# Magneto-Acoustic Imaging in Biology
## 4 Maet
### Recent Advances in MAET  
The last decade has seen a burst of activity in developing MAET. For example, [PERSON], [PERSON], and [PERSON], of the University of the Chinese Academy of Sciences, proposed a new twist to MAET [70]. They applied an ultrasound pulse to the tissue in a magnetic field, inducing current. They did not record this current with electrodes, however, but instead detected its associated magnetic field with coils. They referred to their technique as \"Magneto-Acousto-Electrical Tomography with Magnetic Induction\" (MAET-MI), not to be confused with \"Magneto-Acousto Tomography with Magnetic Induction\" (MAT-MI). In MAT-MI, the ultrasound transducer detects the acoustic signal, whereas in MAET-MI  
Figure 8: A longitudinal wave has a displacement (green arrows) parallel to the direction the wave propagates. A shear wave has a displacement perpendicular to the direction the wave propagates. the transducer generates the acoustic wave. MAET-MI has the advantage over traditional MAET in that it does not require attaching electrodes to the sample; it is non-contact (similar to magnetic resonance imaging). Their computer simulations and gel phantom experiments demonstrated that they could reliably reconstruct the conductivity distribution with MAET-MI. Additionally, other groups have investigated how to implement MAET using coils in place of electrodes [71; 72]. [PERSON] and associates at the University of the Chinese Academy of Sciences examined another topic linked to MAET, which they called the \"secondary process\" [73]. An ultrasonic wave in the presence of a magnetic field produces a current. This current, which is in the same magnetic field, creates an acoustic response, as it does during MAT-MI. This secondary process introduces a conductivity-dependent linear damping of the original ultrasonic wave. [PERSON] et al. show, however, that this effect is tiny and should be negligible in biological tissue. Most measurements of conductivity using MAET are based on the assumption that the electrode is small and makes perfect contact with the sample. [PERSON] has carried out finite element model calculations to examine the consequence of large electrodes with a significant contact impedance [74]. He established that electrode area is the dominant artifact compared to a lossy contact, and recommended ways to minimize the electrode effect. [PERSON] and his collaborators combined MAET with shear wave elasticity imaging using a single linear array transducer, providing a powerful dual-modal imaging method [75]. With their device, they could make simultaneous measurements of a pork sample's electrical and mechanical properties. Much recent research has focused on optimizing the shape for the ultrasound pulse used in MAET. For instance, studies by [PERSON] and collaborators [76; 77] and by [PERSON] and coworkers [78; 79] applied a pulse with a linearly increasing frequency (a \"chirp\"), somewhat similar to the slice-selection radio-frequency pulses used in magnetic resonance imaging. The pulse sweeps a frequency range, resulting in a central peak with side lobes. Such pulses reduce the peak power by about 25 dB (a factor of over 300), which increases the transducer lifetime. Although pulse compression methods can make the pulse have a longer duration, [PERSON] et al. [77] were able to still obtain 1 mm spatial resolution with such methods. Another modification of the excitation waveform is to use a coded pulse. Several groups have implemented a Barker coded excitation, which is a digital coding originally developed for pulse compression of radar [80; 81; 82]. One advantage of such coding is that it increases the signal-to-noise ratio, thereby improving the signal quality. Another advantage is that it can increase the speed of signal processing. In [PERSON] et al. 's original MAET experiment, they had to average the signal 2000 times in order to have adequate signal-to-noise ratio [51].