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Fifth stage
Radiology
Lec-1
.د
زهراء
5/10/2016
Introduction to X-ray and CT –Scan
X-rays were discovered by Wilhelm Konrad Rontgen in 1895 while
he was experimenting with cathode tubes.
Rontgen found out that x-ray was attenuated in a different way
by various kinds of materials and that it could, like light, be
captured on a photographic plate .
This opened up the way for its use in medicine.
The first “Rontgen picture” of a hand was made soon after the
discovery of X-rays.
X-rays:
X-rays : are electromagnetic waves . The wavelength for X-rays is on
the order of Angstroms (10−10 m) The nature of X-rays as short-wave
electromagnetic radiation was established in 1912. Electromagnetic
radiation consists of photons
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X- RAY GENERATION:
X-rays are generated in an X-ray tube, which consists of a vacuum
tube with a cathode (tugestone fillament ) and an anode
(tungestone target) The x-ray beam is produced by bombarding a
tungsten target with an electron beam within an x-ray tube .
X-ray detectors:
The detector s can be :
1- A screen – film combination : in which a film is sandwiched
between two screens , Screen–film detector ( Radiography ) The film
contains an emulsion with silver halide crystals (e.g., AgBr) When
exposed to light, the silver halide grains absorb optical energy and
undergo a complex physical change .
2 - An image intensifier coupled to a camera ( Fluoroscopy ),
3 - A cassette containing a storage phosphor plate ( computed
radiography )
4 - An active matrix flat panel detector or dual-layer detector (digital
radiography )
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Interaction with matter:
When x-ray beam pass through a matter they will interact in the
following ways :
Trasmited : pass through unaffected . A primary or direct
radiation .
Absorbed : transfering to the matter all of their energy (the
photon disappearing completely )
Scattered : diverted in a new direction , with or without loss of
energy and so leave the material as scattered or secondary
radiation .
The X-ray image is formed by the transmitted photons. those that are
absorbed or scattered represent attenuation by matter .
An understanding of how the properties of X-ray and the materials
through which they travel affect the relative amount of attenuation and
transmission gives an understanding of how the X-ray image is formed.
The interactions with tissues , depend on
- the energy of the photon , E = hxf , f = c/Y . In most radiological
examinations the voltage used is typically in the range from 50 to 125
kV.
- the atomic number of the interacting matter ,the higher the atomic
number the more the attenuation , so the contrast media used in
radiography to opacify certain part of the body should have high atomic
number .
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The most important interactions are the following :
photoelectric absorption. A photon can be absorbed by an atom
while its energy excites an electron.
Compton scattering. A second possibility is that the photon
transfers only part of its energy to eject an electron with a certain
kinetic energy ,the electron then escapes in another direction.
According to x- ray attenuation in the tissues ( x – ray penetration) ,the
radiographic appearace can be graded into :
Tranceradiant as gases
Radiolucent or trancelucent as in fatty tissue
Mild radio radio-opague as fluid , muscle ..
Moderate radio-opague as bones and calcifications
Dense radio-opague as metals and contrasts
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Clinical use (DIAGNOSTIC):
Radiography ( static ) : Coventional and digital ( CR and DR )
Fluoroscopy ( dynamic ) Analogue or digital : Used in contrast studies
and interventional procedures .
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CT scan (Computed tomography scan )
Computed tomography or CT is an imaging modality that produces
cross-sectional images representing the X-ray attenuation properties of
the body.
Types and generations :
A- CAT (computed axial tomorgaphy ) scan
Single-slice CT , Circular CT
The most straight forward way to image an entire volume is to scan a
number of consecutive slices by circular tube–detector rotations
alternated with small table shifts.
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B- Spiral CT (Helical CT)
A technique that is widely used nowadays is helical CT. The X-ray tube
rotates continuously around thepatient, just as in 2D CT. At the same
time, the patientis slowly translated through the gantry.
Multidetector spiral CT scan ( 16 slice , 32 slice , 64 ,128 and 256
slice) .
In modern CT scanners, the detector array consists of multiple detector
rows, in order to measure several slices per rotation of the X-ray tube.
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Biologic effects and safety;
X-rays and γ-rays are ionizing waves, Such photons are able to
ionize an atom, i.e., to release an electron from the atom.
Even at very low X-ray doses the energy deposited by ionizing
radiation, such as X-rays, may be sufficient to damage or destroy
cells.
We have two types of effects , Non deterministic effects (stochastic
effects) and Deterministic effects .
Non deterministic effects (stochastic effects) :
The probability always exists that modifications in single cells could lead
to malignancy (cancer) or genetic changes.
Malignant disease and heritable effects, for which the probability but
not the severity is proportional to the dose, without any threshold, are
stochastic effects of radiation .
There is no evidence of a threshold dose below which the proba-
bility would be zero.
Deterministic effects of radiation also exist , They are injuries to a large
population of cells where repair mechanisms fail and the complete
tissue is damaged. Deterministic effects are characterized by a
threshold dose and an increase in the severity of the tissue reaction with
increasing dose .
The deterministic effects of radiation can be acute or chronic
for example, Skin erythema and acute ilness such as diarrhea are acute
While cataract , infirtility are chronic ..ext.
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The SI unit of absorbed dose is the gray (Gy).
One Gy is one joule per kilogram of irradiated material.
the effective dose , is the radiation dose absorbed by the body and is
measured in sieverts (Sv) .
Effective dose is a valuable measure to compare different examinations.
Examples of effective doses for some typical radiographic examinations
are:
dental 0.005–0.02 mSv;
chest 0.01–0.05 mSv;
skull 0.1–0.2 mSv;
pelvis 0.7–1.4 mSv;
lumbar spine 0.5–1.5 mSv.
According to the International Commission on Radiological Protection
(ICRP) , the relative radiation risk for cancer is 5.5% / Sv and
for heritable effects up to the second generation is 0.2% / Sv.
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Radiation protection:
Because of the potential risk of medical irradia-tion, the ICRP
recommends keeping the magnitudeof individual examination doses as
low as reason-ably achievable (ALARA principle).
There are no dose limits for patients, but every exposure should be
justified. This is, to a large extent, a medical decision .
Pregnancy for example, is a state where risks are increased. Special
attention should also be given to children and to high-dose imaging,
such as interventional radiology.
Furthermore, the ICRP recommends limiting allexposed workers from
regulated radiation practices to20 mSv per year when averaged over five
years andthe public to 1 mSv per year.
In particular, physicians may receive a significant exposure when doing
procedures under fluoroscopy, but they too must not exceed 20 mSv per
year.
There are strict protection protocols they have to follow, among which
is the protection of the body the thyroid gland and cornea with a lead
apron ,collar and glasses A dosimeter, which is a small device clipped to
the personnel’s clothing , measures the cumulative absorbed dose .
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