Terminology and Technical Aspects
- 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.
- 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
- 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.
- Sound waves are emitted by the transducer.
- Sound waves penetrate the tissues in the form of a beam.
- 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.
- The echoes return to the transducer.
- 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.
- The CPU processes the electrical signals into images that can be seen on the monitor.
- 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 (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
The interpretation of ultrasound images is done in real time, while the examination is being performed.
- 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.
- Hyperechoic (e.g., surface of bone, 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
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 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.
Applications of Ultrasound
Pros and cons of ultrasound imaging
Indications and contraindications
- Trauma patients:
- Point-of-care ultrasound (POCUS)
- Rapid ultrasound in shock (RUSH)
- Abdominal and cardiac evaluation with sonography in shock (ACES)
- Gallbladder and biliary system:
- Acute cholecystitis
- GI system: appendicitis
- Kidney: hydronephrosis
- Testicular torsion
- Testicular cancer
- Gynecologic imaging:
- Ectopic pregnancy
- Polycystic ovarian syndrome
- Pregnancy assessment:
- Fetal growth monitoring
- Placenta previa
- Cardiac and pulmonary:
- Congestive heart failure
- Pleural effusion
- Blood vessels:
- Carotid artery stenosis
- Deep vein thrombosis
There are no contraindications for ultrasound imaging.
Other Imaging Methods
Comparison with other imaging methods
|Mechanism of acquisition||Ionizing radiation||Ionizing radiation||Acoustic energy||Ferromagnetic pulses|
|Relative cost||Inexpensive||Expensive||Inexpensive||Very expensive|
|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, and vertebral column):
- Radiography is often used to evaluate for fractures of the vertebral column.
- CT is a good choice for head trauma and to exclude intracranial hemorrhage.
- MRI provides more detailed images of the brain and spinal cord, allowing identification of infarction, tumors, disc herniation, and demyelinating disease.
- Pulmonary radiology and imaging of the mediastinum:
- 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.
- 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 function and gastric emptying and for GI bleeding.
- Imaging of the uterus and ovaries:
- Ultrasonography is the most commonly used method to evaluate the ovaries and uterus, including assessing pregnancies and the causes of 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 pathology, including 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, and metabolic bone disease.
- 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§ionid=39669007
- 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§ionid=95875179