Oral Radiology Radiation Physics Part-2 Lecture Notes PDF

Oral Radiology Radiation Physics Part-2 OMD-711n LECTURE ILOs Types of X-Ray machine Describe the components of X-Ray machine Explain the theories of X-Ray Production Explain the function of X-Ray machine components Define the different type of accessories X-ray Machines T...

Oral Radiology Radiation Physics Part-2 OMD-711n LECTURE ILOs Types of X-Ray machine Describe the components of X-Ray machine Explain the theories of X-Ray Production Explain the function of X-Ray machine components Define the different type of accessories X-ray Machines Types X-ray Machine Control panel Extension arm Tube head A) Control Panel: B) Extension Arm: C) Tube Head:  Cathode Tungsten Filament ( Molybdenum cup  Anode Dental x-ray machines have stationary anode Tungsten Target Copper stem  Tungsten (Filament and Target) High atomic number (74) Transfers heat readily High melting point (3370º C) Low vapor pressure  Line Focus Principle The smaller the focal spot (target), the sharper the image (teeth) will be. During x-ray production, a lot of heat is generated. If the target is too small, it will overheat and burn up.  Line Focus Principle Target (Anode) Cathode Apparent (effective) focal spot size 1.5 x 1.5 Actual focal spot size 5.1 x 1.5 PID  The target is at an angle (not perpendicular) to the electron beam from the filament.  Because of this angle, the x-rays that exit through the PID “appear” to come from a smaller focal spot.  Even though the actual focal spot (target) size is larger (to withstand heat buildup, the smaller size of the apparent focal spot provides the sharper image needed for a proper diagnosis. When While the exposure keeping the button exposureis depressed, button the current depressed, the can high- The The The x-ray x-rays length machine pass of through the is exposure plugged the filter is into and selected the electrical collimator with the outlet before timer. flow into the voltage x-rayistubehead. circuit activated This to activates pull the the low-voltage electrons from the (110 exiting volts through usually). the PID. (Click for next slide) circuit which filament heats to the the filament; target, producingthis lasts(Click x-rays. for ½tosecond produce (Click to depress exposure button). x-rays). oil Filament filter Exposure Button  Timer Automatic Manual 1- Direct or immediate timers: Its attached to a long cord to enable the operator to go away from the field of radiation. Operator press on a button just to activate the exposure while the time is pre adjusted and the exposure will stop automatically even if the operator continuous to press the button. 2- Delayed timers: This type provide about 9 second before the start of exposure, so it provides the operator a period to get away from the field of radiation. X-ray Production Bremmstrahlung Characteristic  Bremsstrahlung X-ray Production (Also known as braking radiation or general radiation) Electron slowed down by positive charge of nucelus; energy released in form of x- ray High-speed electron from + filament enters tungsten atom Electron continues on in different direction to interact with other atoms until all of its energy is lost  Bremsstrahlung X-ray Production Maximum energy + High-speed electron from filament enters tungsten atom and strikes target, losing all its energy and disappearing The x-ray produced has energy equal to the energy of the high-speed electron; this is the maximum energy possible  Characteristic X-ray Production Electron in L-shell X-ray with 58 keV drops down to fill Ejected electron of energy vacancy in K-shell leaves atom produced. 70 (binding energy of K-shell electron) vacancy minus 12 (binding M energy of L-shell High-speed electron with L electron) = 58. at least 70 keV of energy K (must be more than the binding energy of k-shell Tungsten atom) strikes electron in the K shell, Recoil electron knocking it out of its orbit (with very little energy) exits atom Only 1% of the interactions between the high-speed electrons and the target atoms result in x-rays. 99 % of the interactions result in heat production. heat Heat Dissipation high melting point of tungsten Good conductivity of Cu stem Cooling property of Insulating oil Good conductivity of Metal Housing Factors Controlling the X-Ray Beam 1- Filtration. 2- Collimation. 3- Source-object distance ( inverse square law). 4- Kilovoltage. 5- Milliampere. 6- Exposure time.  Filtration The process of removing low- energy x-rays from the x-ray beam Filtration Aluminum Filter Added Glass wall of X-ray Total Tube Oil / Metal Barrier Inherent Beryllium Barrier Window PID Material oil Filter Collimator  PID The filter is usually located in the end of the PID which attaches filter to the tube head.  2- Collimation  A collimator is a metallic barrier with an opening in the middle used to shape & restrict the size of the x-ray beam & the volume of tissue irradiated.  The size of the hole determines the final size of the x- ray beam.  Dental x-ray beams are usually collimated to a circle 2.75 inches(7 cm) in diameter at the patient's face. You are looking up through the PID at the collimator (red arrows), which is a circular lead disk with a circular cutout in the middle. This will produce a round x-ray beam. The light gray area in the center is the aluminum filter. Types of collimators: 1. Diaphragm: A thin sheet of lead with an aperture at the centre. 2. Tubular: A tube of lead with one of its ends connected to a diaphragm collimator Advantage: A. Decreases diverging rays B. Increases quality of image by decreasing geometrical unsharpness C. More safety of the patient 3. Rectangular: The beam may be just enough to cover the intraoral film.(reduces the area of the patient’s skin surface exposed by 60%) 4. Slit: Used in panoramic machines  7 cm 6 cm If you switch from a 7 cm round PID to a film 6 cm round (3.5 x 4.5 cm long) PID, the patient receives 25% less radiation.  The shape of the opening in the collimator determines the shape of the x-ray beam. (It affects quantity of x- rays) A) Round, B) Rectangle.  Rectangular collimators further limit the size of the beam to just larger than the intraoral receptor, further reducing patient exposure (patient receiving 60% less radiation when compared to what they would receive with a 7 cm round PID).  3-Source-object distance - Inverse Square Law  For a given beam, the intensity is inversely proportional to the square of the distance from the source, i.e., when the distance from the focal spot is doubled, the intensity of the beam decreases to one quarter.  The reason for this decrease in intensity is that an x-ray beam spreads out as it moves from its source. Cone: Cones are classified according to: 1. Length (measured from the target to end of the cone) 2. Composition 3. Shape