Digitization and Image Types

 
Editor's Note
 
The first four pages of the instrumentation section is a technically detailed discussion of the acquisition, processing, output of the captured images, and compression formats of those images. If you do not require detailed knowledge of these topics, please skim over the sections and proceed to the knobology section. While the initial information on instrumentation is academically interesting, its contribution to clinical echocardiography is more limited than later sections.
 
Objectives
 
 At the completion of this chapter, the student will be able to: 
 
  • Discuss Digital Image Compression
  • Discuss Basic Knobology
  • Discuss 2D Controls
  • Discuss B Mode Controls
  • Discuss M Mode Controls
  • Discuss Color Controls
  • Discuss Pulse Wave Doppler Controls
  • Discuss Continuous Wave Doppler Controls
 
Image Generation and Processing
 
Digital vs. Analog
 
Analog
 
Modern echocardiography primarily performed in a digital fashion. Digitalization of the ultrasound signal offers many advantages over the analog signal. Image 3.1 shows a graphic view of the capturing, manipulation and display of an ultrasound signal. While the original transmission (1) and reception of the ultrasound signal (2) is analog, modern echocardiographic machines convert the received signal to a digital signal(3). After digitization of the signal is completed, various processing (4) of the digital signal is performed. After the processing is completed, the digital signal is either saved (to a disk and/or in computer memory) or it is converted back to an analog signal (5) for display to a monitor screen where it can be recorded to a videocassette tape. This analog-to-digital-to-analog conversion is common to most echocardiographic machines. If the machine is totally digital, the analog conversion is not done and the digital processing results in a digital image/video that is displayed on a digital monitor.
 
Transmission
Reception
Digitization
Processing
Analog Conversion
Display
1-Transmission
2-Reception
3-Digitalization
4-Processing
5-Analog Conversion
6-Display

Image 3.1

 
Digital
 
The most modern echocardiographic machines perform an analog-to-digital conversion whereby the information is stored, displayed, and processed for display from its digital format. The digital format will not degrade when multiple copies are made, it is electronically searchable, it can be transmitted over a network, provides a superior image quality, and it can be processed after the signal is recorded.
 
Digital and Analog Comparison
 
Factor Digital Analog
Image Quality Excellent Fair
Processing after recorded Yes No
Transmission Network Mail
Copy Quality Same Degrades
Searching Random Access Sequential
    Table 3.1.2
 
Binary Representation of Digital Information
 
Digital information, whether it is images, text, or sound are stored in a series of ones and zeros on digital media. The placeholder for the zero or the one is called a bit. Depending upon the computer system used, the bits are stored in groups called bytes. If a byte is a grouping of 4 bits, then it can store four zero's or four one's or any combination of one's and zero's as long as it is 4 bits long. In the binary world, numbers are derived from bytes. In the table below, as we change the bits from zero's to ones, the numbers from 0 to 15 can be represented in a single byte. The bits, since they can only be a zero or a one are a base two representation. Each bit is raised to the power or 2. The first bit on the right is 2 to the power of 0, the second bit on the from the right represents a 2 to the power of 1, the third bit represents a 2 to the power of 2, and the third bit, the one on the left, represents a 2 to the power of 3. For example, if a byte had the four bits equal to 0100, then the number represented would be 4 because the third bit is 22 or 4. The table shows how each number from 0 to 15 are represented in the binary world.
 
Byte
Calculation
Number
0000
0 * 23 + 0 * 22 + 0 * 21 + 0 * 20
0
0001
0 * 23 + 0 * 22 + 0 * 21 + 1 * 20
1
0010
0 * 23 + 0 * 22 + 1 * 21 + 0 * 20
2
0011
0 * 23 + 0 * 22 + 1 * 21 + 1 * 20
3
0100
0 * 23 + 1 * 22 + 0 * 21 + 0 * 20
4
0101
0 * 23 + 1 * 22 + 0 * 21 + 1 * 20
5
0110
0 * 23 + 1 * 22 + 1 * 21 + 0 * 20
6
0111
0 * 23 + 1 * 22 + 1 * 21 + 1 * 20
7
1000
1 * 23 + 0 * 22 + 0 * 21 + 0 * 20
8
1001
1 * 23 + 0 * 22 + 0 * 21 + 1 * 20
9
1010
1 * 23 + 0 * 22 + 1 * 21 + 0 * 20
10
1011
1 * 23 + 0 * 22 + 1 * 21 + 1 * 20
11
1100
1 * 23 + 1 * 22 + 0 * 21 + 0 * 20
12
1101
1 * 23 + 1 * 22 + 0 * 21 + 1 * 20
13
1110
1 * 23 + 1 * 22 + 1 * 21 + 0 * 20
14
1111
1 * 23 + 1 * 22 + 1 * 21 + 1 * 20
15
    Table 3.1.3
 
From this table, a 4 bit system can store numbers from 0 to 15 or a total of 16 numbers (24). Modern systems can process vast amounts of data compared to the 4 bit system. Currently, most systems are 32 bit systems with 64 bit and 128 bit systems coming soon. The next chart shows as the number of bits that can be stored in one byte increases, the amount of data that can be processed increases also.
 
Byte Sizes
 
Byte
Calculation
Largest Number
2 bits
22
4
4 bits
24
16
8 bits
28
256
16 bits
216
65,536
24 bits
224
1,6777,216
32 bits
232
4,294,967,296
64 bits
264
18,446,744,073,709,551,616
128 bits
2128
3.4028236692093846346337460743177 e+38
    Table 3.1.4
As you can see, as the number of bits increases in each byte, the amount of data or bandwidth, that can be processed dramatically increases. Besides computer processing bandwidth, displays or images are described in these terms also. A 16 bit image or display, can display 65,536 different colors. An 8 bit black and white image or display will display 256 shades of gray. A 32 bit display can display over 4 billion colors, which is more colors than the human eye can recognize.
 
Digital Storage
 
Storage of information on media uses a similar language to describe the capacity of the media. Storage media comes in all types, sizes and access speed. After punch cards and magnetic tapes came the floppy disc in a 3 1/2 inch or 5 1/4 inch size. Since then zip disks,flash drives, CDs, DVDs, Magneto Optical Disks, hard drives, digital tape, and RAM have all been invented or improved. Larger capacity is required for digital information because of it's size. To display a saved image, the access time of the media will determine the speed of it's display. The capacity of the media is described in bytes. However, unlike processing or display bytes, a byte of storage media represents a 8 bit wide byte. An system where a byte is 8-bits wide can store a character, such as a letter, a number, or a special character (i.e. decimal point).  The size or capacity of the media is then described not by increasing bits but by the number of 8-bit bytes that can be stored. The table below describes common capacities of storage media:
 
Media
Number of Bytes
Access Speed
5 1/4” Floppy 720 Kilobytes seconds
3 1/2” Floppy 1.44 Megabytes seconds
Zip Disk 100 or 200 Megabytes seconds
RAM 12 Gigabytes nanoseconds
CDROM 650 Megabytes seconds
Magneto Optical Disk 650 Megabytes seconds
Flash Drive 128 Gigabytes milliseconds
DVD 5.4-9.6 Gigabytes seconds
Hard Drive 1-4 Terabyte milliseconds
Digital Tape 1-10 Terabyte minutes
    Table 3.1.5
 
A kilobyte is 1024 (one thousand) bytes. A megabyte is 1,048,576 (one million) bytes. A gigabyte is 1,073,741,824 (one billion bytes), and a terabyte is 1,099,511,627,776 (one trillion) bytes. While some of these capacities seem almost infinite, digital video is also quite large. Uncompressed video will run about 622,080,000 bytes or 600 megabytes per minute (720 x 480 video at 30 frames per second for one minute). If a study takes 5 minutes of digital video, the file will be 3 gigabytes. Even if 10 hard drives were daisy-chained together (a RAID system), the total capacity of the RAID system would be 3 terabytes or about 900 studies. Clearly, a modern echocardiography system will require fast computers, large hard drive RAID systems, and a digital tape for long term storage.

Newer echocardiography machines will sample video at 60 frames per second at a video size up to 1200 x 800 resulting in a video of about 9 gigabytes per study. A higher resolution study will tremendously increase the amount of storage space and network speed that is required for a modern digital echocardiography system.
Storage Relationship
1 Byte = 8 bits
1 Kilobyte = 1024 Bytes
1 Megabyte = 1024 Kilobytes
1 Gigabyte = 1024 Megabytes
1 Terabyte = 1024 Gigabytes
1 Petabyte = 1024 Terabytes
1 Exabyte = 1024 Petabyte
1 Zettabyte = 1024 Exabyte
1 Yottabyte = 1024 Zettabyte*
1 Brontobyte = 1024 Yottabyte*
1 Geobyte = 1024 Brontobyte*
1 Zotzabyte = 1024 Geobyte*
* = Proposed              Table 3.1.6
 
Processing Requirements
 
Most of the current echocardiographic machines have a single CPU. A single processor has limits to its speed of processing the vast amount of bits of information from a video display. The limit in an echocardiographic machine is the number of pulses that can be sent and received in a set amount of time.  This limit is called the pulse repetition frequency.  The speed of sound, c, and the depth of the display, will determine the pulse repetition frequency (PRF).  The formula for a pulse repetition frequency is:
 
PRF = c/2d
 
where c = 1540 m/sec and d is the depth. At a depth of 12 cm the PRF will be:
 
PRF = (154000 cm/sec) / (2 * 12 cm)
 
which is 6,417 cycles/sec or 6.417 kHz. Therefore, at 12 cm the maximum PRF or pulses per scan line is 6417/sec. If a scan has 100 scan lines per frame then the total pulses per frame would be 64,170 pulses per frame per second. If the frame rate is 30 frames per second (fps) then the number of pulses would be 2,139 per frame. The frame rate, scan lines, and pulses per scan line are all interrelated.  If one factor is increased then one or both of the other factors must be decreased.  The relationship between pulses per scan line, scan lines per frame, frames per second, and the PRF is shown below:
 
PRF = pulses/scan line x scan lines/frame x frames/sec
 
Resolution is the ability to discern between two distinct anatomic points. The displayed resolution depends upon the above factors. The number of pulses along a scan line determines the resolution along that line. The number of scan lines per frame determines the resolution between scan lines. The frame rate determines the resolution of cardiac movement or events. Since all of the factors are related to produce a PRF value. Since the PRF determines the maximum of the other factors, manipulating the other factors determines the quality of the image displayed. The best scan will have a high frame rate, a high number of scan lines, and a high number of pulses along the scan line. The frames per second could be increased but at a cost of the number of pulses per second or the number of scan lines per frame. The number of scan lines could be increased but at the cost of the number of pulses or the frame rate. Lastly, the number of pulses could be increased but at a cost of the number of scan lines or the frame rate.
 
Raster vs. Vector Images
 
Images are displayed in two basic formats: raster or vector. Raster images are pixel based. Raster images are drawn and manipulated by changing each pixel. A pixel is a "dot" on the screen that represents the resolution of the image. If an image is 720 x 480 pixels, then it will contain 720 * 480 or 345,600 pixels. Each pixel can represent a color depending upon its bit depth. As noted above if the image is an 8 bit image, then each pixel has the possibility of one of 256 colors. As the number of pixels increases the resolution of the image increases. True photographic quality is 2400 x 2400 pixels whereas digital video is 720 x 480 pixels. Most monitors will display 800 x 600 or higher resolutions. Raster images, if enlarged or shrunken, will appear distorted from the original image. As the image size is changed, the pixels between other pixels must be filled in or deleted. This filling in or deletion phenomena will cause the image to become blurry and distorted because the program must guess the pixel values between the original pixels. Below is a raster image. A portion of the raster image has been blown up in the red circle. Inside the red circle is blown up further and displayed in the enlarged raster image picture. From a distance boundaries will appear sharp, but, under enlarged conditions, the boundaries are blurred as the pixel values change from white to black. When the image is shrunk, boundaries also become blurred because pixel values must be filled in where no original pixel value existed.
 
Image 3.1.7
Normal Raster Image
Image 3.1.8
Enlarged Raster Image
Image 3.1.9
Reduced Raster Image
 
Vector images, on the other hand, do not suffer distortion when their size is changed. Vector images draw the image using a series of mathematical algorithms. If the size of the image is changed, the image is simply redrawn using the mathematical formulas. Vector images always appear sharp at any size. However, Vector images cannot draw each pixel differently from adjacent pixels. All of the pixels in a given area are the result of a mathematical formula. Therefore, for pixel-to-pixel control raster images are superior, but, for image size manipulation, vector images are superior.
 
Image 3.1.10
Normal Vector Image
Image 3.1.11
Enlarged Vector Image
Image 3.1.12
Reduced Vector Image
 
Voxels
 
While an image or video is displayed on a 2D screen, it is displayed in pixels. However, 2D images and video are captured in pixels, whereas, 3D volumes are captured in voxels. A voxel is a unit of volume, similar to a pixel as a unit of area. Voxels are combined to result in a volume set. Pixels are combined to result in an image set. Voxels are raster, not vector based so similar rules apply when altering a voxel based video.  
Voxels