Patient Interactions
• Photoelectric
• Classic Coherent
Scatter
• Compton
Scattering
• Pair Production
• Photodisintegration
Interaction in
The body begin at
the atomic level
Atoms
Molecules
Cells
Tissues
Organ structures
X-ray photons can change
cells
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 xrays 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.)
CASCADE
Photoelectric – Absorption
PHOTOELECTRIC ABSORBTION
IN THE PATIENT
(CASCADE OF ELECTRONS)
• PHOTOELECTRIC
ABSORBTION
IS WHAT GIVES US
THE CONTRAST
ON THE FILM
CLASSICAL SCATTER IN
PATIENT
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 (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
Coherent / Classical Scatter
Classic Coherent Scatter
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.)
Compton Scatter
COMPTON
SCATTERING –
OUTER SHELL
ELECTRON IN
BODY –
INTERACTS
WITH
X-RAY PHOTON
FROM TUBE
(WAVY LINE IN = PHOTON
MUST BE INTERACTION IN THE BODY)
During Fluoro – the patient is the
largest scattering object
XXXXX
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.
Pair Production
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.)
Photodisintegration
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.)
Remember….
When reviewing diagrams
What is coming in (e or photon?
Where is it occurring (the tube or body?)
Keep practicing – you will get it