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 included based on the following criteria: any patient with acute kidney injury, decreased urinary output, or any other reason to suspect abnormal kidney function. Exclusion criteria included patients with an open abdomen. Pain is a relative contraindication to ultrasound exams, especially when the pain is so severe that it prohibits probe placement (e.g., intraabdominal hypertension). The reagents and the equipment used are listed in the Table of Materials.

Terminology: the three planes of the body are called coronal, sagittal, and transverse
This can be confusing because the latter term ("transverse") can also be used to refer to the short axis of a single organ alongside the use of the term "longitudinal" to refer to the long axis of the organ. To minimize confusion, this protocol will exclusively use coronal, sagittal, and transverse to refer to planes in the human body and will use long-axis and short-axis to refer to the planes of the kidney. Further, since the bladder's dynamic shape prevents it from having a permanent long- or short-axis, the two views of the bladder will only be named based on which one of two to the plane of the body that the view aligns with: sagittal or transverse.

1. Transducer selection

1. Select a low-frequency transducer: preferably, this should be the curvilinear transducer (2-5 MHz). If a curvilinear transducer is not available, a sector array (aka "phased array"; 1-5 MHz) can serve as a substitute.
   NOTE: The curvilinear transducer is the best choice for visualizing the kidney through a single view because its footprint creates a large field of view, and its low-frequency piezoelectric crystals permit penetration deep enough into the body. However, the curvilinear probe can be difficult to use when looking at kidneys through the ribs.

2. Machine settings

1. Set the depth so that the kidney will appear in the middle third of the ultrasound screen (the typical setting is between 16 cm and 20 cm).
2. Set the gain such that the renal medullary pyramids appear anechoic (black), the renal sinus complex appears hyperechoic (bright), and the kidney cortex appears intermediate between these extremes.

3. Mode and presets

1. Select two-dimensional (2D) mode, also called brightness mode (B-mode). This is a 2-dimensional greyscale ultrasound mode.
2. After activating the 2D mode, select the abdominal preset.

4. Patient positioning

1. For most of the exam, position the patient supine.
   NOTE: When imaging the right kidney, it may become necessary to reposition the patient in the left lateral recumbent position for better visualization of the right kidney. When imaging the left kidney, it may become necessary to reposition the patient in the right lateral recumbent position for better visualization of the left kidney.
2. Before scanning, expose the patient's lower chest and abdomen.
3. Position the ultrasound machine so that the sonographer's dominant hand can hold the ultrasound probe. This allows for finer manipulation of the ultrasound probe and frees up the non-dominant hand for operating the ultrasound machine.
   NOTE: Right-handed sonographers should position themselves with the patient on their right side and vice versa.

5. Imaging the right kidney

1. Gel application
   1. Apply gel to the ultrasound probe directly to maximize scanning efficiency prior to acquiring each image.
2. Coronal view
   1. Place the probe on the right flank, along the mid-axillary line, 5^th-7^th intercostal space, indicator pointing cranially (Figure 1).
   2. Adjust the probe position (sliding up/down, angling anterior/posterior) until the right kidney is seen in its maximal longitudinal extent (see Figure 2).
   3. Fan through anteriorly to posteriorly to screen for hydronephrosis and other gross abnormalities (Video 1).
   4. Cine clip acquisition: For machines configured for retrospective image acquisition, click on acquire prior to step 5.2.4. For machines configured for prospective image acquisition, click on acquire after step 5.2.4.
3. Kidney long-axis diameter measurement
   1. Center the kidney in the image, click on Freeze, and measure the height of the kidney (Figure 3).
   2. Click on Save (or the equivalent button).
4. Transverse view
   1. After centering on the right kidney, rotate the probe 90 degrees clockwise until the probe marker is facing anteriorly, revealing the kidney in the transverse view (Figure 4).
   2. Adjust the probe position (sliding up/down, angling anterior/posterior) until the right kidney is seen in its maximal size in this transverse plane (Figure 5).
   3. Fan through cranially to caudally to screen for hydronephrosis and other gross abnormalities (Video 2). Repeat the step.

6. Imaging the left kidney

1. Repeat steps 5.1-5.3 for the left kidney.

7. Imaging the bladder

1. Gel application: repeat step 5.1.1.
2. Transverse view
   1. Probe position: position the probe just cranial to the pubic symphysis with the probe indicator pointing toward patient's right side (Figure 6).
   2. Angle the ultrasound beam caudally into the pelvis until the bladder is visible in its maximal size.
   3. Image optimization
      1. Adjust depth until the bladder is visible in the middle third of the screen.
      2. Adjust gain until the bladder lumen is grossly anechoic (black) and the tissue plane directly posterior to the bladder is slightly hyperechoic (bright).
   4. Cine clip acquisition
      1. Fan through the bladder from cranial to caudal to visualize the entire structure. Repeat step 5.2.3.
   5. Transverse bladder dimension measurements
      1. Center the view on the bladder's maximal dimension, click on Freeze and measure the anterior-posterior and lateral-to-medial diameters of the bladder (Figure 7, left panel).
      2. Click on Save (or the equivalent button).
3. Sagittal view
   1. Maintaining the transverse view, center the view on the bladder and rotate the probe 90 degrees clockwise until the probe marker is facing cranially, revealing the kidney in the sagittal plane (Figure 8).
   2. Cine clip acquisition
      1. Fan through the bladder from side to side to visualize the entire structure. Repeat step 5.2.3.
   3. Transverse bladder dimension measurements
      1. Center the view on the bladder's maximal dimension, click on Freeze and measure the cranial-to-caudal diameter of the bladder (Figure 7, right panel).
      2. Click on Save (or the equivalent button).

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

Name	Company	Catalog Number	Comments
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