Page 56 - Libro vascular I

Chap-04.qxd 29~8~04 13:22 Page 47
CREATION OF A COLOR FLOW IMAGE
47
example, a broad-band transducer is used to insonate the contrast agent at a frequency of 3MHz, the scanner will be able to detect a back-scattered sig- nal from the microbubbles at a frequency of 6MHz. The surrounding tissue, however, does not oscillate to the same extent and will therefore not produce as big a back-scattered signal at the higher harmonic frequency. The Doppler shift imposed on the harmonic frequency can be extracted and displayed on a color image or as a Doppler spectrum. This technique, known as harmonic imaging, used in conjunction with contrast agents, may improve the sensitivity of Doppler ultrasound. One of the negative aspects of the use of contrast agents is that the ultrasound examination becomes an invasive procedure.
destroyed as they pass through the lungs and, therefore, when injected intravenously, are only suitable for imaging the right side of the heart. Left heart, or transpulmonary, agents can pass through the lungs and can therefore be used to enhance the back-scattered signal from peripheral arteries. These agents effectively enhance the Doppler sig- nal for approximately 5–10 min, so are only really suitable for investigations that do not take longer than this to perform.
Bubbles insonated with ultrasound will oscillate and will back-scatter ultrasound both at the fre- quency at which they were insonated and at higher frequencies. These higher frequencies are harmon- ics of the original frequency (i.e., they are multiples of the fundamental frequency) (see Ch. 2). If, for
Reference
Ferrara K, DeAngelis G 1997 Color flow mapping. Ultrasound in Medicine and Biology 23(3):321–345
Further reading
Evans D H, McDicken W N 2000 Doppler ultrasound: physics, instrumentation and signal processing. Wiley, Chichester
Hoskins P R, Thrush A, Martin K, Whittingham T (eds) 2003 Diagnostic ultrasound: physics and equipment. Greenwich Medical Media, London
Zagzebski J A 1996 Essentials of ultrasound physics. Mosby, St Louis