JP2015008903A - Magnetic resonance apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a technique capable of reducing contour emphasis.SOLUTION: In a scan SC, sequence units SEto SEare executed. Each sequence unit has a preparation sequence A and an imaging sequence B. The preparation sequence A has a gradient pulse MPG for performing diffusion emphasis. The imaging sequence B collects MR signals Sto Sfrom an imaging site. The imaging sequence B has an RF pulse Xand RF pulses Xto X. The flip angles αto αof the RF pulses Xto Xare preset so that the relation α-α>α-αholds.

Description

  The present invention relates to a magnetic resonance apparatus for executing a sequence having gradient pulses for performing diffusion weighting.

  As a method for acquiring diffusion information, a method using MPG (Motion Probing Gradient) is known (see Patent Document 1).

JP 2012-157687 A

  As an example of a method for obtaining diffusion information using MPG, there is a method called DP (Diffusion Preparation) method in which MPG is applied as a prepulse. The DP method is applied when, for example, a pulse sequence such as FIESTA (Fast Imaging Employing Steady State Acquisition) is executed, and diffusion information can be obtained relatively easily by the DP method.

  However, in the above method, when the MR signal becomes large due to the T1 recovery during data collection, the image quality such as enhancement of the contour of the imaging region may be deteriorated. In particular, as the T1 value of the imaging region is smaller, the phenomenon that the outline is emphasized appears more remarkably. Therefore, a technique capable of reducing contour enhancement is desired.

One aspect of the present invention provides a second sequence having a plurality of RF pulses for generating a magnetic resonance signal arranged in k-space after executing a first sequence having a gradient pulse for performing diffusion weighting. A magnetic resonance apparatus for performing
Of the plurality of RF pulses, a flip angle of a first RF pulse, a flip angle of a second RF pulse applied next to the first RF pulse, and an application next to the second RF pulse. The flip angle of the third RF pulse is a magnetic resonance apparatus set to satisfy the following conditions.
α _n −α _{n + 1} > α _{n + 1} −α _{n + 2} > 0
Here, α _n : flip angle of the first RF pulse α _{n + 1} : flip angle of the second RF pulse α _{n + 2} : flip angle of the third RF pulse

By setting the flip angle so as to satisfy α _n −α _{n + 1} > α _{n + 1} −α _{n + 2} > 0, contour enhancement can be reduced.

It is the schematic of the magnetic resonance apparatus of one form of this invention. It is explanatory drawing of the scan SC performed in order to collect data from an imaging | photography site | part. It is a diagram showing a state to fill the MR signals acquired by the sequence unit _SE 1 _{~SE m} in the k-space. Is an illustration of the RF pulses _x 1 ~x flip angle of _{N α 1 ~α N.} It is a figure which shows the imaging sequence B when flip angle (alpha) _1- alpha _{N is made} into the same value (alpha). Is an explanatory view of a problem in the case of setting the flip angles alpha ₁ to? _N in the same value. It is explanatory drawing of the reason which can reduce outline emphasis. It is a diagram illustrating a flip angle alpha ₁ to? _N in the case of setting the reduced width α _n -α _{n + 1} flip angle so as to satisfy the equation (3). It is explanatory drawing of the reason which cannot fully reduce the outline emphasis of the imaging | photography site | part. It is a figure which shows the flip angle at the time of setting to rd_factor = 1, rd_factor = 0.5, and rd_factor = 0.1. It is a figure which shows the result of simulation E1. It is a figure which shows the result of simulation E2. It is a figure which shows an example of MR apparatus suitable for an operator to select optimal rd_factor. It is a figure which shows an example in case one part flip angle does not satisfy | fill Formula (1).

  Hereinafter, although the form for inventing is demonstrated, this invention is not limited to the following forms.

FIG. 1 is a schematic view of a magnetic resonance apparatus according to one embodiment of the present invention.
A magnetic resonance apparatus (hereinafter referred to as “MR apparatus”, MR: Magnetic Resonance) 100 includes a magnet 2, a table 3, a receiving coil 4, and the like.

  The magnet 2 has a bore 21 in which the subject 11 is accommodated. The magnet 2 includes a superconducting coil, a gradient coil, an RF coil, and the like.

  The table 3 has a cradle 3 a that supports the subject 11. The cradle 3a is configured to be able to move into the bore 21. The subject 11 is transported to the bore 21 by the cradle 3a.

  The receiving coil 4 is attached to the body of the subject 11. The receiving coil 4 receives a magnetic resonance signal from the subject 11.

  The MR apparatus 100 further includes a transmitter 5, a gradient magnetic field power source 6, a receiver 7, a control unit 8, an operation unit 9, a display unit 10, and the like.

  The transmitter 5 supplies current to the RF coil, and the gradient magnetic field power source 6 supplies current to the gradient coil. The receiver 7 performs signal processing such as detection on the signal received from the receiving coil 4.

  The control unit 8 transmits necessary information to the display unit 10 and reconstructs an image based on data received from the receiving coil 4 so as to realize various operations of the MR device 100. Control the operation of each part.

The operation unit 9 is operated by an operator and inputs various information to the control unit 8. The display unit 10 displays various information.
The MR apparatus 100 is configured as described above.

FIG. 2 is an explanatory diagram of the scan SC executed to collect data from the imaging region.
In the scan SC, the sequence parts SE _{1 to} SE _m are executed. The sequence part SE ₁ has a preparation sequence A and an imaging sequence B.

  The preparation sequence A has a gradient pulse MPG for performing diffusion weighting. After executing the preparation sequence A, the imaging sequence B is executed.

The imaging sequence B is a sequence for collecting MR signals S _{1 to SN} from the imaging region. Imaging sequence B has a RF pulses _{x 0} and RF pulses _x 1 ~x _N. RF pulses _{x 0} is the RF pulse for shifting the MR signal in the steady state. Instead of RF pulses x _0, the MR signal with the ramp-up pulse may be migrated to a steady state.

After applying the RF pulses _{x 0,} RF pulses _x 1 ~x _N is applied. RF pulses _x 1 ~x _N flip angle alpha ₁ to? _N of is set to be gradually reduced. Will be described later why the RF pulses _x 1 ~x _N flip angle alpha ₁ to? _N of is set to gradually become smaller. For convenience of explanation, the gradient magnetic field of the imaging sequence B is not shown.

FIG. 2 shows the configuration of the sequence unit SE ₁ , but the other sequence units SE _{2 to} SE _m also have a preparation sequence A and an imaging sequence B, similar to the sequence unit SE ₁ . Therefore, the MR signals S _{1 to} S _N can be collected by executing each of the sequence units SE _{1 to} SE _m .

FIG. 3 is a diagram illustrating a state when MR signals collected by the sequence units SE _{1 to} SE _{m are} embedded in the k space.

MR signals S _{1 to} S _N collected by the sequence unit SE ₁ are arranged in a region of k space of kz = −j. The sequence unit SE ₁ performs MR signal S _{1 to} S _N by centric view ordering (a method of acquiring data alternately between a positive view and a negative view starting from the central view V 0 of kx and in an order close to the view V 0). To collect. After executing the sequence section SE _1, the following sequence unit SE ₂ is executed.

The MR signals S _{1 to} S _N collected by the sequence unit SE ₂ are arranged in a region of k space of kz = − (j−1). The sequence unit SE ₂ also collects the MR signals S _{1 to} S _N by centric view ordering.

Similarly, each sequence unit is executed, and MR signals are collected by centric view ordering. Finally, the sequence part SE _m is executed.

By executing the sequence part SE _m , MR signals S _{1 to} S _N arranged in the k-space region of kz = j are collected. In this way, k-space data is collected.

In the present embodiment, the flip angles α _{1 to} α _N of the RF pulses x _{1 to} x _N of the imaging sequence B are set so as to become gradually smaller (see FIG. 4).

FIG. 4 is an explanatory diagram of the flip angles α _{1 to} α _N of the RF pulses x _{1 to} x _N.
FIG. 4 shows a curve F representing the magnitudes of the flip angles α _{1 to} α _N of the RF pulses x _{1 to} x _N. The flip angles α _{1 to} α _N are set to be gradually reduced. Specifically, it is set to satisfy the following expression.
α _n −α _{n + 1} > α _{n + 1} −α _{n + 2} > 0 (1)
However, 1 ≦ n ≦ N−2

Here, α _n : flip angle of the _nth RF pulse α _{n + 1} : flip angle of the (n + 1) th RF pulse α _{n + 2} : flip angle of the (n + 2) th RF pulse

The flip angle is set so that α _{n + 1} −α _{n + 2} is smaller than α _n −α _{n + 1} . Accordingly, n = 1, that is, the flip angle decrease width α ₁ -α ₂ immediately after the start of the imaging sequence B is large, but as n increases, the flip angle decrease width α _n −α _{n + 1} decreases.

In this embodiment, the flip angles α _{1 to} α _N are decreased so as to satisfy the expression (1). The reason why the flip angles α _{1 to} α _N are reduced will be described below. In explaining this reason, as a comparative example, an example in which the flip angles α _{1 to} α _N are set to the same value without decreasing them is considered, and there is a problem when the flip angles α _{1 to} α _N are set to the same value. Will be described first.

FIG. 5 is a diagram showing an imaging sequence B when the flip angles α _{1 to} α _N are set to the same value α.
In Figure 5, the flip angle alpha ₁ to? _N of the RF pulses _x 1 ~x _N is the same value alpha. Therefore, the flip angles α _{1 to} α _N can be expressed by the following equations.
α _n = α _{n + 1} = α (2)
However, 1 ≦ n <N−1

FIG. 6 is an explanatory diagram of problems when the flip angles α _{1 to} α _N are set to the same value. For convenience of explanation, it is assumed that the imaging region has a large diffusion and that the T1 value of each tissue in the imaging region can be regarded as the same value.

Below the RF pulses x _{1 to} x _N , a T1 recovery curve a of the imaging region and a signal intensity curve b representing a time change of the signal intensity of the MR signals S _{1 to} S _N are schematically shown. Immediately after the start of the imaging sequence B, the T1 recovery speed is fast, but the T1 recovery speed decreases as time progresses. Further, due to the T1 recovery, the signal strength of the MR signals S _{1 to} S _N also increases with time. Thus, since the signal strength of the MR signals S _{1 to} S _N increases with time, a relatively large MR signal is arranged in the high frequency region of the k space. Therefore, when k-space data collected by the imaging sequence B is converted into an image, an image in which the contour of the imaging region is emphasized is obtained. In particular, since the T1 recovery proceeds faster as the T1 value of the imaging region is smaller, the phenomenon that the contour is emphasized appears remarkably. Therefore, the method of setting the flip angles α _{1 to} α _N to the same value α has a problem that the image quality deteriorates.

Therefore, in the present embodiment, the flip angles α _{1 to} α _N are gradually decreased in order to reduce the image quality deterioration as described above. The reason why the edge enhancement can be reduced by gradually decreasing the flip angles α _{1 to} α _N will be described below.

FIG. 7 is an explanatory diagram showing the reason why the edge enhancement can be reduced.
FIG. 7 shows two signal intensity curves b and c. The signal strength curve b schematically represents a time change of the signal strength of the MR signals S _{1 to} S _N when the flip angle is set to the same value α. On the other hand, the signal intensity curve c schematically represents a time change of the MR signals S _{1 to} S _N when the flip angles α _{1 to} α _N are gradually decreased (this embodiment).

When the flip angle is the same value α, the signal strength of the MR signals S _{1 to} S _N increases with time (signal strength curve b). However, in this embodiment, the flip angles α _{1 to} α _N are gradually reduced. By gradually decreasing the flip angles α _{1 to} α _N , an increase in the signal strength of the MR signals S _{1 to} S _N can be suppressed (signal strength curve c). Therefore, according to the method of the present embodiment, it is possible to arrange a relatively small MR signal in the high frequency region of the k space, and thus it is possible to reduce the contour emphasis of the imaging region.

In this embodiment, the flip angle reduction width α _n −α _{n + 1} is set so as to satisfy the expression (1). However, it is also conceivable to set the reduction width α _n −α _{n + 1} of the flip angle so that the following expression (3) holds.
α _n −α _{n + 1} = α _{n + 1} −α _{n + 2} > 0 (3)
However, 1 ≦ n ≦ N−2

FIG. 8 is a diagram showing the flip angles α _{1 to} α _N when the flip angle reduction width α _n −α _{n + 1} is set so as to satisfy the expression (3).

In FIG. 8, a curve F and a straight line L are shown.
A curve F shown in FIG. 8 shows the curve F shown in FIG. 4 (the flip angles α _{1 to} α _N when the flip angle reduction width α _n −α _{n + 1} is set so as to satisfy the expression (1)). . On the other hand, the straight line L indicates the flip angles α _{1 to} α _N when the flip angle decrease width α _n −α _{n + 1} is set so as to satisfy the expression (3).

When the flip angle reduction width α _n −α _{n + 1} is set so that Expression (3) is satisfied (straight line L), contour enhancement of the imaging region may not be sufficiently reduced. The reason for this will be described below with reference to FIG.

FIG. 9 is an explanatory diagram showing the reason why the contour enhancement of the imaging region cannot be sufficiently reduced.
FIG. 9 shows a curve F and a straight line L shown in FIG. Further, below the curve F and the straight line L, a T1 recovery curve a during the execution of the imaging sequence B is shown.

As shown by the T1 recovery curve a, the T1 recovery speed is fast immediately after the start of the imaging sequence B, but the T1 recovery speed becomes slower as time elapses. Accordingly, when the flip angle decrease width α _n −α _{n + 1} is set so as to satisfy the expression (3) (straight line L), the flip angle decrease width α _n −α _{n + 1} is the same regardless of whether the T1 recovery speed is high or low. Therefore, the signal intensity in the high frequency region cannot be sufficiently reduced, and the contour enhancement may not be sufficiently reduced.

On the other hand, in this embodiment, since the flip angle decrease width α _n −α _{n + 1} is set so as to satisfy the expression (1) (curve F), when the T1 recovery speed is high, the flip angle decrease width α _n. When −α _{n + 1} is increased and the T1 recovery speed is low, the flip angle decrease width α _n −α _{n + 1} can be decreased. Therefore, since the reduction width of the flip angle is set in accordance with the characteristics of the T1 recovery speed, the contour emphasis can be sufficiently reduced.

In this embodiment, the flip angle is set so as to satisfy the formula (1). However, as a calculation formula for calculating the flip angle satisfying the formula (1), for example, the following formula can be used.

For example, when α1 = 45 ° and N = 40, the equation (4) can be expressed by the following equation (5).

  FIG. 10 shows the flip angle when setting rd_factor = 1, rd_factor = 0.5, and rd_factor = 0.1 in equation (5).

  Note that how much the edge enhancement is reduced depends on the value of rd_factor and the T1 value of the imaging region. To verify this, two simulations E1 and E2 were performed. Below, each simulation is demonstrated.

(1) About the simulation E1 The simulation conditions of the simulation E1 are as follows.
T1 = 150ms
ADC (Apparent Diffusion Coefficient) = 2.0 * 10 ⁻³ mm ² / sec
b-value = 1000 sec / mm ²

FIG. 11 is a diagram illustrating a result of the simulation E1.
11A shows a simulation result when rd_factor = 1, FIG. 11B shows a simulation result when rd_factor = 0.5, and FIG. 11C shows a simulation result when rd_factor = 0.1. .

11 (a) to 11 (c) show signal strength curves representing temporal changes in signal strength of the MR signal when the imaging sequence B is executed, and the lower row shows the MR signal. An image signal obtained by reconstruction is shown.
Comparing the image signals of FIGS. 11A to 11C, it can be seen that the edge enhancement is reduced as the rd_factor decreases.

(2) About simulation E2 In simulation E2, T1 value was set longer than simulation E1. Specific simulation conditions for the simulation E2 are as follows.
T1 = 920ms
ADC = 0.87 * 10 ⁻³ mm ² / sec
b-value = 1000 sec / mm ²

FIG. 12 is a diagram showing the result of simulation E2.
12A shows a simulation result when rd_factor = 1, FIG. 12B shows a simulation result when rd_factor = 0.5, and FIG. 12C shows a simulation result when rd_factor = 0.1. .
12A to 12C show image signals obtained by reconstructing MR signals.

  Comparing the image signals of FIGS. 12A to 12C, it can be seen that when rd_factor = 0.5, the edge enhancement is reduced. However, when rd_factor = 0.1, it can be seen that the edge of the image signal is curved, so that the outline of the image is blurred.

  Therefore, when the results of simulations E1 and E2 are compared, it is desirable to decrease rd_factor when the T1 value is small, but it is desirable that rd_factor should not be too small when the T1 value is large. It is done.

  Further, as described above, the optimum rd_factor value for reducing the contour enhancement differs depending on the T1 value. Therefore, when scanning the subject, the operator sets the optimum rd_factor according to the T1 value of the imaging region. It is desirable to be able to choose.

FIG. 13 is a diagram illustrating an example of an MR apparatus suitable for an operator to select an optimal rd_factor.
The control unit 8 of the MR apparatus 200 in FIG. 13 has a lookup table 81. The look-up table 81 defines the correspondence between the value of rd_factor and the T1 value. In addition, the control unit 8 includes setting means 82 and calculation means 83. In the MR apparatus 200 of FIG. 13, the operation unit 9 inputs the T1 value of the imaging region in accordance with the operation of the operator. When the T1 value is input by the operation unit 9, the setting unit 82 specifies the value of rd_factor corresponding to the input T1 value from the look-up table 81, and sets the specified value of rd_factor in the formula (4). Set as the value of rd_factor. The calculation unit 83 substitutes the value of rd_factor set by the setting unit 82 into Equation (4), and calculates the flip angles α _{1 to} α _N of the RF pulse of the imaging sequence B. Therefore, the flip angles α _{1 to} α _N of the RF pulse of the imaging sequence B can be set to values suitable for the T1 value of the imaging region. Instead of inputting the T1 value, the value of rd_factor may be directly input.

In this embodiment, the flip angle reduction width α _n −α _{n + 1} is set so that the flip angles α _{1 to} α _N satisfy the expression (1). However, if the edge enhancement can be reduced, it is not always necessary that all the flip angles α _{1 to} α _N satisfy Expression (1), and some of the flip angles do not satisfy Expression (1). Also good. FIG. 14 shows an example in which some flip angles do not satisfy Expression (1). In FIG. 14, the flip angles α _{1 to} α _N-3 are set so as to satisfy the formula (1), but the remaining flip angles α _N-2 , α _N-1 , and α _N are expressed as α _{N -2} = [alpha] _N-1 = [alpha] _N. In FIG. 14, some flip angles do not satisfy Expression (1), but all the flip angles α _{1 to} α _N do not necessarily satisfy Expression (1) as long as the edge enhancement can be reduced.

2 Magnet 3 Table 3a Cradle 4 Receiving coil 5 Transmitter 6 Gradient magnetic field power supply 7 Receiver 8 Control unit 9 Operation unit 10 Display unit 11 Subject 21 Bore 81 Look-up table 82 Setting means 83 Calculation means 100, 200 MR apparatus

Claims (7)

A magnetic resonance apparatus that executes a second sequence having a plurality of RF pulses for generating a magnetic resonance signal arranged in k-space after executing a first sequence having a gradient pulse for performing diffusion weighting There,
Of the plurality of RF pulses, a flip angle of a first RF pulse, a flip angle of a second RF pulse applied next to the first RF pulse, and an application next to the second RF pulse. The flip angle of the third RF pulse is set so as to satisfy the following condition.
α _n −α _{n + 1} > α _{n + 1} −α _{n + 2} > 0
Here, α _n : flip angle of the first RF pulse α _{n + 1} : flip angle of the second RF pulse α _{n + 2} : flip angle of the third RF pulse   The flip angle of each of the first RF pulse, the second RF pulse, and the third RF pulse is set to the flip angle of the RF pulse applied first among the plurality of RF pulses and finally applied. The magnetic resonance apparatus according to claim 1, wherein the magnetic resonance apparatus is set based on a ratio to a flip angle of an RF pulse to be performed.   The magnetic resonance apparatus according to claim 2, further comprising setting means for setting the ratio value. Having an input means for inputting the T1 value of the imaging region;
The setting means includes
The magnetic resonance apparatus according to claim 3, wherein the ratio value is set based on a T1 value input by the input unit.   The magnetic resonance apparatus according to claim 4, further comprising a calculation unit that calculates a flip angle of each of the plurality of RF pulses based on the value of the ratio set by the setting unit. The magnetic resonance apparatus according to claim 3, wherein the flip angle α _n of the first RF pulse is obtained by the following equation.
The magnetic resonance according to claim 1, wherein the second sequence includes another RF pulse for shifting an MR signal to a steady state before the plurality of RF pulses. apparatus.