Miscellaneous imaging II Flashcards
how does scatter reduce Contrast?
contrast = (P2-P1)/P
now with scatter, signal is P+ scatter
So contrast = (P2-P1)/(P+S)
bucky factor
(patient entrance exposre with grid)/(patient entrance exposure without grid)
As kVp increases, bucky factor decreases
grid ratio
(height of grid)/(width of holes in grid)
grid selectivity
ratio of primary transmission to scatter transmission of grid
contrast improvement factor of grid
(contrast achieved with grid)/(contrast achieved without grid)
dark-field xray imaging
-makes scatter useful
-get infor about scattering power of specimen
QA tests in safety code 35
-reproducibility- 10 exposures, small coefficient of variation
-AEC: vary kVp and thickness and ensure that OD varies less than limits
-linearity- |X1-X2|</=0.1(X1+X2)
-spectrum: min value for HVL at each energy
-max exposure rate for fluoro equipment
breast tissues
glandular- makes milk
-fatty tissue
-fibrous tissue- provides support
calcifications in breast
tiny mineral dposits
-small white regions on film
microcalcification
sign of ductal carcinoma
macrocalcification
-usually benign
breast masses
can occur with or without calcifications
-cells clustered together with greater density than other tissues
non-invasive (in situ) breast cancer
-ductal (milk ducts)
-lobula (milk making glands)
invasive breast cNCER
-invasive ductal carcinoma starts in ducts and spreads
energy used in mammo
10-15 keV
-get highest differences between glandular and infiltrating ductal carcinoma
anode for mammo
Mo
what does image quality and radiation dose in mammo depend on?
- xray spectrum: anode material, operating kV, filtration
- anti-scatter technique
- image receptor
limiting factor for detecting microcalcifications
signal difference to noise ratio
what impacts signal difference to noise ratio?
-decreases as Energy increases
-decreases with thicker breasts (more attenuation)
why ground the anode?
-reduces off-focus radiation which just adds dose to patient for nothing
where is cathode in mammo?
at chest wall, to account for heel effect
characteristics of modern mammo
-cathode is at chest wall to account for heel effect
-collimation to give shape (cuts off beam at chest edge)
-compression paddle
-detector is under the screen-film (normally for xrays it is on top). This is because low energy photons in mammo would cast a shadow on the image (high E photons pass through)
focal spot size in mammo
0.3 mm and 0.1 mm
space charge effect
-causes non-linear relationship between filament current and tube current
characteristic radiation of Mo vs Rh
Mo: 17.5 and 19.6 keV
Rh: 20.2 and 22.7 keV
use Rh for thicker patients
SID in mammo
65 cm
how does focal spot change in mammo?
focal spot gets smaller away from chest wall
target/filter combos in mammo
Mo/Mo
Rh/Rh
Mo/Rh
No Rh/Mo
Mo/Mo and Rh/Rh eliminate all energies above characterisitc
Mo/Rh lets in a little bit higher energy than characteristic
what does scatter in mammo depend on?
-scatter increases with increasing breast thickness and breast area, constant with kVp
-reduce scatter by:
-antiscatter grid
-air gaps
-compression
how is mammo grid different than typical xray grid?
-4:1 ratio
-interspace material is carbon fiber not lead (have lower E photons)
-grid frequencies are 30-50 lines/cm for moving grids and 80 lines/cm for stationary
when is grid used in mammo?
contact mammo
why use moving grid?
grid moves while you acquire so you don’t get image of grid lines in image
how does compression help in mammo?
-reduce background (easier to see lesions)
-fewer scattered xrays (better contrast)
-reduces motion
-reduces attenuation (lower dose to breast tissues)
why use magnification in mammo?
-can gain more resolution because you spread over more pixels (resolution increases by magnification factor)
-results in more blur, but can compensate by using smaller focal spot
-if you use smaller focal spot, magnification gets you better MTF
-increase SNR
-reduce scattered radiation and can do without anti-scatter grid
cons of magnification in mammo
-small focal spot limits tube current, and extends exposure times
-therefore get more motion blurring
are parrallax and crossover issues in mammo?
no, because only use 1 screen (not 2 with film sandwiched)
resolution of screen-film system
15-20 lp/mm
intrinsic efficiency 15 %
green light 545 nm
where is AEC in mammo?
under cassette (unlike normal xray)
what does AEC do?
maintains constant mean OD of each mammogram independent of breast thickness, breast density, and selected exposure parameters
mammography quality standards act
monitor dose due to mammo
entrance skin exposure in mammo
500-1000 mR for 4.5 cm breast
HVL in mammo
0.3-0.5 mm Al for 25-30 kVp 1 cm in tissue
how does breast thickness affect entrance skin exposure
if kVp is constant, 1 cm increase in thickness requires double the mAs and entrance skin dose is thus doubled
Dg factor in mammo
Dg = Dgn X entrance skin dose
Dgn converts ESE to Dg
Dgn depends on breast composition, thciness, anode, HVL, kVp, determined by computer
Dgn decreaes as thickness increases because the beam is attenuated more and glandular tissues gets less dose. This is compensated by increase in ESE to get OD..
MSQA annular dose limits
-avg glandular dose limited to 3 mGy/film for compresses breast thickness of 4.2 cm (50% glandular and 50% adipose tissue(
what factors affect breast dose in mammo?
As kVp increases,
-beam penetrability increases
-ESE and Dg decrease
-inherent subject contrast decreases
-Dgn increases
As breast thickness increases,
-dose increases
-ESE increases
-Dgn decreases
effect of thickness and energy on Dgn
as thickness increases, Dgn decreaes
As energy increases, Dgn increases
typical Dgn in mammo
0.2 rad/R