Tissue harmonic imaging with True Echo Harmonics

Patients with certain types of body composition can be difficult to scan with ultrasound. This is due to acoustic noise that occurs when the ultrasound beam is reflected from the tissues surrounding the targeted area.

Tissue harmonic imaging provides a somewhat different view to standard ultrasound. The two imaging modes can therefore supplement each other to provide a better basis for diagnosis. With B-K Medical’s True Echo Harmonics (TEH), scanning of difficult patients can be improved.

Grating lobes disturb the image
Due to the physical construction of an ultrasound transducer, secondary beams called grating lobes split off at an angle to the main ultrasound beam. When grating lobes are reflected from other structures or fatty tissue, they produce unwanted effects on the ultrasound image as they return to the transducer. The problem is often seen in low-intensity areas of the image. Breast scanning and prostate scanning are examples of applications where grating lobes may reduce image clarity.

Harmonics offer a different kind of signal
Conventional ultrasound imaging sends out a fundamental beam and receives essentially the same frequency range back as an echo. However, the sound wave becomes distorted as the tissue expands and compresses in response to the wave. When a certain energy level is reached, this distortion results in the generation of additional frequencies, called harmonics, that are two, three or more times the emitted frequency. The harmonic frequencies return to the transducer together with the fundamental frequency.

Harmonics arise only in the fundamental frequency, not in grating lobes, because the energy level of the grating lobes is not high enough. The second harmonic (twice the frequency of the fundamental) is the frequency used for harmonic imaging.

Using harmonics to reduce the effect of grating lobes
Although the harmonic signal is weaker than the fundamental beam, it better retains its purity by only having to travel one way, from within the tissue (where it is generated) outwards to the receiver. The accumulation of noise and clutter is significantly reduced, because the harmonic signal only has to travel from the tissue back to the transducer.

By electronically filtering out the fundamental frequency and its grating lobes, the harmonic frequency can be isolated and can be used to create an alternative ultrasound image. Compared with standard mode, the harmonic image will typically show enhanced contrast and gray tone differentiation.

Most effective in the mid-range
Because harmonics are generated within the tissue itself, a certain distance from the sender is required in order for the harmonic wave to build up. Near-field imaging does not permit this. Conversely, signal intensity and sensitivity are lost in the far field. Therefore, harmonic imaging is most applicable when scanning structures in the middle range.

Transducers with True Echo Harmonics
Harmonic imaging is available with a number of B-K Medical transducers. Click here for an overview.

Grating lobes occur because secondary beams split off from the main ultrasound signal, due to certain physical characteristics of the ultrasound system and transducer.