MRI using variable ratio of pre-pulses to flip pulses

Details MRI using variable ratio of pre-pulses to flip pulses MRI using variable ratio of pre-pulses to flip pulses

2000-12-19

2002-11-19

Lefkowitz, Edward (Department: 2862)

Electricity: measuring and testing

Particle precession resonance

Using a nuclear resonance spectrometer system

C324S307000

Reexamination Certificate

active

06483307

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a magnetic resonance diagnostic apparatus and, more particularly, to an improvement in an application method of a pre-pulse such as a fat suppression pulse applied before a flip-pulse (exciting RF pulse) is applied.
A typical fat suppression pulse as a pre-pulse reduces an MR signal (Magnetic Resonance signal) from fat by selectively exciting only fat by a flip-pulse narrowed in band in accordance with a chemical shift between fat and water, and then applying a gradient magnetic field (spoiler pulse) to sufficiently dephase the signal. Recently, the fat suppression pulse is indispensable in improving the image quality particularly in MRA (MR Angiography).
As is well known, according to the MRA principle, a still object such as a brain parenchyma or organ within the photographing region is sequentially excited before the longitudinal magnetization satisfactorily recovers. Thus, the signal level gradually decreases. However, fresh blood (water) always flows into the photographing region, so its signal level hardly decreases in comparison with still objects. As a result, an image (bloodstream image) in which the bloodstream is emphasized relatively to still objects is obtained. This MRA suppresses fat by applying a fat suppression pulse as a pre-pulse immediately before application of a flip-pulse.
To narrow the band of the fat suppression pulse, as described above, the pulse width of the fat suppression pulse must be increased from, e.g., ±&pgr; to ±4·&pgr;. A very long standby time is required after application of a pre-pulse to application of a flip-pulse. In many cases, the application period (repetition time TR) of the flip-pulse must be set 2 to 3 times that used when no pre-pulse is applied.
The photographing time required to acquire all data necessary for image generation increases several times. This causes various clinical problems: photographing cannot be completed within one breath-holding time, the burden on a patient increases, and the generation of motion artifacts increases.
BRIEF SUMMARY OF THE INVENTION
It is an object of the present invention to greatly shorten the photographing time while ensuring a necessary image contrast in a magnetic resonance diagnostic apparatus using a pre-pulse such as a fat suppression pulse.
A magnetic resonance imaging apparatus applied flip-pulses and gradient magnetic field pulses to generate a plurality of MR signals and to add encoding information to the MR signals, applies pre-pulses to obtain a predetermined effect, and generates an image on the basis of the MR signals. The MR signals include first MR signals arranged in a first region in a k-space and second MR signals arranged in a second region in the k-space. An applying condition for the second MR signals is differed from that for the first MR signals. The applying condition is defined as a ratio of the number of the pre-pulses to the number of the flip-pulses applied in an unit time. The pulse sequence time can be shortened while good image contrast is ensured.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.


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