Computer application in Radiology - lec -5.pdf

Lecture 5
Cassette-Based Image Acquisition
Dr. Emad Taleb

Computed Radiography Image
Acquisition
Factors associated with image acquisition with
Computed Radiography (cassette-based) Systems:
Part selection
Technical factors
Equipment selection
Collimation
Side/position markers
Exposure indicators
Image data recognition and preprocessing
2

Part Selection
• Once the patient has been positioned and the plate has been
exposed, you must select the exam or body part from the menu
on your workstation.
• If you are performing a skull exam, you select “skull” from
the workstation menu.
3

Part Selection
 Proper part must be selected so
that appropriate image
recognition and interpretation
can take place.
 For example, if a knee exam is
to be performed and the exam
selected is for skull, the
computer will interpret the
exposure for the skull,
resulting in improper density
and contrast and inconsistent
image graininess.
A-Anteroposterior (AP) knee with proper menu selection.
B-AP knee with AP skull selected.
4

Part Selection
• It is not acceptable to select a body part or position
different from that actually being performed simply
because it looks better.
• If the proper exam/part selection results in a
suboptimal image, then service personnel should be
notified of the problem and the problem should be
corrected as soon as possible.
• Improper menu selections may lead to overexposure
of the patient and to repeated exams.
5

Technical Factors
• Kilovoltage peak (kVp) selection
• Milliampere-second (mAs) selection
6

Kilovoltage Peak Selection
 Kilovoltage peak, milliampere-second, and distance
Chosen in exactly the same manner as for conventional
film/screen radiography.
 Kilovoltage peak
Chosen for penetration and the type and amount of contrast
desired.
70 kVp no longer the minimum kVp for CR.
Kilovoltage peak values now range from around 45 to 120.
It is not recommended that kilovoltage peak values less
than 45 or greater than 120 be used because those values
may be inconsistent and may produce too little or too much
excitation of the phosphors.
• The k-edge of phosphor imaging plates ranges from 30
to 50 keV so that exposure ranges of 60 to 110 kVp are
optimum.
7

Milliampere-Second Selection
 Milliampere-second is selected according to the number of electrons
needed for a particular part.
 Too few electrons and no matter what level of kilovoltage peak
is chosen, the result will be a lack of sufficient phosphor
stimulation.
 When insufficient light is produced, the image will be grainy, a
condition known as quantum mottle or quantum noise.
 When converting from film/screen systems to a CR system, it is
critical that the automatic exposure control be recalibrated.
8

Equipment Selection
• Imaging plate selection
• Grid selection
9

Imaging Plate Selection
 Two important factors should be considered when selecting the CR
imaging cassette: type and size.
 Type
• Standard
• High resolution
Cassettes should be marked on the outside to indicate high-
resolution imaging plates.
 Size
• CR digital images are displayed in a matrix of pixels, and the pixel
size is an important factor in determining the resolution of the
displayed image.
• Some vendors vary the pixel size according to the size of the
cassette, but some do not. Therefore those systems that vary the pixel
size, using the smallest imaging plate possible for each exam results
in the highest spatial resolution.
10

Grid Selection
• Moiré pattern: A wavy artifact
caused by grid lines running parallel
to the laser scanning motion.
 The oscillating motion of a moving
grid or Bucky blurs the grid lines
and eliminates the interference.
• Because of the ability of CR
imaging plates to record a very high
number of x-ray photons, the use of
a grid is much more critical than in
film/screen radiography.
11

Grid Frequency
Grid frequency
Refers to the number of grid lines per centimeter or lines per
inch.
• The higher the frequency or the more lines per inch, the
finer the grid lines in the image and the less they interfere
with the image.
Typical grid frequency is between 80 and 152 lines per inch.
Some manufacturers recommend no fewer than 103 lines per
inch and strongly suggest grid frequencies greater than 150
lines per inch.
• The closer the grid frequency is to the laser scanning
frequency, the greater likelihood of frequency harmonics
or matching and the more likely the risk for moiré effects.
12

Grid Ratio
Grid ratio
 The relationship between the height of
the lead strips and the space between
the lead strips
• The higher the ratio, the more
scatter radiation is absorbed.
• The higher the ratio, the more
critical the positioning is, such that
high grid ratio is not the appropriate
choice for mobile radiography.
 A grid ratio of 6:1 would be proper for
mobile radiography, whereas a 12:1
grid ratio would be appropriate for
departmental grids that are more stable
and less likely to be mispositioned,
causing grid cutoff errors.
13

Grid Focus
Focused grids
Consist of lead strips angled to
coincide with the divergence of
the x-ray beam and must be used
within specific distances using a
precisely centered beam
 Parallel grids
Consist of lead strips running
parallel with the image receptor
and are less critical to beam
centering but should not be used
at distances less than 48 inches
14

Collimation
 With increased tissue volume and kVp used, comes increased
production of Compton interactions or scatter.
 By reducing the volume of tissue being irradiated by
collimation, a decrease in scatter production can be seen.
This results in increased contrast because of the reduction
of scatter as fog and reduces the amount of grid cleanup
necessary for increased resolution.
15

Shuttering
• Through postexposure image manipulation known as
shuttering, a black background can be added around the
original collimation edges, virtually eliminating the
distracting white or clear areas.
• This technique is not a replacement for proper pre-exposure
collimation, it is an image aesthetic only and does not
change the amount or angles of scatter created. There is no
substitute for appropriate collimation, for collimation
reduces patient dose.
With
Shuttering
Without
Shuttering
16

Side/Position Markers
 Conventional lead markers should be used the same way they
were used in film/screen systems.
Electronic markers can be easily used to mark images with
left and right side markers or other position or text markers
after the exposure has been made. But . . .
Marking the patient exam at the time of exposure not only
identifies the patient’s side but also identifies the
technologist performing the exam.
If the exam is used in a court case, the marker with the
technologist’s markers allows the possibility of the
technologist’s testimony and lends credibility to his or her
expertise.
17

Exposure Indicators
• The amount of light given off by the imaging plate is a result
of the radiation exposure that the plate has received.
• The light is converted into a signal that is used to calculate
the exposure indicator number, which is a different number
from one vendor to another.
• The base exposure indicator number for all systems
designates the middle of the detector operating range.
18

Exposure Indicators
 For Fuji, Phillips, and Konica systems, the exposure indicator is
known as the S, or sensitivity number.
The S number is the amount of luminescence emitted at 1 mR
at 80 kVp, and it has a value of 200.
The higher the S number with these systems, the lower the
exposure.
• For example, an S number of 400 is half the exposure of an
S number of 200, and an S number of 100 is twice the
exposure of an S number of 200.
The numbers have an inverse relationship to the amount of
exposure so that each change of 200 results in a change in
exposure by a factor of 2.
19

Exposure Indicators
 Carestream uses exposure index, or EI, as the exposure indicator.
A 1 mR exposure at 80 kVp combined with aluminum/copper
filtration yields an EI number of 2000.
• An EI number plus 300 (EI + 300) is equal to a doubling of
exposure, and an EI number of minus 300 (EI − 300) is equal
to a halving of exposure.
The numbers for the Carestream system have a direct
relationship to the amount of exposure so that each change of
300 results in change in exposure by a factor of 2.
This is based on logarithms, only instead of using 0.3 (as is used
in conventional radiographic characteristic curves) as a change
by a factor of 2, the larger number 300 is used.
This is also a direct relationship; the higher the EI, the higher the
exposure. 20

Exposure Indicators
 The term for exposure indicator in an Agfa system is the lgM,
or logarithm of the median exposure.
An exposure of 20 µGy at 75 kVp with copper filtration
yields an lgM number of 2.6.
• Each step of 0.3 above or below 2.6 equals an exposure
factor of 2.
An lgM of 2.9 equals twice the exposure of 2.6 lgM, and an
lgM of 2.3 equals an exposure half that of 2.6.
The relationship between exposure and lgM is direct.
21

Exposure Indicators
• These ranges depend on proper calibration of equipment
and represent the minimum and maximum exposure
numbers that correspond with radiation exposure within
the diagnostic range.
• Exposure numbers outside the range indicate
overexposure and underexposure. Pediatric exam ranges
vary, as do specific body part indices according to
manufacturer.
22

Image Data Recognition and
Preprocessing
 The image recognition phase is important in establishing the
parameters that determine collimation borders and edges and
histogram formation.
All CR systems have this phase, and each has a specific name for
this process.
• Agfa uses the term “collimation”, Carestream uses the term
“segmentation”, and Fuji uses the phrase “exposure data
recognition.”
All systems use a region of interest(ROI) to define the area
where the part to be examined is recognized and the exposure
outside the region of interest is subtracted.
Each vendor has a specific tool for different situations—such as
neck, breasts, and hips, or pediatrics—in which the anatomy
requires some special recognition. 23

Automatic Data Recognition(Fuji)
 Automatic data recognition
The image recording range is automatically determined.
This mode automatically adjusts reading latitude (L) and S
number.
Collimation is automatically recognized, and a complete
histogram analysis occurs.
• Good collimation practices are critical because
overcollimation or undercollimation leads to data recognition
errors that affect the histogram.
Lead markers must be in the exposure area.
24

Automatic Data Recognition(Fuji)
 Automatic data recognition – cont’
Overlapping exposures may have a negative impact on image
display, depending on the manufacturer.
Each of the exposure regions is processed to identify the shape of
the field and the approximate center.
Data recognition then occurs from the center out diagonally, and
when the value of the pixels exceeds a preset threshold, those
points are interpreted as collimation.
Exposure data outside the collimation points is subtracted in the
histogram analysis.
25

Semiautomatic Mode (Fuji)
 Semiautomatic Mode
The L value of the histogram is fixed, and only a small
reading area is used.
No collimation detection, and the proper kilovoltage must be
used to maintain subject contrast because the L value does
not change.
Semiautomatic mode is especially useful for exams of the
odontoid, L5/S1 spot film, sinuses, and any other tightly
collimated exams.
Precautions must be taken when using this mode to carefully
center the part to be examined, and the mode is not
recommended for high-absorption objects such as prostheses.
26

Multiple Manual Selection
Mode(Fuji)
 Also called Semi-X mode
The user selects from nine areas of the imaging plate.
The technologist selects the area of interest, and the image
is derived from the selected areas imaged in semiautomatic
mode.
The same precautions apply as in semiautomatic mode. The
cassette orientation label must be noted with relation to the
area of interest.
This mode is helpful in cross-table exams in which the
body part may not align with automatically selected
imaging plate regions.
27

Fixed Mode (Fuji)
 Fixed mode
S and L values are fixed in that the user selects the S value,
and the L value.
No histogram analysis
No recognition of imaging plate division
using fixed mode is like using film screen
• The density of the image directly reflects the technique
that is used.
This mode is useful when imaging cross-table hips, C7-T1
lateral view of the cervical spine, any body part with a lot of
metal, and parts that cannot be centered.
28

Common CR Image Acquisition
Errors
 As with film screen, artifacts can detract and degrade
images.
Imaging plate artifacts
Plate reader artifacts
Image processing artifacts
Printer artifacts
Operator errors
29

Imaging Plate Artifacts
 As the imaging plate ages, it becomes prone to cracks from the
action of removing and replacing the imaging plate within the reader.
Cracks in the imaging plate appear as areas of lucency on the
image.
The imaging plate must be replaced when cracks occur in
clinically useful areas.
30

• Adhesive tape used to secure lead markers to the cassette can
leave residue on the imaging plate.
31

• If static exists because of low humidity, hair can cling to the
imaging plate.
32

• Backscatter created by x-ray photons transmitted through the
back of the cassette can cause dark line artifacts.
• Areas of the lead coating of the cassette that are worn or cracked
allow scatter to image these weak areas. Proper collimation and
regular cassette inspection help to eliminate this problem. 33

Plate Reader Artifacts
• The intermittent appearance of extraneous line patterns can be
caused by problems in the electronics of the plate reader.
• Reader electronics may have to be replaced to remedy this
problem.
34

 Horizontal white lines may be caused
by dirt on the light guide in the plate
reader. Service personnel need to clean
the light guide.
 If the plate reader loads multiple
imaging plates in a single cassette, only
one of the plates will usually be
extracted, leaving the other to be
exposed multiple times.
 The result is similar to a conventional
film/screen double-exposed cassette.
35

 Incorrect erasure settings result in a residual image left in the
imaging plate before the next exposure.
Results vary depending on how much residual image is left and
where it is located.
 Orientation of a grid so that the grid lines are parallel to the laser
scan lines of the plate reader results in the moiré pattern error. Grids
should be high frequency, and the grid lines should run
perpendicular to the laser scan lines of the plate reader.
Grid lines
parallel
Grid lines
perpendicular
36

Printer Artifacts
• Fine white lines may appear on the image because of
debris on the mirror in the laser printer. Service
personnel need to clean the printer.
37

Operator Errors
 Insufficient collimation results in unattenuated radiation
striking the imaging plate.
The resulting histogram is changed so that it is outside the
normal exposure indicator range for the body part selected.
Three lumbar spine images showing the impact of collimation on contrast. A,
Slight collimation. B, Increased collimation. C, Tight collimation. Note the
increase in contrast due to scatter reduction.
38

Operator Errors
 If the cassette is exposed with the back of a cassette toward the
source, the result is an image with a white grid-type pattern
and white areas that correspond to the hinges.
 Care should be taken to expose only the tube side of the
cassette.
39

Operator Errors
 Contrast reduction due to overexposure. The window width
and window level settings are identical for these images.
Proper amount
of exposure
Grossly overexposed
and could not be
manipulated to the point
where the full range of
soft tissue could be seen
40

Review Questions
1. What is meant by matching the body part to be imaged with the examination
menu selection?
2. How are technical factors chosen for each examination?
3. Why is the size of the imaging plate important?
4. What determines the choice of imaging plate size?
5. How is the grid selected for an examination?
6. Why is preprocessing collimation important?
7. How could a lack of collimation affect the image and the examination?
8. Why is it important to properly mark the patient’s right or left side with
radiographic markers?
9. How do the major equipment manufacturers determine exposure indicators?
What are some potential problems of working with more than one system? 41

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