MAGNETIC RESONANCE METHOD AND SYSTEM TO CORRECT PHASE INFORMATION IN MR IMAGES

• US 20120249138A1
• Filed: 03/16/2012
• Published: 10/04/2012
• Est. Priority Date: 03/28/2011
• Status: Active Grant

• Current Assignee: Siemens Healthineers AG (Siemens AG)
• Original Assignee: Siemens AG
• Inventors: Pfeuffer, Josef
• Granted Patent: US 9,329,254 B2
• Time in Patent Office: Days
• US Class Current: 324/309
• CPC Class Codes: G01R 33/5616 using gradient refocusing, e.g. EPI
• CPC Class Codes: G01R 33/56509 due to motion, displacement or flow, e.g. gradient moment nulling G01R33/567 takes precedence
• 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
• CPC Class Codes: G01R 33/5676 Gating or triggering based on an MR signal, e.g. involving one or more navigator echoes for motion monitoring and correction

Abstract

In a magnetic resonance (MR) method and system for correction of phase information in MR images of a predetermined volume segment of an examination subject, a basic magnetic field is applied and MR data of the predetermined volume segment are acquired and evaluated such that phase information is calculated for each image element of the predetermined volume segment. A navigator signal is acquired that detects an unintentional change of the basic magnetic field that is caused by movements of the examination subject or by the magnetic resonance system itself. The phase information is corrected with this navigator signal.

Claims

1. A method to correct phase information in magnetic resonance (MR) images of a volume segment of an examination subject, comprising;

placing an examination subject in an MR data acquisition unit and operating the MR data acquisition unit to apply a basic magnetic field at least to a predetermined volume segment of the examination subject and to acquire MR data from the predetermined volume segment, said MR data representing image elements of said predetermined volume segment;

in a processor, evaluating said MR data to calculate phase information for each of said image elements of said predetermined volume segment;

operating said MR data acquisition unit to acquire a navigator signal that detects a change in said basic magnetic field caused by a source selected from the group consisting of movement of the examination subject, and said MR data acquisition unit itself; and

in said processor, correcting said phase information dependent on said navigator signal.

2. A method as claimed in claim 1 comprising operating said MR data acquisition unit with an echo planar method to acquire said MR data from said predetermined volume segment, with a selective radio frequency (RF) excitation of said predetermined volume segment and readout of at least one echo signals in said echo planar method.

3. A method as claimed in claim 2 comprising entering said MR data into k-space respectively in segments of k-space, detecting said change of said basic magnetic field with said navigator signal separately during acquisition of said MR data for each of the respective segments of k-space, and correcting said phase information respectively in said segments of k-space, before reconstructing an image from an entirety of the MR data in all segments of k-space.

4. A method as claimed in claim 3 comprising:

determining a first reference phase value by detecting transverse magnetization in said examination subject in a reference measurement in which said navigator signal is not phase coded, using free induction decay, during a first time interval after said RF excitation;

determining a second reference phase value by transverse magnetization of an echo signal resulting from said navigator signal being detected in a center of k-space in said reference measurement at a second time interval after said RF excitation;

determining a first phase value by detecting the transverse magnetization in a measurement with a measurement signal that is not phase coded using the free induction decay after said first time interval after said RF excitation;

determining a second phase value by detecting the transverse magnetization of an echo signal resulting from the measurement signal during said measurement in the center of k-space after a second time interval after the RF excitation; and

correcting said MR data during entry thereof into k-space dependent on said first reference phase value, said second reference phase value, said first phase value and said second phase value.

5. A method as claimed in claim 4 comprising, in said processor:

determining a factor F dependent on time t according to

F(t)=e<sup>−

i(Δ

φ

+Δ

ω

·

t)</sup>,

wherein Δ

φ and

Δ

ω

are calculated according to

Δ

φ

=(T₂·

(φ

_n,1−

φ

_R,1)−

T₁·

(φ

_n,2−

φ

_R,2))/(T₂−

T₁),

Δ

ω

=((φ

_n,2−

φ

_R,2)−

(φ

_n,1−

φ

_R,1))/(T₂−

T₁),

wherein φ

R,1 is the first reference phase value, φ

_R,2 is the second reference phase value, φ

_n,1 is the first phase value, φ

_n,2 is the second phase value, T₁ is the first time interval, and T₂ is the second time interval; and

correcting the MR data by multiplying the MR data with the factor F at time t.

6. A method as claimed in claim 3 comprising:

determining a reference phase value by detecting transverse magnetization in said predetermined volume segment of a resulting echo signal in a reference measurement at a center of k-space at a time interval after said RF excitation;

determining a phase value by detecting the transverse magnetization of a resulting echo signal in said measurement in said center of k-space after said time interval after said RF excitation; and

correcting said MR data during entry of said MR data into k-space dependent on said reference phase value and said phase value.

7. A method as claimed in claim 6 comprising, in said processor:

determining a factor F dependent on time t according to

F=e<sup>−

i·

Δ

ω

·

t</sup>,

wherein Δ

ω

is calculated according to

Δ

ω

=(φ

_R−

φ

_n)/T

wherein φ

_R is the reference phase value, φ

_n is the phase value and T is the time interval; and

correcting the MR data by multiplying the MR data with the factor F at time t.

8. A method as claimed in claim 1 comprising detecting said navigator signal with a device that interacts with the examination subject, said device being selected from the group consisting of a breathing belt and a heart monitor.

9. A method as claimed in claim 1 comprising:

in said processor, evaluating said MR data by calculating amplitude information for each image element of said predetermined volume segment; and

correcting said amplitude information also dependent on said change of said basic magnetic field as detected by said navigator signal.

10. A method as claimed in claim 1 comprising further processing said MR data in a further processing algorithm selected from the group consisting of functional magnetic resonance imaging analysis, magnetic resonance imaging of functional connections, reconstructing a diffusion-weighted MR image, and reconstructing a profusion-weighted MR image.

11. A method as claimed in claim 1 comprising acquiring said MR data by operating said MR data acquisition unit with an echo planar measurement and entering said MR data into k-space a first time followed by a second time with said echo planar method;

compared to said first entry of data into k-space, in said second entry of data into k-space generating a phase coding gradient in said echo planar method that has a smaller gradient moment than for said first entry of said MR data into k-space, with a constant shift in k-space along a direction corresponding to said phase coding gradient being implemented between said first entry of said MR data into k-space and said second entry of said MR data into k-space; and

determining an image element-by-image element difference between said first entry of said MR data into k-space and said second entry of said MR data into k-space that indicates a degree to which a phase value for the respective image element is dependent on said change of said basic magnetic field.

12. A method as claimed in claim 1 comprising acquiring said MR data by operating said MR data acquisition unit with an echo planar method that includes a first echo time and a second echo time;

acquiring said MR data after said first echo time and entering said MR data acquired after said first echo time into k-space;

acquiring MR data after said second echo time and entering said MR data acquired after said second echo time into k-space;

generating a first phase map from said MR data entered into k-space after said first echo time, and generating a second phase map from said MR data entered into k-space after said second echo time, each phase map identifying a phase value for each image element represented by said MR data; and

determining a difference between said first phase map and said second phase map and, from said difference, generating a field map that identifies a strength of said basic magnetic field for each of said image elements.

13. A magnetic resonance (MR) system comprising:

an MR data acquisition unit configured to receive an examination subject therein, said MR data acquisition unit comprising a basic field magnet;

a control unit configured to operate the MR data acquisition unit to apply a basic magnetic field, with said basic field manget, at least to a predetermined volume segment of the examination subject and to acquire MR data from the predetermined volume segment, said MR data representing image elements of said predetermined volume segment;

a processor configured to evaluate said MR data to calculate phase information for each of said image elements of said predetermined volume segment;

said control unit being configured to operate said MR data acquisition unit to acquire a navigator signal that detects a change in said basic magnetic field caused by a source selected from the group consisting of movement of the examination subject, and said MR data acquisition unit itself; and

said processor being configured to correct said phase information dependent on said navigator signal.

14. A non-transitory, computer-readable data storage medium encoded with programming instructions, said data storage medium being loaded into a computerized control and evaluation unit of a magnetic resonance (MR) system that comprises an MR data acquisition unit, having a basic field magnet, and a computerized control and evaluation system, said programming instructions causing said control and evaluation system of said magnetic resonance imaging system to:

operate the MR data acquisition unit to apply a basic magnetic field, with said basic field magnet, at least to a predetermined volume segment of an examination subject in the MR data acquisition unit and to acquire MR data from the predetermined volume segment, said MR data representing image elements of said predetermined volume segment;

evaluate said MR data to calculate phase information for each of said image elements of said predetermined volume segment;

operate said MR data acquisition unit to acquire a navigator signal that detects a change in said basic magnetic field caused by a source selected from the group consisting of movement of the examination subject, and said MR data acquisition unit itself; and

correct said phase information dependent on said navigator signal.