Digital application: Chapter 17- Digital Radiographic Technique
signal
- Image-forming x-rays are those that are transmitted through the patient unattenuated. - represents the difference between x-rays transmitted to the image receptor and x-rays absorbed photoelectrically.
Image Receptor Response
- Is a characteristic curve that shows the responses of a screen-film image and a digital image receptor that relates to the contrast resolution. • It is also called as H & D curve after Hurter & Driffield
Modulation Transfer Function (MTF)
- The ability of an imaging system to render objects of different sizes onto an image. - A measure of the ability of the imaging system to preserve signal contrast as a function of the spatial resolution.
contrast detail curve
- The left side of the contrast-detail curve, that is related to high-contrast objects, is limited by the MTF of the imaging system - The right side of the curve, that relates to low contrast, is noised limited.
edge enhancement
- a process by which the visual system makes edges as visible as possible, facilitating perception of where one object or surface ends in the retinal image and another begins - Accentuation of the interface between different tissues.
high spatial frequency is used for which type of anatomy?
- bone trabeculae, breast microcalcification, and contrast-filled vessels. - difficult to image
What is the contrast detail curve used for?
- comparison of two different imagining system - instructive methods for understanding how digital radiographic techniques and imaging systems factors influence spatial resolution. - help to better understand what influences spatial resolution (MTF) and what influences contrast resolution (SNR) for various systems.
disadvantage of edge enhancement
- easy to over use and over usage creates a "halo" effect. The halo mimics pathology. - noise is enhanced.
what does edge enhancement do to the image?
- increase contrast along the edge - elaborate software function - enhancement is a function of size of structure/pixel
line pair (lp(
- is a two black lines with a white space in the middle - is a high-contrast line that is separated by an interspace of equal width.
Quality control test tools
- is an aluminum phantom with a matrix of holes. - All of the holes in a given row (horizontal) have a constant depth. - All of the holes in a given column (vertical) have a constant diameter.
low spacial frequency is used for which type of anatomy?
- large soft tissue (kidneys, liver and brain) - easy to image
QC test
- measures the amount of blurring as a function of spatial frequency. - measures the modulations of each spatial frequency pattern and use those data to construct an MTF curve.
Contrast Detail Detectability Tool
- numbers on the circle determines the contrast sensitivity. - The low contrast is determined by reporting the number of large circles on the outer rim and high contrast sensitivity is determined by reporting the number of small circles on the inner rim. The inner squares measure spatial frequency
types of film noise
- quantum noise (mottle noise) - structural noise - electronic noises
disadvantage of smoothing
- reduction in contrast - reduces small clinical features - edge is still degraded
spatial frequency
- relates to the number of line pairs in a given length, usually centimeters or millimeters. - line pairs are used as a measure of resolution
Smoothing (low pass filtering)
- results from averaging of the frequency of each pixel with surrounding pixel values to remove high frequency noise.
spatial resolution
- the ability of an imaging system to resolve and render on the image a small high contrast object. - the amount of detail presented on an image
Modulation transfer function
- the ratio of image contrast to object contrast as a function of spatial frequency - It is the ability of an imaging system to render objects to different sizes on an image.
edge enhancement
- when the signal is obtained, averaging of the signal occurs to shorten processing time and storage. - The more pixels involved in the averaging, the smoother the image appears - signal strength of one pixel is averaged with the strength of adjacent pixels or neighboring pixels.
Image Inversion
-Allows the inversion of white (bone) to black -Used to better visualize some pathologies
The ability to capture exposure values from _______ describes the dynamic range.
0.1 µGy to 1,000 µGy
A perfect system will have an MTF of
1 or 100% However, an ideal system does not exist
Radiographic Technique For Screen-Film Imaging Requires:
1. An appropriate kVp be selected on the basis of the anatomy that is being imaged. 2. That the proper mAs be selected to produce proper optical density (OD) on the finished image. For screen-film imaging, kVp controls contrast, and mAs controls OD.
characteristics of a high quality radiograph
1.) adequate brightness (CR/DR only) 2.) good spatial resolution 3.) good contrast resolution 4.) minimal size distortion (unless we intend to magnified that part) 5.) minimal shape distortion unless we intend to distort that part) 6.) no motion blur 7.) no noise
There are several postprocessing functions used to optimize the appearance of a radiographic image:
1.) annotation 2.) window and level 3.) magnification 4.) image flip 5.) image inversion 6.) subtraction (DSA) 7.) pixel shift 8.) region of interest
there are two types of spatial frequency filtering
1.) edge enhancement 2.) smoothing
DQE is dependent on
1.) radiation exposure 2.) spatial frequency 3.) MTF 4.) detector material 5.) quality (voltage and current) of the radiation
digital radiographic technique
1.) spatial resolution - spatial frequency - modulation transfer function 2.) contrast resolution - dynamic range - postprocessing - signal-to-noise ratio 3.) contrast detail curve 4.) patient radiation dose consideration - image receptor response - defective quantum efficiency
Spatial frequency math problem: The image from a nuclear medicine gamma camera can resolve just 1/4 inch. What spatial frequency does this represent?
1/ 4 in × 25.4 mm/in = 6.35 mm It takes two 6.35-mm objects to form a line pair, hence 12.7 mm/lp. The reciprocal is 1 lp/12.7 mm = 0.08 lp/mm = 0.8 lp/cm.
Spatial resolution is spatial frequency at ____% of MTF
10%
the human eye can only discern approximately ______ lp/mm without any aids
10-13 lp/mm
Bar pattern test tools with spatial frequencies up to
20 lp/mm
the human eye can see how much spatial resolution?
200 micrometers
Digital imaging systems have a dynamic range from ______ different shades of gray
256 to 4096
spatial frequency math problem: A digital radiographic imaging system has a spatial resolution of 3.5 lp/mm. How small an object can it resolve?
3.5 lp/mm = 7 objects in 1 mm, or 7/mm (multiply by 2) Therefore the reciprocal is the answer, or 1/7 mm = 0.143 mm = 143 µm
What is the spatial resolution of a 512 × 512 CT image that has a field of view of 30 cm? What spatial frequency does that represent?
512 pixels/30 cm = 512 pixels/300 mm 300 mm/512 pixels = 0.59 mm/pixel Two pixels are required to form a line pair; therefore: 2 × 0.59 mm = 1.2 mm/lp 1 lp/1.2 mm = 0.83 lp/mm = 8.3 lp/cm
How is a contrast detail curve made?
A contrast detail curve is made from observations and reports of exposures made from aluminum or plastic phantoms that have matrix or series of circles of various depths. These phantoms are part of quality assurance protocols in place in all imaging departments to ensure optimal equipment efficiency.
Signal-to-noise ratio (SNR)
A method of describing the strength of the radiation exposure compared with the amount of noise apparent in a digital image
Photoelectric absorption
A photon interacts with an inner shell electron in the atom and removes it from its shell.
WHICH OF THE CURVES IN THE FIGURE REPRESENTS THE DIGITAL IMAGE RECEPTOR REPSONSE? a. A b. B c. C d. D e. E
Answer: a
WHICH OF THE CURVES IN THE FIGURE REPRESENTS A CHARACTERISTIC SCREEN-FILM REPSONSE? a. A b. B c. C d. D e. E
Answer: b
What happens to spatial resolution after 13 lp/mm?
At a certain point, the finer details in a radiographic image begin to blur and the benefits of higher spatial resolutions diminish.
examples of high spatial frequency structures, which are difficult to image.
Bony trabeculae, Breast microcalcifications, Contrast Filled vessels.
Log relative exposure (LRE)
Change in optical density over each exposure interval.
______ is a measured of the combined effects of the signal related to image contrast and noise performance of an imaging system?
DQE
Region of interest
Determine average pixel value for use in quantitative imaging
contrast
Differences in adjacent densities on the radiograph
The principal descriptor for contrast resolution is grayscale is called the
Dynamic range
High contrast
Few gray tones, mainly black and white image; may also be referred to as short-scale contrast
window and level
Image manipulation parameter that changes image brightness on the display monitor, usually through the use of a mouse; Expand the digital grayscale to visible
Compton effect (scattering)
It occurs due to the interaction of the photon (x-ray or gamma) with free electrons or loosely bound valence shell electrons. The resultant incident photon is scattered (changes direction) and gives energy to the electron (recoil electron). The scattered photon will have a different wavelength and thus a different energy.
Annotation
Label the image; adding text to the image to inform clinician about anatomy and diagnosis
examples of low spatial frequency structures in the body, which are easy to image.
Large soft tissues (abdominal tissue), Liver, Kidneys, Brain Tissue.
window level
Location on a digital image number scale at which the levels of grays are assigned. It regulates the optical density of the displayed image and identifies the type of tissue to be imaged.
Spatial resolution is influenced by
MTF
Which of the following postprocessing methods helps improve visualization and spatial resolution?
Magnification
Low contrast
Many gray tones on image; may also be referred to as long-scale contrast
A way to quantify the contribution of each system component to the overall efficiency of the entire system-e.g ratio of the image to the object
Modulation Transfer Function (MTF)
What determines the spatial resolution in CR?
Phosphor layer thickness and pixel size
image Flip
Reorient image presentation ; brings image into standard order
Pixel Shift
Reregister an image to correct for patient motion
Contrast resolution is influenced by
SNR
window width
Specific number of gray levels or digital image numbers assigned to an image. It determines the gray scale rendition of the imaged tissue and therefore the image contrast.
Contrast Resolution
The ability to distinguish many shades of gray from black to white.
Contrast Detail & mAs
The lower curve represents better contrast resolution because tissue with lower subject contrast can be imaged using a higher mAs.
Contrast Detail & Pixel size
This detail curve compares two imaging systems with different pixel sizes. ● Smaller pixel size = better spatial resolution ● Contrast resolution is constant with a constant technique
The postprocessing manipulation called image inversion refers to:
Turning white-black to black-white
A lower SNR generally results in
a grainy appearance to images
spatial resolution improves as: screen blur____ , motion blur____, and geometric blur ______.
all decreases
When mAs is increased (which increases the number of photos), the SNR is ________.
also increased (although at the expense of increased patient radiation dose).
when frequencies of areas of interest are known, they can be _____ and other frequencies can be ______.
amplified; suppressed
The amount of blurring can be represented by the reduced _____ of the representative frequency.
amplitude
amplification (high-pass filtering)
an increase of contrast and edge enhancement
which of these techniques would produce the lowest contrast detail curve? a. 90 kVp, 10 mAs b. 90 kVp, 40 mAs c. 90 kVp, 20 mAs d. 90 kVp, 30 mAs
b. 90 kVp, 40 mAs A high mAs value will produce a lower contrast detail curve.
higher spatial frequency indicates _____ spatial resolution
better
The dynamic range of digital imaging systems is identified by the
bit capacity of each pixel. **The bit depth can be expressed by 2^x, where x is the number of bits. Example: 14 bit dynamic range= 2^14= 16384 shades of grey.
Line pairs become more ______ with increasing spatial frequency.
blurred
Contrast resolution in CT is superior compared to other x-ray imaging systems because of
collimation
noise
compton scatter
Greater the dynamic range (the number of gray a system can reproduce), the better the ________.
contrast resolution
function of window level
controls how bright or dark the image is (brightness).
function of window width
controls the contrast (black & white) of an image.
The MTF curve that represent DR has a distinctive feature of a ________.
cutoff spatial frequency
Contrast ______ as filtration increases
decreases
Changing the window width will display _______.
different levels of contrast of the image.
Another way to increase SNR is seen in
digital subtraction angiography (DSA)
Noise in plain film depends on the number of
discrete x-ray photons reaching the detector.
When the 90-kVp x-ray beam interacts with the patient, most of the x-rays are scattered and are reduced in energy as shown in Figure 2. This results in
even greater absorption of image-forming x-rays.
Dynamic range is the range of ______ that can be captured by a detector
exposures
edge enhancement occurs when
fewer pixels in the neighborhood are included in the signal average
What does it mean when the radiographic image quality is faithful or has fidelity?
fidelity- the image should look as close to the actual anatomy. faithful = high quality radiograph
spatial resolution in screen film radiolography is determined prinicpally by
focal spot size
dose creep
gradual increase in x-ray exposures over time that results in increased radiation dose to the patient.
The smaller the neighbor, the ____ the enhancement.
greater
As kVp is decreased, contrast increases (becomes higher or shorter scale) as a result of
greater absorption of lower energy rays by the anatomical parts (increased photoelectric interaction) Low kVp = high contrast = short-scale contrast = few gray tones (mainly black and white tones)
As filtration is increased, beam becomes _______ (average photon striking the patient has shorter wavelength)
harder
Radiographers strive for_____ SNR by selecting appropriate digital radiographic techniques while keeping with ALARA (as low as reasonably achievable).
high
When object contrast is ____, small objects can be imaged
high
Patient dose in DR should be low because of
high DQE of the image receptor.
Image noise limits contrast resolution, so radiographers strive for
high SNR
A small window width indicates
high contrast with less shades of gray
The upper portion in the dynamic range will be a good image with
high dose
The thinner the phosphor layer, the ______ the resolution
higher
Magnification
improve visualization and spatial resolution; The larger the matrix, smaller the pixels which create better spatial resolution
Subtraction (DSA)
improves image contrast
As mAs is increased, contrast resolution is ____.
increased
Noise in plain radiography can be decreased by
increasing the mAs, which increases the number of photons.
contrast detail curve
is a method for evaluating the spatial resolution and contrast resolution of an imaging system.
Spatial frequency
is a way of expressing object size.
technique creep
is an attempt to reduce patient radiation dose by instituting increased kVp and reduced mAs. This will result in an adequate contrast resolution, constant spatial resolution, and reduced patient radiation dose. However, a very low technique for digital imaging will also result in image noise.
Postprocessing
is editing that is done to a digital radiographic image after it is processed in the imaging system.
function of edge enhancement
is effective for fractures and small, high-contrast tissues.
Spatial Resolution
is the ability of an imaging system to differentiate between two nearby objects. Resolution in space is a measure of how small an object one can see on an image.
Contrast resolution
is the ability to distinguish many shades of gray from black and white.
Image noise
is the deterioration of the image. Image noise may appear speckled, griany, textured, or snowed.
contrast
is the difference in density or difference in the degree of grayness between areas of the radiographic image
Dynamic range
is the number of gray shades that an imaging system can reproduce.
windowing
is the process in which the grayscale of a particular image can be adjusted. This is typically done by the technologist or radiologist to better demonstrate certain anatomy or pathology.
high contrast
is when densities are notable distinguished in an image, black and white.
low contrast
is when there are many shades of gray between adjacent regions in an image.
Contrast can be increased by administering contrast material or lowering the photon energy by reducing the ____.
kVp
______ directly controls contrast by controlling the differential absorption of the x-ray beam in the body; ultimately, image contrast is controlled by window width and bit depth
kVp
suppression of frequencies can be a useful technique for enhancing _______.
large structures such as organs and soft tissue (but it can be noisy).
suppresssion of frequencies (masking) results in
loss of small details
When object contrast is ____, large objects are required for visualization on the image.
low
A large window width indicates that there is a relatively ______________.
low contrast with many shades of gray
The relative value of DQE for various image receptors means that fewer x-rays are required by the higher DQE receptors to produce an image; this translates into
lower patient dose
Primary function of contrast
make detail visible
Bar pattern test tools with microdensitometers
measures density of a small area
The higher frequency that occurs in a set distance results in
more blur
What will reduce contrast resolution?
noise
A radiograph that is more to the center of the dynamic range will be
optimal
No DR imaging system can resolve an object smaller than the ______.
pixel size
digital imaging is limited by
pixel size
spatial resolution in digital imaging system is determined prinicpally by
pixel size
One of the methods to processing a digital image is called
postprocessing
Noise increases in ____ to the square root of the number of images.
proportion
Signal increases in _____ to the number of images integrated.
proportion
Lowest range of an image will result in
quantum mottle
One major advantage of digital radiography is the ability to process procedures with a computer to
reduce image noise, increase visibility of detail and adjust and optimize the image contrast characteristics.
advantage of smoothing
reduction in noise- mottle
Detective Quantum Efficiency (DQE)
refers to the efficiency of a detector in converting incident x-ray energy into an image signal.
As mAs increased, spatial resolution _____.
remains the same
The extreme level of over exposure will be
saturated
As the spatial frequency becomes larger, the object becomes _____.
smaller
DQE is related to the absorption coefficient and to the ______ of the image-forming x-ray beam.
spatial frequency
spatial resolution in DR vs CR
spatial resolution of an a-Se for direct detectors and Csl for indirect detector is higher than CR detectors
A line pair consists of
the line and interspace of the same width as that line.
Spatial frequency refers to
the number of line pairs in a given length, usually measured in line pairs per centimeter or millimeter. This gives it the medical unit of lp/cm or lp/mm.
As kVp is increased, contrast decreases (becomes lower or longer scale) because
the shorter wavelength rays more uniformly penetrate anatomical parts High kVp = low contrast = long-scale contrast = many gray tones
What is the purpose of postprocessing radiographic images?
to alter or change an image to enhance diagnostic interpretation.
Digital subtraction angiography (DSA)
video equipment and a computer produce x-ray images of blood vessels. Radiopaque structures such as bones are eliminated ("subtracted") digitally from the image, which allows for an accurate depiction of the blood vessels.
The postprocessing method used to allow us to see all shades of gray in an image is referred to as:
window and level
kVp Controls differential absorption of the x-ray beam by the body because of its control of
x-ray beam energy
Dose Reduction With Digital Radiography
• Exposures should not be repeated in digital radiography (DR) because of brightness or contrast concerns. • DR systems cannot compensate for excessive noise caused by quantum mottle. • Overexposed images do not have to be repeated and should not become a habit.
response of screen film
• Spatial resolution is determined by Focal -Spot size. • Responds to radiation intensities from less than 1 to greater than 1000 mR. • Exposure is not of interest but the change in optical density over each exposure interval. • Log relative exposure (LRE) used as the scale along the xaxis. • The exposure factor-related repeat rate ranges to approximately 5%. • When overexposed or underexposed, image contrast is reduced. • X-ray intensities must fall into a smaller range to display radiographic densities that can be clear.
response of digital image receptor
• Spatial resolution is determined by pixel size. • Response is linearly related to radiation dose. • This results in greater range densities available for display within the digital image. • Can display many shades of gray in wide range of x-ray intensities exiting the anatomic part. • Can retain more information than radiographic film. • One cannot overexpose or underexpose. • Should never require repeating because of exposure factors. • Because of the ability to display wide range of densities, overexposure can be corrected through post processing. • Contrast is unrelated to dose.
Contrast detail curves of various imaging systems
● Mammography has the best spatial resolution due to small focal spot size ● CT has the best contrast resolution because of x-ray beam collimation and reduced scatter radiation
