Module 6.5 - Medical Imaging

Question 1

Simple scattering

Answer 1

Incident x-ray photon -> scattered x-ray photon with same energy
No change in energy
No absorption of the photon

Question 2

Photoelectric effect

Answer 2

Incident x-ray photon -> photoelectron
Maximum kinetic energy of emitted photoelectrons = incident photon energy
Measuring energy of emitted electrons is a method of detecting/measuring x-ray energy

Question 3

Compton scattering

Answer 3

Incident x-ray photon -> scattered x-ray photon + electron ejected
Electron ejected with small fraction of kinetic energy from incident photon, photon with remaining kinetic energy emitted
Mass-energy conserved
Photon and electron scattered in different directions due to conservation of momentum
If photon is deflected at a large angle, it has lost more energy so has a longer wavelength

Question 4

Pair production

Answer 4

Incident x-ray photon -> positron + electron
Beam of high frequency x-rays causes photon to interact with nucleus
Photon vanishes, spontaneously producing a positron and electron (pair production occurs)

Question 5

Define attenuation

Answer 5

A gradual decrease in intensity

Question 6

2 methods of producing x-rays

Answer 6

1 - fast-moving electrons decelerate rapidly when interacting with a metal target; as an electron interacts with electric field around the nucleus an x-ray photon is emitted; due to conservation of energy the kinetic energy lost by the electron is equal to energy of emitted photon
2 - fast-moving electron ejects inner/local electron in an atom of target metal; electron from higher energy level moves down to replace it; energy released as x-ray photon of specific wavelength corresponding to difference in energy levels

Question 7

How are electrons accelerated and then slowed down in the production of x-rays?

Answer 7

Accelerated - from cathode to anode, high p.d., evacuated chamber/vacuum (no collisions with air particles)
Slowed down - metal target

Question 8

What do the characteristic ‘spikes’ or lines on the graph of intensity against photon energy or wavelength show?

Answer 8

Due to electrons in metal target gaining energy from incoming electrons and moving up energy levels
Electrons then drop back down due to electrostatic attraction to nucleus and emit wave of energy

Question 9

Types of x-ray beam

Answer 9

Point source

Collimated

Question 10

Intensity of x-rays from a point source

Answer 10

Intensity decreases with distance according to inverse square law
I = P / A

Question 11

Intensity of x-rays from a collimated source

Answer 11

Intensity does not change over distance
Intensity decreases with distance when x-rays pass through a substance
Exponential relationship between intensity and distance through a material

Question 12

Define the attenuation coefficient

Answer 12

Constant used to calculate how the intensity of x-rays decreases as they pass through a material

Question 13

Explain contrast media

Answer 13

Bones absorb more x-rays than soft tissues as they have a higher attenuation coefficient, so used for traditional x-ray scans
To see detail of soft tissues which have similar attenuation coefficients, a contrast meal such as barium or iodine is used as they have a higher attenuation coefficient

Question 14

What does CAT scan stand for?

Answer 14

Computerised axial tomography

Question 15

Explain CAT scans

Answer 15

Process using multiple x-ray scans to produce images of ‘slices’ through the body in one plane in order to build up a 3D image
X-ray source and stationary detectors around patient
Patient may have contrast medium (e.g. barium meal)
Thin, fan-shaped x-ray beam produced from point source
X-ray source rotates around patient, producing slices of a cross-sectional image (due to thin beam)
More absorption by dense materials/materials with high acoustic impedance
Process continues in a spiral as patient moves through machine
Computer analyses data, 3D image formed

Question 16

Advantages of CAT scan over x-ray image

Answer 16

Accurate image of position of internal organs without them being obscured by other structures
Sensitive to changes in density so better contrast for different soft tissues
Digital technology allows image to be rotated to view patient’s body from different angles without having to take more images which would expose them to more potentially harmful x-rays
Technology allows areas with density of bone or air to be removed/made transparent

Question 17

Define tracer

Answer 17

A radioactive substance that is either injected or ingested by a patient

Question 18

What must be considered about medical tracers?

Answer 18

Gamma sources must be used (alpha and beta would be absorbed by body, not pass through to detectors and damage body due to higher ionising power)
Half-life must be long enough to carry out investigation but not long enough to cause damage to cells (needs time to be brought from manufacturing site at hospital)
Must not be toxic to humans

Question 19

Components of a gamma camera

Answer 19

Collimator
Scintillator
Photomultiplier tubes
Computer and display

Question 20

Collimator

Answer 20

Collimates gamma photons
Series of hundreds of narrow parallel tubes in block of lead
Ensures all emerging rays are parallel
Lead absorbs non-parallel gamma rays (creates sharper image)

Question 21

Scintillator

Answer 21

Large cylindrical crystal of sodium iodide
Sodium iodide is a fluorescent material and scintillates when it absorbs a gamma photon (emits many photons of visible light)
Connected to several photomultiplier tubes arranged in a hexagonal pattern

Question 22

Photomultiplier tubes

Answer 22

Photon hits start of tube and electron is released via photoelectric effect
As that photoelectron travels down tube, it interacts with the material so multiple electrons are emitted
Current is created by each incident photon
Therefore effect of single photon is amplified
Each tube is connected to the computer

Question 23

Computer and display

Answer 23

Computer receives electrical signals which it uses to build up an overall image shown on the display

Question 24

Technetium-99m

Answer 24

Most common isotope used for tracers in gamma cameras
Can diagnose diseases in thyroid, liver, brain, kidneys, lungs, spleen, heart and circulatory system
Used to study blood flow
Half-life of 6 hours
Chemical properties allow small quantity to be incorporated into other molecules
Not toxic
Gamma emitter

Question 25

What does PET scans stand for?

Answer 25

Positron emission scanning

Question 26

Explain PET scanning

Answer 26

Fluorodeoxyglucose (fluorine-18 added onto glucose) is injected into the patient
Beta-plus decay occurs so a positron is emitted from the nucleus
It is attracted to an electron, resulting in electron-positron annihilation which produces two gamma rays travelling in opposite directions (due to conservation of momentum)
These are detected by a ring detector which can pinpoint where the gamma rays came from
Glucose is used in all respiring cells so fluorodeoxyglucose is used in cells/tissues that respire more (e.g. heart or cancerous cells)
This allows a 3D image to be built up, highlighting any areas with high activity

Question 27

Diagnostic purposes of PET scans

Answer 27

Study blood flow and metabolism
Detect problems with nervous system
Find changes in brain that may lead to epilepsy
Study and analyse certain cancers
Determine how advanced a cancer is or to help choose the best treatment for it

Question 28

Compare PET scans and CAT scans

Answer 28

PET is more expensive
PET can take 2-4 hrs, CAT can take 30 mins
In PET dose of tracer is equal to exposure of standard x-ray at a hospital, in CAT doses are equal to 5 yrs of exposure to background radiation and have been linked to developments of cancers
PET show biological function of part of body, CAT shows detailed tissue and bone structure images
PET uses gamma, CAT uses x-rays
PET uses FDG, CAT uses iodine and barium

Question 29

Define ultrasound

Answer 29

Longitudinal wave with a frequency greater than 20kHz

Question 30

What is the piezoelectric effect?

Answer 30

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

Question 31

Explain how the piezoelectric effect is used with ultrasound

Answer 31

Piezoelectric crystal used in ultrasound transducer which emits and detects ultrasound waves (as waves are sent out in pulses so if it’s constantly emitting the echoes wont be detected)
Ultrasound is produced at a frequency of around 1MHz
Crystal contracts when an alternating p.d. is applied across it
Crystal will oscillate at same frequency as alternating supply
A high frequency alternating p.d. will result in more oscillations and ultrasound waves will be emitted
Reverse process to detect ultrasound

Question 32

Advantages of ultrasound scanning

Answer 32

No known dangers as ultrasound is not ionising radiation
Can obtain real-time images of soft tissue (e.g. heart)
Machines are inexpensive and portable

Question 33

Examples of ultrasound scans

Answer 33

Prenatal scans - determine due date, structural abnormalities, baby’s sex
Echocardiogram - monitors heart function, real-time images

Question 34

Uses of radiation in medicine

Answer 34

Diagnosis (PET, x-rays, CAT, tracers)
Treatment (radiotherapy, palliative care, fluoroscopy)
Sterilisation

Question 35

Define piezoelectricity

Answer 35

The phenomenon of turning mechanical energy into electrical energy and visa versa

Question 36

A-scan

Answer 36

Amplitude scan
Short pulse of ultrasound sent into body at same time as electron beam travels across screen of cathode ray oscilloscope (CRO)
Transducer receives reflected pulses causing vertical spikes on CRO screen
X-axis shows time taken for echo to be detected by transducer, can be used to work out depth/thickness of tissue
No photo but measurements can be taken to determine dimensions

Question 37

B-scan

Answer 37

Brightness scan
Real time 2D/3D image built up from many echoes recorded from several transducers in an array or a transducer moved to different angles/positions around the patient
Greater amplitude of reflected pulse, brighter dot
Range of brightnesses shown where different bone, liquid and soft tissue reflect different proportions of ultrasound beam

Question 38

Define acoustic impedance

Answer 38

Density of material x speed of sound through material

Question 39

Define impedance matching

Answer 39

Reduction in intensity of reflected ultrasound at the boundary between two substances, achieved when the two substances have similar or identical acoustic impedances

Question 40

Explain impedance matching

Answer 40

Gel must be used to couple transducer to skin due to very high value of the fraction of incident ultrasound waves that are reflected when ultrasound passes between two very different materials
Gel has similar acoustic impedance to skin

Question 41

Explain how the Doppler effect can be used in medicine

Answer 41

For a moving object (blood) the reflected pulse will have a different frequency to the one that was emitted
Greater change of frequency mean greater speed
Lower frequency/positive difference means blood is flowing away from source
Higher frequency/negative difference means blood is flowing towards source
Can be used to measure speed of moving object (blood)