The Anode Heel Effect - Assignment Example

1.          Draw and label the major components of an x ray tube. (4 marks) 2.          Explain and diagrammatically represent the line focus principle. (5 marks) The line focus principle is the principle that viewing a sloped surface at an angle reduces its apparent size. In an x-ray tube the angling of the anode results in the effective focal spot being smaller than the actual focal spot. Diagrammatically the line focus principle is represented below: 3.          Explain and diagrammatically represent the anode heel effect. (5 marks) The anode heel effect is the variation of intensity over the cross section of a useful radiographic beam, caused by the angle at which radiographs emerge from beneath the surface of the focal spot, which causes a differential attenuation of photons composing the useful beam. 4.          What is a biangular tube? (2 marks) A x-ray tube that has a small and large focus, one allows for a microscopic view and the other allows a full range view. 5.          What is the purpose of the two cathode filaments? (2 marks) Two cathode filaments produces two different ranges, one for higher intensities/broader focal spot and one for fine focus work. 6.          Use a diagram to derive the equation relating effective focal length to actual focal length. If a 15 degree target angle had an actual focal length of 5mm, what is the effective focal length? Maintaining this effective focal length, what will be the actual focal length if the target angle is reduced to 10 degrees? What are the implications for doing this? (8 marks) effective focal length = actual focal length x sin . A 15 degree target angle that had an actual focal length of 5mm, would equal 5mm(15)=75mm. Thus the effective focal length would be 75mm. If the effective length remained 75mm, then 75mm=(10)7.5mm making the actual focal length would be (10)7.5mm. The implications would be the smaller the anode angle, the better resolution. 7.          What is the role of collimation in xray production? (2 marks) Collimation is the elimination of the more divergent portion of an x-ray beam. 8.          What is the role of the rectifier? Why is this important? (2 marks) A rectifier is an electrical device that converts alternating current (AC) to direct current (DC), a process known as rectification. This is important to have the correct amount of electricity to heat the filaments. 9.          Explain the principles of thermionic emission. (3 marks) Thermionic emissions are emissions of electrons or ions by highly heated substances, the charged particles are thermions. As the temperature rises the more thermions are emitted. The heated substance might be a metal filament. If the heated substances carries a positive or negative charge, the thermions will be of the same charge. 10.          What is the space charge effect? (2 marks) Space charge is where an excess electric charge is treated as a continuum of charge distributed over a region of space instead of a distinct point-like charges. This model typically applies when charge carriers have been emitted from some region of a solid, the cloud of emitted carriers can form a space charge region when sufficiently spread out, or the charged atoms, or molecules left behind in a solid form in a space charge region. Space charge usually only occurs in dielectric media since a conductive medium charge can be rapidly neutralized. Space charges can be negative or positive. 11.          With respect to xray production, define quality and quantity. Discuss the factors that determine xray beam quality and quantity. (10 marks) X-ray production of quality means the penetrating ability of the beam. The higher quality comes from a more penetrating beam, which has to do with the energy spectrum of the x-ray. In respect to x-ray production quantity is the number of x-ray photons in the beam. The quality of the x-ray beam is controlled by the voltage while the millamperes control the quantity. a. X-ray Beam Quality. The quality of the x-ray beam is controlled by the amount of voltage. Voltage provides contrast to the film. The desired contrast appears as various shades of gray, black, and white in the x-ray negative (radiograph). Increased voltage provides less contrast (or more shades of gray). However, the beam has more penetrating power. Decreased voltage, on the other hand, provides more contrast (fewer shades of gray and more black and white shades). However, there is less penetrating power in the low voltage exposure. The technique most commonly used to expose periapical and bite-wing X-rays is a 75 kilovolt peak and 15 milliamperes. b. X-ray Beam Quantity. The x-ray beam quantity is controlled by the milliamperes. The more x-rays (photons) in the x-ray beam, the more dense (dark) the x-ray negative (radiograph) becomes. By increasing the milliamperes, we increase the number of available electrons at the cathode filament. When electrical current (voltage),mm,mlis applied to the x-ray tube, the electrons cross the gap. When they impact on the anode (tungsten target), a greater number of x-rays (photons) are also produced. The more x-rays that are available to penetrate an object, the more dense (dark) is the x-ray negative (radiograph). (Integrated Publishing n.d.) 12.          Calculate the HU for a 90 kVp, 200 mA xray exposure of 100 msec duration for: (2 marks)                 Single phase generators-1,800,000 HU                 Three phase generators-2,430,000 HU 13.        Given your results above, explain why three phase generators might be employed in preference to single phase. (5 marks) The three phase generator gives off more heat, which in turn makes the x ray have a greater energy production. 14.          Explain and diagrammatically represent thermal equilibrium. (5 marks) Thermal equilibrium is achieved when two systems in thermal contact with each other cease to exchange energy by heat. 15.          Using the cooling chart on page 142 of Bushberg et al 2002, make the following determinations. (8 marks)                 You have performed an xray that generated 250000 HU. The anode had a residual HU of 30000. The processor chewed your film. How long before you can repeat the xray? 7 minutes 30 sentences                 You are to perform a sequence of 12 exposures in rapid succession using 90 kVp and 100 mA. The anode has a residual HU of 240000. How long before you can start your sequence if you are to stay within tube rating? Assume HU = kVp x mA. Approximately 11 minutes. Part B: 1.          Comment on the efficiency of the xray production process. Justify your comments.   (5 marks) The x ray production process is efficient. When the unit is turned on, the filament of the cathode is heated to incandescence which causes it to emit electrons. When high voltage is applied, these electrons are drawn across the opening and collide with the focal spot of the anode target resulting in the production of x-rays or photons. This is an efficient way to create x rays. 2.          Define xray beam quality and quantity.   (3 marks) X-ray production of quality means the penetrating ability of the beam. The higher quality comes from a more penetrating beam, which has to do with the energy spectrum of the x-ray. In respect to x-ray production quantity is the number of x-ray photons in the beam. The quality of the x-ray beam is controlled by the voltage while the millamperes control the quantity. a. X-ray Beam Quality. The quality of the x-ray beam is controlled by the amount of voltage. Voltage provides contrast to the film. The desired contrast appears as various shades of gray, black, and white in the x-ray negative (radiograph). Increased voltage provides less contrast (or more shades of gray). However, the beam has more penetrating power. Decreased voltage, on the other hand, provides more contrast (fewer shades of gray and more black and white shades). However, there is less penetrating power in the low voltage exposure. The technique most commonly used to expose periapical and bite-wing X-rays is a 75 kilovolt peak and 15 milliamperes. b. X-ray Beam Quantity. The x-ray beam quantity is controlled by the milliamperes. The more x-rays (photons) in the x-ray beam, the more dense (dark) the x-ray negative (radiograph) becomes. By increasing the milliamperes, we increase the number of available electrons at the cathode filament. When electrical current (voltage),mm,mlis applied to the x-ray tube, the electrons cross the gap. When they impact on the anode (tungsten target), a greater number of x-rays (photons) are also produced. The more x-rays that are available to penetrate an object, the more dense (dark) is the x-ray negative (radiograph). (Integrated Publishing n.d.) 3.          Explain how rectification impacts on the quality and/or quantity of the xray beam.   (4 marks) Rectification impacts the quality and quantity due to the fact low voltage has a less penetrating power, while high voltage allows for deeper penetration. Higher voltage causes less contrast, whereas a lower voltage causes more contrast. 4.          Compare and contrast molybdenum and tungsten as target materials in an xray tube with specific attention to beam quality and quantity.   (10 marks) Tungsten anodes emit a harder spectrum with more high energy x rays than from the molybdenum node. Thus tungsten has less quality, but is more quantity. Molybdenum has less power, but has better quality. 5.         How might filtration of the xray beam improve PET/CT based attenuation correction?   (3 marks) Filtration of the x ray beam allows for absorption of lower energy ray photons before reaching the target, allowing for less scattering of the photons. 6.          Describe the mechanism of, factors affecting and the importance to xray imaging the following interactions:                 Coherent scatter-Forward scatter is dominated by coherent scatter, which is the basis of x-ray diffraction. Its cross section varies with angle and photon energy in a material-specific manner, even for amorphous materials. The dependence on Z and chemical structure allows it to be very useful in distinguishing tissues within the patient. (Johns, Leclair, and Wismayer 2002)                 Photoelectric absorption-the process in which a photon (e.g. X ray photon) impinging on an atom transfers its entire energy to an inner (e.g. K or L) shell electron of the atom. The electron (named photoelectron) is ejected from the atom. The kinetic energy of the ejected photoelectron is equal to the incident X-ray photon energy minus the binding energy of the electron. The vacancy resulting from the electron ejection is filled by an electron from an outer orbit (with lower binding energy), leaving a vacancy in this outer orbit, which in turn is filled by another electron from an orbit even further away from the nucleus. The surplus energy liberated when an electron drops from its outer shell to a shell closer to the nucleus results in emission of characteristic radiation and/or Auger electrons. The energy of the characteristic radiation is equal to the difference in binding energies between the shells. The binding energy of a K-electron increases with increasing atomic number. It is only a few tens of eV in the lightest elements (14 eV in hydrogen), but increases to nearly 100 keV in heavy elements (88 keV in lead). The probability of photoelectric absorption per unit mass of a material is approximately proportional to Z3/E3, where Z is the atomic number of the material and E is the energy of the incident photon. Photoelectric absorption therefore increases with increasing atomic number and decreasing photon energy. (Medcyclopaedia 2009)                 Compton scatter-is the decrease in energy or increase in wavelength of an x ray photon when it interacts with matter. Due to this change in photon energy, an inelastic scattering process occurs. This is important in x ray imaging in terms of quality.                 Pair production-becomes probable with gamma energies exceeding 1.02 MeV, and becomes vital as an absorption mechanism at energies above 5 MeV. Interaction with the electric field of the nucleus, the energy of the incident photon is transformed into the mass of an electron-positron pair. Gamma energy in excess of the equivalent rest mass of the two particles (1.02 MeV) appears as the kinetic energy of the pair and the recoil nucleus. At the end of the positrons array, it combines with a free electron. The entire mass of these two particles is then changed into two gamma photons of at least 0.51 MeV energy each. Use diagrams if necessary. (15 marks) 7.          Describe how scatter can be reduced in xray imaging. (3 marks) The amount of scattered photons encroaching on the detector can be condensed by using the widely accepted absorbing grids or through the separation of the object from the detector plane. 8.          Provide a brief overview of the main characteristics of xray detectors. (5 marks) X ray detectors measures x ray tube peak potential and to simultaneously measure the X-Ray tube peak potential (kVp), and to differentiate the x ray tube spectrum. 9.          If an xray photon (120 keV) undergoes Compton scattering through 24 degrees, what percentage energy remains in the xray photon? (4 marks) 0.010332017792629998 keV 10.        If a thickness of 3.5mm of material reduces an x ray beam intensity by 30%, what is the linear attenuation coefficient? What assumption have you made to determine this? (4 marks) 3.5mm(30)=.105 cm-1.   The assumption is the linear attenuation coefficient is measured by the equation above. 11.          Why is the Lambert-Beers equation important in xray physics? (2 marks) The Lambert-Beers equation is important in x ray physics because it determines the amount of absorption the x ray beams into the mass. 12.          If the air kerma measured with an ionisation chamber is 30 mGy, approximately (to the nearest mGy) what will be the skin dose for xray exposure? (2 marks) 30 mGy. 13.         What is CTDI? What does it indicate? What are its limitations? How can these limitations be overcome? (5 marks) Computed Tomography Dose Index (CTDI) is the average dose imparted by a single axial acquisition to a standard 100-mm pencil chamber dosimeter inside a PMMA phantom over the width of 14 CT slices. The limitations are the CTDI value expresses the concentration of radiation for a precise tissue location. It tells nothing about the total radiation energy delivered. Using a larger slice needs to be examined will help overcome limitations. PART C:     1.          What modification was required to adapt Radons mathematical model for reconstruction to be useful in CT (and SPECT and PET for that matter)?   (4 marks) The adaptation was to invert the Radons to solve the 3D cone beam.     2.          Compare and contrast seven (7) generations of CT scanners.   (10 marks) First Generation Scanners:(Principles of Radiographic Imaging, page 652) The earliest CT scanners were designed to acquire CT images of the patient’s head. These scanners used a thin slit x–ray beam a single x–ray detector to acquire the entire CT image. The x–ray head and detector were mounted on a C–arm frame that could be rotated through and arc of 180o. The x–ray head and detector were mounted onto a synchronized linear scan mechanism. The C–arm would be rotated into position stopped and held still while the linear scanning mechanism moved the thin slit x–ray beam across the anatomical view. Each linear scan produced one view or x–ray image. The C–arm would be positioned in 1o steps to allow 180 linear scans along the 180o arc. The linear scan mechanism allowed 240 detector measurements at 1.7mm intervals to create each of the 180 views. Some scanners had two x–ray beams and a second detector to allow the scanner to acquire two anatomical section scans at the same time. A complete CT scan and image reconstruction took in the order of 10 minutes. First Generation Scanning 2. Second Generation Scanners(Principles of Radiographic Imaging, page 653) A fan–shaped x–ray beam was projected onto a linear array of approximately 30 detectors. The x–ray head and detectors were mounted onto a C-arm frame that could rotate 180o around the patient. The increased number of detectors reduced the number of linear scans required during the 180o arc. The time required for a scan was reduced to less than 90 seconds. Reduced scan times allowed the patient to hold their breath for the entire scan allowing the radiographer to scan the thoracic region of the body. The fan shaped beam increased scatter radiation artifact. This forced the use of lead masks on the detectors to reduce the scatter radiation artifact. Second Generation Scanning 3. Third Generation Scanners(Principles of Radiographic Imaging, page 653) A wider fan–shaped x–ray beam and a curved array of 250–750 detectors. The wider beam and larger detector array allowed the scanner to include the entire body in a single exposure. Eliminated the need for linear scanning to be combined with the x–ray head rotation. Third generation scanners would acquire approximately 700,000 measurements per anatomical section. Scan times reduced to less than 12 seconds. 360o scan arcs were possible and the scan arc could be varied. Shorter scan times allowed sequential scans to approximated dynamic functions with approximately 4 scans per minute. The single detector array made third generation scanners prone to ring artifact. Third Generation Scanning 4. Fourth Generation Scanners(Principles of Radiographic Imaging, page 654) A single projection fan–shaped x–ray beam and 600–2000 stationary detectors. The x–ray head rotates more than 360o around the patient. The detectors are fixed in a circular ring around the patient and x–ray head. Alignment of the x–ray beam to each detector is essential. The x–ray head needs to travel more than 360o in order to provide an acceleration and deceleration zone. Scanning takes place as the x–ray head travels in either direction, clockwise and counter clockwise. Scan rates of approximately 15 scans per minute are achieved. Scan rates are limited by the interscan time used to change the direction of the x–ray head travel. Dynamic scanning modes are available. Overscanning modes exist and use scanning arcs of greater than 360o. Fourth Generation Scanning 5. Helical/Spiral CT Scanners(Principles of Radiographic Imaging, page 654) Advanced slip ring technology is used to allow the x–ray head to travel in one direction indefinitely. The x–ray head does not have to be decelerated and accelerated in between scans. Scanners returned to using moving detector arrays. Approximately 1000 detectors are aligned opposite the x–ray tube. The detector array travels in a circle around the patient at the same time as the x–ray head moves. The patient can be advanced through the CT gantry as the continuously revolving x–ray head circles the patient. The computer acquires measurements from the detectors that result in data representing a continuous helical scan (similar to an apple being peeled). The primary advantage to helical scans is the reduced scan time. The entire abdomen scanned in less than 40 seconds. This results in the use of less contrast media, less motion artifact and greater patient through put. The primary disadvantage is that the entire 360o scan information is not acquired for every anatomical cross section. Computer reconstruction is required to fill in the missing data. Pitch is the ratio of the patient table movement during one rotation of the x–ray head to the section thickness.       I P = pitch P = -- I = table movement mm/sec     S S = section thickness mm   6. Other Scanner Designs(Principles of Radiographic Imaging, page 657) Nutating Detectors. Cardiac Scanner. Dynamic Spatial Reconstructor.   (Biomedical engineering technology 2009)   3.          What are the advantages of multiple detector arrays generations of CT scanners in CT?   (3 marks) Multiple detector arrays in the generations of CT scanners can make use of solid state Detectors. 4.          What is pitch? How does collimator pitch and detector pitch differ?   (4 marks) Pitch is a parameter when helical scan protocols are used. A collimator pitch is a single detector in a multidetector CT group of image receptors. Pitch measures collimator and detector levels. 5.          Describe what interpolation is and why it is necessary in data reconstruction.   (4 marks) Interpolation is a method of constructing new data points between a known set of data points. It is necessary to reconstruct data in the middle to understand the data better.  6.          What is meant by a bone window? What is meant by a soft tissue window? How do they differ?   (5 marks) The bone window is the area where a bone is x-rayed. A soft tissue window is the area where soft tissue is x-rayed. The difference is between the quality and quantity of the beam.     7.         Calculate the CT number using the Hounsfield scale for blood where the linear attenuation coefficient for blood is 0.208 cm-1.   (4 marks) BONE 0.528; BLOOD 0.208; G. MATTER 0.212 ... CT NUMBER SCALE. CORTICAL BONE +1000; MUSCLE +50; WHITE MATTER +45 8.          List and briefly define the factors that affect spatial resolution of CT.   (6 marks) Quality, or the lower level of energy, creates a better spatial resolution of the CT. On the hand quantity, or the higher level of energy, causes the spatial resolution to be of less quality other than the first factor. 9.          List and briefly define the factors that affect contrast resolution of CT.   (4 marks) Quality and Quantity also affect contrast resolution. Quality cause a better resolution of the CT. Quantity causes for darker contrast resolution. 10.          How do spatial and contrast resolution differ in CT?   (2 marks) Spatial resolution is the points of pixels in an x-ray, where as contrast resolution is about the shades on an x-ray. 11.          Describe beam hardening and its impact on the CT image.   (5 marks) Beam hardening is when more heat is applied for more energy causing the beam to harden. The impact is a more penetrating x-ray. 12.          What is the importance of slip-ring technology?   (2 marks) Slip-ring technology allows the continuous rotation of the X-ray tube and other components of a computed tomography 13.         Reflect on your understanding of the xray tube gained in module 1. What special design considerations may need to be accommodated for CT not generally required for general xray? Explain.   (5 marks) The position of the x-ray tube has to be more precise in a CT in order to produce a beam in the correct place. Since CTs are reaching different parts of the body, the x-ray tube for a CT must be more versatile. 14.          What factors affect noise levels in CT scans?   (4 marks) Noise levels in CT scans appear as graining on the image and is caused by a low signal to noise ratio. This occurs more commonly when a thin slice thickness is used. It can also occur when the power supplied to the X-ray tube is insufficient to penetrate the anatomy. 15.          Discuss the advantages and disadvantages of spiral CT with specific reference to dosimetry, data integrity/reconstruction, image quality and instrumentation.   (8 marks) The advantages are a detailed picture of areas inside the body. The pictures are created by a computer linked to an x-ray machine that scans the body in a spiral path. The image quality is better. Disadvantages include data integrity and reconstruction. The instrumentation is also awkward in positioning. 16.          Discuss the issues relating to beam geometry introduced by multi-slice CT. What is the implication for image quality? How are these issues overcome? What are the advantages of multi-slice CT?   (10 marks) The multi-slice CT is efficient for patients with healthy areas being examined. However, for patients that might have that little abnormality in between the multi-slice it is not as good. More tests are needed for those with symptoms that need CT examination. Advantages of the multi-slice are the quality of imagery. 17.          What is bed backlash? How do we check for it? What impact might bed backlash have on a PET/CT scan?   (6 marks) Bed backlash is the beams scattering around the bed or couch. The calibrations on each machine, plus the imagery produced can help check for bed backlash. A PET/CT scan be of poorer quality due to this phenomenon. 18.         What breathing protocol would you suggest to maximise accuracy of registration between PET and CT scans of the chest or abdomen? Explain your choice. What other options might evolve in the future?   (8 marks) Slow even breathing would be my choice to maximize accuracy of registration between PET/CT. This choice would be the less movement through the whole process calms the patient and helps to gain better results from the PET/CT. 19.         Describe the method you think most accurate for minimising CT based attenuation correction errors associated with the different energies of the xrays and photons.   (6 marks) Correction for attenuation is a statistical procedure, due to Spearman, to "rid a correlation coefficient from the weakening effect of measurement error" (Jensen, 1998), a phenomenon also known as regression dilution. Given two random variables X and Y, with correlation rxy, and a known reliability for each variable, rxx and ryy, the correlation between X and Y corrected for attenuation is . How well the variables are measured affects the correlation of X and Y. The correction for attenuation tells you what the correlation would be if you could measure X and Y with perfect reliability. If X and Y are taken to be imperfect measurements of underlying variables X and Y with independent errors, then rxy measures the true correlation between X and Y. (Jensen, 1998) (Jensen, 1998), Bibliography Biomedical engineering technology (2009).Scanner generations. Online. Integrated Publishing. (n.d.). X-Ray Beam Quantity and Quality. Online. Jensen, A.R. (1998). The g Factor. Connecticut, USA: Praeger. Johns, P.C., R.J. Leclair, and M.P. Wismayer. (2002). Medical X-ray imaging with scattered photons. Opto-Canada: SPIE Regional Meeting on Optoelectronics, Photonics, and I Imaging. Online. Medcyclopaedia. (2009). Photoelectric absorption. Medcyclopaedia. Online. Read More