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140 Cards in this Set
- Front
- Back
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What is the function of a grid? |
-Designed to improve contrast by absorbing scatter radiation before it reaches the film -Absorbs scatter radiation produced by x-ray photon interactions with patient |
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Where is a grid used? |
-Place in between the patient and the IR |
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What are the possible photon interactions? |
-Pass thorough unaffected -Absorbed -Scatter (interact and change direction) |
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When is a grid used? |
when body part 10cm or greater or technique above 60 kVp |
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How can you maintain image quality with grid use? |
-Increase technical factors -Increase patient dose |
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How are grids classified? |
-Materials -Grid ratio -Grid frequency |
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What are grids made of? |
-Radiopaque strips – usually lead -Placed side by side -Absorb scatter (and some primary) radiation -Radiolucent inter-space material -Usually aluminum or plastic fiber -Radiation passes thru easily with only absorbing small amount of |
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What is grid ratio? |
-Ratio of the height of the lead strips to the distance between the strips -GR = h/D -h=lead strip height -D = interspace width |
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What happens with a higher grid ratio? |
-More effective at removing scatter -Requires greater accuracy in positioning -Increased possibility of having grid error due to misalignment |
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What are the available grid ratios? |
-5:1, 6:1, 8:1, 10:1,12:1, and 16:1
-most common is 12:1 |
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What is the relationship between distance between lead strips and grid ratio when height of strips remains the same? |
Inverse |
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What are the characteristics of increasing grid ratio? |
-Grid lines may be taller -Higher grid ratio absorbs more radiation -Grid lines may be closer together -Higher grid ratio absorbs more radiation |
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What is grid frequency? |
-Number of grid lines per inch
-Higher grid frequency has thinner lead strips |
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What are the possible grid ranges? |
-Grid ranges -60-196 lines/inch -25-80 lines/inch -Most common -85-103 lines/inch -33-41 lines/inch |
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What is total lead content? |
-Combine info from grid ratio and grid frequency -Determines total lead quantity -Lead content usually greater: -Higher grid ratio -Lower grid frequency |
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What is the result of increased lead content? |
-Increased removal of scatter -Improve contrast -Increase lead content -Decrease scatter reaching film -Increase contrast |
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What are the types of grid pattern? |
Linear
Criss-cross |
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What are the types of linear grid? |
Parallel and Focused |
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What is a criss-cross grid? |
-Two grids sandwiched together -Strips usually both parallel to each other -And at right angles to the long dimension of the grid -Not commonly used |
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What is a parallel grid? |
-All lines parallel to beam -Primary beam will never be aligned with all the strips -Only the strips directly under the x-ray tube in true alignment with primary beam -Not recommended for SID less than 140cm |
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What is a linear parallel grid? |
-Strips don’t coincide with beam divergence -Get some grid cut-off at lateral edges -Best used for long SIDs |
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What is a focused grid? |
-Lead strips are tilted progressively as they move away from center -Lines through each strip converges at a point known as the grid focus -Lead strips match divergence of the x-ray beam -X-ray beams would meet at a line called the convergence line -X-ray tube must be located at the convergence line to eliminate lateral grid cut-off |
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What is grid radius? |
-Distance from grid to the convergence point of a grid -Coincides with useful SID provided by grid -Convergence line the source of x-rays |
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What is focal range? |
-SID range most effective for grid -Distance between grid and x-ray tube for best grid performance |
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What are the factors when choosing a grid? |
-1.Grid conversion factor (dose required) -2.Selectivity -3.Contrast improvement ability |
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What is grid conversion factor? |
-Also called Bucky Factor -Calculates required change in technique when changing from no grid to a grid -GCF = mAs with grid/mAs without grid |
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When does GCF increase? |
with higher grid ratios and higher kVp |
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What is the formula for GCF? |
-GCF = mAs needed with grid/mAs needed without grid
mAs1/mAs2 = GCF1/GCF2 |
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If not given kVp, or chart, what are the grid conversion factors? |
No grid =1 5:1 or 6:1 = 3 8:1 = 4 12:1 = 5 16:1 = 6 |
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What do grid vary in respect to? |
-Grid ratio -Frequency -Lead content |
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What are the factors that influence GCF? |
-Grid ratio -As grid ratio increases, absorbs more scatter -Higher grid ratio = higher bucky factor -KVp -As kVp increases, produces more scatter -Higher kVp – higher bucky factor |
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How can you measure grid performance? |
Selectivity
Contrast Improvement Ability |
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What is selectivity? |
-Amount of the primary beam/scatter a grid absorbs/transmits -Selectivity = %primary radiation transmitted/% scatter transmitted |
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How does lead content and scatter affect selectivity? |
-The better a grid removes scatter, the greater the degree of selectivity -Grids with higher lead content have a greater selectivity -So: increased lead content form higher grid ratios and lower grid frequencies |
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Which of the following has a higher lead content? 10:1 grid with 10-lines/inch or 10:1 grid with 8 lines/inch |
10:1 grid with 8 lines/inch as strips are wider and absorb more radiation |
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What is contrast improvement ability? |
-Measurement of grid performance (contrast) = K -K = contrast with grid/contrast without grid |
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What is contrast improvement ability dependant on? |
-KVp -Field size -Volume – scatter production -Higher K#, greater the contrast improvement |
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What changes were made by Potter-Bucky to moving grids? |
-Improvements to grid use -Realigned strips in 1 direction -Made strips thinner -Moves grid during exposure to reduce visibility of grid lines (perpendicular to grid lines) |
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What are the types of moving grids? |
-Reciprocating
-Oscillating |
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What is a reciprocating grid? |
-Motor drives the grid back and forth during the exposure -Total distance moved is 2-3cm |
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What is an oscillating grid? |
-Oscillating Grid -Electromagnet pulls the grid to one side and then releases it during the exposure -Moves in circular motion |
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What is the air gap technique? |
-Alternate to using grid -Patient placed at greater OID -Creates an air-gap between patient and IR -Less scatter reaches IR -Improves contrast -Disadvantages -Reduced sharpness due to increased OID |
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What air gap is equivalent to a 15:1 grid ratio? |
25cm air-gap |
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What are the types of grid error? |
-Off-level/tilted -Angled against grid lines -Off-center -Off-focus (incorrect SID) -Upside down -Moire effect |
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What is grid cut off? |
with the use of a parallel grid -Undesirable or abnormal absorption |
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What is Off-level or Tilted grid error? |
-Grid cut-off with use of tilted grid -Tube angled across grid lines -Only error possible with parallel grid -Resulting image: -Decrease in density across entire image |
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What is off-center tube alignment error? |
-X-ray tube not centered to center of grid -Laterally de-centered tube -Divergence of beam not matched up to focused grid -Resulting image: -Decreased density across entire image -Increasing lateral de-centering increases loss of density |
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What is tube outside of focal range error? |
-Incorrect SID -Tube is outside grid’s specified focal range -Resulting image: -Grid cut-off along edges of image -Farther from focal range: the greater the cut-off -More noticeable below focal range -Higher grid ratios require more precise centering |
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What is focused grid used inverted error? |
-Focused grid used inverted, upside down -Resulting image: -Severe peripheral cut-off |
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What is Moire effect? |
-In digital systems with stationary grids -Grid lines running in same direction as movement of laser beam scanning, imaging plate visible -To prevent: -Use high frequency grids -103 lines/inch or greater |
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What is beam restriction? |
Collimation |
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What are the factors that influence the amount of scatter produced? |
-Kilovoltage -Volume of material -Type of material |
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Kv effects on scatter? |
-Controls the penetrability of the beam -As KV increases more photons go through the patient -As KV increases, scatter production increases -At a higher KV there are more Compton interactions that photoelectric interactions -More scatter leaves the patient -Good for patient, bad for image |
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Amount of irradiated material's affect on scatter? |
-Affected by the -Atomic number of the material -Volume of the material -The higher the atomic number the less scatter -Volume is affected by the field size and the patient thickness -As volume increases the amount of scatter increases |
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What are the types of beam restrictors? |
-Use beam restrictors to control the field size and patient volume
-Aperture diaphragm -Cones/Cylinders -Light-beam collimator |
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What is an aperture diaphragm? |
-Flat sheet of metal (lead) with a hole in the middle -Different diaphragms for different size receptors and different distances |
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What are the advantages and disadvantages of an aperture diaphragm? |
-Advantages -Low cost, easy to use, simple design
-Disadvantages -Large penumbra on periphery of image, off-focus radiation, no light |
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What are cones/cylinders? |
-Circular aperture diaphragms with metal extensions -Cone = gets larger at the end |
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What are the advantages and disadvantages of cones/cylinders? |
-Advantages: -Inexpensive, easy to use -Disadvantages -Fixed field size, flared cones still have large penumbra on periphery of image and off-focus radiation |
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What are collimators? |
-Added filtration by mirror |
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What are the advantages and disadvantages of collimators? |
-Advantages: -Infinite number of field sizes, light source, reduce penumbra and off-focus radiation -Disadvantages: -More complex, more expensive (than a simple style) |
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What is field light? |
-A light shines on a mirror (45 degrees) in the x-ray beam -Light source and x-ray source are equal distance from mirror -Crosshair in the center to locate middle of the beam -Size of field shown on front of collimator |
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What are do shutters do? |
-Regulate the field size -3 sets of lead plates at right angles to each other -Upper shutters reduce off-focus radiation -Lower 2 pairs reduce penumbra on periphery |
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What are the SC 35 recommendations for collimators? |
-Beam must be size of IR or smaller -Should see edges on all x-ray images -Use smallest IR possible -Restrict beam to area of interest -Irradiating any areas outside of interest = unnecessary radiation -Avoid irradiation to gonads, female breasts, and thyroid |
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When does scatter increase? |
-KV increases = increases -Field size/ volume increases = increases -Thickness of object being imaged/volume increases = increases When scatter increases contrast decreases |
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When does image contrast decrease? |
-KV increases = decreases -Field size/ volume increases = decreases -Thickness of object being imaged/volume increases = decreases When scatter increases contrast decreases |
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What is filtration? |
-Selectively absorbs low energy photons -Absorbed by the patient -Don’t contribute to the image -“Harden” the beam -Measured in thickness of aluminum equivalency (AL/Eq) -Using filtration requires increase in exposure factors, to compensate for decrease in exposure -Overall decrease in patient dose |
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What is inherent filtration? |
From within the design of the tube and housing -0.5 – 1.0mm Al/Eq. -Mirror -1.0mm Al/Eq. |
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What is added filtration? |
-Outside the tube and housing and before the IR (usually before the collimator) -Should absorb many low energy photons and few high energy photons -Collimator -Contributes 1.0mm Al/Eq. |
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What is the SC 35 recommendation for an x-ray unit with a maximum kV at 70 or over? |
2.5mm Al/Eq. |
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What is total filtration? |
-Sum of the inherent and the added filtration -Not any compound or compensating filters -As photon energy increases -The % of photons attenuated decreases even when filtration is increases |
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What happens if there is greater than 3.0mm Al filtration? |
-A reduction in the skin dose doe not warrant the exposure increase -Hard on tube |
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How do we check total filtration? |
-Half Value Layer -The amount of absorbing material that will reduce the intensity of the primary beam to half its original value -Indirect measurement of the total filtration |
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What is compound filtration? |
-Uses 2 or more materials that compliment each other in absorption ability -Each layer absorbs the characteristic photons of the previous layer -K-edge filter -Used often in radiotherapy |
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What are compensating filters? |
-Compensates for unequal absorption within the patient when part is differing thickness and or/composition -Decreases dose, as without filter two images may be needed -Produces a more uniform exposure -Aluminum or leaded plastic -Attach to the collimator or under the patient |
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What is atomically programmed radiography? |
-Radiographic technique chart programmed into x-ray console -Body part indicated with either words or image -Anatomy and body habitus indicated -Technologist can override settings when necessary due to patient condition |
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What is automatic exposure control? |
-Regulates exposure time without technologist involvement -Technologists still control mA and kVp based on patient condition -A radiation detection device measures the quantity of x-rays received by the patient or IR -Value set at installation based on receptor system and density desired |
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What are the types of AEC? |
Phototimer and Ionization chambers |
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What is a phototimer? |
-Older style not commonly in use -Utilizes photomultiplier tube -Detector under cassette |
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What are ionization chambers? |
-Utilizes parallel plate ionization chamber – detectors also called cells or chambers -Referred to as “phototiming” although technology is different -Detector in front of cassette but under patient |
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What is minimum reaction time? |
-Length of time necessary for AEC to respond to radiation and for generator to terminate exposure -Min. reaction time in todays machines usually less than 0.001 secs. -Can’t terminate quickly enough with extremely high speed screen combos |
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What is back-up time? |
-Manually set time to terminate exposure -Can’t exceed tube limit -Wastes tube life -Dramatically increases patient dose
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How should back-up time be set? |
-Should be set at 150% of anticipated manual exposure mAs -Above 50 kVp terminates at 600 mAs -Below 50 kVp terminates at 2000 mAs |
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What causes back-up activation? |
-Wrong detector selected -Wrong tube selected -Incorrect tube – bucky alignment -Tube is not centered to bucky -Prosthesis -Lead shield -Incorrect back-up timer setting -Set too short and prematurely terminates exposure |
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What factors affect AEC? |
-Patient position -Ion chamber selection -Density selector -Collimation -Image receptor speed -Back up timer too short -Prothesis, FB, or lead shield over chamber |
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How does patient position affect AEC? |
-Density based on tissue directly above chamber -Must position patient to place adequate tissue above chamber to reach a desired image on receptor -Positioning must be accurate |
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How does ion chamber position affect AEC? |
-Selection based on tissue and desired density on film -Ex. Lateral chest – center chamber -PA chest: Usually both tight and left chambers, density evened by generator -Shoulders, spines, knees, and abdomen– all have specific chamber selections |
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How does the density selector affect AEC? |
-+3, +2, +1, 0, -1, -2, -3 -Increases or decreases exposure index value -Steps usually set at a 25% increase or reduction in radiation intensity from preset radiation turn off dose -Makes the AEC chamber more or less sensitive to radiation |
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How does collimation affect AEC? |
-Too little, scatter will prematurely terminate exposure -Too much, cut off portion of sensor and exposure will go to back-up to reach set radiation amount |
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How does image receptor speed affect AEC? |
-Calibrated for one speed only -Faster speed systems result in: Too much exposure used -Slower speed systems result in: Too little exposure used |
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When should mobile x-rays be used and why? |
should only be used if patient cannot come to department increased exposure to patient (?) and techs, others |
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What is a capacitor discharge mobile? |
-discharge off high voltage to capacitor supplied by batteries -at exposure switch causes the rectification circuit to charge capacitor in generate not tube -at indicate level, capacitor filled depression of switch discharges power to Xray tube -Xray photons then produced and released |
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What are battery operated mobiles? |
nickel-cadmium batteries , rechargeable provides power to Xray tube DC converter, step up transformer and rectifier to get desired kV Then applied to tube same as mobile units at NAIT |
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How many batteries does a battery operated mobile have? |
two sets: one to drive it one for X-rays |
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What are the advantages of a battery operated mobile? |
batteries can fail recharging can take 2 - 12 hour batteries require maintenance mA value is pre set |
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What needs to be paid careful attention to when using a mobile? |
positioning cassette placement grid use lower grid ratio parallel other peoples whereabouts |
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Mobile safety |
key to turn power on and off front bumper will stop movement if impacted clean it off regularly take precautions against radiation exposure to self and others |
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Mobile Radiation Safety Chart |
-duty to protect everyone, people not required --must leave area -announce intention to expose -carry lead aprons -do not put parts in primary beam (except appropriate patient parts) -provide gonadal shielding -maximize your distance from patient during exposure -avoid repeats |
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What is an artifact? |
-optical density / brightness on an image unrelated t the intended patient anatomy -may require a repeat or lead to misdiagnosis -ID is important to correct the error, identify trends and prevent further artifacts from occurring |
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What are foreign bodies>? |
-are not considered artifacts as they pertain to the patients anatomy at the time of the exam -typically the full foreign body is included on all views |
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What is a prothesis or fixation? |
internal fixtures, external fixation devices and prostheses are also not considered to be artifacts and often must be fully included |
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What is an implanted device? |
implanted devices such as pacemakers, neurostimulators, brea and cochlear implants are also not considered to be artifacts |
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What are the time periods when artifacts can occur? |
exposure and acquisition processing or post acquisition and display handling and storage |
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What is exposure and acquisition artifact |
-those related to radio exam itself -caused by the patient, tech, or the equipment during procedure -easy to identify and correct -can be avoided with use of appropriate care and attention |
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What is a double exposure? |
- two images on one cassette |
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How is scatter (quantum mottle) caused? |
-too few radiation photons at image receptor -grainy, speckly image -how should you decrease noise - increased mAs but that increases the dose |
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What is incorrect cassette orientation? |
tube side not facing the tube
see cassette materials on image |
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What is backscatter artifact? |
due to lack of lead at back of detector |
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What is a phantom image? |
-occurs because ht machine doesn't erase the cassette properly -decreased brightness and contrast, see parts of old image |
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What is a dropout artifact? |
DR pixel array failure small areas where pixel is missing |
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How do defects or scratches on the imaging plate occur and what do they cause? |
- increases image brightness in areas
- scratched IP Dust in IP cassette or DR detector |
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What is tube filter failure? |
-filters are the assembly of the cry tube becoming loose and mispositioned in the cry beam -the filters then attenuate the cray beam non uniformly and result in unwanted attenuation pattern -lines of darker/lighter image |
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What is processing or post acquisition artifact and display? |
-occur after image acquisition for digital systems includes image professors software -malfunctions, IRD problems, printing and display artifacts -dropout artifact, pixels missing i mage, dirt on laser |
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What is histogram error? |
improper image brightness |
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What is indirect collimation? |
not parallel to cassette brightness issues |
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What is halo artifact? |
dark bands at the structure interface where the brightness is very different |
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What is Moire artifact? |
-from aliasing (sampling errors) -the lines of the grid coincide with the lines of the image -the image processor then mistakenly samples -both the grid signal and the image signal therefore sampling the image signal less than twice per cycle |
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What is image reader/scanner error? |
-memory problems -digitization problems -communication problems -dirt in laser / optics system -all can cause: skipped scan lines missing pixels distorted images |
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What do printer errors cause? |
laser misalignment
may cause shading (dark areas)
may cause corduroy effect |
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What is image enhancement artifact? |
-from any erroneous post processing incorrect presets brightness contrast mag edge enhancement spatial filtering |
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What is pixel dropout artifact? |
from computer screen irregularities tiny blank areas (pixels) |
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What is handling/storage artifact? |
these artifacts occur due to the way the IPs are handles and stored
often a result of IPs stored too close to radiation or heat source |
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What is IP fogging? |
-scatter and background radiation decreased contrast -digital image receptors are very sensitive to -radiation fogging -routine erasure of Ip critical to eliminating the occurrence of this artifact |
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What is heat blur? |
IP exposured to heat before read
image is blurry |
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What is post-processing ? |
-There are many changes that can be made to the presentation of the image data -There are also many names for some of these functions, sometimes vendor specific -The digital image displayed is only a small part of the obtained image data; otherwise it would be impossible for us to view it -So each image is only a “window” on the total data range available |
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What is windowing? |
-The process of selecting a segment of the total pixel value range (dynamic range) and then displaying the pixel values (shades of grey) within that segment -The technologist can adjust both the center of the window (window level) and the width of the window (window width) -The combination of these parameters determine the range of pixel values that will be displayed in the image (brightness and contrast) |
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What is window level? |
-Controls image brightness -Direct = Relationship between window level and image brightness -Inverse = Relationship between window level and density -When window level increases, the brightness increases and the density decreases |
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How does window level work? |
-Works by addition or subtraction of each pixel value -Adapted and displayed for specific exams -Default Window Level is different for each area of interest -Information outside chosen window level is lost on image |
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What is window width? |
-Controls image contrast -Gradational enhancement, contrast enhancement, tone scaling, image compression, image expansion… -Window width controls contrast by setting the range of grey shades that are visible |
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Why should you not do any windowing before saving? |
Technologist should avoid adjusting the WW/WL prior to sending the image to PACS. Once the WW/WL is adjusted and saved, the wide dynamic range form the original image is lost, leaving only the range that was saved. The radiologist then has a narrower dynamic range to use when evaluating the image. |
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What is histogram equalization? |
Also used to enhance brightness and/or contrast |
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What is LUT? |
(Look-up table) adjustments -Another way to change brightness and contrast, or invert shades -The LUT for the image is compared to and adjusted to look similar to an LUT stored in the imaging system processor -The stored LUTs each can be selected to produce images with different contrast characteristics |
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What is edge enhancement/sharpening? |
-Can enhance or suppress certain image frequencies (spatial resolution adjustments) -High pass filtering -Increases detail, increases noise -Amplifies the high frequencies or deletes the low frequencies -Small structures can be buried in an enhanced edge -The edges of all structures are enhanced – those of diagnostic value and undesirable edges |
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What is image smoothing? |
-Can enhance or suppress certain image frequencies (spatial resolution adjustments) -Smoothing -Low pass filtering -Amplifies the low frequencies, deletes the high frequencies, or averages pixel values between adjacent pixels -Decreases noise, decreases detail |
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What is image stitching? |
-Combining multiple images into one |
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What is image inversion? |
-Changing white to black and black to white |
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What is image subtraction? |
-Removing part of the anatomy |
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What is image magnification? |
-Zoom -Electronic magnification |
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What is region on interest? |
-To quantify pixel values -For identification |
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What is background removal or shuttering? |
-Removes white background around image -It is more pleasant to look at without the white borders |
