RAD216 ADVANCED IMAGING
MODALITIES & SPECIAL
PROCEDURES
APPLICATION OF
DIGITAL FLUOROSCOPY:
DIGITAL SUBTRACTION
SUBTRACTION
q
which
A technique
removes bony anatomy
which obscures contrastfilled vessels and organs.
PHOTOGRAPHIC SUBTRACTION
(FIRST ORDER)
Involves the use of
ANALOG techniques. The
steps employed in
obtaining subtraction
images are numerous and
require precision and skill.
STEP 1
Obtain the scout film. It is
i
important
t t tto position
iti
the
th patient
ti t
the same way for each of the
subsequent radiographs.
Breathing instructions should be
carefully explained so that they are
replicated with each exposure.
STEP 2
Place the scout film on the glass
surface of the duplication machine.
machine
Place the special-purpose
SUBTRACTION FILM on top of the
scout film. Set the duplication
machine to the correct exposure
time and expose subtraction film.
STEP 3
Process the exposed
subtraction film.
film The
processed film becomes the
DIAPOSITIVE MASK. This
mask is the reversal of the
scout film in that all of the
densities are reversed.
SUBTRACTION FILM
† SIMILAR CHARACTERISTICS TO
DUPLICATION FILM
„ SINGLE EMULSION
„ HIGH CONTRAST
„ SOLARIZED (DENSITY DECREASES WITH
INCREASED EXPOSURE TO LIGHT)
† DISIMILAR CHARACTERISTICS
„ REVERSAL OF DENSITIES
„ DISTINCTIVE NOTCHING ON FILM CORNER
STEP 4
The contrast-enhanced
radiographs are obtained
using positioning and
respiratory control similar to
what was used in obtaining
the scout radiograph.
STEP 5
The contrast-enhanced radiograph
and the mask are positioned
together on the glass of the
duplicating machine so that all
features are precisely
superimposed. This is a very
difficult step and requires patience
and meticulous attention to detail.
STEP 6
After carefully placing the
mask and contrast
radiograph on the
duplicating machine, a
fresh subtraction film is
placed over these.
STEP 7
After exposing the subtraction film,
it is processed.
processed The resulting
subtraction radiograph is obtained.
This method produces contrastfilled vessels and organs free of
bony superimposition. Contrast is
seen as dark whereas other
structures are lighter.
MOCK SUBTRACTION (WITHOUT CONTRAST)
SCOUT
MASK
CONTRAST
CONTRAST - MASK
DISADVANTAGES OF
PHOTOGRAPHIC SUBTRACTION
† TIME CONSUMING
† PRECISION AND SKILL
REQUIRED
† PATIENT FACTORS WILL
DIMINISH QUALITY OF
SUBTRACTION RADIOGRAPHS
(INVOLUNTARY MOTION, ETC.)
DIGITAL SUBTRACTION
A technique
q
employing
p y g
the use of a computer
to remove bony
superimposition from
contrast-filled images.
DIGITAL SUBTRACTION
METHODS
†TEMPORAL SUBTRACTION
„ MASK MODE
„ TIME INTERVAL DIFFERENCE
†ENERGY SUBTRACTION
†HYBRID SUBTRACTION
MASK MODE
The first step involves
converting (digitizing) a
pre-contrast image. This
digitized image will be
used as the mask.
MASK MODE
Subsequent contrast-filled images
are also digitized. The mask is
then used to mathematically
subtract the bony densities from
the contrast images so only the
contrast-filled structures remain.
MASK
M
CONTRAST IMAGES
1
2
3
4
5
SUBTRACTION IMAGES
1-M
2-M
3-M
4-M
5-M
REMEMBER...
When an image is digitized (by means
of an analog
analog-to-digital
to digital converter) all of
the densities of the image are
converted into series of 1’s and 0’s. It
should not be too difficult to see how
bone can be mathematically removed
from soft tissue since this is done pixel
by pixel.
TIME INTERVAL DIFFERENCE
This temporal subtraction technique
utilizes several masks.
masks The first
several images are obtained and
digitized. After a pre-determined time
in which contrast is allowed to fill the
blood vessels, the first images are
subtracted from later images.
TIME INTERVAL DIFFERECE
1
2
3
7-2
8-3
9-4
4
10-5
5
6-1
TIME INTERVAL DIFFERENCE
This technique is especially useful
when the contrast
contrast-filled
filled structures are
susceptible to motion artifacts such as
heart motion. These defects are
called MISREGISTRATION ARTIFACTS.
Misregistration results from the pixels
from each image not lining up
precisely.
ENERGY SUBTRACTION
This technique takes
advantage of the attenuation
differences between tissues
and contrast media when
different x-ray energies are
used.
ENERGY SUBTRACTION
Especially important to this
technique is the k-shell absorption
edge of the iodine. By using two
different x-ray energies on either
side of the k-shell edge of iodine,
photoelectric absorption of iodine is
enhanced or diminished
tremendously in comparison to
surrounding tissues.
ENERGY SUBTRACTION
The difference in
photoelectric absorption of
iodine compared to the
tissues is what makes this
type of subtraction
possible.
ENERGY SUBTRACTION
Two methods can be used to
generate different energy spectra.
spectra
The first is to alter the kVp between
70 and 90 kVp. The other
(preferred) is to introduce two
dissimilar metal filters into the x-ray
beam in alternating fashion (using a
fly wheel).
LIMITATIONS OF DIGITAL
SUBTRACTION
The main disadvantage of digital
subtraction is the small field of
view (FOV). Consequently,
digital subtraction is limited to
relatively small areas of the
body.