# Basic Physics of Nuclear Medicine/Interaction of Radiation with Matter

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We have focussed in previous chapters on the source of radiation and the
types of radiation. We are now in a position to consider what happens
when this radiation interacts with matter. Our main reason for doing
this is to find out what happens to the radiation as it passes through
matter and also to set ourselves up for considering how it interacts
with living tissue and how to detect radiation. Since all radiation
detectors are made from some form of matter it is useful to first of all
know how radiation interacts so that we can exploit the effects in the
design of such detectors in subsequent chapters of this wikibook.

Before we do this let us first remind ourselves of the physical
characteristics of the major types of radiation. We have covered this
information in some detail
earlier
and it is summarised in the table below for convenience.

We will now consider the passage of each type of radiation through
matter with most attention given to gamma-rays because they are the most
common type used in nuclear medicine. One of the main effects that you
will notice irrespective of the type of radiation is that ions are
produced when radiation interacts with matter. It is for this reason
that it is called ionizing radiation.

+----------------+----------------+----------------+----------------+
| Radiation      | Mass           | Electric       | Velocity       |
|                |                | Charge         |                |
+================+================+================+================+
| Alpha          | ```{=html}     | ```{=html}     | ```{=html}     |
| Particles      | <div c         | <div c         | <div c         |
|                | lass="center"> | lass="center"> | lass="center"> |
|                | ```            | ```            | ```            |
|                | relatively     | double         | relatively     |
|                | heavy          | positive       | slow           |
|                |                |                |                |
|                | ```{=html}     | ```{=html}     | ```{=html}     |
|                | </div>         | </div>         | </div>         |
|                | ```            | ```            | ```            |
+----------------+----------------+----------------+----------------+
| Beta Particles | ```{=html}     | ```{=html}     | ```{=html}     |
|                | <div c         | <div c         | <div c         |
|                | lass="center"> | lass="center"> | lass="center"> |
|                | ```            | ```            | ```            |
|                | about 8,000    | negative       | less than the  |
|                | times lighter  |                | velocity of    |
|                |                | ```{=html}     | light          |
|                | ```{=html}     | </div>         |                |
|                | </div>         | ```            | ```{=html}     |
|                | ```            |                | </div>         |
|                |                |                | ```            |
+----------------+----------------+----------------+----------------+
| Gamma Rays     | ```{=html}     | ```{=html}     | ```{=html}     |
|                | <div c         | <div c         | <div c         |
|                | lass="center"> | lass="center"> | lass="center"> |
|                | ```            | ```            | ```            |
|                | None           | None           | 3 x 10^8^ m/s  |
|                |                |                | in free space  |
|                | ```{=html}     | ```{=html}     |                |
|                | </div>         | </div>         | ```{=html}     |
|                | ```            | ```            | </div>         |
|                |                |                | ```            |
+----------------+----------------+----------------+----------------+

Before we start though you might find an analogy useful to help you with
your thinking. This analogy works on the basis of thinking about matter
as an enormous mass of atoms (that is nuclei with orbiting electrons)
and that the radiation is a particle/photon passing through this type of
environment. So the analogy to think about is a spaceship passing
through a meteor storm like you might see in a science-fiction movie
where the spaceship represents the radiation and the meteors represent
the atoms of the material through which the radiation is passing. One
added feature to bring on board however is that our spaceship sometimes
has an electric charge depending on the type of radiation it represents.

## Alpha Particles

We can see from the table above that
alpha-particles have a double positive
charge and we can therefore easily appreciate that they will exert
considerable electrostatic attraction on the outer orbital electrons of
atoms near which they pass. The result is that some electrons will be
attracted away from their parent atoms and that ions will be produced.
In other words ionizations occur.

We can also appreciate from the table that alpha-particles are quite
massive relative to the other types of radiation and also to the
electrons of atoms of the material through which they are passing. As a
result they travel in straight lines through matter except for rare
direct collisions with nuclei of atoms along their path.

A third feature of relevance here is the energy with which they are
emitted. This energy in the case of alpha-particles is always distinct.
For example ^221^Ra emits an alpha-particle with an energy of 6.71 MeV.
Every alpha-particle emitted from this radionuclide has this energy.
Another example is ^230^U which emits three alpha-particles with
energies of 5.66, 5.82, 5.89 MeV.

Finally it is useful to note that alpha-particles are very damaging
biologically and this is one reason why they are not used for in-vivo
diagnostic studies. We will therefore not be considering them in any
great detail in this wikibook.

## Beta Particles

We can see from the table that
beta-particles have a negative electric
charge. Notice that positrons are not considered here since as we noted
in chapter
2 these
particles do not last for very long in matter before they are
annihilated. Beta-minus particles last considerably longer and are
therefore the focus of our attention here.

Because of their negative charge they are attracted by nuclei and
repelled by electron clouds as they pass through matter. The result once
again without going into great detail is ionization.

The path of beta-particles in matter is often described as being
tortuous, since they tend to ricochet from atom to atom.

A final and important point to note is that the energy of beta-particles
is never found to be distinct in contrast to the alpha-particles above.
The energies of the beta-particles from a radioactive source forms a
spectrum up to a maximum energy -- see figure below. Notice from the
figure that a range of energies is present and features such as the mean
energy, E~mean~, or the maximum energy, E~max~, are quoted.

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The question we will consider here is: why should a spectrum of energies
be seen? Surely if a beta-particle is produced inside a nucleus when a
neutron is converted into a proton, a single distinct energy should
result. The answer lies in the fact that two particles are actually
produced in beta-decay. We did not cover this in our treatment in
chapter
2 for
fear of complicating things too much at that stage of this wikibook. But
we will cover it here briefly for the sake of completeness.

The second particle produced in beta-decay is called a
neutrino and was named by Enrico
Fermi. It is quite a mysterious particle
possessing virtually no mass and carrying no charge, though we are still
researching its properties today. The difficulty with them is that they
are very hard to detect and this has greatly limited our knowledge about
them so far.

The beta-particle energy spectrum can be explained by considering that
the energy produced when a neutron is converted to a proton is shared
between the beta-particle and the anti-neutrino. Sometimes all the
energy is given to the beta-particle and it receives the maximum energy,
E~max~. But more often the energy is shared between them so that for
example the beta-particle has the mean energy, E~mean~ and the neutrino
has the remainder of the energy.

Finally it is useful to note that beta-particles are quite damaging
biologically and this is one reason why they are not used for in-vivo
diagnostic studies. We will therefore not consider them in any great
detail in this wikibook.

## Gamma Rays

Since we have been talking about energies above, let us first note that
the energies of gamma-rays emitted from a
radioactive source are always distinct. For example ^99m^Tc emits
gamma-rays which all have an energy of 140 keV and ^51^Cr emits
gamma-rays which have an energy of 320 keV.

Gamma-rays have many modes of interaction with matter. Those which have
little or no relevance to nuclear medicine imaging are:

::\* Mössbauer Effect

::\* Coherent Scattering

::\* Pair Production

and will not be described here.

Those which are very important to nuclear medicine imaging, are the
Photoelectric Effect and the
Compton Effect. We will consider each
of these in turn below. Note that the effects described here are also of
relevance to the interaction of X-rays with matter since as we have
noted before X-rays and gamma-rays are essentially the same entities. So
the treatment below is also of relevance to radiography.

#### Photoelectric Effect

:   When a gamma-ray collides with an orbital electron of an atom of the
    material through which it is passing it can transfer all its energy
    to the electron and cease to exist -- see figure below. On the basis
    of the Principle of Conservation of
    Energy we can deduce that the
    electron will leave the atom with a kinetic energy equal to the
    energy of the gamma-ray less that of the orbital binding energy.
    This electron is called a **photoelectron**.

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:   Note that an ion results when the photoelectron leaves the atom.
    Also note that the gamma-ray energy is totally absorbed in the
    process.

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:   Two subsequent points should also be noted. Firstly the
    photoelectron can cause ionisations along its track in a similar
    manner to a beta-particle. Secondly X-ray
    emission can occur when the
    vacancy left by the photoelectron is filled by an electron from an
    outer shell of the atom. Remember that we came across this type of
    feature before when we dealt with Electron Capture in chapter
    2.

#### Compton Effect

:   This type of effect is somewhat akin to a cue ball hitting a
    coloured ball on a pool table. Here a gamma-ray transfers only part
    of its energy to a valance electron which is essentially free -- see
    figure below. Notice that the electron leaves the atom and may act
    like a beta-particle and that the gamma-ray deflects off in a
    different direction to that with which it approached the atom. This
    deflected or scattered gamma-ray can undergo further Compton Effects
    within the material.

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:   Note that this effect is sometimes called **Compton Scattering**.

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The two effects we have just described give rise to both absorption and
scattering of the radiation beam. The overall effect is referred to as
**attenuation** of gamma-rays. We will investigate this feature from an
analytical perspective in the following chapter. Before we do so, we\'ll
briefly consider the interaction of radiation with living matter.

## Radiation Biology

It is well known that exposure to ionizing radiation can result in
damage to living tissue. We\'ve already described the initial atomic
interactions. What\'s important in radiation biology is that these
interactions may trigger complex chains of biomolecular events and
consequent biological damage.

We\'ve seen above that the primary means by which ionizing radiations
lose their energy in matter is by ejection of orbital electrons. The
loss of orbital electrons from the atom leaves it positively charged.
Other interaction processes lead to **excitation** of the atom rather
than ionization. Here, an outer valence electron receives sufficient
energy to overcome the binding energy of its shell and moves further
away from the nucleus to an orbit that is not normally occupied. This
type of effect alters the chemical force that binds atoms into molecules
and a regrouping of the affected atoms into different molecular
structures can result. That is, excitation is an indirect method of
inducing chemical change through the modification of individual atomic
bonds.

Ionizations and excitations can give rise to unstable chemical species
called free radicals. These are atoms and
molecules in which there are unpaired electrons. They are chemically
very reactive and seek stability by bonding with other atoms and
molecules. Changes to nearby molecules can arise because of their
production.

But, let\'s go back to the interactions themselves for the moment\.....

In the case of X- and gamma-ray interactions, the energy of the photons
is usually transferred by collisions with orbital electrons, e.g. via
photoelectric and Compton effects. These radiations are capable of
penetrating deeply into tissue since their interactions depend on chance
collisions with electrons. Indeed, nuclear medicine imaging is only
possible when the energy of the gamma-rays is sufficient for complete
emission from the body, but low enough to be detected.

The interaction of charged particles (e.g. alpha and beta particles), on
the other hand, can be by collisions with atomic electrons and also via
attractive and repulsive electrostatic forces. The rate at which energy
is lost along the track of a charged particle depends therefore on the
square of the charge on that particle.
That is, the greater the particle charge, the greater the probability of
it generating ion pairs along its track. In addition, a longer period of
time is available for electrostatic forces to act when a charged
particle is moving slowly and the ionization probability is therefore
increased as a result.

The situation is illustrated in the following figure where tracks of
charged particles in water are depicted. Notice that the track of the
relatively massive α-particle is a straight line, as we\'ve discussed
earlier in this
chapter,
with a large number of interactions (indicated by the asterisks) per
unit length. Notice also that the tracks for electrons are tortuous, as
we\'ve also discussed
earlier,
and that the number of interactions per unit length is considerably
less.

!Ionizations and excitations along particle tracks in water, for a 5.4
MeV α-particle (top left), for electrons generated following the
absorption of a 1.5 keV X-ray photon (top right) and electrons generated
during the decay of
iodine-125., for electrons generated following the absorption of a 1.5 keV X-ray photon (top right) and electrons generated during the decay of iodine-125."){width="320"}

The Linear Energy Transfer
(**LET**) is defined as the energy released per unit length of the track
of an ionizing particle. A slowly moving, highly charged particle
therefore has a substantially higher LET than a fast, singly charged
particle. An alpha particle of 5 MeV energy and an electron of 1 MeV
energy have LETs, for instance, of 95 and 0.25 keV/μm, respectively. The
ionization density and hence the energy deposition pattern associated
with the heavier charged particle is very much greater than that arising
from electrons, as illustrated in the figure above.

The energy transferred along the track of a charged particle will vary
because the velocity of the particle is likely to be continuously
decreasing. Each interaction removes a small amount of energy from the
particle so that the LET gradually increases along a particle track with
a dramatic increase (called the Bragg Peak)
occurring just before the particle comes to rest.

The International Commission on Radiation Units and
Measurements (**ICRU**) suggest that **lineal
energy** is a better indicator of relative biological effectiveness
(**RBE**). Although lineal energy has the same units as LET (e.g.
keV/μm), it is defined as the:

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```
`<span style="color:green;">`{=html}ratio of the energy deposited in a
volume of tissue to the average diameter of that volume.`</span>`{=html}

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Since the microscopic deposition of energy may be quite anisotropic,
lineal energy should be a more appropriate measure of potential damage
than that of LET. The ICRU and the ICRP have
accordingly recommended that the radiation effectiveness of a particular
radiation type should be based on lineal energy in a 1 μm diameter
sphere of tissue. The lineal energy can be calculated for any given
radiation type and energy and a Radiation Weighting
Factor,
(w~R~) can then be determined based on the integrated values of lineal
energy along the radiation track.

All living things on this planet have been exposed to ionizing radiation
since the dawn of time. The current situation for humans is summarized
in the following table:

+----------------------+----------------------+----------------------+
| Source               | Effective Dose       | Comment              |
|                      | (mSv/year)           |                      |
+======================+======================+======================+
| Cosmic radiation     | ```{=html}           | ```{=html}           |
|                      | <div class="center"> | <div class="center"> |
|                      | ```                  | ```                  |
|                      | \~0.4                | About 100,000 cosmic |
|                      |                      | ray neutrons and     |
|                      | ```{=html}           | 400,000 secondary    |
|                      | </div>               | cosmic rays          |
|                      | ```                  | penetrate our bodies |
|                      |                      | every hour -- and it |
|                      |                      | increases with       |
|                      |                      | altitude!            |
|                      |                      |                      |
|                      |                      | ```{=html}           |
|                      |                      | </div>               |
|                      |                      | ```                  |
+----------------------+----------------------+----------------------+
| Terrestrial          | ```{=html}           | ```{=html}           |
| radiation            | <div class="center"> | <div class="center"> |
|                      | ```                  | ```                  |
|                      | \~0.5                | Over 200 million     |
|                      |                      | gamma-rays pass      |
|                      | ```{=html}           | through our body     |
|                      | </div>               | every hour from      |
|                      | ```                  | sources such as soil |
|                      |                      | and building         |
|                      |                      | materials            |
|                      |                      |                      |
|                      |                      | ```{=html}           |
|                      |                      | </div>               |
|                      |                      | ```                  |
+----------------------+----------------------+----------------------+
| Internal radiation   | ```{=html}           | ```{=html}           |
|                      | <div class="center"> | <div class="center"> |
|                      | ```                  | ```                  |
|                      | \~0.3                | About 15 million     |
|                      |                      | ^40^K atoms and      |
|                      | ```{=html}           | about 7,000 natural  |
|                      | </div>               | uranium atoms        |
|                      | ```                  | disintegrate inside  |
|                      |                      | our bodies every     |
|                      |                      | hour, primarily from |
|                      |                      | our diet             |
|                      |                      |                      |
|                      |                      | ```{=html}           |
|                      |                      | </div>               |
|                      |                      | ```                  |
+----------------------+----------------------+----------------------+
| Radon and other      | ```{=html}           | ```{=html}           |
| gases                | <div class="center"> | <div class="center"> |
|                      | ```                  | ```                  |
|                      | \~1.3                | About 30,000 atoms   |
|                      |                      | disintegrate inside  |
|                      | ```{=html}           | our lungs every hour |
|                      | </div>               | as a result of       |
|                      | ```                  | breathing            |
|                      |                      |                      |
|                      |                      | ```{=html}           |
|                      |                      | </div>               |
|                      |                      | ```                  |
+----------------------+----------------------+----------------------+

: Adapted with permission from Applied Imaging
Technology.

\
The sum total of this *Natural* Background
Radiation is about 2.5 mSv per year,
with large variations depending on altitude and dietary intake as well
as geological and geographical location.

Its generally considered that repair mechanisms exist in living matter
and that these can be invoked following radiation damage at the
biomolecular level. These mechanisms are likely to have an
evolutionary basis arising as a response to
radiation fluxes generated by natural background sources over the aeons.
Its also known that quite considerable damage to tissues can arise at
quite higher radiation fluxes, even at medical
exposures. Cell death and
transformations to malignant states can result leading to latent periods
of many years before clinical signs of cancer or leukemia, for instance,
become manifest. Further treatment of this vast field of radiation
biology however is beyond our scope
here.

## Practical Radiation Safety

Radiation hazards arise since nuclear medicine involves the handling of
radioactive materials. Although this risk may be small, it remains
important to keep occupational exposures as low as reasonably
achievable. Essential practices for achieving this aim include:

`<b>`{=html}Administration`</b>`{=html}

::\*Maintaining a comprehensive record of all radioactive source
purchases, usage, movement and storage.

::\*Ensuring that any Codes of Safe Practice are adhered to and develop
sensible written protocols and working rules for handling radioisotopes.

::\*Protocols for dealing with minor contamination incidents of the
environment or of staff members must be established. Remember that no
matter how good work practices are, minor accidents or incidents
involving spillage of radioisotopes can take place.
`<b>`{=html}Facility`</b>`{=html}

::\*Storage of radioactive sources in a secure shielded environment.
Specially dedicated facilities are required for the storage, safe
handling, manipulation and dispensing of unsealed radioactive sources.
Storage areas should be designed for both bulk radioisotope and
radioactive waste. Furthermore, radioactive patients should be regarded
as unsealed sources.

::\*Adequate ventilation of any work area. This is particularly
important to minimize the inhalation of Technigas and potentially
volatile radioisotopes such as I-125 and I-131. It is preferable to use
fume hoods when working with volatile materials.

::\*Benches should be manufactured with smooth, hard impervious surfaces
with appropriate splash-backs to allow ready decontamination following
any spillage of radioisotopes. Laboratory work should be performed in
stainless steel trays lined with absorbent paper.

::\*Excretion of radioactive materials by patients may be via faeces,
urine, saliva, blood, exhaled breath or the skin. Provision to deal with
any or all of these potential pathways for contamination must be made.

::\*Provision for collection and possible storage of both liquid and
solid radioactive waste may be necessary in some circumstances. Most
short-lived, water soluble liquid waste can be flushed into the sewers
but longer lived isotopes such as I-131 may have to be stored for decay.
Such waste must be adequately contained and labelled during storage.
`<b>`{=html}Equipment`</b>`{=html}

::\*Ensure that appropriate survey monitors are available to determine
if any contamination has occurred and to assist in decontamination
procedures. Routine monitoring of potentially contaminated areas must be
performed.

::\*Ensure that all potentially exposed staff are issued with individual
personnel monitors.

::\*Protective clothing such as gowns, smocks, overboots and gloves
should be provided and worn to prevent contamination of the personnel
handling the radioactivity. In particular, gloves must be worn when
administering radioactive materials orally or intravenously to patients.
It should be noted that penetration of gloves may occur when handling
some iodine compounds so that wearing a second pair of gloves is
recommended. In any event, gloves should be changed frequently and
discarded ones treated as radioactive waste.
`<b>`{=html}Behaviours`</b>`{=html}

::\*Eating and drinking of food, smoking, and the application of
cosmetics is prohibited in laboratories in which unsealed sources are
utilized.

::\*Mouth pipetting of any radioactive substance is totally prohibited.

::\*Precautions should be taken to avoid punctures, cuts, abrasions and
any other open skin wounds which otherwise might allow egress of
radiopharmaceuticals into the blood stream.
`<b>`{=html}Optimization`</b>`{=html}

::\*Always ensure that there is a net benefit resulting from the patient
procedure. Can the diagnosis or treatment be made by recourse to an
alternative means using non ionizing radiation?

::\*Ensure that all staff, including physicians, technologists, nurses
and interns and other students, who are involved in the practice of
nuclear medicine receive the relevant level of training and education
appropriate to their assigned tasks. The training program could be in
the form of seminars, refresher courses and informal tutorials.

::\*A substantive Quality Assurance (QA) program should be implemented
to ensure that the function of the Dose Calibrator, Gamma Camera,
computer and other ancillary equipment is optimized.

The potential hazards to staff in a nuclear medicine environment
include:

::\*Milking the ^99m^Tc generator, drawing up and measuring the quantity
of radioisotope prior to administration.

::\*Delivering the activity to the patient by injection or other means
and positioning the now radioactive patient in the imaging device.

::\*Removing the patients from the imaging device and returning them to
the ward where they may continue to represent a radiation hazard for
some time. For Tc-99m, a short-lived radionuclide the hazard period will
be only a few hours but for therapeutic isotopes the hazardous period
may be several days.

::\*Disposal of radioactive waste including body fluids, such as blood
and urine, but also swabs, syringes, needles, paper towels etc.

::\*Cleaning up the imaging area after the procedure.

::\*Contamination.

The table below lists the dose rates from patients having nuclear
medicine examinations. In general, the hazards from handling or dealing
with radioactive patients arise in two parts:

::\*External hazard: This will be the case when the radioisotope emits
penetrating gamma-rays. Usually, this hazard can be minimised by
employing shielding and sensible work practices.

::\*Radioactive contamination: This is potentially of more concern as it
may lead to the inhalation or ingestion of radioactive material by
staff. Possible sources of contamination are radioactive blood, urine
and saliva, emanating from a patient, or airborne radioactive vapour.
Sensible work practices, which involve high levels of personal hygiene,
should ensure that contamination is not a major issue.

!Dose rates from patients following
administration.{width="600"}

One of the most common nuclear medicine diagnostic procedures is the
bone scan using the isotope Tc-99m. The exposure rate at 1 metre from a
typical patient will peak at approximately 3 μSv per hour immediately
after injection dropping steadily because of radioactivity decay and
through excretion so that after 2 hours it will be about 1.5 μSv per
hour. Neglecting any further excretion, the total exposure received by
an individual, should that person stand one meter from the patient for
the whole of the first 24 hours, would be \~17 μSv. For a person at 3
meters from the patient this number would reduce to 1.7 μSv and for a
distance of 5 metres it would be \~0.7 μSv. These values have been
estimated on the basis of the inverse square law.

Patients should be encouraged to drink substantial quantities of liquid
following their scan, as this will improve excretion and aid in
minimizing not only their radiation dose but also that of nursing staff.

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