Which of the following is not an example of a method of contrast that qualifies as with contrast

Contrast radiography is a method of studying organs using X-rays and the administration of a special dye, called a contrast medium. This test allows the radiologist to evaluate structures that are not clearly evident on conventional X-ray exams.

X-rays work by passing through the body. Because bones block the X-rays easily, they show up clearly. But organs and other tissue – such as blood vessels, the stomach, and the colon – do not block the X-rays so easily. The contrast medium highlights these specific areas in the body and helps them to be seen in greater detail on the X-ray image.

UT Southwestern specialists are highly trained and experienced in conducting and evaluating contrast radiography scans. We offer advanced imaging tools, many of which are not available at other medical facilities.

Conditions

There are many kinds of contrast radiography procedures. Exams available at UT Southwestern include:

  • Intravenous pyelography, or IVP, which allows the doctor to examine the urinary system, such as the kidneys, ureters, and bladder, and identify tumors, cysts, and stones
  • Upper GI (gastrointestinal) and small bowel series, which are used to examine the esophagus, stomach, and upper small intestine and identify ulcers, obstructions, tumors, or inflammations
  • A barium enema, also called a lower GI series, which is used to examine the colon and rectum and detect polyps, cancer, inflammation, and diverticula (pouches within the colon)
  • Angiography, which allows a doctor to examine the blood vessels and various organs to detect obstructions, tumors, and other problems in the heart, lungs, kidneys, arms, and legs
  • Cardiac catheterization, which is used to evaluate the heart and its vessels 

Contrast Radiology: What to Expect

The type of test a patient receives determines how the exam is conducted. The doctor will give complete instructions prior to the scan.

The patient might be asked to fast before the scan. He or she might also be asked to drink fluid before the test or might have fluid administered through an IV. The patient might also be given a prescription for a laxative or enema to use before arriving at the appointment. Check with the doctor before taking any other medications prior to the exam.

When the patient arrives on the day of the appointment, he or she will be asked to change into a gown and remove items that might interfere with the X-ray, such as:

  • Jewelry
  • Watches
  • Hearing aids

Contrast medium can be given in different ways, depending on what organ or tissue needs to be examined.

  • If the patient receives the contrast medium through an injection in the arm, he or she might feel a warm sensation.
  • If the patient is having an upper GI or small bowel series, he or she will drink a barium solution that looks like a milkshake. Then the patient might be asked to drink a carbonated beverage or medication to produce gas. The gas helps create contrast on the X-ray.
  • If the patient is having a barium enema, or lower GI series, a small tube will be inserted gently into the rectum and barium will flow into the bowel. The patient might feel the urge to empty the bowel but should hold in the barium until asked to release. Another image will be taken after the patient empties his or her bowel.
  • For angiography and cardiac catheterization, the patient will receive a sedative through an IV, and a local anesthetic will be applied to an injection site on the body. A small incision will be made, and a catheter tube will be gently inserted into a blood vessel. Contrast medium will be injected through the catheter.

The technologist will take X-rays at specific intervals. During this time, the X-ray machine might make noises as it shifts position and captures various angles. These sounds are normal.

During various examinations, the patient might be asked to empty his or her bladder, hold his or her breath, or make some other alteration so that new X-rays can be taken for comparison. It is important that the patient remain still during each examination.

After the procedure, most contrast medium will pass through the patient’s body naturally over 24 to 48 hours. Patients should drink plenty of water to help clear the material from the system more quickly. If the patient drank a barium solution or had a barium enema, the stool initially might be light-colored but should return to normal in no more than two to three days.

If the patient had an angiography or cardiac catheterization, he or she might be asked to rest in bed at the clinic for several hours.

The radiologist will review the images and send a report to the doctor, who will notify the patient of any findings. The patient might also request to receive images on CD.

Risks

It is important to note that while contrast radiography is effective and accurate, it does involve exposure to radiation. Some discomfort is associated with various contrast radiography exams, but it is usually minor.

Some people have an allergic reaction to contrast media, such as:

  • Hives
  • Itchiness
  • Nausea
  • Shortness of breath
  • Weakness

Report these symptoms to the doctor, radiologist, or imaging technologist immediately. Tell the technologist if the patient has any known allergies to contrast media or Iodine.

A radiology technologist or radiologist can answer any questions a patient might have about a health condition, including pregnancy, that could affect the exam.

The intent of this Success Criterion is to provide enough contrast between text and its background so that it can be read by people with moderately low vision (who do not use contrast-enhancing assistive technology). For people without color deficiencies, hue and saturation have minimal or no effect on legibility as assessed by reading performance (Knoblauch et al., 1991). Color deficiencies can affect luminance contrast somewhat. Therefore, in the recommendation, the contrast is calculated in such a way that color is not a key factor so that people who have a color vision deficit will also have adequate contrast between the text and the background.

Text that is decorative and conveys no information is excluded. For example, if random words are used to create a background and the words could be rearranged or substituted without changing meaning, then it would be decorative and would not need to meet this criterion.

Text that is larger and has wider character strokes is easier to read at lower contrast. The contrast requirement for larger text is therefore lower. This allows authors to use a wider range of color choices for large text, which is helpful for design of pages, particularly titles. 18 point text or 14 point bold text is judged to be large enough to require a lower contrast ratio. (See The American Printing House for the Blind Guidelines for Large Printing and The Library of Congress Guidelines for Large Print under Resources). "18 point" and "bold" can both have different meanings in different fonts but, except for very thin or unusual fonts, they should be sufficient. Since there are so many different fonts, the general measures are used and a note regarding fancy or thin fonts is included.

Note

When evaluating this success criterion, the font size in points should be obtained from the user agent or calculated on font metrics in the way that user agents do. Point sizes are based on the CSS pt size as defined in CSS3 Values. The ratio between sizes in points and CSS pixels is 1pt = 1.333px, therefore 14pt and 18pt are equivalent to approximately 18.5px and 24px.

Because different image editing applications default to different pixel densities (e.g. 72 PPI or 96 PPI), specifying point sizes for fonts from within an image editing application can be unreliable when it comes to presenting text at a specific size. When creating images of large-scale text, authors should ensure that the text in the resulting image is roughly equivalent to 1.2 and 1.5 em or to 120% or 150% of the default size for body text. For example, for a 72 PPI image, an author would need to use approximately 19 pt and 24 pt font sizes in order to successfully present images of large-scale text to a user.

The 3:1 and 4.5:1 contrast ratios referenced in this Success Criterion are intended to be treated as threshold values. When comparing the computed contrast ratio to the Success Criterion ratio, the computed values should not be rounded (e.g. 4.499:1 would not meet the 4.5:1 threshold).

The previously-mentioned contrast requirements for text also apply to images of text (text that has been rendered into pixels and then stored in an image format) as stated in Success Criterion 1.4.3.

This requirement applies to situations in which images of text were intended to be understood as text. Incidental text, such as in photographs that happen to include a street sign, are not included. Nor is text that for some reason is designed to be invisible to all viewers. Stylized text, such as in corporate logos, should be treated in terms of its function on the page, which may or may not warrant including the content in the text alternative. Corporate visual guidelines beyond logo and logotype are not included in the exception.

In this provision there is an exception that reads "that are part of a picture that contains significant other visual content,". This exception is intended to separate pictures that have text in them from images of text that are done to replace text in order to get a particular look.

Note

Images of text do not scale as well as text because they tend to pixelate. It is also harder to change foreground and background contrast and color combinations for images of text, which is necessary for some users. Therefore, we suggest using text wherever possible, and when not, consider supplying an image of higher resolution.

The minimum contrast success criterion (1.4.3) applies to text in the page, including placeholder text and text that is shown when a pointer is hovering over an object or when an object has keyboard focus. If any of these are used in a page, the text needs to provide sufficient contrast.

Although this Success Criterion only applies to text, similar issues occur for content presented in charts, graphs, diagrams, and other non-text-based information which is covered by Success Criterion 1.4.11 Non-Text Contrast.

See also 1.4.6: Contrast (Enhanced).

Text that is larger and has wider character strokes is easier to read at lower contrast. The contrast requirement for larger text is therefore lower. This allows authors to use a wider range of color choices for large text, which is helpful for design of pages, particularly titles. 18 point text or 14 point bold text is judged to be large enough to require a lower contrast ratio. (See The American Printing House for the Blind Guidelines for Large Printing and The Library of Congress Guidelines for Large Print under Resources). "18 point" and "bold" can both have different meanings in different fonts but, except for very thin or unusual fonts, they should be sufficient. Since there are so many different fonts, the general measures are used and a note regarding fancy or thin fonts is included.

Note

When evaluating this success criterion, the font size in points should be obtained from the user agent or calculated on font metrics in the way that user agents do. Point sizes are based on the CSS pt size as defined in CSS3 Values. The ratio between sizes in points and CSS pixels is 1pt = 1.333px, therefore 14pt and 18pt are equivalent to approximately 18.5px and 24px.

Because different image editing applications default to different pixel densities (e.g. 72 PPI or 96 PPI), specifying point sizes for fonts from within an image editing application can be unreliable when it comes to presenting text at a specific size. When creating images of large-scale text, authors should ensure that the text in the resulting image is roughly equivalent to 1.2 and 1.5 em or to 120% or 150% of the default size for body text as rendered by the browser.

Hues are perceived differently by users with color vision deficiencies (both congenital and acquired) resulting in different colors and relative luminance contrasts than for normally sighted users. Because of this, effective contrast and readability are different for this population. However, color deficiencies are so diverse that prescribing effective general use color pairs (for contrast) based on quantitative data is not feasible. Requiring good luminance contrast accommodates this by requiring contrast that is independent of color perception. Fortunately, most of the luminance contribution is from the mid and long wave receptors which largely overlap in their spectral responses. The result is that effective luminance contrast can generally be computed without regard to specific color deficiency, except for the use of predominantly long wavelength colors against darker colors (generally appearing black) for those who have protanopia. (We provide an advisory technique on avoiding red on black for that reason). For more information see [[ARDITI-KNOBLAUCH-1994]] [[ARDITI-KNOBLAUCH-1996]] [[ARDITI]].

Note

Some people with cognitive disabilities require color combinations or hues that have low contrast, and therefore we allow and encourage authors to provide mechanisms to adjust the foreground and background colors of the content. Some of the combinations that could be chosen may have contrast levels that will be lower than those found in the Success Criteria. This is not a violation of this Success Criterion provided there is a mechanism that will return to the default values set out in the Success Criteria.

A contrast ratio of 3:1 is the minimum level recommended by [[ISO-9241-3]] and [[ANSI-HFES-100-1988]] for standard text and vision. The 4.5:1 ratio is used in this provision to account for the loss in contrast that results from moderately low visual acuity, congenital or acquired color deficiencies, or the loss of contrast sensitivity that typically accompanies aging.

The rationale is based on a) adoption of the 3:1 contrast ratio for minimum acceptable contrast for normal observers, in the ANSI standard, and b) the empirical finding that in the population, visual acuity of 20/40 is associated with a contrast sensitivity loss of roughly 1.5 [[ARDITI-FAYE]]. A user with 20/40 would thus require a contrast ratio of 3 * 1.5 = 4.5 to 1. Following analogous empirical findings and the same logic, the user with 20/80 visual acuity would require contrast of about 7:1.

The contrast ratio of 4.5:1 was chosen for level AA because it compensated for the loss in contrast sensitivity usually experienced by users with vision loss equivalent to approximately 20/40 vision. (20/40 calculates to approximately 4.5:1.) 20/40 is commonly reported as typical visual acuity of elders at roughly age 80. [[GITTINGS-FOZARD]]

The contrast ratio of 7:1 was chosen for level AAA because it compensated for the loss in contrast sensitivity usually experienced by users with vision loss equivalent to approximately 20/80 vision. People with more than this degree of vision loss usually use assistive technologies to access their content (and the assistive technologies usually have contrast enhancing, as well as magnification capability built into them). The 7:1 level therefore generally provides compensation for the loss in contrast sensitivity experienced by users with low vision who do not use assistive technology and provides contrast enhancement for color deficiency as well.

Note

Calculations in [[ISO-9241-3]] and [[ANSI-HFES-100-1988]] are for body text. A relaxed contrast ratio is provided for text that is much larger.

Conversion from nonlinear to linear RGB values is based on IEC/4WD 61966-2-1 [[IEC-4WD]].

The formula (L1/L2) for contrast is based on [[ISO-9241-3]] and [[ANSI-HFES-100-1988]] standards.

The ANSI/HFS 100-1988 standard calls for the contribution from ambient light to be included in the calculation of L1 and L2. The .05 value used is based on Typical Viewing Flare from [[IEC-4WD]].

This Success Criterion and its definitions use the terms "contrast ratio" and "relative luminance" rather than "luminance" to reflect the fact that Web content does not emit light itself. The contrast ratio gives a measure of the relative luminance that would result when displayed. (Because it is a ratio, it is dimensionless.)

Note

Refer to related resources for a list of tools that utilize the contrast ratio to analyze the contrast of Web content.

See also 2.4.7: Focus Visible for techniques for indicating keyboard focus.

It is sometimes helpful for authors to not specify colors for certain sections of a page in order to help users who need to view content with specific color combinations to view the content in their preferred color scheme. Refer to 1.4.5: Images of Text for more information.