97-120 Flashcards
What are the advantages of a digital scan converter over an analog scan converter?
Durability. Unlike analog systems, digital scan converters are long lasting and extremely durable.
Uniformity. The image processing effects of digital scan converters are remarkably uniform over all regions of the image area.
Stability. Digital scan converters are not subject to information error or deterioration, regardless of how long the data is stored.
Accuracy and Speed. Digital converters are actually high-speed computer systems that are extremely accurate.
Image information is stored by a digital scan converter in the form of pixels. What is a pixel?
A pixel is the building block of an image. Each image is made up of thousands of small, distinct pixels or “picture elements.” Each pixel is a tiny part of the image. A pixel is usually a small, square region of an image that has only a single, uniform shade of gray at any instant in time. That is, a pixel is assigned only one shade of gray by the scan converter.
Think of an ultrasound image as a checkerboard, with each square on the board representing a pixel having only one single, uniform color.
Assume that the same image is processed by two scan converters, one analog and the other digital. The analog scan converter contains a dielectric matrix that has 1000 × 1000 elements. The digital scan converter has a 256 x 256 pixel matrix. Which of the two scan converters will produce an image with the highest detail?
The image processed by the analog scan converter will contain more detail, because this image is made up of more picture elements or pixels. Because there are more picture elements in the image processed by the analog converter, each element will be smaller than those in the image processed by the digital scan converter. This is called spatial resolution. The general rule associated with matrix size is as follows: with an image of fixed size, the matrix that provides the greater number of picture elements will produce the image with the greatest degree of detail.
Note: In this example, even though the image detail is superior with the analog converter, the digital converter has other advantages.
Digital scan converters have greater durability, uniformity, stability, accuracy, and speed.
What is a “bit”?
A bit is the smallest amount of computer memory. A single bit can only store either a “O” or a “I.” This is called binary. A collection of bits has the ability to store larger numbers.
What is the function of bits in a digital scan converter?
If a digital scan converter has only one bit for each pixel, the pixel can only be either white or black. The choice is determined by whether the bit is “on” or “off.” In order for a pixel to display gray shades other than white or black, additional bits must be assigned to each pixel. The more bits assigned to each pixel, the greater the selection of gray shades that can be represented in the pixel.
What is a byte?
A byte is comprised of 8 bits
what device controls the bits in a digital scan converter?
a microprocessor controls the computer memory of a digital scan converter
How many different shades of gray can be represented in a pixel with 3 bits of computer memory?
In order to calculate the number of gray shades that can be represented by a collection of bits, simply multiply 2 by as many times as there are bits. In the first example, there are three bits per pixel, so 2 is multiplied by itself three times:
2 x 2 x 2 = 8 possible shades of gray.
How many shades of gray can be represented in a pixel with 5 bits of computer memory?
For five bits per pixel, multiply 2 by itself a total of five times:
2 x 2 x 2 x 2 x 2 = 32 shades of gray
How many different shades of gray can be represented in a pixel with 1 bit of computer memory?
For one bit per pixel, simply 2 shades of gray. This is called a bistable
What is analog-to-digital conversion? Does analog-to-digital conversion occur before or after image data is stored in the digital scan converter?
Simply put, analog-to-digital (A-to-D) conversion is the process of translating real-world (analog) information into computer-world (digital) information.
Analog-to-digital (A-to-D) conversion is the process by which an analog signal is transformed into a digital signal. The signals produced by an ultrasound transducer during the reception phase are analog. However, because computers can manipulate only digital signals, these analog signals must be processed by an analog-to-digital converter into a format that allows them to be manipulated by the digital scan converter.
A-to-D conversion takes place before the image data is stored in the digital scan converter.
