Introduction to Computed
Tomography [CT], and Magnetic
Resonance Imaging [MRI]
Computed Tomography
• Collimated X-ray technique
– Individual slices
– Spiral slicing
Daniel A. Feeney DVM, MS
Professor of Veterinary Radiology
College of Veterinary Medicine
• Two-dimensional display of sectioned
anatomy in “axial” plane with option of
multi-planar reconstruction of contiguous
data
• Window & Level Viewing Options
• Digital Images (DICOM)
University of Minnesota
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Computed Tomography
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Computed Tomography
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2
Computed Tomography
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Standard Computed Tomography
Voxel & Pixel Creation
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(from Feeney, et. al.)
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Computed Tomography
Voxel & Pixel
Computed Tomography
(from Feeney, et. al.)
(from Feeney, et. al.)
CT Numbers  Image
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Computed Tomography
[window width] (from Morgan)
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Computed Tomography
[window level] (from Morgan)
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Computed Tomography
[window/level]
Computed Tomography
• Intravenous Contrast Enhancement:
– Usually scanned as “pre” followed by “post” contrast series using
same scan parameters (e.g. kVp, slice thickness, slice interval,
pitch, etc.)
– Based on the use of sterile iodinated contrast medium:
• Ionic (cheaper)
• Nonionic (preferred for safety)
• Basis of “enhancement” is X-ray attenuation from iodine in vessels or
iodine that has “leaked” into tissues
– Useful to outline vessels for CT angiography
– Abnormalities “enhance” [appear brighter] due to:
Lung
Soft-tissue
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Bone
• Endothelial barrier breakdown, particularly “blood-brain”
• Delayed vascular flow of blood + contrast mixture through tortuous
tumor-induced vessels which lack autoregulation
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Computed Tomography
• Iodinated Contrast Medium:
– Ionic, sterile
• Meglumine and/or Sodium +
Diatrizoate or Iothalamate
• Approved for intravascular use
• Primary uses include:
–
–
–
–
physiologic organ “enhancement”
blood flow detection or tracing
detection of vascular leakage
lower urogenital track (retrograde) opacification
Computed Tomography
• Iodinated Contrast Media:
– Nonionic, sterile
• Iopamidol, Iohexol, Ioversol,..
• Approved for intravascular (some also intrathecal) use
• Primary uses include:
–
–
–
–
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Computed Tomography
physiologic organ “enhancement”
blood flow detection or tracing
detection of vascular leakage
definition of subarachnoid space (if approved)
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Computed Tomography
Spiral CT
• Standard CT: (higher resolution)
– full 360o slice without table motion
– image thickness 1, 2, 5, 10 mm thick made at any interval > 1mm
(e.g. 5 mm thick slices @ 5 mm interval)
• Spiral CT: (higher speed)
– full 360o slice with table motion during the tube/gantry rotation
– image generated as defined slice collimation exposed during
simultaneous tube/gantry rotation and table linear motion
•
o
Pitch = distance traveled by tube/gantry during 360 rotation
slice collimation thickness
– image thickness selected from dataset defined by spiral slicing
reconstruction algorithm
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Computed Tomography
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Computed Tomography Topics
•
•
•
•
•
•
•
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Cranial Neurologic Deficits
Spinal Neurologic Deficits
Cancer Staging
Nasal Disease Clarification
Problematic Lameness
Abdominal Mass Clarification
Complex Vascular Disease
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L3-L4 Disk
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L-S Disk
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182342 Nasal Tumor
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L3-L4 Disk
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L-S Disk
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182342 Nasal Tumor
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182342 Nasal Tumor
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Neurofibrosarcoma
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Neurofibrosarcoma.2
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Neurofibrosarcoma
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Neurofibrosarcoma
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180526 Neurofibrosarcoma.2
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CT References
CT References
• Feeney DA, Fletcher TF and Hardy RM: Atlas of
Correlative Imaging Anatomy of the Normal Dog:
Ultrasound and Computed Tomography. WB
Saunders, Philadelphia, 1991.
• Morgan CL: Basic Principles of Computed
Tomography. University Park Press, Baltimore,
1983.
• Brink JA, Heiken JP, Wang G, et. al.: Helical CT:
Principles and Technical Considerations; Radiographics
1994;14:887-893.
• Dalrymple NC, Prasad SR, Freckleton MW and
Chintapalli KN: Informatics in Radiology: Introduction to
the language of three-dimensional imaging with
multidetector CT. Radiographics 2005;25:1409-1428.
• Rydberg J, Laing Y and Teague SD: Fundamentals of
multichannel CT. Radiol Clin North Am 2003;41:465474.
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Computed Tomography
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Magnetic Resonance Imaging
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Magnetic Resonance Imaging
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Magnetic Resonance Imaging
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Magnetic Resonance Imaging
Magnetic Resonance Imaging
• Deposition in and Retrieval/Localization of
Radiofrequency Energy from Tissue/Organs in
a Static Magnetic Field
• Based on protons (hydrogen nuclei) which when placed
in a magnetic field (0.3 3.0+ Tesla) act like spinning
tops (precessing) around the magnetic field Bo.
• “Resonant Frequency” of Hydrogen [42.6 MHz/T*]
• Individual slices
• Volume imaging [multiple slices]
• Two-dimensional display of sectioned anatomy in
any plane (Digital DICOM Images)
• Varied Pulse Sequences to Emphasize Fat, Water,
or Paramagnetic Contrast Medium
“leakage/presence”
* Tesla (T) = measure of magnetic field strength with earth’s
magnetic field +/- 0.00005 T
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Magnetic Resonance Imaging
In Bo, No RF Input
Magnetic Resonance Imaging
T1 (longitudinal) relaxation
time is when 63% of the
net magnetic vector has
returned to alignment
with Bo [spin-lattice]
In Bo, With RF Input
0.63
Decayed
Vectors
Peak
Vector
0.37
Decayed Magnetic Vector
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Magnetic Resonance Imaging
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T2 (transverse) relaxation
time is when the net
magnetic vector has
decayed to 37% of its
peak due to dephasing
[spin-spin]
• Energy states of
precessing tissue
protons are random
• Bo field promotes
alignment (spin-up
or spin-down)
• RF pulse changes Bo
aligned protons into
alignment at 90o to
180o and a higher
energy state
• As RF-induced
alignment decays,
energy is emitted
and detected
Magnetic Resonance Imaging
• In a magnetic field Bo onto which the following are superimposed:
•Longitudinal magnetic gradient
(slice selection @ Larmor Hz) usually in the “Z” direction
• Frequency encoding gradient (applied during signal readout)
“X” or “Y” direction
•Phase encoding gradient (applied between RF excitation and
signal readout)
“X” or “Y” direction
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Magnetic Resonance Imaging
Magnetic Resonance Imaging
• Patient in Magnet with Slice Selected:
Pixel/Voxel
transverse
Z
longitudinal
Voxels defined in the slice (selected by Bo
gradient) using reversible phase and
frequency gradients 44
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Magnetic Resonance Imaging
Magnetic Resonance Imaging
Spin-echo Pulse Sequence
180o
180o
90o
90o
TR = repetition time
TE = echo time
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Magnetic Resonance Imaging
Image Contrast (weighting)
Short TE Long TE
Short TR
T1W
____
Long TR
PD
T2W
TR = repetition time
TE = echo time
T1W = T1-weighted (bright fat)
T2W = T2-weighted (bright fluid)
PD = Proton Density-weighted (grey-white matter)
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Magnetic Resonance Imaging
• Intravenous Contrast Enhancement:
– Usually scanned as “pre” followed by “post” contrast series using
same scan parameters (e.g. TR, TE, slice thickness,
slice interval, etc.)
– Based on the use of sterile gadolinium-based medium:
•
•
•
•
Ionic (cheaper)
Nonionic (preferred for safety)
Macro-cyclic (most expensive, ? safest)
Basis of “enhancement” is shortening of the T1 relaxation time due to the
gadolinium derivative in vessels or which has “leaked” into tissues
– Useful to outline vessels for MR angiography
– Abnormalities “enhance” [appear brighter] due to:
• Endothelial barrier breakdown, particularly “blood-brain”
• Delayed vascular flow of blood + contrast mixture through tortuous
tumor-induced vessels which lack autoregulation
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Magnetic Resonance Imaging
Brain Tumor
• Paramagnetic Contrast Media:
– Gadolinium:
•
•
•
•
•
Dimeglumine Gadopentate (ionic)
Dimeglumine Gadobendate (ionic)
Gadoteridol (non-ionic, macro-cyclic)
Gadoversetamide (non-ionic)
Gadodiamide (non-ionic)
– For intravascular and, if approved, intrathecal use
– Primary uses include:
• physiologic organ “enhancement”
• blood flow detection or tracing
• detection of vascular leakage
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Disk Herniation
Lead Toxicity [Bald Eagle]
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Vertebral Myeloma
Brachial Plexus Tumor
T2 sag
T1 Gd 
T2 Dorsal
Post Gd
T1 [normal]
T2 Axial
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Pre Gd
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Single, Extra-hepatic Portosystemic
Shunt
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MR References
• Rummeny EJ, Reimer P and Heindel W: MR Imaging of
the Whole Body. Thieme Medical, Stuttgart, 2009.
• Runge V, Nitz W and Schmetts S: The Physics of Clinical
MR Taught Through Images. Thieme Medical, Stuttgart,
2009.
• Lin S and Brown J: MR Contrast Agents: physical and
pharmacologic basics. J Mag Res Imag 2007;25:884-899.
• Westerbrook C: Handbook of MRI Technique. WileyBlackwell, West Sussex, 2008.
• Numerous Authors: Selected topics in MR Neuroimaging.
Mag Res Clin 2006;14:127-285.
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