Large metallic objects such as spinal rods can also markedly attenuate the beam. Such attenuation can lead to 10— fold increases in the intensity of the incident X-ray beam in order to have sufficient photons reaching the detector. The impact of patient thickness on X-ray penetration is dramatic. Small increases in tissue thickness cause large increases in the intensity of the incident X-ray beam. Again this is necessary to have sufficient photons reaching the detector to create useful images.
Since X-rays produced by fluoroscopy units contain photons with a wide range of energies, the lower energy photons tend to be absorbed or scattered by the patient and higher energy photons are more likely to reach the detector. Indeed, since low energy photons are unlikely to reach the detector, aluminum and copper filters are used to preferentially remove them from the beam before they reach the patient.
An interactive simulation that models the attenuation of a polychromatic beam is available [1]. The procedure timeline and event log reveal three separate DSA runs, each approximately 65—75 mGy. The photon then scatters in a different direction with a bit less energy, and the free electron goes about 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 within the patient.
Each time a photon ejects an electron - ionizes an atom - this creates free radicals that can damage DNA and wreak havoc. The energy that a tissue absorbs from photon interactions is referred to as dose. For a discussion of different measures of dose, see the CT Dose section or the briefer discussion in the Radiography section. For this discussion, just remember that dose is energy per kilogram of tissue - and that more of it is bad.
Also, know that another word for dose is kerma kinetic energy released in matter. X-ray beam attenuation. 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 energy photons travel through tissue more easily than low-energy photons i. As E gets larger, the likelihood of interaction drops rapidly. Compton scattering is about constant for different energies although it slowly decreases at higher energies.
We'll discuss the means by which these effects generate tissue contrast later, but just realize here that they are responsible for the different absorption of photons at different energies. Beam hardening. As I mentioned, the low energy x-rays are stopped much more quickly than the high-energy photons. Thus, what comes out of the tissue is mostly the high energy photons see the simulation, left panel.
This is referred to as beam hardening 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. Energy deposition. As the beam is attenuated - and thus deposits energy dose in the patient - the amount of energy left is less, and the average energy is higher so fewer interactions occur.
Both of these effects result in decreasing energy deposition from the skin to the back of the patient. In other words, much of the dose is deposited near the skin. This is why the deterministic effects we are worried about relate to skin injuries the other reason being that the skin is a particularly sensitive organ. Larger patients, who require higher doses to penetrate through the larger body parts, are at higher risk of skin injuries. As you can see from the Beer-Lambert equation, the amount of photons passing through an object drops off rapidly exponentially.
This is illustrated in the middle panel of the simulation. The thicker the patient or body part, the fewer photons escape to the other side. We need a certain number of photons to produce an image. Back in the days of film, you needed a certain exposure to develop a particular type of film this was referred to as the film's ISO. While digital detectors have a much higher dynamic range just like your digital camera, they can operate at basically any ISO , there is still a minimum number of photons necessary to create a useful image.
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 tracking one 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 interacting with 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 that our image is brighter than the noise of fluctuations.
It turns out that in x-rays,. For a more detailed discussion, see the section on signal-to-noise in fluoroscopy. We have already discussed the fact that high-energy photons pass through tissue with less attenuation. While these photons create more damage when they do hit electrons in the tissue, it turns out that this is a relatively small effect. Thus, overall changing an x-ray beam to a higher energy will deposit less dose to the patient.
In addition, because more photons are passing through, there are more photons to make our image - so it will be less noisy. We can either keep those increases and enjoy our pretty picture, or, more commonly and charitably, we can actually drop the number of photons we send through the patient. Radiography and fluoroscopy systems have so-called phototimers that shut off the beam after they get enough photons for the desired noise settings.
Note that x-ray tubes have a maximum mA because they can overheat. The denser the tissue, the more X-rays are attenuated. For example, X-rays are attenuated more by bone than by lung tissue. Contrast within the overall image depends on differences in both the density of structures in the body and the thickness of those structures.
0コメント