Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
1997-12-05
2001-01-09
Patidar, Jay (Department: 2862)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
C600S423000, C324S322000, C324S318000
Reexamination Certificate
active
06171240
ABSTRACT:
This invention relates to radio frequency receiver coils and especially to radio frequency receiver coils for use in interventional nuclear magnetic resonance imaging.
Magnetic resonance imaging (MRI) is used in medicine to produce images of the internal organs of a patient being examined. In MRI, a static magnetic field is applied to the body of the patient to define an equilibrium axis of magnetic alignment in the region of the body being examined. A radio frequency field is then applied to the region being examined in a direction orthogonal to the static magnetic field direction to excite magnetic resonance in the region. This resonance produces signals in r.f coils placed adjacent the body. Normally separate coils are used for excitation and detection although the same coil or coils may be used for both purposes. The detected signals are processed to produce signals representing an image of the patient's body and this image is visually displayed.
In so-called interventional MRI, apparatus may be physically introduced into the body, for example, a catheter through which drugs or other fluids may flow can be inserted into the body. A guide wire can first be inserted until the tip of the wire is at the region of interest in the body. The catheter may be arranged to be co-axial with the guide wire and to surround it so that the catheter can subsequently be introduced into the body along the wire. The guide wire is then withdrawn from the body. Alternatively, the guide wire may be fed inside a partially inserted catheter, to enable it to be guided to a desired region, whereupon the catheter can be withdrawn and an instrument such as a stent for blocking arteries can be slid along the guide wire.
It is desirable that the guide wire and possibly the catheter or any other type of probe used in the body is imaged by the magnetic resonance apparatus so that its position within the body may be determined. Various proposals have been made to achieve this effect.
It is known to employ so-called passive visualisation of catheters and guide-wires but it is difficult to obtain an accurate image of them because of their small size and the fact that their presence itself results in the image being degraded.
Because of these disadvantages so-called active imaging has been proposed. An example of such a proposal is by Dumoulin et al [U.S. Pat. No. 5,318,025 and Magnetic Resonance in Med. 29, 411 (1993)] in which the tip of a catheter is fitted with a small MR coil and an additional small MR measurement sequence is used to determine its position. However, with this proposal there is still the problem of the physical insertion of the catheter within the patient because of the size of the small MR coil referred to earlier.
Subsequently McKinnon et al [Proc 2
nd
Ann. Mtg, SMR, San Francisco, 1994 p.429] proposed the use of a twisted pair stub antenna for the coupling of the electrical field associated with MRI Nuclear Magnetic Resonance. However, problems can arise as a result of coupling to the electrical field.
In another proposal an insulated, current-carrying loop is used. However, the loop works by generating a magnetic field. This is sufficient to spoil the magnetic resonance image and so two images have to be taken; one image while current is flowing through the loop and one image while the current is switched off. A further disadvantage which may be encountered with this proposal is that while the loop is carrying the current it tends to heat up. This is an undesirable effect when the loop is used within the human body. The invention provides a radio frequency coil adapted for use in interventional magnetic resonance imaging, the coil comprising a loop of an elongated electric conductor having outward and return sections arranged to form a twisted pair, and means associated with it for operating the coil both in transmit and in receive mode.
This contrasts with the prior art arrangement referred to earlier in which a stub antenna acts as an electrical field detector, since a loop operates in use as a magnetic field detector, while retaining the advantage that the twisted pair can be made small enough in diameter to pass through needles in common clinical use. The twisted pair does not act as an imaging device with a large field of operation since signals from tissue spaced from the coil integrate to zero. Since the coil has means associated with it for operating in transmit mode, tissue immediately adjacent to it can be excited independently of the rest of the body in which it is located, and so the twisted pair can be tracked without affecting the bulk of the tissue magnetisation. Additionally, the coil of the invention does not couple with the main transmitter coil used for imaging the body. The presence of such coupling would tend to spoil the magnetic resonance image.
The invention also provides a method of visually locating a guide wire, for guiding a catheter for use in interventional MRI, which comprises providing as the guide wire a coil comprising a closed loop of an elongated conductor having outward and return sections arranged to form a twisted pair, and operating the coil both in transmit and receive mode.
The present invention is concerned with providing a radio frequency receiver coil which will give a strong signal as to its position but which will not have the disadvantages referred to earlier in connection with the prior art. In this connection with radio frequency receiver coils for use in NMR systems the image resolution is around 1 mm. Therefore in order to provide an actual image it is necessary to see a number of millimetres. Thus prior art imaging devices of this kind by necessity have to be a certain minimum size. According to the present invention a radio frequency receiver coil for use in an NMR system is below the aforementioned minimum size and is incapable of providing an image except one indicative of its own position.
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patent: 5318025 (1994-06-01), Dumoulin et al.
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patent: 5699801 (1997-12-01), Atalar et al.
patent: 5792055 (1998-08-01), McKinnon
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patent: 0673621 (1994-03-01), None
patent: WO 96/38083 (1996-12-01), None
M. Burl, I.R. Young, “A Novel Coil for Catheters or Guide Wires”, ISMRM, New York, New York; Apr. 1996; vol. 1.
Daniel A. Leung, et al. ; “Intravascular MR Tracking Catheter: Preliminary Experimental Evaluation”; AJR 1995; vol. 164; pp. 1265-1270.
G. C. McKinnon et al. ; “Towards Visible Guidewire Antenna for Interventional MRI”; Proceedings Society of Magnetic Resonance, 2ndMeeting, 1994; p. 429.
Burl Michael
Tanttu Jukka I
Young Ian Robert
Fay Sharpe Fagan Minnich & McKee LLP
Fetzner Tiffany A.
Patidar Jay
Picker International Inc.
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