Source: http://www.google.com/patents/US8148981?dq=7493558
Timestamp: 2017-11-18 23:20:09
Document Index: 563400666

Matched Legal Cases: ['art 41', 'art 41', 'art 41', 'art 41', 'art 41', 'art 413']

Patent US8148981 - MRI apparatus and MRI method for SSFP with center frequency and 1st order ... - Google Patents
A magnetic resonance imaging apparatus includes an input unit, a data acquisition unit and an image generating unit. The input unit inputs information indicating a matter of which resonance frequency is a center frequency of an excitation pulse. The data acquisition unit acquires magnetic resonance data...http://www.google.com/patents/US8148981?utm_source=gb-gplus-sharePatent US8148981 - MRI apparatus and MRI method for SSFP with center frequency and 1st order gradient moments zeroed
Publication number US8148981 B2
Application number US 12/336,708
Also published as US20090160440
Publication number 12336708, 336708, US 8148981 B2, US 8148981B2, US-B2-8148981, US8148981 B2, US8148981B2
Inventors Masao Yui
Patent Citations (23), Referenced by (4), Classifications (11), Legal Events (3)
MRI apparatus and MRI method for SSFP with center frequency and 1st order gradient moments zeroed
US 8148981 B2
an input unit configured to input information indicating an object undergoing magnetic resonance imaging of which the resonance frequency of the object undergoing magnetic resonance imaging is a center frequency of an excitation pulse;
a data acquisition unit configured to acquire magnetic resonance data by obtaining a steady state free precession of a magnetization in a desired object undergoing magnetic resonance imaging by applying plural excitation pulses having a same flip angle with a constant repetition time and gradient magnetic fields to an object, the plural excitation pulses each having a transmission phase varying by a variation amount determined based on a difference between a resonance frequency of a desired object undergoing magnetic resonance imaging and the center frequency defined depending on the object undergoing magnetic resonance imaging; and
an image generating unit configured to generate an image of the desired object undergoing magnetic resonance imaging based on the magnetic resonance data,
wherein a zero order moment of gradient magnetic field from each application time of the plural excitation pulses till a center time of a corresponding echo is zero, and wherein a zero order moment of gradient magnetic field from each center time of echoes till an application time of a following excitation pulse included in the gradient magnetic fields is zero.
a data acquisition unit configured to acquire magnetic resonance data by obtaining a steady state free precession of a magnetization in a desired object undergoing magnetic resonance imaging by applying plural excitation pulses having a same flip angle with a constant repetition time and gradient magnetic fields to an object, the plural excitation pulses each having a transmission phase varying by a variation amount determined based on a phase shift amount due to a fluctuation of a B0 magnetic filed; and
wherein said data acquisition unit is configured to determine respective transmission phases of the plural excitation pulses so as to vary a magnetization of at least one object undergoing magnetic resonance imaging by 2π times an integer number.
wherein said data acquisition unit is configured to apply the plural excitation pulses each having the transmission phase varying by a variation amount determined based on both the difference between the resonance frequency of the desired object undergoing magnetic resonance imaging and the center frequency and the phase shift amount due to the fluctuation of the B0 magnetic filed.
inputting information indicating a object undergoing magnetic resonance imaging of which resonance frequency is a center frequency of an excitation pulse;
acquiring magnetic resonance data by obtaining a steady state free precession of a magnetization in a desired object undergoing magnetic resonance imaging by applying plural excitation pulses having a same flip angle with a constant repetition time and gradient magnetic fields to an object, the plural excitation pulses each having a transmission phase varying by a variation amount determined based on a difference between a resonance frequency of a desired object undergoing magnetic resonance imaging and the center frequency defined depending on the object undergoing magnetic resonance imaging; and
generating an image of the desired object undergoing magnetic resonance imaging based on the magnetic resonance data,
acquiring magnetic resonance data by obtaining a steady state free precession of a magnetization in a desired object undergoing magnetic resonance imaging by applying plural excitation pulses having a same flip angle with a constant repetition time and gradient magnetic fields to an object, the plural excitation pulses each having a transmission phase varying by a variation amount determined based on a phase shift amount due to a fluctuation of a B0 magnetic filed; and
wherein respective transmission phases of the plural excitation pulses are determined so as to vary a magnetization of at least one object undergoing magnetic resonance imaging by 2π times an integer number.
wherein the plural excitation pulses each having the transmission phase varying by a variation amount determined based on both the difference between the resonance frequency of the desired object undergoing magnetic resonance imaging and the center frequency and the phase shift amount due to the fluctuation of the B0 magnetic filed are applied.
As shown in FIG. 1, the conventional SSFP sequence such as the TrueFISP sequence applies a RF excitation pulse repeatedly at a constant and short TR (repetition time) with a same excitation angle (flip angle) α to lead magnetization in a steady state quickly. The gradient magnetic field is adjusted so that the zero-order moment (time integration) becomes zero. The gradient magnetic field in a read out axis direction is controlled so that the polarity inverts several times. As a result, an obtained echo signal has a high signal to noise ratio (SNR) and a signal intensity S depends on a relaxation time of a tissue as shown in the expression (1).
S∝1/(1+T1/T2) (1)
When an angle is controlled so that each excitation angle of continuous RF pulses becomes a, and a phase is controlled so that a phase of continuous RF pulse alternates between zero degree and 180 degrees, the magnetization state alternates between the state (A) and the state (B) as shown in a vectorial representation in FIG. 2.
However, the control technique of the phase angles of excitation pulses in the conventional SSFP sequence is applicable to only the case where signals are acquired from a single matter having a certain chemical shift, and can achieve an effect only in the case in which the center frequency of the excitation pulse set as an imaging condition in the apparatus side is adjusted so as to become same as a resonance frequency of a matter to be an application target. Therefore, when the center frequency of the excitation pulse set in the apparatus side is off a resonance frequency of a matter to be an application target, a magnetization rotates about the static magnetic field direction in an interval between an application of a certain excitation pulse and the application of the next excitation pulse. In such a case, a state different from a steady state as shown in FIG. 2 will be generated.
A phase angle of the (n+1)-th RF excitation pulse differs from a phase angle of the n-th RF excitation pulse by 160 degrees. Therefore, the transverse magnetization rotates into the (n+1) position shown in FIG. 4 immediately after the application of the (n+1)-th RF excitation pulse.
FIG. 8 is a diagram showing a behavior of a transverse magnetization in a matter in case of controlling each phase angle of the respective RF excitation pulses in the SSFP sequence as shown in FIG. 7;
FIG. 17 is a flowchart showing a method for calculating a phase shift amount when automatically adjusting a center frequency of a RF excitation pulse to the resonance frequency of water.
Moreover, in the SSFP sequence shown in FIG. 7, a phase angle of each RP excitation pulse is controlled so that a difference in phase angle between adjacent RF excitation pulses becomes a constant angle π+Δφ1 which is different from π [radian] (180 degrees) This means a phase angle of each RF excitation pulse is controlled so that the relational expression shown in the expression (2) is formed when a phase angle of the n-th RF excitation pulse is denoted by φ(n)
φ(n+1)−φ(n)=π+Δφ1 [radian]
φ(n)±2π=φ(n) (2)
In the expression (2), a shift amount Δφ1, from π, of a phase angle difference it between adjacent RF excitation pulses is determined based on a TR [second] of the RF excitation pulses and a subtraction value Δf [Hz] between a set center frequency of a RF excitation pulse to be and a resonance frequency of a matter to be imaged, as shown in the expression (3) for example.
Δφ1=2π·Δf·TR (3)
When a transverse magnetization of a matter turned to the (n) position shown in FIG. 8 by application of the n-th RF excitation pulse, the transverse magnetization rotates by 2π·Δf·TR immediately before application of the (n+1)-th RF excitation pulse in case where a set center frequency of a RF excitation pulse differs from a resonance frequency of a matter to be imaged by a subtraction value Δf [Hz].
Note that, only a behavior of the transverse magnetization is shown in FIG. 8 but the same holds true for that of a longitudinal magnetization. That is, if the shift amount Δφ1 from π of the phase angle difference between adjacent RF excitation pulses is set to a rotation amount of the longitudinal magnetization due to the center frequency of the excitation pulse not being appropriately adjusted to the resonance frequency of the matter, the size of the longitudinal magnetization also becomes constant and a steady state of the longitudinal magnetization can be maintained.
In FIG. 9, RF denotes RF excitation pulses, SS denotes gradient magnetic field for slice selection in a slice axis direction, PE denotes gradient magnetic field for phase encode in a phase encode axis direction and RO denotes gradient magnetic field for readout in a readout axis direction.
φ(1)−φ(0)=π+Δφ1/2 [radian] (4)
That is, either or both of the phase angle φ(0) of the α/2 pre-pulse and the phase angle φ(1) of the first RF excitation pulse applied subsequent to the α/2 pre-pulse can be controlled so that a phase angle difference between the phase angle φ(0) of the α/2 pre-pulse and the phase angle φ(1) of the first RF excitation pulse applied subsequent to the α/2 pre-pulse becomes π+Δφ1/2. Hereat, it is preferable that a shift amount Δφ1 in phase angle is determined as the expression (3).
Hereat, in contrast to the expression (2) to control the phase angle difference between adjacent RF excitation pulses, 1/2 factor is multiplied by Δφ1 in the expression (4) This is because the time interval from the application time of the α/2 pre-pulse to the application time of the first RF excitation pulse applied subsequent to the α/2 pre-pulse is equal to TR/2 and a phase shift amount of magnetization occurring in the time interval TR/2 is 1/2 of the phase shift amount 2π·Δf·TR of magnetization occurring in the time interval TR between adjacent RP excitation pulses.
In FIG. 10, RF denotes RF excitation pulses, SS denotes gradient magnetic field for slice selection in a slice axis direction, PE denotes gradient magnetic field for phase encode in a phase encode axis direction and RO denotes gradient magnetic field for readout in a readout axis direction.
φ(END)−φ(N)=Δφ1/2 [radian] (5)
In the expression (5), φ(N) denotes a phase angle of the last N-th RF excitation pulse. That is, the phase angle φ(END) of the α/2 post-pulse can be controlled so that a phase angle difference between the phase angle φ(END) of the α/2 post-pulse and the phase angle φ(N) of the N-th RF excitation pulse applied prior to the α/2 post-pulse becomes Δφ1/2. Hereat, it is preferable that a shift amount Δφ1 in phase angle is determined as the expression (3).
As described above, the phase angle φ(n) of the n-th RF excitation pulse, the phase angle φ(0) of the α/2 pre-pulse and the phase angle φ(END) of the α/2 post-pulse can be determined based on the shift amount Δφ1 of the phase angle difference from π between adjacent RF excitation pulses as shown in the expressions (2), (4) and (5) respectively. Moreover, in order to determine the shift amount Δφ1 from the phase angle difference from π between adjacent RF excitation pulses based on the expression (3), it becomes important to accurately obtain a subtraction value Δf [Hz] between the center frequency of RF excitation pulse and the resonance frequency of the matter to be imaged.
Generally, in a MRI apparatus for a human which images a human as an object, a center frequency of a RF excitation pulse is automatically-adjusted to a resonance frequency of a matter to be excited based on a frequency spectrum each time an object P is set on the MRI apparatus. For that reason, also in the magnetic resonance imaging apparatus 20 shown in FIG. 5, a center frequency of a RF excitation pulse is automatically-adjusted to a resonance frequency of a matter to be excited, based on a frequency spectrum as shown in FIG. 11, by the center frequency adjusting part 41A. Therefore, a center frequency of a RF excitation pulse does not become constant but becomes a different value for each object P.
However, it is also possible to determine whether a center frequency of a RF excitation pulse has been automatically-adjusted to a resonance frequency of a different matter by determining whether a signal intensity of fat or water is large or not at a data acquisition timing using the difference between T1 (longitudinal relaxation) times of water and fat. In this case, the center frequency adjusting part 41A is provided with a function to determine an error recognition of a resonance frequency.
Thus, a subtraction value Δf [Hz] between the center frequency of a RF excitation pulse and a resonance frequency of a matter to be imaged can be determined depending on whether the resonance frequency was erroneously recognized, namely whether the center frequency of the RF excitation pulse was adjusted to a resonance frequency of an incorrect matter. For that reason, the imaging condition setting unit 40 is provided with a function to determine a subtraction value Δf [Hz] between the center frequency of a RF excitation pulse and a resonance frequency of a matter to be imaged after receiving information about whether a resonance. frequency was erroneously recognized, from the input device 33.
Especially when a matter to be imaged is water or fat, information about whether an adjusted center frequency of a RF excitation pulse corresponds to the resonance frequency of water or fat, that is, information representing a matter of which resonance frequency is an automatically-adjusted center frequency of an excitation pulse, is input from the input device 33 to the imaging condition setting unit 40. Thus, the imaging condition setting unit 40 can determine a subtraction value Δf [Hz] between the center frequency of a RF excitation pulse and a resonance frequency of a matter to be imaged according to matter information input from the input device 33.
For example, when a matter to be imaged is water, center frequency adjustment result information that the adjusted center frequency f0 of the RF excitation pulse corresponds to the resonance frequency f1 of water can be input from the input device 33 to the imaging condition setting unit 40 in case where the center frequency f0 of the RF excitation pulse is adjusted to the resonance frequency f1 of water. Alternatively, matter information may not input from the input device 33 to the imaging condition setting unit 40. Then, the imaging condition setting unit 40 set a subtraction value Δf [Hz] between the center frequency f0 of a RF excitation pulse and the resonance frequency f1 of water to zero as shown in the expression (6).
In the expression (7), ν denotes the difference value between the chemical shift of water and the chemical shift of fat. A value obtained by multiplying the difference value between the chemical shift of water and the chemical shift of fat by the center frequency f0 of a RF excitation pulse is set as the subtraction value Δf [Hz] between the center frequency f0 of the RF excitation pulse and the resonance frequency f1 of water.
That is, when a matter to be imaged is fat and the center frequency f0 of a RF excitation pulse is adjusted to the resonance frequency f2 of fat, center frequency adjustment result information that the adjusted center frequency f0 of a RF excitation pulse corresponds to the resonance frequency f2 of fat can be input from the input device 33 to the imaging condition setting unit 40. Then, the imaging condition setting unit 40 set a subtraction value Δf [Hz] between the center frequency f0 of the RF excitation pulse and the resonance frequency f2 of fat to zero as shown in the expression (8).
So far, a method for determining a difference between a phase angle φ(n) of the n-th RF excitation pulse and a phase angle φ(n+1) of the (n+1)-th RF excitation pulse based on a subtraction value Δf [Hz] between a resonance frequency of a matter to be imaged and the center frequency of a RF excitation pulse has been described. However, a phase angle difference of RF excitation pulses can be determined depending on a position of an excited slice in case where a resonance frequency of a matter to be imaged varies spatially.
Further, a difference between a phase angle φ(n) of the n-th RF excitation pulse and a phase angle φ(n+1) of the (n+1)-th RF excitation pulse can be determined based on a variation amount of a B0 magnetic field. If a difference between a phase angle φ(n) of the n-th RF excitation pulse and a phase angle φ(n+1) of the (n+1)-th RF excitation pulse is determined based on a variation amount of a B0 magnetic field, a steady state of a magnetization can be maintained satisfactorily with avoiding influence due to a variation of the B0 magnetic field even in a case where the B0 magnetic field changed.
Moreover, it is assumed that the n-th RF excitation pulse and the (n+1)-th RF excitation pulse are applied at t=T(n) and t=T(n+1) respectively, a phase shift amount Δφ2(n+1) of a magnetization occurring between application of the n-th RF excitation pulse and application of the (n+1)-th RF excitation pulse can be calculated as the expression (12) using a variation amount B0(t) of the BC magnetic field,
Δ ϕ 2 ( n + 1 ) = ∫ Υ ( n ) T ( n + 1 ) γ B 0 ( t ) ⅆ t ( 12 )
That is, as shown in the expressions (11) and (12), a variation amount B0(t) of the B0 magnetic field can be estimated based on a schedule G(t) for performing gradient magnetic field pulses and a phase shift amount Δφ2 in a magnetization between adjacent RF excitation pulses can be calculated based on the estimated variation amount B0(t) of the B0 magnetic field. For that reason, the imaging condition setting unit 40 is configured to calculate a phase shift amount Δφ2 of a magnetization between adjacent RF excitation pulses based on a variation amount B0(t) of the B0 magnetic field estimated in the magnetic field variation estimating part 41B and to control a phase angle of a RF excitation pulse using the calculated phase shift amount Δφ2 of the magnetization.
A phase angle of a RF excitation pulse can be controlled so as to satisfy the expression (13) like as the expression (2).
Specifically, a phase angle of each RF excitation pulse is controlled so that a phase angle difference between adjacent RF excitation pulses. becomes an angle π+Δφ2(n+1) that is different from π.
φ(n+1)−φ(n)=π+Δφ1+Δφ2(n+1) [radian] (14)
A phase angle φ(n) of a RF excitation pulse depends on the number Nex of the RF excitation pulses after the start of excitation as described above. Generally, a magnetization has not sufficiently changed to a steady state immediately after the start of excitation. For this reason, dummy RF excitation pulses are applied. Therefore, the number N of a reception signal number n is calculated by subtracting the number Ndummy of dummy RF excitation pulses from the number Nex of the RF excitation pulses after the start of excitation as shown in the expression (15).
A phase Φ(n) for detecting a reception signal to be controlled is determined depending on a phase angle φ(n) of a RF excitation pulse and a phase θ(n) of the reception signal after A/D conversion. Accordingly, a phase Φ(n) for detecting a reception signal, a phase angle φ(n) of a RF excitation pulse and a phase θ(n) of the reception signal after A/D conversion eventually depend on an order of phase. Therefore, a phase Φ(n) of detecting a reception signal differs between the sequential acquisition shown in FIG. 14 and the centric acquisition shown in FIG. 15.
First in step S10, one peak is detected from a frequency spectrum with regard to the object P by the center frequency adjusting part 41A and a center frequency f0 of a RF excitation pulse is automatically-adjusted to the frequency corresponding to the peak. The automatically-adjusted center frequency f0 of the RF excitation pulse and the frequency spectrum are output from the center frequency adjusting part 41A to the display unit 34, and the center frequency f0 of the RF excitation pulse is displayed together with the frequency spectrum on the display unit 34.
Then, when the specified matter M is water, the imaging condition setting unit 40 sets a subtraction value Δf [Hz] between the center frequency of the RF excitation pulse and the resonance frequency of the matter to be imaged to zero in step S13 since proper adjustment of the center frequency f0 has been performed. On the contrary, when the specified matter M is not water, the imaging condition setting unit 40 sets a value obtained by multiplying the difference value v between the chemical shift of water and the chemical shift of fat by the center frequency f0 of the RF excitation pulse as a subtraction value Δf [Hz] between the center frequency of the RF excitation pulse and the resonance frequency of the matter to be imaged as shown in the expression (7) in step S14.
Then in step S15, the imaging condition setting unit 40 calculates a phase shift amount Δφ1 based on the subtraction value Δf [Hz] between the center frequency of the RF excitation pulse and the resonance frequency of the matter to be imaged as shown in the expression (3).
On the other hands, when a phase angle of a RF excitation pulse based on a variation amount B0(t) of a B0 magnetic field is controlled, the variation amount B0(t) of the B0 magnetic field is estimated based on a schedule for performing a pulse sequence by the magnetic field variation estimating part 413, for example by the expression (11) Subsequently, the imaging condition setting unit 40 calculates a phase shift amount Δφ2 of a RF excitation pulse based on the estimated variation amount B0(t) of the B0 magnetic field. Then, by using the calculated phase shift amount Δφ2, an imaging condition with a SSFP sequence is set in step S1 as described above.
That is, the foregoing magnetic resonance imaging apparatus 20 as described above is an apparatus configured so as to determine and control a variation amount of a transmission phase angle of a RF excitation pulse based on a difference value between a center frequency of a RF excitation pulse and a resonance frequency of a matter to be a imaging target and/or a variation amount of a B0 magnetic field so as to maintain a steady state of a magnetization more satisfactorily even if there is a factor disturbing the steady state of the magnetization such as adjustment deviation of the center frequency of the RF excitation pulse and/or a variation of the B0 magnetic field in case of acquiring data with a SSFP sequence.
Therefore, according to the magnetic resonance imaging apparatus 20, both a transmission phase angle of a RF excitation pulse and a phase angle of a magnetization are controlled regularly and steady state free precession of the magnetization can be maintained. In addition, a time until a magnetization transforms to a steady state can be reduced in imaging using a SSFP sequence. As a result, a SSFP image with an improved SNR and/or contrast can be obtained.
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Cooperative Classification G01R33/56527, G01R33/561, G01R33/56563, G01R33/5614, G01R33/543
European Classification G01R33/561, G01R33/561B1