A Noninvasive Ultrasound Based Technique to Identify Treatment Responders in Patients with Portal Hypertension

Abstract

Rationale and Objective:

Subharmonic aided pressure estimation (SHAPE) is based on the inverse relationship between the subharmonic amplitude of ultrasound contrast microbubbles and ambient pressure. The aim of this study was to verify if SHAPE can accurately monitor disease progression in patients identified with portal hypertension.

Materials & Methods:

A modified Logiq 9 scanner with a 4C curvi-linear probe (GE, Waukesha, WI) was used to acquire SHAPE data (transmitting/receiving at 2.5/1.25MHz) using Sonazoid (GE Healthcare, Oslo, Norway; IND 124,465). Twenty-one (median age 59 years; 12 Males) of the 178 patients enrolled in this IRB approved study (14F.113) were identified as having clinically significant portal hypertension based on their hepatic venous pressure gradient (HVPG) results (i.e., HVPG ≥ 10 mmHg). Repeat SHAPE examinations were done every 6.2 months. Liver function tests and clinical indicators were used to establish treatment response.

Results:

Of the 21 portal hypertensive subjects, 11 had successful follow up scans with an average follow up time of 6.2 months. There was a significantly larger SHAPE signal reduction in the group who were classified as treatment responders (n=10; −4.01±3.61 dB) compared to the single non-responder (2.33 dB; p < 0.001). Results for responders matched the corresponding clinical outcomes of improved Model for End Stage Liver Disease (MELD) scores, improvement in underlying cause of portal hypertension, improved liver function tests and reduced ascites.

Conclusion:

SHAPE can potentially monitor disease progression in portal hypertensive patients and hence, may help clinicians in patient management. A larger study would further validate this claim.

Introduction

Clinically significant portal hypertension, defined as a hepatic venous pressure gradient (HVPG) > 10 mmHg, is associated with an increased risk of varices, esophageal hemorrhage, hepatic decompensation, post-operative decompensation, and hepatocellular carcinoma.^1–4 Significant portal hypertension may be present in the absence of signs of clinical decompensation, such as varices or ascites, in up to 50% of patients with chronic liver disease.¹ Hence, diagnosis may require measurement of the HVPG via catheterization, typically performed in conjunction with transvenous biopsy.

Interventions to mitigate portal hypertension include nonselective beta-blockers, antifibrotic agents, anticoagulants for venous thrombosis, and treatment of underlying liver diseases.⁵ Monitoring response to therapy presents a clinical challenge. Direct measurement of the HVPG is the gold standard, but requires specific expertise, is invasive and moderately expensive, and is not universally available – especially not in community practices. Noninvasive ultrasound measures such an elastography for liver stiffness when combined with measurement of spleen size and platelet count are highly predictive for the initial diagnosis of clinically significant portal hypertension but are insensitive to therapeutic reductions in the HPVG.⁶ An improved MELD or Childs-Pugh score, normalization of serum liver function tests, improvement in the underlying cause of portal hypertension, and reduction in ascites and varices qualitatively indicate improvement in portal hypertension. However, none of these measures provide a quantitative measure of the portal pressure. A reliable, cost effective and noninvasive procedure to estimate portal pressure is needed to monitor therapy without the need for repeat catheterizations.

Ultrasound contrast agents are gas filled microbubbles that act as echo-enhancers. When these microbubbles are insonated with ultrasound pressures above 200 kPa, they start to oscillate nonlinearly.^7–9 The nonlinear oscillations occur over a wide range of frequencies from subharmonics (f_o/2) to second harmonics (2f_o) and ultraharmonics (3f_o/2) of the insonation frequency (f_o) as well as its multiples (nf_o, nf_o/2 etc.). Subharmonic imaging transmits at double the resonance frequency and receives at half the transmit frequency i.e., f_o/2.^{8, 10} Our group has proposed the use of subharmonic imaging and contrast agents as pressure sensors in a procedure called subharmonic aided pressure estimation (SHAPE) for noninvasive, quantitative pressure estimation.^11–14 The nonlinear response of microbubbles depends strongly on the incident acoustic pressure of the ultrasound beam, and undergoes three stages: occurrence, growth and saturation.^{15, 16} It is in the growth stage that subharmonic microbubble signals (i.e., SHAPE) have the highest sensitivity to pressure changes and an inverse linear relation with the ambient hydrostatic pressure.^{16, 17}

As part of a multi-center clinical trial, SHAPE was employed to estimate portal pressures in 178 subjects undergoing transjugular liver biopsy with HPVG.¹⁷ The SHAPE gradient between the portal and hepatic veins was in good overall agreement with the HVPG (r = 0.68). Subjects with clinically significant portal hypertension (HVPG ≥ 10 mmHg) had a significantly higher mean subharmonic gradient than subjects with lower HVPGs (0.79 ± 2.53 vs. −4.95 ± 3.44 dB; p<0.001), equivalent to a sensitivity of 90% and a specificity of 80%.¹⁸ This study was designed to assess the sensitivity of serial SHAPE scans for monitoring disease progression in the subset of the study population with clinically significant portal hypertension.

Materials & Methods

Study Participants

This multi-center, prospective, blinded study (Trial registration number: NCT # 02489045) was approved by the institutional review boards of and, as well as by the U.S. Food and Drug Administration (Investigational New Drug number 124,465) and was compliant with the Health Insurance Portability and Accountability Act. Written informed consent was provided by all subjects. The ultrasound contrast agent was provided by GE Healthcare (Oslo, Norway). The full protocol and statistical analysis plan are available at https://clinicaltrials.gov/ct2/show/NCT02489045. Inclusion criteria were adults scheduled for transjugular liver biopsy as part of their clinical care. All authors had access to the study data and reviewed and approved the final manuscript.

Out of the 178 subjects scanned in the first part of the clinical trial, 21 subjects were identified as having clinically significant portal hypertension based on their HVPG results (i.e., HVPG ≥ 10 mmHg). Patient characteristics and baseline findings for these 21 subjects are shown in Table 1.

Table 1:

Patient statistics, transjugular liver biopsy findings and SHAPE scanning parameters at baseline scan

Parameter	Minimum	Mean ± Standard Deviation	Maximum
Patient Statistics
Age (years)	25	58.81± 13.19	84
Body Mass Index (kg/m2)	20.71	29.48±10.54	52.18
MELD Score	7	12.20 ± 4.79	26
Meld-Na Score	7	12.78 ± 5.02	26.56
Hematocrit (%)	25	36.32 ± 5.38	44.6
Bilirubin (mg/dl)	0.5	2.39 ± 3.36	12.8
Serum Creatinine(mg/dl)	0.6	1.23 ± 1.33	6.78
International normalized ratio(sec)	0.97	1.20 ± 0.13	1.47
Sodium (mmol/L)	132	138.15 ± 3.43	144
ALT (IU/L)	13	61.75 ± 48.51	136
AST (IU/L)	22	85.75 ± 62.55	192
SHAPE findings
SHAPE gradient (dB)	−9.95	−1.10 ± 3.13	2.85
PV diameter (cm)	0.41	1.28± 1.00	5.1
HV diameter (cm)	0.16	0.67 ± 0.34	1.30
PV depth (cm)	4.12	6.90 ± 1.94	12.06
HV depth (cm)	0.7	6.68 ± 2.21	9.6
Biopsy findings
Wedge Pressure (mmHg)	12	24.16 ± 7.29	39
Free Pressure (mmHg)	4	13.63 ± 8.40	39
HVPG (mmHg)	10	12.94 ± 2.93	19.6

Subjects identified in the initial examination as having portal hypertension (by HVPG) were monitored by SHAPE for 2 years every 6 months. These subjects had follow-up Computed Tomography (CT) or Magnetic Resonance Imaging (MRI) scans every 6 months to check for liver cancer as well as ascites and at those times a repeat SHAPE examination was performed. Any repeat transjugular biopsies performed in this population also triggered a repeat SHAPE study. Since the patient might not have a repeat HVPG measurement at the follow up time point, different reference points were used to evaluate disease progression. These included liver function tests including the albumin, bilirubin and the coagulation panel, the MELD score and presence of ascites and varices compared to previous study. The endpoint for this study was a new complication (e.g., liver cancer) or a marked worsening in any measured clinical parameters (e.g., MELD scores). The time to reach this endpoint was noted. The main objective was to assess whether serial SHAPE examination differentiates between treatment responders and non-responders.

SHAPE Ultrasound Examination

Subjects were administrated a Sonazoid infusion (0.024 μl/kg/minute) via an intravenous line. A Logiq 9 scanner (GE Healthcare, Milwaukee, WI) with a 4C curvilinear probe was modified to acquire radiofrequency (RF) data during scanning. Once the sonographer confirmed the patency of the portal vein and the hepatic vein and the presence of microbubbles, an automated optimization code to select the optimum acoustic power was activated.^{19, 20} It should be noted that care was taken to select the hepatic vein region away from the inferior vena cava to avoid the influence of retrograde flow. For each participant, portal and hepatic vein regions were selected at similar depths and diameters were measured at this depth.

A region of interest in the portal vein was selected and the automated power control algorithm initiated to determine the optimal acoustic output power for maximum SHAPE sensitivity to account for varying depth and attenuation. This scanner provides dual mode imaging i.e., both the regular B Mode image and the contrast subharmonic mode are displayed at the same time.²¹ The B mode was set to operate at 4.0 MHz and the SHAPE mode was set to transmit four-cycle pulses at 2.5 MHz and to receive subharmonic signals at 1.25 MHz. Examples of the imaging windows are shown in Figure 1.

Figure 1:

Dual Imaging with B mode (black and white) and subharmonic imaging (gold) on the left and right respectively of each image. (a): At baseline scan, a subject with elevated HVPG values, with a bright subharmonic signal in both the portal vein (PV) and the hepatic vein (HV). (b): After 1.2 yrs, same subject with an improved clinical diagnosis and decreased SHAPE gradient having considerable subharmonic signal in portal vein and not in hepatic vein.

Subharmonic data from the microbubbles (i.e., SHAPE) was acquired at the optimal acoustic power setting in 5 s segments during the infusion of the Sonazoid suspension from the portal vein and hepatic vein. All measurements were repeated three times. The average frame rate was 9 fps.

Post Processing

Regions within the hepatic and portal veins previously identified by the sonographer were selected on maximum intensity projection B-mode images (compiled from reconstructed images from the radiofrequency (RF) data) and were fixed throughout the 5-second acquisition (approximately 40–50 frames). The average RF signal over all the frames in the 0.5 MHz bandwidth around 1.25 MHz gave the mean subharmonic signal in each vessel. The RF data from each acquisition was extracted using a proprietary software (GE Global Research). The SHAPE gradient was calculated as the difference in the mean subharmonic signal between the portal vein and the hepatic vein.¹⁴ The SHAPE gradients in the follow up studies were compared to the baseline study. A decrease in the SHAPE gradient (i.e., subharmonic signal in hepatic vein – portal vein) indicated improvement in portal hypertension. We hypothesize that the lack of hepatic vein signal indicating normal pressures is due to increased vascular resistance and/or increased volume of blood flowing through the portal circulation, which could be happening due to difficult outflow of the blood from the portal to the hepatic veins and inferior vena cava. However, more studies are necessary to validate the hypothesis.²² Typical images from SHAPE acquisitions for a portal hypertensive case baseline with improved SHAPE gradient at follow up are presented in Figure 1.

Statistical Analysis

All statistical analysis was conducted using Stata® 15.0 (Stata Corporation, College Station, TX). SHAPE data was collected and processed for comparison with clinical parameters and repeat HVPG measurements when available. The difference between SHAPE gradients for subjects with increased and decreased gradients (i.e. SHAPE non-responders and responders) was assessed using an unpaired Student’s t-test with p < 0.05 considered as significant. The same t-test was employed even for all interim time point evaluation of SHAPE gradients for each individual study being treated as either a responder or non-responder. It should be noted that in this case the individual studies were not independent of each other, but for ease of understanding and to understand the intra-subject variability, this test was employed.

The SHAPE gradient was also compared to clinical outcomes including MELD score, blood bilirubin levels, changes in the underlying cause of portal hypertension, varices and ascites to see if SHAPE results matched the clinical expectations. If the patient had a repeat biopsy, SHAPE was compared to the clinical HVPG measurements. All liver biopsy specimens were interpreted for fibrosis score according to the Batts-Ludwig scoring system.²³ Specificity and sensitivity of SHAPE as a predictor of treatment response was calculated every six months to see if the technique became more robust over time as clinical variability was expected to reduce over time. Patients were classified as clinical responders or non-responders based on their clinical outcomes based on a subjective assessment of their clinical reports by the respective clinical provider who were blinded to the SHAPE results.

Results

Participant Overview

Out of the 178 subjects scanned in the first part of this project, 21 subjects had clinically significant portal hypertension. Of these 21 subjects, 12 (57%) were men and nine (43%) were women. 19 (90.5%) subjects were white and 2 (9.5%) were black. The median age was 59 years ranging from 25 to 84 years (cf., Table 1). Four subjects had undergone a previous liver transplant. Seventeen were diagnosed with cirrhosis. Disease etiology consisted of hepatitis C in eight subjects (38%), nonalcoholic steatohepatitis in six subjects (28.6%), cryptogenic cirrhosis in two subjects (9.5%), drug induced liver injury in four subjects (19.2%) and primary sclerosing cholangitis in one subject (4.7%). Of the 21 subjects, nine had ascites and 7 had varices.

Subjects had a range of 1 to 4 follow up scans. One subject had four follow up scans, two had three, three had two, six had one follow up of which no useful data was saved for one of the subjects due to a technical error. Of the remaining nine, six did not have any follow up and three were lost to follow-up due to death. Overall, 11 subjects underwent successful follow up scans. There were no adverse events for any study. The timeline for follow up scans for each of these 11 subjects in shown in Table 2.

Table 2:

Time points for follow up scans for each individual subject with reference to their baseline scan. There was a variability in the follow up time as these subjects were scanned whenever they came into the healthcare institution as part of their clinical standard of care.

Patient No.	1	2	3	4	5	6	7	8	9	10	11
Timepoint for follow up scans w.r.t baseline scan	1. 6 mth 2. 14 mth 3. 18 mth	1. 6 mth 2. 15 mth 3. 21 mth 4. 27 mth	1. 5 mth 2. 16 mth	1. 15 mth	1. 6 mth 2. 14 mth	1. 6 mth 2. 14 mth 3. 24 mth	1. 5 mth	1. 7 mth 2. 24 mth	1. 7 mth	1. 6 mth	1. 15 mth

Treatment responders and non-responders at final time point – based on SHAPE study results

Figure 2 shows the change in SHAPE gradient for all subjects over time. The SHAPE gradient for two subjects, subjects number 3 and 6, temporarily increased indicating a worsening in the portal hypertension, however for both cases, the final SHAPE gradient was lower than their baseline scan, indicating that they were responding to treatment and their clinical condition was improving as per the estimates of the SHAPE studies. By their final follow up, ten of the subjects were classified as SHAPE responders as their final gradient had reduced since the baseline scan and one subjects was classified as a non-responder, with a final SHAPE gradient that was higher than the baseline gradient.

Figure 2:

SHAPE gradient for all follow up subjects, n=11. Responders are highlighted in a green box i.e. Final SHAPE gradient< Baseline SHAPE Gradient, n=10. Red box indicates the non-responders, i.e. final SHAPE gradient> Baseline SHAPE gradient, n=1. For all subjects, the different colors indicate the SHAPE gradients for different follow up times.

There was a significantly higher reduction in SHAPE gradient in the group who were classified as responders according to the SHAPE study compared to the SHAPE non-responder (p < 0.001). The overall mean change in the SHAPE gradient for the responders was −4.01 ±3.61 dB vs 2.33 dB in the SHAPE non-responder. Figure 3 shows the difference in SHAPE responders (green) and SHAPE non-responders (red) over time.

Figure 3:

Box plot shows the change in SHAPE gradient—the average subharmonic amplitude in the hepatic vein minus that in the portal vein — for SHAPE responders vs non responders (wherever available; none after 1.5 years) over time. Each box plot shows the minimum, quartile 1, median, quartile 3 and the maximum value for that time point.

Treatment responders and non-responders at final time point – based on clinical data

The sensitivity and specificity of SHAPE to distinguish between treatment responders and non-responders was calculated as shown in Table 3. As can be seen, as time progresses, sensitivity improves from 50% to 100%. There are not enough data points to determine the specificity for patients at all time points.

Table 3:

The sensitivity and specificity (in %) of SHAPE to distinguish treatment responders from non-responders

		Time to follow up
	0 – 6 months(n=2)	6 – 12 months (n=7)	1 – 1.5 years (n=7)	1.5 – 2 years (n=2)	≥ 2 years (n=3)
Sensitivity (%)	50 (1/2)	100 (3/3)	100 (4/4)	100 (2/2)	100 (1/1)
Specificity (%)	NA	50 (2/4)	66 (2/3)	NA	50 (1/2)

For subjects who had a follow up from 6 months to 2.25 years of their baselines scans, the results for responders matched their clinical outcomes of improved MELD, improvement in underlying cause of portal hypertension, normal liver function tests and reduced ascites indicating a reduction in portal hypertension. The change in SHAPE gradient and the clinical outcomes for all subjects is shown in Table 4.

Table 4:

Clinical assessments &liver function tests at baseline and final follow up (whenever available) and change in SHAPE gradient for SHAPE responders and non-responders. (Improvements are marked by *)

Scan time	Patient No.	Gender	Etiology/Clinical Assessment	Post transplant	Fibrosis Stage	Meld-Na score	ALT (7–56 units/liter)	AST(10–40units/liters)	Bilirubin(0.1–0.9) mg/dL	Serum Creatinine(0. 7–1.4mg/dL)	INR(0.81–1.19)	Ascites	Varices	SHAPE Gradient
Responders
Baseline	1	Female	Hepatitis C	Yes	3	11	136	159	2.9	0.7	1.05	Yes	No	−1.48 dB
Final Follow Up after1.5 yrs			Ascites has improved. Clinical condition stable, responding well to medications		NA	14	146	159	3.1	1.30	1.05	No*	No	−3.89 dB*
Baseline	2	Female	Hepatitis C	Yes	2	19	17	22	10.2	1.1	1.29	No	No	−0.49dB
Final Follow Up after2.25 yrs			Hepatitis C Cured		NA	8*	13	20	0.6*	0.92	1.18*	No	No	−7.98 dB*
Baseline	3	Female	Hepatitis C with HCC	No	3	10	101	98	0.6	0.7	1.39	No	No	−1.41 dB
Final Follow Up after 1.30 yrs			HCCtreated with ablation, Hepatitis C cured, liver enzymes better as Hepatitis C cured		NA	8.97*	28*	21*	0.5	0.67	1.2	No	No	−3.93 dB*
Baseline	4	Male	Hepatitis C	No	4	14	17	43	1.8	1.1	1.47	Yes	No	1.88 dB
Final Follow Up after1.25 yrs			Ascites absent, liver function tests normal, taken off transplant list		NA	9*	21	29*	0.9*	1.19	1.1	No	No	−1.73 dB*
Baseline	6	Male	Nonalcoholic steatohepatitis	No	4	12.65	43	47	0.8	1.1	1.23	No	No	0.10 dB
Final Follow Up after 2 yrs			MELD reduced to 11 in final follow up		NA	11*	50	57	0.8	1.44	1.1	No	No	−2.34 dB*
Baseline	7	Male	Hepatitis C with HCC	No	3	7	113	192	0.5	0.8	1.09	No	No	−0.56 dB
Final Follow Up after 0.5 yrs			On treatment for Hepatitis C, liver cancer treated, AST better		NA	9	136	93*	0.4	1.06	1.2	No	No	−9.70 dB*
Baseline	8	Female	Nonalcoholic steatohepatitis	No	4	12	13	30	2.13	0.6	1.3	No	No	0.35 dB
Final Follow Up after 2 yrs			Clinically stable, liver function tests normal, responding to medications		NA	15	17	39	2.3	1.1	1.47	No	No	−3.40 dB
Baseline	9	Male	Hepatitis C with HCC	No	4	13.64	49	61	0.5	1.17	1.1	Yes	Yes	−4.05 dB
Final Follow Up after 0.6 yrs			Got chemoembolization. for tumor, labs are same as not treated for hepatitis C		NA	11*	26*	34*	0.7	1.3	1.2	No*	No*	−9.02
Baseline	10	Female	Nonalcoholic steatohepatitis	No	4	15.42	NA	NA	4.5	0.76	1.2	Yes	No	2.85 dB
Final Follow Up			Resolved jaundice, MELD score down to 7, stable condition		NA	7*	NA	NA	NA	NA	NA	NA	NA	2.05 dB
Baseline	11	Male	Primary Sclerosing Cholangitis	Yes	1	12	NA	NA	1.8	0.85	1.3	No	No	−0.65 dB
Final Follow Up			Clinically stable, liver function tests stable		NA	NA	NA	NA	NA	NA	NA	NA	NA	−10.85 dB
Non-Responder
Baseline	5	Female	Nonalcoholic steatohepatitis	No	3	54	95	9	0.5	0.7	1.28	No	No	−1.34 dB
Final Follow Up after 1.2 yrs			Clinically unchanged, got repeat pressure measurement and HVPG was 13.36 similar to baseline value of 13.		NA	NA	NA	NA	NA	0.7	1.20	No	No	0.99 dB

Intra-subject variability

Figure 4 shows the box plot for SHAPE gradients for all follow up studies. Each study was classified based on the subject’s clinical classification of being a treatment responder or non-responder at that time point. As can be seen there was a significant difference in the SHAPE gradient, p<0.005 for all studies classified as responders and non-responders at all time points rather than at their final scan.

Figure 4:

Box plot shows the change in SHAPE gradient—the average subharmonic amplitude in the hepatic vein minus that in the portal vein — for all follow up studies categorized as clinical responders or non-responders based on the subject’s clinical assessment at that particular time point. Please note, all study points are not independent of each other, rather treated as independent for understanding intra-subject variability. Each box plot shows the minimum, quartile 1, median, quartile 3 and the maximum value for that time point.

Discussion

The SHAPE results matched with clinical expectations in both responders and non-responders on their final follow up scan. There was a significant difference in the change in SHAPE gradient for responders compared to the non-responder (p < 0.001).

The treatment regimen is directed towards the etiology and complications rather than portal hypertension itself. There is currently no clinical standard for monitoring portal hypertension, as patients do not get repeat pressure HVPG measurements, which is an invasive procedure, every six months. Hence, there is no time effective and minimally invasive estimate available to monitor portal hypertension. As can be seen in Table 4, there is always a variable component to the blood work of these patients as there is no standardized procedure for their portal hypertension follow up. If the liver function tests begin to normalize, they indicate the liver is functioning better however, this is not conclusive that portal hypertension is improving. Complications such as ascites and varices are again not a clear indicator of the state of the liver and portal pressures. Moreover, the MELD score is relevant for patients with cirrhosis only and not for patients post-transplantation. It has also been shown that MELD does not correlate with patient HVPG implying that liver function and pressures should be measured independently.¹⁴ Recently a modified MELD-Na score was developed to increase its prognostic value by incorporating sodium in addition to serum bilirubin, creatinine, and international normalized ratio.²⁴ There is a high variability in the clinical results and thus, the clinical outcomes are determined by the clinician’s subjective analysis of the results. This is evident even in this study, as for patient 1, even though the MELD-Na score increased by 3, the clinician assessed the patient as a treatment responder as her ascites was resolved. For patient 2, this was based on her MELD-Na score whereas for patient 3, it was based on her alanine transaminase (ALT) and aspartate transaminase (AST) falling into the normal range. A conclusive test like SHAPE providing a significant quantitative difference between responders and non-responders could be a major advancement.

There is a lot of inter-subject variability, i.e. the SHAPE gradient changed with each follow up scan for each subject as is shown in figure 4. However, this may be attributed to the changing portal pressure encountered at different time-points. For example, in subject 6, one can see the subject showed increasing SHAPE gradients for two follow ups with respect to the baseline scan before it reduces to −2.34 dB at the final follow up study. The intermediate increase indicated that the subject’s portal hypertension deteriorated. This increase was also corroborated by the subject’s clinical outcomes. The subject had a repeat HVPG measurements and showed an increase in HVPG from 16 to 24 mmHg. As is clear from this example, the portal pressures increased for this subject and thus, the SHAPE gradient which is an estimate of portal pressures also increased. At the final follow up study the patient was clinically responding to medications and expected to have clinically improved. This was in agreement with the reduced SHAPE gradient as well. Thus, the limited clinical data available at all time points for the subjects supports the hypothesis that the variability seen in the SHAPE gradients for each subject over time is an estimate of that particular patient’s clinical condition specifically portal pressures at that time point and how he/she responded to their treatment at that time point with respect to the baseline scan.

In comparison to transient elastography, which is at best a screening tool but not a confirmatory marker for portal hypertension, SHAPE is a novel ultrasound-based technique, which does not rely on conventional imaging parameters, rather it uses the inverse linear relationship between the subharmonic emissions from ultrasound contrast agents and ambient pressure.

SHAPE can potentially be a cost and time effective, robust and standardized tool for monitoring of portal pressures. In the future, SHAPE may reduce the time to evaluate if beta blockers are working. Also, SHAPE is not only less invasive, but may be more accurate than HVPG since anesthesia may lower pressure values leading to an underestimation of clinically significant portal hypertension.²⁵ SHAPE is an easy to use technique for clinicians with minimal knowledge of the physics to employ having been successfully investigated/explored by various clinicians with varying levels of technical knowledge in the course of the last decade as part of several clinical trials. Moreover, SHAPE measurements are very reproducible (standard deviations < 1 dB), since this is a quantitative technique based on RF data.^{15, 26} This larger study also established SHAPE being useful in a patient population with a wide range of BMI values (16.6–52.2 kg/m²).

The increasing sensitivity with time indicates that SHAPE may become more robust and reliable in determining the clinical status of subjects and distinguishing responders from non-responders. This matches with clinical expectations as well. There is less variability in the clinical parameters (such as the liver function tests, the response for treatment of underlying cause and MELD scores) as time progresses. Even though clinicians follow clinical markers as a surrogate marker for the status of portal hypertension, they are more confident about patient outcome with increasing duration.

Although SHAPE proved to be a reliable technique, this follow up study was based on a small sample size of 11 subjects. Even though we assimilated the available clinical outcomes for each subject at each time point based on the assessment of an experienced gastroenterologist (JF with more than 15 years of experience), the clinical outcomes also were varied and clinical data was unavailable for all time points, because of the different clinical tests for a given subject. Another limitation of this study was the difficulty in tracking subjects and scheduling repeat SHAPE studies every 6 months, as it depended on patient availability and, most importantly, their inclination to undergo another SHAPE study.

In conclusion, this study provided a proof of concept for use of SHAPE as a reliable tool to estimate portal pressures and to differentiate between treatment responders and non-responders. This study provides intriguing results and a larger sample size would further validate this claim. SHAPE can also be a standardized test done for all patients as it is cost effective and noninvasive. Finally, medical therapies for chronic liver disease designed to mitigate liver fibrosis and cirrhosis could in the future be monitored noninvasively with SHAPE.

Abbreviations Used:

SHAPE

Subharmonic aided pressure estimation

HVPG

Hepatic Venous Pressure Gradient

MELD

Model for end stage liver disease

CT

Computed Tomography

MRI

Magnetic resonance imaging

RF

Radiofrequency

ALT

Alanine transaminase

AST

Aspartate transaminase