INTRODUCTION TO RADIOLOGY

what characteristic of the body allows radiographs to be taken

• diagnostic imaging uses radiations

• the human body is partially transparent to those radiations

• this partial transparency allows an image of the body to be formed

what are the major sub-disciplines in imaging

• radiography

• radiology

define radiography

radiography: the acquisition of the image

define radiology

radiology: the interpretation of the image and requires understanding of normal anatomy and disease processes

in which year were X-rays discovered

1895

who is considered the ‘Father of X-rays’

Wilhelm Röntgen

how are X-rays produced

when a bombarding electron interacts with either the nucleus or an inner electron shell of a metal atom such as tungsten

image of a dental X-ray tube

X-ray production process

• X-ray tube is enclosed in a glass tube, in which is a vacuum

• on the opposite end, a tungsten target is embedded in a copper block (ANODE)

• the filament is a thick piece of tungsten wire

• electrical current passed through the filament causes it to heat up to 2220°C and glow white hot (CATHODE)

• electrons are emitted spontaneously from the white hot filament

• this forms a cloud around the cathode

• high voltage between the cathode and anode provides electrons an acceleration force

• this emits them towards the tungsten anode at half the speed of light causing collision with the electrons around the anode

• this leads to the emission of X-rays/ X-ray photons from the anode through the glass vacuum of the X-ray tube

name the processes that produce X-rays in the tungsten atoms of the anode

• braking radiation

• characteristic radiation

describe the process of braking radiation

• incoming electron from the cathode threads its way between the different electron shells of the tungsten atom

• it gets close to the nucleus of the tungsten atom

• electrostatic interaction between -ve electron and +ve proton causes the electron to bend and change its path

• this also causes the electron to slow down

• consequently, the electron gives off some of its energy as an X-ray photon

• this is emitted from the X-ray tube

—

OCCURS NEAR NUCLEUS

describe the process of characteristic radiation

• one incoming electron collides with an inner shell electron of a tungsten atom

• the incoming electron ejects the electron from the tungsten atom which leaves a space where that electron once was

• another electron drops down from a higher shell

• when this happens it gives off characteristic radiation

• this produces X-ray photons which are emitted from the X-ray tube

• the energy of characteristic radiation depends on what metal the target is made from

—

OCCURS AT INNER SHELL OF ELECTRON AROUND NUCLEUS

is it possible to alter the intensity of the X-ray beam?

yes

what factors need to change to alter the intensity of the X-ray beam? (3)

• kV = kilovolt

  • the potential difference between the anode and the cathode

  • controls the penetrating power of the X-ray beam

• mA = milliampere/ milliamp

  • how much current flows through the cathode/ the number of electrons emitted from cathode

• exposure time = the duration the cathode is heated for

how does changing kV alter the intensity of X-ray beams

• kV = the potential difference between the anode and the cathode

  • electrons flowing from cathode have more energy so when they collide with the tungsten anode they give off more energy

  • therefore they release more X-ray photons

  • X-ray film is blacker

how does changing mA alter the intensity of X-ray beams

• mA = how much current flows through the cathode/ the number of electrons emitted from cathode

  • more current = hotter = more electrons emitted from cathode

  • more X-ray photons at anode

  • film is blacker

how does changing the exposure time alter the intensity of X-ray beams

• exposure time = the duration the cathode is heated for (usually milliseconds)

  • increased ET emits more electrons

  • more X-ray photons at anode

  • film is blacker

IN PRACTICE which factor is the only one that can be changed

exposure time

what about kV and mA?

• kV and mA are fixed in practice for dental X-ray sets

• only the exposure time can be changed

what can happen when X-rays enter the human body? (3)

once X-rays pass into matter they can either be

• completely absorbed - give all of its energy into matter

• pass through unchanged i.e. transmitted - has the same energy exiting as it had when entering

• scattered to a new direction with or without loss of energy

how is the radiographic image formed

• the attenuated beam that emerges from the matter (patient) forms the radiographic image

attenuated: some photons completely absorbed, some lose energy, some scattered to a new direction

how else can X-rays be used to produce an image

in some applications X-rays can cause certain materials to fluoresce i.e. emit light

what methods can be used to capture the radiographic image

• film

• digital sensor

capturing the radiographic image: what is the process of film

• X-ray photons that fall onto the emulsion (coating on top of film) ionise the silver bromide contained within it (X-ray sensitive)

• this produces free electrons which attract mobile silver atoms to them to form a grain of metallic silver

• the developing process allows this latent image to be made visible - it gets rid of the silver bromide and leaves behind metallic silver

• the image formed on film is an atomic silver map of how dense the body is to X-rays passing through it

image of film

what does this electron micrograph show

silver bromide crystals = little black rods

capturing the radiographic image: how do digital sensors work

• digital sensors exploit the capacity of certain materials to store X-ray energy within them, then give this out as light

• some have a chemical that trap energy which is revealed in a scanning method

• some uses a technology similar to a phone camera

is the use of film or digital sensors a faster way of producing a radiographic image

• image produced from digital sensor is much faster than film

—

• digital sensor = 5-25 secs

• film ≈ 15 mins

images of digital sensors

what is a collective name for film and digital sensors

image receptor - because they detect X-rays and produces an image

where should the object/ person of interest be in relation to the image receptor when taking an X-ray

• the image receptor and object/ person of interest should be parallel

• the object/ person also has to be as close to the receptor as possible to prevent magnification

at what angle should the X-ray beam hit the image receptor

the X-ray beam should hit the image receptor at 90°

what does this diagram show

• X-ray film is parallel to the long axis of the tooth

• the X-ray beam passes through the tooth and film receptor at 90°

images of X-rays taken wrongly

clinically, what is used to enable the tooth + film to be parallel and the X-ray beam to hit the film at 90°?

• a film holder

• Hawe Super-Bite usually used

what negative effects can ionising radiation cause

• skin blistering

• hair loss

• sunburn

why are X-rays harmful?

• when ionising radiation e.g. X-rays interact with living tissues, some of it is absorbed by the tissue

• chemical changes then occur almost instantly in the tissue

• this can lead to molecular damage in seconds to minutes

• after this, over hours e.g. sunburn to decades, biological damage can occur

what is the primary reason radiation damages tissue

radiation damages tissue primarily because it causes chemical changes in tissue by ionisation

define ionisation

ionisation: the process whereby atoms or molecules gain or lose electrons to acquire a negative or positive charge

what molecules are thought to be ionised

cellular enzymes e.g. those that control cell division and DNA (cannot be observed directly)

what is a direct effect of radiation

direct effect of radiation

• breaking of bonds between atoms

what is an indirect effect of radiation

indirect effect of radiation

• damage by the production of free radicals

• these are powerful oxidisers that can rearrange organic molecules

—

H₂O + radiation  = H₂O⁺ + e^-

  H₂O⁺ » H⁺ + OH^.

• the positive water ion does not exist for very long

• it immediately disintegrates into H+ and the hydroxyl free radical

how does damage to DNA and enzymes affect cells

• inhibits cell division

• causing direct cell death » tissue damage

• causing a cell to transform i.e. uncontrolled cell division

what can cell transformation/ uncontrolled cell division lead to

• development of a radiation-induced cancer

what is the term for the effects that lead to

cell damage » cell death » tissue damage

deterministic effects

what is the term for the effects that lead to cell transformation

stochastic effects

what is the pattern of the occurrence of stochastic effects

• stochastic events are random

• they occur by chance

what is the chief stochastic effect from X-rays

development of a radiation-induced cancer

do stochastic effects have a threshold?

no - there is no safe dose of radiation where your chances of developing a stochastic effect is 0%

what is the correlation between radiation dose and chances of developing a stochastic effect

positive correlation

• as radiation dose increases the chances of developing a stochastic effect increases

• the severity of the effect i.e. tumour does NOT change

list the harmful tissue effects of radiation

• skin reddening/ skin burning

• hair loss

• cataracts - if eye is irradiated

• foetal developmental abnormalities

all harmful tissue effects have a _________

threshold

explain what the threshold dose is

• less radiation than the threshold dose to cause a harmful effect means the effect will not occur

• if the threshold is exceeded then an increasing number of people will show the effect

what is the major risk to patients from dental X-rays

the stochastic (random) triggering of a radiation-induced cancer

what type of cancers are usually radiation-induced

blood cancers

• leukaemia: takes 5-10 yrs to develop

connective tissue cancers

• sarcoma: takes 20-40 yrs to develop

how many cases of radiation-induced cancers occur in the UK annually

≈ 1000 cases

how many of those 1000 cases come from dental X-rays

≈ 10/1000 come from dental X-rays (impossible to tell which patients they are)

explain the term ‘effective dose’

effective dose

• a measure used to estimate the risk of developing a radiation-induced cancer from an imaging technique using ionisation radiation

• unit of measurement = Sievert (Sv)

• milli- and micro- Sv are used in the context of radiation dosage from imaging tests - because one Sv is a huge amount of radiation

can a radiation-induced cancer be differentiated from a non radiation induced cancer i.e. normal disease?

no, radiation-induced cancers look just like normal disease clinically and histologically

_ in _ will die naturally from cancer in the UK

1 in 4 will die naturally from cancer in the UK

what is the current annual exposure of the average UK resident to radiation

2.7mSv per yr - depends where you live

• at least 50% comes from radon gas

list the radiation protection principles developed by the International Commission on Radiological Protection (ICRP)

• justification

• optimisation

• dose limitation

radiation protection principals: explain the ‘justification’ principal

JUSTIFICATION

• ionising radiation should only be used if the benefits of having the test done exceed the risks to the patient being exposed

• easier to justify smaller doses compared to larger doses (risk is proportional to dose)

radiation protection principals: explain the ‘optimisation’ principal

OPTIMISATION

• if exposing the patient to ionisation radiation is justified, then the dose should be as low as reasonable practicable (ALARP principal)

radiation protection principals: explain the ‘dose limitation’ principal

DOSE LIMITATION

• there should be a system of dose limits

• doses greater than the limits cannot be justified - no matter the benefits