6.5 Medical Imaging

Question 1

how do you produce x-rays? what are the two ways?

Answer 1

1. when fast-moving electrons decelerate rapidly by smashing into a metal target, as the electron interacts with the electric field around the nucleus, an X-ray photon is emitted and by the principle of conservation of energy the kinetic energy lost by the electron is equal to energy of the photon emitted
2. when a fast-moving electron ejects an inner electron in an atom of the target metal, another electron from a higher energy level moves to occupy the vacancy created, and this transition to a lower energy level releases an X-ray photon of a specific wavelength corresponding to the difference in energy levels

Question 2

what is an x-ray tube?

Answer 2

x-ray tubes are electrical circuit with a cathode (where electrons are emitted) and an anode (the target metal) as well as a vacuum and a filament

Question 3

how do x-ray tubes produce x rays?

Answer 3

• electrons are emitted from a filament, which is heated by an electric current
• the high p.d accelerates electrons between the filament (cathode) and the positive anode, which is a metal with a high melting point like tungsten
• the electrons would loose energy by colliding with any gas molecules on their path towards the positive anode, so the tube is evacuated
• if the electrons have enough energy when they hit the anode, some x-rays will be emitted

Question 4

what is the simple way of saying how x-rays are produced?

Answer 4

x-rays are produced by bombarding tungsten with high energy electrons

Question 5

what is x-ray attenuation?

Answer 5

the gradual decrease in intensity of x rays when they pass through matter (e.g a person’s body)

Question 6

what is the attenuation coefficient?

Answer 6

the attenuation (or absorption) coefficient is a constant used to calculate how the intensity of x-rays decreases as they pass through a material

Question 7

what are the four main ways x-rays can interact with matter?

Answer 7

• simple scattering
• the photoelectric effect
• the Compton effect
• pair production

Question 8

what is the relationship of attenuation intensity?

Answer 8

exponential, when x-rays pass through matter, they are absorbed and scattered, the intensity of the x-ray beam decreases (attenuates) exponentially with distance from the surface, according to the material’s attenuation coefficient

Question 9

what is the equation for x-ray attenuation?

Answer 9

I = Io x e^-μx

where I = final intensity
Io = initial intensity
μ = attenuation coefficient of the medium through which the x rays are passing
x = the distance through the medium (or the thickness of the medium if you like)

Question 10

what is simple scattering of x-rays? how does it work?

Answer 10

• when low energy x-rays encounter the electrons in an atoms the energy of the x ray photon is not sufficient to cause ionisation
• the photon is scattered (deflected so that its direction is changed) but there is no change in energy and no absorption of the photon
• this is simple scattering and causes ‘noise’ (blurry bits) in the image due to the arrival at the detector of scattered x-rays from several angles as well a from the main beam

Question 11

what is the photoelectric effect way x-rays are attenuated?

Answer 11

• in the same way UV radiation is able to release electrons from a metal x-rays are capable o causing emission of free electrons from atoms
• because the energy of the x-ray photon is so high the work function is so small in comparison it can be ignored when x-rays interact with an atom by the photoelectric effect, the emitted photoelectrons have a maximum kinetic energy equal to the photon energy of the x-rays

Question 12

what is the definition for Compton scattering?

Answer 12

Compton scattering is the effect whereby an x-ray is deflected by interaction with an orbital electron has a longer wavelength than its initial wavelength, the electron is ejected from the atom at high speed

Question 13

wha is Compton scattering? how does it work?

Answer 13

this is when a photon with a high enough energy knocks an electron out of an atom, which causes the photon to loose energy and be scattered, the x-ray photon and electron are scattered in different directions due to the conservation of momentum - the GREATER THE DEFLECTION, the GREATER THE LOSS OF ENERGY OF THE PHOTON and hence the BIGGER ITS WAVELENGTH BECOMES

Question 14

what is pair production in terms of x-ray attenuation?

Answer 14

when a beam of high freq. x-rays causes an x-ray photon to interact with the nucleus of an atom and the x-ray photon vanishes, spontaneously producing a positron and an electron (pair production occurs)

Question 15

do bones absorb x-rays better than flesh?

Answer 15

yes, film remains white, where the x-rays pass through the film turns black (soft tissue)

Question 16

what is a contrast material used for?

Answer 16

to see detail of different types of soft tissue which have similar attenuation coefficients, a contrast media such as barium or iodine that as a large attenuation coefficient, this is due to barium and iodine having a high atomic number

Question 17

what is CAT? (computerised axial tomography)

Answer 17

it is a process using multiple x-ray scans to produce images of ‘slices’ through the body in one plane, in order to produce a 3D image

Question 18

why are CAT scans (also known as CT scans) better than traditional 2D x-ray scans?

Answer 18

• CAT allows for image reconstruction in 3D as it uses multiple cross sections
• provides a very accurate picture of the position of the internal organs without being obscured by other structures in the body
• particularly sensitive to different densities, give much better contrast of soft tissue than x-rays could, for example to detect cancerous tumours
• the digital technology allows the doctor to rotate the image and view the patient’ body from a number of angle without having to take more images of the patient which would expose them to more potentially harmful x-rays

Question 19

how do you produce a CAT scan image?

Answer 19

• to produce a 3D image, many x-ray scans must be taken from different angles to get different views of the same organ
• a thin, fan-shaped x-ray beam is produced which has a very little thickness os only irradiated only a very thin slice of the patient at any one time
• having passed thought the patient, the x-rays are detected by a stationary ring of detectors
• the x-ray source is rotated around the patient and after one revolution, both it and the detectors have moved up about a cm, which means that on the next revolution it looks at the next slice of the body
• a computer then processes this and produces a high quality 3D image that can be rotated, zoomed in and seen from different angles

Question 20

what is a tracer?

Answer 20

a tracer is a radioactive substance either ingested by, or injected into a patient, it emits gamma photons to be detected by a gamma camera

Question 21

what is a gamma camera?

Answer 21

a gamma camera detects gamma photons emitted from a patient given a radioactive tracer, this is used to produce a real-time image of the path of the tracer through the boyd

Question 22

why are gamma sources instead of beta and alpha (as tracers)?

Answer 22

alpha and beta would damage the body due to their great ionising power

Question 23

why are radioactive sources put into the body in the first place?

Answer 23

to diagnose an illness or to treat the illness

Question 24

why are technetium-99m used as a tracer?

Answer 24

technetium-99m has a half life of 6 hours which is an appropriate length of time, because time must e allowed for the source to be brought from its manufacturing site to the hospital

Question 25

how does diagnosis using a gamma camera work?

Answer 25

• a gamma tracer like technetium-99m is put into the body, often injected to the part of the body where it is needed or in many cases attached to a chemical that will go to a certain part of the body (e.g iodine-131 to see functioning of thyroid gland)
• once the tracer is in the body it is possible to monitor it as it is a gamma-emitting source and so a gamma camera can be used

Question 26

what is a gamma camera made up of?

Answer 26

(see diagram page 240)

• a block of lead with tens of thousands of vertical holes is close to the patient, these parallel tubes collimate the beam so only photons travelling along the axis of the collimator can pass through t the detector, photons moving in any other direction will be absorbed by the lead which improves the sharpness of the image as scattered photons are excluded
• once through the collimator the gamma photons strike the scintillator which is a large crystal (made of sodium iodide) and it scintillates when it absorbs a gamma photons (it emits many photons of visible light)
• behind the crystal is an array of photomultiplier tubes arranged in a hexagonal pattern, each tube initially emits one electron for each photon by the photoelectric effect, these tubes amplify the effect to release more electrons giving an electrical pulse output for every incident photons of light
• the electrical pulses are then connect to a computer which processes them and produces an image

Question 27

what is a collimator?

Answer 27

a collimator is a part of a gamma camera, it is a device for producing a parallel-sided (collimated) beam of electromagnetic radiation

Question 28

what is a scintillator?

Answer 28

a scintillator is a part of a gamma camera, it is a material that produces many photons of visible light when struck by a high energy photon

Question 29

what is a photomultiplier tube?

Answer 29

a photomultiplier tube is a part of a gamma camera, it is a device that is used to give a pulse of electrons for each incident photons

Question 30

why are radioactive tracers useful? why are they put into the body when they are harmful?

Answer 30

• show areas of damaged tissue by detecting decreased blood flow
• they can identify active cancer tumors by showing metabolic activity
• show blood flow and activity in the brain, useful for research and treatment of diseases such as Parkinson’s, epilepsy

Question 31

why is flourine-18 used in PET scans?

Answer 31

because it usually undergoes beta plus decay and has a half life of 110 mins meaning the patient is exposed to radioactivity for a much shorter amount of time than with technetium-99m

Question 32

how does diagnosis using PET scans work?

Answer 32

• tracer is put into the bloodstream, usually flourine-18 (which is a beta plus emitter, positrons)
• when source decays the positron annihilates an electron in the desired area two gamma rays are produced at 180 degrees to each other
• a ring of detectors detect the time it takes for the gamma photons to get to the detectors and use the difference in arrival time is used to locate the point where the gamma rays originated
• a computer processes the difference in arrival times and converts them into signals and uses them to produce a 3D image of the organ

Question 33

what are ultrasound waves?

Answer 33

ultrasound waves are longitudinal waves above the upper limit of the audible range, with frequencies greater than 20,000 Hz or 20kHz (higher frequency than humans can hear), for medical purposes frequencies are usually between 1-15MHz

Question 34

what is a transducer?

Answer 34

a transducer is a device, such as a microphone, which converts a non-electrical signal (e.g sound) into an electrical signal - it emits AND receives ultrasound

Question 35

what is the piezoelectric effect?

Answer 35

the piezoelectric effect is the change in volume of a material when a p.d is applied across its opposite faces, alternatively, is is the production of an induced e.m.f when certain crystals are placed under stress

Question 36

what is ultrasound scanning used for?

Answer 36

used particularly when examining a fetus in the uterus of a pregnant woman

Question 37

what are the advantages of ultrasound scanning?

Answer 37

• there are no known dangers associated with ultrasound, as it is not a type of ionising radiation in the way that x-rays and gamma rays are
• you can obtain real time images of soft tissue such as the heart
• ultrasound machines are inexpensive and portable
• the scan is a quick procedure and the patient can move during the scan

Question 38

what are the disadvantages of ultrasound scanning?

Answer 38

• ultrasound doesn’t penetrate bone so it cannot be used to examine fractures or examine the brain
• it cannot give detail on solid masses
• cannot pass through air spaces in the body (due to the mismatch in impedance) so it cannot produce images from behind the lungs

Question 39

how do ultrasound scans work?

Answer 39

• when a ultrasound hits a boundary such as tissue and liquid etc. the waves are reflected, these reflected waves are detected by an ultrasound scanner and are used to generate an image
• the ultrasound waves must be pulsed as the transducer sends out the wave then pauses to allow the reflected waves to return and be detected by the transducer

Question 40

how does the transducer work?

Answer 40

-a transducer contains a piezoelectric crystal and acts a both a transmitter and a receiver of ultrasound

Question 41

explain the piezoelectric effect?

Answer 41

• to produce sound waves with ultrasound frequency of around 1MHz a physical effect called the piezoelectric effect is used
• piezoelectric crystals produce a potential difference when they are deformed (squashed or stretched) - the rearrangement in structure displaces the centres of symmetry and electric charges
• when you apply a p.d across a piezoelectric crystal, the crystal deforms, if the p.d is alternating, the crystal vibrates at the same frequency of the signal and emits ultrasound waves
• because this process can also work in reverse a piezoelectric crystal can act as a receiver of ultrasound, converting sound waves into alternating voltages, and also a transmitter, converting alternating voltages into sound waves

Question 42

what are the two types of ultrasound sans?

Answer 42

A-scan and B-scan

Question 43

what is an A-scan? (ultrasound)

Answer 43

-the amplitude scan (or A-scan) sends a short pulse of ultrasound into the body simultaneously with an electron beam travels across a cathode ray oscilloscope (CRO) screen
-the transducer receives reflected pulses which cause vertical spikes on the CRO screen
-the horizontal axis shows the time that the echo took to be detected by the transducer and can be used to work out the depths of thicknesses of reflecting tissue in the body
-no photo is produced, but measurements can be taken from it to determine dimensions (e.g diameter of baby’s head)
(think of a graph with spikes of different amplitude for A-scan)

Question 44

what is a B-scan? (ultrasound)

Answer 44

• the brightness scan or B-scan is much more commonly used, (in comparison to A-scans the electron beam sweeps down the screen rather than across)
• in this case a real time 2D or 3D image of the area being scanned is built up from many retuning echoes recored from several transducers in an array, or a transducer is moved to different positions or angles around the patient
• the greater the amplitude of the reflected pulse, the brighter each dot will be so a range of brightnesses will be shown in a scan where different bone, liquid and soft tissue reflect different proportions of the transmitted ultrasound beam

Question 45

what is the definition of acoustic impedance? the equation?

Answer 45

acoustic impedance, Z, is defined by the equation Z = ρ x c
where ρ = density of the material
c = speed of sound in the material

Question 46

what is impedance matching?

Answer 46

impedance matching is the reduction in intensity of reflected ultrasound at the boundary between two substances, achieved when the two substances have similar or identical acoustic impedences

Question 47

what does the amount of reflection of the ultrasound waves depend on?

Answer 47

depends on the acoustic impedance, if the two materials at the boundary have a large difference in impedance, then most of the energy is reflected (the intensity of the reflected wave Ir will be high), if the impedance of the two materials is the same then there is no refelction

Question 48

what is the defining ratio equation of the ratio between the reflecting intensity and incident intensity? (ultrasound)

Answer 48

Ir / Io = (Z2 - Z1)^2 / (Z2 + Z1)^2
where Ir = reflected intensity
Io = incident intensity
Z1 = acoustic impedance of first material
Z2 = acoustic impedance of second material

Question 49

why do you need a coupling medium (or a special gel) when performing ultrasound scans?

Answer 49

• soft tissue has a very different acoustic impedance fro air, so almost all the ultrasound will be reflected and will never enter the body at all
• to avoid this you need a coupling gel in between the transducer and tissue to displace the air and to get a match of impedances which will allow for good transmission/reflection values, this is impedance matching

Question 50

what is the equation for determining the speed of blood in the patient using the Doppler effect?

Answer 50

Δf / f = 2vcosθ / c
where
Δf = change in frequency (also known as the beat frequency)
f = initial frequency
v = velocity of the moving cell (e.g blood cells)
θ = the angle between the ultrasound receiver and the direction in which the the cell is moving
c = speed of sound in that medium (e.g blood)

Question 51

how can doctors use the Doppler effect to measure blood flow?

Answer 51

• because ultrasound waves are affected by the Doppler affect they can use the change in frequency (beat frequency) caused by a moving reflector such as blood cells in the body to determine the speed of the blood
• they can use this to create moving pictures that can be colour coded to show directions of blood movement to check if a heart or blood vessel is functioning correctly