Quality Assurance and Digital

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Nov 15, 2013 (3 years and 9 months ago)

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Quality Assurance and Digital
Radiography

Information gathered for this lecture come
from American College of Radiology Practice
Guidelines for Digital Radiography

10/10/07

“Quality Management in The Imaging
Sciences “ Jeffrey Pap Mosby 1998

The Digital Radiography Advantage


Digital Radiography offers many advantages
over film based radiography.


Film response to radiation follows a sigmoidal
response curve while digital response is linear.
This results in a much wider dynamic response
to exposure. The advantages include:


Reduced retakes due to exposure factors.


Improved contrast resolution.


Improved latitude.

The Digital Radiography
Advantages


Much of the quality assurance for film
based radiography is to assure proper
photographic processing of the image.
This is eliminated unless a film printer
or multi
-
format camera is used.


Less space and perhaps less staff is
required because of reduced storage
space requirements. Images can be stored
on a hard disk or CD.

The Digital Radiography
Advantages


If the images are left in digital format, no
darkroom or darkroom personnel is
needed.


No film, processing chemistry and waste
recovery costs.


Computed Radiography can be added to
existing equipment. Direct Digital requires
a total replacement of the image receptor.

Computed Radiography and
Quality Control


Computed radiography still uses conventional
radiographic equipment so basic radiography
quality control testing is still needed. This would
include:


Calibration of the generator


Collimation


Beam Alignment


Collimation


Linearity of exposure


Spatial resolution (focal spot)

Computed Radiography and
Quality Control


Calibration of the generator.


The linear dose response can be a double edged
sword. It can mask calibration problem that can result
in higher patient exposure.


Over exposed images really look good so this has
resulted in dose creep.


Collimation & Beam Alignment


Collimation is critical for the computer to determine
the proper contrast and density scale.


Spatial resolution (focal spot)


Film can record greater spatial resolution than digital
radiography so resolution should be monitored more
often than film based systems.

Computed Radiography and
Quality Control


We have discussed much of the radiographic
quality control earlier.


The end product in radiography is
the image
and accurate report of findings
.


Therefore for film, the processor function, screen
contract and artifact avoidance was critical.


With digital radiography the computer monitor, image
transfer and image receptor plate become the critical
areas for quality assurance.

Monitor Testing


The acquisition
workstation and
secondary displays
should be tested
frequently using a
standard image test
pattern.


The SMPTE RP
-
133
(Society of Motion Picture
and Television
Engineers) or AAPM TG
-
18 QC test patterns are
suitable.

Using the SMPTE Test Pattern


As a image fidelity test,
the test pattern data files
should be sized to
occupy the full area used
to display images.


The overall SMPTE
image appearance
should be inspected to
assure that absence of
gross artifacts.

Using the SMPTE Test Pattern


There should be no
blurring or bleeding of
bright display areas
into dark areas or
aliasing of the spatial
imaging patterns.


All monitors used for
interpretation should
be tested at least
monthly.

Using the SMPTE Test Pattern


For the dynamic
range test, both the
5% and 95% contrast
areas should be
visible and distinctly
different than the
adjacent 0% and
100% areas.


Therefore:

Using the SMPTE Test Pattern


The brightness and
contrast of the monitor is
adequate if the 5%
squares at both ends of
the gray scale are visible.


The gray scale is shown
as a series of squares in
the center of the image
that range from black
(0%) to white (100%).

Using the SMPTE Test Pattern


It may be difficult to get
the monitor to show both
of these inner squares
perfectly.


Most monitors do better
showing the 95% square
than the 5% square.


However, you might try
adjusting the ambient
light to improve the
visibility of the 5%
square.

Spatial Resolution & Aliasing


The spatial resolution
(linearity) and aliasing
(distortion) of the
monitor are within
acceptable limits if the
high contrast bar
patterns in the test
image are distinct as
simple black and
white pairs.

Spatial Resolution & Aliasing


In each corner and
the very center of the
image, inspect the six
squares filled with
varying widths of
alternating
black/white horizontal
and vertical lines.

Spatial Resolution & Aliasing


You should be able to
differentiate all the
lines from fat to
narrow (6 pixels, 4
pixels and 2 pixels)
and both horizontally
and vertically.

BWH Pattern


The BWH pattern is a
test of the continuous
gray scale from the
center to outside of
the pattern.


It was developed at
the Brigham Womens
Hospital Radiology
Department.

BWH Pattern


No concentric ring like
features should be
present in the image.


If they are present,
the images displayed
on your system will be
of less than optimum
quality.