Understanding Axial and Lateral Resolution in Single-Element Transducers

1. Chapter 5 Single-element transducers: transmission and echo reception

2. Key terms • Axial resolution • Beam width • Degree of focusing • Depth of field • Dynamic range • F- number • Far field • Focal length • focal zone • Focusing • Lateral resolution • Near field • Noise • Signal processing • Signal–to–noise ratio (SNR)

3. Axial resolution (also called range resolution and depth resolution) specifies how close together two objects can be along the axis of the beam and yet still be detected as two distinct entities. Depends on several factors such as: SPL, Maching layer, damping, frequency etc.

4. SPL depends on the frequency of the transducer for a constant number of cycles, as the frequency increased, the wavelength and SPL decrease and improves the axial resolution. The best possible axial resolution is the SPL divided by 2. The beam leaves the transducer with a spatial pulse length of 1 SPL and is directed toward two interface spaced SPL/2 apart.

5. Parameters affecting SPL

6. LATERAL RESOLUTION Lateral resolution describes the ability to distinguish as separate entities two objects adjacent to each other oriented perpendicular to the beam axis. It also refers to the ability of the ultrasound beam to detect single small objects perpendicular to the direction of propagation.

7. The ultrasonic field Near field: Far field:

8. Actual intensity distribution

9. Transducer selection The divergence of the beam in the far field is less, which makes the larger transducer preferable for deep lying structures. A similar effect is observed with frequency: that is, with other factors constant, an increased frequency results in a deeper near field and a less diverging far field.

10. Self – focusing Effect At a distance equal to twice the NFD, the NFD, the beam diameter diverges to a size equal to the crystal diameter

11. SIDE LOBES Side lobes are secondary projections of ultrasonic energy that radiate away from the main ultrasound beam. The intensity of side lobes is normally 60 to 100 dB below that of the main ultrasound beam, which usually does not pose significant problems.

12. Focusing Lateral resolution can be improved by focusing the transducer crystal . The focal zone is defined as the region where intensity has a value within 3 dB of maximum along the transducer axis.

13. Focusing Methods • Focused transducer with an acoustic lens. • Focused transducer with a curved piezoelectric.

14. Degree of focusing The degree of focusing can be changed to vary the focal length by increasing the radius of curvature of the crystal or by increasing the curvature of the acoustic lens. The degree of focusing (κ) is expressed quantitatively as the ratio of NFD to focal length.

15. Beam width One of the major advantages of focusing is to reduce the beam width in the focal zone. The beam width in the focal zone (w) is determined by aperture (2a), focal length (f) , and wavelength (λ).

16. F- Number Another parameter used to characterize focusing is the f- number or the ratio of focal length (F) to the aperture size(2a).

17. Depth of field The focal zone is the region within the ultrasonic field that provides the best lateral resolution. The length of focal zone or depth of field is: Depth of filed=7.1λ(f-number)=(7.1 λF2)/4a2

18. Depth dependent of lateral resolution • Lateral resolution is not constant at different depth • Effects on beam with

19. A-scan Main part • Clock pulse repetition frequency (PRF) • Ultrasonic velocity • Depth of investigation • Number of line per image • Filter • Transmitter • Receiver • TGC+Amplifier • Radio Amp. • Video Amp. • Time generator • Processing

20. Flow chart of the signal in Ultrasonic imaging

21. NOISE and SNR Noise is inherent in the measurement process and cannot be totally eliminated. System electronic also contributes to it. The relative amplitude of the signal compared to the noise variation is delineated by the signal – to – to – noise ratio (SNR)