Hello awesomites!
Today I am starting a new series of posts on
radiology. Here I will be mostly dealing with the theory and technical part of
radiology which as an undergraduate student we rarely read. Starting off with
x-rays in this post and CT, USG, MRI to follow in the consecutive ones!
X-rays
A little bit of history,
X-rays were discovered by W. C. Roentgen in Germany
on 8th nov 1895 and for which he received a Nobel Prize in 1901.
First let us know a few technical terms,
collectively known as the ‘exposure factors:
1) kVP:
kilovolt peak
It determines the penetration of the x-ray beam through the body. High
kVP implies more penetration of the body tissues
2) mAS:
milliampere second
It determines the amount of blackening of the film. A high mAS will
cause more blackening of the film for the same amount of x-rays hitting it.
3) Contrast:
It is the contrast shadow that is produced on the film i.e. white for bone and
black for soft tissues. It is influenced mainly by the penetration of the
x-rays i.e. the kVP and partly by mAS.
A low kVP (low penetration) means
high contrast.
Contrast is proportional to 1/kVP
But,
A low mAS (less blackening) means low
contrast.
Contrast is directly proportional to mAS
Let
us take up an example,
In obese and heavily built patients, more penetrance
is needed so we need to increase the kVP but if we do so by increasing the kVP
we are reducing the contrast which is not good.
So to achieve both high contrast and good
penetrance, kVP is increased as well as mAS is increased.Increased kVP will
take care of the required penetrance while high mAS will ensure good contrast.
The general protocol to increase the contrast is
first by reducing the kVP and then if necessary increasing the mAS to desired
levels.
That’s
all for now, more about the actual working of the x-ray machine and different
settings in the next post. Hope you liked it !
Until then,
Keep calm and keep studying
Stay awesome!
-
Ashish G. Gokhale
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