Ultrasound technology is a widely used imaging modality that creates pictures of internal body structures without ionizing radiation. This non-invasive technique transmits high-frequency sound waves, generally in the megahertz (MHz) range, into the body via a handheld device called a transducer. These sound waves travel through tissues and reflect back as echoes when they encounter boundaries between different tissue densities. The transducer captures these returning echoes, which a computer processes to generate a real-time visual image, called a sonogram. This ability to visualize soft tissues and organs makes ultrasound a versatile tool across many medical disciplines.
Imaging by Dimensionality
The most common way to categorize ultrasound is by the spatial dimension of the resulting image. Two-dimensional (2D) ultrasound is the foundational and most traditional form, producing a flat, cross-sectional “slice” of the internal anatomy. This technique provides a grayscale image effective for viewing the structure of organs and vessels in real-time, making it suitable for routine examinations.
Three-dimensional (3D) ultrasound captures volume data by acquiring multiple 2D images from various angles. A computer reconstructs this data to display the anatomy as a three-dimensional surface or shape. This volume rendering is helpful for visualizing complex structures, such as the fetal face or heart chambers, offering a comprehensive view for assessing potential abnormalities.
Four-dimensional (4D) ultrasound takes this volumetric data and adds the element of time. It is essentially a 3D image displayed in motion, providing a real-time video of the internal structure. This dynamic view allows for the observation of movement, such as fetal activity or cardiac valve function. While 2D remains the standard for initial assessment, 3D and 4D imaging offers enhanced spatial detail for specific diagnostic needs.
Imaging by Flow and Velocity
A distinct category of ultrasound relies on the Doppler effect to measure movement, most commonly the flow of blood. The Doppler effect describes the change in frequency of a wave when the source or the receiver is moving. In this application, sound waves reflect off moving red blood cells, and the difference between the transmitted and received frequency, known as the Doppler shift, is directly proportional to the blood flow velocity.
Color Doppler is a visualization technique that superimposes flow information onto the standard 2D grayscale image. It assigns colors, typically red and blue, to indicate the direction of blood flow relative to the transducer, with the color’s intensity representing the flow’s speed. This allows for the rapid assessment of blood vessel patency and flow direction.
Spectral Doppler provides a detailed, quantitative analysis by displaying blood flow velocity over time as a graph. This technique measures specific flow characteristics, such as peak velocity and resistance, aiding in diagnosing conditions like vessel narrowing (stenosis). Power Doppler measures the total strength or amplitude of the Doppler signal, making it highly sensitive to the presence of blood flow, especially in small or slow-flowing vessels. Unlike Color Doppler, Power Doppler does not provide directional information, but its increased sensitivity is advantageous for mapping vascularity within organs or tumors.
Imaging by Access and Function
Imaging by Access
Ultrasound procedures are often categorized by the method used to position the transducer, which influences the clarity and focus of the image. Transabdominal or transcutaneous ultrasound is the standard method where the transducer is placed on the skin, often over the abdomen, to image deeper structures. For structures located deeper within the body cavity, specialized internal probes are used to achieve higher resolution by getting closer to the target organ.
Transvaginal and transrectal ultrasound utilize slender probes inserted into the vagina or rectum to image pelvic organs like the uterus, ovaries, or prostate. The proximity of these endocavitary transducers allows for the use of higher-frequency sound waves, resulting in improved detail and resolution compared to the transabdominal approach. Endoscopic Ultrasound (EUS) integrates a tiny ultrasound probe into a flexible endoscope, enabling imaging from inside the gastrointestinal tract or lungs. This provides high-resolution images of the digestive tract wall and nearby organs like the pancreas.
Functional Techniques
Beyond structural and flow imaging, advanced functional techniques measure specific tissue properties to aid diagnosis. Elastography measures the stiffness of tissue, which is often altered in disease states like fibrosis or cancer. By applying a mechanical force or acoustic radiation force, this method quantifies tissue elasticity and helps differentiate healthy tissue from abnormal, often harder, masses.
Contrast-Enhanced Ultrasound (CEUS) involves injecting a microbubble contrast agent into the bloodstream. These gas-filled microbubbles strongly reflect sound waves, temporarily enhancing the visibility of blood flow within organs or lesions. This assists in characterizing tumors and other vascular irregularities.