Download BD0040

SMU-DDE-Assignments-Scheme of Evaluation
PROGRAM
SEMESTER
SUBJECT CODE &
NAME
BK ID
SESSION
MARKS
B.Sc./DIPLOMA IN MEDICAL IMAGING TECHNOLOGY
III
BD0040/DM0040– PHYSICS OF RADIOGRAPHIC EQUIPMENT
B1054
WINTER 2015
40
Marks
Total Marks
Criteria
Q.No
1.
Explain how a galvanometer can be converted into a voltmeter and an ammeter.
(Unit 3;Pg 30-31)
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2.
The instrument, which is used to measure
current, is known as ammeter, a more sensitive
instrument, which measures thousandths of an
ampere, is called milliammeter.
It is usually connected in series with the circuit
so that the whole of the current, which flows
through it, could be measured.
To increase the sensitivity of the instrument, a
shunt is added in parallel, which diverts or
shunts a fixed fraction of the current from the
meter.
We know a simple galvanometer is a current
measuring device, this can be converted into an
ammeter and milliammeter by connecting a
suitable low resistance across its terminals, this
is known as shunt resistance.
A galvanometer can be converted into a
voltmeter by connecting a high resistance in
series with it. Voltmeters and ammeters are
incorporated in an X-ray machine, to measure
electrical parameters involved in the working of
the machine.
For example AC voltmeter is connected to the
primary winding of the mains voltage
compensating transformer. Pre-reading kV
meter is connected to the kV selector, a
milliammeter is connected in series with the
secondary of the high tension generator.
Explain the magnetic effect of electric current.
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10
(Unit 4;Pg 38-39)
SMU-DDE-Assignments-Scheme of Evaluation
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3.
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When a compass needle is placed near a current
carrying conductor, a deflection is noticed when
the current is switched on.
This proves that, magnetic effect is produced
when current is passed through a conductor.
If the conductor is wound into the shape of a
coil (solenoid) the magnetic effect becomes
stronger.
This is because each of the turns produces a
magnetic field, which reinforces the next.
The strength of a magnetic field can be
increased by:
 Increasing the number of turns on the coil.
 Inserting soft iron core into the coil.
 Increasing the current through the solenoid.
A solenoid with soft iron core which can be
magnetized by passing a current through the
coil is called an electromagnet.
The soft iron core acts as temporary magnet and
does not retain its magnetism after the current is
switched off.
Thus this is a type of magnet which can be
switched on and off as required.
Examples
of
such
applications
of
electromagnets:
 Relay also called contactor in X -ray
generators.
 Electric motor used in X-ray tube whose
function depends upon electromagnetic
induction, which is also called induction
motor.
 Equipment for removing metallic foreign
bodies from the eye.
10
Discuss the properties of x-rays.
Explanation of Fluorescence property of x-rays
Explanation of Photographic property of x-rays
Explanation of Ionization and Excitation property
of x-rays
Explanation of Chemical changes of x-rays
Explanation of Biological effects of x-rays
Explain how radiation transmits through body tissues.
10
(Unit 9;Pg 84)
2
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10
2
2
(Unit 10;Pg 117-119)
SMU-DDE-Assignments-Scheme of Evaluation
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When a beam of X-rays is incident on any
absorbing material, the X-rays can interact
with the atoms of that material by any of the
four mechanisms previously discussed.
The frequency of each mechanism depends on
the atomic number of the target atom and the
X-ray energy.
Out of the four mechanisms only two are
important in diagnostic radiology: the
Compton scattering and the Photoelectric
absorption.
The Compton scattering does not contribute
any useful information rather it results in film
fog, which has to be eliminated by using
devices like grids.
The Photoelectric absorption of X-rays results
in bright areas in the radiograph like those of
bone, the X-rays that penetrate the body and do
not interact result in dark areas of the
radiograph, they pass through body structures
which are termed as radiolucent.
X-ray image results from a difference between
those X -rays absorbed due to photoelectric
absorption and those that are not absorbed at
all. This is called differential absorption.
A general thumb rule we can remember is that
for most of the radiographic examinations less
than 5% of the X-rays which are incident on
the patient reach the film, and less than half
these interact with the film to form the image.
In order to produce a good quality radiograph
proper selection of kVp is necessary so that the
effective X-ray energy results in maximum
differential absorption.
In body tissues, bone has atomic number
approx. 13.8 while soft tissue has atomic
number approx. 7.4. From the formula mass
attenuation coefficient is proportional to cube
of atomic number (Z3), the probability of
photoelectric absorption in bone becomes 7
times greater than in soft tissue [(13.8/7.4)
3 = 6.49].
The probability of Compton scattering for bone
and soft tissue atoms are equal and decrease
with increasing X-ray energy.
At low energies the majority of X-ray
interactions are photoelectric absorption while
at higher energies Compton scattering process
predominates.
Thus to image small differences in soft tissue,
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SMU-DDE-Assignments-Scheme of Evaluation
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one must use low-kVp technique to get
maximum differential absorption.
This is the basis of Mammography for Breast
examination through radiography.
In order to study an anatomical structure which
is adjacent to soft tissue of similar atomic
number, contrast medium is used whose atomic
number is higher thus a differential absorption
is achieved.
Preparations of Iodine (Z= 53) for Intravenous
Pyelograms and Barium (Z= 56) for Barium
meal examination, the probability of
photoelectric interaction in these contrast
material is 400 times compared with the
probability of interaction with the adjacent soft
tissue, thus when barium fills the colon, these
internal organs are readily visible on a
radiograph.
Air was used as a contrast medium for
procedures like ventriculography in the form of
negative contrast but with the advent of
Computerised tomography and Magnetic
resonance imaging this procedure has become
obsolete.
*A-Answer
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