The diagram shows an ultrasound scan of a patient's leg after a sports injury. A probe sends a short pulse into tissue. A reflected pulse from a boundary is...
Question 1 Report
The diagram shows an ultrasound scan of a patient's leg after a sports injury. A probe sends a short pulse into tissue. A reflected pulse from a boundary is detected 0.00016 s later. The speed of ultrasound in the tissue is 1540 m/s.
(a) State the range of frequencies used for ultrasound. [1]
(b) Calculate the depth of the boundary below the probe. [4]
(c) Explain why the calculated distance must be divided by two. [2]
(d) Describe how the scanner uses reflected pulses to produce an image of structures inside the leg. [3]
(e) Explain why gel is placed between the probe and the patient's skin. [2]
(f) State one advantage of using ultrasound rather than X-rays for routine scans of an unborn baby. [2]
Answer Details
(a) Ultrasound has frequencies greater than \(20000\text{ Hz}\), or \(20\text{ kHz}\). [1]
(b) The pulse travels to the boundary and back: \[d_{total}=vt=1540\times0.00016=0.2464\text{ m}\] \[\text{depth}=\frac{0.2464}{2}=0.1232\text{ m}\] Accept \(0.123\text{ m}\). [4]
(c) The measured time includes the outward journey to the boundary and the reflected journey back to the probe. Depth is only the one-way distance, so the total distance is divided by two. [2]
(d) Ultrasound pulses reflect at boundaries between different tissues. The time delay gives the depth of each boundary, while echo strength and position are used to form an image of structures inside the leg. [3]
(e) Gel removes the air gap between probe and skin. This reduces reflection at the surface and allows more ultrasound energy to enter the skin. [2]
(f) Ultrasound is non-ionising, so it does not expose an unborn baby to ionising radiation. It can also produce real-time images. [2]
Download The App On Google Playstore
Everything you need to excel in your exams
