contrast

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Related to positive contrast: negative contrast

con·trast

(kon'trast),
1. A comparison in which differences are demonstrated or enhanced.
2. In radiology, the difference between the image densities of two areas is the contrast between them; this is a function of the number of x-ray photons transmitted or the strength of the signals emitted by the two regions and the response of the recording medium.
4. Performed with a contrast medium.
[L. contra, against, + sto, pp. status, to stand]

contrast

/con·trast/ (kon´trast)
1. the degree to which light and dark areas of an image differ in brightness or in optical density.
2. in radiology, the difference in optical density in a radiograph that results from a difference in radiolucency or penetrability of the subject.

contrast

[kon′trast]
Etymology: L, contra, against, stare, to stand
a measure of the difference in optic density, radiation transmission, pixel brightness, or other parameters between two adjacent areas in a radiographic image. Contrast plays an important role in the ability of a radiologist to perceive image detail.

contrast

Imaging An MRI term for the relative difference of signal intensities in two adjacent regions of an image. Image contrast heavily depends on the chosen imaging technique (i.e., TE, TR, TI), and reflects such parameters as proton density and T1 or T2 relaxation times.

con·trast

(kon'trast)
1. A comparison in which differences are demonstrated or enhanced.
2. radiology The difference between the image densities of two areas.
[L. contra, against, + sto, pp. status, to stand]

Contrast (agent, medium)

A substance injected into the body that illuminates certain structures that would otherwise be hard to see on the radiograph (film).

contrast 

1. Subjective sense: subjective assessment of the difference in appearance of two parts of a field of view seen simultaneously or successively. Hence, luminosity contrast, lightness contrast, colour contrast, simultaneous contrast, successive contrast.
2. 
Objective sense: quantities defined by the formulae for luminance contrast.
Note: Example (c) is better known as luminance ratio (CIE). L2 is the maximum luminance and L1 is the minimum luminance. See spatial frequency; contrast sensitivity; differential threshold.
brightness contrast The enhanced apparent darkening of an area when viewed near, or following a lighter stimulus (or lightening, near or after a darker stimulus).
colour contrast 1. A difference in the appearance of surfaces based on hue or saturation. 2. The enhanced difference in the colour of two surfaces induced by their proximity (simultaneous colour contrast) or successive stimulation (successive colour contrast). The appearance of the smaller area shifts towards the complementary colour of the surround or background. Example: a yellow area appears reddish when surrounded by a green background.
contrast sensitivity function See contrast sensitivity function.
contrast sensitivity test See contrast sensitivity chart; contrast sensitivity.
simultaneous contrast A difference in the appearance of two adjacent areas occurring at the same time.
successive contrast A difference in the appearance of stimuli following each other.
contrast threshold See contrast sensitivity; differential threshold.

con·trast

(kon'trast)
1. A comparison in which differences are demonstrated or enhanced.
2. In radiology, the difference between the image densities of two areas is the contrast between them.
3. Contrast medium.
4. Performed with a contrast medium.
[L. contra, against, + sto, pp. status, to stand]

contrast

radiographically the degree of perceptible difference between two color tones. Black and white images on the one film is said to be high contrast; an all gray film has low or nil contrast.

contrast agents
contrast agents are used for injection into the vascular system for either a local visualization of a system or organ or for outlining an excretory system. Radiolucent (negative) contrast media are gases such as air, oxygen or carbon dioxide. The radiopaque (positive) contrast media include the insoluble salt barium sulfate and a variety of organic iodine compounds.
Barium is used for gastrointestinal studies. Water-soluble, iodinated contrast media excreted by the kidneys are used for many procedures, including all types of angiography and for intravenous and retrograde urography. Those excreted by the liver are used for oral or intravenous cholangiography or cholecystography. New, nonirritant iodine compounds have been developed for myelography. Oily iodinated media are used for lymphangiography and bronchography.
double contrast
the use of two contrast agents or two routes of administration in the one patient. For example, radiopaque dye and then air in the urinary bladder.
contrast materials
see contrast agents (above).
contrast medium
a substance used in radiography to permit visualization of internal body structures. Called also contrast agent, contrast material.
negative contrast
a contrast material that is not radiopaque such as air or carbon dioxide.
contrast pattern
the pattern made by the contrast agent. Includes confined extension patterns in diverticula and similar confined spaces, and unconfined extensions, e.g. in bladder rupture.
positive contrast
the use of a contrast material that is radiopaque such as barium sulfate and iodinated products.
triple contrast
the use of three contrast media or routes in the one patient at the one time.
water-soluble contrast agents
agents used for injection into the vascular system for either a local visualization of a system or organ or for outlining an excretory system. In the past these have consisted mostly of iodine preparations which are irritant and cause tissue damage so that they must be injected intravenously. However, there are now available some recently developed water-soluble iodine preparations that are nonirritant and which can be used in myelography. See also contrast.

Patient discussion about contrast

Q. Has anyone had an allergic reaction to gadolinium dye, MRI contrast agents, I have had a severe reaction. I would like to know the long term effects of this dye. And if anyone else has had or heard of problems and reactions to it. Please answer me. Thank you

A. In 1969 I almost died from the IVP dye. I had no idea I was allergic and when I awoke I was in a "recovery room." The doctor told me to always tell any physicians/paramedics etc of my allergy status regarding the dye. I now have chronic back pain, have a history of cancer in the family and the doctor wants to do a scan (including dye) but when I emphasized that I was allergic he backed off. Now I am wondering if there is anything else that can be done to test the bone (scan) without the dye. Any answers? Thanks

More discussions about contrast
References in periodicals archive ?
For example, the predictions of the theory for positive contrast would not apply if the baseline schedule provided such a low rate of reinforcement that no habituation occurred.
In the positive contrast conditions, positive scores indicate positive contrast while negative scores indicate negative induction.
Higher rates of reinforcement were obtained in the averaged baseline than in the contrast phase for the positive contrast conditions.
Table 1 Rate of Responding and Percent Deviation from Baseline Positive Contrast Session Length Subject Baseline Contrast % Deviation 20 Min 903 46.
The lower axes of Figure 4 show that eliminating reinforcement in the second component of the positive contrast condition decreased the rate of key pecking in that component.
Subjects serving in the positive contrast group received the same procedure with the following exceptions.
The upper axes of Figure 6 show that the positive contrast group usually displayed positive contrast.
The lower axes of Figure 6 show that removing reinforcement from the second component during the positive contrast trial decreased the rate of lever pressing in that component.
Negative contrast analyses were performed on data from alternating and 4% control groups on days when the alternating groups received the 4% solution; positive contrast analyses were performed on data from alternating and 32% control groups on days when the alternating groups received the 32% solution.
Analysis of positive contrast data showed a significant shift x concentration interaction for lick rate, F(4, 124) = 5.
The observed differences in contrast effects between deprived and nondeprived groups in Phase 1 suggest that deprivation state plays a major role in the duration of negative contrast and the occurrence of positive contrast effects.
Shifted groups did not display positive contrast during the upshift periods.