Aberration and reverberation correction
Ultrasound images are for many individuals severely degraded in quality. This is due to spatial variations of tissue parameters affecting sound wave propagation (aberration), and multiple reflections between tissue layers and the ultrasound probe (reverberations). These artifacts reduce the clinical value of the obtained images. This again may lead to poor and uncertain diagnosis, and in the worst case, no possibility of setting a diagnosis at all. Hence, improving the image quality and resolution by correcting for aberration and reverberation in ultrasound images may increase the clinical potential of ultrasound in many applications. Examples are improved diagnosis following a heart attack, or reduced image artifacts in removal of tumors in brain surgery.
Aberration
Aberration (also known as phasefront aberration) due to spatial variations in the ultrasound propagation velocity in tissue, destroys the focusing of the ultrasound beam and increases the sidelobe level. This leads to a reduction of both the spatial and contrast resolution, blurring the image. Aberration is mainly generated in the human body wall, which is composed of skin, fat, muscle and connective tissue. Typical velocities for tissue in the human body wall is 1448 m/s for fat, 1547 m/s for muscle, and 1613 m/s for skin and connective tissue. These constitute the largest sound speed differences in the human body. The effect of aberration may be compared to smearing vaseline on a camera lens, which totally degrades the focusing quality of the lens.
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| Ultrasound images of a tissue mimicking phantom. The leftpane shows an image with no aberration. The right pane shows an image with aberration generated by a 5 mm thick silicon aberrator mimicking the human body wall. |
Reverberations
Strong spatial tissue heterogeneity also creates acoustic noise through multiple scattering known as pulse reverberations. The reverberations produce a tail added to the propagating pulse, and is observed as additive noise in the image. This effect is strongest when imaging through the body wall, as this is composed of irregular mixtures of muscles, fat, and connective tissue. Note that the effect is also important in strongly heterogeneous objects, for example the female breast.
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| Examples of ultrasound imaging of the Carotid artery in the neck. The lumen (inside of the artery) of the artery is corrupted by reverberation noise from the layers above. |
The problems related to aberration and reverberations in medical ultrasound imaging can be summarized as follows:
- Reduced spatial resolution and contrast the images.
- There are large differences in image quality between patients, making it difficult to properly diagnose all cases.
- The diagnose may be very uncertain for some patients, which makes more investigations necessary. This increases the cost for the public health service.
- Increasing obesity in the population will probably increase problems related the aberration and reverberations.
- If solved, may provide more reliable and correct diagnosis for large groups of patients, and may lead to the possibility of using medical ultrasound in new clinical areas.
Research activities at the department
At the Department of Circulation and Imaging there has been a strong research activity during the last 6 years on understanding aberration and reverberations, and finding methods to correct for it. Follow the link Published Research to read more.
| | Professor | Phone: +47 728 28015 |
Svein-Erik Måsøy Post. Doc. Area of interest: ultrasound, reverberation cancellation, nonlinear propagation, | Office: Prinsesse Kristinas gt. 3 , AHL, 3.etg rom 342.03.033, |
