Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
1999-04-20
2004-08-24
Smith, Ruth S. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
C600S413000, C600S419000, C324S306000, C324S307000, C324S309000
Reexamination Certificate
active
06782286
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to magnetic resonance imaging for internally imaging a subject to be examined on the basis of magnetic resonance phenomena occurring in the subject. More particularly, this invention is concerned with an MRI (magnetic resonance imaging) system and MR (magnetic resonance) imaging method directed to MR angiography, in which echo signals are acquired during a shorter imaging period using electrocardiogram gating having an appropriate delay time in order to depict flows of blood, with no contrast medium.
Though the presently described embodiments of this invention have been put into practice to image blood flows in an object, the imaging techniques described can also be applied to image cerebral spinal fluid (CSF) or other fluids that move within the object.
2. Description of the Related Art
Magnetic resonance imaging is based on an imaging technique for magnetically exciting nuclear spins in a subject positioned in a static magnetic field by applying a radio-frequency (RF) signal of the Larmor frequency, and reconstructing an image using MR signals induced by the excitation.
In the field of this magnetic resonance imaging, when blood flow images of the pulmonary field or abdomen are obtained clinically, MR angiography has been used, in which a contrast medium is injected into a subject to produce contrast in blood. However, this contrast MR angiography needs an invasive treatment to inject the contrast medium, thus, first of all, mental and physical burdens on patients become large. Such examination cost is also high. Additionally, there are some cases where the contrast medium cannot be injected on account of patient's physical characteristics.
In cases the contrast medium cannot be injected, other imaging techniques are used alternatively. Such techniques include a time-of-flight (TOF) method and a phase contrast (PC) method. Effects of flow in magnetic resonance imaging are attributed to either of two natures possessed by spins in motion. One is that spins simply move their positions and the other results from phase shift of transverse magnetization caused when spins move in a gradient field. Of these, the former attributed to the position movement is the basis of the TOF technique and the latter attributed to the phase shift is that of phase contrast technique.
However, even when the above TOF technique or phase contrast technique is used, in obtaining MR images in the pulmonary field or the abdomen, the depiction of flows in the superior-inferior direction of such large vessels as the aorta requires imaging to be conducted vertically to its flow direction. That is, axial images are acquired along the slice direction set to the superior-inferior direction. In the three-dimensional imaging based on this, the number of imaging slices becomes large, resulting in that the entire imaging time is substantially longer.
SUMMARY OF THE INVENTION
The present invention attempts to break through the foregoing current situations, and its one object is to, without injection of a contrast medium, obtain MRA images in a non-invasive manner and remarkably shorten the imaging time.
Another object of the present invention is to, without injection of a contrast medium, namely, non-invasively, steadily depict flow of blood pumped out by the heart and remarkably shorten an imaging time to acquire data necessary for this depiction.
Still another object of the present invention is to, without injection of a contrast medium, namely, non-invasively, steadily depict images in which arteries and veins are separated and remarkably shorten an imaging time to acquire data necessary for this depiction.
Still another object of the present invention is to, without injection of a contrast medium, namely, non-invasively, steadily raise a depiction performance of the running direction of vessels and greatly shorten an imaging time to acquire data necessary for this depiction.
MR angiography technique according to the present invention is referred to as an “FBI (Fresh Blood Imaging) technique,” because fresh blood that has just pumped out from the heart can always be scanned. Specifically, this FBI technique is conducted in a manner such that ECG-gating whose delay time is appropriately set is applied in order to restlessly trace fresh and stable, but high speed, blood that has just pumped from the heart every R-wave, the repetition time TR for each slice encode is set to a shorter time so as to bring the longitudinal relaxation of magnetization in stationary parenchyma into an insufficient state on purpose, and, if necessary, IR (inversion recovery) pulses and/or fat suppression pulses are used to suppress signals emanating from fat, all these being included in three-dimensional scanning to suppress signals from parenchyma, depicting flow of blood. This permits vessels (flows of blood) to be depicted steadily with no contrast medium injected.
The three-dimensional scanning used in the present invention is scanning for imaging a volume region of a subject. This scanning is not confined to scanning referred to as the three-dimensional Fourier transform method, but includes scanning based on a multi-slice method by which a plurality of slices are imaged on the two-dimensional Fourier transform method. In the event that the present invention is practiced under the multi-slice method, synchronous timing to a signal indicative of the cardiac temporal phase is set to the same for each slice.
Particularly, it is desirable to set the slice direction so that, for each slice encoding, data can be acquired along a nearly parallel direction to the running direction of blood vessels. It is also desirable to make the phase-encoding direction agree with the running direction of blood vessels. These shorten the imaging time, unlike the TOF or phase contrast technique. Calculating differences of imaged data, for example, twice with changed delay times of the ECG gating is able to provide MRA images of which arteries and veins are separated.
“A shorter repetition time TR” stated in the present invention, which should be compared with a repetition time (approx. 5000 to 8000 ms) used by a FASE (Fast Asymmetric SE) method based on the conventional methods, such as MRCP (MR CholangioPancreatography), used for imaging regions of which T2 time is longer, means that it is shorter than such repetition time. “The shorter repetition time TR” is directed to put the longitudinal magnetization of spins of stationary parenchyma into an unsatisfactory state on purpose. From a comparison with the conventional methods, “the shorter repetition time TR” in the present invention is set to a value not more than four heartbeats (4R-R).
From a detailed construction viewpoint, the fundamental construction of an MRI system according to the present invention is a system that provides a blood flow image from a region to be imaged of the object and is characteristic of comprising temporal phase detecting means for detecting a signal indicative of cardiac temporal phases of an object and imaging scanning means for performing a three-dimensional scanning pulse sequence every slice encoding in synchronism with the signal indicative of the cardiac temporal phases.
Preferably, the pulse sequence includes an RF excitation pulse of which repetition time is set to be shorter. Additionally, the pulse sequence includes a slice-directional gradient for performing data acquisition based on the slice encoding in an approximately parallel direction to a running direction of the blood flow. Furthermore, the pulse sequence includes a phase-encoding directional gradient for applying a phase encode in a direction approximately coinciding with a running direction of the blood flow.
For example, the three-dimensional scanning pulse sequence is a pulse sequence according to a three-dimensional Fourier transform method or a multi-slice method for imaging a volume region.
Further, bay way of example, the temporal phase detecting means are means for acquiring an ECG signal of the object as the
Kabushiki Kaisha Toshiba
Smith Ruth S.
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