Diagnostic Imaging Primer 1 Hour (brief) introduction Sean
Collins Fall 2012 1 Hour (brief) introduction Sean Collins Fall
2012
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Outline Purpose of primer & thread Objectives of primer
Underlying message General Principles & Plain films Computed
Tomography Intro Magnetic Resonance Intro
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Purpose of primer & thread Primer plant a seed of
understanding of diagnostic imaging that will grow throughout many
additional DPT courses during your three years in the program
Thread To meet practice expectations regarding the integration of
diagnostic imaging into physical therapy practice
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Purpose of primer & thread There are many threads
throughout your DPT education. Everything you learn about
examination, evaluation and intervention is technically a thread
through the curriculum (MMT, ROM, Endurance, Functional mobility)
What makes Diagnostic Imaging different? Increased use in practice
is relatively new Response to increased availability & ease of
communication Inclusion into PT education is therefore relatively
new No single course in the curriculum owns the material (neither
do we have a course on MMT)
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Objectives of primer Explain the underlying process of
diagnostic imaging by x-rays, CT scan, MRI How do these
technologies create an image What leads to lightness or darkness in
the image Understand visually the transformation of three-
dimensional anatomy into two-dimensional imaging anatomy (Carried
over into Anatomy & Neuroanatomy course) Define basic terms and
describe basic procedures of covered diagnostic imaging methods
Explain sources of variation in diagnostic images (if presented
with two images explain how they are different and propose
why)
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Underlying message (1) Variation in images is obvious for:
Different anatomical sites Different angles / planes of view
Variation in images is also caused by: 1. Method of imaging x-rays
vs. computer modified images vs. proton signals 2. Interaction of
method of imaging & different tissues You are looking at a 3d
structure in 2d even if there is a 3d reconstruction your film or
screen is only 2d
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General Principles & Plain films Radiation energy
transmitted through space of matter Higher energy (x-ray, gamma
ray) ionize atoms in matter Ionization can disrupt life processes
Diagnostic radiography uses short wavelength ionizing
electromagnetic radiation (therapeutic radiation uses shorter
wavelengths that overlap with gamma rays)
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Plain film process Collimator controls size & shape of
x-ray beam X-ray beam passes through patient and undergoes
attenuation Attenuation is a reduction in # of x-ray photons in the
beam due to interaction with matter and lose of energy through
either scattering or photo-electric absorption Remnant radiation
emerges from patient & contains an aerial image of patient
Remnant radiation is captured by an image receptor Captured image
is latent until processed
Scatter of the beam will result in lower contrast Biederman,
2006
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Radiodensity impacted by thickness despite no change in actual
density
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Need 2 films perpendicul ar to one another to gather accurate
information
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AP View Viewed as if standing in front in anatomical position
Markers: R right L left INT int rota. EXT ext rota WTB standing
DECUB recumbant INSP, EXP
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Biederman, 2006
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Contrast Enhanced Contrast enhanced a contrast medium is
injected or ingested Improves visualization by increasing contrast
in areas with minimal inherence contrast Can be radiopaque or
radiolucent or dual Angiography, mylography (myelogram)
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Nuclear Imaging Based on physiological or functional changes
(usually activity) Radionuclide that emits gamma rays Gamma rays
are detected by gamma camera that transforms into image Static
images, Whole body images, Dynamic images, Positron emission
tomography (PET)
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Computed Tomography Intro CT uses x-rays Same radio densities
as plain films (but not as impacted by other tissues) Difference:
CT creates images based on cross-sectional slices created by up to
1000 projections from different angles Tighter field of view via
collimators that determine slice thickness
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CT Scan Types 3D CT Can be rotated in space on the computer
screen multiplanar reconstruction (MPR) These images are not
adequately viewed in the printed format
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CT Scan Types CT Myelogram Myelogram is most commonly performed
with CT (as opposed to conventional radiographs) Reminder the
injection increases radiolucency or radioopacity of structures CT
myelogram at C4-C5 injection allows radioopacity of spinal
canal
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CT Scan Selective Windowing Windowing refers to the range of
radio densities emphasized in the image Bone Window (top) Soft
tissue allows reader to distinguish between muscles and the fat
between them 1. Glut Medius 2. Glut Maximus 3. Fat between
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CT Scan Imaging Artifacts Hardening: as photons in the x-ray
beam pass through structures such as the skull the beam becomes
harder because they are absorbed more readily. Leads to dark bands
in the image between radiopaque areas Metals: lead to streaking
that can present as bright lines in the image extending radially
from the metal Motion: movements can lead to shading or streaking.
Faster scan times reduce the prevalence of motion artifacts
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CT Scan Pros & Cons Best at: 1.Subtle or complex fractures
2.Degenerative changes 3.First in serious trauma 4.Spinal stenosis
5.Loose bodies in joints Less time & expense than MRI Accurate
measure in any plane Less claustrophobia Limited in use for soft
tissues due to reliance on radio density Relatively high radiation
exposure
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Magnetic Resonance Intro Based on energy emitted from hydrogen
nuclei (protons) following their stimulation by radiofrequency (RF)
waves Energy emitted varies according to tissue characteristics
Therefore, MRI can distinguish between different tissues No radio
density now Signal Intensity SI Greater SI is brighter; less SI is
dark
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Magnetic Resonance Phenomenon MR is process by which nuclei,
aligned in a magnetic field, absorb and release energy While many
molecules display MR, for all practical purposes MRI is based on
signals from hydrogen in water molecules Since hydrogen consists of
1 proton the hydrogen nucleus is referred to as simply the proton
in the context of MRI
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MR Phenomenon First protons are aligned by a strong magnetic
field either in the direction of the field, or the opposite
direction There are slight differences between those in direction
and opposite which results in longitudinal magnetization A pulse of
RF waves is applied at right angles to longitudinal magnetization
The pulse alters the alignment to a transverse plane, and the
energy absorbed in the process brings them to a higher energy
state: transverse magnetization As the protons realign energy is
released this induces a current that gives rise to the data for
creating the MRI
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1. Aligned in magnetic field (longitudinal) 2. RF wave 3.
Altered alignment (transverse, E increased) 4. Gradually return to
alignment (E release)
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T1 & T2 Phenomenon T1 & T2 are different processes
related to the return of the alignment to the main magnetic field
T1 time it takes for protons to gain longitudinal magnetization (T1
Recovery) T2 protons lose their transverse magnetization (T2 Decay)
Two sides of same coin but different processes MRI uses this to
create different images that feature different tissues based on the
protons response to the RF wave TR = time to repetition (time to
repeat RF wave) TE = time to echo (time at which the signal is
captured)
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T1 Recovery Protons lose energy to surrounding molecules Time
of return differs for different tissues Faster recovery (shorter
times short T1) results in stronger signals from the protons of
that tissue
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T2 Decay Transverse magnetization decays because of a loss of
phase coherence, owing to interaction between protons Slower decay
stronger the signal recorded at end of the process
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T1 & T2 Weighted Imaging T1 Weighted Short TR and TE Signal
caught early when difference in relax characteristics for fat has
higher SI Good anatomical detail T2 Weighted Long TR and TE Tissues
that are slow to give up energy are imaged such as water therefore
water has high SI Particularly valuable for detecting
inflammation
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Biederman, 2006
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Image Information Scout image Weighting and/or TR and TE Slice
thickness (4-8 mm) FOV (field of view) Date, Time, facility, body
part, plane
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Protocols Combination of sequences No standard protocols
Combination depends on the body part and the suspected pathology
Two main categories of sequences Spin echo (SE) such as T1 and T2
images Gradient echo (GRE)
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SE Sequences Usually referred to as T1 or T2 weighted with
specific parameters stated Fast SE as it sounds faster Proton
density (PD) Long TR and short TE the contrast is primarily due to
PD, tissues with higher PD have higher SI SI is similar to T1, but
has greater anatomical detail Inversion recovery (STIR short tau
inversion) Inversion pulse cancels out the signal from fat to
further reduce its SI in T2 images
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Biederman, 2006 For better example of differences see Figure
5-4 in McKinnis text
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Biederman, 2006
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GRE Sequences RF wave is applied and only partly flips the
magnetization field (0-90 degrees) and includes a variable flip
angle Allows reformatting to any plane not limited to orthogonal
plan so used for complex anatomy Overall: 1.Fast image acquisition
2.High resolution with thin slices 3.High contrast between fluid
and cartilage
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Use of Contrasts Intravenous gadolinium-containing contrast
agents Gadnolium is a paramagnetic metal ion used for regular MRI,
MR angiography (MRA) and MR arthrography
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Imaging Characteristics of Tissues
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MRI Advantages / Disadvantages Advantages Greater contrast for
soft tissue Image organs surrounded by dense bone No ionizing
radition Less false positives Disadvantages Expensive Not always
available Long imaging times Longer operator time Larger slices
than CT More problems with motion artifact Less resolution for bone
Concern about metal implants