1
Q
What is the associated voltage ripple for each type of generator?
A
- Single phase = 100%
- Three phase
- 6 pulses = 14% (13-25% on image)
- 12 pulses = 4% (3-10% on image)
- High frequcncy = 4-15%
- Constant potential = 2%
2
Q
How do Step up and step down transformers compare?
A
- Step up: Ns>Np
- increases voltage, decreases amperage
- Step down: Ns
- decreases voltage, increases amperage
This is because:
- ratio of voltage output = the ratio of number of wire turns
- N is the numbr of wire turns
- Input power (voltage x current) = output power
3
Q
How is power calculated and what are the si units?
A
- Watt (W) is the si
- W = J/s
- power = voltage (v) x current (i)
Thus
- 1W = 1 volt x 1 ampere (1 ampere = 1 coulomb(C)/s)
- so, V=J/C via algebra
4
Q
What is the SI unit of energy?
A
Joule (J) = 1W x 1 sec
5
Q
How do you calculate heat unit (HU) for three phase and high frequency generators? What is the correction factor multiplier?
A
HU = kVP x mA x s x fcorr
the correction factor (fcorr) = 1.35-1.4
6
Q
What is the relationship b/w the joule and the heat unit?
A
HU = 1.4 x heat input (J)
7
Q
What is the federal leakage regulation?
A
< 0.1 R/hour @ 1 meter from focal spot
8
Q
What is the relationship between the focal spot, focusing cup and filament length?
A
The focusing cup determind the focal spot width
The filament length determines the filament length
9
Q
How do you decrease the heel effect?
A
- Use a larger anode angle
- use a smaller film or collimate more
- Use a larger SID
- position cathode over thicker tissue and anode side over thinner tissue
- (potentially increase kVp according to Watson’s noted)
10
Q
In regard to the space cloud, what does “space charge limited” and “emission limited” mena?
A
- <40 kVp = space charge limited - not all electrons pulled to the anode, tube current insufficient to pull all electrons away from the cloud
- >40 kVp = emission limited - all electrons pulled to anode, total # electrons (tube current) is determined by the filament current - further increase in kVp will not change tube current
11
Q
What increases Bremsstrahlung w-ray production?
A
increased kVp and increased Z
12
Q
What influcences x-ray quantity?
A
- Directly proportional to mAs - mA double, quantity doubles
- increase kVp, incr qauntity (and quality)
13
Q
Accodring to WarMachine, what things increase quantity? page 40
A
- Increased Z (more overall x-rays via Bremsstrahlung)
- Increased kVp
- Increased mAs
- Increased voltage ripple (more overall Bremsstrahlung)
14
Q
What influences quality?
A
- increased kVp
- increased filtration
- decreased voltage waveform ripple (less ripple in tipple = better quality)
15
Q
How do you decrease magnification?
A
- Use a small focal spot
- Use a small anode angle
- Minimize the OFD(OID) → b
- Maximize the FFD(SID) → a+b
16
Q
How do you decrease the prenumbra?
A
- use small focal spot
- small object to image distance
- large focus to object distance
- place ROI on anode side
because,
P = ( F x OID)/SOD → F is the focal spot size
17
Q
How do you calculate true magnification? And when does true mag (M) equal mag (m)?
A
M = m + (m-1)(f/d)
M= true mag
m=mag
f=focal spot size
d=object size
Note: when f<<<
18
Q
WHat influences the probability of photoelectric effect occuring?
A
- increases at energies just above the BE of the k shell (k-edge)
- rapidly decreases at energies much above k-edge
- Inversely proportional to the incident photon energy cubed: 1/E3
- probability directly proportional to the anatomic number cubed: Z3
19
Q
What is the k-edge for iodine?
A
33.2
20
Q
What is tha k-edge for barium?
A
37.4
21
Q
Does compton scatter occur with inner or outer shell electrons?
A
outer shell
22
Q
How do you calculate wavelength change in compton scatter?
What is the relationship between the angle of deflection and the enegery of the scattered photon?
A
- change wavelength = 0.024(1- cosø)
- As the angle of defelction decreases, the energy retained by the scattered photon increases
23
Q
What increases the probability of Compton scatter?
A
- directly proportional to density
- increases with low binding energy electrons
- Dominates above 30 kEv
- PE= compton at 25 kEv
- proportional to 1/E
- INDEPENDENT of Z
24
Q
WHat is a major difference between linear attenuation coefficent and mass attenuation coefficient?
A
LAC is effected by density, MAC is not
25
What increases LAC (µ)?
aka what increases absorption?
* Increased Z of absorber
* increased physical density of absorber
* decreased photon energy
* K-edges
26
What is the half value layer?
thickness of material that attenuated the x-ray beam by 50%
(qauntifies the ability of the beam to penetrate tissue and therefore described the quality of the beam)
27
What factors affect subject contrast?
* Increased kVp - lower contrast
* lower kVp = higher contrast
* Density difference = higher contrast
* Z difference = higher contrast
* Thickness difference = higher contrast
28
How does FWHM (full width at half max) relate to spatial resolution?
the smaller the FWHM, the better the spatial resolution
29
What is the approximate spatial resolution of film (no screen) and film w/ screen?
* film no screen - 100 lp/mm
* screen/film - 5-8 lp/mm
30
* What is an advantage of film screens?
* What are some disadvantages?
Advantage
* captures 60-65% of x-rays, reducing mAs needed and dose to patient
Disadvantages:
* loss of spatial resolution due to lateral liight diffusion
* increased quantum mottle/noise (few photons)
31
What scintillator is now most commonly used in screens (film screen)?
Gadolinium oxysulfate (Gd2O2S)
32
How does light absorbing dye impact total conversion efficiency in screen/film?
reduced lateral speading of light (increases spatial resolution) but decreases conversion efficiency
33
How do reflective layers effect total conversion efficiency in screen film?
reduced spatial resolution but increase conversion efficiency
34
What is QDE?
* quantum detection efficiency = abosrption efficiency
* fraction of incident x-rays that are detected and interact with the screen
35
What is conversion efficiency (film screen)?
the fraction of the absorbed energy that is emitted as light
36
WHat factors affect QDE?
* Z
* E (k-edge)
* packing fraction
* phosphor coat weight
* increased screen thickness - incr QDE (leads to reduced spatial resolution)
37
How do you increase film screen speed?
speed of system = QDE x CE
* increased QDE
* if achieve by using thicker screen, reduced resolution
* increase CE
* leads to more noise if increase CE
38
What is Modulation transfer function?
ratio of output modulation to the input modulation expressed as a function of spatial resolution (bascially what % of the input signal made it through the system)
39
WHat is the useful optical density range?
0. 3 - 2.0
0. 3= 50% transmittance
2. 0 = 1% transmittance
40
How is OD related to transmittance?
OD represents the log reciprocal of transmittance (It/I0):
OD=log10 (I0/It)
41
Which OD/transmittance provides good contrast?
5% = 1.3 OD ⇒ good contrast
42
What is the OD of unexposed film?
0.11-0.15
43
For the film characteristic curves, how to the steep and flatter slopes affect contrast?
* Steeper slope = narrow lattitude (narrower ranges of accepatble exposures
* high contrast film
* low kVp
* Flatter slope = wider range/latitude
* low contrast
* high kVp
44
What effects does increasing the grid ratio have?
* increases degree of collimation
* cleans up more scatter
* allows for more radiation dose to the patient
45
What grid frequency needs to be used to avoid a Moire pattern?
\>60 lp/mm
46
How do you calculate grid ratio?
Heigh/interspace width
47
What is the Bucky factor?
B = incident radiation onto grid ÷ transmitted radiation
48
What affects B factor?
* Increased grid ratio = increase B factor
* e.g. 12:1 grid has B of 5, while 5:1 has B of 2
* increased scatter absorption, need to incr mAs
* Increased kVp= increased scatter = need for increased B factor
Increased B factor = Increased exposure factors = increased patient dose
49
What factors influence the magnitude of lateral decentering grid cutoff?
* increased grid ratio
* decreased focal distance
* increased lateral decentering distance
when exact centering not possible (such as with portable radiography) - use low ratio grid and long focal distance
50
What increases the amount of grid cutoff?
high grid ratios and short grid-focus distances
51
What is the difference between near and far focus grid cut off and which one has greater cut off?
* Near focus grid - target is below the convergent line
* Far focus grid - target is above the convergent line
Near focus will have higher magnitude of cutoff
52
What does the resulting image look like with focus- grid distance decentering?
light periphery and unaffected center
53
In combined lateral and focus-grid distance decentering, how do the images appear for near and far?
* Lateral + far focus-grid decentering
* cutoff is greatest under x-ray tube
* so lighter film right under tube (the near side)
* Lateral + near focus-grid decentering
* cutoff is greatest on the oppposite side from the tube
* so lighter film on the far side
54
In lateral + focus-grid distance decentering, what affects the magnitude of cutoff?
* cutoff propotional to:
* grid ratio
* decentering distance
* cutoff inversely proportional to:
* focal distance
55
Increase in gap width increases blur (reduces spatial resolution). How can you compensate for this?
increase the SID (FFD)
56
What are advantages of pulse mode?
* improves temporal resolution (reduced blur)
* reduces patient exposure
57
In flouroscopy, what is the usual electron gain? (# of photons generated at the output phosphor for every photon generated at the input phosphor)
50
58
In flouro, at the output phosphor, what type of light are electrons converted to? And what is the typical luminence gain?
Green light (530 nm) ⇒ 1 e- produces about 1000-2000 visible light photons
59
What do the charged electrodes in the flouro vaccum do?
* focus the electrons
* accelerates the electrons to the anode creating and electron gain
* intensifies and minifies the electron beam
* Does NOT increase the # of electrons recived at the output phospher
60
In flouro, when magnifying, what happens to dose, spatial resolution and brightness?
* doubling magnification quadruples dose
* increases spatial resolution
* decreases brightness (need ABC to overcome)
61
In flouro, what component is the limiting factor for spacial resolution?
the image intensifyer (about 4-5 lp/mm)
62
How does QDE of an image intensifier compare to flat panel?
1.5 mmAl reduceds II QDE compared to the flat panel
63
How does the contrast and temporal resolution of flouro compare to radiography?
* contrast ⇒ low compared to radiography
* temporal ⇒ high compared to radiography
64
In flouro, what does the contrast ratio measure?
measures the veiling glare - 15:1 to 30:1 in most machines
65
How does increasing kVp effect veiling glare?
Increased kVp will reduce contrast due to increased x-ray and light diffusion inside the II
66
what does a Byte equal?
Byte = 8 bits (28) ⇒ 256 combinations
67
How do you calculate pixel size?
pixel size = FOV ÷ # of pixels
68
What are the ACR recommendations for minimum pixel number and minimum spatial resolution?
* minimum 10 bits per pixel
* minimum spatial resolution of 2.5 lp/mm
69
How many bits per pixel are required for ultrasound?
6-7 bits
70
How many bits per pixel are required for x-ray, CT and MR?
12 bits required to represent full range of CT numbers
71
In CR, what is the photostimulable phospher detector compsed of?
Barium flourohalide (BaFBr and BaFl) doped with europium (Eu)
72
When read out by the red laser, what light energy is released from the CR plate?
blue/blue-green light
73
In indirect flat panel detectors, what determines spatial resolution?
* largely determined by the size of the TFT
* TFT determines the size of the pixel ⇒ the smaller the pixel, the greater the spatial resolution
* also affected by lateral light diffusion in the scintillator
74
What is fill factor and how does this effect contrast resolution?
* ratio of light sensitive area to the entire area of each detector element
* lower the fill factor, the lower the contrast resolution
* smaller TFT (pixel size) causes increased SR but decreased SNR
75
What accounts for Direct flat panels higher QDE?
thick selenium layer → much thicker than the silicon layers in the indirect systems
76
What are advantages of direct DR over indirect DR?
* High directionality of the electrons induced by the applied voltage reduces blurring (no lateral diffusion)
* increased fill factor (incr SNR)
77
What is DQE?
* combines _spatial resolution (MTF)_ and _image noise_ to provide a measure of the signal to noise ratio of the various frequency components of the image
* best objective indicator of image fidelity
78
How do you calculate DQE?
DQE = SNRoutput/SNRinput
79
What is the principal advantage of DR?
Decouples image acquisition and display ⇒ allows for a large dynamic range and linear response to x-ray exposure
80
How do CR and DR compare as far as patient exposure/dose?
* DR can reduse dose 2-3x in comparison to CR
* due to DR's incr. quantum absorption and incr conversion efficiency
* at least equal to 400-800 speed film/screen systemts
81
What are advantanges of flat panel detectors (IDR, DDR) in comparison to CCD DR?
Flat panel DR
* incr dynamic range
* inr spatial resolution (incr MTF)
* dose reduction
CCD DR
* increased SNR (100% fill factor)
* small size of unit
82
What processes involve raw datat manipulation?
* Unsharp masking filter
* Multiscale processing
83
What artifact can occur with Unsharp masking?
edge enhancement artifacts (dark halo)
84
How do unsharp masking and multiscale processing differ?
in MC, image is decomposed into multiple spatial frequncy bands (multiple sub images) based on sturcture size, unlike UM which decomposes into only 2 sub images
85
Regarding kernal sizes, which one (1) results in edge enhancement, (2) results in dynamic range compression and better detail, (3) results in enhancemnt of very small structures?
1. medium
2. large
3. small
86
What tyoes of processing do NOT involve manipulation of raw data?
* Histogram manipulation
* LUT adjustments
87
In regard to post porcessing windowing/leveling, what does viewing with a "wide window" or a "narrow" window really mean?
* wide window - more latitude, less contrast
* narrow window - less latitude, more contrast
88
What is a ACR minimum requirement for monitor luminence?
171.3 cd/m2
89
How does the spatial resolution of medical grade and commecial monitors compare?
* medical grade - 2-5MP (1200x1600 to 2560x2048)
* commercial - 0.75-2MP (1024x768 to 1200x1200)
90
In regoard to monitors, what decreases contrast ratio (dynamic range?
* increased ambient light
* wider angle viewing
91
What is the ACR recommended illuminence (ambient light)?
2-10 lx (lux)
92
WHat does PACS stand for?
Picture archiving and communication systems
93
What are DICOM interconectivity needs/requirements?
* Internet protocol (IP) address
* Application entitiy (AE) title
* Communication port
94
How do you calculate storage capacity?
matix size x #bytes/pixel x # images
95
How do you calculate sensitivity?
TP/(TP + FN) x 100
96
How do you calulate specificity?
TN ÷ (TN + FP) x 100
97
What is sensitivity?
The ability to detect disease - a sensitive test has a low false-negative rate
98
What is specificity?
The ability to identify the abscence of disease - a specific test has a low false-positive rate
99
How do you calculate the PPV?
TP/(TP + FP) x 100
100
How do you calculate the NPV?
TN/(TN + FN) x 100
101
How do you calculate subject contrast and how does kVp effect this?
* Cs = 1 - e-uz
* Decreasing kVp ⇒ inc u (linear attenuation coefficient) ⇒ increases Cs
* (decreasing x-ray energy will result in an increase in subject contrast)
102
What factors affect subject contrast?
* Density difference
* Thickness difference
* Atomic number
* PE creates contrast, predominates at higher Z (and lower kVp)
* Radiation quality
* low kVp - high subject contrast→narrow latitiude
103
OD base + fog should never be above what?
0.2 (usually between 0.13 - 0.18)
104
What is the film gamma?
maximum slope of the H&D curve at its linear portion, primarily determined by film type and processing
105
How do the characterisitc curves of DR and CR compare to film?
NOT sigmoidal
* Flat panel = linear
* contrast level is independent of exposure
* CR = logarithmic
106
How can you reduce scatter?
* air gap
* grid
* reduced FOV
* compression (decrease patient thickness)
* narrow beam geometry
* deacrease kVp
107
What is sampling pitch?
refers to the spacing between discrete detectors (1/2 lp) from central points on the detector element
108
What is the Nyquist frequency?
* If you have a frequency (f), then in order to sample this frequency correctly the sampling frequency must be at least 2 x f
* thus, maximum measurable frequency is half the sampling frequency (pitch)
Nyquist frequency = 1/(2 x sampling ptich)
109
