Week 10 Flashcards
Gold standard for detecting breast lesions
Mammography
purpose of angled tube head
to counter anode heel effect as cathode aligned over chest wall & anode over nipple
purpose of C-arm setup
gantry rotation
what is the FDD
65-66 cm
why is FDD fixed
fixed to balance low patient dose while ensuring high film dose; higher FDD = longer exposure & motion unsharpness, lower FDD = high patient dose
only one type of exam done
compression force used
100 – 150 N (never greater than 200N)
importance of compression
- reducing overlapping anatomy and tissue thickness
- less scatter, geometric blurring, motion, and dose
- increasing contrast and visualisation of tissues
what do screen film mammo use for AEC
photo timers instead behind cassette to prevent over/under exposure
what does the phototimer use
- radiation sensor
- amplifier
- voltage comparator
what happens if there is insufficient dose to trigger comparator switch
backup timer terminates exposure; Retake requires higher energy beam to be selected for greater beam penetration
digital mammo AEC
Voltage and current adjusted based on thickness and composition of breasts
AEC modes of operation
auto time, auto kVp, full automatic
auto time
kVp, target/filter chosen by operator
auto kVp
kVp based on breast thickness
full automatic
kVp, target/filter chosen by unit
anode design for mammo
rotating anode
what does SFM use for anode
dual track Mo/Rh, sometimes with tungsten
why is Mo/Rh mainly used
their characteristics x-ray production near optimum energy for mammo
common dual tracks
Mo/Rh & Mo/W
benefit of dual track anode
More anode material choices
limits of dual track anode
- less reliable
- more expensive
- lower maximum anode heat loading than single tracks
what does DM use for anode
single track Tungsten
when is Mo used for DM
small to avg breast thickness
when is RH & W used for DM
larger breast thickness
benefit of single track
Can deliver high current exposure (2-3x compared to dual track) needed for thicker breasts at appropriate exposure time = reduce motion artifacts
what filters do DM use
W/Rh & W/Al filters
what is the voltage used for mammo
22 - 40 kVp
what is the filament used for mammo
Dual filaments in focusing cup (0.3 mm for contact, 0.1 mm for magnification)
what is the FSS used for mammo
Small FSS to minimize geometric blurring & spatial resolution
what is tube current used for mammo
o 100 mA +/- 25 mA for large 0.3 mm FSS
o 25 mA +/- 10 mA for small 0.1 mm FSS
purpose of anode angulation
changing FSS; tilting used to reduce FSS and mitigate anode heel effect
mammo vs conventional = housing
mammo = metal tube
conventional = glass envelope
mammo vs conventional = anode material
mammo = Grounded Mo, Rh anode
conventional = Tungsten anode
mammo vs conventional = anode angle
mammo = 0 degrees but tube tilted 24 degrees [minimizes effective FSS]
conventional = 7 – 16 degrees
mammo vs conventional = axis of rotation
mammo = Vertical
conventional = Horizontal
mammo vs conventional = filter & function
mammo = Mo/Rh for spectral shaping
conventional = Aluminium for dose reduction
what is needed for high contrast sensitivity
low photon energy
ideal spectrum for mammo
photons of same energy (mono energetic x-rays of 15 – 25 keV) with adjustable energy for different breast conditions
ideal spectrum for mammo achieved via
- target material
- selected filter
- kVp range
how does selected filter achieve ideal spectrum for mammo
Helps remove unnecessary high & low energy bremsstrahlung x-rays to allow transmission of characteristic x-rays and lower patient dose
why is Beryllium used for x-ray tube window
allows transmission of lower energy photons and minimizes radiation absorption in tube
how does low kVp achieve ideal spectrum for mammo
minimize Compton scattering [reduce dose to breast as glandular tissue is radiosensitive]
maximize photoelectric effect for better differential absorption of various tissues in breast
HVL factors
- kVp,
- compression paddle thickness,
- added tube filtration,
- target material
- tube age
HVL increases with __ kVp and __ atomic number targets and filters
higher; higher
HVL dependent on what in breast tissue
strongly dependent on tissue composition (glandular, fibrous, adipose)
how diff is mammo x-ray generator to conventional x-ray generator
Generating power rating is 3 kW
Voltage (22 – 40 kVp)
AEC circuit different
High frequency generators at 6 KHz
where does scatter radiation plateau at
between 25 – 35 kVp
why is scatter radiation bad
Degrades contrast and detail which increases with increased thickness & area of breasts but constant between 25 – 35 kVp
Without scatter rejection, only___ of subject contrast detected
50 – 70%
why is stationary grid not used
compromise image quality as grid may cover tiny details (microcalcifications)
why are oscillating grids used
grid lines eliminated by grid motion