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Physics I
Xray/Fluoro/CT/Mammo
| Question | Answer |
|---|---|
| Subject contrast | contrast encoded in the signal (xray) prior to being measured |
| Ionization | enough energy to eject an electron |
| Xray yield | proportional to Z2 |
| Characteristic xray | inner electron ejected with empty spot filled by outer electron, 69.5 K shell of tungsten |
| Average xray energy | between ½ and 1/3 the max kVp |
| Relationship of quantity in mA | direct, double=double |
| Relationship of quantity in kVp | factor of four, 15% kVp increase→doubling the quantity , double the kVp→increase quantity by four |
| Quantity change by distance | inverse square law |
| Heel effect improves with | larger anode angle, larger source to image distance, smaller FOV (more uniform in the center) |
| Heel effect in mammo | chest wall aligned with cathode |
| Coherent (Classical) Interaction | dominates at low energies, no ionization, no transfer of energy, no contribution to the image, minimal contribution to dose |
| Compton scatter | dominates at higher energies, frees an electron, ionizes the atom and results in a scattered photon (the incoming xray) dependent on the density of the material NOT the Z, major source of scatter and occupational exposure |
| Photoelectric interaction | occurs at 20-120 but dominates at lower energies relative to compton, results in characteristic xray vs auger electron |
| P.E. directly proportional to atomic number | Z3 (atomic number cubed) |
| P.E. inversely proportional to energy cubed | 1/Z3 |
| K-edge | P.E. peaks at the binding energy of the inner shell electron, want to set the kVp above the K edge to get majority of xrays around the k edge with contrast agents in fluoro |
| Low kVp | maximizes contrast by maximizing photoelectric interactions |
| Linear attenuation | fraction of photons interacting per unit thickness of an absorber |
| Linear attenuation goes up | half value layer goes down |
| Linear attenuation goes down | half value layer goes up |
| Collimation improves signal to noise | as long as mA’s go up to compensate for loss of photons |
| Grids | decrease scatter/increase dose due to having to increase mA’s |
| Modulation transfer function | ability of the system to maintain the signal contrast as a function of the spatial resolution |
| Detective Quantum Efficiency | DQE estimate of required exposure level that will be necessary to create an optimal image, high DQE=low dose, low DQE=high dose |
| CR (storage phosphors) | image is stored in the phosphor as an electron in a metastable state, readout laser used to expose image |
| Increasing sampling frequency | increasing the “rate of light sampling” results in smaller pixel pitch→improves spatial resolution |
| How does increasing xrays change the resolution of a CR system | will NOT improve the MAXIMUM spatial resolution |
| Direct flat panel detector | xray to signal, amorphous selenium (a photoconductor), less lateral disperion, fill factor is near 100%, high DQE |
| Indirect flat panel detector | xray to light to signal, cesium iodine (a scintillator), more lateral light dispersion, moderate fill factor, moderate DQE |
| What is the spatial resolution determined by on flat panel detectors | DEL (the detector element, the smaller the better) |
| Which has better spatial resolution DR or CR and why | DR bc the pixel detector is built into the flat panel |
| Fluoro vs spot image | way less current, less spatial resolution, smaller focal spot |
| Flux gain | accelerating electrons between voltage differences, ratio of light at input to light at output |
| Minification gain | electrons from a large surface concentrated on a smaller surface |
| Brightness gain | flux gain x minification gain, light increase from acceleration and concentration processes |
| Conversion gain | how good the I.I. is at turning electrons back to light |
| Geometric mag | increase dose by squaring (inverse square law) |
| Electronic mag | projecting a small FOV onto the matrix of detectors, requires increase in automatic brightness control and dose but will also increase spatial resolution |
| Geometric mag vs Electronic mag | geometric mag greater dose and more focal spot blur, electronic mag increases air kerma (the skin dose) but does not increase the KAP |
| Normal air kerma limit | 87mGy/min (10 Roentgens/min) |
| High level air kerma limit | 176mGy/min (20 Roentgens/min), must have audible and or visual alarms |
| Pulsed fluoro | short pulses with higher mA |
| Reduce pulse rate by 50% | reduces dose by 30% |
| FPD system resolution limited by | detector element size, 2.5-3.0lp/mm |
| I.I. systems are limited by | TV system, |
| Best kVp for IV contrast | 60-80kVp |
| How much dose is distributed in the first 3-5cm of skin | 50% |
| Dose spreading | change the angle of the gantry to expose a broader area of skin |
| Magnification dose what to kerma | increase air kerma but not change KAP |
| Minimal slice thickness on CT | determined by detector element aperture width |
| Pixel size | Field of View/matrix size, smaller pixel→better resolution |
| Pitch of 1 | no overlapping between slices |
| Pitch >1 | table moved faster than the beam, less dose, less resolution |
| Pitch <1 | slow moving table, increased dose, good resolution |
| If HU increases by 10 | then attenuation increased by 1% |
| HU | 1000 x (attenuation of material – attenuation of water)/ attenuation of water |
| Level | the center or midpoint of your gray scale |
| Width | wide width if comparing two very different densities, narrow window for similar densities |
| Increased beam width | increased partial volume (due to beam divergence) faster scan, NO change in radiation dose b/c overall larger area scanned |
| Increased kVp on CT | signal to noise ratio increases, can decrease contrast |
| Iterative reconstruction | “forwards” information which can be compared to actual information to correct image, can correct for noise and lower dose |
| Smooth kernel | less noise and reduced spatial resolution, brain |
| Sharp kernel | more noise and better spatial resolution, bone |
| Prospective Cardiac CT | step and shoot, R-R interval, less radiation, no functional imaging, axial |
| Retrospective Cardaic CT | on the whole time, more dose, functional imaging |
| Relationship of signal to xray flux | direct, twice the xrays, twice the signal |
| Relationship of noise to xray flux | square root, twice the xrays, square root of 2 the noise (doubling the mA→40% increase in signal to noise ratio) |
| What determines spatial resolution on CT in the x-y plane | focal spot |
| What determines spatial resolution on CT in the Z plane | detector size |
| Weighted CTDI | 1/3 the central CTDI and 2/3 the peripheral CTDI in mGy |
| Volume CTDI | weighted CTDI/pitch |
| Dose length product | CTDI – Vol x length of the scan |
| Effective dose | k x DLP (k→body part constant), in Sv |
| Reduce Partial volume artifact CT | make slices thinner |
| Fixing photon starvation | increase tube current and adaptive filtration can be used to smooth data in high attenuation portions |
| Fixing metal artifact | increase kVp, thinner slices |
| Fixing motion artifact | increase scan speed, overscan |
| Fixing ring artifact | replace or recalibrate the CT machine |
| Mammo xray characteristics | 25-35kVp, moly anode, low tube current 100mA, long exposure times, high receptor air kerma 100microGy, beryllium window, small focal spot, low grid ratio 5:1, high optic density |
| Mag mode for mammo | no grid, air gap, small focal spot 0.1mm for spatial resolution, smaller paddle, less mA 25, increased exposure time |
| What do you use for dense breasts and then for intermediate | Rh/Rh then Mo/Rh |
| PPV1-screener call back | 3-8% |
| PPV2-recommended bx | 15-40% |
| PPV3-actual ca detection | 20-45% |
| Processor QC | Daily |
| Darkroom cleanliness | Daily |
| Viewbox conditions | Weekly |
| Phantom evaluation | Weekly |
| Repeat Analysis | Quarterly |
| Compression test | semi-annually |
| darkroom fog | semi-annually |
| screen-film contrast | semi-annually |
| Binning | (fluoro), binning increases pixel size-->improves SNR but reduced spatial resolution |
| ACR CTDI reference values | Adult head- 75mGy, Adult Abd-25mGy, Peds Abd-20mGy |
| Risk of radiation induced ca per dose | Adult-5%/Sv, Child- 15%/Sv |
| Cupping | xrays passing through the middle are more hardened than those in the periphery (darker in the periphery) |
| Streak artifact | difference in attenuation of xrays passing through adjacent dense objects (whether they pass through one vs both) |
| Partial volume | a dense object and low attenuating object both occupying the same voxel-->results in grey box |
| Photon starvation | when the beam travels horizontally through high attenuation region (shoulders) results in streaking |
| Under sampling | insufficient number of photons used to recontstruct CT, view aliasing and ray aliasing artifacts |
| Incomplete projection | parts of pt outside FOV but still attenuating xrays |
| Helical artifact in the axial plane | due to rapidly changing anatomy in the Z plane, why head CT's often use axial scanning instead of helical |
| Stair step artifact | wide collimation over non-overalpping intervals, better with helical scanning 2/2 overlap |
| Zebra artifact | stripes on reformatted images most significant way from axis of rotation |
| Calculation of lp/mm | 1/(2xFWHM) |
| standard deviation calculation | square root of the mean |
| calculation of lp/mm(limiting spatial resolution) based on sampling frequency | 1/2 sampling frequency (sampling frequency= number of items/distance) |
| Entrance skin dose is what compared to the entrance air kerma | 50% more (3mGy entrance air kerma will have an entrance skin dose of 4.5mGy) |
Created by:
pclaiborn08