10.4.1 Ultrasound imaging

• Ultrasound is a longitudinal wave with a frequency above 20 kHz. In medical imaging, it typically ranges between $1$ MHz and $20$ MHz.
• When ultrasound waves interact with human tissues, they can produce internal images. This is because ultrasound waves reflect at boundaries between different tissues, which have varying densities and acoustic properties.

Production of Ultrasound Waves: The Piezoelectric Effect

• Piezoelectric materials (e.g., quartz) are used to generate and detect ultrasound waves:
  • When a potential difference is applied to a piezoelectric material, it undergoes mechanical deformation (changes shape).
  • This deformation generates ultrasound waves through the piezoelectric effect.
• The transducer, a device containing piezoelectric material, is responsible for both transmitting and receiving ultrasound waves.
  • Transmission: Applying an alternating potential difference causes the piezoelectric material to vibrate at the same frequency, generating ultrasound pulses.
  • Reception: When returning ultrasound waves strike the piezoelectric material, they cause it to deform again, producing an electrical signal.

Improving Image Resolution

• Damping is applied to the transducer to produce short pulses of ultrasound waves. This prevents overlapping signals, enhancing resolution.

Reflection and Acoustic Impedance

• Reflection of ultrasound occurs at boundaries between different media due to differences in acoustic impedance (Z):
  • Acoustic Impedance $(Z)$ is defined as $Z = \rho c$, where:
  • $\rho$ is the density of the material.
  • $c$ is the speed of sound in the material.
• The Intensity Reflection Coefficient $\left( \frac{I_r}{I_i} \right)$ determines how much of the ultrasound is reflected at a boundary:

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

where $I_r$ and $I_i$ are the intensities of the reflected and incident waves, and $Z_1$, $Z_2$ are the acoustic impedances of the two media.

Acoustic Impedance of Various Body Tissues

The following table provides values for density, speed of ultrasound, and acoustic impedance for several common materials in the body. These values are essential in determining how ultrasound interacts with each tissue type.

Medium	Density (kg/m³)	Speed of Ultrasound (m/s)	Acoustic Impedance (kg m⁻² s⁻¹)
Air	$1.3$	$330$	$429$
Water	$1000$	$1500$	$1.5 \times 10^6$
Fat	$925$	$1450$	$1.34 \times 10^6$
Muscle	$1075$	$1590$	$1.70 \times 10^6$
Bone	$1400–1900$	$4080$	$5.7 \times 10^6\ to\ 7.8 \times 10^6$

Coupling Medium for Effective Imaging

• To ensure efficient transmission of ultrasound into the body, a coupling medium (like a gel) is used between the transducer and the skin. This removes air, which would otherwise reflect most of the waves.

Attenuation

• Attenuation refers to the reduction in the amplitude of ultrasound waves as they travel through tissue, mainly due to scattering and absorption.

Types of Ultrasound Scans

1. A-Scan (Amplitude Scan)

• Used for simple measurements like the distance from skin to a particular structure.
• The time delay between sending the ultrasound pulse and receiving the reflected signal determines the depth of structures.
• Typically used in ophthalmology (e.g., measuring eye length).

2. B-Scan (Brightness Scan)

• Involves moving the transducer across the patient to build a 2D image.
• The amplitude of the reflected signal determines the brightness of each point in the image.
• Used for imaging more complex structures, such as foetal development and internal organs.

Advantages of Ultrasound Imaging

• Non-invasive and does not use ionising radiation, making it safer.
• Portable and quick: It provides real-time imaging.
• Effective for soft tissue imaging, such as monitoring foetal health.

Disadvantages of Ultrasound Imaging

• Limited by bone and air: Ultrasound cannot effectively penetrate bones or air-filled regions like the lungs.
• Lower resolution compared to other imaging techniques like X-rays.
• Operator-dependent: Image quality relies on the skill of the technician.