JP2013192578A - Magnetic resonance imaging apparatus and program - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an MRI apparatus capable of three-dimensional imaging to compute a T1 value as a time constant for the recovery of longitudinal magnetization.SOLUTION: The magnetic resonance imaging apparatus (10) collects echo data from a subject and reconstructs a three-dimensional image. The MRI apparatus includes: a first sequence (S01 and S02) for acquiring first image data of the subject in a steady state in which a constant longitudinal magnetization signal intensity is generated; a second sequence (S03 and S04) for acquiring second image data of the subject with a plurality of high-frequency excitation pulses following one high-frequency inversion pulse; and a T1 computing part (173) for computing the T1 value based on difference (S05) between the first image data and the second image data.

Description

  The present invention relates to an MRI (magnetic resonance imaging) apparatus that repeats a data acquisition sequence for collecting echoes and generates an image, and a program installed in the magnetic resonance imaging apparatus.

  The MRI apparatus generates an image of a subject using the contrast of an image, such as a T1 (vertical relaxation) weighted image, a T2 (lateral relaxation) weighted image, or a PD (proton density) weighted image. Since these images are contrasts and the absolute values of T1, T2, and PD are not known, diagnosis requires knowledge and experience of a radiologist. Therefore, a method for calculating the T1 value that is a time constant of longitudinal magnetization recovery is required. A method of calculating the T1 value with a thick slice of 2D (Dimention) is already known for relatively homogeneous sites such as the heart and liver. For example, in the MRI apparatus disclosed in Patent Document 1, in order to calculate the T1 value, a method is proposed in which a signal intensity that is actually a negative value is returned from the longitudinal magnetization signal intensity based on the absolute value. Further, as described in the background art of Patent Document 1, in order to calculate the T1 value, a Look-Locker that actually images until the longitudinal magnetization recovers to T1 after a 180 degree pulse (IR pulse: inversion recovery pulse). There is a law.

JP 2006-149563 A

  However, in the method disclosed in Patent Document 1, it is necessary to apply a 180-degree pulse (IR pulse) after sufficiently waiting for longitudinal magnetization recovery, and imaging takes time. Therefore, the method disclosed in Patent Document 1 is limited to 2D imaging. In particular, in a region such as an intracranial region, a cervical region, or a limb joint, 3D imaging is required because a partial volume effect becomes a problem. In order to perform 3D shooting, it is necessary to shorten the shooting time.

  Accordingly, the present invention provides an MRI apparatus capable of 3D imaging in order to calculate a T1 value that is a time constant of longitudinal magnetization recovery. The program necessary for the shooting is also provided.

  A magnetic resonance imaging (MRI) apparatus according to a first aspect collects echo data from a subject and reconstructs a two-dimensional image or a three-dimensional image. The MRI apparatus includes a first sequence for acquiring first image data of a subject in a steady state in which a constant longitudinal magnetization signal intensity is generated, and a subject with a plurality of high-frequency excitation pulses following a single high-frequency inversion pulse. A second sequence for acquiring the second image data, and a T1 calculation unit for calculating a T1 value based on the difference between the first image data and the second image data.

In the MRI apparatus according to the second aspect, the high-frequency excitation pulse has a flip angle of 3 ° to 30 °.
In the MRI apparatus of the third aspect, the steady state is a state in which the signal intensity is constant by applying a high-frequency excitation pulse having a predetermined flip angle a plurality of times.

The T1 calculation unit of the MRI apparatus according to the fourth aspect includes a time reduction effect by a plurality of high-frequency excitation pulses by using a difference between the first image data and the plurality of second image data and logarithmizing and linear least squares. A logarithmic calculator that calculates the constant T1 * value and a correction calculator that corrects and calculates the time constant T1 * value to calculate the T1 value.
The correction calculation unit of the MRI apparatus according to the fifth aspect corrects the time constant T1 * value based on the flip angle of the high-frequency excitation pulse.
The correction calculation unit of the MRI apparatus of the sixth aspect corrects the time constant T1 * value based on the first flip angle α of the high-frequency excitation pulse and the second flip angle r × α that is a predetermined multiple of the first flip angle. .

The correction calculation unit of the MRI apparatus according to the seventh aspect corrects the time constant T1 * value based on the actually obtained flip angle of the high frequency excitation pulse.
The correction calculation unit of the MRI apparatus according to the eighth aspect provides a first image of the subject in two steady states at a first flip angle α of the high-frequency excitation pulse and a second flip angle r × α that is a predetermined multiple of the first flip angle. The time constant T1 * value is corrected based on the data. The different flip angles may be 3 or more, and the first image data of the subject in different steady states may be 3 or more. In the correction calculation unit, the different flip angles are 3 ° to 30 °.

The MRI apparatus of the ninth aspect reconstructs an image using a part of the nth second image data when reconstructing the n + 1th second image of the subject.
In the MRI apparatus of the tenth aspect, the echo time for acquiring the second image data of the subject by a plurality of high-frequency excitation pulses is equal to the time when the phase due to the chemical shift of fat overlaps with water.

In the MRI apparatus according to the eleventh aspect, the IR interval between the nth high frequency inversion pulse and the n + 1 th high frequency inversion pulse is set equal to or less than the expected T1 value.
In the MRI apparatus of the twelfth aspect, the time interval between the nth radio frequency excitation pulse and the (n + 1) th radio frequency excitation pulse is 4 msec to 30 msec.
In the MRI apparatus according to the thirteenth aspect, the time interval between the nth second image and the (n + 1) th second image is variable, and the time interval is shortened when the change in the longitudinal magnetization signal intensity per unit time is large. When the change in the longitudinal magnetization signal intensity per unit time is small, the time interval is lengthened.

In the MRI apparatus according to the fourteenth aspect, the time interval between the high frequency inversion pulse and the first high frequency excitation pulse is 100 milliseconds or more.
The MRI apparatus according to the fifteenth aspect applies a killer pulse that generates a gradient magnetic field that causes transverse magnetization to disappear before or after the high-frequency inversion pulse.
The flip angle of the high frequency inversion pulse is 80 ° to 280 °. The flip angle of the high frequency inversion pulse is 180 °.

  A program for an MRI apparatus according to a sixteenth aspect is installed in a calculation unit of a magnetic resonance imaging apparatus that collects echo data from a subject and reconstructs a three-dimensional image. The program includes a first acquisition process for acquiring first image data of a subject in a steady state in which a constant longitudinal magnetization signal intensity is generated, and a plurality of high-frequency excitation pulses followed by a single high-frequency inversion pulse. The calculation unit is caused to execute a second acquisition process for acquiring the second image data of the specimen and a T1 calculation process for calculating a T1 value based on a difference between the first image data and the second image data.

  According to the magnetic resonance imaging apparatus and program of the present invention, by measuring the T1 value, which is the time constant of longitudinal magnetization recovery, the identity of the tissue of the subject can be determined from the absolute value of the T1 value. It can be.

1 is a schematic configuration diagram of a magnetic resonance imaging apparatus 10 of the present embodiment. 10 is a flowchart of a T1 value calculation unit 173. It is a first sequence pulse SQ1 of a gradient echo for obtaining a steady-state signal intensity Sstst. (A) is the second sequence pulse SQ2 of the gradient echo. (B) is a conceptual diagram for calculating the signal intensity S (TI (n)) based on a plurality of echo data. It is a graph of T1 relaxation time which returns to the steady state stst from the excitation state start by 2nd sequence SQ2. (A) is a second sequence SQ2 including a pulse having a flip angle α0 degrees and a pulse having a flip angle r × α0 degrees. (B) is a second sequence SQ2 including a pulse having a TR interval TR3 (TR0) and a pulse having a TR interval TR5 (r × TR0). The first sequence SQ1 includes a flip angle r1 × α ₀ degrees and a flip angle r2 × α ₀ degrees. It is the figure which showed view sharing. It is a conceptual diagram where the inversion time interval TS between the signal strength S (TI _(n) ) and the signal strength S (TI _{(n + 1)} ) gradually increases. It is a conceptual diagram where the inversion time interval TS between the signal intensity S (TI _(n) ) and the signal intensity S (TI _{(n + 1)} ) gradually decreases.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. The technical scope of the present invention is not limited to these forms.
<Configuration of magnetic resonance imaging apparatus>

  FIG. 1 is a schematic configuration diagram of a magnetic resonance imaging apparatus 10 of the present embodiment. With reference to FIG. 1, the configuration and basic operation of the magnetic resonance imaging apparatus 10 of the present embodiment will be described.

  The magnetic resonance imaging apparatus 10 of the present embodiment includes a magnet system 100, a gradient coil drive unit 130, an RF coil drive unit 140, a data collection unit 150, a sequence control unit 160, a calculation unit 170, a display unit 180, and an operation unit 190. .

  The magnet system 100 includes a main magnetic field coil unit 102, a gradient coil unit 106, and an RF coil unit 108. Each of these coil portions has a substantially cylindrical shape, and is arranged coaxially with each other in a substantially columnar bore. The subject HB is placed on the bed 110 in the bore, and the bed 110 can move in the bore in the magnet system 100 according to the imaging region.

  The main magnetic field coil unit 102 forms a static magnetic field in the internal space of the magnet system 100. The direction of the static magnetic field is generally parallel to the direction of the body axis of the subject HB and forms a horizontal magnetic field. The main magnetic field coil unit 102 is normally configured using a superconducting coil, but may be configured using a permanent magnet or the like without being limited to the superconducting coil.

  The gradient coil unit 106 has three types of gradients for imparting gradients to the static magnetic field strength formed by the main magnetic field coil unit 102 in the three axes orthogonal to each other, that is, in the direction of the slice axis, the phase axis, and the frequency axis. Generate a magnetic field. The gradient coil unit 106 has three gradient coils (not shown) for the slice axis, the phase axis, and the frequency axis. In order to enable the generation of such a gradient magnetic field, the gradient coil unit 106 is connected to a gradient coil drive unit 130 having three systems of drive circuits. The gradient coil driving unit 130 gives a drive signal to the gradient coil unit 106 to generate a gradient magnetic field.

  The gradient magnetic field in the slice axis direction is called a slice gradient magnetic field, the gradient magnetic field in the phase axis direction is called a phase encode gradient magnetic field (or phase encode gradient magnetic field), and the gradient magnetic field in the frequency axis direction is read out gradient magnetic field (or frequency encode gradient). Magnetic field).

  In the three-dimensional orthogonal coordinate system, when coordinate axes orthogonal to each other in the static magnetic field space are an X axis, a Y axis, and a Z axis, any of the axes can be a slice axis. In the present embodiment, the slice axis is the direction of the body axis of the subject HB is the Z-axis direction, one of the remaining two axes is the phase axis, and the other is the frequency axis. Note that the slice axis, the phase axis, and the frequency axis can have an arbitrary inclination with respect to the X, Y, and Z axes while maintaining the orthogonality therebetween.

  The RF coil unit 108 forms a high-frequency magnetic field for exciting spins in the body of the subject HB in the static magnetic field space. Formation of a high-frequency magnetic field is called transmission of an RF excitation signal, and the RF excitation signal is called an RF pulse. An RF coil drive unit 140 is connected to the RF coil unit 108, and the RF coil drive unit 140 sends a drive signal to the RF coil unit 108 to transmit an RF pulse. An electromagnetic wave generated by the excited spin, that is, a magnetic resonance signal is received by the RF coil unit 108. A data collection unit 150 is connected to the RF coil unit 108. The data collection unit 150 collects echo data (or MR reception signal) received by the RF coil unit 108 as digital data.

  The magnetic resonance signal detected by the RF coil unit 108 and collected by the data collecting unit 150 becomes a signal in the frequency domain (frequency domain), for example, Fourier space. Since the magnetic resonance signal is encoded in two axes by the gradient in the phase axis direction and the frequency axis direction, the magnetic resonance signal is obtained as a signal in a two-dimensional Fourier space, for example, when the frequency space is illustrated in Fourier space. The two-dimensional Fourier space is also referred to as k-space. The phase encoding and magnetic field encoding (readout) gradient fields determine the sampling position of the signal in two-dimensional Fourier space. If the magnetic resonance signal is encoded in three axes, it can be obtained as a signal in a three-dimensional Fourier space. 3D imaging is performed by acquiring the k-space of the three-dimensional Fourier space. An image created from the 3D k-space can be measured with an accurate absolute value of the T1 value with little per share volume effect.

  The sequence control unit 160 is connected to the gradient coil drive unit 130, the RF coil drive unit 140, and the data collection unit 150. The sequence control unit 160 drives the gradient coil driving unit 130 and the RF coil driving unit 140 in accordance with the imaging conditions input by the operator, that is, the imaging protocol.

  The display unit 180 is configured by a graphic display or the like. The display unit 180 is connected to the calculation unit 170. The display unit 180 can display an operation screen, an image reconstructed image, and the like.

  The operation unit 190 includes a keyboard or the like equipped with a pointing device. The operation unit 190 is connected to the calculation unit 170. The operation unit 190 is operated by the operator via the display unit 180. The operation unit 190 may arrange a touch panel on the display unit 180 instead of a keyboard or the like.

  The main configuration of the calculation unit 170 includes a storage unit 172 and a T1 value calculation unit 173, and various data processing, T1 value calculation, and program execution are performed. The storage unit 172 stores the calculated value or stores a predetermined RF pulse sequence. That is, a program for calculating the T1 value is installed in the calculation unit 170, and the magnetic resonance imaging apparatus 10 collects echo data from the subject and reconstructs a three-dimensional image.

  The T1 value calculation unit 173 includes a logarithmic calculation unit 173L and a correction calculation unit 173R. The T1 value calculation unit 173 calculates a T1 value from three-dimensional k-space data obtained in a predetermined sequence. Three-dimensional k-space data, various imaging protocols, and various programs are stored in the storage unit 172, and are read and written as appropriate. The following shows a method for calculating the T1 value by the T1 value calculating unit 173 of the present embodiment.

<Calculation method of T1 value>
FIG. 2 is a flowchart of the T1 value calculation unit 173.

  In step S01, the arithmetic unit 170 reads the first sequence SQ1, and applies an RF pulse via the RF coil driving unit 140 (see FIG. 1). The first sequence SQ1 is a gradient echo system sequence in which a 180-degree pulse (IR pulse) is not applied. FIG. 3 shows a pulse sequence of a gradient echo. As shown in the drawing, for example, a pulse PL1 having a flip angle of α1 degree is used, and the TR (repetition time) interval is TR1 for repeated imaging. TR1 is set to 30 msec or less.

  The first sequence SQ1 uses a 3D gradient echo imaging sequence to collect three-dimensional k-space data. The sequence of the 3D gradient echo system is, for example, GRASS (gradient recalled acquisition with steady state), SPGR (spoiled GRASS), or balancedSSFP (steady state free precession).

  Further, the flip angle α1 degree of the high frequency excitation pulse PL1 of the first sequence SQ1 is, for example, 3 ° to 30 °. If the flip angle of the pulse PL1 of the first sequence is too small, the signal-to-noise ratio (SNR) decreases. If the flip angle is too large, the flip angle deviates from the ernst angle. For this reason, the flip angle of the α-degree pulse is preferably 3 ° to 30 °, and more preferably 5 ° to 10 °.

  In the lower stage, the timing of the phase encoding PE1 is illustrated. The phase encoding PE1 is performed twice for each pulse PL1, ie, the phase encoding PE1a and the phase encoding PE1b. The phase encode PE1a and the phase encode PE1b are reversed in phase, and the opposite phase phase encode PE1b is performed so that the first phase encode PE1a does not affect the next pulse PL1. The first sequence SQ1 can also use a pulse PL11 of r × α1 degrees with the flip angle set to r times.

  Returning to FIG. 2, in step S02, the T1 value calculation unit 173 calculates the longitudinal magnetization signal strength of the signal strength S in a steady state (steady state) such as a flip angle of α1 degree or r × α1 degree according to the first sequence. . The T1 value calculation unit 173 can calculate the longitudinal magnetization signal strength of the steady-state signal strength Sstst in a short time. The calculated longitudinal magnetization signal strength of the steady-state signal strength Sstst is stored in the storage unit 172.

  In step S03, the T1 value calculation unit 173 calls the second sequence SQ2. The second sequence SQ2 is a gradient echo pulse sequence as shown in FIG. As illustrated, the second sequence SQ2 uses a 180-degree pulse IRp and a high-frequency excitation pulse PL2 having a flip angle of α2 degrees. The 180 degree high frequency inversion pulse IRp is repeatedly transmitted at an IR (inversion recovery) interval IRt, and the pulse PL2 is repeatedly transmitted at a TR interval TR2. Let WT be the waiting time until the pulse PL2 to be transmitted next to the 180-degree pulse IRp. In the lower stage, the timing of the phase encoding PE2 is illustrated. The second sequence SQ2 uses the same pulse as the first sequence SQ1. The first sequence SQ1 and the second sequence SQ2 are the difference in presence / absence of the 180-degree pulse IRp and the difference in the number of pulses PL1 or the number of pulses PL2.

  The flip angle α2 degrees of the pulse PL2 of the second sequence SQ2 is also set similarly to the flip angle α1 of the first sequence SQ1.

  In addition, an IR (inversion recovery) interval IRt, which is an interval between the 180 degree pulse IRp and the 180 degree pulse IRp, is a short time, which is about the same as or less than the T1 value (for example, about 1 second). Since the IR interval IRt is short, the TR interval TR2 is inevitably short, and TR2 is set to 30 msec or less. TR2 is desirably a time sufficiently shorter than the T1 value, for example, 4 to 30 msec, and is preferably 4 to 8 msec so that the T1 shortening effect does not become too strong. For this reason, the second sequence enables high-speed shooting. In addition, the interval WT1 between the 180 degree pulse IRp and the first pulse PL2 is preferably 100 msec or more in order to reduce the influence of transverse magnetization. The interval WT2 between the last pulse PL2 and the next 180-degree pulse IRp is preferably 100 msec or more in order to reduce the influence of transverse magnetization.

  In the second sequence, the pulse PL2 is applied to the subject many times without waiting for sufficient recovery of T1, and therefore the T1 value becomes small due to the T1 shortening effect. This reduced T1 value is defined as a T1 * value that is a time constant for longitudinal magnetization recovery. The correction calculation unit 173R of the T1 value calculation unit 173 corrects the T1 * value and calculates an accurate T1 value. A method of correcting the T1 * value and calculating an accurate T1 value will be described later.

  Although not shown, TE (echo time) of the second sequence avoids the influence of fat chemical shift and acquires an in-phase image in which the phases of water and fat are aligned. For this reason, it is preferable that the time when the phase due to the chemical shift of fat overlaps with water is 4.6 msec.

Returning to FIG. 2, in step S04, the T1 value calculator 173 calculates the signal strength S (TI (n)). Specifically, as shown in FIG. 4B, echo data is acquired every time the pulse PL2 is applied to the subject in the second sequence. That is, the RF coil unit 108 applies the RF pulse PL2 based on the second sequence SQ2, and the data collection unit 150 collects echo data based on the RF pulse PL2. Then, the T1 value calculation unit 173 calculates the signal intensity S (TI (n)) based on the plurality of echo data. In FIG. 4B, the T1 value calculation unit 173 fills the k space based on the three echo data, and longitudinal magnetization signal strengths S (TI ₁ ), S (TI ₂ ), S (TI ₃ ),. S (TI (n))... Is calculated. The calculated longitudinal magnetization signal strengths S (TI ₁ ), S (TI ₂ ), etc. are equal in the inversion time interval TS between them.

  FIG. 5 is a graph of T1 relaxation time for returning from the excited state to the steady state by the second sequence SQ2. The vertical axis represents the longitudinal magnetization signal intensity Sstst in the steady state from the longitudinal magnetization signal intensity Sstart in the excited state, and the horizontal axis represents time t. In the transition state Stra, the longitudinal magnetization signal intensity S (TI (n)) is calculated many times. For example, an image is reconstructed based on echo data obtained by three times of the pulses PL2, and the longitudinal magnetization signal intensity S (TI (n)) at the inversion time TI (n) is calculated. The echo data obtained by the pulse PL2 of the inversion time TI (n-1) and the inversion time TI (n + 1) fills the signal around the k space, and the echo data obtained by the pulse PL2 of the inversion time TI (n) is k. Fill the signal in the middle of the space.

Returning to FIG. 2, in step S05, the T1 value calculation unit 173 subtracts the signal intensity S (TI (n)) calculated in step S04 from the steady-state signal intensity Sstst calculated in step S02.
In step S06, the T1 value calculation unit 173 calculates a T1 * value.
In step S07, the T1 value calculation unit 173 corrects the T1 * value and calculates the T1 value.

  The following shows details of the T1 value calculation method of the T1 value calculation unit 173 in steps S05 to S07.

The signal intensity S (TI (n)) at the nth inversion time TI (n) can be expressed by Equation 01. Equation 01 is a Look-Locker signal strength equation. The T1 * value is a value obtained by reducing the T1 value due to the shortening effect.

... Formula 01

  In the T1 value calculation method of this embodiment, the steady-state signal strength Sstst is calculated in the first sequence SQ1 in step S02 of FIG. Further, the signal intensity S (TI (n)) is calculated in the second sequence of step S04.

In step S05 of FIG. 2, the logarithmic calculation unit 173L of the T1 value calculation unit 173 subtracts the signal intensity S (TI (n)) of the second sequence from the steady-state signal intensity Sstst of the first sequence, and calculates the difference. Take a log of 01. Then, Formula 01 becomes Formula 02.

... Formula 02

  In Equation 02, the steady-state signal strength Sstst is known. For this reason, in step S06, the T1 value calculation unit 173 can easily calculate the excited state Sstart and the T1 * value by using the two-variable linear least square method using Equation 02. The signal intensity Sstart in the excited state may be the same value for each IR interval IRt, and it is not necessary to take much time for T1 to recover. Further, even if the signal intensity S (TI (n)) is small, Sstst-S (TI (n)) in Expression 02 has a large value, and thus the signal noise rate SNR is good. Accordingly, the T1 value calculation unit 173 can calculate the T1 * value using the second sequence having a short IR interval IRt.

If the T1 * value is calculated without obtaining the steady-state signal strength Sstst in the first sequence with the formula 01, the following problem occurs.
When calculating the signal intensity S (TI (n)), if the value of the signal intensity S (TI (n)) is close to 0, the measurement error is large, so the inversion time TI needs to be as long as possible. For example, it is necessary to calculate the signal strength S (TI (n + m)) close to the steady state signal strength Sstst at the inversion time TI (n + m). Since the inversion time TI (n + m) is a time for sufficiently recovering T1, the IR interval IRt shown in FIG. 4 is about 3 to 5 times the T1 value. When the IR interval IRt is increased, the imaging time is increased, which is not suitable for 3D imaging. In addition, since the three variables of the excited state Sstart, the steady state signal strength Sstst, and the T1 * value are unknown, it is necessary to obtain Equation 01 by a nonlinear least square method or the like. In addition, since the mathematical formula 01 has three variables, there is a disadvantage that it is easily affected by measurement errors.

<Correction method>
The calculated T1 * value is affected by the T1 shortening effect and is not an accurate T1 value. Therefore, the correction calculation unit 173R of the T1 value calculation unit 173 corrects the T1 * value and calculates an accurate T1 value. Hereinafter, the correction method performed in step S07 will be described.

For example, when the 3D gradient echo sequence is SPGR, the T1 * value that has received the T1 shortening effect in the transition state can be expressed by Equation 03.

... Formula 03

When the α angle of the flip angle is sufficiently small, Expression 03 becomes Expression 04 by linear approximation.

... Formula 04
Equation 05 can be derived from Equation 03 and Equation 04.

... Formula 05

  Since the calculated T1 * value, α angle of the flip angle, and TR interval of the RF pulse are known, the correction calculation unit 173R can calculate the T1 value based on Expression 05.

When the 3D gradient echo system sequence is Balanced SSFP, the time constant T1 * of the longitudinal magnetization recovery subjected to the T1 shortening effect is expressed by Equation 06. Formula 06 is a formula when there is no static magnetic field inhomogeneity and RF is chopped, and the flip angle α degree is sufficiently small. Here, T2 is a lateral relaxation time.

... Formula 06
If α and T2 are already known from Equation 06, the T1 value can be calculated.

Similarly, when the sequence of the 3D gradient echo system is GRASS, the time constant T1 * value of longitudinal magnetization recovery subjected to the T1 shortening effect is expressed by Expression 07. Formula 07 is a formula when the α degree of the flip angle is sufficiently small and the TR interval is sufficiently smaller than T1.

... Formula 07
The T1 value can be calculated from Equation 07. C1 is a variable that depends on the T1 value and the T2 value that do not depend on the flip angle.

  As shown in Formula 05, Formula 06, and Formula 07, in SPGR, BalancedSSFP, and GRASS, the T1 value can be calculated by correcting the T1 * value. However, in the case of Balanced SSFP, in the case of off resonant in which static magnetic field inhomogeneity occurs, vibration occurs in the transition state Stra, and the T1 * value is not constant. In addition, when large transverse magnetization exists, vibration occurs in the SPGR transition state Stra, and the T1 value is not constant.

  However, in the case of SPGR, the vibration of the transition state Stra can be suppressed. Specifically, in order to reduce the influence of transverse magnetization, the waiting time WT1 and WT2 before and after the 180 degree pulse (IR pulse) is lengthened and the transverse magnetization is attenuated by T2, before and after the 180 degree pulse (IR pulse). The vibration of the transition state Stra can be suppressed by, for example, a method in which a large killer gradient is placed and the transverse magnetization is dephased.

  In the method in which the transverse magnetization is attenuated by T2, the waiting times WT1 and WT2 before and after the 180-degree pulse IRp are desirably about 100 msec. Further, in the method of dephasing transverse magnetization, it is desirable to dephase transverse magnetization to about 4 to 8π at one pixel before and after the IR pulse.

  Similarly, in the case of GRASS and SPGR, when TI (n) is close to 0 or the T2 value is long, vibration occurs in the transition state Stra, and the T1 * value is not constant. That is, when the 3D gradient echo system sequence is BalancedSSFP, GRASS, and SPGR, it is difficult to simply calculate the T1 value using the above-described Expression 05, Expression 06, and Expression 07. Therefore, the following correction methods 1 to 3 are applied.

(Correction method 1)
In the correction method 1, a T1 value is obtained from two types of T1 * values using sequences having different flip angles. This correction method 1 can be used without distinction between SPGR, BalancedSSFP, and GRASS.

For example, in the correction method 1, in step S <b> 03 of FIG. 2, shooting is performed with the second sequence SQ <b> 2 including the second A sequence SQ <b> 2 a and the second B sequence SQ <b> 2 b. FIG. 6A shows the second sequence SQ2 (second A sequence SQ2a, second B sequence SQ2b). In the second A sequence SQ2a, the flip angle of the pulse PL3 is α ₀ degrees, and in the second B sequence SQ2b, the flip angle of the pulse PL4 is r × α ₀ degrees. For example, r is a natural number. The second A sequence SQ2a and the second B sequence SQ2b have the same 180 degree pulse IRp, IR interval IRt, TR interval TR3, and waiting times WT3 and WT4.

The flip angle α ₀ degree of the second A sequence SQ2a and the flip angle r × α ₀ degree of the second B sequence SQ2b are assigned to the expression 05 for SPGR, the expression 06 for BalancedSSFP, or the expression 07 for GRASS, respectively. To do. Then, in Formula 05, Formula 06, or Formula 07, Formula 08 and Formula 09 are established, respectively.

... Formula 08

... Formula 09

By substituting C1 of Formula 08 into Formula 09 from the common term of C1 of Formula 08 and C1 of Formula 09, Formula 10 is obtained.

... Formula 10

  The correction calculation unit 173R can obtain the T1 value from the T1 * value based on Expression 10. In the case of balancedSSFP, the T1 value can be obtained from this equation.

Further, in the correction method 1, it is also possible to take a picture in the second sequence SQ2 including the second A sequence SQ2a and the second C sequence SQ2c in step S03 of FIG. FIG. 6B shows the second sequence SQ2 (second A sequence SQ2a, second C sequence SQ2c). In the second A sequence SQ2a, the pulse PL3 is a TR interval TR3 (TR ₀ ), and in the second C sequence SQ2c, the pulse PL5 is a TR interval TR5 (r × TR ₀ ). r is a natural number. The second A sequence SQ2a and the second C sequence SQ2c have the same 180 degree pulse IRp, IR interval IRt, flip angle α, and waiting times WT3 and WT4.

Substituting TR ₀ into TR in Equation 05 yields Equation 11.

... Formula 11
Further, when r × TR ₀ is substituted into TR of Expression 05, Expression 12 is obtained.

... Formula 12
Equation 13 can be derived from Equation 11 and Equation 12.

... Formula 13
The correction calculation unit 173R can calculate the T1 value based on Equation 13 from the calculated T1 * _0r value and T1 * ₀ value.

(Correction method 2)
The correction method 2 is a method for calculating the T1 value by obtaining the α degree of the flip angle. The flip angle has a position dependency due to the slice shape of the RF pulse in the case of 3D, the transmission sensitivity distribution of the RF transmitter coil, and the MRI apparatus of 3T (Tesla) or more due to a dielectric effect, etc. It is known.

  For example, in the case of SPGR, when the actual flip angle α degree is obtained, the variable of Equation 03 is only T1, and therefore the correction calculation unit 173R can obtain the T1 value from the T1 * value. As a method for obtaining the actual flip angle α degree, there is a double TR method. In the double TR method, the flip angle α degree can be obtained by applying a sequence in which two identical RF pulses are applied at two different TR intervals.

  Specifically, Magnetic Resonance in Medicine 57: 192-200 (2007) “Actual Flip-Angle Imaging in the Pulsed Steady State: A Method for Rapid Three-Dimensional Mapping of the Transmitted Radiofrequency Field” (author Vasily L. Yarnykh ).

(Correction method 3)
The correction method 3 is a method for calculating T1 by imaging two or more steady-state signal strengths Sstst. For example, in the correction method 3, in step S01 of FIG. 2, the first sequence SQ1 including the first A sequence SQ1a and the second B sequence SQ1b is photographed. FIG. 7 is a pulse diagram of the first A sequence SQ1a and the first B sequence SQ1b. The pulse PL6 of the first A sequence SQ1a uses a flip angle of r1 × α ₀ degrees, and the pulse PL7 of the first B sequence SQ1b uses a flip angle of r2 × α ₀ degrees. Pulse PL6 is at TR interval TR6, and pulse PL7 is also at TR interval TR6. R1 and r2 are preferably natural numbers.

Formula 14 is a signal strength formula of the signal strength Sstst in a steady state where the flip angle in SPGR is r × α ₀ degrees. M0 is the magnetization intensity of the ground state.

... Formula 14

Assuming that TR is sufficiently smaller than T1 and α ₀ degree of the flip angle is sufficiently small in Expression 14, the terms in the denominator and numerator of Expression 14 can be approximated to Expression 15, Expression 16, and Expression 17.

... Formula 15

... Formula 16

... Formula 17

When Formula 15, Formula 16, and Formula 17 are substituted into Formula 14, and approximated, Formula 18 is obtained.

... Formula 18

When formula 05 is substituted for formula 18, formula 19 is obtained.

... Formula 19

It is assumed that the first A sequence SQ1a has a flip angle of 1 × α ₀ degrees and the first B sequence SQ1b has a flip angle of r × α ₀ degrees and is photographed in two steady states. Then, when the two steady states are put into Equation 19 and their ratio is taken, the ground state magnetization intensity M0 and the angle α ₀ degrees can be erased, and Equation 20 is established.

... Formula 20

Equation 20 can be transformed into Equation 21.

... Formula 21

  The correction calculation unit 173R can obtain the T1 value from the T1 * value based on Equation 21.

(Correction method 4)
In the above-described correction method 3, and the 1A sequence SQ1a flip angle of alpha _0, flip angle has been described a case of using two kinds of flip angle of the 1B sequence SQ1b of r × α _0. Correction Method 4, the 1A sequence SQ1a the flip angle r1 × alpha _0, flip angle r2 × alpha ₀ ° of the 1B sequence SQ1b, and flip angle three flip of the 1C sequence SQ1c of r3 × alpha ₀ This is the case when using an angle. Note that r1, r2, and r3 are natural numbers.

When r shown in Equation 19 becomes three types, r1, r2, and r3, T1 / T1 * and r are included in the denominator of Equation 19, so two nonlinear least squares methods must be used. Not in. This is susceptible to measurement errors. The mathematical formula is changed so that r1, r2, and r3 are deleted. Specifically, the two variables x1 and x2 are replaced by Equation 22 and Equation 23.

... Formula 22

... Formula 23

Substituting Equations 22 and 23 into Equation 19 and reversing the numerator and denominator yields Equation 24.

... Formula 24
A cost function F of the least square method is set as shown in Equation 25.

... Formula 25

From Equation 25, there are two unknown variables, x1 and x2, which can be replaced by the linear least square method of the two variables. In this case, since there is a steady-state signal strength Sstst (r) on the numerator side of the cost function F, no divergence occurs even if the steady-state signal strength Sstst (r) is small. Therefore, it is possible to obtain x1 and x2 at which the result of partial differentiation of the cost function F by x1 and x2 is 0. Using these x1 and x2, the T1 value can be obtained from Equation 26.

... Formula 26

  Three types of flip angles, r1 × α0 degrees, r2 × α0 degrees, and r3 × α0 degrees, were used. However, if the flip angle is too small, the SNR decreases. It is preferable to use three types of sequences such that the degree is 5 degrees, r2 × α0 degrees is 10 degrees, and r3 × α0 degrees is about 15 degrees.

  As described above, the T1 value calculation method according to the correction method 3 and the correction method 4 can obtain the T1 value from the T1 * value without knowing the exact flip angle.

  <Modification 1>

  The T1 value calculation method described above may use a view sharing technique in order to shorten the shooting time.

FIG. 8 is a diagram showing view sharing.
It is assumed that the T1 value calculation unit 173 calculates the signal intensity S (TI (n)) based on, for example, echo data obtained with five pulses PL2. Based on the echo data for five times from the inversion time TI _1-2 to the inversion time TI ₁₂ , the T1 value calculation unit 173 calculates the signal intensity S (TI ₁ ). Next, the T1 value calculation unit 173 calculates the signal intensity S (TI ₂ ) based on the echo data for five times from the inversion time TI ₁₁ to the inversion time TI ₂₂ . Next, the T1 value calculation unit 173 calculates the signal intensity S (TI ₃ ) based on the echo data for five times from the inversion time TI ₂₁ to the inversion time TI ₃₂ . Similarly, the T1 value calculation unit 173 calculates the signal strength S (TI ₄ ).

The echo data of the inversion time TI ₁₁ and the inversion time TI ₁₂ is also used for calculating the signal strength S (TI ₁ ) and is also used for calculating the signal strength S (TI ₂ ). The echo data of the inversion time TI ₂₁ and the inversion time TI ₂₂ is also used for calculating the signal strength S (TI ₂ ), and is also used for calculating the signal strength S (TI ₃ ). Similarly, the echo data of the inversion time TI ₃₁ and the inversion time TI ₃₂ is used for calculating the signal strength S (TI ₃ ), and is also used for calculating the signal strength S (TI ₄ ).

Inversion interval TS of the signal strength of the signal strength S (TI ₁₎ and the inversion time _{TI 20} of the inversion time _{TI 10} S _{(TI 2),} the signal intensity S (TI ₂₎ and inversion time inversion time _{TI 20 TI 30} Is equal to the inversion time interval TS with respect to the signal strength S (TI ₃ ).

Thus, the echo data is shared when calculating the adjacent signal strength S (TI _(n) ) and signal strength S (TI _{(n + 1)} ). In this way, the imaging time for calculating the signal intensity S (TI _(n) ) is shortened.

<Modification 2>
In the second modification, T1 using the second C sequence SQd or the second D sequence SQe in which the inversion time interval TS between the adjacent signal strength S (TI _(n) ) and the signal strength S (TI _{(n + 1)} ) is not equal. Calculate the value.

  In each of the second sequences SQ shown in the above embodiment, the inversion time intervals TS are equally spaced. FIG. 9 is a conceptual diagram in which the inversion time interval TS is gradually shortened.

In the second modification, the second C sequence SQ2d is taken in step S03 of FIG. The second C sequence SQ2d is a sequence of a pulse PL8 with a TR interval TR8. The RF coil unit 108 applies an RF pulse PL8 based on the second sequence SQ2d, and the data collection unit 150 collects echo data based on the RF pulse PL8. The T1 value calculation unit 173 fills the k space based on the seven echo data, and longitudinal magnetization signal strengths S (TI ₁ ), S (TI ₂ ), S (TI ₃ ),... S (TI (n)) … Is calculated.

Echo data is acquired each time the pulse PL8 is applied to the subject in the second sequence SQ2d. Then, the T1 value calculation unit 173 calculates the signal intensity S (TI (n)) based on a plurality of echo data using the view sharing method shown in FIG. The longitudinal magnetization signal intensity S (TI ₂ ) shares the four echo data used in the longitudinal magnetization signal intensity S (TI ₁ ). The longitudinal magnetization signal intensity S (TI ₃ ) shares the three echo data used in the longitudinal magnetization signal intensity S (TI _{2 2} ). The longitudinal magnetization signal intensity S (TI ₄ ) shares the two echo data used in the longitudinal magnetization signal intensity S (TI ₃ ). By gradually reducing the echo data to be shared in this way, the reversal time interval TSb between the longitudinal magnetization signal strengths S (TI ₂ ) and S (TI ₃ ) becomes the longitudinal magnetization signal strength S (TI ₁ ). And S (TI ₁ ) are longer than the inversion time interval TSa. When the inversion time TI is long (far) from the 180-degree pulse IRp, the signal strength of Sstst-S (TI) expressed by Formula 02 is low and the signal change is small, so that the acceleration of parallel imaging is reduced. For example, it is preferable to increase the SNR by widening the inversion time interval TS.

FIG. 10 is a conceptual diagram in which the inversion time interval TS is gradually shortened. The second C sequence SQ2e is a sequence of the pulse PL9 at the TR interval TR9. The RF coil unit 108 applies an RF pulse PL9 based on the second sequence SQ2e, and the data collection unit 150 collects echo data based on the RF pulse PL9. The T1 value calculation unit 173 fills the k space based on the seven echo data, and longitudinal magnetization signal strengths S (TI ₁ ), S (TI ₂ ), S (TI ₃ ),... S (TI (n)) … Is calculated.

Echo data is acquired every time the pulse PL9 is applied to the subject in the second sequence SQ2e. Then, the T1 value calculation unit 173 calculates the signal intensity S (TI (n)) based on a plurality of echo data using the view sharing method shown in FIG. The longitudinal magnetization signal intensity S (TI ₂ ) does not use the echo data used in the longitudinal magnetization signal intensity S (TI ₁ ). The longitudinal magnetization signal intensity S (TI ₃ ) shares one echo data used in the longitudinal magnetization signal intensity S (TI _{2 2} ). The longitudinal magnetization signal intensity S (TI ₄ ) shares the two echo data used in the longitudinal magnetization signal intensity S (TI ₃ ). By gradually increasing the echo data to be shared in this way, the reversal time interval TSv between the longitudinal magnetization signal strengths S (TI ₂ ) and S (TI ₃ ) becomes the longitudinal magnetization signal strength S (TI ₁ ). And S (TI ₁ ) are shorter than the inversion time interval TS. When the inversion time TI is short (near) from the 180-degree pulse IRp, the signal strength of Sstst-S (TI) expressed by Equation 02 is high and the signal change is large. It is preferable to increase the time resolution by narrowing.

<Modification 3>
The MRI apparatus shown in the first embodiment has a 1.5T magnetic field and the IR pulse has a flip angle of 180 degrees, but there is also a high magnetic field MRI apparatus such as 7T. In a high magnetic field MRI apparatus such as 7T, the flip angle varies depending on the imaging position due to RF non-uniformity, and there are cases where an adiabatic pulse cannot be used due to restriction of human body heat generation. For this reason, in the high magnetic field MRI apparatus, the flip angle of the IR pulse is preferably 80 to 280 degrees.

DESCRIPTION OF SYMBOLS 10 ... Magnetic resonance imaging apparatus 100 ... Magnet system (102 ... Main magnetic field coil part, 106 ... Gradient coil part 108 ... RF coil part)
110 ... bed 130 ... gradient coil drive unit 140 ... RF coil drive unit 150 ... data collection unit 160 ... sequence control unit 170 ... calculation unit 172 ... storage unit 173 ... T1 value calculation unit (173L ... logarithmic calculation unit, 173R ... correction calculation Part)
180 ... Display unit 190 ... Operation unit

Claims (20)

A magnetic resonance imaging apparatus that collects echo data from a subject and reconstructs a two-dimensional or three-dimensional image,
A first sequence for acquiring first image data of the subject in a steady state in which a constant longitudinal magnetization signal intensity is generated;
A second sequence for acquiring second image data of the subject with a plurality of high-frequency excitation pulses following a single high-frequency inversion pulse;
A T1 calculator that calculates a T1 value based on a difference between the first image data and the second image data;
A magnetic resonance imaging apparatus comprising:   The magnetic resonance imaging apparatus according to claim 1, wherein the high-frequency excitation pulse has a flip angle of 3 ° to 30 °.   3. The magnetic resonance imaging apparatus according to claim 1, wherein the steady state is a state in which a high-frequency excitation pulse having a predetermined flip angle is applied a plurality of times to make the signal intensity constant. The T1 calculator is
A logarithmic calculator for calculating a time constant T1 * value including a time shortening effect by the plurality of high-frequency excitation pulses by using the difference between the first image data and the plurality of second image data as a logarithm and linear least squares. When,
A correction calculation unit for correcting and calculating the time constant T1 * value in order to calculate the T1 value;
The magnetic resonance imaging apparatus according to any one of claims 1 to 3, further comprising:   The magnetic resonance imaging apparatus according to claim 4, wherein the correction calculation unit corrects the time constant T1 * value based on a flip angle of the high-frequency excitation pulse.   The correction calculation unit corrects the time constant T1 * value based on a first flip angle α of the high-frequency excitation pulse and a second flip angle r × α that is a predetermined multiple of the first flip angle. The magnetic resonance imaging apparatus described.   The magnetic resonance imaging apparatus according to claim 5, wherein the correction calculation unit corrects the time constant T1 * value based on a flip angle of the high-frequency excitation pulse actually obtained.   The correction calculation unit is based on first image data of the subject in two steady states at a first flip angle α of the high-frequency excitation pulse and a second flip angle r × α that is a predetermined multiple of the first flip angle. The magnetic resonance imaging apparatus according to claim 4, wherein the time constant T1 * value is corrected.   The magnetic resonance imaging apparatus according to claim 8, wherein the different flip angles are 3 or more, and the first image data of the subject in different steady states is 3 or more.   The magnetic resonance imaging apparatus according to claim 8, wherein the correction calculation unit has the different flip angle of 3 ° to 30 °.   5. The image reconstruction according to claim 1, wherein when the n + 1-th second image of the subject is reconstructed, the image is reconstructed using a part of the n-th second image data. The magnetic resonance imaging apparatus described. n is an integer of 1 or more.   12. The echo time for acquiring the second image data of the subject by the plurality of high-frequency excitation pulses is equal to the time when the phase due to the chemical shift of fat overlaps with water. Magnetic resonance imaging device.   The magnetic resonance according to any one of claims 1 to 12, wherein an IR interval between the nth high frequency inversion pulse and the n + 1th high frequency inversion pulse is set to be equal to or less than an expected T1 value. Imaging device. n is an integer of 1 or more.   The magnetic resonance imaging apparatus according to claim 1, wherein a time interval between the nth radio frequency excitation pulse and the (n + 1) th radio frequency excitation pulse is 4 msec to 30 msec. n is an integer of 1 or more.   The time interval between the n-th high-frequency excitation pulse and the n + 1-th high-frequency excitation pulse is variable, and when the change in the longitudinal magnetization signal intensity per unit time is large, the time interval is shortened, and the longitudinal magnetization The magnetic resonance imaging apparatus according to claim 14, wherein the time interval is increased when a change in signal intensity per unit time is small.   The magnetic resonance imaging apparatus according to any one of claims 1 to 15, wherein a time interval between the high frequency inversion pulse and the first high frequency excitation pulse is 100 milliseconds or more.   The magnetic resonance imaging apparatus according to any one of claims 1 to 15, wherein a killer pulse that generates a gradient magnetic field that causes transverse magnetization to disappear is applied to one or both of the high-frequency inversion pulse.   The magnetic resonance imaging apparatus according to any one of claims 1 to 15, wherein a flip angle of the high-frequency inversion pulse is 80 ° to 280 °.   The magnetic resonance imaging apparatus according to claim 17, wherein a flip angle of the high frequency inversion pulse is 180 °. A program installed in a calculation unit of a magnetic resonance imaging apparatus that collects echo data from a subject and reconstructs a three-dimensional image,
A first acquisition process for acquiring first image data of the subject in a steady state in which a constant longitudinal magnetization signal intensity is generated;
A second acquisition process of acquiring second image data of the subject with a plurality of high frequency excitation pulses following a single high frequency inversion pulse;
T1 calculation processing for calculating a T1 value based on the difference between the first image data and the second image data;
A program for causing the arithmetic unit to execute.