Slice orientation selection arrangement

Details Slice orientation selection arrangement Slice orientation selection arrangement

1999-01-21

2001-08-14

Oda, Christine (Department: 2862)

Electricity: measuring and testing

Particle precession resonance

Using a nuclear resonance spectrometer system

C324S307000

Reissue Patent

active

RE037325

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to Magnetic Resonance Imaging (“MRI”) systems and more particularly to slice selection arrangements for use with such MRI systems.
BACKGROUND OF THE INVENTION
One of the advantages of MRI systems is a lack of moving parts. In contrast to CT imaging systems, there are no detectors or radiation sources which translate and/or rotate about the object or patient. Instead of the rotating parts, slices to be images are selected by manipulating field gradients and pulse sequences to provide images in the usual sagittal, coronal or transverse (axial) planes. Thus it is known to vary the static magnetic field with gradient pulses applied during the application of RF (Larmor) frequency pulses to select the imaging planes.
For example, consider an MRI system laid out according to the X, Y, and Z cartesian coordinates with the static field applied extending in the direction of the Z axis. The patient is oriented longtitudinally coaxially with the Z axis. Generally speaking, sagittal, coronal or transverse imaging planes are selected by applying the RF pulse simultaneously with the Y gradient pulse, an X gradient pulse or a Z gradient pulse, respectively.
In contrast to this, in other modalities using equipment such as CT scanners, the detector and/or the X-ray source are rotated. Usually rotation and thus data acquisition is accomplished for mechanical reasons in a transverse plane, about the patient's body. It is true that the source of X-ray energy and/or the detectors can be made to swivel so as to image planes at angles to the transverse planes. Nevertheless, the imaging capability is certainly mechanically limited. With MRI systems there is no such mechanical limitations; and therefore, theoretically, it is possible to acquire image data from any direction or in any plane. However, instead of mechanical limitations, there are practical, mathematical and processing limitations to obtaining images in non-orthoganal planes. Accordingly, such images in the non-orthoganal planes have not been used. Those skilled in the art know that when more than one gradient is simulateously applied during the excitation procedure, the imaging process will be unduly complicated. The actual gradients and the read or data collect gradient will also have to comprise multiple gradients. The selection of each of the gradients is further complicated by its relationship to the other gradients.
Certain set procedures are used in the prior art to obtain the exact type of image wanted in MRI systems. For example, the procedure followed in obtaining spin echo images in an orthogonal plane is to apply a plane selection gradient during the application of a shaped, selected RF saturation pulse signal. The position plane selection gradient is followed by a defocusing negative of the plane selecting gradient. After a time period “Ta” following the saturation RF pulse, an inversion RF pulse is applied. Again, a plane selecting gradient is applied during the echo inducing signal application period. Prior to the application of the read gradient, an encoding gradient signal is applied. The field generated by the encoding gradient signal is mutually orthogonal to each of the fields generated by the plane selecting gradient and the read gradient. After another time period Ta another inversion pulse is applied during the application of the plane selecting gradient pulse. A second echo pulse is generated at a time period Ta following the application of the second inversion pulse. The process is repeated as long as meaningful “echos” are obtained.
To select a non-orthogonal plane (herein a plane neither parallel nor normal to more than one of the orthogonal XY, XZ and YZ planes) in the object, at least two orthogonal gradient pulses have to be a simultaneously applied. Consequently, the encoding and reading gradients each also require at least two simultaneously applied gradient fields of proper amplitude and width to select the encoding and reading gradients. The complications involved in such a method have deterred imaging in any but non-orthogonal planes until now. Accordingly, there is a need for the efficient imaging of non-orthogonal planes with NMR equipment.
It is known in the X-ray medical imaging art to use means for visibly indicating the location on the patient of the imaging slices. Thus, for example, in CT imaging, it is known to apply external light lines to the patient for aiding in aligning the imaging equipment. See for example U.S. Pat. No. 4,385,397. Such indicating means have not been applied in the magnetic resonance imaging art.
BRIEF DESCRIPTION OF THE INVENTION
The slice orientation selection arrangement taught herein is ideally suited for usage in cooperation with visible orientation indications aligned with the patient's organs. The aligning of the visible orientation indication automatically selects the plane or slice that will encompass a portion of the patient or an organ of the patient.
Accordingly it is an object of the present invention to provide means and methods for imaging in non-orthogonal planes with MRI equipment and for automatically selecting such planes with reliability and facility.
According to the present invention a method of imaging an object is provided wherein the slice selection is at any desired angle to the usual XYZ coordinates of an MRI system, the method comprises the steps of:
subjecting the object to a static strong magnetic field along the Z coordinate in order to generate magnetic moments extending in the Z direction,
applying an RF magnetic signal rotating at the Larmor frequency to preturb said magnetic moments and to generate FID signals,
varying said static field during the application of the RF field using a first and second magnetic gradient for selection an imaging plane not necessarily orthogonal to either said X, said Y or said Z planes, and
rotating the coordinate system prior to applying the gradient signals so that location of the magnetic used to select the non-orthogonal imaging plane is automatically accomplished.
A feature of the invention utilizes as the angle of rotation of the XYZ coordinates an angle obtained using visible indicators or the subject to determine the desired plane of imaging.


REFERENCES:
patent: 4318043 (1982-03-01), Crooks
patent: 4339716 (1982-07-01), Young
patent: 4374360 (1983-02-01), Stepponen
patent: 4385397 (1983-05-01), Verro
patent: 4390840 (1983-06-01), Ganssen
patent: 4626784 (1986-12-01), Sepponen

Affiliated with

Also associated with

Rating

Slice orientation selection arrangement does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Slice orientation selection arrangement, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Slice orientation selection arrangement will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2550871