k-Wave User Forum » Topic: TGC in B-Mode Example Script

k-Wave User Forum » Topic: TGC in B-Mode Example Script http://www.k-wave.org/forum/topic/tgc-in-b-mode-example-script Support for the k-Wave MATLAB toolbox en-US Wed, 13 May 2026 01:49:26 +0000 http://bbpress.org/?v=1.0.2 <![CDATA[Search]]> q http://www.k-wave.org/forum/search.php Bradley Treeby on "TGC in B-Mode Example Script" http://www.k-wave.org/forum/topic/tgc-in-b-mode-example-script#post-8252 Wed, 21 Jul 2021 15:59:52 +0000 Bradley Treeby 8252@http://www.k-wave.org/forum/ Hi Neo, Yes, the factor of 2 in the k-Wave example is to account for the round trip to a depth of r. You can think of it as calculating the correct distance the wave has travelled, rather than a correction to the attenuation factor. Brad. Neo on "TGC in B-Mode Example Script" http://www.k-wave.org/forum/topic/tgc-in-b-mode-example-script#post-8159 Wed, 26 May 2021 18:43:24 +0000 Neo 8159@http://www.k-wave.org/forum/ I can't edit my post, so I have to write another answer.... In my second post the factor <code>r</code> is missing in both <code>tgc</code> equations. I had an error in my code, the TGC with factor 2 as written in the example now works better than my proposed TGC. Do we have to include the additional factor 2 because <code>alpha_coeff</code> is the "one-way" attenuation? If we measure the signal arriving from a depth <code>r</code> with a transducer, do we have to use the "two-way" attenuation <code>2*alpha_coeff</code>? Neo on "TGC in B-Mode Example Script" http://www.k-wave.org/forum/topic/tgc-in-b-mode-example-script#post-8158 Wed, 26 May 2021 12:48:37 +0000 Neo 8158@http://www.k-wave.org/forum/ To make things easier... Here is the complete code that I refer to in the "Simulating B-mode Ultrasound Images" example: <code>t0 = length(input_signal)*kgrid.dt/2;</code> <code>r = c0 * ( (1:length(kgrid.t_array)) * kgrid.dt / 2 -t0);</code> <code>tgc_alpha_db_cm = medium.alpha_coeff * (tone_burst_freq * 1e-6)^medium.alpha_power;</code> <code>tgc_alpha_np_m = tgc_alpha_db_cm / 8.686 * 100;</code> <code>tgc = exp(tgc_alpha_np_m * 2 * r);</code> If we substitute everything the tgc would then be defined as <code>tgc = exp((1/4.343)*alpha_coeff*100*(f/1MHz)^alpha_power)</code> instead of <code>tgc = exp((1/2)*(1/4.343)*alpha_coeff*100*(f/1MHz)^alpha_power)</code> Neo on "TGC in B-Mode Example Script" http://www.k-wave.org/forum/topic/tgc-in-b-mode-example-script#post-8157 Wed, 26 May 2021 12:42:36 +0000 Neo 8157@http://www.k-wave.org/forum/ Hi, I would like to discuss the time gain compensation that is presented in the Example "Simulating B-mode Ultrasound Images". I saw that you corrected/modified the time gain compensation in the last release and you added the convertion of the tgc alpha from dB to Np. Is it possible that there is still a mistake in the tgc calculation? I think that the factor 2 in <code>tgc = exp(tgc_alpha_np_m * 2 * r)</code> is not correct. Angelsen writes in his book "Ultrasound Imaging Waves Signals and Signal Processing (Vol I)" that the attenuation in dB is defined as <code>alpha_dB = (1/r)*10log(I0/I(r))</code> where <code>I0</code> is the intensity in the incident wave and <code>I(r)</code> is the intensity at depth <code>r</code>. The intensity at depth <code>r</code> is then again: <code>I(r) = I0*exp(-µ(f)*r)</code> where <code>µ(f)</code> is the intensity absorption coefficient: <code>µ(f)=A*(f/f1)^m</code> with <code>A</code> in Np/length element and <code>m</code> beeing the equivalent to alpha_power in k-Wave. For pressure we can use the amplitude absorption coefficient <code>k(f)</code>: <code>p(r) = p0*exp(-k(f)*r)</code> where <code>p0</code> is the pressure amplitude at <code>r=0</code> and <code>p(r)</code> is the pressure at depth <code>r</code>. Since the intensity is the square of the amplitude we can see that <code>µ(f)=2k(f)</code> and we can calculate the attenuation in dB as: <code>alpha_dB = (1/r)*10log(I0/I(r)) = (1/r)*20log(p0/p(r)) = 4.34*A*(f/f1)^m</code> We can rewrite the equation of <code>p(r)</code> as: <code>p(r) = p0*exp(-k(f)*r) = p0*exp(-(1/2)*µ(f)*r) = p0*exp(-(1/2)*A*(f/f1)^m*r)</code> with <code>A</code> in units of Neper. If we want to insert <code>alpha_coeff</code> in dB we have to write: <code>p(r) = p0*exp(-(1/2)*(1/4.34)*alpha_coeff*100*(f/1MHz)^m*r)</code> (with [alpha_coeff]=dB/(cmMHz)) Thus the TGC has to be: <code>tgc = exp(+(1/2)*(1/4.34)*alpha_coeff*100*(f/1MHz)^m*r)</code> This equals <code>tgc = exp(tgc_alpha_np_m * 2 * r)</code> as it is written in the example except for the factor 2. I think the correct code is <code>tgc = exp(tgc_alpha_np_m * r)</code> without factor 2. A test of both TGCs also shows that Time Gain Compensation without the factor of 2 provides better results.