Factors Affecting Contrast In An Xray Image

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Factors Affecting Contrast In An Xray Image Essay

To prove declaration, a line brace guage is used. To mensurate MTF in an x-ray system, the sine wave equivalent of a line brace gage is used. The closest distinguishable brace of rhythms determines the best MTF, it is quoted in rhythms per millimeter [ 2 ] .
A common manner to show the system declaration is to cite the frequence where the MTF is reduced to either 3 % , 5 % or 10 % of the original tallness [ 3 ] . MTF and spacial frequence are related by MTF curves. Examples of MTF curves are given below ;
Mammography System:
Fig 2: MTF curve for a mammography system [ 4 ] .
Chest Radiography System:
Fig 3: MTF curve for a typical thorax X ray. Three different sensor types are indicated on the secret plan [ 5 ] .
Fluoroscopy System:
Fig 4: MTF curve for a Fluoroscopy system with secret plans shown for single constituents of the imagination procedure. The movie and the optics have first-class declaration. The MTF of the imge intensive is shown to hold a confining declaration of about 4.8 cycles/mm.Factors Affecting Contrast In An Xray Image Essay. The Television camera is the worst in the series, it limits the MTF of the overall image during unrecorded fluoroscopy and videotaped imaging. [ 6 ]
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Question 2:
Contrast is the fluctuation in brightness or optical denseness across an image. Factors impacting contrast in an x-ray image include the tubing end product, or the kVp. This is a step of the energy of the x-ray beam go forthing the x-ray tubing and passing through the patient to organize an image. X raies with higher kVp can perforate deeper into stuffs. In an image with the right kVp bone is white and soft tissues and air are gray/black. If the kVp is excessively high, the X raies will go through through even dense bone, making an image that is largely black with identical characteristics [ 7 ] . The opposite occurs when a kVp which is excessively low is used. The most suited kVp depends on the characteristic under probe. Besides among the factors impacting the image contrast is the patient. The denseness, the atomic figure Z and the thickness of the portion of the patient being imaged. Denser tissue, tissue with higher Omega or tissue of a greater thickness consequences in lighter countries on the image because they have blocked the X ray from exposing the image receptor. Variation in contrast occurs because tissues in the organic structure faded X raies otherwise. The human oculus can percieve a difference of about 2 % in contrast between next countries [ 8 ] . The concluding influence on image contrast to be discussed here is the image receptor. In movie imagination, the contrast of the attendant image depends on the sensitiveness of the movie used. To bring forth an image with the right contrast, a movie with corrresponding sensitiveness must be chosen before imaging. In digital imagination, there is no fixed sensitiveness. It has the advantage of being able to enter the full scope of exposures and digital processing after imaging can be used to better the contrast in the image. Factors Affecting Contrast In An Xray Image Essay.
Image contrast can be evaluated utilizing a densiometer. This device emits visible radiation of a known energy. The visible radiation is reflected back from the image and detected by the densitometer. The difference in energy between emitted and detected visible radiation is used to calculate the optical denseness ( inkiness ) in that country. Since contrast is the fluctuation in optical denseness, this method can be used to study the contrast in the image.
Question 3:
The undermentioned description is based on an article from the NDT database [ 9 ] .
Spatial declaration of an x-ray system is limitied by the size of the focal topographic point. Fourier analysis can be used to cipher the focal topographic point size. X raies are passed through a trial object with a known form. This trial object is placed between the x-ray beginning and sensor, the agreement is shown in the figure below. The focal topographic point of the X ray is non assumed to be point-like, as the sensor is moved off from the beginning, the detected focal topographic point appears larger. Blurring of the image by the sensor is included, this blurring is related to the point spread funtion ( Popular Struggle Front ) of the sensor. Otherwise, an ideal sensor is assumed. Image impairment due to noise is besides factored into the description.
Fig 5: Apparatus for determing the focal topographic point size. The X-ray beginning, the level object, and the strength distribution measured at the sensor system lie in different planes for which different co-ordinate systems with the variables ( ten, Y ) , ( x ‘ , y ‘ ) and ( ten ” , y ” ) severally, are used. This is done in order to include magnification effects in the computations.
The measuring of the x-ray transmittal, T, is derived mathematically. This is done by convoluting the strength distribution of the focal topographic point degree Fahrenheit with the transmittal profile of the level object g and the sensor point spread map d. Besides, t is deteriorated by noise, which is taken into consideration by add-on of a noise term N to the consequence of the whirl.Factors Affecting Contrast In An Xray Image Essay. In order to take into history the geometrical magnification, V, of the apparatus, these maps are represented in one of these planes ( here the plane of the sensor ) , whereby the physical magnification effects of the apparatus were observed before the whirl is accomplished, this is shown in the 2nd portion of the equation below. The magnification is the distance between the beginning and the sensor system divided by the distance between the beginning and the object.
The Convolution Theorem states that the Fourier transform of a whirl is the merchandise of the Fourier transforms. Conversely, the Fourier transform of a merchandise is the whirl of the Fourier transforms. Using the above equation, a deconvolution of T with giˆ ( iˆ vitamin D outputs an estimation of degree Fahrenheit.
In a technique like this, a suited trial object is measured. The resulting image corresponds to a whirl of the trial object with the strength distribution of the focal topographic point and other factors.
Information on the focal topographic point is derived from this measuring utilizing cognition on the trial
object and other act uponing values which means that the whirl procedure is undone to
a certain extent. Besides, with the presented method an arbitrary two dimensional strength distribution can be measured, irrespective of form.
Harmonizing to the whirl theorem, a whirl in the spacial sphere corresponds to a point-by-point generation in the corresponding Fourier sphere. Factors Affecting Contrast In An Xray Image Essay. Furthermore, harmonizing to the add-on theorem, an add-on in the spacial sphere corresponds to an add-on in the corresponding Fourier sphere. ( Note: lower instance letters represent maps and upper instance letters represent the Fourier transforms of the tantamount maps. ) The initial equation now becomes ;
At certain spacial frequences | N | can be significantly higher than| F i?-iˆ P | . At these spacial frequences division of T by P chiefly increases noise and deteriorates the image quality. This is due to the fact, that information on F is lost at these spacial frequences. For that ground, independently of the deconvolution method applied, all spacial frequences which are contained with high strength in | F | should be contained with high strength in |P| in order that | Fi?- P | is significantly larger than | N | . This means that the trial object ( in combination with the sensor imaging belongingss ) should incorporate the major spatial frequences which are required to depict the focal topographic point with sufficient strength. In this instance F can be restored good at these spacial frequences, which yields a good estimation of degree Fahrenheit.
Question 4:
Using a saloon apparition similar to that used for finding declaration can take to an mistake finding the focal topographic point size. This is because the line braces are aligned in one way merely. For accurate measuring of the focal topographic point size, many images with the saloon apparition at different angles would be necessary [ 10 ] . To get the better of this job, a star apparition is used. This is a phonograph record of jumping Lead radiuss and x-ray crystalline stuff. At a peculiar diameter of the focal topographic point the image of the radiuss fuzzs, i.e. , next radiuss can non be distinguished from each other. The diameter of the fuzz is an indicant of the focal topographic point size [ 11 ] .
Fig 6: Star form for proving focal topographic point size [ 12 ]
Question 5:
5a.
The most obvious parts of a CT scanner are the traveling patient tabular array and the gauntry or ‘tube ‘ . Conventional projection skiagraphy is limited because it collapses 3D objects onto 2D images. CT has a revolving system of emanation and sensing and so it can give accurate 3D diagnostic information about the distribution of constructions inside the organic structure. Inside the gauntry there is the X-ray tubing, x-ray sensors and slip-rings. The X-ray beam is collimated and radiates in a ‘fan-beam ‘ form. The x-ray emitter and sensors rotate in the gauntry to mensurate projections that form an image that is a ‘slice ‘ though the organic structure. Factors Affecting Contrast In An Xray Image Essay.There are coppices around the revolving slip-rings to convey signals. In CT, the additive fading coefficient, I? is measured. This tells how much strength is lost as the beam travels through the medium. This distribution of I? is the footing of image formation. There are two distinguishable gestures of the x-ray beam relation to the patient ‘s organic structure during CT imaging. One gesture is the scanning of the beam around the organic structure. The other gesture is the motion of the beam along the length of the body.A This is achieved by traveling the organic structure through the beam as it is revolving about
Fig 7: External visual aspect of a CT scanner. [ 13 ]
Fig 8: Basic schematic of the building of a CT scanner.
Fig 9: CT image quality and electromechanical credence trials. The ‘Priority ‘ column indicates which of the trials are the most of import. [ 14 ]
5b.
CT images are formed by multiple decussate projections. This is illustrated in the figure on the left. In the bottom right subdivision, it is seen that the combination of the projections causes film overing in the concluding image. The blurring goes as 1/r, i.e. , it is relative to the distance from the Centre point. The 2D Fourier transform of 1/r is 1/I? . Since the opposite of 1/I? is |I?| , it is possible to calculate the 2D Fourier Transform of the bleary image, multiply the Fourier transform of the attendant image by |I?| and the calculate the opposite Fourier transform to obtain a sharper image [ 15 ] .
Fig 10: On the left, unfiltered back projections and on the right, filtered back projections. The transition is done in the spacial sphere by convoluting the projection with the IFT of |I?| .
Fig 11: The mathematics of the image Reconstruction procedure, can be expressed compactly in the above equation, where the footings have been grouped to reflect the filtered-back-projection ” attack [ 15 ] . Factors Affecting Contrast In An Xray Image Essay.
5c.
The cardinal system public presentation indexs are CT figure, declaration, noise, and patient dose.26 The truth of CT Numberss is measured by scanning a water-filled apparition at least monthly. The CT figure for H2O should be zero over a 20-cm-diameter apparition, with a fluctuation of less than 1 CT figure. Deviation from the expected CT figure of 0 for H2O at any energy is
adjusted by using a rectification factor for the pel value. Constancy of the value
should be monitored with a day-to-day scan. An overall cheque of system public presentation is obtained from biannual measurings of CT image noise, defined as the standard divergence of CT Numberss in a part of involvement. Constancy of public presentation is checked by rating of the standard divergence in the day-to-day H2O scan mentioned antecedently. Resolution is measured by scanning apparitions on a monthly footing. Of peculiar importance is low contrast declaration, which is a sensitive index of alterations in component public presentation as they affect noise. Patient dosage is evaluated biyearly. Specially designed ionisation Chamberss provide measurings from which the dosage may be calculated for the exposure conditions ( narrow beam, variable piece thickness ) used in CT. The values should hold with maker ‘s specifications to within 20 % . [ hendee ]
To test resolution, a line pair guage is used. To measure MTF in an x-ray system, the sine wave equivalent of a line pair gauge is used. The closest distinguishable pair of cycles determines the best MTF, it is quoted in cycles per mm [2]. Factors Affecting Contrast In An Xray Image Essay.

A common way to express the system resolution is to quote the frequency where the MTF is reduced to either 3%, 5% or 10% of the original height [3]. MTF and spatial frequency are related by MTF curves. Examples of MTF curves are given below;

Mammography System:

Fig 2: MTF curve for a mammography system [4].

Chest Radiography System:

Fig 3: MTF curve for a typical chest X-ray. Three different detector types are indicated on the plot [5].

Fluoroscopy System:

Fig 4: MTF curve for a Fluoroscopy system with plots shown for individual components of the imaging process. The film and the optics have excellent resolution. The MTF of the imge intensifier is shown to have a limiting resolution of approximately 4.8 cycles/mm. The TV camera is the worst in the series, it limits the MTF of the overall image during live fluoroscopy and videotaped imaging. [6]

Question 2:
Contrast is the variation in brightness or optical density across an image. Factors affecting contrast in an x-ray image include the tube output, or the kVp. This is a measure of the energy of the x-ray beam leaving the x-ray tube and passing through the patient to form an image. X-rays with higher kVp can penetrate deeper into materials. In an image with the correct kVp bone is white and soft tissues and air are gray/black. If the kVp is too high, the x-rays will pass through even dense bone, creating an image that is mostly black with indistinguishable features [7]. The opposite occurs when a kVp which is too low is used. The most suitable kVp depends on the feature under investigation. Also among the factors affecting the image contrast is the patient. The density, the atomic number Z and the thickness of the part of the patient being imaged. Denser tissue, tissue with higher Z or tissue of a greater thickness results in lighter areas on the image because they have blocked the x-ray from exposing the image receptor. Variation in contrast occurs because tissues in the body attenuate x-rays differently. Factors Affecting Contrast In An Xray Image Essay. The human eye can percieve a difference of approximately 2% in contrast between adjacent areas [8]. The final influence on image contrast to be discussed here is the image receptor. In film imaging, the contrast of the resultant image depends on the sensitivity of the film used. To produce an image with the correct contrast, a film with corrresponding sensitivity must be chosen before imaging. In digital imaging, there is no fixed sensitivity. It has the advantage of being able to record the full range of exposures and digital processing after imaging can be used to improve the contrast in the image.

Image contrast can be evaluated using a densiometer. This device emits light of a known energy. The light is reflected back from the image and detected by the densitometer. The difference in energy between emitted and detected light is used to compute the optical density (blackness) in that area. Since contrast is the variation in optical density, this method can be used to survey the contrast in the image.

Question 3:
The following description is based on an article from the NDT database [9].

Spatial resolution of an x-ray system is limitied by the size of the focal spot. Fourier analysis can be used to calculate the focal spot size. X-rays are passed through a test object with a known pattern. This test object is placed between the x-ray source and detector, the arrangement is shown in the figure below. The focal spot of the x-ray is not assumed to be point-like, as the detector is moved away from the source, the detected focal spot appears larger. Blurring of the image by the detector is included, this blurring is related to the point spread funtion (psf) of the detector. Otherwise, an ideal detector is assumed. Image deterioration due to noise is also factored into the description.

Fig 5: Setup for determing the focal spot size. The X-ray source, the flat object, and the intensity distribution measured at the detector system lie in different planes for which different coordinate systems with the variables (x, y), (x’, y’) and (x”, y”) respectively, are used. This is done in order to include magnification effects in the calculations. Factors Affecting Contrast In An Xray Image Essay.

The measurement of the x-ray transmission, t, is derived mathematically. This is done by convolving the intensity distribution of the focal spot f with the transmission profile of the flat object g and the detector point spread function d. Also, t is deteriorated by noise, which is taken into consideration by addition of a noise term n to the result of the convolution. In order to take into account the geometrical magnification, V, of the setup, these functions are represented in one of these planes (here the plane of the detector), whereby the physical magnification effects of the setup were observed before the convolution is accomplished, this is shown in the second part of the equation below. The magnification is the distance between the source and the detector system divided by the distance between the source and the object.

The Convolution Theorem states that the Fourier transform of a convolution is the product of the Fourier transforms. Conversely, the Fourier transform of a product is the convolution of the Fourier transforms. Using the above equation, a deconvolution of t with g€ (€ d yields an estimate of f.

In a technique like this, a suitable test object is measured. The resulting image corresponds to a convolution of the test object with the intensity distribution of the focal spot and other factors.

Information on the focal spot is derived from this measurement using knowledge on the test

object and other influencing values which means that the convolution process is undone to

a certain extent. Also, with the presented method an arbitrary two dimensional intensity distribution can be measured, regardless of shape.

According to the convolution theorem, a convolution in the spatial domain corresponds to a point-by-point multiplication in the corresponding Fourier domain.Factors Affecting Contrast In An Xray Image Essay. Furthermore, according to the addition theorem, an addition in the spatial domain corresponds to an addition in the corresponding Fourier domain. (Note: lower case letters represent functions and upper case letters represent the Fourier transforms of the equivalent functions.) The initial equation now becomes;

At certain spatial frequencies | N | can be significantly higher than| F ƒ-€ P |. At these spatial frequencies division of T by P mainly increases noise and deteriorates the image quality. This is due to the fact, that information on F is lost at these spatial frequencies. For that reason, independently of the deconvolution method applied, all spatial frequencies which are contained with high intensity in | F | should be contained with high intensity in |P| in order that | Fƒ- P | is significantly larger than | N |. This means that the test object (in combination with the detector imaging properties) should contain the major spatial frequencies which are required to describe the focal spot with sufficient intensity. In this case F can be restored well at these spatial frequencies, which yields a good estimate of f.

Question 4:
Using a bar phantom similar to that used for determining resolution can lead to an error determining the focal spot size. This is because the line pairs are aligned in one direction only. For accurate measurement of the focal spot size, many images with the bar phantom at different angles would be necessary [10]. To overcome this problem, a star phantom is used. This is a disc of alternating Lead spokes and x-ray transparent material. At a particular diameter of the focal spot the image of the spokes blurs, i.e., adjacent spokes cannot be distinguished from each other. The diameter of the blur is an indication of the focal spot size [11]. Factors Affecting Contrast In An Xray Image Essay.

Fig 6: Star pattern for testing focal spot size [12]

Question 5:
5a.
The most obvious parts of a CT scanner are the moving patient table and the gantry or ‘tube’. Conventional projection radiography is limited because it collapses 3D objects onto 2D images. CT has a rotating system of emission and detection and so it can give accurate 3D diagnostic information about the distribution of structures inside the body. Inside the gantry there is the X-ray tube, x-ray detectors and slip-rings. The X-ray beam is collimated and radiates in a ‘fan-beam’ shape. The x-ray emitter and detectors rotate in the gantry to measure projections that form an image that is a ‘slice’ though the body. There are brushes around the rotating slip-rings to transmit signals. In CT, the linear attenuation coefficient, μ is measured. This tells how much intensity is lost as the beam travels through the medium. This distribution of μ is the basis of image formation. There are two distinct motions of the x-ray beam relative to the patient’s body during CT imaging. One motion is the scanning of the beam around the body. The other motion is the movement of the beam along the length of the body. This is achieved by moving the body through the beam as it is rotating around

Fig 7: External appearance of a CT scanner. [13]

Fig 8: Basic schematic of the construction of a CT scanner.

Fig 9: CT image quality and electromechanical acceptance tests. The ‘Priority’ column indicates which of the tests are the most important. [14] Factors Affecting Contrast In An Xray Image Essay.

5b.
CT images are formed by multiple intersecting projections. This is illustrated in the figure on the left. In the bottom right section, it is seen that the combination of the projections causes blurring in the final image. The blurring goes as 1/r, i.e., it is proportional to the distance from the centre point. The 2D Fourier transform of 1/r is 1/ρ. Since the inverse of 1/ρ is |ρ|, it is possible to compute the 2D Fourier Transform of the blurred image, multiply the Fourier transform of the resultant image by |ρ| and the calculate the inverse Fourier transform to obtain a sharper image [15].

Fig 10: On the left, unfiltered back projections and on the right, filtered back projections. Factors Affecting Contrast In An Xray Image Essay.The conversion is done in the spatial domain by convolving the projection with the IFT of |ρ|.

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Fig 11: The mathematics of the image reconstruction process, can be expressed compactly in the above equation, where the terms have been grouped to reflect the “filtered-back-projection” approach [15].

5c.
The fundamental system performance indicators are CT number, resolution, noise, and patient dose.26 The accuracy of CT numbers is measured by scanning a water-filled phantom at least monthly. The CT number for water should be zero over a 20-cm-diameter phantom, with a variation of less than 1 CT number. Deviation from the expected CT number of 0 for water at any energy is

adjusted by applying a correction factor for the pixel value. Constancy of the value

should be monitored with a daily scan. An overall check of system performance is obtained from semiannual measurements of CT image noise, defined as the standard deviation of CT numbers in a region of interest. Constancy of performance is checked by evaluation of the standard deviation in the daily water scan mentioned previously. Resolution is measured by scanning phantoms on a monthly basis. Of particular importance is low contrast resolution, which is a sensitive indicator of changes in component performance as they affect noise. Patient dose is evaluated semiannually. Specially designed ionization chambers provide measurements from which the dose may be calculated for the exposure conditions (narrow beam, variable slice thickness) used in CT. The values should agree with manufacturer’s specifications to within 20%. [hendee]

Factors Affecting Contrast In An Xray Image Essay
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