Medical imaging techniques: Practice Questions

When using an endoscope to examine internal organs, coherent bundles of optical fibres are primarily used for which of the following purposes?

Which of the following statements comparing Computed Tomography (CT) and conventional X-ray radiography is/are correct?

(1) CT scans provide 3D section images while conventional X-rays provide 2D shadow images.
(2) CT images represent a map of the distribution of radioisotopes in the body.
(3) The effective dose of radiation for a patient is generally higher in a CT scan than in a single chest X-ray.

A B-scan image of a fetus is formed using ultrasound. If the axial resolution is determined by the pulse duration, which of the following changes would improve (decrease) the axial resolution limit?
(1) Using a higher frequency ultrasound transducer.
(2) Using shorter ultrasound pulses.
(3) Increasing the gain of the amplifier.

In medical ultrasound imaging, which of the following is the main function of the coupling gel applied between the transducer and the patient's skin?

In an ultrasound scan, an ultrasound beam is directed from fat tissue (\(Z_1 = 1.38 \times 10^6 \text{ kg m}^{-2} \text{ s}^{-1}\)) towards muscle tissue (\(Z_2 = 1.70 \times 10^6 \text{ kg m}^{-2} \text{ s}^{-1}\)). What is the percentage of the incident intensity that is reflected at the fat-muscle interface?

State the definition of acoustic impedance \( Z \) of a medium and explain how the difference in acoustic impedance between two media affects the intensity reflection coefficient of ultrasound waves at their interface.

In an ultrasound examination, a coupling gel is applied between the transducer and the patient's skin. In terms of acoustic impedance, explain why this gel is necessary for a successful scan.

Calculate the intensity reflection coefficient \(\alpha\) at the interface between muscle (\(Z_1 = 1.70 \times 10^6 \text{ kg m}^{-2} \text{ s}^{-1}\)) and bone (\(Z_2 = 7.80 \times 10^6 \text{ kg m}^{-2} \text{ s}^{-1}\)). Why is it difficult to image structures behind bone using ultrasound?

Ultrasound Reflection at Tissue Interfaces

A medical ultrasound probe is used to examine the boundary between fat and muscle tissue. The acoustic impedance of fat is \(1.38 \times 10^6\text{ kg m}^{-2}\text{s}^{-1}\) and the acoustic impedance of muscle is \(1.70 \times 10^6\text{ kg m}^{-2}\text{s}^{-1}\).

(a) Calculate the intensity reflection coefficient \(\alpha\) at the fat-muscle interface when the ultrasound beam is incident normally.

(b) What percentage of the incident ultrasound intensity is transmitted into the muscle layer?

(c) Explain why a coupling gel is required between the ultrasound transducer and the patient's skin, referring to acoustic impedance in your answer.

In medical ultrasound imaging, the acoustic impedance \(Z\) of a medium is a key factor in determining the reflection of waves at boundaries. Consider an ultrasound beam traveling through soft tissue (muscle) with an acoustic impedance \(Z_1 = 1.70 \times 10^6 \text{ kg m}^{-2}\text{ s}^{-1}\) and reaching an interface with air in the lungs, where \(Z_2 = 430 \text{ kg m}^{-2}\text{ s}^{-1}\).

(a) Calculate the intensity reflection coefficient \(\alpha\) at the muscle-air interface. (2 points)

(b) Based on your result in (a), explain why it is virtually impossible to image the internal structure of the lungs using ultrasound. (1 point)

(c) When a transducer is placed on a patient's skin, a coupling gel is always applied. Explain the physics behind this practice in terms of acoustic impedance matching. (1 point)