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Superharmonic imaging

Harmonic imaging exploits the generation of harmonic frequencies as an ultrasound wave propagates through tissue.

SUPERHARMONIC IMAGING

In the past years, there have been dramatic technological improvements in the applications of echocardiography. Among the results are high-resolution images, blood flow measurements, real-time 3D imaging and ultrasound contrast imaging. Despite these improvements and the efforts of sonographers and operators, the images were still inadequate in many situations. Overweight patients, elderly patients, patients with dense muscle structure and those recovering from surgery or radiation therapy increase the likelihood of technically inadequate images. This class of patients, considered as patients with poor echocardiographic windows or technically difficult to image, have not benefited as much from these advances as have patients with already good image quality. These factors create haze and other image artefacts and may obscure vital information such as the presence of clots and severe wall motion abnormalities. Recently, much improvement has been gained using harmonic imaging.

Harmonic imaging:

Harmonic imaging exploits the generation of harmonic frequencies as an ultrasound wave propagates through tissue. The generation of harmonic frequencies is related to wave distortion due to slight nonlinearities in sound propagation, which gradually deform the shape of the wave and result in development of harmonic frequencies not present in the initial wave (fig. 1).

Fig. 1: Transmitted ultrasound wave, a nonlinearly deformed wave and their associated spectra.

Second harmonic imaging

The standard imaging mode utilizing the nonlinearity of the wave propagation is second harmonic imaging. Among its advantages are a considerable reduction in near field artefacts, a reduction of the size of the side lobes of the produced ultrasound beam and a narrower beam width compared to fundamental imaging implying a higher lateral resolution (fig. 2). Literature reports that selectively imaging patients using second harmonic imaging significantly improves ultrasound images in several applications compared to fundamental imaging, especially for patients considered technically difficult to image. It represents a major advance that reduces artefacts and enables physicians to make better diagnoses (1).

Nonetheless, second harmonic imaging has some drawbacks, such as the requirement for the ultrasound receiver to have a high enough sensitivity to obtain a sufficient signal-to-noise ratio at the second harmonic frequency. Moreover, even though the –6 dB axial beam width of the second harmonic is smaller than the fundamental beam width, artefacts and reverberations from strong scatterers near the edges of the beam are not entirely suppressed. Furthermore, if axial receive filters are used to distinguish the received second harmonic bandwidth from the transmitted fundamental bandwidth, the axial resolution is reduced. In recent years, a technique such as pulse inversion has been developed to overcome the aforementioned problem. However, this could introduce new drawbacks, such as a reduction of the imaging frame rate (1). Super harmonic Tissue Imaging

In order to alleviate the drawbacks of second harmonic imaging and to further improve the advantages of harmonic imaging a new imaging technique dubbed super harmonic imaging has been suggested by Bouakaz and de Jong (1,2). Super harmonic imaging utilizes the combined effects of the harmonics higher than the second harmonic. The principle is to combine and incorporate the generated higher harmonics into a single component. It is achieved using a very wide band receive filter that selects the desired higher harmonic frequencies ranging from the third to the fifth.

Published experiments have demonstrated the beneficial effects of super harmonic imaging by producing images with exceptionally improved clarity and yielding a dramatically cleaner and sharper contrast between the different structures being imaged. In addition to increased signal-to-noise ratio, super harmonic imaging also showed better contrast and axial resolution as well as acceptable penetration depth (1).

Some of the advantages of super harmonic imaging are improved lateral, axial and temporal resolution compared to fundamental and second harmonic imaging, and a better suppression of reverberations and artefacts at the edges of the beam compared to fundamental and second harmonic imaging (1).

 

Fig 2: simulated beam profiles from the fundamental up to the fifth harmonic for a focused
single element transducer, produced by solving the KZK equation on a spreading grid.

Super harmonic imaging and ultrasound contrast agents.

Originally fundamental imaging was used to visualize and detect ultrasound contrast agents. In this mode, ultrasound contrast agents simply enhance the back-scattered signal, which translates into an increased grey scale level. However, in many situations it turned out to be impossible to detect the micro bubbles in the presence of tissue, especially in very small vessels. When micro bubbles are insonified using sufficiently high acoustic pressure waves, the backscattered signal from the bubble will contain higher harmonics. In practice, micro bubbles generate higher harmonics than tissue does. Therefore, higher harmonic imaging has the potential to produce a better contrast-to-tissue ratio than fundamental or second harmonic imaging and will hence produce a better detection of flow in small vessels like those in the myocardium. The feasibility of super harmonic imaging has been established in earlier work by this department (2).

Project goal The benefits of super harmonic imaging are realized with an appropriate combination of power and frequency in order to ensure acceptable signal-to-noise ratio, higher axial and contrast resolution and permissible penetration depth. In order to fully realize the potential of super harmonic imaging research has to be conducted in very wide band transducers and new signal processing techniques.

Simulating the wave propagation and the transducer response to an electrical signal are also important to fully understand the mechanism involved in super harmonic imaging (fig. 2 and 3).

Fig 3: continuous wave simulation assuming linear wave propagation, displayed using a linear scale.

Literature

1) A. Bouakaz, N. de Jong, ‘Native Tissue Imaging at Superharmonic Frequencies’, IEEE Trans. Ultrason. Ferroelec., Freq Contr, 50(5):496-506, 2003.

2) A. Bouakaz, S. Frigstad, F.J. Ten Cate, N. de Jong, ‘Super harmonic imaging: a new imaging technique for Improved contrast detection’, Ultrasound in Med. & Biol., 28(1):59-68, 2002.