DIAGNOSTIC ULTRASOUND

SUSANNA L. OVEL

OUTLINE

Principles of diagnostic ultrasound

Historical development

Physical principles

Anatomic relationships and landmarks

Clinical applications

Cardiologic applications

Conclusion

Definition of terms

Principles of Diagnostic Ultrasound

Diagnostic medical sonography is a general term used to encompass abdominal, breast, cardiac, gynecologic, obstetric, and vascular sonography. Registered diagnostic medical sonographers (RDMS) specialize in abdominal sonography, obstetrics/gynecology imaging, breast sonography, or neonatal neurosonography. Registered diagnostic cardiac sonographers (RDCS) specialize in fetal, pediatric, or adult echocardiography. Registered vascular technologists (RVT) specialize in abdominal vasculature imaging, imaging of arteries and veins of the upper and lower extremities, imaging of extracranial arteries and veins, transcranial duplex sonography, and physiologic vascular testing in pediatric and adult patients. One overall physics examination is required encompassing sonographic principles, hemodynamics, and instrumentation for all these specialties.

Diagnostic medical sonography employs high-frequency transducers ranging from 2.0 to 30.0 MHz. The transducer emits short pulses of ultrasound (pulse waves) into the human body. Real-time reflections or frequency shifts from structures or vessels along the sound waves path are received by the transducer and displayed as a gray-scale, color Doppler, spectral, or duplex image. Velocity of the red blood cells can be calculated using the Doppler technique. Pulse wave, continuous wave, and color Doppler techniques show blood flow direction, flow resistance and turbulence within the vessel, and regurgitation of the cardiac chamber.

Diagnostic medical sonography has evolved into a unique imaging tool. Sonography was previously thought to be a completely noninvasive technique; however, with the introduction of intracavity and intraluminal transducers, collection of diagnostic data of the pelvic and cardiovascular regions has been shown to improve patient management and care.

CHARACTERISTICS OF DIAGNOSTIC MEDICAL SONOGRAPHERS

The diagnostic medical sonographer uses complicated equipment, independent judgment, and systematic problem-solving skills to acquire quality images and technical data for assistance in a patient’s diagnosis, management, and care. Integrity and honesty are important qualities in all medical professionals. In sonography, these character traits are crucial because almost 90% of observed data is discarded. A technical report is provided to the reading physician by the sonographer after each examination detailing the size and description of normal and abnormal anatomy or hemodynamics along with possible differential diagnostic considerations.

Physical requirements play an additional role in sonography. Sonographers must be able to aid in moving patients and medical equipment. Attention to the use of proper body mechanics is essential (Fig. 33-1). Repetitive usage injury or syndrome of the neck, shoulder, elbow, wrist, and back have been documented. The sonographer should be in good physical, emotional, and nutritional health and possess a dedication to continual learning. The career can be exciting and rewarding as well as stressful, demanding, frustrating, and occasionally depressing.

Fig. 33-1 Sonographer performing an ultrasound examination. (Courtesy Philips Medical Systems.)

RESOURCE ORGANIZATIONS

Resource organizations devoted exclusively to ultrasound include the American Society of Echocardiography (ASE), the Society of Diagnostic Medical Sonography (SDMS), the American Institute of Ultrasound in Medicine (AIUM), the Society of Radiologists in Ultrasound (SRU), and the Society of Vascular Ultrasound (SVU).

The development of sonar^* was the precursor to medical ultrasound. Sonar equipment was initially constructed for defense efforts during World War II to detect the presence of submarines. Various investigators later proved that ultrasound had a valid contribution to make to medicine.

In 1947, Dussick positioned two transducers on opposite sides of the head to measure ultrasound transmission profiles. He also discovered that tumors and other intracranial lesions could be detected by this technique. In the early 1950s, Dussick with Heuter, Bolt, and Ballantyne continued to use through-transmission techniques and computer analysis to aid in diagnosis of brain lesions in the intact skull. They discontinued their studies, however, after concluding that the technique was too complicated for routine clinical use.

In the late 1940s, Howry (a radiologist), Wild (a diagnostician interested in tissue characterization), and Ludwig (interested in reflections from gallstones) independently showed that when ultrasound waves generated by a piezoelectric crystal transducer were transmitted into the human body, these waves would be returned to the transducer from tissue interfaces of different acoustic impedances. At this time, research efforts were directed toward transforming naval sonar equipment into a clinically useful diagnostic tool. In 1948, Howry developed the first ultrasound scanner, consisting of a cattle watering tank with a wooden rail anchored along the side. The transducer carriage moved along the rail in a horizontal plane, and the object to be scanned and the transducer were positioned inside the water tank.

Echocardiographic techniques were developed by Hertz and Edler in 1954 in Sweden. These investigators were able to distinguish normal heart valve motion from the thickened, calcified valve motion seen in patients with rheumatic heart disease. In 1957, an early obstetric contact-compound scanner was built by Brown and Donald in Scotland. This scanner was used primarily to evaluate the location of the placenta and to determine the gestational age of the fetus.

Further developments resulted in the real-time ultrasound instrumentation used in hospitals and clinics at the present time. High-frequency transducers with improved resolution now allow the sonographer to accumulate several images per second at a rate of up to 30 frames per second. Diagnostic ultrasound as used in clinical medicine has not been associated with any harmful biologic effects and is generally accepted as a safe modality.

Physical Principles

PROPERTIES OF SOUND WAVES

Sound waves are traveling variations of pressure, density, and particle motion. Matter must be present for sound to travel; it cannot travel through a vacuum. Sound carries energy, not matter, from one place to another. Vibrations from one molecule carry to the next molecule along the same axis. These oscillations continue until friction causes the vibrations to cease.

Ultrasound refers to sound waves beyond the audible range (>20 kHz). Diagnostic medical sonography can use frequencies of 2 to 30 MHz.

Acoustic impedance

Sound travels through tissues at different speeds depending on the density and stiffness of the medium. Acoustic impedance of a medium determines how much of the wave transmits to the next medium (Fig. 33-2).

Fig. 33-2 Relationship among incident, reflected, and transmitted waves.

Velocity of sound

Propagation speed is the speed with which a sound wave travels through a medium. It is determined by the density and stiffness of a medium. In soft tissue, the propagation speed of sound is 1540 m/sec. Bone shows a very high propagation speed (4080 m/sec), whereas air shows the lowest propagation speed (330 m/sec).

TRANSDUCER SELECTION

Imaging transducers routinely operate in a frequency range of 2 to 16 MHz. Transducers may be linear, convex, sector, or vector in construction (Fig. 33-3). Higher frequencies are used in intracavity and intraluminal transducers and for visualizing the extremities or superficial structures. Lower frequencies are needed for deeper structures of the thoracic cavity, abdomen, and pelvis. Lower frequencies provide necessary penetration depth at the expense of detail resolution.

Fig. 33-3 Various ultrasound transducers. (Courtesy Philips Medical Systems.)

Pulse wave transducers convert electrical energy into acoustic energy during transmission and acoustic energy into electrical energy for reception. A continuous wave transducer produces a continuous wave of sound and is composed of a separate transmit and receiver element within a single transducer assembly. Diagnostic ultrasound transducers operate on the principle of piezoelectricity. The piezoelectric effect states that some materials produce a voltage when deformed by an applied pressure.

VOLUME SCANNING AND THREE-DIMENSIONAL AND FOUR-DIMENSIONAL IMAGING

Volume scanning allows for quick “sweeps” of specific areas of the body or fetus. These sweeps give volume data that can be rendered even after the patient has left the ultrasound department. Threedimensional imaging systems allow the sonographer to acquire volume data. The sonographer can reconstruct these data into a three-dimensional image on the ultrasound machine or at a workstation. With four-dimensional imaging, the ultrasound system is able to acquire and display three-dimensional images in real time.

Anatomic Relationships and Landmarks

The use of anatomic landmarks to define specific areas of the human body is an important part of the imaging and orientation skills of the sonographer. The middle hepatic vein is a sonographic landmark used to locate the division between the left and right hepatic lobes (Fig. 33-4, A). The main lobar fissure is used to locate the gallbladder fossa (Fig. 33-4, B). The ovaries are located medial and anterior to the iliac vessels (Fig. 33-4, C). The use of anatomic landmarks is a routine part of many sonographic examinations.

Fig. 33-4 A, Transverse sonogram of liver showing middle hepatic vein (MHV) dividing left and right hepatic lobes. LHV, left hepatic vein; RHV, right hepatic vein. B, Sagittal image of main lobar fissure (MLF) and its relationship to gallbladder (GB). C, Sagittal color Doppler image of right ovary lying anterior and medial to iliac vessels. (Courtesy Paul Aks, BS, RDMS, RVT.)

Clinical Applications

CHARACTERISTICS OF THE SONOGRAPHIC IMAGE

The sonographer uses specific terms to characterize the sonographic image. If the echo pattern is similar throughout a structure or mass, it is termed homogeneous (Fig. 33-5, A). If the echo pattern is dissimilar throughout a structure or mass, it is termed heterogeneous (Fig. 33-5, B). Internal composition of a structure or mass is described using the terms anechoic (without internal echoes), echogenic (with internal echoes), and complex (containing anechoic and echogenic regions) (Fig. 33-6). Descriptive terms are also used by the sonographer to describe the borders of a mass. Are the borders smooth or irregular; thin or thick; calcified or dilated?

Fig. 33-5 A, Sagittal sonogram of normal homogeneous liver. B, Transverse sonogram of heterogeneous hepatic lobe in a patient with a history of colon carcinoma. (Courtesy Paul Aks, BS, RDMS, RVT.)

Fig. 33-6 A, Endovaginal sonogram of anechoic ovarian cyst. B, Echogenic mass is measured in upper inner quadrant of right breast. C, Transverse sonogram of complex thyroid mass. (Courtesy Paul Aks, BS, RDMS, RVT.)

Imaging artifacts are an additional concern for the sonographer. Acoustic artifacts include reflections that are missing; not real; improperly positioned; or of improper brightness, number, shape, or size (Fig. 33-7). Understanding the assumptions of the ultrasound system and the physical principles of sound waves, the sonographer is better able to comprehend the real-time images.

Fig. 33-7 A, Breast carcinoma showing posterior acoustic shadowing (arrow). B, Mirror image of anterior tibial artery.

ABDOMEN AND RETROPERITONEUM

The abdominal ultrasound examination generally includes a survey of the liver, pancreas, gallbladder, spleen, great vessels, and kidneys in the sagittal and transverse planes (Figs. 33-8 and 33-9). Specific protocols are followed to image size, shape, and echogenicity of the organ parenchyma and anatomic relationships of the surrounding structures. Doppler flow patterns of the upper abdominal blood vessels may be included. Patients are examined in two different body positions (i.e., supine and decubitus). The use of two positions shows mobility of gallstones and repositions interfering bowel gas. Air reflects most of the sound wave, making visualization of the abdominal and retroperitoneal structures difficult. Abdominal examinations are typically scheduled in the morning with the patient fasting 6 to 8 hours before the sonogram.

Fig. 33-8 A, Transverse sonogram of right upper quadrant over right lobe of liver. B, Line drawing of gross anatomic section. C, Gross anatomic section at approximately same level as A.

Fig. 33-9 A, Sagittal sonogram of right upper quadrant over medial segment of left lobe of the liver, hepatic vein, and inferior vena cava. B, Line drawing of gross anatomic section. C, Gross anatomic section at approximately same level as A.

The retroperitoneal ultrasound examination includes a survey of the great vessels, kidneys, and bladder in the sagittal and transverse planes before and after voiding. Specific protocols are followed to image the size, shape, cortical thickness, and echogenicity of the renal parenchyma. Diameters of the inferior vena cava, aorta, and common iliac arteries are measured and documented. Doppler flow patterns of the great vessels and kidneys may be included. Retroperitoneum examinations can be scheduled in the morning or afternoon with the patient drinking 8 to 16 oz. of water 1 hour before the sonogram.

To produce an adequate survey of the abdominal and retroperitoneal cavities, the sonographer must have an understanding of the patient’s clinical history. Although ultrasound cannot diagnose the specific pathology of a lesion or condition, a complete clinical picture may lead to more specific differential diagnostic considerations.

Liver and biliary tree

Sonographic examinations of the liver and biliary tree are generally requested in patients with right upper quadrant pain or elevations in liver function laboratory tests. The liver is assessed for size and echogenicity of the parenchyma. Under normal circumstances, the liver parenchyma appears moderately echogenic and homogeneous. Some types of liver pathologies shown on ultrasound include fatty infiltration, cirrhosis, cavernous hemangioma, and hepatoma (Fig. 33-10). Doppler evaluation of the hepatic artery, hepatic veins, and portal veins is included with a patient history or suspicion of cirrhosis, portal hypertension, portal vein thrombosis, and Budd-Chiari syndrome.

Fig. 33-10 A, Transverse sonogram of liver shows a complex mass in the left lobe. B, Sonogram of liver shows fatty infiltration with small area of normal liver parenchyma anterior to porta hepatis (arrow). (A, Courtesy Paul Aks, BS, RDMS, RVT.)

The biliary tree includes the gallbladder and the intrahepatic and extrahepatic bile ducts. The gallbladder is evaluated for size, wall thickness, and absence of internal echoes. Under normal circumstances, the gallbladder is a pear-shaped anechoic structure located in the gallbladder fossa on the posterior surface of the liver (Fig. 33-11). The intrahepatic biliary ducts converge near the porta hepatis forming the common hepatic duct. The cystic duct joins the common hepatic duct to form the extrahepatic common bile duct. The biliary tree is evaluated for size and evidence of intraductal stones or masses. Some abnormalities of the biliary tree shown on ultrasound include intrahepatic and extrahepatic biliary obstruction, cholelithiasis, and cholecystitis (Fig. 33-12).

Fig. 33-11 Sagittal sonogram of normal gallbladder (GB).

Fig. 33-12 A, Sagittal sonogram of gallbladder showing multiple small gallstones with posterior acoustic shadowing. B, Transverse sonogram of acute cholecystitis.

Pancreas

The pancreas is an elongated organ oriented in a transverse oblique plane in the epigastric and left hypochondriac regions of the retroperitoneal cavity. The head of the pancreas lies in the descending portion of the duodenum and lateral to the superior mesenteric artery. The body is the largest portion, lying anterior to the superior mesenteric artery and splenic vein (Fig. 33-13). The tail is the superiormost portion lying posterior to the antrum of the stomach and generally extends toward the splenic hilum. The echogenicity of the pancreas varies depending on the amount of fat but should appear homogeneous throughout the organ. Ultrasound examinations of the pancreas are requested in patients with a history of unexplained weight loss, epigastric pain, and elevation in pancreatic enzymes or liver function laboratory tests. The pancreas is evaluated for size and echogenicity of the parenchyma. The distal common bile duct is routinely measured in the head of the pancreas. Some abnormalities of the pancreas shown on ultrasound include inflammation, calcifications, tumor, or abscess formation (Fig. 33-14).