Ultrasound technology in medicine continues to improve the ability of health care workers to obtain images of internal human structures. Not only are images obtained quickly, they also are delivered with minimal harm and discomfort to the patient. One current example of this technology is the real-time two-dimensional picture of the human fetus frequently obtained by expectant mothers when they get a sonogram by an obstetrician. These images are pie-shaped forms with the apex at the skin surface and showing ever deeper layers radiating out from the apex, namely, the wall of the uterus, the fetus, and then the opposite wall of the uterus. The depth of the image is limited by the frequency at which the sound is emitted, its amplitude and the currently available technology.
While visualizing the fetus may be the best known use of this technology in medicine, it is also used to look at the skin, the liver, the ovaries, the kidneys, and the heart. In looking at the heart it is invaluable because it provides a functional study. By this is meant that a real-time moving image of the heart (as it contracts in front of the observer) is obtained, yielding information about the contractions. This knowledge is readily obtained from watching the moving image on a computer monitor. Some examples of the interesting information that can be obtained are how much each chamber of the heart pumps with each beat, and what percentage of blood is expelled from each chamber with each contraction.
A simple explanation of ultrasound technology is as follows. An ultrasound transducer uses a piezoelectric crystal or similar device to convert electrical energy to sound vibrations and vice versa. High frequency sound energy is emitted by the transducer into a sound conductive medium. Since different structures in the human body conduct and reflect sound energy differently, the various times and intensity at which the sound wave energy is reflected and returned to the transducer can be used to reconstruct an image, using available computerized techniques. A simple analogy is the use of sonar to determine the range, bearing and size of a submarine located in the ocean. Sonar uses essentially the same principles of measuring reflected sound. In the study of the human body, the ranges to structures are much smaller, allowing hundreds of images to be obtained each second. This allows for the real-time visualization of moving structures.
The most commonly manifested use of ultrasound technology in medicine employs a phased array or other ultrasound transducer to obtain two-dimensional tomographic images of human structures. In other words, the images represent planar "slices" through the body. Different two-dimensional slices are obtained by adjusting the orientation and position of the transducer. An analogy would be the result obtained by a butcher when using a slicing machine to obtain thin sheets of ham. If the ham were rotated in the machine before each slice, different shaped slices could be obtained.
Following in this thought is the fact that if the sequence of the slices of ham are known, a reasonable approximation of the original ham can be obtained by restacking the original slices. A similar premise lies behind presently available three-dimensional ultrasound imaging, wherein a series of two-dimensional images of a structure in the human body is used to recreate pictorially a three-dimensional image of that structure. This is not a new idea, with several approaches currently being summarized in patents and the scientific literature. The basic premise of the current art is that if the time of imaging and the location of the ultrasound transducer are known, the order and position of individual two-dimensional slices is also known. A relatively simple computer algorithm then "stacks," or reassembles, these slices into a three-dimensional picture. The present application of this technology is limited by constraints in the accuracy of assumptions made about the position of the transducer as the individual two-dimensional images are taken. For example, it may be assumed that the position of the transducer is moving linearly, while, in fact, extraneous movements of the patient and the like may cause it to move non-linearly. Consequently, when the images are assembled ("stacked") according to such inaccurate positional assumptions, loss of focus and other image distortion results. In other cases, a constant velocity of movement is assumed, but this also is difficult to achieve in practice.
Sound waves are poorly transmitted by bone and air. This prevents adequate images from being obtained of, for example, the heart when attempting to look through ribs, the breastbone (sternum), or the lungs. In looking at the heart, there are two primary ways in which ultrasound beams can be directed. One way to avoid bone is by looking from the upper abdomen, just below the breastbone, and up at the heart. This is known as the transabdominal or subxiphoid view. The second way is by inserting a transducer probe into the esophagus--a structure which is in close proximity to the heart during much of its course from pharynx to stomach. This transesophageal view is widely regarded as allowing for superior ultrasound views of the heart. Different areas of the heart can be seen simply by sliding the transducer up and down in the esophagus. Obviously, this latter approach is less well tolerated by the patient, requiring sedation while the flexible probe (often 1 to 1.5 cm. in diameter) is manipulated in his or her esophagus. In addition, there are greater risks to the patient, including but not limited to, tears of the esophagus. The larger the probe and the longer it must remain in place, the greater are the discomfort and risks.
Prior developments in this field may be generally illustrated by reference to the following information disclosure statement:
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