Research Interest

Tissue characterisation for HIFU treatments

G terHaar, L Retat, I Rivens, in collaboration with B Zeqiri, P Gelat & C Bickley, National Physical Laboratory.  EPSRC

Measurement of thermal conductivity

One of the aims of this project is to determine the thermal conductivity of soft tissue as a function of temperature, to serve as an input to our thermal model.

Two main limitations of previously published methods have been established. Firstly, the method used – a one-dimensional heat flow system- is based indirectly on the assumption that the whole tissue volume under investigation is homogeneous in terms of constitutive properties. This has not previously been taken into account, and therefore we have addressed it. 

A tinctorial stain - Haematoxylin and Eosin (H&E) has been used to provide histological information about the samples used. A Matlab-based code has been written to quantify the histological data obtained in terms of density of nuclei, nuclear and cytoplasmic damage (Figure 1).  

Secondly, previous analyses have not taken into account air trapped in the system. A semi-automatic Matlab-based code using a VisualSonics 3-D B-mode ultrasound scan of each sample has been written to determine the volume of air trapped in each sample (Figures 2,3 ).

Figure 1: Red (top), green (middle), and blue (bottom) weighted images of the control (left) and experimental (right) sections.

Figure 2: Typical screen capture from Visualsonics scanner, showing scanning parameters and B-mode image

Figure 3: Mask used for segmentation of the trapped air (white) in the system well (red)

Figure 4: Determination of volume of air trapped threshold for which a correction to the thermal conductivity vale is needed, from analysis of the curve of the change in thermal conductivity (%) versus the amount of air trapped in the system (%). The green area represents the experiments that don’t need correction; the orange area represents experiment where modelling correction is needed

Acoustic Measurements

One of the aims of this project is to determine soft tissue attenuation coefficient, absorption coefficient and speed of sound, in order to aid prediction of the effects of HIFU treatments.
Fibre optic (NPL & UCL, Paul Morris) and large pyroelectric sensors have good potential as calibration devices for HIFU transducers.  

Simultaneous measurement of temperature and acoustic pressure, which is possible with the fibre optic system and measurement of the voltage resulting from temperature rise in the pyroelectric sensor, provide essential parameters for quality assurance. 

Consequently, an investigation of the possible use of these two systems for measuring attenuation, absorption coefficients, and speed of sound has been instigated. Firstly, the sensor responses were compared using two co-axially, co-aligned Imasonic transducers in a method known as Finite Amplitude Insertion Substitution (FAIS); secondly, the  FAIS and thermometric methods were compared. The studies used porcine skin based gelatine with low attenuation, two thicknesses of Tissue Mimicking Materials (2% w/v AGAR) and a highly attenuating 2 MHz attenuator. The sources of error for each method, each device and each reference material have been determined.  

An important aim of this project is to be able to determine the thermal and acoustic properties in a single sample.  We have therefore designed an “all in one” measurement system to achieve this aim.