Method and magnetic resonance apparatus for suppressing undesired coherence pathways
In a method and apparatus for recording MR signals with an image-recording sequence with which preparation gradients for preparing the MR signals are switched before the signal readout and readout gradients, in order to predominantly record the MR signal components with a desired signal coherence pathway in the readout segment, the signal components with an undesired signal coherence pathway are suppressed by dephasing gradients. The dephasing gradients are determined independently of the time intervals in which the preparation gradients are switched, and only in dependence on the effective size of the gradient moments of the preparation gradients.
1. A method for acquiring magnetic resonance (MR) signals, comprising:
from a control computer, operating an MR scanner, while a subject is situated in the MR scanner, to acquire the MR signals from the subject by executing a data acquisition sequence comprising a time interval in which a plurality of preparation gradients are activated for preparing the MR signals, followed by a readout segment in which readout gradients are activated for reading out the MR signals, and wherein said data acquisition sequence comprises radiation of radio frequency (RF) pulses that, when radiated, produce components of said MR signal having a desired signal coherence pathway and components of said MR signal having undesired signal coherence pathways, and in said readout segment, predominantly acquiring said MR signal components with said desired signal coherence pathway;
in said control computer, determining said MR signal components having said undesired signal coherence pathways in said data acquisition sequence;
from said control computer, operating said MR scanner in said data acquisition sequence to reduce acquisition of said MR signal components having said undesired signal coherence pathways by activating dephasing gradients dependent on said preparation gradients and dependent on the undesired signal coherence pathways, with said dephasing gradients having respective gradient moments that are determined in said control computer so that a dephasing gradient moment for each undesired signal coherence pathway is greater than a predetermined threshold value, and in said control computer, determining said dephasing gradients independently of said time interval and dependent only on an effective size of the respective gradient moments of said preparation gradients; and
from said control computer, making the MR signals acquired with said data acquisition sequence in which said dephasing gradients were activated, available in electronic form as a data file.
2. The method as claimed in claim 1 comprising, from said control computer, operating said MR scanner with a double spin-echo sequence as said data acquisition sequence and, in said double spin-echo sequence, radiating one RF excitation pulse and two RF refocusing pulses and activating a first of said preparation gradients, with a first effective gradient moment, between the RF excitation pulse and a first of said two RF refocusing pulses, and activating at least a second of said preparation gradients, with a second effective gradient moment, between the two RF refocusing pulses and, in said control computer, determining said dephasing gradients dependent on a size of said first effective gradient moment relative to a size of said second effective gradient moment.
3. The method as claimed in claim 2 comprising, from said computer, activating said dephasing gradients with said second effective gradient moment more than twice as large as said first effective gradient moment.
4. The method as claimed in claim 2 comprising, from said control computer, operating said MR scanner to apply said dephasing gradients with said second effective gradient moment smaller than twice said first effective gradient moment.
5. The method as claimed in claim 2 comprising, from said control computer, operating said MR scanner to apply said dephasing gradients with said first effective gradient moment larger than said second effective gradient moment.
6. The method as claimed in claim 2 comprising, from said control computer, operating said MR scanner to apply said dephasing gradients with half of said first effective gradient moment larger than said second effective gradient moment.
7. The method as claimed in claim 2 comprising, from said control computer, operating said MR scanner to apply said dephasing gradients with said second effective gradient moment smaller than zero.
8. The method as claimed in claim 1 comprising, from said control computer, operating said MR scanner with a double spin-echo sequence as said data acquisition sequence and, in said double spin-echo sequence, radiating one RF excitation pulse and two RF refocusing pulses and activating a first of said preparation gradients, with a first effective gradient moment, between the RF excitation pulse and a first of said two RF refocusing pulses, and activating at least a second of said preparation gradients, with a second effective gradient moment, between the two RF refocusing pulses, and wherein a third of said preparation gradients, with a third effective gradient moment, is activated after a second of said two RF refocusing pulses and, in said control computer, determining said dephasing gradients dependent on a size of only two of said first, second and third effective gradient moments relative to each other.
9. The method as claimed in claim 8 comprising, from said computer, activating said dephasing gradients with said second effective gradient moment more than twice as large as said first effective gradient moment.
10. The method as claimed in claim 8 comprising, from said control computer, operating said MR scanner to apply said dephasing gradients with said second effective gradient moment smaller than twice said first effective gradient moment.
11. The method as claimed in claim 8 comprising, from said control computer, operating said MR scanner to apply said dephasing gradients with said first effective gradient moment larger than said second effective gradient moment.
12. The method as claimed in claim 8 comprising, from said control computer, operating said MR scanner to apply said dephasing gradients with half of said first effective gradient moment larger than said second effective gradient moment.
13. The method as claimed in claim 8 comprising, from said control computer, operating said MR scanner to apply said dephasing gradients with said second effective gradient moment smaller than zero.
14. The method as claimed in claim 8 comprising determining said dephasing gradients dependent only on a size of said first effective gradient moment relative to a size of said second effective gradient moment.
15. The method as claimed in claim 1 comprising, from said control computer, operating said MR scanner to apply one of said preparation gradients with a gradient moment produced by at least two chronologically separate gradients.
16. A magnetic resonance (MR) apparatus comprising:
an MR scanner;
a control computer configured to operate said MR scanner, while a subject is situated in the MR scanner, to acquire MR signals from the subject by executing a data acquisition sequence comprising a time interval in which a plurality of preparation gradients are activated for preparing the MR signals, followed by a readout segment in which readout gradients are activated for reading out the MR signals, and wherein said data acquisition sequence comprises radiation of radio frequency (RF) pulses that, when radiated, produce components of said MR signal having a desired signal coherence pathway and components of said MR signal having undesired signal coherence pathways, and in said readout segment, predominantly acquiring said MR signal components with said desired signal coherence pathway;
said control computer being configured to determine said MR signal components having said undesired signal coherence pathways in said data acquisition sequence;
said control computer being configured to operate said MR scanner in said data acquisition sequence to reduce acquisition of said MR signal components having said undesired signal coherence pathways by activating dephasing gradients dependent on said preparation gradients and dependent on the undesired signal coherence pathways, with said dephasing gradients having respective gradient moments that are determined in said control computer so that a dephasing gradient moment for each undesired signal coherence pathway is greater than a predetermined threshold value, and in said control computer, determining said dephasing gradients independently of said time interval and dependent only on an effective size of the respective gradient moments of said preparation gradients; and
said control computer being configured to make the MR signals acquired with said data acquisition sequence, in which said dephasing gradients were activated, available in electronic form as a data file.