Method and system for tissue imaging and analysis

1. A method for distinguishing different tissue in a region of tissue based on their different responses to heating, comprising:

a) making a first measurement of ultrasound backscattered from the region;

b) heating the different tissues in the region, at least after the first measurement, causing the temperature at each point in the region to rise by no more than 4 degrees Celsius;

c) making one or more additional measurements of ultrasound backscattered from the region after some or all of the heating; and

d) analyzing the measurements to distinguish the different tissue by finding differences in changes, caused by the heating, of one or more characteristics of ultrasound backscattering, between the tissue; wherein said analyzing comprises calculating a frequency shift of backscattered ultrasound as a function of position from which it is scattered;

wherein the region is greater than 1 cm in diameter.

2. A method according to claim 1 , wherein the heating heats both a tissue in the region that is no more than 1 centimeter in diameter in its shortest dimension, and a different surrounding tissue in the region, so that the two tissues rise in temperature by amounts that differ from each other, and the measurements and analysis detect the different temperature rises, to distinguish the two tissues.

3. A method according to claim 1 , wherein analyzing comprises calculating one or more characteristics of a distribution of amplitudes of backscattered ultrasound as a function of position from which it is scattered.

4. A method according to claim 1 , wherein heating comprises heating with ultrasound.

5. A method according to claim 4 , wherein heating with ultrasound comprises using ultrasound generated by a different ultrasound transducer than a transducer used to generate the backscattered ultrasound measured in the first and additional measurements.

6. A method according to claim 5 , also comprising not running the transducer used to generate the ultrasound used for heating, while making the first and additional measurements.

7. A method according to claim 4 , wherein heating comprises using ultrasound power that does not exceed spatial peak temporal-average (Ispta) of 720 mW/cm^2.

8. A method according to claim 1 , wherein analyzing comprises calculating a temperature change as a function of position in the region, as a result of the heating, from the changes in the one or more ultrasound backscattering characteristics.

9. A method according to claim 8 , wherein the calculated differences in the temperature change, for at least some different positions in the region, are accurate to within less than 2 degrees Celsius, with a spatial resolution of 1 centimeter or better.

10. A method according to claim 9 , wherein the calculated temperature change, as a function of position in the region, is accurate to within 2 degrees Celsius, with a spatial resolution of 1 centimeter or better.

11. A method according to claim 1 , wherein making one of more additional measurements comprises making at least two additional measurements, and analyzing comprises calculating an ultrasound absorption rate and a thermal equilibration rate as a function of position in the region.

12. A method according to claim 1 , wherein the different tissues comprise abnormal tissue and healthy tissue.

13. A method according to claim 1 , wherein the heating heats both a tissue in the region and a different surrounding tissue in the region, so that the two tissues rise in temperature by amounts that differ by no more than a factor of 3 from each other, and the measurements and analysis detect the different temperature rises, to distinguish the two tissues.

14. A system for distinguishing different tissue in a region of tissue, comprising:

a) a diagnostic ultrasound transducer and detector;

b) a tissue heating element, the same as or different from the diagnostic ultrasound transducer, configured for heating a region of tissue greater than 1 cm in diameter; and

c) a controller programmed to:

i) control the tissue heating element to heat the different tissues in the region by no more than 4 degrees Celsius;

ii) control the diagnostic ultrasound transducer and detector to make at least two measurements of backscattered ultrasound from the region, respectively before and after at least one time interval when the tissue heating element is heating the tissue in the region;

iii) analyze results of the measurements to find differences, at different positions in the region, in one or more characteristics of ultrasound backscattering before and after the time interval, that distinguish the different tissue, wherein said characteristics comprise a frequency shift of backscattered ultrasound as a function of position from which is scattered; and

iv) using the differences in frequency shift to identify which parts of the region are which of the different tissue.

15. A system according to claim 14 , wherein the tissue heating element comprises an ultrasound transducer, the same as or different from the diagnostic ultrasound transducer.

16. A system according to claim 15 , wherein the heating ultrasound transducer is different from the diagnostic ultrasound transducer.

17. A system according to claim 14 , wherein the controller is programmed to use the differences in one or more characteristics of ultrasound backscattering before and after the time interval, to find differences in one or both of a heating rate of the tissue, and a temperature equilibration rate of the tissue.

18. A system according to claim 17 , wherein the controller is programmed to use the differences in one or more characteristics of ultrasound backscattering to find differences in both the heating rate of the tissue, and the temperature equilibration rate of the tissue.

19. A system according to claim 14 , wherein the controller is programmed to analyze results of the measurements to find differences that distinguish an abnormal tissue from healthy tissue, and to use the differences in frequency shift to identify which parts of the region are the abnormal tissue and which parts are healthy tissue.