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shear wave elastography ( swe ) measures the shear wave velocity ( swv ) generated by the acoustic radiation force impulse ( arfi ) .
thus , swe can be used to evaluate tissue stiffness , both quantitatively and objectively .
some authors have described the utility of swe for differentiating thyroid nodules from normal surrounding tissue [ 28 ] ; however , since the factors that affect swv in thyroid lesions are unclear , the appropriate use of swe for this purpose remains to be determined .
virtual touch tissue quantification ( vtq ) was originally used to evaluate the extent of liver fibrosis ; the technique is well established in this context [ 9 , 10 ] . since the same principles can be applied to the thyroid , we hypothesize that swe would be useful for diagnosing chronic autoimmune thyroiditis ( cat ) , which causes fibrosis in the thyroid parenchyma .
since the arfi is not usually impaired in superficial tissues such as the thyroid , we hypothesize that shear waves will be more stable in the thyroid than in the liver .
conventional strain elastography measures the stiffness of the target tissue relative to that of the surrounding tissue ; therefore , it can only estimate the tissue strain induced by compression [ 1114 ] .
thus , it is difficult to estimate the stiffness of a diffuse lesion using strain imaging because the technology can not really estimate minor differences in stiffness .
however , swe may be helpful in the evaluation of patients for cat by determining the extent of fibrosis .
the aims of the present study were to examine the diagnostic accuracy of swe for cat and to verify the correlation between fibrosis and swv in the thyroid .
study subjects included 229 individuals , including 253 normal thyroid lobes ( controls ) and 150 lobes with cat , who underwent ultrasonography of the thyroid gland in the department of otolaryngology - head and neck surgery of tottori university .
this study , which was performed between november 2011 and october 2014 , was approved by the ethics committee and the institutional review board of tottori university .
the normal thyroid group had normal ultrasound findings , as well as normal serum levels of thyroid - stimulating hormone , free triiodothyronine , and free thyroxine .
a diagnosis of cat was based on elevated levels of thyroglobulin antibodies , anti - thyroperoxidase antibodies , or both , as well as a diffuse hypoechoic pattern in the thyroid tissue on ultrasonography [ 15 , 16 ] .
the study population included patients receiving l - thyroxine therapy , patients not receiving any therapy , euthyroid patients , and patients with subclinical or overt hypothyroidism , because ruchala et al .
cat patients with goiter , normal thyroid volume , or an atrophic thyroid were included .
patients with grave 's disease , a history of radiation therapy to the neck , a history of using medications that may cause secondary cat , and a history of neck surgery that has altered the blood flow within the remnant thyroid were excluded .
generally , the left lobe is smaller than the right lobe . when the left lobe was not thick enough to encompass the region of interest ( roi ) , it was excluded from the analysis . in patients who underwent surgery for another indication , such as carcinoma , and received a pathological diagnosis of cat , the degree of fibrosis was evaluated pathologically .
we divided the degree of fibrosis into three groups : no fibrosis , moderate fibrosis , and severe fibrosis .
we used imagej software ( version 1.48 , wayne rasband , national institutes of health , usa ) to calculate the degree of fibrosis .
severe fibrosis was defined as extended fibrosis in over 50% of the area of 5 mm microscope fields .
each lobe of the thyroid gland in control and cat subjects was measured using a 9 mhz b - mode - arfi combination linear transducer ( acuson 2000 , siemens medical systems , forchheim , germany ) .
generally , vtq was performed on b - mode ultrasonographic images as follows : first , a target region was identified within a roi of 5  5  mm .
an acoustic push pulse was then transmitted , with a shear wave generated within the target region .
this shear wave was detected by sonographic detection pulses , and numerical swv values were displayed .
swv was measured in each lobe of the thyroid glands by pressing the probe lightly against the neck in an axial direction .
the roi ( 5  5  mm ) was completely circumscribed by the thyroid lesion ( figure 1 ) .
sporea et al . reported that five separate measurements are sufficient to assess thyroid stiffness ; therefore , five measurements were performed in the same location , and the average was recorded for comparison .
this information was used to derive the optimal swv cut - off value . the correlation between swv and various factors associated with cat , such as serum antibody titers and thyroid isthmus thickness , was evaluated using spearman 's correlation analysis .
statistical analysis was performed using spss software , version 22 ( ibm corporation , armonk , ny ) .
a total of 229 patients ( 90 males and 139 females ; mean age , 58.2 years ; range , 1091 years ) who were eligible according to the inclusion and exclusion criteria were examined .
there were 253 normal thyroid lobes in 145 patients ( 83 males and 62 females ; mean age , 57.5 years ) and 150 cat lobes in 84 patients ( 7 men and 77 women ; mean age , 60.5 years ) .
the mean swv for cat was 2.47  0.57  m / s , which was significantly higher than the mean swv for normal controls ( 1.59  0.41  m / s ) ( p < 0.001 ; figure 2 ) .
the area under the roc curve ( auc ) was 0.899 , and the optimal swv cut - off value was 1.96  m / s . the sensitivity , specificity , positive predictive value , negative predictive value , and diagnostic accuracy were 87.4% , 78.7% , 74.2% , 94.0% , and 85.1% , respectively .
there were one patient with no fibrosis and swv of 2.05  m / s , 11 patients with moderate fibrosis ( mean swv , 2.44  m / s ) , and three patients with severe fibrosis ( mean swv , 3.12  m / s ) .
there were 111 patients positive for anti - thyroperoxidase antibodies and 39 patients negative for anti - thyroperoxidase antibodies .
there were 117 patients positive for anti - thyroglobulin antibodies and 33 patients negative for anti - thyroglobulin antibodies .
we found a weak positive correlation between swv and serum levels of anti - thyroperoxidase antibodies ( spearman 's correlation coefficient , 0.311 ; figure 4 ) and between swv and thyroid isthmus thickness ( spearman 's  = 0.441 ; figure 5 ) .
there was no correlation between swv and levels of serum anti - thyroglobulin antibodies ( spearman 's  = 0.101 ) .
the mean swv for cat lobes with positive anti - thyroperoxidase antibodies was 2.56  0.57  m / s , significantly higher than that for cat lobes without detectable anti - thyroperoxidase antibodies ( 2.27  0.51  m / s ) ( p = 0.002 ; figure 6(a ) ) .
there were no significant differences in swv between cat lobes with anti - thyroglobulin antibodies ( 2.49  0.61  m / s ) and without anti - thyroglobulin antibodies ( 2.46  0.39  m / s ) ( figure 6(b ) ) .
the finding of higher swv in cat than in control lobes shows that it is possible to use swe to evaluate the fibrosis associated with cat .
the swv in lobes with cat correlated with both the thickness of the thyroid isthmus and the presence of anti - thyroperoxidase antibodies ; both are parameters that increase with the duration of disease .
the pathological features of cat include interstitial infiltration by lymphocytes and a variable degree of fibrosis in the interstitium [ 15 , 16 , 19 ] .
it is thought that fibrosis accelerates the propagation of the shear wave ; therefore , the swv in cat should depend on the level of fibrosis in the thyroid tissue .
conventional strain elastography measures the stiffness of the target tissue relative to that of the surrounding tissue ; therefore , it can only estimate the tissue strain induced by compression [ 1114 ] .
thus , it is difficult to estimate the stiffness of a diffuse lesion using strain imaging because the technology can not really estimate minor differences in stiffness .
by contrast , swe uses a push pulse , known as the arfi , which generates a shear wave in a focused area within the roi .
we surmise that the swv only reflects the characteristics of the tissue inside the roi ; thus swe , but not strain elastography , may be able to diagnose diffuse lesions such as cat . in other words , swe can examine the pathological structures within the target region inside the roi .
previous studies have shown that the duration of inflammation correlates with the degree of sonographic hypoechogenicity [ 15 , 20 ] .
thus , it is plausible that cat associated with hypoechogenicity on ultrasonography reflects the presence of fibrosis in the present study .
these results indicate that a combination of b - mode scanning and swe could be used to evaluate the degree of fibrosis in cat .
swv in lobes with cat was positively correlated with the thickness of the thyroid isthmus .
since cat is an inflammatory disease , the duration of disease affects the progression of fibrosis and the size of diffuse lesions [ 15 , 16 ] .
we think that this explains the positive correlation between swv and the thickness of the thyroid isthmus
. the presence of anti - thyroperoxidase antibodies may be characteristic of a late adaptive immune response , whereas anti - thyroglobulin antibodies may reflect an early immune response [ 16 , 20 ] .
anti - thyroperoxidase antibodies are closely associated with overt thyroid dysfunction , and their presence tends to correlate with the degree of thyroid damage and lymphocytic infiltration .
therefore , anti - thyroperoxidase antibody titers may correlate with the degree of fibrosis in the thyroid , which may explain the observed positive correlation between swv and serum titers of anti - thyroperoxidase antibodies . on the other hand , anti - thyroglobulin antibodies are thought to reflect an early immune response and the relationship between anti - thyroglobulin antibodies and the duration of disease is unclear .
fibrosis does not progress during the early stages of the immune response , which may explain why serum titers of anti - thyroglobulin antibodies were not correlated with swv .
previous studies have reported that some thyroid nodules are unmeasurable [ 1 , 4 , 2123 ] .
however , we were able to quantitatively measure all normal and cat thyroids in the present study .
some authors have reported that tumors with heterogeneous histopathology within a given roi were unmeasurable , because swv can not be calculated when the swv varies within the roi .
it is well known that some types of papillary thyroid carcinoma , which are composed of solid cells , fibrosis , adipose tissue , and other components , are unmeasurable .
this study shows that quantitative swe is useful for making measurements in diffuse targets stable .
a limitation of this study was that pathological examination results are not available for all patients .
the aims of this study were to evaluate the utility of shear wave elastography ( swe ) using acoustic radiation force impulse ( arfi ) for diagnosing chronic autoimmune thyroiditis ( cat ) and to verify the effect of fibrotic thyroid tissue on shear wave velocity ( swv ) . 
 the subjects were 229 patients with 253 normal thyroid lobes ( controls ) and 150 cat lobes . 
 the swv for cat ( 2.47  0.57  m / s ) was significantly higher than that for controls ( 1.59  0.41  m / s ) ( p < 0.001 ) . 
 the area under the receiver operating characteristics ( roc ) curve for cat was 0.899 , and the swv cut - off value was 1.96  m / s . 
 the sensitivity , specificity , and diagnostic accuracy were 87.4% , 78.7% , and 85.1% , respectively . 
 levels of anti - thyroperoxidase antibodies and thyroid isthmus thickness were correlated with tissue stiffness in cat . however , there was no correlation between levels of anti - thyroglobulin antibodies and tissue stiffness . 
 quantitative swe is useful for diagnosing cat , and it is possible that swe can be used to evaluate the degree of fibrosis in patients with cat .