Contrast to noise ratio matlab tutorial pdf
Keywords: contrast , contrast-to-noise ratio , noise , tube output , tube voltage. Radiologists are responsible for the manner in which radiologic examinations, including radiographs, mammograms, and CT images, are obtained.
Radiographic protocol parameters should be selected to ensure adequate diagnostic performance, and radiologists therefore need to understand the image creation process, in addition to interpreting radiologic examinations [ 1 — 4 ]. The choice of protocol parameter also affects the amount of radiation received by the patient, and radiologists need to ensure that patients are not being subjected to unnecessary radiation exposure.
The most important aspect of the choice of protocol parameters is to ensure that the image quality will be sufficient for a given diagnostic imaging task [ 5 ]. It is therefore essential for radiologists to understand how choices for each protocol parameter will affect the resultant image being generated. Key x-ray choices include the voltage across the x-ray tube kilovoltage , the size of the x-ray tube current milliamperes , and the imaging exposure time seconds [ 6 , 7 ].
Although artifacts in radiographic imaging are of obvious importance for image quality, these are beyond the scope of this article [ 8 ]. Important determinants of x-ray beam quantity are the choice of tube current milliamperes and the corresponding x-ray beam exposure times seconds [ 9 — 11 ].
The product of the tube current and exposure time, known as the milliampere-second value, is the primary indicator of the x-ray beam intensity. The milliampere-second value indicates the relative radiation output for a given x-ray tube when operated at a specified tube voltage, but does not account for differences between xray systems using a variety of tubes and filtration.
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For this reason, it is generally not very helpful or informative to describe any radio-graphic examination as being performed at a given milliampere-second value. The intensity of an x-ray beam is quantified by an air kerma, which relates to the kinetic energy released per unit mass when x-rays interact with air [ 12 ]. Air kerma can be thought of as the number of x-rays per unit area, with the photon energies of minimal concern. The air kerma in any x-ray beam is directly proportional to both the tube current and the corresponding exposure time.
Because air kerma is well defined and universally understood, it is the metric of choice for specifying x-ray beam intensity.
The higher the air kerma, the higher the x-ray beam's intensity, and the more photons will be incident on the patient. The kerma-area product is the total amount of radiation incident on the patient and will affect the energy deposited into the patient. Air kerma is used to estimate the entrance skin dose and thereby the likelihood of a deterministic skin radiation risk.
CCD Signal-To-Noise Ratio
Kerma-area product quantifies the total amount of radiation incident on the patient and is most closely related to the total stochastic patient risk. For most patients, the stochastic risk can be taken as the carcinogenic risk. Table 1 shows representative values of kerma-area product for complete examinations in radiologic imaging [ 13 ]. The quality of an x-ray beam refers to the penetrating power of the beam and is normally expressed as a half-value layer HVL of aluminum in millimeters.
A typical x-ray beam used in abdominal radiography would likely have an HVL of 3 mm of aluminum. Figure 1 shows that 0. When the average x-ray beam energy increases, meaning the x-ray beam has increased penetrating power, the HVL also increases. Notice dramatic reduction in material thickness as atomic number increases. Illustrations by Abrahams RB. In radiography, the two important determinants of x-ray beam quality are tube voltage and beam filtration. Increasing the tube voltage or the total amount of filtration or both will increase the average photon energy and thereby the x-ray beam penetrating power.
It is important to note that adding filters always reduces the x-ray tube output quantity but increases x-ray beam penetrating power quality.
The most common filter material used in radiography is aluminum, with a typical filtration in routine radiography being about 3 mm of aluminum.
Copper is also used in modalities for which dose issues are important, specifically in pediatrics and interventional radiology. In mammography, the most common filter materials are molybdenum, rhodium, and silver. CT makes use of relatively heavy filtration, with aluminum, copper, and titanium being the most frequently used materials.
A typical radiographic x-ray beam in abdominal radiography will have an x-ray tube voltage of 80 kV and a total filtration of about 3 mm of aluminum. The x-ray beam quality is likely to be also 3 mm of aluminum, which is easily confused with the filtration because they are often quantitatively very similar.
Beam qualities in mammography are very low and of the order of 0.
A typical CT x-ray tube voltage is kV, and the filtration can consist of 2 mm of aluminum with an additional 0. The resultant HVLs will be of the order of 6 mm of aluminum or even higher, markedly greater than those encountered in radiography. When the amount of radiation needed to generate an image is to be increased, this will generally be achieved by increasing the milliampere-second value, as shown in Figure 2.
It is important to note that changing the milliampere-second value does not modify the x-ray beam spectrum. The average photon energy remains constant, resulting in the same percentage transmission through the patient. When possible, increasing the milliampere-second value should be done by increasing the tube current milliamperes. In practice, this may not be possible because of tube heating limitations or because the maximum tube current available is already being used.
In such cases, increasing the milliampere-second value will generally require longer exposures, thereby increasing the likelihood of motion blur. When tube voltage kV increases, more photons are produced, and average photon energy also increases right. Increasing milliampere-second value only increases quantity, whereas increasing kilovoltage increases quantity and quality.
In radiography, the x-ray tube output air kerma is generally taken to be proportional to kilovoltage squared, whereas in CT the x-ray tube output is taken to be proportional to kilovoltage to the 2.
However, the choice of x-ray tube voltage is generally determined by considering patient penetration, image contrast, and dynamic range requirements. Only very rarely, if ever, is the kilovoltage adjusted simply to change the amount of radiation in any xray examination. When kilovoltage is modified, it is always necessary to evaluate other factors, including patient penetration, scattered radiation intensities, x-ray transmission through a grid, and changes in photon absorption efficiencies in the x-ray detectors.
Protocol design requires the initial selection of an appropriate x-ray tube voltage and a subsequent adjustment of the x-ray tube milliampere-second to ensure that the radiation intensity at the image receptor is the one that is desired for image quality. Figure 3 left panel shows a spine that is invisible too white on the image , requiring the tube voltage to be increased to improve beam penetration Fig.
The increase in kilovoltage achieves the correct amount of penetration, but now the image is too dark Fig.
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Because tube voltage affects output and penetration, as depicted in Figure 2 , the x-ray tube current in any x-ray examination the xray intensity milliampere-second must always be adjusted when kilovoltage has been modified to ensure the appropriate amount of radiation at the image receptor. Chest radiograph left is underpenetrated, requiring increase in x-ray tube voltage. After voltage kV has been increased middle , radiograph is too dark. After x-ray tube output milliampere-second [mAs] value has been reduced right , radiograph has correct image density.
Consider a lesion that appears in any radiologic image, where the lesion intensity is given by I lesion.
Whether the contrast is positive or negative is determined by the lesion's characteristics and is not important from an image quality perspective. For example, adding iodine or carbon dioxide to a blood vessel will make the vasculature visible but of differing polarity because iodine absorbs more x-rays, whereas carbon dioxide will absorb fewer x-rays than adjacent tissues.
Contrast is independent of the amount of radiation used to generate an image, as shown in Figure 4. Images shown in Figure 4 illustrate the only effect of reducing the milliampere-second, is to increase the amount of image mottle.
When fewer photons are used to make image right , contrast remains exactly the same, but image mottle increases. The most important factor affecting contrast is the average photon energy used to generate the image, which is determined by the choice of x-ray tube voltage and the amount of x-ray beam filtration. As the photon energy decreases, the differential attenuation between the lesion and surrounding tissues increases.
Accordingly, low energy is usually associated with higher contrast and vice versa, as shown in Figure 5. Although this is generally true, it is important to note that the improvement in lesion contrast at low energies also depends on the lesion's atomic number.
Reducing photon energy improves the contrast i. In this example, automatic exposure control was used, so milliampere-second value will increase when selected kilovoltage is reduced. By keeping air kerma at image receptor same in both images, image mottle is unchanged. Two additional factors need to be considered regarding contrast relating to scatter and digital image display.
Increased scatter will generally reduce contrast, which can be appreciated by looking at an abdominal radiograph obtained without the use of a scatter removal grid. Image manipulation in digital radiography can be used to modify both the brightness and contrast of any given feature. When the window width is increased, display contrast is reduced, and vice versa.
Accordingly, it is possible to increase display contrast by use of narrow window widths. However, when narrow window widths are used, the range of pixel values tissues that are visible is also reduced. Therefore, there is a trade-off between display contrast and type of anatomy that is visible in the displayed image.
A good example is the use of lung and mediastinum display settings while reading a chest CT. Consider a digital x-ray detector that is exposed to a uniform x-ray beam, with the average number of photons detected in a pixel being The statistical nature of x-ray emission and detection means that not every pixel will detect exactly photons.
Some pixels have more x-rays and appear darker, whereas other pixels have fewer than photons and will appear lighter. The distribution of these darker and lighter pixels is random, and the image will have a mottled appearance salt-and-pepper distribution. Image mottle is negligible in high milliampere-second mAs value image left , but is very high in low milliampere-second value image right.
Contrast Enhancement Techniques
Because digital data in high milliampere-second value image will be much higher than low milliampere-second value image, adjustment of digital display settings window and level are always necessary. Because noise is random, the amount of noise does not increase in a linear manner when the amount of radiation used to generate an image increases.
When the number of photons increases by a factor of 4, the relative SD will be halved i. It is thus useful to always remember that quadrupling of the number of photons used to generate any radiographic image will halve the amount of image noise, and vice versa.
Noise in virtually all x-ray imaging modalities is dominated by quantum mottle, where the latter relates to the total number of x-rays used to generate an image.
Matlab Image Signal to Noise Ratio
Quantum mottle is dominant in all radiography, mammography, fluoroscopy, and CT examinations. This is important because the only technical way to reduce noise using any x-ray imaging modality is to use more x-rays.
X-ray detection efficiencies in most x-ray imaging systems are relatively high, so improvements are not likely. A corollary of the fact that virtually all of medical imaging is quantum mottle limited is that replacing components in the imaging chain, such as charge coupled device CCD for television camera in fluoroscopy, cannot result in dose reductions if image quality is to be maintained. When the contrast alone increases or noise alone is reduced, a lesion will clearly become more visible.
However, the visibility of a lesion is not determined by just the noise or the lesion contrast alone. The visibility of any lesion in a radiologic image needs to take into account both the amount of lesion contrast and the corresponding amount of noise. The amount of lesion contrast relative to the amount of noise mottle is the key determinant of the visibility of a given lesion. The ratio of lesion contrast to image mottle is known as the contrast-to-noise ratio CNR.