Overview Speed of Sound from High-Speed Images ...

Characterising Focused Ultrasound via High Speed Shadowgraphic Imaging at 10 Million Frames per Second Kristoffer Johansen, Jae Hee Song and Paul Pren�ce CavLab, Cluster of Ultrasound Science, Technology and Engineering Research University of Glasgow, UK

3

Overview Ultra-high speed imaging of focused ultrasound propaga�on at 10 million frames per second with 256 frames per sequence. Synchronized 10 ns laser illumina�on provides shadowgraphic capabilites. Pressure fluctua�ons can be directly observed. 1 3 High intensity focused ultrasound (HIFU) characterisa�on [1] Speed of sound. 2 Field informa�on at needle hydrophone (N.H.) �p Phase characterisa�on of a N.H. 4

1

Field Perturba�on at 3.4 MHz

3

Fig. 3 (left) Direct observation of field perturbation from N.H. (right) Pixel data along propagatin axis

Op�cal and acous�c measurement of a 3.4 MHz HIFU. High-speed shadowgraphic imaging show construc�ve and destruc�ve interference, highlighted by rectangles ( and ). Also depicted in the plot (right), where N.H. output ( ), and varia�on in gray scale ( and ) [2] along line centered at N.H..

Field Characterisa�on at 1.1 MHz (b)

(a)

High-Speed Camera

�p

Needle Hydrophone

Monozoom 7 Synchronized 10 ns laser pulse

1mm

HIFU field

(b) Beam profiles

In (a), high-speed shadowgraphic imaging of a 1.1 MHz HIFU field. Red line indicates pixels that are sampled for speed of sound es�ma�on and phase characterisa�on of needle hydrophone. Blue line shows where beam profile measurement was undertaken. In (b), op�cally measured beam profile ( ) from (a), fiber-op�c hydrophone ( ), and theore�cal predic�on ( ) [3].

4

Phase Characterisa�on of Needle Hydrophone 30 (a)

1 0 -1 50

20 [°]

Speed of Sound from High-Speed Images

(c)

25

60

70

80

NH

Normalised amplitude [-]

2

Single element focused transducer

90

15

φ

Fig.1 (a) 1.1 MHz HIFU

(b)

1 0

5

-1 70

10

70.5

71

71.5

72

0

0.7

0.8

0.9

1

1.1

1.2

Time [µs] Frequency [MHz] Fig. 4 (a,b) Propagation time delay estimation (c) Calibrated and estimated phase of N.H. 76.5

77.0 Axial [mm]

77.5

Fig. 2 Speed of sound in water from images

Speed of sound es�mated ( ), along the red line in line in Fig. 1 (a) from cross-correla�on of pixels that are separated by λ/2 , where ( ) is the averaged es�mated speed sound, and( blue is the theore�cal predic�on [5].

Conclusion

)

Acous�c and op�cal measurement of ultrasonic field at 1.1 MHz. In (a), deconvolved needle hydrophone measurement ( ) [4], pixel sampled at midpoint ( ) of red line Fig. 1 (a), and propaga�on �me delayed pixel data ( ) rela�ve to deconvolved needle. In (b), propaga�on �me delayed ( ), and convolved needle hydrophone measurement ( ). In (c), phase characterisa�on of needle hydrophone along red line in Fig. 1 (a) using high-speed shadowgraphic imaging, where ( ) is Na�onal Physical Laboratory calibrated phase with uncertain�es, ( ) is averaged es�maed phase, and ( ) is average es�mated phase phase from acous�c focus, Fig. 1 (a). Uncertain�es are represented by the standard devia�on of es�mated phase for respec�ve regions.

Unprecedented field characterisa�on with shadowgraphic imaging. Speed of sound es�ma�on by cross-correla�on of op�cal measurements. Op�cal observa�on of field perturba�on from reflec�on during hydrophone measurement. Phase characterisa�on of needle hydrophone by correla�on of op�cal and acous�c data.

Refrences

Acknowledgment The research leading to these results has reveived funding from the European Research Council under the European Unions Seventh Framework Programme (FP/2007 - 2013)/ERC Grant Agreement no. 336189(TheraCav). The authors acknowledge Keith Johnston, Grame Casey and Miriam Jiménez García for technical assistance.

[1] N. Kudo, "A Simple Technique for Visualizing Ultrasound Field Without Schlieren Op�cs," Ultrasound Med. Biol., vol. 41, pp. 2071-2081, 2015. [2] I. Thormählen et al., "Refrac�ve Index of Water and Its Dependence on Wavelength, Temperature, and Density," J. Phys. Chem. Ref. Data, vol. 14, pp. 933 - 945, 1985. [3] E Radulescu et al., "Hydrophone Spa�al Averaging Correc�on from 1 to 40 MHz," IEEE Trans. Ultrason. Ferroelectr. Freq. Control, vol. 48, pp. 1575 - 1580, 2001 [4] A. Hurrell, "Voltage to pressure conversion: are you ge�ng "phased" by the problem? ," J. Phys. Conf. Ser., vol. 1, pp. 57 - 62. [5] H. Medwin, "Speed of sound in water: A simple equa�on for realis�c parameters," J. Acoust. Soc. Am., vol. 58, pp. 1318 - 1319, 1975 ä