US-12618926-B2 - Magnetic resonance imaging apparatus and control method thereof

US12618926B2US 12618926 B2US12618926 B2US 12618926B2US-12618926-B2

Abstract

Preventing blood flow artifacts from being increased in an imaging plane allows reduction of the blood flow artifacts in the cross section subsequently excited. In the imaging using a spin-echo (SE) pulse sequence, an excitation width of a 90° pulse is extended from an imaging plane to one side. This one side indicates a downstream side of the blood flow with respect to a vessel of interest. In imaging multiple slices, the order of measuring the multiple imaging slices is set along the direction in which the width is extended.

Inventors

Kazuho Kamba
Masahiro Takizawa
Taisei Ueda
Nobuyuki Yoshizawa
Atsushi KURATANI
Kosuke Ito

Assignees

FUJIFILM HEALTHCARE CORPORATION

Dates

Publication Date: 20260505
Application Date: 20230915
Priority Date: 20220930

Claims (11)

1 . A magnetic resonance imaging apparatus comprising, a measurement unit configured to apply an RF pulse to excite a predetermined cross section of an examination subject and gradient magnetic field pulses, and to collect nuclear magnetic resonance signals generated from the predetermined cross section, and a measurement control unit configured to control the measurement unit so that the measurement unit collects the nuclear magnetic resonance signals according to a spin-echo pulse sequence, and to control at least either of an excitation pulse and the gradient magnetic field pulse included in the spin-echo pulse sequence in such a manner that a width in a thickness direction of the predetermined cross section excited by the excitation pulse in the spin-echo pulse sequence is extended from an imaging plane of the predetermined cross section only to a downstream side of a liquid that flows into the predetermined cross section, so that the width includes the predetermined cross section and a region adjacent to the predetermined cross section on the downstream side, and the width, after being extended, does not include any region adjacent to the predetermined cross section to an upstream side of the liquid that flows into the predetermined cross section.
2 . The magnetic resonance imaging apparatus according to claim 1 , wherein the measurement control unit shifts a center frequency of the excitation pulse to a further downstream side than a center of the predetermined cross section in the thickness direction.
3 . The magnetic resonance imaging apparatus according to claim 2 , wherein the measurement control unit provides as an excitation profile of the excitation pulse, a trapezoidal-shaped profile throughout the region that is excited by the excitation pulse.
4 . The magnetic resonance imaging apparatus according to claim 1 , wherein the excitation pulse excites two cross sections simultaneously, and the measurement control unit controls a position for excitation of the excitation pulse in such a manner that one of the two cross sections corresponds to the predetermined cross section and the other corresponds to a region on the downstream side.
5 . The magnetic resonance imaging apparatus according to claim 1 , wherein the measurement control unit sets a center frequency of the excitation pulse to be positioned at a center of the predetermined cross section in the thickness direction, and provides an excitation profile of the excitation pulse, the profile having excitation intensity that is different (a) within the predetermined cross section as compared to (b) within the region on the downstream side.
6 . The magnetic resonance imaging apparatus according to claim 5 , wherein the measurement control unit provides the excitation profile in which the excitation intensity increases monotonously from an end of the predetermined cross section to an end of the region on the downstream side.
7 . The magnetic resonance imaging apparatus according to claim 5 , wherein the measurement control unit provides the excitation profile in which the excitation intensity is maximized in the predetermined cross section and decreases toward an end of the region on the downstream side.
8 . The magnetic resonance imaging apparatus according to claim 1 , further comprising a UI unit configured to receive user's designation of the predetermined cross section, wherein the measurement control unit determines the region on the downstream side of the predetermined cross section based on a measurement order, when the UI unit receives designation of the measurement order as to multiple cross sections.
9 . The magnetic resonance imaging apparatus according to claim 1 , wherein the measurement control unit controls the measurement unit so that the nuclear magnetic resonance signals are sequentially collected as to multiple predetermined cross sections, and the measurement control unit also controls a measurement order in such a manner that the predetermined cross sections adjacent in time for collecting nuclear magnetic resonance signals are spatially apart.
10 . A control method of a magnetic resonance imaging apparatus provided with an RF magnetic field generation unit configured to generate an RF magnetic field to be applied to the examination subject, a gradient magnetic field generation unit configured to generate a gradient magnetic field in a space in which the examination subject is placed, and a measurement unit configured to measure a nuclear magnetic resonance signal generated from the examination subject, the magnetic resonance imaging apparatus being operated according to a predetermined pulse sequence, and the control method comprising: using as the pulse sequence, a spin-echo pulse sequence that includes application of a 90° pulse and application of a 180° pulse and measures a spin echo from a predetermined cross section of the examination subject; controlling one or more of an excitation pulse and the gradient magnetic field pulse included in the spin-echo pulse sequence in such a manner that a width in a thickness direction of the predetermined cross section excited by the excitation pulse in the spin-echo pulse sequence is extended from an imaging plane of the predetermined cross section only to a downstream side of a liquid that flows into the predetermined cross section, so that the width includes the predetermined cross section and a region adjacent to the predetermined cross section on the downstream side, and the width, after being extended, does not include any other region adjacent to the predetermined cross section to an upstream side of the liquid that flows into the predetermined cross section; and collecting the nuclear magnetic resonance signals from the predetermined cross section, with setting a range excited by the 90° pulse as a range that includes the predetermined cross section and the region adjacent to the predetermined cross section and located on the downstream side of a fluid flowing into the predetermined cross section, and setting a range to apply the 180° pulse to coincide with the range of the predetermined cross section.
11 . The control method of the magnetic resonance imaging apparatus according to claim 10 , further comprising repeating collection of the nuclear magnetic resonance signals as to multiple cross sections while shifting the predetermined cross section to the downstream side.

Description

TECHNICAL FIELD The present invention relates to a magnetic resonance imaging apparatus (hereinafter, referred to as an MRI apparatus), and more particularly, the present invention relates to control of a pulse sequence for measuring a nuclear magnetic resonance signal in the MRI apparatus. DESCRIPTION OF THE RELATED ART In an MRI apparatus, nuclear magnetic resonance signals generated from an examination subject caused by nuclear magnetic resonance are collected to create an image of the examination subject. The nuclear magnetic resonance signals are collected by operating a pulse sequence. In the pulse sequence, there are determined application intensity and the order of application to apply pulses, such as RF pulses for exciting nuclear spins (normal protons) of atoms in the tissue of the examination subject and gradient magnetic field pulses for adding position information to the nuclear magnetic resonance signals. Even when the nuclear magnetic resonance signals are generated from the same nuclear spin, the signal intensity and phase vary depending on the tissue where the nuclei are present. Thus, imaging is performed with settings of the pulse sequence and imaging conditions that allow enhancement of ability for visualizing the tissue, in response to properties of the tissue as a target of the imaging. A typical one of pulse sequences used in the MRI apparatus is a spin echo (SE) type pulse sequence (hereinafter, also referred to as an “SE pulse sequence”) that uses an excitation pulse (90° pulse) and a 180° pulse to measure a nuclear magnetic resonance signal generated in the form of a spin echo (echo signal). For example, this pulse sequence is utilized for taking a T1 enhanced image reflecting a time constant of a longitudinal relaxation time of tissue. In the imaging using the SE pulse sequence, since nuclear spins in blood that flows through the imaging plane depend on the velocity of the blood flow, there occurs a phase change different from that of the nuclear spins from stationary tissue in the imaging plane. Thus this causes a flow artifact along the phase-encoding direction in a reconstructed image. In general, there are widely known methods for reducing this kind of flow artifact, including following methods; a method of adding a flow compensation pulse for canceling the phase change of the spins of fluid at the time of collecting echo signals, and a method of applying a pre-saturation pulse for previously saturating spins in a nearby region outside the imaging target region to control the signals. Chinese Patent No. 107536609 (hereinafter, referred to as Patent Literature 1) suggests that in the imaging using the SE pulse sequence, an excitation thickness is increased in either of a 90° pulse and a 180° pulse. In this technique, the center frequency positions of the 90° pulse and the 180° pulse are not changed, and one excitation width is extended to be wider than the other, thereby preventing the fluid, e.g., blood, excited in one cross section, from flowing into the next cross section and interfering with image formation of the next cross section, resulting in that this reduces the flow artifact. SUMMARY OF THE INVENTION Technical Problem In the technique disclosed in Patent Literature 1, the regions on both sides adjacent to the imaging plane are excited by the 90° pulse or the 180° pulse, so that magnetization of the excited regions does not contribute to forming an image of the imaging plane, and the magnetization is reduced in the next cross section. However, when focusing on the imaging plane, the blood located on the upstream side of the cross section and excited by the 90° pulse or the 180° pulse, that is, the blood having extra magnetization, flows into the imaging plane, resulting in that this increases artifacts. An object of the present invention is to prevent the increase of the blood flow artifacts in the imaging plane, so that the blood flow artifacts can be reduced in the cross section to be excited next. Solution to Problem In order to solve the above-described problem, in the imaging using an SE pulse sequence, the excitation width of a 90° pulse is extended to one side from the imaging plane. The one side is assumed as the downstream side of a blood flow with respect to a vessel of interest. When imaging of multiple cross sections is performed, the order of measuring the imaging planes is set along the direction in which the width is extended. That is, an MRI apparatus of the present invention comprises a measurement unit configured to apply an RF pulse for exciting a predetermined cross section of the examination subject and a gradient magnetic field pulse, and to collect nuclear magnetic resonance signals generated from the predetermined cross section, and a measurement control unit configured to control the measurement unit so that the measurement unit collects the nuclear magnetic resonance signals according to a spin-echo pulse sequence. The measurement control uni