function rw = weinmann(tw)
% see Tofts, JMRI January 1997
a1 = 3.99; % kg/L
a2 = 4.78; % kg/L
m1 = 0.144; % min^-1
m2 = 0.0111; % min^-1
D = 0.2; % mmol/kg
rw = D * ( a1 * exp(-m1*tw) + a2 * exp(-m2*tw) );
rw(tw<0)=0; % before injection concentration is zero
endfunction
