Point-of-Care Kidney and Genitourinary Ultrasound in Adults: Image Acquisition

Summary

Point-of-care ultrasound (POCUS) of the renal and genitourinary (renal-GU) system can help to screen for certain causes of kidney dysfunction. However, despite its clinical utility, renal-GU POCUS remains underutilized due to a lack of training among clinicians. To address this gap, this article describes renal-GU image acquisition and interpretation.

Abstract

A range of conditions involving the kidneys and urinary bladder can cause organ-threatening complications that are preventable if diagnosed promptly with diagnostic imaging. Common imaging modalities include either computed tomography or diagnostic ultrasound. Traditionally, ultrasound of the kidney-genitourinary system has required consultative teams consisting of a sonographer performing image acquisition and a radiologist performing image interpretation. However, diagnostic point-of-care ultrasound (POCUS) has recently emerged as a useful tool to troubleshoot acute kidney injury at the bedside. Studies have shown that non-radiologists can be trained to perform diagnostic POCUS of the kidneys and bladder with high accuracy for a set number of important conditions. Currently, diagnostic POCUS of the kidney-genitourinary system remains underused in actual clinical practice. This is likely because image acquisition for this organ system is unfamiliar to most clinicians in specialties that encounter acute kidney injury, including primary care, emergency medicine, intensive care, anesthesiology, nephrology, and urology. To address this multi-specialty educational gap, this narrative review was developed by a multi-disciplinary group to provide a specialty-agnostic framework for kidney-genitourinary POCUS image acquisition: indications/contraindications, patient positioning, transducer selection, acquisition sequence, and exam limitations. Finally, we describe foundational concepts in kidney-genitourinary ultrasound image interpretation, including key abnormal findings that every bedside clinician performing this modality should know.

Introduction

Acute kidney injury (AKI), resulting from a variety of etiologies, is a frequent medical diagnosis identified in hospitalized patients. AKI precipitates an abrupt decrease in kidney function that leads to an accumulation of extracellular fluid, urea, and other nitrogenous waste products, along with the dysregulation of electrolytes. Moreover, the diagnosis of AKI portends worse short-term and long-term outcomes and is associated with the consumption of greater healthcare resources¹. According to the United States Renal Data System (USRDS), among Medicare Fee-For-Service beneficiaries in 2020, the rate of hospitalization with AKI was 62 admissions per 1000 patient years². Furthermore, a systematic review of 154 studies that adopted the Kidney Disease Improving Global Outcomes (KDIGO) AKI diagnostic criteria found that among the 3,585,911 people in these trials, primarily from North America, Northern Europe, and Eastern Asia, the incidence of AKI in different inpatient settings ranged from 20%-32%³. In the inpatient setting, AKI is commonly identified in the intensive care unit and is associated with increased mortality⁴. Point-of-care ultrasound (POCUS) machines are readily available in settings such as the ICU, but diagnostic kidney-GU ultrasound is often underused despite being able to quickly evaluate several etiologies of AKI⁵.

Compared to consultative ultrasound, in which a patient’s primary provider orders a formal ultrasound to be performed by a radiology technician and read by a radiologist, diagnostic POCUS is performed and interpreted by a patient’s primary provider at the point of care⁶. There is growing evidence that non-radiologists can effectively and accurately use diagnostic POCUS for a variety of conditions⁷. For instance, the 2019 Hospitalist-Operated Compression Ultrasonography: a Point-of-Care Ultrasound Study (HOCUS-POCUS) prospective cohort study compared hospitalist-performed deep vein thrombosis (DVT) compressive POCUS to consultative technician/radiologist performed vascular DVT ultrasound. The study showed similar accuracy of hospitalist-performed POCUS to technician/radiologist-performed consultative vascular ultrasound with sensitivity of 100% and specificity of 96%⁸. Similarly, a 2020 study found kidney-GU POCUS performed by Emergency Department providers of varied experience had moderate accuracy (specificity of 72% and sensitivity of 77%) in detecting hydronephrosis when compared to computed tomography scan⁹. Importantly, primary-provider performed POCUS allows for more timely diagnosis and intervention compared to technician/radiologist imaging.

Causes of AKI can be divided into pre-renal (hemodynamically mediated injury), intra-renal (glomerular or interstitial pathologies), and post-renal (urologic etiologies, most commonly obstructive uropathy). The latter, especially, can be diagnosed with POCUS. Obstructive uropathy has an annual incidence of 1.7 per 1000 people and has been estimated to account for about 10% of acute and chronic kidney disease¹⁰. From prostatic hyperplasia to nephrolithiasis, there are many causes of urinary obstruction. The main pathologic manifestation of these conditions at the level of the kidney is hydronephrosis. This is easily visualized on POCUS, and speed in diagnosis can be critical in treating acutely ill patients with kidney failure.

Beyond AKI, POCUS remains a cost-effective and safe modality for evaluating chronic kidney disease. POCUS can be used to identify lesions indicative of renal cell carcinoma, given the ability to visualize cysts greater than or equal to 3 cm and to discriminate characteristics deemed concerning for malignancy¹¹. POCUS allows for the rapid evaluation of autosomal dominant polycystic kidney disease (ADPKD), avoiding unnecessary scheduling of kidney biopsies and expensive lab work. Additionally, ultrasonographic measurement of kidney length was shown to prognosticate risk of progression in early ADPKD when compared to the gold standard magnetic resonance-based, height-adjusted total kidney volume measurement (htTKV)¹².

While computed tomography scans have an advantage in detecting neoplasms, stones, and calcifications, there has been no proven benefit of computed tomography over ultrasonography in the diagnosis of AKI etiologies¹³. Furthermore, some patients may be too sick to move out of their rooms, precluding transport to the CT scanner or even the radiology suite for a technician/radiologist to perform the consultative ultrasound. In these cases, POCUS provides a safe and reliable diagnostic alternative. Despite this, diagnostic kidney-GU POCUS remains an underused tool, likely due to a lack of training among frontline clinicians¹⁴. To address this knowledge gap, this narrative review brings together the expertise of multiple specialties (hospital medicine, critical care, anesthesiology, and nephrology) to propose an evidence-based kidney-GU POCUS image acquisition protocol, including indications/contraindications, patient positioning, transducer selection, acquisition sequence, and exam limitations. Finally, we describe foundational concepts in kidney-GU ultrasound image interpretation, including key abnormal findings that every bedside clinician performing this modality should know.

Protocol

All procedures performed in this study involving human participants were in accordance with the ethical standards of the Duke University Health System institutional research committee and with the 1964 Helsinki Declaration and its later amendments or comparable ethical standards. Images included were performed on the authors themselves for normal images and as part of routine educational ultrasound scans done for teaching purposes for positive images, with preceding consent per institutional standards. Patients were incl…

Representative Results

Sonographically normal exam Normal kidney ultrasound The echogenicity of the kidney capsule and limited anatomic variability (except for the occasional pelvic kidney or the even more rare horseshoe kidney) allow for easy identification of the kidneys with bedside POCUS. The kidneys will have a typical bean-shaped appearance, measuring on average 9-13 cm, though size varies with patient height and weight (Figure 2, Vi…

Discussion

AKI commonly manifests in critically ill hospitalized patients, amplifying the risk of mortality. To proficiently execute the steps outlined above and differentiate normal from pathologic findings, a comprehensive understanding of normal anatomy and ultrasonographic appearances is essential, along with precise adherence to the protocol's specific steps.

Critical anatomy/steps in the protocol
Kidney– The kidneys are retroperitoneal organs that lie in an obliq…

Materials

Curvilinear Transducer	Philips	C5-2 USB	2-5 MHz, also called the abdominal probe
Curvilinear Transducer	SonoSite	C5-1	1-5 MHz, also called the abdominal probe
Edge 1 ultrasound machine	SonoSite		Used to obtain a subset of the Figures and Videos
Phased-Array Transducer	Philips		1-5 MHz, also called the cardiac probe
Phased-Array Transducer	SonoSite	P5-1	1-5 MHz, also called the cardiac probe
Ultrasound system	Philips	Affiniti30	Used to obtain a subset of the Figures and Videos