Magnetic resonance imaging apparatus and magnetic resonance imaging method

1. A magnetic resonance imaging (MRI) apparatus comprising:

an MRI gantry and associated control circuits including at least one computer configured to collect magnetic resonance signals from a subject by applying a uniform static magnetic field, radio-frequency magnetic fields, and gradient magnetic fields to the subject in accordance with a predetermined pulse sequence;

said control circuits including at least one computer configured to reconstruct an image concerning the subject based on the collected magnetic resonance signals;

said control circuits being configured to detect a respiration level of the subject;

said control circuits also being configured to control image reconstruction based on the collected magnetic resonance signals when a peak of the detected respiration level falls within an allowable range; and

wherein said control circuits are configured to change the allowable range during an MRI scan which collects MRI signals from the subject in response to a change in a plurality of peak values of detected respiration levels.

2. The magnetic resonance imaging apparatus according to claim 1 , wherein the control circuits are configured to shift a central level of the allowable range.

3. The magnetic resonance imaging apparatus according to claim 1 , wherein the control circuits are configured to change a width of the allowable range.

4. The magnetic resonance imaging apparatus according to claim 1 , wherein the control circuits are configured to change the allowable range based on a change in respiration peak level of the subject obtained from the detected respiration level.

5. The magnetic resonance imaging apparatus according to claim 1 , wherein the respiration level is repeatedly detected, and

the allowable range is changed based on an average value of peak values of the respiration level based on N, where N is an integer greater than or equal to 2, detected respiration levels.

6. The magnetic resonance imaging apparatus according to claim 5 , wherein said control circuits are further configured:

to obtain an index value for a fluctuation in respiratory status of the subject; and

to change the value N based on the obtained index value.

7. The magnetic resonance imaging apparatus according to claim 6 , wherein the index value is one of: (a) a variance of the respiration levels detected during a predetermined period, (b) an average value or a difference value of a variance of the respiration levels in predetermined two periods, and (c) the number of peak levels included in the respiration levels detected in a unit time.

8. The magnetic resonance imaging apparatus according to claim 1 , wherein the image is reconstructed based on the collected magnetic resonance signals when a peak of the detected respiration level falls within the allowable range during a period that the subject holds his/her breath.

9. The magnetic resonance imaging apparatus according to claim 1 , wherein the image is reconstructed based on the collected magnetic resonance signals when a peak of the detected respiration level falls within the allowable range irrespective of a period that the subject holds his/her breath or a period that subject naturally breathes.

10. A magnetic resonance imaging apparatus comprising:

an MRI gantry and associated control circuits including at least one computer configured to collect magnetic resonance signals from a subject by applying a uniform static magnetic field, radio-frequency magnetic fields, and gradient magnetic fields to the subject in accordance with a predetermined pulse sequence;

said control circuits including at least one computer configured to reconstruct an image concerning the subject based on the collected magnetic resonance signals;

said control circuits being configured to detect a respiration level of the subject; and

said control circuits also being configured to control collection and reconstruction of the magnetic resonance signals during an MRI scan which collects MRI signals from the subject when the rate of change of the detected respiration level is less than or equal to a predetermined rate, and to reconstruct the image in response to the thus collected magnetic resonance signals.

11. A magnetic resonance imaging apparatus comprising:

an MRI gantry and associated control circuits including at least one computer configured to collect magnetic resonance signals from a subject by applying a uniform static magnetic field, radio-frequency magnetic fields, and gradient magnetic fields to the subject in accordance with a predetermined pulse sequence;

said control circuits including at least one computer configured to reconstruct an image concerning the subject based on the collected magnetic resonance signals;

said control circuits being configured to repeatedly detect a respiration level of the subject;

said control circuits being configured to control the image reconstruction in response to the collected magnetic resonance signals when a peak of the detected respiration level falls within an allowable range;

said control circuits being configured to repeatedly detect peak values of the detected respiration level in one cycle of a respiratory motion of the subject;

said control circuits being configured to set the allowable range based on an average value of the peak values during a first period that the plurality of respiration levels are detected; and

said control circuits being configured to control the allowable range setting frequency during an MRI scan which collects MRI signals from the subject in accordance with a frequency of the respiratory motion of the subject and to set the allowable range at the setting frequency.

12. The magnetic resonance imaging apparatus according to claim 11 , wherein the control circuits are further configured to:

calculate a variation of a plurality of peak values based on the plurality of detected peak values during each of a plurality of second periods which are set to cause the plurality of detected peak values and these second periods to be shifted from each other in terms of time; and

set a length of the first period in accordance with a frequency that the calculated variation exceeds a reference value during a third period longer than the second period.

13. The magnetic resonance imaging apparatus according to claim 11 , wherein a central level of the allowable range is shifted.

14. The magnetic resonance imaging apparatus according to claim 11 , wherein a width of the allowable range is changed.

15. The magnetic resonance imaging apparatus according to claim 11 , wherein a period during which two peak values are detected is determined as the second period, and the variation as a difference between the two peak values is calculated.

16. The magnetic resonance imaging apparatus according to claim 11 , wherein said control circuits are configured:

to accept input of a detection signal output every predetermined time phase by an electrocardiographic monitor which detects the predetermined time phase of a heartbeat concerning the subject,

to detect the respiration level in synchronization with the input detection signal, and

to determine a period during which a predetermined number of respiration levels are detected as the third period,

to measure a length of the third period based on intervals at which the detection signal is input during the third period, and

to calculate the frequency as a ratio of the length and a number of times that the calculated variation exceeds the reference value during the third period.

17. A magnetic resonance imaging apparatus, comprising:

an MRI gantry and associated control circuits including at least one computer configured to collect magnetic resonance signals from a subject by applying a uniform static magnetic field, radio-frequency magnetic fields, and gradient magnetic fields to the subject in accordance with a predetermined pulse sequence;

said control circuits including at least one computer configured to reconstruct an image concerning the subject based on the collected magnetic resonance signals;

said control circuits being configured to repeatedly detect a respiration level of the subject;

said control circuits being configured to image reconstruction based on the collected magnetic resonance signals when a peak of the detected respiration level falls within an allowable range;

said control circuits being configured to set a trackable range in response to the detected respiration level when or before starting collecting of the magnetic resonance signals which are used for image; and

said control circuits being configured to set the allowable range within the trackable range during an MRI scan which collects MRI signals from the subject based on an average value of the peak values during a period that the plurality of respiration levels are detected.

18. A magnetic resonance imaging method comprising:

collecting magnetic resonance signals from a subject by applying a uniform static magnetic field, radio-frequency magnetic fields, and gradient magnetic fields to the subject in accordance with a predetermined pulse sequence;

repeatedly detecting a respiration level of the subject during execution of said predetermined pulse sequence;

controlling image reconstruction based on magnetic resonance signals collected when a peak of the detected respiration level falls within an allowable range; and

changing the allowable range during an MRI scan which collects MRI signals from the subject in response to a change in a plurality of peak values of a plurality of detected respiration levels.