Page 31 - Libro vascular I

Chap-02.qxd 29~8~04 13:20 Page 22
22
PERIPHERAL VASCULAR ULTRASOUND
AB
Figure 2.25 Tissue harmonic imaging can provide improved image quality as seen by comparing conventional imaging (A) with tissue harmonic imaging (B) of an aorta.
harmonics with center frequency 2f. As nonlinear propagation only occurs in high-amplitude pulses, harmonics are not present in lower amplitude echoes produced, for example, by multiply reflected pulses, reverberations, grating lobes or side lobes. In conventional imaging it is these spurious echoes that produce noisy images. With THI these spurious echoes will contain little or no harmonics and there- fore will not be detected. Figure 2.25 shows the improvement when imaging an aorta using har- monic imaging compared to conventional imaging.
used in THI is a lower frequency than that used in conventional imaging. For example, in an abdominal application, a center frequency of 3.5 MHz would typically be used in conventional imaging whereas a center frequency of 1.75 MHz would be used for THI, producing a second harmonic at 3.5MHz. Improvements in transducer sensitivity over the years have enabled the production of broad-band transducers with large bandwidths, allowing the transducer to transmit ultrasound with a center frequency f and selectively receive the returning
References
Fish P 1990 Physics and instrumentation of diagnostic medical ultrasound. Wiley, Chichester
McDicken W N 1981 Diagnostic ultrasonics: principles and use of instruments, 2nd edn. Wiley, New York
Whittingham T A 1999 Tissue harmonic imaging. European Radiology 9(Suppl 3): S323–S326
Further reading
Hendrick W R, Hykes D L, Starchman D E 1995 Ultrasound: physics and instrumentation. Mosby, St Louis Kremkau F W 1998 Diagnostic ultrasound—principles and
instruments, 5th edn. WB Saunders, Philadelphia Whittingham T A 1997 New and future developments in
ultrasound imaging. British Journal of Radiology 70: S119–S132
Whittingham T A 2003 Transducers and beam-forming. In: Hoskins P R, Thrush A, Martin K, Whittingham T A (eds) Diagnostic ultrasound: physics and equipment. Greenwich Medical Media Ltd, London, pp 23–48
Whittingham T A 1999 Section I: New transducers. European Radiology 9(Suppl 3): S298–S303
Whittingham T A 1999 Section II: Digital technology. European Radiology 9(Suppl 3): S307–S311