The results of the current study found a strong correlation between MRI-EL and US-EL. Blant–Altmant analysis showed low biases in all the measurements. These biases were more pronounced in RKV (7.5%, p = 0.0211) and TKV measurements (6.2%, p = 0.0328), while were very low in the LKV measurements (2.3%, p = 0.4927). It should be noted that if the p-value is less than 0.05, it indicates the presence of a consistent bias, but this does not automatically imply that the methods are not comparable. As noted by Bland and Altman [28], a consistent bias can be easily corrected, if needed, by subtracting the mean difference from the measurements obtained by the US-EL method. Furthermore, it is important to highlight that these differences are independent of renal volume, as the mean slopes did not significantly deviate from zero.

In addition, Passing–Bablok regression analysis comparing MRI-EL and US-EL found strong correlation in RKV (95% CI for intercept − 123.6 to 42.1 and for slope 0.83 to 1.16; ρ = 0.96), LKV (95% CI for intercept − 84.6 to 35.8 and for slope 0.88 to 1.16; ρ = 0.91), and TKV (95% CI for intercept − 184.9 to 84.0 and for slope 0.85 to 1.15; ρ = 0.94). These results clearly indicated that both methods were interchangeable.

Finally, kidney volume concordance between MRI-EL and US-EL, assessed by CCC, found a good agreement in RKV (0.95) and TKV (0.94), but slightly lower concordance in LKV (0.89).

One possible explanation is the different anatomical relationships of the two kidneys. For example, the anatomical proximity of the liver to the right kidney often results in acoustic shadowing during ultrasound examinations. Furthermore, the presence of polycystic liver disease, which is the most common extrarenal manifestation of ADPKD, might influence both imaging and measurements accuracy [29]. In addition, healthy liver parenchyma shows homogeneous echo texture and similar echogenicity compared to the right kidney, which might potentially impact on imaging and measurements [30]. In contrast, the left kidney has fewer surrounding structures that cause such interference, allowing for clearer imaging and more accurate volume measurements [31].

In ADPKD, the cystic burden is most accurately represented by TKV measurements obtained via MRI. Additionally, TKV is currently the most robust predictor of future renal insufficiency in ADPKD [7, 9, 11].

Kidney volume has been evaluated in numerous experimental and clinical studies employing various imaging techniques. MRI provides consistently reproducible measurements of kidney volume, as well as low inter- and intra-operator variability [32], while ultrasound is frequently used due to its accessibility and non-invasive nature [14,15,16].

While CT and MRI provide superior resolution for detecting small cysts, US remains the preferred initial method due to its accessibility, lower cost, and absence of radiation or contrast exposure. US demonstrates good reproducibility for TKV measurements, correlating well with CT, despite slightly lower accuracy and sensitivity [33]. Additionally, Advances in three-dimensional (3D) US technology have further enhanced diagnostic precision, enabling improved cyst detection and accurate volume measurements [34, 35]. Additionally, artificial intelligence (AI)-assisted 3D US systems show performance comparable to MRI, offering a promising alternative for routine clinical use [35]. These developments underscore the potential of US, particularly 3D and AI-enhanced systems, as accessible and effective tools for monitoring TKV and assessing treatment efficacy in ADPKD [33,34,35,36].

Despite being more cost-effective and readily accessible, ultrasound-derived kidney volume measurements are generally considered to be less accurate than those obtained from MRI ellipsoid analysis [34, 37]. Indeed, previous studies have found current US methods are still vulnerable to underestimation compared with MRI- and CT-based estimates [33, 34, 38, 39]. In agreement with these findings, compared with MRI-EL, US-EL displayed systematic bias for underestimating RKV, LKV, and TKV (mean bias of − 7.5%, − 2.3%, and − 6.2%, respectively). Nevertheless, the results of our study (Passing–Bablok regression analysis) showed that the measurement of renal volumes with US-EL was interchangeable with MRI-EL. Therefore, the clinical significance of this underestimation may not be relevant. The increase in kidney size enables clinicians to identify patients experiencing rapid disease worsening, thus supporting timely intervention aimed at slowing disease progression. However, to the best of our knowledge, there is currently no data available in the literature regarding the recommended frequency for performing MRI scans.

Consistent with this hypothesis, Breysem et al. [39] proposed that while US-EL measurements tend to underestimate kidney volume, they still offer a valuable alternative to MRI for the assessment of early ADPKD.

In addition, Bhutani et al. [40] observed that TKV measurements obtained by ultrasound and MRI were comparable, particularly in kidneys of normal to moderate size (< 17 cm). This is likely attributable to the ability to capture the entire kidney within a single imaging plane. Moreover, this study also found that a single measurement of kidney length, either with US or MRI, can reliably predict the development of CKD stage 3 within an 8-year timeframe. This approach effectively reduces healthcare costs while delivering essential prognostic insights into potential outcomes and complications associated with ADPKD [40].

Furthermore, Braconnier et al. [41], reported a strong correlation between ultrasound-measured renal length and MRI-measured renal length in both patients with and without chronic kidney disease (CKD). However, the correlation between MRI and ultrasound measurements for kidney volume, while statistically significant, was notably weaker. Consequently, renal volume assessments should be interpreted with caution [41].

Finally, this study found an inverse correlation between renal function, either assessed by mGFR or eGFR, and TKV, regardless of the method used for determining TKV. Our findings align with those of previous studies, which have demonstrated an inverse correlation between kidney volume and renal function [7, 42,43,44]. However, these studies were performed evaluating renal volume with MRI, while ours used both MRI and ultrasound, finding no significant differences between both methods. These findings support the use of US-EL for determining kidney volume in clinical practice.

This study has several limitations that should be considered when interpreting its findings. A key limitation of this study is its small sample size of only 32 patients, which restricts the ability to draw generalizable conclusions and limits the broader applicability of the findings. The second major limitation is the time interval between the MRI and ultrasound examinations, which raises the possibility of kidney volume changes occurring during this period. The timing discrepancy between these imaging modalities could influence the findings, as prior research suggests that kidney condition progression is time-sensitive, potentially impacting the consistency of measurements [45]. In our specific case, this delay might be primarily attributed to limited access to MRI facilities. Nevertheless, all patients included in this study had an estimated GFR greater than 60 mL/min (CKD-EPI), indicating early-stage disease, and their clinical stability was maintained throughout the study. Notably, for most patients (18 of 32), the interval between measurements was less than 2 months, with only five patients exceeding 4 months. While renal volume changes cannot be entirely ruled out, no significant clinical alterations were observed that might have influenced the results. Another limitation is that we did not evaluate intraobserver variability of both MRI-EL and US-EL. This study was conducted by a single expert radiologist to ensure consistency and reproducibility. Although US is an operator-dependent technique, and it is advisable that radiologists undergo at least 6 months of specialized training, both techniques have shown low intraobserver variability [39, 41], although such variability may be slightly greater with US-EL [46]. In addition, US may offer other advantages such as low cost, high availability, no radiation exposure, and minimal patient discomfort. Additionally, US is quicker and less expensive than MRI (US takes between 20–30 min and the MRI between 30–50 min) [47].

The primary strength of this study lies in its execution under real-world clinical conditions, providing a more accurate reflection of how these diagnostic tools perform in routine clinical practice, outside of controlled research settings.