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Monitoring of the formation of lesions in HIFU treatments on the basis of radiated shear waves

13.01.2011

When using pulsed HIFU (High Intensity Focused Ultrasound), shear waves are radiated which contain information on the process of the formation of lesions. These can be used to monitor the progress of the therapy for the early detection and avoidance of irregularities.

HIFU is mainly used to destroy tumour tissue, but increasingly its use is being studied for fields of application in neurology, too. By locally heating up an area by means of HIFU, tissue is necrotised in a clearly delimited area (ellipsoid). Besides the temperature increase, also cavitation and boiling bubbles can occur which also contribute to the formation of lesions. In the occurrence of such bubbles, the ultrasound is reflected by them and the area lying beneath the bubbles is shielded. This leads to a shift of the treatment zone – i.e. a "migration" of the focus towards the HIFU transducer. Furthermore, the ellipsoidal lesion assumes a shape resembling a tadpole. This irregularity must be detected early to avoid uncontrolled necrotisation due to shifting and deformation.

When the HIFU is switched on, the tissue is shifted away from the HIFU transducer due to the acoustic radiation force. The tissue relaxes again as soon as the HIFU is switched off. In both cases, a shear wave radiates into the neighbouring tissue. It was recently shown that the denaturation of proteins is clearly reflected by a change in the elasticity of the tissue. Since the deflection – and, thus, the magnitude of the shear waves – depends on the elasticity of the treated and of the neighbouring tissues, a change in the radiated shear wave in the course of the necrotisation process is also to be expected.

In ex vivo measurements on porcine tissue, lesions were induced by means of a HIFU transducer. Thereby, the HIFU exposure (8 kW/cm²) was periodically intermitted for 4 ms each time, so that the interrupted radiation force caused a shear wave. During this relaxation time of the tissue, high-frequency echo signals (A-scans) were recorded by a single-element transducer at a distance of 5 mm from the HIFU focus (pulse repetition frequency: 8 kHz). A sketch of the measuring arrangement is represented in Figure 1. Using a correlation procedure, the displacement of the tissue was determined from the successive A-scans.

Measuring arrangement for the detection of the emitted shear waves with pulsed HIFU: since the tissue shifts when the HIFU is switched on, due to the acoustic radiation force, and since the tissue relaxes again when the HIFU is switched off, shear waves are emitted into the neighbouring tissue. These shear waves can be detected with a succession of A-scans; they contain information on the evolution of the lesion formation.

Figure 1: Measuring arrangement for the detection of the emitted shear waves with pulsed HIFU: since the tissue shifts when the HIFU is switched on, due to the acoustic radiation force, and since the tissue relaxes again when the HIFU is switched off, shear waves are emitted into the neighbouring tissue. These shear waves can be detected with a succession of A-scans; they contain information on the evolution of the lesion formation.

If the tissue displacement is plotted over time and tissue depth, it is possible to represent the shear wave which passes through the area of the A-scan (see Fig. 2). Based on this, the magnitude of the shear wave in different tissue depths and the corresponding propagation times in the course of the lesion formation were analysed. It was demonstrated that a focus shift leads to a decrease of the shear wave magnitude in deeper tissue areas. A deformation of the lesion is accompanied by considerable changes both of the magnitude and of the propagation time of the shear wave. These results prove that it is possible to obtain information on the formation of lesions from the radiated shear waves. Since this method can be realised with minimum equipment, it is a good alternative to real-time monitoring.

Representation of the shear wave running through the A-scan. The tissue shift is plotted as a function of time and tissue depth.

Figure 2: Representation of the shear wave running through the A-scan. The tissue shift is plotted as a function of time and tissue depth.

Contact person:

Stefanie Dencks, Dept. 1.6, WG 1.62, e-mail: stefanie.dencks@ptb.de