MAGNETIC RESONANCE SYSTEM AND METHOD TO ADJUST AT LEAST ONE SHIM CURRENT AND AN ASSOCIATED RF CENTER FREQUENCY DURING AN INTERLEAVED MULTISLICE MR MEASUREMENT OF A MOVING EXAMINATION SUBJECT

• US 20120235681A1
• Filed: 03/19/2012
• Published: 09/20/2012
• Est. Priority Date: 03/17/2011
• Status: Active Grant

• Current Assignee: Siemens Healthineers AG (Siemens AG)
• Original Assignee: Siemens AG
• Inventors: Stemmer, Alto
• Granted Patent: US 8,994,373 B2
• Time in Patent Office: Days
• US Class Current: 324/307
• CPC Class Codes: G01R 33/4835 of multiple slices
• CPC Class Codes: G01R 33/5613 Generating steady state signals, e.g. low flip angle sequences [FLASH]
• CPC Class Codes: G01R 33/5614 using a fully balanced steady-state free precession [bSSFP] pulse sequence, e.g. trueFISP
• CPC Class Codes: G01R 33/56383 involving motion of the sample as a whole, e.g. multistation MR or MR with continuous table motion
• CPC Class Codes: G01R 33/56563 caused by a distortion of the main magnetic field B0, e.g. temporal variation of the magnitude or spatial inhomogeneity of B0 G01R33/56509, G01R33/56518, G01R33/56536 take precedence

Abstract

In a method and system to adjust at least one shim current in a shim channel of a magnetic resonance apparatus and an associated RF center frequency for the radio-frequency system of the magnetic resonance apparatus during an interleaved multislice MR measurement of a moving examination subject, in which MR measurement at least two excitations are implemented for complete acquisition of the desired data of a slice of the examination subject that is to be measured, the multislice MR measurement is implemented such that phase discontinuities between measurement data acquired after individual excitations are avoided.

Claims

1. A method for implementing an interleaved multislice magnetic resonance (MR) measurement of a moving examination subject, comprising:

continuously moving an examination subject through an MR data acquisition unit;

while continuously moving the examination subject through the MR data acquisition unit, acquiring MR data from the subject in an interleaved multislice MR data acquisition procedure in which a basic magnetic field is generated in a measurement volume of the MR data acquisition unit and in which at least two successive excitations of nuclear spins in a slice of the examination subject are produced by radiation of respective RF pulses by an RF system of the MR data acquisition unit, said RF system having an RF center frequency, and acquiring MR data from said slice after said at least two successive excitations of the nuclear spins in said slice; and

shimming said basic magnetic field to achieve a desired homogeneity of said basic magnetic field at least in said measurement volume by, via a computerized control unit, adjusting a shim current in a shim channel of said MR data acquisition unit, together with adjusting said RF center frequency, in order to avoid phase discontinuities between measurement data respectively acquired from said slice after each of said at least two successive excitations.

2. A method as claimed in claim 1 comprising:

providing, to said processor, an adjustment value for a shim current in said shim channel and an adjustment value for an associated RF center frequency for at least two different positions of the continuously moving examination subject in the measurement volume;

thereafter starting said interleaved multislice MR measurement of said moving examination subject; and

selecting and adjusting the shim current in said shim channel and selecting and adjusting an associated RF center frequency during said interleaved multislice MR measurement dependent on the provided adjustment value for the shim current and the provided adjustment value for the associated RF center frequency for said at least two different positions, in order to cause a defined slice of the examination subject always to be measured with a same shim current and RF center frequency during said interleaved multislice MR measurement.

3. A method as claimed in claim 2 comprising setting said shim current in said shim channel and setting an associated RF center frequency to cause the set shim current and the set associated RF center frequency to be active no later than a beginning of a respective excitation of nuclear spins in said defined slice, among said at least two successive excitations, and at least until an end of readout of MR data from said defined slice resulting from said respective excitation.

4. A method as claimed in claim 2 comprising entering said MR data into an electronic memory in communication with said processor that represents k-space having a k-space center, and adjusting said shim current in said shim channel and an associated RF center frequency by selecting respective values for said shim current and said associated RF center frequency that are optimal for improving said homogeneity of said basic magnetic field at least in said measurement volume during acquisition of MR data from said defined slice in said interleaved multislice MR measurement that include said k-space center.

5. A method as claimed in claim 2 comprising operating said MR data acquisition unit with a pulse sequence in said multislice MR measurement that is configured to suppress an unwanted signal component and that comprises at least one spectrally selective RF pulse and comprising, for a duration of said at least one spectrally selective RF pulse, adjusting said shim current in said shim channel and the associated RF center frequency to respective values that are optimal for homogenizing said basic magnetic field at least in said measurement volume, for a current position of the continuously moving examination subject during the radiation of said at least one spectrally selective RF pulse.

6. A method as claimed in claim 2 comprising implementing said interleaved multislice MR measurement using a sliding multislice technique.

7. A method as claimed in claim 1 comprising implementing said interleaved multislice MR measurement with a pulse sequence that comprises at least one gradient pulse between each of said at least two successive excitations and readout of MR signals resulting from the respective excitation, configured to compensate for phase discontinuities in said MR data that occur due to switching of said shim current.

8. A magnetic resonance apparatus comprising:

a magnetic resonance data acquisition unit;

a patient bed operable to move an examination subject thereon continuously through a measurement volume of said MR data acquisition apparatus;

said MR data acquisition apparatus comprising a radio frequency (RF) system having an RF center frequency, and a basic field magnet that generates a basic magnetic field in said measurement volume, and a shimming system comprising of at least one shim channel, a control unit configured to operate said patient bed and said MR data acquisition unit to continuously move an examination subject through an MR data acquisition unit on the patient bed and, while continuously moving the examination subject through the MR data acquisition unit, acquire MR data from the subject in an interleaved multislice MR data acquisition procedure in which a basic magnetic field is generated by said basic field magnet in a measurement volume of the MR data acquisition unit and in which at least two successive excitations of nuclear spins in a slice of the examination subject are produced by radiation of respective RF pulses by said RF system, and acquire MR data from said slice after said at least two successive excitations of the nuclear spins in said slice; and

said control unit being configures to operate said shimming system to shim said basic magnetic field to achieve a desired homogeneity of said basic magnetic field at least in said measurement volume by, via a computerized control unit, adjusting a shim current in said at least one shim channel together with adjusting said RF center frequency, in order to avoid phase discontinuities between measurement data respectively acquired from said slice after each of said at least two successive excitations.

9. A non-transitory, computer-readable data storage medium encoded with programming instructions that, when said data storage medium is loaded into a computerized control unit of a magnetic resonance apparatus, that comprises a magnetic resonance data acquisition unit with a patient bed configured to continuously move an examination subject thereon through the MR data acquisition unit, a radio-frequency (RF) system having an RF center frequency, and a basic field magnet that generates a basic magnetic field in a measurement volume in said MR data acquisition unit, and a shimming system comprising at least one shim channel, and said programming instructions causing said computerized control system to:

continuously moving an examination subject through an MR data acquisition unit on the patient bed;

while continuously moving the examination subject through the MR data acquisition unit, acquire MR data from the subject in an interleaved multislice MR data acquisition procedure in which a basic magnetic field is generated by said basic field magnet in a measurement volume of the MR data acquisition unit and in which at least two successive excitations of nuclear spins in a slice of the examination subject are produced by radiation of respective RF pulses by said RF system, and acquire MR data from said slice after said at least two successive excitations of the nuclear spins in said slice; and

shim said basic magnetic field to achieve a desired homogeneity of said basic magnetic field at least in said measurement volume by adjusting a shim current in said at least one shim channel together with adjusting said RF center frequency, in order to avoid phase discontinuities between measurement data respectively acquired from said slice after each of said at least two successive excitations.