0885–3010/$25.00 © 2009 ieee1327ieee transactions on ultrasonics, ferroelectrics, and frequency control, vol. 56, no. 7, july 2009Abstract—A method for improving the contrast resolution of B-mode images is proposed by combining the speckle-reduc-tion technique of frequency compounding (FC) and the coded-excitation and pulse-compression technique called resolution enhancement compression (REC). FC suppresses speckle but at the expense of a reduction in axial resolution. Using REC, the axial resolution and bandwidth of the imaging system was doubled. Therefore, by combining REC with FC (REC-FC), the tradeoff between axial resolution and contrast enhance-ment was extended significantly. Simulations and experimental measurements were conducted with a single-element transduc-er (f/2.66) having a center frequency of 2.25 MHz and a −3-dB bandwidth of 50%. Simulations and measurements of hyper-echoic (+6 dB) tissue-mimicking targets were imaged. Four FC cases were evaluated: full-, half-, third-, and fourth-width of the true impulse response bandwidth. The image quality metrics used to compare REC-FC to conventional pulsing (CP) and CP-FC were contrast-to-noise ratio (CNR), speckle signal-to-noise ratio, histogram pixel intensity, and lesion signal-to-noise ratio. Increases in CNR of 121%, 231%, 302%, and 391% were obtained in experiments when comparing REC-FC for the full-, half-, third-, and fourth-width cases to CP. Further-more, smaller increases in CNR of 112%, 233%, and 309% were obtained in experiments when comparing CP-FC for the half-, third-, and fourth-width cases to CP. Improved lesion detect-ability was observed by using REC-FC.i. introductionspeckle, the granular structure in ultrasonic images, reduces the ability to detect low-contrast targets. speckle is formed by subresolution scatterers that cause constructive and destructive interference of backscattered ultrasonic signals within the resolution cell volume of an ultrasonic source [1]. furthermore, speckle is considered to be a deterministic process because, when an object is im-aged under the same operating conditions, no changes in the speckle pattern occur. Because of this nature, speckle is not reduced by signal averaging. therefore, a consider-able amount of work and effort has been spent over the last few decades developing techniques to reduce speckle in ultrasound images.in imaging, small structures or an object could be iden-tified if the contrast between the target and the surround-ing area are different [2]. in ultrasound, the difference in contrast between different soft tissues could be as small as 1%. therefore, by using speckle-reduction techniques, image contrast can be improved by removing the interfer-ence pattern created by speckle, which is also known as speckle noise, and enhance the detectability of structures with low contrast from the background [3]. consequently, speckle-reduction techniques have been used clinically and applied to commercial systems to improve the ability to detect small structures and lesions.speckle-reduction techniques can be classified into 2 categories: postprocessing techniques and compounding methods. examples of some of the postprocessing tech-niques are adaptive filtering (linear and nonlinear) [4]–[8], deconvolution [9], [10], and wavelet despeckling [11], [12]. the compounding speckle-reduction methods include spa-tial and frequency compounding. these schemes rely on making separate images that have uncorrelated or par-tially correlated speckle patterns and then are averaged to reduce the speckle but at the expense of spatial resolution. originally for spatial compounding, the source aperture was translated laterally or at different angles to make im-ages from different orientations [13], [14]. the main draw-backs of these techniques are loss in lateral resolution and image alignment due to motion that causes image arti-facts. also, the need for multiple images would mean a de-crease in the frame rate. recently, however, manufacturers have employed receive aperture-only spatial compounding that is not subject to frame rate losses or motion-based image registration errors. other advances in spatial-com-pounding [15], [16] use electronic-beam steering to obtain images at different angles and to overcome some of these tradeoffs by using advanced image registration.another method, known as frequency compounding (fc), can be applied on transmit mode by using mul-tiple sources at different frequencies or on receive mode as a postprocessing speckle-reduction technique by divid-ing the spectrum of the radio-frequency (rf) echoes into subbands to make separate images [17]–[23]. the latter instance is also known as frequency diversity [18], [23], or split spectrum processing [21]. the main disadvantage introduced by using frequency compounding is the inher-ent tradeoff between axial and contrast resolution. con-sequently, if the axial resolution and the bandwidth of An Ultrasonic Imaging Speckle-Suppression and Contrast-Enhancement Technique by Means of Frequency Compounding and Coded Excitationjose r. sanchez, Student Member, IEEE, and Michael l. oelze, Member, IEEEManuscript received november 16, 2008; accepted april 15, 2009. this work was supported by a grant from the niH (r21 eB006741).the authors are with the department of electrical and computer engineering, university of illinois at urbana-champaign, urbana, il (e-mail: [email protected]).digital object identifier 10.1109/tuffc.2009.1189an ultrasonic imaging system could be increased, these tradeoffs between axial and contrast resolution could be extended. the first researchers that applied fc to medi-cal ultrasound were abbott and thurstone [17]. in this study, they compared laser speckle to ultrasound speckle and suggested several techniques to generate independent speckle patterns including fc. Magnin et al. [19] observed that decorrelation of speckle patterns is dependent on the excitation bandwidth and obtained increases in speckle signal-to-noise ratio (ssnr) of 26% in B-mode images us-ing a phased array system. Melton et al. [20] developed a model to predict the amount of speckle reduction in a-mode scans based on correlation coefficients. trahey et al. [14] discussed a method for optimal speckle reduc-tion based on the