Selective cutting (e.g. – punching) – With means to impose randomly actuated control of... – Embodying means to detect indicium in work or pattern
Reexamination Certificate
2003-02-21
2004-11-09
Shrivastav, Brij B. (Department: 2859)
Selective cutting (e.g., punching)
With means to impose randomly actuated control of...
Embodying means to detect indicium in work or pattern
C324S318000
Reexamination Certificate
active
06814280
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of Japanese Application No. 2002-048533 filed Feb. 25, 2002.
BACKGROUND OF THE INVENTION
The present invention relates to an MRI (magnetic resonance imaging) apparatus and MRA (magnetic resonance angiography) imaging method, and more particularly to an MRI apparatus and MRA imaging method that can reduce degradation of image quality due to attenuation of signal intensity and can satisfactorily render blood flow even when fast blood flow and slow blood flow are simultaneously present in an imaged region.
A TOF (time-of-flight) technique is known as an example of conventional MRA imaging techniques for rendering blood flow.
In general, the TOF technique renders blood flow in white by utilizing an in-flow effect, by which NMR (nuclear magnetic resonance) signals from unsaturated blood flow that flows into a thick slab and that is not saturated by an RF (radio frequency) pulse is intensified as compared with NMR signals corresponding to surrounding tissue that is saturated by an RF pulse.
Examples of the MRA imaging according to the TOF technique will be outlined below.
FIG. 8
is a prior art explanatory diagram showing a relationship among an imaged region A, a slab S′, and a flip angle &agr; in imaging blood flow in the head H of a subject.
The thickness L of the imaged region A is 15 cm, for example.
The slab S′ has a thickness equal to that of the imaged region A.
As indicated by a flip angle profile P
61
, the flip angle &agr; has a constant value &agr;v with respect to the thickness direction Z of the slab S′.
If fast blood flow is to be mainly imaged, the flip angle &agr;v is set to a small value (e.g., 20°); and if slow blood flow is to be mainly imaged, the flip angle &agr; is set to a large value (e.g., 40°).
FIG. 9
is a prior art explantory diagram showing a flip angle profile P
71
disclosed in Japanese Patent Application Laid Open No. H5-154132.
In the flip angle profile P
71
, the flip angle &agr; varies with the position in the thickness direction Z. Specifically, the flip angle &agr;s is small near the neck in which blood flow is fast, and the flip angle &agr;e is large near the top of the head in which blood flow is slow, resulting in the linearly varying flip angle &agr;.
FIG. 10
is a prior art explanatory diagram showing yet another flip angle profile P
81
.
In the flip angle profile P
81
, the imaged region A is divided into a plurality of slabs Sa-Sf each having a thickness &tgr; smaller than the thickness L of the imaged region A. The thickness &tgr; is 2.5 cm, for example. The flip angle &agr; has a constant value &agr;v with respect to the thickness direction Z.
In the conventional MRA imaging method described with reference to
FIG. 8
, the imaging time is reduced because the imaged region is defined as one slab S′; however, since the entire imaged region A has a constant flip angle &agr;v, the method poses the problem that it is difficult to satisfactorily render the whole blood flow when fast blood flow and slow blood flow are simultaneously present in the imaged region A.
On the other hand, in the conventional MRA imaging method described with reference to
FIG. 9
, the imaging time is reduced because the imaged region is defined as one slab S′; however, since the residence time of blood flow in the slab S′ is long, the signal intensity in the distal portion attenuates, leading to a problem of degraded image quality.
Further, in the conventional MRA imaging method described with reference to
FIG. 10
, degradation of image quality due to attenuation of signal intensity is prevented because the imaged region is divided into a plurality of thin slabs Sa-Sf and the time during which blood flow resides in each slab Sa-Sf is reduced: however, since the entire imaged region A has a constant flip angle &agr;v, the method poses the problem that it is difficult to satisfactorily render the whole blood flow when fast blood flow and slow blood flow are simultaneously present in the imaged region A.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention is to provide an MRI apparatus and MRA imaging method that can reduce degradation of image quality due to attenuation of signal intensity, and can satisfactorily render blood flow even when fast blood flow and slow blood flow are simultaneously present in an imaged region.
In accordance with its first aspect, the present invention provides an MRI apparatus characterized in comprising: static magnetic field generating means for generating a static magnetic field; gradient magnetic field generating means for generating a gradient magnetic field; RF pulse transmitting means for transmitting RF pulses with a flip angle profile whose flip angle varies with respect to the thickness direction in each of a plurality of adjacent slabs formed by dividing an imaged region and whose average flip angle differs for each slab; NMR signal receiving means for receiving NMR signals from a subject; and blood flow imaging means for conducting blood flow imaging based on said NMR signals.
In the MRI apparatus of the first aspect, the time during which blood flow resides in each slab can be reduced by dividing an imaged region into thin slabs, and thus, attenuation of signal intensity is reduced and image quality is improved. Moreover, since the flip angle is varied in each slab and further the average flip angle is differentiated for each slab, the slabs can be excited by respective flip angles fit to local variation of blood flow conditions; thus, the whole imaged region can be satisfactorily rendered even when fast blood flow and slow blood flow are simultaneously present in the imaged region.
In accordance with its second aspect, the present invention provides the MRI apparatus having the aforementioned configuration, characterized in that the total number of slabs divided by said RF pulse transmitting means is in the range of 3 to 100.
In the MRI apparatus of the second aspect, since the lower limit of the total number of slabs is “3”, the blood flow residence time is reduced to ⅓ or less as compared with a case in which the imaged region is defined as one slab. Moreover, since the upper limit of the total number of slabs is “100”, the problem of an extremely lengthened total imaging time is avoided.
In accordance with its third aspect, the present invention provides the MRI apparatus having the aforementioned configuration, characterized in that said RF pulse transmitting means transmits ramped RF pulses in which said flip angle linearly varies.
In the MRI apparatus of the third aspect, since the flip angle is linearly varied using ramped RF pulses, RF pulses can be generated and transmitted by relatively simple processing.
In accordance with its fourth aspect, the present invention provides the MRI apparatus having the aforementioned configuration, characterized in that said MRI apparatus comprises flip angle specifying means for specifying flip angles at both ends of said imaged region or at both ends of each said slab, and said RF pulse transmitting means transmits RF pulses whose flip angle linearly varies from the flip angle at one end to the flip angle at the other end.
In the MRI apparatus of the fourth aspect, by an operator etc. simply specifying flip angles at both ends of an imaged region, it is possible to define the property linearly varying from a flip angle at one end to a flip angle at the other end, and therefore, determination of the flip angles involves no cumbersome operation. Moreover, if the flip angles at both ends of each slab are specified, the flip angle property can be minutely defined for each slab.
In accordance with its fifth aspect, the present invention provides the MRI apparatus having the aforementioned configuration, characterized in that said MRI apparatus comprises flip angle specifying means for specifying flip angles at both ends of said imaged region or at both ends of each said slab, and said RF pulse transmitting means transmits RF pulses whose flip
Kosugi Susumu
Miyoshi Mitsuharu
Armstrong Teasdale LLP
GE Medical Systems Global Technology Company LLC
Horton Esq. Carl B.
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