An ultrasound investigation was used to identify a small volume of an unknown substance in a patient. It is suspected that this substance is either a cyst or a tumour.
During the ultrasound investigation, an ultrasound pulse of frequency 5 MHz is passed through soft tissue and then into the small volume of the unknown substance.
The ratio of the reflected intensity to the incident intensity for the ultrasound pulse reflected at the boundary was found to be 7.54 × 10−5.
Table 1.1 shows some information about the density and speed of ultrasound in soft tissue, cysts and tumours.
Table 1.1
material |
density / kg m−3 |
ultrasound velocity / m s−1 |
soft tissue |
1065 |
1530 |
cyst |
1020 |
1570 |
tumour |
990 |
1565 |
(i)
Use the information in Table 1.1 to show that the unknown medium is a cyst.
[4]
(ii)
A pulse of ultrasound reflected from the front surface of the cyst was detected 27.5 µs later and a pulse from the rear surface was detected 43.2 µs later.
Determine the length of the cyst, in cm.
[2]