Technique and radiation

No specific preparation is required for US sonography. It is beyond the scope of this book to describe the vast array of sono-graphic abnormalities, and therefore only brief principles will be discussed.

• Kidneys and retroperitoneum can be examined in the supine, prone, or decubitus position

• The right kidney is best approached with the patient in a supine position and using the liver as a "viewing window"

• The left kidney is optimally seen with the patient in the right decubitus position with the spleen as an acoustic window

• All other urinary organs, except for the prostate, can be scanned with the patient in the prone position. Trans-rectal USS of the prostate is discussed in a later chapter

The use of color doppler sonography can demonstrate pathologies affecting arterial and venous flow, such as malformations, occlusions, and aneurysms. This is particularly useful in investigation of—

• Kidneys (renal blood flow, renal artery stenosis, renal vein, tumor thrombus). (NB. Fasting for 6-8 hours is helpful for examination of renal arterial flow)

• Scrotal contents (testicular artery blood flow)

• Penile vasculature (cavernosal blood flow)

Finally, USS poses no radiation risk and is the investigation of choice for repeated examinations.

Contrast-enhanced ultrasound

Contrast-enhanced ultrasound (CEUS) is a rapidly expanding field and has been extensively used in the diagnosis and characterization of hepatic lesions. However, due to the inherent vascular nature of the kidneys, CEUS allows excellent spatial and temporal resolution, as well as assessment of renal perfusion.

Contrast agents (SonoVue, Levovist, Optison, Definity, Son-azoid) are intravenously injected compounds which consist of microbubbles containing a gas (air or hydrophobic gas) stabilized by a shell. Thus the agents acting as vascular tracers neither leave the blood vessels nor are subjected to renal filtration, and therefore dramatically increase the signal intensity from blood. Although modern contrast agents are more stable, the micro-bubble shells are thin and fragile, breaking easily as they oscillate under the influence of the ultrasonic accoustic pressure wave. Even the destruction of the microbubbles can aid in their detection and allow for quantification of renal perfusion. Contrast agents are—

• Can be given in a small bolus dose or as a continuous infusion

• Independent of renal function

• Non-nephrotoxic

• Rarely cause allergic reactions

It is likely that given time, CEUS will become more readily available than CT or MR. Disadvantages include the requirement of dedicated contrast-specific software to exploit the many possibilities of CEUS fully and the inability of contrast agents to concentrate in urine, as in IVU and CT. Possible urological applications include—

• Vascular applications

1. Renal artery stenosis (RAS)—the use of CEUS, rather than doppler will readily demonstrate the entire renal vascula-ture, including small parenchymal vessels, and has been shown to improve the diagnosis of RAS by enhancing Doppler signals, shortening examination time and decreasing the number of unsuccessful studies

2. Renal vein thrombosis—a small thrombus overlooked by Doppler, may be better demonstrated by CEUS

3. To demonstrate renal blood flow and quantify renal perfusion in the transplanted kidney as well as following suspected renal trauma

4. Parenchymal perfusion—a well perfused parenchyma will exhibit intense contrast-aided enhancement. Therefore CEUS is highly sensitive for the detection of non-perfused lesions including renal cysts, lacerations, infarctions (local underperfusion) and severe pyelonephritis (global underperfusion)

• Focal renal lesions—Small and/or indeterminate focal renal lesions detected by CT or MR are not uncommon in urological practice. CEUS may be superior to conventional USS in the evaluation of such lesions. The absence of any enhancement in a clearly defined cystic lesion would be virtually diagnostic of a benign pathology, due to the lack of blood vessels. Contrast enhancement within renal tumors is often indistinguishable from surrounding parenchyma, but abnormal vascularity frequently seen in tumors is readily identified

• Pediatric—Contrast-enhanced voiding ultrasonography using intravesical Levovist has been demonstrated to have a diagnostic efficacy comparable to standard micturiting cysto-urethrogram (concordance rate 91%), while avoiding the need for ionizing radiation

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