Calculating specific absorption rate (SAR) with magnetic resonance signals

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

an MRI gantry including a static magnetic field coil, gradient magnetic field coils, at least one radio frequency (RF) coil configured to transmit RF nuclear excitation pulses into an imaging volume, and to receive nuclear magnetic resonance (NMR) RF signals from a patient located in the imaging volume;

at least one reference phantom arranged at a location within the at least one RF coil, the location being substantially unaffected by a scattered magnetic field created by the patient; and

an MRI control system having at least one computer configured to:

determine a specific absorption rate (SAR) for the patient based on at least (a) NMR signal strength at the phantom when the at least one RF coil is loaded with the patient for an MRI scan, and (b) NMR signal strength at the phantom when the at least one RF coil is not loaded with the patient for an MRI scan.

2. An MRI system as in claim 1 , wherein said location is spatially displaced away from the patient towards an end of the at least one RF coil to reduce scattered RF coil field effects.

3. An MRI system as in claim 2 , wherein said location is substantially close to a surface of the at least one RF coil.

4. An MRI system as in claim 1 , wherein comprising plural reference phantoms arranged at respective different locations within the FOV of the at least one RF coil and wherein said SAR is determined based on relative unloaded and loaded NMR signals from said plural phantoms.

5. An MRI system as in claim 1 , wherein said reference phantom is a non-loading object filled with an off water resonance substance.

6. An MRI system as in claim 1 , wherein the NMR signal strength at the phantom when the at least one RF coil is loaded with the patient for an MRI scan, and the NMR signal strength at the phantom when the at least one RF coil is not loaded with the patient are determined by applying an NMR pulse sequence with small flip angle pulses.

7. An MRI system as in claim 6 , wherein the pulse sequence is a field echo (FE) pulse sequence.

8. An MRI system as in claim 6 , wherein the pulse sequence includes pulses at an NMR resonance frequency of the reference phantom which is different from an NMR resonance frequency of water.

9. An MRI system as in claim 1 , wherein determining NMR signal strength at the phantom when the at least one RF coil is loaded with the patient for an MRI scan is performed responsive to a locator pulse sequence preceding a diagnostic MRI scan.

10. An MRI system as in claim 1 , wherein determining NMR signal strength at the phantom when the at least one RF coil is not loaded with the patient for an MRI scan includes accessing a previously measured and stored value representing the NMR signal strength at the phantom when the at least one RF coil is not loaded.

11. An MRI system as in claim 1 , wherein said SAR is calculated using at least: (a) patient weight; (b) NMR signal strength at the phantom when the at least one RF coil is loaded with the patient for an MRI scan; (c) NMR signal strength at the phantom when the at least one RF coil is not loaded with the patient for an MRI scan; and (d) a comparison of power outputs when the at least one RF coil is and is not loaded with the patient for an MRI scan.

12. An MRI system as in claim 1 , wherein said SAR is calculated using the following formula:

SAR

=

1

weight

⁢

{

P

scan

-

P

unloaded

·

(

S

scan

S

unloaded

)

2

}

where:

weight=patient weight in kilograms;

P scan =RF power transmitted to the RF coil when loaded with the patient for an MRI scan;

P unloaded =RF power transmitted to the RF coil when not loaded with the patient for an MRI scan;

S scan =NMR signal strength at the reference phantom when RF coil is loaded with patient for an MRI scan; and

S unloaded =NMR signal strength at the reference phantom when RF coil is not loaded with patient for an MRI scan.

13. An MRI system as in claim 12 , wherein calibration values for P unloaded and S unloaded are pre-determined and pre-stored in memory for ready use in calculating SAR during subsequent MRI patient imaging procedures.

14. A magnetic resonance imaging (MRI) method comprising:

configuring and using an MRI system having static and gradient magnetic field generators, at least one radio frequency (RF) coil configured to transmit RF nuclear excitation pulses into an imaging volume, and to receive nuclear magnetic resonance (NMR) RF signals from a patient located in the imaging volume, at least one reference phantom arranged at a location being within the at least one RF coil, the location substantially unaffected by a scattered magnetic field created by the patient, and an MRI control system having at least one configurable computer to:

determine a specific absorption rate (SAR) for the patient based on at least: (a) NMR signal strength at the phantom when the at least one RF coil is loaded with the patient for an MRI scan, and (b) NMR signal strength at the phantom when the at least one RF coil is not loaded with the patient for an MRI scan.

15. The method as in claim 14 , wherein said location is spatially displaced away from the patient towards an end of the at least one RF coil to reduce scattered RF coil field effects.

16. The method as in claim 14 , wherein the determining NMR signal strength at the phantom without the patient in the imaging volume comprises:

applying an NMR pulse sequence to the imaging volume without the patient; and

measuring NMR signal strength responsive to the pulse sequence at the phantom.

17. The method as in claim 14 , determining NMR signal strength at a location with a patient in an imaging volume comprises:

applying an NMR pulse sequence to the imaging volume with the patient being present in the at least one RF coil; and

measuring NMR signal strength responsive to the NMR pulse sequence at the phantom.

18. An MRI method as in claim 14 , wherein said SAR is machine calculated using at least: (a) patient weight; (b) NMR signal strength at the phantom when the at least one RF coil is loaded with the patient for an MRI scan; (c) NMR signal strength at the phantom when the at least one RF coil is not loaded with the patient for an MRI scan; and (d) a comparison of power outputs when the at least one RF coil is and is not loaded with the patient for an MRI scan.

19. An MRI method as in claim 14 , wherein said SAR is calculated using the following formula:

SAR

=

1

weight

⁢

{

P

scan

-

P

unloaded

·

(

S

scan

S

unloaded

)

2

}

where:

weight=patient weight in kilograms;

P scan =RF power transmitted to the RF coil when loaded with the patient for an MRI scan;

P unloaded =RF power transmitted to the RF coil when not loaded with the patient for an MRI scan;

S scan =NMR signal strength at the reference phantom when RF coil is loaded with patient for an MRI scan; and

S unloaded =NMR signal strength at the reference phantom when RF coil is not loaded with patient for an MRI scan.

20. A non-transitory computer readable storage medium having instructions stored thereon that, when executed by a computer of an MRI system having static and gradient magnetic field generators, at least one radio frequency (RF) coil configured to transmit RF nuclear excitation pulses into an imaging volume, and to receive nuclear magnetic resonance (NMR) RF signals from a patient located in the imaging volume, at least one reference phantom arranged at a location within the at least one RF coil, the location being substantially unaffected by a scattered magnetic field created by the patient, causes the computer to:

determine a specific absorption rate (SAR) for the patient based on at least: (a) NMR signal strength at the phantom when the at least one RF coil is loaded with the patient for an MRI scan, and (b) NMR signal strength at the phantom when the at least one RF coil is not loaded with the patient for an MRI scan.