Short-lag spatial coherence of backscattered echoes: imaging characteristics

Abstract

Conventional ultrasound images are formed by delay-and-sum beamforming of the backscattered echoes received by individual elements of the transducer aperture. Although the delay-and-sum beamformer is well suited for ultrasound image formation, it is corrupted by speckle noise and challenged by acoustic clutter and phase aberration. We propose an alternative method of imaging utilizing the short-lag spatial coherence (SLSC) of the backscattered echoes. Compared with matched B-mode images, SLSC images demonstrate superior SNR and contrast-to-noise ratio in simulated and experimental speckle-generating phantom targets, but are shown to be challenged by limited point target conspicuity. Matched B-mode and SLSC images of a human thyroid are presented. The challenges and opportunities of real-time implementation of SLSC imaging are discussed.

Fig. 1

Examples of coherence functions in a point target and speckle background, as well as an experimental coherence function from in vivo thyroid tissue. The abscissa represents the lag, or spacing between receive elements. The ordinate represents inter-element RF echo correlation.

Fig. 2

Simulated B-mode images of 3-mm lesions with contrasts of anechoic, −24 dB, −18 dB from left to right in the first row, and −12, −6, and 6 dB from left to right in the second row. The corresponding SLSC images created with Q=20.8% are shown in rows 3 and 4. B-mode and SLSC images are shown with 40 dB of dynamic range. The boxes in the upper-left image indicate ROIs used to calculate the contrast, CNR, SNR, and point target conspicuity.

Fig. 3

Mean (a) contrast and (b) CNR observed in the lesions of the simulated B-mode and SLSC images, as a function of the intrinsic lesion contrast. Contrast of the B-mode images are a close match to ideal values. SLSC imaging suffers a significant decrease in contrast when Q=5.2%, but is more similar to B-mode imaging when Q=20.8%. SLSC imaging with Q=5.2% and 20.8% shows considerably higher CNR for hypoechoic lesions than B-mode imaging. Error bars indicate one standard deviation.

Fig. 4

Point target conspicuity increases as a function of target brightness for B-mode imaging, but remains flat for SLSC imaging regardless of brightness or Q. Error bars indicate one standard deviation.

Fig. 5

Theoretical calculations of the short-lag spatial coherence image compared to the simulated B-mode and SLSC images for a lateral slice through the center of a spherical 3mm anechoic lesion with −24 dB contrast.

Fig. 6

(a) Contrast and (b) CNR for the −24 dB lesion, (c) SNR, and (d) lateral resolution as a function of Q. Q indicates the size of the receive aperture expressed as a percentage of the transmit aperture used to create the SLSC and B-mode images. Error bars indicate one standard deviation for six simulations.

Fig. 7

Matched B-mode (a) and SLSC (b-d) images of 4 mm spherical anechoic lesions in a tissue-mimicking phantom. The SLSC images were created with Q equal to 7.8, 15.6, and 23.4%, from left to right, respectively. The SLSC images show improved CNR and SNR and increased depth-of-field effects for smaller Q. Resolution differences are observable with increasing Q.

Fig. 8

Matched B-mode (left) and SLSC (right) images of 1-cm lesions, formed from simulated data without noise (top) and experimental data (bottom). Q is equal to 20.8% and 20.3% in simulated and experimental data, respectively. The boxes indicate ROIs used to calculate the contrast, CNR, and SNR. B-mode and SLSC images are shown with 50 dB dynamic range.

Fig. 9

Theoretical calculations of the short-lag spatial coherence image. A lateral slice through the center of a spherical 1cm lesion with −12 dB contrast is compared to simulated data with no noise (a), simulated data with noise, and experimental data (b). (c) Theoretical −12 dB contrast lesions of varying sizes. (d) Theoretical 12mm lesions of varying contrasts, with the vertical lines denoting lesion boundaries. Q is equal to 20.8% in theory and simulations and 20.3% in experimental data.

Fig. 10

(a) In vivo B-mode image of a cyst at 1.5 cm depth in a human thyroid. SLSC images of the thyroid formed with (b) Q=10.4% (c) Q=20.8%, and (d) Q=31.2%. (e) A spatial-compounded image of the thyroid. The SLSC images show improved CNR of the cyst and improved SNR of the thyroid tissue compared to the B-mode and spatial-compounded images. The SLSC images also show improved resolution compared to the spatial-compounded image.