How a 4-D Ultrasound Works
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What is an Ultrasound?
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Ultrasound, or ultrasonography, is an imaging technique frequently employed in medicine that uses the echoes of high frequency sound waves emitted from a transducer to produce an image of the subcutaneous structure being examined. Common to obstetrics, 4-D ultrasound arranges a rapid succession of 3-D images across time (the fourth dimension) to produce near-real-time video, which can be recorded and stored in digital format. This technique provides physicians and expectant parents with valuable information on fetal development.
Principle
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The fundamental principle of ultrasound is to calculate the distance between two things by emitting a sound of known frequency from the first source, bouncing it off the second source and measuring how long the echo takes to come back. This works with all types of waveforms. Laser scanning and radar work on this same principle, but they use light waves. Ultrasound uses frequencies above the range of human hearing, which tops out at a little more than 20kHz, or 20,000 cycles per second. The range of frequency used by ultrasound machines is adjusted by the physician and varies from roughly 2 to 13 MHz.
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The Signal Chain
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Ultrasound machines emit a sound wave of known frequency from a transducer, which passes through a beamformer and resonates. Then it is received by a signal processor, interpreted by a scan converter and displayed on the monitor. In the past, these were all individual processes and machines, but recent advances in computer processing power have transformed this into an entirely digital process. As computers are able to process data faster, they can return more images in a shorter amount of time. Today, this ranges from about 25 to 40 frames per second. Displayed in rapid succession, a 4-D movie can be produced.
Two-Dimensional Array, 3-D Imaging, and the Fourth Dimension
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Two-dimensional arrays are used in modern digital transducers and emit the sound from an arrangement of beamformers. The beamformers, arranged like pixels on a television, each return a 3-D volume of information, called a voxel. The computer interprets each voxel separately and constructs a full 3-D image of the entire scan.
A typical 2-D array has 1024 elements per square millimeter over 75 to 100mm. Rectangular arrays that are arranged in matrices of 16x512 because of their unevenness and are referred to as 1.5-D.
The waves travel out from the beamformers in a conical or pyramidal shape. Therefore, in a true 2-D array, there are 1024 pyramidal-shaped pieces of data, resonating at a frequency corresponding to the depth of the structure being imaged.
Arrays come in four types: annular (concentric circles), curved (along a convex curve), linear and rectangular. The arrangement of the pixels determines the shape of the scan plane. Scan planes can be linear, rectangular, pie-shaped or a sector. For example, annular arrays allowed for more focused imaging, such as that needed to peer between the ribs to image the heart. Curved arrays are standard in obstetrics for wide-field view and to fit the curve of the body. Curved arrays also can scan more area with a smaller array, at the cost of resolution.
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