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Diagnostic methods in medicine

Chapter 16

As we saw in the last chapter, X-rays are used to

diagnose problems in a patient because they can

be generated outsidethe body and then directed

through the patients body to produce an image. An

alternative approach is to place a source of radiation

insidethe patient and use the radiation that emerges

from inside the body to produce an image.

An example of this approach is where a patient

undergoes a bone scan (to see whether there iscancerous tissue in their bones). The patient is

given an injection containing the radioactive

substance. They then have a few hours in which

to relax while the material circulates around their

body. Next, they are placed in a machine called

a gamma camera (Figure 16.1) which detects

-rays coming from inside their body. The result

is an image showing points in the body where the

radioisotope has accumulated (Figure 16.2). After

the scan, the patient must take care to ush the

toilet twice after use and to avoid kissing otherpeople this is because the radioisotope is still

Using radioisotopes

active for several hours and it is present in their

saliva and urine.

In this chapter, we will look in detail at some

ways in which radioisotopes (also known as

radionuclides) are used for diagnosis. Radioisotopes

are also used in treatment, where the radiation

they produce is used to destroy harmful tissue,

particularly cancerous tumours.

Figure 16.1 A female patient undergoing a bone

scan in a gamma camera. There are two detectors,

one above the patient and one below.

Figure 16.2 Bone scan images of a healthy patient.

The radionuclide has been taken up by the patients

bones; there is a lot of it in the bladder also.

e-Learning

Objectives

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Chapter 16: Diagnostic methods in medicine

Figure 16.3shows the system used for the

extraction (elution) of Tc-99m from the container

of Mo-99. A saline solution is passed through the

container, and this dissolves out the Tc-99m.

Radiopharmaceuticals

To ensure that the radioisotope reaches the

correct organ, it must be converted into aradiopharmaceutical. This means that it is

chemically combined with other elements to produce

a substance which will be taken up by the tissue of

interest. For example, for a bone scan, Tc-99 m is

combined with a phosphorus-containing compound

and the patient injected with a dose of activity about

600 MBq. This is taken up by bone tissue, particularly

cancerous tissue where there is a high rate of

metabolism as new cells are being formed.

Because radiopharmaceuticals are designed to

target particular organs or tissues in the body, they are

often described as tracers.

A summary of a few radioisotopes used by

hospitals and their diagnostic use is given in

Table 16.1.

Choosing a radionuclideAll radioactive substances decay, some more quickly

than others. There are several hundred different

radionuclides which are found in Nature or which

can be created in a nuclear reactor or a small linear

particle accelerator (linac). So which ones are

suitable for medical purposes?

The radionuclide is put into the patients body and

its radiation detected. This requires that the substance

chosen should be a gamma emitter. (An alpha or beta

source is not suitable because the body will absorb

the - or -particles. These types of radiation are

also extremely lethal inside the body because of their

strong ionising properties.)

The radionuclide should also have a short half-life.

There are two reasons for this.

It will give out its radiation quickly, so that only1 a small amount is needed to form an image in the

gamma camera.

Any radionuclide that remains in the patient2

will soon decay away, ensuring that they are not

exposed to hazardous levels of radiation.

The problem then is that, if a hospital buys a batch of

a short-lived radioisotope, it has bought something

that is rapidly decaying away. What can be done to

stop it decaying before the hospital has time to make

use of it?

One solution is illustrated by a radioisotope calledtechnetium-99m. This is an isotope of the element

technetium (Tc) with nucleon number 99. Tc-99m is

produced when molybdenum-99 undergoes decay.

This happens in two stages:

9942 Mo

9943 Tc

m+ 01 e + half-life 67 h

9943 Tc

m 9943 Tc + half-life 6 h

9943 Tc decays by emission half-life 2.110

5years

The m indicates that 9943 Tcmis metastable, that is,

it remains in an energetic state for some time before

decaying by emission. Each -ray photon has an

energy of 140 keV. How does this solve the hospitals

problem? The nuclear medicine department of the

hospital buys a supply of Mo-99, which is produced

in a nuclear reactor. The Mo-99 then produces

Tc-99m at a predictable rate, and this can then be

extracted for use with patients.

reduced

pressure

elution vial

sodiumpertechnetate

solution

terminalfilter

lead shielding

salinereservoir

one wayair filter

Mo adsorbed on toalumina column

99

Figure 16.3 A simplied diagram of a technetium-

99m generator. It is designed to minimise the risk

that the technician will be exposed to radiation. An

evacuated collection vial on the output side draws

saline from the reservoir through the column. Here

the saline dissolves the technetium to form a solution

of sodium pertechnetate.

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SAQ

1 Mo-99 is used to produce Tc-99m. The half-life of

molybdenum-99 is 67 h.

a Explain why a hospital will require supplies of

this substance to be delivered each week.b Explain why it would be inconvenient if the

half-life of Mo-99 was much longer than

67 h, and if it was much shorter

than 67 h.

The gamma cameraIt is over 50 years since the gamma camera

(Figure 16.1) was rst invented, and it is now the

major imaging device used in diagnostic nuclear

medicine. It detects -ray photons coming fromsources such as technetium-99m inside the patient.

Inside the camera is a single, very large crystal of

sodium iodide with about 0.5% of thallium iodide,

typically between 400 and 500 mm in diameter and

912 mm thick. This crystal is a scintillator; that is, a

gamma photon incident on this material may produce

a ash of visible light in the crystal.

Figure 16.4shows how the gamma camera

constructs an image of the patients insides from these

ashes of light.

The gamma photons pass upwards through the collimator. The collimator consists of ahoneycomb of cylindrical tubes in a lead plate.

The scintillator detects only photons travelling

along the axis of these tubes. It therefore cuts

out any -rays travelling at an angle to the

scintillator.

Radioisotope Uses

uorine-18 (189 F) bone imaging

technetium-99m (9943 Tcm) bone growth

blood circulation in lung, brain and liver

function of heart and liver

iodine-123 (12353 I) function of thyroid

function of kidney

xenon-133 (13354 Xe) function of lung

Table 16.1 Some radioisotopes used in hospitals and their uses.

computer

hexagonal array

photomultipliertubes

lightguide

scintillator

collimator

display

y

a

b

x

Figure 16.4 aThe structure of a gamma camera.

bThe arrangement of photomultipliers.

Answer

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avalanche of electrons. These eventually give rise

to an electrical pulse at the last electrode. Thus

a single electrical pulse is produced by a single

photon of light incident on the photocathode.

The electrical signals from the photomultipliersare processed electronically by a computer to

produce a high-quality image on a screen. The

output from each photomultiplier corresponds to a

single point or pixel on the screen.

Uses of the gamma camera

A gamma camera is used in a bone scan. This is an

example of a static study, in which a single image is

produced a suitable time after the injection of

the tracer.

Another use is in situations where it is desired

to see the progress of the tracer through the body.An example is a kidney scan (a renogram

see Figure 16.6). The patient is given a

radiopharmaceutical which will pass through their

system and be excreted by the kidneys. A series of

images of the kidneys are made over a period of time

to see the process of excretion as it happens. This is

an example of a dynamic study.

The beam of gamma photons then strikes thescintillator crystal, where each photon produces

a ash of light. About 10% of the incident -ray

photon energy is converted into visible light.

The light is detected by one or more of thephotomultiplier tubeswhich produce an

electrical pulse for each photon of light they

receive. The photomultipliers are arranged in a

hexagonal array over the surface of the crystal.

Figure 16.5shows a single photomultiplier tube.

The incident gamma photon strikes the scintillatorcrystal to produce photons of visible light. A

single light photon releases a single electron from

the photocathode by the process of photoelectric

effect. This electron is accelerated to the +100 V

electrode (dynode) and on impact releases two

or three secondary electrons. This process isrepeated at each electrode and soon there is an

scintillation crystal

+600 V

+800 V

+400 V

+200 V

0 V

incident gamma-ray photon

+500 V

+700 V

+300 V

+100 V

single electron

dynode

photocathode

Figure 16.5 Details of a photomultiplier tube.

Figure 16.6 A gamma camera image of a patients

kidneys. The image, called a scintigram, has been

coloured to show how the intensity of the -rays

varies. The kidney on the left is functioning normally

while the one on the right has very limited blood ow

through it.

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242

However, it is not the gamma photon emitted in

this decay that we are interested in. Rather, it is the

positron 0+1 e that is useful. Once emitted, it soon

collides with an electron and the two are annihilated.

Their mass is released as energy in the form of two

gamma photons. These are emitted at 180 to each

other (see Figure 16.7). In PET scanning, it is these

two gamma photons that are detected.

PET scanner

A PET scanner looks similar to a CAT scanner but,

of course, it is detecting -rays, not X-rays. The

principle is illustrated in Figure 16.8.

The patient is injected with a positron-emitting

radiopharmaceutical, in this case a form of glucose

(sugar) tagged with uorine-18. This tends to

accumulate in tissues with a high rate of respiration.

In this case, we imagine that the doctors are looking

at brain function, so the tracer will be taken up most

by active cells in the brain.

The patient is surrounded by a ring of gamma

detectors (similar to those in a gamma camera). These

detect pairs of -rays coming from inside the patient

and travelling in oppositedirections. The times at

which they arrive at the detectors are compared and

SAQ

2 A gamma camera can be adjusted by changing

the collimator.

a The collimator is changed to one with lead

tubes of larger diameter. Explain why this will

allow a shorter exposure time, but will give a

less well-dened image.

b If the collimator is changed to one with lead

tubes of longer length, more -rays will be

cut out as they cannot pass through to the

scintillator crystal. How will this affect the

exposure time and the

denition of the image?

Positron emission tomography

PET scanning is another technique which uses thefact that gamma rays can emerge from a source inside

the body. The name is an abbreviation of positron

emission tomography. As with CAT scanning,

the word tomographyimplies that images of slices

through the body can be obtained (using computer

manipulation of the image data).

The radiopharmaceuticals used in a PET scan

contain radioisotopes that emit positrons; that is, they

are +emitters. An example is uorine-18, which

decays like this:

189 F

188 O +

0+1 e + +

gamma

detectors

e+

e

Figure 16.8 A patient undergoing a PET scan. In

this case, -rays emitted by the tracer in the brain

are being detected.

Figure 16.7 Positronelectron annihilation. The

masses of the positron e+and electron eappear as

the energy of the two gamma photons.

Answer

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SAQ

3 Fluorine-18 is a +-emitting radioisotope

commonly used in PET scanning. Its half-life

is 110 minutes. Suggest why this

makes it a good choice for

this purpose.

4 Tomography is a type of imaging.

a Describe how tomography differs from

conventional imaging.

b Describe how a PET scan

forms a tomographic image.

Magnetic resonance imagingMagnetic resonance imaging, or MRI, is another

technique from nuclear medicine. However, it does

not rely on nuclides that are radioactive; rather, it

relies on the fact that some atomic nuclei behave like

tiny magnets in an external magnetic eld.

(MRI was originally known as nuclear magnetic

resonance imaging, but the word nuclear was

dropped because it was associated in patients minds

with bombs and power stations. To emphasise: MRI

does not involve radioactive decay, ssion or fusion.)As in CAT scanning, PET scanning and the gamma

camera, MRI scanning involves electromagnetic

radiation, in this case radio frequency (RF)

electromagnetic waves. The patient lies on a bed in

a strong magnetic eld (Figure 16.10), RF waves are

sent into their body, and the RF waves that emerge are

detected. From this, a picture of the patients insides

can be built up by computer. As we will see, MRI gives

rather different information from that obtained by the

other non-invasive techniques we have been looking at.

from this the position at which they were emitted

can be determined. Because gamma photons travel

at the speed of light, the time interval that must be

measured is very small.

Gradually, a three-dimensional image of the

distribution of radioactive tracer in the patient is built

up and, from this, any abnormal functioning can be

deduced. An image of a slice through the patient can

be viewed on a computer screen.

Uses of PET scanning

PET scanning is an important diagnostic tool, in

particular for showing up cancerous tissue. However,

it has also proved very useful in showing up aspects

of normal bodily functions, such as brain activity.

Figure 16.9shows scans of a persons brain when

they were reading aloud and then silently. Differentareas of the brain are active in these apparently

similar situations.

Figure 16.9 Articially coloured PET scans of a

human brain when the subject was reading aloud

(top) and silently (bottom). Reading aloud requires

extra areas of the brain to be active in order to

control the mouth and tongue and to listen to the

sounds produced.

Answer

Answer

Extension

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244

of hydrogen atoms that are studied, since hydrogen

atoms are present in all tissues. A hydrogen nucleus is

a proton, so we will consider protons from now on.

A proton has positive charge. Because it spins, it

behaves like a tiny magnet with N and S poles. Figure

16.11ashows a number of protons aligned randomly.

When a very strong external magnetic eld is

applied, the protons respond by lining up in the eld

(just as plotting compasses line up to show the direction

of a magnetic eld). Most line up with their N poles

facing the S pole of the external eld, a low energy

state; a few line up the other way round, which is an

unstable, higher energy state (Figure 16.11b).

A proton does not align itself directly along the

external eld. In practice, its magnetic axis rotates

around the direction of the external eld (Figure

16.12), just like the axis of a spinning top. Thisrotation or gyration action is known as precession.Principles of nuclear magnetic resonanceThe nuclei of certain atoms have a property called

spin, and this causes them to behave as tiny magnets

in a magnetic eld. In MRI, it is usually the nuclei

N

SN

S

N

S

NS

NS

S

N

N

S

S

N

N

S

S

N

N

S N

SS

N

S

N

S

N

S

N

S

N

S

N

S

N

S

N

S

N

N

S

S

N

N

SN

S

NS

NS

NS

NS

NS

NS

NS N

S

N S

N S

N

S

b

a

electromagnet

external

field

N

S

Figure 16.11 How protons behave in a strong

magnetic eld. aProtons are randomly directed when

there is no external magnetic eld. bBecause protons

are magnetic, a strong external magnetic eld causes

most of them to align themselves with the eld.

gravitational field

spin

spin

axis of

spin

path of

precession

path of

precession

magnetic field

axis of spin

Figure 16.12 A spinning top (left) rotates about its

axis; at the same time, its axis precesses about the

vertical, which is the direction of the gravitational

eld. In a similar way, a proton (right) spins and its

axis of rotation precesses about the direction of the

external magnetic eld.

Figure 16.10 A patient undergoing an MRI scan of

the brain. This is a form of tomography; the display

shows different slices through the patients brain.

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In Figure 16.13, you can see that the relaxation

of the protons follows an exponential decay pattern.

Curves like this are characterised by two relaxation

times:

T 1,the spinlattice relaxation time, where the

energy of the spinning nuclei is transferred to thesurrounding lattice of nearby atoms;

T 2,the spinspin relaxation time, where the energy

is transferred to other spinning nuclei.

These relaxation times depend on the environment

of the nuclei. For biological materials, it depends on

their water content:

Water and watery tissues (e.g. cerebrospinal uid)have relaxation times of several seconds.

Fatty tissues (e.g. white matter in the brain)have shorter relaxation times, several hundred

milliseconds.

Cancerous tissues have intermediate relaxationtimes.

This means that different tissues can be distinguished

by the different rates at which they release energy

after they have been forced to resonate. That is the

basis of medical applications of nuclear magnetic

resonance.

The angular frequency of precession is called

the Larmor frequency0, and depends on the

individual nucleus and the magnetic ux densityB0

of the magnetic eld:

0= B0

So, the stronger the external eld, the faster the

protons precess about it. The quantity is called the

gyromagnetic ratio for the nucleus in question and is

a measure of its magnetism. (Note that the Larmor

frequency is measured in radians per second. This

means that, strictly speaking, it is not a frequency.)

For protons, has the approximate value

2.68108rad s1T1. To determine the frequencyf0

of the precessing nuclei, we can use the equation

0= 2f0

Therefore:

f0=B0

2

In an MRI scanner, the external magnetic eld is

very strong, of the order of 1.5 T (thousands of times

the strength of the Earths eld). The precession

frequencyf0is

f0=2.68 1081.5

2= 6.4 107Hz = 64 MHz

This frequency lies in the radio frequency (RF) region

of the electromagnetic spectrum.

You should recall that resonancerequires a

system with a natural frequency of vibration; when

it is stimulated with energy of the same frequency, it

absorbs energy. In MRI, protons precessing about the

strong external eld are exposed to a burst or pulse

of RF waves whose frequency equals the frequency

of precession. Each proton absorbs a photon of RF

energy and ips up into the higher energy state; this is

nuclear magnetic resonance (Figure 16.13).

Now we come to the useful bit. The RF waves are

switched off and the protons gradually relax into their

lower energy state. As they do so, they release their

excess energy in the form of RF waves. These can be

detected, and the rate of relaxationtells us something

about the environment of the protons.

absorption

Time00

Energy

ofprotons

relaxation

Figure 16.13 In nuclear magnetic resonance, a

spinning nucleus is ipped into a higher energy state

when it absorbs a photon of RF energy; then it relaxes

back to its lower energy state.

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246

produce an additional external magnetic eld that

varies across the patients body. These coils are

arranged such that they alter the magnitude of the

magnetic ux density across the length, depth and

width of the patient. This ensures that the Larmor

frequency of the nuclei within the patient will be

slightly different for each part of the body. This

means that only a small volume of the body is at

exactly the right eld value for resonance and so

the computer can precisely locate the source of theRF signal within the patients body and construct

an image.

A computer that controls the gradient coils and RFpulses, and which stores and analyses the received

data, producing and displaying images.

Procedure

The patient lies on a bed which is moved into the bore

of the electromagnet. The central imaging section is

about 0.9 m long and 0.6 m in diameter. The magnetic

eld is very uniform, with variations smaller than 50

parts per million in its strength. The gradient eld

is superimposed on this xed eld. An RF pulse is

then transmitted into the body, causing protons to

ip (resonate). Then the receiving coils pick up the

relaxation signal and pass it to the computer.

SAQ

5 Protons precess at a frequency of 42.6 MHz in an

external eld of magnetic ux density 1.0 T.

a Determine the frequency at

which will they precess in a

eld of magnetic ux density 2.5 T.

b State the frequency of RF radiation that will

cause the protons to resonate

in this stronger magnetic eld.

6 Figure 16.14shows how the amplitude of RF

waves coming from watery tissue varies after

resonance. Copy the graph and add lines and

labels to show the graphs you would expect

to see for cancerous and

fatty tissues.

Figure 16.14 See SAQ 6.

MRI scanner

Figure 16.15shows the main components of MRI

scanner. The main features are:

A large superconducting magnet which producesthe external magnetic eld (up to 2.0 T) needed to

align the protons. Superconducting magnets are

cooled to 4.2 K (269 C) using liquid helium.

An RF coil that transmits RF pulses into the body.An RF coil that detects the signal emitted by therelaxing protons.

A set of gradient coils. (For clarity, only one pairof gradient coils in shown in Figure 16.14.) These

Time

watery tissue

00

Amplitude

large

external

magnet

RF receiving coil

RF transmitting coil

computer

longitudinal

gradient coil

Figure 16.15 The main components of an MRI

scanner.

Hint

Answer

Answer

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One disadvantage of MRI is that any metallic objects

in the patient, such as surgical pins, can become

heated. Also, heart pacemakers can be affected, so

patients with such items cannot undergo MRI scans.

Loose steel objects must not be left in the room as

these will be attracted to the magnet, and the room

must be shielded from external radio elds.

Figure 16.17shows how an MRI scan can becombined with a CAT scan to show detail of both

bone and soft tissue, allowing medical staff to see

how the two are related. Compare this with

Figure 16.16.

SAQ

7 An MRI scan might be considered a safer

procedure than a CAT scan.

a Explain why it might be considered to be safer.

b Why might a CAT scan be chosen in preference

to an MRI scan?

c Explain why MRI is described

as non-invasive.

The result is an image like the one shown in

Figure 16.16. This image has been coloured to show

up the different tissues, which are identied by their

different relaxation times.

Advantages and disadvantages of MRI

MRI has several advantages compared to other

scanning techniques:It does not use ionising radiation which causes ahazard to patients and staff.

There are no moving mechanisms, just changingcurrents and magnetic elds.

The patient feels nothing during a scan (althoughthe gradient coils are noisy as they are switched),

and there are no after-effects.

MRI gives better soft-tissue contrast than a CATscan, although it does not show bone as clearly.

Computer images can be generated showing any

section through the volume scanned, or as a three-dimensional image.

Figure 16.16 MRI scan through a healthy human

head. Different tissues, identied by their differentrelaxation times, are coloured differently.

Figure 16.17 A combined CAT scan and MRI scan,

showing how the tissues revealed by MRI relate tothe bone structure shown by X-rays.

Answer

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248

Summary

Medical tracers such as technetium-99m are used to diagnose the function of organs.A gamma camera detects gamma radiation coming from the medical tracer in the body and deducesits position.

The main components of a gamma camera are: collimator, scintillator crystal, photomultiplier tubesand computer.

In PET scanning, a +emitter is used as a tracer; -rays produced as the positrons annihilate with electronsare used to determine the position of the tracer.

In MRI scanning, spinning, precessing protons are forced to resonate using radio frequency pulses. RFradiation from relaxing protons is used to obtain diagnostic information about internal organs, particularly

soft tissues.

The main components of an MRI scanner are: superconducting magnet, RF transmitter coil, RF receivercoil, set of gradient coils and computer.

Questions

a1 State one application of technetium-99m as a tracer. [1]

Technetium-99m nuclei are produced when radioactive nuclei of molybdenum-99b

emit -particles.

Complete the nuclear reactions below:i

9942 Mo99

? Tcm+ 01e + ?

9943 Tcm 9943 Tc + ? [2]

Suggest why technetium-99m is suitable as a tracer. [2]ii

Molybdenum-99 has a half-life of 67 h. The initial activity of a sample ofiii

molybdenum is 600 MBq. For this sample of molybdenum, calculate:

the decay constant in s1 1 [1]

the initial mass in grams of the sample [3]2

the activity of the sample after 30 h. [2]3

[Total 11]

2 This question is about magnetic resonance imaging (MRI).

Explain what is meant by:a

the Larmor frequencyi

0 of nuclei [2] relaxation time of nuclei. [3]ii

Describeb two advantages of MRI compared with a CAT scan. [2]

Outline some of the main components of an MRI scanner. [5]c

[Total 12]

continued

Glossary

Hint

Hint

Answer

Answer

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a3 The diagram below shows the key components of a gamma camera.

A-ray

photonsB C

photomultiplier

tubes

Name each component A, B and C and state its function. [6]

A photomultiplier tube has 10 electrodes known as dynodes. An electron emittedb

from the photocathode is accelerated towards the rst dynode. On impact, it

produces, on average, three secondary electrons. These are accelerated towards

the second dynode and the whole process is repeated. An electrical pulse lasting

for 2.0 ns at the tenth dynode is produced. Calculate:

the total number of electrons at the tenth dynode as a result of a singlei

electron impacting the rst dynode [2]

the average current from the last dynode. [3]ii [Total 11]

Hint

Hint

Answer

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