Production of X-rays

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Chapter 9 Production of X-rays

Patti Ward

CHAPTER CONTENTS

Introduction 104
Electron production 104
Target interactions 104
Emission spectrum 106
X-ray quality and quantity 107
Exposure manipulation 109

KEY POINTS

image X-rays are produced when high-speed projectile electrons collide with the X-ray tube target.
image The kinetic energy of projectile electrons transfers to target atoms. Approximately 99% of the energy converts into heat and only about 1% converts into X-rays.
image The production of X-rays comes from two interactions: bremsstrahlung and characteristic.
image A bremsstrahlung interaction involves projectile electrons that emit radiation as they slow down when passing close to the nucleus of target atoms.
image Most diagnostic X-rays are the product of bremsstrahlung interactions.
image A characteristic interaction involves the emission of radiation following a collision between projectile electrons and the orbital electrons of target atoms.
image X-ray beam quality and quantity are affected by the target material, beam filtration, distance, and prime exposure factors (kVp, mA, and exposure time).
image The target material affects both the quality and quantity of the X-ray beam. Tungsten is the standard target material due to its high proton number.
image Beam filtration affects the beam’s quality and quantity by removing low energy X-rays.
image The inverse square law governs the relationship between X-ray quantity and distance.
image The quality of radiation in an X-ray beam is the penetrating ability of the beam.
image The quantity of radiation in an X-ray beam is the number of photons in the beam.
image Peak kilovoltage (kVp) controls the quality of the beam. As kVp increases, X-rays are more penetrating and as kVp decreases, X-rays become less penetrating.
image kVp affects the quantity of X-rays produced. An increase in kVp rapidly increases the number of X-ray photons and a decrease in kVp rapidly decreases the number of X-rays. The amount of radiation produced is proportional to the square of the ratio of the kVp.
image 15% rule: a 15% increase in kVp doubles radiographic density; a 15% decrease in kVp halves radiographic density.
image mA s (product of mA and exposure time) controls the quantity of X-rays produced.
image The relationship between the quantity of X-rays and mA s is directly proportional.
image Reciprocity law: any combinations of mA and time that give the same mA s will result in the same quantity of X-ray photons.

INTRODUCTION

Diagnostic X-rays are produced in the target of the anode when high-energy projectile electrons are rapidly decelerated. Diagnostic X-ray imaging equipment provides the means for practitioners to control the quality and quantity of the X-ray beam. Consequently, it is important to understand the process of X-ray production and the factors that influence the characteristics of the beam. Practitioners familiar with the concepts and factors that influence quality and quantity are better able to control exposure factors to produce optimal radiographic images while minimizing patient dose.

ELECTRON PRODUCTION

Four conditions are necessary for the production of diagnostic X-rays:

1 A source of free electrons.
2 A means to provide the electrons with high kinetic (motion) energy.
3 A method to concentrate the electrons into a beam.
4 A suitable material to rapidly decelerate the electrons.

In the X-ray tube, the purpose of the filament is to provide the free electrons necessary for X-ray production. As the rotor is activated the current passing through the filament heats to the point where electrons boil off. This process is referred to as thermionic emission. At this point, a space charge (cloud of electrons) forms around the filament. The focussing cup temporarily concentrates the free electrons and helps form them into a beam.

When the exposure begins, the primary circuit closes and a high voltage is applied across the anode (positively charged) and cathode (negatively charged). This causes electrons to stream towards the anode at a high rate of speed. The potential energy of each electron is one kiloelectron volt (keV) of energy for each kilovolt (kV) of voltage set for the exposure. Electrons (sometimes called projectile electrons) that travel from the cathode to anode make up the tube current.

TARGET INTERACTIONS

When the high-speed projectile electrons collide with the X-ray tube target they interact with the orbital electrons or the nuclear field of the target atoms. Kinetic energy transferred from the projectile electrons to the target atoms converts into heat or X-rays. When projectile electrons strike outer target shell electrons it puts them in an excited state and as a result, infrared (heat) radiation is emitted. Approximately 99% of the energy of projectile electrons converts into heat. Only about 1% of the energy converts into X-ray photons. Two types of interaction produce X-ray photons: bremsstrahlung interactions and characteristic interactions.

BREMSSTRAHLUNG INTERACTIONS

Bremsstrahlung in German means ‘to brake radiation’ or braking radiation. Bremsstrahlung target interaction occurs when projectile electrons pass by outer shell electrons of target atoms and interact with the force field of the nucleus of the atom. Because atomic nuclei are positively charged and electrons are negatively charged, there is a mutual attraction between them. The nuclear force field causes the entering electron to slow down (or brake) and change direction. The loss of kinetic energy that occurs when a projectile electron slows down is emitted as an X-ray photon. These X-ray photons are known as bremsstrahlung photons or brems radiation (Fig. 9.1). In the diagnostic range, approximately 85% of X-ray emissions are the result of bremsstrahlung interactions.

image

Figure 9.1 Production of bremsstrahlung radiation.

CHARACTERISTIC INTERACTIONS

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