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 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.)

CASCADE

Photoelectric – Absorption

PHOTOELECTRIC ABSORBTION

IN THE PATIENT

(CASCADE OF ELECTRONS)

• PHOTOELECTRIC

ABSORBTION

IS WHAT GIVES US

THE CONTRAST

ON THE FILM

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

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