Notes from booklet Flashcards
1
Q
Mass number is what
A
protons and neutrons
2
Q
Atomic number is
A
protons
3
Q
Tungstan has a K shell energy of what?
A
69.5keV
4
Q
What are properties of isotopes
A
same chemical properties as same protons
5
Q
How does Iodine 123 decay?
A
by 160keV Gamma Ray
6
Q
Isotope vs isotone?
A
Isotope has same number of protons. Isotone same number of neutrons.
7
Q
what is a bacquerel?
A
Number of decays per second
8
Q
What is emitted with a beta plus decay?
A
A positron
9
Q
when a positron and electonr anihilate what is the energy of the resulting photons?
A
511kEV each
10
Q
What is emitted in beta minus decay?
A
electron
11
Q
EM waves travel in what way?
A
perpendicular to direction of travel and 90 degrees to each other
12
Q
what is plancks constant?
A
the constant connecting Energy and frequency.
13
Q
relationship of frequency and wavelength
A
f = 1/ wavelength
14
Q
equation for contrast
A
propotional to (u1 - u2) x thickness of material
15
Q
in CT a reduced FOV and increased matrix size causes what for the noise
A
smaller pixels, therefore reduced SNR.
16
Q
in a single slice scanner - does pitch affect the noise?
A
no, only the spatial resolution due to partial voluming.
17
Q
in a multislice scanner does pitch incease noise
A
yes
18
Q
relationship of SNR and slice width
A
SNR is proportional to the square root of slice width.
to doulbe your SNR you have to quadruple the slice width.
19
Q
relationship of SNR and mA
A
SNR is proportional to the square root of mA
20
Q
How does windowing affect noise
A
Decreasing the windowing will increase noise. However the SNR is still the same.
21
Q
What type of noise is the most significant
A
Quantum, electronic the least
22
Q
CT - cupping is the effect of what?
A
Beam hardening
23
Q
Ring artefacts are less suceptible in multi or single slice
A
In multi
24
Q
XR source to patient distance will affect what
A
spatial resolution
25
CTDI depends on what?
mA, kV and filtering.
26
DLP is what?
CTDI (vol) xL
27
effective dose and DLP relationship
E = DLP multiplied by the constant for body region and scanner design.
28
in fluoro - scatter of light in the output window will affect what>
contrast
29
in fluoro - a single photon will cause how many electrons to be stimulated in the photocathode?
1. II input window
Convex metal shield that covers the input face of the II
Usually made of aluminium or titanium foil (low Z metal) to allow x-ray beam to enter with minimum attenuation
Provides protection for sensitive input components of the tube and maintains the vacuum
2. Input phosphor
Layer of sodium activated caesium iodide (CsI:Na) for good x-ray absorption efficiency (70-90%)
Channelled into tiny needle-like crystals (5µm in diameter) with fibreoptic-like characteristics
Deposited on a thin aluminium substrate
CsI:Na usually 400-500µm thick
Each x-ray photon produces ~3000 light photons in the blue spectrum
3. Photocathode
Fluorescent emission from phosphor then absorbed in a light-activated photocathode comprising a very thin layer of antimony caesium (SbCs3) alloy that has a spectral sensitivity well matched to the blue light emission of CsI:Na
Absorption of the fluorescent light photons releases a pattern of electrons in the body of the II tube
Approximately 200 electrons released per absorbed x-ray photon
30
what is DQE?
out brightness squared / input signal squared
31
fluoro - what happens to DAP on magnification
DAP will decrease.
32
Grid controlled tubes are used in what kind of fluoro
Pulsed mode
33
in mammography what kind of focal spot size is used
smaller
34
as the tissue thicness changes in fluoro what happens to the dose rate?
It will stay the same as kV and mA are changed to keep output the same.
35
in fluoro: what is the quation for minification?
input diameter squared /output diameter squared
Minification gain = (Dinput / Doutput)2 (where D is diameter of input and output screen respectively)
36
Brightness gain =
minification x flux gain
37
in fluoro for kids what type of kV do we use?
High kV --> but the resulting image is poorer.
38
in fluoro - good bone to soft tissue is achieved with high or low kV
low kV
39
what is the benefit of compression?
it reduces gemoetrical and movement unsharpness
40
decreased electrons at the edges of the edges of the fluoro II causes increased
distrotion and decreased brightness
41
in ct: Beam p =
movement / collimation
42
in Ct: slice p =
movement /slice width
43
relationship between collimation and slice width
collimation is greater than or equal to slice width.
44
relationship between slice p and beam pitch. (in ct)
slice p greater than or equal to beam pitch.
45
in ct what is the spatial resolution
1 / (2 x pixel size)
46
in ct - what is the pixel size
FOV / matrix
47
which body is contacted if a high dose is given to a patient
CQC
48
List some gamma emitters
Tc99m
I 123
Ga 67
In 111
49
In a fission reactor Uranium becomes what?
Mo99.
50
in NM imaging - what happens to the image if the collimator has wider holes?
sensitivity goes up
| resolution goes down
51
In NM imaging when is high sensitivity particularly good?
When short frame rate is needed for dynamic studies.
52
why do we not use a collimator in in PET?
want to increase the sensitivity
53
T2 is loss of net WHAT magnetisation
transverse
54
in MRI: relationship between Lamor frequencyt and T
they are proportional. Ie At 1.5T the L is 64 MHz and increases with increasing T.
55
in MRI - why do lesions have a longer relaxation time
they have a greater content of just water than most tissues.
So a longer T1.
56
Describe the relationship of T1 loss of magnetisation
Spin lattice - loss to external molecules.
Pure liquid - lots of movement, no time to interact so LONG relaxation
Viscous - less rapid movement, time to interact. Short relaxation.
Solids - the molecules are fixed so can't interact. Therefore T1 is LONG again .
57
Describe the T2 loss of coherance
T2, spin spin, loss to each other.
They affect each other - lose coherance.
In liquids - move freely, magnetic field evens out. T2 is long. LONG T2
In solids - can't move, field is fixed so affects it. Dephase quickly. SHORT T2.
58
WHich is shorter T2 or T1
T2 is always shorter than T1.
59
What affect does the Tesla field strength have on T1 and T2
T2 is unaffected (transverse)
T1 timings increase.
60
US: how long are wavelengths normally
0.1 - 1.5mm
61
Abdo US frequency
3.5 to 5MHz
62
How much is reflected from a muslce /bone interface
40%
63
What is the equation for impedance?
Velocity x density
64
What is the units of impedance
kg/m-2/s
65
What are the units of intensity?
W / mm2
66
What is inensity proportional to?
amplitude squared
67
resonant frequency is what relation to PZT?
twice the thickness
68
Damping causes what?
decrease in amplitude
69
High damping causes what?
decreased amplitude but a wider band of frequencies and a short pulse.
70
Low damping
higher amplitude, longer pulse.
71
High Q is which damping - high or low
low
72
continuous mode has what kind of frequency?
single frequency
73
pulsed mode is what kind of frequency?
range of frequencies
74
near field equation is
radius squared / wavelength.
75
what does the impednace matching layer do
provides further damping in addition to achieving max energy transfer
76
for a fixed transduceer radius how does the near field change if the frequency is decreased?
decreased.
due to Near field = r squared / wavelength
77
beam width =
focal length x wavelength
78
axial resolution =
half the spatial pulse length
79
lateral resolution =
ultrasound beam width.
80
which is better - axial or lateral resolution?
axial resolution is better
81
how does diamter of transducer affect the resolution in the near field
if its a smaller diameter it will be a better resolution in the near field.
82
What things can change the lateral resolution?
frequency, focusing, transducer diamter, distance from the transducer.
83
Doppler shift frequency is proportional to
cosine angle of US
84
what is the maximum doppler shift frequency that can be detected?
PRF / 2
85
what can a high PRF create?
range ambiguity as echoes are generated generated deeper will be registered as being more superficial.
86
speed is = to what (relationship of rigidity and density)
= square root of (k/p)
87
time is
2d / c
88
Us is what kind of wave?
Longitudinal
89
How fast are waves?
1540cm/s
90
the matching layer serves what purpose?
More is transmitted, less is reflected
91
What is Rayleigh scattering?
When particles are smaller than the size of the wavelength grainy
92
what is compound imaging?
look behind structures. good fro aorta
93
What is harmonic imaging?
Non linear propogation of waves to increase resolution
94
what kind of frequency does the harmonic imaging use?
Higher frequency
95
Three types of doppler?
Continuous, pusled and colour doppler
96
Does continous doppler offer depth?
No
97
Pulsed wave offers what
Depth but lots of alisaing.
98
What does A, B and M modes mean
A for amplitude - depth of boundaries.
B
M - motion of structures like heart valves.
99
What temperature rise a risk to the foetus?
1.5 rise
100
Which is higher energy the pulsed doppler or B mode?
Pulsed doppler
101
Near field is proportional to what of D
D squared
102
Lateral resolution is best where? and why?
in the focal zone due to beam narrowing.
103
pulse length =
number of cycles x wavelength
104
Q factor equation =
F / bandwith
105
Intensity reflected =
Z2 - Z1 / Z2 plus Z1 all squared
106
F doppler
= 2 Ft v cos angle / speed
107
backscatter is Frequency.....
dependant
108
doppler is best at what angle
less than 60 degrees due to cosine
109
does continuous doppler have a velocity limit
no
110
does pulsed dopper have a velocity limit
yes
111
what value of MI can cause damage?
>0.3
112
What values are disoplayed for MI and TI
if MI greater than 1
| if TI is greater than 0.4
113
In basic sciences - valence band gives what properties
the chemical properties, thermal, optical and electrical properties
114
what is the speed of EM waves in a vacuum
travels at speed of light
115
what kind of radiation do you get in Internal conversion?
characteristic radiation as the electrons get replaced.
116
List some crystals that are used
Silver bromide
Sodium iodide
Caesium iodide
117
What is a coulomb
1 a / s
118
what is an eV
it is the energy required to move an electron through one volt.
119
What happens to the binding energy as the Z increases?
the binding energy increases
120
how does quantum radiation travel
in a straight line as packets of ernegy
121
how does the wave aspect of radiation travel
in a transverse waves, sinusoidally varying.
122
In electromagnetic radiation - travel at...
right angles to each other and the direction of travel.
123
how does the constant, wavelength and frequency are linked
constant = wavelength x frequency
124
what is photon fluence?
amount of photons that pass through an area in space multiplied by all their energies
125
what is energy fluence rate?
it is the photon fluence / second
126
air kerma
air kerma is measured as a substitute for photon fluence.
127
equation of air kerma and distance
air kerma B / air kerma point A = distance of A squared / distance of b squared
128
How do the electrons in the tube lose their energy
Multiple little enrgy losses - heat
large ernegy losses with inner shell or the nucleus.
129
Why does the energy of the electron need to be greater for higher atomic numbers
What affect does this have on the emitted photons
The nuclear force holding the K shell electron is stronger.
The photons after this interaction are therefore of a higher energy.
130
Does tube voltage affect the photons emitted from charachteristic interactions?
It does in order for the electron energy to be over the K binding energy. However the energy of the photons will be changed based on the atomic number.
131
Intensity of the brehmstrauling radiation is proportional to
KV squared x mA (number of electrons)
132
efficiency of XR production =
XR output / electrical power supplied.
133
what is the electrical power supplied =
proporitonal to kV x mA
134
Intensity of XR production
proportional kV squared x mA
135
interaction of XR with matter - what are the three ways
Transmitted (unaffected), absorbed (lose all their eneregy) and scattered
136
what is half value thickness?
thickness to reduce by 50%
137
What is the relationship between LAC and HVL
LAC = 0.693 / HVL
138
How can you increase the LAC?
increase the density or the atomic number
139
Intensity of beam equation
I = I E to the pwoer of u d
140
why do we consider LAC with a thin beam?
the wide beam will have scatter that will give a false higher reading of radiation
141
Attenutation - low energy or high energy are attenuated more?
Low energy are attenuated more
142
Which causes is worse for image quality - high energy or low energy beam?
High ernegy - higher energy scatter gets through.
143
What are the types of attenuation?
Compton (inelastic)
Phtoelectric - TOTAL absorption
Elastic.
144
In what direction can compton scatter be in?
in all direction
145
Compton - what direction will the elctron go in?
sideways or forwards (not backwards)
146
probability of compton is proportional to what?
proportional to physical density (ie mass and electron density)
147
which has more energy - a forward scattered electron or a side scattered electron?
Forward scattered has more
148
relationship of compton scatter to density and energy
C = density / E
149
what is the equation for Photoelectric effect
PE = density x Zcubed / E cubed
150
describe an absorption edge?
as the energy increase the chances of PE decrease, however when it hits a new electron level the chances suddenly jump up as electrons can be hit from this level.
151
Which kind of effect, compton or PE, is affected by Z and energy of the photons?
PE is affected most by both.
Compton is NOT related to Z number (only electron density)
PE is more sensitive to energy as it is cubed
152
sift tusses are affected by compton or PE
compton
153
heavy stuff is affected by compton or PE
PE (lead ect)
154
the ionizing path of secondary radiation (within tissues) is what ratio
proprotional to 1 / density
155
when aluminium is used to filter a beam - what interaction is occurring?
Low energy photons are taken out by the metal through PE interaction.
156
How much aluminium is used?
2.5mm at 70kv
157
at diagnostic energies - absorbed dose and air kerma are
the same
158
how can you improve the quality / intensity of the XR beam?
INcrease the kv
increase mA
intensity proportional to Ma x kV square / Fsquared
159
which happens quicker phospholuminescence or fluorescence?
Fluoro
160
what is the usual angle of the anode?
7 - 20 degrees
161
what type of vapour pressure is desirable for the thermionic emission?
low
162
high kV will affect scatter how?
increase, more scatter closer to the film
163
mass attenuation coefficient is what
LAC / density
164
what are the units of mass attenuation?
cm squared / g
| basically LAC per unit of density
165
why is LAc better than MAC?
describes the depth of tissues rather than the mass
166
what is LAC?
the fractional reduction in intensity per unit length
167
units of LAC
mm -1 (per mm)
168
Total absorption happens in which?
Compton or Photoelectric?
169
in XR production - what is inherant filtration?
| How much is there?
XR passing the tube housing ect
| total filter desired is 2.5mm, inherant is 0.5 to 1mm
170
what is the tube rating?
a high tube rating means it can lose the heat generated efficiently
171
which type of badge uses a double emulsion film?
Film badges
172
Beta radiation can travel
a few mm's
173
Difference between kerma and absoroption
Kerma - energy transferred
Absoroption - energy absorbed.
.
174
WHich radiation ionizes directly?
XR and photons?
175
LET is what
amount of energy transferred in a distance
176
Relative Radiological Efectiveness is what
ratio of absorbed doses required to produce the same biological end point for two RADIATION types.
177
Linear no threshold theory exists for which?
Stochastic
178
radon decays giving out what
Alpha particles
179
Entrance surface dose can be measured with what?
TLD
180
Which badge has a linear response over a wide range
TLD
181
Which has better sensitivity - film or TLD
trick - both the same
182
most precise dosimeters
electric ones
183
In MRI the field strength causes what affect on the body
PNS from the gradient coils going on and off
184
RF pulse will cause what?
Heating
185
Which is shorter T2 or T2*
T2*, decays quicker due to inhomogeneities
186
In MRI - what are used to reduce breathing arefacts?
Saturation bands
187
frequency encoding is done in how many echoes?
one echo
188
should supervised areas have warning signs
yes
189
critical examination of installed equipement is usually done by
the installer but also the RPA can do this
190
Who should be contacted prior to equipement installation
The RPA
191
who chooses the designated areas?
RPA
192
Tube leakage from the housing should be what
less than 1 mGy/hr @ 1m
193
chance of fatal cancer from 1mSv
1 in 20 000
194
As photon energy increases what happens to attenuation
it decreases
195
photoelectric effect cause what kind of radiation?
characrteristic
196
what kind of interaction is an incident energy completely absorbed
photoelectric
197
describe an Auger electron?
charachteristic radiation is immediately absorbed in LOW atomic number material. This can result in another photoelectron being ejected. This is an auger electron
198
how does reducing the tube voltage affect side scatter and penetrating scatter?
increase side scatter
| reduce penetrating scatter
199
probaability of compton effect is proportional to what
the density of the material and the electron density
| NOT THE ATOMIC NUMBER
200
what is the atomic number of Tungesten and K shell binding energy?
74 atomic number
| K shell is 70
201
with XR filters, what is the relationship between the peak kV and the absorption edge.
The peak kV should be the right (or the high energy side) of the absorption edge.
202
the range of secondary electrons is inversely proportional to what
the material density
203
which has a higher atomic number copper or aluminium?
Copper - greater photoelectric absorption efficiency.
204
filtration on an xr beam will cause what to the minimum and effective photons?
increase
205
do photons have mass
no
206
what kind of filter for xr beam is used in mammography?
malibdium
207
how do you compensate for having a grid in radiography
photons get absorbed so need more mA across filament to produce more electrons and thereefore more photons.
208
what is the cost of having a large air gap?
though less noise, also less sharp image.
overcome this by smaller focal spot or bigger focus to object distance.
209
What does the CT number represent?
The average linear attenuation coefficient in the voxel
210
what is the HU of white matter?
20 - 30
211
Partial volume effect:
| thin high contrast object at an oblique angle will show as what
it will appear larger.
212
why is the anod cathode parallel to the z axis
minimise the anode heel effect
213
what are some features of 4th gen scanners?
larger focus to patient distance - means higher dose needed.
Detectors all the way round - more expensive.
214
In CT - detectors should have what properties?
fast response, negligible afterglow
| wide dynamic range
215
Solid state detectors are worse than ionization chambers for what one thing?
they are less sensitive.
| but smaller and more stable
216
what are the features of scanograms?
minimal scatter but poor resolution
217
Radionuclides
| why is it that the material from a cyclotron can easily be seperated?
They have different atomic numbers so different physical properites.
Protons are added
218
do isomers have the smae half life
no
219
what type of ernegy does beta decay emit?
a continuous spectrum - antineutrino and beta split the enregy in different amounts.
220
MRI: when is chemical shift used?
fat sat techniques an opposed phase imaging
221
gradient echo in MRI has what type of weighting?
T2*
222
MRI: what is the brightness of fat in different PD T1 adnT2
bright in PD and T1. Bright in T2 but not as bright as water.
223
what causes a low T2 signal?
melanin, calcification, fibrous tissue, high protein content and flow void.
224
phase encoding direction should be in what direction acorss the imaged object
the shortest
225
what is the ernst angle?
at this anlge get the miximum the magnetic resonance signal for a given tissue T1. (and at a given TR)
226
in MRI what is the acquisition matirx normlly?
64 x 64
227
In STIR - what else in addiiotn to fat is supressed?
Other tissues with similar T1.
- mucoid tissue, haemorrhage, protein rich fluid, gad
melanin (sometimes)
T1 times Mucoid, haemorrhage, protein, melanin.
228
motion artefacts in MRI occur in what direction?
the phase encoding direction
229
SNR i sproportional to what of the bandwith in MRI
inversely proportional to the quare root of bandwith
230
which gives a stronger signal spin echo or gradient?
spin echo. Gradient angle reduces the signal .
231
what thickness of aluminiium do filters need to be
2.5mm
232
At the anode - elastic interactoin happens if kV is less than
10eV
233
At the anode - inelastic interactoin happens in what three ways
heat
characteristic
brehamstrauling
234
Which of characteristic and brehmstrauling is the most?
Bremstrauling is 80%
235
what is meant by beam quality?
Ability to penetrate
236
Beam quanitty means what?
number of photons
237
What is the angle of deflection in Compton
between straight ahead and down to the angle of deflection.
238
Is compton coherant or incoherant?
incoherant
239
if high energy of photon to electron - what kind of angle of deflection of photon occurs
high angle of deflection
240
What is the compton attenuating coeeficient?
Prob of a compton reaction
241
What does compton rely on
density of material and electron density.
242
Apart from hydrogen, in every gram of material there are the same number of
free electrons
243
Photoelectric interact - what happens to the energy of the photon
completely absorbed
244
when does the auger electron get released?
Low Z, get complete absorption
245
equation for PE
p x Zcubed / energy cubed
246
LAC equation
0.693 / HVl
247
what is the atomic number of bone
13.8
248
what is the atomic number of soft tissues
7.4
249
LAC is combination of
PE and compton
250
What is the mass energy absorption coefficient MAC =
LAC / density
251
Benefit of MAC
removes all density, can compare number which relies more on Z
252
Beam quality equation
kVp + HVL
253
how does a wider beam affect scatter? and HVL?
higher scatter therefore a misread increased HVL as some of the scatter is registeredas the primary beam.
254
DQE using SNR equation
DQUE = ( SNR out / SNR in ) asquared
255
if spatial frequency increases what happens to the MTF
decreases
256
contrast equation
proprotional to (U1 - U2 ) x thickness
257
what impact does scatter have on image
reduces contrast
258
how to overcome scatter
large air gap or scatter grid
259
MTF of 1 means
range in the image is the same
260
MTF < 1
lower range in image
261
MTF = 0
No info in image
262
Noise relative to signal equation
1 / square root of Number of photons
263
Noise equation
square root of number of photons
264
SNR =
N / square root of N
also means just square root of Number of photons
265
What are the causes of unsharpness
Geometric
image receptor
movement
edge
266
Geometric unsharpenss happens how?
penumbra
267
Penumbra is proportional to what
XR focal spot size x Distance of object to film / distance from source to object
268
plain film radiography - magnification is what?
M = image size / object size = distacne to image from source / distance to object from source
269
what are units of MAC
cm squared / kg
270
how can we measure the size of the focal spot?
pin hole imaging
271
In fluoroscopy the output screen is what material ?
silver activated zinc cadmium sulphide
272
Where are dynodes used?
Electron multipliers?
273
Where are focusing electrodes used?
II
274
What is brightness gain?
ratio of output phosphr brightness to input phosphor brightness
275
What is the readout of CCD in Fluoro?
30 f/s
276
What is the normal tube current in fluro?
lower than others, 25Kv
277
ABC is picked up wher eon the image?
centre of the output screen
278
vignetting causes what?
magnification at the edges of the film
279
What happens to SNR after DSA?
SNA goes down as noise is added
280
How much is the lag in Image intensifiers?
1 ms
281
Which is better in fluoro - flate plate detectors or image intensifiers with regard to quantum efficieny?
both the same
282
Why does a grid increase dose in fluoro?
less signal so more dose is given to patient to ensure enough brightness
283
Which is smaller a photospot camera or a spot film?
photospot is smaller
284
Continous or pulsed gives better resolution in fluoro?
pulsed
285
What is the contributer to stray radiation?
scatter from the patient
286
Operator at 1m gets how much of the dose?
0.1%
287
Beta rays emit energies in WHAT
a continous spectrum of energy (due to antineutrino)
288
In US how big are the microbubbles?
1 - 10 micrometeres
289
doppler shift equation is what
F = 2vF cos0 / c
290
Us what is the equation of MI
peak rarefractional pressure / square root of frrequency
291
how can you increase the MI in US
lower the frequency or increase the rarefractional pressure.
292
What are the three types of pulsed doppler?
Colour
- give a box and direction
Spectral
- sonogram
Power
- good for low flow
293
Why are the edges of a fluro darker?
vignetting
- less focusing of electrons
- magnified at edges therefore darker image here also
294
what is the Fluro input energy?
1 miro Gray / second
295
How much should vignetting be kept below?
below 25%
296
in fluro - a single photon at the input causes how many electrons and then how many photons?
goes to 3000 electrons
| then 100 000 light photons at the output (green)
297
input phospher is made of what in fluro?
Caesium idodide
298
in fluro the focus to skin distance is usually
less than 45cm
299
With regards to MRI and the coils
| - Permanent coil offers what?
0.3T
300
Resistive offers what (MRI coil)
0.5T
301
What does the faraday cage do in MRI?
protects from external magnetic forces
302
What is the maximum T to staff limbs?
5T
303
in CT what is the minimum detectable contrast?
0.5%
304
Which has a higher CT number contrast or bone?
Contrast
305
What decides the size of the voxel?
The width of the XR beamm
306
in CT - what happens to quantum noise as you increase the FOV
the noise decreases
307
what is CTDI?
the amount of radiation hitting the phantom
308
CTDIw
accounts for a cylindrical person and beam hardnein.
Middle third
Outside two thirds
309
CTDIvolume
CTDIw / pitch
310
Dose legnth product
CTDI vol x ????
311
what is the atomic number and K edge of Iodine?
Atomic number is 53 and K edge is 33
312
How big is normal focal spot in radiography?
1mm
313
How many chambers does an AEC have?
3
314
critical angle of refraction in Ultrasound
when the refracted beam is parallel to the border
315
Ultrasound intensity and amplitude are proportional how?
I prop to amp squared
316
a low kV in film causes scatter to
decrease
317
what is the minimum dose needed for XR
3 microGray
318
in radiography - low kV vs high kV and their proportions to the output
in low kV - output is prop to kV CUBED (mammography)
in high kV - output is prop to kV squared
