Image Analysis: Medicine

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Image Analysis: Medicine

The evolution of computers has dramatically affected the quality of human life in the field of medicine. Many developments during the last thirty years of the twentieth century have taken place in the field of diagnostic radiology, which is used to diagnose and sometimes treat illnesses. Among the image analysis tools now available that are derived from advances in computer technology are the ultrasound, computed tomography, nuclear medicine, and magnetic resonance imaging.

Ultrasound

Ultrasound is an example of technology that was first developed to serve military purposes. The principle behind ultrasonography was first used during World War II. Called SONAR, it helped detect the presence of underwater submarines. Ultrasound was not widely used in medicine until the early 1970s. Diagnostic ultrasound uses sound waves that are too high for the human ear to detect (between one million and 20 million cycles per second). These sound waves are used to evaluate the soft tissues of the body, such as muscles and organs.

The method is simple. First, a piezoelectric crystal is "pulsed" with electricity, making the crystal vibrate in a hand-held unit, called a transducer. The result is a sound wave. When the transducer is applied to the skin with a special "electrophoretic" gel, the sound waves are sent through tissue until they encounter "acoustical resistance." Such resistance makes some portion of the sound wave reflect back to its source. After pulsing, the transducer stops and "listens" for the sound waves reflected back to the crystal. Once that happens, the crystal vibrates again, producing electricity to indicate the pressure, amplitude, frequency, and speed of the returning wave. The computer then records the results. With this information, the computer is able to construct an image of the variety and depth of tissues through which the wave has passed. The use of ultrasound is limited, however. Tissues with too much acoustical resistance, for example bone and air, make this tool useless when the tissue to be examined lies behind a bone or a lung.

Computed Tomography

Computed Tomography, or CT, which became widely used in the late 1970s, would be impossible without computers. In CT, X-rays are transmitted through a body at various angles. The X-rays are then measured on the other side of the body by detectors. Depending on the percentage of X-rays that are successfully transmitted through the body, the computer is able to create a matrix defining the structure of tissues that absorbed the X-rays. The resulting images are usually displayed as slices, however, complex two-and three-dimensional images may be constructed by the computer if additional information is needed by physicians. The CT scanner usually takes multiple slices that are a prescribed distance apart, from millimeters to centimeters.

Modern CT scanners use slices made up of millions of pixels . These pixels are assigned a number on the grayscale somewhere in between very black and very white, and given a numerical value. With CT, the computer can adjust the grayscale after the study is complete to allow the trained observer to study a particular tissue density. For instance, the grayscale is adjusted to a very low level to look inside the lung, which is mostly air. With slight differences of grayscale, the image could show subtle differences in lung tissue, but with that particular scale, muscle and bone surrounding the lung would both look white. Conversely, if the goal of the study is to look for a small metal fragment imbedded in the spine, the grayscale would be adjusted to allow the image to differentiate between the density of metal versus the slightly less dense bone. Air, muscles, and other tissues in that study would look uniformly black. Both examinations can be done, without repeating the study, simply by varying the grayscale level of a given result.

Nuclear Medicine

Nuclear medicine is another type of diagnostic study that requires computers. These studies deal with the body's function more than with evaluating anatomy. Special radioactive molecules are tailored to target an organ's function. These radioactive materials are usually injected into the patient. After a given period of time, the patient is examined with a special device that measures radioactivity from the body. The computers measure the amount of radioactivity, help determine the source of that activity, and most importantly, are used to filter out misleading radioactivity.

Magnetic Resonance Imaging

Magnetic resonance imaging, or MRI, is very different. A strong magnet is utilized to align protons in the human body. The MRI machine then briefly sends radio waves through the body, exciting the protons. The unit then "listens" for the radio waves that the excited protons emit. The machine measures the number of hydrogen nuclei, or protons, in a given part of the body. Different body tissues, and even the different chemicals in the tissues, emit different radio waves.

After the study is done using the desired frequency, the information is collected and digitized by the computer, so the information can be presented and analyzed. Since the frequency of the radio waves is chosen for a specific task, the process might be repeated several times using different frequencies in a single study.

Evolution of Medical Technology

All of these techniques would be impossible without advances in both physics and computers. The results from these technologies, as well as regular (called plain-film) X-rays, can be digitized, and therefore analyzed, manipulated, transported, stored, and presented in multiple ways.

When these tools were first introduced, the computers that ran them were mainframe computers requiring their own customized room with extensive cooling systems. Over the years, they became smaller as computers became more powerful. By the end of the twentieth century most of these studies were being done with computers only slightly larger than PCs. As with everything else in the computer field, these tools, software applications, and technologies continue to evolve.

With great technological advances, uses for computer image analysis are limited only by the imagination. Computer color analysis is used to measure the success of bleaching teeth. It is also used to analyze lesion borders in various types of skin cancer as well as to examine the back of the eye through digitized retinal analysis. Image analysis can be used to evaluate wound healing that is undetectable to the human eye. Also, image analysis computer programs are invaluable in guiding surgeons in microscopic surgery.

Outside of medicine, uses for image analysis abound in the sciences, from the development of agricultural products, to the study of the activity and reproduction of plant and animal cells. Image analysis tools are used for the genetic analysis of various living entities. Computer image analysis is also becoming an economical and accurate method in the teaching of biology, microbiology, and other fields of science.

see also Medical Systems; Molecular Biology; Scientific Visualization.

Anwer H. Puthawala and Mary McIver Puthawala