Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
2000-03-31
2003-05-13
Lateef, Marvin M. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
C324S307000, C324S309000
Reexamination Certificate
active
06564080
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention is related to an MRI (magnetic resonance imaging) system and an MR imaging method both of which make use of the magnetic resonance phenomenon of nuclear spins present in a subject's body, in particular, to the MRI system and the MR imaging method for conducting an ASL (Arterial Spin Labeling) method capable of providing images of perfusion (blood flows in tissue) or blood vessels.
Especially, in the present invention, the present inventor made an invention on the ASL method, which should be refereed to as an ASTAR (modified STAR using Asymmetric Inversion slabs) method based on a STAR (Signal Targeting Alternating Radio frequency) technique known as one ASL method.
Magnetic resonance imaging is a technique for magnetically exciting nuclear spins of a subject placed in a static magnetic field by applying a radio-frequency signal with the Larmor frequency, and obtaining images using FID (free-induction decay) signals or echo signals induced with the excitation.
One category of the magnetic resonance imaging is ASL (Arterial Spin Labeling) imaging. This imaging provides perfusion (tissue blood) images in which blood vessels and microcirculation of a subject are reflected, without injecting contrast medium into the subject, i.e., with non-invasiveness.
This ASL method includes a “continuous ASL (CASL) technique” and a “dynamic ASL (DASL) technique.” The CASL technique is a way of applying a largely continuous adiabatic RF wave, while the DASL technique is that of applying a pulsed adiabatic RF wave that can easily be practiced by a clinical MRI system.
The DASL technique includes two main techniques of STAR (Signal Targeting with Alternating Radio frequency) and FAIR (Flow sensitive alternating Inversion Recovery). These two techniques are further deformed into the following various modes.
The STAR technique is a way of imaging one-way flow (normally, an inflow direction of the arteries) with the use of a tagging RF pulse spatially offset from an imaging plane, as proposed by “Nishimura et al., MRM 7:472-484 (1988)” and “Edelman et al., MRM 31:233-238 (1994).” However, in this case, owing to differences in MT (magnetization transfer) effects which will be caused by the tagging RF pulse, there occur signal errors of larger scales than the flow. Particularly, since the tissue blood flow is imaged based on flow signal components provided by differences in minute signals corresponding to orders that are 2% or less of the original signal, differences in the MT effects have a large influence on it.
A way to eliminate the differences in the MT effects is proposed by “Edelman et al., Radiology, 192, 513-520(1994),” which is an imaging method known as an EPISTAR (echo-planar imaging and signal targeting with alternating radio frequency) technique. In this technique, to try to eliminate differences in the MT effects, RF pulses symmetric in both thickness and offset amount are applied to the upstream and downstream locations of blood flows (artery flows) passing through an imaging plane, respectively. This application enables differences in the MT effects in the imaging plane to be eliminated or lowered, but, like the FAIR technique, resulting in that the blood flows in both the inflow directions into the imaging plane are imaged as well. Thus the effect of prohibiting the veins from being imaged, which is known as vein suppression, will be lost.
Meanwhile, compared to the STAR technique, the FAIR (flow-sensitive alternation inversion recovery) technique proposed by “Kwong et al., MRM 34, 878-887 (1995),” for instance, hardly occur differences in the MT effects, thus lessening a transit delay time, because on-resonance IR pulses are employed as controlling/tagging RF pulses. However, because it is not impossible to separate blood inflow directions into an imaging slice, there arises a problem that using this technique alone cannot accomplish the vein suppression. In addition, when an inflow direction of a dominant blood vessel into a region of interest is desired to be determined, this is also impossible.
In the ASL technique, it is therefore important that the conflicting problems of canceling the MT effects and imaging only one-way flow are solved. A manner of solving or improving these two problems is proposed as, for example, techniques of “New EPI-STAR” and “ASI-STAR.” Of these, the New EPI-STAR technique is described by Mai et al., ISMRM 1998, p1205,” for example. In this technique, the nature of an adiabatic pulse is utilized to apply an IR pulse of 360 degrees to a tagging side and to apply two IR pulses of 180 degrees to the identical location in the controlling side to that in the tagging side, so that the MT effects are cancelled. This way corresponds to an improved EPISTAR technique, which allows flows from the tagging side to be imaged and enables multislice imaging.
Moreover, the ASI-STAR technique is considered an improved FAIR technique. This technique is performed such that a non-selective IR pulse is applied to a thickened region and a tagging pulse causes a larger offset in an inflow side. This allows the vein inflow side to be located approximately by a selective IR pulse.
However, the foregoing New EPI-STAR technique and ASI-STAR technique have drawbacks that will be described below. Where the New EPI-STAR technique is used to conduct single-slice imaging, two 180-degree pulses consecutively applied cannot provide a completely restored longitudinal magnetization Mz in the controlling-side region. Compared to a condition under which no pulse is applied, influence of an incomplete cancellation of MT effects appears as an amount that cannot be ignored and the RF power increases, leading to a larger SAR. In cases where multislice imaging is conducted, it basically increases a transit delay time, which is unfavorable to quantification.
On one hand, in imaging with the ASI-STAR technique, a frequency offset is given for only one side region. Hence, even if an amount of the offset is small, a difference in the MT effects between the controlling and tagging applications is left as an amount that is small, but cannot be canceled well, which cannot therefore be ignored completely. This amount becomes a major error factor in detecting blood that flows slowly. Moreover, as to the profiles of both vein-side tagging and controlling slabs, their slopes do not coincide with each other completely or approximately completely. As a result, veins that flow at slower speeds are excited, making it impossible to completely cancel the differences between the tagging and controlling applications.
SUMMARY OF THE INVENTION
The present invention is made to consider the foregoing problems caused by the prior art techniques. An object of the present invention is to provide highly quantified perfusion images or blood flow images (MRA) in which not only MT effects in an imaging region are mutually canceled steadily so as to lower difference errors due to signals from stationary tissue but also sensitivity is given to only one-way blood flows so as to extremely reduce the influence of veins, for example, a flow component composed of almost arteries being produced, without largely raising the power of RF waves and/or increasing SAR (RF exposure) excessively.
The present invention is to provide a technique for obtaining perfusion (tissue blood flows) or blood flow images (MRA) on the basis of the ASL technique; those images are non-invasively provided with no contrast medium injected.
In order to accomplish the foregoing objects, the present invention adopts two types of imaging techniques both belonging to the ASL method. One is an approach on a novel ASL method, which is referred to as an ASTAR (Signal Targeting with Alternated Radio frequency using Asymmetric Inversion Slab) technique by the present inventor, while the other one is an approach on novel signal processing conducted with the foregoing EPISTAR method.
1. Approach on ASTAR Technique
First, the ASTAR technique will be now be described.
The ASTAR technique according to the present invent
Kabushiki Kaisha Toshiba
Shah Devaang
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