medical equipment lec 6 CT scan

Medical Equipment III
Computed Tomography

In short…
 CT scanners are
complex X-ray
machines attached
to very clever
computers for the
processing of many
X-ray measurements
taken from different
angles to produce
cross-sectional
images
“tomograms” of
specific areas of the
scanned object.

X ray tube
X ray detector
Patient
X Rays are
produced in an
X ray tube, pass
through the
patient and are
detected by the
detector
The scanner
rotates the X ray
tube and
detector so the
patient is
scanned from all
angles

What are we measuring?
 The average linear attenuation, between
the tube and detectors
 Attenuation coefficient
reflects the degree to
which the x-ray intensity
is reduced by a material.

Ray – Imaginary line
between Tube &
Detector
Ray Sum – Average
attenuation along a Ray
View – The set of ray
sums in one direction
Attenuation profile –
The plot of ray sums as
a function of their
position.
View Attenuation
profile
Ray
Ray sums
Ray, Ray sum, View & Attenuation
Profile

 Attenuation of objects
having different
densities changes the
attenuation profile
Object with low
attenuation
Object with high
attenuation
Attenuation Profile of different
Structures

Attenuation Profile at different
positions
 In a translate –rotate
CT, after a view is
recorded, the tube
and detector rotate a
small angle and the
entire process is
repeated until many
views are recorded
for the same slice

1
2
3
4
View & Attenuation Profiles for
a slice

Projections
 2D views  1D projections at angles all
the way round the patient
 Measured attenuation at each detector
(attenuation profile) represents
projection data.
 Rotate the tube and detectors
a small amount to take another
projection.

Instrumentation for CT
 Most commercial scanners are so called ‘third
generation scanners’ which use a wide X-ray fan-
beam and between 512 and 768 detectors.
 The tube and detectors have to be fixed to a heavy
gantry, which rotates very rapidly.
 Two separate collimators are used in front of the
source.
 The first collimator: restricts the beam to an angular
width of 45o–60o to limit patient dose.
 The second collimator : placed perpendicular to the
first, restricts the beam to the desired slice
thickness, typically 1–5 mm, in the patient head/foot 10

Contrast resolution is improved in CT
imaging compared with conventional
radiographic imaging.(why?)
 Collimation serves to reduce scattered
radiation to less than 1% of the primary
beam intensity, therefore leading to
better contrast images.
Medical Equipment II Spring 2015 Inas A.
Yassine 11

Instrumentation for CT
12
 The detectors used in CT are solid-state devices,
based upon converting the X-ray energy into light
using a scintillator, then light is further converted
into a voltage using a photodiode.

CT NUMBER
 A CT image does not actually display a map of the
spatially-dependent tissue attenuation coefficients,
but rather a map of the tissue CT numbers, defined
by:
 CT number values are expressed in Hounsfield units
(HU)
1000


Water
WaterTissue
numberCT



CT NUMBER
 CT numbers normalized in this manner provide a
range of several CT numbers for a 1% change in
attenuation coefficient.
Water = 0
Air = -1000
Bone = 1000
 Attenuation coefficients of various
tissues for 60 keV x-rays
are shown in table.
Yassine 15

CT numbers of different tissues at
70 keV
16
Medical Equipment IV Spring 2018 Shereen M. El-Metwally and Inas A.
Yassine

CT # vs Gray Level
+ 1000
-1000
A contrast enhancement
feature in the display
device maps the range of
CT numbers of diagnostic
interest to the shades of
gray available in the
display device (i.e., the
dynamic range of the
display)
In a CT image, higher CT numbers are brighter and lower CT numbers are
darker.

18
X-ray attenuation data at four positions of the window level of the
contrast enhancement control.

Exponential Attenuation of X-
ray

 The image is created by reflecting the
attenuation profiles back in the same direction
they were obtained.
 This process
is called
BACK-PROJECTION
Image Reconstruction from
Projections

Back-Projection Method
 Start from a projection value and back-project a
ray of equal pixel values that would sum to the
same value.
 Back-projected ray is added to the estimated
image and the process is repeated for all
projection values at all angles.
 With sufficient projection angles, structures can
be somewhat restored.
22

Example
23
Yassine

 The resultant image
closely resembles the
original object
 But it shows star-
shaped patterns
around objects and
streaks
 These are called
‘Star’ and ‘streak’
artifacts
Drawbacks in Back propagation
‘Star’ and ‘Streak’ Artifacts

 Consider a scan
of a single high
density object
suspended in air
Formation of Star and streak
artifacts

 The
attenuation
profile for this
object has a
single impulse
signal
Attenuation Profile

The back projections
obtained
Take the form
of a stripe
through the
center of the
object

Back projections are formed for each
profile

Final back projection
Star and Streak Artifacts
 Addition of the
attenuation profiles
creates an image with
star and streak
artifacts.
 Low-pass emphasis in
back-projection
method results in
severe blurring in the

Filtered Back-Projection
 It is the principal reconstruction algorithm used in
CT scanners. Often referred to as the “convolution
method”.
 A deblurring function is first convolved with the
projections data to remove most of the blurring
before data is back-projected.
 Several types of high-pass filters can be used.
 Filtered back-projection removes
the star-like blurring seen in simple
back-projection method. 30

Spiral (Helical) CT
 Use of multiple detector
rows along the head/foot
direction enables CT
images to be acquired
much more rapidly.
 With a single gantry
rotation, CT slices of a
larger volume of the
patient are obtained.
 The effective slice
thickness is dictated by
the dimensions of the
individual detectors. 32Multi-slice spiral CT
64-slice scanner

Spiral (Helical) CT
 Pitch (p) is defined as the table feed (d) per rotation of the
X-ray tube divided by the collimated slice thickness (S).
 Low pitch (i.e., small increments of table movement) yields
improved spatial resolution along the long axis (Z axis) of
the patient, but also results in higher patient doses and
longer imaging times.
 For pitches greater than unity, patient doses decrease, but
data must be interpolated so that resolution along the Z axis33

Spiral (Helical) CT
Small and large pitch values:
 A pitch of one yields a contiguous spiral.
 A pitch of two yields an extended spiral.
 A pitch of 1/2 yields an overlapping spiral.
 The principal advantage of spiral CT is its ability to image
a larger volume of tissue in a relatively short time. With
spiral CT, for example, the entire torso can be imaged in
a single breathold.
 Multi-slice CT allows for hundreds of CT images to be
accumulated in a single study, resulting in high patient
doses and massive amounts of digital imaging data.
34

Data Flow in CT
REFERENCE DETECTOR
ADC
PREPROCESSOR
COMPUTER
RAW DATA
CONVOLVED DATA
BACK
PROJECTORRECONSTRUCTED DATA
PROCESSORS
DISK TAPE
DAC CRT DISPLAY

Patient Dose
 The relationship between resolution and dose can
be approximated as:
where: D is the patient dose, s is the signal/noise
ratio (or contrast resolution), e is the spatial
resolution, b is the slice thickness, and a is a
constant.
36
Yassine

Patient Dose
From equation, it is apparent that:
1. A twofold improvement in the signal-to-noise
ratio (contrast resolution) requires a fourfold
increase in patient dose.
2. A twofold improvement in spatial resolution
requires an eightfold increase in patient
dose.
3. A twofold reduction in slice thickness
requires a twofold increase in patient dose.
Yassine 37

Advantages of CT scans
 Images are like “slices”
 CT machines are quite cheap compared with other
scanners (MRI and PET).
 Imaging of soft tissues is improved compared to
conventional X-Ray.
 Digital processing ability
 Spiral CT-single breath hold studies
 CT assists in radiation therapy
 Bone scan package
 Xenon-enhanced CT used for noninvasive
measurement of cerebral blood flow
 Perfusion CT for rapid assessment of acute stroke

Disadvantages of CT
 Still use X rays that can damage healthy tissues (in
large uncontrolled doses).
 Imaging of soft tissues is improved but still not
always as detailed as doctors require.
 Unable to differentiate between tissues with slight
contrast differences < 1%.

medical equipment lec 6 CT scan

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