Diagnostic X-ray

أستاذ المادة: المدرس ناهدة حمود الجراح
Lecture one : physics of diagnostic x – rays
1- What are X-rays ?
2- Discovery ?
3- Characteristics of X-Rays ?
4- X- ray Production ?
6- How X- ray are absorbed
7- Uses of X-rays ?
1- What are X-rays ?
X-rays are electromagnetic waves like radio and light waves. These all
travel at the speed of light, which is (300 000)km per second. The
wavelength of an X-ray however is one hundredth that of the light rays
that you can see, at around (10¯¹?) m. This means that they have a lot
more energy. X-ray fluorescence is the emission of X-rays from excited
atoms produced by the impact of high-energy electrons, other particles or
a primary beam of other X-rays. Atoms of all the elements emit a
characteristic X-ray spectrum when they are bombarded with
electrons. The X-ray photons are emitted when the incident electrons
knock an inner orbital electron out of an atom. When this happens, an
outer energy electron falls into the inner shell to replace it, losing
potential energy in doing so. X-rays can pass through many forms of
matter and therefore have many uses. X-rays are produced for these
purposes by an X-ray tube.
2- Discovery :
The discovery of X-rays was by a German physicist called
Wilhelm Roentgen. In 1895 Roentgen discovered X-rays almost by
accident. Whilst doing some experiments in which he passed an electric
current through tubes (special tubes containing a cathode and electrode
from which the air has been removed), Roentgen noticed that
photographic plates nearby began to grow fogged. To discover why this
occurred he placed black paper on the tube and then switched on the
current. Nearby a screen coated with barium began to glow. This caused
Roentgen to believe that unknown rays produced inside the tube were
passing through the paper to make this fluorescent substance give out
light. These he named X-rays since x is a scientific number for anything
that is unknown.
3- Characteristics of X-Rays :
1- X-rays are invisible.
2- X-rays are electrically neutral. They have neither a positive nor a
negative charge. They cannot be accelerated or made to change direction
by a magnet or electrical field.
3- X-rays have no mass.
4-X-rays travel at the speed of light in a vacuum.
5- X-rays cannot be optically focused.
6- X-rays form a poly energetic or heterogeneous beam.
7- The x-ray beam used in diagnostic radiography comprises many
photons that have many different energies.
8- X-rays travel in straight lines.
9 - X-rays can cause some substances to fluoresce.
10-X-rays cause chemical changes to occur in radiographic and
photographic film.
11- X-rays can be absorbed or scattered by tissues in the human body.
12- X-rays can produce secondary radiation.
13- X-rays can cause chemical and biologic damage to living tissue.
4 - X-ray Production :
The production of x-rays requires a rapidly moving stream of electrons
that are suddenly decelerated or stopped. The source of electrons is the
cathode, or negative electrode. Electrons are stopped or decelerated by
the anode, or positive electrode. Electrons move between the cathode and
the anode because there is a potential difference in charge between the
electrodes. The components of the X-ray tube include a glass envelope
containing a high vacuum. A cathode or negative electrode which
contains a tungsten filament , which when heated emits electrons in a
process called thermionic emission . The cathode also has a focussing
cup to better direct the emitted electrons across the vacuum to hit the
target. The anode or positive electrode is a thick copper rod with a small
tungsten target at the end. Tungsten is required as it has a high atomic
number to improve the efficiency of bremsstrahlung x-ray production
(see below) , and a high melting point.. , A potential difference , or
voltage is applied between the cathode and anode. The tungsten filament
(cathode) is heated by an independent current emitted electrons are
accelerated across the potential difference to a high velocity before
striking the tungsten target. The high vacuum is needed to reduce the
electron /atom collisions. The electrons that hit the tungsten target
undergo sudden deflection because of the interactions with the tungsten
nucleus. Some additions to this basic set up include the anode made of
copper. The window is thin and made of beryllium. Beryllium is chosen
because it is a metal which has little effect on the photon beam and can
effectively maintain the vacuum.
5 -Types of X-rays :
There are two types of X-rays, according to their photon energy:
Soft X-rays
These x-rays are defined by having photon energies below 10keV. They
have less energy than the hard x-rays , therefore they have higher
wavelength. Soft X-rays are used in radiography to take pictures of bones
and internal organs. Because they have a relatively low frequency, they
do not cause much damage to tissues, unless they are repeated too often .
Hard X-rays
Hard X-rays have photon energies above 10keV. They have smaller
wavelength than the soft x-rays. These X-rays are used in radiotherapy, a
treatment for cancer. Due to their high frequency, they destroy molecules
within specific cells, thus destroying tissue. Another use for these X-rays
is airport security. Hard X-rays are used in security scanners to examine
baggage.
6- How X- Ray Are Absorbed
X- ray are not absorbed equally well by all materials if they were , they
would not be very useful in diagnosis . Heavy elements such as calcium
are much better absorbers of x- ray than light elements such as carbon ,
oxygen , and hydrogen , and as a result , structures containing heavy
elements , like the bones . The soft tissues – fat muscles and tumors all
absorb about equally well and are thus difficult to distinguish from each
other on an x-ray image . of course , air is a poor absorber of x- ray.
7- Uses of X-rays :
The main use of X-rays is in medicine. A common application is in the
form of X-ray machines, which take photos of a patient’s body. If an arm
or leg were broken for example, then this limb would be put in front of
the X-ray with a piece of photographic film behind. The X –ray is turned
on briefly and goes through to the film. The rays go through the skin and
flesh easily, showing up as dark areas on the film, but with more
difficulty through bone. They are slowed down and so these areas are
much lighter. X-rays can also be used to kill cancer cells, but also kill
healthy cells, so must be used with much care.
Other uses are in industry, at airports to check customers and
baggage. X-ray diffraction is also very important in spectroscopy and as
a basis for X-ray crystallography. The diffraction of X-rays by a crystal
where the wavelength of X-rays is comparable in size to the distances
between atoms in most crystals is used to disperse X-rays in a
spectrometer and to determine the structure of crystals or molecules.
About Medical X-Rays
X-rays have two important uses in medicine: imaging and radiation
therapy. In X-ray imaging, X-rays are sent through a patient s body to a
sheet of film or an X-ray detector. While some of the X-rays manage to
pass through tissue, most of them are blocked by bone. The patient s
bones form a shadow image on the film behind them. In X-ray radiation
therapy, the Xrays are again sent through a patient s body, but now their
interaction with diseased tissue is what s important. The X-rays deposit
some of their energy in this tissue and kill it.
X-ray of hands and fingers.
Medical x-rays are used to see what is happening inside the body. X-rays
pass through objects, including internal organs, body tissue, and clothing ,
and project a picture onto film or a detector linked to a computer monitor.
In general, denser objects , like bones, absorb more radiation, reducing
the amount of radiation that passes through to the detector. This is why
bones appear white on x-ray images. A conventional (or regular) medical
x-ray produces a two-dimensional picture that can help find fractures
(broken bones), tumors and foreign objects. Medical x-rays are also used
in other types of examinations and procedures, including CT scans and
fluoroscopy.
Dental X-Rays :
Dental x-rays let your dentist see the condition of your teeth from the
crown to the roots. They also show the bones of the jaw and the overall
condition of the bones of your face. During a dental x-ray, radiation
passes through your cheek and teeth to strike the special x- ray film
clamped between your teeth. Newer x-ray machines use digital imaging
instead of film. All types of dental x-rays use a small amount of radiation
to take the pictures.
CT Scans :
Computed tomography scans (also known as CT scans, CAT scans or
computed axial tomography scans) are advanced x-ray procedures that
create cross-sectional views and three dimensional images of a patient s
internal organs. When a person has a CT scan, many x-rays of their body
or body part are taken at nearly the same time. CT scans are useful
because they help doctors diagnose problems by creating very clear
images of internal organs. The detailed images help identify problems
inside the body, such as tumors or damage to organs. CT scans can also
help doctors prepare for surgery by providing a map of the inside of the
patient that surgeons can follow when operating . Medical imaging is a
very powerful and valuable technique that can provide important and life
saving information.
Fluroscopy :
It can show the movement of a body part (like the heart) or dye (contrast
agent) takes as it travels through the body. Unlike conventional x-rays,
fluoroscopy uses an x-ray beam that is passed continuously through the
body. The image is transmitted to a monitor so that doctors can see the
body part. A medical instrument or dye is then added to show the
function of the body part. Fluoroscopy is used in many types of
examinations and procedures, including:
• Viewing movement of materials through the stomach and
intestines.
• Directing the placement of a catheter during heart surgery.
• Visualizing blood flow to organs.
• Helping doctors properly set broken bones.