A distinction was made between the measurable in situ "exposure," and "dose" which is not measurable. The difficulty in attempting to calculate the in situ exposure of ultrasound at the location of the developing embryo within the gestational sac was presented. On the basis of tissue properties the calculated total attenuation of a 3.5 MHz ultrasound beam transmitted from the transducer to the gestational sac is 4.1 dB, shown in Table 6. However, this value varies considerably from published in vivo measurements in different patients, and the subject requires further investigation. At frequencies used in pregnancy diagnosis the presence in the sound field of spheres with dimensions equivalent to the gestational sac can significantly increase pressure within the sphere. Also a small change (5%) in density or velocity can give rise to an increase by a factor of 20 in p2 compared to a situation where there is no sphere. Therefore the potential for a maximum pressure "hot spot" exists for a position at the posterior part of the sphere which also corresponds to the position in the gestational sac occupied by a 6-7 wk fetus. These factors indicate the difficulty in attempting to determine dose in situ if exposure parameters alone are used.
ASJC Scopus subject areas
- Radiological and Ultrasound Technology
- Acoustics and Ultrasonics