Physics Flashcards
what factors affect image resolution?
- signal to noise ratio (increases with # of photons– ie ^ mAs)
- kVp (contast slightly decreases with increased kVp–higher energy photons in beam, more penetrating, less photoelectric effect)
- size of lesion (sharper reconstruction kernel will increase contrast in small lesions, BUT will increase noise)
- increasing slice thickness
how does harmonic US work
The transducer sends a lower frequency pulse, but only receives the 1st order harmonic (double the incident frequency) echoes. This leads to:
increased lateral resolution
reduction of side lobe artifacts and reverberation artifacts caused by tissues close to the transducer.
Electron binding energy is what
amount of energy needed to remove electron completely from the atom
(also called ionization potential)
measured in eV (1 eV = 1.6 x 10-19 J)
directly proportional to the atomic number (Z)
indirectly proportional to the distance from nucleus, i.e. inner-shell electrons have greater binding energy than outer-shell electrons
An electron can only be removed from an atom if the applied energy is greater than its electron binding energy. When an inner-shell electron is ejected, the vacancy will be filled by an electron from an outer shell. The excess energy from this shift is emitted as electromagnetic radiation.
Coherent/Raleigh scatter
unmodified, classic, elastic scatter
Upon interacting with the attenuating medium, photon does not have enough energy to liberate the electron from its bound state (i.e. the photon energy is well below the binding energy of the electron) so no energy transfer occurs. No energy deposition, thus no dose from coherent scattering.
Film screen systems
create image by? (exposure)
Dose required?
image exposed by light photons created from x rays
decrease required dose but at expense of spatial resolution (light exposes larger area of the film per photon)
syringe shields
- for radiopharmaceuticals
- Fall under principle of ALARA
- syringe and vial must be labeled with radiopharmaceutical, also shield unless can see labels on the vial/syringe
inverse square law?
I ~ 1/d2
When X rays enter the patient tissue, what interactions are possible?
- Coherent scatter (doesn’t account for many interactions in Dx Rads, happens at lower energy radiation)
- Compton scatter
- Photoelectric absorption (what we want)
kVp?
Kilovoltage peak
voltage applied across x ray tube (cathode–> anode)
determines penetration of x ray beam and affects image contrast by determining maximum energy of x ray photons produced (ie applying 60 kV would result in xray spectrum with max energy 60 keV
if kVp is 100, what is the average energy (keV) of emitted photons in the x ray beam
~33-45 keV
(on average, 1/3-1/2 the energy of the kVp)
What happens at higher kVp/higher keV?
Radiography: Dose ∝ kVp2
CT: Dose ∝ kVp2.6
greater intensity of X ray beam–> more rays penetrate patient –> giving LESS contrast between densities
(this is why mammo uses lower kVp–need better contrast between tissue densities but photons can travel less far before being attenuated by tissue)
doubling kVp QUADruples dose
what is the photoelectric effect(aka photoelectric absorption/PEA)?
one of the 3 forms of photon interaction in tissues
a low energy photon interacts with an electron in the atom and removes it from its shell
Most likely to occur when energy of the incident photon is equal to or just greater than the binding energy of the electron in its shell (k edge) and the electron is tightly bound (as in K shell)
electron that is removed is called a photoelectron and incident photon is completely absorbed. Hence, the photoelectric effect contributes to the attenuation of the x-ray beam as it passes through matter/tissue
To stabilize the atom an outer shell electron fills the vacancy in the inner shell. The energy which is lost by this electron as it drops to the inner shell is emitted as characteristic radiation (an x-ray photon) or as an Auger electron
why is lower kVp better for contrast?
and higher kVp bad for contrast?
more photoelectric effect
higher kVp–> more compton scatter
mA/s?
milli-amperes (/second)
the current applied across the xray tube mA X exposure time s
determines # of electrons produced at the cathode per second
thus high mA increases number of photons(radiation) produced
directly proportional to dose (dose ~ mA)
if you increase kVp, what will x ray machine do automatically?
decrease mA
- so you produce fewer electrons–> producing fewer photons–> BUT they will be higher intensity/more penetrating
- decreases dose to patient
- decreases contrast in image
what is absorbed dose?
the amount of radiation absorbed per unit mass of tissue
(not all rays will be absorbed bc scatter)
was measured in rads, now measured in Gy
1 Gy = 1 J/kg
(~3mGy average annual background dose, ~5Gy acutely pretty much fatal)
Equivalent dose?
not all x rays have same degree of negative effect on our cells
Alpha particles + neutrons more damaging than x rays, gamma rays, electrons
multiply absorbed dose by weighting factor WR
In diagnostic rads, WR is 1, measured in Sv
1 Sv= 1 J/kg = 100mrem
what is effective dose
some tissues receive higher dose than others
accounting for stochastic effects in different organs
weighting factor WT for each tissue, all of these sum to 1 (ie if you radiate the whole body)
measured in Sv
effective dose=equivalent dose if you radiate entire body
**effective dose in CT is DLP x WT for imaged area: eg WT for head is 0.0015, for abdomen = 0.0021
Rad? Rem? Gy? Sv?
relationships?
Rad= absorbed dose, now measured in Gy
100 rad = 1 Gy
Rem= equivalent dose, now measured in Sv
100 rem = 1 Sv
X ray production occurs in the tube, how?
Characteristic and bremstrahhlung
interaction with K-shell electron: causes the production of characteristic radiation
interaction with nucleus: causes bremsstrahlung radiation
interaction with outer shell electrons: causes line spectrum
**less than 1% of the colliding electron energy on the anode creates x rays–>99% dissipates as heat (this is why we use materials with high melting point like tungsten)
what is Compton scatter?
one of the 3 forms of photon interaction
interaction of the photon (x-ray or gamma) with free electrons (unattached to atoms) or loosely bound valence shell (outer shell) electrons. The resultant incident photon is scattered (changes direction) and imparts energy to the electron (recoil electron)
scattered photon has a different wavelength and thus a different energy (E=hc/λ)
Depends on:
- directly proportional to number of outer shell electrons, i.e. the electron density, physical density of the material
- inversely proportional to photon energy
- does not depend on atomic number (unlike photoelectric effect and pair production)
Becomes dominant process in human tissues at 30 keV to 30 meV (doses used in dx and therapeutic rads)
what does the photoelectric effect depend on/is it related to?
- atomic # of attenuating tissue (Z)
- energy of the incident photon (E)
- physical density of attenuating tissue (p)
- Z3 p/E3
if Z doubles, PEA ^ by a factor of 8, and if E doubles, PEA decreases by a factor of 8, meaning small changes in Z or E have big effects on PEA
(this is why lead which has a high Z is a good shield)
Finite size of x ray focal spot gives rise to?
Penumbra (geographic blurring)
–> this is why we use a smaller focal spot in magnification images
space charge effect?
produced electrons in a cloud as they are emitted from the cathode will tend to repel each other and this leads to divergence
Anode angle and anode heel effect
X rays produced on the focal spot of the anode only (small, rectangular)–
Most have anode angle 12-15*
Smaller anode angle= smaller focal spot (more heat generated)
Heel effect: because of the beveled surface, the rays that must travel farther in and back out of the anode//target material to reach the exit window and contribute to the beam will have lower energy (and their emission angles are more perpendicular to the electron beam). Also there will be fewer of them in general as more are absorbed by target material, which leads to higher intensity of beam at cathode end-
- anode angle: by increasing the angle, the amount of target material perpendicular to the anode is decreased resulting in less resorption of x-rays produced.
- target-to-film distance: increase in distance reduces heel effect by allowing more divergence of the beam which produces a more uniform image.
- field size: the field will be more uniform at the center (i.e. smaller field size) due to the collimator absorbing the peripheral variations.
- positioning: by aligning higher attenuating material towards the cathode and lower attenuating material towards the anode the resulting field is more uniform
what is x ray filtration?
the process of preferentially removing low energy x-ray photons from a polychromatic radiation beam (which otherwise really would only contribute to skin dose (ESD))
inherent–within the tube itself, like the housing etc
added– metal sheets placed in x ray beam path before it reaches the patient that attenuate lower energy photons. specific metals used for different energy absorption levels (k edge)–beryllium in mammo, aluminum/copper/tin in XR
- reduces x-ray intensity (quantity) and quality (shape of beam spectrum), but not the maximum energy of the x-ray beam spectrum. The change in the shape of beam spectrum with filtration is referred to as beam hardening.
What is off focus radiation?
photons that originate outside the focal spot
form of scatter which can cause blurring
largely shielded by design of x ray tube itself:
metal housing of the tube absorbs many
outside the tube, the metal collimator shapes the x ray field and
What is the line focus principle?
beveling/angulation of anode surface helps to create a smaller apparent focal spot size
–increasing anode angle will result in a larger apparent focal spot size and vice versa
what considerations are made when determining kVp?
- need enough photons at a high enough energy to get to the detector to make an image (exposure)
- larger body parts/obese patients need higher kV - at higher energies, more compton scatter (which is less variable between tissues than PEA) and more photons will penetrate the patient completely (both decreasing contrast)
- dose: When other factors left alone, dose increases with increasing kVp by a factor of 2 for XR and 2.6 for CT (decreasing mAs is by a 5th power relation which is more than enough to balance at the expense of lower contrast)
how much of an increase in kVp will double the film density/exposure?
15% (the 15% rule)
equivalent to doubling the mAs
What is pair production?
photons with energy at least 1.022meV interact with the nucleus and create matter from energy –1 positron and 1 electron
(mass of electron/positron expressed in energy units is 511 keV (0.511 meV)); if higher energy photon, they share the excess energy as kinetic energy
electron is quickly absorbed, but positron creates two new photons when it comes to rest and interacts with a nearby electron -(annihilation reaction)
advantage of bucky grids?
moving grids / Potter-Bucky or reciprocating grids)
eliminate the fine grid lines that may appear on the image when focused or parallel grids are used; cannot be used for portable films
focused grids (most grids): strips are slightly angled so that they focus in space so must be used at specified focal distances
parallel grid: used for short fields or long distances
what is the bucky factor? (1)
Contrast improvement factor? (2)
the ratio of radiation on the grid to the transmitted radiation.
It indicates the increase in patient dose due to the use of a grid. It is typically two to six.
the ratio between the contrast with a grid and without a grid.
it is typically two. Image contrast can be improved by increasing the grid ratio via increasing the height of the lead strips or reducing the interspace. However, this leads to increased x-ray tube loading and radiation exposure to the patient
what is a grid ratio?
describes working ability of a grid
Higher grid ratio= better image contrast at the expense of patient dose
ratio of the height of the lead strips (h) to the distance between two strips, i.e. the interspace (D). Grid ratio of 8:1 is generally used for 70-90 kVp technique and 12:1 is used for >90 kVp technique. The strip line density (number of strips per cm) is 1/(D+d), where d is the thickness of the strip. This is typically 20-60 strips per cm.
what does an intensifying screen do?
- intensify the effect of the x-ray photon by producing a larger number of light photons.
- decreases the mAs required to produce a particular density and hence decreases the patient dose significantly.
- also reduces motion blur and x-ray tube loading by reducing exposure time
color of the light emitted (wavelength) must match the light sensitivity of the film used. This is known as spectral matching: conventional films: sensitive to ultraviolet and blue lights, orthochromatic films: sensitive to ultraviolet, blue and green lights
what is this and what effect does this have on noise and image quality?
decreased source to object distance (SOD)
also decreases SID (source to image distance) –SID = SOD + ODD
Detector and patient are closer to the source = patient receives higher dose of radiation, image noise is reduced
practical applications?
ie what will increase unsharpness
- larger focal spot
- patient farther from detector (increased ODD-also decreases dose)
- focal spot closer to patient (decreased SOD)
Left- more blur with decreased SOD Middle- less blur with increased SOD Right theoretical, no blur with point source as focal spot
applications?
Patient farther from detector = more magnification
Focal spot closer to patient = more magnification
E.g. If SOD is 1 meter, and ODD is 2 meters, then SID must be 3 meters, so 3/1 = 3x mag
image production with Computed Radiography?
infrequently used now
A storage phosphor housed in the image cassette absorbs the incident x-ray energy and converts it into metastable energy state that can exist for several days
Shooting a red laser beam at the plate frees the trapped energy (electrons) from the metastable state, resulting in emission of blue-green light
A detector (photomultiplier or photodiode) measures the blue-green light to make an image
Digital radiography
Charged-Coupled Detectors (not used as much anymore)
Flat Panel Thin Film Transistor (TFT) Detectors
What is fogging artifact?
Addition of unintended charge to the detector from something other than the original x-ray beam, causing increased “blackening” of the exposed portion of the image
ie: patient with implanted radioactive seeds to treat a lung tumor in the right lower lung results in extra radiation exposure to the detector in this area causing fogging
What is over penetration? underpenetration?
Penetration is determined by kVp (energy of the x-rays)
Underpenetration is due to kVp which is too low –densest structures are attenuated and will not be visible (e.g. bones)
Overpenetration is due to kVp which is too high – least dense structures will be too black, obscuring detail (e.g. lungs)
Higher kVp = more penetration (photons can get through denser structures)
What causes improper exposure?
Exposure is determined by mAs (how many x-rays reach the detector)
Underexposure is due to low mAs = image looks too white on film cassettes and demonstrates quantum mottle on digital detectors
Quantum mottle is a noisy appearance due to underexposure (not enough “data” reaches the detector)
Overexposure is due to high mAs = image looks too black on film cassettes but can look normal on digital detectors
Digital detectors automatically normalize the images, thus overexposures 5-10 times normal will still appear properly exposed
Higher mAs = more exposure (more photons get through all structures)
how to differentiate underpenetrated from underexposed x ray?
both appear ‘whiter’
Differentiate from penetration by looking at the relative visibility of different structures (e.g. a chest x-ray will appear whiter if underpenetrated or underexposed, but the vertebral bodies will not be visible in an underpenetrated chest x-ray, and will be visible in an underexposed x-ray)
–underexposed will also have quantum mottle from too little photons in general
overexposed image (on Film-screen system)
Won’t see on Digital system as it is corrected for
What factors decrease patient radiation exposure in fluoro?
- using pulsed fluoroscopy with low frame rates
- minimizing electronic magnification, collimating the beam
- minimizing fluoroscopy time
- shortening the distance between the patient and the image receptor (image intensifier or flat panel detector)
Reduce peak skin dose by using multiple gantry angles and maximizing the distance between the patient and the x-ray source
Dose limits for workers, pregnant and non-pregnant
Non-pregnant workers: 5 rem
Pregnant workers: 500mrem (0.5rem)
What does Nuclear Regulatory Commission (NRC) regulation 10 CFR Part 20 deal with
Standards for protection against radiation
10 CFR Part 20 is titled “Standards for Protection Against Radiation,” and is a key regulation for defining radiation dose limits, general regulations for the use of radioactive materials, and defining radiation use areas.
to decrease dose to patient during fluoro, do what?
position them close to the II/image receptor
far from the detector
use pulsed fluoroscopy when possible
increase collimation
minimize time
Advantages of flat panel detectors over film screen?
higher quantum detection efficiency (although lower spatial resolution)
may directly convert x-rays into electric charge or indirectly via a scintillator that converts detected x-rays into visible light, then to a charge through a charge-coupled device (CCD) or photodiode
What is exposure and it’s SI units?
quantity of charge in the air due to ionization from x- or gamma rays
measured in C/kg in SI units (Roentgen historically)
Threshold dose for permanent sterility ?
Males vs females>
6Gy in males
3 Gy in females
per oocyte, LD50 is <2Gy (understood cause for radiation induced infertility)
Sterility more often occurs in older women with fewer oocytes
What increases SPR?
(scatter to primary ratio)
This is the ratio of energy deposited on a detector by SCATTERED vs NONSCATTERED photons-is SPR of 1 means 50% of energy is from scattered photons and useless
increased with: increasing thickness of patient
increased FOV
decreased air gap
NOT using a grid
