Transcript Computed Tomography

Computed Tomography
History:
• Godfrey Hounsfield of EMI, LTD
demonstrated the principle for computed
tomography in 1970.
• Alan Cormack developed the mathematics
used to reconstruct the CT images.
• They shared the 1982 Nobel Prize for
physics.
Computed Tomography
• Computed Tomography is the most
significant development in radiology in the
past 40 years.
• MRI and Ultrasound are also significant
developments but they do not use x-ray to
produce the image.
• The x-ray tube spins around the patient.
Basic CT Principles
• Instead of film, radiation detectors
measure the radiation attenuation as the
beam passes through the body.
• The detectors are connected to a
computer that uses algorithms to process
the data into useful images that are then
recorded on film and viewed on a
computer monitor.
Basic CT Principles
• Conventional
tomography has the
image parallel to the
long axis of the body.
This is referred to as
Axial Tomography.
Basic CT Principles
• Computed
Tomography has the
x-ray tube move
across the axis so the
image is called a
transverse image or
one perpendicular
to the long axis of
the body.
Computed Tomography
Development
• Computed tomography has gone through
five major design advancements since
1970
• Each development improved both scan
time and resolution or image quality.
• Scan time have been reduced from 5
minutes to 50 ms.
• First scanner used a very tightly collimated
pencil beam.
First Generation CT Scanner
• Pencil Beam
• Translate-Rotate
Design
• 180 one degree
images or
translations.
• One or two detectors.
• 5 minutes scan time
Second Generation CT Scanner
• Translate-Rotate
• Fan beam collimation
so there is more
scatter radiation.
• 5 to 30 detectors
• 10 degrees
/translation 18 per
scan.
• 30 second scan times
• Faster scan time
Third Generation CT Scanner
• Rotate-Rotate
• Fan shaped beam of
30 to 60° for full
patient coverage.
• Constant Source to
detector distance due
to curvilinear detector
array.
Third Generation CT Scanner
• If one detector fails, a
ring artifact appears.
• 1 second scan times
• Superior
reconstruction and
resolution.
Fourth Generation CT Scanner
• The tube rotates
around a stationary
ring of detectors.
• Fan beam
• Variable slice
thickness with pre
and post patient
collimation.
Fourth Generation CT Scanner
• As many as 8000
detectors.
• 1 second scan time.
• Auto-detector
calibration so no ring
artifact.
• High radiation dose
compared to earlier
scanners.
Fifth Generation
CT Scanner
• This is the latest
generation of CT.
• Allows for continuous
rotation of the tube for
spiral CT.
• 5th Generation also
includes two novel
designs:
Fifth Generation CT Scanner
• Toshiba maintains the
same SID by
wobbling the
detectors.
• Heartscan by Imatron
used an electron
beam instead of x-ray
tube and 50 ms scan
times.
Fifth Generation CT Scanner
• Spiral CT scanners
allow for contiguous
or even overlapping
data acquisition.
• As the tube spins, the
table moves.
• On earlier units, the
table moved between
scans.
Spiral CT Scanner
• Spiral CT is made
possible by slip-ring
technology. The tube
can continuously
rotate 360 degrees,
where it must stop
after each rotation
with conventional CT.
Spiral CT Scanner
• The detector array
may contain as many
as 14,600 detectors
that are 1.25mm
wide.
• This allows multiple
slice to be made with
one scan and more
tissue volume to be
imaged.
Benefits of Spiral CT
• Less motion artifacts
• Improve lesion detection because the
reconstructed image can be at arbitrary
intervals.
• Reduced partial volume because of
overlapping reconstruction intervals.
• Reduced scan time.
Benefits of Spiral CT
• Advances in
computer processing
allows for multi-planar
reconstruction and
even 3D
reconstruction.
Basic CT Scanner Components
• Gantry includes: the
– Pedestal or table
– Tube, Collimators,
Detectors & High
Voltage Generator
– Mechanical Supports
• Operators Console
• Computer
Basic CT Scanner Components
• Multi-format laser
camera using either
dry chemical images
or conventional laser
film.
• Viewing station for
radiologist (optional)
CT Components
• Table, pedestal or couch holds patient
and is motor driven to move the patient
into the scanner at the correct rate and
distance.
• X-ray tube with very high heat capacity,
measured in millions of heat units.
Two Collimators in CT
• Prepatient collimator
determines slice
thickness
• Predetector
collimators reduce
scatter radiation to
improve contrast.
Large Computer
• A very large and fast
computer is needed
to perform over
250,000 calculations
per image.
• Newer scanners use
an array processor so
the calculations are
done simultaneously.
CT Image Characteristic
• Image matrix: Original EMI format was
80 x 80 so there were 6400 cells of
information called pixels.
• Today the format is 512 x 512 resulting in
262,144 pixels.
• The numerical number in each pixel is a
CT number or Hounsfeld Number.
CT Image Characteristic
• CT number or Hounsfeld
Number represents the
tissue volume in the pixel.
• Field of View (FOV)is the
diameter of the
reconstructed image. As
the FOV increases, the
size of the pixel
increases.
• Voxel: is the square of
the matrix times the
thickness of the slice.
Hounsfeld or CT Number
• The precise CT number is related to the
attenuation of the tissue contained in the voxel.
• Bone = +1000
• Muscle= +50
• Water= 0
• Fat=-100
• Lung= -200
• Air = -1000
Image Quality
• Spatial Resolution: The motion of CT
tends to blur the image compared to the
actual object. (The ability to discriminate objects)
• The ability of the scanner to reproduce
high contrast or sharp edges (edge
response function) is measured as
Modulation Transfer Function (MTF).
(MTF: is a measure of the ability of the scanner to reproduce high
contrast or sharp edges.)
(Image contrast: the intensity of image produced)
Image Quality
• The best possible resolution is equal to the
pixel size. In terms of line pairs, 1 would
be two pixels.
• Items that impact spatial resolution include
collimation, detector size and
concentration and the mechanical gantry
control. Much like conventional
radiography.
Image Quality
• Contrast Resolution: The ability to
distinguish one soft tissue from another is
contrast resolution. This is where CT
excels.
• The absorption or attenuation
characteristics is affected by the atomic
number and the mass density of the
tissue.
Contrast Resolution
• Conventional
Radiography has
relatively poor
contrast resolution.
• CT can amplify the
tissue characteristics
to provide superior
contrast resolution.
Contrast Resolution
• The Contrast Resolution is improved
because of the predetector collimation.
• The contrast resolution for low contrast
tissues is limited by the size and uniformity
of the object and the noise in the system.
• Noise is determined by the number of xrays used by the detector to make the
image.
Computed Tomography
Problems
• CT scans require significantly higher
doses of radiation compared to
conventional radiography. Therefore the
risks of the radiation and the benefits of
the information gained by the scan must
be factored when determining the need for
Computed Tomography.
Computed Tomography
Problems
• If a chest x-ray is equal to the amount of
radiation received in 10 days from our
natural environment, a CT of the brain is
equal to 8 months exposure and CT
abdomen, chest or lumbar spine is equal
to 3 years each.
• Do they mention this when they advertise
total body CT scanning?
Computed Tomography
Problems
• Computed Tomography equipment are
expensive and have high service costs.
• Computed Tomography is expensive for
the patient or insurance. As much as
$1,000 per exam. HMO’s require
preauthorization
End of Lecture