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The SAR calculator (SARcalc) is designed to let you see how much power
is actually produced by your sequence. It can be useful for quick
calculations before a study so you do not waste time resetting should the
RF monitor trip. You can quickly see how much energy is present, and how
long you should make the TR period to minimize heating. Configuring the
calculator is not difficult, but requires adding a bit of code to your
pulse sequence source as shown below. The subroutines for the calculation
are located in sarcalc.h.
Instructions for using SARcalc in your pulse sequence (e.g. ase.c):
1) Include the header file into your C program with the line:
#include <sarcalc.h>
It should be at or near the top of your source, after the other #include
directive(s).
2)You must include the energy of each pulse using sarcalc().
The syntax for sarcalc() is as follows:
sarcalc(pulsepat, tpwr, pulsewidth, TT(or DD), ntimes);
TT specifies tn as the nucleus and using the Tx channel for
normal pulses. DD specifies EITHER using the decouple channel
for a 2nd coil (e.g. if you have both a C13 and a proton coil) OR it may
be used in conjunction with sardecouple().
The SARcalc calibrations are for the normal observe & decouple
amplifiers. If you change the amplifiers, the calibrations need to
be adjusted, or you will not get good calculations.
Arrayed Powers
You may use SARcalc when you have arrayed variables for pulsewidth
or power. However, it will only compute the deposited energy for the maximum
values in your arrays, to avoid huge and unnecesary output. Arrays for
pulse shape are not supported at this time.
ntimes means the number of times this exact
pulse appears in the sequence. See example
below.
Another function, sargetpwr() is available. It's syntax is:
sargetpwr(pulsepat, tpwr, pulsewidth, TT(or DD), return_variable)
and instead of adding this energy to the SARcalc total, it just returns
the value into a variable of your choice (return_variable). This
is not used as commonly as the normal, sarcalc() function, but may be useful
for calculating power from a specific modual, like water suppression seperately.
To estimate the power required for decoupling
use
sardecouple(dm[n]);
where n is an integer such that dm[0] indicates status A, dm[1] status
B, etc. Therefore, if your decoupling is indicated by status C, dm='nny'
and you should use:
sardecouple(dm[2]);
This call assumes that the decoupling pattern is square (or nearly
square), and approximates it's power as dpwr * at. Most types of
decoupling are adequately approximated by a square pulse, even adiabatic
decoupling sequences based off BIR-4 shapes are 90% of the power of a square
pulse. If you are not using dpwr or a longer period than at an alternative
construct can be used:
if((dm[2] = = 'y') || (dm[2] = = 'Y') || (dm[2]
= = 'a') || (dm[2] = = 'A') || (dm[2] = = 's') || (dm[2] = = 'S'))
{ sarcalc("hard", dpwrc, at, DD, 1);
}
Note that neither of these constructs can accomidate changing dm='nny'
to dm='y'. The first turns the decoupler channel on only during status
C. The second turns the decoupler on for the entire sequence, but
the check for dm[2] will return '' and not calculate any decoupling power.
These programs are useful tools, but they can't catch all errors and the
operator must still use common sense and reasonable SAR hardware trip levels
in conjunction with these tools.
3) After any and all sarcalc() calls, you need to specify a time constant,
for averaging the power over. Often, a variable such as tr is quite appropriate
for this purpose, but you may use any variable/real number you desire.
You specify this as follows
saravgtime(variable);
This is important. It also displays the final results. You must include
it.
/* HEADER: EXAMPLE.c A sequence that demonstrates sar calculator
*/
/* This is a simple double spin-echo sequence with an optional inversion
*/
/* recovery pulse. Note conditional ir pulse, and multiplication
*/
/* used to simplify calculation and save time.
*/
#include <standard.h>
#include <sarcalc.h>
...
pulsesequence() {
...
...
/* SAR Calcuations *************************************/
getstr("sar", sar);
if (is_y(sar[0])) {
if (ix = = 1) {
sarcalc(p1pat, tpwr1, p1, TT, 1); /*pulse
goes only once*/
sarcalc(p2pat, tpwr2, p2, TT, (2*ne));
/*pulse twice per echo*/
sardecouple(dm[1]);
if (is_y(ir[0])) sarcalc(pipat, tpwri, pi, TT, 1);
/*optional IR*/
saravgtime(tr);
}
}
/* PULSE SEQUENCE ELEMENTS********************************/
...
...
}
***Important: This is ONLY an example. You can paste this
code into your PSG, but you may need to modify some of it. You need
to alter the variable-names to agree with those used in your sequence.
When finished, this section of code should resemble a stripped down version
of the sequence to follow. It should contain similar conditions and/or
loops, and each pulse() or shapedpulse() should be represented here.
5) Create a string variable in VNMR called 'sar', by typing: create('sar','string')
6) Set sar='y' when the calculations must be performed, and sar='n'
when they must be turned off.
7) Calculations should be done on execution of either "dps" or "go"
commands in VNMR.
System Administration Notes:
Calibration tables for SAR calculator may be found in /vnmr/imaging/sartables.
These files are named like so:
sarcalib.tn.H1 is the transmit channel, proton table.
sarcalib.dn.P31 is the decouple channel, Phosphorous 31 table,
etc.
Tables consist of exactly two columns of numeric values. The
first is tpwr values in ascending order, and the second is the corresponding
power values in watts. No comments or headings are allowed in these
files. Different tables are used on ely (4T) and superior (7T). When
amplifiers are changed, tables must be changed to match, if SAR calculator
is to be used.
The file sarcalc.h ought to be in /vnmr/psg, and seqgen will find it
there.