Liver Tumors: Utility of Characterization at Dual-Frequency US

IEEE Trans. Ultrason., Ferroelect., Freq. Contr.

2010

By analyzing backscattered echo signal power spectra and thereby obtaining backscatter coefficient vs. frequency data, the size of sub-resolution scatterers contributing to echo signals can be estimated.Here we investigate tradeoffs in data acquisition and processing parameters for reference phantom based backscatter and scatterer size estimations. RF echo data from a tissue-mimicking test phantom were acquired using a clinical scanner equipped with linear array transducers. One array has a nominal frequency bandwidth of 5-13 MHz and the other 4-9 MHz. Comparison of spectral estimation methods showed that the Welch method provided spectra yielding more accurate and precise backscatter coefficient and scatterer size estimations than spectra computed by applying rectangular, Hanning, or Hamming windows and much reduced computational load than if using the multitaper method. For small echo signal data block sizes, moderate improvements in scatterer size estimations were obtained using a multitaper method but this significantly increases the computational burden.It is critical to average power spectra from lateral A-lines for the improvement of scatterer size estimation. Averaging approximately 10 independent A-lines laterally, with an axial window length 10 times the center frequency wavelength optimized tradeoffs between spatial resolution and the variance of scatterer size estimates. Applying the concept of a time-bandwidth product this suggests using analysis blocks that contain at least 30 independent samples of the echo signal.The estimation accuracy and precision depend on the ka range where k is the wave number and a is the effective scatterer size. This introduces an ROI depth dependency to the accuracy and precision because of preferential attenuation of higher frequency sound waves in tissue-like media. With the 5-13 MHz transducer ka ranged from 0.5 -1.6 for scatterers in the test phantom, which is a favorable range, and the accuracy and precision of scatterer size estimations were both within ~5% using optimal analysis block dimensions. When the 4-9 MHz transducer was used, the ka value ranged from 0.3 to 0.8 -1.1 for the experimental conditions, and the accuracy and precision were found to be ~10% and 10% -25%, respectively. Although the experiments were done with two specific models of transducers on the test phantom, the results can be generalized to similar clinical arrays available from a variety of manufacturers and/or for different size of scatterers with similar ka range.

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Trade-offs in data acquisition and processing parameters for backscatter and scatterer size estimations

Liu

IEEE Trans. Ultrason., Ferroelect., Freq. Contr.

2010

“…With the advent of faster and more robust technology implemented in the hardware and software of modern machines, the knowledge of certain characteristics of a scanner-transducer pair is more critical. For instance, the frequency dependence of scattering has been shown to be a useful diagnostic tool in certain diseases, [1][2][3][4][5][6][7][8][9][10][11][12][13][14] but the information related to the actual operating frequency of a scanner is usually inadequately presented to the clinician to enable assessment of any relationship between echo signal levels and ultrasonic frequency. 15,16 The increased use of harmonic imaging in static and dynamic procedures is on the rise, and the use of this modality could benefit from knowledge of the frequency content of the scanner at various positions in the ultrasonic field.…”

mentioning

confidence: 99%

A Test Phantom for Estimating Changes in the Effective Frequency of an Ultrasonic Scanner

Wilson

Journal of Ultrasound in Medicine

2002

The test phantom performs as predicted by computer simulations and theoretical calculations of backscatter versus frequency. Contrast on images of the test phantom produced by clinical systems correlates with scanner frequency settings, showing feasibility. Relative shifts in effective frequency and operating bandwidth can be assessed from these contrast differences between settings with this test phantom.

“…Therefore, scatterer size information may be very useful to differentiate them. Sommer et al [6] showed B-mode image changes with frequency that are consistent with this idea.…”

Section: In Vivo Human Liver Hemangioma Imagesmentioning

confidence: 62%

2G-5 Methods for Improvement on Liver Scatterer Size Estimates

Liu

2006 IEEE Ultrasonics Symposium

Gerig

et al. 2006

For ultrasound scatterer size imaging, a large estimator variance caused by statistical fluctuations in echoes from random media is an important problem. This paper reviews causes of scatterer size estimator variance when using a reference phantom to measure backscatter and discusses methods to reduce the variance.A Siemens Antares machine equipped with a VFX9-4 linear array transducer is used to acquire RF data both from a reference phantom and excised human livers. The ultrasound bandwidth is in the range from 2.5 MHz to 5.5 MHz. Scatterer sizes are estimated from the frequency dependence of the backscatter coefficient using the reference phantom method. Welch's averaging method with 20 -30 independent 10 -15 wavelength gated reference data segments provides a reasonable power spectrum. The sample power spectra were also computed using 20 -30 averages of the spectral estimates for 10 -15 wavelength gated windows which provide optimum estimation of the scatterer size. Angular compounding of partially uncorrelated scatterer size estimates acquired from different angular insonifications is one method to reduce the variance. A 2 -3 degree step size is a good choice for angular compounding with this transducer setting. Phantom studies show that the correlation between echo power spectrum estimates drops to less than 0.5 for this step size. Thus, beam steered RF data acquisition from -10 to +10 degree along with compounding plus averaging across 2 -3 uncorrelated Alines laterally was chosen for acquiring sample data.Another error source of scatterer size estimates is due to frequency dependent attenuation. In large homogeneous regions the size estimates should not change significantly with depth. Our experiments show that this criterion can be used to improve the estimates of the attenuation value.Differentiation of benign from malignant liver masses, particularly liver hemangiomas from metastases is clinically important. We are acquiring and analyze liver tumor data from excised human liver. Our experiments show that the effective scatterer diameter in human liver is 140 -190 µm over the measurement bandwidth. Preliminary in vivo results suggest that ultrasound scatterer size imaging may be an effective processing method for imaging liver hemangiomas.