Physics 2 test 2 Flashcards
Quality Control is referring to
Equiptment
Quality Asurance is referring to
Patients
These are consistent with what "Quality" People oriented Necessary and appropriate procedures Producing desired information Accurately interpreted with ALARA as the goal for exposure, cost and inconvenience: scheduling prep instructions report accuracy & distribution image interpretation
Quality Assurance
Began as a war against unacceptable repeat rates
Screen-Film Quality Control
Equiptment oriented-image production, procesing, image evaluation & critique
Screen-Film Quality Control
Planned, Continuous, Documented, Deomstrate adaptations
Screen-Film Quality Control
Best possible image obtained with respect for patient safety
Screen-Film Quality Control
Sort rejects on positioning, motion, density, etc.
Reject analysis (QC)
Goal for reject analysis:
Less than 2% per 250 patients
Who does quality control testing?
Physicist or designated technologist
When and who does Acceptance testing?
Upon installation and it’s done independently
PM
Preventative Maintenance
Routine Performance Evaluation:
Periodic or upon major repair/part replacement
Another reasons why Quality Control testing would be needed
Error Correction Consistency Reproducibility Predictability Confidence
3 benchmarks for QC Guidelines
Tolerance
method
Frequency
QC guidelines for Filtration
2.5 mm Al minimum
Tested by HVL
Annually
QC guidelines for Collimation
Light field coincides withing 2% of SID
Test each size film
Semiannually
QC guidelines for Effective Focal Spot
Tested with slit camera, pinhole camera or star pattern
Variance fairly large
Annually
QC guidelines for kVp
+/- 10 kVp diagnostic (+/- 1 kVp mammo)
Dx: 3% kVp variance will alter image density, radiographic contrast, and patient exposure
Annually
QC guidelines for Exposure Control
Time has a big influence on patient exposure & radiographic density +/- 5%
Test actual time set and AEC systems
backup timer must stop exposure at 6 sec or 600 mAs
Annually
QC guidelines for Linearity
Constant output for any mA/sec resulting constant mAs +/- 10% (reciprocity law)
Measure mR/mAs by varying mA (so timer inaccuracy doesnt flaw test) *only want one variable
Annually
QC guidelines for Reproducibility
Appropriate denstiy & contrast for technique factors selected +/- 5%
half value layer
pieces of equiptment: step wedge, etc
QC guidelines for Film contact
Wire mesh test
semi or annually
QC guidelines for Scren film cleaning
Depends on volume (how many x-rays your performing)
Actually emtpy the cassets and clean them
QC guidelines for Lead protective apparel
45-50 kVp range (not too much go right through-looking for a tear or whole in apron-will look black)
Annually
QC guidelines for Film Illuminators
view box-light is measured in Lumens
Measures averagle light intensity
Annually
Special QC for Fluro
Exposure Rate
Cant exceed (but shouldn't need it anyway) 10 rad/min for ABS 5 rad/min for manual
Special QC for Fluro
Spots
Entrance skin does highest for cassette loaded spots- spots off image intensifier much less exposure smaller mode (mag) image inensifier is highest patient dose *Patient exposure increases from fluro to still spots
Special QC for Fluro
AEC
Constant image brightnes
Measured by seeing if input phosphor receives constant dose regardless of whats being penetrated
Conventional Tomography QC
Tomo
Constant patient exposure
Cut height accurate +/- 5 mm
Patient movement between slices accurate +/- 2 mm
Computed Tomography QC
CT
Ongoing Preventive Maintence Periodic testing of noise/uniformity Linearity Contrast resolution Slice thickness Table incrementation Light localization Patient dose
`Processor QC-Radiographic film processor
DOCUMENTATION: No Doc, it didn’t happen
Digital QC
SMPTE pattern adopted by ACR for evaluation of digital monitors
DICOM Digital Imagning Communication
Line pairs per millimeter
blank spot and line together is a “line pair”
AAPM TG 18 Test
American Association of Physicians: Test patterns and related procedures-for digital display devices
Digital QC Display Resolution
Display separable images with high fidelity
Digital QC Display Noise
Fluctuations-quantified with a TB test pattern
Continuous Digital QC Program
State has to approve for usage
Acceptance testing
Routine use of TG 18-QC test pattern by QC Tech
Periodic review by Medical Physicist
Annual and Post-Repair Performance evaluations
Fluro Cones
Centrally located
Better for fine detail
Best in bright light
Fluro Rods
More abundant at periphery
Less acuity
Best in dim light/darkness
Fluor Image Intensifier Tubes
Convert the x-ray image into a small bright optical image, which can then be recorded using a TV camera.
Fluro Image Intensifier Tube Conversion
X-rays to light
Light to electrons (input phospher)
Electrons cross the tube crossing sides
Electrons convert to light (output phospher)
What is special about the image that is displayed from the tube?
It must be inverted to get an accurate anatomy.
Comparison of image quality between the input/output phosphors
DQE Detective Quantum Efficiencies:
want at least 60-70% quality
what you lose from input to output
Fluorscopy Intensifier Tube Sequence of events
1-3
- Remnant Beam
- Glass envelope of Image Intensifier
- Input Phosphor-converted to visible light (Cesium Iodide-CsI)
Fluorscopy Intensifier Tube Sequence of events
4-6
- Photocathode-think metal: Cesium and antimony compunds. Light here causes electron emission (photoemission) Many light photons required to produce one electron
- Electrons are focused down 50 cm tube by electrostatic focusing lenses (electrodes of increasing voltage all along); accelerated by 25 kV potential difference
- Electrons cross anode neck, inverting image
Fluorscopy Intensifier Tube Sequence of events
7-9
- Through thin aluminum layer (allows electrons through but prevents light from output phosphor from going back toward cathode end of tube.
- Electrons stike the output phosphor (zinc cadmium sulfide crystals) each electron produces 50-75 x’s the light required to produce it.
- For every electron that strikes the Intensifier tube here 50-75 X’s as many light photons are emitted
Intensifier Tube Flux gain, Minification gain, Brightness gain
Flux gain: # of light photons at output phosphor/ # of x-ray photons at input phosphor x-ray photons to electrons-lose gain
Minification gain: (input size)2/(output size)2 how many light photons get from # of electrons-gain
Brightness gain: Minification gain X’s Flux gain # >1
Multifield Image Intensification
- May be operated using full or partial input phosphor
- In less than full field modes: Reduced field of view, magnified image and increased patient dose to THAT anatomy
- Amount of magnification: base/top
- Amount of patient dose: base/top squared
TV Camera Tube/Charge Coupled Devices (CCD’s)
*Conventional TV Camera Tube
- i.e. Vidicon, Plumbicon*
- Light from the output phosphor of image-intensifier tube strikes window
- Light goes through window
- Light goes through signal plate to target (photoconductive layer)
- Electron beam from cathode strikes illuminated portion of target, electrons are conducted, video signal goes out. If photoconductive layer is not illuminated, no video signal is produced.
- Electronic scanning converts imge into electronic signal which is transmitted to TV monitor
Video capture is where on the tube?
Output Phosphor
Static is referring to…
Stationary Image
Spots in Fluro
mAs goes up as well as pt dose. Versus spots off the image intensifier
Chain of Mechanics of Image Intensified Fluoro
xray tube Patient Cassette loaded spot film device (optional) Image intensifier Coupling Device (fiber optics, mirrors or lenses) TV Camera tube/CCD (Vidicon/Plumbicon) TV Picture Tube (CRT) Image on Fluorescent Screen
Coupling Devices
- Fiber optics
- Mirrors or Lenses
Fiber optics: only allows cassette loaded spot film
Digital Fluro
Smaller pixel size equals better spatial resolution
Comparison between conventional versus Digital
digital
- Faster image acquisition
- More post-processing enhancement options
- mA in the 100s versus 4-5 mA conventional
- Pulse progressive fluro in digital versus conventional
- Flat panel image receptor versus Image Intensifier in conventional, flat panerl much better less distortion smaller pixels and better spatial resolution
Image Wisely
Came first, reduce repeats
Image Gently
Came second, pediatric patients need less technical factors than adults
High Frequency Generators
Capable of switching on and off rapidly.
have interrogation and extinction times of less than 1 ms
Interrogation Time
The time required for the x-ray tube to be switched on and reach selected levels of kVp and mA
Extinction Time
The time required for the x-ray tube to be switched off.
Duty Cycle
The fraction of time the x-ray tube is energized, what percentage of time the beam is ACTUALLY on
CCD replaces TV Camera Tube
*Advantages
- 1024X1024 large matrix @ 10 lp/mm- Spatial resolution
- Eliminates pin cushion/barrel artifact (curves at the edge)
CCD Compared to TV Camera Tube
- More DQE (light sensitivity) Quantum Efficiency
- Less noise
- More SNR (signal to noise ratio)
- More Contrast Resolution
- Less Patient Exposure
Flat Panel Image Receptor
- Small, light, flexible
- Insensitive to magnetism
- Shape match to Image Intensifier
FPIR’s
- Need SNR 1000:1
- Progressive mode: e-beam sweep
- Signal from FPIR doesn’t have to be digitized, it already is!
Digital Subtraction Angiography
Image before Contrast
Then contrast Image
computer superimposes the images and subtracts anything that is the same, removes it.
In Digital Subtraction the Mask is…
The original image
Subtraction TID
Time Interval Difference:
Different masks required throughout the study, ie Cardiac
Subtraction Mis-registration
Patient motion between mask and acquisition (same pixel contains different anatomy)
Energy Subtraction
Using x-ray beams of alternating energy (pulse energy) to take advantage of the k-edge absorption difference for contrast media (est. 33 keV)
Beams are not Monoenergetic
Hybrid: energy subtraction 1st followed by temporal subraction
Roadmapping
Subtraction image following images with a catheter and a final/new “mask”
Patient Exposure in Digital Fluro
Beam is pulsed
Camera tube and CCD are more sensitive
Ease of acquisition makes “extra” images tempting
An acceptable QC program consists of three steps:
Acceptance testing
Routine performance monitoring
Maintenance
Misalignment must not exceed
2% of the SID
Three tools are used for measurement of focal-spot size:
Pinhole camera
Star pattern
Slit camera
Screen Film: Measured kVp should be within (blank) % of the indicated kVp
10%
Screen Film: Exposure timer accuracy should be within (blank) % of the indicated time for exposure times great than (blank) ms
5%
10 ms
Screen Film: Exposure linearity must be within (blank)% for adjacent mA stations
10%
Screen Film: Sequential radiation exposures should be reproducible to within
+- 5%
What measurements and observations should be instituted for all digital display devices
AAPM TG 18
Photometric evaluation of digital display devices and ambient light levels is essential to what QC
Digital QC
Coupling Devices
-Mirrors or Lenses
Mirrors or Lenses, auxillary devices may be added here:
- Spot film camera (70 mm, 90 mm, 105 mm)
- Cine Camera (16 or 35 mm)
