【００２６】
図６に示したこの発明のアダマール符号化形の実施例では、応答信号４５１の様な信号４個が図１の作像及び追跡装置１７０に記憶される。４個の応答信号Ｐ１，Ｐ２，Ｐ３，Ｐ４の各々からの位置が、図５及び図７（後述）に示すパルス順序について述べるのと同様にして、応答信号のフーリエ変換を計算することによって計算される。次に、４つの位置Ｐ１，Ｐ２，Ｐ３，Ｐ４の線形の組合せを計算して、Ｘ，Ｙ及びＺ磁界勾配軸に対する装置の位置に敏感な３つの処理済み応答信号を求める。ＮＥＸ＝４であるアダマール符号化形実施例では、下に示す線形の組合せが役立つ。
【００２７】
Ｘ位置＝Ｐ１−Ｐ２−Ｐ３＋Ｐ４
Ｙ位置＝Ｐ１−Ｐ２＋Ｐ３−Ｐ４
Ｚ位置＝Ｐ１＋Ｐ２−Ｐ３−Ｐ４
図５に概略的に述べた「ゼロ基準ＭＲ追跡順序」又は図６について概略を述べた「アダマールＭＲ追跡順序」を用いて得られた図１の装置１５０の位置は、装置が異なる種類の組織を通過する時、又は送信器の周波数の調節不良の様な共鳴のオフセット状態がある時に起こり得る化学シフトの差に影響されない。
【００２８】
図７には、検出された信号から図１の装置１５０の位置を決定する為に図１の作像及び追跡装置１７０によって実行される段階が示されている。図５又は図６の何れかに示したパルス順序に応答して、信号５００がＭＲ装置によって検出される。信号５００は、印加された磁界勾配の方向に於ける装置の位置に関する情報を持っている。信号をフーリエ変換（ＦＴ）にかけて、データの時間依存性を周波数依存性に変換することにより、この周波数情報が抽出される。周波数依存性を持つデータの組５１０は、印加された磁界勾配の方向に於ける図２のＲＦコイル２００の位置に対応する１個の最大値を持っている。データの組に於ける最大値の場所を抽出し、それを表示手段１８０（図１）に送って、オペレータに表示する。
【００２９】
希望によっては、ＭＲ作像及び装置の追跡は、大部分のハードウエアを同じにした装置で実施することが出来る。像の収集と追跡とをインターリーブにして、両方が大体同時に行なわれる様にすることも可能である。この代りに、作像手順の勾配波形と、装置１５０（図２）内にあるＲＦコイル２００によって検出されたＭＲ応答信号を解析して、装置１５０の場所を決定することにより、インターリーブをせずに、追跡及び作像を同時に行なうことが出来る。
【００３０】
この発明の好ましい実施例では、装置１５０内にあるＲＦコイル２００が受信機能をする。然し、送信及び受信コイルの間には相反性があり、装置１５０にあるＲＦコイル２００を使ってＲＦエネルギを送信し、外部コイル１４０を使ってＭＲ応答信号を受信する様な追跡装置も可能である。
この発明の別の実施例ではＲＦコイル２００を使って、図８に示す様に、交互にＲＦエネルギの送信及び受信が出来る。制御器９００が、使われるＭＲ順序に従ってスイッチ９０３を作動し、コイル２００を送信器９３０に接続して、被検体にＲＦエネルギを送り込む。逆に、制御器９００がスイッチ９０３を作動して、コイル２００を受信器９４０に接続し、被検体からのＲＦエネルギを受信する。
【００３１】
現在好ましいと考えられる幾つかの実施例のＭＲ追跡装置を詳しく説明したが、当業者には種々の変更が考えられよう。従って、特許請求の範囲は、この発明の範囲内に属するこの様な全ての変更を包括するものであることを承知されたい。
【図面の簡単な説明】
【図１】被検体内にある装置の場所を追跡する動作中のこの発明の実施例の一部分を切欠いた斜視図。
【図２】被検体の身体に挿入しようとする装置に設けられたＲＦコイルの概略構成図。
【図３】この発明による図２の装置を追跡するのに適したＭＲ作像装置のブロック図。
【図４】印加磁界勾配が存在する時のＭＲ応答周波数を１つの軸に沿った位置に対して示すグラフ。
【図５】この発明の「ゼロ基準ＭＲ追跡順序」を表す波形図であって、ＲＦパルス、磁界勾配パルス、データ収集及び検出された信号の間の関係を示す。
【図６】この発明の「アダマールＭＲ追跡順序」を表す波形図であって、ＲＦパルス、磁界勾配パルス、データ収集及び検出された信号の間の関係を示す。
【図７】印加された磁界勾配の方向に沿ったＲＦコイルの場所を決定する為に必要な段階を示す流れ図。
【図８】この発明による別の実施例ＭＲ追跡装置の部分的なブロック図。
【符号の説明】
１００ 被検体
１２５ 永久磁石
１４０ 外部コイル
１５０ 追跡される装置
１８０ 表示手段
２００ ＲＦコイル
９００ 制御器
９１０ 勾配増幅器
９３０ 送信器
９４０ 受信器
９５０ 計算手段[0001]
[Related application]
No. 07 / 861,718, filed Apr. 1, 1992, entitled "Tracking Device and Pulse Sequence for Monitoring Device Position Using Magnetic Resonance," 07 / 861,662, Title of the Invention, "Tracking Device for Monitoring the Position and Orientation of the Device Using Magnetic Resonance Detection of Samples Stored in the Device", and 07 / 861,690, Title of the Invention It has an association with “a tracking device that monitors the position and orientation of the device using multiplexed magnetic resonance detection”.
[0002]
FIELD OF THE INVENTION
The present invention relates to medical procedures for inserting a device into a body, and more particularly, to tracking the device using magnetic resonance signals.
[0003]
[Description of Related Technology]
Several methods for tracking devices within the body using radio frequency (RF) signals are disclosed in US patent application Ser. No. 07 / 753,565, filed Sep. 3, 1991, all of which are incorporated herein by reference. No. 07 / 753,563, entitled "Tracking Device for Tracking Position and Orientation of Device Using Radio Frequency Magnetic Field", entitled "Tracking Device for Tracking Device Position and Orientation Using Radio Frequency Magnetic Field Gradient" No. 07 / 753,564, title of the invention, "stereoscopic X-ray fluoroscopy apparatus using radio frequency magnetic field", and title of the same, 07 / 753,567, title of the invention, "Automatic gantry positioning with respect to image forming apparatus." No. 07 / 753,566, entitled "Multiplanar X-ray Fluoroscopy Using Radio Frequency Magnetic Fields". These methods use RF transmitting and receiving devices that track a coil attached to a device that is in vivo.
[0004]
Determine the location of the device within the body, as described in U.S. Patent Application Serial Nos. 07 / 861,718, 07 / 861,662 and 07 / 861,690, cited above. To do this, a magnetic resonance signal is used. Position information is determined in three orthogonal directions by successively measuring data in each of the three orthogonal dimensions. Data from each dimension is determined twice, once for each polarity of the applied magnetic field gradient, to correct for artifacts resulting from resonant offset conditions such as transmitter misregulation or permeability effects. . Thus, the method described above requires six measurements to locate the device in three dimensions.
[0005]
Currently, a need has arisen for a tracking device that tracks devices in a subject within a magnetic resonance (MR) imaging device at near real-time speeds without the need for substantial extra equipment.
[0006]
SUMMARY OF THE INVENTION
Tracking catheters and other devices placed in the body using a magnetic resonance (MR) imaging device consisting of magnets, pulsed magnetic field gradient devices, radio frequency transmitters, radio frequency receivers and controllers Is achieved by: The device to be tracked (the tracked device) is modified by mounting a small radio frequency (RF) coil near its end. The subject is placed in the bore of the magnet and the device is introduced into the subject. An MR device generates a series of RF and magnetic field gradient pulses that are delivered to the subject, which induces a resonant MR response signal from selected nuclear spins in the subject. This response signal induces a current in the RF coil attached to the device. Because the RF coil is small, the area in which it is sensitive is limited. Thus, only nuclear spins that are very close to the RF coil are detected by the RF coil. A receiver receives the detected MR response signal, demodulates, amplifies, filters and digitizes the MR response signal, which is then stored as data by the controller.
[0007]
Data collection is performed while applying magnetic field gradients in three mutually orthogonal directions. With these gradients, the frequency of the detected signal is directly proportional to the position of the RF coil along each applied gradient. Data collection is still performed in response to different combinations of polarity and intensity of the magnetic field gradient pulse. A linear combination of the collected data is calculated to extract position information along three mutually orthogonal axes. Thereafter, the digitized data is processed using a Fourier transform to calculate the position of the RF coil in three dimensions. This position information can be superimposed on a medical diagnostic image of the region of interest from the imaging means.
[0008]
[Object of the invention]
It is an object of the present invention to provide a method for tracking a device that is in vivo during a magnetic resonance (MR) examination.
It is another object of the present invention to provide an interactive display of the position of the device over a medical image.
[0009]
It is another object of the present invention to provide a method for tracking an in-vivo device using multiplexed detection of MR signals.
While the novel features of the invention are set forth in the following claims, the structure, operation, and other objects and advantages of the invention itself will be best understood from the following description of the drawings.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
In FIG. 1, a subject 100 on a support table 110 is placed in a homogeneous magnetic field generated by a magnet 125 in a magnet housing 120. The magnet 125 and the magnet housing 120 have a cylindrical symmetry, and are shown in half to indicate the position of the subject 100. The area of the subject 100 into which the device 150, shown as a catheter in the drawing, is inserted is defined approximately at the center of the bore of the magnet 125. The subject 100 is surrounded by a set of cylindrical magnetic field gradient coils 130 (shown in half) that create a magnetic field gradient having a predetermined strength at a predetermined time. The gradient coil 130 generates magnetic field gradients in three directions orthogonal to each other.
[0011]
The external coil 140 also surrounds the region of interest of the subject 100. Coil 140 is shown as a cylindrical outer coil (having been split in half) with a diameter large enough to encompass the entire subject. Other shapes, such as smaller cylinders specifically designed to image the head or end, may be used instead. Alternatively, a non-cylindrical external coil such as a surface coil may be used. An external coil 140 radiates radio frequency energy to the subject 100 at a scheduled time at a scheduled frequency to nutate the nuclear magnetic spins of the nuclei of the subject 100 in a known manner. Due to the nutation of the spin, the spin resonates at the Larmor frequency. The Larmor frequency for each spin is directly proportional to the strength of the magnetic field experienced by the spin. This magnetic field strength is the sum of the static magnetic field generated by the magnet 125 and the local magnetic field generated by the magnetic field gradient coil 130.
[0012]
Device 150 is inserted into subject 100 by operator 160, but may be a guidewire, catheter, endoscope, laparoscopic mirror, biopsy needle, or similar device. This device has an RF coil that detects an MR signal generated in a subject in response to a radio frequency magnetic field generated by an external coil 140. Since the RF coil is small, the area having sensitivity is also small. Thus, the detected signal has a Larmor frequency caused only by the strength of the magnetic field in the immediate vicinity of the coil. These detected signals are sent to an imaging and tracking device 170 where they are analyzed. The position of the device 150 is determined by the imaging and tracking device 170 and displayed on the display means 180. In a preferred embodiment of the present invention, the position of device 150 is displayed on display means 180 by superimposing a graphic symbol on a normal MR image driven by superposition means within imaging and tracking device 170. In another embodiment of the invention, a graphic symbol representing the device 150 is superimposed on the diagnostic image obtained by the imaging means 190. The imaging means may be X-ray, computed tomography (CT), positron emission tomography or an ultrasound imaging device. In another embodiment of the present invention, the position of the device is displayed as a number or a graphic symbol without using the diagnostic image as a reference.
[0013]
One embodiment of the apparatus 150 is shown in detail in FIG. A small RF coil 200 is electrically coupled to the MR device via conductors 210,220. In a preferred embodiment of the present invention, conductors 210 and 220 form a coaxial pair. Conductors 210, 220 and RF coil 200 are encapsulated within outer shell 230 of device 150. An MR signal originating from the tissue surrounding the device 150 is detected by the coil 200.
[0014]
FIG. 3 is a block diagram of an MR apparatus suitable for imaging and tracking the apparatus. The apparatus has a controller 900 that generates control signals for a set of magnetic field gradient amplifiers 910. These amplifiers drive the magnetic field gradient coils 130 within the magnet housing 120 (also shown in FIG. 1). The gradient coil 130 can generate magnetic field gradients in three directions orthogonal to each other. Controller 900 also generates a signal that is sent to transmitter means 930. These signals may correspond to a “zero-reference” magnetic resonance tracking order or a “Hadamard” magnetic resonance tracking order. A signal from controller 900 causes transmitter means 930 to generate RF pulses of a selected frequency at a power suitable to nutate selected spins in a region of the subject within external coil 140. , This external coil is in the bore of the magnet 125. An MR signal is induced in the RF coil 200 (also shown in FIG. 2) connected to the receiver means 940. Receiver means 940 processes the MR signal by amplifying, demodulating, filtering and digitizing it. Controller 900 collects the signal from receiver means 940 and communicates it to computing means 950 where it is processed. Calculation means 950 applies a Fourier transform to the signal received from controller 900 to determine the location of coil 200. The image of the subject is supplied to the controller 900 by the imaging means 190. These images can be generated by ultrasound, x-ray, positron emission tomography or computed tomography imaging equipment. At the position corresponding to the location of the coil 200 calculated by the calculation means 950, the symbol is positioned on the image of the image display means 180.
[0015]
FIG. 4 shows that when a magnetic field gradient is applied, the Larmor frequency of the spin is approximately proportional to its position. Spin at the center point 300 of the gradient coil 130 (FIG. 1) has a Larmor frequency _{f 0.} The Larmor frequency f _{0 at} point 300 is determined solely by the static magnetic field generated by magnet 125 (FIG. 1). The spin at position 310 has a Larmor frequency f ₁ determined by the sum of the static magnetic field and another magnetic field generated at that location by the magnetic field gradient coil 130 (FIG. 1). Since the gradient coil response 320 is substantially linear, the spin Larmor frequency is substantially proportional to position.
[0016]
As shown in FIG. 2, the MR response signal detected by the RF coil 200 enclosed within the device 150 is generated in response to the RF and magnetic field gradient pulses of the MR device. The pulse timing of the presently preferred embodiment is shown in FIG. 5 and is referred to hereinafter as the "zero reference magnetic resonance tracking sequence". In this time diagram, a zero reference broadband RF pulse 400 is applied. Next, a data acquisition signal 440 is generated to digitize the zero reference MR response signal 450 for storage in the imaging and tracking unit 170 of FIG. A zero reference MR response signal 450 is detected when no magnetic field gradient is present. Thus, the position defined by the frequency of the zero reference MR response signal 450 is the detected center of the imaging device. However, when there is a resonance offset state such as a transmitter frequency misadjustment, a magnetic permeability effect, etc., the detected position is the actual center of the imaging device by an amount proportional to the magnitude of the resonance offset. And different. The measured error due to the resonance offset is subtracted from each of the three orthogonal positions calculated in response to the first, second, and third data acquisition signals 440x, 440y, 440z.
[0017]
The first broadband RF pulse 400x excites all spins of the subject within the external coil 140 of FIG. After the first broadband RF pulse 400x, a first magnetic field gradient pulse 410x is applied in a predetermined direction. The gradient pulse 410x dephases the magnetization of the spin to an extent proportional to the position of the spin along the applied magnetic field gradient (here shown as the X direction). Subsequent to the gradient pulse 410x, a second magnetic field gradient pulse 420x having the opposite polarity is issued to form a two-lobe magnetic field gradient pulse. The product of the magnitude of the magnetic field gradient and the duration of the gradient pulse (ie, the shaded area) is selected to be substantially the same for the first and second gradient pulses. Next, the amplitude of the second magnetic field gradient pulse 420x is maintained for a duration approximately equal to that of the second magnetic field gradient pulse 420x, and a third magnetic field having substantially the same area as the second pulse 420x is effectively formed. Pulse 430x is generated. In effect, the second and third gradient pulses 420x, 430x form a single pulse which is split into two pulses for clarity. At the end of the second gradient pulse, all spins in the subject are substantially in phase. The third gradient pulse 430x causes additional dephasing of the MR signal.
[0018]
During the second gradient pulse 420x and the third gradient pulse 430x, the data acquisition signal 440x causes the RF coil 200 (FIG. 2) to receive the first MR response signal 450x. The MR response signal 450x is digitized and stored in the imaging and tracking device 170 (FIG. 1). The MR response signal 450x reaches a maximum amplitude approximately at the end of the second gradient pulse 420x and has a Larmor frequency that is approximately proportional to the position of the device 150 (FIG. 1) along the direction of the applied magnetic field gradient. Using a frequency of the MR response signal 450x, it determines the position of the in device 150 (FIG. 1) in a first direction parallel to the direction of the applied magnetic field _{G x.}
[0019]
A second broadband RF pulse 400y is applied immediately after acquiring the first MR response signal 450x. In a manner similar to determining the position of the device 150 of FIG. 1 in the first direction, the fourth, fifth, and sixth gradient pulses 410y, 420y, 430y are applied to the fourth direction substantially perpendicular to the first direction. 2 (in this case, shown as the Y direction). A data acquisition signal 440y is generated during the fifth and sixth gradient pulses 420y, 430y, and a second MR response signal 450y is digitized and stored in the imaging and tracking unit 170 of FIG. Like The frequency of the MR response signal 450y is used to determine the position of the device 150 (FIG. 1) in the second direction y. After detecting the MR response signal 450y, a third broadband RF pulse 400z is applied, and the seventh, eighth, and ninth gradient pulses 410z, 420z, 430z are applied to the third broadband RF pulse 400z, which is substantially orthogonal to the first and second directions. (Shown as Z direction in the figure). A data acquisition signal 440z is generated during the eighth and ninth gradient pulses so that the third MR response signal 450z is digitized and stored in the imaging and tracking unit 170 of FIG. . The frequency of the MR response signal 450z is used to determine the position of the device 150 (FIG. 1) in the third direction Z.
[0020]
After the detection of the third MR response signal 450z, the measurement error due to the resonance offset obtained by the zero reference MR response signal 450 is subtracted from the respective X, Y and Z positions by the imaging and tracking device 170, and Is determined, and this location is displayed on the display means 180. Thereafter, the entire pulse sequence shown in FIG. 5 is repeated until no more tracking of the device is required. Instead, the entire pulse sequence shown in FIG. 5 is periodically interleaved with an imaging pulse sequence that collects MR response signals from a normal imaging RF coil to provide for imaging of the subject and tracking of the device. Can be performed almost simultaneously.
[0021]
In another embodiment of the invention, the duration of the third, sixth and ninth gradient pulses 430x, 430y, 430z is extended so that the signal is completely out of phase before applying the next broadband RF pulse. Guaranteed to be This minimizes artifacts caused by phase coherence of spins from multiple RF pulses. A second way to minimize phase coherence is to use a random phase in the RF receiver and transmitter of the MR device for each RF pulse.
[0022]
In another embodiment of the present invention, the amplitude and / or duration of the first, fourth and seventh gradient pulses 410x, 410y, 410z is reduced, while the remaining gradient pulses remain unchanged. This reduces the degree of out-of-phase that each signal experiences prior to the data collection period, thus changing the instant of the maximum signal, but not its frequency. Reducing the duration of the first, fourth, and seventh gradient pulses 410x, 410y, 410z can advantageously reduce the RF pulse interval.
[0023]
The pulse order shown in FIG. 5 can be changed so as to become the “Hadamard magnetic resonance tracking order” shown in FIG. In this embodiment of the invention, a broadband RF pulse 460 is used. Three out-of-phase magnetic field gradient pulses 470x, 470y, 470z are applied at approximately the same time to de-phase spins along three mutually orthogonal axes. Readout magnetic field gradient pulses 480x, 480y, 480z are applied simultaneously after the out-of-phase gradient pulse. A data acquisition signal 441 is applied in a manner similar to the pulse sequence of FIG. 5, such that a response signal 451 is acquired by the MR device. It will be appreciated that each set of out-of-phase and readout field gradient pulses forms a two-lobe gradient pulse.
[0024]
After detecting the response signal 451, the pulse sequence shown in FIG. 6 is repeated using the magnetic field gradient pulses 470x, 470y, 470z, 480x, 480y, 480z, 490x, 490y, and 490z having different polarities. In a preferred embodiment of the invention, the polarity of the magnetic field gradient pulse is chosen according to a Hadamard coding matrix. An example of a Hadamard encoding matrix when the number of excitations is four is as follows.
[0025]
[Table 1]

Here, "+" indicates a magnetic field gradient pulse applied in a selected direction, and "-" indicates that substantially the same gradient pulse of opposite polarity is applied along the same axis.
[0026]
In the Hadamard-encoded embodiment of the invention shown in FIG. 6, four signals, such as response signals 451, are stored in the imaging and tracking unit 170 of FIG. The positions from each of the four response signals P1, P2, P3, and P4 are calculated by calculating the Fourier transform of the response signals in the same manner as described for the pulse order shown in FIGS. Is done. Next, a linear combination of the four positions P1, P2, P3, P4 is calculated to determine three processed response signals that are sensitive to the position of the device with respect to the X, Y and Z field gradient axes. In a Hadamard coded embodiment where NEX = 4, the linear combinations shown below are useful.
[0027]
X position = P1-P2-P3 + P4
Y position = P1-P2 + P3-P4
Z position = P1 + P2-P3-P4
The position of the device 150 of FIG. 1 obtained using the “zero reference MR tracking order” outlined in FIG. 5 or the “Hadamard MR tracking order” outlined in FIG. And is not affected by differences in chemical shifts that can occur when there is a resonance offset condition, such as a transmitter frequency misadjustment.
[0028]
FIG. 7 shows the steps performed by the imaging and tracking device 170 of FIG. 1 to determine the position of the device 150 of FIG. 1 from the detected signals. In response to the pulse sequence shown in either FIG. 5 or FIG. 6, a signal 500 is detected by the MR device. Signal 500 carries information regarding the position of the device in the direction of the applied magnetic field gradient. This frequency information is extracted by subjecting the signal to a Fourier transform (FT) to convert the time dependency of the data into the frequency dependency. The frequency dependent data set 510 has one maximum value corresponding to the position of the RF coil 200 of FIG. 2 in the direction of the applied magnetic field gradient. The location of the maximum value in the data set is extracted and sent to display means 180 (FIG. 1) for display to the operator.
[0029]
If desired, MR imaging and device tracking can be performed on most hardware-equivalent devices. It is also possible to interleave the image acquisition and tracking so that both occur at substantially the same time. Instead, by analyzing the gradient waveform of the imaging procedure and the MR response signal detected by the RF coil 200 in the device 150 (FIG. 2), the location of the device 150 is determined so that no interleaving is performed. In addition, tracking and image formation can be performed simultaneously.
[0030]
In the preferred embodiment of the present invention, the RF coil 200 in the device 150 performs the receiving function. However, there is a reciprocity between the transmit and receive coils, and a tracking device is possible that transmits RF energy using the RF coil 200 in the device 150 and receives the MR response signal using the external coil 140. is there.
In another embodiment of the present invention, an RF coil 200 can be used to alternately transmit and receive RF energy, as shown in FIG. The controller 900 activates the switch 903 according to the MR sequence used and connects the coil 200 to the transmitter 930 to deliver RF energy to the subject. Conversely, controller 900 activates switch 903 to connect coil 200 to receiver 940 and receive RF energy from the subject.
[0031]
While several embodiments of the MR tracker that are presently preferred are described in detail, various modifications will occur to those skilled in the art. It is therefore to be understood that the appended claims are intended to cover all such modifications that fall within the scope of the invention.
[Brief description of the drawings]
FIG. 1 is a cutaway perspective view of an embodiment of the present invention in operation to track the location of a device within a subject.
FIG. 2 is a schematic configuration diagram of an RF coil provided in a device to be inserted into a body of a subject.
FIG. 3 is a block diagram of an MR imaging apparatus suitable for tracking the apparatus of FIG. 2 according to the present invention.
FIG. 4 is a graph illustrating the MR response frequency with respect to a position along one axis when an applied magnetic field gradient is present.
FIG. 5 is a waveform diagram illustrating the “zero reference MR tracking sequence” of the present invention, showing the relationship between RF pulses, magnetic field gradient pulses, data acquisition and detected signals.
FIG. 6 is a waveform diagram illustrating the “Hadamard MR tracking order” of the present invention, showing the relationship between RF pulses, magnetic field gradient pulses, data acquisition and detected signals.
FIG. 7 is a flow diagram showing the steps required to determine the location of an RF coil along the direction of an applied magnetic field gradient.
FIG. 8 is a partial block diagram of another embodiment of an MR tracking apparatus according to the present invention.
[Explanation of symbols]
100 Subject 125 Permanent magnet 140 External coil 150 Device to be tracked 180 Display means 200 RF coil 900 Controller 910 Gradient amplifier 930 Transmitter 940 Receiver 950 Calculation means

Claims (11)

In a magnetic resonance tracking system that monitors the location of a tracked device within a subject,
(A) a tracked device inserted into the subject;
(B) magnetic field means for applying a homogeneous magnetic field having a substantially uniform amplitude over the subject;
(C) delivering radio frequency (RF) energy having a selected duration, amplitude and frequency to the subject to cause nutation of a selected set of nuclear spins within the subject; ) Transmitter means;
(D) gradient means for varying the amplitude of the magnetic field in a selected number of spatial dimensions to form a magnetic field gradient in a selected direction;
(E) detection means mounted on the tracked device for detecting a magnetic resonance (MR) response signal from a selected set of spins;
(F) calculating means for calculating the location of the tracked device from the detected MR response signal in response to the detecting means;
(G) controller means coupled to said transmitter means, detection means, calculation means and gradient means for operating said transmitter means, detection means, calculation means and gradient means in a zero reference magnetic resonance tracking order;
(H) responsive to said calculating means, magnetic resonance tracking system and a display means for displaying the location of the tracked device to the operator. 2. An image forming means for acquiring a medical diagnostic image of the subject, and superimposing means for superimposing a symbol on the medical diagnostic image at a position representing the calculated location of the tracked device. Magnetic resonance tracking system. 3. The magnetic resonance tracking of claim 2, wherein said imaging means is one of the group consisting of magnetic resonance, x-ray, computed tomography (CT), positron emission tomography, and ultrasound imaging. system. An apparatus for tracking the location of a tracked device in a subject using magnetic resonance,
(A) a tracked device inserted into the subject;
(B) transmitter means for applying a homogeneous magnetic field having a substantially uniform amplitude over the subject,
(C) transmitter means for delivering a zero reference radio frequency (RF) pulse to the subject to cause nutation of a selected set of nuclear spins;
(D) detecting means for detecting a zero-reference magnetic resonance (MR) response signal from the set in which nutation of the nuclear spin has occurred;
(E) calculating means for calculating a location offset from the zero reference MR response signal;
Including
(F) the transmitter means sends a first RF pulse to the subject to cause nutation of the selected set of nuclear spins;
(G) applying a first two-lobe readout magnetic field gradient pulse to the subject in a first selected direction;
(H) the detecting means detects a magnetic resonance (MR) response signal from a selected set of spins via a coil attached to the tracked device;
(I) the calculating means calculates an approximate location in the first direction from the first MR response signal;
(J) calculating the location of the tracked device along the direction of the applied magnetic field gradient by subtracting the location offset from the approximate location in the first direction;
apparatus. (K) collecting a medical diagnostic image of the subject;
5. The apparatus of claim 4, further comprising: (l) display means for superimposing a symbol on said medical diagnostic image at a position representing the calculated location of said tracked device. 6. The apparatus according to claim 5, further comprising superimposing means connected to the display means for acquiring a medical diagnostic image of the subject. 5. The apparatus of claim 4, wherein said detecting means detects said first MR response signal and said transmitter means simultaneously applies a first readout magnetic field gradient pulse in said first selected direction. 5. The apparatus according to claim 4, wherein said calculating means performs a Fourier transform of the MR response signal from a time dependency to a frequency dependency, and maps the frequency dependency to an approximate location. 5. The method according to claim 4, wherein said calculating means subtracts an offset of said location from an approximate location in said first direction to enable localization in said first selected direction. apparatus. (A) the transmitter means sends a second non-selective RF pulse to the subject;
(B) applying a second readout magnetic field gradient pulse having two opposite polarity lobes to the subject, directed to the subject in a second direction substantially orthogonal to the first direction; And
(C) the detecting means detects a second MR response signal simultaneously with the application of the second readout magnetic field gradient pulse to the subject;
(D) the calculating means calculates an approximate location in the second direction from the second MR response signal;
5. The method of claim 4, further comprising the step of: (e) subtracting the offset of the location from the approximate location in the second direction to allow localization in the first and second directions. apparatus. (A) the transmitter means sends a third non-selective RF pulse to the subject;
(B) subjecting a third readout magnetic field gradient pulse having two opposite polarity lobes to a subject oriented in a third direction substantially orthogonal to the first and second directions; Applied to the specimen,
(C) the detecting means detects a third MR response signal at the same time that the third readout magnetic field gradient pulse is applied to the subject;
(D) the calculating means calculates an approximate location in the third direction from the third MR response signal;
10. The method of claim 9, further comprising: (e) subtracting the location offset from the approximate location in the third direction to enable localization in the first through third directions. apparatus.

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