RAD 114 RADIATION
PROTECTION
X-RAY INTERACTIONS
WITH MATTER
X-RAY PRODUCTION
z ELECTRON
BOMBARDMENT OF
TUNGSTEN OR RHENIUMTUNGSTEN ALLOY ANODE
z KILOVOLTAGE (KVP)
DETERMINES ENERGY OF
MOST ENERGETIC PHOTON
z X-RAY BEAM IS
HETEROGENEOUS
1
X-RAY ATTENUATION
THE RESULT OF
ABSORPTION OR WIDEANGLE SCATTERING OF
THE PRIMARY BEAM
(PHOTONS) AS IT PASSES
THROUGH THE PATIENT
2
X-RAY ATTENUATION
THE PHOTONS WHICH DO
NOT REACH THE FILM
ARE SAID TO BE
ATTENUATED. THOSE
PHOTONS THAT REACH
THE FILM ARE CALLED
EXIT OR IMAGEFORMATION PHOTONS
ATTENUATED PHOTONS
DEPENDING ON THE ENERGY OF
THE PRIMARY PHOTONS,
ABSORBED PHOTONS WILL
INTERACT WITH THE ATOMS OF
THE ABSORBING TISSUES IN A
NUMBER OF POSSIBLE WAYS
ATTENUATED PHOTONS
3
COHERENT SCATTERING
A PHOTON WITH AN ENERGY OF
BETWEEN 1 AND 50 KEV MAY BE
ABSORBED BY AN ATOM. THE
RESULTING INTERACTION
CAUSES THE ELECTRONS TO
VIBRATE, CREATING
ELECTROMAGNETIC RADIATION
OF THE SAME ENERGY AS THE
INCIDENT PHOTON.
COHERENT SCATTERING
THE RESULTING PHOTON MAY
EXHIBIT A SLIGHT CHANGE OF
DIRECTION FROM THE INCIDENT
PHOTON, HENCE IT IS A
SCATTERED PHOTON.
COHERENT SCATTER IS ALSO
CALLED RAYLEIGH SCATTERING
AND CAN CONTRIBUTE TO
RADIOGRAPHIC FOG
25 KeV X-ray
25 K
eV X
-ray
4
PHOTOELECTRIC
ABSORPTION (PE)
AN X-RAY PHOTON POSSESSING
AN ENERGY BETWEEN 1 AND
50 KEV CAN BE TOTALLY
ABSORBED BY AN INNERSHELL (K) ELECTRON. THIS
ELECTRON IS EJECTED FROM
ITS ORBIT AROUND THE
TARGET ATOM
PHOTOELECTRIC
ABSORPTION
THE EJECTED INNER-SHELL
ELECTRON LEAVES AN
OPENING FOR AN OUTER-SHELL
ELECTRON TO FILL. THE
OUTER-SHELL ELECTRON FILLS
THE INNER-SHELL VACANCY BY
RELEASING X-RAY ENERGY
PHOTOELECTRIC
ABSORPTION
A CASCADE OF
CHARACTERISTIC
RADIATION RESULTS AS
THE OUTER-SHELL
ELECTRONS FILL THE
INNER-SHELL VACANCIES
5
X-ray Enters Patient
Inner-Shell Electron Ejected
Characteristic X-ray
6
PHOTOELECTRIC ABSORPTION VS.
CHARACTERISTIC X-RAY PRODUCTION
PROBABILITY OF PE
ABSORPTION INCREASES
WITH:
z DECREASING
X-RAY ENERGY
(KVP)
z INCREASING ATOMIC NUMBER
(Z) OF ABSORBER
z INCREASING TISSUE DENSITY
(MASS/UNIT VOLUME)
PHOTOELECTRIC
ABSORPTION
IS THE PRINCIPAL CONTRIBUTOR
TO RADIOGRAPHIC CONTRAST
(THE ABILITY TO DISTINGUISH
BETWEEN TISSUES ON A
RADIOGRAPH DUE TO
DIFFERENCES IN RADIOGRAPHIC
DENSITY)
7
PHOTOELECTRIC ABSORPTION
PHOTOELECTRIC
ABSORPTION
ALTHOUGH CONTRIBUTING
TO IMAGE CONTRAST, PE
IS CHIEFLY RESPONSIBLE
FOR PATIENT RADIATION
DOSE.
COMPTON SCATTER
THE LIKELIHOOD OF COMPTON
SCATTER PRODUCTION
INCREASES SIGNIFICANTLY
ABOVE 35 KEV. IN THIS
INTERACTION, AN INCIDENT
PHOTON INTERACTS WITH AN
OUTER-SHELL ELECTRON
8
COMPTON SCATTER
BECAUSE THE INCIDENT
PHOTON’S ENERGY IS GREATER
THAN THE BINDING ENERGY OF
THE OUTER-SHELL ELECTRON,
THE OUTER-SHELL ELECTRON
IS EJECTED WITH ENERGY LEFT
OVER.
COMPTON SCATTER
THIS LEFT-OVER ENERGY IS THE
SCATTERED X-RAY PHOTON,
DEVIATING FROM THE PATH IT
ORIGINALLY HAD BEFORE
COLLIDING WITH THE OUTERSHELL ELECTRON
X-ray Enters Patient
9
Outer-shell electron ejected
Compton Scatter
Ejected Electron
COMPTON SCATTER
COMPTON SCATTER CAN
DEGRADE THE RADIOGRAPH BY
PRODUCING IMAGE FOG.
OTHERWISE, IT CAN EMERGE
FROM THE PATIENT AND
CONTRIBUTE TO
OCCUPATIONAL DOSE
10
COMPTON SCATTER
COMPTON SCATTER
PROBABILITY OF COMPTON
SCATTER INCREASES WITH:
z INCREASING
KVP
z INCREASING TISSUE VOLUME
z INCREASING FIELD SIZE
11
PAIR PRODUCTION
OCCURS WHEN USING X-RAY
ENERGY OF AT LEAST 1.022
MILLION ELECTRON VOLTS
(MEV). THIS IS WELL BEYOND
THE DIAGNOSTIC RANGE, BUT
WITHIN THE THERAPEUTIC
RANGE.
PAIR PRODUCTION
AN INCIDENT X-RAY PHOTON OF
AT LEAST 1.022 MEV
INTERACTS WITH AN THE
NUCLEUS OF AN ATOM. THE
RESULT IS THE PRODUCTION
OF 2 SUBATOMIC PARTICLES: A
POSITRON AND NEGATRON
PAIR PRODUCTION
THE POSITRON COLLIDES WITH
THE NEAREST ORBITAL
ELECTRON. THE RESULT IS THE
ANNIHILATION OF THE
PARTICLES AND THE CREATION
OF 2 X-RAY PHOTONS, EACH
POSSESSING 0.511 MEV AND
MOVING IN OPPOSITE
DIRECTIONS
12
Positron (+)
1.02 Mev X-ray
Negatron (-)
0.51 MeV X-ray
+
0.51 MeV X-ray
PAIR PRODUCTION
THE NEGATRON, HAVING
THE SAME MASS AND
CHARGE AS AN
ELECTRON, COMBINES
WITH AN ATOM NEEDING
AN ELECTRON
13
SUMMARY
THE TYPE OF INTERACTION WITH
MATTER DEPENDS ON THE XRAY BEAM’S AVERAGE ENERGY.
THE CHOICE OF KVP WILL
INFLUENCE RADIOGRAPHIC
CONTRAST, SCATTER
PRODUCTION AND
PATIENT/OCCUPATIONAL DOSE
SUMMARY
ALWAYS USE TECHNIQUES THAT
ARE OPTIMAL (STRIKE AN
ACCEPTABLE BALANCE
BETWEEN NEEDED
DIAGNOSTIC INFORMATION
AND PATIENT DOSE) IN
KEEPING WITH ALARA
NEXT:
RADIATION QUANTITIES
AND UNITS (CHAPTER 3)
WITH PRACTICE
PROBLEMS WE CAN DO
IN CLASS
14