Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
1998-06-25
2001-04-24
Lateef, Marvin M. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
C324S318000
Reexamination Certificate
active
06223065
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to magnetic resonance imaging (“MRI”) and, more particularly, to a method and an apparatus for selectively enabling coils in an MRI host device.
Initially, MRI systems used whole body coils to image subjects, such as human patients. The whole body receive coils of these systems had the advantage that sensitivity was, to a first approximation, substantially constant over the entire region being imaged. While this uniformity in sensitivity was not strictly characteristic of such whole body receive coils, the sensitivity was substantially constant to a degree that most reconstruction techniques assumed a constant coil sensitivity. Due to their large volume, however, the whole body receive coils suffer from a relative insensitivity to, individual spins.
For certain applications, a surface coil is preferable to a whole body receive coil in MRI systems. For an example of a surface receiving coil, see U.S. Pat. No. 4,793,356 to Misic et al. Surface coils can be made much smaller in geometry than whole body receive coils, and for medical diagnostic use they can be applied near, on, or inside the body of a patient. This is especially important where attention is directed to imaging a small region within the patient, rather than an entire anatomical cross section. The use of a surface coil in MRI systems also reduces the noise contribution from electrical losses in the body in comparison to a corresponding whole body receive coil, while maximizing the desired signal. MRI systems thus typically use small surface coils for localized high resolution imaging.
A disadvantage of surface coils, however, is their limited field of view. A single surface coil can only effectively image a region of a subject having lateral dimensions comparable to the surface coil diameter. Therefore, surface coils necessarily restrict the field of view, and inevitably lead to a tradeoff between resolution and field of view. Generally, large surface coils generate more noise due to their exposure to greater patient sample losses and therefore have a larger noise component relative to the signal, while smaller coils have lower noise but in turn restrict the field of view to a smaller region.
One technique for extending the field-of-view limitation of a surface coil is described in U.S. Pat. No. 4,825,162, entitled “Nuclear Magnetic Resonance (NMR) Imaging with Multiple Surface Coils,” issued to Roemer et al. Roemer et al. describes a set of surface coils arrayed with overlapping fields of view. Each of the surface coils is positioned to have substantially no interaction with any adjacent surface coils. A different response signal is received at each different surface coil from an associated portion of the sample that was enclosed within an imaging volume defined by the array. Each different response signal is used to construct a different one of a like plurality of different images of the sample. The different images are then combined to produce a single composite image of the sample. Roemer et al. describes a four coil array for imaging a human spine.
While an increased number of surface coils can be used in this manner to increase the field of view of MRI systems, MRI system scanners typically have a limited number of simultaneous data acquisition channels or receivers, and a limited number of selectable inputs. The number of selectable inputs is typically equal to the number of receivers. In some cases the number of selectable inputs is double the number of receivers, each receiver being capable of selectively receiving from either of two inputs. The number of data acquisition channels and separate inputs is therefore a design limitation on the number of phased array surface coils that can be used in an MRI system. A disadvantage of conventional phased array surface coils, therefore, is that the surface coil array can include only the number of surface coils that can be directly connected to the phased array inputs of the system scanner. The number of simultaneous data acquisition channels, or receivers, can be a further restriction on the utility of surface coil arrays.
To overcome the limitations of MRI system scanners imposed by the limited number of data acquisition channels or receivers, and the limited number of inputs, MRI technicians have resorted to physically moving the surface coils or manually switching selected groups of coils after successive scans to obtain MRI images. As can be appreciated, these techniques require excessive scan room intervention by personnel operating the MRI systems. That is, after each scan a technician must enter the scan room to physically reposition the coils, or manipulate a local selector switch to reconfigure the active coils of a large array to those needed to cover the desired patient anatomy. These scan room intervention techniques increase examination time and the likelihood of a patient rejecting the procedure.
SUMMARY OF THE INVENTION
The present invention provides automatic selection of phased array coil elements appropriate for an anatomical region being scanned, without scan room intervention by MRI personnel. In one aspect of the present invention, coil elements of the array may be automatically selected to image anatomical regions of a patient according to the locations of the coil elements relative to the isocenter of the MRI system. In another aspect, coil elements appropriate for an anatomical region to be scanned may be selected by moving the target anatomy of the patient to the magnetic isocenter of the MRI system.
According to a first preferred embodiment of the present invention, a method for magnetic resonance imaging of a subject in an imaging system having a magnetic field includes: providing a plurality of receive coils movable with respect to the magnetic field; providing a sensor having a known position with respect to the subject; and sensing the magnetic field with the sensor. At least one receive coil of the plurality of receive coils is selected in accordance with the sensing by the sensor. In response, the selected receive coil is enabled to form an image of the anatomical region of the subject.
In a preferred embodiment, the position of the phased array coil relative to (1) the isocenter of the system main magnet that creates the static [B
0
] magnetic field, and (2) the coils creating the transmit RF and/or gradient time varying [B
1
] magnetic fields is determined. A switch may be used to selectively enable the appropriate coil elements and to connect them to the phased array coil inputs of the host MRI system. The selected receive coil elements are used to form an image of the region of anatomy desired for a particular scan operation.
According to a second preferred embodiment of the present invention, a magnetic resonance system having a magnetic field includes a plurality of receive coils movable with respect to the magnetic field for imaging a subject, a sensor having a known position with respect to the subject for sensing the magnetic field, and a switching device for selecting one or more of the receive coils in accordance with the sensing of the magnetic field by the sensor.
According to another preferred embodiment of the present invention, a magnetic resonance system for determining the position of a subject within a magnetic field includes a sensor device having two or more spaced sensors movable with respect to the magnetic field. The sensors sense the amplitudes of the magnetic field at different locations within the magnetic resonance system and determine the position of the subject in accordance with the sensed amplitudes.
By providing automatic coil selection, the present invention eliminates the need for scan room intervention by MRI technicians to physically reposition the surface coils or to manually switch selected groups of coils after successive scans to image desired patient anatomical regions. Thus, the present invention can decrease both examination time and the likelihood of a patient rejecting the procedure.
The present invention, alo
Misic George J.
Watral J. Michael
Lateef Marvin M.
Medrad Inc.
Shaw Shawna J
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