Magnetic resonance imaging apparatus, Q-value calculation method, and specific absorption rate management method
To avoid the complication of an MRI apparatus and avoid the overestimation of a calculated value of SAR without extending a processing time and to perform accurate SAR management. To this end, the MRI apparatus is equipped with a high frequency antenna which has a plurality of channels and resonates at a predetermined frequency, and a measuring instrument which measures the amplitudes of a forward traveling and reflected waves of each high frequency signal supplied to the high frequency antenna. In the MRI apparatus, a reflection matrix S is determined based on the measured amplitudes. Diagonal terms of the determined reflection matrix S are used to calculate Q values for each of the channels. Each non-diagonal term of the reflection matrix S is used to correct the calculated Q value. The corrected Q value is used to calculate irradiation power consumed in a subject among irradiation power from the high frequency signals supplied to the high frequency antenna when imaging to thereby manage a specific absorption rate.
1. A magnetic resonance imaging apparatus comprising:
a high frequency antenna which has a plurality of channels and resonates at a predetermined frequency;
a supply unit which supplies a plurality of high frequency signals having different frequencies to the high frequency antenna;
a measuring instrument which measures the amplitudes of a forward traveling wave and a reflected wave of each of the high frequency signals supplied from the supply unit to the high frequency antenna;
a Q-value calculation unit which calculates a Q value for each of the channels by fitting an absolute value of each diagonal term of a reflection matrix S calculated based on the amplitudes measured by the measuring instrument to a predetermined circuit model; and
a Q-value correction unit which corrects the Q value calculated by the Q-value calculation unit using an absolute value of each non-diagonal term of the reflection matrix S.
2. The magnetic resonance imaging apparatus according to claim 1 , wherein the Q-value correction unit estimates the ratios of energy consumed by the high frequency antenna, of energy of the high frequency signals input to the high frequency antenna, and energy thereof consumed in a subject disposed near the high frequency antenna to thereby correct the Q value.
3. The magnetic resonance imaging apparatus according to claim 1 , wherein the Q-value correction unit corrects the Q value Q appear calculated by the Q-value calculation unit in accordance with the following formula to thereby acquire a Q value Q 0i :
Q
0
i
=
Q
appear
i
(
1
+
∑
k
=
1
,
(
k
≠
i
)
k
=
M
A
∑
j
=
1
M
S
jj
2
-
n
∑
m
=
1
M
S
mm
-
n
+
B
Q
a
p
p
e
a
r
i
Q
a
p
p
e
a
r
k
|
S
k
i
2
|
1
-
|
S
ii
2
|
)
where A, B, and n are constants determined according to the subject, in which n is a number greater than 0, and A and B are positive numbers. Further, i, j, k, and m represent channel numbers of the high frequency antenna, M represents a channel number, |S mm |, |S jj |, and |S ii | represent the absolute values of the diagonal terms of the reflection matrix, and |S ki | represents the absolute value of the non-diagonal term of the reflection matrix.
4. The magnetic resonance imaging apparatus according to claim 1 , further comprising a specific absorption rate management unit which calculates, using the Q-value corrected by the Q-value correction unit, irradiation power consumed in the subject among irradiation power by the high frequency signals supplied to the high frequency antenna at the time of imaging to manage a specific absorption rate.
5. A Q-value calculating method comprising:
a reflection coefficient calculation step of measuring the amplitudes of a forward traveling wave and a reflected wave of each high frequency signal supplied to a high frequency antenna having a plurality of channels to calculate the absolute values of diagonal and non-diagonal terms of a reflection matrix of the high frequency signals for each of the channels;
a Q-value calculation step of fitting the absolute value of each diagonal term of the reflection matrix to a predetermined circuit model to thereby calculate a Q value for each of the channels; and
a correction step of correcting the Q value calculated by the Q-value calculation step using the absolute value of each non-diagonal term of the reflection matrix.
6. A specific absorption rate managing method comprising:
a reflection coefficient calculation step of measuring the amplitudes of a forward traveling wave and a reflected wave of each high frequency signal supplied to a high frequency antenna having a plurality of channels to calculate the absolute values of diagonal and non-diagonal terms of a reflection matrix of the high frequency signals for each of the channels;
a Q-value calculation step of fitting the absolute value of each diagonal term of the reflection matrix to a predetermined circuit model to thereby calculate a Q value for each of the channels;
a correction step of correcting the Q value calculated by the Q-value calculation step using the absolute value of each non-diagonal term of the reflection matrix; and
a specific absorption rate management step of calculating, using the corrected Q value, irradiation power consumed in a subject among irradiation power by the high frequency signals supplied to the high frequency antenna to manage a specific absorption rate.