Automatic time gain compensation and dynamic range control in ultrasound imaging systems

Transcription

1 Automatic time gain compensation and dynamic range control in ultrasound imaging systems Duhgoon Lee, Yong Sun Kim, and Jong Beom Ra * Dept. of EECS, KAIST, Guseong-dong, Yuseong-gu, Daejeon, Korea ABSTRACT For efficient and accurate diagnosis of ultrasound images, the appropriate time gain compensation (TGC) and dynamic range (DR) control of ultrasound echo signals are important. TGC is used for compensating the attenuation of ultrasound echo signals along the depth, and DR is for controlling the image contrast. In recent ultrasound systems, those two factors are automatically set by a system and/or manually adjusted by an operator to obtain the desired image quality on the screen. In this paper, we propose an algorithm to find the optimized parameter values for TGC and DR automatically. In TGC optimization, we determine the degree of attenuation compensation along the depth by reliably estimating the attenuation characteristic of ultrasound signals. For DR optimization, we define a novel cost function by properly using the characteristics of ultrasound images. Experimental results are obtained by applying the proposed algorithm to a real ultrasound (US) imaging system. The results prove that the proposed algorithm automatically sets values of TGC and DR in real-time so that the subjective quality of the enhanced ultrasound images may become good enough for efficient and accurate diagnosis. Keywords: ultrasound imaging, time gain compensation, dynamic range control. INTRODUCTION The ultrasound imaging system is popularly used in clinical diagnosis because the imaging is non-invasive and can be performed in real-time. However, there exist several disadvantages compared to X-CT (X-ray computed tomography) and MRI (magnetic resonance imaging) images. One of them is the presence of speckle noise in ultrasound images. The noise induces quality deterioration in the images and provides a negative impact on clinical diagnosis. Therefore, many algorithms have been continually developed to reduce the speckle noise. In consequence, the image quality for some applications is close to that of MR images. As the other disadvantage, an ultrasound imaging system needs many interactions with an operator to obtain a desired image quality. In the process of obtaining an ultrasound image, the operator actually needs to adjust several basic parameters of the system and the image quality is sensitive to their values. Among them, the parameters of TGC and DR control are to be frequently adjusted by the operator. Here, TGC is for compensating the attenuation of ultrasound echo signal along the depth and DR control is for determining a proper range to be displayed from a wide range of echo signal. Hence, it is considered meaningful to automatically find the optimized parameter values for TGC and DR in real-time without operator s interaction. In the conventional ultrasound imaging system, the time gain compensation for an ultrasound signal is done by using a compensation curve, and the curve can be adjusted by a user. An ultrasound image is composed of the arrays of hundreds of scan lines, and each scan line represents a sampled ultrasound echo signal along the depth. Several algorithms have been proposed for compensating the intensity of ultrasound signals along the depth []-[5]. These methods focus on the individual compensation of each scan line. Also, some algorithms are considered mainly at the radio frequency signal level, thereby they can be implemented only through the modification of system hardware. Dynamic range control in ultrasound systems is used to determine a range to be displayed as an 8-bit B/W image from a wide range of log-compressed echo signal. The relationship between a log-compressed ultrasound signal and the B/W ultrasound image after the DR control is usually modeled as in Fig.. Here, the range of 8 bits is mainly due to the * jbra@ee.kaist.ac.kr; phone ; fax: ; www-isl.kaist.ac.kr Medical Imaging 2006: Ultrasonic Imaging and Signal Processing edited by Stanislav Emelianov, William F. Walker, Proc. of SPIE Vol. 647, 64708, (2006) /06/$5 doi: 0.7/ Proc. of SPIE Vol

2 limitation of display system. Based on the relationship given in Fig., we can adjust the image contrast by changing the value of DR. In this case, the initial point of the DR is fixed and only the width of DR is changed. Note here that the image contrast affects the roughness of soft tissue and the edgeness near the tissue boundary, thereby strongly affects the clinical subjective quality. Intensity 2 8 US image 0 2 DR 5 Intensity Log-compressed echo signal Fig.. Relationship between a log-compressed echo signal and the corresponding ultrasound image Many algorithms for improving the ultrasound image contrast have been developed. Among them, one method tried to increase the CNR of lesions to the background by analyzing the frequency spectra [6]. However, this method is not appropriate for finding the optimal DR because it deals with only lesions rather than the overall image. As the other approach, the gray-scale image enhancement algorithm has been proposed [7]. But this method cannot find the optimal value of DR because the individual transformation of each pixel value does not yield the global transformation of DR as shown in Fig.. In this paper, we aim to develop an algorithm to automatically find the optimal parameters values for TGC and DR control in real-time without operator s interaction. The proposed algorithm is designed on the basis of a simplified model of the ultrasound imaging system without a loss of generality. Therefore, the algorithm differs from the early attempts and can be easily applied to a practical ultrasound imaging system. This paper is organized as follows. In Section 2, the proposed algorithm is described. Section 2. describes a TGC optimization scheme based on the attenuation estimation and compensation along the depth. In Section 2.2, we introduce a DR optimization scheme by using the proposed measures of edge contrast and soft tissue roughness. Experimental results are shown in Section 3. Finally, conclusions are drawn in Section PROPOSED ALGORITHM The proposed algorithm consists of two procedures, TGC and DR optimization. As an input to the algorithm, we use the log-compressed digital echo data whose intensities are not compensated along the depth. We perform the optimal TGC based on this input data. After compensating for the intensities of the input data along the depth, the DR optimization is performed. Finally the optimized parameter values for TGC and DR control are applied to the system. 2. TGC optimization It is known that ultrasound echoes attenuate exponentially with the propagation distance from the interface. Hence, logcompressed ultrasound echoes linearly decay with the distance. The degree of attenuation of echoes depends on the propagation medium. If we assume that the attenuation in an ultrasound imaging system occurs due to the one major medium in the body, the attenuation of log-compressed echoes can be modeled as a straight line. In order to alleviate undesirable errors in estimating a straight line and achieve statistically reliable TGC results, we propose the following Proc. of SPIE Vol

3 method. 2.. Vertical profiles and straight line modeling To estimate the attenuation, we need to measure the intensity variation along the depth. It is usually difficult to estimate the attenuation by using a log-compressed echo signal because of the noise. In order to overcome this problem, we may use a vertical profile which represents the average of several neighboring echo signals. To obtain the profiles, we divide an input echo image into M subsequent vertical strips. If each vertical strip consists of N scan lines, the corresponding vertical profile v k (n) can be written as vk ( n) = N N k m= u ( m, n) () where u k (m, n) denotes the intensity at (m, n) within the kth strip and the range of k is to M. As mentioned above, the attenuation function of log-compressed echoes in a vertical strip can be modeled as a straight line. And this straight line can be modeled by using the averaged vertical profile v k (n). In approximating the vertical profile to a line profile, we adopt the least squares fit technique. Fig. 2 shows one of the vertical profiles and its approximated line profile to estimate the attenuation. We can see in the graph that the approximated line profile well represents the attenuation of the vertical profile along the depth. Vertical profile Approximated line profile 96.4 Intensity Depth [pixel] Fig. 2. Average intensity profile of a vertical strip along the depth in an ultrasound image (solid line) and its approximated line profile (dashed line) 2..2 Compensation In an ultrasound image, there can be local dark regions such as a vessel and mass in the scanned area. And these dark regions may disturb a correct estimation of the attenuation line because they tend to make the slope of the approximated line profile steeper. Therefore, to avoid this problem, we exclude the line profiles with steeper slopes in averaging M line profiles. Then, the remained line profiles are averaged to obtain the attenuation line function for the ultrasound image. In the attenuation line, the attenuation value at the zero depth is converted to one for normalization. Based on the slope of the attenuation line, we perform TGC, or compensate for the intensities along the depth. In this case, we can consider that the TGC is being done by adjusting the slope of the attenuation line to zero. However, if we simply make the slope zero, the noise will be considerably amplified in the far field area. In the clinical sense, such noise Proc. of SPIE Vol

4 amplification is not desirable. Therefore, in order to prevent the unwanted noise amplification in the far field area, we need to alleviate the degree of TGC. Fig. 3 shows an estimated attenuation line a(n) and its compensated versions. In the figure, α denotes the normalized attenuation factor at the deepest point of the image and has a value in the range of 0 to. For TGC, we use a value of α which is smaller than, while the value is to be for ideal compensation as in c i (n) in the figure. Practically, TGC can be performed by multiplying the intensities of echoes by a proper gain according to the depth. However, in the ultrasound image, the multiplicative model is changed into the additive model because the input echo signals are log-compressed in the system. Therefore, the image intensity u k (m, n) can be compensated as follows; c u ( m, n) = u ( m, n) + β ( c( n) a( n)), (2) k k where a(n) denotes the estimated attenuation line and c(n) denotes the desired attenuation line after the compensation. Also, β denotes the intensity value at the zero depth of the estimated attenuation line before normalization. Note in Eq. (2) that TGC is simply done by the addition. Intensity α Ideal compensation Practical compensation Estimated attenuation line c i (n) c(n) a(n) Depth [pixel] Fig. 3. Estimated attenuation line (solid) and desired attenuation lines (dashed) 2.2 DR optimization The ultrasound imaging system displays an 8-bit B/W image which is selected from a log-compressed echo data with a wide range of intensity by using a given DR. A DR optimization algorithm is to find a value of DR that provides the best quality of 8-bit images displayed on the screen. The overall structure of the proposed DR optimization algorithm is given in Fig. 4. The proposed algorithm introduces a cost function as a quantitative measure representing the quality of the displayed image. And the algorithm tries to find the value of DR that minimizes the cost function. The cost function consists of two measures that are obtained on the basis of detected edges in a B/W image. For efficient calculation, we first obtain an edge map for the image in advance. Then, we calculate the measures only on edge regions described in the edge map Edge detection To detect edges, we adopt the coherent nonlinear anisotropic diffusion model which is known to be effective [8]. In the adopted diffusion model, a pixel locating on an edge has a large difference between its two eigenvalues. And the edge direction is determined as one of the two eigenvectors which represent the normal and tangential directions. However, a pixel in a homogeneous region where speckle noise is dominant, has a small difference between its two eigenvalues. Based on these characteristics of eigenvectors, we generate an edge map which includes the edgeness and direction at Proc. of SPIE Vol

5 each edge pixel. Start Initialize DR Edge map Convert input data into a B/W image Measure the characteristics Calculate the cost Change DR Minimum? No Stop Fig. 4. Proposed DR optimization algorithm Two proposed measures The DR directly affects the boundary contrast and the roughness of soft tissue. Hence, we define two measures, namely, edge contrast (EC) and soft tissue roughness (STR) based on the edge map. The edge contrast represents the intensity difference between two regions within a window, which are separated by the edge line centered at an edge pixel. It can be obtained by using the following equation; EC = K ( i, E ( f ( i, m + f ( i, ), A m B (3) where E denotes a set of edge pixels in an ultrasound image and K denotes the total number of edge pixels. And f(i, represents the average of intensity values of the pixels on the edge line, which passes through an edge pixel, (i,, and locates in a window centered at the edge pixel. Here, the edge line is determined by using the direction vector of the edge pixel, and the intensity value of each point on the edge line is calculated by interpolating its neighboring pixels. Also, m A and m B denote the averages of the intensity values of the two regions separated by the edge line in the window, respectively. The soft tissue roughness is then defined as a standard deviation of the intensities of non-edge pixels in the image. Since soft tissue regions in the image should be homogeneous, the soft tissue roughness becomes to represent the roughness caused by speckle noise, and can be obtained by using the equation, STR = L ( u( i, L ( i, E ( i, E u( i, ) 2. (4) Here, L denotes the total number of non-edge pixels and u(i, denotes the intensity at pixel (i,. Proc. of SPIE Vol

6 2.2.3 Cost function An image with a high EC and a low STR is clinically desirable. And the optimal condition may be obtained by adjusting parameter DR. Hence, we propose a cost function to find the optimal value of DR in a clinical sense. The cost function is defined as a weighted sum of STR and the inverse of EC, i.e., λ J ( DR) = + STR. (5) EC + ε Here, ε is a constant of a small value that prevents the denominator of the first term of the right side in the equation from being zero, and λ is a weighting factor which will be selected empirically. The proposed cost J gets lower as EC increases and/or STR decreases. Therefore, we can expect that it has the minimum value when DR is optimized. Fig. 5 shows a plot of the cost versus the value of DR. To find a value of DR which makes the cost minimum, we adopt the downhill simplex method, which reliably guarantees the convergence within a capture range where the global minimum exists Proposed cost Value of DR [x0 5 ] Fig. 5. The cost versus the value of DR 3. EXPERIMENTAL RESULTS To test the performance of the proposed algorithm, we implement the algorithm to a practical ultrasound imaging system which includes a PC with a CPU of 2.0GHz and GB memory. The ultrasound imaging system takes the optimized parameter values of TGC and DR based on the proposed algorithm and displays the enhanced images. The processing time to determine the parameter values depends on the characteristic of the input image, but is approximately between 00ms to 300ms without software code optimization. So, the proposed algorithm can be considered fast enough for realtime processing. The variation of the processing time is mainly due to the number of iterations to minimize the cost in DR optimization. We obtain experimental results for thyroid and carotid which are most frequently scanned in the clinical diagnosis. In the experiments, values of α and λ are determined on the basis of the opinions of clinicians. For thyroid and carotid, α and λ are set to 0.95 and 450, respectively. In the experiment, images before and after parameter optimization are not scanned at the same time. Since the images are obtained by scanning a real body, a certain time interval is involved before obtaining the enhanced image. During this interval, the initial image is stored and processed to obtain the optimal parameters values of TGC and DR. Since this interval is small, we can obtain quite similar images before and after parameter optimization, which are still good enough Proc. of SPIE Vol

7 for examining the performance of the proposed algorithm. Fig. 6 shows experimental results for the thyroid. Fig. 6(a) is an initial image in which the parameters of TGC and DR are not properly adjusted. The result after TGC optimization by using the input echo signal is given in Fig. 6(b). Compared to Fig. 6(a), Fig. 6(b) shows a more homogeneous image because it is compensated along the depth. The image after the DR optimization is shown in Fig. 6(c). In this figure, we can verify that the contrast of the input image is improved and the structures look better. Finally, the image after both TGC and DR optimization is shown in Fig. 6(d). As expected, the image shows the best quality compared to the other images. Experimental results for carotid are given in Fig. 7. Fig. 7(a) shows that the image intensity rapidly attenuates along the depth and the image looks rough due to high contrast. Meanwhile, Fig. 7(b) shows the image after the TGC optimization. In this image, the intensity is compensated but the contrast is still too high. In the image after the DR optimization, however, the contrast becomes lower and the image looks smoother as shown in Fig. 7(c). Of course, we can obtain the best result as shown in Fig. 7(d) after both TGC and DR optimization. Fig. 6. Experimental results for Thyroid: (a) Initial image and the images after (b) TGC optimization, (c) DR optimization, and (d) TGC and DR optimization, respectively. Proc. of SPIE Vol

8 Fig. 7. Experimental results for Carotid: (a) Initial image and the images after (b) TGC optimization, (c) DR optimization, and (d) TGC and DR optimization, respectively. 4. CONCLUSIONS In this paper, we present an algorithm to optimize parameters TGC and DR which strongly affect the image quality in ultrasound imaging systems. In the proposed algorithm, in order to find the optimized parameters for TGC, we divide an ultrasound image into consecutive vertical strips and obtain vertical profiles from the strips. Then, we approximate every vertical profile to a straight line, and estimate the degree of attenuation along the depth by averaging the straight lines. The parameter values for TGC are obtained by using the estimated attenuation line. For DR optimization, we define two measures of edge contrast and soft tissue roughness which are the functions of DR. For the optimal value of DR, the corresponding image is expected to have high edge contrast and low soft tissue roughness. Hence, the proposed cost function is defined as a weighted sum of the soft tissue roughness and the inverse of edge contrast, and it has the minimum value when DR is optimized. This cost function enables a robust DR optimization. By applying the proposed Proc. of SPIE Vol

9 algorithm to a practical ultrasound system, we showed that it is possible to automatically optimize the parameters for TGC and DR control in real-time. We prove that the subject quality of the image with the optimized parameters is clearly improved for clinical diagnosis. REFERENCES. D. I. Hughes and F. A. Duck, Automatic attenuation compensation for ultrasonic imaging, Ultrasound Med. Biol., vol. 23, pp , W. D. Richard, A new time-gain correction method for standard B-mode ultrasound imaging, IEEE Trans. Med. Imag., vol. 8, pp , T. Herrick and A. Bryant, Automated gain control for medical diagnostic ultrasound imaging, in Proc. IEEE Circuits and Systems, A. Bryant and T. J. Herrick, Adaptive gain control and contrast improvement for medical diagnostic ultrasound B- mode imaging system using charge-couples devices, in Proc. IEEE Circuits and Systems, B. Richard and O. Charliac, A new digital adaptive time gain correction for B-mode ultrasonic imaging, in Proc. IEEE Engineering in Medicine, P. F. Stetson, F. G. Sommer, and A. Macovski, Lesion contrast enhancement in medical ultrasound imaging, IEEE Trans. Med. Imag., vol. 6, pp , August C. Munteanu and A. Rosa, Gray-scale image enhancement as an automatic process driven by evolution, IEEE Trans. Systems, Man, and Cybernetics, vol. 34, pp , April Y. S. Kim and J. B. Ra, "Improvement of ultrasound image based on wavelet transform: speckle reduction and edge enhancement," in Proc. SPIE Medical Imaging 2005, Vol. 5747, pp. 085~092, Proc. of SPIE Vol