Transcript The A-scan

2.7 Imaging Techniques
You should be able to:
2.7.1 Describe the flexible endoscope in terms of structure,
technique and applications
2.7.2 Describe ultrasonic A scans and B scans in terms of
physical principles, basic equipment, technique and application
2.7.3 Describe CT scans in terms of physics principles, basic
equipment, technique and applications
2.7.4 Describe MRI scans in terms of physical principles, basic
equipment, technique and applications.
The Endoscope
The fibre optic endoscope is a
flexible tube for looking inside
the body. It can be used to
remove samples of suspect
tissue (biopsy), to inject fluids
into the body or surgery
(known as key hole surgery)
to remove gallstones or
foreign bodies from the
oesophagus.
The main shaft is constructed from metal mesh and is covered in
plastic to ease its insertion into the body. It is about 10mm in
diameter and can be up to 2m long.
Obtaining the Picture
Two separate bundles of optical fibres are used in the
endoscope:
(1) A non coherent bundle which is used to transmit light to the object being
viewed (called the illumination bundle)
(2) A coherent bundle which is used to carry back the reflected light with the
image (called the image bundle).
Recap: Total Internal Reflection
When light is shone down an optical fibre at an angle of
incidence which is greater than the critical angle of the glass
and cladding, total internal reflection occurs.
Light is then transmitted through the optical fibre through
flashes and these flashes are converted to a digital signal at
the end.
The Optical Fibre bundles
(1) The Coherent Bundle: The image bundle
• Fibres are arranged in parallel to each other. This means
they can transmit an ‘in phase’ image.
• The narrower the individual fibres, the greater the fraction
of the cross sectional area of the whole bundle that is
occupied by the core (rather than the cladding) and the
greater the resolution of the image. The diameter of a
single fibre is therefore ~0.01mm and ~10,000 of these
fibres are packed together to form a bundle a few mm in
diameter.
(2) The Non Coherent Bundle: The light bundle
• do not need to be in the same relative position and can be
thicker since resolution is not important.
• this makes them more efficient at transmitting light
because there are fewer reflections per unit path length.
The Structure of the Endoscope: The Shaft
The Shaft consists of:
•The non coherent bundle
•The coherent bundle
•An irrigation channel
• An operations channel to carry out the required task
•Control cables to control the direction of the bending of the shaft
The Structure of the Endoscope: The Shaft
The viewing end consists of:
•An eye piece which controls the focus and allows a camera
attachment
•Distal tip deflection controls to control the bending of the shaft
• Objective lens control to focus the lens at the distal tip
•Valve controls to control air aspiration (to suction body fluids
through the operations channel) or the irrigation channel.
•Operating valve which controls the entry for catheters,
electrodes, biopsy forceps and other flexible devices.
Advantages and Disadvantages
The Advantages of using the endoscope
(1) The endoscope is much less invasive than open surgery
because only a small incision in the body is required where
as open surgery requires deep incisions. This also means
that recovery is quicker and there is less swelling, scarring
and risk of infection.
(2) Endoscopes can be used by an outpatients department
and does not need to be done by a hospital. This reduces
costs.
The Disadvantages of using the endoscope
•It can only see the surface of the organ
•Can only view a small area at a time
Endoscope Images
This image was taken of a patient with
hematemesis. This image revealed that it
was due to a bleeding ulcer in the
oesophagus.
This image shows an elastic hair tie that
was swallowed by a patient.
This is an image of a sessile polyp in a
patient's duodenum. By using biopsy, it was
found that it was a tubulovillous adenoma
Past Paper Question No. 3
June ‘09 AS3
5(a) A flexible endoscope contains two different bundles of optical
fibres. One bundle is said to be coherent and the other non
coherent.
(i) Explain the meaning of the term non coherent in this context
•The fibres are arranged randomly at each end or no spatial
alignment of the fibres at the ends [1]
(ii) Explain the purpose of each type of bundle in the endoscope
•Non-coherent: provides illumination for viewing [1]
•Coherent: transmits an image from one end to the other [1]
2.7 Imaging Techniques
You should be able to:
2.7.1 Describe the flexible endoscope in terms of structure,
technique and applications 
2.7.2 Describe ultrasonic A scans and B scans in terms of
physical principles, basic equipment, technique and application
2.7.3 Describe CT scans in terms of physics principles, basic
equipment, technique and applications
2.7.4 Describe MRI scans in terms of physical principles, basic
equipment, technique and applications.
Piezoelectric Crystals
It was found that a piezoelectric crystal (e.g. quartz) changes
shape and vibrates when a potential difference is applied. At a
high frequency the vibrations from the crystal generates
ultrasound waves.
The reverse is also true:
If an ultrasound beam is incident
on the crystal it will resonate when
the frequency of the beam
matches the natural frequency of
vibration of the crystal. This
resonance enables the crystals to
generate an alternating potential
difference
The Piezoelectric transducer
This means that a piezoelectric crystal can be used as a:
•Ultrasound generator – by applying an alternating pd of
appropriate ultrasound frequency, thereby causing crystal
vibrations and the emission of ultrasound waves
Alternating pd applied crystals emit ultrasound waves
•Ultrasound detector – by monitoring the piezoelectric pd
developed across the crystal when it is forced to vibrate by
incoming ultrasonic waves.
Crystal vibrates by incoming ultrasound waves
transducer picks up pd
The Ultrasound Transducer
• The ultrasound produces short pulses so the crystal is backed by
a material which damps the vibrations quickly (the backing block).
• A thin disc of piezoelectric crystal has silver electrodes attached
to each face. One electrode is earthed and the other is connected
to a coaxial cable.
• The coax cable is then connected to a high voltage power supply
when the probe is used as a transmitter (to produce a vibration in
the crystals); and to an amplifier (and CRO) when used as a
receiver (receiving the alternating potential differences).
Partial Reflections and Energy
When a wave travels from one medium into another not all of the ray
passes into the second material. There will also be a weak reflected
ray, which will obey the laws of reflection. This is called partial
reflection.
Incident
wave
Weak
reflected
wave
i
r
refracted
wave
This means that some of the energy from the incident wave has
been transferred to the reflected wave (~10%), and the rest to the
refracted wave (~90%). The exact amount of energy transferred
depends upon how different the two materials on each side of the
boundary are.
frequencies of 1 -5MHz  at each tissue boundary the
The
Scanning
process
ultrasonic pulse is partially reflected (about 1% of the
input induced energy)  this reflected wave is detected
by the sensitive detector in the probe to form the image
 the echoes are converted into electrical signals which
can be displayed on a CRO (oscilloscope).
• The remainder of the pulse then travels deeper into the body for
more reflections to occur at the other boundaries.
• All of the echo pulses received from the various organs are
converted into an electrical signal which can be displayed as the
image on the CRO screen.
When the ultrasound probe is initially placed on skin the air/body
boundary produces excessive reflection and there is not sufficient
input induced energy travelling into the body. In order to reduce
this reflection a film of oil or jelly is used as a coupling medium.
The coupling medium
When the ultrasound probe is initially placed on skin the
air/body boundary produces excessive reflection and there
is not sufficient input induced energy travelling into the body.
In order to reduce this reflection a film of oil or jelly is used
as a coupling medium.
Obtaining the information
Information obtained by the ultrasound probe can be gained
from two different methods: (1) the Amplitude, A-scan and (2)
the Brightness, B-scan.
The A-scan
The A –scan is used to take detailed
measurements of the eye or the size of the
foetal skull and identification of tumours.
Can be used in order to measure distances – it does
not produce pictures. A transducer emits an
ultrasonic pulse into the body and the time taken for
the pulse to bounce off an object and come back is
measured in order to determine how far away the
object is. The CRO is used to accurately measure
this time difference – the speed of ultrasound is
known, so the distance travelled can easily be found.
A-Scan: Example
The time delay between the two traces on the CRO (shown
by the arrow in the diagram) is 60μs and the speed of
ultrasound in the body is 1500ms-1. Find the size of the
eyeball.
Time it takes for ultrasound to go to the eyeball and back =
60μs
Speed = 1500ms-1.
Using speed = distance/time => 1500 = distance/60 x 10-6
Distance = 0.09m. This is time to get there and back=>
Eyeball is (0.09/2) = 0.045m.
(2) B-Scan
B-scans can be used to take an image
of a cross-section through the body.
The transducer is swept across the
area and the time taken for pulses
to return is used to determine
distances.
The amplitude of the reflected pulse is also measured and are
plotted as a series of dots of different brightness to produce a
picture. B-Scans will give two-dimensional information about
the cross-section.
B-scans are used to produce pictures of a developing foetus;
it is also used to detect cysts, abscesses and tumours as well
as assisting surgeons with minor operations.
Advantages and Disadvantages
Advantages of ultrasound
• Portable
• Easy to use
• Good for soft tissues
• Relatively cheap
Disadvantages of ultrasound
• Unsuitable for imaging the lungs or bowel
• Resolution is not as good as other methods
Ireland 1999:
America 2003:
Ultrasound Images
Past Paper Question No. 2
Jun 08 AS3
(a) (i) Describe the basic principle of operation of an ultrasonic
generator when used to produce ultrasound.[2]
(a) (i) (piezoelectric) crystal connected to a.c. generator/source [1]
(adjust frequency to) resonance/cause change in shape/
vibrations [1] [2]
(ii) There is a distinct difference between the information that can
be obtained from an ultrasonic A-scan and an ultrasonic B-scan.
State this difference. [2]
(ii) A scan – measures distances/depths/sizes [1]
B scan – provides pictures or images of organs/foetus etc [1] [2]
Past Paper Question No. 4
(a) Transducer used in ultrasound = piezoelectric crystal
(b) Distance = speed x time = 4000x9.6x10-6 = 38.4mm =>
thickness = 19.2mm
(c) There would be too much reflection at the surface of the
skin and this would reduce the intensity of the signal
reflected from the organ under test
(ii) Coupling medium/ gel
(d) (i) 1. B scans are swept across the cross section of the
body / B scans collect information on intensity of reflected
wave to build up picture
2. A scans just emit a pulse and wait for a reflection to be
detected/ can only measure distances
(ii) Foetal scanning/detection of cysts or tumours
2.7 Imaging Techniques
You should be able to:
2.7.1 Describe the flexible endoscope in terms of structure,
technique and applications 
2.7.2 Describe ultrasonic A scans and B scans in terms of
physical principles, basic equipment, technique and application
2.7.3 Describe CT scans in terms of physics principles, basic
equipment, technique and applications
2.7.4 Describe MRI scans in terms of physical principles, basic
equipment, technique and applications.

CT Scans
Computed Tomography (CT) imaging is also
known as "CAT scanning" (Computed Axial
Tomography)
CT was invented in 1972 by British engineer Godfrey Hounsfield
of EMI Laboratories, England and by South Africa-born physicist
Allan Cormack of Tufts University, Massachusetts. Hounsfield
and Cormack were later awarded the Nobel Peace Prize for
their contributions to medicine and science.
The CT scan produces a set of 2D images of different slices of
the body. A computer can then use the different 2D images to
produce a 3D image displayed on a computer. The CT scan is
used to produce detailed images of the brain, liver and kidneys.
It can also be used to measure bone mineral density for the
detection of osteoporosis.
The Scan process
In a conventional X ray
both the emitter and
detector are stationary, but
in a CT (or CAT) scan the
detectors are arranged in a
circular arc.
The patient is placed on a bed and both the X-ray tube and the
2000 tiny detectors are rotated about the centre of the slice being
imaged. The complete scan takes place in around 5 seconds,
which is much safer for the patient than conventional X rays.
The production of the image is a complex process which takes
place using powerful computers. Each pixel of the image involves
1 million calculations.
Advantages and disadvantages
Advantages of the CT scan
• Images can be scored in computer memory
• The computer can also be used to construct a slice in a
different plane using other visual data
• CT scans give good contrast images
Disadvantages of the CT scan
• Risk to the patient because of the high radiation dose
• Very expensive
CT Scan Images
CAT scan ‘slice’ of brain
CAT scan of heart
CAT scan of brain
Past Paper Question No. 2
Jun 08 AS3
(ii) In what way does a conventional X-ray image differ from an
image produced by X-ray tomography? [1]
(ii) X-ray image – 2d/shadow picture, X-ray tomography
cross-section of body/3d [1]
Past Paper Question No. 2
(c) Both ultrasound B-scans and computerised tomography are
widely used in medical diagnosis. Briefly compare the two
types of imaging.
In your answer you should refer to
(i) similarities and differences in the basic techniques used,
(ii) advantages and disadvantages of each method.
• both produce sections of body /CT measures intensity
transmitted and US measures intensity reflected
•X-rays harmful/ionising radiation or ultrasound harmless
•US cannot see through bone (or air as too much reflection)
•US used for foetal scanning and CT for brain scanning
•Both ultrasound B-scan and tomography use computers to
interpret image
• other relevant points, e.g. CT more expensive
Not allowed: portability, resolution any 4
Quality of written communication [1]
2.7 Imaging Techniques
You should be able to:
2.7.1 Describe the flexible endoscope in terms of structure,
technique and applications 
2.7.2 Describe ultrasonic A scans and B scans in terms of
physical principles, basic equipment, technique and application
2.7.3 Describe CT scans in terms of physics principles, basic
equipment, technique and applications

2.7.4 Describe MRI scans in terms of physical principles, basic
equipment, technique and applications.

MRI Scan
The MRI scan works on the
principle of Nuclear Magnetic
Resonance or NMR. It uses
the fact that the nucleus of
atoms spin and have an
electric charge produces a
magnetic field (just like
current in a magnet).
The MRI scan has only been used for medical imaging since the
1970s but is now used for many applications such as brain scans
and provide more detailed images than conventional X rays.
MRI Scan
MRI scanners contain either permanent magnets or
electromagnets (which are superconducting) typically able to
produce a magnetic field of 1.5T, which is 30,000 times
greater than the Earth’s magnetic field. The human body is
almost 65% water, and MRI images are obtained from the
proton within each hydrogen atom in this water.
resistance
transition
temperature
temperature
The Spinning Protons
Each of the hydrogen protons in the body spins about an
imaginary axis like a spinning top, which makes the proton
magnetic.
The proton spins points in random directions until a body is placed
in a scanner. Under the influence of the high magnetic field
created by the scanner magnet, the proton spins align either
parallel or anti-parallel to the field. Slightly more protons position
themselves parallel to the field, creating a net magnetisation.
Randomly
spinning
protons
Protons
aligned to
create net
magnetism
The Larmour Frequency
A radio frequency pulse, called the Larmour Frequency (a timevarying magnetic field) is directed at the area of the body that is to
be studied. This changes the magnetisation of the protons in that
region. Once this pulse is switched off, the protons relax into the
state they were in before it was turned on.
A conducting coil works as an antenna and detects – via
electromagnetic induction – the small changes in magnetic field
that the relaxation produces. As the type of tissue surrounding
a proton influences its behaviour after the radio frequency pulse,
computer software can analyse the detected signals and build up
a detailed image of the body that shows different types of tissue.
Graphical processors, originally developed by the computer
gaming industry are now used by some MRI systems to
reconstruct and display images quickly.
The MRI Scanning Process
The patient is placed inside the coil and
surrounding the patient is the magnetic
field produced by a super conductor
magnet. The table is then moved inside
the large cavity and a pulse of radio
waves is emitted by the coils
surrounding the patient. This causes the
hydrogen nuclei within the patient to flip.
Like the CT scan, the image is produced using complicated
computer software. This means the image cannot be obtained
immediately.
Other MRI Facts
• Usually superconducting magnets are used because they
produce large fields in a small space. In order to maintain
their superconducting temperatures (-269oC) they are
immersed in liquid helium. This is expensive to replenish and
adds to the cost of the scan procedure.
• The magnet itself is very heavy and it needs to be placed in
rooms where the floors have been reinforced to take the
weight.
• A metal cage is used to screen the magnetic field. Although
there is no radiation risk, precautions need to be taken to
ensure that there are no loose metal objects in the fields.
Patients with metal implants or pacemakers cannot use MRI
scanners.
Advantages and disadvantages
Advantages of MRI Scan:
•No ionising radiation is used, which can be dangerous
•It provides superior images to help distinguish types of tissue
over CT scan
•It is non invasive
Disadvantages of MRI scan:
•It is extremely expensive – most expensive of all scan
techniques
•Care is needed with metal objects around the scanner
•The room needs to be screened to prevent damage to
electronic equipment due to the strong magnetic field.
MRI Images
2.7 Imaging Techniques
You should be able to:
2.7.1 Describe the flexible endoscope in terms of structure,
technique and applications 
2.7.2 Describe ultrasonic A scans and B scans in terms of
physical principles, basic equipment, technique and application
2.7.3 Describe CT scans in terms of physics principles, basic
equipment, technique and applications

2.7.4 Describe MRI scans in terms of physical principles, basic
equipment, technique and applications. 

MRI Scan Past Paper Question (No. 3)
(b) Give an account of the general principles of magnetic
resonance imaging (MRI). Your account should mention some
of the problems associated with this technique, and the
precautions taken when operating it.
1. External r.f. radiation scanned [1]
2. Resonance in hydrogen atoms in body [1]
3. Hydrogen atoms change orientation in magnetic field [1]
4. Changes in magnetic field detected [1]
5. r.f. receiver detects changes [1]
6. Signals applied to computer for analysis [1]
7. High magnetic fields (2T) required
This is a difficulty in magnitude and/or cost [1]
8. Special accommodation required for magnet [1]
9. Precautions must be taken by persons in proximity of magnet, e.g. jewellery, cr
cards, pacemakers [1]
Only [1] maximum for precaution
10. or any other valid point, e.g. scanner moves round to give 3-D image
claustrophobia
Any [8] Quality of written communication [1]
General Scan Past Paper Question (No. 1)
June 07 AS 3B
5 (a) Computed Tomography (CT) and Magnetic Resonance
Imaging (MRI) are powerful diagnostic tools that use computers
to produce detailed images of the body.
Both techniques required electromagnetic radiation in order to
operate.
(i) Name the region of the electromagnetic spectrum used in
CT scans
X-rays [1]
(i) Name the region of the electromagnetic spectrum used in
MRI scans
Radio Waves (RF Wave)
Scan Past Paper Question
(b) Explain briefly the role played by the electromagnetic
radiation in the resonance process of MRI [2]
Match frequency [1]
Absorb energy/causing change of orientation/protons “flip” [1]
(c) (i) In ultrasonic imaging a piezoelectric crystal is used both to
generate and detect ultrasonic waves. Describe briefly how
ultrasonic waves are generated using the piezoelectric effect.[2]
Resonance/large vibration amplitude/
large ultrasound amplitude/frequency of
applied p.d. = applied frequency [2]
(Detection, 0/2)
ii) Fig. 5.1 shows the printout from an ultrasound A-scan
from an investigation of a patient’s eye.
a: Initial spike (probe tip and cornea)
b: Front lens surface
c: Back lens surface
d: Retina
e: Sclera
f: Orbital fat
Ultrasound velocity in lens = 1641ms–1.
Use the data from the A-scan, and associated information
above, to calculate the thickness of the eye lens.
s = ut in any form [1]
Identify t in range 4.5–5.5 (μs) [1]
Echo awareness [1]
Ans in range 3.7 mm–4.5 mm [1]
[10n error, –1 No echo awareness (→ 8.2) 2/4
Uses wrong peaks, can score 1, 0, 1, 0 = 2/4