Influence of insonification angle on echogenicity of B-mode images of atherosclerotic plaque in vitro

A newly developed (off-line) spatial compound scanner was used to scan formalin-fixed atherosclerotic carotid plaques. Fony-eight B-mode images were recorded using 7 insonification angles. All calculations were done on the envelope-detected image data. The mean amplitude level (MALJ in (relative) volts was calculated for the plaque region in each image. for each of the 7 angles between 0.12 V and 0.18 V. For The standard deviation over the 48 MAL values were each scan plane, the standard deviation was also calcuof these 48 numbers were 0.07 Vand 0.04 V, respectively. luted over the 7 images. The mean and standard deviation Thus based on these data, the variation from scan plane angle for a single scan plane. to scan plane is normally larger than the variation over 1 . Introduction For the purpose of assessing the risk of stroke, several the relative amount of soft materials in atherosclerotic in vivo studies".','] have investigated the relation between plaque removed by surgery and the mean echogenicity of tion is quite weak, and one of the reasons might be the in vivo ultrasound images of these plaques. The correlanon-soft tissues. variation in echogenicity with insonification angle for In an attempt to investigate this, a newly developed spatial compound scanner was used lo scan formalin-fixed tomy). The mean amplitude level was calculated for the atherosclerotic plaque removed by surgery (endarterecplaque region in all images and the variation of this level over scan plane and angle was calculated and analyzed. 2 . Ultrasound System linear m a y transducer with low pitch connected to a The ultrasound system consisted of a 192-element 64-channel (off-line) ultrasound system, as depicted in the lower part of Figure 1. The system was controlled by a real-time scan controller and operated from a workstation. Additional description of the system can be found elsewhere in this pr~ceeding.'~' The system operates by emitting beams in a specified direction (below 3V)[4' from an aperture of 64 elements and then recording the received single-element signal, each element at a time, from the same 64 element aperture. Specifically, each of the 64 single-element signals are preamplified by its own preamplifier and then one signal is selected with a switch, amplified with Time Gain Compensation (TGC) and eventually digitized with a resolution of 12 bits at up to 60 MHz sampling rate. All signal processing is performed in software, including the dynamic focusing at receive. Subsequent signal processing consists of bandpass filtertion, scaling and display. ing, envelope detection, scan conversion, logarithmizaThe implementation of the signal processing in softsignals are available at all instances in the signal ware has the advantage that very high quality ultrasound processing chain. In this paper, all calculations are based on the raw (envelope-detected) image data. while all image displays show the logarithmic images Y . . . . . . . . . . . . . . . . . . . . . . . . . ,