Computed Tomography in the Diagnostic Radiography Curriculum

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Computed Tomography in the Diagnostic Radiography
Curriculum
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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.

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The Premise

• I look at CT within the curriculum as a two-fold
  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.

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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.

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• 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 Lecture Topic Reading Assignment
• Aug 28 Orientation/Principles of CT B. Ch 29,
  M v3 Ch 33
• Sep 4 HOLIDAY
• Sep 11 Components of a CT scanner
• Sep 18 Data Acquisition technology B. Ch 30
• Sep 25 Spiral CT
• Oct 2 Image reconstruction
• Oct 9 Image quality
• Oct 16 Image manipulation M. Ch 36
• Oct 23 MRI physics and equipment
• Oct 30 MRI image acquisition M. v. 3 Ch 36
• Nov 6 Computer literacy and its relevance B. Ch
  26
• Nov 13 Basic concepts of digital imaging B. Ch
  27, M v3 Ch 34
• Nov 20 Digital fluoroscopy M v 3 Ch 35, B. Ch
  28
• Nov 27 Digital fluoroscopy
• Dec 4 Ultrasound and Nuc. Med. Applications M
  v3 Ch 3738
• Dec 11 FINAL

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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.

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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.

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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.

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The Proposed CT Curriculum

• CT Generations
• Components, Operations, and Processes
• Radiation Protection Practices

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CT Generations

• This is really the only area that has limited
  value in the diagnostic curriculum.

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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.

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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.

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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.

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Fifth Generation

• Electron beam CT
• EBCT
• Ultrafast

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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.

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• 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.

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Components, Operations, and Processes

• Most of these topics have direct correlation to
  diagnostic radiography.
• Data acquisition
• Factors controlling image appearance
• Anatomical structures
• Post-processing

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Data Acquisition

• Methods
• Slice by slice
• Contiguous
• Volumetric
• Spiral/helical

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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.

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Data Acquisition system (DAS)Components

• Tube
• Detectors
• Filters
• Collimators
• ADC

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CT Tubes

• Much higher heat loading than conventional tubes
• 8MHU and up
• Generally have two focal spots

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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.

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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.

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Post-patient Collimation

• Controls slice thickness.
• Additionally, it serves to define the slice
  profile which provides a sort of grid effect.
• Scatter rejection

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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.

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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.

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Encoding into Binary Data

• The data from the views is converted into
  attenuation coefficients using the formula
• The attenuation coefficients are then sent to the
  ADC.

1
___
lnIo/I

x
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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.

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• 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.

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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.

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CT Numbers

• CT numbers are calculated by comparing the
  attenuation coefficients of water and tissue.
• The formula is
• CT

__
.
__________
t
w
K

w
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• 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.

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Examples of Tissue Attenuation Coefficients and
Their CT Numbers
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Factors Affecting Attenuation

• Photon energy
• Selected kVp
• Filtration
• Tissue effective atomic number
• Tissue mass density

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Selectable Scan Factors

• Field of View
• Scan
• Display
• Matrix size
• Slice thickness
• Algorithm
• Scan time and rotational arc

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• Tube output
• mAs
• Region of Interest (ROI)
• Magnification
• FSS and Tube geometry

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Scan FoV

• The total area from which raw data is acquired

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Display FoV

• Determines how much raw data is used in
  displaying the acquired image.

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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.

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Algorithm

• Mathematical formula applied to the raw data in
  order to produce a specific image outcome.

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Scan time and Rotational Arc
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Radiographic Tube Output

• mAs

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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.

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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.

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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.

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Factors Affecting Image Quality

• Spatial resolution
• Contrast resolution
• Noise
• Radiation dose
• Artifacts

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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

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Geometric Factors

• FSS
• Detector aperture width
• Slice thickness
• SID
• SOD distance to isocenter
• Sampling distance
• Number of projections

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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

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Potential Spatial Resolution
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• Can easily be demonstrated on CR/DR as well as CT
• Examples here

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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

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Contrast Resolution

• Affected by several factors
• Photon flux
• Slice thickness
• Patient size
• Detector sensitivity
• Reconstruction algorithm
• Image display
• noise

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Photon flux

• Basically, the number of photons available
• kVp
• mAs
• Beam filtration
• Patient size also affects photon flux
• Larger patients attenuate more photons

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Slice Thickness

• Slice thickness is controlled by post-patient
  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.

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Detector Sensitivity

• The more sensitive the detector the more
  variation in photon energy it will resolve

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Reconstruction Algorithm

• Smooth algorithms improve contrast resolution
• A rule of thumb
• Increase spatial resolution decrease contrast
  resolution

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Grayscale Manipulation
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Distortion
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Noise
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Spatial Resolution
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Post-Processing

• Image Reformation
• Image smoothing
• Edge enhancement
• Grayscale manipulation

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Radiation Dose

• Technical factor selection
• Adjustments for children
• Scanner dosimetry survey
• Reducing scatter to the technologist

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Data Acquisition

• In CT data is acquired from either scintillation
  or gas-filled detectors.

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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

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Indirect Digital Radiography

• The intensifying screen is made up of
  cesium-iodide crystals and the photodetector is
  made up of amorphous silicon.

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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.

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http//w4.siemens.de/FuI/en/archiv/zeitschrift/hef
t1_97/artikel03/index.html
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http//www.impactscan.org/rsna2001.htm
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Contrast Media
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Photon Tissue Interactions

• PE
• CE

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Scatter Control
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Filtration

• Compensating
• Required
• Effects on beam energy

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Anode Heel Effect

• Line focus principle

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Exposure Creep

• Look for article about pediatric overexposure in
  CT

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Sensitivity of Image Receptor

• Differences providing the ability to visualize
  different structures

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Quantum Mottle

• Along for fluoroscopy an excellent modality to
  demonstrate the effects of it.
• Now possible with CR/DR

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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.

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Equipment

• Detectors
• Tubes
• FSS
• Filtration
• Collimation

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Concepts

• Spatial resolution
• Contrast resolution
• Image matrix
• FoV

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Patient Care

• Contrast Media
• Patient prep
• Reactions
• Dose rates
• Venipuncture
• Ionic v. non-ionic
• Atomic number
• Concentration
• Barium versus iodine

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Tubes

• Anode heel effect
• Line focus principle

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Collimation

• Total
• Compensating
• Pre and post
• grid and patient dose

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Tissue Interactions

• Photoelectric effect
• Absorption
• Compton effect
• Scatter
• Attenuation

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PE

• Absorption
• High contrast
• Plain film radiography

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CE

• Low contrast
• Scatter
• High energy photons
• More likely forward scatter
• High energy photons
• Less absorption (charts/graphs here)

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Contrast resolution

• This will be new
• Gray scale
• Dynamic range
• High and low contrast
• Count anatomical structures

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Radiation Protection

• Dose versus Image Quality

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Quantum Mottle

• Easily demonstrated
• CR/DR applicable
• Particularly when using appropriate techniques
• Fluoroscopy applicable

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Technique selection

• No penalty for overexposure
• Similar to CR/DR
• Too little exposure is trouble
• Quantum mottle
• Exposure creep

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Anatomy and Pathology

• Opportunity to review diseases again

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Spine

• CSP
• LSP
• Intervertebral foramen
• Zygo joints
• Myelograms
• Discograms
• In some facilities this may be the only
  opportunity to see these exams

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Stomach

• Location
• Position
• Structures
• Pathology
• contrast

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Kidney

• Mention in last years student bowl
• Position and angulation

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Colon

• Flexures and their position
• Pathology
• Appendicitis

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Skull

• Skull types
• Angles
• Visibility of structures

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Extremities

• Positional relationships between structures
• Angles
• Non-linear reconstructions

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Patient Prep

• Contrast
• Instructions
• Post-procedural care
• Biopsies
• Myelograms
• Etc.

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Review of Lab Values

• Vital Signs
• Hemoglobin
• RBC
• Platelets
• O2
• Prothrombin
• Partial thromboplastin time

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Several labs will only be done in CT
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Consents
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Postural hypotension
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