Download RAD 216 ADVANCED IMAGING MODALITIES

RAD 216 ADVANCED
IMAGING MODALITIES
Introduction to Computed
Tomography
COMPUTED TOMOGRAPHY
Also called CT, was
marketed in the 1970’s
by EMI, a British
corporation. CT’s
inventor electrical
inventor,
engineer Dr. Godfrey
Hounsfield, was
awarded a Nobel Prize
(1979) for his work in
producing the first CT
images in 1967.
FIRST PROTOTYPE CT SCANNER
The first “CAT”
scanners were
limited to the
head.
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CT SCANNERS
The scanner consists of the housing
(gantry) that contains the x-ray tube
and detector elements. A scan table
is situated so that the patient can be
positioned within the gantry’s
opening.
CT SCANNERS
A minicomputer is used to process
data using an array processor.
Rather than a single microprocessor
(CPU), array processors use several.
This enables image processing to be
divided among the microprocessors in
order to shorten scan time.
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CT SCAN DEVELOPMENT:
FIRST GENERATION
Early scanners were limited to the
head. An x-ray tube and single
detector was used. During the scan,
the tube and detector moved in a
series of sweeps and rotations called
translate-rotate.
CT SCAN DEVELOPMENT:
FIRST GENERATION
The single detector system required
long scan times, a single slice
requiring 5 minutes to complete.
1st GENERATION CT
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CT SCAN DEVELOPMENT:
SECOND GENERATION
By adding more detectors, the beam
could be directed to cover a broader
area, called a fan beam. The beam
was thin enough to define thin slices.
The scan still utilized a translaterotate pattern, but scan time was
reduced.
2nd GENERATION CT
CT SCAN DEVELOPMENT:
THIRD GENERATION
A curved detector array was
introduced with an increased number
of gas-filled detectors. The x-ray
tube and detector array rotate during
the scan (a departure from the
translate-rotate mode.
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CT SCAN DEVELOPMENT:
THIRD GENERATION
Scan times, although considerably
shorter than in previous generation
scanners, could take 20 seconds to
complete (per slice).
3rd GENERATION CT
CT SCAN DEVELOPMENT:
FOURTH GENERATION
The major innovation was the use of
a fixed detector array system. Only
the x-ray tube rotates during the
scan. The x-ray tube moves along a
track, called a slip ring.
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CT SCAN DEVELOPMENT:
FOURTH GENERATION
By using a slip ring, the x-ray tube
can rotate continuously without
snagging power cables. Depending
on design, the high voltage
transformer rotates along with the xray tube.
FOURTH GENERATION CT
CT SCAN DEVELOPMENT:
FOURTH GENERATION
Short scan times (1 slice per second)
and continuous tube rotation makes
helical or spiral CT possible.
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HELICAL (SPIRAL) CT
Helical CT involves the rotation of the
x-ray tube with continuous
movement of the scan table. The
process results in a scanning pattern
similar to a coil.
HELICAL (SPIRAL) CT
Because the slices are connected in a
coil-like pattern, individual slices
must be created. Part of the image
reconstruction process, called
interpolation, is used.
HELICAL (SPIRAL) CT
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HELICAL (SPIRAL) CT
MULTIDETECTOR CT (MDCT)
Instead of using a single ring of
detectors,, MDCT adds several rows,,
permitting acquisition of several
slices in one sweep of the x-ray tube.
Scan times are very short, permitting
entire body scanning in just a matter
of seconds. The number of detector
rows and detector configurations vary
with each system and manufacturer.
HELICAL (SPIRAL) CT:
ADVANTAGES
† ENTIRE ORGANS CAN BE SCANNED
IN A SHORT PERIOD
† LESS LIKELIHOOD OF
MISREGISTRATION ARTIFACTS
† MULTIPLANAR AND 3D
RECONSTRUCTION USING IMAGE
DATA IS POSSIBLE
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5TH GENERATION CT
Electron beam computed tomography
(EBCT), also called ultra fast CT, was
a major innovation in scanner design.
Rather than using a rotating x-ray
tube, the scanner itself acts as an xray tube.
ELECTRON BEAM CT
An electron gun in the back of the
scanner directs a fine stream of
electrons to a set of target rings in
the gantry. The electron-target
collision produces several x-ray
beams which pass through the
patient. There are no moving parts
other than the table.
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ELECTRON BEAM CT
EBCT is useful in producing highresolution cardiac images with scan
times of between 50 and 100 ms.
Cine and flow studies are possible
CT DETECTOR ARRAYS
Third generation scanners used gas
gasfilled detectors. Gas detectors use
compressed xenon (Xe). Each
detector is only 1 mm wide.
Ionization caused by the capture of
x-ray photons is converted into
electrical signals which are sent to an
array processor.
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GAS-FILLED DETECTORS
CT DETECTOR ARRAYS
Fourth generation scanners use solidgas detectors. Solid-state
state or g
detectors detect radiation by
interaction with photoemissive
crystals. The light given off these
crystals stimulate photodiodes,
converting light into electrical
impulses that are relayed to an array
processor.
SOLID-STATE DETECTOR
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CONTROL OF SCATTER
RADIATION
Scatter radiation is controlled by
means of proper selection of kVp and
the use of pre- and post-patient
collimation.
PREPATIENT COLLIMATION
Prepatient collimation controls the
thickness of each scanned slice.
Scatter production is reduced when
using thinner slices
POSTPATIENT COLLIMATION
Collimators positioned in front of the
detector array keeps scatter that
emerges from the patient from
reaching the detectors. This helps
increase signal-to-noise ratio.
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IMAGE RECONSTRUCTION
During a CT scan, an x-ray beam
passes through the patient at various
angles. The attenuated beam that
emerges at each angle (called a
projection) is captured by the
detector array.
IMAGE RECONSTRUCTION
The attenuation data (called raw
data) is digitized and manipulated by
the array processor using a
mathematical technique called
filtered back projection.
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IMAGE RECONSTRUCTION
Back projection is a summation
q
that is used to add and
technique
average attenuation data. Filtering is
used to smooth image irregularities
resulting from the scanning process.
Unless these irregularities are
corrected, the resulting image will
have rough edges and streak
artifacts.
IMAGE DISPLAY
CT images are arrayed on a matrix.
Each of the pixels within the matrix
has a shade of gray that correlates
with a numerical value representative
of a tissue, fluid or air. These
numbers are called CT numbers.
CT NUMBERS
CT numbers depend on the kVp which
affects the attenuation coefficient of
the tissues. Attenuation coefficient
(cm-1) is the amount of attenuation
experienced by x-rays as they go
through each linear centimeter of
material.
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CT NUMBERS
μt − μ w
• 1000
μw
CT REFERENCE NUMBERS
†
†
†
†
†
†
WATER = 0
AIR = -1000
FAT = -80
MOST SOFT TISSUES = +30 TO +80
CALCIUM > +100
SOLID BONE = +1000
DISPLAYED IMAGE & FIELD
OF VIEW
The technologist determines at the
time of the scan how large or small
the displayed image should look.
This is called the reconstruction field
of view (FOV).
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DISPLAYED IMAGE & FIELD
OF VIEW
A small field of view means that the
reconstructed image will depict a
close-up of the anatomy in question.
A large field of view will depict more
of the anatomy making details
appear smaller on the monitor.
CT NECK
CT ABDOMEN
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FIELD OF VIEW & PIXEL SIZE
Pixel Size =
Field of View
Matrix Size
FIELD OF VIEW & PIXEL SIZE
Given a field of view (FOV) of 20 cm
and a 512 x 512 matrix size, what is
the size of each pixel?
FIELD OF VIEW & PIXEL SIZE
Answer:
20 cm x 10 = 200 mm
200 mm ÷ 512 = 0.39 mm
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MEASURES OF CT IMAGE QUALITY
†
†
†
†
†
†
†
SPATIAL RESOLUTION
LOW CONTRAST RESOLUTION
SPATIAL FREQUENCY
MODULATION TRANSFER FUNCTION
NOISE
LINEARITY
SPATIAL UNIFORMITY
SPATIAL RESOLUTION
The ability to clearly distinguish the
borders between tissues. Abrupt
changes in tissue composition tend to
create loss of detail on CT images.
LOW CONTRAST
RESOLUTION
The ability to distinguish subtle
differences in tissue composition. At
a 0.5% difference in tissue
composition, CT is capable of
distinguishing between objects that
are 4 mm in size or larger.
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SPATIAL FREQUENCY
The ability of an imaging system to
detect small details as measured with
a test pattern. For example, if a
scanner can resolve a test pattern
with a spatial frequency of 12 line
pairs per cm (12 lp/cm), what is the
smallest size object that the scanner
can “see” clearly?
SPATIAL FREQUENCY
SPATIAL FREQUENCY
Answer:
12 lp/cm = 24 bars and spaces/cm
1 cm = 10 mm
1/24 x 10 mm = 0.42 mm
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MODULAR TRANSFER
FUNCTION
A measure of how well a scanner is
able to faithfully represent the
anatomy scanned (image fidelity).
Generally, the smaller the objects
scanned, the less faithful the
representation of the image
NOISE
Any number of factors that will
diminish the amount of useful
information (signal). Slice thickness,
kVp, mA and matrix size affect noise.
It manifests itself as variation in CT
numbers, leading to incorrect
information.
QUALITY CONTROL
† WATER BATH PHANTOM (STANDARD
DEVIATION TEST)
† WATER BATH PHANTOM (SPATIAL
UNIFORMITY HISTOGRAM)
† AAPM TEST (LINEARITY)
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STANDARD DEVIATION
A statistical measure of variation.
The degree to which individual
measurements stray from the
average measurement is called the
standard deviation
STANDARD DEVIATION
( x-x ) 2
Std. Dev. = ∑
n −1
SPATIAL UNIFORMITY
Similar to the test of standard
deviation, the results are depicted in
a bar chart generated by the CT
computer.
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LINEARITY
A test to determine the degree of
accuracy of a CT system by
identifying objects of varying
attenuation coefficients. When the
system is functioning correctly, the
graphical plot of the CT numbers will
form a straight line.
FACTORS THAT INFLUENCE
PATIENT DOSE IN CT
†
†
†
†
†
†
VOLUME OF COVERAGE
MA AND KVP
SLICE THICKNESS
SCAN PITCH
FILTERING (CONVOLUTION)
2D vs. 3D IMAGING
MULTIPLE SCAN AVERAGE
DOSE (MSAD) CONCEPT
Calculating patient dose in CT is a
fairly complex exercise when
compared to diagnostic radiography.
This is due to the many parameters
and imaging techniques used in CT
and the fact that many image slices
are taken during scanning.
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MULTIPLE SCAN AVERAGE
DOSE (MSAD) CONCEPT
In the 1980’s the FDA (Center for Devices
and
dR
Radiologic
di l i H
Health)
lth) d
devised
i d a method
th d tto
determine dose from CT scanning. The
dose taken at the center of a slice is called
the CT Dose Index (CTDI). The average
dose from multiple slices within a volume of
tissue is called the Multiple Scan Average
dose (MSAD).
MULTIPLE SCAN AVERAGE
DOSE (MSAD) CONCEPT
When a scan is taken with a pitch of 1
(
(meaning
i
th
thatt all
ll slices
li
are ttaken
k
contiguously) then the MSAD is equal
to the CTDI. As scan pitch increases,
MSAD decreases consistent with the
formula:
MSAD = CTDI (INVERSE OF SCAN PITCH)
PROBLEM
Assume a CTDI of 1000 mrem. If
spiral CT images are obtained
using a table increment (BI) of
10mm per tube rotation and a
slice thickness (SW) of 2.5 mm,
what is the MSAD?
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ANSWER
† CTDI = 1000 MREM
† PITCH = 10 mm ÷ 2.5 mm = 4
† MSAD = CTDI x INVERSE OF
PITCH
† MSAD = 1000 x (1/4) = 250
mrem
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