UNIT 4: Review Material Flashcards
T or F: Spatial Resolution and Contrast Resolution are the most important characteristics of medical imaging quality
True
Contrast resolution is primarily controlled by
the amount of scatter radiation present on the IR
T or F: Collimation is the area in which the radiographer has the least control over contrast resolution
False, the most
Spatial Resolution is
the amount of detail present on any image or the ability to render small high contrast objects
Spatial frequency is expressed as
lp/mm or lp/cm
Higher spatial frequency indicates worse or better spatial resolution?
Better
Spatial resolution of a system is measured by
spatial frequency
Spatial Resolution of Imaging Systems
This demonstrates that mammography has the greatest spatial resolution – this is due to small focal spots used (0.3-0.6mm) as compared to radiography (1.0-2.0 mm), Film radiography has better spatial resolution than digital, but digital has superior contrast resolution
DQE is the
measure of how sensitive and accurate incoming data is converted to output viewing. DQE of 1 = 100% or no loss of information. No system can represent the object with 100% accuracy, therefore DQE is always less than 1
Film/screen systems have the ________ DQE and DR has the _________
Lowest, highest
K-shell absorption edge
When the x-ray energy equals the K-shell electron binding energy, the two K-shell
electrons become available for photoelectric interaction. Consequently, at this energy, the probability of photoelectric absorption increases abruptly. This abrupt increase in absorption at this energy level is called the K-shell absorption edge, and it is followed by another rapid reduction in photoelectric absorption with increasing x-ray energy
Lanthanum oxysulfide (LaOS) and gadolinium oxysulfide (GdOS) are used in
film/screen
Barium fluorobromide (BaFBr), cesium iodide (CsI), and amorphous selenium (a-Se) are used with
digital image receptors
more or less photoelectric absorption occurs with the DR receptors than film/screen?
-More, this translates to higher DQE and more importantly to less patient dose needed to produce an acceptable image
Modulation Transfer Function (MTF) measures
the accuracy of a recorded image compared to the original object. In other words it measures recorded detail, (sharpness or resolution) of a system as a function of image contrast. MTF is plotted on a scale of 0-1. Zero is no image and 1 is perfect fidelity
As spatial frequency increases, MTF
Decreases
Explanation: This is due to increased image blur with increase spatial frequency.
At low spatial frequencies (large objects), good reproduction is noted on the image.
However, as the spatial frequency of the object increases (the objects get smaller), the faithful reproduction of the object on the image gets worse. Higher spatial frequency means smaller objects but smaller objects are harder to see and to image due to image blur. MTF measures how true the recorded image is to the actual image. This is called fidelity. There is no system that can image an object exactly, therefor MTF is always less than 1.
Quality control test objects and tools have been designed to measure
the amount of blurring as a function of spatial frequency.
These are usually plastic-encased lead bar patterns that are designed to construct a modulation transfer function (MTF) curve. Such tools used with a microdensitometer (film) or special software (digital) can measure the modulation of each spatial frequency pattern and can use those data to construct an MTF curve
MTF Curve
-MTF Curve plots the fidelity of the object (MTF) against the size of the object ( spatial frequency). This is just a fancy way to prove that large objects are easier to image than small objects.This type of curve is most useful when comparing the fidelity of two or more imaging systems.
•Large objects = low spatial frequency- MTF closer to 1.0- are easier to image faithfully
•Small objects = high spatial frequency – MTF closer to 0 – they are harder to image
faithfully
•Contrast is lost at high spatial frequencies
Dynamic range and Exposure latitude
-Dynamic Range is the number of shades of gray the imaging system is capable of
representing. With film screen imaging the dynamic range is about 1000 shades of
gray. Even though the range of Optical Density (OD) is from 0.0 to 3.0, the useful
ranges are between the toe and shoulder of the H & D curve (see image on the left)
With digital imaging the dynamic range can up to about 65,000 (216) shades of gray for digital mammography. For digital radiography about 214 or 16,384 shades of gray are available. This is based on the pixel bit depth of the imaging system.
-The exposure latitude is the range of exposures than can be used to produce an
acceptable image. Digital imaging systems have a greater range of exposures that will produce an acceptable image than film/screen. This is reason that most repeats in digital are due to errors in positioning rather than for technical factor selection. In
film/screen, because of the very narrow exposure latitude about half of all repeats were due to technical factor selection.
Performance standards have been developed for monitors used to view medical images. Examples of test patterns used to verify performance of medical displays were developed by the
-The Society of Motion Picture and Television Engineers (SMPTE) called appropriately the SMPTE pattern (left image)
-Another set of QC patterns (right image) was developed by the American Association of Physicists in Medicine (AAMP) called the task group (TG) report 18
