Physics: Rapid Review Flashcards
What is the approximate average energy of an xray beam?
1/3 kVp
Note: The beam is mostly (80%) due to Bremsstrahlung interactions creating xrays.
Which electron is ejected from an atom during characteristic xray production?
An inner shell electron
What is the binding energy for Tungsten that creates a characteristic xray peak?
70 KeV
What is the purpose of the glass enclosure/envelop of an xray tube?
- Maintain a vacuum
- Allow the amount and speed of the electrons to be controlled independently
What does the xray tube cathode do?
It is the filament that emits electrons to be accelerated towards the anode
What does the xray tube focusing cup do?
Help the electron beam strike an appropriately sized focus on the anode
What does the anode do?
Provide the target material (e.g. tungston) to be hit by the electron beam to make xrays
What happens if you increase the target anode atomic number (Z)?
You increase the quantity and quality of xrays produced
What happens if you increase the xray tube kVp?
You increase the quality and quantity of xrays produced
What happens if you increase the xray tube mAs?
You increase the quantity of xrays produced (but not their average energy)
What happens if you increase the voltage ripple in an xray tube?
You decrease the quantity and quality of xrays produced
What happens if you add filtration to an xray tube?
You increase the quality but decrease the quantity of xrays produced
On which side should you place thicker parts to minimize heel effect?
The cathode side
How should you change the xray tube angle to minimize heel effect?
Increase angle
Note: Smaller angles are associated with worse heel effect.
How does the focus to film distance (FFD) affect the heel effect?
Smaller FFD means less heel effect
How does the field of view affect the heel effect?
Smaller field of view (film) means less heel effect
For mammography, you should put the chest side on the…
Cathode side (to minimize heel effect)
How does increasing mAs affect the average energy of xrays produced?
It doesn’t change the average energy (it only results in a higher quantity of xrays produced)
How can you determine the kVp for a given target curve?
It is the maximum energy of xray photon produced (where the curve falls back to 0 on the x axis)
You lower the kVp and now no longer see a characteristic peak on the target curve…
You lowered the kVp below the binding energy for the target (no characteristic xrays can be produced)
What happened if the target curve now has a different characteristic xray peak?
You changed target material
Note: Tungston will always have a characteristic peak at ~70 KeV (as long as the kVp is set to 70 or above).
How does k shell binding energy change with atomic number?
Higher atomic number means higher k shell binding energy (need a higher kVp to produce characteristic xrays)
Compton interactions are a main contributor to…
Scatter/noise
Note: Compton scatter involves the outer shell electrons, whereas the photoelectric effect (which contributes to desired contrast) involves inner shell electrons.
Magnification is proportional to
SID/SOD
Note: Increased source-image distance and decreased source-object distance both increase magnification.
How can you decrease blur in radiographs?
- Smaller focal spot
- Bring detector as close as possible to pt (and source as far away as possible)
Detective quantum efficiency
An estimate of the required exposure level necessary to produce an optimal image (a measure of how efficient a detector is compared to an ideal detector)
Note: Higher DQE detectors will result in decreased doses to the pt.
Modulation transfer function
A measure of the relationship between sharpness (i.e. edge detection) and resolution (i.e. ability to discriminate between two close points)
Note: The higher the MTF, the higher the detective quantum efficiency of the detector.
As detective quantum efficiency increases, signal to noise ratio ______
Decreases (inversely proportional)
Detective quantum efficiency is better at low/high spatial resolution
DQE is better at LOW spatial resolution
What is the approximate detective quantum efficiency of digital radiography?
0.45 (almost half of the xrays hitting the detector are recorded)
Note: The DQE for plain film is worse (0.25).
If a tech wants to noticeably increase the brightness of a radiograph, they should…
Increase mAs by 30%
How does changing the kVp change the radiograph?
Lower kVp -> higher contrast
How does changing the mAs change the radiograph?
Higher mAs -> higher radiographic density (brighter image)
If you increase mAs by 50%, what should you change to maintain the same radiographic density?
Decrease the kVp by 15%
What is the main downside to using a grid in radiography?
Increased dose to the pt (more xrays needed to produce the same exposure)
What are common scenarios where you would not use a grid for radiography?
- Babies
- Extremities
- Magnification (in mammography)
How can you reduce scatter (improve contrast in radiography)?
- Collimate
- Compress the body part
- Lower kVp
- Use a grid or air gap
What are the benefits of using collimation in radiography?
- Increased contrast
- Decreased scatter
- Decreased dose (kerma area product)
Note: The main down side to collimation is decreased field of view.
What is the typical standard of care resolution for a digital display in radiology?
3 Mega pixels (3,000,000 pixels)
What is the major determinate of image contrast in film radiography?
kVp (lower kVp -> higher contrast)
What is the major determinate of image contrast in digital radiography?
Look up tables
Note: kVp still influences contrast, but since digital detectors have such a high dynamic range you can adjust the contrast by windowing.
What is the difference between the detector response curves in film vs digital radiography?
DR has a linear response curve (high dynamic range)
FR has a curvilear response curve (has a hard time discriminating between really low energy and really high energy photons, good contrast is only achieved in a narrow range of photon energies)
What are the major determinates of spatial resolution in digital radiography
Better resolution if:
- Smaller pixels (detector elements)
- Decreased pixel pitch (distance between pixels)
Indirect digital detectors
Indirect detectors (scintillators) convert the xrays into visible light, which then gets converted into electrical charge
Note: Direct detectors (photoconductors) convert xrays directly into electrical charge.
What is used as the scintillator in indirect digital detectors?
Thallium doped Cesium Iodide (CsI)
Note: This converts the xrays into visible light. The added step also decreases resolution (the thicker the crystal the more the light can scatter before actually reaching the digital detector).
What component converts xrays to electrical charge in direct digital detectors?
Amorphous selenium
What factors can improve spatial resolution for computed radiography?
- Smaller laser size
- Thinner phosphor plate (less light spreading)
- Increased sampling frequency (which results in a smaller pixel pitch)
- Smaller imaging plate (for a fixed matrix size CR system)
Note: Increasing the number of xrays available will not improve maximum spatial resolution.
Which has better spatial resolution: computed radiography or digital radiography?
Digital radiography (because the pixel detector is built into the DR flat panel)
What factors can improve spatial resolution for digital radiography?
Solely dependent on the size of the detector elements (smaller detector elements, better resolution)
What is the fill factor for direct conversion digital radiography?
Nearly 100%
Note: Indirect detectors only have a moderate fill factor.
What is the kVp used for mammography?
Low energy (25-25 kVp)
What is the kVp used for general radiography?
High energy (50-120 kVp)
What is the most common anode used in mammography?
Molybdenum
What is the most common anode used in general radiography?
Tungston
What is the mAs used in mammography?
100 mAs (low tube current)
What is the mAs used in general radiography?
500 mAs (high tube current)
What is a typical exposure time for mammography?
1 second/1000 ms (long)
What is the typical exposure time in general radiography?
50 ms (short)
How does the receptor air kerma differ in mammography compared to general radiography?
Higher receptor air kerma (100 micro Gy)
Note: General radiography is 5 micro Gy.
Instead of the pyrex glass window used in general radiography, mammography uses…
A beryllium window
How does the focal spot differ in mammography compared to general radiography?
Smaller focal spot than used in general radiography
How does the grid ratio differ in mammography compared to general radiography?
Lower grid ratio (5:1) than used in general radiography (10:1)
How does the optic density differ in mammography compared to general radiography?
Higher optic density than used in general radiography
How do the view boxes differ in mammography compared to general radiography?
Brighter (3000 cd/m^2) than in general radiography (1500 cd/m^2)
How do processing times differ in mammography compared to general radiography?
Longer processing times than in general radiography
What is used to reduce scatter in mammography?
- A grid
- Air gap without grid (for mag views)
What is the focal spot used for mag views in mammography?
0.1 mm (smaller than the 0.3 mm used for general mammography)
What changes for mag views in mammography?
- Air gap instead of grid
- Smaller focal spot (0.3 mm -> 0.1 mm)
- Lower mAs (100 mAs -> 25 mAs)
- Increased exposure time (1 sec -> 3 sec)
What target/filter should you use for larger/dense breasts?
Rh/Rh or Mo/Al
What target/filter should you use for intermediate density breasts?
Mo/Rh
What target/filter should you use for low density breasts?
Mo/Mo
What target/filter combination should never be used?
Rh/Mo
Note: Rh produces characteristic xrays of 21 kEv, which would get filtered by a molybdenum filter (K edge of 20 KeV).
MQSA requirement for PPV1 (abnormal screener call back)
Benchmark 4.4% (acceptable range is 3-8%)
MQSA requirement for PPV2 (recomendations for biopsy)
Benchmark 25.4% (acceptable range is 15-40%)
Note: The acceptable range increases if there is a palpable mass (to 25-50%).
MQSA requirement for PPV3 (percentage of biopsies performed that show cancer)
Benchmark 31% (acceptable range is 20-45%)
Note: The acceptable range increases if there is a palpable mass (to 30-55%).
What body is tasked with MQSA enforcement?
FDA
MQSA QA frequency for processor quality control
Daily
MQSA QA frequency for darkroom cleanliness
Daily
MQSA QA frequency for viewbox conditions
Weekly
MQSA QA frequency for phantom evaluation
Weekly
MQSA QA frequency for repeat analysis
Quarterly
MQSA QA frequency for compression test
Semi-annually (twice yearly)
MQSA QA frequency for darkroom fog
Semi-annually (twice yearly)
MQSA QA frequency for screen-film contrast
Semi-annually (twice yearly)
What is the appropriate target range for MQSA recall rate for a medical audit?
5-7%
What is the appropriate target range for MQSA # of cancers/1000 screened?
3-8
How many mammograms do you have to read during training for MQSA?
240
How long is the MQSA formal training requirement for mammography?
3 months (and 60 documented hours)
Describe the MQSA phantom breast
- 50% glandular tissue (50% fat)
- 4.2 cm thick (compressed)
What is the target dose for the MQSA breast phantom?
< 3 mGy (300 millirads)
Note: The phantom dose is performed with a grid.
What is the typical mAs used for fluoro?
0-5 mAs
Note: This is much less than the 200-800 mAs used for general radiography.
What is the typical kVp used for fluoro?
50-120 kVp (same as for general radiography)
What is the typical focal spot used for fluoroscopy?
0.3-0.6 mm (smaller than the 1.0-1.2 mm used for general radiography)
Fluoroscopy uses ______ exposure times compared to general radiography
Fluoroscopy uses longer exposure times
Compared to a spot film, a “last image hold” in fluoroscopy has…
- Lower dose to pt
- More quantum mottle (less photons)
- Lower spatial resolution (~2 line pairs/mm compared to ~3 line pairs/mm for a spot film)
Note: A digital spot is the same as a film spot (but doesn’t need a cassett).
How do you achieve geometric magnification?
You decrease source-object distance
Note: SID/SOD (you could also increase source-image distance).
How does radiation dose change as you move the pt closer to the source?
Increases by the square of the change in distance (1/3 the distance means 9x the radiation)
How does operator dose change if you move the pt closer to the source?
Increases (scatter radiation isn’t blocked as efficiently)
How does geometric magnification affect image quality?
- Increased blurring
- Decreased scatter (due to added air gap)
How do you achieve electric magnification (i.e. zoom)?
You project a smaller field of view on the same matrix of detector elements (resulting in minification gain)
How does electronic magnification affect pt radiation dose?
Increases air kerma (due to automatic brightness control compensating for fewer photons)
Note: Air kerma product does not change.
How does electronic magnification affect image quality?
Increased resolution
Note: There is no increased blurring like there is with geometric magnification.
What is the best pt positioning for fluoroscopy?
As close to the detector (II/image intensifier) as possible
As far away from the source (xray tube) as possible
Where should the operator stand during fluoro?
On the same side of the pt as the detector (II)
i.e. opposite side as the xray tube/source
How does doubling your distance from the xray source change your radiation dose?
Decreases it by a factor of 4 (inverse square law)
What is the normal air kerma limit for fluoroscopy?
87 mGy/min (10 roentgens per min)
What is the air kerma limit if using high level control fluoroscopy (for very large pts)?
176 mGy/min (20 roentgens per min)
Note: This is double the normal limit.
What must you have in addition to normal precautions when using high level control fluoroscopy?
Audible or visual alarms (in addition to the normal timer alarm)
What is pulse fluoro?
Using pulses of xrays rather than a continuous current (less radiation and less motion blur)
Pulse fluoro results in less radiation dose as long as…
Frame rate is below 30 frames/sec
If you lower the fluoroscopy frame rate from 30 to 15 fps, then radiation dose…
Decreases by 30%
Note: It is not a 1:1 relationship because lower frame rates have higher mAs per pulse.
What is the best kVp to use during IR cases using IV contrast?
60-80 kVp (average beams hit the k-edge for iodinated contrast well)
To maximize spatial resolution during IR fluoro cases…
- Smaller focal spots
- Smaller anode angles
Are grids used for IR fluoroscopy?
Usually (not if pediatric pts or extremities)
50% of the radiation dose during IR fluoroscopy is…
Delivered to the superficial 3-5 cm of skin/fat
What is the concept of dose spreading?
Changing the angle of the gantry during fluoroscopy cases in order to spread out the radiation dose over a larger volume of tissue
How does magnification affect radiation dose to the pt during IR fluoroscopy?
Increases air kerma (but not air kerma product)
What percentage of the radiation dose received by the pt does an interventional radiologist receive standing 1 m away from the pt (not wearing lead)?
0.1% (1/1000)
How do the mAs used in CT differ from general radiology?
Higher (up to 1000 mAs) compared to general radiography (200-800 mAs)
How do the kVps used in CT compare to general radiography?
Higher but similar (80-120 kVp) compared to general radiography (50-120 kVp)
What focal spot sizes are used for CT?
0.6-1.2 mm, often smaller than that for general radiography (1.0-1.2 mm)
What kind of xrays are used for CT?
Highly filtered, high keV xrays (average energy is 75 keV)
Why are bow tie filters used in CT?
- Compensate for uneven filtration
- Reduces scatter
- Reduces dose to the pt
What are the “septa” used in CT?
A grid
What determines the minimal slice thickness for a CT?
The detector element aperture width
How can you calculate the pixel size that a CT will create?
field of view/matrix size
How will decreasing the kVp from 140 to 80 change the HU of a contrast-enhanced vessel?
Increased HU
Note: This increased HU with increased kVp is only seen with high atomic number substances (e.g. iodine) due to the higher k-edge.
CT pitch of 1
No overlap between slices
CT pitch greater than 1
Gaps between slices (the table moves faster than the xray beam), resulting in decreased spatial resolution
CT pitch less than 1
Some overlap between slices (table moves slower than the xray beam), resulting in increased spatial resolution and increased radiation dose to the pt
What windowing value should you change to change image brightness?
Window level (midpoint of the gray scale display)
Note: Set the window level at the attenuation level of the thing you want to see (e.g. high level for bone).
What windowing value should you change to change image contrast?
Window width
Note: Narrow windows are better for comparing things of similar attenuation (e.g. white and gray matter).
How do you increase contrast by windowing?
Make the window width more narrow
Brain window values
Level +40 (width 80)
Stroke window values
Level +30 (width 30)
Lung window values
Level -400 (width 1500)
Abdomen window values
Level +50 (width 400)
Bone window values
Level +500 (width 1600)
What is the benefit of prospective cardiac gating?
Reduced radiation dose (CT is only on at specific points in the cardiac cycle)
What are the cons of prospective cardiac gating?
- More motion artifact
- No functional imaging
- Need a slow heart rate
Can you use a helical CT for cardiac gating?
Not for prospective cardiac gating (only axial CT)
What dose of metoprolol is used for cardiac CT to reduce heart rate?
2.5-5.0 mg IV metoprolol
What dose of nitroglycerine is used for prospective cardiac CT to dilate coronary arteries?
0.8-1.2 mg glycerol trinitrate
OR
5 mg isosorbide dinitrate
What is the required heart rate for prospective cardiac CT?
50-65 bpm
Contraindications for metoprolol use in cardiac CT
- Bradycardia (HR < 60)
- SBP < 100
- Decompensated heart failure
- Asthma (and on beta-agonist inhaler/albuterol)
- Active bronchospasm
- Severe COPD
- 2nd or 3rd degree AV block
Treatment for metoprolol overdose during cardiac CT
- Fluids (careful if CHF)
- Atropine
- Glucagon
What dose of atropine should be used for metoprolol overdose during cardiac CT?
0.5 mg IV atropine (can be repeated up to a total of 3 mg)
What dose of glucagon should be used for metoprolol overdose during cardiac CT?
50 micrograms/kg IV loading dose, followed by 1-15 mg/hour infusion
Contraindications for nitroglycerine during cardiac CT?
- Viagra/cialis within 48 hours
- Severe aortic stenosis
- Hypertrophic cardiomyopathy
PDE-5 inhibitors should be held for ______ prior to a cardiac CT
48 hours (2 days)
e.g. sildenafil/tadalafil
How will decreasing the CT field of view affect resolution?
- Improved spatial resolution
- Decreased contrast resolution (fewer photons per detector element)
Note: Smaller FOV means smaller pixel size (pixel size = FOV/matrix size).
How do reconstruction filters affect resolution?
Bone algorithm gives better spatial resolution
Soft tissue algorithm gives better contrast resolution
A ______ focal spot will increase spatial resolution
Smaller focal spot
A ______ detector element width will increase spatial resolution
Smaller detector element width
Note: This determines spatial resolution in the craniocaudal dimension.
A ______ CT pitch will increase spatial resolution
lower (<1) CT pitch
Note: If >1 there will be gaps between slices.
A ______ CT pitch will increase slice sensitivity profile
higher (>1)
What is the downside to a lower CT pitch?
Increased radiation dose (more slice overlap)
How can you improve contrast resolution by changing tube voltage and current?
- Increase mAs (more signal, less mottle)
- Decrease kVp (less scatter, less noise)
Decreasing CT mAs by 50% will ______ radiation dose by ______
Decrease radiation dose by 50%
Increasing CT pitch by 50% will ______ radiation dose by _______
Decrease radiation dose by 50%
Which type of reconstruction method allows for reduced pt radiation doses?
Iterative reconstruction (handles noise better allowing for lower dose protocols)
Increased CT rotational speed leads to _______ radiation dose
Decreased radiation dose
How do you fix this artifact?
Call manufacturer to replace the detector (Ring artifact is a detector problem)
How can you decrease partial volume artifact?
Thinner slices (decrease beam width, increase beam collimation)
How can you decrease stair step artifact?
- Thinner slices (decrease beam width, increase beam collimation)
- Reconstruction with overlapping intervals
How can you fix this artifact?
- Reposition (e.g. move arms up)
- Increase kVp
Note: Beam hardening artifact is due to the selection for higher energy xrays as the beam moves through the pt.
How can you decrease metal artifact?
- Increase kVp
- Thinner slices
- Interpolation software
How can you decrease photon starvation artifact?
- Increase automatic tube current modulation (i.e. increase mAs)
- Use adaptive filtration (to smooth the data)
Note: Photon starvation artifact is increased quantum mottle because not enough xrays are reaching the detector.
How can you decrease motion artifact?
- Decrease scan acquisition time
- Over scan (add an extra 10% to each 360 degree rotation and take the average)
- ECG gating
Ultrasound reflection occurs at
The interface between tissues with different acoustic impedences
Ultrasound refraction occurs at
The interface between tissues with different speeds of sound
Ultrasound refraction is dependent on…
- The change in speed of sound
- The angle of incidence
How do higher frequency probes affect scatter?
More scatter
How do higher frequency probes affect attenuation?
More attenuation (can’t see as deep)
The functional part of the ultrasound probe is the…
Piezoelectric crystal
A thicker piezoelectric crystal results in a…
Lower frequency US probe
Why would you use a high Q dampening block for ultrasound?
To preserve velocity information during Doppler ultrasound
Why would you use a low Q dampening block for ultrasound?
For higher axial resolution (heavy dampening with broad bandwidth)
What does the matching layer do in ultrasound?
Minimizes the acoustic impedence differences between the transducer and the pt
A higher frequency transducer has a ____ near field
Longer near field
The focal zone on an ultrasound machine maximizes…
Lateral resolution
How can you improve tissue penetration during ultrasound?
Switch to a lower frequency probe
How can you brighten deep structures during US?
Increase time gain compensation
How can you improve axial US resolution?
- Shorter pulses (smaller spatial pulse length)
- Greater dampening (Low Q)
- Higher frequency probe
How can you improve lateral ultrasound resolution?
- Adjust focal zone
- Increase the line density of the probe
- Higher frequency probes (less beam spreading)
How can you improve elevational ultrasound resolution?
- Use a thinner crystal
- Minimize slice thickness
- Use a fixed focal length across the surface of the array
What is the purpose of the stand off pad in ultrasound?
Helps place superficial things in the focal zone (improving lateral resolution)
Axial resolution depends on…
Spatial pulse length
Lateral resolution depends on…
Transducer element width
Elevation resolution depends on…
Transducer element height/thickness
What type of ultrasound resolution gets worse the deeper you go?
Lateral resolution (due to beam spreading)
Note: Axial resolution stays the same.
What type of resolution is improved by using harmonics?
Lateral resolution
How does ultrasound harmonics work?
Transmitting at one frequency and receiving at another (harmonic) frequency
How does ultrasound compound imaging work?
Using electronic US beam steering to image an object in multiple different directions
Where will US harmonics not be helpful?
Near field (US waves haven’t traveled far enough)
What is the downside to using US harmonics?
Decreased depth penetration (higher frequencies attenuate more quickly)
What technique can you use to decrease posterior acoustic shadowing?
Compound imaging
Note: Compound imaging will also make a cyst look more solid.
How does ultrasound compound imaging affect edges?
It sharpens them
What is the ultrasound thermal index?
The maximum temperature rise in tissue secondary to energy absorption
What is the ultrasound mechanical index?
How likely that cavitation will occur (considering peak rarefaction pressure and frequency)
What should be avoided with neonatal imaging?
Pulsed spectral doppler (high risk of cavitation)
Note: Use M-Mode instead to document fetal heart rate.
The ultrasound thermal index should be under…
1.0 (some say under 0.7)
What is the ideal Doppler angle?
30-60 degrees (but the lower the better)
Doppler angle of 90 degrees
Will not show any flow
If you are looking for slow flow using Doppler…
- Use power Doppler
- Use a low pulse repetition frequency
Power doppler does not depend on…
The Doppler angle
What artifact will not be demonstrated on power Doppler?
Aliasing
Note: Color and spectral Doppler both can show aliasing.
When does ultrasound aliasing occur?
When the doppler shift is too high (greater than the Nyquist frequency)
How can you reduce ultrasound aliasing artifact?
- Increase the scale
- Increase the pulse repetition frequency
- Use a lower frequency transducer
- Increase the Doppler angle (closer to 90)
Reverberation artifact is due to
Two parallel highly reflective surfaces
Comet tail artifact is due to…
Two parallel highly reflective surfaces that are very close together (<1/2 the spatial pulse length)
Ring down artifact is due to…
Fluid trapped between a tetrahedron of air bubbles
Mirror image artifact is due to…
US waves getting trapped behind a strong reflector
