HARMONIC IMAGING

spi

HARMONIC IMAGING
The second new technology improving visualization, harmonic imaging,9 was developed in tandem with contrast media, following the realization that ultrasound has some nonlinear properties. Returning echoes produced by the media are not only at the original fundamental generated by the transducer but at several different frequencies-multiples of the original one and secondary to vibrations of the contrast agent bubbles.
Insonated tissues will also vibrate under the influence of the changing pressures induced by the incident ultrasound wave, and they will reflect echoes at different frequencies. Whereas this was once considered noise or artifact and was suppressed or was assumed to be too weak to be measured, the information has now been captured and turned into meaningful data. A low-frequency transducer may be used (affording better penetration) but the image resolution is improved since the returning frequency is twice as high.
Harmonics are generated while the ultrasound wave travels through the tissues during the transmit phase of the pulse-echo cycle. The returning echoes, at higher frequency, travel only one way-back to the transducer, thereby reducing potential confusing information. Only echoes not at the "right" frequencies are canceled upon reception, thus reducing artifacts. Depending on the equipment vendor, this is called tissue harmonic imaging, native tissue harmonics, and so forth. The result is better signal-to-noise ratio with improved contrast and spatial resolutions. Since time (which equals depth, in ultrasound) is necessary for generation of the harmonics, they are helpful in larger, harder-to-image patients.
The procedure has been shown to improve imaging in patients pregnant above 22 weeks and weighing more than 200 pounds.10 Its advantages are less obvious in patients easier to scan, although some practitioners turn it on simply because of the better contrast. Improved imaging of the liver, gallbladder, pancreas, pelvis, kidneys, and retroperitoneal lymph nodes is on record.11-13 At the moment, only the second harmonic (twice the original frequency) is displayed, but research in multiple harmonics is under way, with the aim of obtaining even better contrast and improved information.
 
 
 
CONTRAST AGENTS
 
The third new wrinkle in ultrasound is the contrast agent.14 The idea of using contrast enhancement goes back more than 30 years, when Gramiak and Shah, working at the University of Rochester, noticed strong echoes in the aortic root when injecting saline through an intra-aortic catheter.
Ultrasound images depend on echoes being produced by the insonated structures (acoustic backscatter). It is therefore easy to conceptualize that increasing the amount of echo-producing substance in the insonated area will create additional echoes and thus, if properly processed, additional information. This may be important when dealing with tiny vessels beyond the resolution of gray-scale ultrasound, color imaging, or power Doppler.
The literature on UCM is already extensive, especially in adult echocardiography for imaging of cardiac cavities and valves, visualization of the coronary arteries, and analysis of myocardial perfusion and tissue viability.15 Other areas where UCM have been clinically used include the hepatic and renal vascular bed.16
In oncology UCM may offer tremendous advantages. Neoangiogenesis (creation of new blood vessels) is common to all malignant tumors, and these new vessels are usually abnormal-irregular in size, branching, and distribution, with flow in bizarre directions. Ultrasound alone cannot detect these small vessels but with the addition of UCM, they may be visualized. This has already been demonstrated in breast cancer17 and undoubtedly will move into other areas like ovarian cancer screening.
In obstetrics and gynecology, the use of UCM is limited,18 but placental perfusion19 and ovarian tumors are potential areas for this modality.