Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
1998-11-24
2001-06-12
Lateef, Marvin M. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
C600S412000, C607S115000, C606S034000, C606S134000
Reexamination Certificate
active
06246896
ABSTRACT:
BACKGROUND OF THE INVENTION
The field of the invention is nuclear magnetic resonance imaging (MRI) methods and systems. More particularly, the invention relates to the tracking of RF ablation devices using MRI methods.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B
0
), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B
1
) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M
z
, may be rotated, or “tipped”,into the x-y plane to produce a net transverse magnetic moment M
t
. A signal is emitted by the excited spins after the excitation signal B
1
is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G
x
, G
y
and G
z
) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles, or “views”,in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Intra-operative MR imaging is employed during a medical procedure to assist the doctor in guiding an Instrument. For example, during a needle biopsy the MRI system is operated in a real-time mode in which image frames are produced at a high rate so that the doctor can monitor the location of the needle as it is inserted. A locator device such as that described in U.S. Pat. Nos. 5,622,170 and 5,617,857 may be used to track the location of the instrument and provide coordinate values to the MRI system which enable it to mark the location of the instrument in each reconstructed image. The medical instrument is attached to a handpiece that is manipulated by the physician and whose position is detected by surrounding sensors. For example, the handpiece may emit light from two or more light emitting diodes which is sensed by three stationary cameras.
Tracking devices which employ the MRI system to locate markers in the medical device have also been developed. As described in U.S. Pat. Nos. 5,271,400; 5,307,808; 5,318,025; 5,353,795 and 5,715,822, such tracking systems employ a small coil attached to a catheter or other medical device to be tracked. An MR pulse sequence is performed using the tracking coil to acquire a signal which indicates the location of the tracked device. The location of the tracking coil is determined and is superimposed at the corresponding location in a medical image acquired with the same MRI system.
To accurately locate the tracking coil, position information is obtained in three orthogonal directions that require at least three separate measurement pulse sequences. To correct for errors arising from resonance offset conditions, such as transmitter misadjustment and susceptibility effects, two measurements may be made in each direction with the polarity of the readout gradient reversed in one measurement. This tracking method requires that six separate measurement pulse sequences be performed to acquire the tracking coil location. As disclosed in U.S. Pat. No. 5,353,795, these separate measurements can be reduced to four in number by altering the readout gradients in a Hadamard magnetic resonance tracking sequence.
One of the primary medical procedures which employs intra-operative MR imaging is ablation therapy. As described, for example, in U.S. Pat. Nos. 5,443,489; 5,551,426; 5,697,925; 5,718,701 and 5,800,428, ablation devices are precisely guided into position against target tissue. Radio frequency power is applied to the ablation device and this energy creates electric fields in the tissue. These fields cause electrical currents to flow in the tissue since the tissue is electrically conductive. If the currents are strong enough, heating occurs and the temperature can be high enough to destroy tissue cells. Needless to say, it is important that such ablation devices be accurately positioned and the radio frequency power be precisely controlled to only destroy the target tissues.
SUMMARY OF THE INVENTION
The present invention is an ablation system which employs an ablation device that is guided into position by a physician and then energized with radio frequency power to treat target tissues. More particularly, the system includes: an ablation device that contains a tracking coil; an MRI system which is operable to acquire image data from a patient being treated with the ablation device and being operable to acquire NMR tracking data from the tracking coil; and an ablation control for providing radio frequency power to the tracking coil to treat target tissues. The tracking coil serves dual purposes. It provides NMR tracking data for the MRI system which is used to help the physician guide the ablation device into the proper treatment position, and then it is used by the ablation system to deliver the heat generating energy to the target tissues.
One aspect of the present invention is the use of a tracking coil in an ablation device to provide NMR tracking data to an MRI system and to receive RF power from an ablation system. A switch operated by the MRI system couples the tracking coil to a receiver in the MRI system when it performs a position measurement NMR pulse sequence. The same switch may be used to couple the ablation control to the tracking coil at other times during the procedure, or the ablation control can be inductively coupled to the tracking coil.
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Darrow Robert David
Dumoulin Charles Lucian
General Electric Company
Ingraham Donald S.
Lateef Marvin M.
Lin Jeoyuh
Testa Jean K.
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