X-Ray Technical Document PDF

# Transformers ## What Are Transformers? - Transformers are electrical devices used to convert or "transform" AC voltage from one level to another. (high to low or low to high) - Input and output are AC. - They do this by the principle of electromagnetic induction. ## Parts of a Transformer A tran...

# Transformers ## What Are Transformers? - Transformers are electrical devices used to convert or "transform" AC voltage from one level to another. (high to low or low to high) - Input and output are AC. - They do this by the principle of electromagnetic induction. ## Parts of a Transformer A transformer consists of 3 basic components. - **Primary Coil or Primary Winding:** It is an electrical wire wrapped around the core on the input side. - **Secondary Coil or Secondary Winding It:** Is an electrical wire wrapped around the core on the output side. - **Core:** A ferromagnetic material that can conduct a magnetic field through it. Example: Iron. ## Transformer Operation - An electrical transformer normally consists of a ferromagnetic core and two coils called "windings." - A transformer uses the principle of mutual inductance to create an AC voltage in the secondary coil from the alternating electric current flowing through the primary coil. - The voltage induced in the secondary can be used to drive a load. ## Transformer Types ### Step-down Transformer - #primary coils > # secondary coils - primary voltage > secondary voltage ### Step-up Transformer - # primary coils < # secondary coils - primary voltage < secondary voltage ## Transformer Construction - Transformers have 2 coils of wire. - no electrical contact between coils. - When electric current passed through one coil. - magnetic field develops around first coil. - second coil near enough to feel magnetic field. ## Rectification - Changes alternating current output of high voltage transformer to direct current. - Allows current flow in one direction only. - X-ray tube is a rectifier because current will not flow from anode to cathode. - no source of free electrons at anode. ## Transformer Types - **Step-down Transformer** - #primary coils > # secondary coils - primary voltage > secondary voltage - **Step-up Transformer** - # primary coils < # secondary coils - primary voltage < secondary voltage ## High Voltage Circuit ### High Voltage Transformer - Supplies high voltage for x-ray tube. - Step-up transformer. - primary from autotransformer. - secondary to rectifier circuit. - mA monitored at the center grounded point of secondary. ### High Voltage Transformer - Grounded metal box. - filled with oil. - electrical insulator. - **Function:** - increases or decreases alternating voltage. - **Also contains rectifier circuit:** - changes alternating current into direct current. ## Full-wave Rectification - **Rectifiers:** - Four diode "bridge” configuration used with single phase. - both + & - half cycle of high tension transformer used. - efficient. - circuit reverses negative half cycle & applies to x-ray tube. ## X-ray Generator - Supplies electrical power to x-ray tube. - high voltage between anode & cathode. - filament voltage. - Controls exposure timing. - Turns exposure on and off. - High voltage switched on and off. - Filament heated before exposure. ## Generator Components - **control console** - kVp adjust - mA adjust or mAs adjust - time adjust - transformer - high voltage (step up) - filament - low voltage (step down) - electronics cabinet - support circuitry ## Battery-Powered Generators - **Batteries used for:** - x-ray - transport - **Independent of power line during exposure.** - **Disadvantages:** - Batteries must be charged. - Batteries maintenance. - Heavy. - **Battery's DC converted to AC for high voltage transformer to operate.** ## Medium Frequency Generators - **Advantages:** - conducive to computer control. - immune to power line fluctuations. - low ripple. - small size of electronics & transformer. - small enough to spin on CT gantry. - Today's trend in generators. ## Rotating Anode - The anode of rotating anode tube consists of a large disc of tungsten, or an alloy of tungsten, which theoretically rotates at a speed of about 3600 rpm. - The purpose of rotating anode is to spread the heat produced during an exposure over a large area of anode. ## Operating Console - **Control unit.** - Manages current, voltage and timer. - **Current control** allows adjustment of tube current to vary radiation intensity. - **Voltage control** allows adjustments in anode to change energy of radiation. - **Timer** determines the duration of exposure. ## Advantages of a Operating Console - It is non-invasive. - It can be made available as a portable x-ray unit. - It is less costly when compared to the other imaging modalities like MRI or CT. ## Limitations of an Operating Console - X-ray uses radiation, which can cause cancer if you are exposed to it over a prolonged period of time. - Do not provide as much information as MRI and CT. ## Bucky Tray - The bucky tray can be found: - In the table or - Chest board - Holds the cassettes. - CR - Film Screen ## "Technique" kVp , mAS (mA x s) - mA- is the current in combination with the time - determines HOW LONG the beam will stay on. - Controls the density on the film/image. ## Tilting Tables - Diagnostic and fluoroscopic work. - 90 degrees in one direction. - 15 – 30 degrees in the other direction. - Ancillary equipment: - Footboard, shoulder support, handgrips, compression bands ## Fluoroscopic with image intensifiers ### What is an Image Intensifier? A complex electronic imaging device that receives the remnant beam and converts it to light and increases the intensity of the light. The image intensifier tube is contained in a glass envelope in a vacuum and mounted in a metallic container which provides protection for the components. ## "Technique" kVp, mAs (mA x s) - What is set at the control panel. - How the "image" is created on the "film" or Image receptor (digital). - kVp controls the “ENERGY" of the beam. - The Higher kVp – more penetrating. - Ranges is 50 -110 in Diagnostic x-ray. ## Exposure Timing - **Manual** - Operator sets time. - **Automatic (Phototimed)** - Equipment measures exposure. - Terminates exposure when designated exposure is measured. ## Photocathode - Thin metal layer bonded directly to the input phosphor. - Usually made of Cesium and Antimony compounds that respond to light stimulation. - Responsible for Photoemission. - Electron emission after light stimulation. - The number of electrons emitted is directly proportional to the intensity of light intensity of the incident x-ray photon. - Converts the pattern of light photons emitted by the input phosphor into electrons. ## Electrostatic Focusing Lenses - A series of lenses inside the II tube to maintain proper focus of the photoelectrons emitted from the photocathode. - They contain a positive charge. - They are located along the length of the II tube. - The focusing lenses assist in maintaining the kinetic energy of the photoelectrons to the output phosphor. ## Output Phosphor - Usually constructed of zinc cadmium sulfide crystals. - Serves to increase illumination of the images by converting photoelectrons to light photons. ## Spot Film Device - Used to make permanent images during the radiographic examination. - Film is positioned b/w the patient and the image intensifier. - When the film is needed, the radiologist actuates the control that brings the cassette in position. This changes the tube from fluoroscopic mA to radiographic mA. - During fluoroscopy, the tube is operated at less than 5 mA. ## TV Monitors - This practical and efficient viewing system was employed because of the limitations of the mirror optic viewing system. - **TV monitors:** - Enables viewing by multiple persons. - Monitors may be located in remote locations other than the radiographic room. - Image brightness and contrast can be manipulated. - Images may be stored on different medium for reviewing at a later time. ## Fluoroscopy -Modes of operation - **Manual Mode** - Allow the use to select the exact MA and KVp required. - **AEC Mode** - Allow the unit to drive the KVp and MA to optimize dose and image quality. - **Pulsed Digital mode** - Modifies the fluoroscopic output by cutting by cutting out exposure between pulses. - With the pulsed mode, it can be set to produce less than the conventional 25 or 30 images per second. This reduces the exposure rate. ## Fluoroscopic X-Ray Tubes - mA range: 1 – 5.0 mA. - KVP: 70 – 110. - depending on exam and contrast used. - 15” minimum SOD in fixed fluoroscopic equipment. - Foot switch (dead man). ## Principle of Fluoroscopic X-Ray Tubes - Is an ionization detector which main function is to measure the ion pairs created when ionization mediation through the gas. - The ion pairs are attached either in the positive or negative electrodes which are then connected the battery therefore reading measurement of ionizing radiation (AKA ionization induced electric). ## Advantages of Gas Filled Detectors - Accurate and precise. - Recommended for beam calibration. - Instant readout. - using in N.M (hot lab) - For important structure in dose collimator. ## Disadvantages of Gas Filled Detectors - Connecting cables required. - High voltage supply required. - Many corrections required for high energy dosimetry. ## GM Counter - **Characteristics:** - large dead time (~ 100µs), saturation. - has no energy info. - high sensitivity (100% for each ionizing event). - measures low exposure rates (~0.1 mR / hr). ## Gm Counter - **Advantages:** - Higher currents = cheaper electronics required. - Easily replaceable tubes. - Audible and visible outputs. - Not susceptible to atmospheric changes – humidity etc. - Sensitive – best used as a contamination monitor. - **Disadvantages:** - Cannot be used as a doserate monitor. - Requires quenching to stop runaway cascade. - Fragile end windows. - No energy discrimination. ## Gm Counter - Operation similar to ion chamber – gas ionisation - **Major Differences:** - Low pressure gas ~ 10-3 Atm. - High voltage across electrodes > 500 V. - HV causes avalanche / cascade effect. ## Scintillation Detectors - Phosphors (Nal(TI), CsF, BGO, LSO) - Photomultiplier Tube (PMT) - dynodes, counting chain, spectra - Liquid Scintillation Counting (“wipes”) ## Film Badge Dosimeters - **Advantages:** - Cheap. - Permanent record. - Sensitive to low energy gammas < 20 keV. - Discrimination. - **Disadvantages:** - Messy developing chemicals. - Not as accurate as other techniques. - Degradation of film. - Only by used once. ## Scintillator Characteristics - Phosphors (Nal(TI), CsF, BGO, LSO) - Photoelectric interaction ~ Z4 - Nal(TI): reference, decay const. ~ 1μς. - CsF : faster than Nal(TI), TOF PET. - BGO: slower but more efficient, PET. - LSO: very fast (~ 1ns), high res. PET. ## TLD Dosimeters - Gamma and beta exposure from 20mR to 106R. - No energy dependence. - Linear dose response over wide range. - Tissue equivalence. - Thermal neutron dose measure possible - (Li-6, Li-7). - Wearing period of up to one year.