6 Medical Imaging

Question 1

The nature of xrays

Answer 1

Experiments performed on the newly discovered xrays showed that they could be polarised, were diffracted by atoms in crystals, and had extremely short wavelengths. They are electromagnetic waves and therefore travel through a vacuum at the speed of light
Xrays are harmful to living cells and can kill them

Question 2

Production of xrays

Answer 2

X-ray photons are produced when fast moving electrons are decelerated by interaction with atoms of a metal such as tungsten. The KE of the electrons is transformed into X-ray photons

Question 3

X-ray tube

Answer 3

Consists of an evacuated tube containing two electrodes. The tube is evacuated so that electrons pass through the tube without interacting with gas atoms 
An external power supply is used to create a large pd between these electrodes
The cathode (neg) is a heater, which produces electrons by thermionic emission. These electrons are accelerated towards the anode
The anode is made from a metal, known as the target metal, such as tungsten, that has a high melting point

Question 4

Production of xrays from an X-ray tube

Answer 4

X-ray photons are produced when the electrons are decelerated by hitting the anode
The energy output of xrays is less than 1% of the kinetic energy of the incident electrons
The remainder of the energy is transformed into thermal energy of the anode
In many X-ray tubes, oil is circulated to cool the anode, or the anode is rotated to spread the heat over a large SA
The anode is shaped so that the xrays are emitted in the desired direction through a window
The X-ray tube is lined with lead to shield the radiographer from any xrays emitted in other direction

Question 5

X-ray equations

Answer 5

An electrons accelerated through a pd V gains kinetic energy eV
Max energy of X-ray photon= max KE of electron
eV=E
eV=hf
eV= hc/WL
WL= hc/eV

Question 6

X-ray attenuation mechanisms

Answer 6

The decrease in the intensity of EM radiation as it passes through matter and/or space. 4 mechanisms:
Simple scatter
Photoelectric effect
Compton scattering
Pair production

Question 7

Simple scatter

Answer 7

Important for xray photons with energy in the range 1-20keV
The X-ray photon interacts with an electron in the atom, but has less energy than the energy required to remove the electron, so the X-ray photon simply bounces off (is scattered) without any change to its energy
The X-ray machines used in hospitals use pds greater than 20kV, so this type of mechanisms is insignificant for hospital radiography

Question 8

Photoelectric effect

Answer 8

This mechanisms is significant for X-ray photons with energy less than 100keV
The X-ray photon is absorbed by one of the electrons in the atom
The electron uses this energy to escape from the atom
Attenuation of xrays by this type of mechanism is dominant when an X-ray image is taken, because hospital xray machines typically use 30-100kV supplies

Question 9

Compton scattering

Answer 9

This mechanisms is significant for X-ray photons with energy in the range 0.5-5MeV
The incoming X-ray photon interacts with an electron within the atom
The electron is ejected from the atom, but the X-ray photon doesn’t disappear completely- instead it is scattered with reduced energy
In the interaction, both energy and momentum are conserved

Question 10

Pair production

Answer 10

This mechanism only occurs when X-ray photons have energy equal to or greater than 1.02MeV
An X-ray photon interacts with the nucleus of the atom
It disappears and the EM energy of the photon is used to create an electron and its antiparticle, a positron

Question 11

Attenuation coefficients

Answer 11

The transmitter intensity of xrays depends on the energy of the photons and on the thickness and type of the substance. For a given substance and energy of photons, the intensity falls exponentially with thickness of substance.
Intensity= initial intensity x e^-attenuation/ absorption coefficient x thickness
I=I0 e^-ux

Question 12

Absorption coefficient

Answer 12

A measure of the absorption of X-ray photons by a substance, also known as attenuation coefficient

Question 13

Contrast medium

Answer 13

Soft tissues have low absorption coefficients, so a contrast medium is used to improve the visibility of their internal structures in X-ray images
The two most common are iodine and barium compounds, both of which are relatively harmless to humans
Both are elements with large atomic number Z. For X-ray imaging, the predominant interaction mechanism is the PE effect, for which the attenuation coefficient is proportional to the cube of the atomic number (u~ Z^3). The average atomic mass for soft tissues is about 7. This means that iodine and barium are much more absorbent than soft tissues

Question 14

Iodine contrast medium

Answer 14

Used as a CM in liquids, eg, to view blood flow
An organic compound of iodine is injected into the blood vessels so that doctors can diagnose blockages in the blood vessels and the structure of organs such as the heart from the X-ray image

Question 15

Barium Sulfate contrast medium

Answer 15

Used to image digestive systems
It is given to a patient in the form of a white liquid mixture (a ‘barium meal’), which the patient swallows before an xray image is taken

Question 16

Therapeutic use of xrays

Answer 16

Xrays are also used for therapy rather than imaging
Specialised X-ray machines, called linacs (linear accelerators), are used to create high energy X-ray photons
These photons are used to kill off cancerous cells
They do so by the mechanisms of Compton scattering and pair production

Question 17

Two dimensional imaging

Answer 17

A conventional X-ray image provides a quick and cheap way to examine patient’s internal structures
Xrays pass through the patient, and the intensity of the transmitted xrays is recorded as a two dimensional image on an electronic plate
Overlapping bones and tissues cannot be differentiated, and without the use of a contrast medium, different soft tissues are difficult to distinguish

Question 18

CAT scan

Answer 18

Computerised axial tomography
CAT scanner records a large number of X-ray images from diff angles and assembles them into a 3D image with the help of a sophisticated software

Question 19

CAT scan structure and procedure

Answer 19

The gantry houses an X-ray tube on one side and an array of electronic X-ray detectors on the opposite side
The X-ray tube and the detectors opposite it rotate around within the gantry
The X-ray tube produces a fan shaped beam of xrays that is typically only 1-10mm thick
The thin beam irradiates a thin slice of the patient, and the X-rays are attenuated by diff amounts by diff tissues
The intensity of the transmitted xrays is recorded by the detectors, which send electrical signals to a computer
Each time the X-ray tube and detectors make a 360 rotation, a two dimensional image or slice is acquired
By the time the X-ray tube has made one complete revolution, the table has moved 1cm through the ring.
The radiographer can view each 2D slice through the patient. The slices can be manipulated by sophisticated software to produce a 3D image of the patient

Question 20

Advantages of CAT scans

Answer 20

Can be used to create 3D images of the patient that helps doctors to assess the shape, size and position of disorders such as tumours.
Can distinguish between soft tissues of similar attenuation coefficients

Question 21

X-ray advantages

Answer 21

Quicker and cheaper that CAT scans

Question 22

CAT scans disadvantages

Answer 22

Some CAT scans can be quite prolonged and so expose the patients to a radiation dose equivalent to several years of background radiation, much more than a simple X-ray
Patients have to remain very still during the scanning process

Question 23

X-ray disadvantages

Answer 23

2D Images
Xrays are very ionising radiation and such are harmful
Patients have to remain very still during the scanning process

Question 24

Radioactive isotopes in medicine

Answer 24

Used in both diagnosis and therapy
In diagnosis doctors try to find out what is wrong with the patient
In radiation therapy doctors attempt to cure the patient using ionising radiation. Tumours can be targeted by gamma radiation or high energy xrays from outside the patient or by using a radioactive source implanted in or next to the tumour inside the patient

Question 25

Gamma camera

Answer 25

Diagnostic tool

Detector of gamma photons emitted from radioactive nuclei injected into the patient

Question 26

why are gamma emitting sources ideal radioisotopes?

Answer 26

gamma photons are the least ionising and can penetrate through the patient to be detected externally.

Question 27

choosing the right radioisotopes heading and weight

Answer 27

gamma emitting sources

short half life

Question 28

name 2 medical tracers

Answer 28

fluorine-18

technetium-99m

Question 29

basic structure of xray tube

Answer 29

components – heater (cathode), anode, target

metal and high voltage supply

Question 30

CAT scanners components

Answer 30

– rotating X-tube producing a
thin fan-shaped X-ray beam, ring of detectors,
computer software and display

Question 31

gamma camera components

Answer 31

– collimator,
scintillator, photomultiplier tubes, computer and
display; formation of image

Question 32

medical tracers used in diagnosis

Answer 32

to ensure the radioisotope reaches the correct organ or tumour, it is chemically combined with elements that will target the desired tissues to make a radiopharmaceutical, also known as medical tracer
its progress through the body can be traced using a gamma camera as medical tracers emit gamma photons

Question 33

gamma camera use and procedure

Answer 33

detects the gamma photons emitted from the medical tracer injected into the patient, and an image is constructed indicating the conc of the tracer within the patient’s body
the gamma photons travel towards the collimator, a honeycomb of long, thin tubes made from lead. any photons arriving at an angle to the axis of the tubes are absorbed by the tubes, so only those travelling along the axis of the tubes reach the scintillator.
a single gamma photon striking the scintillator produces throusands of photons of visible light. not all the gamma photons produce these tiny flashes, because the chance of a gamma photon interacting with the scintillator is about 1 in 10.
the photos of visible light travel through the light guide into the photomultiplier tubes. these tubes are arranged in a hexagonal pattern. a single photon of light entering a photomultiplier tube is converted into an electrical pulse. the outputs of all the PM tubes are connected to a computer. a software processes the electrical signals from the tubes to locate the impacts of the gamma photons on the scintillator. these impact positions are used to construct a high quality image that shows the concs of the medical tracer within the patient’s body. the final image is displayed on a screen

Question 34

how does a gamma camera differ from an xray imaging technique

Answer 34

gamma camera produces an image that shows the function and processes of the body rather than its anatomy

Question 35

technetium-99m

Answer 35

extremely versatile radioisotope that can be used to monitor the function of major organs such as the heart, liver, lungs, kidneys and brain
the isotope is produced from the natural radioactive decay of molybdenum-99
the Mo-99 isotope decays by beta-minus emission with a half life of 67 hours
Tc-99m is a daughter nucleus in this decay, and it too is unstable
the Tc-99m isotope loses energy by emitting a gamma photon with energy of exactly 140keV, with a half life of about 6 hrs

Question 36

fluorine-18

Answer 36

versatile radiopharmaceutical (medical tracer) used in positron emission tomography (PET)
the isotope is a positron emitter with a half life of about 110 mins
a nucleus of fluorine-18 decays into a nucleus of oxygen-18, a positron, a neutrino and a gamma photon
F-18 has to be made either on-site or in a specialist lab near the hospital with a particle accelerator

Question 37

diagnosis using PET scans

Answer 37

like a CAT scan, produces slices through the body that can be used to construct a detailed 3D image, but gamma radiation is used instead of xrays
most use a medical tracer called fluorodeoxyglucose, which is similar to naturally occurring glucose but is tagged with a radioactive F-18 atom in place of one oxygen atom
an advantage of using FDG is that our bodies treat it like normal glucose. when it is injected into the patient it accumulates in tissues with a high rate of respiration
the activity from FDG in the body is moitores using gamma detectors

Question 38

carbon monoxide use in PET scans

Answer 38

another medical tracer used for PET scanning is carbon monoxide made using the carbon-11 isotope. this isotope emits a positron and has a half life of about 20 mins
carbon monoxide is v good at clinging onto haemoglobin molecules in the rbcs, so it can be transported through the body and the concs of carbon monoxide can be monitored in a PET scan

Question 39

the PET scanner

Answer 39

the patient lies on a horizontal table and is surrounded by a ring of gamma detectors. each detector consists of a photomultiplier tube and a sodium iodide scintillator, and produces a voltage pulse or signal for every gamma photon incident at its scintillator. the detectors are connected to a high speed computer.
the patient is injected with FDG. the PET scanner detects the gamma photons emitted when the positrons from decaying fluorine-18 nuclei annihilate with electrons inside the patient
the annihilation positron and an electron produces two gamma photons travelling in opposite directions, so momentum is conserved
the computer can determine the point of annihilation from the diff in the arrival times of these photons at the two diametrically opposite detectors and the speed of the photons
the voltage signals from all the detectors are fed into the computer, which analyses and manipulates these signals to generate an image on a display screen in which diff concs of the tracer show up as areas of diff colours and brightness.

Question 40

advantages of PET

Answer 40

a non invasive technique (patient is not subjected to the risks of surgery)
used to help diagnose diff types of cancers, to help plan complex heart surgery, and to observe the function of the brain
it can help doctors identify the onset of certain disorders of the brain, such as Alzheimers disease
PET scans are also being used to assess the effect of new medicines and drugs on organs

Question 41

disadvantages of PET

Answer 41

technique is very expensive because of facilities required to produce the medical tracers
PET scanners are found only at larger hospitals, and only patients with complex health problems are recommended for PET scans

Question 42

ultrasound scans

Answer 42

ultrasounds used for medical imaging has frequencies in the range of 1-15MHz. it can be refracted as it travels between substances, reflected at the boundary between two substances, and diffracted by small structures or apertures.

Question 43

ultrasound

Answer 43

longitudinal sound wave

frequency greater than 20kHz

Question 44

benefits of using ultrasound to form images of the internal structures of the body

Answer 44

it is non ionising and therefore harmless, it is non invasive (no surgery) and it is quick

Question 45

human hearing

Answer 45

we can hear sounds with frequencies in the range from 20Hz to 20kHz

Question 46

ultrasound transducer

Answer 46

a device used both to generate and to receive ultrasound

it changes electrical energy into sound and sound into electrical energy, by means of the piezoelectric effect

Question 47

piezoelectric effect

Answer 47

the production of an emf by some crystals, such as quartz, when they are compressed, stretched, twisted or distorted

Question 48

A-scans

Answer 48

The simplest type of ultrasound scan is called and an A-scan. A single transducer is used to record along a straight line through the patient.
An A-scan can be used to determine the thickness of bone or the distance between the lens and the retina in the eye.
This technique is being superseded by more elaborate techniques such as the B scan, but it provides a useful insight into the principles of using ultrasound to scan internal structures.
Consider a transducer sending ultrasound pulses into the body of a patient. Each pulse of ultrasound will be partly reflected and partly transmitted at the boundary between any two different tissues. The reflected or echo pulse will be received at the transducer. It will have less energy than the original pulse because of energy losses within the body and also because some of the energy of the original pulse is transmitted through the Boundary
The pulsed voltage at the ultrasound transducer is displayed on an oscilloscope screen or computer screen as a voltage against time plot.
The time interval is the time taken for the ultrasound pulse to travel from the front of the transducer to the retina and then back to the transducer. The total distance travelled by the ultrasound pulse is 2L, where L is the distance between the transducer and the retina.

Question 49

B-scans

Answer 49

When you see images of an ultrasound scan, they are most likely to be a B-scan, which provides a 2D Image on the screen. In a B-scan, the transducer is moved over the patient’s skin. The output of the transducer is connected to a high-speed computer. For each position of the transducer, the computer produces a row of dots on the digital screen- each dot corresponds to the boundary between two tissues
The brightness of the dot is proportional to the intensity of the reflected ultrasound pulse. The collection of dots produces correspond to the diff positions of the transducer over the patient, making a 2D image of a section through the patient

Question 50

What happens at a boundary when a uniform beam of ultrasound is incident at a boundary between two substances?

Answer 50

Proportion of its intensity will be reflected and the remainder will be refracted. The fraction of the ultrasound intensity reflected at the boundary depends on the acoustic impedance of both media

Question 51

Acoustic impedance

Answer 51

It describes how much resistance an ultrasound beam encounters as it passes through a tissue
The acoustic impedance Z of a substance is defined as the product of the density p of the substance and the speed c of ultrasound in that substance
Z=pc
The SI Unit of acoustic impedance is kg m-2 s-1

Question 52

Intensity reflection coefficient

Answer 52

The ratio of reflected intensity over incident intensity for ultrasound incident at a Boundary
Ir/I0=(Z2-Z1)^2/(Z2+Z1)^2
Z= acoustic impedance
Ir= reflected intensity
I0= incident intensity

Question 53

Acoustic matching – coupling gel

Answer 53

When an ultrasound transducer is placed on the skin of a patient air pockets will always be trapped between the transducer and the skin. The air-skin boundary means that about 99.9% of the incident ultrasound will be reflected before it even enters the patient.
To overcome this problem, a special gel, called a coupling gel, with acoustic impedance similar to that of skin is smeared onto the skin and the transducer.
The gel fills air gaps between the transducer and the skin and ensures that almost all the ultrasound enters the patient’s body.

Question 54

Impedance matching/acoustic matching

Answer 54

Terms used when two substances (e.g. coupling gel and skin) have similar values of acoustic impedance. In this case, negligible reflection occurs at the boundary between two substances

Question 55

Doppler ultrasound

Answer 55

The frequency of ultrasound changes when it’s reflected off a moving object – the Doppler effect. Doppler ultrasound, and non-invasive technique, uses the reflection of ultrasound from iron rich blood cells to help doctors to evaluate bloodflow through major arteries and veins, such as those in the arms, legs, neck and even the heart.
The technique can be used to reveal blood clots (thrombosis), identify the narrowing of walls caused by accumulation of fatty deposits (atheroma), and evaluate the amount of bloodflow to a transplanted kidney or liver.

Question 56

Colour Doppler scans

Answer 56

During Doppler ultrasound, the ultrasound transducer is pressed lightly over the skin above the blood vessels. The Transducer sends pulses of ultrasound and receives the reflected pulses from inside the patient. Ultrasound reflected off tissues will return with the same frequency and wavelength, but that reflected off the many moving blood cells will have a changed frequency.
The frequency is increased when the blood is moving towards the transducer and decreased when the blood is receding from the transducer. The frequency shift or change in frequency is directly proportional to the speed (of approach or recession) of the blood.
The transducer is connected to a computer that produces a colour-coded image to show the direction and speed of the bloodflow on a screen.

Question 57

Determining the speed of blood

Answer 57

Ultrasound in scans has a frequency in the range 5 to 15 MHz, and in blood flow analysis this can give a Doppler shift up to 3KHz.
Change in f= 2fvcos(-)/c
f is the original ultrasound frequency
v is the speed of the moving blood cells
c is the speed of the ultrasound in the blood
Theta is The angle that the axis of the probe is held to the blood vessel