many factors such as image parameters ( e.g. , tr , te , ti , flip angle ) , image weighting ( e.g. , t1 , t2 , t2 * , pd weighted ) , the magnetic susceptibility of the contrast agent , the magnetic field strength , the pulse sequence parameters , the dose of the contrast agent , the injection rate and bolus volume , the cardiac output and blood volume and the tissue topology may affect the si ( 1 ) . the paramagnetic - metal contrast agents such as gadolinium - diethylenetriamine - pantaacetic acid ( gd - dtpa ) are administered to improve the contrast of the mri images ; while the paramagnetic contrast medium passes through the tissue , it produces a local magnetic field in the homogeneities leading to a reduction in the transverse relaxation time ( t2 ) and longitudinal relaxation time ( t1 ) of the bulk of the tissue . a decrease in t1 typically causes an increase in the signal intensity ( si ) , whereas a decrease in t2 causes a decrease in the si ( 2 ) . the t1-shortening effect is dominant at low concentrations of gd - dtpa , and the t2-shortening effect is dominant at high concentrations and leads to a decrease in the si . the mr signal intensity will depend on the image parameters , the type of imaging sequence used and the magnetic field strength ( 3 - 5 ) . inversion recovery ( ir ) sequences are commonly used to suppress the si of a specific tissue such as cerebrospinal fluid ( csf ) or fat . ir pulse sequences can generate t1-weighted images with a different range of inversion times ( tis of 851238 ms ) to suppress or null the si for a specified tissue ( 6 - 8 ) . previous studies have shown that the inversion time ( ti ) , repetition time ( tr ) , echo time ( te ) , different sequences ( e.g. , inversion recovery and saturation recovery ) , inflow , the phase - encoding scheme , the dose of contrast agent concentration and the magnetic field strength can affect the relationship between the changes in the si and in the concentration of the t1-weighted images ( 2 - 5 , 9 - 15 ) . in this work , we aimed to study the effect of ti on the concentration of the contrast agent which leads to a minimum si using the ir t1-weighted 3-dimensional fast - gradient echo imaging sequence. theory : the mr sequence can influence the connection between t1 and the si , which in turn is dependent on the concentration of the contrast agent ( 16 ) . ( 1 ) expresses the standard ir sequence : 
 
 ( 1)s(t)=s0(1(1cosinv)exptit1+exptrt1 ) , 
 
 where s ( t ) is the si after the administration of the contrast agent , and s0 is the observed si when no magnetization preparation pre - pulses are applied or there is no contrast agent ; inv 
 denotes the flip angle of the inversion pulse . if inv = 180 in the ir sequences or at the higher concentrations of the contrast agent , eq.(1 ) can be written as follows : ( 4 , 17 , 18 ) 
 
 ( 2)s(t)=s0(12exptit1+exptrt1)exp(tet2 ) , 
 
 eq . ( 2 ) which contains the concentration of the contrast agent at time t ( c ( t ) ) can be described as ( 4 ) : 
 
 ( 3)s(t)=s0(12exp(ti(c(t)k+1t1pr  e))+exp(tr(c(t)k1t1pr  e)))exp(tet2 ) , 
 
 where t1pre is the longitudinal relaxation times before the contrast application ; k is a constant that depends on the contrast medium ( 13 , 19 ) . 
 
 the null point of a tissue therefore , for the null point of a tissue , s ( t ) = 0 , so : 
 
 ( 4)12exp(ti(c(t)k+1t1pr  e))=exp(tr(c(t)k+1t1pr  e ) ) , 
 
 ( 5)12exp(ti(c(t)k+1t1pr  e))=exp(tr(c(t)k+1t1pr  e ) ) . 
 
 and assuming that tr>>t1 , the null point is 
 
 ( 6)ti(c(t)k+1t1pr  e)=ln(12)ti=(0.96c(t)k+1t1pr  e ) . 
 
 or , considering eq . ( 2 ) , the null point of a tissue or fluid can be expressed as : 
 
 ( 7)12exptit1=exptrt1 
 
 ( 8)ti = t1ln(12)=0.693t1 . therefore , ti can be adjusted to optimize the contrast between the suppressed region of interest ( e g. , tissue ) and its surroundings . phantom : to assess the effect of different ti values on a concentration that leads to a minimum si , a phantom was designed with vials containing a variety of concentrations of the contrast agent . the shape of the phantom was approximately cubic and was made of perspex ; its length , width and height were 20 , 18 and 20 cm , respectively . the phantom contained 25 vials ( glass tubes , inner diameter approximately 15 mm ) and was filled with different concentrations of gd - dtpa ( magnevist , schering health care ltd , west sussex , uk ) ) . the concentration of gd - dtpa varied between 0 and 19.77 mmol / l ( 0.00 , 0.30 , 0.45 , 0.60 , 0.75 , 0.90 , 1.20 , 1.50 , 1.80 , 2.10 , 2.39 , 2.69 , 2.99 , 3.28 , 3.58 , 3.98 , 4.96 , 5.95 , 7.93 , 9.90 , 13.85 , and 19.77 mmol / l ) . the vials were set vertically , and the axes of the vials were perpendicular to the image plane ( coronal image ) . 1 displays a coronal image of the phantom with different concentrations of the contrast agent . 
 
 the coronal image of the phantom is shown in figure 1 , and the position of different concentrations 
 ( mmol / l ) inside the vials can be seen in this figure . 
 

 image acquisition : 
  the phantom was situated within the coil . all studies were performed on a 1.5 t clinical mr scanner ( vision , siemens medical , erhlangen , germany ) . we used ir t1-weighted 3-dimensional fast gradient echo images to measure the si in vials with varying concentrations of the contrast agent . the imaging parameters were as follows : 
 
 matrix size = 256256 , tr = 5000ms , te = 3 ms , ti was varied between 100 and 3000 ms ( 100 , 150 , 200 , 250 , 300 , 400 , 500 , 600 , 700 , 800 , 1000 , 1200 , 1500 , 2000 , 2500 , and 3000 ms ) , pixel size = 11 mm , slice thickness = 10 mm , and flip angle = 15. each image was repeated 16 times . image analysis : 
  the images were processed by the transfer of the image data from the mr scanner to a personal computer , and they were then computerized by an image - processing software in an interactive data language ( idl , research systems , inc . ; programs were written to find the followings automatically : 
 
 the mean image of 16 acquisitions primarily used for the improvement of the signal - to - noise ratio . the mean si and standard deviation of the 9 innermost pixels of the vial to avoid the partial volume effects . 
 
 the concentration at which si is minimized . these programs can be run on either a unix workstation or a personal computer . 2 - 8 demonstrate some typical results for the mean si from the 9 innermost pixels of the vials versus the concentration of the contrast agent at ti values of 100 , 300 , 500 , 700 , 1000 , 2000 and 3000 ms , respectively . demonstrates the concentration of the contrast agent versus the mean si from the 9 innermost pixels of the vials at ti = 100 ms . .   displays the concentration of the contrast agent versus the mean si from the 9 innermost pixels of the vials at ti = 300 ms . shows the concentration of the contrast agent versus the mean si from the 9 innermost pixels of the vials at ti = 500 ms . exhibits the concentration of the contrast agent versus the mean si from the 9 innermost pixels of the vials at ti = 700 ms . demonstrates the concentration of the contrast agent versus the mean si from the 9 innermost pixels of the vials at ti = 1000 ms . shows the concentration of the contrast agent versus the mean si from the 9 innermost pixels of the vials at ti = 2000 ms . demonstrates the concentration of the contrast agent versus the mean si from the 9 innermost pixels of the vials at ti = 3000 ms . fig figs 9 shows that the minimum si was obtained at different concentrations to obtain different values of ti . exhibits ti versus the concentration of the contrast ; a concentration which leads to a minimum si will be decreased when ti is increased ; the concentration vanishes to zero at higher ti ( more than 1200 ms ) . the figure indicates that the minimum si was nearly the same , and the slope of the curve was low at lower ti values . at higher ti ( more than 1500 ms ) , the minimum si would increase , and the slope of the curve would be very high . the figures show that the t1-shortening effect is dominant at the low concentrations of gd - dtpa based on eq . ( 1 ) , whereas the t2-shortening effect is dominant at high concentrations and leads to a decrease in si ( see the ir sequences will be started with a 180-degree inversion radiofrequency pulse , and the pulse will flip the longitudinal magnetization vector through the x - y plane and into the z- direction . the longitudinal magnetization gradually increases after the external inversion pulse is turned off ; this is called t1 recovery . the reconstruction process in ir images is only sensitive to the magnitude of the magnetization of a tissue , not whether it is positive or negative . the positive or negative magnetization values depend on the ti and tissue t1 values and also on the time at which the readout sequence is applied . therefore , the image contrast is dependent on the ti value for a specific tissue when other image parameters are constant ( 6 ) . ( 20 ) stated that a short ti ( 80 - 180 ms ) can be used for the null si for fat , white matter and liver . in addition , they reported that a medium ti ( 200 - 800 ms ) and a long ti ( 1600 - 2800 ms ) are suitable for nulling of blood and csf , respectively . the sign of longitudinal magnetization is mixed negative and positive for blood and positive for csf . amano et al . ( 21 ) investigated the differences in null points between the left ventricle ( lv ) and right ventricle ( rv ) in patients with cardiac disease using a contrast - enhanced inversion recovery mr imaging . they reported that the null points of the rv myocardium were shorter than those of the lv myocardium in some patients with cardiac disease . nakamura et al . ( 22 ) investigated the effect of ti values ( 100 , 200 , 300 , 400 , and 500 ms ) to differentiate the ne structures in the brain s gray and white matter ( gray - gray and white - white differentiation ) . they reported that a ti of 400 ms was the most suitable for this purpose . wagner et al . ( 23 ) investigated the influence of ti on acute myocardial infarct size measurements and the difference in image intensities between the infarcted and normal myocardium based on mri delayed contrast enhancement . they found that ti has an effect on the image intensity and should be chosen very prudently . 
 
 in clinical studies , normally ti= 800ms will be used in ir t1-weighted turbo fast low angle shot ( turbo flash ) images because of the null signal from the blood ( 24 ) . the results of this study revealed that the t1 recovery curve did not cross the x- axis for tis of 100 - 1000 ms ; a minimum si can be seen in   figs . 9 and 10 ) the concentration of the contrast agent , which leads to a minimum si , is dependent on ti and that the minimum si will be increased at higher ti . the results of this study also revealed that the t1 effect , which leads to an increased si , will appear at lower ti at different concentrations . in addition , the t2 effect , which leads to a decrease in si , will appear from ti = 250 ms at higher concentrations . 
 
 according to the results , ti can have an effect on the concentration of the contrast agent , which leads to a minimum si . therefore , when using an ir sequence with a high signal - to - noise ratio or when suppressing si on an image , the image parameters such as ti should be chosen with great caution . the effect of ti on the si of ir t1-weighted images was evaluated in this study . the result of this study revealed that a difference in ti can have an effect on the strength of the si , which is dependent on the concentration of the contrast agent . in conclusion , to suppress the si of specified tissues in clinical studies ( e.g. , fat suppression or blood suppression in perfusion measurements ) , the ti parameter should be chosen very cautiously .
background : inversion recovery ( ir ) pulse sequences can generate t1-weighted images with a different range 
 of inversion time ( ti ) to suppress or null the signal intensity ( si ) for a specified tissue . in this study , we aimed 
 to investigate the effect of ti values on the concentration of the contrast agent , which leads to a minimum signal 
 intensity , using an inversion recovery t1-weighted 3-dimensional fast - gradient echo imaging sequence . 
 methods : a phantom was designed to hold 25 vials which contained different ( between 0 and 19.77mmol / l of 
 ( gd - dtpa ) ) concentrations of the contrast agent . 
 we used the vials of different concentrations to measure si 
 using ir sequences with different inversion times ( ti , 100 - 3000ms ) . 
 results : the results of this study revealed that the t1 recovery curve did not cross the x- axis for the lower ti . 
 therefore , a minimum si can be observed from the concentration of the contrast agent versus si curves . 
 the 
 findings of this study also revealed that the concentration of the contrast agent , which leads to a minimum si , is 
 dependent on the ti and the minimum si will be increased at higher ti concentrations . 
 conclusion : in conclusion , when the ti parameter is used to suppress the si of the specified tissues in clinical 
 studies ( e.g. , fat suppression or blood suppression in perfusion measurements ) , it should be chosen with great 
 caution .