CH 13 RTC 100 Test 11/27

long and short scale contrast

- A low contrast radiograph will display many more shades of gray and few blacks and whites. This is called long scale contrast. - A high contrast radiograph will appear mostly black and white with few shades of gray in between. This is called short scale contrast.

long and short scale contrast

- A low contrast radiograph will display many more shades of gray and few blacks and whites. This is called long scale contrast. - A high contrast radiograph will appear mostly black and white with few shades of gray in between. This is called short scale contrast.

Determine image contrast and scale of grays

- As radiographs have varying regions of density, one cannot simply make assumptions based on a small region of interest. It is due to this that the radiograph contrast of an entire image is referred to as 'long scale' or 'short scale.' - Short-scale radiographs are considered 'high-contrast' whereby density differences are greater, overall possess fewer in density steps (lesser shades of gray). - Long-scale radiographs are considered 'lower-contrast' whereby density differences are less noticeable however posses many more shades of grey. Long-scale radiographs are preferred while examining the lung fields, where subtle changes in density are pertinent to a diagnostic image.

Determine the effect of part thickness on the final radiographic image

- Subject Contrast is affected by the following factors: Thickness difference: when two different thickness of the same material attenuate an x-ray beam, the thickness part will attenuate more x-rays than the thinner part. - Increasing part thickness lowers radiographic contrast because of more scatter radiation reaching the image receptor; decreasing part thickness increases radiographic contrast because of less scatter radiation reaching the image receptor.

Describe the x-ray emission spectrum

- The x-ray spectrum is defined as the energy distribution of the radiation produced by an x-ray exposure. The x-ray spectrum has a major impact on image quality and radiation dose delivered to patients. - X-ray emission spectrum. The number of x-rays can be plotted as a function of the energy of each individual x-ray. Number and energy of the x-rays are used to plot the x-ray emission spectrum.

Continuation of Analyzing relationships of factors that control and affect image exposure:

- mAs : Measures to increase the number of photons, such as increasing the mAs (tube current, exposure time and product), therefore increasing the signal to noise ratio. - quantum mottle: The noise results from random variations in the number of photons (the quanta of light) that are generated, pass through the patient, and interact with the detector; this noise is referred to as quantum mottle. - Another term to describe Quantum Mottle is photo starvation (the image is starving for photoelectrons.

15% rule

A 15% increase in kVp causes a doubling of exposure. An increase in kVp by 15%, the mAs is reduced by a factor of two which is multiplying it by 1.15 A 15% decrease in kVp causes a halving of exposure. A decrease in kVp by 15%, the mAs is increased by a factor of two which is multiplied by 0.5

Explain the difference between a negative and positive image display

A negative picture is the inverse of a normal, or positive, image. In a negative picture, areas that are white or light appear dark, and darker areas appear to be light. Usually, photo negatives are turned into positive images in a darkroom.

High and low contrast

High Contrast is more black and white shades and low contrast on the other hand is shades of gray.

Identify image characteristics that are essential for image visibility

In addition to contrast sensitivity and blurring there are visual noise, artifacts, and geometric distortion. The three (contrast sensitivity, blurring, visual noise) referred to here as "the critical three", are highly significant because they apply to all images.

Analyze the relationship of factors that control and affect radiographic/subject contrast

Subject Contrast is caused by the range of absorption of radiation by the subject, i.e., the material being radiographed. This is the wavelength (a) the mass of the subject, including the atomic number and the thickness, and (b) the penetrating power of the radiation source, which is the wavelength of the radiation used.

Describe conditions necessary to produce x-radiation

The three things needed to create x-rays are a source of electrons, a means of accelerating the electrons to high speeds, and a target for the accelerated electron to interact with. - X-rays are produced when the free electrons cause energy to be released as they interact with the atomic particles in the target.

Discuss various photon interaction with matter

The two most common forms of interactions are the photoelectric effect and Compton scattering. - The probability of these events depends on the absorbing medium and the photon energy. The photoelectric effect predominates for low energy photons (less than 100 keV). - In photoelectric (PE) interaction, the photon is absorbed by an inner shell(eg: K Shell) electron of an atom. All the photon energy is transferred to the electron so that the photon disappears. - PE interactions are desirable in shields for protons since the photons are completely absorbed.

Explain the factors that affect the x-ray emission spectrum

The x-ray spectrum has a major impact on image quality and radiation dose delivered to patients. - The authors explored the effects of three key factors on x-ray spectra: generator type, peak-type potential, and filtration. - Different generator types are characterized by the amount of ripple in the kV (kilovoltage) waveform.

Densitometer

Tool used to read density that it is on a black part which is on an x-ray film.

Critiquing the radiographic contrast within various radiographic images

Which can be quantum mottle

Analyze relationships of factors that control and affect image exposure: - Identify recognizability functions in an image

a. Define Image Brightness: An Area of high density (low brightness) is where the x-ray was transmitted, and an area of low density (high brightness) is where the x-ray beam was absorbed. b. Image contrast: radiographic contrast is the density difference between neighboring regions on a plain radiograph. High radiographic contrast is observed in radiographs where density differences are notably distinguished (black to white). c. Define image noise: The common types of noise that are present in x-ray images are Poison noise, salt and pepper noise, and speckle noise. - The salt and pepper noise is seen in the image as white and black pixels respectively - Poison noise is as a result of uneven distribution of x-rays over the receptor surface. - Speckle noise on the other hand occurs as a granular appearance in an image which is produced as a result or random fluctuation in the return signal from an object which is not found to be bigger than a single image processing element. d. Define image artifact: X-ray artifacts can present in a variety of ways including abnormal shadows noted on a radiograph or degraded image quality, and have been produced by artificial means from hardware failure, operator error and software (post-processing) artifacts. e. Define signal to noise ratio: Signal to Noise Ratio(SNR) always has a desirable high signal to noise ratio for the image to be ideal. In radiography, the signal to noise ratio (SNR) and thus the apparent nosiness of the image, is proportional to the amount of contrast and the square root of the number of photons transmitted.

Determine of part thickness and the beam interaction are the following -

a. Photo electric: The dependence of photoelectric absorption on Z # and Electrons means that it is the major contributor to beam attenuation up to approximately 30 keV when human tissues (Z# = 7.4) are exposed. At beam energies above this, the Compton effect predominates. b. Compton Scatter: Compton scattering leads to a background haze in an x-ray image. It can lead to different more structured artifacts in CT image. Impact on Dose: Coherent scattering does not have a significant impact on patient dose. Impact on patient dose: the electrons deposit the energy locally c. Absorption: The absorption of x-rays by a tissue depends on the quality of the x-ray beam, the character of the atoms in the tissue being examined as well as the density and thickness of the tissue. The amount of x-ray absorption by the tissues defines the density of the shadow cast on the radiograph d. Attenuation: Is the reduction of intensity of an x-ray beam as it transverses matter. The reduction may be caused by absorption or by deflection (scatter) of photons from the beam and can be be affected by different factors such as beam energy and atomic number or the absorber. e. Transmission: Traditional transmission x-ray images result when x-rays pass through an object to a detector located on the far side of the object. Objects with greater density block or absorb x-ray than object with lesser density.

Calculate mAs.

mA x time -The mAs is the product of the mA and is calculated by multiplying the mA by the time. 500 mA x 0.01 seconds = 5 mAs 200 mA x 1/5 seconds = 40 mAs 300 mA x 50 milliseconds = 15 mAs - When time is stated in milliseconds, the milliseconds number must always be converted to seconds be multiplied by the mA. To calculate the mAs, multiply the mA by the time in seconds.

Density maintenance formula

mAs 1/mAs 2 = (SID1)2/(SID2)2