Re: [odin-devel] sinusoidal gradients

[Lazar F. <laz...@gm...>]

Dear Thies,

On Fri, Apr 3, 2009 at 12:30 PM, Thies Jochimsen <th...@jo...> wrote:
> Dear Lazar,
>
> can you please post the relevant part of your code?
>
> Thank you, Thies
>

Here is the line of code which I am using.

I am trying to set a sinusoidal gradient with amplitude equal to the 0.8 of max
and which has a period (full sinus) of 1 ms.

The relationship between the gradient integral (under one sinus lobe) with the
maximum amlitude is: intergral = gmax*period/pi., the ramp duration
is, obviously,
period/4. Of course, this is limited by the slew rate, so basically, I
can not have
very high gradient amplitudes with very short periods.

However, when this gradient is simulated, it has relatively large
horizontal component.

// integral = gmax/pi * period of oscillation, and the ramps are equal
to the quater period, try for oscillation period of 1 ms
// slew rate limit sets limit on max oscillation frequency to use
sinusoidal gradients and reach maximum amplitude:
// gmax*omega < slewrate limit
     GxyzOscillator1 =
SeqGradTrapezParallel("Oscillat1",0.8*systemInfo->get_max_grad()/M_PI*1.0,

0.8*systemInfo->get_max_grad()/M_PI*1.0,

0.8*systemInfo->get_max_grad()/M_PI*1.0,

0.8*systemInfo->get_max_grad(),

0.01,half_sinusoidal,1.0/4.0);


Am I doing something wrong?

Thanks

Lazar
>
> On Freitag, 3. April 2009, Lazar Fleysher wrote:
>> Hello everybody,
>>
>> I can not seem to be able to make sinusoidal-shaped gradients.
>> I was thinking about using trapesoidal gradients and set the rapmtype
>> to "half_sinusoidal".
>> Then I made a calculation for the integral, max gradient and
>> rampduration so that the
>> shape of the gradient will be a sinus (with horizontal part of the
>> trapezoid equal to zero, only ramps).
>> Instead, I seem to observe that some horizontal part of the gradient
>> is kept and the gradient amplitude
>> is reduced to produce correct gradient intergral. I know that if I set
>> the total length of the gradient to
>> a very short one, I will exceed the allowed gradient slewrate and this
>> would happen. But I have done
>> checking and the slewrate is 1/3 of the maximum. This indicates that
>> there is some sort of hard limit
>> on the minimum duration of the horizontal part of the gradient which
>> is substantial. I could not find where
>> it was set in the code.
>>
>> Am I trying to do something illegal? maybe it is a design feature
>> based on some limits which I do not understand,
>> but then, how to make sinusoidal gradients?
>>
>> Thanks a lot for your help
>>
>> Lazar
>>
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