Unsharpness Calculations
and
Resolution
By Professor Stelmark
Source-to-object distance (SOD)
refers to the distance from the x-ray source (focal spot) to the object
being radiographed. SOD can be expressed mathematically as follows:
Focal spot size FSS = Dimensions of the nominal focal spot in millimeters (mm)
OID
= The distance between the object (area of interest) and the image receptor
SOD
= The distance between the focal spot (source) and object (area of interest)
SOD
= SID−OID
Calculating Geometric Unsharpness
The amount of geometric unsharpness can be calculated for each of the following images to
determine which image has more geometric unsharpness.
Image 1
Image 2
Focal spot size=0.6 mm SID=40 inches
OID=0.25 inch
Focal spot size=1.2 mm SID=56 inches
OID=4.0 inches
Geometric unsharpness of Image 1=0.004 mm
Geometric unsharpness of Image 2=0.09 mm
Image 2 has the greater amount of unsharpness.
TEMPORAL
RESOLUTION
SPATIAL RESOLUTION
CONTRAST RESOLUTION
RADIATION
DOSE
NOISE
IMAGE ARTIFACTS
SPATIAL RESOLUTION
The degree of sharpness in the image
The degree of blurring in the image
Spatial resolution (also known as high-contrast resolution) is the ability of an
imaging system to create separate images of closely spaced high-contrast (black
and white) objects (as with contrast resolution, spatial resolution also applies to
all imaging modalities). In other words, do the two objects appear sharp and clear,
or do they blur together?
The spatial resolution is specified in terms of line pairs per centimeter (lp/cm)
or line pairs per millimeter (lp/mm). A line pair is a pair of equal-sized blackwhite bars. The number of line pairs in digital imaging is specified as spatial
frequency.
7 lp/mm
17 lp/mm
11 lp/mm
11 lp/mm
The most common method of measuring spatial resolution is to use a
value known as spatial frequency. The unit of spatial frequency is the line
pairs per millimeter (lp/mm) and is obtained with a resolution chart (Fig.
3-13). A line pair includes an opaque line and a radiolucent space. In the
resolution chart, one lp/mm would have a 0.5 mm lead bar separated by
0.5 mm of radiolucent material. Two lp/mm would have 0.25 mm lead
bars separated by 0.25 mm of radiolucent material and so the greater the
lp/mm value, the smaller the object that can be imaged and the better the
spatial resolution. The limiting spatial resolution (also known as the
Nyquist frequency) is the maximum number of lp/mm that can be
recorded by the imaging system.
The resolving power of the unaided human eye is approximately 30
lp/mm when inspecting an image up close, and at normal reading
distance (about 25 cm), it is about 5 lp/mm.
Imaging System
Analog non screen film system
Analog screen film system
Limited Spatial Resolution
50 – 100 lp/mm
6 -10 lp/mm
Computed Radiography (CR)
2-5 lp/mm
Digital Radiography (DR)
8-10 lp/mm
Smallest
Object
Resolved
SPATIAL FREQUENCY- 15 lp/mm.
WHAT IS THE SMALLEST SIZE OF AN OBJECT
THE IMAGING SYSTEM CAN RESOLVE
1/15 = 0.06 mm/lp
0.06 /2 = 0.03 mm object
Spatial Resolution Influencing Factors in Digital
Imaging
• FOV
• Matrix size
• Type of Algorithm
Contrast Resolution
The ability of an imaging system to distinguish structures with similar x-ray
transmission as separate entities (the term not only applies to intensifying
screens, but all imaging systems including computed radiography (CR),
digital radiography (DR), conventional and digital fluoroscopy, computed
tomography, magnetic resonance imaging, and sonography).
Contrast Resolution Influencing Factors in Digital
Imaging
•
•
•
•
SNR
Dynamic Range
Pixel Depth
Type of Algorithm
TEMPORAL RESOLUTION
Temporal resolution is an indication of a X-ray system's ability to freeze motions
of the exposed object. An oversimplified analogy is the “shutter” speed of a
camera. When a photo is taken at a sports event, a higher shutter speed should
be used to reduce the blurring effects caused by the moving athletes.