X-ray Imaging and Computed Tomography PDF

24 X-ray imaging and Computed Tomography Dr. Mohammad Abu Mhareb good  Introduction 5 Izmet X-ray planar radiography is one of the mainstays i of a radiology department, providing a first ‘screening’ for both acu...

24 X-ray imaging and Computed Tomography Dr. Mohammad Abu Mhareb good  Introduction 5 Izmet X-ray planar radiography is one of the mainstays i of a radiology department, providing a first ‘screening’ for both acute injuries and suspected em chronic diseases.I 55 Planar radiography (general X-ray) is widely used to assess the degree of bone fracture in an acute injury, the presence of masses in lung cancer/emphysema and other airway pathologies, the presence of kidney stones, and diseases of the gastrointestinal (GI) tract.  X-rays generated from a source are directed towards the patient, as shown in Figure 1(a). X-rays which pass through the patient are detected using a solid-state flat panel detectorI which is placed just below the patient. 1 I If  I The detected X-ray energy is first converted 2 into light, then into a voltage and finally is digitized. There is very high contrast, for example, between the bones (white) which absorb X- rays, and the lung tissue (dark) which absorbs very few X-rays. IT a dark in x 2 whytheLung tissue appear as ray imaging  1. X-ray tube and X-ray energy spectrum For both planar radiography and CT the X-ray source is a specialized piece of equipment known as an X-ray tube. A i A photograph of an X-ray tube and a schematic diagram of its major components are shown in Figure 2.  2 44 o Principle components of an X ray tube are an cathode Electron Source from a heated tungsten filament g with a focusing cup serving as the tube Cathode, an Anode or Target and a Tube Envelope to is maintain an interior vacuum target pmetal gsEm iit. m pic from potential III difference 25.140kV sutteeofelectron  All of the components of the X-ray system are contained within an evacuated vessel. 154m's a giveme thereason whythe x raystubesurroundedbyoil The evacuated vessel is surrounded by oil for both cooling and electrical isolation. IT The whole assembly is surrounded by a lead shield with a glass window, through which the X-ray beam is emitted. ms  withPI If X-rays are produced by a beam of high energy electrons striking the surface of a metal target. DID A negatively-charged cathode acts as the source of these electrons, and consists of a small helix of thin tungsten wire, through which an electric current is passed. A potential difference between the anode and cathode of between 25 and 140 kV (depending upon the particular type of clinical study) is applied. y target y et E 2 v note  This potential difference is known as the accelerating voltage, or kVp. si target The metal anode must be able to produce X- rays efficiently,y and also be able to withstand s the very high temperatures generated. high 2 In terms of efficiency, the higher the atomic number of the metal in the target, the higher the efficiency of X-ray production. e  WEI The most commonly used metal is tungsten which has a high atomic number, 74, and a melting point of 3370 °C.JAIL 2 gie me thereasonyky tungsten th 1 wide In addition, it has good thermal conductivity É and a low vapour pressure, which allows a strong vacuum to be established within the X- 1s ray tube, thusI providing the electrons with an unimpeded path between cathode and anode. a reason 14 a give me themetal target why vnot unblocked  a In practice, a tungsten–rhenium (2–10%0 rhenium) alloy is used for extra mechanical stability of the target. FLY because As mentioned earlier, digital mammography requires very low energy X-rays, and for these types of application the metal in the anode is molybdenum rather than tungsten. 2 why vnot  e In order to achieve a well-defined small area in which the X-rays are created, the anode is bevelled at an angle between 8 and 17°, with Anode 12–15° being the usual range. The smaller the angle, the smaller the effective focal spot size (f), shown in Figure 3(a), given by: et where θ is the bevel angle and F the width of the electron beam. beam IF  Values of the effective focal spot size range from 0.3 mm for digital mammography to between 0.6 and 1.2 mm for planar radiography and computed tomography. The bevel angle also affects the coverage of F the X-ray beam, as shown in Figure 3(b), which is given by: if II forbysher 31,0  t.gg et a explain 46 In practice, the X-ray beam has a higher intensity at the ‘cathode-end’ than at the ‘anode-end’, a phenomenon known as the Heel effect. This effect is due to differences in the distances that X-rays have to travel through the target itself. Ñ teetdf.at I hydrawing g 2 give me commal 2 I  There are three parameters that can be chosen by the operator for X-ray imaging: 1. The accelerating voltage (kVp), 2. The tube current (mA), and 3. The exposure time. The current that passes from the cathode to the anode is typically between 50 and 400 mA for planar radiography, and up to 1000 mA for CT.  The value of the kVp varies from ~25 kV for digital mammography to ~140 kV for bone and chest applications. X-ray tubes produce X-rays with a wide range of energies, up to a maximum value given by the kVp, as shown in Figure 4. kilovoltagepeak  There are two separate mechanisms by which X-rays are produced: 1. Bremsstrahlung X-ray: 2. Characteristics X-ray: 8 MY  2 1. Bremsstrahlung X-ray: The first mechanism involves an Ek s electron passing close to the e nucleus of an atom of the metal abreak tendation forming the anode, and being few deflected from its original trajectory by the attractive I forces from the positively charged nucleus. This I 2 deflection results in a loss of electron kinetic energy, and this energy loss is converted into an me X-ray. hase2ws 2. Characteristic X-ray: If an electron accelerated from the cathode collides with a tightly bound K- shell electron in the c e anode, this bound ejected electron is ejected and the resulting ‘hole’ in the hole K K-shell is filled by an iM electron from an outer (L or M) shell. in inter gendered i