Background and Indications for Examination
The urinary system is very amenable to imaging with ultrasound. The liver and spleen provide windows through which the kidneys can be visualized. The bladder is located directly behind the pubic symphysis and is readily seen from the suprapubic approach, particularly when full. Ultrasound of the kidneys and bladder can play a vital role in the effective management of patients in the emergent and critical care setting. Focused renal and urinary ultrasound can determine the presence of hydronephrosis, directly visualize stones, and measure bladder volume. Ultrasound easily identifies renal cysts, and may identify masses or clots in the kidneys or bladder. Ultrasound may guide urinary procedures such as ensuring correct Foley catheter placement, and suprapubic bladder aspiration.
Focused renal ultrasound should be incorporated into the examination of any patient with undifferentiated flank or abdominal pain. Ureteral stones can be difficult to visualize because they are retroperitoneal, but the presence of hydronephrosis on the side of the pain is very specific for ureterolithiasis as the cause of the patient’s symptoms. Patients with abdominal, back, or flank pain may also have more serious diagnoses such as abdominal aortic aneurysm or dissection, cholecystitis, ovarian torsion, ruptured ectopic pregnancy, and others. Bedside ultrasound may be helpful in diagnosing other causes of pain although other imaging modalities such as consultant-performed ultrasound or CT may be required if the diagnosis remains uncertain.
Bedside ultrasound evaluation of the urinary system should be performed in patients presenting with the following:
Probe Selection and Technical Considerations
When imaging the kidneys and bladder, the curvilinear probe with a frequency of 3.0–5.0 MHz should be utilized. Alternatively, a phased-array probe with a frequency of 2.0–4.0 MHz can be used to visualize the kidneys through the intercostal spaces. Lower frequencies, which improve penetration, can provide better visualization in obese patients.
Another ultrasound machine setting that can be of particular use in renal imaging is tissue harmonics. Renal stones, which are often made up of calcium, block the penetration of the ultrasound beam and produce an acoustic shadow. Tissue harmonics enhance acoustic shadowing and may improve the identification of nephrolithiasis.
The sonographer should start with adequate depth in order to not miss abnormalities in the far field. This is important for findings such as free fluid around the bladder, which is easily missed if subtle. Once the far field is fully interrogated, the depth can be decreased in order to make sure the kidneys or bladder are taking up most of the screen.
The overall gain can be adjusted in order to brighten the image if needed. In patients with difficult anatomy or obesity, the far field is often what is difficult to visualize. In these patients, the far gain or time-gain compensation can be turned up to improve the deeper parts of the image. However, with a full bladder the sound travels so easily that the brightness in the far field can “wash out” the image and cover up pathological findings, especially free fluid. This is known as posterior acoustic enhancement and reducing the far-field gain may help minimize this issue.
To optimize the image, the focal zone (area of maximal lateral resolution) should be positioned at the depth of the kidney or bladder.
Color-flow Doppler may help distinguish renal vasculature from hydronephrosis, both of which can appear as anechoic areas in the renal hilum. Vessels should show color flow while hydronephrosis does not. Power Doppler, which shows the presence of flow only without regard to velocity and direction, is good at picking up low-flow states. Doppler can help to evaluate the presence of urinary bladder jets in ruling out obstruction, which is discussed later in this chapter. Doppler placed over kidney stones may display a “twinkling” artifact that can help identify stones.
Normal Ultrasound Anatomy
The kidneys lie within the retroperitoneum, in a space that also includes the adrenal glands, proximal collecting system, and perinephric fat. Typically, the left kidney is positioned more superiorly and posteriorly than the right kidney, while the right kidney is often slightly larger in size. A difference in size up to 2 cm between the kidneys may be normal. The kidneys generally lie between T12 and L4 or between the eighth and eleventh ribs. Their average size is 9–13 cm in length, 4–6 cm in width, and 2.5–3.5 cm in depth (Fig. 10-1). The right kidney is surrounded by the liver anteriorly, the diaphragm superiorly, and the psoas and quadratus lumborum muscles posteriorly. The left kidney is surrounded by the spleen, large and small bowel, and stomach anteriorly, the diaphragm superiorly, and the psoas and quadratus lumborum muscles posteriorly.
A longitudinal view of a normal right kidney. This image was obtained with the probe in a coronal position, pointed toward the patient’s head, in the mid-to-anterior axillary line. Also visualized are the liver and right hemidiaphragm. Within the architecture of the liver is a black anechoic stripe representing a rib shadow. Rib shadows can obscure important findings, and therefore, the sonographer must attempt to scan around them. This can be achieved by asking the patient to take a deep breath in and out, which will move the liver and kidneys out from beneath the ribs. In addition, the probe can be moved down a rib space, it can be angled more obliquely in between the ribs, or a probe with a smaller footprint may be used.
Each kidney is surrounded by three outer layers: closest to the kidney lies the true fibrous capsule, next is a layer of perirenal fat, and finally, Gerota fascia, which also encloses the adrenal glands. The true fibrous capsule is quite echogenic, is easily visualized on ultrasound, and outlines the kidney. The kidney itself can be further subdivided into the renal parenchyma and the renal sinus. The renal parenchyma is considered the body of the kidney and consists of the renal cortex and the renal medulla. The cortex and medulla are divided by an imaginary line running along the outside of the medullary pyramids. The cortex contains the filtration apparatus of the nephrons and appears more hypoechoic and less echogenic than either the liver or spleen. The medullary pyramids have their base toward the cortex while the papillae drain into the renal fornices. The fornices, in turn, empty into the calyces, which then merge to form the renal pelvis. As the urine-producing segment of the kidney, the renal medulla typically appears less echogenic than the renal cortex. The renal sinus is the central portion of the kidney and consists of the calyces, vasculature, lymphatics, and peripelvic fat. Due to the peripelvic fat, the renal sinus usually appears echogenic or bright. In general, there are 8–18 minor calyces that coalesce to form 2–3 major calyces. The major calyces empty into the renal pelvis and proximal ureter. The renal hilum can be defined as the entrance to the renal sinus and is occupied by the renal artery, renal vein, and the proximal ureter. The renal vasculature consists of the main renal artery, which branches into interlobar arteries, followed by arcuate arteries, and finally interlobular arteries. The arcuate arteries course along the base of the medullary pyramids and can be important sonographic landmarks. The renal veins run parallel to the course of the arteries.
The ureters drain the renal pelvis and are retroperitoneal structures that run along the anterior and medial surface of the psoas muscle. In the pelvis, they course anterior to the iliac vessels and enter the bladder at a postero-inferior angle. They are approximately 6 mm in diameter and can be difficult to image through their entire course as they often become obscured by bowel gas. Typically, the ureters are not well visualized unless they are markedly dilated. The three most common places for ureteral obstruction due to kidney stones are: (1) where the renal pelvis begins to drain into the ureter (the ureteropelvic junction or UPJ), (2) as the ureter crosses the pelvic brim, and (3) where the ureter enters the bladder (the uretero-vesicular junction or UVJ).
Ureteral jets are intermittent streams of urine expressed into the bladder by the normal peristalsis of the ureters. While they may be seen with B-mode or gray-scale imaging, power or color-flow Doppler can more easily visualize these jets. When using Doppler, the pulse repetition frequency, also known as scale, needs to be decreased in order to detect these lower-flow velocities. Ureteral jets are seen as bursts of color entering from the base of the bladder and flowing toward its center. These jets occur at regular intervals, approximately every 15–20 seconds in a well-hydrated patient, and typically last under a second (Fig. 10-2). When scanning for ureteral jets, it is important to scan slowly through the bladder, focusing over the trigone, looking for these intermittent pulsatile contractions. If ureteral jets are not visualized, this can suggest the presence of obstruction, particularly if a unilateral jet is seen on the unaffected side. Conversely, documenting the presence of bilateral ureteral jets suggests the lack of significant obstruction.
The bladder is a triangular structure located within the pelvis, directly behind the pubic symphysis. The apex or vertex of the bladder is located anteriorly and is connected to the umbilicus by the median umbilical ligament, which is the remnant of the urachus. The bladder can be further divided into the fundus, body, trigone, and neck. The fundus or base of the bladder faces posteriorly toward the rectum, along the back of which run the seminal vesicles and terminal portions of the vas deferens. The neck of the bladder leads to the urethra. The ureters enter the bladder at a postero-inferior angle. In most people, the bladder is a thin-walled, anechoic, urine-filled structure with a wall that is 3–6 mm thick (see Fig. 10-2). In males, the prostate is located posterior and inferior to the bladder and in females the cervix occupies this posterior position. The prostate normally measures less than 5 cm in its maximum dimension. A diffusely enlarged prostate may be consistent with benign prostatic hypertrophy, while an asymmetric or irregular deformity of the prostate may be more suggestive of possible malignancy. Ultrasound does not reliably differentiate between hypertrophy and carcinoma; therefore, any concerning abnormalities should be referred for further management.
Imaging Tips and Protocol
In a patient with suspected pathology, the sonographer should begin with the unaffected side, in order to have a baseline image to compare the potentially abnormal kidney to. The patient should be placed in a supine or lateral decubitus position to help bring the kidneys out from beneath the ribs. The kidneys should be viewed primarily in a coronal plane, although other planes may be helpful. The right kidney is more easily visualized due to the large acoustic window provided by the liver. In order to obtain a coronal view, the probe should be placed in the mid-to-anterior axillary line between the eighth and eleventh rib spaces with the transducer pointing toward the patient’s head (Fig. 10-3a). The left kidney is more difficult to visualize due to its slightly more posterior and superior location. The probe position on this side should be adjusted accordingly and placed in the posterior axillary line between the sixth and ninth rib spaces (Fig. 10-3b). Oftentimes, the lateral decubitus position or the use of a phased-array probe to image between the ribs is required to adequately image the left kidney. The kidneys often lie somewhat obliquely; therefore, in order to obtain an adequate long-axis view the transducer may need to be rotated slightly from the coronal plane. The transducer may be rotated 90° from the coronal position in order to obtain a transverse view. The kidney should be visualized in its entirety from the superior pole down to the inferior pole and from anterior to posterior surfaces.