# US 2010/0167668 A1 · Method and device for monitoring a radio-frequency transmitter device in a magnetic resonance tomography system

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- Number: US 2010/0167668 A1
- Document ID: US20100167668A1
- Status: Published application
- Kind: A1
- Application number: 12/641,651
- Filed: 2009-12-18
- Published: 2010-07-01
- Earliest claimed: 2008-12-18
- Assignees: Siemens Aktiengesellschaft
- Inventors: Juergen Nistler; Markus Vester
- Web page: https://patentgrep.com/patent/US20100167668A1

## CPC

- G01R 33/48: NMR imaging systems
- G01R 33/288: {Provisions within MR facilities for enhancing safety during MR, e.g. reduction of the specific absorption rate [SAR], detection of ferromagnetic objects in the scanner room}
- G01R 33/34076: {Birdcage coils}
- G01R 33/3415: comprising arrays of sub-coils {, i.e. phased-array coils with flexible receiver channels}
- G01R 33/3628: {Tuning/matching of the transmit/receive coil}
- G01R: MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES (indicating correct tuning of resonant circuits H03J3/12)

## Abstract

In a method and device for monitoring a radio-frequency transmit device of a magnetic resonance tomography system, having a transmitter antenna system that has a number of transmit channels, during a magnetic resonance measurement of an examination subject, a reference scattering parameter matrix of the transmitter antenna system is determined in the unloaded state, and a subject-specific scattering parameter matrix of the transmitter antenna system is determined in a state loaded with the subject of examination. Moreover, transmitter amplitude vectors are determined in time-dependent fashion that represent the radio-frequency voltage amplitudes on the individual transmit channels. On the basis of the subject-specific scattering parameter matrix, the reference scattering parameter matrix, and the transmit amplitude vectors, radio-frequency power values absorbed at particular transmit times in the subject are determined. Based on a large number of the determined radio-frequency power values, a number of monitoring values are formed. The radio-frequency transmit device is limited in its functioning when a monitoring value reaches or exceeds a prespecified boundary monitoring value.

## Claims

### Claim 1

A method for monitoring a radio-frequency transmitter device in a magnetic resonance tomography apparatus, during acquisition of magnetic resonance data, involving the transmitter device, in the magnetic resonance system, said radio-frequency transmitter device comprising a transmitter antenna system comprising a plurality of transmitter channels, said method comprising the steps of:
prior to introducing an examination subject into the magnetic resonance apparatus, determining a reference scattering parameter matrix of the transmitter antenna system in an unloaded state;
after introduction of an examination subject into the magnetic resonance apparatus, determining a subject-specific scattering parameter matrix of the transmitter antenna system in a state loaded by the examination subject;
in a processor, automatically determining time-dependent transmitter amplitude vectors for each of said transmitter channels that represent radio-frequency voltage amplitudes in the respective transmitter channels;
in said processor, using said subject-specific scattering parameter matrix and said reference scattering parameter matrix and said transmitter amplitude vectors to determine radio-frequency power values that are absorbed by the examination subject at respective transmit times;
in said processor, forming a plurality of monitoring values based on the determined radio-frequency power values; and
from said processor, limiting operation of said radio-frequency transmitter device during the acquisition of said magnetic resonance data when one of said monitoring values equals or exceeds a predetermined boundary monitoring value.

### Claim 2 (depends on 1)

A method as claimed in claim 1 comprising:
in said processor, automatically determining a subject-specific conductance matrix from said subject-specific scattering parameter matrix;
in said processor, automatically determining a reference conductance matrix of said transmitter antenna system from said reference scattering parameter matrix; and
in said processor, forming said radio-frequency power values from said subject-specific conductance matrix and said reference conductance matrix.

### Claim 3 (depends on 2)

A method as claimed in claim 2 comprising forming said radio-frequency power values at respective times (t) according to:
Po(UF(t))=Re{ ((UcUF(t))T·(Ytot−Yc)·(UcUF(t)))}
wherein PO is the radio-frequency power value, UF is a transmitter amplitude vector formed from voltage values that drop in the respective transmitter channels of an emitted signal at time (t), UC is an effective voltage vector that is a function of UF, Ytot is the subject-specific conductance matrix, and YC is the reference conductance matrix.

### Claim 4 (depends on 3)

A method as claimed in claim 3 comprising, in said processor, calculating said effective voltage amplitude vector UC from the transmitter amplitude vector UF according to
Uc(UF(t))=(E+Stot)·UF(t)
wherein Stot is the subject-specific scattering parameter matrix and E is the unit matrix.

### Claim 5 (depends on 3)

A method as claimed in claim 3 wherein said radio-frequency transmit device comprises a matching circuit having a matching impedance, and comprising, in said processor, calculating said effective voltage amplitude vector UC from the transmitter amplitude vector UF as follows:
U
c

(
U
F

(
t
)
)
=
(
E
·
(
1
-
1
jϖ


C
S

Z
0
)
+
S
tot
·
(
1
+
1
j


ϖ


C
S

Z
0
)
)
·
U
F

(
t
)
wherein Stot is the subject-specific scattering parameter matrix, E is the unit matrix, j is √−1, ω is the transmitter frequency at which said radio-frequency transmitter device is operating, CS is the matching capacitance, and Z0 is a reference impedance.

### Claim 6 (depends on 1)

A method as claimed in claim 1 comprising determining said scattering parameter matrix by emitting respective radio-frequency signals in chronological succession from each of said transmitter channels and detecting a voltage value of a returning signal in each of said transmitter channels.

### Claim 7 (depends on 6)

A method as claimed in claim 6 comprising measuring said voltage values using a directional coupler connected to each transmitter channel.

### Claim 8 (depends on 6)

A method as claimed in claim 6 wherein said voltage value is a first voltage value, and determining a first voltage value in each of said transmitter channels for each emitted signal, and determining respective entries in said scattering parameter matrix as respective relationships between said first voltage values and said second voltage values.

### Claim 9 (depends on 8)

A method as claimed in claim 8 comprising measuring each of said first and second voltage values in respective directional couplers connected to said transmitter channels.

### Claim 10 (depends on 1)

A method as claimed in claim 1 comprising determining said transmitter amplitude vectors by measuring voltage values at respective chronological intervals in each of said transmitter channels.

### Claim 11 (depends on 8)

A method as claimed in claim 8 comprising, from said processor, limiting said operation of said radio-frequency transmitter device when one of said monitoring values equals or exceeds a predetermined boundary monitoring value by limiting said second voltages.

### Claim 12 (depends on 1)

A method as claimed in claim 1 comprising forming each monitoring value as an average of a plurality of said radio-frequency power values.

### Claim 13 (depends on 1)

A method as claimed in claim 1 comprising selecting said plurality of radio-frequency power values that are used to forms aid average value as radio-frequency power values that fall within a time window that slides over said radio-frequency power values in the time domain.

### Claim 14

A radio-frequency transmitter device for a magnetic resonance tomography apparatus, comprising:
a transmitter antenna system comprising a plurality of transmitter channels; and
a monitoring device that monitors transmission by said transmitter antenna system during acquisition of magnetic resonance data, said monitoring device comprising a processor configured to open at said transmission antenna system prior to introducing an examination subject into the magnetic resonance apparatus, to determine a reference scattering parameter matrix of the transmitter antenna system in an unloaded state, and to operate said transmission antenna system after introduction of an examination subject into the magnetic resonance apparatus, to determine a subject-specific scattering parameter matrix of the transmitter antenna system in a state loaded by the examination subject, and to automatically determine time-dependent transmitter amplitude vectors for each of said transmitter channels that represent radio-frequency voltage amplitudes in the respective transmitter channels, and to use said subject-specific scattering parameter matrix and said reference scattering parameter matrix and said transmitter amplitude vectors to determine radio-frequency power values that are absorbed by the examination subject at respective transmit times, and to form a plurality of monitoring values based on the determined radio-frequency power values, and to limit operation of said radio-frequency transmitter device during the acquisition of said magnetic resonance data when one of said monitoring values equals or exceeds a predetermined boundary monitoring value.

### Claim 15 (depends on 14)

A radio-frequency transmitter device as claimed in claim 14 wherein said monitoring device comprises a measurement device comprising directional couplers respectively connected to said transmitter channels that measure said radio-frequency voltage amplitudes in the respective transmitter channels.

### Claim 16

A radio-frequency monitoring device for monitoring a radio-frequency transmitter device in a magnetic resonance tomography apparatus, during acquisition of magnetic resonance data, involving the transmitter device, in the magnetic resonance system, said radio-frequency transmitter device comprising a transmitter antenna system comprising a plurality of transmitter channels, said monitoring device comprising:
a measurement device that, prior to introducing an examination subject into the magnetic resonance apparatus, determines a reference scattering parameter matrix of the transmitter antenna system in an unloaded state;
said measurement device, after introduction of an examination subject into the magnetic resonance apparatus, determining a subject-specific scattering parameter matrix of the transmitter antenna system in a state loaded by the examination subject;
a processor that automatically determines time-dependent transmitter amplitude vectors for each of said transmitter channels that represent radio-frequency voltage amplitudes in the respective transmitter channels;
said processor, using said subject-specific scattering parameter matrix and said reference scattering parameter matrix and said transmitter amplitude vectors, determining radio-frequency power values that are absorbed by the examination subject at respective transmit times;
said processor forming a plurality of monitoring values based on the determined radio-frequency power values; and
said processor limiting operation of said radio-frequency transmitter device during the acquisition of said magnetic resonance data when one of said monitoring values equals or exceeds a predetermined boundary monitoring value.

### Claim 17

A magnetic resonance tomography system comprising:
a magnetic resonance data acquisition unit;
a radio-frequency transmitter device in said magnetic resonance data acquisition unit, comprising a transmitter antenna system comprising a plurality of transmitter channels; and
a monitoring device that monitors transmission by said transmitter antenna system during acquisition of magnetic resonance data, said monitoring device comprising a processor configured to open at said transmission antenna system prior to introducing an examination subject into the magnetic resonance data acquisition unit, to determine a reference scattering parameter matrix of the transmitter antenna system in an unloaded state, and to operate said transmission antenna system after introduction of an examination subject into the magnetic resonance data acquisition unit, to determine a subject-specific scattering parameter matrix of the transmitter antenna system in a state loaded by the examination subject, and to automatically determine time-dependent transmitter amplitude vectors for each of said transmitter channels that represent radio-frequency voltage amplitudes in the respective transmitter channels, and to use said subject-specific scattering parameter matrix and said reference scattering parameter matrix and said transmitter amplitude vectors to determine radio-frequency power values that are absorbed by the examination subject at respective transmit times, and to form a plurality of monitoring values based on the determined radio-frequency power values, and to limit operation of said radio-frequency transmitter device during the acquisition of said magnetic resonance data when one of said monitoring values equals or exceeds a predetermined boundary monitoring value.