Patent Publication Number: US-2018028158-A1

Title: Ultrasound observation apparatus, method for operating ultrasound observation apparatus, and computer-readable recording medium

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     This application is a continuation of PCT international application Ser. No. PCT/JP2015/083938, filed on Dec. 2, 2015 which designates the United States, incorporated herein by reference, and which claims the benefit of priority from Japanese Patent Application No. 2015-072726, filed on Mar. 31, 2015, incorporated herein by reference. 
    
    
     BACKGROUND 
     1. Technical Field 
     The disclosure relates to an ultrasound observation apparatus for observing tissues as an observation target by using ultrasound waves, a method for operating the ultrasound observation apparatus, and a computer-readable recording medium. 
     2. Related Art 
     In order to observe the characteristics of body tissues or material as an observation target, there are cases where ultrasound waves are used. More specifically, by transmitting an ultrasound wave to an observation target and performing predetermined signal processing for an ultrasound echo reflected from the observation target, information relating to the characteristics of the observation target is acquired. 
     The intensity of an ultrasound wave attenuates when propagating through an observation target. Conventionally, a technology for determining the characteristics of the material of an observation target by using such attenuation is known (for example, see WO 2007/003058). According to such a technology, an electric signal corresponding to an ultrasound echo is transformed into an amplitude spectrum of the frequency domain, an attenuation amount is calculated by comparing the amplitude spectrum with a predetermined reference amplitude spectrum, and the attenuation amount is fitted to an attenuation model that depends on the characteristics of the material, whereby the characteristics of the material are determined. 
     SUMMARY 
     In accordance with some embodiments, an ultrasound observation apparatus, a method for operating the ultrasound observation apparatus, and a computer-readable recording medium are provided. 
     In some embodiments, an ultrasound observation apparatus includes: a frequency analyzing unit configured to calculate a plurality of frequency spectra by analyzing a frequency of a signal generated based on an echo signal acquired by converting an ultrasound echo into an electric signal, the ultrasound echo being obtained by irradiating an observation target with an ultrasound wave and receiving the ultrasound wave reflected from the observation target; an approximation unit configured to calculate features of the plurality of frequency spectra; an attenuation correcting unit configured to perform an attenuation correction for excluding an influence of attenuation of the ultrasound wave, on each of the features of the plurality of frequency spectra using each of at least three attenuation rate candidate values giving different attenuation characteristics in propagating the ultrasound wave through the observation target, thereby calculating corrected features of the plurality of frequency spectra; an optimal attenuation rate setting unit configured to: calculate a statistical dispersion of the corrected features for each of the at least three attenuation rate candidate values; generate a quadratic function based on the statistical dispersion; and set one of the at least three attenuation rate candidate values which gives a minimum statistical dispersion in the quadratic function, as an optimal attenuation rate; and a feature image data generating unit configured to generate feature image data based on the corrected features calculated by the attenuation correcting unit using the optimal attenuation rate set by the optimal attenuation rate setting unit. 
     In some embodiments, a method for operating an ultrasound observation apparatus includes: by a frequency analyzing unit, calculating a plurality of frequency spectra by analyzing a frequency of a signal generated based on an echo signal acquired by converting an ultrasound echo into an electric signal, the ultrasound echo being obtained by irradiating an observation target with an ultrasound wave and receiving the ultrasound wave reflected from the observation target; by an approximation unit, calculating features of the plurality of frequency spectra; by an attenuation correcting unit, performing an attenuation correction for excluding an influence of attenuation of the ultrasound wave, on each of the features of the plurality of frequency spectra using each of at least three attenuation rate candidate values giving different attenuation characteristics in propagating the ultrasound wave through the observation target, thereby calculating corrected features of the plurality of frequency spectra; by an optimal attenuation rate setting unit: calculating a statistical dispersion of the corrected features for each of the at least three attenuation rate candidate values; generating a quadratic function based on the statistical dispersion; and setting one of the at least three attenuation rate candidate values which gives a minimum statistical dispersion in the quadratic function, as an optimal attenuation rate; and by a feature image data generating unit, generating feature image data based on the corrected features calculated by the attenuation correcting unit using the optimal attenuation rate set by the optimal attenuation rate setting unit. 
     In some embodiments, provided is a non-transitory computer-readable recording medium with an executable program stored thereon. The program causes an ultrasound observation apparatus to execute: by a frequency analyzing unit, calculating a plurality of frequency spectra by analyzing a frequency of a signal generated based on an echo signal acquired by converting an ultrasound echo into an electric signal, the ultrasound echo being obtained by irradiating an observation target with an ultrasound wave and receiving the ultrasound wave reflected from the observation target; by an approximation unit, calculating features of the plurality of frequency spectra; by an attenuation correcting unit, performing an attenuation correction for excluding an influence of attenuation of the ultrasound wave, on each of the features of the plurality of frequency spectra using each of at least three attenuation rate candidate values giving different attenuation characteristics in propagating the ultrasound wave through the observation target, thereby calculating corrected features of the plurality of frequency spectra; by an optimal attenuation rate setting unit: calculating a statistical dispersion of the corrected features for each of the at least three attenuation rate candidate values; generating a quadratic function based on the statistical dispersion; and setting one of the at least three attenuation rate candidate values which gives a minimum statistical dispersion in the quadratic function, as an optimal attenuation rate; and by a feature image data generating unit, generating feature image data based on the corrected features calculated by the attenuation correcting unit using the optimal attenuation rate set by the optimal attenuation rate setting unit. 
     The above and other features, advantages and technical and industrial significance of this invention will be better understood by reading the following detailed description of presently preferred embodiments of the invention, when considered in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating the configuration of an ultrasound observation system including an ultrasound observation apparatus according to an embodiment of the present invention; 
         FIG. 2  is a graph illustrating a relation between a reception depth and an amplification factor in an amplification process performed by a signal amplifying unit of an ultrasound observation apparatus according to the embodiment of the present invention; 
         FIG. 3  is a graph illustrating a relation between a reception depth and an amplification factor in an amplification correcting process performed by an amplification correcting unit of an ultrasound observation apparatus according to the embodiment of the present invention; 
         FIG. 4  is a schematic diagram illustrating a data arrangement in one sound ray of an ultrasound signal; 
         FIG. 5  is a graph illustrating an example of a frequency spectrum calculated by a frequency analyzing unit of an ultrasound observation apparatus according to the embodiment of the present invention; 
         FIG. 6  is a graph illustrating a straight line having, as a parameter, a corrected feature obtained by an attenuation correcting unit of an ultrasound observation apparatus according to the embodiment of the present invention; 
         FIG. 7  is a graph illustrating an example of the distribution of corrected features obtained by performing attenuation correction based on two different attenuation rate candidate values for a same observation target; 
         FIG. 8  is a graph illustrating a relation between attenuation rate candidate values and the dispersion of corrected features obtained by performing the attenuation correction based on the attenuation rate candidate values; 
         FIG. 9  is a flowchart illustrating an overview of a process performed by an ultrasound observation apparatus according to the embodiment of the present invention; 
         FIG. 10  is a flowchart illustrating an overview of a process performed by a frequency analyzing unit of an ultrasound observation apparatus according to the embodiment of the present invention; and 
         FIG. 11  is a schematic diagram illustrating an example of a feature image on a display device of an ultrasound observation system according to the embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Exemplary embodiments of the present invention will be described with reference to the attached drawings. 
       FIG. 1  is a block diagram illustrating the configuration of an ultrasound observation system including an ultrasound observation apparatus according to an embodiment of the present invention. An ultrasound observation system  1  illustrated in the drawing includes: an ultrasound endoscope  2  that transmits an ultrasound wave to a subject as an observation target and receives an ultrasound wave reflected from the subject; an ultrasound observation apparatus  3  that generates an ultrasound image based on an ultrasound signal acquired by the ultrasound endoscope  2 ; and a display device  4  that displays the ultrasound image generated by the ultrasound observation apparatus  3 . 
     The ultrasound endoscope  2 , in a tip end portion thereof, includes an ultrasound transducer  21  that converts an electric pulse signal received from the ultrasound observation apparatus  3  into an ultrasound pulse (acoustic pulse) and emits the ultrasound pulse to a subject and converts an ultrasound echo reflected from the subject into an electric echo signal represented using a voltage change and outputs the echo signal. 
     The ultrasound endoscope  2  generally includes an imaging optical system and an imaging device, is inserted into a digestive tract (esophagus, stomach, duodenum, or large intestine) or respiratory organs (trachea or a bronchial tube) of a subject, and is capable of imaging the digestive tract, the respiratory organs, and peripheral organs thereof (pancreas, gallbladder, a bile duct, a biliary tract, a lymph node, mediastinum organs, blood vessels, and the like). In addition, the ultrasound endoscope  2  includes a light guide that guides illumination light to be emitted to a subject at the time of imaging. This light guide has a tip end portion extending up to a tip end of a subject insertion portion of the ultrasound endoscope  2  and a base end portion connected to a light source device generating illumination light. 
     The ultrasound observation apparatus  3  includes: a transmitting and receiving unit  31  that is electrically connected to the ultrasound endoscope  2 , transmits a transmission signal (pulse signal) configured by a high-voltage pulse to the ultrasound transducer  21  based on a predetermined waveform and transmission timing, receives an echo signal that is an electric reception signal from the ultrasound transducer  21 , and generates and outputs data of a digital high frequency (radio frequency (RF)) signal (hereinafter, referred to as RF data); a signal processing unit  32  that generates digital B-mode reception data based on the RF data received from the transmitting and receiving unit  31 ; a computing unit  33  that performs a predetermined arithmetic operation for the RF data received from the transmitting and receiving unit  31 ; an image processing unit  34  that generates various kinds of image data; an input unit  35  that is realized by using a user interface such as a keyboard, a mouse, a touch panel, or the like and receives input of various kinds of information; a control unit  36  that controls the overall operation of the ultrasound observation system  1 ; and a storage unit  37  that stores various kinds of information necessary for the operation of the ultrasound observation apparatus  3 . 
     The transmitting and receiving unit  31  includes a signal amplifying unit  311  that amplifies an echo signal. The signal amplifying unit  311  performs a sensitivity time control (STC) correction for amplification with a higher amplification factor for an echo signal having a large reception depth.  FIG. 2  is a graph illustrating a relation between a reception depth and an amplification factor in an amplification process performed by the signal amplifying unit  311 . A reception depth z illustrated in  FIG. 2  is an amount calculated based on a time elapsing from a reception start time point of an ultrasound wave. As illustrated in  FIG. 2 , in a case where the reception depth z is smaller than a threshold z th , an amplification factor β (dB) is linearly increased from β 0  to β th  (&gt;β 0 ) in accordance with an increase in the reception depth z. On the other hand, in a case where the reception depth z is the threshold z th  or more, the amplification factor β (dB) takes a constant value β th . The value of the threshold z th  is a value in which an ultrasound signal received from an observation target almost attenuates, and a noise is dominant. More generally, the amplification factor β may be monotonously increased according to an increase in the reception depth z in a case where the reception depth z is smaller than the threshold z th  . The relation illustrated in  FIG. 2  is stored in the storage unit  37  in advance. 
     After performing a filtering process and the like for an echo signal amplified by the signal amplifying unit  311 , the transmitting and receiving unit  31  generates RF data of the time domain by performing an A/D conversion and outputs the generated RF data to the signal processing unit  32  and the computing unit  33 . In a case where the ultrasound endoscope  2  has a configuration for electronically scanning the ultrasound transducer  21  in which a plurality of elements are disposed in an array pattern, the transmitting and receiving unit  31  includes a multi-channel circuit used for beam synthesis corresponding to the plurality of elements. 
     The frequency band of a pulse signal transmitted by the transmitting and receiving unit  31  may be a broadband that almost covers the linear response frequency band of the electric acoustic conversion of a pulse signal into an ultrasound pulse in the ultrasound transducer  21 . The frequency bands of various processes of an echo signal performed in the signal amplifying unit  311  may be a broadband that almost covers the linear response frequency band of the acoustic electric conversion of an ultrasound echo into an echo signal that is performed by the ultrasound transducer  21 . Accordingly, when an approximation process of a frequency spectrum to be described later is performed, approximation having high accuracy can be performed. 
     The transmitting and receiving unit  31  also has a function for transmitting various control signals output by the control unit  36  to the ultrasound endoscope  2  and receiving various kinds of information including an identification ID from the ultrasound endoscope  2  and transmitting the received information to the control unit  36 . 
     The signal processing unit  32  performs known processes such as band pass filtering, envelope detection, and a logarithmic conversion for RF data, thereby generating digital B-mode reception data. In the logarithmic conversion, a common logarithm of a quantity acquired by dividing RF data by a reference voltage V c  is taken and is represented in a decibel value. The signal processing unit  32  outputs the generated B-mode reception data to the image processing unit  34 . The signal processing unit  32  is realized by using a central processing unit (CPU), various arithmetic operation circuits, and the like. 
     The computing unit  33  includes: an amplification correcting unit  331  that performs an amplification correction such that the amplification factor β is constant for the RF data generated by the transmitting and receiving unit  31  regardless of the reception depth; a frequency analyzing unit  332  that calculates a frequency spectrum by performing a frequency analysis by performing a fast Fourier transform (FFT) for the RF data for which the amplification correction has been performed; and a feature calculating unit  333  that calculates a feature of the frequency spectrum. The computing unit  33  is realized by a central processing unit (CPU), various arithmetic operation circuits, and the like. 
       FIG. 3  is a graph illustrating a relation between a reception depth and an amplification factor in an amplification correcting process performed by the amplification correcting unit  331 . As illustrated in  FIG. 3 , the amplification factor β (dB) in the amplification correcting process performed by the amplification correcting unit  331  takes a maximum value β th -β 0  when the reception depth z is zero, linearly decreases until the reception depth z reaches the threshold z th  from zero, and is zero when the reception depth z is the threshold z th  or more. The amplification correcting unit  331  performs amplification correction on a digital RF signal by using the amplification factor determined in this way to offset the effect of an STC correction performed by the signal processing unit  32 , which makes it possible to output a signal of a constant amplification factor β th . Obviously, the relation between the reception depth z and the amplification factor β in the amplification correcting process performed by the amplification correcting unit  331  differs depending on the relation between the reception depth and the amplification factor in the signal processing unit  32 . 
     The reason for performing the amplification correction will be described. The STC correction is a correction process for excluding the influence of attenuation from the amplitude of an analog signal waveform by uniformly amplifying the amplitude of the analog signal waveform over the whole frequency band and amplifying the depth by using an amplification factor that is monotonously increased. For this reason, in a case where a B-mode image to be displayed is generated by converting the amplitude of an echo signal into the luminance and, in a case where a uniform tissue is scanned, by performing the STC correction, the luminance value becomes constant regardless of the depth. In other words, an effect of excluding the influence of attenuation from the luminance value of the B-mode image can be acquired. 
     On the other hand, in a case where an analysis result acquired by calculating the frequency spectrum of an ultrasound wave is used as in the embodiment, the influence of attenuation associated with the propagation of the ultrasound wave cannot be accurately excluded even by the STC correction. The reason for this is that, generally, while the attenuation amount is different according to the frequency (see Equation (1) to be described later), the amplification factor of the STC correction changes according to the distance and has no dependency on the frequency. 
     In order to address the situation described above, in other words, the situation that the influence of attenuation associated with the propagation of the ultrasound wave is not accurately excluded even by the STC correction if an analysis result acquired by calculating the frequency spectrum of an ultrasound wave is used, a method may be employed in which a reception signal for which the STC correction is performed is output when a B-mode image is generated, and, when an image that is based on the frequency spectrum is generated, new transmission other than transmission for generating the B-mode image is performed, and a reception signal for which the STC correction is not performed is output. However, in such a case, the frame rate of image data generated based on a reception signal may be decreased. 
     Thus, in the embodiment, in order to exclude the influence of the STC correction for a signal for which the STC correction is performed for a B-mode image while the frame rate of generated image data is maintained, the amplification factor is corrected by the amplification correcting unit  331 . 
     The frequency analyzing unit  332  performs sampling of RF data (line data) of each sound ray for which an amplification correction is performed by the amplification correcting unit  331  at a predetermined time interval, thereby generating sample data. The frequency analyzing unit  332  performs an FFT process for a sample data group, thereby calculating a frequency spectrum at a plurality of positions (data positions) on the RF data. 
       FIG. 4  is a schematic diagram illustrating a data arrangement in one sound ray of an ultrasound signal. In a sound ray SR k  illustrated in the drawing, a white rectangle or a black rectangle represents data at one sample point. In the sound ray SR k , as data is positioned on a further right side, the data is sample data of a deeper position in a case where the position is measured from the ultrasound transducer  21  along the sound ray SR k . The sound ray SR k  is configured to be discrete at a time interval corresponding to a sampling frequency (for example, 50 MHz) of an A/D conversion performed by the transmitting and receiving unit  31 . In  FIG. 4 , while a case is illustrated in which an eighth data position of the sound ray SR k  of a number k is set as an initial value Z (k)   0  in the direction of the reception depth z, the position of the initial value may be arbitrarily set. The result of the calculation acquired by the frequency analyzing unit  332  is acquired as a complex number and is stored in the storage unit  37 . 
     A data group F j  (j=1, 2, . . . , K) illustrated in  FIG. 4  is a sample data group that is a target for an FFT process. Generally, in order to perform FFT, the number of data in a sample data group needs to be power of two. Hence, while a sample data group F j  (j=1, 2, . . . , K-1) is a normal data group having 16 (=2 4 ) data, a sample data group F K  is an abnormal data group because the number of data is 12. In order to perform FFT on an abnormal data group, a process of generating a normal sample data group is performed by inserting zero data to cover the shortfall. This point will be described in detail when the process performed by the frequency analyzing unit  332  is described (see  FIG. 9 ). 
       FIG. 5  is a graph illustrating an example of a frequency spectrum calculated by the frequency analyzing unit  332 . The “frequency spectrum” represents a “frequency distribution of intensities for a certain reception depth z” acquired by performing FFT on a sample data group. The “intensity” described here, for example, represents a parameter of a voltage of an echo signal, power of an echo signal, sound pressure of an ultrasound echo, acoustic energy of an ultrasound echo, or the like, the amplitude or the time integration value of the parameter, or a combination thereof. 
     In  FIG. 5 , the horizontal axis is the frequency f. In addition, in  FIG. 5 , the vertical axis is a common logarithm (represented in decibels) I=10log 10 (I 0 /I c ) of a quantity acquired by dividing the intensity I 0  by a reference intensity I c  (constant). A straight line L 10  illustrated in  FIG. 5  will be described later. In the embodiment, a curve or a straight line is configured by a set of discrete points. 
     In a frequency spectrum C 1  illustrated in  FIG. 5 , a lower limit frequency f L  and an upper limit frequency f H  of a frequency band used for the arithmetic operation performed thereafter are parameters that are determined based on the frequency band of the ultrasound transducer  21 , the frequency band of a pulse signal transmitted by the transmitting and receiving unit  31 , and the like. Hereinafter, as illustrated in  FIG. 5 , a frequency band set by the lower limit frequency f L  and the upper limit frequency f H  will be referred to as a “frequency band U”. 
     Generally, in a case where an observation target is a body tissue, a frequency spectrum represents a different tendency in accordance with characteristics of body tissues scanned by an ultrasound wave. The reason for this is that the frequency spectrum has a correlation with the size, the number density, the acoustic impedance, and the like of a scattering body scattering an ultrasound wave. The “characteristics of body tissues” described here, for example, are a malignant tumor, a benign tumor, an endocrine tumor, a mucinous tumor, a normal tissue, a cyst, a vessel, and the like. 
     The feature calculating unit  333  calculates the feature of each of a plurality of frequency spectra, calculates corrected feature of each frequency spectrum by performing an attenuation correction for excluding the influence of attenuation of an ultrasound wave for feature (hereinafter, referred to as pre-correction feature) of each frequency spectrum for each of a plurality of attenuation rate candidate values giving different attenuation characteristics at a time when an ultrasound wave propagates through an observation target, and sets an attenuation rate that is optimal for the observation target among the plurality of attenuation rate candidate values by using the corrected feature. 
     The feature calculating unit  333  includes: an approximation unit  333   a  that calculates a pre-correction feature of a frequency spectrum by approximating the frequency spectrum by a straight line; an attenuation correcting unit  333   b  that calculates corrected feature by performing an attenuation correction based on each of a plurality of attenuation rate candidate values for the pre-correction feature calculated by the approximation unit  333   a ; and an optimal attenuation rate setting unit  333   c  that sets an optimal attenuation rate among the plurality of attenuation rate candidate values based on a statistical dispersion of the corrected feature calculated by the attenuation correcting unit  333   b  for all the frequency spectra. 
     The approximation unit  333   a  approximates a frequency spectrum by a linear expression (regression line) by performing a regression analysis on the frequency spectrum in a predetermined frequency band, thereby obtaining pre-correction features which define the linear expression. For example, in the case of the frequency spectrum C 1  illustrated in  FIG. 5 , the approximation unit  333   a  acquires a regression line L 10  by approximating the frequency spectrum C 1  by a linear expression by performing the regression analysis on a frequency band U. In other words, the approximation unit  333   a  calculates, as the pre-correction features, a slope a 0  and an intercept b 0  of the regression line L 10  and a mid-band fit c 0 =a 0 f M +b 0  that is a value on the regression line of the center frequency f M =(f L +f H )/2 of the frequency band U. 
     Among three pre-correction features, the slope a 0  has a correlation with the size of a scattering body of an ultrasound wave and, generally, the slope is considered to have a smaller value as the size of the scattering body is larger. The intercept b 0  has correlations with the size of a scattering body, a difference in the acoustic impedance, the number density (density) of the scattering body, and the like. More specifically, it is considered that the intercept b 0  has a larger value as the size of the scattering body is larger, has a larger value as the difference in the acoustic impedance is larger, and has a larger value as the number density of the scattering body is larger. The mid-band fit c 0  is an indirect parameter that is derived from the slope a 0  and the intercept b 0  and gives the intensity of the spectrum disposed at the center within an effective frequency band. For this reason, the mid-band fit c 0  is considered to have a correlation with the luminance of a B-mode image to some degree in addition to the size of the scattering body, the difference in the acoustic impedance, and the number density of the scattering body. The feature calculating unit  333  may approximate the frequency spectrum by a second-order polynomial or higher-order polynomial using regression analysis. 
     The correction performed by the attenuation correcting unit  333   b  will be described. Generally, the attenuation amount A(f, z) of an ultrasound wave is attenuation occurring while the ultrasound wave reciprocates between a reception depth  0  and a reception depth z and is defined as a change (a difference represented in decibel) in the intensity before and after the reciprocation. The attenuation amount A(f, z) is empirically known to be proportional to the frequency within a uniform tissue and is represented in the following Equation (1). 
         A ( f, z )=2α zf    (1)
 
     Here, a proportion constant α is a quantity called an attenuation rate. In addition, z represents a reception depth of an ultrasound wave, and f represents a frequency. In a case where the observation target is a living body, a specific value of the attenuation rate α is determined according to a portion of the living body. The unit of the attenuation rate α, for example, is dB/cm/MHz. In the embodiment, the attenuation correcting unit  333   b,  in order to set a most appropriate attenuation rate (optimal attenuation rate), performs an attenuation correction for each of a plurality of attenuation rate candidate values. The plurality of attenuation rate candidate values will be described in detail later with reference to  FIG. 8 . 
     The attenuation correcting unit  333   b  calculates corrected features a, b, and c by performing an attenuation correction on the pre-correction features (the slope a 0  the intercept b 0 , and the mid-band fit c 0 ) extracted by the approximation unit  333   a  using Equations (2) to (4) represented below. 
         a=a   0 +2α z    (2)
 
       b=b 0    (3)
 
         c=c   0   +A ( f   M   , z )= c   0 +2α zf   M (= af   M   +b )   (4)
 
     As is clear from Equations (2) and (4), the attenuation correcting unit  333   b  performs a correction having a larger correction amount as the reception depth z of the ultrasound wave is larger. According to Equation (3), a correction for the intercept is an identical transformation. The reason for this is that the intercept is a frequency component corresponding to a frequency 0 (Hz) and does not receive the influence of the attenuation. 
       FIG. 6  is a graph illustrating a straight line having corrected features a, b, and c obtained by the attenuation correcting unit  333   b  as parameters. The equation of the straight line L 1  is represented as below. 
         I=af+b =( a   0 +2α z ) f+b   0    (5)
 
     As is clear from the Equation (5), the straight line L 1  has a larger slope (a&gt;a 0 ) than that of the straight line L 10  before the attenuation correction and has a same intercept (b=b 0 ) as that of the straight line L 10  before the attenuation correction. 
     The optimal attenuation rate setting unit  333   c  sets, as an optimal attenuation rate, an attenuation rate candidate value which gives a minimum statistical dispersion of the corrected feature calculated by the attenuation correcting unit  333   b  for each attenuation rate candidate value for all the frequency spectra. In the embodiment, as a quantity representing a statistical dispersion, a dispersion is applied. In this case, the optimal attenuation rate setting unit  333   c  sets, as then optimal attenuation rate, an attenuation rate candidate value which gives the minimum dispersion. Among the three corrected features a, b, and c, two pieces are independent. The corrected feature b does not depend on the attenuation rate. Accordingly, in a case where an optimal attenuation rate is set for the corrected features a and c, the optimal attenuation rate setting unit  333   c  may calculate the dispersion of one of the corrected features a and c. 
     However, the corrected feature used when the optimal attenuation rate is set by the optimal attenuation rate setting unit  333   c  is preferably a same type as that of the corrected feature used when feature image data is generated by a feature image data generating unit  342 . In other words, it is preferable that the dispersion of the corrected feature a is applied in a case where the feature image data generating unit  342  generates feature image data by using a slope as the corrected feature, and the dispersion of the corrected feature c is applied in a case where the feature image data generating unit  342  generates feature image data by using a mid-band fit as the corrected feature. The reason for this is that Equation (1) giving the attenuation amount A(f, z) merely represents an ideal case, and practically, the following Equation (6) is appropriate. 
         A ( f, z )=2α zf+ 2α 1   z    (6)
 
     α 1  represented in the second term of the right-hand side represented in Equation (6) is a coefficient that represents a magnitude of a change in the signal intensity in proportion to the reception depth z of an ultrasound wave and is a coefficient that represents a change in the signal intensity occurring due to the non-uniformity of a tissue that is an observation target, a change in the number of channels at the time of beam synthesis, or the like. Since the second term of the right-hand side of Equation (6) is present, in a case where feature image data is generated using a mid-band fit as the corrected feature, the attenuation can be accurately corrected in a case where an optimal attenuation rate is set by using the dispersion of the corrected feature c (see Equation (4)). On the other hand, in a case where feature image data is generated using a slope that is a coefficient proportional to the frequency f, attenuation can be accurately corrected by excluding the influence of the second term of the right-hand side in a case where an optimal attenuation rate is set using the dispersion of the corrected feature a. For example, in a case where the unit of the attenuation rate α is dB/cm/MHz, the unit of the coefficient α 1  is dB/cm. 
     Here, the reason why an optimal attenuation rate can be set based on the statistical dispersion will be described. In a case where an optimal attenuation rate is applied to an observation target, it is considered that the feature converges to a value that is unique to the observation target regardless of a distance between the observation target and the ultrasound transducer  21 , and a statistical dispersion is decreased. On the other hand, in a case where an attenuation rate candidate value that is not appropriate for the observation target is set as an optimal attenuation rate, the attenuation correction is excessive or insufficient, and accordingly, it is considered that a deviation occurs in the feature in accordance with a distance to the ultrasound transducer  21 , and the feature is statistically irregular. Accordingly, an attenuation rate candidate value which gives the smallest statistical dispersion can be regarded as an optimal attenuation rate for the observation target. 
       FIG. 7  is a graph illustrating an example of the distribution of corrected features obtained by performing the attenuation correction based on two different attenuation rate candidate values for a same observation target. In  FIG. 7 , the horizontal axis is the corrected feature, and the vertical axis is the frequency. Two distribution curves N 1  and N 2  illustrated in  FIG. 7  are the same as a total sum of frequencies. In the case illustrated in  FIG. 7 , the distribution curve N 1  has a statistical dispersion of the feature smaller than that of the distribution curve N 2  (smaller dispersion) and forms a shape having a peak steeper than that of the distribution curve N 2 . Thus, in a case where an optimal attenuation rate is set from two attenuation rate candidate values corresponding to these two distribution curves N 1  and N 2 , the optimal attenuation rate setting unit  333   c  sets an attenuation rate candidate value corresponding to the distribution curve N 1  as an optimal attenuation rate. 
     Generally, it is known that, for attenuation rate candidate values and the dispersion that is a statistical dispersion of the corrected feature calculated for each attenuation rate candidate value, one quadratic function is determined for each frame. In the embodiment, the optimal attenuation rate setting unit  333   c  acquires a minimal value (extreme value) in the quadratic function generated based on the dispersion of corrected features c obtained by performing the attenuation correction based on a plurality of attenuation rate candidate values (three attenuation rate candidate values in the embodiment) and sets an attenuation rate candidate value corresponding to the extreme value as an optimal attenuation rate. An optimal attenuation rate is set based on the extreme value by the fact that a true value of an attenuation rate is identical to an attenuation rate α which gives a minimal dispersion if an observation target is uniform. While four or more attenuation rate candidate values may be set, from the viewpoint of decreasing the load according to the arithmetic operation process, three attenuation rate candidate values are preferable. 
     In the embodiment, three attenuation rate candidate values (attenuation rate candidate values α 1 , α 2 , and α 3 ) are stored in the storage unit  37  in advance, and the optimal attenuation rate setting unit  333   c  sets an optimal attenuation rate by using these three attenuation rate candidate values. The attenuation rate candidate values α 1 , α 2 , and α 3  are values of 0.0 or more, and, in a case where the observation target is a body tissue, the attenuation rate of the body tissues is generally near 0.6, and accordingly, it is preferable that a smallest attenuation rate candidate value among the three attenuation rate candidate values is 0.6 or less, and a largest value thereof is 0.6 or more. 
       FIG. 8  is a graph illustrating a relation between attenuation rate candidate values α 1 , α 2 , and α 3  and a quadratic function Q generated based on dispersions S(α 1 ), S(α 2 ), and S(α 3 ) of corrected features obtained by performing the attenuation correction based on the attenuation rate candidate values α 1 , α 2 , and α 3 . The optimal attenuation rate setting unit  333   c  acquires dispersions S(α 1 ), S(α 2 ), and S(α 3 ) of the corrected features c obtained by performing the attenuation correction based on the preset attenuation rate candidate values α 1 , α 2 , and α 3  and generates a quadratic function Q passing through the acquired dispersions S(α 1 ), S(α 2 ), S(α 3 ). The quadratic function acquired at this time is a function that is convex downward. The optimal attenuation rate setting unit  333   c  acquires an extreme value of the generated quadratic function Q and sets an attenuation rate candidate value α s  corresponding to the extreme value as an optimal attenuation rate. 
     Here, in RF data of a same frame, the reason why the dispersions that are based on a plurality of attenuation rate candidate values are present on a same quadratic function, and the quadratic function is convex downward will be described. In description presented below, the dispersion of corrected feature c (mid-band fit) will be described as an example. When the dispersion of the corrected feature c is denoted by V c (α), the following Equation (7) is derived from Equation (4) described above. In Equation (7), i is a subscript used for identifying a sample point, and dispersion V c (α) is calculated by acquiring a sum of the square of a difference between each corrected feature c 1  and an arithmetic mean of corrected feature. 
         V   c (α)=Σ i ( c   i   − c   ) 2    (7)
 
     From Equation (7) described above, the following Equation (8) is derived. In the following Equation (8), a reciprocation distance is L (L=2z). 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           
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     In Equation (8), the coefficient of α 2  in V c (α) has a positive value. Thus, V c (α) is a quadratic function that is convex downward. 
     For the dispersion of corrected feature a, when the dispersion of the corrected feature a is denoted by V a (α), the following Equation (9) is derived from Equation (2) described above. 
         V   a (α)=Σ i ( a   i   −ā ) 2    (9)
 
     From Equation (9) described above, by performing calculation similar to the dispersion V c (α) described above, the following Equation (10) is derived. 
     
       
         
           
             
               
                 
                   
                     
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     In Equation (10), the coefficient of α 2  in V a (α) has a positive value. Thus, also for V a (α), a quadratic function that is convex downward is acquired. 
     The image processing unit  34  includes: a B-mode image data generating unit  341  that generates a B-mode image data that is an ultrasound image converting the amplitude of an echo signal into luminance and displaying the luminance; and a feature image data generating unit  342  that generates feature image data displaying feature that is based on an optimal attenuation rate set by the optimal attenuation rate setting unit  333   c  in association with visual information together with the B-mode image. 
     The B-mode image data generating unit  341 , for B-mode reception data received from the signal processing unit  32 , performs signal processing using known technologies such as gain processing and contrast processing and performs data interpolation according to a data step width determined based on the display range of an image in the display device  4 , and the like, thereby generating a B-mode image data. The B-mode image is a gray scale image in which the values of R (red), G (green), and B (blue), which are variables in a case where a RGB color system is employed as a color space, match each other. 
     The B-mode image data generating unit  341 , after performing a coordinate conversion of rearrangement for the B-mode reception data transmitted from the signal processing unit  32  such that the scanning range can be correctly spatially represented, performs an interpolation process for the B-mode reception data, and fills a gap between the B-mode reception data, thereby generating B-mode image data. The B-mode image data generating unit  341  outputs the generated B-mode image data to the feature image data generating unit  342 . 
     The feature image data generating unit  342  superimposes visual information relating to the feature calculated by the feature calculating unit  333  on each pixel of an image of B-mode image data, thereby generating feature image data. The feature image data generating unit  342 , for example, assigns visual information corresponding to feature of a frequency spectrum calculated from one sample data group F j  (here, j=1, 2, . . . , K) to a pixel area corresponding to a data amount of the sample data group F j  illustrated in  FIG. 4 . The feature image data generating unit  342 , for example, associates hue as visual information with one of the slope, intercept, and the mid-band fit described above, thereby generating a feature image. The feature image data generating unit  342  may generate feature image data by associating hue with one of two features selected from the slope, the intercept, and the mid-band fit and associating shading with each other. As the visual information relating to the feature, for example, there are hue, saturation, brightness, a luminance value, and variables of a color space configuring a predetermined display color system such as R (red), G (green), and B (blue). 
     The control unit  36  is realized by a central processing unit (CPU) having an arithmetic operation and control function and various arithmetic operation circuits, and the like. The control unit  36  reads information stored by the storage unit  37  from the storage unit  37  and performs various arithmetic operation processes relating to an operation method of the ultrasound observation apparatus  3 , thereby performs an overall control of the ultrasound observation apparatus  3 . The control unit  36  may be configured by using a CPU and the like that are common to the signal processing unit  32  and the computing unit  33 . 
     The storage unit  37  includes a feature information storing unit  371  that stores the attenuation rate candidate values α 1 , α 2 , and α 3 , a plurality of features calculated in accordance with attenuation rate candidate values by the attenuation correcting unit  333   b  for each frequency spectrum, and a dispersion giving a statistical dispersion of the plurality of features in association with the attenuation rate candidate values. 
     In addition to the information described above, the storage unit  37  stores, for example information (the relation between the amplification factor and the reception depth illustrated in  FIG. 2 ) required for an amplification process, information (the relation between the amplification factor and the reception depth illustrated in  FIG. 3 ) required for an amplification correcting process, information (see Equation (1)) required for the attenuation correcting process, and information of a window function (such as Hamming, Hanning, and Blackman) required for a frequency analyzing process. 
     The storage unit  37  further stores various programs including an operation program for performing the operation method of the ultrasound observation apparatus  3 . The operation program may be recorded on a computer-readable recoding medium such as a hard disk, a flash memory, a CD-ROM, a DVD-ROM, or a flexible disk and be widely distributed. The various programs described above may be acquired by being downloaded through a communication network. The communication network described here, for example, is realized by an existing public circuit network, a local area network (LAN), a wide area network (WAN), or the like and may be either a wired network or a wireless network. 
     The storage unit  37  having the configuration described above is realized by a read only memory (ROM) in which various programs are installed in advance, a random access memory (RAM) storing arithmetic operation parameters and data of each process. 
     In the embodiment, in the attenuation correcting unit  333   b,  it is preferable that the dynamic range of the corrected feature is set in accordance with a range that can be set in the gain processing, the contrast processing, and the like performed by the image processing unit  34 . More specifically, the attenuation rate candidate values α 1 , α 2 , and α 3  are three values set between 0.0 and 2.0 such that the feature displayed as a feature image is within a dynamic range relating to display according to a range that can be set in the gain process, the contrast process, and the like described above. For example, in a case where the attenuation rate candidate value is larger than 2.0, there are cases where the calculated corrected feature exceeds the dynamic range and cannot be maintained as corrected feature. Accordingly, the values of the statistical dispersions become different, and, for example, a function that is generated based on the dispersions is not a quadratic function, and the attenuation rate cannot be set to an optimal value. In other words, the attenuation rate candidate values α 1 , α 2 , and α 3  relating to the embodiment are values set in the range of 0.0 to 2.0, and it is preferable that a smallest attenuation rate candidate value among the three attenuation rate candidate values is 0.6 or less, and a largest value thereof is 0.6 or more. Particularly, if the observation target is body tissues and a fixed-point system is used, it is more preferable that three attenuation rate candidate values are respectively set to be around 0.6 in order to suppress a decrease in the calculation precision due to clip. 
       FIG. 9  is a flowchart illustrating an overview of a process performed by the ultrasound observation apparatus  3  having the configuration described above. First, the ultrasound observation apparatus  3  receives an echo signal as a result of the measurement of an observation target that is performed by the ultrasound transducer  21  from the ultrasound endoscope  2  (Step S 1 ). 
     The signal amplifying unit  311  that has received the echo signal from the ultrasound transducer  21  amplifies the echo signal (Step S 2 ). Here, the signal amplifying unit  311 , for example, amplifies (STC correction) of the echo signal based on the relation between the amplification factor and the reception depth illustrated in  FIG. 2 . 
     Subsequently, the B-mode image data generating unit  341  generates B-mode image data by using the echo signal amplified by the signal amplifying unit  311  and outputs the generated B-mode image data to the display device  4  (Step S 3 ). The display device  4  that has received the B-mode image data displays a B-mode image corresponding to the B-mode image data (Step S 4 ). 
     The amplification correcting unit  331  performs an amplification correction having a constant amplification factor regardless of the reception depth for a signal output from the transmitting and receiving unit  31  (Step S 5 ). Here, the amplification correcting unit  331 , for example, performs an amplification correction such that the relation between the amplification factor and the reception depth illustrated in  FIG. 3  is satisfied. 
     After the amplification correction, the frequency analyzing unit  332  calculates a frequency spectrum for all the sample data groups within a focused area that is an area for which feature image data is generated by performing a frequency analysis through the FFT operation (Step S 6 : frequency analyzing step).  FIG. 10  is a flowchart illustrating an overview of a process performed by the frequency analyzing unit  332  in Step S 6 . Hereinafter, the frequency analyzing process will be described in detail with reference to the flowchart illustrated in  FIG. 10 . 
     First, the frequency analyzing unit  332  sets a counter k used for identifying a sound ray that is an analysis target to k 0  (Step S 21 ). 
     Subsequently, the frequency analyzing unit  332  sets an initial value Z (k)   0  of the data position (corresponding to the reception depth) Z (k)  representing a series of data groups (sample data groups) acquired for the FFT operation (Step S 22 ). For example,  FIG. 4 , as described above, illustrates a case where an eighth data position of the sound ray SR k  is set as the initial value Z (k)   0 . 
     Thereafter, the frequency analyzing unit  332  acquires a sample data group (Step S 23 ) and applies a window function stored in the storage unit  37  to the acquired sample data group (Step S 24 ). In this way, by applying the window function to the sample data group, the sample data group is not discontinuous at the boundary, and the occurrence of an artifact can be prevented. 
     Subsequently, the frequency analyzing unit  332  determines whether or not the sample data group of a data position Z (k)  is a normal data group (Step S 25 ). As described with reference to  FIG. 4 , a sample data group needs to have the number of data that is power of two. Hereinafter, the number of data of a normal sample data group will be 2 n  (n is a positive integer). In the embodiment, the data position Z (k)  is set to be the center of a sample data group to which Z (k)  belongs as possibly as can. More specifically, since the number of data of the sample data group is 2 n , Z (k)  is set to a 2 n /2 (=2 n-1 )-th position close to the center of the sample data group. In such a case, a sample data group being normal represents that 2 n-1 −1 (=N) pieces of data are present on the front side of the data position Z (k) , and 2 n-1  (=M) pieces of data are present on the rear side of the data position Z (k) . In the case illustrated in  FIG. 4 , the sample data groups F j  (j=1, 2, . . . , K−1) are normal altogether.  FIG. 4  illustrates a case where n=4 (N=7 and M=8). 
     As a result of the determination acquired in Step S 25 , in a case where the sample data group of the data position Z (k)  is normal (Step S 25 : Yes), the frequency analyzing unit  332  proceeds to Step S 27  to be described later. 
     As a result of the determination acquired in Step S 25 , in a case where the sample data group of the data position Z (k)  is not normal (Step S 25 : No), the frequency analyzing unit  332  generates a normal sample data group by inserting zero data to cover the shortfall (Step S 26 ). A window function is applied to a sample data group (for example, a sample data group F K  illustrated in  FIG. 4 ) determined not to be normal in Step S 25  before the insertion of zero data. For this reason, even when zero data is inserted to the sample data group, discontinuity of data does not occur. After Step S 26 , the frequency analyzing unit  332  proceeds to Step S 27  to be described later. 
     In Step S 27 , the frequency analyzing unit  332  acquires a frequency spectrum that is a frequency distribution of amplitudes by performing the FFT by using the sample data group (Step S 27 ). The frequency spectrum C 1  illustrated in  FIG. 5  is an example of the frequency spectrum acquired as a result of Step S 27 . 
     Subsequently, the frequency analyzing unit  332  changes the data position Z (k)  by a step width D (Step S 28 ). The step width D is assumed to be stored in the storage unit  37  in advance.  FIG. 4  illustrates a case where D=15. While the step width D preferably coincides with a data step width used when the B-mode image data generating unit  341  generates a B-mode image data, in a case where the amount of calculation performed by the frequency analyzing unit  332  is desired to be decreased, a value lager than the data step width may be set as the step width D. 
     Thereafter, the frequency analyzing unit  332  determines whether or not the data position Z (k)  is larger than a maximum value Z (k)   max  in the sound ray SR k  (Step S 29 ). In a case where the data position Z (k)  is larger than the maximum value Z (k)   max  (Step S 29 : Yes), the frequency analyzing unit  332  increases the counter k by one (Step S 30 ). This represents that the process proceeds to a next sound ray. On the other hand, in a case where the data position Z (k)  is the maximum value Z (k)   max  or less (Step S 29 : No), the frequency analyzing unit  332  causes the process to be returned to Step S 23 . In this way, the frequency analyzing unit  332  performs the FFT on [(Z (k)   max −Z (k)   0 +1)/D+1] sample data groups for the sound ray SR k . Here, [X] represents a maximum integer not exceeding X. 
     After Step S 30 , the frequency analyzing unit  332  determines whether or not the counter k is larger than the maximum value k max  (Step S 31 ). In a case where the counter k is larger than the maximum value k max  (Step S 31 : Yes), the frequency analyzing unit  332  ends a series of frequency analyzing processes. On the other hand, in a case where the counter k is the maximum value k max  or less (Step S 31 : No), the frequency analyzing unit  332  causes the process to be returned to Step S 22 . This maximum value k max  is a value that is arbitrarily directed and input through the input unit  35  by a user such as an operator or a value that is set in the storage unit  37  in advance. 
     In this way, the frequency analyzing unit  332  performs FFT multiple times on each of (k max −k 0 +1) sound rays within the analysis target area. The results of the FFT are stored in the storage unit  37  together with the reception depth and the reception direction. 
     In the description presented above, while the frequency analyzing process is performed only within the set focused area, the frequency analyzing unit  332  may be configured to perform the frequency analyzing process for all the areas from which an ultrasound signal is received. 
     Following the frequency analyzing process of Step S 6  described above, the feature calculating unit  333  calculates pre-correction features of each of a plurality of frequency spectra, for each of a plurality of attenuation rate candidate values giving different attenuation characteristics when an ultrasound wave propagates through an observation target, calculates corrected feature of each frequency spectrum by performing an attenuation correction excluding the influence of attenuation of an ultrasound wave for the pre-correction features of each frequency spectrum, calculates a dispersion of each attenuation rate candidate value by using the corrected features, and sets an optimal attenuation rate for the observation target by generating a quadratic function representing the relation between the attenuation rate candidate value and the dispersion and acquiring an extreme value (Steps S 7  to S 12 : feature calculating step). Hereinafter, the process of Steps S 7  to S 12  will be described in detail. 
     In Step S 7 , the approximation unit  333   a  performs a regression analysis of each of the plurality of frequency spectra calculated by the frequency analyzing unit  332 , thereby calculating a pre-correction feature that corresponds to each frequency spectrum of a divided area as the attenuation rate setting target (Step S 7 ). More specifically, the approximation unit  333   a  performs regression analysis on each frequency spectrum to approximate each frequency spectrum by a linear expression, and obtain a slope a 0 , an intercept b 0  and a mid-band fit c 0 , as the pre-correction feature. For example, the straight line L 10  illustrated in  FIG. 5  is a regression line obtained by approximating the frequency spectrum C 1  in the frequency band U using regression analysis performed by the approximation unit  333   a.    
     Thereafter, the optimal attenuation rate setting unit  333   c  sets values of attenuation rate candidate values applied when an attenuation correction to be described later is performed to predetermined set values α 1 , α 2 , and α 3 . It may be configured such that the values of the set values α 1 , α 2 , and α 3  are stored in the feature information storing unit  371  in advance, and the optimal attenuation rate setting unit  333   c  refers to the feature information storing unit  371 . 
     Subsequently, the attenuation correcting unit  333   b  performs attenuation correction on the pre-correction feature obtained by the approximation unit  333   a  by approximating each frequency spectrum, using the set values α 1 , α 2 , and α 3  as attenuation rate candidate values to calculate corrected features, and stores the calculated corrected features in the feature information storing unit  371  together with the set values α 1 , α 2 , and α 3  (Step S 8 ). A straight line L 1  illustrated in  FIG. 6  is an example of a straight line acquired by the attenuation correcting unit  333   b  performing the attenuation correcting process. 
     In Step S 8 , the attenuation correcting unit  333   b  calculates the corrected features by substituting the reception depth z in Equations (2) and (4) with the data position Z=(f sp /2v s )Dn acquired by using a data arrangement of sound rays of an ultrasound signal. Here, f sp  represents the sampling frequency of data, v s  represents the speed of sound, D represents a data step width, and n represents the number of data steps from first data of a sound ray up to the data position of a sample data group that is a processing target. For example, when the sampling frequency f sp  of data is 50 MHz, the speed of sound v s  is 1,530 m/sec, and the step width D is 15 by employing the data arrangement illustrated in  FIG. 6 , Z=0.2295·n (mm). 
     The optimal attenuation rate setting unit  333   c  calculates the dispersion of representative corrected feature among a plurality of corrected features acquired by the attenuation correcting unit  333   b  performing an attenuation correction for each frequency spectrum and stores the calculated dispersions in the feature information storing unit  371  in association with the set values α 1 , α 2 , and α 3  (Step S 9 ). In a case where the corrected feature is a slope a and a mid-band fit c, like the case illustrated in  FIG. 8  described above, the optimal attenuation rate setting unit  333   c,  for example, calculates the dispersion of the corrected feature c. In Step S 19 , it is preferable that the optimal attenuation rate setting unit  333   c  applies the dispersion of the feature a in a case where the feature image data generating unit  342  generates feature image data by using the slope and applies the dispersion of the corrected feature c in a case where feature image data is generated by using the mid-band fit. 
     Thereafter, the optimal attenuation rate setting unit  333   c  generates a quadratic function based on the dispersion of the corrected features c after the attenuation correction is performed based on the set values α 1 , α 2 , and α 3  (Step S 10 ). The optimal attenuation rate setting unit  333   c  acquires an extreme value of the generated quadratic function (Step S 11 ) and sets, as an optimal attenuation rate, an attenuation rate candidate value corresponding to the extreme value (Step S 12 ). 
     As illustrated in  FIG. 8 , in a case where the dispersion takes an extreme value S(α) min  when the attenuation rate candidate value α s  is 0.65 (dB/cm/MHz), the optimal attenuation rate setting unit  333   c  sets α=0.65 (dB/cm/MHz) as an optimal attenuation rate. 
     The feature image data generating unit  342  superimposes visual information (for example, hue) associated with the corrected feature that is based on the optimal attenuation rate specified in Step S 12  on each pixel in the B-mode image data generated by the B-mode image data generating unit  341  and adds information of the optimal attenuation rate, thereby generating feature image data (Step S 13 : feature image data generating step). 
     Thereafter, the display device  4 , under the control of the control unit  36 , displays a feature image corresponding to the feature image data generated by the feature image data generating unit  342  (Step S 14 ).  FIG. 11  is a schematic diagram illustrating an example of display of a feature image in the display device  4 . A feature image  201  illustrated in the drawing includes: a superimposed image display section  202  displaying an image in which visual information relating to feature is superimposed on a B-mode image; and an information display section  203  displaying identification information of an observation target and information of an attenuation rate candidate value set as the optimal attenuation rate. In the information display section  203 , the information of the feature, the information of the approximation equation, the information of a quadratic function generated based on the dispersion of the corrected features obtained by performing the attenuation correction based on the set values α 1 , α 2 , and α 3 , image information of a gain, contrast, and the like may be further displayed. A B-mode image corresponding to a feature image may be displayed so as to be aligned with a feature image. Furthermore, the input unit  35  may be configured to receive a direction signal representing where or not the information of the attenuation rate candidate value is displayed. 
     In the series of processes (Steps S 1  to S 14 ) described above, the process of Step S 4  and the process of Steps S 5  to S 12  may be configured to be performed in parallel. 
     According to the embodiment of the present invention described above, each corrected feature is calculated by performing an attenuation correction by using three set values (set values α 1 , α 2 , and α 3 ) set in advance as attenuation rate candidate values, an extreme value of a quadratic function generated based on the dispersion of the corrected feature is acquired, and an attenuation rate candidate value corresponding to the extreme value is set as an optimal attenuation rate. Accordingly, the attenuation characteristics of an ultrasound wave that are appropriate for an observation target can be acquired at a high speed through simple calculation, and an observation using the attenuation characteristics can be performed. 
     In addition, according to the embodiment, an optimal partial attenuation rate is set based on a statistical dispersion of corrected feature acquired by performing an attenuation correction of each frequency spectrum, and accordingly, the amount of calculation can be smaller than that of a conventional case where fitting with a plurality of attenuation models is performed. 
     Furthermore, according to the embodiment, even if an attenuation rate that is appropriate for an observation target is unknown, it is possible to set an optimal attenuation rate. 
     In addition, according to the embodiment, by configuring the set values α 1 , α 2 , and α 3  as the attenuation rate candidate values to have values of 0.0 or more, configuring a smallest attenuation rate among the three attenuation rate candidate values to be 0.6 or less, and configuring a largest value thereof to be 0.6 or more, the calculation accuracy of the optimal attenuation rate acquired when a body tissue generally having an attenuation rate near 0.6 is set as an observation target can be improved. 
     Furthermore, according to the embodiment, an attenuation rate corresponding to an extreme value of the quadratic function is set as the optimal attenuation rate. Accordingly, a numerical value having digits more than the attenuation rate candidate values set in advance can be set as the attenuation rate, whereby the calculation accuracy of the optimal attenuation rate can be improved. 
     In addition, according to the embodiment, by setting the attenuation rate candidate values α 1 , α 2 , α 3  to be in the range of 0.0 to 2.0 such that feature displayed as a feature image is within the dynamic range, the calculation accuracy of the optimal attenuation rate acquired when a body tissue is set as an observation target can be improved. 
     Furthermore, according to the embodiment, by further displaying the information on a quadratic function generated based on the dispersion of corrected features obtained by performing the attenuation correction based on the set values α 1 , α 2 , α 3  on the information display section  203 , an error or the like occurring in the generation of the quadratic function can be checked and acquired by a user. 
     Modified Example of Embodiments 
     Next, a modified example of the embodiment of the present invention will be described. In the modified example, the optimal attenuation rate setting unit  333   c  sets an optimal attenuation rate in a dynamic range wider than a dynamic range at the time of displaying a feature image. 
     More specifically, when the display dynamic range of an image generated by the feature image data generating unit  342  is 70 dB, the feature calculating unit  333  performs an attenuation calculation process with a dynamic range (for example, 100 dB) larger than this dynamic range (70 dB). For example, while the feature image data generating unit  342  uses a fixed-point system of eight bits, the feature calculating unit  333  performs the attenuation calculating process including the calculation of feature to the setting of an optimal attenuation rate by using a floating-point system of 32 bits. 
     According to the modified example, compared to the attenuation calculating process using a fixed-point system, the calculation accuracy can be improved. By performing the generation of a quadratic function that is based on the dispersion from the calculation of the pre-correction feature with further higher accuracy, an optimal attenuation rate can be calculated with high accuracy. 
     Although the modes carrying out the present invention has been described, the present invention is not limited only by the embodiments described above. For example, the optimal attenuation rate setting unit  333   c  may calculate each optimal attenuation rate correspondence value corresponding to an optimal attenuation rate for all the frames of an ultrasound image and set a mean value, a median value, or a maximum frequency of a predetermined number of optimal attenuation rate correspondence values including an optimal attenuation rate correspondence value of a latest frame as an optimal attenuation rate. In such a case, a change in the optimal attenuation rate is smaller than that of a case where an optimal attenuation rate is set in each frame, and accordingly, the value thereof can be stabilized. 
     The optimal attenuation rate setting unit  333   c  may set an optimal attenuation rate at a predetermined frame interval of an ultrasound image. In such a case, the amount of calculation can be decreased to a large extent. In such a case, until an optimal attenuation rate is set next time, the value of the optimal attenuation rate that is set late may be used. 
     The statistical dispersion may be calculated on a target area for each sound ray or on an area with the reception depth being a predetermined value or more. The input unit  35  may be configured to receive a setting of such an area. 
     The input unit  35  may be configured to receive an input of a setting change of the set values α 1 , α 2 , and α 3  of the attenuation rate candidate values. 
     As a quantity giving a statistical dispersion, for example, any one of a standard deviation, a difference between a maximum value and a minimum value of feature in a population, a half-value width of the distribution of feature may be applied. As a quantity giving a statistical dispersion, a reciprocal of the dispersion may be applied. However, in such a case, it is apparent that the quadratic function thereof is convex upward, and an attenuation rate candidate value corresponding to the extreme value thereof is set as an optimal attenuation rate. 
     The optimal attenuation rate setting unit  333   c  may calculate each statistical dispersion of a plurality of kinds of corrected feature and set an attenuation rate candidate value corresponding to an extreme value of a quadratic function generated based on the statistical dispersion as an optimal partial attenuation rate. 
     Furthermore, an ultrasound miniature probe having a small diameter having no optical system may be applied as an ultrasound probe. The ultrasound miniature probe, generally, is inserted into a bile duct, a biliary tract, a pancreatic duct, trachea, a bronchial tube, urethra, or a urinary duct and is used when peripheral organs thereof (pancreas, lung, prostate, urinary bladder, a lymph node, and the like) are observed. 
     As the ultrasound probe, an external-type ultrasound probe emitting an ultrasound wave from the surface of a subject may be employed. The external-type ultrasound probe, generally, is used when organs in the abdomen (liver, gallbladder, or urinary bladder), mamma (particularly, mammary gland), or thyroid gland is observed. 
     Furthermore, the ultrasound transducer may be a linear transducer, a radial transducer, or a convex transducer. In a case where the ultrasound transducer is a linear transducer, the scanning area thereof forms a rectangle (a rectangle or a square). On the other hand, in a case where the ultrasound transducer is a radial transducer or a convex transducer, the scanning area thereof forms a linear shape or a circular shape. The ultrasound endoscope may allow the ultrasound transducer to perform mechanical scanning or to perform electronic scanning by arranging a plurality of elements as ultrasound transducers in an array pattern and electronically performing switching among the elements relating to transmission/reception or applying a delay to the transmission/reception of each element. 
     According to some embodiments, it is possible to acquire attenuation characteristics of an ultrasound wave suitable for an observation target through simple calculation and to perform an observation using the attenuation characteristics. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.