Chi Hyung Seo

Sidelobe suppression in ultrasound imaging using dual apodization with cross-correlation

This paper introduces a novel sidelobe and clutter suppression method in ultrasound imaging called dual apodization with cross-correlation or DAX. DAX dramatically improves the contrast-to-noise ratio (CNR) allowing for easier visualization of anechoic cysts and blood vessels. This technique uses dual apodization or weighting strategies that are effective in removing or minimizing clutter and efficient in terms of computational load and hardware/software needs. This dual apodization allows us to determine the amount of mainlobe versus clutter contribution in a signal by cross-correlating RF data acquired from 2 apodization functions. Simulation results using a 128 element 5 MHz linear array show an improvement in CNR of 139% compared with standard beamformed data with uniform apodization in a 3 mm diameter anechoic cylindrical cyst. Experimental CNR using a tissue-mimicking phantom with the same sized cyst shows an improvement of 123% in a DAX processed image.

Evaluating the robustness of dual apodization with cross-correlation

We have recently presented a new method to suppress side lobes and clutter in ultrasound imaging called dual apodization with cross-correlation (DAX). However, due to the random nature of speckle, artifactual black spots may arise with DAX-processed images. In this paper, we present one possible solution, called dynamic DAX, to reduce these black spots. We also evaluate the robustness of dynamic DAX in the presence of phase aberration and noise. Simulation results using a 5 MHz, 128-element linear array are presented using dynamic DAX with aberrator strengths ranging from 25 ns root-mean-square (RMS) to 45 ns RMS and correlation lengths of 3 mm and 5 mm. When simulating a 3 mm diameter anechoic cyst, at least 100% improvement in the contrast-to-noise ratio (CNR) compared with standard beamforming is seen using dynamic DAX, except in the most severe case. Layers of pig skin, fat, and muscle were used as experimental aberrators. Simulation and experimental results are also presented using dynamic DAX in the presence of noise. With a system signal-to-noise ratio (SNR) of at least 15 dB, we have a CNR improvement of more than 100% compared with standard beamforming. This work shows that dynamic DAX is able to improve the contrast-to-noise ratio reliably in the presence of phase aberration and noise.

A dual-layer transducer array for 3-D rectilinear imaging

Very large element counts (16,000-65,000) are required for 2-D arrays for 3-D rectilinear imaging. The difficulties in fabricating and interconnecting 2-D arrays with a large number of elements (>5,000) have limited the development of suitable transducers for 3-D rectilinear imaging. In this paper, we propose an alternative solution to this problem by using a dual-layer transducer array design. This design consists of 2 perpendicular 1-D arrays for clinical 3-D imaging of targets near the transducer. These targets include the breast, carotid artery, and musculoskeletal system. This transducer design reduces the fabrication complexity and the channel count, making 3-D rectilinear imaging more realizable. With this design, an effective N times N 2-D array can be developed using only N transmitters and N receivers. This benefit becomes very significant when N becomes greater than 128, for example. To demonstrate feasibility, we constructed a 4 times 4 cm prototype dual-layer array. The transmit array uses diced PZT-5H elements, and the receive array is a single sheet of un-diced P[VDF-TrFE] copolymer. The receive elements are defined by the copper traces on the flexible interconnect circuit. The measured -6 dB fractional bandwidth was 80% with a center frequency of 4.8 MHz. At 5 MHz, the nearest neighbor crosstalk of the PZT array and PVDF array was -30.4 plusmn 3.1 dB and -28.8 plusmn 3.7 dB, respectively. This dual-layer transducer was interfaced with an Ultrasonix Sonix RP system, and a synthetic aperture 3-D data set was acquired. We then performed offline 3-D beamforming to obtain volumes of nylon wire targets. The theoretical lateral beamwidth was 0.52 mm compared with measured beamwidths of 0.65 mm and 0.67 mm in azimuth and elevation, respectively. Then, 3-D images of an 8 mm diameter anechoic cyst phantom were also acquired.

P5J-4 256x256 2-D Array Transducer with Row-Column Addressing for 3-D Imaging

In previous work, we presented experimental results using a prototype 64 x 64 2-D array with row-column methods. The main benefits of this row-column design are a reduced number of interconnects, a T/R switching scheme with a simple diode circuit, and an ability to perform transmit beamforming in azimuth direction and receive beamforming in elevational direction for a volumetric imaging of targets near the transducer. In this paper, we present experimental results acquired from a new 256 times 256 2-D array transducer. The center frequency was 6.4 MHz with a -6 dB bandwidth of 57 %. The series resonant impedance was 120 Ohms near 5.8 MHz. The mean crosstalk of the adjacent element was -22 dB in the frequency range of interest. For a wire target, in azimuth direction with transmit beamforming, we measured the -6 dB beamwidth to be 0.75 mm at a depth of 25.5 mm compared to a theoretical beamwidth of 0.68 mm. We successfully acquired 3-D images of a wire target and a cylindrical anechoic cyst of 10 mm in diameter made of graphite scatterers in gelatin.

5A-5 64 × 64 2-D Array Transducer with Row-Column Addressing

We present experimental results from a prototype 5 MHz, 64 times 64 (4096 elements, 16 mm times 16 mm) 2D array transducer with row-column addressing. The main benefits of our design are a reduced number of interconnects, a T/R switching scheme with a simple diode circuit, and an ability to perform transmit and receive beamforming. With transmit beamforming in elevational direction and receive beamforming in azimuth direction, it is possible to perform 3D imaging of targets near the transducer. The series resonant impedance was measured at 120 Ohms near 5.5 MHz. The spectrum of the pulse has a center frequency of 5.6 MHz and a -6 dB fractional bandwidth of 43%. In azimuth direction with receive beamforming, we measured the -6 dB width to be 1.22 mm at a depth of 55 mm compared to a theoretical beamwidth of 0.91 mm. In elevational direction with transmit beamforming, -6 dB width was 0.92 mm at a depth of 27 mm compared to a theoretical beamwidth of 0.45 mm. Finally, we introduce results from 266 times 266 2D array transducer

The effect of different cross-correlation methods on the dual apodization with cross-correlation algorithm

We have recently presented a new method to suppress side lobes and clutter in ultrasound imaging called dual apodization with cross-correlation or DAX. This method uses two apodization functions in receive mode which yield point spread functions with very similar main lobes but different side lobes and clutter. These similarities and differences are quantified by using normalized cross-correlation of the radio frequency (RF) data in the axial direction. These cross-correlation coefficients serve as a pixel-by-pixel weighting or filter to pass main lobe dominated signals and suppress clutter dominated signals. In this paper, we investigate the effect of different cross-correlation methods and cross-correlation segment size on contrast-to-noise ratio (CNR) using the DAX algorithm. When using 1-D axial cross-correlation, an axial segment size of 1.73 mm gave us the highest CNR with 125% improvement. Using 1-D lateral cross-correlation showed a 91% improvement in CNR with a segment size of 1.05 mm. 2-D cross-correlation showed a 145% improvement with segment size of 1.2 mm axially by 0.45 mm laterally. A simulation using a cylindrical 1.5 mm diameter anechoic cyst located at 30 mm depth embedded in a 3-D phantom of scatterers gave us a CNR improvement of 52% with 2-D cross-correlation. Lastly, using an excised sheep heart, DAX was able to improve CNR by at least 77%.

Recent results using a 256 × 256 2-D array transducer for 3-D rectilinear imaging

In previous work, we presented initial experimental results using a 256 times 256 2-D array with row-column methods. The main benefits of this design are a reduced number of interconnects, a T/R switching scheme with a simple diode circuit, and an ability to perform transmit beamforming in azimuth and receive beamforming in elevation for volumetric imaging of targets near the transducer. In this paper, we present 3-D images of axial wires embedded in a clear gelatin phantom, of an 8 mm diameter cylindrical anechoic cyst phantom and of a 10 mm diameter spherical anechoic cyst phantom acquired from a new 256 times 256 2-D array transducer. DAX processing was used to reduce clutter due to 1-way beamforming.

P5J-1 Dual-Layer Transducer Array for 3-D Imaging

The difficulty in fabricating and connecting large numbers of 2-D array elements has limited the development of 2-D transducer array with more than 5000 elements. In this paper, we propose a dual-layer transducer array design which uses perpendicular 1-D arrays for 3-D imaging. This transducer design reduces the complexity of fabrication and the number of channels while maintaining adequate performance compared to a fully sampled 2-D transducer array. To demonstrate feasibility, we constructed a 4 cm times 4 cm prototype dual-layer transducer array which consists of 256 elements for each layer operating at 4 MHz. Both layers used P[VDF-TrFE] material. The -6 dB fractional bandwidth was 130% without matching layer. The measured crosstalk of P[VDF-TrFE] layer was -24 dB in the frequency range of 3 MHz-10 MHz. We also verified the performance of this transducer using a wire target phantom. The theoretical beamwidth was 0.62 mm compared to the measured beamwidth of 0.53 mm using coherence factor when f-number is 1.6.

Dual apodization with cross-correlation in the presence of phase aberration and noise

We have recently presented a new method to suppress side lobes and clutter in ultrasound imaging called dual apodization with cross-correlation or DAX. This method uses two apodization functions in receive mode which yield point spread functions with very similar main lobes but different side lobes and clutter. These similarities and differences are quantified by using normalized cross-correlation of the radio frequency (RF) data in the axial direction. These cross-correlation coefficients serve as a pixel-by-pixel weighting or filter to pass main lobe dominated signals and suppress clutter dominated signals. In this paper, we evaluate the robustness of DAX in the presence of phase aberration and noise. Aberrating layers of pig skin, fat, and muscle were used experimentally to mimic different level of aberrators. Experimental results are also presented using DAX in the presence of noise. With system signal-to-noise ratio (SNR) of at least 15 dB, we have a contrast-to-noise ratio (CNR) improvement of over 100 % compared to standard beamforming. This work shows that DAX is able to reliably improve contrast-to-noise ratio in the presence of phase aberration and noise.

A PZT-P[VDF-TrFE] dual-layer transducer for 3-D rectilinear imaging

The difficulties associated with fabricating and connecting 2-D arrays with large numbers of elements have limited the development of arrays with more than 5000 elements. However, 2-D arrays for rectilinear imaging of targets such as the breast, carotid artery, and musculoskeletal system require 128times128=16,384 to 256times256=65,536 elements. To simplify transducer design and system requirements, we propose a PZT-P[VDF-TrFE] dual-layer transducer array design which uses perpendicular 1-D arrays for 3-D imaging of targets near the transducer. This transducer design reduces the fabrication complexity and the channel count.