Ultrasound to assess lipid content in salmon muscle
In this thesis, ultrasound pulse transit time measurement techniques are applied to aquaculture, specifically to measure the intramuscular fat in salmon muscle tissue. The main advantages of this technique are that it is noninvasive and that it uses low-cost components. Fat in salmon muscle exists as oil dispersed throughout the tissue. Therefore, a phantom was built to empirically model a dispersed fat system. The phantom was a mixture of low-fat milk and high-fat double cream. By varying the quantities of each component, the fat level of the phantom could be controlled. A trend of increasing speed of sound and attenuation with fat content was observed. Prom velocity measurements at a single temperature, it was possible to predict the fat content of the mixture to within ±1.5% fat. A measurement system was created to measure the sample thickness and the speed of sound through a sample at the same time. Velocity and attenuation measurements were made on fifty samples of salmon muscle tissue containing two distinct fat ranges. A trend of decreasing speed of sound with fat content was observed. Further measurements were taken on twelve more samples and compared to the results of chemical fat analysis to determine the strength of the correlation between fat content and speed of sound through the samples. Again, a trend of decreasing speed of sound with increasing fat content was observed (r=0.73, 71=12). This trend was not as strong as that observed for the phantom due to natural variation in the structure of the tissue. A conclusion drawn from this part of the research is that it may be possible to group the data into "high fat", "medium fat" and "low fat" categories. Attenuation measurements proved too dependent on muscle structure to yield a correlation between attenuation and fat content. Ray-tracing techniques were used to model the propagation velocity of a wavefront travelling through a single salmon sample. The model provided an insight into how variations in temperature, fat content, myoseptum thickness and myosepta configuration affect measured velocity. This thesis provides an insight into how ultrasound velocity measurement may be used to assess the fat content of salmon white muscle tissue. It also provides a starting point for future work in which these techniques may be combined with a vision system to enable similar measurements on live fish.