Ultrasound (Sonography)

Ultrasonography is an imaging technique used in medicine for the imaging of subcutaneous body structures, blood vessels, joints, and internal organs to exclude structural pathologies. This technique is based on the utilization of ultrasound (or high-frequency, inaudible sound waves). In medical imaging, the sound waves have a frequency of 2–18 megahertz (MHz). The equipment utilizes a transducer acting as the emitter and receptor of sound waves, and a central computer processes the electrical signals to generate the image. The general advantages of this type of imaging is its low cost, availability, and safety. Some specialties that rely heavily on ultrasound examination are cardiology, nephrology, general surgery, gastroenterology, emergency medicine, and obstetrics.

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Editorial responsibility: Stanley Oiseth, Lindsay Jones, Evelin Maza

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Terminology and Technical Aspects

Definitions

  • Ultrasound: inaudible sound waves with a frequency of 2–18 megahertz (MHz) when used for medical imaging
  • Ultrasound imaging: the use of ultrasound to generate anatomical images

Core components of an ultrasound machine

  • Transducer (or probe):
    • A device placed on the patient’s body to visualize a target 
    • Acts as an emitter and receptor of sound waves
    • Contains piezoelectric crystals that convert electrical signals into sound waves
    • The reflected sound waves (echoes) travel back to the probe and are converted to electrical signals.
    • Types:
      • Convex (used in fetal imaging)
      • Micro-convex (used in gynecologic imaging)
      • Linear (used in vascular imaging)
      • Phased array (used in thoracic imaging)
    • Frequency is inversely related to wavelength and depth of tissue penetration.
    • Higher frequencies →  detailed image
  • Central processing unit (CPU): processes electrical signals to generate an image
  • Console: 
    • Allows for the manipulation of the images coming from the transducer
    • Activation of M-mode and Doppler

Generation of images with ultrasound

The main principle behind ultrasound imaging is the transmission and reflection of sound waves through the tissues.

  1. Sound waves are emitted by the transducer.
  2. Sound waves penetrate the tissues in the form of a beam.
  3. As the beam travels, it is reflected by structures in the tissues (or echoes) back to the transducer, with some energy being absorbed by the tissues.
    • The amplitude of the echoes depends on the degree of energy absorption of the emitted beam.
    • Absorbed energy from the beam is later released as heat.
  4. The echoes return to the transducer.
  5. The sound waves are turned into electrical signals and then amplified in the console. The signals are assigned a shade of gray depending on the amplitude of the echo produced by the tissue after interacting with the piezoelectric crystals.
    • Higher amplitudes are assigned shades closer to white.
    • Lower amplitudes are assigned shades closer to black.
  6. The CPU processes the electrical signals into images that can be seen on the monitor.
Interaction between ultrasound waves and the tissues (sonography)

Ultrasound waves and the tissues:
The diagram shows that as the ultrasound wave beam (blue horizontal bar) penetrates the tissues, a percentage is reflected back (left arrows) toward the transducer while another continues to go deeper into the tissues (right arrow), losing some energy to the parenchyma as it goes.

Image by Lecturio.

Images

Imaging planes:

  • Sagittal (or longitudinal): along the long axis of the structure being evaluated
  • Transverse: perpendicular to the sagittal plane

Types of images:

  • Static images (photographs)
  • Cine images: captured during real-time scanning

Image definition or sharpness of the image generated can be characterized in terms of:

  • Axial definition: 
    • Differentiation of 2 objects close to each other, parallel to the beam
    • Determines the depth of the ultrasound beam; quality impacted by beam penetration
  • Lateral definition: 
    • Differentiation of 2 objects on a plane perpendicular to the beam
    • Determines the ability of the probe to distinguish structures perpendicular to the beam
    • Primarily determined by the beam width

Image definition is also determined by how close objects are to the transducer; according to their frequencies, probes have a near and a far field of “vision”:

  • Near field: the focal point of the probe with the greatest lateral definition
  • Far field: greater axial definition at the expense of lateral definition

Doppler ultrasound

Doppler ultrasound (or just “Doppler”) is a widely used ultrasound method based on the principle of sound-wave compression and dilation relative to the receptor. Doppler ultrasound is most commonly used to visualize blood flow.

  • When the ultrasound beam comes into contact with the blood, its frequency is shifted, either becoming:
    • Compressed (frequency is increased) by a flow that is coming toward the transducer
    • Dilated (frequency is decreased) by a flow away from the transducer
  • There are different Doppler methods:
    • Spectral Doppler: demonstrates the direction and waveform of flow
    • Power Doppler: a single color is assigned to all areas of flow, which has increased sensitivity to detect slow/lower flow
    • Continuous-wave Doppler: used for measuring high-velocity flow
    • Pulse-wave Doppler: makes measurements on a small segment of the ultrasound beam

Interpretation

The interpretation of ultrasound images is done in real time, while the examination is being performed.

Ultrasound evaluation

  • The sonographer needs to be well familiarized with the presentation of the anatomy in the particular ultrasound method they employ.
  • The sequence of evaluation also depends on its purpose, for example:
    • FAST follows a specific sequence of anatomical landmarks within the abdomen and thorax very quickly in the emergency setting.
    • A biophysical profile is performed in fetuses with suspected growth restriction by measuring their biometric parameters and checking where they lie in the growth curves. 

Terminology

  • Hyperechoic (e.g., surface of bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Structure of Bones, urinary tract Urinary tract The urinary tract is located in the abdomen and pelvis and consists of the kidneys, ureters, urinary bladder, and urethra. The structures permit the excretion of urine from the body. Urine flows from the kidneys through the ureters to the urinary bladder and out through the urethra. Urinary Tract calculi, fat-containing lesions): a structure that produces a high-amplitude echo (lighter grays and white)
  • Hypoechoic (e.g., abscesses without gas, solid tumors without calcifications or fat): a structure that produces a low-amplitude echo (darker grays)
  • Anechoic (e.g., simple cysts): a structure that produces no echo at all (looks completely black)
  • Isoechoic: a structure that produces an echo of a very similar amplitude to its environment and is very difficult to distinguish

Doppler ultrasound

By convention, in color Doppler:

  • Blood flow that is coming toward the probe (compressed sound waves) is depicted in red.
  • Blood flow away from the probe (dilated sound waves) is depicted as blue.

Artifacts

Artifacts are artificial objects produced by the equipment’s misinterpretation of sound-wave data coming back from the tissues that do not represent actual structures. 

Some examples of artifacts are:

  • Enhancement: Echoes from structures behind hypoechoic/anechoic objects appear brighter.
  • Shadowing: Echoes from structures behind denser objects appear darker or are not visualized at all. 
  • Reverberation: Echos are trapped between two hyperechoic objects and bounce back and forth several times.
Ultrasound (sonography) of acute cholecystitis

Ultrasound from a patient with acute cholecystitis Cholecystitis Cholecystitis is the inflammation of the gallbladder (GB) usually caused by the obstruction of the cystic duct (acute cholecystitis). Mechanical irritation by gallstones can also produce chronic GB inflammation. Cholecystitis is one of the most common complications of cholelithiasis but inflammation without gallstones can occur in a minority of patients. Cholecystitis:
Multiple gallstones are visualized within the gallbladder Gallbladder The gallbladder is a pear-shaped sac, located directly beneath the liver, that sits on top of the superior part of the duodenum. The primary functions of the gallbladder include concentrating and storing up to 50 mL of bile. Gallbladder and Biliary Tract lumen with gallbladder Gallbladder The gallbladder is a pear-shaped sac, located directly beneath the liver, that sits on top of the superior part of the duodenum. The primary functions of the gallbladder include concentrating and storing up to 50 mL of bile. Gallbladder and Biliary Tract wall thickening and pericholecystic fluid. Shadowing can be seen behind the gallstone.

Image: “Ultrasound of a patient with acute cholecystitis Cholecystitis Cholecystitis is the inflammation of the gallbladder (GB) usually caused by the obstruction of the cystic duct (acute cholecystitis). Mechanical irritation by gallstones can also produce chronic GB inflammation. Cholecystitis is one of the most common complications of cholelithiasis but inflammation without gallstones can occur in a minority of patients. Cholecystitis” by Spangler R et al. License: CC BY 4.0

Applications of Ultrasound

Pros and cons of ultrasound imaging

Table: Pros and cons of ultrasound imaging
Pros Cons
  • No radiation (safest imaging method)
  • Portability
  • Can be used in multiple settings
  • Relatively low cost
  • Visualization of soft tissues
  • Can differentiate cystic from solid lesions
  • Can assess moving structures (e.g., heart)
  • Patient comfort during test
  • Dependent on operator training
  • Difficulty in evaluating deep tissues (e.g., retroperitoneal vessels)
  • Gas collections (e.g., in the bowels) interfere with visualization.
  • Sound waves cannot penetrate bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Structure of Bones and metal.

Indications and contraindications

Indications:

  • Trauma patients: 
    • FAST
    • Point-of-care ultrasound (POCUS)
    • Rapid ultrasound in shock Shock Shock is a life-threatening condition associated with impaired circulation that results in tissue hypoxia. The different types of shock are based on the underlying cause: distributive (↑ cardiac output (CO), ↓ systemic vascular resistance (SVR)), cardiogenic (↓ CO, ↑ SVR), hypovolemic (↓ CO, ↑ SVR), obstructive (↓ CO), and mixed. Types of Shock ( RUSH RUSH Abdominal and cardiac evaluation with sonography in shock (ACES) and rapid ultrasound for shock and hypotension (RUSH) are point-of-care ultrasound (POCUS) examinations indicated in cases of nontraumatic, undifferentiated hypotension, or shock. ACES and RUSH: Resuscitation Ultrasound Protocols)
    • Abdominal and cardiac evaluation with sonography in shock Shock Shock is a life-threatening condition associated with impaired circulation that results in tissue hypoxia. The different types of shock are based on the underlying cause: distributive (↑ cardiac output (CO), ↓ systemic vascular resistance (SVR)), cardiogenic (↓ CO, ↑ SVR), hypovolemic (↓ CO, ↑ SVR), obstructive (↓ CO), and mixed. Types of Shock ( ACES ACES Abdominal and cardiac evaluation with sonography in shock (ACES) and rapid ultrasound for shock and hypotension (RUSH) are point-of-care ultrasound (POCUS) examinations indicated in cases of nontraumatic, undifferentiated hypotension, or shock. ACES and RUSH: Resuscitation Ultrasound Protocols)
  • Gallbladder and biliary system:
    • Acute cholecystitis Cholecystitis Cholecystitis is the inflammation of the gallbladder (GB) usually caused by the obstruction of the cystic duct (acute cholecystitis). Mechanical irritation by gallstones can also produce chronic GB inflammation. Cholecystitis is one of the most common complications of cholelithiasis but inflammation without gallstones can occur in a minority of patients. Cholecystitis
    • Cholelithiasis Cholelithiasis Cholelithiasis (gallstones) is the presence of stones in the gallbladder. Most gallstones are cholesterol stones, while the rest are composed of bilirubin (pigment stones) and other mixed components. Patients are commonly asymptomatic but may present with biliary colic (intermittent pain in the right upper quadrant). Cholelithiasis
  • GI system: appendicitis Appendicitis Appendicitis is the acute inflammation of the vermiform appendix and the most common abdominal surgical emergency globally. The condition has a lifetime risk of 8%. Characteristic features include periumbilical abdominal pain that migrates to the right lower quadrant, fever, anorexia, nausea, and vomiting. Appendicitis
  • Kidney: hydronephrosis Hydronephrosis Hydronephrosis is dilation of the renal collecting system as a result of the obstruction of urine outflow. Hydronephrosis can be unilateral or bilateral. Nephrolithiasis is the most common cause of hydronephrosis in young adults, while prostatic hyperplasia and neoplasm are seen in older patients. Hydronephrosis
  • Scrotum:
    • Testicular torsion Testicular torsion Testicular torsion is the sudden rotation of the testicle, specifically the spermatic cord, around its axis in the inguinal canal or below. The acute rotation results in compromised blood flow to and from the testicle, which puts the testicle at risk for necrosis. Testicular Torsion
    • Testicular cancer Testicular cancer Testicular cancer is the most common solid malignancy affecting men 15-35 years of age. Most of the testicular cancers are of the germ cell tumor type, and they can be classified as seminomas and nonseminomas. The most common presentation of testicular cancer is a painless testicular mass. Testicular Cancer
  • Gynecologic imaging:
    • Ectopic pregnancy Ectopic pregnancy Ectopic pregnancy refers to the implantation of a fertilized egg (embryo) outside the uterine cavity. The main cause is disruption of the normal anatomy of the fallopian tube. Ectopic Pregnancy
    • Polycystic ovarian syndrome Polycystic ovarian syndrome Polycystic ovarian syndrome (PCOS) is the most common endocrine disorder of reproductive-age women, affecting nearly 5%-10% of women in the age group. It is characterized by hyperandrogenism, chronic anovulation leading to oligomenorrhea (or amenorrhea), and metabolic dysfunction. Polycystic Ovarian Syndrome
  • Pregnancy assessment:
    • Diagnosis
    • Fetal growth monitoring
    • Placenta Placenta The placenta consists of a fetal side and a maternal side, and it provides a vascular communication between the mother and the fetus. This communication allows the mother to provide nutrients to the fetus and allows for removal of waste products from fetal blood. Placenta, Umbilical Cord, and Amniotic Cavity previa 
  • Cardiac and pulmonary:
    • Congestive heart failure Congestive heart failure Congestive heart failure refers to the inability of the heart to supply the body with normal cardiac output to meet metabolic needs. Echocardiography can confirm the diagnosis and give information about the ejection fraction. Congestive Heart Failure
    • Pleural effusion Pleural Effusion Pleural effusion refers to the accumulation of fluid between the layers of the parietal and visceral pleura. Common causes of this condition include infection, malignancy, autoimmune disorders, or volume overload. Clinical manifestations include chest pain, cough, and dyspnea. Pleural Effusion
  • Blood vessels:
    • Carotid artery stenosis Carotid artery stenosis Carotid artery stenosis is a chronic atherosclerotic disease resulting in narrowing of the common and internal carotid arteries. Common risk factors include family history, advanced age, hyperlipidemia, smoking, and diabetes mellitus. Patients may present with or without symptoms of decreased cerebral perfusion. Carotid Artery Stenosis
    • Deep vein thrombosis Deep vein thrombosis Deep vein thrombosis (DVT) usually occurs in the deep veins of the lower extremities. The affected veins include the femoral, popliteal, iliofemoral, and pelvic veins. Proximal DVT is more likely to cause a pulmonary embolism (PE) and is generally considered more serious. Deep Vein Thrombosis

There are no contraindications for ultrasound imaging.

Other Imaging Methods

Comparison with other imaging methods

Table: Comparison of imaging methods
Radiography CT Ultrasound MRI
Mechanism of acquisition Ionizing radiation Ionizing radiation Acoustic energy Ferromagnetic pulses
Relative cost Inexpensive Expensive Inexpensive Very expensive
Portable Yes No Yes No
Length of exam Seconds < 1 minute Seconds Approximately 1 hour
Contrast No May be needed May be needed May be needed

Imaging method options by system

  • Imaging of the CNS (brain, spinal cord Spinal cord The spinal cord is the major conduction pathway connecting the brain to the body; it is part of the CNS. In cross section, the spinal cord is divided into an H-shaped area of gray matter (consisting of synapsing neuronal cell bodies) and a surrounding area of white matter (consisting of ascending and descending tracts of myelinated axons). Spinal Cord, and vertebral column Vertebral column The human spine, or vertebral column, is the most important anatomical and functional axis of the human body. It consists of 7 cervical vertebrae, 12 thoracic vertebrae, and 5 lumbar vertebrae and is limited cranially by the skull and caudally by the sacrum. Vertebral Column): 
    • Radiography is often used to evaluate for fractures of the vertebral column Vertebral column The human spine, or vertebral column, is the most important anatomical and functional axis of the human body. It consists of 7 cervical vertebrae, 12 thoracic vertebrae, and 5 lumbar vertebrae and is limited cranially by the skull and caudally by the sacrum. Vertebral Column
    • CT is a good choice for head trauma Head trauma Head trauma occurs when external forces are directed to the skull and brain structures, resulting in damage to the skull, brain, and intracranial structures. Head injuries can be classified as open (penetrating) or closed (blunt), and primary (from the initial trauma) or secondary (indirect brain injury), and range from mild to severe and life-threatening. Head Trauma and to exclude intracranial hemorrhage. 
    • MRI provides more detailed images of the brain and spinal cord Spinal cord The spinal cord is the major conduction pathway connecting the brain to the body; it is part of the CNS. In cross section, the spinal cord is divided into an H-shaped area of gray matter (consisting of synapsing neuronal cell bodies) and a surrounding area of white matter (consisting of ascending and descending tracts of myelinated axons). Spinal Cord, allowing identification of infarction, tumors, disc herniation, and demyelinating disease.
  • Pulmonary radiology Pulmonary Radiology Pulmonary, or chest, imaging includes imaging of the lungs and surrounding structures in the thorax. Imaging of the chest represents a substantial portion of the imaging tests that are routinely performed. Common imaging methods include X-ray, CT, MRI, and ultrasonography (US). Pulmonary Radiology and imaging of the mediastinum Mediastinum The mediastinum is the thoracic area between the 2 pleural cavities. The mediastinum contains vital structures of the circulatory, respiratory, digestive, and nervous systems including the heart and esophagus, and major thoracic vessels. Mediastinum and Great Vessels
    • Radiography is the preferred initial imaging study for viewing lung pathology. 
    • CT provides more detailed views of the lung parenchyma, mediastinal structures, and vasculature. 
    • MRI is not often used, but may be employed for evaluating malignancies and cardiac disease. 
    • Ultrasonography can be used for rapid bedside trauma assessment and for guiding procedures such as thoracentesis.
  • Breast imaging: 
    • Mammography is often the initial choice for breast cancer screening Breast cancer screening Breast cancer is the most common cancer in women and the 2nd-leading cause of cancer-related deaths in women in the United States. Early detection and improved pathology-specific treatments have resulted in a decrease in death rates. Breast Cancer Screening
    • MRI may be used to further evaluate and stage breast cancer. 
    • Ultrasonography is helpful for evaluating lymph nodes and to guide biopsy.
  • Imaging of the abdomen and renal imaging: 
    • Radiography is often used to evaluate for kidney stones, bowel obstruction, and pneumoperitoneum. In addition, barium may be used to assess swallowing and bowel function. 
    • CT and MRI provide more detailed assessments of the abdominal viscera and vasculature. 
    • Nuclear medicine can be used to assess gallbladder Gallbladder The gallbladder is a pear-shaped sac, located directly beneath the liver, that sits on top of the superior part of the duodenum. The primary functions of the gallbladder include concentrating and storing up to 50 mL of bile. Gallbladder and Biliary Tract function and gastric emptying and for GI bleeding.
  • Imaging of the uterus and ovaries Ovaries Ovaries are the paired gonads of the female reproductive system that contain haploid gametes known as oocytes. The ovaries are located intraperitoneally in the pelvis, just posterior to the broad ligament, and are connected to the pelvic sidewall and to the uterus by ligaments. These organs function to secrete hormones (estrogen and progesterone) and to produce the female germ cells (oocytes). Ovaries
    • Ultrasonography is the most commonly used method to evaluate the ovaries Ovaries Ovaries are the paired gonads of the female reproductive system that contain haploid gametes known as oocytes. The ovaries are located intraperitoneally in the pelvis, just posterior to the broad ligament, and are connected to the pelvic sidewall and to the uterus by ligaments. These organs function to secrete hormones (estrogen and progesterone) and to produce the female germ cells (oocytes). Ovaries and uterus, including assessing pregnancies and the causes of abnormal uterine bleeding Abnormal Uterine Bleeding Abnormal uterine bleeding is the medical term for abnormalities in the frequency, volume, duration, and regularity of the menstrual cycle. Abnormal uterine bleeding is classified using the acronym PALM-COEIN, with PALM representing the structural causes and COEIN indicating the non-structural causes. Abnormal Uterine Bleeding
    • CT and MRI provide more detailed views and are often useful in assessing cysts, malignancies, and benign masses.
  • Imaging of the musculoskeletal system: 
    • Radiography is often used to exclude fractures. 
    • CT is more sensitive to bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Structure of Bones pathology, including osteomyelitis Osteomyelitis Osteomyelitis is an infection of the bone that results from the spread of microorganisms from the blood (hematogenous), nearby infected tissue, or open wounds (non-hematogenous). Infections are most commonly caused by Staphylococcus aureus. Osteomyelitis
    • MRI is preferred for a soft tissue evaluation, such as assessing for malignancy and myositis. 
    • Bone scanning can be useful in finding occult fractures, osteomyelitis Osteomyelitis Osteomyelitis is an infection of the bone that results from the spread of microorganisms from the blood (hematogenous), nearby infected tissue, or open wounds (non-hematogenous). Infections are most commonly caused by Staphylococcus aureus. Osteomyelitis, and metabolic bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Structure of Bones disease.

References

  1. Chen MM, Whitlow CT. (2011). Chapter 1. Scope of diagnostic imaging. In Chen MM, Pope TL, Ott DJ (Eds.). Basic Radiology, 2nd ed., Chapter 1. McGraw-Hill. https://accessmedicine-mhmedical-com.ezproxy.unbosque.edu.co/content.aspx?bookid=360&sectionid=39669007
  2. Zaer NF, Amini B, Elsayes KM. (2014). Overview of diagnostic modalities and contrast agents. In Elsayes KM, Oldham SA (Eds.). Introduction to Diagnostic Radiology. McGraw-Hill. https://accessmedicine-mhmedical-com.ezproxy.unbosque.edu.co/content.aspx?bookid=1562&sectionid=95875179

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