Exam QuestionsRevision NotesVideo TutorialsPast Papers

Non-Ionising Imaging

01Non-Ionising Imaging

Ultrasound Imaging

This section covers piezoelectric ultrasound transducers, acoustic impedance, reflection and attenuation, pulse-echo imaging, A-scans and B-scans.

Generating and detecting ultrasound

Ultrasound is sound with a frequency above the upper limit of human hearing, 20 kHz20\,\text{kHz}. Medical ultrasound commonly uses frequencies in the MHz range.

A medical ultrasound transducer contains a piezoelectric crystal. The same crystal can act as both transmitter and receiver:

  • An alternating potential difference applied across the crystal makes it repeatedly deform and vibrate at the frequency of the applied signal.
  • The crystal is cut to a suitable size so that its natural frequency corresponds to the required ultrasound frequency and resonance can occur.
  • At resonance the vibration amplitude is large and ultrasound is emitted efficiently.
  • A returning ultrasound wave deforms the crystal and generates an alternating potential difference across it.
  • The received electrical signal can then be amplified and processed.

The crystal is heavily damped, for example using a backing material, so that it produces short pulses rather than continuing to vibrate. Short pulses prevent transmitted and received signals from overlapping and improve the ability to distinguish nearby reflecting boundaries.

transmissiona.c. p.d.crystalultrasounddetectioncrystala.c. signal

Pulse-echo principle

The transducer sends a short pulse into the patient. At a boundary between tissues, some ultrasound is reflected while the remainder is transmitted.

The returning echo is detected by the same transducer. Its time delay provides information about the depth of the boundary, while the strength of the echo provides information about the nature of the boundary.

For a boundary at depth dd, the pulse travels to the boundary and back, giving a total distance 2d2d:

d=ct2d=\frac{ct}{2}

where cc is the speed of ultrasound in the tissue and tt is the round-trip time.

Acoustic impedance

The acoustic impedance, ZZ, of a material is the product of its density and the speed of sound in it:

Z=ρcZ=\rho c

where ZZ is measured in kg m−2 s−1\mathrm{kg\,m^{-2}\,s^{-1}}, ρ\rho is density in kg m−3\mathrm{kg\,m^{-3}}, and cc is the speed of sound in m s−1\mathrm{m\,s^{-1}}.

At a boundary between materials with different acoustic impedances, some ultrasound is reflected and some transmitted. Increasing the difference between the two impedances increases the fraction reflected.

The intensity reflection coefficient is:

α=IrI0=(Z2−Z1Z2+Z1)2\alpha=\frac{I_r}{I_0} =\left(\frac{Z_2-Z_1}{Z_2+Z_1}\right)^2

where IrI_r is reflected intensity and I0I_0 is incident intensity.

  • If Z1Z_1 and Z2Z_2 are very similar, little ultrasound is reflected.
  • If their impedances differ greatly, a much larger fraction is reflected.
  • The reflected and transmitted intensities together account for the incident intensity at the boundary.

Coupling gel and attenuation

Air and body tissue have very different acoustic impedances. An air gap between the transducer and skin would therefore cause almost complete reflection before the pulse entered the body.

A coupling gel removes the air layer and has an acoustic impedance closer to that of tissue, so a much greater fraction of the pulse is transmitted into the patient.

As ultrasound moves through the body, energy is lost through absorption and scattering. This decrease in signal strength is attenuation. Echoes that have travelled farther through the body consequently return with smaller amplitudes.

A swept-gain amplifier can compensate by applying greater amplification to signals received later, because these echoes have travelled farther through the tissue and experienced greater attenuation.

Worked example: Bone has an acoustic impedance of 7.0×106 kg m−2 s−17.0\times10^6\,\mathrm{kg\,m^{-2}\,s^{-1}} and the speed of ultrasound in bone is 4100 m s−14100\,\mathrm{m\,s^{-1}}. Calculate the density.

From Z=ρcZ=\rho c:

ρ=Zc\rho=\frac{Z}{c}

ρ=7.0×1064100≈1.7×103 kg m−3\rho=\frac{7.0\times10^6}{4100} \approx1.7\times10^3\,\mathrm{kg\,m^{-3}}

A-scans and B-scans

Feature A-scan B-scan
Basic method A pulse is sent along one direction and returning echoes are displayed against time. The ultrasound beam is moved across the patient, or measurements are taken in many directions.
Signal display An echo produces a vertical pulse; its horizontal position represents its time delay and hence the depth of the boundary. The amplitude of each echo determines the brightness of a displayed point, while transducer position determines where that point is placed.
Result A one-dimensional measurement of distances or depths. Many measurements are combined to form a two-dimensional image; multiple measurements can also be used to build more detailed images.
Examples Useful for measuring distances such as dimensions in the eye. Useful for imaging internal structures and monitoring a developing foetus.

Higher-frequency ultrasound has a shorter wavelength and can resolve smaller structures. However, increasing resolution comes at the expense of penetration depth.

Advantages and disadvantages

Advantages Disadvantages
Does not use ionising radiation. Image resolution is lower than techniques such as MR or X-ray imaging.
Non-invasive and suitable for imaging a developing foetus. Structures behind bone or gas cannot usually be imaged effectively.
Can produce real-time images of moving structures. Image quality depends strongly on correct transducer positioning and operator skill.
Relatively quick, portable and inexpensive compared with CT or MR scanning. Penetration depth and resolution cannot both be maximised.

Exam Tip: In a longer ultrasound-imaging explanation, give the complete chain: alternating p.d. → piezoelectric crystal vibrates → pulse enters tissue → partial reflection at an impedance boundary → echo deforms crystal → alternating p.d. is produced → time delay gives depth and echo strength gives information about the boundary.

Create a free account to continue

Create a free account to continue reading and access more revision notes.