RADIOLOGY
RADIATION UNITS
In the SI system (the internationally agreed system of units), three main units are
used to measure radiation levels - the becquerel, the gray, and the sievert. These
three units are defined below, along with two other radiation units (the rad and
rem) that have now been largely superseded but are still occasionally used for
some purposes.
Becquerel
The SI unit of radioactivity. One becquerel (symbol Bq) is defined as one disintegration (or other
nuclear transformation) per second. Although the number of becquerels is a measure of how
strongly radioactive a particular source is, it takes no account of the different effects of different
types of radiation on tissue; for medical purposes, the sievert is generally more useful.
Gray
The SI unit of absorbed dose of ionizing radiation, the gray (symbol Gy) has superseded the rad.
One gray is defined as an energy absorption of 1 joule per kilogram of irradiated material. One
gray is equivalent to 100 rads.
Rad
An acronym for radiation absorbed dose, the rad is a unit of absorbed dose of ionizing radiation.
One rad is equal to an energy absorption of 100 ergs (an erg is a unit of work or energy) per gram
of irradiated material. The rad has been superseded by the gray (the corresponding SI unit);
1 rad is equivalent to 0.01 grays.
Rem
An acronym for roentgen equivalent man, the rem is the absorbed dose of ionizing radiation that
produces the same biological effect as 1 rad ofX-rays or gamma rays. The rem was introduced as a
result of the observation that some types of ionizing radiation, such as neutrons, produce a
greater biological effect for an equivalent amount of absorbed energy than X-rays or gamma rays.
In short, the rem is a measure ofthe biological effectiveness of irradiation. For X-rays and gamma
rays, the rem is equal to the rad. For other types of radiation, the number or rems equals the
number of rads multiplied by a special factor (called the quality factor or relative biological
effectiveness) that depends on the type of radiation involved. The rem has been superseded by
the sievert in the SI system of units; 1 rem is equivalent to 0.01 sieverts.
Sievert
The SI unit of equivalent absorbed dose of ionizing radiation, the sievert (symbol Sv) has
superseded the rem. One sievert is the absorbed dose of radiation that produces the same
biological effect as 1 gray ofX-rays or gamma rays. One sievert is equivalent to 100 rems.
R
ra d io lo g y
T h e
m e d i c a l
s p e c i a l i t y
t h a t
u s e s
X -ra ys,
u ltra s o u n d sc a n n in g , M R I
( m a g n e t i c r e s o n -
a n c e i m
a g i n g ) , a n d
r a d io n u c lid e s c a n n in g
t o i n v e s t i g a t e , d i a g n o s e , a n d t r e a t d i s e a s e .
R a d i o l o g i c a l m e t h o d s p r o v i d e i m a g e s
o f t h e b o d y i n a
n o n in v a s iv e
w a y s o t h a t
e x p l o r a t o r y
s u r g e r y
i s
n o t
r e q u i r e d .
T h e t e c h n i q u e s a l s o e n a b l e i n s t r u m e n t s
( s u c h a s
n e e d l e s a n d
c a t h e t e rs
)
t o
b e
a c c u r a t e l y g u i d e d i n t o d i f f e r e n t p a r t s o f
t h e
b o d y
b o t h
f o r
d i a g n o s i s
a n d
f o r
t r e a t m e n t ,
a
s u b s p e c i a l i t y
k n o w n
a s
i n t e r v e n t i o n a l r a d i o l o g y .
ra d io lu c e n t
A t e r m r e f e r r i n g t o a n y t h i n g t h a t i s a l m o s t
t r a n s p a r e n t t o
ra d ia tio n ,
e s p e c i a l l y t o
X -rays
a n d g a m m a r a y s . S o m e X - r a y i m a g i n g p r o -
c e d u r e s r e q u i r e t h e i n t r o d u c t i o n o f
ra d io -
p a q u e
s u b s t a n c e s i n t o t h e b o d y t o m a k e
ra d io tra n sp a re n t
a n d
r a d i o l u c e n t
o r g a n s
a n d / o r b o d y t i s s u e s s t a n d o u t m o r e c l e a r l y
ra d io n u c lid e s c a n n in g
A d i a g n o s t i c t e c h n i q u e t h a t i s b a s e d o n
t h e
d e t e c t i o n
o f
ra d ia t io n
e m i t t e d
b y
r a d i o a c t i v e
s u b s t a n c e s
i n t r o d u c e d i n t o
t h e b o d y . D i f f e r e n t s u b s t a n c e s a r e t a k e n
u p i n d i f f e r e n t c o n c e n t r a t i o n s b y d i f f e r -
e n t t i s s u e s , a l l o w i n g s p e c i f i c o r g a n s t o
Radionuclide bone scan
Here, the radionuclide has been absorbed by bone.
Abnormalities (such as metastases) absorb it in
greater amounts and appear as ‘hot spots’.
b e s t u d i e d . F o r e x a m p l e , i o d i n e i s t a k e n
u p m a i n l y b y t h e t h y r o i d g l a n d , s o b y
“ t a g g i n g ”
a
s a m p l e
o f i o d i n e
w i t h
a
r a d i o a c t i v e
m a r k e r
( r a d i o n u c l i d e ) ,
t h e
t h y r o i d g l a n d ’s u p t a k e o f i o d i n e c a n b e
m
o n i t o r e d t o i n v e s t i g a t e t h e f u n c t i o n -
i n g o f t h e g l a n d .
HOW IT WORKS
T h e r a d i o n u c l i d e i s s w a l l o w e d o r a l t e r -
n a t i v e l y i n j e c t e d
i n t o
t h e b l o o d s t r e a m
a n d
t h e n
a c c u m u l a t e s
i n
t h e
t a r g e t
o r g a n . T h e o r g a n e m i t s r a d i a t i o n i n t h e
f o r m
o f
g a m m a
r a d i a t i o n ,
w h i c h
i s
d e t e c t e d b y a g a m m a c a m e r a a n d t h e n
a n i m a g e i s p r o d u c e d .
C r o s s - s e c t i o n a l i m a g e s
( “ s l i c e s ” )
c a n
b e o b t a i n e d b y u s i n g a c o m
p u t e r - c o n -
t r o l l e d
g a m m a
c a m e r a
t h a t
r o t a t e s
a r o u n d
t h e
p a t i e n t .
T h i s
s p e c i a l i z e d
f o r m o f r a d i o n u c l i d e s c a n n i n g i s k n o w n
a s S P E C T ( s i n g l e p h o t o n e m i s s i o n c o m
-
p u t e d t o m o g r a p h y ) . M o v i n g i m a g e s c a n
a l s o b e t a k e n w i t h t h e u s e o f a c o m p u t -
e r ;
a s e r i e s
o f i m a g e s
i s r e c o r d e d i m
-
m e d i a t e l y f o l l o w i n g
t h e a d m i n i s t r a t i o n
o f t h e r a d i o n u c l i d e .
WHY IT IS DONE
R a d i o n u c l i d e
s c a n n i n g
i s
c a p a b l e
o f
d e t e c t i n g s o m e
d i s o r d e r s a t a n e a r l i e r
s t a g e t h a n o t h e r t y p e s o f
im a g in g te ch -
n iq u e s
b e c a u s e
c h a n g e s
i n
t h e
f u n c -
t i o n i n g o f a n o r g a n o f t e n o c c u r b e f o r e
t h e s t r u c t u r e
o f t h e
o r g a n i s a f f e c t e d .
T h e t e c h n i q u e i s a l s o u s e d t o d e t e c t d i s -
o r d e r s t h a t a f f e c t o n l y t h e f u n c t i o n o f
a n o r g a n . M o v i n g i m a g e s c a n p r o v i d e
i n f o r m a t i o n
a b o u t
b l o o d
f l o w ,
t h e
m o v e m e n t o f t h e h e a r t w a l l s , t h e f l o w
o f u r i n e t h r o u g h t h e k i d n e y s , a n d b i l e
f l o w t h r o u g h t h e l i v e r .
RISKS
R a d i o n u c l i d e
s c a n n i n g
i s
a s a f e
p r o c -
e d u r e , r e q u i r i n g o n l y m i n u t e d o s e s o f
r a d i a t i o n t h a t a r e e x c r e t e d w i t h i n h o u r s .
T h e r a d i o n u c l i d e s c a r r y v i r t u a l l y n o r i s k
o f t o x i c i t y o r
h y p e rs e n s itiv ity .
ra d io p a q u e
A t e r m d e s c r i b i n g a n y t h i n g t h a t i s a b l e
t o b l o c k
ra d ia t io n ,
e s p e c i a l l y
X -ra y s
a n d
g a m m a r a y s . A s m a n y b o d y t i s s u e s a r e
ra d io lu c e n t
( a l m o s t
t r a n s p a r e n t
t o
X -
r a y s ) , s o m e X - r a y i m a g i n g p r o c e d u r e s
r e q u i r e t h e i n t r o d u c t i o n o f r a d i o p a q u e
s u b s t a n c e s
i n t o
t h e
b o d y
i n
o r d e r
t o
m a k e t h e o r g a n s s t a n d o u t c l e a r l y
( S e e
a l s o
ra d io tra n s p a re n t
. )
ra d io th e ra p y
T r e a t m e n t
o f
c a n c e r
a n d ,
o c c a s i o n a l l y ,
s o m e
n o n c a n c e r o u s
t u m o u r s ,
b y
t h e
u s e
o f
X -ra y s
o r
o t h e r
ra d ia t io n .
R a d i o -
646
previous page 645 BMA A Z Family Medical Encyclopedia   2004 read online next page 647 BMA A Z Family Medical Encyclopedia   2004 read online Home Toggle text on/off