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.

2.7 Imaging Techniques

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 Endoscope

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 PictureTwo 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

Images of internal organs obtained using an endoscope

                                 

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

This image shows an elastic hair tie that was swallowed by a patient.

This image was taken of a patient with hematemesis. This image revealed that it was due to a bleeding ulcer in the oesophagus.

Endoscope Images

Images of internal organs obtained using an endoscope

                                 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

Past Paper Question No. 3

•The fibres are arranged randomly at each end or no spatialalignment of the fibres at the ends [1]

•Non-coherent: provides illumination for viewing [1]•Coherent: transmits an image from one end to the other [1]

(ii) Explain the purpose of each type of bundle in the endoscope

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.

2.7 Imaging Techniques

Piezoelectric Crystals

 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

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.

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 Piezoelectric 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).

The Ultrasound Transducer

Partial Reflections and EnergyWhen 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.

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.

Incidentwave

Weakreflectedwave

refractedwave

i

r

Ultrasound pulses are transmitted into the body at frequencies of 1 -5MHz at each tissue boundary the 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 Scanning process

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

Information obtained by the ultrasound probe can be gained from two different methods: (1) the Amplitude, A-scan and (2) the Brightness, B-scan.

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.

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.

Obtaining the information

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.

A-Scan: Example

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.

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.

(2) B-Scan

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 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

Advantages and Disadvantages

Ultrasound ImagesIreland 1999:

America 2003:

Past Paper Question No. 2Jun 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 picture2. 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

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.

2.7 Imaging Techniques

Computed Tomography (CT) imaging is also known as "CAT scanning" (Computed Axial Tomography)

CT Scans

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.

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 Scan process

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 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

Advantages and disadvantages

CAT scan of brain

CAT scan of heart

CAT scan ‘slice’ of brain

CT Scan Images

(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 tomographycross-section of body/3d [1]

Past Paper Question No. 2Jun 08 AS3

(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 expensiveNot allowed: portability, resolution any 4 Quality of written communication [1]

Past Paper Question No. 2

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.

2.7 Imaging Techniques

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 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

temperaturetransitiontemperature

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.

The Spinning Protons

Randomly spinning protons

Protons aligned to create net magnetism

A radio frequency pulse, called the Larmour Frequency (a time-varying 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 Larmour Frequency

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.

The MRI Scanning Process

Like the CT scan, the image is produced using complicated computer software. This means the image cannot be obtained immediately.

• 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.

Other MRI Facts

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.

Advantages and disadvantages

MRI Images

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.

2.7 Imaging Techniques

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) requiredThis 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, credit cards, pacemakers [1]Only [1] maximum for precaution10. or any other valid point, e.g. scanner moves round to give 3-D image claustrophobiaAny [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 ofapplied 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.

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.

a: Initial spike (probe tip and cornea)b: Front lens surfacec: Back lens surfaced: Retinae: Scleraf: Orbital fat

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) 2/4Uses wrong peaks, can score 1, 0, 1, 0 = 2/4