Ch. 28 The Digital Image

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Spatial Resolution
Ability to image small objects that have high subject contrast.
Resolution in space
A measure of how small an object one can see on an image.
Spatial frequency
*Measure of resolution; relates to the number of line pairs in a given length; usually expressed in line pairs per millimeter (lp/mm).
*It is low and easy to image for large soft tissue such as liver, kidney and brain but it would be high and more difficult to image for bone trabeculae, breast microcalcifications and contrast-filled vessels.
Smaller
As spatial frequency becomes larger, the object becomes ________.
Line Pair
One bar(high-contrast line) and its interspace of equal width.
Size of the pixel
Spatial resolution in ALL of the digital imaging modalities is limited by what?
One pixel
No digital imaging system can image an object smaller than what?
Modulation transfer function (MTF)
Mathematical procedure for measuring resolution; the ratio of image to object as a function of spatial frequency.
Increase
When a line pair pattern is imaged, the line pairs become more blurred with the __________ of spatial frequency.
Computed Tomography because of x-ray beam collimation and the resultant reduction in scatter radiation
Which x-ray imaging system has the best contrast resolution and why?
Focal-spot size
In screen-film radiology, spatial resolution is determined by what?
pixel size
In digital imaging, spatial resolution is determined by what?
Digital image receptor response is linearly related to radiation dose, image contrast does not change with dose.
Why can't an overexposure or underexposure of a digital image occur and is also the reason why a digital image should never require repeating for reasons of exposure factors?
143µ
(It takes 7 objects to form a line pair. 7 objects in 1mm [7/mm]→ reciprocal 1/7mm= .143mm convert to µm→ .143 × 10⁻³ =1.43E⁻⁴......1.43E⁻⁴ ÷ 10⁻⁶=143µm)
A digital radiographic imaging system has a spatial resolution of 3.5 lp/mm. How small an object can it resolve?
Higher
An imaging system with ____________ spatial frequency has better spatial resolution.
200 µm
Most people can see objects as small as how many micrometers?
1
A system would have an MTF equal to what if the system were able to produce an image that appeared exactly as the object?
10 lp/mm
(50µm→ 1 single object. It takes two objects to form a single line pair→ 50 µm × 2= 100µm [1lp=100µm or 1lp/100µm] convert to mm→ 100×10⁻⁶= 1E⁻⁴......1E⁻⁴÷10⁻³= .1mm→ 1lp/.1mm= 10lp/mm)
A screen-film mammography imaging system operating in the magnification mode can image high-contrast microcalcifications as small as 50 µm. What spatial frequency does this represent?
...
The use of two screens amplifies the contrast of large objects with little blur
...
The single screen and smaller focal spot result in better spatial resolution with mammography.
Expanded dynamic range of the DR and its higher detective quantum efficiency (DQE)
Why is it that DR has higher MTF at low spatial frequencies?
black and white
What is 100% contrast?
The ability to distinguish many shades of gray from black to white.
What is contrast resolution?
Dynamic Range
The principal descriptor for contrast resolution is grayscale. What can it also be called?
The number of gray shades that an imaging system can reproduce.
What is dynamic range?
By the bit capacity of each pixel
How is the dynamic range of a digital imaging system identified?
12-bit dynamic range
What is the bit capacity of a computed tomography and magnetic resonance imaging system?
14-bit dynamic range
What is the bit capacity of a DR?
16-bit dynamic range
What is the bit capacity in mammography?
0 - 3.0
Dynamic range of screen-film radiograph is essentially three orders of magnitude, from an optical density of how much?
30 shades
An OD of 0 - 3.0 represents a dynamic range of 1000 but how many shades of gray can a human visualize?
To preprocess and postprocess the image for the purpose of extracting even more information.
What is the principal advantage of digital imaging?
Contrast; Spatial
_________ resolution is more important than __________ resolution when soft tissue is imaged.
Postprocessing
What tool is used to allow visualization of all shades of gray?
Signal
Represents the difference between those xrays transmitted to the image receptor and those absorbed photoelectically.
mAs
As _________ is increased, the SNR also is increased.
the number of images integrated
Another way to increase SNR is seen in digital subtraction angiography (DSA). Signal increases in proportion to _________________________; noise increases in proportion to the square root of the number of images.
Contrast-detail curve
The method for evaluating the spatial resolution and contrast resolution of an imaging system.
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