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Announcement
Mid-term exam : 9 March 2015
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Computed Tomography: Principle and Applications
Prof. Defeng Wang
Department of Imaging and Interventional Radiology,
The Chinese University of Hong Kong
Email: [email protected]
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Overview
CT image principle
CT data acquisition
Helical CT
Multi-slice CT
Image formation
Image quality
Image reconstruction and visualization
Surface rendering
MIP, MinIP and Volume rendering
Virtual endoscopy
Clinical applications
Conventional CT
CT Perfusion
CTA1/93
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CT image principle
100 years ago, a German scientist Roentgen discovered X-ray,
which
enables people to
view the anatomic structure of human body without operation
Disadvantages:
Superimposed image
Couldnt view soft tissues generally
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In 1972, Godfrey N. Hounsfield developed the first
clinically
useful CT scanner
Based on the mathematical and experimental methods developed
by A. M. Cormack;
Both shared the Nobel Prize in physiology or medicine in 1979.On
the day he won the Nobel prize in 1979, Hounsfield had some
home-spun words of advice for all would-be Nobel
prizewinners: "Don't worry too much if you don't pass exams, so
long as you feel you have understood the subject. It's
amazing what you can get by the ability to reason things out by
conventional methods, getting down to the basics of what
is happening."
In 1974, Robert S. Ledley developed the first whole-body CT
scanner.
History of CT scanner
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With CT scanner, we could view
1. Tomographic or slice anatomy
2. Density difference
The helical and multi-slice CT scanners were introduced in
1989 and 1998 respectively, so that CT has opened the way
to 3D images of the heart and dynamic (4D) studies.
Cont.
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Computed tomography (or computerized axial
tomography) is an examination that uses X-ray and
computer to obtain a cross-sectional image of the human
body.
Notion of CT:
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Computed tomography (CT):
an image modality that produces cross-sectional image
representing the X-ray attenuation properties of the body.
Cross-sectional image formation is based on the following
procedure:
I. X-ray tube produces x-rays
II. X-rays are attenuated when going through the body
III.X-rays are measured by an X-ray detector
Characters
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CT device:
(a) Schematic representation (b) CT scanner
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CT contrast
For clear visualization of vessels or lymphatic system, general
CT
scanning may produce misdiagnosis.
Sometimes CT contrast is essential before CT scanning.
Urografin
Omnipaque (iodine compound)
However, contrast injection may cause side-effect:
1. Emesis
2. Palpitation
3. Urticaria
4. Edema
5. Spasm
6. Shock 8/93
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X-ray
X-ray beams, a set of lines, which covers the entire field
of
view (FOV);
Repeat scanning for a large number of angles and generate
line
attenuation measurements for all possible angels and
distances
from the center;
The actual attenuation at each point of the scanned slices can
be
reconstructed from all the previous measurements.
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X-ray beams
(a)Parallel x-ray beams;(b) fanned x-ray beams;(c)repeated
process of (a) with
rotation;(d)repeated process of (b) with rotation.10/93
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X-ray Attenuation
When X-ray passes through objects, it will be attenuated (energy
reduction)
by two ways:
Absorption
Scattering (not considered in CT)
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The X-ray absorption is proportional to the density of
object.
More attenuation Less attenuation
Lambert-Beer Law:
where I is the transmitted X-ray energy after absorption, Iois
the incident X-
ray, is absorption coefficient, is the object thickness.
Cont.
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For an object with n voxels, there is
With scanning times of n or more than n,
could be computed and the CT values could be obtained.
+
Cont.
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Iodine is the usual contrast dye. Some patients are allergic
to
iodine and may experience a reaction that may include
nausea,
breathing difficulty, or other symptoms.
Radiation exposure during pregnancy may lead to birth
defects.
The amount of radiation during a CT procedure should be
reduced to produce the least harm to people; CT scan should
be
carried out when it is really necessary.
Risks of CT
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CT number
In reconstructed CT images, the value of each pixel
(intensity
value) represents the CT number which is defined as
following
: the linear attenuation coefficient
: Hounsfield unit (HU)
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CT number of different organs and tissues
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Window and Level
Window size: the width of the displayed gray level interval
Window level: the center of the displayed gray level
interval
(a) (b)
CT image of chest:(a) Window size=1600,level=-600;(b) window
size=400,level=40
For soft tissuesFor lungs
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CT data acquisition design
Helical CT
Widely used nowadays
Table translates and the x-ray tube rotates continuously
around the patient
table feed (TF)
TF=axial translation per tube rotation
z=slice thickness
pitch:
pitch = TF/z
Typical pitch ratio: 0.5, 1.0, 1.375, 1.5, 2.0
Larger pitch?
Smaller pitch?
Faster scanning, worse quality
Better quality, slower scanning18/93
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Continuous source rotation with the patient translation
through X-ray beam
Patient couch moves as X-ray tube rotates
Cont.
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Continuously rotating tube/detector system
Continuously generating X-ray
Continuously table feed
Continuously data acquisition
The 4C of Helical CT
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Image reconstruction continuously
{
Increment
Slice Thickness
TF=thickness
1. No overlap
2. No gap
{
Scanning mode
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Overlapping image reconstruction
TF
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Image reconstruction with gaps
TF>thickness
1. Gaps between slices
2. Less images created
{Increment
}Slice Thickness
Cont.
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Advantages:
A. Examination time is greatly reduced and patient comfort is
much
improved.
B. Image noise is usually less with helical CT.
C. Helical CT has no inter-scan delay.
D. Helical CT misses no anatomy in the scanned volume for
no-gaps
scanning.
Limitations:
As more data is acquire in helical CT, image reconstruction
takes more time
(interpolation needs more time than conventional CT).
Advantages and limitations of helical CT:
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Multi-slice CT
Multi-slice CT is a development of conventional helical CT, with
multiple
detectors on the opposite side of X-ray beams, which enables
multiple images
acquisitions. 25/93
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Advantages:
Same acquisition in short time
Thin slices give better z-axis resolution
Scan larger volumes in the same time
Cont.
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More than one detector, while traditional only have one
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A CT slice of the chest showing the lungs using single-slice and
multi-
slice CT scanners. The image acquired from multi-slice CT gives
better
quality.
Cont.
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Fixed detector length: flexible combination, convenient
thickness change
Variable detector length: less detectors number, less X-ray
absorption
Detectors
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Different X-ray detectors
Scintillation crystal with photomultiplier tube (PMT)(
)
(scintillator: material that converts ionizing radiation
into
pulses of light)
high absorption efficiency
low packing density
PMT used only in the early CT scanners
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Gas ionization chambers()
replace PMT
X-rays cause ionization of gas molecules in chamber
ionization results in free electrons/ions
these drift to anode/cathode and yield a measurable electric
signal
lower absorption efficiency than PMT systems, but higher
packing density
Cont.
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Resolution can be improved
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Scintillation crystals with photodiode ()
current technology
Scintillator material converts X-rays into visible light,
which
hits the photodiode, causing it to produce measureable
electric
current
high absorption efficiency
very fast response time
Cont.
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Photon counting detectors()
recent detector
based on direct conversion
Direct conversion material (cadmium telluride or cadmium-
zinc-telluride) converts x-ray photons into electronic
charges
proportional to photon energy
Produced charge is 10 times larger than that produced by the
scintillator/ photodiode combination
Electronic noise no longer dominates the signal
Cont.
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Image formation
The Fourier-slice theorem
The formula states that the FT of a projection is a slice
(orthogonal to the projection
direction) from the 2-D FT of the original image.
where F(u,v) is the Fourier transform of f(x,y), is the
Fourier
transform of one CT projection whose direction is pi/2+ relative
to x-axis.
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Illustration of the Fourier-slice theorem.
Cont.
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Can reduce the dosage
After break, talk a little before T1
Know the meaning and 90 degree projection
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CT image reconstruction
For a image f(x,y), it can be expressed as
By letting and
From Fourier-slice theorem,
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Cont.
With the theory of integral calculus, the final result is as
following:
The inner expression is in the form of an inverse 1-D FT, with
the
added term .
Usually a window is applied to the ramp filter .
Practically,
the hamming function is used.
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(a) Frequency domain plot of filter |w| after band limit with a
box filter.(b)
Spatial domain representation.(c) Hamming windowing function.(d)
Windowed
ramp filter, formed as the product of (a) and (c).(e) Spatial
representation of the
product (note the decrease in ringing).
Cont.
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CT reconstruction procedure:
1. Compute the 1-D FT of each projection.
2. Multiply each FT by the filter function which has been
multiplied by a suitable (e.g., Hamming) window.
3. Obtain the inverse 1-D FT of each resulting filtered
transform.
4. Integrate (sum) all the 1-D inverse transforms from step
3.
Cont.
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Inside is 1D, outside is 2D
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Image quality
The spatial resolution of a CT image depends on the
following factors:
size of focal spot
If the focal spot size increases, more geometric unsharpness
introduced, thus decreasing spatial resolution.
detector width
Higher spatial resolution is able to be obtained for smaller
detector element sizes.
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Number of projections
If there are more projections, more data are available for
image reconstruction and improvement on spatial resolution
Cont.
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Slice thickness
Smaller slice thickness improves spatial resolution, since
partial volume effect is less.
CT images of lungs with different slice thickness
Cont.
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Pixel matrix
The number of pixels used to reconstruct the CT image has
a direct influence on spatial resolution under a fixed FOV.
Increasing matrix size for fixed FOV can improve spatial
resolution (e.g. 512*512->1024*1024)
Cont.
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Noise
quantum noise or statistical noise, electronic noise
quantum noise is dominant
Number of noise rely on:
the total exposure
increasing power reduces noise but increases patient dose
the reconstruction algorithm
Both the applied filters and the interpolation methods
influence
the image noise
Cont.
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Cont.
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Artifact-free reconstruction of a
simulated water bowl with iron
rod.
Same slice reconstructed after
noise was added.
Cont.
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Contrast between an object and its background depends
primarily on:
1. their attenuation properties
2. a variety of physical factors
the spectrum of the X-ray tube
the amount of beam hardening
a number of low energies x-rays are absorbed
scatter
Cont.
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Artifacts
Normal phantom (simulated water
with iron rod)
Aliasing artifacts when the number
of detector samples is too small
(ringing at sharp edges)
Cont.
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Normal phantom (simulated plexiglas
plate with three amalgam fillings)
Beam hardening artifacts
Scatter
Cont.
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Motion artifacts
movement of an object
Normal phantom (simulated plexiglas
plate with three amalgam fillings)Motion artifacts caused by a
short
movement of the iron rod
Cont.
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Stairstep artifacts
happen when the helical pitch is too large.
The stairstep artifact is visible in 3D images as a helical
winding along inclined surfaces.
Cont.
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Other artifacts:
Metal artifacts are due to a combination of beam hardening,
scatter, nonlinear partial volume effect, and noise
CT image of a slice through the
prosthesis showing steak artifacts
due to the metallic implant.
Cont.
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Image reconstruction & visualization
Image reconstruction and visualization are important:
The development of CT technique makes it urgent and
essential to visualize 3-D organs and tissues.
3-D reconstruction enables better data visualization and
diagnosis.
3-D visualization avoids doctor from 2-D data sea,
which may cause misdiagnosis due to mass 2-D images.
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Surface rendering-Surface shaded display (SSD)
SSD recognizes tissue by its intensity and shows the surface of
the organs
as an opaque object.
Predefined thresholding value is necessary in SSD.
Using shading technique for visualization.
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For surface rendering,
The 8 voxels from neighboring slices are used to form a
cubic.
Voxels with intensity values larger than the preset
thresholding value are assigned as inner(outer) voxels;
otherwise, they are outer(inner) voxels.
The iso-surface can be constructed with triangulations
according to the distribution in the former step.
Marching cubes algorithm
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A 2-D example. Each point of the grid has a weigh
(intensity) and the thresholding value is 5 here.
Cont.
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Possible iso-surfaces in 3-D reconstruction.
Cont.
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A 3-D surface rendering example:
Cont.
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Advantages:
Produces binary images, which is convenient for interaction.
Strong sense of reality.
Completely provide 3D anatomic morphometry.
Disadvantages:
SSD doesnt provide any densitometric information.
The rendering result is sensitive to the thresholding value.
Advantages and disadvantages of surface rendering
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MIP, MinIP and volume rendering
Maximum intensity projection (MIP)
If the pixel value of the projected image is equal to the voxel
that has the
highest value along the way, the result is MIP image.
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The contrast of MIP is high
and it is widely used in
structures and tissues with
high density, like vessel,
bone, lung tumor
Cont.
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A disadvantage of MIP is that the voxels whose value is not the
highest along
the way are not represented.
Hypo-intense structures within hyper-intense structures can be
masked because
only the material with the highest intensity along the projected
ray is
represented.
(a) show clearly the dissected flap. Performing MIP with
increasing thickness,the
dissected flap disappears (b-c).
Cont.
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Minimum intensity projection (MinIP)
If the pixel value of the projected image is equal to the voxel
that has the lowest
value along the way, the result is MinIP image.
0
MinIP
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MinIP is mainly used in the visualization of airway, and
sometimes for that
of bile duct in liver.
Cont.
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Volume rendering (VR)
Volume rendering utilizes the entire volume data (for MIP or
MinIP, only 10%
are used), calculates the contributions of each voxel along a
line from the
viewers eye through the data set, and displays the resulting
composition for
each pixel of the display.
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VR involves 3 principle parts:
1. The forming of an RGBA volume from the data
2. Reconstruction of a continuous function from this discrete
data set.
3. Projecting the result onto the 2-D viewing plane from the
desired point of
view.
The opacity contribution may range from 100% to 0%, which has
an
significant impact to the result.
Cont.
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First row: VR result
Second row: opacity function
Cont.
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Advantages:
Thresholding value is not necessary in VR and all the voxels are
used.
Voxels classification can be fuzzy (i.e., gradual changed
opacity function).
VR can be used on data with unapparent boundary.
Disadvantages:
As the semitransparent projection with overlapping, VR is
awkward to determine
spatial relationship.
The reconstruction is slow as the entire data is used.
VR advantages and disadvantages
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Virtual endoscopy
Endoscopy is a way to see inside the body to screen and
cure.
Conventional endoscopy
Advantages:
Minimal invasive
High resolution
interactivity
Disadvantages:
Painful and uncomfortable
Limited exploration
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Virtual endoscopy is the navigation of a virtual camera
through
the 3D reconstruction of a patients anatomy .
Adjusting the parameters
based on the volume
rendering result:
Thresholding value
Opacity
Lighting
Perspective direction
Cont.
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Virtual endoscopy combines strengths of previous alternatives
on
patient-specific dataset:
-spatial exploration
-cross-correlation with original volume
It is compact and intuitive to explore huge
amount of information.
Clinical studies:
Planning and post-operation: generates views
that are not observable in actual endoscopic
examinations
color coding algorithms give supplemental
information(e.g. curvature)
Cont.
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Clinical Applications
Conventional CT
head and neck
Subsequent CT slices through the brain show a subdural
hemorrhage as a
hyperdense region along the inner skull wall (short arrows).
This blood collection
causes an increased pressure on the brain structures with an
important displacement
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thorax
CT of the chest. (a) Mediastinal and (b) lung window/level
settings, and (c) coronal
resliced image. The images show a congenital malformation of the
lung located in
the left lower lobe. Notice the two components of the lesion: a
dense multilobular
opacity (arrow) surrounded by an area of decreased lung
attenuation (arrow heads)
Cont.
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urogenital tract
(a) Axial CT slice through the kidney showing a perirenal
liposarcoma in
the nephrographic phase after intravenous injection of contrast
medium.
(b) Reformatted coronal CT slice at the level of the aorta of
the same
patient
Cont.
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abdomen
(a) A CT slice through the colon shows a polyp (arrow).
(b) A virtual colonoscopy program creates a depth view of the
colon with
polyp (arrow) and allows the clinician to navigate automatically
along the
inner wall.
Cont.
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musculoskeleton
(a) On a sagittal reformatted CT image, an anterior-posterior
course of an
acetabular fracture is visible.
(b) A 3D view on the acetabular surface more clearly localizes
the course of the
fracture extending into the posterior column
Cont.
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Cont.
Liver
Dual phase liver exam
Arterial phase Venous phase
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Post-processing
3D segmentation 3D bronchoscopy
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CT perfusion
Perfusion is the passage of fluid through the lymphatic system
or blood
vessels to an organ or a tissue. The practice of CT perfusion is
the process
by which this perfusion can be observed , recorded and
quantified.
No contrast enhancement is seen within the first 9 s. At 18 s
early contrast is seen
within a CT Spot Sign, peaking at 36 s. Dissipation of contrast
material is seen on
delayed image at 36 s .
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Why CT perfusion is needed?
Brain infarction/thrombosis diagnosis.
When should CT perfusion be performed?
After standard brain scan and no bleeding
is perceived.
Suggested by neurologist and radiologist.
Cont.
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Images to the left show a brain scan where the arrows point to a
dark area.
Image to the right is the same as on the left, but it has been
converted to a color
map.
The red area indicates low perfusion in this part of the
brain.
Cont.
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Upper picture to the left shows a normal CT image using
perfusion protocol.
The other images show different parameters. E.g., BF (blood
flow),
BV(blood volume),MTT(mean transit time)...
Cont.
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CT angiography
CTA uses a CT scanner to produce detailed images of both blood
vessels
and tissues in various parts of the body.
An iodine-rich contrast material (dye) is usually injected
through a small
catheter placed in a vein of the arm.
A CT scan is then performed while the contrast flows through the
blood
vessels to the various organs of the body.
After scanning, the raw data will be processed using computer
and
reviewed in different planes and projections.
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CT angiography of the head in sagittal view.
Cont.
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Circle of Willis
Aneurysms
Vascular Malformations
Head CTA
Cont.
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Neck CTA
Carotid
bifurcations
Vertebral
arteries
Aortic arch
Cont.
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Benefits
examine blood vessels in many key areas of the body, including
the brain,
kidneys, pelvis, and the lungs.
displays the blood vessels more precisely than MRI or
ultrasound.
a useful way of screening for arterial disease
safer and much less time-consuming
Risks
may cause allergic reaction
should be avoided in patients with kidney disease or severe
diabetes
significant dose of ionizing radiation with repeated
examinations
Cont.(CTA)
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CT development tendency
Fast scanning with thin slice
The development of CT scanner enables thin slice scanning.
-multi-slice CT
For the same scanning target:
CT scanner Time
4-detector CT 15s
16-detector CT
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Operation navigation
Surgery Navigation for Hearing Aid Implant
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References
[1]http://ebooks.cambridge.org/ebook.jsf?bid=CBO9780511596803
[2]Suetens P. Fundamentals of medical imaging[M]. Cambridge
University Press, 2009.
[3]Gonzalez R C, Woods R E. Digital image processing[J].
2002.
[4]http://users.polytech.unice.fr/~lingrand/MarchingCubes/applet.html
[5]http://www.clg.niigata-u.ac.jp/~tsai/home-page/lecture/3D_reconstruction.htm
[6]http://www.radiologyinfo.org/en/info.cfm?pg=angioct
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Thank you!
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