Full-Field-of-View Time-Harmonic Elastography of the Native Kidney

Explorative study using ultrasound time-harmonic elastography for stiffness-based quantification of skeletal muscle function

Time-harmonic elastography (THE) is an emerging ultrasound imaging technique that allows full-field mapping of the stiffness of deep biological tissues. THE's unique ability to rapidly capture stiffness in multiple tissues has never been applied for imaging skeletal muscle. Therefore, we addressed the lack of data on temporal changes in skeletal muscle stiffness while simultaneously covering stiffness of different muscles. Acquiring repeated THE scans every five seconds we quantified shear-wave speed (SWS) as a marker of stiffness of the long head (LHB) and short head (SHB) of biceps brachii and of the brachialis muscle (B) in ten healthy volunteers. SWS was continuously acquired during a 3-min isometric preloading phase, a 3-min loading phase with different weights (4, 8, and 12 kg), and a 9-min postloading phase. In addition, we analyzed temporal SWS standard deviation (SD) as a marker of muscle contraction regulation. Our results (median [min, max]) showed both SWS at preloading (LHB: 1.04 [0.94, 1.12] m/s, SHB: 0.86 [0.78, 0.94] m/s, B: 0.96 [0.87, 1.09] m/s, p < 0.001) and the increase in SWS with loading weight to be muscle-specific (LHB: 0.010 [0.002, 0.019] m/s/kg, SHB: 0.022 [0.017, 0.042] m/s/kg, B: 0.039 [0.019, 0.062] m/s/kg, p < 0.001). Additionally, SWS during loading increased continuously over time by 0.022 [0.004, 0.051] m/s/min (p < 0.01). Using an exponential decay model, we found an average relaxation time of 27 seconds during postloading. Analogously, SWS SD at preloading was also muscle-specific (LHB: 0.018 [0.011, 0.029] m/s, SHB: 0.021 [0.015, 0.027] m/s, B: 0.024 [0.018, 0.037] m/s, p < 0.05) and increased by 0.005 [0.003, 0.008] m/s/kg (p < 0.01) with loading. SWS SD did not change over loading time and decreased immediately in the postloading phase. Taken together, THE of skeletal muscle is a promising imaging technique for in vivo quantification of stiffness and stiffness changes in multiple muscle groups within seconds. Both the magnitude of stiffness changes and their temporal variation during isometric exercise may reflect the functional status of skeletal muscle and provide additional information to the morphological measures obtained by conventional imaging modalities.

Ultrasound elastography in chronic kidney disease: a systematic review and meta-analysis

Ultrasound elastography (USE) is a noninvasive technique for assessing tissue elasticity, and its application in nephrology has aroused growing interest in recent years. The purpose of this article is to systematically review the clinical application of USE in patients with chronic kidney disease (CKD), including native and transplanted kidneys, and quantitatively investigate differences in elasticity values between healthy individuals and CKD patients. Furthermore, we provide a qualitative analysis of the studies included, discussing the potential interplay between renal stiffness, estimated glomerular filtration rate, and fibrosis. In January 2022, a systematic search was carried out on the MEDLINE (PubMed) database, concerning studies on the application of USE in patients with CKD, including patients with transplanted kidneys. The results of the included studies were extracted by two independent researchers and presented mainly through a formal narrative summary. A meta-analysis of nine study parts from six studies was performed. A total of 647 studies were screened for eligibility and, after applying the exclusion and inclusion criteria, 69 studies were included, for a total of 6728 patients. The studies proved very heterogeneous in terms of design and results. The shear wave velocity difference of - 0.82 m/s (95% CI: - 1.72-0.07) between CKD patients and controls was not significant. This result agrees with the qualitative evaluation of included studies that found controversial results for the relationship between renal stiffness and glomerular filtration rate. On the contrary, a clear relationship seems to emerge between USE values and the degree of fibrosis. At present, due to the heterogeneity of results and technical challenges, large-scale application in the monitoring of CKD patients remains controversial.

Are the Currently Available Elastography Methods Useful in the Assessment of Chronic Kidney Disease? A Systematic Review and a Meta-Analysis

Background: We require an quantitative imaging technique for the diagnosis and assessment of chronic kidney disease (CKD). Renal elastography has been widely used in recent years in different studies; however, the results across them are not consistent and, as a result, we conducted a meta-analysis of the published literature on this topic. Methods: The databases of PubMed, Medscape, Medline were searched for all studies published in English from 2010 until November 2021 that evaluated kidney shear wave speed (SWS) by elastography in patients with CKD. Trial design, methodological information, patient characteristics, interventions, results, and outcome data were all collected from each study according to a set protocol. Results: We found 37 publications, yet only 18 studies that utilized point shear wave elastography (Virtual Touch Quantification—VTQ system) were compared because the values achieved using different types of elastography are not evaluable. Finally, 1995 attendees (1241 patients with CKD versus 781 healthy subjects as the control group) were included. When comparing mean values of kidney SWS between studies we found increased heterogeneity Q = 513.133; DF = 10; p < 0001, I2 (inconsistency) = 98.12% (95% CI for I2 97.52–98.57%). With a standardized mean difference of −0.216, patients with CKD have a lower kidney SWS than healthy controls. A positive association between kidney SWS and eGFR was also discovered across the presented studies, with a pooled correlation coefficient of 0.38 (Z = 10.3, p < 0.001), Q = 73.3, DF = 5, p < 000.1, I2 = 93.18% (95% CI for I2 87.86 to 96.18). The pooled area under the ROC curve for kidney SWS to predict chronic kidney disease was 0.831 (95% CI, p < 0.001), Q = 28.32, DF = 6, p = 0.0001, I2 = 78.8% (95% CI for I2 56.37 to 89.72). In the four articles that used the Elast-PQ method, the data presented were insufficient for statistical analysis: area under the curve (AUC) values are used to compare distinct characteristics (differentiating kidney SWS between mildly and moderately impaired kidneys, between non-diabetic/prediabetic/diabetic patients, or kidney SWS between the CKD and control group), therefore not being suitable for further evaluation. Conclusions: The results show that patients with CKD have a lower kidney SWS than healthy controls. However, the number of studies involving renal elastography that have been published is limited and show an increased heterogeneity. Further research is needed to determine which factors actually influence kidney SWS in CKD patients and, as a result, to specify the role and indication of renal elastography in clinical practice.

Emphysematous Pyelonephritis Presenting as Pneumaturia and the Use of Point-of-Care Ultrasound in the Emergency Department

Emphysematous pyelonephritis (EPN) is a rare form of pyelonephritis causing a severe infection of the renal system that includes gas in the renal parenchyma, collecting system and surrounding tissue often presenting with sepsis. We report the case of a 60-year-old male with poorly controlled insulin dependent diabetes mellitus who presented with abdominal pain, nausea, vomiting, and “peeing air.” CT scan revealed air extending from the left renal parenchyma, perinephric fat and into the bladder, consistent with emphysematous pyelonephritis. Bedside point-of-care ultrasound (POCUS) subsequently revealed dirty shadowing and reverberation artifacts in the left kidney and the bladder consistent with gas in the urinary collecting system. By understanding the identifying artifacts seen with EPN, reflective shadow and reverberation artifact, the emergency physician may be alerted to the diagnosis sooner. Often this illness presents similarly to simple, acute pyelonephritis or undifferentiated sepsis. Therefore, POCUS allows for real time consideration of this condition while in the emergency department and thus prompter time to treatment.

Non-invasive structure-function assessment of the liver by 2D time-harmonic elastography and the dynamic Liver MAximum capacity (LiMAx) test

BACKGROUND AND AIM

Accurate assessment of structural and functional characteristics of the liver could improve the diagnosis and the clinical management of patients with chronic liver diseases. However, the structure-function relationship in the progression of chronic liver disease remains elusive. The aim of this study is the combined measurement of liver function by the ¹³ C-methacetin Liver MAximum capacity (LiMAx) test and tissue-structure related stiffness by 2D time-harmonic elastography for the assessment of liver disease progression.

METHODS

LiMAx test and time-harmonic elastography were applied, and the serological scores fibrosis 4 index and aspartate aminotransferase to platelet ratio index were calculated in patients with chronic liver diseases (n = 75) and healthy control subjects (n = 22). In 47 patients who underwent surgery, fibrosis was graded by histological examination of the resected liver tissue.

RESULTS

LiMAx values correlated negatively with liver stiffness (r = -0.747), aminotransferase to platelet ratio index (r = -0.604), and fibrosis 4 (r = -0.573). Median (interquartile range) LiMAx values decreased with fibrosis progression from 395 μg/kg/h (371-460 μg/kg/h) in participants with no fibrosis to 173 μg/kg/h (126-309 μg/kg/h) in patients with severe fibrosis. Median liver stiffness increased progressively with the stage of fibrosis from no fibrosis (1.56 m/s [1.52-1.63 m/s]) to moderate fibrosis (1.60 m/s [1.54-1.67 m/s]) to severe fibrosis (1.85 m/s [1.76-1.92 m/s]).

CONCLUSION

Our findings show that structural changes in the liver due to progressing liver diseases and reflected by increased tissue stiffness correlate with a functional decline of the organ as reflected by a decreased metabolic capacity of the liver.

Intra- and inter-operator reproducibility of US point shear-wave elastography in various organs: evaluation in phantoms and healthy volunteers

PURPOSE

This study was conducted in order to assess the intra- and interoperator reproducibility of shear-wave speed (SWS) measurement on elasticity phantoms and healthy volunteers using ultrasound-based point shear-wave elastography.

MATERIALS AND METHODS

This study was approved by the institutional review board. Two operators measured the SWS of five elasticity phantoms and seven organs (thyroid, lymph node, muscle, spleen, kidney, pancreas, and liver) of 30 healthy volunteers with 1.0-4.5 MHz convex (4C1) and 4.0-9.0 MHz linear (9L4) transducers. The phantom measurements were repeated ten times, while the volunteer measurements were performed five times each. Intra- and interoperator reproducibility was assessed. Interoperator reproducibility was also evaluated with the 95% Bland-Altman limits of agreement (LOA).

RESULTS

In phantoms, all intraclass correlation coefficients (ICCs) were above 0.90 and the 95% LOA between the two operators were less than ± 18%. In volunteers, intraoperator ICCs were > 0.75 for all regions except the pancreas. Interoperator ICC was above 0.75 for the right lobe of the liver (depth 4 cm) and the kidney, but the 95% LOA was less than ± 25% only for the liver.

CONCLUSION

Although excellent in phantoms, interoperator reproducibility was insufficient for all regions in the volunteers other than the right hepatic lobe at a depth of 4 cm. Clinicians should be aware of the 95% LOA when using SWS in patients.

KEY POINTS

• Our phantom study indicated a high reproducibility for shear-wave speed (SWS) measurements with point shear-wave elastography (pSWE). • In volunteers, intraoperator reproducibility was generally high, but the interoperator reproducibility was not high enough except for the right hepatic lobe at 4 cm depth. • To evaluate interoperator reproducibility, the 95% limits of agreement (LOA) between operators should be considered in addition to the intraclass correlation coefficient (ICC).

In vivo time-harmonic ultrasound elastography of the human brain detects acute cerebral stiffness changes induced by intracranial pressure variations

Cerebral stiffness (CS) reflects the biophysical environment in which neurons grow and function. While long-term CS changes can occur in the course of chronic neurological disorders and aging, little is known about acute variations of CS induced by intracranial pressure variations. Current gold standard methods for CS and intracranial pressure such as magnetic resonance elastography and direct pressure recordings are either expensive and slow or invasive. The study objective was to develop a real-time method for in vivo CS measurement and to demonstrate its sensitivity to physiological aging and intracranial pressure variations induced by the Valsalva maneuver in healthy volunteers. We used trans-temporal ultrasound time-harmonic elastography (THE) with external shear-wave stimulation by continuous and superimposed vibrations in the frequency range from 27 to 56 Hz. Multifrequency wave inversion generated maps of shear wave speed (SWS) as a surrogate maker of CS. On average, cerebral SWS was 1.56 ± 0.08 m/s with a tendency to reduce with age (R = -0.76, p < 0.0001) while Valsalva maneuver induced an immediate stiffening of the brain as reflected by a 10.8 ± 2.5% increase (p < 0.0001) in SWS. Our results suggest that CS is tightly linked to intracranial pressure and might be used in the future as non-invasive surrogate marker for intracranial pressure, which otherwise requires invasive measurements.