Physics Flashcards
What does kV dictate?
Energy of each photon aka the quality of the Xray beam
What does mAs dictate?
of photons emitted aka the quantity of photons in the beam
In direct radiography, what components are replaced by the digital imaging sensor?
Cassette, film & screen
Describe how computed radiography works.
CR cassette contains photostimulable phosphor plate. Phospor absorbs Xray energy. Exposed cassette fed into reading device & laser releases stored energy in the form of light. Photomultiplier tube transform light into electrical signals which results in a digital picture based on the different light intensities.
T or F, film-screen radiographs have poorer spatial resolution than digital radiographs?
False
What is a negative of using a barium sulphate suspension to assess the esophagus?
Does not adhere well to mucosa
What contrast agent should be chosen if an esophageal perforation or bronchoesophageal fistula is suspected?
Non-ionic iodinated contrast media alone (no food/kibble, etc)
What are two potential negative S/E of ionic (hyperosmolar) iodinated contrast media?
Pulmonary edema (if aspirated)
Severe dehydration in an already volume-depleted animal
At what age does the thymus typically involute?
~6 months (most prominent ~ 4mos)
What are 3 radiographic changes that may occur to the cardiovascular structures & pulmonary parenchyma in an emaciated animal?
Hyperlucent lung fields
+/- visualization of the azygous vein
Microcardia
Wavelength of diagnostic X-rays is long or short?
Extremely short! (usually between 1 and 0.1 angstroms , angstrom = 10^-10 m)
Amount of energy carried by each photon depends on what?
The frequency of the radiation
Technically it’s freq x Plank’s constant ( E = hv)
What is the unit used to measure photon energy?
electron volt (eV)
What is the eV minimum to be considered ionizing radiation?
15 eV (gamma, X-rays, & some UV rays are all types of ionizing radiation)
What is the SI unit of radionuclide activity?
Becquerel (Bq)
What is the SI unit of absorbed dose of radiation?
Gray (Gy)
What is the comparable SI unit of a roentgen?
Coulomb/kg
Where are electrons produced?
Cathode (negative electrode/filament)
Electrons flow from _____ to _____.
Cathode –> anode
What does current refer to?
Refers to the NUMBER of electrons flowing per second from filament to target (measured in milliamperes, mA)
List 4 reasons why tungsten is used as an emitting material
1) High melting point
2) High atomic # (74) –> efficient X-ray producer
2) Little tendency to vaporize
3) Can be molded into thin, strong wire
What is the space charge effect?
Tendency to limit emission of electrons from filament.
(Space charge concept: electrons emitted from tungsten filament form a negative cloud around filament. This is called the space charge. The negative charge cloud prevents other electrons from being emitted until they acquire enough thermal energy to overcome the force caused by space charge)
What prevents the electron stream from spreading out via mutual repulsion/bombardment at the filament site?
Focusing cup
(Cup is designed so that it’s forces cause electron stream to coverage onto the anode in the required size and shape)
What is the focusing cup usually made of?
Nickel
A larger filament is generally used for _______?
Larger exposures
As anode angle becomes smaller, the effective focal spot size becomes _______.
Smaller
What limits the degree to which the anode angle can be changed?
Heel effect (point of anode cutoff)
Above 40kVP, further increases in kVp produce very little change in tube current. What is this principle called?
Saturation voltage
(Below 40kVp, the current flowing in the tube is limited by space charge effect)
Intensity of the X-ray beam depends on the angle at which the xrays are emitted from the focal spot. This variation is termed what?
Heel effect
During the heel effect, the intensity of the beam toward the _____side is larger.
Cathode
Because of the heel effect, __thicker or thinner__parts of the body should be placed toward the __anode or cathode____side of the X-ray tube
Thicker; cathode
What change to the focus film distance will make the heel effect less noticeable?
Larger distance
For equal target film distances, the heel effect will be less for ________ films.
Smaller (b/c xray beam intensity near the center is more uniform than towards periphery)
What is the max leakage radiation of a tube at a distance of 1m from the source?
Should NOT exceed 100mR (milliroentgens) in an hour
What is the source of off focus radiation?
Produced by X-ray tube when high speed electrons interact with metal surfaces other than the anode track.
Main source = electron backscatter from anode
(can hit anode a 2nd time and produce xrays)
What can be used to decrease off focus radiation?
Place a collimator or lead diaphragm as close to the X-ray tube as possible
What is the atomic number (Z)?
Number of protons in the nucleus
The total number of protons & neutrons in a nucleus is called what?
Mass number (A)
What is an isotope?
Same number of protons, but different number of neutrons (therefore different mass number)
What is the electron shell closest to the nucleus?
K shell (K shell binding force is greater than L)
What is the K shell binding energy of tungsten?
70 keV
In a single phase, fully rectified circuit, the voltage varies from 0 to what?
The maximum kVp value selected
(Applied voltage pulsates between lower & maximum values)
What are the two different processes by which Xrays are generated in the X-ray tube?
1) Bremsstrahlung aka general radiation (electrons interact with tungsten nucleus)
2) Characteristic radiation (electron collides with electrons in shell of target tungsten atoms
How is bremsstrahlung radiation produced?
“Braking” radiation. Electron attracted towards nucleus and is deflected from original direction which slows it down –> direct photon emission
What do the highest and lowest energy photons leaving the X-ray tube depend on?
Highest E photon dependent upon kVp used
Lowest E photon determined by the filter used (or by absorption of low E xrays by the tube envelope if no filter is used)
The energy of a photon of radiation is ______related to its wavelength?
Inversely (wavelength = 12.4 / kVp
What is an isomer?
molecule w/ same total A (mass #: # of protons & neutrons) but differing nuclear energy state
What is an isotope?
An isotope of an element has the same number of protons, but a different number of neutrons
Ratio of protons: neutrons defines what?
The stability of the element
What is the heaviest stable element and what is it’s atomic number?
Lead, Z = 82
What is the half life of Tc-99m?
6 hours
What is the equation for radionuclide decay?
A(t) = A(0) x e ^( - decay constant x t)
What are the 4 different types of magnetic properties?
1) Ferromagnetism = materials with a large positive magnetic susceptibility (e.g. iron, nickel, cobalt)
2) Paramagnetism = materials with some ions with unpaired electrons (e.g. ions of various metals such as iron, magnesium, gadolinium)
3) Super-paramagnetism = materials made of discrete individual domains of elements that when in bulk have ferromagnetic properties
4) Diamagnetism = materials that do not possess intrinsic atomic magnetic moment but if placed in a strong magnetic field slightly repel the field –> negative magnetic susceptibility (e.g. water & most biologic tissues)
What are the 4 things that determine the magnitude of the net magnetization vector?
1) Amplitude of the main magnetic field
2) Proton density of the sample
3) Gyromagnetic ratio of the element at hand
4) Temperature
What 2 things happen when an RF pulse is applied….
1) More E into system -> protons move from spin up to spin down orientation -> DECREASED longitudinal magnetization component
2) Precession motions of protons become more in phase -> INCREASED transverse magnetization component
The net value of the flip angle depends on what 3 factors?
1) Gyromagnetic ratio
2) Magnitude of the rotating magnetic field B(1) [RF pulse]
3) Time that B(1) is applied for
The longer the RF pulse is applied for, the ________the flip angle.
Larger
In order to maintain minimal image acquisition times in practice stronger/weaker pulses are used to increase/decrease flip angles?
Stronger
Increase
Longitudinal T1 relaxation is also called what?
Spin lattice relaxation -relies on E exchange between protons & environmental molecules
Transverse T2 relaxation is also called what?
Spin-spin relaxation
-relaxation is due to interactions between protons due to different molecular environments they’re in (protons dephasing)
The rate of regrowth of M(z) is called what?
T1 longitudinal relaxation time
-time it takes for longitudinal relaxation to reach 63% of its original amplitude
What is the rate of M(xy) decay called?
T2 transverse relaxation time
-time it takes for the transverse magnetization to decrease by 63%
Which is shorter in biologic tissues, T1 or T2?
T2 (typically about 10x shorter)
-the shorter the T2 the faster the transverse magnetization will disappear
T1 tends to be shorter or longer in fluids vs solids and fatty vs non-fatty tissues
- T1 longer in fluids > solids (high collisional freq of water molecules»_space; than protons Larmor freq) …….hence why with Inc water content (e.g. brain edema) T1 becomes DARKER
-T1 shorter in fatty > non-fatty
Describe the relationship between T1 relaxation time with respect to the strength of the magnetic field.
As B(0) [the magnetic field] increases so does the Larmor frequency of protons in that field —> increases differences btw Larmor freq & Brownian collisional freq —> increased T1 relaxation times
Describe how T2 transverse relaxation can become longer?
When there’s high mobility of small molecules in tissues the fast motion averages out local magnetic field inhomogenities -> more homogeneous magnetic field —> decreased spin spin interactions = longer T2 relaxation
What is the amplitude of a newly created M(xy2) have less amplitude that the initial one at t=0 in spin echo sequences?
Because the 180 degree pulse only canceled out the fixed magnetic field inhomogenities (T2*) but NOT the intrinsic spin-spin interactions
What is the difference between matrix size and FOV?
Matrix size = the # of lines & columns in the image
FOV = the actual dimensions in height & width of the image frame obtained
The number of lines x the number of columns in the image = what?
of voxels in the slice being imaged
If matrix size increases, what is the subsequent change to the individual pixels & spatial resolution?
Inc matrix size = Inc number of lines & columns —> decreased pixel size —> increased spatial resolution
What determines the volume of an individual voxel?
Pixel Surface area x slice thickness
When decomposing a spatial domain signal into its frequency components, what is true of the information gained from low frequencies & high frequencies?
Low frequencies = general shape & gross variations in signal intensity aka general shape & contrast
High frequencies = edge/sharp details aka detail & spatial resolution
What is the relationship between transmission bandwidth of the RF pulse & slice thickness?
The wider the bandwidth (shallow gradient slope) the thicker the slice
Describe the concept of frequency encoding (FE) gradient.
Precession frequencies are varied in the FE direction(e.g. x-axis).
- the collected MR signal contains the sum of these frequencies which is sorted out using the Fourier transform
