Reviewer Power Rating to APR PDF

high frequency generators require about ½ the mAs used by single phase equipment to obtain similar image density. - techniques for High frequency generators are roughly equivalent to those for 3𝜑 equipment. - single-phase generation - three-phase equ...

high frequency generators require about ½ the mAs used by single phase equipment to obtain similar image density. - techniques for High frequency generators are roughly equivalent to those for 3𝜑 equipment. - single-phase generation - three-phase equipment - battery-powered mobile units POWER RATING - energy/unit time that can be supplied by the x-ray generator or received by the x-ray tube during operation Power = current x potential P = IV Watts = amperes x volts High voltage generator power (kW) = maximum x-ray tube current (mA) at 100kVp and 100ms PROBLEM: When a system with low voltage ripple is energized at 100kVp, 100ms the maximum possible tube current is 800mA. What is power rating? 3𝜑 & HF power Power rating (kW) = mA (kVp) 1000 An international system is capable of 1200mA when operated in 100kVp, 100ms. What is the power rating? 1𝜑 generators 𝑚𝐴(𝑘𝑉𝑝) Power rating = (0.7) 1000 X-RAY GENERATOR USAGE & POWER RATINGS Generator Type Power Level Usage Applications Typical Power Requirements (kW) Single-pulse, 1𝜑 Very low Dental & handheld fluoroscopy units 10 & 30 & 50 & 80 & 2 & 1.5mm Al Radiography >70 >2.5mm Al High-voltage studies (chest) >100 Cu/Al High-energy imaging >200 Sn/Cu/Al Compensating filters – useful in maintaining image quality Wedge filter – used when radiographing a part that varies considerably in thickness Trough filter – thin central region positioned over mediastinum while the lateral thick portions shadow the lung fields Bow-tie – used w/ some CT imaging system Conic filters – used in digital fluoroscopy where the IR, image intensifier tube is located FACTORS AFFECTING X-RAY QUALITY & QUANTITY An Increase in X-ray Quality X-ray Quantity mAs None Increase kVp Increase increase SID None reduce filtration Increase reduce EXPOSURE TECHNIQUE CHARTS - standardized the selection of exposure factors for the typical patient or average patient so that the quality of radiographic image is consistent. - for each radiographic procedure, technologist consults the technique chart for the recommended exposure variables. - properly designed & used technique chart - decrease the number of repeat radiographic studies needed & therefore decrease patient’s exposure  CONDITIONS NECESSARY FOR AN EFFECTIVE TECHNIQUE CHARTS: Radiographic equipment must be calibrated Image processing must be consistent throughout the department CONTENTS STANDARDIZES IN A TECHNIQUE CHART Anatomic part Grid ratio AEC detector selection kVp If applicable mA CR location Part thickness & measuring point Type of IR Position or projection Focal spot size SID BASIC SYSTEM OF RADIATION EXOSURE TECHNIQUE 1. Variable kVp – accurate measurement of part thickness is critical to the effective use of this type of technique chart. - SID remain constant, but kVp are varied to compensate for various thickness & density of part being radiographed - in general, exposure made provide radiographs of shorter contrast scale. kVp = 2x thickness of anatomy + constant Constant HF – 23 1𝜑 – 30 3𝜑 - 25 Anatomic part knee IR 400 speed Projection AP Table top/bucky bucky Measuring point midpatella Grid ratio 12:1 SID 100cm Focal spot size small  Thickness(cm) kVp mAs 2. Fixed kVp – developed by Arthur W. Fuchs 10 63 20 - uses the concept of selecting an optimal kVp value 11 65 20 that is required for the radiographic examination & 12 67 20 adjusting the mAs for variations in part thickness 13 69 20 - optimal kVp for each part indicated, & mAs is 14 71 20 varied as a function of part thickness 15 73 20 - accurate measurement of the anatomic part is 16 75 20 important but less critical 17 77 20 - part size is grouped as small, medium or large 18 78 20 Abdomen - AP Part thickness (cm) Required mAs kVp : 80 small: 14-20 56 SID : 100cm medium: 21-25 80 IR Speed: 200 large: 26-31 104 3. High kVp – ideal for barium studies & routine chest x-ray to improve visualization of various tissue mass densities present in the lung fields & mediastinum - procedure in preparing this technique chart is the same as fixed kVp technique chart - exposures for a particular anatomic part would use the same kVp, the mAs value would be much less. EXPOSURE TECHNIQUE CHART CHARACTERICTICS Design Type Part Contrast Radiographic Patient Dose Tube Heart Measurement Scale Contrast Load Variable kVp/fixed Critical Shorter Variable Higher Increased mAs Fixed Less critical Longer Standardized Lower decreased kVp/variable mAs BASIC RULES FOR USING TECHNIQUE CONVERSIONS: 1. Selection of mA must necessarily depend on the capacity of the x-ray generator & the x-ray tube loading the requirements of the focal spot dimensions. 2. Time of exposure should be reduced to a minimum to counteract the effects of motion 3. kVp changes must be interpolated with respect to mAs values PREPARATION FOR FORMULATING THE TECHNIQUE CHART 1. Radiographic equipment 2. Grids 3. Screen speed 4. Collimator light-field/radiation-field alignment 5. Elimination of variables CONTROL OF SCATTERED RADIATION Factors that affects the amount of scattered radiation: 1. kVp 2. Field size 3. Patient thickness Devices that controls scattered radiation 1. Beam limiting device 2. Radiographic grid BEAM LIMITING DEVICE - device attached to the opening of the x-ray tube housing w/c regulates the size & shape of the primary beam TYPES: 1. Aperture diaphragms or lead diaphragms - Flat piece of lead that has a hole on it - Aperture opening is designed to cover precisely the size of film employed in a particular procedure. - A properly designed aperture diaphragm Aperture opening = field size or size of film source to diaphragm SID distance Problem: If a 20cm sq. film is to be imaged to 100cm SID & the diaphragm is placed 10cm from the target, what should be the dimension of one side of the diaphragm opening. To leave an unexposed border of 1.0cm on each side, we must reduce the beam size to 19. 2. Cones & cylinders - an aperture diaphragm that has an extended flange attached it. - cones and cylinders limit unsharpness surrounding the radiographic image more than diaphragms do. - disadvantages 3. Collimator (variable aperture collimator) - considered the best type of beam-restricting device available for radiography - it has 2 sets of adjustable lead shutters - field size produced is always rectangular or square unless an aperture diaphragm, core or cylinder is used in conjunction with it - equipped with a white light source & a mirror to protect a light field onto the patient. - plastic template w/ crosshair is affixed to the bottom of the collimator automatic collimator or positive beam limiting device (PBL) CONVERSION FACTORS FOR CYLINDER CONE extension cylinder collapsed increase mAs 40% or +5kVp extension cylinder extended increase mAs 60% or +10kV CONVERSION FACTORS FOR RECTANGULAR COLLIMATOR FIELD SIZE To Convert to a: IMPORTANT RELATIONSHIP REGARDING 10x12” field increase mAs 40% RESTRICTION OF THE PRIMARY BEAM 8x10” field increase mAs 60% Increased Result Factors Collimation Patient dose increase Scatter radiation increase Radiographic contrast increase Radiographic density increase Field size Patient dose increase Scatter radiation increase Radiographic contrast increase Radiographic density increase RADIOGRAPHIC GRIDS Two (2) types: 1. Stationary Grid – Gustave Bucky in 1913 2. Moving Grid – Dr. Hollis Potter of Chicago - device placed between the patient & IR w/c serves the purpose of absorbing radiation. - large enough to cover the biggest size of a cassette GRID CONSTRUCTION – contain thin lead strips or lines that have a precise height, thickness & more than 60kVp is needed for the examination. GRID CHARACTERISTICS: 1. Grid Ratio – ratio of height of the lead strips to the distance between them 3 important dimension on a grid: thickness of the grid material (T) Thickness of grid interspace material (D) Height of grid (h) Grid ratio = h D Problem: What is the grid ratio when the lead strips are 3.2mm high & separated by 0.2mm? Grid ratios range from 4:1 to 16:1 High-ratio grids remove, or clean, more scatter radiation than lower-ratio grids & thus further increase radiographic contrast high ratio are more difficult to manufacture than low ratio grids 2. Grid Frequency – number of grid strips or grid lines/ inch or cm - most grids have frequencies in the range of 25 to 45 lines/cm Grid frequency = 10,000𝜇m/cm (T+D) um/line pair Problem: What id the grid frequency of a grid having a grid strip width of 30𝜇m & an interspace width of 300𝜇m? 3. Interspace material – approx. 0.33mm thick interspace material – organic material (plastic fiber) - inorganic material (aluminum) 4. Grid strip – should be infinitely thin & high absorption properties 5. Grid casing – encased completely by a thin cover of aluminum GRID PERFORMANCE: 1. Contrast improvement factor (k) – ratio of the contrast of a radiograph made w/ a grid to the contrast of a radiograph made w/o a grid - k of 1.0 indicate no improvement - most grids have k of between 1.5 to 2.5 k = radiographic contrast w/ grid Radiographic contrast w/o grid Problem: An aluminum step wedge is placed on a tissue phantom 20cm thick & a radiograph is made. W/o a grid , analysis of the radiograph shows an average gradient of 1.1. w/ 12:1 grid, radiographic contrast is 2.8. What is the k of this grid? 2. Bucky factor (B) (grid factor) – measure the penetration of both primary & scatter radiation through the grid. B = incident remnant radiation Transmitted image forming radiation - the higher the grid ration, the higher the bucky factor Approximate Bucky Factor Values for Popular Grids Grid Ratio 70kVp 90kVp 120kVp Average No Grid 1 1 1 1 5:1 2 2.5 3 2 8:1 3 3.5 4 4 12:1 3.5 4 5 5 16:1 4 5 6 6 3. Selectivity (Σ) – ratio of transmitted primary radiation to transmitted scatter radiation Σ = primary radiation transmitted through grid Scatter radiation transmitted through grid - primary a function of the construction characteristics of the grid rather than the x-ray beam. GRID CHARACTERISTICS ❖ High grid ratio have high contrast improvement factors ❖ High-frequency grids have low contrast improvement factors ❖ Heavy grids have high sensitivity high contrast improvement factors GRID PATTERNS 1. Parallel grid or non-focused lead lines – lead lines run parallel to each other - easy to manufacture and simplest type - OD reaches a maximum along the center line of the IR & decreases toward the side GRID CUTOFF Distance to Cutoff = SID Grid ratio Problem: A 16:1 parallel grid is positioned for chest radiography at 180cm SID. What is the distance from central axis to complete grid cutoff? Will the image satisfactory cover a 35x42cm IR? 2. Cross grid or cross-hatched – have lead strips running parallel to both the long & short axes of the grid - not difficult to manufacture - much more efficient than linear in cleaning up scatter radiation - 16:1 constructed with two (2) 6:1 linear grids 3. Focused grid – lead lines are angled to approximately match the angle of divergence of the primary beam - more difficult to manufacture than parallel grids - convergent point and convergent line Types of Grids 1. stationary 1.1 water grid 1.2 grid cassette 2. moving 2.1 reciprocating 2.2 oscillating ❖ Stroboscopic effect – synchronization between x-ray pulsation & grid movement FOCUSED - GRID ALIGNMENT Type of Grid Misalignment Result Off-level Grid cutoff across image; underexposed, light image Off-center Grid cutoff across image; Underexposed, light image Off-focus Grid cutoff toward edge of image Upside-down Severe grid cut-off toward edge of image GRID CONVERSIOB FACTORS IN From non-grid to: mAs kVp 5:1 1.5x +8kVp 6:1 2x 12kVp 8:1 3x 20kVp 12:1 3.5x 23kVp 16:1 4x 25kVp Grid Conversion Factors (GCF) Grid Ration 5:1 2 6:1 3 8:1 4 12:1 5 16:1 6  GCF = mAs w/ grid mAs1 = GCF1 mAs w/o grid mAs2 GCF2 Problem: If a technologist produced a knee radiograph w/ a non-grid exposure using 10mAs & next wanted to use an 8:1 grid, what mAs should be used to produce a radiograph w/ the same density? (GCF for 8:1 = 4) GRID RATIOS & kVp Upper kVp limits Grid Ratio Bones Studies Barium studies 8:1 90 100 12:1 100 120 16:1 150 150 Chest studies Grid Ratio kVp range to avoid excessive contrast 16:1 100-120 8:1 140 10:1 150 GRID SELECTION - In most situation it is appropriate to design radiographic procedures around moving grids. - selection of grid with proper ratio depends on three (3) interrelated factors: 1. High kVp 2. High grid ratio Grid Selection Factors 3. High patient dose - Patient dose increase w/ increasing grid ratio - use of one grid - High-ratio grids are usually used for high-kVp examinations - Patient dose at high kVp is more than that at low kVp In general, grid ratios up to 8:1 are satisfactory at tube potentials below 90kVp. Grid ratios above 8:1 are used when kVp exceeds 90kVp Radiographic Grids Increase factors Result Grid Ratio Contrast increases Patient dose increase The likelihood of grid cutoff increases All dedicated mammographic imaging system are equipped with a 4:1 or 5:1 ratio moving grid. Even at low kVp used for mammography, considerable scatter radiation occurs. Approximate change in Radiographic Technique for Standard Grids Grid Ratio mAs Increased kVp increase No grid 1x 0 5:1 2x +8 to 10 8:1 4x +13 to 15 12:1 5x +20 to 25 16: 6x +30 to 40 AIR-GAP TECHNIQUE - based on the simple concept that much of the scatter will miss the IR if there is increased distance between the patient & IR - this technique has found application particularly in areas of chest radiography & cerebral angiography - magnification usually acceptable - limited in its usefulness because the necessary OID results in decreased record detail - not normally as effective w/ high kVp radiography, in which direction of scattered radiation is more forward AUTOMATIC EXPOSURE CONTROL (AEC) - one method for setting exposure factors that a quality radiographic image is produced. - designed to produce radiograph with optimal density by controlling the amount of radiation exposure reaching the film. Principle of Operation: Once the predetermined amount of radiation is transmitted through the patient, the x-ray exposure is terminated. This determines the exposure time & therefore the resulting density. Clinical operation: exposure time is set to the backup time Two (2) types of AEC system: 1. Photomultiplier tube system (PMT) 2. Ionization chamber  Sensors chambers cells radiation-measuring devices that measures radiation exposure reaching the IR pick –ups detectors PMT Systems – electronic device that converts visible light energy into electrical energy - exit type devices - light paddles serve as the detectors, and the radiation interacts with the paddles, producing visible light Ionization Chamber System or Ion Chamber - hollow cell that contains air & is connected to the timer circuit via an electrical wire. - considered entrance-type devices - when ionization chamber is exposed to radiation from a radiographic exposure - compared w/ PMT, ion chambers are less sophisticated & less accurate, but are less prone to failure - most of today’s AEC systems use ionization chambers TECHNICAL CONSIDERATIONS WITH AEC - to use AEC to its best advantage, RT must be aware of some important technical considerations peculiar to AEC systems. 1. Centering of part – anatomic area of interest must be centered properly over the detector(s) 2. Detector selection – select the detector(s) that will be superimposed by the anatomic structures that are of greatest interest & need to be visualized. ACCURATE CENTERING & DETECTOR SELECTION Accurate centering & detector selections are critical w/ AEC systems because the radiograph will demonstrate optimal density of the anatomy located directly over the detector. If area of radiographic interest is not directly over the selected, that area will be overexposed or underexposed. 3. kVp & mA selections – select kVp values that provides appropriate scale of contrast & is at least the minimum kVp to penetrate the part. - kVp selected should be high enough to produce the radiographic contrast appropriate to the part being examined while keeping the patient’s exposure as low as possible.  - mA value selected has a direct effect on the exposure time needed by the AEC device. 4. Density selections - density controls that allow the RT to fine-tune radiographic density produce by the unit - actual numbers presented on density controls vary, but each of these buttons changes exposure time by some predetermined amount or increment expressed as percentage. 5. Collimation – additional scatter radiation produced by failure to accurately restrict the beam may cause the detector to terminate exposure prematurely. - detector is unable to distinguish transmitted radiation from scatters radiation and as always, ends the exposure when a preset amount of exposure has been reached. - RT should open the collimator to the extent that the part being radiographed is imaged appropriately, but not so much as to cause the AEC device to stop the exposure before the area being imaged is properly exposed. 6. Backup time – maximum length of time the x-ray exposure will continue where using an AEC system - may be set by the RT or controlled automatically by the radiographic unit. - may be set as backup time or as back up mAs - role of backup time FUCNTION OF BACKUP TIME Backup time, the maximum exposure time allowed during an AEC examination, serves as a safety mechanism when the AEC is not functioning properly. - chest x-ray done at upright bucky but RT set the control panel for table bucky - when controlled by the RT, backup time should be set high enough to be greater than the exposure needed but low enough to protect patient from excessive exposure in case of a problem. - setting backup time at 150% to 200% of expected exposure time is appropriate. - if backup timer is routinely or periodically terminates exposure, higher mA values should be used to shorten the exposure time  SETTING BACKUP TIME Backup time should be set at 150% to 200% of the expect exposure time This allows the properly used AEC system to appropriately terminate the exposure but protects the patient & tube from excessive exposure if problem occurs 7. The Patient - some patient require greater technical consideration when AEC is used for their radiographic procedures - if area of radiographic interest dose not completely cover the detector, the resulting density may be inappropriate - if detector must be very close to the edge of the patient’s body, the detector must be covered completely by the anatomic area of interest. - if portion of detector is exposed directly by the x-ray beam, the radiation exposure level necessary to terminate the exposure is reached almost immediately, resulting in underexposure of area of interest.  THE PATIENT & AEC If the anatomic area directly over the detector does not represent the anatomic area of interest, inappropriate density may result. This can happen when anatomic area over the detector contains a foreign object a pocket of air, or if the anatomic area does not completely cover the detector - using density control buttons may work in some cases, whereas in others it may be necessary to recenter the patient or part. 8. Bucky selection - if more than one bucky per radiographic units uses AEC, the technologist must be certain to select the correct bucky before making exposure. - failure to use the proper setting - cross table, table top or stretcher/ wheelchair studies AEC & NON-BUCKY STUDIES RT should be certain to deactivate the AEC system & use a manual technique when performing any radiographic study where the IR is located outside of the bucky 9. mAs Readout – actual amount of mAs used for a particular procedure is displayed after the exposure - there may be studies with different positions where AEC & manual techniques are combined because of difficulty with accurate centering. AEC & mAs READOUT If the radiographic unit has a mAs readout display, the technologist should be sure to notice the reading after the exposure is made LIMITATIONS OF AEC SYSTEM: 1. Interchangeability of film-screen systems – different film-screen systems cannot be interchanged easily once an AEC device is calibrated to produce specific densities. - when AEC is calibrated - some radiographic units have AEC devices that can accommodate more than one speed of film-screen system. 2. Minimum response time – refers to the shortest time that the system can produce - longer with AEC systems than with other types of radiographic timers 3. Lack of calibration – AEC device must be calibrated to accepted standards ANATOMIC PROGRAMMING OR ANATOMICALLY PROGRAMMED RADIOGRAPHY (APR) - preprogrammed set of exposure factors is displayed & selected for use - controlled by ab integrated circuit or computer chip that has been programmed with exposure factors for different projections & positions of different anatomic parts. - once an anatomic part & projection or position has been selected, the RT can adjust the exposure factors that are displayed. - RT can use APR to select a projection or position for a specific anatomic part & view the kVp, mA & exposure time for an equivalent manual technique.