| |
| Resolution |
| |
| Resolution of an image is dependent upon the number
of pixels an image contains. A pixel is a dot or square that is
assigned a color. The sum of all of the pixels, which make up the
image, will determine the resolution of the image. As the size of
the total pixel number increases, the resolution or clarity of the
image will increase. If a small resolution image is enlarged, the
image will appear grainy or blurry. An image that is a high resolution
image, when enlarged, will retain it's clarity. Below are some common
picture sizes and the resolution size of the image. |
| |
| Width (pixels) |
Height (pixels) |
Total Number of Pixels |
Resolution Name |
Quality |
| 640 |
480 |
307,200 |
0.3 Megapixel |
Newspaper |
| 720 |
480 |
345,600 |
Digital Video |
Digital Video |
| 1280 |
1280 |
1,638,400 |
1.5 Megapixel |
Poor Photo Quality* |
| 2400 |
2400 |
5,760,000 |
5 Megapixel |
Near Photo Quality* |
| 2400 |
3000 |
7,200,00 |
7.2 Megapixel |
Photo Quality * |
3264
|
2448 |
7,990,272
|
8.0 Megapixel
|
Photo Quality *
|
| 3600 |
3600 |
12,960,000 |
12 Megapixel |
Super Photo Quality * |
| |
|
|
|
Table 3.2.1 |
|
*Standard 4 x 6 print at 300 dpi
|
| |
| An image's width and height can be defined by the number of pixels or dots per inch (dpi). Dots per inch (dpi) is commonly used for describing the resolution of a printed image. A printed image has a width and height, but the number of dots per inch describes the resolution. A screen image may be 720 pixels wide and 480 pixels wide, but if it's printed resolution is 80 dpi, then it's width and height will be 9 inches by 6 inches. If the printed resolution is increased to 160 dpi, then the width and height changes to 4.5 inches by 3 inches.
For true photo quality, a printed image will have 300 dpi which will require a several megapixel resolution image for a standard print size. If an 8 by 10 inch true photographic quality photo is required then a 2400 by 3000 pixel image is required. To the right is a table of image pixel requirements for standard print sizes. |
| 300 dpi Print Size |
Megapixels |
| 4 x 6 inch |
7.2 |
| 6 x 8 inch |
14.4 |
| 8 x 10 inch |
24 |
| 16 x 20 inch |
96 |
| |
Table 3.2.2 |
|
|
| |
| Aspect Ratio |
| |
| The aspect ratio is the ratio between the width and the height of the image. An image that has a resolution of 3000 x 2400 pixels will have an aspect ratio of 3:2. An image that has a resolution of 3000 x 3000 is a square image with an aspect ratio of 1:1. When a captured image with an aspect ratio of 1:1 is printed with an aspect ratio of 3:2 the image can either be cropped, where the extra 600 pixels are just deleted, or the image can be re-sized. Resizing an image to match the printed aspect ratio can result in loss of the image resolution, image orientation, or pixels may be added or combined (interpolated) to achieve the proper aspect ratio. Straight lines on the monitor may not appear straight on the printed image. Therefore, when printed image quality is critical it is important that the image be re-sized on the monitor before being printed to ensure that the loss of clarity is not significant. |
| |
| Pixel Mapping and Color Mapping |
| |
| For example an image that is 720 pixels wide and 480 pixels high (the standard digital video format), will have 720 columns of pixels and 480 rows of pixels. An x,y coordinate map to the image all pixels could be described as having a column value (x) from 0 to 719 and a row value (y) from 0 to 479. For each pixel at an (x,y) value, the pixel could be assigned a color value. If the image were an 8-bit image, there would be 256 color values to choose from. If the color map was a black-to-white map, then each color value would be assigned a gray scale color from 0 to 255. If the color map were a sepia map, then each of the values are still 0 to 255 but instead of a gray scale color, each color would be a variant of the color sepia. Notice that while the color value remains unchanged, the color displayed can change depending upon the color map used. |
| |
 |
 |
 |
Image 3.2.3
Pixel Coordinate Map |
Image 3.2.4
Color Map |
Image 3.2.5
Pixel Color Assigned |
|
 |
 |
Image 3.2.6
Pixel Coordinate Map |
Image 3.2.7
Color Map |
Image 3.2.8
Pixel Color Assigned |
|
| |
| Brightness |
| |
| Brightness of a pixel refers to
the intensity of the pixel or it's luminescence. For a color map, a
color's intensity or brightness depends upon the amount of "whiteness"
in the color. For example, a red pixel with almost no "whiteness"
will appear to be a deep red color, whereas a red pixel with a significant
amount of whiteness added will appear pink. From the primary colors
and the amount of intensity of those colors, almost all shades of
colors can be produced. Some pixel formats will separate the intensity
into it's own value (YUV, HSV) whereas other pixel formats will incorporate
the intensity into the color value (RGB). |
| |
| Gray Scale vs. Color |
| |
| As noted above, the displayed color, whether gray scale or color depends upon the color map. For 2D echo, a gray scale map or some variance of a singe color (i.e. sepia) is used. For Doppler, another color map, usually blue to red, is used to display the color values. In 2D echocardiography, the color value is dependent upon the amplitude of the reflected wave. Reflected echoes with a high amplitudes are assigned a higher color value than low amplitude reflections. Therefore, when utilizing the color map, the higher values are assigned a brighter color than the lower values. In Doppler echocardiography, the frequency shift decides the color value. A higher frequency shift is assigned a higher color value than a lower frequency shift. A positive frequency shift is assigned a color value while a negative frequency shift is assigned another color value, depending upon the map used. Typically, in a red-blue map, a positive frequency shift is assigned a red value, while a negative frequency shift is assigned a blue value. If a different map were applied to the Doppler signal, the original values assigned to each pixel would not change, only the display of those pixel values would change. |
| |
 |
 |
 |
Image 3.2.9
2D Signal Amplitude |
Image 3.2.10
Map |
Image 3.2.11
Pixel Assigned Color |
|
| |
 |
 |
 |
Image 3.2.12
CFD Signal Velocity |
Image 3.2.13
Map |
Image 3.2.14
Pixel Assigned Color |
|
| |
| Color Systems |
| |
| Two types of color systems exist, additive and subtractive. An additive
color system will display white when all of the primary colors are
added together. Black is created by the absence of any of the primary
colors. An example of an additive color system is the RGB or Red-Green-Blue
color system. Subtractive color systems create white by the absence
of the primary colors. To create black in a subtractive color system,
all of the primary colors are added together. An example of a subtractive
color system is the CYM (Cyan-Yellow-Magenta) color system. Additive
color systems are used for display systems where the color is lumina ted
to produce an image. Subtractive color systems are used where the
colors are reflected (i.e. printed paper). |
|
|
|
|
Image 3.2.15
Additive Color System (RGB)
|
Image 3.2.16
Subtractive Color System (CYM)
|
|
| |
| Color Formats |
| |
| There are different methods for representing color
and intensity information in an image. The format used to store
this information is also known as the color format. There are four
major types of color formats. They are classified as RGB, YUV
, CYM, and HSV formats. When the
color format is applied to a (self-illuminating) display system,
the color is applied to a pixel. When the color format is applied
to a paint format, usually white paper, the color is applied to
a dot. Each dot or pixel will contain only one color. Since the
human eye cannot see each individual pixel or dots in high resolution
images, the human eye will combine adjacent pixel or dot colors
to form a new color. RGB and YUV are commonly used for pixels. CYM
and HSV are commonly used for dots. |
| |
| System |
1st Letter |
2nd Letter |
3rd Letter |
Uses: |
| RGB |
Red |
Green |
Blue |
Monitors |
| YUV |
Luminance |
Chrominance |
Chrominance |
TV |
| CYM |
Cyan |
Yellow |
Magenta |
Printed Paper |
| HSV |
Hue |
Saturation |
Value |
Paint |
| |
|
|
|
Table 3.2.17 |
|
| |
| RGB Pixel Format |
| |
| The RGB format is the most common pixel format
used today. The RGB is an additive color system used in the display
of images, including videos. The primary colors in color video are
red, green, and blue (RGB). RGB is often used to describe a type of
video color-recording scheme and the type of equipment that uses it.
It also describes a type of computer color display output signal comprising
separately controllable red, green, and blue signals (as opposed to
composite video, in which signals are combined before output). RGB
monitors typically offer higher resolution than composite monitors.
Each pixel is assigned a number from 0 to 255 which represents each
shade of red, blue or green. For example, a pure blue RGB pixel would
have the value 0,0,255. Various combinations of the RGB values are
used to display the pixel color on the monitor. |
| |
| YUV Pixel Format |
| |
| YUV Color System is a color-encoding
scheme for natural pictures in which the luminance (the black-and-white
component of a video signal that controls the light intensity) and
chrominance (the color component of the video signal) are separate.
The human eye is less sensitive to color variations than to intensity
variations, so YUV allows the encoding of luminance (Y) information
at full bandwidth and chrominance (UV) information at half bandwidth.
The YUV format is a linear transformation of the RGB format and
is most widely used for television transmission. It is not a true
color system since there is not a combination of colors to produce
a color. The RGB format is linearized and the linearized value is used for
the color value (UV) in the YUV pixel format. Variations of the
YUV pixel format are the UCbCr and YPbPr formats.
YUV formats are subdivided into two more groups: packed and planar.
In the packed format, the Y, U, and V components are stored in a
single array. The three components packed into what is known as
a macropixel (two pixels stored in one unsigned integer value (an
unsigned integer is an integer that is positive)). Conversely, the
planar format stores these components in three separate arrays and
combines the planes to form the image.
The variations in the different YUV samples are based on how data
is sampled, both in the horizontal and vertical directions. The
horizontal subsampling interval describes how frequently across
a line that a sample of that component is taken, and the vertical
interval describes on which lines samples are taken. For example,
if the format has a horizontal subsampling period of 2 for both
the U and V components, it indicates that U and V samples are taken
for every second pixel across a line. If the vertical subsampling
period is 1, it indicates that U and V samples are taken on each
line of the image. |
| |
| CMY Pixel Format |
| |
| CMY (Cyan-Magenta-Yellow) is a subtractive color system.
Therefore, it is used mostly by printers and photographers. The combination
of the primary colors are used to produce shades of colors on white
paper. When illuminated the color reflected is the the result of of
the other colors being absorbed. For example, a combination of cyan
and magenta will absorb cyan and magenta from the illuminating light,
leaving only yellow to be reflected. A variant of the CMY color system
is the CMYK color system. The K stands for black. Since producing
black from the combination of cyan-magenta-yellow could be very expensive,
a black cartridge is added to the printer and the amount of black
to be added is determined by the K value. |
| |
| HSV Pixel Format |
| |
| HSV (Hue-Saturation-Value) is a color system
that is neither additive nor subtractive. HSV varies the degree of
properties of colors to create new colors rather than using a mixture
of primary colors. Hue specifies the color value. Saturation or chroma
refers to the amount of white in a hue. Value or brightness specifies
the amount of self-luminescence of a color. Self luminescence is the
amount of light a color emits. The HSV format is most commonly used
by painters and other artists. Variations of the HSV format are HSL (Hue-Saturation-Luminosity)
and HBL (Hue-Brightness-Luminosity). |
| |
| Image
File Formats |
| |
An image file format contains the information
about each pixel, how they are arranged, and a compression scheme
for the image. Each of the formats will contain the values for each
pixel, the map used, the pixel format, and information about the
file, such as how many rows and columns. The information about the
pixels is placed into a file header, while the raw pixel information
is contained in the main body of the file. Commonly used formats
are TIFF, BMP, GIF, JPEG, and DICOM.
While each of the formats has their advantages and disadvantages,
the universal standard for echocardiography is the DICOM format.
The DICOM (Digital Imaging and Communications in Medicine) standard was by developed by the American College of Radiology (ACR) and the National electrical Manufacturers Association (NEMA) in a joint committee. Version 3.0 was released in 1993 under the name of DICOM and included numerous improvements as well as TCP/IP compatibility. |
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