MR imaging with pre-sequence including plural saturation pulses

Details MR imaging with pre-sequence including plural saturation pulses MR imaging with pre-sequence including plural saturation pulses

1998-03-26

2001-11-20

Williams, Hezron (Department: 2862)

Electricity: measuring and testing

Particle precession resonance

Determine fluid flow rate

C324S307000, C324S309000

Reexamination Certificate

active

06320377

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magnetic resonance imaging (MRI) technique referred to as magnetic resonance (MR) angiography, which acquires images of blood vessels of subjects, based on a magnetic resonance phenomenon occurring in the subjects. More particularly, the present invention is concerned with a magnetic resonance imaging system and magnetic resonance imaging method based on an improved MR angiography technique using a plurality of saturation pulses applied with slice-selective gradient pulses, the saturation pulses being used for separation between arteries and veins, suppression of body motion artifacts, and the like.
2. Description of the Related Art
Magnetic resonance imaging is a technique for magnetically exciting nuclear spins existing in a subject positioned in a static magnetic field by applying a radio-frequency signal with the Larmor frequency, and reconstructing an image using an MR signal induced with the excitation or producing a spectrum of the MR signal.
In the field of MRI, MR angiography techniques for imaging flows of blood within a subject or measuring the flow speed thereof have already been in practical use in medical examination. One of the MR angiography techniques uses a saturation pulse, applied with a slice-selective gradient, causing proton spins of flows of blood to be pre-excited and saturated at the time of acquisition of MR signals.
In the conventional saturation pulse based technique, a signal saturation pulse is applied to a slice positioned upstream or downstream across flows of blood passing through an imaging slice. For example, in the case of imaging the inferior limb, since the directions of flows of blood are opposite to each other between the arteries and veins, a single saturation pulse is applied to a preliminary slice set at an upstream or downstream side to an objective imaging slice, and then echo signals are acquired from the imaging slice using, for example, an FE method. Since the spins (dipoles of magnetization) of the arteries or veins inflowing the imaging slice have already been excited and saturated, they are no longer excited by the succeeding MR data acquisition sequence, thereby reducing the strength of MR signals induced from the blood flow. By contrast, the saturation pulse has not been applied to arteries or arteries inflowing the imaging slice from the opposite side, MR signals of higher strength are acquired from those blood vessels. It may therefore be expected that the arteries and veins be separated from each other on a reconstructed MR image.
However, speeds of blood flows in an imaged region are sometimes very fast or slow (particularly, in the inferior limb, they are slow). In such cases, owing to the fact that a flow-void phenomenon occurs or the inflow of saturated spins becomes extremely slow, the effectiveness of applying the saturation pulse is not enough, thereby providing no images where arteries and veins are distinctly visual-separated from each other.
Additionally, for the limbs, in general, the speeds of pulsated blood flows become remarkably slow depending on distances along peripheral vessels. To be specific, when the sequence incorporating the conventional saturation pulse is used, the blood flows of the inferior limb themselves can hardly be imaged because of the time of flight effect is less. Yet, for example, for knee diseases, to meet a clinical demand that artery-vein clearly separated images are desired is beyond the conventional imaging technique.
Another MR angiography technique different from the sequence using the saturation pulse is under research, which is trying the visual separation between arteries and veins. According to this research, MR contrast medium is injected into a patient and arteries and veins are visually separated based on differences between temporal changes in contrast for arteries and veins.
For MR angiography using MR contrast medium, however, invasiveness due to the injection of MR contrast medium is very large, requiring patients to endure it.
Further in the case of using MR contrast medium, since the actual contrast effect for inferior limb is low, differences in contrast peak times between arteries and veins are small. Thus, for the present, no clear separation images are provided.
Therefore, any conventional MR angiography technique is not suitable for such regions as the limbs where the speeds of blood flows are extremely low. It is almost impossible to provide high-quality artery-vein separated images with non-invasive treatment.
SUMMARY OF THE INVENTION
The present invention attempts to break through the foregoing current situation of known arts. Specifically, an object of the present invention is to provide, with sustaining non-invasiveness which is inherent to MR imaging, MRA images of higher contrast between blood and parenchyma even when the speeds of blood flows are extremely large or small.
Another object of the present invention is to provide, with higher contrast between blood flows and parenchyma, MRA images where arteries and veins are visually separated in a steady manner.
Still another object of the present invention is to create a situation in which a variety of types of scan sequences can be executed with higher contrast between blood flows and parenchyma as well as a steady visual separation of arteries and veins.
For accomplishing the above objects, one aspect of the present invention relates to an magnetic resonance system obtaining an image representing a blood vessel in an objective imaging slice of a subject, comprising: first means for performing a pre-sequence including a plurality of saturation pulses time-sequentially applied to a pre-saturated slice positionally different from the imaging slice; and second means for performing a data acquisition sequence for acquiring an MR signal from the imaging slice after the performance of the pre-sequence.
It is preferred that the pre-sequence includes a slice gradient pulse for being applied in parallel with the plurality of saturation pulses and used for a spatial position of the pre-saturated slice. As one example, the slice gradient pulse consists of a plurality of slice gradient pulses each applied in parallel with each of the plurality of slice gradient pulses. Still another example is that the plurality of saturation pulses are determined to have a time interval of zero between two adjacent pulses of the saturation pulses.
It is also preferred that the slice gradient pulse consists of a single slice gradient pulse continuously applied in parallel with all the plurality of saturation pulses.
It is still preferred that pre-sequence includes a plurality of gradient spoilers each applied to the subject after each of the plurality of saturation pulses.
Further preferred is that the pre-sequence includes a gradient spoiler applied to the subject after application of a last temporal saturation pulse in a train consisting of the plurality of saturation pulses. By way of example, the gradient spoiler includes at least a gradient spoiler applied in the slice direction. Another example is that the gradient spoiler includes three gradient spoiler pulses applied in three directions, respectively, consisting of the slice direction and a phase-encoding and read-out directions perpendicular to the slice direction.
Preferably, each of the plurality of saturation pulses is given a flip angle of spins less than 100 degrees.
Still preferably, at least one of the plurality of saturation pulses is given a flip angle of spins determined differently from its remaining saturation pulses. For example, each of the plurality of saturation pulses is given a flip angle of spins different from one another. It may be employed that the flip angle of each of the plurality of saturation pulses lowers gradually as going forward in an application time.
Preferably the slice gradient is formed such that both the pre-saturated slice and the imaging slice become parallel or thereabout to each other.
Still preferred is that the data acquisition sequence is a p

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