Shear wave elastography using high-frequency linear probe for kidney transplant monitoring: A methodological study

Abstract

OBJECTIVES:

To investigate the influencing factors of the image quality of shear wave elastography (SWE) performed using a high-frequency probe and its reproducibility for renal allografts.

METHODS:

A total of 211 patients with transplanted kidneys who underwent SWE examination performed using high-frequency or low-frequency probes were recruited for the study. The reproducibility of inter- and intraobserver agreements were analysed by using the intraclass correlation coefficient (ICC). According to the colour filling of the area of interest and imaging noise when conducting SWE, the image quality was classified as three grades: “good”, “common”, and “poor”. A logistic regression was used to analyse the independent factors for SWE quality.

RESULTS:

In the comparative analysis, high frequency, transection measurement and middle pole were selected as the appropriate measurement methods. Regarding reproducibility, the ICCs) of the intra- and interobserver agreements were 0.85 and 0.77, respectively. Multivariate analysis indicated that only the skin allograft distance and kidney width were independent variables for SWE quality. In the subgroup analysis of the skin-allograft distance, the “good” and “common” rates of images decreased as the distance increased, but the CV (coefficients of variation) showed the opposite trend. The SWE quality of kidney width <5.4 cm was significantly better than that of kidney width ≥5.4 cm.

CONCLUSIONS:

High-frequency SWE can be used in the evaluation of transplanted kidneys due to its good repeatability and high successful measurement rate, but we should pay attention to the influence of the skin-allograft distance and kidney width on SWE quality.

Keywords

Shear wave elastography renal allograft image quality repeatability high-frequency probe

1 Introduction

Kidney transplantation is one of the most effective methods for the treatment of end-stage renal disease [1]. Studies have shown that chronic changes such as interstitial fibrosis (IF) and tubular atrophy (TA) are major threats to the long-term survival of the graft and the recipient [2, 3]. Early diagnosis of renal fibrosis enables proper treatment to prevent further damage to the renal allograft [4].

In the present study, biopsy remains the gold standard for assessing renal allograft dysfunction. However, as an invasive examination, biopsy carries a series of risks and complications, such as haemorrhage, haematuria, perirenal haematoma and arteriovenous fistula [5, 6]. Shear wave elastography (SWE) is a noninvasive imaging technique that is effective in the assessment of tissue stiffness [7, 8]. SWE has been widely used in liver evaluation because of its advantages of stable measurement and high repeatability [9].

Currently, Elastography has been utilized for the evaluation of numerous renal diseases in scientific publications [10, 11]. However, the application of low-frequency SWE in renal transplantation (RT) is still controversial. Some studies mentioned that SWE in the medulla of transplanted kidneys is related to the degree of fibrosis [12, 13] while other studies showed that renal cortical stiffness was not correlated with interstitial fibrosis [14, 15]. Furthermore, many factors can impact the SWE imaging quality of kidney transplantation, such as anisotropy, blood vessel and ureter pressure, probe pressure, body mass index(BMI), etc. [16]. Compared to low-frequency ultrasound, high-frequency ultrasound has the advantage of high resolution and is suitable for inspection of renal allografts. Additionally, a high-frequency linear array probe might produce less probe pressure than a low-frequency convex array probe [17]. Furthermore, a high-frequency probe can help to avoid the medulla, large vessels, etc. that may affect SWE measurement. Due to the advantages of high-frequency probes, high-frequency SWE has been developed and introduced to thyroid and breast imaging [18, 19].

However, few existing studies have used high-frequency SWE to evaluate renal allograft dysfunction [20]. Therefore, the purpose of this study was to investigate the intra- and interobserver concordance and explore the factors that affect SWE imaging quality performed using a high-frequency probe.

2 Materials and methods

2.1 Participants

From September 2019 to October 2020, 240 RT patients with clinical indications (i.e., rising creatinine or proteinuria) in our institution were prospectively enrolled. All consenting patients underwent a conventional ultrasound protocol including B-mode, Doppler ultrasound and SWE measurements one day before allograft biopsy. Patients were excluded using the following criteria: (1) transplantation time <3 months; (2) postoperative complications such as urinary tract obstruction, perirenal haematoma/effusion, and local infection; and (3) presence of documented renal artery stenosis or other renal vascular disease. Considering the above, patients with transplantation time <3 months (n = 24), perirenal effusion (n = 4), or renal artery stenosis (n = 1) were excluded. Finally, 211 patients participated in the present study. The study group consisted of 130 men (61.6%) and 81 women (38.4%). The mean age was 38.6±12.2 years (range, 5–68 years). The inclusion and exclusion criteria are shown in Fig. 1. Written informed consent was obtained from all patients before biopsy procedures, and the Institutional Ethics Committee approved this single-centre prospective study (Approval ID:[2020]364).

Fig. 1

Flowchart shows patient enrolment in study.

2.2 Conventional ultrasound examination

All ultrasound examinations were performed using an Aixplorer machine with a 1–6 MHz (low frequency) convex transducer and an 8–12 MHz (high frequency) linear transducer (Supersonic Imagine, Aixen-Provence, France). Scanning was performed with the patients lying in a supine position. Before starting SWE, a “check” US examination was performed to evaluate the allograft morphologic characteristics and vascularity and perigraft collection. A low-frequency transducer was used to evaluate the graft size, parenchymal echo, colour blood flow grading, peak systolic velocity (PSV) and resistance index (RI) of the renal hilar artery and segment artery. Then, after switching to a high-frequency transducer, the parenchymal thickness, skin-allograft distance and cortical blood flow grade were recorded.

2.3 SWE examination

Immediately after conventional ultrasound examinations, a whole view of the kidney allograft was obtained; and the mid pole of the graft kidney, where the depth of the kidney capsule from the skin surface was the smallest, was targeted. To reduce external pressure, the transducer was placed on the targeted area with the gentlest pressure to acquire a good fill in the elastographic box without evidence of a compression artefact [12]. Furthermore, the sound beam was maintained as perpendicular to the kidney as possible. Then, a real-time colour scale elasticity map displayed on a scale of 0–80 kPa was positioned to cover the anterior cortex. ROIs with a standardized diameter of 7–11 mm were placed within the covered cortex. To obtain stable measurement, the acquisitions were performed during separate breath holdings of 5 s or more. At least five measurements were performed in each renal allograft, and the mean SWE values were calculated. SWE examinations were performed by one or two experienced radiologists blinded to the patients’ clinical and laboratory information. All measurements were recorded and used for subsequent statistical analyses. The research process was conducted in the following phases:

2.4 First: Effect of transducer frequency on SWE measurements

A total of 80 patients underwent SWE measurements performed using both high-frequency and low-frequency transducers to assess the effect of transducer frequency on SWE quality. First, a low-frequency transducer was used to perform the SWE measurement, and this was followed by high-frequency transducers. Both of the measurements targeted the same location in the middle pole of the transplanted kidney (Fig. 2).

Fig. 2

Effect of transducer frequency on SWE measurements. A. high frequency transducer B. Low frequency transducer.

2.5 Second: Effect of transducer direction on SWE measurements

To assess the effect of anisotropy on SWE quality, eighty patients were selected to receive high-frequency SWE measurements in both the longitudinal and transverse directions. In longitudinal measurement, the probe direction was parallel to the long axis of the transplanted kidney and placed in the middle pole. However, in transverse measurement, the probe direction was parallel to the short axis of the transplanted kidney (Fig. 3).

Fig. 3

Effect of transducer direction on SWE measurements. A. Transverse measurement B. Longitudinal measurement.

2.6 Third: Effect of measurement pole on SWE measurements

Eighty patients were selected to explore the difference in SWE quality between measurement poles, including the middle pole and upper pole. High-frequency transducers were placed in the middle and upper poles successively to obtain transverse SWE, as shown in Fig. 4.

Fig. 4

Effect of measurement pole on SWE measurements. A. Middle pole B. Upper pole.

2.7 Fourth: Intra- and interobserver correlation

To assess intraobserver variability of high-frequency SWE measurements in the transverse direction, 30 patients underwent SWE using two operators, and another 30 consecutive patients underwent SWE using one operator at different times.

2.8 Fifth: Influencing factors of the image quality of SWE measurement

A total of 211 patients were included in the study to evaluate the influencing factors of SWE measurement image quality. All of the patients underwent transverse SWE examination using one observer.

2.9 Image quality classification

Two researchers read all the SWE images and provided the quality grading. When the results were inconsistent, another senior physician with more than 5 years of SWE examination experience made the final decision. According to the colour filling of the area of interest and imaging noise when conducting SWE, the image quality was classified as three grades: “good”, “common”, and “poor”. “Good” was defined as the colour filling area in the SWE frame being 100% and the distribution being uniform without obvious noise. “Common” was defined as the colour filling area in the SWE being more than 60% without obvious noise or the colour filling area in the SWE frame being 100% with some noise, which does not affect the measurement. “Poor” was defined as the colour filling in the SWE frame being more than 60% with obvious noise or the colour filling area in the SWE frame being less than 60% (Fig. 5). When the image quality is “good” or “common”, SWE measurement is successful. The SWE measurement was considered to have failed when the image quality was “poor” [21]. When the scale of Color Doppler was set at 4 cm/s without any overflow, Parenchymal blood flow grade can be seen in Supplement Figure 1.

Fig. 5

Image quality classification. A. Good B. Common C. Poor.

2.10 Statistical analysis

Categorical data are presented as numbers (percentages) for dichotomous variables and means (standard deviations) for normally distributed variables or medians (ranges) for nonnormally distributed variables. Differences between groups were analysed using t-tests and Mann-Whitney U-tests or chi-squared tests for continuous and dichotomous variables, respectively. The intra- and interobserver agreement of the SWE measurement was assessed by the intraclass correlation coefficient (ICC). Agreement was classified as poor (ICC <0.4), moderate (ICC = 0.40–0.75), or excellent (ICC >0.75) [22]. The interobserver variability was expressed using Bland-Altman plots. Single and multiple logistic regression analyses were performed to select the independent variables of SWE images, and then independent variables were used for subsequent analysis. A two-sided P < 0.05 was considered statistically significant. Statistical analysis was performed using GraphPad Prism 8.0 (Prism, San Diego, CA, USA), MedCalc10.1 (MedCalc Software, Mariaker, Belgium) and SPSS 25.0 (SPSS, Chicago, Illinois, USA).

3 Results

3.1 The effects of the transducer frequency, measurement direction and renal pole on SWE quality

The SWE image quality of the high-frequency probe was “good” in 63 (78.8%) cases, “common” in 12 (15%) cases and “poor” in 5 (6.2%) cases; furthermore, the low-frequency probe was “good” in 71 (88.8%) cases, “common” in 7 (8.7%) cases, and “poor” in 2 (2.5%) cases. There was no significant difference between the two probes (P = 0.850).

In the transection measurement, the image quality was “good” in 61 (76.3%) cases, “common” in 13 (16.3%) cases, and “poor” in 6 (7.5%) cases. As for longitudinal measurement, the image quality was “good” in 54 (67.5%) cases, “common” in 20 (25%) cases and “poor” in 6 (7.5%) cases. The difference between the two measurement directions was statistically significant (P = 0.001). The details are shown in Table 1.

Table 1
Comparison of SWE image quality

SWE measurement Good Common Poor P value

Transducer frequency 0.850

High frequency 63(78.8%) 12(15%) 5(6.2%)

Low frequency 71(88.8%) 7(8.7%) 2(2.5%)

Measurement direction 0.001

Transection measurement 61(76.3%) 13(16.3%) 6(7.5%)

Longitude measurement 54(67.5%) 20(25%) 6(7.5%)

Measurement pole <0.001

Middle pole 54(67.5%) 21(26.3%) 5(6.2%)

Upper pole 39(48.8%) 26(32.5%) 15(18.7%)

SWE measurement	Good	Common	Poor	P value
Transducer frequency				0.850
High frequency	63(78.8%)	12(15%)	5(6.2%)
Low frequency	71(88.8%)	7(8.7%)	2(2.5%)
Measurement direction				0.001
Transection measurement	61(76.3%)	13(16.3%)	6(7.5%)
Longitude measurement	54(67.5%)	20(25%)	6(7.5%)
Measurement pole				<0.001
Middle pole	54(67.5%)	21(26.3%)	5(6.2%)
Upper pole	39(48.8%)	26(32.5%)	15(18.7%)

Of the eighty patients receiving SWE measurements of both the middle and upper poles, there were significant differences in the image quality between the two groups (P < 0.001). In the SWE measurement of the middle pole, the image quality was “good” in 54 (67.5%) cases, “common” in 21 (26.3%), and “poor” in 5 (6.2%) cases. In the SWE measurement of the upper pole, the image quality was “good” in 39 (48.8%) cases, “common” in 26 (32.5%) cases, and “poor” in 15 (18.7%) cases. As shown in Table 1, the image quality of the middle pole was better than that of the upper pole.

3.2 Intra- and interobserver correlation of SWE measurement

The mean cortical stiffness of SWE measurements at different times by the same observer were 22.2±7.2 kPa and 23.1±6.3 kPa, respectively. In addition, the mean cortical stiffness by the two observers was 22.3±6.0 kPa and 21.9±7.1 kPa, respectively. Both intraobserver and interobserver correlations were “excellent”, and the ICCs for SWE measurement were 0.85 and 0.77, respectively. Bland-Altman plots for measuring cortical stiffness of intra- and interobservers are shown in Supplement Figure 2.

3.3 Baseline characteristics

A total of 211 kidney transplant recipients were included. There were 191 cases in the successful measurement group (140 cases of “good” and 41 cases of “common”) and 20 cases in the unsuccessful group, with a success rate of 91.8%. The baseline characteristics of these 211 patients are summarized in Table 2.

Table 2
Baseline characteristics of image quality

Characteristics Total Good & Common Poor P value

Clinical index

Age(year) 38.6±12.2 38.0±12.0 44.6±12.6 0.691

Gender(male/female) 130/81 119/72 11/9 0.727

Transplantation time(month) 27.0(13.0–65.0) 26.0(13.0–61.7) 29.0(9.0–156.0) 0.499

Urine volume(mL) 2000(2000–2600) 2000(2000–2600) 2000(1975–2650) 0.834

BMI(kg/m²) 20.4(18.7–23.3) 20.2(18.4–23.1) 23.1(21.9–27.5) <0.001

Systolic pressure(mmHg) 130(127–140) 130(126–145) 130(130–140) 0.971

Ultrasound index

Skin-allograft distance(cm) 1.02(0.75–1.39) 0.96(0.72–1.32) 2.03(1.40–2.66) <0.001

Mild hydronephrosis(cm) 49 45 4 0.814

Parenchymal blood flow grade 0.009

I 124 119 5

II 60 51 9

III 27 22 5

Kidney length(cm) 10.9±1.2 10.9±1.1 11.6±1.5 0.044

Kidney width(cm) 4.8(4.2–5.5) 4.7(4.1–5.4) 6.2(5.0–6.7) <0.001

Parenchymal thickness(cm) 1.6(1.4–1.8) 1.6(1.4–1.8) 1.9(1.6–2.2) <0.001

Characteristics	Total	Good & Common	Poor	P value
Clinical index
Age(year)	38.6±12.2	38.0±12.0	44.6±12.6	0.691
Gender(male/female)	130/81	119/72	11/9	0.727
Transplantation time(month)	27.0(13.0–65.0)	26.0(13.0–61.7)	29.0(9.0–156.0)	0.499
Urine volume(mL)	2000(2000–2600)	2000(2000–2600)	2000(1975–2650)	0.834
BMI(kg/m²)	20.4(18.7–23.3)	20.2(18.4–23.1)	23.1(21.9–27.5)	<0.001
Systolic pressure(mmHg)	130(127–140)	130(126–145)	130(130–140)	0.971
Ultrasound index
Skin-allograft distance(cm)	1.02(0.75–1.39)	0.96(0.72–1.32)	2.03(1.40–2.66)	<0.001
Mild hydronephrosis(cm)	49	45	4	0.814
Parenchymal blood flow grade				0.009
I	124	119	5
II	60	51	9
III	27	22	5
Kidney length(cm)	10.9±1.2	10.9±1.1	11.6±1.5	0.044
Kidney width(cm)	4.8(4.2–5.5)	4.7(4.1–5.4)	6.2(5.0–6.7)	<0.001
Parenchymal thickness(cm)	1.6(1.4–1.8)	1.6(1.4–1.8)	1.9(1.6–2.2)	<0.001

3.4 Independent variables affecting the image quality of SWE

Table 3 summarizes the influencing factors of SWE image quality. In the univariate logistic regression analysis, a number of significant factors for poor image quality were identified: age [OR 95% CI, 1.044 (1.004–1.086), P = 0.032], BMI [OR 95% CI, 1.285 (1.140–1.451), P < 0.001], skin-allograft distance [OR 95% CI, 7.226 (3.315–15.750), P < 0.001], parenchymal blood flow grade [OR 95% CI, 0.185 (0.049–0.692), P = 0.012], kidney length [OR 95% CI, 1.766 (1.163–2.680), P = 0.008], kidney width [OR 95% CI, 3.529 (2.064–6.034), P < 0.001] and parenchymal thickness [OR 95% CI, 11.243 (2.802–45.112), P = 0.001]. Multivariate analysis found that the independent predictors for poor image quality were skin-allograft distance [OR 95% CI, 5.074 (2.265–11.370), P < 0.001] and kidney width [OR 95% CI, 2.654 (1.458–4.833), P = 0.001].

Table 3
Independent variables affecting image quality of SWE

Characteristics Univariate Multivariate

OR(95% CI) P value OR(95% CI) P value

Clinical index

Age(year) 1.044(1.004–1.086) 0.032 – –

Gender(male/female) 1.186(0.456–3.085) 0.727

Transplantation time(month) 1.006(0.999–1.014) 0.088

Urine volume(mL) 1.000(1.000–1.001) 0.392

BMI(kg/m²) 1.285(1.140–1.451) <0.001 – –

Systolic pressure(mmHg) 1.001(0.970–1.034) 0.926

Ultrasound index

Skin-allograft distance(cm) 7.226(3.315–15.750) <0.001 5.074(2.265–11.370) <0.001

Mild hydronephrosis(cm) 0.871(0.275–2.758) 0.814

Parenchymal blood flow grade

I – –

II 0.776(0.233–2.583) 0.680

III 0.185(0.049–0.692) 0.012 – –

Kidney length(cm) 1.766(1.163–2.680) 0.008 – –

Kidney width(cm) 3.529(2.064–6.034) <0.001 2.654(1.458–4.833) 0.001

Parenchymal thickness(cm) 11.243(2.802–45.112) 0.001 – –

Characteristics	Univariate	Multivariate
Clinical index
Age(year)	1.044(1.004–1.086)	0.032	–	–
Gender(male/female)	1.186(0.456–3.085)	0.727
Transplantation time(month)	1.006(0.999–1.014)	0.088
Urine volume(mL)	1.000(1.000–1.001)	0.392
BMI(kg/m²)	1.285(1.140–1.451)	<0.001	–	–
Systolic pressure(mmHg)	1.001(0.970–1.034)	0.926
Ultrasound index
Skin-allograft distance(cm)	7.226(3.315–15.750)	<0.001	5.074(2.265–11.370)	<0.001
Mild hydronephrosis(cm)	0.871(0.275–2.758)	0.814
Parenchymal blood flow grade
I	–	–
II	0.776(0.233–2.583)	0.680
III	0.185(0.049–0.692)	0.012	–	–
Kidney length(cm)	1.766(1.163–2.680)	0.008	–	–
Kidney width(cm)	3.529(2.064–6.034)	<0.001	2.654(1.458–4.833)	0.001
Parenchymal thickness(cm)	11.243(2.802–45.112)	0.001	–	–

BMI: body mass index.

3.5 Subgroup analysis of the skin-allograft distance on SWE

As shown in Fig. 6A, we noticed that poor image quality tended to have a larger skin allograft distance (good vs. common vs. poor: 0.93±0.34 vs. 1.53±0.62 vs. 2.07±0.92 cm, P < 0.001). To further investigate the effect of the skin allograft distance on SWE quality, the skin allograft distance was divided into the following subgroups: 0–0.49 cm, 0.50–0.99 cm, 1.0–1.49 cm, 1.50–1.99 cm, 2.0–2.49 cm and ≥2.50 cm. The distribution of the number of patients with “good”, “common” and “poor” image quality at different skin-allograft distances is shown in Table 4. The rate of “good” image quality gradually decreased as the distance increased. When the skin allograft distance was 0–0.5 cm, the rate of “good” image quality reached up to 93.3%, but it decreased to 0 with a distance ≥2.5 cm. The success rate of SWE measurement also decreased from 100% to 40.0% as the distance increased from 0–0.5 cm to ≥2.5 cm.

Fig. 6

Subgroup analysis of skin-allograft distance and kidney width on SWE. A. skin allograft distance B. CV C. Kidney width. CV: coefficients of variation.

Table 4

Subgroup analysis of skin-allograft distance on SWE

Depth(cm)	Good(n)	Common(n)	Poor(n)	Good rate(%)	Good & Common rate(%)	CV
0–0.5	14	1	0	93.3	100	0.16
0.5–1.0	79	8	2	88.8	97.7	0.20
1.0–1.5	49	10	5	76.6	92.2	0.24
1.5–2.0	7	14	2	30.4	91.3	0.32
2.0–2.5	1	4	5	10.0	50.0	0.43
>2.5	0	4	6	0	40.0	0.54

CV: coefficients of variation.

To assess the measurement reliability, the coefficients of variation of the SWE value were calculated as follows: CV = (SD/mean)×100% [23], which indicates the degree of separation of the SWE value. A greater CV indicates greater noise interference on the measurement, which leads to the unreliability of the SWE value. The CV values were significantly different among the different SWE qualities (good vs. common vs. poor = 0.19±0.07 vs. 0.31±0.11 vs. 0.48±0.11, P < 0.001, seen in Fig. 6B). Table 4 shows that the CV increased as the skin allograft distance increased. When the skin allograft distance increased from 0–0.5 cm to ≥2.5 cm, the CV increased from 0.16 to 0.54. These results indicated that a high-frequency probe is no longer suitable for renal graft SWE measurement when the skin allograft distance is ≥2.5 cm.

3.6 Subgroup analysis of the kidney width on SWE

Regarding the kidney width, there were significant differences between the three SWE qualities (good vs. common vs. poor = 4.65±0.80 vs. 5.29±1.01 vs. 5.95±1.10 cm, P < 0.001, see Fig. 6C), indicating that a wider kidney width may contribute to poor SWE quality. According to the optimal threshold value, the kidney width was divided into two groups: <5.4 cm and ≥5.4 cm. In the comparison of the rate of “good” image quality, the difference between the two groups was statistically significant (<5.4 cm vs. ≥5.4 cm: 83.3% vs. 41.0%, P < 0.001). There was also a significant difference when comparing the success rate of SWE measurement (<5.4 cm vs. ≥5.4 cm: 95.3% vs. 78.7%, P < 0.001).

4 Discussion

In the past, a low-frequency probe was often used to perform SWE measurements [12, 24] while the use of a high-frequency probe was rarely reported. To date, there is no general consensus on the use of high-frequency SWE for renal allografts. Therefore, the purpose of this study was to investigate the application of high-frequency probe shear wave elastography to transplanted kidneys. Methodological studies of high-frequency SWE include the assessment of the repeatability and influencing factors. In this study, both intraoperator and interoperator ICC values were greater than 0.75, indicating good consistency in the measurement of SWE. We further reached a conclusion that performing a transection SWE measurement at the middle pole of the renal allograft is more likely to attain good SWE quality. It was found that the skin allograft distance and kidney width were the main factors affecting SWE image quality. As the skin-allograft distance and kidney width increased, the success rate of SWE measurement gradually decreased.

The effect of the high-frequency measurement direction of renal graft elasticity was investigated in this study. The results showed that the SWE quality of the transverse measurement was better than that of the longitudinal measurement. This may be related to the anisotropy of the kidney and the fact that the longitudinal measurement can cause too many pressure artefacts, resulting in poor image quality. However, more studies are needed to confirm this finding.

In the comparison of image quality between the high-frequency and low-frequency probes, there was no statistically significant difference between them. Low-frequency SWE has been widely used in the evaluation of transplanted kidneys [13, 24]. Maggie et al. reported that the intraclass correlation coefficient of the cortex could reach up to 0.84 [13]. Our results indicated that the transplanted kidney, located in the pelvic cavity, is sufficiently superficial for high-frequency probes to obtain successful SWE measurements. Furthermore, because of the high image resolution, a high-frequency probe is able to more accurately measure structural details. For example, the cortex, medulla and renal blood vessels of the renal allograft can be clearly discriminated, which helps to avoid the large vessels that can affect SWE measurement. Additionally, it has been reported that a higher frequency is equivalent to more precise measurements of shear wave speeds [20].

Repeatability is an important factor affecting the clinical application of elastography technology. Until now, no studies have reported on the elastic repeatability of transplanted kidneys using high-frequency probes. Thus, efforts to explore the elastic repeatability of high-frequency probes is warranted. In this study, the intra- and interobserver correlations were 0.85 and 0.77, respectively, which indicated that the high-frequency probe had good consistency in evaluating the elasticity of transplanted kidneys.

In the study, the skin-allograft distance was the most important factor affecting the quality of the elastography performed by the high-frequency probe. Distance is a factor leading to poor image quality from elastography, which has been mentioned in many studies, including research on the quality of the elastography of transplanted kidneys and livers [23, 25]. Factors such as the effects of the kidney capsule and thicker subcutaneous fat may be the reasons for this poor performance of SWE at increased distances. Additionally, as the distance increases, ultrasonic waves gradually attenuate, which leads to the degradation of the image quality [26]. Moreover, the attenuation of the high-frequency probe is more serious than that of the low-frequency probe. When the skin allograft distance is greater than 2.5 cm, it is difficult to obtain successful measurements using SWE. In the study, the rate of successful SWE measurement was 91.8%, indicating that except for a small proportion of obese patients, the vast majority of transplanted kidney patients can be examined using high-frequency probe SWE. Located under the iliac fossa, transplanted kidneys are very suitable for SWE measurement.

In addition, the width of the kidney allograft was also one of the influencing factors of SWE quality. When the width of the transplanted kidney increased, the SWE quality decreased. Increased transplanted kidney width usually occurs in the case of acute rejection, with lymphocyte infiltration and parenchymal swelling, which is equivalent to artificial pressure and an increase in image pressure artefacts, thus resulting in poor SWE quality. This was confirmed by the results of Chen et al. [27].

There are some limitations to our study. First, this was a single-centre study with a relatively small population. Another limitation is that we did not make comparisons between SWE quality and additional factors, such as the anisotropy and probe pressure. Furthermore, intra- and interobserver correlations were conducted on 30 consecutive patients. However, these patients had different aetiological compositions, which may have a certain impact on the ICC; and a more rigorous design needs to be developed in future studies. Lastly, though parenchymal blood flow was graded 3 levels according to Doppler ultrasound in the study, contrast enhanced ultrasound(CEUS) is more sensitive to renal microcirculation.

5 Conclusion

In conclusion, the application of a high-frequency probe in the SWE measurement of transplanted kidneys provides a new method for the monitoring of transplanted kidneys, and its good repeatability and high success rate are of great significance for clinical application. Although a small number of renal allograft patients fail to receive high-frequency SWE technology due to obesity, parenchyma swelling or other reasons, SWE could be a promising technique for allograft monitoring, such as renal fibrosis.

Footnotes

Acknowledgments

The authors are grateful for the financial support provided by Guangdong Basic and Applied Basic Research Foundation 2020A1515010653.

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