Figure 1 shows an ultrasound transducer used to perform medical scans - AQA - A-Level Physics - Question 4 - 2020 - Paper 6

The ultrasound pulse travels through the body and reflects off various tissues with differing acoustic impedances. When the pulse encounters a boundary, part of the wave is reflected back towards the transducer. The transducer is designed to detect these reflected pulses through the same piezoelectric crystal that generated the initial pulse. It converts the mechanical vibrations of the returning sound waves back into electrical signals for processing.

The transducer's ability to both transmit and receive ultrasound pulses is due to its design, allowing for rapid switching between the transmitting and receiving states. After sending out an ultrasound pulse, it can revert to a receiving state almost instantaneously, capturing the echoes from any surrounding tissues. This dual functionality is critical in medical diagnostics, as it enables real-time imaging.

The resolution of the scan can be estimated using the formula:

ext{Resolution} = rac{1}{2} imes rac{v}{f}

where $v$ is the speed of sound in the liver tissue (1600 m/s) and $f$ is the frequency of the ultrasound (1.0 MHz or $1.0 imes 10^6$ Hz).

So,

ext{Resolution} = rac{1}{2} imes rac{1600}{1.0 imes 10^6} = 0.0008 ext{ m} = 0.8 ext{ mm}

To calculate the percentage of incident ultrasound intensity transmitted into the lung tissue, we can use the equation for intensities at the boundary:

$Z_1 = \rho_1 v_1$ $Z_2 = \rho_2 v_2$

Where:

• $Z_1$ is the acoustic impedance of air.
• $Z_2$ is the acoustic impedance of lung tissue.

Substituting values:

• $Z_1 = 1.3 imes 330 \quad (kg/m^3)$
• $Z_2 = 1075 imes 1580 \quad (kg/m^3)$

Using these to find the percentage of intensity transmitted:

$I_t = I_i \left( \frac{Z_2}{Z_1 + Z_2} \right)^2$

Numberical evaluation is required to find the percentage transmitted.

Ultrasound scans can be useful for evaluating soft tissues and structures within the body; however, their effectiveness can vary when it comes to certain conditions like tumours within the lung. The air-filled spaces in the lungs can cause significant reflection and scattering of ultrasound waves, which may hinder their ability to reach and adequately image the target area. As such, while an ultrasound might provide some information, other imaging modalities such as CT or MRI could be more suitable for investigating tumours inside a lung due to their ability to provide clearer and more detailed images through denser structures.