scatter radiation in digital ..
A fast scatter field estimator for Digital Breast Tomosynthesis
Mammography - Massachusetts General Hospital, …
As the x-ray beam passes through tissue, photons get absorbed so there is less energy; this is known as attenuation. It turns out that higher energyphotons travel through tissue more easily than low-energy photons (i.e. the higher energy photons are less likely to interact with matter). Much of this effectis related to the photoelectric effect; the probability of photoelectric absorption is approximately proportional to (Z/E)3, where Z is the atomicnumber of the tissue atom and E is the photon energy. As E gets larger, the likelihood of interaction drops rapidly. Compton scattering is about constantfor different energies although it slowly decreases at higher energies. We'll discuss the means by which these effects generate tissue contrast later, butjust realize here that they are responsible for the different absorption of photons at different energies.
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This simulator illustrates the effects of changing tube voltage in fluoroscopy or radiography on patient dose and image contrast. You can also alter the body part thickness. Air kerma (Kar) reflects the change in photon flux necessary to obtain the sameexposure (image noise). Dose is the dose to the patient for the same image noise; note that this doseis a first-order approximation and does not take into account secondary scatter events. The x-ray tube is a (very) gross approximationof a tungsten anode with 2.5 mm Al filtration.
The Abington-Jefferson Advantage
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As we mentioned, far fewer photons will make it through a thicker patient - exponentially so. Therefore, we have to sendmore photons through in the first place (the phototimer will just wait). More photons means more likelihood to hit the tissue and thus more dose. Also, largerpatients have more tissue for scattered photons to hit a second or third time, increasing dose. See how increasing the kV would really help in large patients -we have fewer tissue interaction events and better penetration, so we can use fewer photons and have less dose. As we will see below, though, it's not a freelunch - you have to trade-off between kV and image contrast. Play with the simulation to see how dose changes with kV and patient thickness.
Why do we need a certain number of photons to make an image? Realize that each photon being produced is a random event (we're not trackingone photon that goes into the patient to see if it comes out the other side - we just see how many photons come out). The possibility of a photon interactingwith the tissue and scattering is random as well. Thus, our image is actually made up of millions of random events. In any random process, there are fluctuations(that's why it's random!). We need a large enough number of photons to be sure of what our picture looks like - in other words, we need enough photons so thatour image is brighter than the of fluctuations. It turns out that in x-rays,
Digital radiography: The bottom line comparison of CR …
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In addition to the low kV, we'd like a more efficient way of producing low-keV x-rays. Traditionally, this was done by using a molybdenum instead of tungstenanode. In order to understand why, let's go back to how x-rays are . One component is the Bremsstrahlung radiation, which dependssimply on the tube voltage. The other component, which is more efficient, is the characteristic radiation. Characteristic x-rays represent those produced whenan electron from the cathode knocks out an electron in an anode atom; another electron from a higher-energy orbital takes its place. This new electron releasesan x-ray with the energy of the difference in binding energies of the two orbitals.
As I mentioned, the low energy x-rays are stopped much more quickly than the high-energy photons. Thus, what comes out of the tissueis mostly the high energy photons (see the simulation, left panel). This is referred to as (a soft beam has mostly low energies).The amount of radiation that passes through tissue is given by the Beer-Lambert law, which says that
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Computed Tomography — An Increasing Source of Radiation Exposure
Digital Breast Tomosynthesis QC Phantom - CIRS 021
Mammography - Wikipedia
The CIRS Digital Breast Tomosynthesis QC Phantom is designed to ..
It covers the relationship between patient size, attenuation, kV, and dose
Digital Breast Tomosynthesis QC ..
The phantom consists of eight homogeneous slabs made from breast-equivalent material in 50/50 ratio of gland and adipose tissue (BR50/50). Optional swirled slabs, that can be purchased separately, are comprised of 100% adipose and 100% gland tissues together in an approximate 50/50 ratio by weight. Three homogeneous slabs include imaging targets. Each semicircular shaped slab measures 100 x 180 x 10 mm. This allows for using a combination of homogeneous and swirled slabs to test the influence of scatter radiation on image quality in more clinically-relevant, inhomogeneous conditions.
Digital Mammography Machine - Mammomat …
(2012)A fast scatter field estimator for Digital Breast Tomosynthesis In: Conference on Medical Imaging - Physics of Medical Imaging, 2012-02-05 - 2012-02-08, San Diego, CA.
ASSESS BREAST TOMOSYNTHESIS IMAGE ..
Left: X-ray spectra; Tube beam spectrum. transmitted beam spectrum (scaled to have the same intensity).Middle: X-ray intensity passing through the patient (white = strongest radiation, black = weakest).Right: Simulated image contrast with fat, muscle, and iodinated contrast in a vessel. This contrast does not take into account the effects of scatter.
Dedicated breast radiation imaging ..
Again, without going into the gory detail here, we need to know that there are two major ways in which diagnostic x-raysinteract with tissue. The first is the photoelectric effect, where a photon uses up all of its energy to eject an electron from an atom; while the electron willmove around and ionize neighboring atoms, there are no scatter photons. The second major effect is Compton (incoherent) scatter, where a photon hits an atom andionizes an electron but does not use up all of its energy. The photon then scatters in a different direction with a bit less energy, and the free electron goesabout doing damage. Scattered photons can travel back towards the tube, pass through the patient and hit the detector from any odd angle, or scatter again withinthe patient.
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