# INTRODUCTION TO QUALITY CONTROL

Urban and Civil

Nov 15, 2013 (4 years and 8 months ago)

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INTRODUCTION TO QUALITY CONTROL

ANTHONY VO
|
14846227

Review questions
-

Part A

1)

Determine if the LBD of the room you are using is with in acceptable
limits.

If not what could cause the
problems, if it is what might be
sources for LBD in
-
congruency.

The congruence

is measured
using the

percentage of the distance of the
corresponding edges of the x
-
ray field and the light field, divided by the
SID
(source to image distance) used.

An allow
ance of +/
-

1% is acceptable.

Congruency (Height) = percentage change of height/
SID

{
[(
8.001
-
7.569)/
8.001]

x
100}
/ 1 =
5.399
%
(3dp)

Congruency (width)= percentage change of width/SID

{
[(
5.606
-
5.488)/5.606] x 100}/ 1=
2.1%

(1 s
.
f)

The source of
in
congruency

is mainly due to the fact that the mirror at the
centre of the collimator which is mainly used to reflect light from the bulb
through the opening of the shutter shifts positions. Another factor may be due
to the malfunction of the actual shutter
s themselves.

2)

What are the issues that can be attributed to possessing an inaccurate
LBD.

Image quality

An inaccurate LBD is an
serious

breach in radiation safety and image quality.
Poor application of the collimator may result in various undiagnostic images
which will in most cases lead to an excess of radiation dose for the patient due
to repeated x
-
rays. For example if the size of the
field exceeds what is
necessary,
more scattered radiation is produced which can be further
absorbed by the patient. If the collimation is too small, the image may miss out
on various important anatomical features that are required to diagnose the
patient.
This may very well lead to a repeat examination and hence a larger
absorbed dose for the patient.
It

is evident that an inaccurate LBD will lead to
breaches in radiation safety and image quality.

Review questions
-

Part B

1)

Compare the entrance dose and the
transmission dose, for the 70kvp
and
90kvp exposures in steps 3 and 4
. What are some explanations for
the results obtained?

Entrance dose (STEP 3): 12.22 mGy

Entrance dose (STEP 4): 5.536 mGy

Transmission dose

(STEP 3): 79
.32 μGy

Transmission dose (STEP 4
): 63.72μGy

In both steps, the entrance dose is significantly larger then the transmission
dose. This is due to the fact that the radiation is attenuated inside the
phantom as it passes through, this will leave only a portion of the radiation to
pass thro
ugh, hence resulting in a lower transmission dose.

Step 3 and 4 are significantly different in the fact that step 3 is set with a lower
Kvp but higher mAs, however step 4 is is at a higher Kvp but lower mAs.
Theoretically a higher Kvp in step 4 would resu
lt in a more penetrating and
harder beam
, meaning that it should be able to produce a higher transmission
dose, as there would be less attenuation. This however is not the case as is
shown above. This is due to the fact that step 3 has a higher mAs which
subsequently increases the dose due to the increased quantity of x
-
ray
photons.

2)

C
ompare

the values obtained from the
10x10cm field size from steps 5
and 6
.

With those obtained with the 35 x 43 cm field size from steps 3
and 4.
How can any differences be e
xplained?

Entrance dose (STEP 5
):
10.17 mGy

Entrance dose (STEP 6
):
4.436 m Gy

Transmission dose
(STEP 5
):
19.6

μGy

Transmission dose (STEP 6
):
0.28
μGy

It is evident that the doses from the 10x10 field size are considerably less then
the doses from
the 45x43 size. This is due to the fact that a smaller field size
will equate to less scattered radiation reaching the patient.

3)

On the basis of the data,

which combination has the greatest impact on
patient exposure?

a)

High kilovolt(peak), low milliampere
-
se
cond factors

b)

Variation in collimation size

By analysing the data and distinguishing which combination would actually
reduce the absorbed dose the most, it would have to be the combination of
high kilovolt and low mA factors.