Ultrasound of Kidneys/Ureters/Bladder
Ultrasonography of Urinary Tract and Male General System
Ultrasonography is a painless and noninvasive method of using high-frequency sound waves to image anatomic structures and provides valuable information regarding renal morphology and perfusion. A brief pulse of high-frequency sound energy produced by a transducer is transmitted into the patient. The sound waves interact with the tissue and are either reflected, refracted, or absorbed, depending on the type of tissue involved. For example, 100% is reflected by air and 100% is absorbed by bone. The transducer then becomes a receiver detecting the returning echoes of sound energy from tissue. The depth of any particular echo is determined by measuring the round-trip time of flight for the transmitted pulse and the returning echo and by calculating the depth of the reflecting tissue interface by assuming an average speed of transmission of sound in body tissue of 1540 m/sec. The composite image is computer generated by interrogating tissue in the field of view with multiple, closely spaced ultrasound pulses. Sonographic images may be produced in any anatomic plane by adjusting the orientation and angulation of the transducer and the position of the patient. Ultrasound signals may be displayed in several ways including gray-scale, duplex Doppler, and color and power-mode Doppler to provide diagnostic information. Usually, a midrange (3.5 to 5 MHz) sector or convex transducer is used in adults. Higher-frequency transducers (>,5 MHz) yield the greatest spatial resolution, although restricted by limited penetration, and are effective in most children and for the evaluation of superficial structures such as the scrotum.
Ultrasonography has many advantages including availability, flexibility, lack of ionizing radiation, and accurate anatomic and, sometimes, physiologic information obtained without the need for intravascular contrast agents. Ultrasonography is not function dependent. Characteristics of blood flow can be determined. Ultrasonography also has limitations, with respect to the kidney, it has inferior resolution compared with EXU. It also provides no functional information. Certain structures cannot be visualized, for example, the nondilated abdominal ureter and most of the retroperitoneum are not easily visualized. Ultrasonography depends on both the operator`s experience and the quality of the ultrasonographic equipment to produce consistently high-quality studies. The examination may be limited by the patient`s body habitus and the presence of intervening bowel gas shadows and ribs, particularly in imaging of the left inferior renal polar region. The examination may also be limited by surgical wounds, dressings, and skin lesions, which preclude firm transducer contact with the skin.
The mainstay of ultrasound imaging is a real-time, two-dimensional, gray-scale display. With this technique, variations in the amplitudes of the echoes arising from tissues are displayed as varied shades of gray on a monitor. Fluid-containing structures such as cysts, dilated calyces, and ureters, and the distended bladder characteristically demonstrate well-defined walls, an absence of internal echoes (anechoic), and distal acoustic enhancement. Solid tissue typically demonstrates a speckled pattern of tissue texture with definable blood vessels. Solid organs such as kidneys demonstrate lower echo levels of echogenicity (lower-intensity echoes), whereas fat is usually highly echogenic. Lesions within or arising from organs demonstrate a mass effect with alteration of organ contour and displacement of blood vessels and with alteration in tissue texture. Lesions of lower echogenicity than surrounding parenchyma are termed hypoechoic, lesions of greater echogenicity than surrounding parenchyma are termedhyperechoic, and lesions of echogenicity similar to surrounding parenchyma are termed isoechoic. The vast majority of renal masses in adults are simple renal cysts. If the sonographic criteria of a simple cyst are present, no further work-up is necessary. In other renal masses, there is an overlap in echo characteristics between benign and malignant processes.
Duplex, Color, and Power Doppler
Doppler ultrasonography is an important adjunct to real-time gray-scale imaging. The Doppler effect is a physiologic phenomenon that demonstrates an apparent shift in frequency of a wave reflecting from a moving object and is proportionate to the speed of that object relative to the observer. The Doppler effect allows measurement of the velocity of a moving object, such as blood flow, and determines the flow direction by applying an ultrasound beam. Pulsed duplex Doppler ultrasonography allows the operator to display the flow in an area on the corresponding gray-scale image as a continuous time-velocity waveform (spectrum). Doppler ultrasound waveform analysis of the renal artery is used for the evaluation of renal arterial stenosis. A delay in the systolic upstroke downstream is referred to as the tardus-parvus waveform, indicative of renal artery stenosis. When a color-flow device is used, a color-encoded Doppler signal is superimposed on a gray-scale image (color Doppler). Power (energy) Doppler sonography, a modification of standard color Doppler ultrasonography, is a newer technique that determines the amplitude of the Doppler frequency shift instead of the mean frequency shift, as in conventional color Doppler imaging. The color sensitivity is improved without compromising image qualityand is three to five times more sensitive in depicting flow in the intrarenal arteries than conventional color Doppler ultrasonography. However, in contrast to color Doppler ultrasonography, power Doppler ultrasonography does not provide any information relative to velocity or the direction of blood flow.
Ultrasound contrast agents are injectable agents that undergo a phase change from a liquid to a gas when injected into the blood stream, thus improving the detection of Doppler signals and the depiction of normal and abnormal vascularity. These agents are under investigation for genitourinary applications and have shown promise in characterizing renal vascularity.
Three-dimensional (3D) ultrasonography is a technique that uses a mechanically or manually driven transducer to acquire a volume of data that is then reconstructed and displayed as a 3D reconstruction in any of three orthogonal planes (sagittal, transverse, or coronal) or in any arbitrary oblique plane. 3D ultrasonography has the potential for a more accurate and repeatable evaluation of anatomic structures, and for improved detection of LOCAL lesions. 3D ultrasonography can make accurate volume assessments for radiation treatment planning or for estimating prostate-specific antigen levels. 3D power Doppler imaging is useful in angiographic applications.
In many clinical situations, ultrasonography remains the modality of choice for the kidneys. Renal sonography provides information on the number, size, shape, and location of the kidneys. Hydronephrosis appears as an anechoic or hypoechoic fluid collection that splits the bright central echo of the renal sinus and often assumes the shape of the calyces and renal pelvis (pelvicaliectasis). The presence of a dilated renal pelvis and calyces usually indicates obstructive uropathy in an appropriate clinical setting. False-positive studies, however, are not uncommon and may be due to a large extrarenal pelvis. A large parapyelic cyst may be difficult to differentiate from hydronephrosis, however, careful film analysis with oblique views should make a correct diagnosis possible. In addition, sonography is not a functional study and cannot distinguish obstructive from nonobstructive hydronephrosis. In some cases, further evaluation with functional studies such as EXU or nuclear renal scanning may be indicated. False-negative studies may occur when hydronephrosis has not yet developed in patients with acute extrarenal obstruction. Some authors have advocated the use of spectral Doppler ultrasonography to diagnose both acute and chronic obstruction on the theory that renal obstruction produces a vasoconstriction that can be detected by measuring the resistive index of the intrarenal arteries.
Ultrasonography can detect the presence of a fluid-filled renal mass with greater than 98% accuracy. Because most renal masses are either tumors or cysts, ultrasonography is extremely valuable in their evaluation. The patterns for cysts and solid masses are quite distinctive. Ultrasonography is very sensitive in detecting solid masses, particularly those measuring 2 to 2.5 cm or more in largest diameter. The detection of solid neoplasms is aided by demonstrating blood flow within them by means of color and energy Doppler. This modality also has value in demonstrating IV and intracaval tumor extension of renal neoplasms.
Ultrasonography can outline the kidney and ascertain its depth below the skin, thus facilitating percutaneous renal biopsy with an aspiration biopsy needle guide. Ultrasonography is also very helpful in guiding needles into renal cysts or dilated renal collecting systems for antegrade pyelography in preparation for percutaneous nephrostomy.
Renal pelvic filling defects can be easily assessed by sonography. These are most commonly caused by nonopaque calculi, urothelial neoplasms, blood clots, and inflammatory debris. Renal calculi are markedly echogenic, produce acoustic shadowing, and are readily detected by ultrasonography and therefore easily differentiated from soft tissue such as clot and tumor. The presence of no shadowing debris in the dilated collecting system in patients with urosepsis most likely represents pyonephrosis (infected hydronephrosis).
Ultrasonography can be used to evaluate perinephric fluid collections. Fluid collections around the kidney consist of abscesses, urinomas, hematomas, or, in the case of a transplanted kidney, lymphoceles. Although these fluid collections can be recognized as echo-free masses in the perinephric space or around the kidney, differentiating one from the other is usually not possible by ultrasonography alone.
Sonography is an excellent means of renal surveillance given its low cost, easy availability, and minimal patient inconvenience. Kidneys at risk for the development of specific diseases can be periodically monitored by ultrasonography. Examples are monitoring the contralateral kidney after nephrectomy for Wilms` tumor and evaluation of family members of patients with hereditary disorders such as polycystic disease, tuberous sclerosis, and von Hippel–Lindau disease. The status of existing diseases such as hydronephrosis may also be monitored to determine whether the process is static, improving, or worsening.
Color (and power) Doppler sonography can, for the most part, demonstrate arterial and venous lesions (e.g., occlusions, stenoses, aneurysms, fistulas, malformations) of the extra- and intrarenal arteries and veins, and flow velocities can be estimated by duplex Doppler sonography. In renal arterial stenosis, duplex Doppler ultrasonography demonstrates a high-velocity jet at, or immediately distal to, the area of narrowing in the main renal artery. The Doppler waveform pattern in the intrarenal arteries may show slowed systolic acceleration and diminished systolic velocity peak (tardus and parvus). The Doppler technique also allows ready demonstration of bland and tumor thrombus in the renal vein and inferior vena cava, which are seen as filling defects.
Ultrasonography is the adrenal imaging of choice in infants and children, especially for the detection of neonatal adrenal hemorrhage.
Ultrasonography has limited usefulness in most ureteral disorders but is very successful in a few isolated circumstances. Dilated ureters can be visualized, as can ureteroceles demonstrated within the bladder, and small calculi can be imaged, especially in the ureteropelvic or ureterovesical junction. Color and power Doppler demonstration of asymmetrical jets of urine from the ureteral orifices into the bladder often indicates the presence of ureteral obstruction.
Uses of sonography include the measurement of residual urine, guidance for suprapubic aspiration, staging of bladder tumors, and evaluation of intravesical masses and diverticula. Sonographic evaluation of the bladder both before and after voiding is helpful for determining the presence of debris, masses, and trabeculation.
Prostate and Seminal Vesicles
The prostate can be imaged transabdominally and seen as a rounded homogeneous structure beneath the bladder. Endorectal ultrasonography of the prostate, however, provides the best perspective for imaging. Prostatic ultrasonography is most often performed during image-guided biopsy of the prostate.
The seminal vesicles are paired tubular structures about 3 cm long and 1.5 cm wide. They lie above the base of the prostate and behind the bladder. The seminal vesicles are routinely imaged during prostatic sonography and in the investigation of male infertility.
Scrotum, Testis, and Penis
Ultrasonography of the scrotum is useful in evaluating various acute and chronic conditions. Scrotal ultrasonography aids in the evaluation of testicular torsion, cryptorchidism, trauma, testicular tumors, and epididymitis. Color Doppler sonography provides information on blood flow and can diagnose or rule out testicular torsion and document hyperemia associated with epididymitis and or epididymo-orchitis. Scrotal ultrasonography is also useful in the evaluation of varicoceles. Cryptorchid testes below the inguinal ligament may be evaluated by sonography.
In males with erectile dysfunction, flow in the cavernosal arteries is readily identified, and this technique has become a standard part of the evaluation of the vasculogenic impotence. In priapism, both noninvasive perineal Doppler ultrasound evaluation of the cavernosal arteries and cavernosal blood-gas determinations often obviate the need for internal pudendal arteriography. In patients with priapism, Doppler ultrasonography can differentiate between the nonischemic high-flow type and the ischemic low-flow type and can be repeated for reassessment after treatment.