Physics Flashcards
Increasing which factors would increase radiation dose to patient? (3)
- mA
- kVp
- scan length (anatomic coverage)
Increasing what factor would decrease radiation dose to patient?
pitch
What is the effect of increasing screen thickness in film-screen system?
Increased chance of xray interacting with the screen
Increasing kVp from 100 to 140 on CT abd will have what effect?
- increase radiation dose
- decrease image noise
What is beam hardening?
xray beam passes thru matter –> low energy photons are removed from xray beam
What effect does beam hardening have on xray beam?
higher effective energy
What will lessen fluoroscopic radiation dose to a patient, in terms of distance of xray source and image receptor?
- xray source: far from patient
- image receptor: close
Fluoroscopy –> does electronic magnification have any effect of radiation dose to patient?
Yes –> increase radiation
Annual whole-body occupational effective dose limit?
5 rem (50 mSv)
Max dose/yr (whole gestation) for pregnant worker?
0.5 rem (5 mSv)
What is the threshold for cataracts caused from an acute exposure to the eye (onset > 20 years)
0.5 Gy (50 rads)
how often is constancy checked on a radioisotope dose calibrator?
daily
White I radioactive label –> maximum permissible surface exposure rate per hour?
0.5 mrem
What is the minimum luminance of grayscale monitors required by the American College of Radiology?
50 foot-lamberts (171 cd/m^2)
medical imaging –> avg annual radiation exposure –> how did it change from 1980s to 2006?
increased 6x
- what is DI?
- DI of 0 indicates what?
- positive DI indicates what?
- negative?
deviation index: regarding whether appropriate exposure used
0: intended exposure was used
positive: overexposed
negative: underexposed
what is mAs?
milliampere-seconds: amt of radiation per second
what is air kerma?
xray photons per unit area
how do mAs and air kerma relate to each other? ie. if mAs is increased x2, what happens to air kerma?
linearly proportional
mAs increase 2x –> air kerma increase 2x
- what is SID?
- SOD?
SID: source to image distance
SOD: source to object distance
formula for x-ray magnification?
M = SID/SOD
what is calibration phantom?
what is it for?
cylinder or plate –> contain densities of known value –> used in quality control to ensure images are reconstructing density to correct values
quality control –> calibration phantom –> needs to be done how often?
wkly
velocity encoded value –> low –> what kind of artifact may occur?
aliasing
principal reason that iodine contrast increases blood’s ability to attenuate xrays?
higher atomic # –> increase photoelectric interactions
trauma –> pregnant F –> CT chest –> fetus –> radiation exposure –> primarily derived from what type of radiation?
internal scatter
fxal MRI utilitizes what kind of imaging technique?
blood oxygen level dependent (BOLD)
increase skin to image receptor distance –> entrance skin dose –> increase/decrease? why?
increase
xray source –> need to increase radiation output to maintain constant exposure to image receptor to maintain same image quality
what physical property allows contrast media to provide enhanced contrast at certain xray energies?
K-absorption edge
what is K-absorption edge?
property of a material that provides preferential absoprtion of xrays for energies near the K-shell energy –> provides increased contrast where the media is located
area under the curve (AUC) value from receiver operator characteristic (ROC) curve = 0.5 –> indicates what?
inputs are random guesses
what is minimum inherent threshold energy required for positron emission decay process to occur?
1.022 MeV
PET/CT –> typical uptake time?
60 min
US liver –> acoustic radiation force impulse (ARFI) –> hepatic fibrosis or stiffness estimated by what?
speed of the shear wave
radioisotope dose calibrator –> quality control –> accuracy –> how often?
annual
radioisotope dose calibrator –> quality control –> linearity –> how often?
quarter
control room –> MRI zone?
3
MRI aliasing (wrap-around artifact) –> how fix?
- increase FOV
- increase velocity encoded value
annual occupational dose limit for eye?
0.5 Gy
calcium –> has what magnetic property?
diamagnetic
diamagnetic material –> magnetic field properties?
induced internal magnetic field –> opposes external field –> weakly repulsed
what substances are diamagnetic? (2)
- water
- calcium
paramagnetic material –> magnetic field properties?
internal field –> enhance local magnetic field –> weakly attracted
what substances are paramagnetic? (2)
- gadolinium at body temp
- deoxyhemoglobin
ferromagnetic material –> magnetic field properties?
internal field –> strongly augment external magnetic field –> strongly attracted
what substances are ferromagnetic? (2)
- iron
- cobalt
superparamagnetic material –> magnetic field properties?
- strongly augment external magnetic field
- but not magnetic when removed from external field
what are isotones?
atoms w same #neutrons; diff # protons
100 kVp x-ray beam –> avg energy (KeV)?
1/3-1/2 of kVp
Compared to standard contact imaging, what may compromise image quality of magnification views in mammography
increased motion blur
The digital radiography system that employs direct conversion of x-rays into a signal (electrons) –> what substance?
Amorphous selenium – TFT flat panel array
A CT scan is performed at 120 kV and 100 mAs. A region-of-interest measurement from the CSF in the ventricles yields an average CT number of 10 with a standard deviation of 8. The scan is repeated with 400 mAs. What is the expected CT number and standard deviation of the CSF?
CT number of 10 with a standard deviation of 4
In what direction do chemical shift artifacts manifest in MRI
frequency encoded
Which radionuclide would require the use of a medium energy collimator to image a patient on a gamma camera ?
Ga-67
A chief advantage of digital radiography systems over screen film systems is
wider exposure latitude
acquisition time for a standard spin - echo pulse sequence –> formula?
TR x # phase encode steps x # averages
What is the annual permissible effective dose equivalent for the general public in the United States
1 mSv (0.1 rem)
During the preimplantation stage of pregnancy, what is the most likely adverse effect due to a radiation exposure of 200 mGy ( 20 rads)?
embryonic death
What is the average annual natural background radiation level in the United States?
3.1 mSv/yr
Portable radiographs taken with a film - screen system utilizing a fixed radiographic grid tend to have less contrast than radiographs taken in radiography rooms because of what?
Use of lower grid ratio radiographic grid
A computed radiography image with a 10-bit pixel depth will have how many possible shades of gray?
1024
Dynamic range refers to the ratio of the highest to the lowest amplitude displayed on the screen in decibels –> T/F?
T
What is the latent image in computed radiography (CR)?
Electrons trapped in semi-stable energy wells
US –> axial resolution is better than lateral resolution –> T/F?
T
Concerning the inverse square law in chest fluoroscopy, how much does the entrance dose change if the source - to - image distance (SID) is increased from 90 cm to 120 cm while maintaining a similar source - to - skin distance (SSD) –> inc or dec? by a factor of how much?
Increases by a factor of approximately 1.8
Concerning ultrasound interaction with matter, an ultrasound beam reflected from small (2 mm) kidney stones would a non-specular reflection –> T/F?
T
MRI –> which sequence generates the highest signal intensity in a conventional spin - echo pulse sequence?
proton density
