Image Quality, digital tehnology

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Image Quality Factors—Film-Screen Systems

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Exposure Factors (Technique)
Kilovoltage (kV)
Milliamperage (mA)
Exposure time
(seconds)
– mAs (milliampere- seconds)

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Density

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Density Change Rule

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Density Change Rules
Density adjustment rule:
25% to 30% increase in mAs (minimum change)
Density repeat rule:
Doubling mAs (to correct density on repeats)

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Anode Heel Effect
More intense under cathode
Increase with
– Small focal spot
– Shorter SID
– Larger IR size
Application
– Thicker parts at cathode

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Compensating Filters
Compensating filters filter out a portion of the primary beam toward the thin or less dense part of the body that is being imaged.
Types of compensating filters include the following:
Wedge filter
Trough
Boomerang

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Compensating Filters
Wedge filter
Boomerang filter

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Benefits of Compensating Filter

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Radiographic Contrast
Differences (variation) in density
Controlling factor—kV

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High vs. Low Contrast
High contrast:
– Short scale
– 50 kV (800 mAs)
Low contrast:
– Long scale
– 110 kV (20 mAs)

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Off-Center Grid Cutoff

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Off-Level Grid Cutoff

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Off-Focus Grid Cutoff

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Upside-Down Grid Cutoff

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Resolution (Definition)

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Motion
Voluntary motion (breathing)
Involuntary motion (peristalsis)

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Image Quality Summary Chart

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Focal Spot Size

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Distortion (Magnification)
Misrepresentation of object size or shape
X-ray beam divergence

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Distortion
Controlling factors
SID (source image receptor distance)
OID
Object IR alignment
CR alignment

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Distortion
Controlling factors
SID
OID (object image receptor distance)

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Distortion
Controlling factors
SID
OID
Object IR alignment

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Digits parallel—joint open
Digits not parallel—joints not open

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Distortion
Controlling factors
SID
OID
Object IR alignment
CR alignment

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CR Alignment
CR parallel to joint
CR not parallel to joint

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Quiz Me
Which of the following is NOT a quality factor for film-based radiography?
Density
Focal spot size
Contrast
Distortion
Resolution

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Quiz Me
The primary controlling factor for density is
mAs
kV
SID
OID

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Quiz Me
When the anode heel rule is applied, the thicker aspect of the anatomy should be placed under the cathode end of the x-ray tube.
True
False

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Quiz Me
Which of the following is not a type of compensating filter?
Wedge
Boomerang
Slotted
Trough

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Quiz Me
What is the primary controlling factor for radiographic contrast?
mAs
kV
SID
Focal spot size

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Quiz Me
What type of grid cutoff will occur if a shorter SID is used than what is specified for a particular grid?
Off-center
Off-level
Off-focus
Off-distance

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Image Quality in Digital Radiography

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Processing digital image
Exposure controls—kV, mAs, and time

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Image Quality in Digital Radiography
Brightness
Contrast
Resolution
Distortion
Exposure index
Noise

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Brightness
AP shoulder—high brightness
AP shoulder—less brightness
Intensity of light representing individual pixels in image

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Contrast
Differences in brightness between light and dark areas of image
AP shoulder—higher contrast
AP shoulder—lower contrast

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Resolution
Controlling factors:
Acquisition pixel size
– Inherent to the digital imaging detector
Display matrix
– Dependent on capabilities of the display monitor
Recorded Sharpness of Structures on Image

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Distortion
Controlling factors:
SID
OID
CR alignment
Misrepresentation of Object Size or Shape

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Exposure Index (referred to by some manufacturers as sensitivity “S” numbers)
A numeric value that is representative of the exposure the image receptor received
May be inversely or directly proportional to radiation striking the image receptor
Key in verifying optimal digital image is obtained with least dose to patient

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Exposure Index
Dependent on the intensity of the radiation striking the detector, which is the effect of
– mAs
– kV
– Total detector area irradiated
– Objects exposed (air, metal implants, patient anatomy)

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Low exposure index (high “S” number)—Underexposed
Acceptable exposure index

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High exposure index (low “S” number)
Acceptable exposure index

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Noise
Random disturbance that obscures image clarity
High signal-to-noise ratio (SNR) is desirable.
Low signal-to-noise ratio (SNR) is undesirable.

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Acceptable SNR (good image quality)
Low SNR (poor image quality)

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Post-processing
Changing or enhancing the electronic image to improve diagnostic quality
Algorithms applied to improve diagnostic quality of image
Post-processing cannot improve low SNR image.

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Post-processing Options
Windowing
Smoothing
Magnification
Edge enhancement
Subtraction
Image reversal
Annotation

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Chest without any post-processing
Chest with image reversal

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Nonsubtracted AP shoulder
Subtracted and magnified shoulder angiogram

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Quiz Me
Highly complex mathematical formulas are called
Binary codes
Exposure indices
Equalization filters
Algorithms

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Quiz Me
The intensity of light that represents the individual pixels in the digital image on the monitor is the definition for
Brightness
Contrast
Density
Noise

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Quiz Me
Random disturbance that obscures or reduces clarity is the definition for
Noise
Resolution
SNR
Distortion

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Quiz Me
A low SNR digital image can be enhanced through post-processing techniques.
True
False

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Applications of Digital Technology
Computed tomography (CT), one of the first applications of computers in radiography

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Computed Radiography (CR)

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Image Plate Reader/Processor and Workstation

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Why is it important to collimate and use lead blockers with CR?

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Direct Digital Radiography (DR)

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DR Chest Imaging System

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DR Mammography Unit

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Steps of Image Production Comparison
Film-screen
Computed radiography (CR)
Direct digital radiography (DR)

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PACS Network

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Provide Definitions for the Following Digital-Related Acronyms:
PACS
RIS
HIS
HL7
DICOM
IP
IR
SNR

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Review
What is the difference between “window level” and “window width”?
What is the difference between “density” and “brightness”?
Define the term “noise.”

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Radiation Protection
Patient
Fellow workers
Self

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Units of Radiation

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Units of Radiation
(traditional and SI units)

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Dose-Limiting Recommendations
Replaced MPD (maximum permissible dose) in 1994

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Pregnant Technologists
What is the dose limit for a pregnant technologist per month?
What is it for the entire gestational period?

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Personnel Monitoring
Film badge
TLD (thermoluminescent dosimeter)
OSL (optically stimulated
luminescence)
*Worn at waist or chest level
or
on collar during fluoroscopy
* Consult RSO.

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ALARA Principles
Always wear a personnel monitor.
Radiology personnel should not restrain patients.
Sound radiographic exposure factors
Cardinal rules of radiation protection:
– Time
– Distance
– Shielding
Mobile fluoroscopy or C-arm

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Fluoroscopy exposure patterns (without tower drape shields in place)

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Worker Protection During Fluoroscopy
Fluoroscopy exposure patterns
(with tower drape shields in place)

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Fluoroscopy Safety Practices
Bucky slot cover
Lead drape
.5 mm lead apron
Exposure limit: 10 R/min

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Thyroid shield with protective apron

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Patient Protection
Minimum repeat radiographs
– Clear instructions
– Positioning and exposure factors

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Radiation Protection Practices
Minimum repeat radiographs
Correct filtration
– Inherent and added
– 2.5 mm Al total

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Close four-sided collimation: One of the best ways of reducing patient exposure! (Remember divergence of x-ray beam.)

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Radiation Protection Practices
Minimum repeat radiographs
Correct filtration
Accurate collimation
Types of collimators
– Manual type
– Positive-beam limitation
(PBL)

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Radiation Protection
Minimum repeat radiographs
Correct filtration
Accurate collimation
Specific area shielding
– Shadow shields
– Contact shields

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Vinyl-covered lead shield

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Radiation Protection
Gonadal contact shields
– 1 mm lead equivalent
– Reduces dose 50% to 90%

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Radiation Protection
Minimum repeat radiographs
Correct filtration
Accurate collimation
Specific area shielding
Protection for pregnancies

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Quiz Me
The SI unit equivalent for Rad is
Coulombs/kg of air
Gray
Sievert
Curie

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Quiz Me
What is the annual dose limit for a technologist per year?
5 mSv
15 mSv
50 mSv
500 mSv

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Quiz Me
What minimum lead thickness or equivalency must a protective apron possess when worn for a fluoroscopy procedure?
.5 mm Pb/Eq
1.0 mm Pb/Eq
1.5 mm Pb/Eq
2.5 mm Pb/Eq

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Quiz Me
What is the most effective way to reduce patient dose?
Use of high kV
Increase in added filtration in x-ray tube
Use of gonadal shielding
Close four-sided collimation

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Image Quality, Digital Technology, and Radiation Protection
- The End -
Chapter 2

Copyright © 2010 by Mosby, Inc., an affiliate of Elsevier Inc.
Image Quality, Digital Technology, and Radiation Protection
- The End -
Chapter 2