Hi,
I am trying to take into account absorption effects on ultrasound propagation through a skull.
I am wondering whether alpha_max should be in Np/MHz/cm or dB/MHz/cm.
I am considering a heterogeneous absorption map, linearly related to density with a maximum value alpha_max=3.5 and minimum 0 in water around the skull. I understand that the unit is supposed to be dB/MHz/cm; however, with a bone thickness of about 3 mm at 800kHz, I obtained a transmission reduction of 66% compared to the same simulation with alpha_max=0. This result would match a mean alpha_coeff of 1.7 Np/MHz/cm = 15 dB.MHz/cm; as the modelled skull is very porous, the mean absorption coefficient is about half the max coefficient so it seems that the software is actually considering that alpha_max=3.5 Np/MHz/cm instead of 3.5 dB/MHz/cm.
When searching for more information on the website, I found this code (http://www.k-wave.org/documentation/example_us_bmode_linear_transducer.php) where again, it looks like the absorption coefficient unit is Np/MHz/cm and not dB/MHz/cm in the equation "p=p0 exp(-alpha*f*d)
where alpha is the attenuation coefficient in dB/(MHz cm)":
"[p=p0 exp(-alpha*f*d)]
where alpha is the attenuation coefficient in dB/(MHz cm), f is the frequency in MHz (chosen here as the transmit frequency), and d is the total (round-trip) distance travelled in cm. Here the distance variable is created assuming that t = 0 corresponds to the middle of the transmitted tone burst.
% create radius variable assuming that t0 corresponds to the middle of the
% input signal
t0 = length(input_signal)*kgrid.dt/2;
r = c0*( (1:length(kgrid.t_array))*kgrid.dt/2 - t0); % [m]
% create time gain compensation function based on attenuation value,
% transmit frequency, and round trip distance
tgc_alpha = 0.25; % [dB/(MHz cm)]
tgc = exp(2*tgc_alpha*tone_burst_freq/1e6*r*100);"
Could you please clarify this?
Thanks a lot!