Physics Flashcards
Target (anode) material
-molybdenum (Mo)
-rhodium (Rh)
-tungsten (W)
Filter material is inserted into xray beam to
Enhance contrast
Filters are most ofter made of
-molybdenum (Mo)
-rhodium (Rh)
-silver (Ag)
-aluminum (Al)
Filter type will be selected by machine based on breast thickness
Exit window of tube housing made of
Beryllium
Roles of added filtration
1) removes soft energy photons which add dose
2) removes high energy photons which reduces contrast
Grids
-used to absorb scatter and improve contrast
-absorb 75-85% of scatter
Grid ratio in mammography
3:1 to 5:1
Linear/focused grid
-reduce scatter from 1 direction
-interspace material is carbon
Honeycomb/HTC grid
-reduces scatter in 2 directions
-better contrast
-interspace material is air
How far will light field come?
All the way to chest wall and not exceed any edge by more than 2%
Radiation shield must have the equivalent attenuation of at least
0.08 mm lead at 35 kVp
Line focus principle
The steeper the angle the smaller the effective focal spot and the better the detail
Large focal spot measures
0.3 mm and is used for standard imaging
Small focal spot measures
0.1 mm and is used for magnification imaging
The anode acts as a what
A filter; called self-filtration
The xrays towards the cathode end of tube have
-More intensity (less of anode to travel through)
-dose is more at cathode end
The xrays toward the anode side are
-less intense (more of the anode to travel and more absorbed)
-less dose at anode end
Half field geometry
Lead is used to block “half” of the beam from and prevent it from going into lungs
kVp
-controls wavelength of xray beam
-penetrating power
-influences contrast and exposure latitude
-average range in mammo is 25-35
mAs
-will control xray beam intensity
-used to control image blackening
Typical mA selection
20-100 mA
Typical time in sections
0.4 to over 1 second
Backup timers
-600 mAs for grid techniques
-300 mAs for non-grid work
What are the two types of radiation produced in mammo xray spectrum?
1) characteristic radiation
2) bremsstrahlung radiation
Characteristic radiation
-2 energy spikes (determined by anode material)
-important for determining xray beam penetration and giving optimal constant
Bremsstrahlung radiation
-xray produced with multiple energies (keV)
-det. by selected kVp
Anode material molybdenum (characteristic radiation)
-produces the characteristic rad ,2 strikes, optimal to penetrate breast
-moly anode prod. char. rad. spikes of 17.6 kaV & 19.7 keV
Anode material rhodium (characteristic radiation)
-Rh prod. char. rad. spike of 20.3 and 22.7 keV
Molybdenum filter (bremsstrahlung radiation)
-Mo’s k-edge is 20 keV
-it will filter (slow down) xrays with energies above 20 keV, which decreases contrast
Rhodium filter (bremsstrahlung radiation)
-Rh has k-edge of 23 keV
-it will filter (slow down) xrays with energies about 23 and get them closer down to 23
-good for thicker breast
Pt dose with and without grid (one view)
-with grid: 3mGy
-w/out grid: 1mGy
What is average breast made of?
-50% glandular 50% fat
-compress to 4.2 cm
Average SID in mammography machine
65-66 cm
Magnification increases possibility of unsharpness, so what do you used to help gain detail?
Small focal spot
Common magnification factors
1.5x or 2x, some units offer 1.8x
