decreases, more uses of digital image processing will appear in all facets of life. Some people have predicted that by the turn of the century at least 50% of the images we handle in our private and professional lives will have been processed on a computer. Image Capture a digital image is nothing more than a matrix of numbers. The question is how does this matrix represent a real image that one sees on a computer screen? K Like all imaging processes, whether they are analog or digital, one first starts with a sensor(or transducer) that converts the original imaging energy into an electrical signal. These sensors, for instance, could be the photomultiplier tubes used in an x-ray system that converts the x-ray energy into a known electrical voltage. The transducer system used in ultrasound imaging is an example where sound pressure is converted to electrical energy, a simple Tv camera is perhaps the most ubiquitous example. An important fact to note is that the process of conversion from one energy form to an electrical signal is not necessarily a linear process. In other words, a proportional charge in the input energy to the sensor will not always cause the same proportional charge in the output electrical signal. In many cases calibration data are obtained in the laboratory so that the relationship between the input energy and output electrical signal is known. These data are necessary because istics change with age and other usage facto The sensor is not the only thing needed to form an image in an imaging system. The sensor must have some spatial extent before an image is formed By spatial extent we mean that the sensor must not be a simple point source examining only one location of energy output. To explain this further, let us examine two types of imaging sensors used in imaging: a CCD video camera and the ultrasound transducer used in many medical imaging applications The CCD camera consists of an array of light sensors known as charge-coupled devices. The image is formed by examining the output of each sensor in a preset order for a finite time. The electronics of the system then forms an electrical signal which produces an image that is shown on a cathode-ray tube(Crt) display. The image formed because there is an array of sensors, each one examining only one spatial location of the region to be sensed. The process of sampling the output of the sensor array in a particular order is known as scanning. Scanning the typical method used to convert a two-dimensional energy signal or image to a one-dimensional electrical ignal that can be handled by the computer. (An image can be thought of as an energy field with spatial extent. Another form of scanning is used in ultrasonic imaging. In this application there is only one sensor instead of an array of sensors. The ultrasound transducer is moved or steered (either mechanically or electrically)to various spatial locations on the patient's chest or stomach. As the sensor is moved to each location, the output electrical signal of the sensor is sampled and the electronics of the system then form a television-like signal which is displayed. Nearly all the transducers used in imaging form an image by either using an array of sensors or a single sensor that is moved to each spatial location. One immediately observes that both of the approaches discussed above are equivalent in that the energy is sensed at various spatial locations of the object to be imaged. This energy is then converted to an electrical signal by the transducer. The image formation processes just described are classical analog image formation, with the distance between the sensor locations limiting the spatial resolution in the system. In the array sensors, resolution is determined by how close the sensors are located in the array In the single-sensor approach, the spatial resolution is limited by how far the sensor is moved. In an actual system spatial resolution is also determined by the performance characteristics of the sensor. Here we are assuming for our purposes perfect sensors. In digital image formation one is concerned about two processes: spatial sampling and quantization. Sam pling is quite similar to scanning in analog image formation. The second process is known as quantization or analog-to-digital conversion, whereby at each spatial location a number is assigned to the amount of energy the transducer observes at that location. This number is usually proportional to the electrical signal at the output of the transducer. The overall process of sampling and quantization is known as digitization. Sometimes the digitization process is just referred to as analog-to-digital conversion, or A/D conversion; however, the reader should remember that digitization also includes spatial sampling The digital image formulation process is summarized in Fig 17. 1. The spatial sampling process can be considered as overlaying a grid on the object, with the sensor examining the energy output from each grid box C 2000 by CRC Press LLC© 2000 by CRC Press LLC decreases, more uses of digital image processing will appear in all facets of life. Some people have predicted that by the turn of the century at least 50% of the images we handle in our private and professional lives will have been processed on a computer. Image Capture A digital image is nothing more than a matrix of numbers. The question is how does this matrix represent a real image that one sees on a computer screen? Like all imaging processes, whether they are analog or digital, one first starts with a sensor (or transducer) that converts the original imaging energy into an electrical signal. These sensors, for instance, could be the photomultiplier tubes used in an x-ray system that converts the x-ray energy into a known electrical voltage. The transducer system used in ultrasound imaging is an example where sound pressure is converted to electrical energy; a simple TV camera is perhaps the most ubiquitous example. An important fact to note is that the process of conversion from one energy form to an electrical signal is not necessarily a linear process. In other words, a proportional charge in the input energy to the sensor will not always cause the same proportional charge in the output electrical signal. In many cases calibration data are obtained in the laboratory so that the relationship between the input energy and output electrical signal is known. These data are necessary because some transducer performance characteristics change with age and other usage factors. The sensor is not the only thing needed to form an image in an imaging system. The sensor must have some spatial extent before an image is formed. By spatial extent we mean that the sensor must not be a simple point source examining only one location of energy output. To explain this further, let us examine two types of imaging sensors used in imaging: a CCD video camera and the ultrasound transducer used in many medical imaging applications. The CCD camera consists of an array of light sensors known as charge-coupled devices. The image is formed by examining the output of each sensor in a preset order for a finite time. The electronics of the system then forms an electrical signal which produces an image that is shown on a cathode-ray tube (CRT) display. The image is formed because there is an array of sensors, each one examining only one spatial location of the region to be sensed. The process of sampling the output of the sensor array in a particular order is known as scanning. Scanning is the typical method used to convert a two-dimensional energy signal or image to a one-dimensional electrical signal that can be handled by the computer. (An image can be thought of as an energy field with spatial extent.) Another form of scanning is used in ultrasonic imaging. In this application there is only one sensor instead of an array of sensors. The ultrasound transducer is moved or steered (either mechanically or electrically) to various spatial locations on the patient’s chest or stomach. As the sensor is moved to each location, the output electrical signal of the sensor is sampled and the electronics of the system then form a television-like signal which is displayed. Nearly all the transducers used in imaging form an image by either using an array of sensors or a single sensor that is moved to each spatial location. One immediately observes that both of the approaches discussed above are equivalent in that the energy is sensed at various spatial locations of the object to be imaged. This energy is then converted to an electrical signal by the transducer. The image formation processes just described are classical analog image formation, with the distance between the sensor locations limiting the spatial resolution in the system. In the array sensors, resolution is determined by how close the sensors are located in the array. In the single-sensor approach, the spatial resolution is limited by how far the sensor is moved. In an actual system spatial resolution is also determined by the performance characteristics of the sensor. Here we are assuming for our purposes perfect sensors. In digital image formation one is concerned about two processes: spatial sampling and quantization. Sam￾pling is quite similar to scanning in analog image formation. The second process is known as quantization or analog-to-digital conversion, whereby at each spatial location a number is assigned to the amount of energy the transducer observes at that location. This number is usually proportional to the electrical signal at the output of the transducer. The overall process of sampling and quantization is known as digitization. Sometimes the digitization process is just referred to as analog-to-digital conversion, or A/D conversion; however, the reader should remember that digitization also includes spatial sampling. The digital image formulation process is summarized in Fig. 17.1. The spatial sampling process can be considered as overlaying a grid on the object, with the sensor examining the energy output from each grid box