Correlation of ultrasound attenuation with proton density fat fraction across multiple organs in healthy volunteers.

BACKGROUND

Local determination of the attenuation in biological tissues has been the focus of extensive research over the past decades. Its characterization is of major interest for both noninvasive thermal therapies and diagnostic applications. Numerous ultrasound (US)-based methods have been investigated in recent years to assess attenuation in vivo.

PURPOSE: In this article, we aim to map US attenuation in four organs of human volunteers: liver, pancreas, kidney, and breast. We propose to compare the mean attenuation values obtained in each tissue with the corresponding proton density fat fraction (PDFF) derived from quantitative magnetic resonance imaging (qMRI). This comparison allows us to (i) present a direct calculation method of the local US attenuation and (ii) investigate the relationship between the average fat content of each organ and its global US attenuation.

METHODS

The ultrasonic measurement method is in line with techniques originating from the spectral difference method. Here, the attenuation coefficient (AC) is estimated by insonifying tissues with a single plane wave and acquiring the backscattered echoes. This approach avoids the need for a reference medium to compensate for diffraction and focusing effects. The method is characterized and validated on a calibrated phantom and compared with two commonly used techniques on ex vivo liver tissues. Subsequently, attenuation maps and average values obtained from US imaging in healthy volunteers are compared with PDFF values measured by qMRI. Hepatic ( n = 12 $n = 12$ ) renal ( n = 12 $n = 12$ ), pancreatic ( n = 8 $n = 8$ ) and breast ( n = 3 $n = 3$ ) tissues were analyzed. Statistical significance was assessed using a paired t-test. To account for multiple comparisons, a Bonferroni correction was applied, resulting in an adjusted 5% significance threshold of p < 0.0125 $p<0.0125$ . Effect sizes were also reported using Cohen's d $d$ parameter. Effect sizes were considered large for d ≥ 0.8 $d \ge 0.8$ .

RESULTS

Measurements on the calibrated phantom showed relative errors between the measured mean values and the manufacturer values of 2% and 9%, respectively. Average AC of each organ was included in the confidence interval of the corresponding literature value. The Pearson correlation coefficient between log $\log $ (PDFF) and AC slope is R 2 = 0.73 $R^2 = 0.73$ ( p < 0.00025 $p<0.00025$ ). When each organ was considered separately, no significant correlation was observed between PDFF average values and global US attenuation, as variations between volunteers were found of the same order of magnitude as the standard deviation around each average value.

CONCLUSIONS

This work presents an alternative method for in vivo characterization of US attenuation based on the emission of a plane wave, and highlights the impact of fat density on inter-organ attenuation variations. Together, these results provide new insights into the relationship between tissue microstructure and US attenuation.