Transcript Computed Tomography in the Diagnostic Radiography Curriculum

Computed Tomography in the
Diagnostic Radiography
Curriculum
My Disclaimer
• My position on CT in the Diagnostic
Curriculum is that it is more beneficial than
harmful.
• I am not suggesting that students graduate
from our Programs as CT techs.
• I AM suggesting that they have an
understanding of the modality, its basic
concepts, and focused clinical
opportunities.
The Premise
• I look at CT within the curriculum as a twofold activity from the student perspective.
– One, provides students a basic overview of
what CT is, how it works, and why its ‘better’
for some diagnoses.
– Two, CT provides an excellent means of
review for general radiography principles that
may be old hat for some, boring for others, or
just offers a different perspective than the
original explanations.
When to Present CT
• CT has to be in the second year or later. There
needs to be a foundation of relevance and
understanding.
• In our Program, CT is officially taught in the Rad
T 265 course, first semester second year.
• Clinical rotations begin in the middle of the first
semester second year.
• Unofficially CT is found throughout our second
year curriculum.
• Radiologic Technology 265
• Principles of Digital Imaging and Computer
Applications (2)
Prerequisite: Radiologic Technology 165. Introduction to
computer aided medical imaging's as used in
radiography departments. Applications include computed
and digital radiography (CR/DR), CT, MRI, and other
modalities. Basic imaging principles are applied,
including physics, imaging protocols, and systems
electronics. Software and display strategies for varying
modalities will be discussed.
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Date
Aug 28
Sep 4
Sep 11
Sep 18
Sep 25
Oct 2
Oct 9
Oct 16
Oct 23
Oct 30
Nov 6
Nov 13
Nov 20
Nov 27
Dec 4
Dec 11
Lecture Topic
Orientation/Principles of CT
HOLIDAY
Components of a CT scanner
Data Acquisition technology
Spiral CT
Image reconstruction
Image quality
Image manipulation
MRI physics and equipment
MRI image acquisition
Computer literacy and its relevance
Basic concepts of digital imaging
Digital fluoroscopy
Digital fluoroscopy
Ultrasound and Nuc. Med. Applications
FINAL
Reading Assignment
B. Ch 29, M v3 Ch 33
B. Ch 30
M. Ch 36
M. v. 3 Ch 36
B. Ch 26
B. Ch 27, M v3 Ch 34
M v 3 Ch 35, B. Ch 28
M v3 Ch 37&38
Why the importance of teaching
CT?
• Provides a break from the regular routine.
• Offers ‘new’ technology or info that may be
exciting.
• Reviews existing (hopefully) knowledge.
– For example, Photon/tissue interactions
• Great way to review anatomy and
pathology as seen clinically.
• Provides an excellent opportunity to
experience a modality first hand.
The Clinical Component
• We began a clinical affiliation this year with
a free-standing imaging center.
• Last year, we had an observational
agreement that allowed students to visit
and only watch.
• This year students have clinical
expectations based on the time they
spend there.
Clinical continued
• Students are allowed to pick a three week
optional rotation.
– We chose this in order to have students doing
something that interested them thereby
decreasing the possibility of discontent.
– Also, students looking for additional
education, therapy or nuclear medicine, could
get their observational requirements met.
The Proposed CT Curriculum
• CT Generations
• Components, Operations, and Processes
• Radiation Protection Practices
CT Generations
• This is really the only area that has limited
value in the diagnostic curriculum.
First and Second Generation CT
• The first and second generations of CT were
very similar.
• Both used a scanning technique called
translate/rotate in order to move around the
patient.
• The first generation scanner used a single
detector and thin beam. While the second
generation scanner use several detectors and a
fan beam.
– These changes resulted in a significantly faster
scanner.
Third Generation
• The big change here was that the tube
was in constant motion throughout the
exposure, no more stops and starts.
• The detectors were also moving during the
exposure and more detectors were added.
• As before, we now have an even faster
scanner.
Fourth Generation
• It became obvious that moving detectors
introduces noise into the image.
• Now the detectors are fixed in a ring
around the patient and only the tube
moves.
• Thousands of detectors are now needed
to generate an image.
• Faster imaging with increased spatial
resolution.
Fifth Generation
• Electron beam CT
– EBCT
– Ultrafast
Spiral
• Slip-ring technology eliminates power
cables.
• Constant power to moving tube.
• Continuous exposure
• Patient moves through the beam during
exposure
• A stream a data is generated (spiral) as
opposed to a series of individual slices.
• CT scanner generations have limited value
outside of understanding CT. However, it
does provide a mechanism to see the
development of a modality.
• Additionally, the advantages of each
generation and its evolution illustrates the
thought processes that go into learning
and adapting.
Components, Operations, and
Processes
• Most of these topics have direct
correlation to diagnostic radiography.
– Data acquisition
– Factors controlling image appearance
– Anatomical structures
– Post-processing
Data Acquisition
• Methods
– Slice by slice
• Contiguous
– Volumetric
• Spiral/helical
Beam Geometry
• Parallel
• Fan
– The traditional beam geometry, it is opened
along the width of the patient.
• Spiral
– The beam is continuously on allowing for
more anatomical coverage in a shorter time.
Data Acquisition system (DAS)
Components
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Tube
Detectors
Filters
Collimators
ADC
CT Tubes
• Much higher heat loading than
conventional tubes
– 8MHU and up
• Generally have two focal spots
Filters
• Again CT filtration is similar to diagnostic
radiography
• All tubes are required to have minimum filtration
– Primary purpose is patient protection
– Also, in CT the filter is used to harden the beam;
thereby, decreasing absoption
• Compensating filters
– ‘Bow-tie’
– Uniform beam intensity at the detectors
• Think ‘wedge ‘ filter in diagnostic radiography.
CT Collimators
• CT consists of both pre and post-patient
collimation
• Pre-patient collimation is analogous to the
collimation we already know.
– Controls beam coverage or amount of
anatomy exposed.
Post-patient Collimation
• Controls slice thickness.
• Additionally, it serves to define the slice
profile which provides a sort of grid effect.
– Scatter rejection
Analog-to-Digital Convertor (ADC)
• Converts the analog signal from the
detectors to a digital signal for processing.
• Rated by bits
– Most scanners today are 16-bit systems
– Produce 4096 data points
• The more data points, the better the gray scale
(contrast) resolution.
Measurement of the Transmitted
Beam
• A ray
– Basically, the detected value of a single
photon
• Several rays combine to form a view.
– The data from multiple photons hitting the
detector during a single translation.
• Profile
– The electrical signal produced by the detector.
Encoding into Binary Data
• The data from the views is converted into
attenuation coefficients using the formula:
=
1
___ lnI /I
o
x
• The attenuation coefficients are then sent to the
ADC.
Data Transmission to the Computer
• Data processing begins
– The raw (detector) data is preprocessed to
remove bad data sectors.
• The reformatted raw data is now sent to
the array processors.
– The array processors are using filter
algorithms to produce the desire image
appearance, i.e. soft tissue, bone, high-res.
• After the array processors, the data is then
subjected to a reconstruction algorithm
that produces the cross-sectional image
we see.
• The most common reconstruction
algorithm today is the filtered back
projection.
• The data is now image data and available
for image manipulation.
The CT Image
• Any digital image, including CT, is
comprised of picture elements (pixels).
• The pixels are 2-dimensional elements
that represent volume elements (voxels).
• Pixels are displayed in a matrix.
• The brightness of each pixel is determined
by the CT number it represents.
CT Numbers
• CT numbers are calculated by comparing
the attenuation coefficients of water and
tissue.
• The formula is:
__
CT #
=
t
w
__________
.
w
K
• The CT number of water is ‘0’.
• Now, if you look at the formula you can
see that tissues attenuate more than water
will have a positive CT number.
• Conversely, tissues less attenuate less
have negative CT numbers.
Examples of Tissue Attenuation
Coefficients and Their CT Numbers
Attenuation
Coefficient
CT
Number
Bone
0.528
1000
White
matter
0.213
45
Blood
0208
20
Water
0.206
0
Fat
0.185
-100
Tissue
Factors Affecting Attenuation
• Photon energy
– Selected kVp
– Filtration
• Tissue effective atomic number
• Tissue mass density
Selectable Scan Factors
• Field of View
– Scan
– Display
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Matrix size
Slice thickness
Algorithm
Scan time and rotational arc
• Tube output
– mAs
• Region of Interest (ROI)
• Magnification
• FSS and Tube geometry
Scan FoV
• The total area from which raw data is
acquired
Display FoV
• Determines how much raw data is used in
displaying the acquired image.
Matrix Size
• Basically, the number of pixels displayed.
• Affects spatial resolution
– The bigger the matrix the more pixels.
– Given that image size stays the same the
pixels have to be smaller; therefore, spatial
resolution increases.
• Generally, the larger the image matrix the
higher the patient dose.
Algorithm
• Mathematical formula applied to the raw
data in order to produce a specific image
outcome.
Scan time and Rotational Arc
Radiographic Tube Output
• mAs
ROI
• Allows the technologist to select a specific
area of interest for image reconstruction.
• Uses the raw data for the reconstruction
instead of using image data
– The result is a better quality image.
Magnification
• Defined as a post-processing activity.
– Magnification uses image data not raw data,
so the final product has less spatial resolution
than when using ROI.
FSS and Tube Geometry
• FSS
– In CT, FSS selection has the same
connotations it has in diagnostic radiography.
• A smaller FSS has more detail (resolution) than a
larger one. However, due to digital imaging issues
(monitor and matrices) the effects of a small
versus large FSS are not as apparent.
Factors Affecting Image Quality
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Spatial resolution
Contrast resolution
Noise
Radiation dose
Artifacts
Spatial Resolution
• The degree of blurring within the image
• Ability to discriminate objects of varying
density a small distance apart.
• CT spatial resolution is affected by
– Geometric factors
– Reconstruction algorithm
Geometric Factors
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FSS
Detector aperture width
Slice thickness
SID
SOD – distance to isocenter
Sampling distance
– Number of projections
Reconstruction algorithms
• Several different types of convolution
algorithms are available.
– Edge enhancement
– Smoothing
– Soft tissue
– Bone
• Matrix size is also going to play a role in
spatial resolution
Potential Spatial Resolution
Algorithm
Spatial
Resolution
Ultrahigh
15
Brain soft tissue
9.5
Abdomen soft
tissue
Abdomen low
detail
10
6
• Can easily be demonstrated on CR/DR as
well as CT
– Examples here
Spatial resolution
• FoV
– Amount of anatomy displayed
– Also an issue with fluoroscopy
– Affects on patient dose
• Matrix
– Affects on spatial resolution and patient dose
• Pixel
• Voxel
• Slice thickness
– Opportunity to demonstrate partial voluming and
superimposition
Contrast Resolution
• Affected by several factors
– Photon flux
– Slice thickness
– Patient size
– Detector sensitivity
– Reconstruction algorithm
– Image display
– noise
Photon flux
• Basically, the number of photons available
– kVp
– mAs
– Beam filtration
• Patient size also affects photon flux
– Larger patients attenuate more photons
Slice Thickness
• Slice thickness is controlled by postpatient collimation
• Tight collimation decreases the number of
scattered photons that can strike the
detectors
– Fewer scatter photons, more contrast
• Essentially, post-patient collimation works
like a grid.
Detector Sensitivity
• The more sensitive the detector the more
variation in photon energy it will resolve
Reconstruction Algorithm
• Smooth algorithms improve contrast
resolution
– A rule of thumb
• Increase spatial resolution decrease contrast
resolution
Grayscale Manipulation
Distortion
Noise
Spatial Resolution
Post-Processing
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Image Reformation
Image smoothing
Edge enhancement
Grayscale manipulation
Radiation Dose
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Technical factor selection
Adjustments for children
Scanner dosimetry survey
Reducing scatter to the technologist
Data Acquisition
• In CT data is acquired from either
scintillation or gas-filled detectors.
Scintillation or Solid-state detectors
• Various materials are coupled to
photodiodes to record photon activity.
– Examples of materials include:
• Cadmium tungstate
• Ceramics doped with gadolinium or yttrium
Indirect Digital Radiography
The intensifying screen is made up of
cesium-iodide crystals and the
photodetector is made up of amorphous
silicon.
p--layer
SiO2 (0.1 μm )
n-layer
n+-layer
SiO2 ( 0.5 μm )
passivation layer
( 1.0 μm )
Al
Al-layer
Another Positive in the CT Debate
• During the past several years there has
been an ongoing discussion about how do
we get people interested in being faculty.
• Adding CT brings another group of
potential faculty members to the table.
– Certainly, we increase the probability of
adjunct faculty to teach the CT component.
• Also, we increase the exposure of our
students to potential employers.
http://w4.siemens.de/FuI/en/archiv/zeitschrift/heft1_97/artikel03/inde
x.html
http://www.impactscan.org/rsna2001.htm
Contrast Media
Photon Tissue Interactions
• PE
• CE
Scatter Control
Filtration
• Compensating
• Required
• Effects on beam energy
Anode Heel Effect
• Line focus principle
Exposure Creep
• Look for article about pediatric
overexposure in CT
Sensitivity of Image Receptor
• Differences providing the ability to
visualize different structures
Quantum Mottle
• Along for fluoroscopy an excellent
modality to demonstrate the effects of it.
• Now possible with CR/DR
Cross-sectional anatomy
• Provides further review for students
• Allows them to learn about something they
frequently see in the department and
hospital.
• Certainly helps with positioning and
pathology review.
Equipment
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Detectors
Tubes
FSS
Filtration
Collimation
Concepts
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Spatial resolution
Contrast resolution
Image matrix
FoV
Patient Care
• Contrast Media
– Patient prep
– Reactions
– Dose rates
– Venipuncture
– Ionic v. non-ionic
– Atomic number
• Concentration
• Barium versus iodine
Tubes
• Anode heel effect
• Line focus principle
Collimation
• Total
• Compensating
• Pre and post
– grid and patient dose
Tissue Interactions
• Photoelectric effect
– Absorption
• Compton effect
– Scatter
– Attenuation
PE
• Absorption
• High contrast
• Plain film radiography
CE
• Low contrast
• Scatter
• High energy photons
– More likely forward scatter
• High energy photons
– Less absorption (charts/graphs here)
Contrast resolution
• This will be new
– Gray scale
– Dynamic range
– High and low contrast
– Count anatomical structures
Radiation Protection
• Dose versus Image Quality
Quantum Mottle
• Easily demonstrated
– CR/DR applicable
• Particularly when using appropriate techniques
– Fluoroscopy applicable
Technique selection
• No penalty for overexposure
– Similar to CR/DR
• Too little exposure is trouble
– Quantum mottle
• Exposure creep
Anatomy and Pathology
• Opportunity to review diseases again
Spine
• CSP
• LSP
– Intervertebral foramen
– Zygo joints
• Myelograms
• Discograms
– In some facilities this may be the only
opportunity to see these exams
Stomach
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Location
Position
Structures
Pathology
contrast
Kidney
• Mention in last years student bowl
• Position and angulation
Colon
• Flexures and their position
• Pathology
• Appendicitis
Skull
• Skull types
– Angles
– Visibility of structures
Extremities
• Positional relationships between structures
• Angles
• Non-linear reconstructions
Patient Prep
• Contrast
• Instructions
• Post-procedural care
– Biopsies
– Myelograms
– Etc.
Review of Lab Values
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Vital Signs
Hemoglobin
RBC
Platelets
O2
Prothrombin
Partial thromboplastin time
Several labs will only be done in
CT
Consents
Postural hypotension