Patient Interactions Photoelectric Classic Coherent Scatter Compton Scattering Pair Production...
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Transcript of Patient Interactions Photoelectric Classic Coherent Scatter Compton Scattering Pair Production...
Patient Interactions
• Photoelectric
• Classic Coherent Scatter
• Compton Scattering
• Pair Production
• Photodisintegration
Some radiations are energetic enough to rearrange atoms in materials through which
they pass, and can therefore he hazardous to living tissue.
1913
EM Interactions with Matter
• General interactions with matter include– scatter (w or w/o partial absorption)– absorption (full attenuation)
Interactions of X-rays with matter
• No interaction: X-ray passes completely and get to film
• Complete absorption: no x-rays get to film
• Partial absorption with scatter
Photoelectric effect• Low energy (low kVp) x-ray photon ejects inner
shell electron (energy absorbed)
• Leaving an orbital vacancy. As vacancy is filled a photon is produced
• More likely to occur in absorbers of high atomic number (eg, bone, positive contrast media)
• Contributes significantly to patient dose,
• As all the photon energy is absorbed by the patient (and for the latter reason, is responsible for the production of short-scale contrast).
FIG. 9–3 Photoelectric absorption interaction.
(Modified from Carlton RC, Adler AM: Principles of radiographic imaging, an art and a science, ed 4, Thomson Delmar Learning, 2006, Albany, NY. Reprinted with permission of Delmar Learning, a division of Thomson Learning: http://www.thomsonrights.com.
Fax 800-730-2215.)
8 p+ + 8e- = neutral atom
INCOMING PHOTONS FROM TUBE
Pass by the ELECTRONS IN THE PATIENT
Do not interact with e–
Causes them to VIBRATE – RELEASING SMALL AMOUNTS OF HEAT
CLASSICAL SCATTER IN PATIENT
Classical (Coherent) ScatteringClassical (Coherent) Scattering
Excitation of the total complement of atomic electrons occurs as a result of interaction with the incident photon
No ionization takes place Electrons in shells “vibrate” Small heat is released The photon is scattered in
different directions Energies below 10K keV
FIG. 9–2 Classic coherent scatter interaction.
(Modified from Carlton RC, Adler AM: Principles of radiographic imaging, an art and a science, ed 4, Thomson Delmar Learning, 2006, Albany, NY. Reprinted with permission of Delmar Learning, a division of Thomson Learning: http://www.thomsonrights.com.
Fax 800-730-2215.)
Compton scatter• High energy (high kVp) x-ray photon ejects an
outer shell electron. • Energy is divided between scattered photon and
the compton electron (ejected e-)• Scattered photon has sufficient energy to exit
body. • Since the scattered photon exits the body, it
does not pose a radiation hazard to the patient. • Can increase film fog (reduces contrast)• Radiation hazard to personnel
FIG. 9–4 Compton scatter interaction.
(Modified from Carlton RC, Adler AM: Principles of radiographic imaging, an art and a science, ed 4, Thomson Delmar Learning, 2006, Albany, NY. Reprinted with permission of Delmar Learning, a division of Thomson Learning: http://www.thomsonrights.com.
Fax 800-730-2215.)
Differential Absorbtion
• Results from the differences between xrays being abosorbed and those transmitted to the image receptor
– Compton Scattering– Photoelectric Effect – X-rays transmitted with no interaction
Compton and Differential Absorbtion
• Provides no useful info to the image
• Produces image fog, a generalized dulling of the image by optical densities not representing diagnostic information
• At high energies
Photoelectric and Differential Absorbtion
• Provides diagnostic information
• X-rays do not reach film because they are absorbed
• Low energies (more differential absorbtion)
• Gives us the contrast on our image
No interactions with Image Receptor and Differential
Absorbtion
• No interaction
• Usually high kVp
• Goes through body
• Hits image receptor
• Usually represents areas of radiolucency (low atomic numbers)
• Results in dark areas on the film
• The probability of radiation interaction is a function of tissue electron density, tissue thickness, and X-ray energy (kVp).
• Dense material like bone and contrast dye attenuates more X-rays from the beam than less dense material (muscle, fat, air).
• The differential rate of attenuation provides the contrast necessary to form an image.
FIG. 9–5 Pair production interaction.
(Modified from Carlton RC, Adler AM: Principles of radiographic imaging, an art and a science, ed 4, Thomson Delmar Learning, 2006, Albany, NY. Reprinted with permission of Delmar Learning, a division of Thomson Learning: http://www.thomsonrights.com.
Fax 800-730-2215.)
FIG. 9–6 Photodisintegration interaction.
(Modified from Carlton RC, Adler AM: Principles of radiographic imaging, an art and a science, ed 4, Thomson Delmar Learning, 2006, Albany, NY. Reprinted with permission of Delmar Learning, a division of Thomson Learning: http://www.thomsonrights.com.
Fax 800-730-2215.)