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
typical entrance skin exposure in mammo
1000 mR
advtantages of screen-film in mammo
-high resolution
-inexpensive
cons of screen-film in mammo
-limited latitude
-film granularity contributes to nouse
-processing
-can’t post process
digital has ___ latitude compared to screen film
wide
digital mammo targets and filters
-tungsten target
-Rh or Ag filter
advantages of digital mammo
-improved latitude
-use window and level to improve contrast
-processing analysis and software
-digital storage, can be transmitted for 2nd opinions
-higher kVp = shorter exposure time
-no film processing
disdadvantages of digital mammo
-5-10 lp/mm vs 20 for film
-(but digital has better contrast resolution)
-4X price of screen film
per clinical trials, which is better, screen film or digital mammo?
the same
how do CR plates work?
-absorbed xray energy is stored
-imaging plate is exposed and read
-scanned by laser beam, which causes stored energy to be released as light
-light is collected by light guide and sent to PMT
-image plate can be reused by exposing it to bright light
how do charged coupled devices work?
-has discrete pixel electronics
-when visible light falls on the pixels, electrons are liberated and stored in each pixel and read out later
-light released by intensifying screen is focused onto CCD surface by fiber optic tapers
how do indirect detection flat panel detectors work?
-CsI converts xrays to light, which is then detected by flat panel detector
-detector elements (photodiodes) build up charge when exposed to light
-charge is held by capacitor and can be read out by electronics
how do direct amorphous selenium detectors work?
-when xrays stroke the selenium layer, electrons are released and they travel to the detector elements under direction of electric field
-no blurring- path of electrons is controlled by electric field
pros and cons of flat panel detectors in mammo
-at lower spatial frequency, flat panels have a higher DQE than CR plate and screen-film
-expensive
tomosynthesis in mammo
-no grid
-taken at different angles
-helps resolve overlapping tissues
-low dose exposures
-needs high DQE selenium detector
-images acquired from different angles separate structures at differing heights
-reconstruct image using backprojection
stereo breast biopsy
-use 2 xray images at angles to get depth of lesion
-needle targets coordinates
dual energy contrast enhanced digital mammo
-images are acquired pre and post contrast
-exploit differences in iodine u below and above iodine k-edge
-low energy- 26-30 keV
-high energy- 45 keV
-images are subtracted by weighted log subtraction
mammo image quality QA tests
-artifact evaluation
-HVL evaluation
-resolution evaluation
-dose evaluation
-image quality evaluation
-SNR and CNR evaluation
HVL requirement for mammo
-HVL > kVp/100 + some constant
constant 0.03-0.3 mm depending on filter
how to do dose evaluation in mammo
-Use ACR mammo phantom
-stimulates xray attenuation of a 4.2 cm compressed breast (50% glandular, 50% adipose)
-measure ESE
-use conversion tables to get Dg
-Dg< 3 mGy
image quality evaluation in mammo
-use ACR phantom
-must see at least 4 largest fibres, 3 largest speck groups, and 3 largest masses
MRI for breast imaging
-breast MRI and mammo recommended for women at risk
-MRI more useful for detecting invasive breast cancer but less so for microcalcifications
Is US useful in breast cancer screening?
no, because it cannot detect microcalcifications
it can detected:
-distinguish solid lump from cyst
-guide biopsy needles
-detect metastases in lymph nodes
what is US useful for?
-imaging soft tissues that are radiologically similar
US- what converts electrical signal into mechanical vibration?
transducer
US typical pulse duration and pulse repetition time
-duration = 1 us
-repetition = 1 ms
propagation of US through homogeneous medium vs through heterogeneous medium
homogeneous= attenuation
heterogeneous = reflection or refraction
what info does time span of echo give?
-info about depth/distance of boundary
what info does amplitude of echo give?
-info about degree of physical difference between the 2 media
sounds velocity in tissue, air, bone
tissue: 1540 m/s
air: 330 m/s
bone: 4100 m/s
equation for frequency bw vs pulse duration in US
delta f * delta t = 1/(2pi)
velocity of sound in a medium
c = lambda f
decibel
dB = 10log10(I2/I1)
-3dB = 0.5
+ 3 dB = 2
dB/cm loss
4.343 * u
frequency dependence of rate of attenuation in US
dB/cm = dB/CM at 1 MHz * f
for muscle, blood, soft tissue
dB/cm = dB/cm at 1 MHz * f^2
for water, bone
in US, what do incident, reflected, and refracted waves have in common?
frequency
Snell’s law
(lamda1/lambda2) = c1/c2
sin(theta1)/sin(theta2) = c1/c2
-if c1=c2, no refraction
-if theta1=theta2, no refraction
US fraction of energy reflected (R) and transmitted (T)
R = [(z1/cos(thetat) - z1/cos(thetai))/((z2/cost(thetat)+z1/cost(thetai))] ^2
T = 1-R
Z = density * c
what is transducer made from?
piezoelectric crystal
-crystal with no bulk polarization, but with local zones of polarization
US axial resolution
axial resolution = N * lambda
-N = # of complete cycles of wavelength lambda contained in a single pulse
US lateral resolution
-lateral resolution = lambda * F /(2a)
F= focal length
2a = aprerture of transducer
DOF in US
-defines volume for which the beam provides beam echoes from boundaries and max lateral resolution
-within DOF, beam intensity is -3dB (or 50%) of max intensity or greater
trade-off between resolution and penetration in US
-axial and lateral resolution are improved by increasing frequency
-but dB/cm is proportional to f or f^2
-penetration thus decreases with increasing f
optimal transducer material
-would have impedance = skin impedance so less energy is reflected at transucer-skin interface
-the more energy is reflected at the interface, the less energy is available for producing echoes
-reflection increases with differences between acoustic impedances
impedance matching gel
helps to match impedance of transducer to that of kin
Zmatch = square root (ZtransducerZskin)
natural resonance frequecy in US
-Q value
–frequency at which electrical energy is most efficiently converted into mechanical energy
freson= Ct/2t
Ct= velocity of sound in crystal
t= thickness of crystal
characteristic frequency in US
-tau
-time it takes for crystal to decay from resonant state to unexcited state
-want short tau so that after excitation, crystal can quickly detect echoes
-tau = Q/(2pifres)
-tau can be decreases with electronic or mechanical damping
fresnel zone in US
-near zone
-zone over which beam retains the cross-sectional area of the crystal
fraunhoffer zone
-far zone
-beam diversges with phi= arcsin(1.2lambd/D)
D= diameter of transducer
3 measures to quantify intensity of US beam
It= W/A = time-averaged power/effective area of transducer surface
Tspta = energy flow through area through time, at pt of peak intensity within beam, as average over long period- spatial peak temporal averaged
Isppa= intensity at pt of peak instensity within beam as average over single pulse- spatial peak pulsed average
focused transducers in US
-improves lateral resolution, beam penetration, and echo strength
-use curved piezoelectric crystal- get constructive interference at a pt
-or use flat crystal with focused acoustic lens
-drawback is that focal pt is fixed…
phased array crystal in US
-array of crystals that can be excited independently
-by exciting crystals with certain time delays, depth focusing (increasing or decreasing focal length) and steering (moving focal pt laterally) can be achieved
scattering vs reflection in US dependence on object dimension (d)
-d»> lambda- get strong echoes
-d«_space;lambda- get scattering (speckle)
-d~lambda- get graininess- adds structure to the texture
A-mode
-fast
-only shows depth at which boundaries occur along single line in medium
-no image
-depth of echo = C * echo time /2
M-mode
-only single line through medium
-image rapid movement
-stationary boundaries make horizontal lines
-moving boundaries make periodic oscillations
-
B-mode
2D image in real time
-intensity of pixel is proportional to echo intensity received corresponding to that pixel
US echo signal processing
-don’t want attenuation effect, what is of value is ratio of reflected to transmitted
-echoes at deeper regions produce weaker intensity due to attentuation and reflection at boundary
-echo may be many orders of magnitude smaller than transmitted pulse and therefore echo signals are subjected to log amplification instead of linear amplification
time gain compensation
in US, compensates for attenuation effects by amplifying echoes by amt. proportional o time since the pulse transmission
-log amplification and time gain compensation reduce the effects of attenuation, but cannot improve the loss of signal due to noise from echoes from deep structures
noise rejection electronics
-can be used to remove weaker signals from real echoes
-risk rejecting weak echoes
doppler US
-measure flow rates for vessels
-frequency shift in echo results from interaction of pulse with a moving structure
-not like M-mode, which measures motion of large organs
formula for doppler US
delta f / ftrans = +/- 2 (vblood/c) * cos(theta)
theta = angle between beam direction and that of flow
+2 if towards transducer, minus if away
continuous vs pulsed US beams for doppler US
-both can be used
-simpler to demodulate a continuous beam (composed of a single frequency), than to demodulate a pulse containing many frequencies
sound waves cannot…
travel in vaccuum
frequency of sound waves
infrasouns < 15 Hz
audible 15Hz-20 kHz
ultra sound > 20 kHz
US - 0.8- 15 MHz
to get final attenuation in US…
add up dBs
what is Z in US
z = density * c
“Rayl”
acoustic impedance
does everyone but patient wear Pb apron in fluoro?
yes
x-ray equipment must abide by what?
FDA
radiation emitting devices act
how to determine CT slice thickness for helical scanning
measure FWHM of bead senstivity profile as functiun of z position
how do you QA the function of AEC?
by measuring resulting OD
dose limit for techs and students training on DI equipment
1 mSv/year
max leakage radiation from x-ray tube
1 mGy/h at 1 m from focal spot
how to measure CT uniformity
subtract CT number at middle from that of periphery
-should be within 2 HU of baseline and baseline should be within 5 HU
shielding calcs for CT
NCRP 147
secondary only
radiation symbols on CT equipment
xrays-attention-rayonx
-trefoil symbol or xray tube in upside down triangle
