Investigating the efficacy of subharmonic aided pressure estimation for portal vein pressures and portal hypertension monitoring

Abstract

The efficacy of using subharmonic emissions from Sonazoid microbubbles (GE Healthcare, Oslo, Norway) to track portal vein pressures and pressure changes was investigated in 14 canines using either slow- or high-flow models of portal hypertension (PH). A modified Logiq 9 scanner (GE Healthcare, Milwaukee, WI, USA) operating in subharmonic mode (f(transmit): 2.5 MHz, f(receive): 1.25 MHz) was used to collect radiofrequency data at 10-40% incident acoustic power levels with 2-4 transmit cycles (in triplicate) before and after inducing PH. A pressure catheter (Millar Instruments, Inc., Houston, TX, USA) provided reference portal vein pressures. At optimum insonification, subharmonic signal amplitude changes correlated with portal vein pressure changes; r ranged from -0.82 to -0.94 and from -0.70 to -0.73 for PH models considered separately or together, respectively. The subharmonic signal amplitudes correlated with absolute portal vein pressures (r: -0.71 to -0.79). Statistically significant differences between subharmonic amplitudes, before and after inducing PH, were noted (p ≤ 0.01). Portal vein pressures estimated using subharmonic aided pressure estimation did not reveal significant differences (p > 0.05) with respect to the pressures obtained using the Millar pressure catheter. Subharmonic-aided pressure estimation may be useful clinically for portal vein pressure monitoring.

Fig. 1

Grayscale B-mode and color Doppler images acquired during data acquisition. (a) The locations of the surgical inlet (red-dotted ring) and the pressure catheter (red ring) are indicated. (b) Color Doppler image verifying the presence of pressure catheter (red ring) in the portal vein (PV); inferior vena cava (IVC) is shown as well.

Fig. 2

Schematic representing experimental setup.

Fig. 3

Data acquisition process. (a) Standard imaging mode where the ROI (region of interest with portal vein) is selected (yellow outline). (b) RF data acquisition mode.

Fig. 4

Maximum intensity projection (MIP) subharmonic image obtained from a canine. Note that the abscissa represents the width or the lateral span of the ROI shown in Fig. 3b. The dotted lines delineate the portal vein.

Fig. 5

Boxplot of the subharmonic signal amplitudes obtained at 10 %, 20 % and 40 % incident acoustic power (IAP) levels with 2, 3 and 4 transmit cycles. Minor and major outliers are indicated with circles and asterisks, respectively. Note, that the subharmonic signal amplitudes obtained with 10 % IAP and with 2 cycles are relatively low as compared to the amplitudes obtained with 20 % and 40 % IAP, for 3 and 4 transmit cycles.

Fig. 6

Maximum intensity projection (MIP) subharmonic images of the data obtained from ROI at 10 % (a, d, g), 20 %, (b, e, h) and 40 % (c, f, i) incident acoustic power (IAP) with 2 (a-c), 3 (d-f), and 4 (g-i) transmit cycles. Note, that subharmonic signal amplitude increases with an increase in IAP levels; but is relatively low for 2 transmit cycles.

Fig 7

Mean changes in the portal vein pressures are plotted with mean changes in subharmonic signal amplitudes acquired with 3 transmit cycles along with the best-fit line and the correlation coefficient is indicated for both the Gelfoam and AV-fistula portal hypertension models. (a) and (c) represent data acquired at 20 % incident acoustic power (IAP) levels for subharmonic signal analyzed from the MIP image (SH_MIP) and as the mean value from all the frames (SH_{All_Frames}), respectively. (b) and (d) represent the corresponding data acquired at 40 % IAP.

Fig 8

Mean changes in the portal vein pressures are plotted with mean changes in subharmonic signal amplitudes acquired with 4 transmit cycles along with the best-fit line and the correlation coefficient is indicated for both the Gelfoam and AV-fistula portal hypertension models. (a) and (c) represent data acquired at 20 % incident acoustic power (IAP) for subharmonic signal analyzed from the MIP image (SH_MIP) and as the mean value from all the frames (SH_{All_Frames}), respectively. (b) and (d) represent the corresponding data acquired at 40 % IAP.

Fig 9

Absolute portal vein pressures are plotted with absolute subharmonic signal amplitudes acquired with 4 transmit cycles (note, these are mean values obtained from each canine). (a-b) and (c-d) represent data acquired at 20 % and 40 % incident acoustic powers (IAPs), respectively. The subharmonic amplitudes obtained from the maximum intensity projection images (SH_MIP) and from the mean signal combined from all the frames (SH_{All_Frames}) are shown separately in each panel. The best-fit line along with its gradient is indicated in (a) and (c). Boxplots in (b) and (d) indicate the interquartile range (vertical span of the box), the median value (horizontal line within the box), the range (whiskers) and minor outliers (circles) for the subharmonic signal amplitudes. Note that the SH_{All_Frames} is relatively less than the SH_MIP (as expected).