Diagnostic Ultrasound Student Handout PDF

DIAGNOSTIC ULTRASOUND STUDENT HANDOUT 1. HISTORICAL DEVELOPMENT OF ULTRASOUND The development of sonar (Sound Navigation and Ranging) was the precursor to medical ultrasound. Sonar equipment was initially constructed for the defense effort during World War II to detect...

DIAGNOSTIC ULTRASOUND STUDENT HANDOUT 1. HISTORICAL DEVELOPMENT OF ULTRASOUND The development of sonar (Sound Navigation and Ranging) was the precursor to medical ultrasound. Sonar equipment was initially constructed for the defense effort during World War II to detect the presence of submarines. Various investigators later proved that ultrasound had a valid contribution to make in medicine. In 1947 Dr. Karl Theodore Dussick positioned two transducers on opposite sides of the head to measure ultrasound transmission profile. He also discovered that tumors and other intracranial lesions could be detected by this technique. Initial clinical applications monitored changes in the propagation of pulses through the brain to detect intracerebral hematoma and brain tumors based on the displacement of the midline. In the early 1950s Dussick, with Heuter, Bolt, and Ballantyne, continued to use through-transmission techniques and computer analysis to diagnose brain lesions in the intact skull. However, they discontinued their studies after concluding that the technique was too complicated for routine clinical use. In the late 1940s Douglas Howry (a radiologist), John Wild (a diagnostician interested in tissue characterization), and George Ludwig (interested in reflections from gallstones) independently demonstrated 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 interface 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, while 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. Then in 1957 an early obstetric contact-compound scanner was built by Tom Brown and Ian 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 have resulted in the real-time ultrasound instrumentation used in hospitals and clinics today. 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. Ultrasound rapidly progressed through the 1960s from simple "A-mode" scans to "B- mode" applications and compound "B-scan" images using analog electronics. Advances in equipment design, data acquisition techniques, and data processing capabilities have led to electronic transducer arrays, digital electronics, and real- time image display. This progress is changing the scope of ultrasound and its applications in diagnostic radiology and other areas of medicine. High-resolution, real- time imaging, harmonic imaging, 3D data acquisition, and power Doppler are a few of the innovations introduced into clinical practice. Contrast agents for better delineation of the anatomy, measurement of tissue perfusion, precise drug delivery mechanisms, and determination of elastic properties of the tissues are topics of current research. 2. IMPORTANCE OF ULTRASOUND IN DIAGNOSIS OF DISEASES  Like therapeutic ultrasound, diagnostic ultrasound involves transmission of high- frequency sound waves (5 to 10 MHz) into the tissues by a transducer through a coupling agent (Gel) with calculation of the time it takes for the echo to return to the transducer from different interfaces.  It has the advantage of providing dynamic (moving) real-time images; tissues can be visualized as they move. It also allows localization of any tenderness or palpable mass.  Therefore, it is used to assess soft-tissue injury, such as tendon, ligament, or muscle pathology, soft-tissue masses (e.g., tumor, ganglion, cyst, inflamed bursa), effusion, and congenital dislocation of the hip, and it allows dynamic visualization of muscle.  Clinical Applications:  1. Abdomen and Retroperitoneum Application: The upper abdominal ultrasound examination generally includes a survey of the abdominal cavity from the diaphragm to the level of the umbilicus. Specific protocols are followed to image the texture, borders, anatomic relationships, and blood flow patterns within the liver, biliary system, pancreas, spleen, vascular structures, retroperitoneum, and kidneys.  2. Superficial Structures Applications- such as the thyroid, breast, scrotum, and penis are imaged very well with ultrasound using high-frequency transducer.  3. Neonatal Neurosonography Applications- The premature infant is susceptible to intracranial hemorrhage during the stress of delivery and the struggle to survive.  Sonography is the preferred clinical diagnostic tool to evaluate the premature infant for intracranial hemorrhage, infection, or hunt drainage for ventriculomegaly. Ultrasound examination can detect within the neonatal skull include meningomyelocele, Arnold- Chiari deformity, hydrocephalus or ventriculomegaly, Dandy Walker deformity, agenesis of the corpus callosum, and arteriovenous malformation.  4. Gynecologic Applications- A complete transabdominal examination of the female pelvis includes visualization of the distended urinary bladder, uterus, cervix, endometrial canal, vagina, ovaries, and supporting pelvic musculature.  5. Obstetric Applications- Endovaginal ultrasound is the procedure of choice during the first trimester of pregnancy to delineate the gestational sac with the embryo, yolk sac, chorion, and amniotic cavities. The gestational sac may be visualized as early as 4 weeks from the date of conception with endovaginal ultrasound. The embryo, heartbeat, and site of the placenta may be seen at 5 weeks of gestation.  6. Vascular Applications- The use of ultrasound and color flow Doppler has enhanced the ability to image peripheral vascular structure in the body. The ultrasound facilitates good visualization of the common femoral artery and vein and their branches as they extend into the calf. Thrombus within a distended venous structure is identified when the sonographer is unable to compress the vein with the transducer. Color flow Doppler is also useful for denoting an absence of flow within a vessel.  7. Cardiologic Applications- Real-time echocardiography of the fetal, neonatal, pediatric, and adult heart has proven to be a tremendous diagnostic aid for the cardiologist and internist. Echocardiography is used to evaluate many cardiac conditions. Atherosclerosis or previous rheumatic fever may lead to scarring, calcification, and thickening of the valve leaflets. With these conditions, valve tissue destruction continues, causing stenosis and regurgitation of the leaflets and subsequent chamber enlargement. Echocardiography has been used to diagnose congenital lesions of the heart in fetus, neonates, and young children. The cardiac sonographer is able to assess abnormalities of the four cardiac valves, determine the size of the cardiac chambers, assess the interatrial and interventricular septum for the presence of shunt flow, and identify the continuity of the aorta and pulmonary artery with the ventricular chambers to look for abnormal attachment relationships  Doppler ultrasound may be used for vascular assessment. The full bladder helps to push the small bowel superiorly out of the pelvic cavity, flattens the body of the uterus, and serves as a sonic window to image the pelvic structures. Sonography of the pelvis is clinically useful for imaging normal anatomy, identifying the size of ovarian follicles as part of an infertility workup, measuring endometrial thickness, evaluating the texture of the myometrium, determining if a pregnancy is intrauterine or extrauterine, detecting tumors or abscess formations, and localizing an intrauterine contraceptive device  A Doppler ultrasound is a test that uses high-frequency sound waves to measure the amount of blood flow through your arteries and veins, usually those that supply blood to your arms and legs. Vascular flow studies, also known as blood flow studies, can detect abnormal flow within an artery or blood vessel.   The disadvantages of diagnostic ultrasound include limited contrast resolution, limited depth of penetration, small viewing field, and lack of penetration of bone. 3. BIOLOGICAL EFFECTS Diagnostic ultrasound has established a remarkable safety record. Significant deleterious bioeffects on either patients or operators of diagnostic ultrasound imaging procedures have not been reported in the literature. Despite the lack of evidence that any harm can be caused by diagnostic intensities of ultrasound, it is prudent and indeed an obligation of the physician to consider issues of benefit versus risk when performing an ultrasound exam, and to take all precautions to ensure maximal benefit with minimal risk. At high intensities, ultrasound can cause biologic effects by thermal and mechanical mechanisms. Biologic tissues absorb ultrasound energy, which is converted into heat; thus, heat will be generated at all parts of the ultrasonic field in the tissue. Thermal effects are dependent not only on the rate of heat deposition in a particular volume of the body, but also on how fast the heat is removed by blood flow and other means of heat dissipation. Thermal and mechanical indices of ultrasound operation are now the accepted method of determining power levels for real-time instruments that provide the operator with quantitative estimates of power deposition in the patient. These indices are selected for their relevance to risks from biologic effects and are displayed on the monitor during real-time scanning. The sonographer can use these indices to minimize power deposition to the patient (and fetus) consistent with obtaining useful clinical images in the spirit of the ALARA (as low as reasonably achievable) concept. The thermal index (TI), is the ratio of the acoustic power produced by the transducer to the power required to raise tissue in the beam area by 1°C. Cavitation is a consequence of the negative pressures (rarefaction of the mechanical wave) that induce bubble formation from the extraction of dissolved gases in the medium. The mechanical index (MI) is a value that estimates the likelihood of cavitation by the ultrasound beam.