Sound is the mechanical propagation of pressure changes, or waves, through a deformable medium. A wave frequency of one cycle/second (cps) is called a hertz (Hz). Sound frequencies greater than 20 kHz are beyond the range of human hearing and are called ultrasound. Medical sonography uses ultrasound to produce images (Figures 6–22, 6–23, 6–24, 6–25, 6–26, and 6–27). The frequencies commonly used in medical sonography are between 3.5 and 10 MHz.
Ultrasound waves for imaging are generated by transducers, devices that convert electrical energy to sound energy and vice versa. These transducers are special piezoelectric crystals that emit ultrasonic waves when they are deformed by an electrical voltage and, conversely, generate an electrical potential when struck by reflected sound waves. Thus, they act as both sonic transmitters and detectors. In general concept, medical sonography resembles naval submarine sonar. Ultrasound images are reflection images formed when part of the sound that was emitted by the transducer bounces back from tissue interfaces to the transducer. The sound reflected by stationary tissues forms the data set for anatomic gray-scale images. The sound reflected by moving structures (eg, flowing blood in a vessel) has an altered frequency due to the Doppler effect. By determining the Doppler shift, vascular flow direction and velocity can be encoded graphically (spectral Doppler) or by color (color Doppler). A more sensitive method of detecting flow, called power mode Doppler, is available on modern equipment. This technique displays the integrated power of the Doppler signal rather than the mean Doppler frequency shift. Direction or velocity of flow is not displayed in the power mode.
Reflected sound received by the transducer is converted into electrical signals that are analyzed by computer algorithms and rapidly converted into video images viewed directly on a real-time display. Images are rapidly updated on the display, giving an integrated cross-sectional anatomic depiction of the site studied. Individual frames may be frozen during an examination for motion-free analysis and recording, or continuous images may be recorded as digital or conventional video.
Ultrasound is commonly used for the evaluation of the kidney, urinary bladder, prostate, testis, and penis.
Ultrasound is useful for assessing renal size and growth. It is also helpful in triage for patients with renal failure. For example, small echogenic kidneys suggest renal parenchymal (medical) disease, whereas a dilated pelvocaliceal system indicates an obstructive, and potentially reversible, cause of renal failure.
Renal ultrasound is useful in detection and characterization of renal masses. Ultrasound provides an effective method of distinguishing benign cortical cysts from potentially malignant solid renal lesions. Since the most common renal lesion is a simple cortical cyst, ultrasound is a cost-effective method to confirm this diagnosis. Ultrasound may also be used to follow up mildly complicated cysts detected on CT; for example, hyperdense cysts or cysts with thin septations.
The differential diagnosis for echogenic renal masses includes renal stones, angiomyolipomas, renal cell carcinomas, and, less commonly, abscesses and hematomas. All echogenic renal masses should be correlated with clinical history and, if necessary, confirmed with another imaging modality or follow-up ultrasound. Thin-section CT showing fat within the renal lesion characterizes it as a benign angiomyolipoma, and no further investigation is required. Echogenic lesions smaller than 1 cm are more difficult to characterize by CT owing to partial volume averaging; in the correct clinical setting, follow-up ultrasound rather than repeat CT scan may be more useful.
Doppler sonography is useful for the evaluation of renal vessels, vascularity of renal masses, and complications following renal transplant. It can detect renal vein thrombosis, renal artery stenosis, and ureteral obstruction prior to the development of hydronephrosis, arteriovenous fistulas, and pseudoaneurysms. Perinephric fluid collections following renal transplantation, extracorporeal shock wave lithotripsy, or acute obstructions are reliably detected by ultrasound.
Developments in other imaging modalities have decreased the use of ultrasound in several clinical scenarios. Most patients with suspected renovascular hypertension are evaluated with CTA, MRA, or radionuclide renography rather than Doppler ultrasonography. Unenhanced helical CT is now the initial procedure of choice for the evaluation of the patient with acute flank pain and suspected urolithiasis. In addition to rapidly and sensitively detecting renal stones without the need for intravenous contrast medium, helical CT also has the potential for identifying other causes of flank pain, such as appendicitis and diverticulitis. In the past, a combination of KUB and ultrasound was advocated for the evaluation of hematuria, but recent studies indicate that IVU, CT (CTU), or both are the preferred modalities to evaluate this common clinical problem.
Applications of bladder sonography include assessment of bladder volume and bladder wall thickness and detection of bladder calculi and tumors. The suprapubic, transabdominal approach is most commonly used. The transurethral approach during cystoscopy has been recommended for tumor detection and staging.
Ultrasound examination of the testis has become an extension of the physical examination. The superficial location of the testis allows the use of a high-frequency transducer (5–10 MHz), which produces excellent spatial resolution. The addition of color Doppler sonography provides simultaneous display of morphology and blood flow. Normal intratesticular arterial blood flow is consistently detected with power or color Doppler. Sonography is highly accurate in differentiating intratesticular from extratesticular disease and in the detection of intratesticular pathology. Ultrasound is commonly used to evaluate acute conditions of the scrotum. It can distinguish between inflammatory processes, inguinal hernias, and acute testicular torsion. In addition, epididymitis not responding to antibiotics within 2 weeks should be investigated further with scrotal ultrasonography.
Advantages & Disadvantages
The main advantages of ultrasound are ease of use, high patient tolerance, noninvasiveness, lack of ionizing radiation, low relative cost, and wide availability. Disadvantages include a relatively low signal-to-noise level, tissue nonspecificity, limited field of view, and dependence on the operator's skill and the patient's habitus.