Download APPLICATION OF THE ABOVE TO RADIOGRAPHY

APPLICATION
OF
THE
ABOVE
TO
RADIOGRAPHY
In radiography the X-rays are going to pass right through parts of the body,
in order to create images of the inside. Softer components of the total beam
leaving the tube, say in the 20 keV region, would have a linear attenuation
coefficient of about 79 m-1 in soft tissue. From this you can show that only
about 0.04% of these photons would penetrate through 10 cm of the body.
(The equivalent figures for 150 keV photons are 15 m -1 and 22%) These
photons would be worse than useless because their ionising effect on the
tissue would produce damage. Therefore we want to make the radiation
harder by filtration before it reaches the body of the patient. Some of the
lower energy photons will not get through the wall of the tube, but more
filtering is needed. To get the filtering effect, we use the fact that
photoelectric absorption increases as Z3, while simple and Compton
scattering increase less rapidly. Hence we choose a filter with large enough
Z so that most of the absorption in it will be by the photoelectric effect Aluminium turns out to be suitable. Since photoelectric absorption decreases
very rapidly with photon energy, the higher energy photons will tend to
survive. The energy range we try to establish is up to about 30 keV. (This
requires that we have a tube voltage of about 900 kV; the glass wall is
equivalent to about 1 mm of Aluminium, and another few mm of
Aluminium
are
needed
as
the
rest
of
the
filter)
The X-rays will pass far more easily through air spaces in the body, because
as the density of the medium decreases we have seen that attenuation
decreases - there are less atoms present per unit volume for the photons to
hit.
The average proton number Z of renal tissue and of fatty tissue, are
approximately the same - about 6 or 7. But since absorption increases with
density we can distinguish the kidney from the superficial layer of fat
surrounding
it.
Muscle has a larger Z than the above soft tissues - about 7.5, while bone is
larger still (about 13 to 14), as well as being denser, so their attenuation
coefficient is further enlarged - they show up very clearly on the radiograph.
Above 30 keV photon energy, Compton scattering is the dominant process
of absorption, which is roughly independent of Z so discrimination would
rely on density differences alone. But if we can use photons filtered into the
region below 30 keV (see above), then simple scattering and the
photoelectric effect will predominate; these increase rapidly as Z increases,
and therefore will discriminate well between bones and soft tissues.
In the radiogram, regions of high attenuation (bone) appear white, medium
(tissue)
appear
grey,
and
negligible
(air)
appear
black.
Natural contrast is sufficient for the diagnosis of fractures and dislocations
of bones. If the natural contrast offered by the relevant body parts under
study is insufficient, artificial contrast agents may be used. For example, the
radio-opaque barium sulphate in an aqueous suspension is widely
administered
for
gastrointestinal
tract
examinations.
When the diagnosis concerns a hand, it can be immobilised and an exposure
of several seconds is possible without movement, and hence without
blurring. But when a stomach is to be investigated, its involuntary
movements would cause the radiograph to be uselessly blurred unless the
exposure time is less than 0.5 s. To achieve this and still get good contrast
on the exposure, the intensity of the beam has to be increased, which means
increasing
the
current
through
the
tube.
Grid
to
reduce
detection
of
scattered
radiation
Compton scattering of X-rays from tissue and bones leads to a generalised
spread of radiation uniformly across the film. This is useless for diagnosis; it
merely reduces the contrast between the dark and light regions on the film.
The simplest way to prevent these scattered X-rays reaching the film is to
arrange a grid of lead sheets, so arranged that only rays which are travelling
more or less in the original direction can get through.
Since the grid inevitably also absorbs some of the directed X-rays, the
intensity of the original beam unfortunately needs to be increased to
compensate.
The grid is made of many long parallel strips of lead held together by an
interspace material transparent to X-rays; there are about three strips of lead
per mm., each one being about 0.05 mm thick and 5 mm deep (this depth is
vital to the function of absorbing radiation that is going in the 'wrong'
direction.
X-RAYS FOR TREATMENT
X-ray Megavoltage Radiotherapy: The 70 keV X-rays are used for
diagnosis , hoping to affect the person's body as little as possible. X-rays
produced by electrons that have been accelerated through more than 1 MV
will be energetic enough to pass through the body without showing up
useful detail of bones and muscle. Instead of diagnosis, they can be used for
treatment , for therapy - in particular, for the destruction of cancers.
The beam of X-rays is collimated, using, say, lead cylinders with holes
drilled in them. Then it is aimed at the cancer, within the body. The beam
will damage and kill the cancerous cells, but unfortunately it will equally
damage and kill the healthy cells that it meets along the path through the
body. Various methods of reducing this collateral damage are used:
(i) Several beams can be used simultaneously, aiming at the cancer from
different
directions,
only
crossing
at
the
cancer.
(ii) The critical dose is carefully assessed; the patient is given the treatment
which is only just enough to kill the cancer cells.
(iii) The body is given time to recover, to repair damage, caused by the Xrays to previously healthy cells; clearly this is a compromise, since the
cancer
may
also
recover
(iv) The patient, or the X-ray machine, can be rotated , so that the beam is
aimed at the cancer from various directions, producing the same effect as (i).
It might seem much easier to move the machine, but a megavoltage X-ray
source is a sizeably machine; the patient can quite easily be strapped onto a
couch
and
moved.
Note that the energy of these X-ray photons is similar to that of g-ray
photons; the two photons only differ in the way they are produced.
{Grolier's Encyclopedia (CD-Rom): Malignant tissues are more sensitive
than normal tissues to radiation exposure and can be treated if they have not
spread throughout the body and are not surrounded by normal tissue that is
especially sensitive to radiation, such as the spinal cord. Sophisticated
physical and biological techniques are used for radiation therapy, often
accompanied by computer analyses. A radiation therapist develops a
treatment plan that permits the absorption of a fatal amount of radiation by
all tumor cells but causes relatively minor damage to normal tissue. The
usual mode of therapy is an external high-energy beam directed at the tumor
site for a few minutes a day for 2 to 6 weeks, depending on the type of
malignancy. X-rays, gamma rays, and such isotopes as cobalt-60 and iodine131 are often used}
Nuclear Magnetic Resonance Imaging (NMR)
This is a method of scanning parts of the body, including the brain, without
using X-rays or Ultrasound. Very strong magnetic fields are produced using
electromagnets in which the current-carrying coils are cooled low enough to
become superconductors. The patient is put into a combination of a strong
uniform magnetic field, and a non-uniform magnetic field which
increases in strength across her body. Each nucleus of the atoms of the
various elements in the body has a magnetic 'moment'; it behaves as a small
magnet, and tends, therefore, to line up in the resultant magnetic field. The
nucleus is now like a little compass, held in one position by the field of a
magnet; it has a set of natural frequencies at which it would oscillate, if
given the energy to do so (these are quantised, like the energy levels of an
electron in an atom). If then a pulse of electromagnetic waves of various
frequencies are projected into the body, the nucleus will 'pick up', absorb,
and then re-radiate, just those photons of the electromagnetic radiation
which are at the right energy. Thus the frequency absorbed and re-radiated is
characteristic of the element. The absorption spectrum can be detected, and
analysed to identify which kind of elements are present.
Because the strength of the magnetic field varies across the body, the
frequency absorbed and re-emitted by the same element also varies, in a
predictable way. Therefore the position of the nucleus can also be worked
out and displayed.
By computer analysis, given, for example, the known combination of
elements in various types of tissue, the cocktail of elements located at
various positions can be converted into images on a computer monitor of the
various tissues in the body. The different tissues can be given artificial
colours
by
the
computer,
to
aid
visual
recognition.
{Grolier's Encyclopedia (CD-ROM): Magnetic resonance imaging (MRI) is
a sophisticated medical diagnostic technique based on the principles of
Nuclear Magnetic Resonance imaging. A patient is placed inside a cylinder
that contains a strong magnet. Radio waves are then introduced into the
cylinder, which cause the atoms of the body to resonate. Each type of body
tissue emits characteristic signals from the nuclei of its atoms, and a
computer translates these signals into a two-dimensional picture.
Unlike traditional X rays or CAT scans {which use low-energy X-rays}
used in Radiology, MRI does not use ionizing radiation. It also does not
require the use of radioactively labeled dyes. In addition, MRI can see
through bone and produce images of blood vessels, cerebrospinal fluid,
cartilage, bone marrow, muscles, and ligaments. MRI is particularly useful
to detect tumors in the posterior fossa (the region at the back of the brain
between the ears), lesions associated with multiple sclerosis, joint injuries,
and herniated disks. MRI is a harmless procedure except for persons with
metal objects implanted in their bodies, such as pacemakers, joint pins, or
artificial heart valves. These objects may be dislodged by the powerful
magnetic
field}
{New Scientist CD-ROM: MRI works by subjecting the body to an intense
magnetic field which causes the hydrogen nuclei in water in the body to line
up like bar magnets. The nuclei are then slightly disturbed using pulses of
radio waves. When the radio waves are removed, the nuclei relax back to
their original state, giving off signals that depend on their chemical
environment
and
their
magnetic
properties.
Deoxygenated haemoglobin in the blood is paramagnetic, and so slightly
distorts the magnetic field around it. Oxygenated haemoglobin is not
paramagnetic, so appears different to deoxyhaemoglobin in an MRI image.
This difference allows a type of scan called blood oxygen level dependent
imaging,
or
BOLD.
Seiji Ogawa at AT&T discovered BOLD in 1988. In 1991 researchers at
Massachusetts General Hospital in Charlestown found they could image the
brain by injecting a paramagnetic substance into the bloodstream. But this
substance can prove toxic if used more than a few times. In these latest
experiments
the
blood
itself
is
imaged.
David Tank of AT&T's biological computation research department says
that when an area in the brain becomes more active, the blood flow to it
increases. But the uptake of oxygen does not seem to change, so the amount
of oxyhaemoglobin in the veins increases, and this is imaged by MRI.