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
Types of transducers:
Image by Lecturio.
Note that decreasing the frequency increases the depth to which the ultrasound wave travels. However, this comes at the cost of image resolution.
An ultrasound machine and different probes
Image: “Photos of a sonography system and typical transducers.” by Kieran Maher. License: Public Domain
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.
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.
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
Near and far fields of a transducer in ultrasound imaging:
Image by Lecturio.
Note how, in the near field, the transducer has an adequate lateral definition, being able to distinguish between 2 points. On the other hand, in the far field, increasing axial definition (depth) sacrifices lateral definition.
Different planes of fetal ultrasound imaging:
Image: “Different planes of fetal ultrasound imaging” by Lei B et al. License: CC BY 4.0
Top left: Sagittal plane
Top right: Coronal plane
Bottom left: Axial plane
Bottom right: Non–fetal facial standard plane (FFSP)
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, 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
A: Urinary ultrasound showing right-sided hydronephrosis and the irregular upper section of the right ureter
.. Image: “Urinary and Gynecological Ultrasound (Sonography)” by Ting Yan et al. License: CC BY 4.0
B: Gynecologic ultrasound showing a cystic mass (8.8 X 7.5 X 8.6 cm) in the right rear uterus
Note that both findings are hypoechoic to anechoic because of their liquid content
Transvaginal ultrasound showing a polycystic ovary:
Image: “Transvaginal ultrasound scan of polycystic ovary” by Schomynv. License: CC0 1.0
Note the multiple cysts on the periphery of the ovary (white arrow)
Example of a nonspecific hypoechoic structure on an ultrasound of the right breast, which could represent a malignancy, an abscess, or a benign tumor, depending on the clinical scenario
Image by Hetal Verma.
Example of a thyroid nodule containing internal calcifications, depicted as hyperechoic foci on an ultrasound of the thyroid gland
Image by Hetal Verma.
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.
Color Doppler and spectral Doppler examination of the left internal carotid artery revealing stenoses of about 70%
Image: “Color Doppler and spectral Doppler examination of the left ICA” by Christian Arning et al. License: CC BY 2.0
Doppler ultrasound of the carotid arteries showing carotid stenosis:
Image: “Two Plaque types with different Plaque scores” by Wasser K et al. License: CC BY 2.0
A plaque can be seen obstructing < 40% of the lumen, and hyperechoic foci within the vessel wall can be seen, probably due to calcification
Doppler ultrasound of the right testicle:
Image by Hetal Verma.
Note the difference in color for flow coming toward the transducer (red) and flow away from the transducer (blue).
Spectral Doppler ultrasound of the right testicle:
Image by Hetal Verma.
A graph of the flow appears at the bottom of the image.
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 from a patient with acute cholecystitis:
Multiple gallstones are visualized within the gallbladder lumen with gallbladder wall thickening and pericholecystic fluid. Shadowing can be seen behind the gallstone.
Applications of Ultrasound
Pros and cons of ultrasound imaging
| Pros | Cons |
|---|---|
|
|
Indications and contraindications
Indications:
- Trauma patients:
- FAST
- 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
- Cholelithiasis
- GI system: appendicitis
- Kidney: hydronephrosis
- Scrotum:
- Testicular torsion
- Testicular cancer
- Gynecologic imaging:
- Ectopic pregnancy
- Polycystic ovarian syndrome
- Pregnancy assessment:
- Diagnosis
- 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
| 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, 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.
References
- 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
