JP2002248092A - Magnetic resonance imaging apparatus and magnetic resonance imaging method - Google Patents

Abstract

(57) Abstract: To provide a sufficient MT effect by applying an MT pulse and shorten a scan time. An MT pulse P _mt having a frequency that is off-resonance with respect to an imaging region of the subject is applied to the subject, and after applying the MT pulse P _mt , a spoiler pulse P _slr is applied, and a spoiler pulse P _mt is applied. After applying the _slr , an echo signal is collected from the imaging region. The application time of the MT pulse is set short. This application time is preferably a value of 10 [msec] or less. MT pulse P _mt
Is set to a required value equivalent to that of the related art. The MT pulse P _mt can be suitably applied to a two-dimensional scan of a multi-slice method using an MT pulse and a three-dimensional scan using an MT pulse.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic resonance imaging for obtaining an image of a blood flow or a substantial part inside a subject based on a magnetic resonance phenomenon of nuclear spin of the subject, and in particular, to MT (magnetization transf).
er) A magnetic resonance imaging apparatus and a magnetic resonance imaging method for obtaining an image with improved contrast between blood (or blood flow) and a substantial part using a pulse.

[0002] As used herein, "blood (or blood flow)" is used to mean "fluid" representing cerebrospinal fluid, blood (blood flow), and the like flowing in a subject.

[0003]

2. Description of the Related Art In magnetic resonance imaging (MRI), a nuclear spin of a subject placed in a static magnetic field is magnetically excited by a high frequency signal of a Larmor frequency, and an image is generated based on an MR signal generated by the excitation. This is an imaging method for obtaining.

[0004] One field of this magnetic resonance imaging is MR angiography (MR angiography).
One of the conventional methods of the MR angiography is an MT effect (MTC (magnetization trans).
In recent years, a technique of obtaining a blood flow image with a contrast between blood (blood flow) and a substantial part using the “fer contrast” effect) has been actively performed. One example is described in US Patent Publication No. 5,050,609.
No. (Magnetization Transfer)
Contrast and Proton Relax
ationand Use thereof in M
acoustic Resonance Imagin
g).

[0005] The study of the MT effect is described in Forsen &
ST (saturation based on Hoffman's
study of transfer method (Forsen et a
l. , Journal of Chemical P
physics, Vol. 39 (11) pp. 28
92-2901 (1963)), and a chemical exchange between protons between free water and a polymer as a plurality of kinds of nuclear pools (chemical ex).
change and / or cross mitigation
relaxation).

Several methods have been proposed for the conventional MR angiography utilizing the MT effect as follows.

[0007] As is well known, the spectra of free water and protons of a polymer are T2 relaxation (lateral relaxation).
Free water with a long time (T2 is about 100 msec) and a polymer with a short T2 relaxation time (T2 is about 0.1 to 0.2 msec)
There is a region where c) resonates at the same frequency. Since the T2 relaxation time of the free water signal value is long, the signal value after the Fourier transform shows a peak with a narrow half width. In contrast, the restriction of movement between macromolecules such as proteins (restrict)
Since the signal value of the ed) proton is short in T2 relaxation time, the signal value after Fourier transform has a wide half-value width and hardly appears as a peak value on the spectrum.

When the resonance peak frequency f ₀ of free water is set as the center frequency, a frequency selection pulse is applied as an MT pulse from the center frequency f _{0 of} free water to 500 Hz, for example.
Excitation of shifted frequency band (off-resonance)
(Excitation). Thereby, between the magnetization Hf of the free water in the equilibrium state and the magnetization Hr of the polymer, the magnetization Hf of the free water moves to the magnetization Hr of the polymer. As a result, the MR signal value from the protons of free water decreases, while the MR signal value from the protons of the polymer decreases at a higher rate. Therefore, there is a difference in the signal value between the site where the chemical exchange and / or cross-relaxation between free water and the polymer is reflected and the site where the cross-relaxation is not reflected. There is a contrast difference between the two parts, and a blood flow image can be obtained.

At present, the MR angiography method based on the MT effect is largely spatially non-selective (s
It is possible to use a non-selective imaging method and a slice-sel method.
active) imaging method.

As the former, for example, “GB Pik
e, MRM 25, 327-379, 1992 ", a frequency-selective binomial pulse is used as the MT pulse.
se) is applied spatially and non-selectively to obtain a contrast between the parenchyma and blood flow based on "MT effect of parenchyma> MT effect of blood flow".

[0011] As an example of the latter, “M.
zaki, MRM 32, 52-59, 1994 "
The proposed method is known. A slice-selective MT pulse is formed by an RF excitation pulse having a long application time and a gradient magnetic field spoiler pulse, and by applying this pulse,
The signal from the substantial part (stationary part) of the imaging surface is greatly reduced by the MT effect from the blood flow, and the MT effect of the blood flow flowing into the imaging surface is reduced (the signal from the blood flow is reduced by the substantial part). Less), to extract contrast between blood flow and parenchyma.

In time-of-flight (TOF) angiography based on a three-dimensional scan of the head,
The application time of the MT pulse is usually set to about 15 [msec].

Further, regarding the MT effect in the multi-slice method, see “PS Melkiand RV Mu”.
lkern, Magnetization Tran
sfer Effects in Multislic
e RARE Sequence ”, Magn Re
son Med 24, 189-195 (199
2) "," AD Elster, Radiolog
y 1994; 190: 541-551 "," DA
Finem, Radiology 1994; 1
90: 553-559 ". According to these reports, an MT pulse having a wavelength of 10 to 16 msec is used.

[0014]

However, in any of the MR imaging using the above-described MT pulse, the wavelength of the MT pulse, that is, the application time is set to a long value. The ratio of the application time of the MT pulse in one repetition time TR is, for example, about 35%.
This is based on the historical recognition that if the application time is not long, a sufficient waveform area of the MT pulse cannot be secured, and a high MT effect cannot be secured.

For this reason, there is a reality that the entire scanning time (imaging time) becomes longer. Further, if the multi-slice method is performed using the MT pulse having a long application time without changing the scan time, the number of slices is reduced.

SUMMARY OF THE INVENTION The present invention has been made to overcome the above-mentioned current situation, and provides at least the same MT effect as that of the related art and shortens the scan time caused by the application of the MT pulse as compared with the related art. This is the first purpose.

Further, according to the present invention, at the time of performing two-dimensional MR imaging by a multi-slice method using an MT pulse, it is possible to obtain at least an MT effect equivalent to the conventional one, and to reduce a scan time caused by the application of the MT pulse. A second object is to reduce the number of multi-slices in comparison with the conventional method and to maintain the number of multi-slices at a value equivalent to the conventional method.

Further, the present invention provides a method using an MT pulse.
A third object is to obtain at least the same MT effect as that of the related art when performing MR imaging in a dimensional scan, and to reduce the scan time due to the application of the MT pulse as compared with the related art.

[0019]

In order to achieve the above-mentioned first object, a magnetic resonance imaging apparatus according to the present invention provides an MT pulse having a frequency which is off-resonance with respect to an imaging region of a subject. MT pulse applying means for applying to the specimen, spoiler applying means for applying a gradient magnetic field spoiler pulse after applying the MT pulse, and scanning means for performing a scan for collecting echo signals from the imaging region after applying the spoiler pulse With
The application time of the MT pulse is set to be short.

As an example, the application time of the MT pulse is 10 [msec] or less. In particular, the application time of the MT pulse may be 6 [msec] or less.

Preferably, the MT pulse has a waveform area almost equal to or smaller than that of a conventional MT pulse having a long application time.

In order to achieve the second object,
The scanning unit may be configured as a unit that scans the imaging area based on a multi-slice method.

Further, in order to achieve the third object, the scanning means can be constituted as a means for three-dimensionally scanning the imaging area. In this case, preferably,
The apparatus may further include a region selection unit that applies a gradient magnetic field pulse together with the MT pulse to select an application region of the MT pulse to the subject at a position different from the imaging region.

In the magnetic resonance imaging apparatus according to the present invention, the application time of the MT pulse is set so short that the longitudinal magnetization of the magnetization spin in the subject hardly relaxes during the application time. Configuration.

On the other hand, in order to achieve the first object, the magnetic resonance imaging method according to the present invention provides a magnetic resonance imaging method, wherein at least two of the specimens having a binding relationship based on at least one of a chemical conversion phenomenon and a cross relaxation phenomenon. A magnetic resonance imaging method for collecting echo signals based on magnetic resonance phenomena of two kinds of nuclear pools, wherein an MT pulse having a short application time is applied to the subject to decouple a coupling relationship between the at least two kinds of nuclear pools. Then, a gradient magnetic field spoiler pulse is applied to the decoupled nucleus pool, and thereafter, echo signals of the imaging region of the subject are collected.

For example, the two types of nucleus pools are a free water nucleus pool and a polymer nucleus pool. In order to achieve the second or third object, the echo signal of the imaging region is collected by a two-dimensional scan based on a multi-slice method using an MT pulse or a three-dimensional scan using an MT pulse. You may. Further, as an example, the application time of the MT pulse is 10
[Msec] or less.

[0027]

Embodiments of the present invention will be described below with reference to the accompanying drawings.

(First Embodiment) FIG. 1 shows a first embodiment.
9 will be described.

FIG. 1 shows a schematic configuration of a magnetic resonance imaging (MRI) apparatus according to this embodiment.

This magnetic resonance imaging apparatus comprises a bed on which a subject P to be imaged is placed, a static magnetic field generator for generating a static magnetic field, a gradient magnetic field generator for adding positional information to the static magnetic field, A transmitting / receiving unit for transmitting and receiving high-frequency signals, a control / arithmetic unit for controlling the entire system and image reconstruction, an electrocardiographic measuring unit for measuring an ECG signal of the subject P, and instructing the subject to temporarily hold his / her breath. And a breath holding command unit.

The static magnetic field generating section includes, for example, a superconducting magnet 1 and a static magnetic field power supply 2 for supplying a current to the magnet 1, and has a cylindrical opening (diagnostic space) into which the subject P is loosely inserted. the axial direction (Z axis direction) to generate a static magnetic field H _0.
Note that a shim coil 14 is provided in this magnet portion. The shim coil 14 is supplied with a current for homogenizing a static magnetic field from a shim coil power supply 15 under the control of a host computer described later. The couch part can retreatably insert the top plate on which the subject P is placed into the opening of the magnet 1.

The gradient magnetic field generator has a gradient coil unit 3 incorporated in the magnet 1. The gradient magnetic field coil unit 3 includes three sets (types) of x, y, and z coils 3x to 3z for generating gradient magnetic fields in the X, Y, and Z-axis directions as physical axes of the gantry, which are orthogonal to each other. .
The gradient magnetic field unit further includes a gradient magnetic field power supply 4 for supplying a current to the x, y, z coils 3x to 3z. The gradient power supply 4 controls x, y, and x under the control of a sequencer 5 described later.
A pulse current for generating a gradient magnetic field is supplied to the z coils 3x to 3z.

The x, y, z coils 3x from the gradient magnetic field power supply 4
By controlling the pulse current supplied to ~ 3z,
A physical axis X, Y, by combining the gradient magnetic field in the Z direction, slice direction gradient magnetic field G _S, the phase encode direction gradient magnetic field G _E, and readout direction (frequency encode direction) arbitrarily gradient G _R set and Can be changed. Slice direction, phase encoding direction and readout direction are logic-axis directions perpendicular to each other, the gradient of each direction are superimposed on the static magnetic field H _0.

The transmitting / receiving section includes an RF coil 7 disposed near the subject P in the diagnostic space in the magnet 1, and a transmitter 8 T and a receiver 8 R connected to the coil 7.
The transmitter 8T and the receiver 8R operate under the control of the sequencer 5 described later. The transmitter 8T supplies the RF coil 7 with an RF current pulse having a Larmor frequency for causing magnetic resonance (MR). The receiver 8R is RF
The MR signal (high-frequency signal) received by the coil 7 is fetched and subjected to various kinds of signal processing to generate digital MR data (original data).

The control / arithmetic unit includes a sequencer (also called a sequence controller) 5, a host computer 6, an arithmetic unit 10, a storage unit 11, and a display unit 1.
2, an input device 13 and a sound generator 16 are provided.

The host computer 6 has a function of instructing the sequencer 5 on pulse sequence information and controlling the operation of the entire apparatus by a software procedure stored in advance.

The sequencer 5 has a CPU and a memory, stores the pulse sequence information sent from the host computer 6, and controls the operation of the gradient magnetic field power supply 4, the transmitter 8T, and the receiver 8R according to the information. At the same time, the receiver 8R is configured to temporarily receive the decibel data of the MR signal output from the receiver 8R and transfer the same to the arithmetic unit 10.

Here, the pulse sequence information is all information necessary for operating the gradient magnetic field power supply 4, the transmitter 8T and the receiver 8R in accordance with a series of pulse sequences, for example, x, y, z coils. Information about the intensity of the pulse current applied to 3x to 3z, the application time, the application timing, and the like are included.

The pulse sequence is a two-dimensional scan or three-dimensional scan pulse sequence.
The form of the pulse train includes an SE (spin echo) method, an FE (field echo) method, and an FSE (high-speed S
E) method, FFE (high-speed FE) method, FASE (high-speed Asy)
Any pulse train for imaging, such as a metric SE) method or an EPI (echo planar imaging) method, may be used.

The arithmetic unit 10 receives the digital echo data output from the receiver 8R, arranges the digital echo data in a Fourier space (also called a k-space or a frequency space) formed by a built-in memory, and sets this echo data set. Is subjected to a two-dimensional or three-dimensional Fourier transform to reconstruct a real space image.

The storage unit 11 stores echo data and reconstructed image data. The display 12 displays an image. Information such as scan conditions and pulse sequences can be input to the host computer 6 via the input device 13.

The voice generator 14 can emit a breath-hold start and a breath-hold end message as voice when instructed by the host computer 6.

Further, the electrocardiogram measuring section is provided with an ECG sensor 17 which is attached to the body surface of the subject to detect an ECG signal as an electric signal, and performs various processes including digitization on the sensor signal to perform sequencer 5 processing. And an ECG unit 18 for outputting to the The measurement signal by this electrocardiograph is
It is used by the sequencer 5 as a timing signal when executing a scan sequence. As a result, the synchronization timing for ECG synchronization can be appropriately set, and an ECG synchronization scan based on the set synchronization timing can be performed to collect echo data.

FIG. 2 shows an example of the timing and pulse sequence of echo acquisition based on the two-dimensional multi-slice method. Each of the plurality of n slices is RF-excited and echo-collected at each repetition time TR, and the remaining slices are sequentially similarly excited and echo-collected until the next repetition. As a result, one of the slices is always
Echoes are collected at a repetition time (period), also called "im TR".

As shown in FIG. 2, the pulse sequence used for this echo acquisition is an MT pulse P having a short application time.
_mt and a spoiler pulse P applied in any or all gradient magnetic field directions of slice, phase encode, and readout to dephase the transverse magnetization spin
_{The slr} and the imaging pulse train S _ima are formed in this order in the time series direction. FIG. 3 shows a pulse sequence using a conventional MT pulse P _mt ′ for comparison.

The MT pulse P _mt of this embodiment is represented by Sin
A high-frequency pulse is generated by modulating a waveform of the c-function or the Gaussian function with a high-frequency signal of a predetermined frequency, and the frequency is set to, for example, a value offset from the resonance center frequency of the water component of the subject by a predetermined value. This MT pulse P _mt
Is shorter than a conventional value of about 15 msec, that is, a value of less than 10 msec (for example, 1.5 msec) obtained by confirmation of an experiment or the like as described later.
Is set to

However, the peak value (intensity) of the MT pulse _{Pmt according to} the present embodiment and the peak value (intensity) are set high in consideration of the shortened application time. However, the peak value is MT
Pulse P waveform area of _mt may be increased until the same comparable conventional MT pulse P _{mt ',} but not necessarily increased to there. The reason is that the contribution of the improvement in the MT effect due to the reduction in the application time of the MT pulse is greater. Further, for example, when a two-dimensional scan based on the multi-slice method is performed, the MT effect obtained when another slice is excited contributes.

In the present embodiment, the MT pulse P _mt is applied non-slice-selectively without a slice gradient magnetic field, but may be applied slice-selectively as needed.

According to an experiment described later by the present inventor, the application time of the MT pulse P _{mt according to} the present embodiment is set to a value of 10 msec or less when the two-dimensional multi-slice method is used as the imaging pulse train S _ima. By shortening,
M is considerably higher than that of the conventional MT pulse having a long application time.
It has been found that the T effect can be obtained. Further, it is also known that by setting the application time of the MT pulse P _mt to 6 msec or less, a significantly higher MT effect can be obtained as compared with the conventional MT pulse.

For this reason, according to the present invention, M
The T pulse means an application time of 10 msec or more, and the MT pulse having a short application time means an application time of less than 10 msec.

The application time of the spoiler pulse P _slr is set to about 2 msec as in the prior _art . further,
The imaging pulse train S _ima is formed of an SE or FE system pulse train, and the application time varies depending on the type.

In this embodiment, the RF coil 7, the transmitter 8T, the sequencer 5, and the host computer 6 are the M
The main part of the T pulse applying means is formed, and the gradient magnetic field coil unit 3, the gradient magnetic field power supply 4, the sequencer 5, and the host computer 6 form the main part of the spoiler applying means of the present invention. The device 8R, the sequencer 5, and the host computer 6 form the main part of the scanning means.

Next, the operation and effect of the magnetic resonance imaging apparatus according to this embodiment will be described.

For example, if the imaging part of the subject is the head, the desired part of the head is scanned according to the two-dimensional multi-slice method. A desired part is selectively excited by dividing it into a plurality of slices, and an echo signal is collected from each slice at every repetition time TR.

In this acquisition, first, an MT pulse P _mt having a short application time (for example, 1.5 msec) is applied. As a result, the longitudinal magnetization component of the nuclear spin at the excitation site is reversed in the −z′-axis direction as shown in FIG. 4A, but since the application time of the MT pulse P _mt is short, the longitudinal magnetization The rate at which the component returns to the initial state in the z'-axis direction is small or negligible. In other words, the "shortening" effect of the longitudinal magnetization component is large, and this promotes chemical exchange and / or cross relaxation between protons between the polymer and free water.

Thereafter, after the spoiler pulse _Pspl is applied and the transverse magnetization component is sufficiently _dephased , an echo signal is collected by applying a predetermined imaging pulse train _Sima . At this time, since the application time of the MT pulse is short, the MT effect due to the application of the MT pulse and the MT effect due to the application of the imaging pulse train _{Sima work} synergistically.

As described above, chemical exchange and cross-relaxation of protons between the polymer and free water are greatly promoted by the application of the MT pulse for a short application time. The value will be lower. That is, the MT effect per unit time is improved, and a larger MT effect is obtained.

This will be described in comparison with a conventional case using an MT pulse having a long application time. In the conventional case, the time during which the MT pulse is applied is long (for example, 15 msec).
c), as schematically shown in FIG. 4B, a component that returns in the + z′-axis direction starts to appear during the application, and the net longitudinal magnetization component remaining in the −z′-axis direction at the end of the application is:
The number is smaller than when the MT pulse is applied for a short time (see FIG. 4A). For this reason, compared to this embodiment in which the application time of the MT pulse is short, chemical exchange and cross relaxation of protons between the polymer and free water are less, and thus the signal value of the collected echo signal is smaller in this embodiment. Larger than the case.
That is, only a small MT effect can be obtained.

For this reason, an echo signal collected using an MT pulse with a short application time as in the present embodiment and an echo signal collected using an MT pulse with a long application time as in the prior art are represented by their intensities SI. By comparison, qualitatively, FIG.
Are represented as shown in FIG. The figure shows an MR contrast agent (eg, g
(adolinium-DTPA) to obtain a cross-sectional image of the head. Since the contrast agent originally has a low rate of receiving the MT effect, when the contrast agent passes through a lesion D such as a cancer, the lesion D is drawn with a high signal value. As described above, since the normal tissue portion T uses the MT pulse with a short application time, the signal value is greatly reduced due to a large MT effect. That is, the contrast between the lesion D and the normal tissue T becomes higher than in the conventional case.

On the other hand, in the conventional case, since the application time of the MT pulse is long, the MT effect of the normal tissue portion T is not so large due to the above-mentioned reason, and the amount of decrease in the signal value is small. For this reason, the contrast between the lesion portion D and the normal tissue portion T is lower than in the case of the present embodiment.

For this reason, by using the MT pulse whose application time is set short and a required amount of waveform area is secured as in the present embodiment, a portion between the portion greatly affected by the MT effect and the portion not affected by the MT effect can be obtained. Can be further enhanced, and the ability to depict lesions and the like can be significantly improved. Therefore, it is suitable for a 2D multi-slice T1-weighted image or an image that depicts cartilage.

Particularly, the so-called “minimum TR”
In the case of the two-dimensional multi-slice method, in which the repetition time between slices is relatively short, the synergistic effect due to off-resonance excitation between multi-slices (potential MT Effect) is also obtained, so that a high MT effect can be reliably obtained as a whole. Conversely, since there is such a synergistic effect, M
Even when the waveform area of the T pulse is set slightly lower, a sufficient MT effect can be obtained.

In addition, since the application time of the MT pulse is short, the scan time for each excitation is short, and the entire imaging time can be shortened.

Further, since the scan time for each excitation is short, when securing the same number of multi-slices as in the prior art, the entire imaging time can be reduced. Conversely,
If it is allowed to set the same imaging time as in the related art, it is possible to set a large number of such multi-slices.

An example of the imaging time and the number of multi-slices will be described. Multi-slice method (slice thickness = 7m)
m), the pulse train for imaging = SE method, min
imum TR = 28 msec, TR = 500 msec
In this case, 17 slices can be secured. When an MT pulse having an application time of 17 msec (including a spoiler pulse) according to the conventional method is added to the pulse train of the SE method, the number of slices is reduced to 11 sheets. Conversely, to secure 17 slices, the repetition time TR had to be extended to 765 msec.

On the other hand, when the above-described imaging is performed by applying the present invention, the application time of the MT pulse is 3.5 msec.
(Including a spoiler pulse), and the other conditions are the same, the repetition time TR = 536 msec, and the number of slices = 17 can be secured only by slightly extending the repetition time TR.

Here, FIGS. 6 to 9 show the results of experiments performed by the present inventors to quantitatively verify the effects of the present invention.

FIG. 6 shows the results of Experiment 1. In this experiment, the number of multi-slices required for the head T1-weighted image = 17, the flip angle = 500 °, and the multi-slice method were used.
The signal was collected while the MT offset frequency was set to -1500 Hz, the MT pulse waveform was set to sinc (1π, 1π), and the pulse length of the MT pulse was changed from 10 to 1.5 msec. Note that the PVA signal value and the noise are indicated by numerical values in arbitrary units. Also, pulse length (total length) = τ
Length × 2.

According to this, the sequence corresponding to the present invention: SE15-MTC750 (the pulse length of the MT pulse = 10 msec) compared to the sequence corresponding to the conventional method: the SE15-MTC5000 (the pulse length of the MT pulse = 10 msec).
At 1.5 msec), the MTR (MT ratio) is improved by about 45% in S / N, and the number of slices =
The repetition time TR at 17 was reduced to less than half.

A sequence corresponding to the conventional method: SE
15-MTC5000 and SE15-MTC3000 (M
Comparing with (pulse length of T pulse = 6 msec), it can be seen that also in this case, the MTR is improved (that is, the S / N is reduced). From this, it is understood that by setting the pulse length of the MT pulse to a value less than 10 msec, the effect of the “MT pulse with a short application time”, which is equivalent to the present invention, is already exhibited. Therefore, the range of the application time of the MT pulse according to the present invention is 10 msec.
Interpreted as a value less than c.

FIG. 7 shows the results of Experiment 2. In this experiment, signal acquisition was performed while the repetition time TR was fixed at 500 msec and the pulse length of the MT pulse was changed from 10 to 1.5 msec. Other imaging conditions are the same as described above.

From this experiment, it was found that the normal imaging time (TR = 5
00 msec), the pulse length of the MT pulse is set to 1
It was found that by setting the length to less than 0 msec, a larger MT effect (lower S / N) and a larger number of slices can be obtained than in the case of the sequence corresponding to the conventional method: SE15-MTC5000. This effect is, as shown by the numerical values of S / N and the number, the pulse length of the MT pulse = 1.
It can be seen that the effect is already exhibited when the time is less than 0 msec.

Experiment 3 is shown in FIG. This experiment shows the change in S / N when the number of slices and the flip angle are fixed and the frequency offset amount of the MT pulse is made variable. From this experiment, a range in which the MT effect is exerted on the plus side and the minus side with respect to the resonance peak frequency of free water was confirmed.

Further, Experiment 4 is shown in FIG. In this experiment, when the offset frequency of the MT pulse is set to the same numerical value (1200 Hz, -1200 Hz) on the + side and the − side, and these parameters are used as parameters, S / N changes. Thereby, the pulse intensity range in which the MT effect is exhibited and the symmetry thereof were confirmed.

Although the above-described experiment is performed on a PVA phantom, it is known that, when a PVA phantom is compared with a human head, the MT effect is generally larger in the human head. Therefore, when the imaging according to the present invention is performed on the human head, the effect of the “MT pulse with a short application time” becomes more remarkable, and this also demonstrates the effectiveness of the present invention. .

(Second Embodiment) A second embodiment according to the magnetic resonance imaging apparatus of the present invention will be described with reference to FIGS.

In the second embodiment, instead of the two-dimensional scan based on the two-dimensional multi-slice method using the above-described MT pulse, the above-mentioned MT pulse is replaced with, for example, the one described in US Pat. No. 5,627,468. A magnetic resonance imaging apparatus that performs imaging by applying to a three-dimensional scanning method is provided. Since the hardware configuration of this magnetic resonance imaging apparatus is the same as that of the first embodiment, a detailed description thereof will be omitted.

The pulse sequence shown in FIG. 10 is executed by the sequencer 5 of the magnetic resonance imaging apparatus as an example of a three-dimensional scan.

[0079] As can be seen from the figure, this pulse sequence, a slice magnetic gradient G _S and accompanied to the MT pulse P
_mt is applied first, and then the slice-direction gradient magnetic field G
_S, phase encode direction gradient magnetic field _{G E,} and readout direction (frequency encode direction) to each of the gradient _{G R} spoiler pulses _{P slr} is applied. Next, as an example, an imaging pulse train S for a three-dimensional scan employing the FE method
_ima is applied.

Among them, the method of shortening the application time according to the present invention is applied to the MT pulse _Pmt . That is, the application time T _dur is reduced to a value less than 10 msec as in the first embodiment. Also, this MT pulse P _mt
Is expressed by the strength of the high-frequency magnetic field,
To 32 μT, preferably 19 to 26 μT. According to the angle display, the strength of the high-frequency magnetic field is 6 μT, 3
2 μT is 90 degrees and 500 degrees, respectively.

This MT pulse P_mtIs applied in parallel with
Slice direction gradient magnetic field G_SIs, as shown in FIG.
Desired imaging region R_imaPre-excitation region R different from_mt
For example, the intensity is set in advance so that the position can be selected.
Have been. Thereby, for example, a desired imaging region R
_imaIs the head as shown in FIG.
R _mtIs located at an imaging region R with respect to the heart._imaFarther than
Is set at the top of the head. As a result, the MT pulse P_mt
Is the pre-excitation region R on the top of the head_mtLimited to
And the imaging region R_imaBlood flow from the heart side
There is no excitation on the inflow side.

The MT in which the application time is set to be short as described above
When the application of the pulse _Pmt is completed, for example, the slice direction,
A spoiler pulse P _slr is applied in each of the phase encoding direction and the reading direction.

Thereafter, the imaging pulse train S
Each pulse of the _ima is applied to the imaging region _{R ima,} echo signals are acquired from the imaging region _{R ima.} This echo signal includes a strong echo signal obtained from the blood flow. In other words, the artery AR that has flowed into the head from the heart generates a strong echo signal because it is excited by the imaging pulse train S _ima only after entering the imaging region R _ima , but the vein VE returning to the heart via the top of the head is _Are already excited in the pre-excitation region R _mt set on the parietal side, so that the imaging region R
The intensity of the echo signal generated at _ima becomes smaller.

Therefore, the signal level from the vein VE can be suppressed, and the signal level from the artery AR, which is clinically more significant, can be relatively increased. At the same time, the pre-excitation region R _mt is set near the imaging region R _ima, and the MT pulse P _mt is applied to the region R _mt. Therefore, the rate of reduction of the echo signal generated from the substantial part in the imaging region R _ima . Is greater than that from the bloodstream (ie, the MT effect). The increase in the MT effect becomes more remarkable by the MT pulse _{Pmt having} a short application time according to the present embodiment. Accordingly, MR angiography for imaging peripheral blood vessels with high accuracy can be performed.

Of course, if necessary, the pre-excitation region R _mt
Can be set at a position closer to the heart than the imaging region R _ima , whereby MR angiography image data in which the vein VE is emphasized more than the artery AR can be obtained.

The contents described in the above-described embodiment are as follows.
It is only an exemplary mode for carrying out the invention described in the claims, and it is needless to say that those skilled in the art can change and modify various modes without departing from the spirit of the present invention.

[0087]

As described above, according to the present invention,
Since an MT pulse with a short application time is used, an MT effect equivalent to that of the conventional method can be obtained, and the scan time (imaging time) caused by the application of the MT pulse can be significantly reduced as compared with the conventional method.

Further, by performing the MR imaging of the two-dimensional scan based on the multi-slice method using the MT pulse having a short application time, the number of multi-slices can be maintained at the same value as the conventional method, and the scan time is minimized. And an MT effect comparable to or higher than that of the conventional method can be obtained to increase the contrast of the image and improve the rendering ability of the MR image. Similarly, an MT pulse with a short application time can be adopted for MR imaging of a three-dimensional scan using an MT pulse, and both a favorable MT effect and a reduction in scan time can be achieved.

[Brief description of the drawings]

FIG. 1 is a functional block diagram showing a schematic configuration of a magnetic resonance imaging apparatus according to first and second embodiments of the present invention.

FIG. 2 is a flowchart showing an outline of a pulse sequence of a two-dimensional scan based on a multi-slice method and an application timing according to the first embodiment of the present invention.

FIG. 3 is a pulse sequence representing an MT pulse according to a conventional method described for comparison.

FIG. 4 is a diagram schematically illustrating the behavior of a longitudinal magnetization component of spin caused by the application of an MT pulse in comparison with the conventional method and the method of the present invention.

FIG. 5 is a diagram schematically illustrating the superiority and inferiority of the MT effect in comparison with the conventional method and the method of the present invention.

FIG. 6 is a view showing experimental results.

FIG. 7 is a view showing an experimental result.

FIG. 8 is a view showing an experimental result.

FIG. 9 is a view showing experimental results.

FIG. 10 is a flowchart illustrating an outline of a pulse sequence of a three-dimensional scan according to the second embodiment of the present invention.

FIG. 11 is an explanatory diagram exemplifying a positional relationship between a pre-excitation region for applying an MT pulse and an imaging region for applying an imaging pulse train in the second embodiment.

[Explanation of symbols]

 Reference Signs List 1 magnet 2 static magnetic field power supply 3 gradient magnetic field coil unit 4 gradient magnetic field power supply 5 sequencer 6 host computer 7 RF coil 8T transmitter 8R receiver 10 arithmetic unit 11 storage unit 12 display 13 input unit

Claims (19)

[Claims] 1. An MT pulse applying means for applying an MT pulse having a frequency which is off-resonance to an imaging region of an object to the object, and a spoiler for applying a gradient magnetic field spoiler pulse after applying the MT pulse. A magnetic resonance imaging apparatus comprising: an application unit; and a scanning unit that performs a scan for collecting an echo signal from the imaging region after applying the spoiler pulse, wherein the application time of the MT pulse is set to be short. 2. The magnetic resonance imaging apparatus according to claim 1, wherein the application time of the MT pulse is 10 [msec] or less. 3. The magnetic resonance imaging apparatus according to claim 2, wherein the application time of the MT pulse is 6 [msec] or less. 4. The magnetic resonance imaging apparatus according to claim 1, wherein the MT pulse has substantially the same waveform area as a conventional MT pulse having a long application time. Magnetic resonance imaging device. 5. The magnetic resonance imaging apparatus according to claim 1, wherein the MT pulse has a smaller waveform area than a conventional MT pulse having a longer application time. Resonance imaging device. 6. The magnetic resonance imaging apparatus according to claim 1, wherein the scanning unit is a unit that two-dimensionally scans the imaging region based on a multi-slice method. Magnetic resonance imaging device. 7. The magnetic resonance imaging apparatus according to claim 1, wherein the scanning unit is a unit that three-dimensionally scans the imaging region. 8. The magnetic resonance imaging apparatus according to claim 7, wherein a gradient magnetic field pulse for selecting an application region of the MT pulse to the subject at a position different from the imaging region is added to the MT pulse. A magnetic resonance imaging apparatus further comprising a region selecting means for applying a voltage to the magnetic resonance imaging apparatus. 9. An MT pulse applying means for applying an MT pulse having a frequency which is off-resonance to an imaging region of the subject to the subject, and a spoiler for applying a gradient magnetic field spoiler pulse after applying the MT pulse. Application means, and scanning means for performing a scan for collecting an echo signal from the imaging area after applying the spoiler pulse. The application time of the MT pulse is determined by the longitudinal magnetization of the magnetization spin in the subject. A magnetic resonance imaging apparatus characterized in that it is set so as to be hardly relaxed during time. 10. The magnetic resonance imaging apparatus according to claim 9, wherein the application time of the MT pulse is 10 [msec] or less. 11. The magnetic resonance imaging apparatus according to claim 10, wherein the scanning unit is a unit that two-dimensionally scans the imaging region based on a multi-slice method. 12. The magnetic resonance imaging apparatus according to claim 10, wherein the scanning means is a means for three-dimensionally scanning the imaging area. 13. The magnetic resonance imaging apparatus according to claim 12, wherein a gradient magnetic field pulse for selecting an application region of the MT pulse to the subject at a position different from the imaging region is added. A magnetic resonance imaging apparatus further comprising a region selecting means for applying a voltage to the magnetic resonance imaging apparatus. 14. A magnetic resonance imaging method for collecting echo signals based on magnetic resonance phenomena of at least two types of nucleus pools in a subject having a binding relationship based on at least one of a chemical conversion phenomenon and a cross relaxation phenomenon. Applying an MT pulse with a short application time to the subject to decouple the coupling relationship between the at least two types of nuclear pools, applying a gradient magnetic field spoiler pulse to the decoupled nuclear pool, and thereafter, A magnetic resonance imaging method, wherein the echo signal of an imaging region of a subject is collected. 15. The magnetic resonance imaging method according to claim 14, wherein the two types of nuclear pools are a free water nuclear pool and a polymer nuclear pool. 16. The magnetic resonance imaging method according to claim 15, wherein the echo signals of the imaging region are collected by a two-dimensional scan based on a multi-slice method. 17. The magnetic resonance imaging method according to claim 15, wherein echo signals of the imaging region are collected by a three-dimensional scan. 18. The magnetic resonance imaging method according to claim 17, wherein a gradient magnetic field pulse for selecting an application region of the MT pulse to the subject at a position different from the imaging region is added. A magnetic resonance imaging method characterized in that the method is applied to a magnetic field. 19. The magnetic resonance imaging method according to claim 14, wherein the application time of the MT pulse is 10 [msec] or less.