Gradient magnetic field measurement method and MRI apparatus

Details Gradient magnetic field measurement method and MRI apparatus Gradient magnetic field measurement method and MRI apparatus

2000-07-10

2002-03-26

Williams, Hezron (Department: 2862)

Electricity: measuring and testing

Particle precession resonance

Using a nuclear resonance spectrometer system

C324S318000, C324S309000

Reexamination Certificate

active

06362621

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a gradient magnetic field measurement method and an MRI (magnetic resonance imaging) apparatus, and more particularly to a gradient magnetic field measurement method and an MRI apparatus that can accurately measure a gradient magnetic field actually applied.
FIG. 1
is a diagram for explaining a gradient magnetic field measurement pulse sequence for use in a gradient magnetic field measurement method disclosed in an article entitled “Novel k-space Trajectory Measurement Technique” by Y. Zhang et al., in
Magnetic Resonance in Medicine
, 39: 999-1004 (1998).
The gradient magnetic field measurement pulse sequence J applies an excitation RF pulse R and a slice selective pulse G
s
, applies a rephasing pulse G
r
, and collects data S(
1
)-S(T) from an FID signal while applying an encoding pulse G
e
having a spiral gradient waveform.
Next, data D(
1
)-D(T−1), which have a phase difference &Dgr;&phgr; as an angle, are obtained from the collected data S(
1
)-S(T). In particular, the following calculation is performed:
D(t)=S(t)·S(t+1)*,
wherein S(t+1)* represents the conjugate complex of S(t+1).
Then, gradient magnetic field differences &Dgr;G(
1
)-&Dgr;G(T−1) are obtained from the data D(
1
)-D(T−1) having a phase difference &Dgr;&phgr; as an angle. In particular, the following calculation is performed:
Δ
⁢
 
⁢
G
⁡
(
t
)
=
arctan
⁢
 
⁢
{
D
⁡
(
t
)
}
2
⁢
π
·
γ
·
z
·
Δ
⁢
 
⁢
t
,
wherein arctan{} is the arc tangent function, &ggr; is the gyromagnetic ratio, z is the slice position on the gradient axis, and &Dgr;t is the time difference between the data S(t) and S(t+1).
Next, the gradient magnetic field differences &Dgr;G(
1
)-AG(T−1) are integrated to obtain the gradient magnetic field G(
1
)-G(T−1). In particular, the following calculation is performed:
G
⁡
(
τ
)
=
∑
t
=
1
τ
⁢
Δ
⁢
 
⁢
G
⁡
(
t
)
.
The result of the gradient magnetic field measurement is used for correcting the encoding pulse G
e
. Moreover, it is used for analyzing eddy current or remanence.
Ideally, the result of the gradient magnetic field measurement with respect to the encoding pulse G
e
shown in
FIG. 1
would be such as shown in FIG.
2
.
However, such a neat result as shown in
FIG. 2
is not obtained in practice. Particularly, randomness will occur in a latter portion indicated by a broken line in FIG.
2
. This is because a larger encoding pulse G
e
increases the difference of gradient magnetic field strength within a sample, resulting in an observed FID signal reduced due to a phase shift generated within the sample.
FIG. 3
shows the temporal variation of an FID signal. Basically, the FID signal exponentially decreases with time, but a lot of further smaller minimum portions appear because of the phase shift generated within the sample.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a gradient magnetic field measurement method and an MRI apparatus that can accurately measure a gradient magnetic field actually applied.
In accordance with a first aspect of the invention, there is provided a gradient magnetic field measurement method comprising the steps of: applying an excitation RF pulse, applying a pre-encoding pulse P
k
, collecting data S(k,
1
)-S(k, T) from an FID signal while applying an encoding pulse G
e
having a gradient waveform to be measured, and repeating these steps K times with the magnitude of the pre-encoding pulse P
k
varied; obtaining data D(
1
,
1
)-D(
1
, T−1), D(
2
,
1
)-D(
2
, T−1), . . . , D(K,
1
)-D(K, T−1) having a phase difference &Dgr;&phgr; as an angle from the collected data S(
1
,
1
)-S(
1
, T), S(
2
,
1
)-S(
2
, T), . . . , S(K,
1
)-S(K, T); adding data having corresponding magnitudes of the encoding pulse G
e
to obtain added data d(
1
)-d(T−1); obtaining gradient magnetic field differences &Dgr;G(
1
)-&Dgr;G(T−1) from the added data d(
1
)-d(T−1); and integrating the gradient magnetic field differences &Dgr;G(
1
)-&Dgr;G(T−1) to obtain a gradient magnetic field G(
1
)-G(T−1).
According to the gradient magnetic field measurement method of the first aspect, because the pre-encoding pulse P
k
is varied in the collected data S(
1
, t), . . . , S(K, t), the magnitudes of phase are different. However, if the collected data are converted into data D(
1
, t), . . . , D(K, t) having a phase difference &Dgr;&phgr; as an angle, the data come to have corresponding magnitudes of the encoding pulse G
e
. On the other hand, because the pre-encoding pulse P
k
is varied, the magnitude of phase shift within a sample is varied and portions at which an FID signal is reduced due to the phase shift are different among the data S(
1
, t), . . . , S(K, t). That is, although an FID signal observed in a certain portion in certain data is small, it is not small in the corresponding portion in other data. Then, adding these data gives d(
1
)-d(T−1). Since a gradient magnetic field G(
1
)-G(T−1) is obtained based on such added data d(
1
)-d(T−1), the gradient magnetic field can be accurately measured.
In accordance with a second aspect of the invention, there is provided an MRI apparatus comprising RF pulse transmitting means, gradient pulse applying means, NMR signal receiving means and data processing means, wherein the RF pulse transmitting means applies an excitation RF pulse, the gradient pulse applying means applies a pre-encoding pulse P
k
followed by an encoding pulse G
e
having a gradient waveform to be measured, the NMR signal receiving means receives an FID signal while applying the encoding pulse G
e
to collect data S(k,
1
)-S(k, T), and, from the data S(
1
,
1
)-S(
1
, T), S(
2
,
1
)-S(
2
, T), . . . , S(K,
1
)-S(K, T) collected by repeating the above operation K times with the magnitude of the pre-encoding pulse P
k
varied, the data processing means obtains data D(
1
,
1
)-D(
1
, T−1), D(
2
,
1
)-D(
2
, T−1), . . . , D(K,
1
)-D(K, T−1) having a phase difference &Dgr;&phgr; as an angle, adds data having corresponding magnitudes of the encoding pulse G
e
to obtain added data d(
1
)-d(T−1), obtains gradient magnetic field differences &Dgr;G(
1
)-&Dgr;G(T−1) from the added data d(
1
)-d(T−1), and integrates the gradient magnetic field differences &Dgr;G(
1
)-&Dgr;G(T−1) to obtain a gradient magnetic field G(
1
)-G(T−1).
The MRI apparatus of the second aspect is capable of suitably implementing the gradient magnetic field measurement method as described regarding the first aspect.
In accordance with a third aspect of the invention, there is provided the gradient magnetic field measurement method as described regarding the first aspect, comprising the steps of: defining time points J
k
(k=1, . . . , K) dispersedly within a period of the encoding pulse G
e
having a gradient waveform to be measured; and determining a magnitude of the pre-encoding pulse P
k
so as to cancel an integral value of the encoding pulse G
e
from its start time point to a time point J
k
.
Checking on the time points at which an observed FID signal is reduced due to a phase shift within a sample, it is found that the time points are not concentrated at one location but are distributed over a plurality of locations.
The gradient magnetic field measurement method of the third aspect therefore defines a plurality of time points J
k
(k=1, . . . , K) distributed within a period of the encoding pulse G
e
, and determines the magnitude of the pre-encoding pulse P
k
so as to eliminate the phase shift at each time point J
k
. Thus, time points at which the FID signal is reduced due to a phase shift within the sample are differentiated among the pre-encoding pulses P
k
, and therefore the gradient magnetic field can be accurately measured from the added data.
In accordance with a fourth aspec

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