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Ebook Clark''s essential physics in imaging for radiographers: Part 2

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Ebook Clark''s essential physics in imaging for radiographers: Part 2

CHAPTER 6__________PRINCIPLES OF RADIATION DETECTION AND IMAGE FORMATIONINTRODUCTIONThe aim of this chapter is to explore how radiation is detected, m

Ebook Clark''s essential physics in imaging for radiographers: Part 2measured, quantified and used in order to produce images.There are various types of radiation detector which are designed for different purposes withi

n medical imaging. There are automatic exposure devices and computed tomography (CT) detectors, as well as those used within general radiographic and Ebook Clark''s essential physics in imaging for radiographers: Part 2

fluoroscopic imaging.This chapter will begin by looking generally at the types of detector we may come across in the radiography department, but the f

Ebook Clark''s essential physics in imaging for radiographers: Part 2

ocus and bias later in the chapter revolves specifically around large field detectors used in general radiography.Learning objectivesThe students shou

CHAPTER 6__________PRINCIPLES OF RADIATION DETECTION AND IMAGE FORMATIONINTRODUCTIONThe aim of this chapter is to explore how radiation is detected, m

Ebook Clark''s essential physics in imaging for radiographers: Part 2us detectors and how they are used for different clinical purposes.■Discuss the benefits and limitations of various detector types used within differe

nt imaging systems.Principles of Radiation Detection and Image FormationDESIRABLE CHARACTERISTICS OF RADIATION DETECTORSThere are a number of characte Ebook Clark''s essential physics in imaging for radiographers: Part 2

ristics which are considered for any kind of radiation detector. The main ones include:■Absorption efficiency is clearly desirable that a detector is

Ebook Clark''s essential physics in imaging for radiographers: Part 2

able to absorb as many of the incident X-rays as possible. The overall absorption is dependent on the physical density (atomic number, size, thickness

CHAPTER 6__________PRINCIPLES OF RADIATION DETECTION AND IMAGE FORMATIONINTRODUCTIONThe aim of this chapter is to explore how radiation is detected, m

Ebook Clark''s essential physics in imaging for radiographers: Part 2 is dependent on the physical area of the face plate minus the interspace between individual detectors and side and end walls.■Dose efficiency is infl

uenced by both conversion and capture efficiency. Typical dose efficiency is anywhere between 50 and 80 per cent for individual detector designs, but Ebook Clark''s essential physics in imaging for radiographers: Part 2

nearer 30-60 per cent for flat panel detectors.■Temporal response should be as fast as possible and is the time it takes the detector to absorb the ra

Ebook Clark''s essential physics in imaging for radiographers: Part 2

diation, send a signal and prepare for the next reading.■Phosphorescence or afterglow affects temporal response; until the detector has stopped giving

CHAPTER 6__________PRINCIPLES OF RADIATION DETECTION AND IMAGE FORMATIONINTRODUCTIONThe aim of this chapter is to explore how radiation is detected, m

Ebook Clark''s essential physics in imaging for radiographers: Part 2sitive to.■High reproducibility and stability help avoid drift and resultant detector fluctuation or noise variation.DETECTIVE QUANTUM EFFICIENCYDetec

tive quantum efficiency (DQE) is often a measure that is quoted in order to make comparisons between various imaging systems and takes account of all Ebook Clark''s essential physics in imaging for radiographers: Part 2

the characteristics mentioned above.Detective Quantum EfficiencyThe DQE describes how well an imaging system performs, essentially based on its overal

Ebook Clark''s essential physics in imaging for radiographers: Part 2

l signal-to-noise ratio (SNR) when compared against a theoretical ideal detector. It is essentially a measure of how much of the available signal is d

CHAPTER 6__________PRINCIPLES OF RADIATION DETECTION AND IMAGE FORMATIONINTRODUCTIONThe aim of this chapter is to explore how radiation is detected, m

Ebook Clark''s essential physics in imaging for radiographers: Part 2e detector system. A DQE ol 50 per cent means that approximately 50 per cent oi the available quanta is used by the system (compared to an ideal syste

m) to produce a signal.If we consider two imaging systems with different DQEs, but the same SNR, the one with the higher DQE would require less signal Ebook Clark''s essential physics in imaging for radiographers: Part 2

and consequently less radiation exposure for the same eventual image quality. So, in some ways, it can almost be used as a measure of dose efficiency

Ebook Clark''s essential physics in imaging for radiographers: Part 2

.The actual measures of true DQEs are a little more complex as DQE is also affected by spatial frequency. The DQE of a particular system can also vary

CHAPTER 6__________PRINCIPLES OF RADIATION DETECTION AND IMAGE FORMATIONINTRODUCTIONThe aim of this chapter is to explore how radiation is detected, m

Ebook Clark''s essential physics in imaging for radiographers: Part 2The same system will probably have a slightly different DQE for different kv values. As such, manufacturers often supply a series of graphs of DQE plo

tted against spatial frequency and kv.Figure 6.1 illustrates the complex relationships involved in assessing DQE. The main reason it is often quoted i Ebook Clark''s essential physics in imaging for radiographers: Part 2

s that it is a helpful measure of detector performance but, if taken at face value, can mislead without careful consideration of how it is derived.Ion

Ebook Clark''s essential physics in imaging for radiographers: Part 2

isation chambersIn their simplest configuration, ionisation chambers consist of a positive (anode) and a negative (cathode) electrode plate which are

CHAPTER 6__________PRINCIPLES OF RADIATION DETECTION AND IMAGE FORMATIONINTRODUCTIONThe aim of this chapter is to explore how radiation is detected, m

Ebook Clark''s essential physics in imaging for radiographers: Part 2the electrodes forms the sensitive volume and this is filled with a gas, such as air.The electrodes are supplied with a voltage, but as the chamber is

made of an insulating material and the air in between the electrode plates is also naturally a good insulator; then a current will not flow between t Ebook Clark''s essential physics in imaging for radiographers: Part 2

he electrodes.I lowever, when X-rays pass through the chamber, some of them interact with the outer shell electrons of the atoms that make up air

CHAPTER 6__________PRINCIPLES OF RADIATION DETECTION AND IMAGE FORMATIONINTRODUCTIONThe aim of this chapter is to explore how radiation is detected, m

CHAPTER 6__________PRINCIPLES OF RADIATION DETECTION AND IMAGE FORMATIONINTRODUCTIONThe aim of this chapter is to explore how radiation is detected, m

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