Transcript 35.CT Physics Module H

Module H
Computed Tomography
Physics, Instrumentation,
and Imaging
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Conventional CT
Slice-by-slice
One row of detectors
Patient moves through gantry according to
slice parameters established by the
Technologist.
10 x 10 means the couch will travel 10 mm
after each 10 mm slice is imaged.
Conventional CT is defined as contiguous
acquisition
Conventional CT
10 x 5 would be a 10 mm slice would be
imaged every 5mm’s.
With the above protocol
Overlapping Slices cause the
tissue within the overlapped area
to be irradiated TWICE.
Conventional CT
Images acquired slice-by-slice
X-ray tube rotates around the patient
CT detectors (a single-row) capture the xray attenuation measurements.
CT couch increments to the location of the
next slice location
Filtered-back-projection used a
reconstruction algorithm
Spiral CT
Spiral CT developed in 1989
Also called Helical CT
Helical scanning acquires data in volume
rather than slice-by-slice
X-ray tube travels in a continuous
rotational scanning method
Slip-ring design
Slip Ring design
Electromechanical devices that have
circular electrical conductive rings
Conductive rings transmit energy across a
rotating surface.
Slip-ring eliminated the cable wrap-round
process of conventional CT
Generator is located inside the gantry
Slip Ring designs
Slip Ring Disk
Conductive rings are
positioned in
concentric circles to
lie within the
rotational plane
Slip Ring Cylinder
Conductive rings are
placed along the axis
of rotation, forming a
cylinder
Brushes
Brushes are used to transmit electrical
power to the CT scanner components
Brushes glide in contact groves along the
slip-ring
Two types of Brushes
– Wire
– composite
Wire / Composite Brushes
Wire Brush
Conductive wire
One or more wires
arranged so as to
function as a cantilever
spring
2 brushes per ring are
often used to increase
communication reliability
or current-carrying
capacity
Composite Brush
A block of conductive
material used as a sliding
contact
– Silver-graphite alloy
spring designs include:
– cantilever
– constant- force
– compression
Two brushes per ring are
often used
Low-Voltage / High-Voltage
Low-voltage slip ring
scanners
480 AC power
Low-voltage brushes
The slip ring provides
power to the high voltage
transformer
Generator, tube, and
other controls rotate
continuously
High-voltage slip ring
scanners
AC delivers power to the
high-voltage generator,
located in the gantry
High-voltage generator
supplies voltage to the
slip-ring
High-voltage form the slip
ring is transferred to the
x-ray tube.
In High-voltage slip ring
scanners the generator
does not rotate
Types of Spiral CT Scanners
Single-Slice Volume CT introduced 1990
– Also called Single detector-row spiral CT
– Patient is translated at a constant speed
through the gantry
– Tube makes a continuous exposure (HU)
– Faster than conventional CT
– Data from patient received by the detectors
– Detectors send the data to array processors
– Volume data (Not single-image data like in
conventional CT)
Volume Data
Spiral CT scanners send volume
data to the array processor
Standard algorithms can not be
used for image reconstruction
Single slice volume CT
Single slice volume CT have a 1-D
detector array because of the single-row of
detectors.
Fan-beam profile is used in both
conventional CT and single slice volume
CT’s.
There are pre patient and post patient
collimators to deliver a constant beamwidth at the detectors.
Single slice volume CT
Single-slice volume CT scanners:
– Analytic reconstruction algorithms were
developed to improve the imaging limitations
of back projection.
Filtered back projection reconstruction algorithms
(also called Convolution method)
Convolution removes the blurring (see Seeram page
105-108, Fig.6-11, 6-12)
– Single-slice volume scanning uses 360degree linear interpolation algorithm (LIA)
Single slice volume
In order to improve poor image quality and
artifact production, created by the 360degree LIA, the 180-degree LIA is applied.
The 180-degree LIA
– Maintains the detail (slice sensitivity and
spatial resolution)
– Creates more noise in the image
Single slice volume CT
In CT NO slices are produced by the
scanning process!
Images are produced by the Computer.
CT is a digital process
Single slice volume CT
Advantages
Improvements over
conventional CT
include speed and
volume coverage
Disadvantages
Long exposures create
high heat units (HU)
High heat units limited
the volume of scan
area (z-axis)
High pitch for volume
coverage degraded
slice sensitivity profile
(detail).
Single slice volume CT
Single slice volume CT have a 1-D
detector array because of the single-row of
detectors.
Fan-beam profile is used in both
conventional CT and single slice volume
CT’s.
There are pre patient and post patient
collimators to deliver a constant beamwidth at the detectors.
Pitch
….defined as the distance of couch top
travel, per one revolution of the x-ray tube,
divided by the slice thickness or the beam
collimation.
Normal pitch is 1:1
– The table travels 1cm with a 1cm slice
thickness or 5mm with a 5mm slice thickness
etc…….
Multi-slice CT scanners
Developed late 1990’s
Also called Multi-detector-row CT
Had 4 detector rows (quad)
Cone-beam profile is used in these
scanners
Detector rows keep increasing…..
8, 16, 32, and 64
Recently developed 128 detector-row CT scanner
Multi-slice or Multi-detector CT
scanners
Because more detectors are available at
any given point in the scanning area, more
data can be collected.
Therefore, more data can be derived from
the anatomy covered
Multi-slice or Multi-detector CT
scanners / Pitch
The definition of pitch for multi-slice
scanners differs from that in single slice
CT.
Definition varies according to
manufacturer. MDCT Multi-Detector
Computer Tomography can be:
Slice thickness when using single collimation, or
Detector-row collimation, or
Beam-width at the center of rotation
Multi-slice or Multi-detector CT
scanners / Slice Thickness
There is a difference in slice-thickness
determination for MDCT.
The slice thickness is determined by the:
Beam-width (BW)
Pitch
Shape and width of the reconstruction filter in the
Z-axis
Detector row and beam collimation
relationship
d = detector-row collimation
D = beam collimation
N = number of detector rows
d(mm) = D(mm)/N
Image reconstruction for MDCT
Several choices based on one of the
following:
Interlaced Sampling
Longitudinal Interpolation by z-axis filtering
Fan beam reconstruction
Multi-slice/Multi-detector
advantages
Greater volume coverage (more detectors collecting data
at one time)
Allows sub-second scanning (anatomy is over each
detector row for less than a second)
Greater detail (increased spatial resolution)
More efficient use of the beam profile
The acquisition of sub-millimeter slices (currently .5mm)
Reduction in radiation dose to the patient
Greater accuracy in needle localizations for CT guided
biopsies)
More effective use of IV contrast media
Current applications
Pulmonary embolus protocols (.5mm to 1.25mm slices)
Trauma protocols with increased volume coverage
More detailed and accurate 3-D reformations, surface shading and
volume rendering
Multiplanar reconstructions
Computed Tomography Angiography (CTA)
CT Fluoroscopy
CT Interactive Cine
CT Colonography
Virtual Reality CT (colonography, cystography, and endography)
Calcium Scoring
ECG/EKG Gated Cardiac Studies
Coronary Artery CT