Equation for magnification
SID / SOD
What color resembles greater optical density, white or black?
What about greater relative exposure?
Black for both
Equation for transmittance
T = I / I0
Equation relating optical density to transmittance
OD = -log10T
T = 10-OD
What does the reciprocity law state and when is it not true?
Relationship between exposure and OD should remain constant regardless of exposure rate.
Fails at long and short exposure times. OD is not constant
On the Hunter and Driffield curve, what represents the film contrast?
Steepness (average gradient) of the linear region.
Makes sense, for a change of exposure, there should be a greater change in OD
What regions of the H&D curve have poor contract (small slopes on the curve)
Toe and Shoulder region
What is the usuable range of the H&D curve? What is the range of x-ray exposures within this usuable range?
Usuable range: Area where exposure and OD are linearly related
Latitude gives the exposure range
Fast films require __________ exposure to achieve a given OD
(more or less)
Faster SF systems result in lower patient dose, but in general exhibit more ____________
By how much does using film-sceen versus film only reduce the radiation dose to the patient?
By up to 50x!
Much more x-rays are detected. Resolution is degraded though
What is the conversion efficiency of a phosphor?
Fraction of absorbed energy emitted as UV or visible light
Difference between Conversion efficiency (CE) and absorprtion efficiency (QDE)
CE - ability of screen to convert energy deposited by absorbed x-rays into film darkening
QDE - how efficiency the screen detects the x-ray photons incident upon it
Main difference between direct and indirect digital film
Direct: signal comes straight from the beam x-rays
Indirect: there is an intermediate conversion step
Which of these are untrue about digital
Wider dynamic range than screen film
Reduces repeat rates
Higher patient throughput but lower dose efficiency
How many bits are in a byte?
Equation for bit depth
where n is the number of bits
What is the difference between the H&D curve of digital detectors vs analog?
Digital holds the linearity much longer than analog
Remember, resolution is always defined by the bigger dimension.
Example, you have 1000 lp
for a 24 x 30 cm plate
resolution = 1000 lp/ 300 mm = 3.3 lp/mm
What is the process in which a laser scans a imaging plate and PSP processes data?
Which of the following is not a posisble cause of blurring at the borders of high contrast objects?
Large focal spot
Trick question, they all are
What is the major concern of optical coupling?
Optical coupling: occurs from a gap between scintillator and focusing lens to detector
Large loss of light!
What is the fill factor of a thin-film transistor
Light sensitive area / detector element area
Which gives better absorption, thick or thin screen?
What about better resolution?
Better absorption - Thick screen
Better resolution - Thin screen
If 2 mGy of dose is delivered to the surface, approximately how much is delivered to center of chest? What about posterior end?
0.22 mGy to center
0.02 mGy to other side
What is the relationship between output kerma and typical x-ray tube voltages?
Roughly linear. Maybe slightly exponential
Dose at depth d in a patient given entrance skin dose, SSD and attenuation coefficient
Dd = ESD e-µd (SSD/(SSD+d))2