An alternative to the unfocused high-frame methods is multi-line transmission (MLT). Similar to Space Division Multiple Access (SDMA) used in telecommunications, this method is based on focusing narrow beams in different directions separated in space
Fig. 21. Comparison between conventional focused imaging and MLT [Courtesy to Damien Garcia] The emitted signal applied to each element ሺݏሺݐሻ) used for such a multi-line transmission is achieved by linearly superposing the excitation signals that we would use in order to emit each focused beam successively. The resulting emitted signal can be written as:
ݏሺݐሻ ൌ ݓ௧ ݏ൫ݐ െ ߬௧൯ ேಾಽ
ୀଵ
(2.3) where ܰெ் stands for the number of beams sent simultaneously and the emission delays ߬௧ for each beam k are computed using (1.10). Since ܰெ் beams are emitted in parallel, it takes ܰெ் times less emission events in order to reconstruct as many image lines as in conventional focused imaging. Therefore, the frame rate is increased by a factor of ܰெ் compared to conventional imaging.
An example of combining 3 such focused transmissions for obtaining 3 MLT is shown in Fig. 22. As it can be observed on the right side, the waveform applied to each element in MLT is not just a delayed version of an original signal as it is the case for the other ultrafast methods. Superposing several transmit waveform may result in an irregular complicated signal. For this reason, the implementation of this method in practice requires a certain complexity.
Despite the complexity required by MLT in practice, Tong et al. showed in simulations the potential of this method in increasing the frame rate while preserving an image quality competitive with conventional focused imaging (Tong, Gao, et al. 2013). The practical implementation was later possible thanks to the Arbitrary Waveform Generator (AWG) provided by the ultrasound research system ULA-OP (Ricci et al. 2007).
Fig. 22. The waveforms applied normally to focus in 3 different directions in conventional imaging (left) are superposed in one single transmission in MLT (right)
Although the experimental validation of this method in 2D cardiac imaging showed promising results (Tong et al. 2014), the main limitation of this technique is the presence of cross-talk artefacts. The transmit cross-talk appears when the energy of the side lobes of the transmit beam is picked up by the main lobe of the received beam corresponding to a different direction (Fig. 23 A). On the other hand, the receive cross- talk is created when the energy of the main lobe belonging to the transmit beam is picked up by the side lobes of the received beam corresponding to a different direction (Fig. 23 B). The two-way cross-talk appears when the energy from the side lobes belonging to the transmit beam is picked up by the side lobes belonging to the received beam corresponding to a different direction (Fig. 23 C).
The influence of the cross-talk can also be observed by analytically writing the signal received by the element j of the transducer as:
ݎ݂ሺݐሻ ൌ ൫ݐ െ ߬௧൯ כ ݄௧ሺݐሻ כ ݉ሺݐሻ כ ݉ሺݐሻ כ ݄ሺݐሻ ݊ሺݐሻ ൌ ே ୀଵ ேಾು ୀଵ ேಾಽ ୀଵ ൌ ሺݐ െ ߬௧௧ሻ כ ݄௧ሺݐሻ כ ݉ሺݐሻ כ ݉ሺݐሻ כ ݄ሺݐሻ ݊ሺݐሻ ே ୀଵ ேಾು ୀଵ ൫ݐ െ ߬௧൯ כ ݄ ௧ሺݐሻ כ ݉ሺݐሻ כ ݉ሺݐሻ כ ݄ሺݐሻ ݊ሺݐሻ ே ୀଵ ேಾು ୀଵ ேಾಽ ୀଵǡஷ௧ (2.4)
Compared to (1.14), the received signal showed in equation (2.4) contains a combination of ܰெ் signals, received separately in conventional ultrasound. Thus, if we try to use ݎ݂ሺݐሻ to reconstruct a single line corresponding to a specific focused beam, the influence from the other (ܰெ்െ ͳ ) simultaneously unwanted focused beams will be taken into account, which will lead to cross-talk artefacts.
Fig. 23. MLT cross-talk: The transmit cross-talk appears when the energy of the side lobes of the transmit beam is picked up by the main lobe of the received beam corresponding to a different direction (Fig. 23 A). On the other hand, the receive cross-talk is created when the energy of the main
lobe belonging to the transmit beam is picked up by the side lobes of the received beam corresponding to a different direction (Fig. 23 B). The two-way cross-talk appears when the energy
from the side lobes belonging to the transmit beam is picked up by the side lobes belonging to the received beam corresponding to a different direction (Fig. 23 C). [Retrieved from (Dénarié 2014)]
Several methods have been proposed to reduce these artefacts. One possibility would be to divide the bandwidth of the transducer into multiple sub-bands that correspond with different simultaneous transmissions (Demi et al. 2013). Each of the parallels transmits pulses are generated in a different bandwidth. However, the main limitation of this technique is the degradation of the axial resolution as a result of the bandwidth division. Cross-talk reduction with less impact on axial resolution can be achieved by using a Tukey apodization in both transmit and receive as proposed in (Tong, Gao, et al. 2013). When compared to the Minimum Variance (MV) Beamforming introduced by (Rabinovich et al. 2015), the Tukey apodization provides a better contrast to noise ratio (CNR) and cross-talk reduction. However, the limitation of this method is the degradation of the lateral resolution. Filtered-Delay Multiply and Sum Beamforming (F-DMAS) has been proposed for improving the lateral resolution while providing a better suppression of the cross-talk (Matrone et al. 2017). A drawback of this method is the reduced CNR when compared with non-apodized DAS. Another strategy that allows obtaining an improved lateral resolution compared to Tukey apodization proposes using a predefined set of apodizations based on low complexity adaptive beamforming (LCA) (Zurakhov et al. 2018).
When cross-talk reduction methods are employed, the minimum number of simultaneous focused beams that allow obtaining an acceptable image quality is reduced compared to conventional MLT imaging. While some authors found that 6 parallel transmissions could offer a good image quality (Tong et al. 2014), (Zurakhov et al. 2018), others investigated further increases in frame rate, proving that up to 8 transmissions could provide good in vivo cardiac results (Matrone et al. 2017).
However, forming a full image with just one MLT transmission would not be possible without significant artefacts as it would be the case for PW/DW imaging. On the other hand, transmitting a single unfocused beam would definitely affect the image quality. Although among the methods discussed in this subchapter, MLT and DW/PW imaging offer a very promising gain in frame rate, each one comes with its own compromise between temporal resolution and image quality. A question that one could be posing is which method offers the best compromise for cardiac imaging? Since an important aspect in echocardiography is heart dynamics, the answer should be discussed depending on the particularities of the employed velocity estimation methods.